Niemann-Pick disease type C yields possible clue for why cerebellar neurons do not form neurofibrillary tangles

Distinct sets of lysosomal genes define synucleinopathy and tauopathy

Neurodegenerative diseases are characterized by distinct protein aggregates, such as those of α-synuclein and tau. Lysosomal defect is a key contributor to the accumulation and propagation of aberrant protein aggregates in these diseases. The discoveries of common proteinopathies in multiple forms of lysosomal storage diseases (LSDs) and the identification of some LSD genes as susceptible genes for those proteinopathies suggest causative links between LSDs and the proteinopathies. The present study hypothesized that defects in lysosomal genes will differentially affect the propagation of α-synuclein and tau proteins, thereby determining the progression of a specific proteinopathy. We established an imaging-based high-contents screening (HCS) system in Caenorhabditis elegans (C. elegans) model, by which the propagation of α-synuclein or tau is measured by fluorescence intensity. Using this system, we performed RNA interference (RNAi) screening to induce a wide range of lysosomal malfunction through knock down of 79 LSD genes, and to obtain the candidate genes with significant change in protein propagation. While some LSD genes commonly affected both α-synuclein and tau propagation, our study identified the distinct sets of LSD genes that differentially regulate the propagation of either α-synuclein or tau. The specificity and efficacy of these LSD genes were retained in the disease-related phenotypes, such as pharyngeal pumping behavior and life span. This study suggests that distinct lysosomal genes differentially regulate the propagation of α-synuclein and tau, and offer a steppingstone to understanding disease specificity. [BMB Reports 2023; 56(12): 657-662].

Autophagy is a novel pathway for neurofilament protein degradation in vivo

How macroautophagy/autophagy influences neurofilament (NF) proteins in neurons, a frequent target in neurodegenerative diseases and injury, is not known. NFs in axons have exceptionally long half-lives in vivo enabling formation of large stable supporting networks, but they can be rapidly degraded during Wallerian degeneration initiated by a limited calpain cleavage. Here, we identify autophagy as a previously unrecognized pathway for NF subunit protein degradation that modulates constitutive and inducible NF turnover in vivo. Levels of NEFL/NF-L, NEFM/NF-M, and NEFH/NF-H subunits rise substantially in neuroblastoma (N2a) cells after blocking autophagy either with the phosphatidylinositol 3-kinase (PtdIns3K) inhibitor 3-methyladenine (3-MA), by depleting ATG5 expression with shRNA, or by using both treatments. In contrast, activating autophagy with rapamycin significantly lowers NF levels in N2a cells. In the mouse brain, NF subunit levels increase in vivo after intracerebroventricular infusion of 3-MA. Furthermore, using tomographic confocal microscopy, immunoelectron microscopy, and biochemical fractionation, we demonstrate the presence of NF proteins intra-lumenally within autophagosomes (APs), autolysosomes (ALs), and lysosomes (LYs). Our findings establish a prominent role for autophagy in NF proteolysis. Autophagy may regulate axon cytoskeleton size and responses of the NF cytoskeleton to injury and disease.

Tau Protein and Zebrafish Models for Tau-Induced Neurodegeneration

Tauopathies are a specific type of slow and progressive neurodegeneration, which involves intracellular deposition of fibrillar material composed of abnormal hyperphosphorylation of the microtubule associated protein (MAP) tau. Despite many years of intensive research, our understanding of the molecular events that lead to neurodegeneration is far from complete. No effective therapeutic treatments have been defined, and questions surround the validity and utility of existing animal models. It is an urgent need to develop a novel animal model to study the underlying neurodegenerative mechanisms of tauopathies. Zebrafish models of tauopathies could complement existing models by providing an in vivo platform for genetic and chemical screens in order to identify new therapeutic targets and compounds, meanwhile zebrafish models have permitted discovery of unique characteristics of these genes that could have been difficultly observed in other models. Novel transgenic zebrafish models expressing wild-type or mutant forms of human 4R-tau in neurons have recently been reported. These studies show disease-relevant changes including tau hyperphosphorylation, aggregation and somato-dendritic relocalization. This review highlights the availability of transgenic tau zebrafish models that allow more detailed biochemical studies of tau in the zebrafish CNS to characterize solubility, fibril morphology and further clarify phosphorylation proceedings. Furthermore, a deeper knowledge of the zebrafish brain and a better characterization of tau caused by alterations in neurodegenerative disorders are needed.

Changes in global gene expression indicate disordered autophagy, apoptosis and inflammatory processes and downregulation of cytoskeletal signalling and neuronal development in patients with Niemann-Pick C disease

Changes in gene expression profiles were investigated in 23 patients with Niemann-Pick C1 disease (NPC). cDNA expression microarrays with subsequent validation by qRT-PCR were used. Comparison of NPC to control samples revealed upregulation of genes involved in inflammation (MMP3, THBS4), cytokine signalling (MMP3), extracellular matrix degradation (MMP3, CTSK), autophagy and apoptosis (CTSK, GPNMB, PTGIS), immune response (AKR1C3, RCAN2, PTGIS) and processes of neuronal development (RCAN2). Downregulated genes were associated with cytoskeletal signalling (ACTG2, CNN1); inflammation and oxidative stress (CNN1); inhibition of cell proliferation, migration and differentiation; ERK-MAPK pathway (COL4A1, COL4A2, CPA4); cell adhesion (IGFBP7); autophagy and apoptosis (CDH2, IGFBP7, COL4A2); neuronal function and development (CSRP1); and extracellular matrix stability (PLOD2). When comparing NPC and Gaucher patients together versus controls, upregulation of SERPINB2 and IL13RA2 and downregulation of CSRP1 and CNN1 were characteristic. Notably, in NPC patients, the expression of PTGIS is upregulated while the expression of PLOD2 is downregulated when compared to Gaucher patients or controls and potentially could serve to differentiate these patients. Interestingly, in NPC patients with (i) jaundice, splenomegaly and cognitive impairment/psychomotor delay-the expression of ACTG2 was especially downregulated; (ii) ataxia-the expression of ACTG2 and IGFBP5 was especially downregulated; and (iii) VSGP, dysarthria, dysphagia and epilepsy-the expression of AKR1C3 was especially upregulated while the expression of ACTG2 was downregulated. These results indicate disordered apoptosis, autophagy and cytoskeleton remodelling as well as upregulation of immune response and inflammation to play an important role in the pathogenesis of NPC in humans.

