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Recently, missense mutations in the gene TARDBP encoding TDP-43 have been linked to familial ALS. The discovery of genes encoding these RNA binding proteins, such as TDP-43 and FUS/TLS, raised the notion that altered RNA metabolism is a major factor underlying the pathogenesis of ALS. To begin to unravel how mutations in TDP-43 cause dysfunction and death of motor neurons, investigators have employed both gain- and loss-of-function studies in rodent model systems. Here, we will summarize major findings from the initial sets of TDP-43 transgenic and knockout rodent models, identify their limitations, and point to future directions toward clarification of disease mechanism(s) and testing of therapeutic strategies that ultimately may lead to novel therapy for this devastating disease. This article is part of a Special Issue entitled RNA-Binding Proteins.

γ-Secretase, a multisubunit transmembrane protease comprised of presenilin, nicastrin, presenilin enhancer 2, and anterior pharynx-defective one, participates in the regulated intramembrane proteolysis of Type I membrane proteins including the amyloid precursor protein (APP). Although Aph-1 is thought to play a structural role in the assembly of γ-secretase complex and several transmembrane domains (TMDs) of Aph-1 have been shown to be critical for its function, the importance of the other domains of Aph-1 remains elusive. We screened a series of Aph-1 mutants and focused on nine mutations distributed in six different TMDs of human APH-1aS, assessing their ability to complement mouse embryonic fibroblasts lacking Aph-1. We showed that mutations in TMD4 (G126) and TMD5 (H171) of Aph-1aS prevented the formation of the Nct/Aph-1 subcomplex. Importantly, although mutations in TMD3 (Q83/E84/R85) and TMD6 (H197) of APH-1aS did not affect Nct/Aph-1 subcomplex formation, both mutations prevented further association/endoproteolysis of PS1. We propose a model that identifies critical TMDs of Aph-1 for associations with Nct and PS for the stepwise assembly of γ-secretase components.

Over the past three decades, significant progress has been made in understanding the neurobiology of Alzheimer's disease. In recent years, the first attempts to implement novel mechanism-based treatments brought rather disappointing results, with low, if any, drug efficacy and significant side effects. A discrepancy between our expectations based on preclinical models and the results of clinical trials calls for a revision of our theoretical views and questions every stage of translation—from how we model the disease to how we run clinical trials. In the following sections, we will use some specific examples of the therapeutics from acetylcholinesterase inhibitors to recent anti-Aβ immunization and γ-secretase inhibition to discuss whether preclinical studies could predict the limitations in efficacy and side effects that we were so disappointed to observe in recent clinical trials. We discuss ways to improve both the predictive validity of mouse models and the translation of knowledge between preclinical and clinical stages of drug development.

TAR DNA-binding protein-43 (TDP-43), a DNA/RNA-binding protein involved in RNA transcription and splicing, has been associated with the pathophysiology of neurodegenerative diseases, including ALS. However, the function of TDP-43 in motor neurons remains undefined. Here we use both gain- and loss-of-function approaches to determine roles of TDP-43 in motor neurons. Mice expressing human TDP-43 in neurons exhibited growth retardation and premature death that are characterized by abnormal intranuclear inclusions composed of TDP-43 and fused in sarcoma/translocated in liposarcoma (FUS/TLS), and massive accumulation of mitochondria in TDP-43-negative cytoplasmic inclusions in motor neurons, lack of mitochondria in motor axon terminals, and immature neuromuscular junctions. Whereas an elevated level of TDP-43 disrupts the normal nuclear distribution of survival motor neuron (SMN)-associated Gemini of coiled bodies (GEMs) in motor neurons, its absence prevents the formation of GEMs in the nuclei of these cells. Moreover, transcriptome-wide deep sequencing analysis revealed that a decrease in abundance of neurofilament transcripts contributed to the reduction of caliber of motor axons in TDP-43 mice. In concert, our findings indicate that TDP-43 participates in pathways critical for motor neuron physiology, including those that regulate the normal distributions of SMN-associated GEMs in the nucleus and mitochondria in the cytoplasm.

A transmembrane aspartyl protease termed β-site APP cleavage enzyme 1 (BACE1) that cleaves the amyloid-β precursor protein (APP), which is abundant in neurons, is required for the generation of amyloid-β (Aβ) peptides implicated in the pathogenesis of Alzheimer's disease (AD). We now demonstrate that BACE1, enriched in neurons of the CNS, is a major determinant that predisposes the brain to Aβ amyloidogenesis. The physiologically high levels of BACE1 activity coupled with low levels of BACE2 and α-secretase anti-amyloidogenic activities in neurons is a major contributor to the accumulation of Aβ in the CNS, whereas other organs are spared. Significantly, deletion ofBACE1inAPPswe;PS1ΔE9mice prevents both Aβ deposition and age-associated cognitive abnormalities that occur in this model of Aβ amyloidosis. Moreover, Aβ deposits are sensitive toBACE1dosage and can be efficiently cleared from the CNS whenBACE1is silenced. However,BACE1null mice manifest alterations in hippocampal synaptic plasticity as well as in performance on tests of cognition and emotion. Importantly, memory deficits but not emotional alterations inBACE1–/–mice are prevented by coexpressingAPPswe;PS1ΔE9transgenes, indicating that other potential substrates of BACE1 may affect neural circuits related to emotion. Our results establish BACE1 and APP processing pathways as critical for cognitive, emotional, and synaptic functions, and future studies should be alert to potential mechanism-based side effects that may occur with BACE1 inhibitors designed to ameliorate Aβ amyloidosis in AD.

Copper-mediated oxidative damage is proposed to play a critical role in the pathogenesis of Cu/Zn superoxide dismutase (SOD1)-linked familial amyotrophic lateral sclerosis (FALS). We tested this hypothesis by ablating the gene encoding the copper chaperone for SOD1 (CCS) in a series of FALS-linked SOD1 mutant mice. Metabolic 64Cu labeling in SOD1-mutant mice lacking the CCS showed that the incorporation of copper into mutant SOD1 was significantly diminished in the absence of CCS. Motor neurons in CCS-/- mice showed increased rate of death after facial nerve axotomy, a response documented for SOD1-/- mice. Thus, CCS is necessary for the efficient incorporation of copper into SOD1 in motor neurons. Although the absence of CCS led to a significant reduction in the amount of copper-loaded mutant SOD1, however, it did not modify the onset and progression of motor neuron disease in SOD1-mutant mice. Hence, CCS-dependent copper loading of mutant SOD1 plays no role in the pathogenesis of motor neuron disease in these mouse models.

