Search

The microtubule-associated protein tau (tau) forms hyperphosphorylated aggregates in the brains of tauopathy patients that can be pathologically and biochemically defined as distinct tau strains. Recent studies show that these tau strains exhibit strain-specific biological activities, also referred to as pathogenicities, in the tau spreading models. Currently, the specific pathogenicity of human-derived tau strains cannot be fully recapitulated by synthetic tau preformed fibrils (pffs), which are generated from recombinant tau protein. Reproducing disease-relevant tau pathology in cell and animal models necessitates the use of human brain-derived tau seeds. However, the availability of human-derived tau is extremely limited. Generation of tau variants that can mimic the pathogenicity of human-derived tau seeds would significantly extend the scale of experimental design within the field of tauopathy research. Previous studies have demonstrated that in vitro seeding reactions can amplify the beta-sheet structure of tau protein from a minute quantity of human-derived tau. However, whether the strain-specific pathogenicities of the original, human-derived tau seeds are conserved in the amplified tau strains has yet to be experimentally validated. Here, we used biochemically enriched brain-derived tau seeds from Alzheimer’s disease (AD), corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP) patient brains with a modified seeding protocol to template the recruitment of recombinant 2N4R (T40) tau in vitro. We quantitatively interrogated efficacy of the amplification reactions and the pathogenic fidelity of the amplified material to the original tau seeds using recently developed sporadic tau spreading models. Our data suggest that different tau strains can be faithfully amplified in vitro from tau isolated from different tauopathy brains and that the amplified tau variants retain their strain-dependent pathogenic characteristics. Supplementary Information The online version contains supplementary material available at 10.1007/s00401-020-02253-4.

Aim The heterogeneity in the distribution and morphological features of TAR DNA-binding protein-43 (TDP-43) pathology in the brains of frontotemporal lobar degeneration (FTLD-TDP) patients and their different clinical manifestations suggest that distinct pathological TDP-43 strains could play a role in this heterogeneity between different FTLD-TDP subtypes (A-E). Our aim was to evaluate the existence of distinct TDP-43 strains in the brains of different FTLD-TDP subtypes and characterise their specific seeding properties in vitro and in vivo. Methods and results We used an inducible stable cell line expressing a mutant cytoplasmic TDP-43 (iGFP-NLSm) to evaluate the seeding properties of distinct pathological TDP-43 strains. Brain-derived TDP-43 protein extracts from FTLD-TDP types A (n = 6) and B (n = 3) cases induced the formation of round/spherical phosphorylated TDP-43 aggregates that morphologically differed from the linear and wavy wisps and bigger heterogeneous filamentous (skein-like) aggregates induced by type E (n = 3) cases. These morphological differences correlated with distinct biochemical banding patterns of sarkosyl-insoluble TDP-43 protein recovered from the transduced cells. Moreover, brain-derived TDP-43 extracts from type E cases showed higher susceptibility to PK digestion of full-length TDP-43 and the most abundant C-terminal fragments that characterise type E extracts. Finally, we showed that intracerebral injections of different TDP-43 strains induced a distinctive morphological and subcellular distribution of TDP-43 pathology and different spreading patterns in the brains of CamKIIa-hTDP-43NLSm Tg mice. Conclusions We show the existence of distinct TDP-43 strains in the brain of different FTLD-TDP subtypes with distinctive seeding and spreading properties in the brains of experimental animal models.

Alpha-synuclein (aSyn) participates in synaptic vesicle trafficking and synaptic transmission, but its misfolding is also strongly implicated in Parkinson’s disease (PD) and other neurodegenerative disorders known as synucleinopathies where misfolded aSyn accumulates in different regions of the central and peripheral nervous systems. Although increased aSyn expression levels or altered aggregation propensities likely underlie familial PD with SNCA amplification or mutations, the majority of synucleinopathies arise sporadically, indicating that disease can develop under normal levels of wildtype aSyn. We report here the development and characterization of a mouse line expressing an aSyn-GFP fusion protein under the control of native Snca regulatory elements. Regional and subcellular localization of the aSyn-GFP fusion protein in brains and peripheral tissues of knock-in (KI) mice are indistinguishable from that of wildtype littermates. Importantly, similar to wildtype aSyn, aSyn-GFP disperses from synaptic vesicles upon membrane depolarization, indicating that the tag does not alter normal aSyn dynamics at synapses. In addition, intracerebral injection of aSyn pre-formed fibrils into KI mice induced the formation of aSyn-GFP inclusions with a distribution pattern similar to that observed in wildtype mice, albeit with attenuated kinetics due to the GFP tag. We anticipate that this new mouse model will facilitate in vitro and in vivo studies requiring in situ detection of endogenous aSyn, therefore providing new insights into aSyn function in health and disease.Significance StatementAlpha-synuclein (aSyn) participates in synaptic vesicle function and represents a major component of the Lewy pathology found in Parkinson’s and related neurodegenerative diseases. The function of aSyn and the sequence of events leading to its aggregation and neurotoxicity are not fully understood. Here we present a new mouse model in which Enhanced Green Fluorescence Protein (GFP) has been knocked-in at the C-terminal of the Snca gene. The resulting fusion protein shows identical expression and localization to that of wildtype animals, is functional, and is incorporated into pathological aggregates in vitro and in vivo. This new tool allows for monitoring aSyn under a variety of physiological and pathological conditions, and may uncover additional insights into its function and dysfunction.

In Lewy body diseases-including Parkinson's disease, without or with dementia, dementia with Lewy bodies, and Alzheimer's disease with Lewy body co-pathology 1 -α-synuclein (α-Syn) aggregates in neurons as Lewy bodies and Lewy neurites 2 . By contrast, in multiple system atrophy α-Syn accumulates mainly in oligodendrocytes as glial cytoplasmic inclusions (GCIs) 3 . Here we report that pathological α-Syn in GCIs and Lewy bodies (GCI-α-Syn and LB-α-Syn, respectively) is conformationally and biologically distinct. GCI-α-Syn forms structures that are more compact and it is about 1,000-fold more potent than LB-α-Syn in seeding α-Syn aggregation, consistent with the highly aggressive nature of multiple system atrophy. GCI-α-Syn and LB-α-Syn show no cell-type preference in seeding α-Syn pathology, which raises the question of why they demonstrate different cell-type distributions in Lewy body disease versus multiple system atrophy. We found that oligodendrocytes but not neurons transform misfolded α-Syn into a GCI-like strain, highlighting the fact that distinct α-Syn strains are generated by different intracellular milieus. Moreover, GCI-α-Syn maintains its high seeding activity when propagated in neurons. Thus, α-Syn strains are determined by both misfolded seeds and intracellular environments.

Alzheimer’s disease (AD) is characterized by extracellular amyloid-β (Aβ) plaques and intracellular tau inclusions. However, the exact mechanistic link between these two AD lesions remains enigmatic. Through injection of human AD-brain-derived pathological tau (AD-tau) into Aβ plaque–bearing mouse models that do not overexpress tau, we recapitulated the formation of three major types of AD-relevant tau pathologies: tau aggregates in dystrophic neurites surrounding Aβ plaques (NP tau), AD-like neurofibrillary tangles (NFTs) and neuropil threads (NTs). These distinct tau pathologies have different temporal onsets and functional consequences on neural activity and behavior. Notably, we found that Aβ plaques created a unique environment that facilitated the rapid amplification of proteopathic AD-tau seeds into large tau aggregates, initially appearing as NP tau, which was followed by the formation and spread of NFTs and NTs, likely through secondary seeding events. Our study provides insights into a new multistep mechanism underlying Aβ plaque–associated tau pathogenesis.