Psychiatric and Cognitive Symptoms Associated with Niemann-Pick Type C Disease: Neurobiology and Management

Niemann-Pick disease type C (NPC) is a lysosomal storage disorder that presents with a spectrum of clinical manifestations from infancy and childhood or in early or mid-adulthood. Progressive neurological symptoms including ataxia, dystonia and vertical gaze palsy are a hallmark of the disease, and psychiatric symptoms such as psychosis and mood disorders are common. These latter symptoms often present early in the course of NPC and thus these patients are often diagnosed with a major psychotic or affective disorder before neurological and cognitive signs present and the diagnosis is revised. The commonalities and characteristics of psychotic symptoms in both NPC and schizophrenia may share neuronal pathways and mechanisms and provide potential targets for research in both disorders. The neurobiology of NPC and its relationship to the pattern of neuropsychiatric and cognitive symptoms is described in this review. A number of neurobiological models are proposed as mechanisms by which NPC causes psychiatric and cognitive symptoms, informed from models proposed in schizophrenia and other metabolic disorders. There are a number of symptomatic and illness-modifying treatments for NPC currently available. The current evidence is discussed; focussing on two medications which have shown promise, miglustat and hydroxypropyl-β-cyclodextrin.

Aberrant activation of Cdc2/cyclin B1 is involved in initiation of cytoskeletal pathology in murine Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a fatal, neurovisceral lipid storage disease, neuropathologically characterized by cytoplasmic sequestration of glycolipids in neurons, progressive neuronal loss, neurofibrillary tangles (NFTs) formation, and axonal spheroids (AS). Cytoskeletal pathology including accumulation of hyperphosphorylated cytoskeletal proteins is a neuropathological hallmark of the mouse model of NPC (npc mice). With a goal of elucidating the mechanisms underlying the lesion formation, we investigated the temporal and spatial characteristics of cytoskeletal lesions and the roles of cdc2, cdk4, and cdk5 in lesion formation in young npc mice. Cytoskeletal lesions were detectable in npc mice at three weeks of age. Importantly, concomitant activation of cdc2/cyclin B1 kinase and accumulation of a subsequently generated cohort of phospho-epitopes were detected. The activation of cdk4/cyclin D1 and cdk5/p25 kinases was observed during the fourth week of life in npc mice, and this activation contributed to the lesion formation. We concluded that the progression of cytoskeletal pathology in npc mice older than four weeks is accelerated by the cumulative effect of cdc2, cdk4, and cdk5 activation. Furthermore, cdc2/cyclin B1 may act as a key initial player one week earlier. Targeting cell cycle activation may be beneficial to slow down the NPC pathogenesis.

Cognition and anatomy of adult Niemann-Pick disease type C: Insights for the Alzheimer field

Niemann-Pick disease type C (NPC) is a rare lysosomal storage disorder causing an intracellular lipid trafficking defect and varying damage to the spleen, liver, and central nervous system. The adult form, representing approximately 20% of the cases, is associated with progressive cognitive decline. Intriguingly, brains of adult NPC patients exhibit neurofibrillary tangles, a characteristic hallmark of Alzheimer's disease (AD). However, the cognitive, psychiatric, and neuropathological features of adult NPC and their relation to AD have yet to be explored. We systematically reviewed the literature on adult NPC with a particular focus on cognitive and neuroanatomical abnormalities. The careful study of cognition in adult NPC allows drawing critical insights in our understanding of the pathophysiology of AD as well as normal cognition.

Accumulation of amyloid-β in the cerebellar cortex of essential tremor patients

The accumulation of insoluble amyloid-beta (Aβ) peptides is associated with neurodegenerative disorders, such as Alzheimer's disease (AD). As essential tremor (ET) could involve neurodegenerative processes in the cerebellum, we quantified soluble and insoluble Aβ in cerebellar cortices from patients diagnosed with ET (n=9), compared to Controls (n=16) or individuals with Parkinson's disease (n=10). Although ante-mortem cognitive performance was not documented, all individuals included had the diagnosis of AD ruled out by a neuropathologist. ELISA-determined concentrations of insoluble Aβ42 in ET patients displayed a bimodal distribution, with a median 246-fold higher than in Controls (P<0.01, Kruskal-Wallis). Higher Aβ42 concentrations were measured in the parietal cortex of the same ET patients, compared to Controls (107-fold median increase, P<0.01, Kruskal-Wallis), but similar phosphorylated tau levels were detected. The rise in cerebellar insoluble Aβ42 concentrations is not associated to APP expression and processing or the ApoE4 status. However, Aβ42 levels in ET individuals were correlated with cerebellar insoluble phosphorylated tau (r(2)=0.71, P=0.005), unphosphorylated neurofilament heavy chain (NF-H; r(2)=0.50, P=0.030) and Lingo-1 (r(2)=0.73, P=0.007), indicative of a generalized neurodegenerative process involving the cerebellum. Our results suggest prevalent accumulations of insoluble Aβ42 in the cerebellum of ET, but not in age-matched PD. Whether this anomaly plays a role in ET symptoms warrants further investigations.

Bidirectional links between Alzheimer's disease and Niemann-Pick type C disease

Alzheimer's disease (AD) and Niemann-Pick type C (NPC) disease are progressive neurodegenerative diseases with very different epidemiology and etiology. AD is a common cause of dementia with a complex polyfactorial etiology, including both genetic and environmental risk factors, while NPC is a very rare autosomal recessive disease. However, the diseases share some disease-related molecular pathways, including abnormal cholesterol metabolism, and involvement of amyloid-β (Aβ) and tau pathology. Here we review recent studies on these pathological traits, focusing on studies of Aβ and tau pathology in NPC, and the importance of the NPC1 gene in AD. Further studies of similarities and differences between AD and NPC may be useful to increase the understanding of both these devastating neurological diseases.

Complex proteinopathy with accumulations of prion protein, hyperphosphorylated tau, α-synuclein and ubiquitin in experimental bovine spongiform encephalopathy of monkeys

Proteins aggregate in several slowly progressive neurodegenerative diseases called 'proteinopathies'. Studies with cell cultures and transgenic mice overexpressing mutated proteins suggested that aggregates of one protein induced misfolding and aggregation of other proteins as well - a possible common mechanism for some neurodegenerative diseases. However, most proteinopathies are 'sporadic', without gene mutation or overexpression. Thus, proteinopathies in WT animals genetically close to humans might be informative. Squirrel monkeys infected with the classical bovine spongiform encephalopathy agent developed an encephalopathy resembling variant Creutzfeldt-Jakob disease with accumulations not only of abnormal prion protein (PrP(TSE)), but also three other proteins: hyperphosphorylated tau (p-tau), α-synuclein and ubiquitin; β-amyloid protein (Aβ) did not accumulate. Severity of brain lesions correlated with spongiform degeneration. No amyloid was detected. These results suggested that PrP(TSE) enhanced formation of p-tau and aggregation of α-synuclein and ubiquitin, but not Aβ, providing a new experimental model for neurodegenerative diseases associated with complex proteinopathies.