The mechanisms leading to neurodegeneration in ALS (amyotrophic lateral sclerosis) are not well understood, but cytosolic protein aggregates appear to be common in sporadic and familial ALS as well as transgenic mouse models expressing mutant Cu/Zn superoxide dismutase (SOD1). In this study, we systematically evaluated the presence of these aggregates in three different mouse models (G93A, G85R, and G37R SOD1) and compared these aggregates to those seen in cases of sporadic and familial ALS. Inclusions and loss of motor neurons were observed in spinal cords of all of these three mutant transgenic lines. Since a copper-mediated toxicity hypothesis has been proposed to explain the cytotoxic gain-of-function of mutant SOD1, we sought to determine the involvement of the copper chaperone for SOD1 (CCS) in the formation of protein aggregates. Although all aggregates contained CCS, SOD1 was not uniformly found in the inclusions. Similarly, CCS-positive skein-like inclusions were rarely seen in ALS neurons. These studies do not provide strong evidence for a causal role of CCS in aggregate formation, but they do suggest that protein aggregation is a common event in all animal models of the disease. Selected proteins, such as the glutamate transporter GLT-1, were not typically observed within the inclusions. Most inclusions were positively stained with antibodies recognizing ubiquitin, proteasome, Hsc70 in transgenic lines, and some Hsc70-positive inclusions were detected in sporadic ALS cases. Overall, these observations suggest that inclusions might be sequestered into ubiquitin-proteasome pathway and some chaperone proteins such as Hsc70 may be involved in formation and/or degradation of these inclusions.

Significant progress has been made in the identification of genes and chromosomal loci associated with several types of motor neuron disease. Of particular interest is recent work on the pathogenic mechanisms underlying these diseases, especially studies in in vitro model systems and in transgenic and gene-targeted mice.

The neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating illnesses in medicine. Some of these disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, the prion diseases, and Parkinson's disease, can occur sporadically and, in some instances, are caused by inheritance of gene mutations. Huntington's disease is acquired in an entirely genetic manner. Transgenic mice that express disease-causing genes recapitulate many features of these diseases. This review provides an overview of transgenic mouse models of familial amyotrophic lateral sclerosis, familial Alzheimer's disease, and Huntington's disease and the emerging insights relevant to the underlying molecular mechanisms of these diseases.

To investigate the role of neurofilaments in motor neuron disease caused by superoxide dismutase (SOD1) mutations, transgenic mice expressing a amyotrophic lateral sclerosis-linked SOD1 mutant (SOD1 G37R ) were mated with transgenic mice expressing human neurofilament heavy (NF-H) subunits. Unexpectedly, expression of human NF-H transgenes increased by up to 65%, the mean lifespan of SOD1 G37R mice. Microscopic examination corroborated the protective effect of NF-H protein against SOD1 toxicity. Although massive neurodegeneration occurred in 1-yr-old mice expressing SOD1 G37R alone, spinal root axons and motor neurons were remarkably spared in doubly SOD1 G37R ;NF-H-transgenic littermates.

Superoxide dismutase 1 (SOD1), a ubiquitously expressed enzyme, detoxifies superoxide radicals and participates in copper homeostasis. Mutations in this enzyme have been linked to a subset of autosomal dominant cases of familial amyotrophic lateral sclerosis (FALS), a disorder characterized by selective degeneration of motor neurons. Transgenic mice expressing FALS mutant human (Hu) SOD1 at high levels develop a motor neuron disease, indicating that mutant Hu SOD1 gains properties that are particularly toxic to motor neurons. In this report, we demonstrate that transgenic mice expressing Hu SOD1 with the G37R FALS mutation, but not mice expressing wild-type enzyme, develop focal increases in immunoreactivity in the proximal axons of spinal motor neurons. This SOD1 immunoreactivity and immunoreactivity to hypophosphorylated neurofilament H epitopes are found adjacent to small vacuoles in axons. Using metabolic radiolabeling methods, we show that mutant G37R HuSOD1 as well as endogenous mouse SOD1 are transported anterograde in slow component b in motor and sensory axons of the sciatic nerve. Together, these findings suggest that anterogradely transported mutant SOD1 may act locally to damage motor axons. r 1998 Academic Press

Mutations in presenilin 1 (PS1) cosegregate with approximately 25% of early onset familial Alzheimer's disease (FAD) pedigrees. A variety of in vitro and in vivo paradigms have established that one mechanism by which PS1 variants cause AD is by elevating the production of highly amyloidogenic Abeta1-42/43 peptides. PS1 is homologous to sel-12, a C. elegans protein that facilitates signaling mediated by the Notch/lin-12 family of receptors. Wild-type human PS1 complements an egg-laying defect in C. elegans lacking sel-12, while FAD-linked PS1 variants exhibit reduced rescue activity. These data suggested that mutant PS1 may cause disease as a result of reduction in PS1 function. To test the function of FAD-linked PS1 in mammals, we examined the ability of the A246E PS1 variant to complement the embryonic lethality and axial skeletal defects in mice lacking PS1. Finally, to examine the influence of reduced PS1 levels on Abeta production, we quantified Abeta1-42/43 peptide levels in PS1 heterozygous null mice (PS1[+/-] mice). We now report that both human wild-type and A246E PS1 efficiently rescue the phenotypes observed in PS1(-/-) embryos, findings consistent with the view that FAD-linked PS1 mutants retain sufficient normal function during mammalian embryonic development. Moreover, the levels of Abeta1-42/43 and Abeta1-40 peptides between PS1(+/-) and control mice are indistinguishable. Collectively, these data lead us to conclude that mutant PS1 causes AD not by loss of normal PS1 function but by influencing amyloid precursor protein (APP) processing in a manner that elevates Abeta1-42/43 production.

Huntington's disease (HD) is caused by CAG triplet repeat expansion inIT15which leads to polyglutamine stretches in the HD protein product, huntingtin. The pathological hallmark of HD is the degeneration of subsets of neurons, primarily those in the striatum and neocortex. Specific morphological markers of affected cells have not been identified in patients with HD, although a unique intranuclear inclusion was recently reported in neurons of transgenic animals expressing a construct encoding the N-terminal part (including the glutamine repeat) of huntingtin (Davieset al., 1997). In order to understand the importance of this finding, we sought for comparable nuclear abnormalities in autopsy material from patients with HD. In all 20 HD cases examined, anti-ubiquitin and N-terminal huntingtin antibodies identified intranuclear inclusions in neurons and the frequency of these lesions correlated with the length of the CAG repeat inIT15. In addition, examination of material from the related HD-like triplet repeat disorder, dentatorubral and pallidoluysian atrophy, also revealed intranuclear neuronal inclusions. These findings suggest that intranuclear inclusions containing protein aggregates may be a common feature of the pathogenesis of glutamine repeat neurodegenerative disorders.