Pathological tau aggregates occur in Alzheimer's disease (AD) and other neurodegenerative tauopathies. It is not clearly understood why tauopathies vary greatly in the neuroanatomical and histopathological patterns of tau aggregation, which contribute to clinical heterogeneity in these disorders. Recent studies have shown that tau aggregates may form distinct structural conformations, known as tau strains. Here, we developed a novel model to test the hypothesis that cell-to-cell transmission of different tau strains occurs in nontransgenic (non-Tg) mice, and to investigate whether there are strain-specific differences in the pattern of tau transmission. By injecting pathological tau extracted from postmortem brains of AD (AD-tau), progressive supranuclear palsy (PSP-tau), and corticobasal degeneration (CBD-tau) patients into different brain regions of female non-Tg mice, we demonstrated the induction and propagation of endogenous mouse tau aggregates. Specifically, we identified differences in tau strain potency between AD-tau, CBD-tau, and PSP-tau in non-Tg mice. Moreover, differences in cell-type specificity of tau aggregate transmission were observed between tau strains such that only PSP-tau and CBD-tau strains induce astroglial and oligodendroglial tau inclusions, recapitulating the diversity of neuropathology in human tauopathies. Furthermore, we demonstrated that the neuronal connectome, but not the tau strain, determines which brain regions develop tau pathology. Finally, CBD-tau- and PSP-tau-injected mice showed spatiotemporal transmission of glial tau pathology, suggesting glial tau transmission contributes to the progression of tauopathies. Together, our data suggest that different tau strains determine seeding potency and cell-type specificity of tau aggregation that underlie the diversity of human tauopathies.SIGNIFICANCE STATEMENTTauopathies show great clinical and neuropathological heterogeneity, despite the fact that tau aggregates in each disease. This heterogeneity could be due to tau aggregates forming distinct structural conformations, or strains. We now report the development of a sporadic tauopathy model to study human tau strains by intracerebrally injecting nontransgenic mice with pathological tau enriched from human tauopathy brains. We show human tau strains seed different types and cellular distributions of tau neuropathology in our model that recapitulate the heterogeneity seen in these human diseases.

Tauopathies are characterized by abnormal accumulation of tau protein in neurons and glia. In Alzheimer’s disease (AD), tau aggregates in neurons, while in corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP), tau also aggregates in astrocytes and oligodendrocytes. We previously demonstrated that human CBD and PSP tauopathy lysates (CBD-tau and PSP-tau) contain distinct tau strains that propagate neuronal and glial tau aggregates in nontransgenic (nonTg) mouse brain. Yet the mechanism of glial tau transmission is unknown. Here, we developed a novel mouse model to knock down tau in neurons to test for glial tau transmission. While oligodendroglial tau pathology propagated across the mouse brain in the absence of neuronal tau pathology, astrocytic tau pathology did not. Oligodendroglial tau aggregates propagated along white matter tracts independently of neuronal axons, and resulted in oligodendrocyte cell loss. Thus, glial tau pathology has significant functional consequences independent of neuronal tau pathology.

Intracerebral inoculation of tau fibrils from AD brains results in the induction and propagation of tau inclusions in WT mice., Filamentous tau aggregates are hallmark lesions in numerous neurodegenerative diseases, including Alzheimer’s disease (AD). Cell culture and animal studies showed that tau fibrils can undergo cell-to-cell transmission and seed aggregation of soluble tau, but this phenomenon was only robustly demonstrated in models overexpressing tau. In this study, we found that intracerebral inoculation of tau fibrils purified from AD brains (AD-tau), but not synthetic tau fibrils, resulted in the formation of abundant tau inclusions in anatomically connected brain regions in nontransgenic mice. Recombinant human tau seeded by AD-tau revealed unique conformational features that are distinct from synthetic tau fibrils, which could underlie the differential potency in seeding physiological levels of tau to aggregate. Therefore, our study establishes a mouse model of sporadic tauopathies and points to important differences between tau fibrils that are generated artificially and authentic ones that develop in AD brains.

Motor neurons (MNs) are the neuronal class that is principally affected in amyotrophic lateral sclerosis (ALS), but it is widely known that individual motor pools do not succumb to degeneration simultaneously. Because >90% of ALS patients have an accumulation of cytoplasmic TDP-43 aggregates in postmortem brain and spinal cord (SC), it has been suggested that these inclusions in a given population may trigger its death. We investigated seven MN pools in our new inducible rNLS8 transgenic (Tg) mouse model of TDP-43 proteinopathy and found striking differences in MN responses to TDP-43 pathology. Despite widespread neuronal expression of cytoplasmic human TDP-43, only MNs in the hypoglossal nucleus and the SC are lost after 8 weeks of transgene expression, whereas those in the oculomotor, trigeminal, and facial nuclei are spared. Within the SC, slow MNs survive to end stage, whereas fast fatigable MNs are lost. Correspondingly, axonal dieback occurs first from fast-twitch muscle fibers, whereas slow-twitch fibers remain innervated. Individual pools show differences in the downregulation of endogenous nuclear TDP-43, but this does not fully account for vulnerability to degenerate. After transgene suppression, resistant MNs sprout collaterals to reinnervate previously denervated neuromuscular junctions concurrently with expression of matrix metalloproteinase 9 (MMP-9), a marker of fast MNs. Therefore, although pathological TDP-43 is linked to MN degeneration, the process is not stochastic and mirrors the highly selective patterns of MN degeneration observed in ALS patients. SIGNIFICANCE STATEMENT Because TDP-43 is the major pathological hallmark of amyotrophic lateral sclerosis (ALS), we generated mice in which mutant human TDP-43 expression causes progressive neuron loss. We show that these rNLS8 mice have a pattern of axonal dieback and cell death that mirrors that often observed in human patients. This finding demonstrates the diversity of motor neuron (MN) populations in their response to pathological TDP-43. Furthermore, we demonstrate that resistant MNs are able to compensate for the loss of their more vulnerable counterparts and change their phenotype in the process. These findings are important because using a mouse model that closely models human ALS in both the disease pathology and the pattern of degeneration is critical to studying and eventually treating progressive paralysis in ALS patients.

In order to treat progressive paralysis in ALS patients, it is critical to develop a mouse that closely models human ALS in both pathology and also in the timing of these events. We have recently generated new TDP-43 bigenic mice (called rNLS8) with doxycycline (Dox)-suppressible expression of human TDP-43 (hTDP-43) harboring a defective nuclear localization signal (hTDP-43∆NLS) under the control of the NEFH promoter. Our previous studies characterized the pathology and disease course in young rNLS8 mice following induction of neuronal hTDP-43ΔNLS. We now seek to examine if the order and timing of pathologic events are changed in aged mice. We found that the expression of hTDP-43∆NLS in 12+ month old mice did not accelerate the appearance of neuromuscular abnormalities or motor neuron (MN) death in the lumbar spinal cord (SC), though disease progression was accelerated. However, following suppression of the transgene, important differences between young and aged rNLS8 mice emerged in functional motor recovery. We found that recovery was incomplete in aged mice relative to their younger treatment matched counterparts based on gross behavioral measures and physiological recordings from the animals’ gastrocnemius (GC) muscles, despite muscle reinnervation by surviving MNs. This is likely because the reinnervation most often only resulted in partial nerve and endplate connections and the muscle’s junctional folds were much more disorganized in aged rNLS8 mice. We believe that these studies will be an important basis for the future design and evaluation of therapies designed to slow denervation and promote re-innervation in adult ALS patients.