Cerebellar volume correlates with saccadic gain and ataxia in adult Niemann-Pick type C

BACKGROUND AND PURPOSE

Cerebellar Purkinje cells are known to be highly vulnerable to neuronal pathology in Niemann-Pick type C (NPC), a disease where widespread white matter changes have also been reported. We sought to determine the relationship between white and grey matter cerebellar changes and clinical variables in NPC.

MATERIALS AND METHODS

Ten adult patients with NPC were matched to control subjects (n=27) on age and gender. Patients were rated for symptom duration and severity, degree of ataxia, and were assessed for saccadic eye measures. Cerebellar white and grey matter volumes were automatically segmented using the Freesurfer software package.

RESULTS

NPC patients had a significant reduction in both grey and white matter volumes. Volume did not correlate with symptom duration or severity, but did correlate with saccadic gain and ataxia measures.

CONCLUSIONS

Both cerebellar grey and white matter volume decreases in adult NPC, and these changes are associated with impairments in saccadic gain and in motor control.

Modulating molecular chaperones improves sensory fiber recovery and mitochondrial function in diabetic peripheral neuropathy

Quantification of intra-epidermal nerve fibers (iENFs) is an important approach to stage diabetic peripheral neuropathy (DPN) and is a promising clinical endpoint for identifying beneficial therapeutics. Mechanistically, diabetes decreases neuronal mitochondrial function and enhancing mitochondrial respiratory capacity may aid neuronal recovery from glucotoxic insults. We have proposed that modulating the activity and expression of heat shock proteins (Hsp) may be of benefit in treating DPN. KU-32 is a C-terminal Hsp90 inhibitor that improved thermal hypoalgesia in diabetic C57Bl/6 mice but it was not determined if this was associated with an increase in iENF density and mitochondrial function. After 16 weeks of diabetes, Swiss Webster mice showed decreased electrophysiological and psychosensory responses and a >30% loss of iENFs. Treatment of the mice with ten weekly doses of 20mg/kg KU-32 significantly reversed pre-existing deficits in nerve conduction velocity and responses to mechanical and thermal stimuli. KU-32 therapy significantly reversed the pre-existing loss of iENFs despite the identification of a sub-group of drug-treated diabetic mice that showed improved thermal sensitivity but no increase in iENF density. To determine if the improved clinical indices correlated with enhanced mitochondrial activity, sensory neurons were isolated and mitochondrial bioenergetics assessed ex vivo using extracellular flux technology. Diabetes decreased maximal respiratory capacity in sensory neurons and this deficit was improved following KU-32 treatment. In conclusion, KU-32 improved physiological and morphologic markers of degenerative neuropathy and drug efficacy may be related to enhanced mitochondrial bioenergetics in sensory neurons.

The many faces of tau

While the microtubule-binding capacity of the protein tau has been known for many years, new functions of tau in signaling and cytoskeletal organization have recently emerged. In this review, we highlight these functions and the potential roles of tau in neurodegenerative disease. We also discuss the therapeutic potential of drugs targeting various aspects of tau biology.

Interplay between glycogen synthase kinase-3β and tau in the cerebellum of Hsp27 transgenic mouse

The association between heat shock protein 27 (Hsp27) and hyperphosphorylated tau has gained attention for more than a decade, but it has never been explored in vivo. In the present study, we found that tau phosphorylated at S396/404 (PHF-1) and S262 sites was significantly increased in the cerebellum of Hsp27 transgenic mice, which was concomitant with increased glycogen synthase kinase-3β (GSK3β) phosphorylated at Y216 and decreased GSK3β phosphorylated at S9. Neither 70-kDa ribosomal protein S6 kinase (p70S6K; total p70S6K, p70S6K at T389, and p70S6K at T421/S424) nor protein phosphatase PP2A (total PP2A, PP2A at Y307, methylated or demethylated PP2A) was changed. This suggests that the increased tau phosphorylation at S396/404 and S262 sites may be induced by Hsp27 through enhancement of GSK3β activity in the mouse cerebellum.

Loss of amyloid precursor protein in a mouse model of Niemann-Pick type C disease exacerbates its phenotype and disrupts tau homeostasis

Niemann-Pick type C disease (NPC) is a lysosomal storage disorder which, at the cellular level, shows amyloid Aβ and tau pathologies comparable to those seen in the AD brain. Here, we have investigated, in a mouse model of NPC, the impact of removing the source of Aβ, namely APP, on the disease phenotype and on the expression levels and phosphorylation patterns of tau. We reasoned that removing APP from the NPC brain might help to unveil its impact on the disease phenotype and shed light on the mechanisms governing the interaction, both physiological and pathological, between APP function and tau homeostasis, at least in NPC. We show that, unexpectedly, loss of APP in NPC mice leads to poorer neuromuscular coordination and cumulative survival rates; exacerbation of their cholesterol abnormalities; higher levels of astrocytosis and dysregulation of tau homeostasis. Our results are consistent with a mechanism of neurodegeneration in the NPC and AD brains in which cholesterol dysregulation is a key early pathogenic event affecting tau homeostasis in parallel with, and independently of, amyloid accumulation.

Tau, prions and Aβ: the triad of neurodegeneration

This article highlights the features that connect prion diseases with other cerebral amyloidoses and how these relate to neurodegeneration, with focus on tau phosphorylation. It also discusses similarities between prion disease and Alzheimer's disease: mechanisms of amyloid formation, neurotoxicity, pathways involved in triggering tau phosphorylation, links to cell cycle pathways and neuronal apoptosis. We review previous evidence of prion diseases triggering hyperphosphorylation of tau, and complement these findings with cases from our collection of genetic, sporadic and transmitted forms of prion diseases. This includes the novel finding that tau phosphorylation consistently occurs in sporadic CJD, in the absence of amyloid plaques.