Spinal muscular atrophy (SMA), the second most common fatal, autosomal recessive disease of infants, manifests as generalized muscle weakness. The most severe form (Type I, Werdnig-Hoffmann disease) is associated with quadriplegia, respiratory muscle paralysis and death in infancy. Less severe forms are classified as Type II and Type III, based on age of onset and ultimate motor disability. Some spinal motor neurons show chromatolysis and the number of these cells is decreased. Recently, SMA has been mapped to chromosome 5q11.2-13.3 (Gilliam et al., 1990), a region that contains three candidate genes: Survival Motor Neuron (SMN) (Lefebvre et al., 1995); Neuronal Apoptosis Inhibitory Protein (NAIP) (Roy et al., 1995); and p44, a subunit of transcription factor II H (TFIIH) (Carter et al., 1995; Bürglen et al., 1997). Homozygous deletions or deleterious mutations in SMN are present in all SMA patients, and in some affected individuals, deletions have been identified in one or both of the other genes. These extensive deletions may be associated with a more severe phenotype. We have identified and characterized the mouse homologue of SMN, MoSMN, which is 82% identical to SMN at the amino-acid level. Unlike the duplicated human SMN, MoSMN is present in single copy. Like its human counterpart, MoSMN is ubiquitously expressed, but unlike SMN, MoSMN does not appear to be alternatively spliced. In-situ hybridization analysis of the mouse nervous system revealed that MoSMN mRNA is expressed in spinal cord and throughout the brain, with relatively higher levels of expression in the hippocampus and cerebellum.

Mutations in two related genes, PS1and PS2, account for the majority of early onset cases of familial Alzheimer’s disease. PS1 and PS2 are homologous polytopic membrane proteins that are processed endoproteolytically into two fragments in vivo. In the present report we examine the fate of endogenous PS1 and PS2 after overexpression of human PS1 or PS2 in mouse N2a neuroblastoma cell lines and human PS1 in transgenic mice. Remarkably, in N2a cell lines and in brains of transgenic mice expressing human PS1, accumulation of human PS1 derivatives is accompanied by a compensatory, and highly selective, decrease in the steady-state levels of murine PS1 and PS2 derivatives. Similarly, the levels of murine PS1 derivatives are diminished in cultured cells overexpressing human PS2. To define the minimal sequence requirements for “replacement” we expressed familial Alzheimer’s disease-linked and experimental deletion variants of PS1. These studies revealed that compromised accumulation of murine PS1 and PS2 derivatives resulting from overexpression of human PS1 occurs in a manner independent of endoproteolytic cleavage. Our results are consistent with a model in which the abundance of PS1 and PS2 fragments is regulated coordinately by competition for limiting cellular factor(s).

Missense mutations in the beta-amyloid precursor protein gene (APP) co-segregate with a small subset of autosomal dominant familial Alzheimer's disease (FAD) cases wherein deposition of the 39-43 amino acid beta-amyloid (A beta) peptide and neurodegeneration are principal neuropathological hallmarks. To accurately examine the effect of missense mutations on APP metabolism and A beta production in vivo, we have introduced yeast artificial chromosomes (YACs) containing the entire approximately 400 kbp human APP gene encoding APP harboring either the asparagine for lysine and leucine for methionine FAD substitution at codons 670 and 671 (APP(K670N/M671L)), the isoleucine for valine FAD substitution at codon 717 (APP(V7171)) or a combination of both substitutions into transgenic mice. We demonstrate that, relative to YAC transgenic mice expressing wild-type APP, high levels of A beta peptides are detected in the brains of YAC transgenic mice expressing human APP(K670N/M671L) that is associated with a concomitant diminution in the levels of apha-secretase-generated soluble APP derivatives. Moreover, the levels of longer A beta peptides (species terminating at amino acids 42/43) are elevated in YAC transgenic mice expressing human APP(V7171). These mice should prove valuable for detailed analysis of the in vivo effects of the APP FAD mutations in a variety of tissues and throughout aging and for testing therapeutic agents that specifically alter APP metabolism and A beta production.

We tested the hypothesis that synaptic defects in the hippocampus of individuals with Alzheimer disease (AD) correlate with the severity of cognitive impairment. Three postmortem groups were studied: controls with normal and stable cognition; cognitively intact subjects with senile plaque densities diagnostic for possible AD (p-AD) and neurofibrillary changes characteristic of early AD (Braak stage III); and individuals with definite AD and neurofibrillary changes typical of incipient to severe AD (Braak stage III, V, or VI). Synaptophysin (a presynaptic vesicle protein) levels were quantified by immunoblotting of synaptic membrane fractions isolated from hippocampus, entorhinal cortex, caudate nucleus, and occipital cortex. Average synaptophysin levels were reduced in hippocampus when comparing definite AD to controls (55%, p < 0.0001), p-AD to control (25%, p < 0.005), and definite AD to p-AD (30%, p < 0.05), but levels in entorhinal cortex, occipital cortex, and caudate nucleus were either unchanged or less significantly altered than in hippocampus. By univariate analysis, hippocampal synaptophysin levels correlated with neuropsychological measurements, including Mini-mental state examination scores (r = 0.83, p < 0.0001) and Blessed scores (r = 0.74, P < 0.001), and with senile plaque densities (r = 0.89, p < 0.0001). We conclude that synaptic abnormalities in the hippocampus correlate with the severity of neuropathology and memory deficit in individuals with AD, and that this defect may predate neuropsychological evidence for cognitive impairment early in AD.

Nerve growth factor (NGF) infusion significantly reduces spatial recent memory deficits in aged rats, an effect that has great relevance to the treatment of memory impairments characteristic of patients with Alzheimer’s disease. The present study was designed to examine whether this NGF-induced improvement in spatial recent memory persists after the discontinuation of NGF treatment, an issue of crucial importance for the potential clinical use of this compound. Spatial recent memory was tested in a Morris water maze delayed nonmatch-to-position task. In addition to memory, sensorimotor skills were also examined. Four- and 22-month-old rats were tested preoperatively, infused intraventricularly with recombinant human NGF or vehicle, and tested both during the 4 week infusion period and during the 4 weeks after discontinuation of the infusion. NGF significantly improved spatial recent memory in 22-month-old rats only, during the 4th week of infusion and for up to 4 weeks after discontinuation of the infusion. Although NGF did not affect overall sensorimotor skills during infusion in either age group, sensorimotor skills were significantly improved both 2 and 4 weeks after discontinuation of infusion in 22-month-old rats. These findings demonstrate that the beneficial effects of NGF on spatial recent memory can persist for up to 1 month after discontinuation of infusion and suggest that NGF can be used intermittently for the treatment of age-associated memory dysfunction and Alzheimer’s disease.