SummaryInclusions composed of α-synuclein (α-syn), i.e., Lewy bodies (LBs) and Lewy neurites (LNs), define synucleinopathies including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Here, we demonstrate that preformed fibrils generated from full-length and truncated recombinant α-syn enter primary neurons, probably by adsorptive-mediated endocytosis, and promote recruitment of soluble endogenous α-syn into insoluble PD-like LBs and LNs. Remarkably, endogenous α-syn was sufficient for formation of these aggregates, and overexpression of wild-type or mutant α-syn was not required. LN-like pathology first developed in axons and propagated to form LB-like inclusions in perikarya. Accumulation of pathologic α-syn led to selective decreases in synaptic proteins, progressive impairments in neuronal excitability and connectivity, and, eventually, neuron death. Thus, our data contribute important insights into the etiology and pathogenesis of PD-like α-syn inclusions and their impact on neuronal functions, and they provide a model for discovering therapeutics targeting pathologic α-syn-mediated neurodegeneration.

Cytoplasmic inclusions containing alpha-synuclein (alpha-Syn) fibrils, referred to as Lewy bodies (LBs), are the signature neuropathological hallmarks of Parkinson's disease (PD). Although alpha-Syn fibrils can be generated from recombinant alpha-Syn protein in vitro, the production of fibrillar alpha-Syn inclusions similar to authentic LBs in cultured cells has not been achieved. We show here that intracellular alpha-Syn aggregation can be triggered by the introduction of exogenously produced recombinant alpha-Syn fibrils into cultured cells engineered to overexpress alpha-Syn. Unlike unassembled alpha-Syn, these alpha-Syn fibrils "seeded" recruitment of endogenous soluble alpha-Syn protein and their conversion into insoluble, hyperphosphorylated, and ubiquitinated pathological species. Thus, this cell model recapitulates key features of LBs in human PD brains. Also, these findings support the concept that intracellular alpha-Syn aggregation is normally limited by the number of active nucleation sites present in the cytoplasm and that small quantities of alpha-Syn fibrils can alter this balance by acting as seeds for aggregation.

Aggregated alpha-synuclein (alpha-syn) fibrils form Lewy bodies (LBs), the signature lesions of Parkinson's disease (PD) and related synucleinopathies, but the pathogenesis and neurodegenerative effects of LBs remain enigmatic. Recent studies have shown that when overexpressed in Saccharomyces cerevisiae, alpha-syn localizes to plasma membranes and forms cytoplasmic accumulations similar to human alpha-syn inclusions. However, the exact nature, composition, temporal evolution, and underlying mechanisms of yeast alpha-syn accumulations and their relevance to human synucleinopathies are unknown. Here we provide ultrastructural evidence that alpha-syn accumulations are not comprised of LB-like fibrils, but are associated with clusters of vesicles. Live-cell imaging showed alpha-syn initially localized to the plasma membrane and subsequently formed accumulations in association with vesicles. Imaging of truncated and mutant forms of alpha-syn revealed the molecular determinants and vesicular trafficking pathways underlying this pathological process. Because vesicular clustering is also found in LB-containing neurons of PD brains, alpha-syn-mediated vesicular accumulation in yeast represents a model system to study specific aspects of neurodegeneration in PD and related synucleinopathies.

TAR DNA-binding protein 43 (TDP-43) is a major pathological protein of sporadic and familial frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions (FTLD-U) with or without motor neuron disease (MND). Thus, TDP-43 defines a novel class of neurodegenerative diseases called TDP-43 proteinopathies. We performed ubiquitin and TDP-43 immunohistochemistry on 193 cases of familial and sporadic FTLD with or without MND. On selected cases, immunoelectron microscopy and biochemistry were performed. Clinically defined frontotemporal dementias (FTDs) included four groups: 1) familial FTD with mutations in progranulin (n = 36), valosin-containing protein (n = 5), charged multivesicular body protein 2B (n = 4), and linked to chromosome 9p (n = 7); 2) familial cases of FTD with unknown gene association (n = 29); 3) sporadic FTD (n = 72); and 4) familial and sporadic FTD with MND (n = 40). Our studies confirm that the spectrum of TDP-43 proteinopathies includes most cases of sporadic and familial FTLD-U with and without MND and expand this disease spectrum to include reported families with FTD linked to chromosome 9p but not FTD with charged multivesicular body protein 2B mutations. Thus, despite significant clinical, genetic, and neuropathological heterogeneity of FTLD-U, TDP-43 is a common pathological substrate underlying a large subset of these disorders, thereby implicating TDP-43 in novel and unifying mechanisms of FTLD pathogenesis.

Fragmentation of the neuronal Golgi apparatus (GA) was reported in amyotrophic lateral sclerosis (ALS), corticobasal degeneration, Alzheimer's and Creutzfeldt-Jacob disease, and in spinocerebelar ataxia type 2 (SCA2). In transgenic mice expressing the G93A mutant of Cu/Zn superoxide dismutase (SOD1) of familial ALS (fALS), fragmentation of the GA of spinal cord motor neurons and aggregation of mutant protein were detected months before the onset of paralysis. Moreover, cells that expressed the G93A and G85R mutants of SOD1 showed fragmentation of the GA and decreased viability without apoptosis. We summarize here mechanisms involved in Golgi fragmentation implicating: (a) the dysregulation by mutant SOD1of the microtubule-destabilizing protein Stathmin, (b) the disruption by mutant SOD1of the neuronal cytoplasmic dynein, (c) the coprecipitation of mutant SOD1 with Hsp25 and Hsp27, (d) the reduction of detyrosinated microtubules by aggregated tau which resulted in non-apoptotic cell death and (e) the disruption by mutant growth hormone of the trafficking from the rough endoplasmic reticulum to the GA. The data indicate that neuronal Golgi fragmentation is an early and probably irreversible lesion in neurodegeneration, caused by a variety of mechanisms. Golgi fragmentation is not secondary to apoptosis but it may "trigger" apoptosis.

Gain-of-function mutations of the Cu/Zn superoxide dismutase (SOD1) gene cause dominantly inherited familial amyotrophic lateral sclerosis. The identification of differentially regulated proteins in spinal cords of paralyzed mice expressing SOD1(G93A) may contribute to understanding mechanisms of toxicity by mutant SOD1. Protein profiling showed dysregulation of Stathmin with a marked decrease of its most acidic and phosphorylated isoform, and up-regulation of heat shock proteins 25 and 27, peroxiredoxin 6, phosphatidylinositol transfer protein-alpha, apolipoprotein E, and ferritin heavy chain. Stathmin accumulated in the cytoplasm of 30% of spinal cord motor neurons with fragmented Golgi apparatus. Overexpression of Stathmin in HeLa cells was associated with collapse of microtubule networks and Golgi fragmentation. These results, together with the decrease of one Stathmin isoform, suggest a role of the protein in Golgi fragmentation. Mutant SOD1 co-precipitated and co-localized with Hsp25 in neurons and astrocytes. Mutant SOD1 may thus deprive cells of the anti-apoptotic and other protective activities of Hsp25. Astrocytes contained peroxiredoxin 6, a unique nonredundant antioxidant. The up-regulation of peroxiredoxin 6 probably constitutes a defense to oxidative stress induced by SOD1(G93A). Direct effects of SOD1(G93A) or sequential reactions triggered by the mutant may cause the protein changes.