Defective cholesterol trafficking in Niemann-Pick C-deficient cells

Pathways of intracellular cholesterol trafficking are poorly understood at the molecular level. Mutations in Niemann-Pick C (NPC) proteins, NPC1 and NPC2, however, have led to insights into the mechanism by which endocytosed cholesterol is exported from late endosomes/lysosomes (LE/L). Mutations in NPC1, a multi-spanning membrane protein of LE/L, or mutations in NPC2, a soluble luminal protein of LE/L, cause the neurodegenerative disorder NPC disease. This review focuses on data supporting a model in which movement of cholesterol out of LE/L is mediated by the sequential action of the two NPC proteins. We also discuss potential therapies for NPC disease, including evidence that treatment of NPC-deficient mice with the cholesterol-binding compound, cyclodextrin, markedly attenuates neurodegeneration, and increases life-span, of NPC1-deficient mice.

A mutation affecting the sodium/proton exchanger, SLC9A6, causes mental retardation with tau deposition

We have studied a family with severe mental retardation characterized by the virtual absence of speech, autism spectrum disorder, epilepsy, late-onset ataxia, weakness and dystonia. Post-mortem examination of two males revealed widespread neuronal loss, with the most striking finding being neuronal and glial tau deposition in a pattern reminiscent of corticobasal degeneration. Electron microscopic examination of isolated tau filaments demonstrated paired helical filaments and ribbon-like structures. Biochemical studies of tau demonstrated a preponderance of 4R tau isoforms. The phenotype was linked to Xq26.3, and further analysis identified an in-frame 9 base pair deletion in the solute carrier family 9, isoform A6 (SLC9A6 gene), which encodes sodium/hydrogen exchanger-6 localized to endosomal vesicles. Sodium/hydrogen exchanger-6 is thought to participate in the targeting of intracellular vesicles and may be involved in recycling synaptic vesicles. The striking tau deposition in our subjects reveals a probable interaction between sodium/proton exchangers and cytoskeletal elements involved in vesicular transport, and raises the possibility that abnormalities of vesicular targeting may play an important role in more common disorders such as Alzheimer's disease and autism spectrum disorders.

Tau normal function influences Niemann-Pick type C disease pathogenesis in mice and modulates autophagy in NPC1-deficient cells

The tauopathies are a diverse class of devastating neurodegenerative disorders, characterized by the hyperphosphorylation and aggregation of the microtubule binding protein tau. Niemann-Pick type C disease (NPC) is a tauopathy that affects children,and is caused by mutations in intracellular lipid and cholesterol trafficking proteins. Loss-of-function mutations in the NPC1gene are responsible for 95 percent of all NPC cases, and lead to progressive neurodegeneration and early death. To assess the extent to which tau affects NPC pathology, we generated mice that lack both NPC1 and tau. NPC1/tau double-null mutants exhibit an exacerbated NPC phenotype, including severe systemic manifestations,and die significantly earlier than NPC1 single-null mutants.Since autophagy has been previously implicated in NPC pathogenesis,we investigated the impact of tau deletion on this pathway.Acute reductions of tau in NPC1-deficient fibroblasts significantly decrease autophagic induction and flux, while having no effect on the autophagic pathway in control cells. Here we propose a model in which tau's normal function is critical to the induction of autophagy in NPC1 deficiency, and suggest that this novel mechanism contributes to cellular dysfunction in the tauopathies.

The neuroprotective effects of cyclin-dependent kinase-5 inhibition in mice with Niemann-Pick disease type C

In order to investigate the neuroprotective effects of cyclin-dependent kinase-5 (cdk-5) inhibition in mice with Niemann-Pick disease type C (NPC) (npc(-/-)), recombinant adeno-associated virus (rAAV) carrying the small interfering RNA (siRNA) specific for cdk-5 gene was injected into 3-day-old npc(-/-) mice intracerebroventricularly. The rAAV-GFP-injected age-matched npc(-/-) mice and non-surgery age-matched npc(-/-) mice were employed as controls (n=6-10/group). From the 4th to 8th week after the treatment, mice were weighed, and evaluated for limb motor activity by using the coat hanger test once a week. Eight-week-old npc(-/-) mice were sacrificed by decapitation, and brains were quickly dissected and halved sagittally. Immunohistochemistry, Western blotting, and HE staining were used to evaluate the neuropathology in npc(-/-) mice. The results showed that rAAV-cdk-5-siRNA-GFP significantly reduced the number of axonal spheroids, delayed the death of Purkinje neurons, ameliorated motor defects in npc(-/-) mice, and significantly attenuated the hyperphosphorylation of tau proteins. These data suggested that inhibition of cdk-5 activity has neuroprotective effect on neurons in NPC mice.

Mitotic epitopes are incorporated into age-dependent neurofibrillary tangles in Niemann-Pick disease type C

The mechanism underlying neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) and other neurodegenerative disorders remains elusive. Niemann-Pick disease type C (NPC) is a kind of genetic neurovisceral disorder in which the intracellular sequestration of cholesterol and other lipids in neurons, NFT formation and neuronal degeneration in brain are the neuropathology hallmarks. The age of onset and progression of the disease vary dramatically. We have analyzed the hippocampus from 17 NPC cases, aged from 7 months to 55 years, to depict the temporal characteristics of NFT formation. Unexpectedly, classic NFT was observed in about 4-year-old NPC brain, suggesting that NFT is not aging dependent, and that juvenile brain neurons satisfy the requirements for NFT formation. NFT in the hippocampus of NPC was significantly increased in number with the advance of age. More importantly, multiple mitotic phase markers, which are not usually found in normal mature neurons, were abundant in the affected neurons and incorporated into NFT. The unusual activation of cdc2/cyclin B kinase and downstream mitotic indices are closely associated with the age-dependent NFT formation, signifying the contribution of abortive cell cycle to neurodegeneration. The cdc2 inhibitors may be therapeutically used for early intervention of neurodegeneration and NFT formation in NPC.

