Search

Autophagy, the major lysosomal pathway for degrading damaged or obsolete constituents, protects neurons by eliminating toxic organelles and peptides, restoring nutrient and energy homeostasis, and inhibiting apoptosis. These functions are especially vital in neurons, which are postmitotic and must survive for many decades while confronting mounting challenges of cell aging. Autophagy failure, especially related to the declining lysosomal ("phagy") functions, heightens the neuron's vulnerability to genetic and environmental factors underlying Alzheimer's disease (AD) and other late-age onset neurodegenerative diseases. Components of the global autophagy–lysosomal pathway and the closely integrated endolysosomal system are increasingly implicated as primary targets of these disorders. In AD, an imbalance between heightened autophagy induction and diminished lysosomal function in highly vulnerable pyramidal neuron populations yields an intracellular lysosomal build-up of undegraded substrates, including APP-βCTF, an inhibitor of lysosomal acidification, and membrane-damaging Aβ peptide. In the most compromised of these neurons, β-amyloid accumulates intraneuronally in plaque-like aggregates that become extracellular senile plaques when these neurons die, reflecting an "inside-out" origin of amyloid plaques seen in human AD brain and in mouse models of AD pathology. In this review, the author describes the importance of lysosomal-dependent neuronal cell death in AD associated with uniquely extreme autophagy pathology (PANTHOS) which is described as triggered by lysosomal membrane permeability during the earliest "intraneuronal" stage of AD. Effectors of other cell death cascades, notably calcium-activated calpains and protein kinases, contribute to lysosomal injury that induces leakage of cathepsins and activation of additional death cascades. Subsequent events in AD, such as microglial invasion and neuroinflammation, induce further cytotoxicity. In major neurodegenerative disease models, neuronal death and ensuing neuropathologies are substantially remediable by reversing underlying primary lysosomal deficits, thus implicating lysosomal failure and autophagy dysfunction as primary triggers of lysosomal-dependent cell death and AD pathogenesis and as promising therapeutic targets. [ABSTRACT FROM AUTHOR]

Autophagy is a lysosomal degradative process used to recycle obsolete cellular constituents and eliminate damaged organelles and protein aggregates. These substrates reach lysosomes by several distinct mechanisms, including delivery within endosomes as well as autophagosomes. Completion of digestion involves dynamic interactions among compartments of the autophagic and endocytic pathways. Neurons are particularly vulnerable to disruptions of these interactions, especially as the brain ages. Not surprisingly, mutations of genes regulating autophagy cause neurodegenerative diseases across the age spectrum with exceptional frequency. In late-onset disorders such as Alzheimer's disease, amyotrophic lateral sclerosis and familial Parkinson's disease, defects arise at different stages of the autophagy pathway and have different implications for pathogenesis and therapy. This Review provides an overview of the role of autophagy in neurodegenerative disease, focusing particularly on less frequently considered lysosomal clearance mechanisms and their considerable impact on disease. Various therapeutic strategies for modulating specific stages of autophagy and the current state of drug development for this purpose are also evaluated. [ABSTRACT FROM AUTHOR]

This review critically examines progress in understanding the link between Alzheimer's disease (AD) molecular pathogenesis and behavior, with an emphasis on the impact of amyloid-β. We present the argument that the AD research field requires more multifaceted analyses into the impacts of Alzheimer's pathogenesis which combine simultaneous molecular-, circuit-, and behavior-level approaches. Supporting this argument is a review of particular research utilizing similar, 'systems-level' methods in mouse models of AD. Related to this, a critique of common physiological and behavioral models is made-highlighting the likely usefulness of more refined and specific tools in understanding the relationship between candidate molecular pathologies and behavioral dysfunction. Finally, we propose challenges for future research which, if met, may greatly extend our current understanding of how AD molecular pathology impacts neural network function and behavior and possibly may lead to refinements in disease therapeutics. [ABSTRACT FROM AUTHOR]

Cystatin C, an inhibitor of cysteine proteases, colocalizes with amyloid β (Aβ) in parenchymal and vascular amyloid deposits in brains of Alzheimer’s disease (AD) patients, suggesting that cystatin C has a role in AD. Cystatin C also colocalizes with β amyloid precursor protein (βAPP) in transfected cultured cells. In vitro analysis of the association between the two proteins revealed that binding of cystatin C to full-length βAPP does not affect the level of Aβ secretion. Here we studied the effect of in vivo overexpression of cystatin C on the levels of endogenous brain Aβ. We have generated lines of transgenic mice expressing either wild-type human cystatin C or the Leu68Gln variant that forms amyloid deposits in the cerebral vessels of Icelandic patients with hereditary cerebral hemorrhage, under control sequences of the human cystatin C gene. Western blot analysis of brain homogenates was used to select lines of mice expressing various levels of the transgene. Analysis of Aβ40 and Aβ42 concentrations in the brain showed no difference between transgenic mice and their nontransgenic littermates. Thus, in vivo overexpression of human cystatin C does not affect Aβ levels in mice that do not deposit Aβ. [ABSTRACT FROM AUTHOR]