High levels of familial Amyotrophic Lateral Sclerosis (ALS)-linked SOD1 mutants G93A and G37R were previously shown to mediate disease in mice through an acquired toxic property. We report here that even low levels of another mutant, G85R, cause motor neuron disease characterized by an extremely rapid clinical progression, without changes in SOD1 activity. Initial indicators of disease are astrocytic inclusions that stain intensely with SOD1 antibodies and ubiquitin and SOD1-containing aggregates in motor neurons, features common with some cases of SOD1 mutant- mediated ALS. Astrocytic inclusions escalate markedly as disease progresses, concomitant with a decrease in the glial glutamate transporter (GLT-1). Thus, the G85R SOD1 mutant mediates direct damage to astrocytes, which may promote the nearly synchronous degeneration of motor neurons.

Mutations in genes encoding related proteins, termed presenilin 1 (PS1) and presenilin 2 (PS2), are linked to the majority of cases with early-onset familial Alzheimer’s disease (FAD). To clarify potential function(s) of presenilins and relationships of presenilin expression to pathogenesis of AD, we examined the expression of PS1 and PS2 mRNA and PS1 protein in human and mouse. Semi-quantitative PCR of reverse-transcribed RNA (RT-PCR) analysis revealed that PS1 and PS2 mRNA are expressed ubiquitously and at comparable levels in most human and mouse tissues, including adult brain. However, PS1 mRNA is expressed at significantly higher levels in developing brain.In situhybridization studies of mouse embryos revealed widespread expression of PS1 mRNA with a neural expression pattern that, in part, overlaps that reported for mRNA encoding specific Notch homologs.In situhybridization analysis in adult mouse brain revealed that PS1 and PS2 mRNAs are enriched in neurons of the hippocampal formation and entorhinal cortex. Although PS1 and PS2 mRNA are expressed most prominently in neurons, lower but significant levels of PS1 and PS2 transcripts are also detected in white matter glial cells. Moreover, cultured neurons and astrocytes express PS1 and PS2 mRNAs. Using PS1-specific antibodies in immunoblot analysis, we demonstrate that PS1 accumulates as ∼28 kDa N-terminal and ∼18 kDa C-terminal fragments in brain. Immunocytochemical studies of mouse brain reveal that PS1 protein accumulates in a variety of neuronal populations with enrichment in somatodendritic and neuropil compartments.

An expression plasmid (MoPrP.Xho), for use in transgenic mice, was developed from the promoter, 5' intronic, and 3' untranslated sequences of the murine prion protein gene. Analyses of mice harboring the MoPrP.Xho construct with cDNA genes encoding the amyloid precursor protein (APP) and human presenilin 1 demonstrated that this vector provides relatively high levels of transgene-encoded polypeptides in brains and hearts of transgenic mice. The MoPrP.Xho vector should be very useful in strategies designed to overexpress a variety of wild-type and disease related mutant transgenes in the heart and brain.

Mutations in the presenilin 1 (PS1) and presenilin 2 genes cosegregate with the majority of early-onset familial Alzheimer's disease (FAD) pedigrees. We now document that the Abeta1-42(43)/Abeta1-40 ratio in the conditioned media of independent N2a cell lines expressing three FAD-linked PS1 variants is uniformly elevated relative to cells expressing similar levels of wild-type PS1. Similarly, the Abeta1-42(43)/Abeta1-40 ratio is elevated in the brains of young transgenic animals coexpressing a chimeric amyloid precursor protein (APP) and an FAD-linked PS1 variant compared with brains of transgenic mice expressing APP alone or transgenic mice coexpressing wild-type human PS1 and APP. These studies provide compelling support for the view that one mechanism by which these mutant PS1 cause AD is by increasing the extracellular concentration of Abeta peptides terminating at 42(43), species that foster Abeta deposition.

Axonal transection provides very useful paradigms to study cellular responses to injury, mechanisms of regeneration and plasticity, and processes that lead to nerve cell degeneration. Moreover, models of axotomy are valuable for testing experimental therapeutic approaches. Lesions can be made with great precision, and, depending on the neural system, location of the lesion, and age of the animal, these models allow the opportunity to examine a range of neuronal responses. Many parameters influence the character, evolution, and outcomes of axotomy-related processes. The most severe outcome of axotomy is cell death, very common in lesions of neurons of the central nervous system (CNS), although neurons of the peripheral nervous system (PNS) may also die if the transection is sufficiently close to the neuronal cell body or if lesions are performed in young animals. Studies of axotomy models have provided clues into the cellular/molecular events associated with neuronal death and the ways in which interventions can delay or prevent processes that lead to cell death. In this review, we select examples, primarily from our own work, to illustrate how axotomy models have enhanced our understanding of neuronal responses to injury, clarified mechanisms of both regeneration/plasticity and degeneration/ cell death, and allowed assessments of the utility of therapeutic approaches.

The majority of early-onset cases of familial Alzheimer's disease (FAD) are linked to mutations in two related genes, PS1 and PS2, located on chromosome 14 and 1, respectively. Using two highly specific antibodies against nonoverlapping epitopes of the PS1-encoded polypeptide, termed presenilin 1 (PS1), we document that the preponderant PS1-related species that accumulate in cultured mammalian cells, and in the brains of rodents, primates, and humans are approximately 27-28 kDa N-terminal and approximately 16-17 kDa C-terminal derivatives. Notably, a FAD-linked PS1 variant that lacks exon 9 is not subject to endoproteolytic cleavage. In brains of transgenic mice expressing human PS1, approximately 17 kDa and approximately 27 kDa PS1 derivatives accumulate to saturable levels, and at approximately 1:1 stoichiometry, independent of transgene-derived mRNA. We conclude that PS1 is subject to endoproteolytic processing in vivo.

Transgenic FVB/N mice overexpressing human (Hu) or mouse (Mo) Alzheimer amyloid precursor protein (APP695) die early and develop a CNS disorder that includes neophobia and impaired spatial alternation, with diminished glucose utilization and astrogliosis mainly in the cerebrum. Age at onset of neophobia and age at death decrease with increasing levels of brain APP. HuAPP transgenes induce death much earlier than MoAPP transgenes expressed at similar levels. No extracellular amyloid was detected, indicating that some deleterious processes related to APP overexpression are dissociated from formation of amyloid. A similar clinical syndrome occurs spontaneously in approximately 20% of nontransgenic mice when they reach mid- to late-adult life, suggesting that APP overexpression may accelerate a naturally occurring age-related CNS disorder in FVB/N mice.