Lentivector-mediated RNAi efficiently downregulates expression of murine TNF-alpha gene in vitro and in vivo

In order to explore the role of TNF-alpha in Niemann-Pick type C (NPC) disease, lentiviral-delivered RNA interference (RNAi) was used to silence the expression of murine TNF-alpha gene in vitro and in npc mice. Interference efficiency of the lentivirus expressing TNF-alpha-siRNA, previously constructed with the concentration of 2 x 10(8) ifu/mL, was determined by RT-PCR and ELISA in BV-2 cells and astrocytes. At the same time, the constructed Lenti-TNF-alpha-siRNA was intracerebroventricularly infused into 4-week old npc mice for a 4-week period, and the mice were divided into 3 groups: Lenti-TNF-alpha-siRNA (n=6), control lentivirus (n=6), and NPC mice without any intervention (n=4). By using immunohistochemistry and real-time PCR, the down-regulation of the target genes was detected. The Lenti-TNF-alpha-siRNA downregulated the expression of murine TNF-alpha gene efficiently in vitro and the interference efficiency was 66.7%. Lentivirus could be expressed stably for long-term in the npc mice brain. Immunohistochemistry and real-time PCR revealed that, as compared with non-intervention group and Lenti-control group, Lenti-TNF-alpha-siRNA efficiently down-regulated the expression of murine TNF-alpha gene with the interference efficiency being 66.9%. TNF-alpha-siRNA down-regulated the expression of TNF-alpha gene in vitro and in vivo, which provided a potential tool for studying and treating neurodegenerative diseases and TNF-alpha-related diseases.

Mechanisms and consequences of impaired lipid trafficking in Niemann-Pick type C1-deficient mammalian cells

Niemann-Pick C disease is a fatal progressive neurodegenerative disorder caused in 95% of cases by mutations in the NPC1 gene; the remaining 5% of cases result from mutations in the NPC2 gene. The major biochemical manifestation of NPC1 deficiency is an abnormal sequestration of lipids, including cholesterol and glycosphingolipids, in late endosomes/lysosomes (LE/L) of all cells. In this review, we summarize the current knowledge of the NPC1 protein in mammalian cells with particular focus on how defects in NPC1 alter lipid trafficking and neuronal functions. NPC1 is a protein of LE/L and is predicted to contain thirteen transmembrane domains, five of which constitute a sterol-sensing domain. The precise function of NPC1, and the mechanism by which NPC1 and NPC2 (both cholesterol binding proteins) act together to promote the movement of cholesterol and other lipids out of the LE/L, have not yet been established. Recent evidence suggests that the sequestration of cholesterol in LE/L of cells of the brain (neurons and glial cells) contributes to the widespread death and dysfunction of neurons in the brain. Potential therapies include treatments that promote the removal of cholesterol and glycosphingolipids from LE/L. Currently, the most promising approach for extending life-span and improving the quality of life for NPC patients is a combination of several treatments each of which individually modestly slows disease progression.

Astrocyte-only Npc1 reduces neuronal cholesterol and triples life span of Npc1-/- mice

Niemann-Pick type C (NPC) disease is an autosomal recessive, lethal neurodegenerative disorder. Although neurodegeneration of Purkinje cells in the mouse model (Npc1(-/-)) is thought to be autonomous, the basis of neuronal death in other regions of the brain remains elusive. We addressed this issue in vivo by using the glial fibrillary acidic protein (GFAP) promoter to direct astrocyte-specific, replacement expression of Npc1 in Npc1(-/-) mice. These mice showed enhanced survival, decreased neuronal storage of cholesterol associated with less accumulation of axonal spheroids, lower numbers of degenerated neurons and reactive astrocytes, and restoration of myelin tracts. Their death was not associated with the usual terminal decline in weight but instead with a loss of Purkinje cells and motor coordination. We conclude that neurodegeneration of Npc1(-/-) mice is greatly affected by the loss of fibrillary astrocyte function.

Tau deletion exacerbates the phenotype of Niemann-Pick type C mice and implicates autophagy in pathogenesis

Hyperphosphorylation and aggregation of the microtubule-binding protein tau characterize a diverse array of neurodegenerative disorders. Most of these lack mutations in the encoding MAPT gene, and the role of tau in disease pathogenesis remains controversial. Among these tauopathies is Niemann-Pick type C disease (NPC), a lysosomal storage disorder characterized by progressive neurodegeneration and premature death, most often caused by an inherited deficiency in the intracellular lipid trafficking protein NPC1. To determine the extent to which tau affects NPC pathogenesis, we generated Npc1-/- mice deficient in tau. Unexpectedly, NPC1/tau double null mutants are generated in markedly smaller litters, exhibit an enhanced systemic phenotype and die significantly earlier than NPC1 single null mutants. As autophagy is up-regulated in NPC and protein degradation through this pathway depends on movement along microtubules, we knocked down MAPT expression in NPC1-deficient human fibroblasts and examined effects on this pathway. We show that an acute reduction of tau expression in a cellular model of NPC decreases induction and flux through the autophagic pathway. Our data establish that MAPT deletion exacerbates the NPC phenotype through a mechanism independent of tau protein aggregation and identifies a critical role for tau in the regulation of autophagy in NPC1-deficient cells.

Mitogen-activated protein kinase activity may not be necessary for the neuropathology of Niemann-Pick type C mice

Hyperphosphorylation of neurofilament and tau, and formation of cytoskeletal lesions, are notable features of several human neurodegenerative diseases, including Niemann-Pick Disease Type C (NPC). Previous studies suggested that the MAPKs, extracellular signal regulated kinase 1 and 2 (ERK1/2) may play a significant role in this aspect of NPC. To test this idea, we treated npc mice with PD98059, a specific and potent inhibitor of MAPK activation. Although activity of ERK1/2 was inhibited by 40%, a 2-week intracerebroventricular infusion of PD98059 just prior to onset of cytoskeletal pathology and symptoms in npc mice did not delay or inhibit prominent hallmarks of NPC. Unexpectedly, ERK1/2 inhibition led to aggravation of tau hyperphosphorylation, particularly in oligodendroctyes, in a manner similar to that of certain human tauopathies. Our results suggest that ERK1/2 does not play a major role in NPC neuropathology, and therefore, that MAPK inhibition is unlikely to be a useful strategy for managing the disease.

p35/Cyclin-dependent kinase 5 is required for protection against beta-amyloid-induced cell death but not tau phosphorylation by ceramide

Ceramide is a bioactive sphingolipid that can prevent calpain activation and beta-amyloid (A beta) neurotoxicity in cortical neurons. Recent evidence supports A beta induction of a calpain-dependent cleavage of the cyclin-dependent kinase 5 (cdk5) regulatory protein p35 that contributes to tau hyperphosphorylation and neuronal death. Using cortical neurons isolated from wild-type and p35 knockout mice, we investigated whether ceramide required p35/cdk5 to protect against A beta-induced cell death and tau phosphorylation. Ceramide inhibited A beta-induced calpain activation and cdk5 activity in wild-type neurons and protected against neuronal death and tau hyperphosphorylation. Interestingly, A beta also increased cdk5 activity in p35-/- neurons, suggesting that the alternate cdk5 regulatory protein, p39, might mediate this effect. In p35 null neurons, ceramide blocked A beta-induced calpain activation but did not inhibit cdk5 activity or cell death. However, ceramide blocked tau hyperphosphorylation potentially via inhibition of glycogen synthase kinase-3beta. These data suggest that ceramide can regulate A beta cell toxicity in a p35/cdk5-dependent manner.