The Golgi apparatus-complex (GA), is a key organelle involved in several posttranslational modifications of polypeptides destined for lysosomes, plasma membranes and secretion. As reported from this laboratory, certain astrocytes in rat brain contain cisternae of the GA not only in perikarya, but also in processes. In order to further investigate which type of astrocytes contain GA in processes we conducted the present study using primary cultures of rat astrocytes and organelle specific antibodies against the GA and the rough endoplasmic reticulum (RER). While the perikarya of all cells contained elements of the GA, only a single process of a subset of type I astrocytes, negative to antibodies A2B5 and HNK-1, contained GA. In contrast, elements of the RER were found within perikarya and all processes. In order to confirm that the immunostained structures in processes indeed represent the GA, we exposed cultures to Brefeldin A (BFA), a secretion blocker which disperses the GA and redistributes it to the RER. We observed that BFA disrupted the GA of both perikarya and processes. However, astrocytes were resistant to prolonged incubations with BFA, while a similar treatment killed cultured fibroblasts and PC-12 cells. Furthermore, in astrocytes exposed to BFA for several days, the delicate network of glial fibrillary acidic protein (GFAP), was replaced by large perinuclear masses of the protein. These observations demonstrate that a subset of type I astrocytes have a single process with elements of the GA. We suggest that this specialization of the GA may be related to yet unrecognized secretory or protein processing functions of these cells. The resistance of astrocytes to BFA and the striking changes in their cytoskeleton induced by the drug, may contribute to studies on the mechanism(s) of action of BFA.

Mice expressing the G93A and other mutations of Cu,Zn superoxide dismutase (SOD1G93A) are valid models for the familial form of amyotrophic lateral sclerosis (FALS) with SOD1 mutations and, probably, for sporadic ALS. Adult mice become progressively paralyzed and show most of the histopathological lesions reported in sporadic ALS, i.e. neuronal loss, astrogliosis, ubiquitin and Lewy body-like inclusions, dystrophic axons and fragmentation of the Golgi apparatus (GA) of motor neurons. In transgenic mice, the mutant protein and ubiquitin aggregate within pathological 13 nm thick filaments [Stieber A, Gonatas JO, Gonatas NK. J Neurol Sci 2000;173:53–62]. This immunocytochemical and quantitative study of mice expressing SOD1G93A establishes the chronological order and cellular localization of aggregates of SOD1 and their correlation with fragmentation of the GA. Young asymptomatic mice expressing SOD1G93A showed aggregates of mutant SOD1 within neurites, prior to the detection of SOD1 in the perikarya of spinal cord motor neurons and astrocytes. Both dendrites and the periaxonal oligodendroglial cytoplasm, surrounding atrophic axons, contained SOD1 as revealed by immunoelectron microscopy The perikarya of a small percentage of spinal cord motor neurons contained both fragmented GA and aggregates of SOD1; however, about 50% of motor neurons with fragmented GA did not contain SOD1 in the perikaryon, suggesting that aggregates of mutant protein may not directly cause fragmentation of the GA. The mechanism of the putative toxic effect by the mutant protein remains to be clarified. The isolation and biochemical characterization of the filamentous aggregates of mutant protein and ubiquitin from spinal cords of transgenic mice expressing mutations of the SOD1 gene may offer clues on pathogenetic mechanisms.

In this study we examined the Golgi apparatus of avian notochord transplants excised from 2-day-old (E2) chick embryos and grafted isochronically into a chick host either in a medial-ventral position, next to the host notochord, or in a superficial position under the ectoderm laterally or dorsally to the neural tube. The operated embryos were examined from E2 to E8. The diameters, the cytoplasmic vacuolization and the immunostained Golgi apparatus were identical between the endogenous and ventrally grafted notochords, as well as between host-and superficially transplanted notochords when observed at E2. In contrast, from E4 to E8, the size of the notochords grafted dorsally or laterally to the neural tube was significantly smaller than the host, while the cytoplasmic vacuolization and the degree of fragmentation of the Golgi apparatus were significantly less than in the host notochords. These results show that environmental and position-specific factors influence the developmental program and the secretory activity of the notochordal cells.

The Golgi apparatus (GA) of spinal cord motor neurons is fragmented in sporadic amyotrophic lateral sclerosis (ALS), and in asymptomatic and symptomatic transgenic mice expressing the G93A mutation of the gene of the human Cu,Zn superoxide dismutase, found in certain cases of familial ALS (FALS) [Gonatas NK (1994) Am J Pathol 145:751-761; Mourelatos Z, et al. (1996) Proc Natl Acad Sci USA 93:5472-5477]. A similar fragmentation of the GA has been described in cells treated with microtubule-depolymerizing drugs, where the organelle is functional and contains both Golgi stacks and trans-Golgi network (TGN), the compartment of exit and targeting of proteins processed by the GA. To gain a better definition of the structure of the fragmented neuronal GA in ALS, four cases of sporadic ALS with numerous Bunina bodies in spinal cord motor neurons were stained with antibodies against human TGN and against the lumenal and cytoplasmic domains of MG160, a protein of the medial cisternae of the GA. The fragmented GA was stained with the three antibodies, indicating the presence of both Golgi stacks and TGN. Furthermore, the staining of the fragmented GA by the antiserum against the cytoplasmic domain of MG160 indicates that the fragmentation of the GA is not the result of a terminal and global cytoplasmic lytic event. The Bunina bodies were not stained by the anti-MG160 antibodies, suggesting that they are not derived from the GA. The perikarya of neurons with fragmented GA showed normal immunoreactivity with antibodies against the heavy neurofilament subunit and alpha-tubulin. However, because of the lack of appropriate antibodies the localization of proteins such as spectrin, ankyrin, centractin and others which link the microtubules with the GA were not done. The findings support the hypothesis that, in ALS, the fragmented neuronal GA is functional. Additional work with animal models of ALS may establish whether the fragmentation of the GA is a sign of early degeneration or a compensatory reaction of the injured motor neuron.

MG160, a type I cysteine-rich membrane sialoglycoprotein residing in the medial cisternae of the rat Golgi apparatus, is highly homologous to CFR, a fibroblast growth factor receptor, and ESL-1, an E-selectin ligand located at the cell surface of mouse myeloid cells and recently detected in the Golgi apparatus as well. The mechanism for the transport of MG160 from the Golgi apparatus to the cell surface is unknown. In this study we found that differential processing of the carboxy-terminal cytoplasmic domain (CD), consisting of amino acids Arg1159 Ile Thr Lys Arg Val Thr Arg Glu Leu Lys Asp Arg1171, resulted in the partial transport of the protein to the plasma membrane and filopodia. In Chinese hamster ovary cells (CHO), stably transfected with the entire cDNA encoding MG160, the protein was localized in the Golgi apparatus. However, when the terminal Arg1171 or up to nine distal amino acids were deleted, the protein was distributed to the plasma membrane and filopodia as well as the Golgi apparatus. This report shows that the CD of an endogenous type I Golgi protein is important for its efficient retention and identifies a unique residue preference in this process. Cleavage within the CD of MG160 may constitute a regulatory mechanism for the partial export of the protein from the Golgi apparatus to the plasma membrane and filopodia.