The presence of amyloid deposits in the parenchyma of the amygdala, hippocampus, and neocortex is a major histopathological hallmark of Alzheimer's disease. The principal component of amyloid is amyloid beta, a 39–43 amino acid peptide comprised of a portion of the transmembrane domain and the extracellular domain of the amyloid precursor proteins. Amyloid precursor proteins occur as several amyloid beta-containing isoforms of 695, 751, and 770 amino acids. In cultured cells, amyloid precursor proteins mature through the constitutive secretory pathway, and some cell-surface-bound amyloid precursor proteins are cleaved by an enzyme, designated as alpha-secretase, within the amyloid beta domain, an event that precludes amyloid beta amyloidogenesis. Two additional pathways of amyloid precursor protein processing include an endosomal/lysosomal pathway that generates a complex set of amyloid precursor protein-related membrane-bound fragments, some of which contain the entire amyloid beta sequence; and, by mechanisms not fully understood, secretion of amyloid beta 1–40 into the conditioned medium in vitro and its presence in cerebrospinal fluid in vivo. The intracellular sites of enzymes responsible for proteolytic cleavage at the amino- and carboxyl-termini of amyloid beta, termed gamma- and beta-secretase, respectively, have not been identified. Molecular genetic investigations have identified a variety of mutations in the amyloid precursor protein gene that segregate with early-onset familial Alzheimer's disease and with hereditary cerebral hemorrhage with amyloid, Dutch type. Several of these mutations appear to influence amyloid precursor protein processing and result in the production of higher levels or longer amyloid beta-related peptides that are inherently more fibrillogenic. Although a variety of lines of evidence implicate amyloid precursor protein and amyloid beta in Alzheimer's disease, the mechanism(s) by which amyloid beta influences the biology and vulnerability of neural cells is not fully clear. Amyloid beta toxicity is being explored in vitro and in vivo. Finally, recent progress has been made in the generation of transgenic mice expressing amyloid precursor protein or amyloid beta that recapitulate a subset of the pathology observed in Alzheimer's disease.

Mutations in Cu/Zn superoxide dismutase (SOD1) cause a subset of cases of familial amyotrophic lateral sclerosis. Four linesof mice accumulating oneof these mutant proteins (G37R) develop severe, progressive motor neuron disease. At lower levels of mutant accumulation, pathology is restricted to lower motor neurons, whereas higher levels cause more severe abnormalities and affect a variety of other neuronal populations. The most obvious cellular abnormality is the presence in axons and dendrites of membrane-bounded vacuoles, which appear to be derived from degenerating mitochondria. Since multiple lines of mice expressing wild-type human SOD1 at similar and higher levels do not show disease, the disease in mice expressing the G37R mutant SOD1 must arise from the acquisition of an adverse property by the mutant enzyme, rather than elevation or loss of SOD1 activity.

Huntington disease (HD) is an inherited neurodegenerative disorder characterized by selective death of striatal medium spiny neurons. Intrastriatal injections of glutamate receptor agonists (excitotoxins) recapitulate some neuropathological features of this disorder. Although this model suggests that excitotoxic injury may be involved in HD, the exact mechanisms of cell death in HD and its models are unknown. The present study was designed to test the hypothesis that HD can develop via the activation of an apoptotic mechanism of cell death and to examine whether excitotoxic striatal lesions with quinolinic acid in rats represent accurate models of HD. To characterize cell death, we employed DNA electrophoresis, electron microscopy (EM), and the terminal transferase-mediated (TdT) deoxyuridine triphosphate (d-UTP)- biotin nick end labeling (TUNEL) method for the in situ detection of DNA strand breaks. In the neostriatum of individuals with HD, patterns of distribution of TUNEL-positive neurons and glia were reminiscent of those seen in apoptotic cell death during normal development of the nervous system; in the same areas, nonrandom DNA fragmentation was detected occasionally. Following excitotoxic injury of the rat striatum, internucleosomal DNA fragmentation (evidence of apoptosis) was seen at early time intervals and random DNA fragmentation (evidence of necrosis) at later time points. In addition, EM detected necrotic profiles of medium spiny neurons in the lesioned rats. In concert, these results suggest that apoptosis occurs in both HD and excitotoxic animal models and that apoptotic and necrotic mechanisms of neuronal death may occur simultaneously within individual dying cells in the excitotoxically injured brain. However, the distribution of dying neurons in the neostriatum, the degree of glial degeneration, and the involvement of striatofugal pathways are very different between HD and excitotoxically damaged striatum. The present study suggests that multiple methods should be employed for a proper characterization of neuronal cell death in vivo.

A major histopathological hallmark of Alzheimer's disease (AD) is the presence of amyloid deposits in the parenchyma of the amygdala, hippocampus, and neocortex. The principal component of amyloid is the beta-amyloid protein (A beta), a 39-43 amino acid peptide composed of a portion of the transmembrane domain and the extracellular domain of the amyloid precursor protein (APP). APP occurs as several A beta-containing isoforms of 695, 751, and 770 amino acids, with the latter two APP containing a domain that shares structural and functional homologies with Kunitz serine protease inhibitors. In cultured cells, APP mature through the constitutive secretory pathway, and some cell surface-bound APP are cleaved by an enzyme, designated as alpha-secretase, within the A beta domain, an event that precludes A beta amyloidogenesis. Several studies have delineated two additional pathways of APP processing: first, an endosomal/lysosomal pathway generates a complex set of APP-related membrane-bound fragments, some of which contain the entire A beta sequence; and, second, by mechanisms which are not fully understood, A beta 1-40 is secreted into the conditioned medium in vitro and is present in cerebrospinal fluid in vivo. The intracellular sites of enzymes responsible for proteolytic cleavage at the NH2 and COOH termini of A beta, termed gamma- and beta-secretase, respectively, have not been identified. Finally, recent molecular genetic investigations have identified a variety of mutations in APP that segregate with early-onset familial AD and with hereditary cerebral hemorrhage with amyloid, Dutch type (HCHWA-D). Several of these mutations appear to influence APP processing and result in the production of higher levels or longer A beta-related peptides that are inherently more fibrillogenic. Although a variety of lines of evidence implicates APP/A beta in AD, the mechanisms by which A beta influences the biology and vulnerability of neural cells are not fully understood but are very active areas of investigation. This review focuses on the present state of our understanding of APP and A beta in the context of AD.

Mutations in superoxide dismutase 1 (SOD1) have been linked to familial amyotrophic lateral sclerosis, a dominantly inherited motor neuron disorder of midlife. Because SOD1 is a homodimeric enzyme, dimerization of mutant and wild-type SOD1 subunits could dominantly alter the activity, stability, or localization of wild-type SOD1 subunits. To explore these possibilities, we used transient and stable gene transfection to express high levels of either of two mutant human SOD1 subunits in the presence of limited levels of wild-type mouse and/or human SOD1 subunits. Although both mutant subunits displayed diminished half-lives and free radical scavenging activities, their presence caused no change in the half-life or activity of wild-type SOD1 subunits. Our data indicate that mutant subunits do not dominantly affect the function of wild-type SOD1 subunits. These findings, together with observations that many mutant SOD1 subunits retain significant stability and activity, suggest that motor neuron damage in familial amyotrophic lateral sclerosis is caused by the acquisition of injurious properties by mutant SOD1 subunits.