Hyperphosphorylation of tau and neurofilaments and activation of CDK5 and ERK1/2 in PTEN-deficient cerebella

Inherited mutations to the tumor suppressor PTEN sporadically lead to cerebellar gangliocytoma characterized by migration defects. This has been modeled by CNS-specific PTEN ablation in mice, but the underlying mechanism cannot be explained by the known role of PTEN in Akt/PKB inactivation. Here we show that the loss of PTEN in mouse cerebellar neurons causes neurodegeneration by hyperphosphorylation of tau and neurofilaments, and activation of Cdk5 and pERK1/2, suggesting that dysregulation of the PTEN/pAkt pathway can mediate neurodegeneration.

p35/p25 is not essential for tau and cytoskeletal pathology or neuronal loss in Niemann-Pick type C disease

Hyperactivation of the cyclin-dependent kinase 5 (cdk5), triggered by proteolytic conversion of its neuronal activator, p35, to a more potent byproduct, p25, has been implicated in Alzheimer's disease (AD), amyotrophic lateral sclerosis, and Niemann-Pick type C disease (NPC). This mechanism is thought to lead to the development of neuropathological hallmarks, i.e., hyperphosphorylated cytoskeletal proteins, neuronal inclusions, and neurodegeneration, that are common to all three diseases. This pathological ensemble is recapitulated in a single model, the npc-1 (npc(-/-)) mutant mouse. Previously, we showed that pharmacological cdk inhibitors dramatically reduced hyperphosphorylation, lesion formation, and locomotor defects in npc(-/-) mice, suggesting that cdk activity is required for NPC pathogenesis. Here, we used genetic ablation of the p35 gene to examine the specific involvement of p35, p25, and hence cdk5 activation in NPC neuropathogenesis. We found that lack of p35/p25 does not slow the onset or progression or improve the neuropathology of NPC. Our results provide direct evidence that p35/p25-mediated cdk5 deregulation is not essential for NPC pathology and suggest that similar pathology in AD may also be cdk5 independent.

Tau phosphorylation increases in symptomatic mice overexpressing A30P alpha-synuclein

Mice overexpressing mutant alpha-synuclein develop a progressive loss of motor function associated with the accumulation of aggregated alpha-synuclein in neurons of the brainstem. Recent reports suggest that tau pathology might also be associated with Parkinson disease (PD) and aggregation of alpha-synuclein. We now report that mice overexpressing A30P alpha-synuclein develop abnormally phosphorylated tau in parallel with the accumulation of aggregated alpha-synuclein. Enhanced phosphorylation of tau occurs only in symptomatic mice that also harbor abundant aggregated alpha-synuclein. The increased phosphorylation of tau occurs at S396/404 and S202 as shown by immunoblotting and immunocytochemical studies with the antibodies PHF-1 and AT8. Neurons that accumulated alpha-synuclein occurred in the dorsal brainstem and did not show strong colocalization with neurons that showed abnormal tau phosphorylation, which largely occurred in the ventral brainstem. Aggregation of alpha-synuclein and phosphorylation of tau are associated with increased levels of phosphorylated c-jun kinase (JNK), which is a stress kinase known to phosphorylate tau protein. These results suggest that alpha-synuclein pathology can stimulate early pathological changes in tau.

Cholesterol homeostasis in neurons and glial cells

Cholesterol is highly enriched in the brain compared to other tissues. Essentially all cholesterol in the brain is synthesized endogenously since plasma lipoproteins are unable to cross the blood-brain barrier. Cholesterol is transported within the central nervous system in the form of apolipoprotein E-containing lipoprotein particles that are secreted mainly by glial cells. Cholesterol is excreted from the brain in the form of 24-hydroxycholesterol. Apolipoprotein E and cholesterol have been implicated in the formation of amyloid plaques in Alzheimer's disease. In addition, the progressive neurodegenerative disorder Niemann-Pick C disease is characterized by defects in intracellular trafficking of cholesterol.

Before the loss: neuronal dysfunction in Niemann-Pick Type C disease

Niemann-Pick Type C (NPC) disease is an autosomal recessive disorder caused by mutations in either the NPC1 or HE1 genes. Hallmarks of this presently incurable disease include abnormal intracellular accumulation of cholesterol and glycosphingolipids, progressive neuropathology and neurodegeneration, and premature death. There have been increased efforts to understand the effects of NPC disease on neurons of the brain, in part due to the recent development of improved research tools and reagents, and in part due to the rapidly growing appreciation of the importance of cholesterol and lipoproteins in the brain during neuronal development, function, and degeneration. Here, we highlight fundamental aspects of neurons that appear to be affected by NPC disease, including their morphology, metabolism, intracellular transport, electrical signaling, and response to environmental factors, and suggest other potentially important areas for future investigation. This provides a framework for acquiring additional insight to this disorder and shaping new therapeutic approaches to NPC disease.

Consequences of NPC1 and NPC2 loss of function in mammalian neurons

Genetic deficiency of NPC1 or NPC2 results in a devastating cholesterol-glycosphingolipidosis of brain and other organs known as Niemann-Pick type C (NPC) disease. While NPC1 is a transmembrane protein believed involved in retroendocytic shuttling of substrate(s) to the Golgi and possibly elsewhere in cells as part of an essential recycling/homeostatic control mechanism, NPC2 is a soluble lysosomal protein known to bind cholesterol. The precise role(s) of NPC1 and NPC2 in endosomal-lysosomal function remain unclear, nor is it known whether the two proteins directly interact as part of this function. The pathologic features of NPC disease, however, are well documented. Brain cells undergo massive intracellular accumulation of glycosphingolipids (lactosylceramide, glucosylceramide, GM2 and GM3 gangliosides) and cholesterol and concomitant distortion of neuron shape (meganeurite formation). In neurons from humans with NPC disease the metabolic defects and storage often lead to extensive growth of new, ectopic dendrites (possibly linked to ganglioside sequestration) as well as formation of neurofibrillary tangles (NFTs) (possibly linked to dysregulation of cholesterol metabolism). Other features of cellular pathology in NPC disease include fragmentation of the Golgi apparatus and neuroaxonal dystrophy, though reasons for these changes remain largely unknown. As the disease progresses, neurodegeneration is also apparent for neurons in some brain regions, particularly Purkinje cells of the cerebellum, but the basis of this selective neuronal vulnerability is unknown. The NPC1 protein is evolutionarily conserved with homologues reported in yeast to humans; NPC2 is reported in C. elegans to humans. While neurons in mammalian models of NPC1 and NPC2 diseases exhibit many changes that are remarkably similar to those in humans (e.g., endosomal/lysosomal storage, Golgi fragmentation, neuroaxonal dystrophy, neurodegeneration), a reduced degree of ectopic dendritogenesis and an absence of NFTs in these species suggest important differences in the way lower mammalian neurons respond to NPC1/NPC2 loss of function.