SummaryMany neurodegenerative diseases are characterized by the accumulation of insoluble protein aggregates, including neurofibrillary tangles comprised of tau in Alzheimer’s disease and Lewy bodies composed of α-synuclein in Parkinson’s disease. Moreover, different pathological proteins frequently codeposit in disease brains. To test whether aggregated α-synuclein can directly cross-seed tau fibrillization, we administered preformed α-synuclein fibrils assembled from recombinant protein to primary neurons and transgenic mice. Remarkably, we discovered two distinct strains of synthetic α-synuclein fibrils that demonstrated striking differences in the efficiency of cross-seeding tau aggregation, both in neuron cultures and in vivo. Proteinase K digestion revealed conformational differences between the two synthetic α-synuclein strains and also between sarkosyl-insoluble α-synuclein extracted from two subgroups of Parkinson’s disease brains. We speculate that distinct strains of pathological α-synuclein likely exist in neurodegenerative disease brains and may underlie the tremendous heterogeneity of synucleinopathies.

Dominant mutations of the SOD1 gene encoding Cu,Zn superoxide dismutase have been found in members of certain families with familial amyotrophic lateral sclerosis (ALS). To better understand the contribution of SOD1 mutations in the pathogenesis of familial ALS, we developed transgenic mice expressing one of the mutations found in familial ALS. These animals display clinical and pathological features closely resembling human ALS. Early changes observed in these animals were intra-axonal and dendritic vacuoles due to dilatation of the endoplasmic reticulum and vacuolar degeneration of mitochondria. We have reported that the Golgi apparatus of spinal cord motor neurons in patients with sporadic ALS is fragmented and atrophic. In this study we show that spinal cord motor neurons of transgenic mice for an SOD1 mutation display a lesion of the Golgi apparatus identical to that found in humans with sporadic ALS. In these mice, the stacks of the cisternae of the fragmented Golgi apparatus are shorter than in the normal organelle, and there is a reduction in Golgi-associated vesicles and adjacent cisternae of the rough endoplasmic reticulum. Furthermore, the fragmentation of the Golgi apparatus occurs in an early, presymptomatic stage and usually precedes the development of the vacuolar changes. Transgenic mice overexpressing the wild-type human superoxide dismutase are normal. In familial ALS, an early lesion of the Golgi apparatus of motor neurons may have adverse functional effects, because newly synthesized proteins destined for fast axoplasmic transport pass through the Golgi apparatus.

Aggregation of TDP-43 proteins to form intracellular inclusions is the primary pathology in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) with TDP-43 inclusions (FTLD-TDP). Histologically, in the cerebral cortex and limbic regions of affected ALS and FTLD-TDP patients, these pathologies occur as a variety of cytoplasmic, neuritic and intranuclear TDP-43 inclusions. In the spinal cord and lower brainstem of ALS patients, the lesions form cytoplasmic dashes or complex filamentous and spherical profiles in addition to skein-like inclusions (SLI). Ultrastructurally, the morphology of TDP-43 inclusions is heterogeneous but mainly composed of loose bundles of 10- to 20-nm-diameter straight filaments associated with electron-dense granular material. All of these TDP-43 inclusions are generally described as disordered amorphous aggregations unlike the amyloid fibrils that characterize protein accumulations in neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. We here report that Thioflavin-S positive SLI are present in a subset of ALS cases, while TDP-43 inclusions outside the spinal cord lack the chemical properties of amyloid. Further, we examine the differential enrichment of fibrillar profiles in SLI of ALS cases by TDP-43 immuno-electron microscopy (immuno-EM). The demonstration that pathological TDP-43 can be amyloidogenic in situ suggests the following conclusions: (1) the conformational changes associated with TDP-43 aggregation are more complex than previously thought; (2) Thioflavin-S positive SLI may be composed primarily of filamentous ultrastructures.

We have reported that MG160, an intrinsic membrane sialoglycoprotein of the Golgi apparatus (GA), resides in the medial cisternae of the organelle (Gonatas et al. (1989) J. Biol. Chem. 264, 646–653). In order to resolve the question whether MG160 acquires sialic acid residues in the trans cisternae or trans-Golgi network (TGN) prior to its retrograde transport, we have examined the effects of brefeldin A (BFA) on the post-translational processing of MG160, and the distribution of internalized wheat germ agglutinin covalently linked with HRP (WGA-HRP), which labels the TGN (Gonatas et al. (1977) J. Cell Biol. 73, 1–13). In BFA-treated PC12 cells, MG160 acquires resistance to endo H, but fails to be sialylated. This effect occurs in parallel with the redistribution of MG160 into an ER compartment dispersed throughout the cytoplasm including the nuclear envelope, and the collapse of the WGA-HRP-labelled TGN into vesicles and tubules surrounding the centriole. These results suggest that MG160 is not sialylated in BFA-treated cells because it is sequestered from the sialyltransferase enzyme(s), presumably located in the TGN, and provide evidence supporting the hypothesis for a retrograde transport pathway that recycles resident GA proteins, including MG160, between the Golgi cisternae and the TGN. To examine further the above hypothesis we studied cells treated with BFA and then allowed to recover from the effect of the drug for various lengths of time. After 15 minutes of recovery, cisternae of the Golgi apparatus, typically found in the pericentriolar region, are labeled by both MG160 and WGA-HRP.(ABSTRACT TRUNCATED AT 250 WORDS)

Over the last 20 years specific membrane polypeptides, such as receptors, enzymes, and organelle-specific antigens, have been visualized in the electron microscope with 'pre-embedding' or 'postembedding' methods of tissue processing (Anderson et al., 1978; Louvard et al., 1982; Tokuyasu, 1986; Brown and Farquhar, 1989; Gonatas, J., et al., 1989; Roth, 1989; Chen et al., 1989). With 'pre-embedding' methods, tissue sections or cells are fixed and 'permeabilized' with a suitable detergent prior to the application of the labeling probe; the labeling probe is then visualized in the light and electron microscopes with horseradish peroxidase (HRP), which may be coupled directly to the labeling probe, or to a second ligand (Guesdon et al., 1979; Brown and Farquhar, 1989). The detection of HRP in the light and electron microscopes is then accomplished with the classic diaminobenzidine (DAB) reaction of Graham and Karnovsky (1966). In 'postembedding' immunocytochemistry, ceils or