Calpains modulate processes that govern the function and metabolism of proteins key to the pathogenesis of Alzheimer’s disease, including tau and amyloid precursor protein. Because activation of the calpain system might contribute to the impairment of synaptic transmission in Alzheimer’s disease, we are currently testing the hypotheses that a treatment with calpain inhibitors might restore normal cognition and synaptic transmission in a transgenic model of Alzheimer’s disease, the APP (K670N:M671L)/PS1(M146L) mouse. Findings derived from these studies will provide a novel approach to cognitive enhancement in Alzheimer’s disease. [ABSTRACT FROM AUTHOR]

Neurofilament proteins synthesized in the cell body of neurons are assembled and transported into axons, where they influence axon radial growth, axonal transport, and nerve conduction velocities. In diseased states, neurofilaments accumulate in cell bodies and proximal axons of affected neurons, and these lesions are characteristic of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), spinal muscular atrophy (SMA), Charcot-Marie-Tooth disease type 2 (CMT2), and hereditary sensory motor neuropathy. Although the molecular mechanisms that contribute to these accumulations are not yet identified, transgenic mouse models are beginning to provide insight into the role of neurofilament transport in disease-related dysfunction of neurons. This review addresses axonal transport in mouse models of ALS and the special significance of neurofilament transport in this disease. [ABSTRACT FROM AUTHOR]

Activation of the calpain system might contribute to the impairment of synaptic transmission in Alzheimer’s disease (AD) (Liu et al., 1999; Rapoport, 1999; Selkoe, 1994). Calpains regulate the function of many proteins by limited proteolysis and initiate the complete degradation of other proteins. In particular, they modulate processes that govern the function and metabolism of proteins key to the pathogenesis of AD, including tau and amyloid precursor protein (APP). (Xie and Johnson, 1998; Wang, 2000). We have found that overexpression of APP(K670M:N671L) and PS1(M146L) proteins in hippocampal cultures derived from transgenic mice causes an increase in the frequency of spontaneous release of neurotransmitter. We have also found that calpain immunoreactive clusters are co-localized with immunoreactivity for the vesicle-associated presynaptic marker, synaptophysin. Moreover, application of calpain inhibitor reduces the frequency of spontaneous release of neurotransmitter. Therefore, we have hypothesized that calpains might contribute to the increase in transmitter release. Based on this hypothesis, we propose to test whether it is possible to restore normal synaptic transmission between cells derived from the transgenic model of AD by using calpain inhibitors. The transgenic mouse model also shows spatial learning impairment, a phenomenon that is thought to be associated with plastic changes at synaptic level. Therefore, we will also test whether we can rescue the learning impairment through a treatment with calpain inhibitors. [ABSTRACT FROM AUTHOR]

The calcium-activated protease calpain cleaves a variety of biologically important proteins and serves, therefore, as a key regulator of many cellular functions. Activation of both main isoforms, calpain 1 and calpain 2, was demonstrated previously in Alzheimer's disease. In this report, antibodies specifically recognizing the active form of calpain 2 were used to investigate calpain 2 activation in a broad range of neurodegenerative diseases, utilizing multiple-label confocal immunofluorescence imaging. With rare exceptions, the active form of calpain 2 was found in colocalization with hyperphosphorylated tau protein. Aggregates of mutated huntingtin, α-synuclein, or unidentified protein in motor neuron disease type of frontotemporal dementia were always negative. These findings indicate that calpain 2 activation is not a general response to protein aggregation. In tauopathies, more pathological inclusions were labeled for hyperphosphorylated tau than for activated calpain 2. The extent of colocalization varied in both a disease-specific and cell-type specific manner. The active form of calpain 2 was detected in 50–75% of tau neurofibrillary pathology in Alzheimer's disease, Alzheimer neurofibrillary changes and Down's syndrome, as well as in the accompanying Alzheimer-type tau pathology in diffuse Lewy bodies disease, progressive supranuclear palsy, and corticobasal degeneration. For glial cells, only 10–25% of tuft-shaped astrocytes, glial plaques, or coiled bodies contained activated calpain 2. The majority of Pick bodies were negative. The association of calpain 2 activation with hyperphosphorylated tau might be the result of an attempt by the calpain proteolytic system to degrade the tau protein aggregates. Alternatively, calpain 2 could be directly involved in tau hyperphosphorylation by modulating protein kinase activities. Overall, these results provide evidence of the important role of the calpain proteolytic system in the pathogenesis of neurodegenerative diseases with tau neurofibrillary pathology. [ABSTRACT FROM AUTHOR]

Shows that amyloid beta immunization reduces both deposition of cerebral fibrillar amyloid beta and cognitive dysfunction in the TgCRND8 murine model of Alzheimer's disease without altering total levels of amyloid beta in the brain. Implication that either a fifty percent reduction in dense-cored amyloid beta plaques is sufficient to affect cognition or that vaccination may modulate the activity/abundance of a small subpopulation of especially toxic amyloid beta species.