Cyclin-dependent kinase inhibitors attenuate protein hyperphosphorylation, cytoskeletal lesion formation, and motor defects in Niemann-Pick Type C mice

Dysregulation of cyclin-dependent kinases (cdks) and cytoskeletal protein hyperphosphorylation characterizes a subset of human neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, and Niemann-Pick Type C (NPC). It is thought that these cytoskeletal changes lead eventually to development of hallmark cytoskeletal lesions such as neurofibrillary tangles and axonal spheroids. Although many studies support an involvement of cdks in these neurodegenerative cascades, it is not known whether cdk activity is essential. The naturally occurring npc-1 mutant mouse mimics human NPC, in displaying activation of cdk5, mitotic cdc2, and cdk4, with concomitant cytoskeletal pathology and neurodegeneration. We availed of this model and specific pharmacological inhibitors of cdk activity, to determine whether cdks are necessary for NPC neuropathology. The inhibitors were infused intracerebroventricularly for a 2-week period, initiated at a pathologically incipient stage. While an inactive stereoisomer, iso-olomoucine, was ineffective, two potent inhibitors, roscovitine and olomoucine, attenuated significantly the hyperphosphorylation of neurofilament, tau, and mitotic proteins, reduced the number of spheroids, modulated Purkinje neuron death, and ameliorated motor defects in npc mice. These results suggest that cdk activity is required for neuropathology and subsequent motor impairment in NPC. Studies aimed at knocking down individual cdks in these mice will help identify the specific cdk(s) that are essential, and delineate their precise roles in the neurodegenerative process.

The cellular biochemistry of cholesterol and statins: insights into the pathophysiology and therapy of Alzheimer's disease

The causes of late onset Alzheimer disease (AD) are poorly understood. Although beta-amyloid (Abeta) is thought to play a critical role in the pathophysiology of AD, no genetic evidence directly ties Abeta to late onset AD. This suggests that the accumulation of Abeta and neurodegeneration associated with AD might result from an abnormality that indirectly affects Abeta production or accumulation. Increasing evidence suggests that abnormalities in the metabolism of cholesterol and related molecules, such as cholseterol esters and 24(S) hydroxycholesterol might contribute to the pathophysiology of late onset AD by increasing production of Abeta. 24(S) Hydroxycholesterol is a member of a family of oxidized cholesterol catabolites, termed oxysterols, which function to regulate export of cholesterol from the cell and transcription of genes related to cholesterol metabolism. Cholesterol esters are cholesterol derivatives used for cholesterol storage. Levels of 24(S) hydroxycholesterol increase with AD. Polymorphisms in several different genes important for cholesterol physiology are associated with an increased load or level of Abeta in AD. These genes include apolipoprotein E, cholesterol 24 hydroxylase (Cyp46), acyl-CoA:cholesterol acetyltransferase (ACAT), and the cholesterol transporter ABCA1. Other studies show that levels of cholesterol, or its precursors, are elevated in subjects early in the course of AD. Finally, studies of the processing of amyloid precursor protein show that cholesterol and its catabolites modulate amyloid precursor protein processing and Abeta production. These lines of evidence raise the possibility that genetic abnormalities in cholesterol metabolism might contribute to the pathophysiology of AD.

Differential expression of cholesterol hydroxylases in Alzheimer's disease

Cholesterol is eliminated from neurons by oxidization, which generates oxysterols. Cholesterol oxidation is mediated by the enzymes cholesterol 24-hydroxylase (CYP46A1) and cholesterol 27-hydroxylase (CYP27A1). Immunocytochemical studies show that CYP46A1 and CYP27A1 are expressed in neurons and some astrocytes in the normal brain, and CYP27A1 is present in oligodendrocytes. In Alzheimer's disease (AD), CYP46A1 shows prominent expression in astrocytes and around amyloid plaques, whereas CYP27A1 expression decreases in neurons and is not apparent around amyloid plaques but increases in oligodendrocytes. Although previous studies have examined the effects of synthetic oxysterols on the processing of amyloid precursor protein (APP), the actions of the naturally occurring oxysterols have yet to be examined. To understand the role of cholesterol oxidation in AD, we compared the effects of 24(S)- and 27-hydroxycholesterol on the processing of APP and analyzed the cell-specific expression patterns of the two cholesterol hydroxylases in the human brain. Both oxysterols inhibited production of Abeta in neurons, but 24(S)-hydroxycholesterol was approximately 1000-fold more potent than 27-hydroxycholesterol. The IC(50) of 24(S)-hydroxycholesterol for inhibiting Abeta secretion was approximately 1 nm. Both oxysterols induced ABCA1 expression with IC(50) values similar to that for inhibition of A beta secretion, suggesting the involvement of liver X receptor. Oxysterols also inhibited protein kinase C activity and APP secretion following stimulation of protein kinase C. The selective expression of CYP46A1 around neuritic plaques and the potent inhibition of APP processing in neurons by 24(S)-hydroxycholesterol suggests that CYP46A1 affects the pathophysiology of AD and provides insight into how polymorphisms in the CYP46A1 gene might influence the pathophysiology of this prevalent disease.

Role of tau protein in both physiological and pathological conditions

The morphology of a neuron is determined by its cytoskeletal scaffolding. Thus proteins that associate with the principal cytoskeletal components such as the microtubules have a strong influence on both the morphology and physiology of neurons. Tau is a microtubule-associated protein that stabilizes neuronal microtubules under normal physiological conditions. However, in certain pathological situations, tau protein may undergo modifications, mainly through phosphorylation, that can result in the generation of aberrant aggregates that are toxic to neurons. This process occurs in a number of neurological disorders collectively known as tauopathies, the most commonly recognized of which is Alzheimer's disease. The purpose of this review is to define the role of tau protein under normal physiological conditions and to highlight the role of the protein in different tauopathies.