We used a monoclonal antibody (10A8), derived from mice immunized with fractions enriched in Golgi apparatus of rat brain neurons, to isolate an intrinsic membrane sialoglycoprotein of 160 KD from rat brain. By immunoelectron microscopy the sialoglycoprotein, named MG-160, was localized in medical cisternae of the Golgi apparatus of neurons, glia, adenohypophysis, and cultured rat pheochromocytoma (PC 12). The monoclonal antibody (MAb) reacted only with rat tissues. Because the epitope(s) recognized by a monoclonal antibody may be restricted, localization of an antigen by a single MAb may not reflect the extent of the distribution of antigen in various species and tissues. Therefore, to further investigate the presence and localization of MG-160 or of an antigenically related protein in several species and tissues, we used a polyclonal antiserum raised against MG-160 purified by antibody (10A8) affinity chromatography. Immunoblots of crude microsomal fractions from rat brain probed with the antiserum against MG-160 showed two to three prominent bands of approximately 160, 150, and 68 KD. Immunoblots of crude microsomal fractions from human, chicken, and frog brains showed prominent bands of 130-140 and 68 KD. Immunoblots of crude membrane fractions from Saccharomyces cerevisiae showed prominent bands of approximately 110-120 and 80 KD. Light microscopic immunocytochemical studies with frog, chicken, mouse, rat, rabbit, bovine, and human brains and with several other rat and human tissues showed a staining pattern consistent with the Golgi apparatus. Immunoelectron microscopy with rat and human brain and with rat myocardium and pituitary showed prominent and exclusive staining of cis, medial, and occasionally trans cisternae of the Golgi apparatus. The cisternae of the trans Golgi network were not stained. These findings are consistent with the hypothesis that a polypeptide related to MG-160 is present in the Golgi apparatus of several tissues in human, rodents, chicken, and frog and possibly in Saccharomyces cerevisiae. The antiserum to MG-160 represents a reliable reagent for immunohistochemical visualization of the Golgi apparatus in brain and several other human tissues obtained at autopsy, fixed with Bouin's, and embedded in paraffin.

The Golgi apparatus of motor neurons (GA) is fragmented in sporadic amyotrophic lateral sclerosis (ALS), in familial ALS with SOD1 mutations, and in mice that express SOD1G93A of familial ALS, in which it was detected months before paralysis. In paralyzed transgenic mice expressing SOD1G93A or SOD1G85R, mutant proteins aggregated not only in the cytoplasm of motor neurons, but also in astrocytes and oligodendrocytes. Furthermore, aggregation of the G85R protein damaged astrocytes and was associated with rapidly progressing disease. In order to gain insight into the functional state of the fragmented GA, we examined the effects of S0D1 mutants G93A and G85R in Chinese Hamster Ovary Cells (CHO). In contrast to cells expressing the wt and G93A, the G85R expressers had no SOD1 activity. However, cells expressing both mutants, and to a lesser degree the wt, showed decreased survival, fragmentation of the GA, and dysfunction of the secretory pathway, which was assessed by measuring the amount of cell surface co-expressed CD4, a glycoprotein processed through the GA. The G93A and wt proteins were partially recovered in detergent insoluble fractions; while the recovery of G85R was minimal. Both mutants showed equal reductions of cell survival and function of the secretory pathway, in comparison to the wt and cells expressing mutant alsin, a protein found in rare cases of fALS. These results are consistent with the conclusion that the two SOD1 mutants, by an unknown mechanism, promote the dispersion of the GA and the dysfunction of the secretory pathway. This and other in vitro models of mutant SOD1 toxicity may prove useful in the elucidation of pathogenetic mechanisms.

Transgenic mice that express the G93A mutation of human Cu,Zn superoxide dismutase (SOD1 G93A ), found in familial amyotrophic lateral sclerosis (FALS), showed clinical symptoms and histopathological changes of sporadic ALS, including fragmentation of the neuronal Golgi apparatus (GA). The finding of fragmented neuronal GA in asymptomatic mice, months before the onset of paralysis, suggests that the GA is an early target of the pathological processes causing neuronal degeneration. Transgenic mice expressing human SOD1 G93A have aggregates of mutant protein and ubiquitin in neuronal and glial cytoplasm; they appeared first in the neuropil and later in the perikarya of motor neurons, where they were adjacent to fragmented GA. The aggregates of SOD1 G93A appeared in neuronal perikarya of asymptomatic mice containing fragmented GA. The numbers of neurons with deposits of SOD1 G93A and fragmented GA progressively increased with age. Immuno-electron microscopy using colloidal gold showed labeling of ubiquitin and SOD1 over 13 nm thick cytoplasmic filaments. Spinal cord extracts showed a 20-fold increase of SOD1 G93A in transgenic mice compared to the wild-type protein in controls. The results suggest a causal relationship between the aggregation of mutant SOD1 and ubiquitin, fragmentation of the Golgi apparatus of motor neurons and neurodegeneration.

Fragmentation of the Golgi apparatus (GA) of motor neurons was first described in sporadic amyotrophic lateral sclerosis (ALS) and later confirmed in transgenic mice expressing the G93A mutation of the gene encoding the enzyme Cu,Zn superoxide dismutase (SOD1 G93A ) found in some cases of familial ALS. In these transgenic mice, however, the fragmentation of the neuronal GA was associated with cytoplasmic and mitochondrial vacuoles not seen in ALS. The present new series of transgenic mice expressing 14–17 trans gene copies of SOD1 G93A , compared to 25 copies in the mice we studied previously, showed consistent fragmentation of the GA of spinal cord motor neurons, axonal swellings, Lewy-like body inclusions in neurons and glia, but none of the cytoplasmic or mitochondrial vacuoles originally reported. Thus, this animal model recapitulates the clinical and most neuropathological findings of sporadic ALS. Neurofilaments (NF) accumulate in axons and, less often, in neuronal perikarya in most cases of sporadic ALS and they have been implicated in its pathogenesis. In order to investigate whether fragmentation of the neuronal GA also occurs in association with accumulation of perikaryal NFs, we studied the organelle in transgenic mice expressing the heavy subunit of human neurofilaments (NF-H) which developed a motor neuronopathy resembling ALS. The neuronal GA of mice expressing NF-H, however, was intact despite massive accumulation of NFs in both perikarya and axons of motor neurons. In contrast, in transgenic mice expressing SOD1 G93A , the GA was fragmented despite the absence of accumulation of perikaryal NFs. These findings suggest that, in transgenic mice with neuronopathies caused by the expression of mutant SOD1 G93A or the human NF-H, the GA and the perikaryal NFs are independently involved in the pathogenesis. The evidence suggests that the GA plays a central role in the pathogenesis of the vast majority of sporadic ALS and in FALS with SOD1 mutations.

The Golgi apparatus (GA) of innervated rat and chicken skeletal muscle is present in a typical perinuclear location, and in subsynaptic areas where it disperses after denervation. It was suggested that the subsynaptic segments of the GA are linked with functions involved in the maturation and targeting of synaptic proteins. Similarly, the GA of rat myocardium is found in a perinuclear location and between myofibrils, adjacent to the T system of tubules. These findings raise the question whether the GA of polarized cells is present in a typical perinuclear location, for the performance of general "housekeeping" functions, and in distal areas, for the mediation of specialized functions. Astrocytes may contain GA within their long cytoplasmic processes which are difficult to identify in thin sections. To ensure the astrocytic origin of GA in otherwise unidentifiable small processes, we used transgenic mice expressing the rat MG160 medial Golgi sialoglycoprotein only in the GA of astrocytes, and visualized the GA with monoclonal antibody 10A8 (mAb10A8) which reacts only with rat MG160. Thus, we identified cisternae of the GA in distal perisynaptic and subependymal processes, in perivascular foot plates of cerebral astrocytes, and in processes of the Muller glia in the retina. A similar strategy may be adopted in future investigations aiming at the detection of elements of the GA in distal processes of neurons and oligodendrocytes. The functional implications of GA in perisynaptic astrocytic processes and other processes are unknown. However, the isolation and molecular characterization of the perisynaptic subset of astrocytic Golgi may be feasible, since others have purified the astrocytic glutamate transporter 1 (GLT1) from crude synaptosomal fractions in which astrocytic processes are probably unavoidable contaminants.