A prominent feature of brain pathology in Alzheimer's disease is a robust activation of the neuronal lysosomal system and major cellular pathways converging on the lysosome, namely, endocytosis and autophagy. Recent studies that identify a disturbance of the endocytic pathway as one of the earliest known manifestation of Alzheimer's disease provide insight into how β-amyloidogenesis might be promoted in sporadic Alzheimer's disease, the most prevalent and least well understood form of the disease. Primary lysosomal dysfunction has historically been linked to neurodegeneration. New data now directly implicate cathepsins as proteases capable of initiating, as well as executing, cell death programs in certain pathologic states. These and other studies support the view that the progressive alterations of lysosomal function observed during aging and Alzheimer's disease contribute importantly to the neurodegenerative process in Alzheimer's disease. [ABSTRACT FROM AUTHOR]

Newly synthesized neurofilament proteins become highly phosphorylated within axons. Within 2 days after intravitreously injecting normal adult mice with [P]orthophosphate, we observed that neurofilaments along the entire length of optic axons were radiolabeled by a solubleP-carrier that was axonally transported faster than neurofilaments.P-incorporation into neurofilament proteins synthesized at the time of injection was comparatively low and minimally influenced the labeling pattern along axons.P-incorporation into axonal neurofilaments was considerably higher in the middle region of the optic axons. This characteristic non-uniform distribution of radiolabel remained nearly unchanged for at least 22 days. During this interval, less than 10% of the totalP-labeled neurofilaments redistributed from the optic nerve to the optic tract. By contrast, newly synthesized neurofilaments were selectively pulse-labeled in ganglion cell bodies by intravitreous injection of [S]methionine and about 60% of this pool translocated by slow axoplasmic transport to the optic tract during the same time interval. These findings indicate that the steady-state or resident pool of neurofilaments in axons is not identical to the newly synthesized neurofilament pool, the major portion of which moves at the slowest rate of axoplasmic transport. Taken together with earlier studies, these results support the idea that, depending in part on their phosphorylation state, transported neurofilaments can interact for short or very long periods with a stationary but dynamic neurofilament lattice in axons. [ABSTRACT FROM AUTHOR]

Growing appreciation of the multiple functions of proteolytic enzymes in intracellular protein degradation and post-translational modification, in the release of biologically active macromolecules and peptides from precursors and in cellular protein regulation and quality control has stimulated interest in proteases in neurobiology and neuropathology. In this article, the proteinases and peptidases thus far studied in the human central nervous system are reviewed with respect to their enzymology, anatomical and cytological distributions and contributions to neurological and psychiatric disease states. Though information concerning brain proteases in man is fragmentary, it suffices to establish the importance of these complex systems for advancing knowledge of human cerebral function in health and disease. [ABSTRACT FROM AUTHOR]

Endo- and exopeptidase activities have been measured in normal post-mortem human prefrontal cortex and subjacent white matter to estimate their relative capabilities for protein and peptide degradation. Cathepsin D and three dipeptidases versus leucyl-glycine, glycyl- l-leucine and glycyl-glycine) were assayed in serial, microtome prepared frozen sections (± 125 μg fresh weight) and related to histological composition (Nissl stain), dry weight, total protein, and DNA content. RNA concentrations were similarly determined, serving as approximate indices of protein synthetic potential. Cathepsin D activity and RNA concentration were, respectively, threefold and twofold greater in cortical gray than in subcortical white matter. Each dipeptidase showed somewhat higher activity in white matter than in cortex. In both tissues the order of activities were: glycyl-leucine> glycyl-glycine>leucyl-glycine dipeptidase. The results are consistent with preferential localizations of cathepsin D in cortical neurons and dipeptidases in neuroglia. None of the four enzymes showed differences in activity in comparable cortex from six patients with chronic schizophrenia. [ABSTRACT FROM AUTHOR]

Using transgenic mice expressing human cystatin C (encoded by CST3), we show that cystatin C binds soluble amyloid-β peptide and inhibits cerebral amyloid deposition in amyloid-β precursor protein (APP) transgenic mice. Cystatin C expression twice that of the endogenous mouse cystatin C was sufficient to substantially diminish amyloid-β deposition. Thus, cystatin C has a protective role in Alzheimer's disease pathogenesis, and modulation of cystatin C concentrations may have therapeutic implications for the disease. [ABSTRACT FROM AUTHOR]

We investigated the relative inhibition of purified human μCANP and mCANP by five cysteine proteinase inhibitors including N-acetyl-Leu-Leu-nor-leucinal (C-I) and N-acetyl-Leu-Leu-methioninal (C-II), calpeptin, E64, and leupeptin. Based on IC measurements, calpeptin and C-I were stronger inhibitors by one to two orders of magnitude than C-II, leupeptin or E64. None of the five inhibitors, however, exhibited greater specificity for human μCANP or mCANP. These results indicate that, although the inhibition of a given cellular event by these compounds may suggest CANP involvement, effects on μCANP cannot be discriminated from those on mCANP. [ABSTRACT FROM AUTHOR]

The article discusses possible functions for prion proteins. The protein, which is known to cause fatal neurodegenerative diseases, called transmissible spongiform encephalopathies, has not been studied extensively. Resistance to proteolytic degradation, mitral cells, olfactory sensory neurons, and the olfactory cortex are discussed.