Intracellular accumulation of amyloidogenic fragments of amyloid-beta precursor protein in neurons with Niemann-Pick type C defects is associated with endosomal abnormalities

Niemann-Pick type C disease (NPC) is characterized by neurodegeneration secondary to impaired cholesterol trafficking and excessive glycosphingolipid storage. Abnormal cholesterol and ganglioside metabolism may influence the generation and aggregation of amyloidogenic fragments (ie, C99 and Abeta) from amyloid-beta precursor protein (APP), crucial factors causing neurodegeneration in Alzheimer's disease. To reveal whether abnormal accumulation and aggregation of APP fragments also occurs in NPC, we studied their expression in cultured cortical neurons treated with U18666A, a compound widely used to induce NPC defects, and also in brain tissues from NPC patients. U18666A treatment resulted in increased intraneuronal levels of C99 and insoluble Abeta42, which were distributed among early and late endosomes, in compartments distinct from where endogenous cholesterol accumulates. Analyses of NPC brains revealed that C99 or other APP C-terminal fragments (APP-CTF), but not Abeta42, accumulated in Purkinje cells, mainly in early endosomes. In contrast, in hippocampal pyramidal neurons, the major accumulated species was Abeta42, in late endosomes. Similar to what has been shown in Alzheimer's disease, cathepsin D, a lysosomal hydrolase, was redistributed to early endosomes in NPC Purkinje cells, where it co-localized with C99/APP-CTF. Our results suggest that endosomal abnormalities related to abnormal lipid trafficking in NPC may contribute to abnormal APP processing and Abeta42/C99/APP-CTF deposition.

Neuronal cyclin-dependent kinase 5: role in nervous system function and its specific inhibition by the Cdk5 inhibitory peptide

Cyclin-dependent kinase 5 (Cdk5) is a member of the cyclin-dependent kinase family that is involved in the regulation of the cell cycle. As their name suggests, the Cdks require association with activator proteins called cyclins for their activity. Cdk5, however, is unique to this family of proline-directed serine/threonine kinases on two accounts. Firstly, Cdk5 has not been found to function in the cell cycle and, although expressed in a number of tissues, its activity is restricted to the nervous system. Secondly, unlike the other members of the Cdk family, Cdk5 is not activated by association with a cyclin, although it can bind them. Instead, Cdk5 is activated by the activator proteins p35 and p39 that are structurally distinct from cyclins and have, for the most part, a neuronal-specific expression pattern. In the past decade of research on Cdk5, it is now established that Cdk5 activity is critical for the proper formation and function of the brain. Moreover, its role as a central kinase, phosphorylating its substrates in its 'cross-talk' control of other kinase and signal transduction pathways, has also been determined. In addition to the normal physiological role of Cdk5, the kinase has been implicated in certain neurodegenerative disorders. For example, Cdk5 associates with the proteolytic, more active p25 fragment that is derived through the cleavage of p35. In turn, the p25/Cdk5 complex aberrantly phosphorylates its substrates tau and neurofilaments, which has been implicated in the pathogenesis of these disorders. Here, we attempt to review the past decade of research on Cdk5 from our laboratory and others, on the roles of Cdk5 in nervous system function. Additionally, our research has recently uncovered a possible therapeutic avenue of research, focusing on inhibition of aberrant Cdk5 hyperactivity which may well be used to treat the symptoms of a number of neurodegenerative diseases. The elucidation of a specific inhibitor of p25/Cdk5, termed CIP, also inhibits p25/Cdk5-mediated tau phosphorylation. This may well provide us with avenues of research focusing on the inhibition of pathologically damaging p25/Cdk5 species.

Plasticity and the spread of Alzheimer's disease-like changes

Tangles are a major histopathological feature of Alzheimer's disease and their regional location and number correlate significantly with the individual's cognitive decline. Intriguingly, these tangles are formed only in a small subset of nerve cell types and are practically absent in most animal species examined so far. In humans, tangle formation seemingly starts decades before clinical signs of dementia are seen and spread over cortical areas in a regular manner described by the Braak classification. In the present article the role of plasticity-related molecules and mechanisms are discussed considering their putative role in neuronal vulnerability and spread of tangles. Special emphasis is given to some aspects of lipid metabolism, that is, apolipoprotein E polymorphism, statin effects, and lysosomal dysfunction in Alzheimer's and Niemann-Pick C's diseases.

Cell cycle regulation of neuronal apoptosis in development and disease

Apoptosis of neurons is indispensable to the normal development of the nervous system and contributes to neuronal loss in neurologic injury and disease. Life and death decisions are imposed upon neurons by extracellular and intracellular stimuli including the lack of trophic support, exposure to neurotoxins, oxidative stress, and DNA damage. These stimuli induce signaling pathways that are integrated at the mitochondrial apoptotic machinery culminating in cell survival or death. Growing evidence suggests that cell cycle proteins are expressed in dying neurons in the developing and adult brain. However, the role and mechanisms by which re-activation of cell cycle pathways in postmitotic neurons propagates an apoptotic signal to the cell death machinery are just beginning to be characterized. Here, we will review the molecular mechanisms of neuronal cell death and survival with a focus on recent findings on cell cycle regulation of neuronal apoptosis in primary cultures of neurons, mouse models of neuronal diseases, and human neurodegenerative diseases.

Cyclin-dependent kinase-5 is involved in neuregulin-dependent activation of phosphatidylinositol 3-kinase and Akt activity mediating neuronal survival

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway plays an important role in mediating survival signals in wide variety of neurons and cells. Recent studies show that Akt also regulates metabolic pathways to regulate cell survival. In this study, we reported that cyclin-dependent kinase-5 (Cdk5) regulates Akt activity and cell survival through the neuregulin-mediated PI 3-kinase signaling pathway. We found that brain extracts of Cdk5-/-mice display a lower PI 3-kinase activity and phosphorylation of Akt compared with that in wild type mice. Moreover, we demonstrated that Cdk5 phosphorylated Ser-1176 in the neuregulin receptor ErbB2 and phosphorylated Thr-871 and Ser-1120 in the ErbB3 receptor. We identified the Ser-1120 sequence RSRSPR in ErbB3 as a novel phosphorylation consensus sequence of Cdk5. Finally, we found that Cdk5 activity is involved in neuregulin-induced Akt activity and neuregulin-mediated neuronal survival. These findings suggest that Cdk5 may exert a key role in promoting neuronal survival by regulating Akt activity through the neuregulin/PI 3-kinase signaling pathway.