Several diseases involving a variety of cells and tissues are associated with defective enzymes of the Golgi apparatus (GA). An intact GA of neurons is crucial for the physiological function of axons and presynaptic terminals since proteins destined for fast axoplasmic transport are processed by the organelle. Despite the obvious importance of the GA of neurons, its function and involvement in pathological reactions have not been studied systematically. The purpose of this paper is to draw attention to the contribution of the neuronal GA in pathology using two paradigms: (1) the involvement of the neuronal GA in the pathogeneses of amyotrophic lateral sclerosis (ALS), in an animal model of ALS, and in Alzheimer's disease; and (2) the elucidation of a retrograde transport pathway involving the neuronal transgolgi network, in vitro and in vivo, and the participation of this pathway in the toxicity and/or endocytosis of ricin and other toxic or non-toxic ligands.

MG-160, a membrane sialoglycoprotein of the medial cisternae of the rat Golgi apparatus, was identified and isolated with the use of an organelle specific antibody (Gonatas et al. 1989). A polyclonal antiserum against MG-160, purified by immunoaffinity chromatography from rat brain, reacted with the Golgi apparatus of several species including human (Croul et al. 1990).

A polyclonal antiserum raised against the carboxy-terminal 17 amino acids of the rat p185c-neu (anct) reacted with a 140 kDa polypeptide in membranes of synaptosome fractions from neocortex and hippocampus of 11-day-old and adult rats. The same antiserum reacted with a 185 kDa polypeptide in microsome membranes from rat pheochromocytoma cells (PC12). By light microscopic immunocytochemistry, the anct antibodies against the 140 kDa protein were localized in the neuropile of brain, cerebellum and spinal cord of 11-day-old and adult rats. Especially prominent staining was obtained in the CA2-CA3 zones of the hippocampus, and in the substantia gelatinosa in the spinal cord. The finely granular and diffuse pattern of the immunostain was consistent with synaptic localizations. Interestingly, antibodies against the entire endodomain of p185c-neu (a-Bacneu) were localized in granular structures, probably representing axo-somatic and axo-dendritic synapses, on a subset of pyramidal neurons of the CA3 zone. By immunoelectron terminals in the giant mossy fiber type in the CA3 and CA4 regions. The immunolocalization of the anct antibodies was restricted in segments of the presynaptic membrane facing the synaptic cleft which include the active zone. The identify and function of the 140 kDa membrane protein of rat brain presynaptic terminals, detected by the anct antibodies, is unknown. The 140 kDa protein may be related to p185c-neu, a tyrosine kinase, or to other known or unknown kinases.

The Golgi apparatus-complex (GA) plays a seminal role in the transport, processing and targeting of polypeptides synthesized in the rough endoplasmic reticulum (RER). Important insight into the function of the GA has been gained with the use of viruses or their coat glycoproteins which are processed through the GA. Coronavirus mouse hepatitis virus (MHV), possesses a membrane protein (M) which is targeted to the trans-Golgi network (TGN) 1.

Lately, we have identified two polypeptides of 92–94 kDa (GRL1) and 45–60 kDa (GRL2), expressed in cytoplasmic granules of chicken granulocytes and thrombocytes. Here, we report that GRL1 and GRL2 are widely distributed in all exocrine and several endocrine cell types, but not in neurons of the central nervous system, during late stages of embryonic development, as well as in newly hatched and two-month-old chickens. Immunogold studies in ultrathin frozen sections of pancreatic acinar cells show that GRL1 and GRL2 are co-localized at the periphery of zymogen granules, in granules fused with apical acinar membranes and on apical membranes of acini, while the pregranular compartments of the secretory pathway are weakly or not labeled. Semiquantitative morphometric studies indicate that GRL1 and GRL2 are equally distributed in secretory granules. A variety of physical and metabolic studies reveal that GRL2, a highly N-glycosylated polypeptide, is an intrinsic membrane protein, while GRL1 is a peripheral membrane polypeptide released by Na2CO3 treatment of granulocyte membranes. In all hematopoietic, exocrine or endocrine cells examinated, GRL1 shows identical electrophoretic patterns, while GRL2 is identified as a diffuse band, at 40–65 kDa, in hematopoietic and pancreatic cells. Taken together, the morphological and biochemical studies indicate that GRL1 and GRL2 are components of the secretory granule membrane in chicken exocrine, endocrine and hemopoietic cell types.

Monoclonal antibody (MAb) 2H1, raised in mice immunized with membrane fractions from cultured rat pheochromocytoma cells (clonal line PC-12), detects a polypeptide from rat brain and PC-12 cell membranes of 60-65 KD apparent molecular mass. The polypeptide has been localized by immunoelectron microscopy in the rough endoplasmic reticulum (RER) of neurons. By light microscopic immunocytochemistry, several rat tissues and two rat-derived cultured cell types show selective patterns of staining with 2H1. In the central nervous system, the antibody stains neuronal cytoplasm; in the spleen, staining is seen only in certain cells of the marginal zone of the white pulp, and in lymph nodes, in plasma cells, and in areas populated by monocytes and macrophages. Whereas astrocytes and adrenal medullary cells in situ are virtually unstained with 2H1, primary cultures of astrocytes and PC-12 cells, which are derived from adrenal medullary cells, stain intensely with 2H1. The strong staining of cultured astrocytes and PC-12 cells with 2H1 suggests that the levels of the 60-65 KD polypeptide are up-regulated during cell proliferation and growth. Only a few hepatocytes stain with 2H1; intestinal epithelial and pancreatic cells are not stained with 2H1. The organelle-specific antibody 2H1 may prove a useful probe in structural and functional studies of membranes of the rough endoplasmic reticulum in neurons, and in certain cells of the immune system.

Cholera toxin (CT), covalently attached to horseradish peroxidase (HRP), is a specific cytochemical marker for GM1 ganglioside (GM1) and retains the ability of the native toxin to raise levels of cyclic AMP in avian erythrocytes. Using a cytochemical stain for HRP, we found that 9% of control cultured murine neuroblastoma cells bound cholera toxin-horseradish peroxidase conjugates (CT-HRP) on their surfaces after incubations for 1 h at 4 degrees C. Exogenous GM1, the natural receptor of CT, becomes associated in the culture medium with the plasma membranes of these cells so that 96% of cells are stained. Cells preincubated with GM1 at 4 degrees C were exposed to CT-HRP for 1 h at 4 degrees C. After washing, cells were incubated at 37 degrees C for 30 min-24 h. Endocytosis of CT-HRP occurred within 30 min and CT-HRP remained, throughout the 24-h period, in tubules, vesicles, and cisternae often found near the Golgi apparatus; this aggregate of peroxidase-positive elements probably corresponds to Golgi apparatus-endoplasmic reticulum-lysosomes (GERL) of neurons. In metaphase cells, CT-HRP was observed in aggregates of vesicles and tubules clustered near the centriole. Conjugates of HRP with subunit B, the GM1 binding component of CT, were internalized by cells pretreated with GM1 as was CT-HRP. The 9% of neuroblastoma cells binding CT-HRP in the absence of exogenous GM1 internalized the ligand in a manner indistinguishable from that of the treated cells. These findings indicate that, in neuroblastoma cells, a system of vesicles, tubules, and cisternae, analogous to GERL of neurons, is the primary recipient of adsorptive endocytosis of CT bound to endogenous or exogenously introduced GM1.