The present study establishes that populations of neurons in the lumbosacral cord, which innervate pelvic striated muscles, express p75NGFR throughout their life spans. These neuronal groups comprise the Onuf's nucleus in humans and its principal rat homologues, dorsolateral (DL) and dorsomedial (DM) nuclei, as well as the cremasteric (CRE) nucleus. The p75NGFR in these neurons is localized in the rough endoplasmic reticulum, Golgi complex, and lysosomes. Almost all neurons that project to striated perineal muscles in the male rat express p75NGFR; very low levels of p75NGFR are detected in neurons that innervate perineal sphincters of the female. In the female rat, p75NGFR expression is masculinized with perinatal androgen treatment. In addition, the expression of p75NGFR in DM and DL neurons in the adult is up-regulated by injury (i. e., pudendal axotomy) but is not influenced by gonadectomy. The results of this study establish that neurons of Onuf s nucleus and its rat homologues differ from general somatic motor neurons in that they express p75NGFR from early postnatal life (i. e., when all motor neurons express p75NGFR) into the adult (when the former, but not the latter, express the receptor). In view of growing evidence for the role of neurotrophins in the physiology of motor neurons, the above differentiating feature between general somatic and sexually dimorphic motor neurons suggests that p75NGFR may be involved in motor neuron plasticity and may participate in mechanisms by which neurons can protect themselves from degenerative insults. © 1994 Wiley-Liss, Inc.

Alzheimer disease and the prion disorders/spongiform encephalopathies share many common features. These chronic, progressive, sometimes familial diseases of the central nervous system are characterized by the presence of different types of amyloid deposits in the brain. This review provides a perspective on these two types of neurodegenerative disorders.

SummaryAssemblies of β-amyloid (Aβ) peptides are pathological mediators of Alzheimer's Disease (AD) and are produced by the sequential cleavages of amyloid precursor protein (APP) by β-secretase (BACE1) and γ-secretase. The generation of Aβ is coupled to neuronal activity, but the molecular basis is unknown. Here, we report that the immediate early gene Arc is required for activity-dependent generation of Aβ. Arc is a postsynaptic protein that recruits endophilin2/3 and dynamin to early/recycling endosomes that traffic AMPA receptors to reduce synaptic strength in both Hebbian and non-Hebbian forms of plasticity. The Arc-endosome also traffics APP and BACE1, and Arc physically associates with presenilin1 (PS1) to regulate γ-secretase trafficking and confer activity dependence. Genetic deletion of Arc reduces Aβ load in a transgenic mouse model of AD. In concert with the finding that patients with AD can express anomalously high levels of Arc, we hypothesize that Arc participates in the pathogenesis of AD.

As only symptomatic treatments are now available for Alzheimer's disease (AD), safe and effective mechanism-based therapies remain a great unmet need for patients with this neurodegenerative disease. Although gamma-secretase and BACE1 [beta-site beta-amyloid (Abeta) precursor protein (APP) cleaving enzyme 1] are well-recognized therapeutic targets for AD, untoward side effects associated with strong inhibition or reductions in amounts of these aspartyl proteases have raised concerns regarding their therapeutic potential. Although moderate decreases of either gamma-secretase or BACE1 are not associated with mechanism-based toxicities, they provide only modest benefits in reducing Abeta in the brains of APPswe/PS1DeltaE9 mice. Because the processing of APP to generate Abeta requires both gamma-secretase and BACE1, it is possible that moderate reductions of both enzymes would provide additive and significant protection against Abeta amyloidosis. Here, we test this hypothesis and assess the value of this novel anti-amyloid combination therapy in mutant mice. We demonstrate that genetic reductions of both BACE1 and gamma-secretase additively attenuate the amyloid burden and ameliorate cognitive deficits occurring in aged APPswe/PS1DeltaE9 animals. No evidence of mechanism-based toxicities was associated with such decreases in amounts of both enzymes. Thus, we propose that targeting both gamma-secretase and BACE1 may be an effective and safe treatment strategy for AD.

A principal neuropathological hallmark of Alzheimer's disease is deposition of beta-amyloid, composed primarily of a 4 kD peptide, A beta. This peptide is derived from larger amyloid precursor proteins. The mechanisms that are responsible for A beta formation in vivo are unknown. Recently, transgenic strategies have been employed to test several hypothetical mechanisms in order to reproduce Alzheimer's disease-specific pathology in rodents.

Recombinant baculovirus was used to overexpress human Alzheimer β/A4-amyloid precursor protein (APP) in Spodoptera frugiperda (Sf9) cells. Lysates of these cells were then analyzed for the presence of carboxyl-terminal fragments of APP by an immuno-blotting assay using either an antibody against the APP cytoplasmic domain (rabbit anti-human 695APP645–694 or an antibody against the amino terminus of β/A4-amyloid (rabbit anti-human 695APP586–606). Anti-human 695APP645–694 identified APP holoprotein, a 25-kDa species, and a prominent group of carboxyl-terminal fragments of 17, 16, and 14 kDa, whereas anti-human 695APP586–606 identified APP holoprotein and a single prominent low-molecular-mass protein species comigrating with the 17-kDa carboxyl-terminal fragment identified by anti-human 695APP645–694. No immunoreactive species was detected at these molecular mass positions when either antibody was used for analysis of lysates of either uninfected Sf 9 cells or Sf 9 cells infected with wild-type Autographa californica baculovirus. For each antibody, specific immunoreactivity was abolished by preabsorption with the corresponding peptide immunogen. The incorporation of a β/A4-amyloid amino-terminal epitope into a 17-kDa fragment of APP suggests that, in the baculoviral overexpression system, the electrophoretic microheterogeneity of APP carboxyl-terminal fragments is due, at least in part, to alternative proteolysis of APP. If such carboxyl-terminal fragments of APP containing an intact β/A4-amyloid domain are produced in human brain, then they may represent intermediates in the conversion of APP to deposited β/A4-amyloid. The identification of potentially amyloidogenic fragments in recom-binantly engineered Sf 9 cells may provide a useful experimental system for determination of alternative sites of APP proteolysis and investigation of the processing mechanisms involved.

The Baltimore Longitudinal Study of Aging (BLSA) was established in 1958 and is one the oldest prospective studies of aging in the USA and the world. The BLSA is supported by the National Institute of Aging (NIA) and its mission is to learn what happens to people as they get old and how to sort out changes due to aging and from those due to disease or other causes. In 1986, an autopsy program combined with comprehensive neurologic and cognitive evaluations was established in collaboration with the Johns Hopkins University Alzheimer’s Disease Research Center (ADRC). Since then, 211 subjects have undergone autopsy. Here we review the key clinical neuropathological correlations from this autopsy series. The focus is on the morphological and biochemical changes that occur in normal aging, and the early neuropathological changes of neurodegenerative diseases, especially Alzheimer’s disease (AD). We highlight the combined clinical, pathologic, morphometric, and biochemical evidence of asymptomatic AD, a state characterized by normal clinical evaluations in subjects with abundant AD pathology. We conclude that in some individuals, successful cognitive aging results from compensatory mechanisms that occur at the neuronal level (i.e., neuronal hypertrophy and synaptic plasticity) whereas a failure of compensation may culminate in disease.