Cultured rat oligodendrocytes were studied by transmission electron microscopy. Cells were identified by an immunocytochemical stain for galactocerebroside, a specific cell surface marker for oligodendroglial cells. Oligodendroglial cell perikarya contained numerous ribosomal rosettes, microtubules, prominent networks of cisternae of the Golgi apparatus and residual bodies. Glycogen and intermediate filaments were absent. Oligodendrocytes gave rise to numerous processes. Pentalaminar and heptalaminar profiles, consistent with tight and gap junctions, were seen between plasma membranes of processes and between perikarya and processes. The cell surface of processes showed numerous gross ruffles which stained for galactocerebroside. Similar membranous profiles appeared in the vicinity of oligodendroglial processes and suggested to us that the plasma membrane of certain of its components may be released into the medium. We concluded that cultured rat oligodendrocytes maintain many similarities with oligodendrocytes in situ and, therefore, are valid models for morphologic, physiologic and biochemical studies.

We studied with morphometric methods the endocytosis by pheochromocytoma cells of a conjugate of wheat germ agglutinin with ferritin (WGA-Ft) and of horseradish peroxidase (HRP). Quantitative studies indicated that WGA-Ft was cleared slowly from cell surfaces and that it was not recycled to the surface. Cells labeled with WGA-Ft for 15 min at room temperature were washed and incubated in medium containing HRP for 15 or 30 min at 37 degrees C. The greatest proportion of labeled vesicles and tubules contained only WGA-Ft (83.4% at 15 min and 85.3% at 30 min). A very small fraction of labeled vesicles and tubules contained only HRP (0.2% at 15 min and 0.9% at 30 min). Vesicles and tubules at the Golgi apparatus were labeled almost exclusively with WGA-Ft (97% at 15 min and 30 min); the rest had both labels. Most labeled lysosomes contained both labels (80.1% at 15 min and 80.8% at 30 min). Of the remainder more contained WGA-Ft alone (20% at 15 min and 10.9% at 30 min), then HRP alone (none at 15 min and 8.2% at 30 min). In contrast to the various and varying patterns of labeling with WGA-Ft and HRP of the other organelles studied, the vast majority of endosomes contained both markers (94.1% at 15 min and 100% at 30 min); the rest contained WGA-Ft only. These results demonstrate that endosomes are recipients of both fluid phase and adsorptive endocytosis markers; these findings are consistent with the hypothesis that endosomes mediate the sorting out and subsequent intracellular traffic of membrane bound and fluid phase markers. Cisterns of the Golgi apparatus did not contain WGA-Ft; in sharp contrast, when WGA-HRP was used, the cisterns of the Golgi apparatus consistently contained HRP.

Intracellular immunoglobulins of paraformaldehyde-fixed rat lymph node cells were identified with ultrastructural autoradiography of 125I Fab of sheep antirat Ig antibody. We observed excellent intracellular penetration and specific localization of the iodinated Fab fragments of the antirat Ig antibody. In contrast, 125I sheep antirat IgG or conjugates of peroxidase and Fab-antibody fragments did not show satisfactory intracellular penetration. Simultaneous detection of plasma membrane and intracellular immunoglobulins has been achieved with the use of iodinated Fab of sheep antirat Ig and peroxidase-labeled Fab of sheep antirat Ig and subsequent processing of the cells for cytochemical (peroxidase) and autoradiographic study. The simultaneous demonstration of specific (antiperoxidase) antibody and Ig molecules is also feasible.

The endocytosis of [125I]nerve growth factor (NGF) by cultured rat pheochromocytoma cells, the PC12 line, was studied by ultrastructural quantitative autoradiography. Cells previously grown in the absence of NGF were incubated at 37 degrees C with [125I]NGF for periods of up to 26 h. Under these conditions, PC12 cells have not yet shown outgrowth of neurites. Heavy labeling of the plasma membrane was observed at 2 h. At 6 and 26 h, lower but significant levels of labeling of the plasma membrane were still noted. After 2, 6 and 26 h, endocytosis of [125I]NGF was detected. Low breakdown of [125I]NGF was observed only after 26 h. At 26 h, grain density distributions of [125I]NGF showed significant labeling of lysosomes, while nuclei and rough endoplasmic reticulum showed the lowest levels of labeling. Significant apparent labeling of vesicles of smooth endoplasmic reticulum and of various cytoplasmic components, including cytoskeletal elements, was also observed. These findings indicate that [125I]NGF undergoes endocytosis quite slowly. During the initial phase of the interaction between NGF and PC12 cells, plasma membrane moieties are constantly labeled while lysosomes show progressively increasing uptake of NGF. The pathway of endocytosis of [125I]NGF included vesicles of the smooth endoplasmic reticulum but the Golgi apparatus was not unequivocally labeled. The findings do not support the hypothesis that NGF is transported to the nucleus for the initiation of transcriptional events. Our morphologic observations are consistent with the hypothesis that NGF constantly occupies sites on or adjacent to the plasma membrane, and that it slowly undergoes endocytosis into lysosomes.

A covalent conjugate of wheat germ agglutinin (WGA) with horseradish peroxidase (HRP) was used for a morphologic study of its adsorptive endocytosis by cultured human fibroblasts. Initial binding at 4 degrees C of the conjugate was observed over the entire plasma membrane, including "coated" and smooth pits. Endocytosis of HRP and the WGA-HRP conjugate was observed in lysosomes, but only the conjugate was seen in a cisterna of the Golgi apparatus (GERL), and in adjacent coated vesicles.

Rat spleen lymphocytes were iodinated (125 I) with lactoperoxidase. Quantitative autoradiographic studies on cells fixed immediately after iodination showed 19-24% of intracytoplasmic grains at 3HD and over from the plasma membrane. Normalization of grain density distribution and comparison of resulting curves with the universal curve of grain scatter of 125 I showed that a significant percentage of intracytoplasmic grains (36%) originates from intracytoplasmic labeled sources rather than from scattering from the heavily labeled plasma membrane. Damaged cells had a threefold grain density than intact cells. Radioactivity counts in sliced polyacrylamide gels of iodinated cells revealed 65-72% of total radioactivity in five peaks of apparent mol wt of 44, 50, 57, 90 and 195 thousand daltons. Segregation and internalization of anti-immunoglobulin-Ig-horseradish peroxidase (HRP) complexes from the iodinated plasma membrane proteins of lymphocytes was studied with quantitative autoradiography (125 I) and peroxidase cytochemistry; 64% of grains at 1.5HD (1,500 A) from the plasma membrane were within the cap zone, and 36% of grains remained outside the capped immunoglobulins; 45-57% of grains internalized together with Fab-anti-Ig-Ig-HRP, and 68% of grains internalized together with anti-Ig-Ig-HRP. These studies indicate that (a) iodination of rat spleen lymphocytes results in a significant internal labeling and that (b) immunoglobulins segregate into caps and internalize together with other iodinated plasma membrane proteins while a significant percentage of iodinated proteins (36%) are excluded from the immunoglobulin caps or internalization sites (32-55%).