Alzheimer's disease is characterized by the presence of abundant neurofibrillary tangles and beta-amyloid deposits in neocortex, hippocampus and amygdala. The major protein components of tangles and plaques have recently been identified. These findings, briefly reviewed here, will allow researchers to design investigations that will lead to an understanding of the pathogenesis of the disease and to the development of new therapeutic approaches that may result in an effective treatment.

In the brains of individuals with Alzheimer's disease (AD) and aged monkeys, the serine protease inhibitor alpha 1-antichymotrypsin (ACT) is selectively associated with deposits of amyloid found in senile plaques and in the walls of blood vessels. The origin of ACT in the brains of these aged subjects is unclear. In this study, ribonucleic acid (RNA) blots of human brains show that ACT messenger RNA (mRNA) increases during development. Levels of mRNA were negligible in fetuses and young adults but were increased slightly in normal aged individuals and highest in individuals with AD. In situ hybridization detected ACT transcripts in astrocytes of the cortex, subependymal region, and superficial white matter. The expression of ACT mRNA was highest in subjects with AD, in an adult with Down's syndrome, in an individual with Pick's disease, and in cases of Huntington's disease. In the brains of adult monkeys, ACT expression was detected primarily in astrocytes of the subependyma and white matter. Thus the presence of ACT appears to be related to the response of astrocytes to the brain abnormalities seen in these conditions.

Cholinergic neurons in the basal forebrain magnocellular complex (BFMC) respond to nerve growth factor (NGF) during development and in adult life, and it has been suggested that the administration of NGF might ameliorate some of the abnormalities that occur in neurological disorders associated with degeneration of this population of neurons. A prerequisite for the introduction of NGF in clinical trials is the availability of active recombinant human NGF (rhNGF). The present investigation was designed to test, in vivo, the efficacy of a preparation of rhNGF. Axons of cholinergic neurons of the BFMC in the rat were transected in the fimbria-fornix; this manipulation alters the phenotype and, eventually, causes retrograde degeneration of these neurons. Our investigation utilized two lesion paradigms (resection and partial transection of fibers in the fimbria-fornix), two different strains of rats, and two delivery systems. Following lesions, animals were allowed to survive for 2 weeks, during which time one group received intraventricular mouse NGF (mNGF), a second group received rhNGF, and a third group received vehicle alone. In animals receiving vehicle, there was a significant reduction in the number (resection: 70%; transection: 50%) and some reduction in size of choline acetyltransferase- or NGF receptor-immunoreactive cell bodies within the medial septal nucleus ipsilateral to the lesion. Treatment with either mNGF or rhNGF completely prevented these alterations in the number and size of cholinergic neurons. The rhNGF was shown to be equivalent in efficacy with mNGF. Thus, rhNGF is effective in preventing axotomy-induced degenerative changes in cholinergic neurons of the BFMC. Our results, taken together with the in vitro effects of rhNGF (42), indicate that an active rhNGF is now available for further in vivo studies in rodents and primates with experimentally induced or age-associated lesions of basal forebrain cholinergic neurons. These investigations provide essential information for the consideration of future utilization of rhNGF for treatment of human neurological disorders, including Alzheimer's disease.

We compared hippocampal lesions in three pedigrees of Familial Alzheimer's Disease (FAD). In these pedigrees, the disease is inherited as an autosomal dominant disorder and has been linked to DNA markers on chromosome 21. In eight cases of FAD (four from one pedigree and two each from two others) we quantified neurofibrillary tangles (NFT) and senile plaques (SP) in hippocampal subdivision CA1-4, subiculum, presubiculum, and dentate gyrus. We observed consistent patterns of the distribution of lesions: The highest density of NFT and SP was present in CA1-2; virtually no SP or NFT were present in presubiculum; SP diameter was consistently greatest in CA4. We found no overall differences among pedigrees in total densities of NFT and SP, but statistical analyses disclosed that an uncommon type of SP was disproportionately present in two pedigrees. This type of SP was usually restricted to CA4, had a marked amyloid core devoid of argyrophilic neurites. These studies also disclosed inter- and intrafamilial heterogeneity of lesion distribution (including congophilic angiopathy and cerebellar plaques) in these three pedigrees.

Galanin, a 29-amino acid peptide, has been shown by immunocytochemistry to occur in most large acetylcholinergic neurons of the complex that includes the nucleus basalis of Meynert and the nucleus of the diagonal band of Broca in nonhuman primates. In contrast, several studies have reported that most large neurons of the human nucleus basalis of Meynert complex appear to lack galanin immunoreactivity. We investigated this apparent species-difference by hybridization histochemistry for galanin messenger ribonucleic acid (mRNA) in humans and baboons. The results confirm previous immunocytochemical data; very few large neurons of the nucleus basalis of Meynert complex in humans contained detectable galanin messenger RNA, whereas most such cells in baboons were labeled by the oligodeoxynucleotide probe. The few labeled neurons in humans were primarily medial or ventral to the main body of the nucleus basalis of Meynert and corresponded in location to a minor population of relatively intensely labeled cells in baboons. These findings indicate that the indetectability of immunoreactive galanin in most cells of the nucleus basalis of Meynert complex in humans is due to a paucity or an absence of galanin messenger RNA and not to differences in posttranslational processing or transport of the peptide. Inasmuch as the probe labeled neurons in several other nuclei of both species, it is unlikely that differences in galanin messenger RNA sequences underlie the species-related disparity in hybridization in the nucleus basalis of Meynert complex. The indetectability of galanin messenger RNA in most cells of the human nucleus basalis of Meynert complex indicates that the expression of the galanin gene is regulated by as yet unidentified influences that differ in human and nonhuman primates. The varying phenotypes of galanin in primates suggest potentially important species-differences in the function of galanin in neurons of the nucleus basalis of Meynert complex.

Amyotrophic lateral sclerosis (ALS), a fatal and progressive neurodegenerative disorder characterized by weakness, muscle atrophy, and spasticity, is the most common adult-onset motor neuron disease. Although the majority of ALS cases are sporadic, ∼5–10% are familial, including those linked to mutations in SOD1 (Cu/Zn superoxide dismutase). Missense mutations in a dynactin gene (DCTN1) encoding the p150Gluedsubunit of dynactin have been linked to both familial and sporadic ALS. To determine the molecular mechanism whereby mutant dynactin p150Gluedcauses selective degeneration of motor neurons, we generated and characterized mice expressing either wild-type or mutant human dynactin p150Glued. Neuronal expression of mutant, but not wild type, dynactin p150Gluedcauses motor neuron disease in these animals that are characterized by defects in vesicular transport in cell bodies of motor neurons, axonal swelling and axo-terminal degeneration. Importantly, we provide evidence that autophagic cell death is implicated in the pathogenesis of mutant p150Gluedmice. This novel mouse model will be instrumental for not only clarifying disease mechanisms in ALS, but also for testing therapeutic strategies to ameliorate this devastating disease.

NGF is a neurotrophic factor for basal forebrain cholinergic neurons and may serve to counteract the cholinergic deficits that are observed in Alzheimer's disease. Prior to the introduction of clinical trials, it is essential that recombinant human NGF (rhNGF) be produced and that its actions on target cells in the CNS be demonstrated. We prepared rhNGF and examined its actions on fetal rat brain neurons in culture including, in particular, the cholinergic neurons of the basal forebrain. rhNGF was more potent in increasing choline acetyltransferase (ChAT) activity in septal cultures than NGF purified from mouse salivary glands (mNGF). ED50s of the beta-NGF dimers were 4.9 pM for rhNGF and 12.4 pM for mNGF. The maximal ChAT activity response was achieved at approximately 35 pM with both NGFs and their efficacies were not significantly different. The two NGFs were not additive in effect. Identical to the results with mNGF, rhNGF strongly enhanced the intensity of ChAT immunostaining in septal cultures. Neither rhNGF nor mNGF affected the appearance of the cultures under phase-contrast illumination. Survival of cells at very low plating density on polyornithine/laminin-coated culture dishes was not affected by rhNGF or mNGF. Protein content and the uptake of GABA were also unaffected. At concentrations of up to 10 micrograms/ml, rhNGF did not significantly increase uptake of dopamine into cultures of ventral mesencephalon. We conclude that rhNGF produces potent and selective actions on cholinergic neurons of the basal forebrain as previously shown for mNGF.

Antisera to human dopamine β-hydroxylase (DBH) was used to stain noradrenergic axons in the inferior olivary complex (IOC) of human controls. A modest plexus of thin, beaded, immunoreactive fibers was present in all subdivisions of the IOC, and many fibers coursed in the rostrocaudal axis. This noradrenergic plexus is similar to that of monkey, provides complementary evidence to reports of β-adrenergic receptors in human IOC, and supports the concept of noradrenergic projections to IOC.

In the brains of aged humans and cases of Alzheimer disease, deposits of amyloid in senile plaques are located in proximity to nerve processes. The principal component of this extracellular amyloid is beta/A4, a peptide derived from a larger amyloid precursor protein (APP), which is actively expressed in brain and systemic organs. Mechanisms that result in the proteolysis of APP to form beta/A4, previously termed beta-amyloid protein, and the subsequent deposition of the peptide in brain are unknown. If beta/A4 in senile plaques is derived from neuronally synthesized APP and deposited at locations remote from sites of synthesis, then APP must be transported from neuronal cell bodies to distal nerve processes in proximity to deposits of amyloid. In this study, using several immunodetection methods, we demonstrate that APP is transported axonally in neurons of the rat peripheral nervous system. Moreover, our investigations show that APP is transported by means of the fast anterograde component. These findings are consistent with the hypothesis of a neuronal origin of beta/A4, in which amyloid is deposited in the brain parenchyma of aged individuals and cases of Alzheimer disease. In this setting, we suggest that APP is synthesized in neurons and delivered to dystrophic nerve endings, where subsequent alterations of local processing of APP result in deposits of brain amyloid.

Senile plaques are a characteristic feature in brains of individuals with Alzheimer's disease (AD) and aged monkeys. The principal component of amyloid in senile plaques is β/A4, a peptide derived from a larger amyloid precursor protein (APP). To date, several alternatively spliced APP transcripts have been described. The relationship between levels of these APP mRNAs and amyloid deposition is unclear. In this study, we directly measured the relative levels of APP transcripts that lack the protease inhibitor domain (APP-695) and transcripts that encode the inhibitor sequences (APP-751/770). Our results indicate that the expression of APP mRNAs is not selectively altered in AD cortex. Moreover, the differential expression of APP transcripts is not correlated with the deposition of amyloid in cases of AD and aged monkeys. These findings suggest that other factors, not directly related to the relative expression of APP mRNAs, may contribute to amyloidogenesis in the brain.

Although gamma-secretase is recognized as a therapeutic target for Alzheimer's disease, side effects associated with strong inhibition of this aspartyl protease raised serious concerns regarding this therapeutic strategy. However, it is not known whether moderate inhibition of this enzyme will allow dissociation of beneficial effects in the CNS from mechanism-based toxicities in the periphery. We tested this possibility by using a series of mice with genetic reduction of gamma-secretase (levels ranging from 25 to 64% of control mice). Here, we document that even 30% reduction of gamma-secretase can effectively ameliorate amyloid burden in the CNS. However, global reduction of this enzyme below a threshold level increased the risk of developing squamous cell carcinoma as well as abnormal proliferation of granulocytes in a gamma-secretase dosage-dependent manner. Importantly, we demonstrate that there exists a critical gamma-secretase level that reduces the risk of amyloidosis in the CNS and limits tumorigenesis in epithelia. Our findings suggest that moderate inhibition of gamma-secretase represents an attractive anti-amyloid therapy for Alzheimer's disease.

Gamma-secretase, a unique aspartyl protease, is required for the regulated intramembrane proteolysis of Notch and APP, pathways that are implicated, respectively, in the pathogenesis of cancer and Alzheimer disease. However, the mechanism whereby reduction of gamma-secretase causes tumors such as squamous cell carcinoma (SCC) remains poorly understood. Here, we demonstrate that gamma-secretase functions in epithelia as a tumor suppressor in an enzyme activity-dependent manner. Notch signaling is down-regulated and epidermal growth factor receptor (EGFR) is activated in SCC caused by genetic reduction of gamma-secretase. Moreover, the level of EGFR is inversely correlated with the level of gamma-secretase in fibroblasts, suggesting that the up-regulation of EGFR stimulates hyperproliferation in epithelia of mice with genetic reduction of gamma-secretase. Supporting this notion is our finding that the proliferative response of fibroblasts lacking gamma-secretase activity is more sensitive when challenged by either EGF or an inhibitor of EGFR as ompared with wild type cells. Interestingly, the up-regulation of EGFR is independent of Notch signaling, suggesting that the EGFR pathway functions in parallel with Notch in the tumorigenesis of SCC. Collectively, our results establish a novel mechanism linking the EGFR pathway to the tumor suppressor role of gamma-secretase and that mice with genetic reduction of gamma-secretase represent an excellent rodent model for clarifying pathogenesis of SCC and for testing therapeutic strategy to ameliorate this type of human cancer.