Commitment of 1-methyl-4-phenylpyrinidinium ion-induced neuronal cell death by proteasome-mediated degradation of p35 cyclin-dependent kinase 5 activator

Update on the association between alpha-synuclein and tau with mitochondrial dysfunction: Implications for Parkinson's disease

The critical role of mitochondrial dysfunction in the pathological mechanisms of neurodegenerative disorders, particularly Parkinson's disease (PD), is well established. Compelling evidence indicates that Parkinson's proteins (e.g., α-synuclein, Parkin, PINK1, DJ-1, and LRRK2) are associated with mitochondrial dysfunction and oxidative stress in PD. Significantly, there is a possible central role of alpha-synuclein (α-Syn) in the occurrence of mitochondrial dysfunction and oxidative stress by the mediation of different signaling pathways. Also, tau, traditionally considered as the main component of neurofibrillary tangles, aggregates and amplifies the neurotoxic effects on mitochondria by interacting with α-Syn. Moreover, oxidative stress caused by mitochondrial dysfunction favors assembly of both α-Syn and tau and also plays a key role in the formation of protein aggregates. In this review, we provide an overview of the relationship between these two pathological proteins and mitochondrial dysfunction in PD, and also summarize the underlying mechanisms in the interplay of α-Syn aggregation and phosphorylated tau targeting the mitochondria, to find new strategies to prevent PD processing.

CDK5: Key Regulator of Apoptosis and Cell Survival

The atypical cyclin-dependent kinase 5 (CDK5) is considered as a neuron-specific kinase that plays important roles in many cellular functions including cell motility and survival. The activation of CDK5 is dependent on interaction with its activator p35, p39, or p25. These activators share a CDK5-binding domain and form a tertiary structure similar to that of cyclins. Upon activation, CDK5/p35 complexes localize primarily in the plasma membrane, cytosol, and perinuclear region. Although other CDKs are activated by cyclins, binding of cyclin D and E showed no effect on CDK5 activation. However, it has been shown that CDK5 can be activated by cyclin I, which results in anti-apoptotic functions due to the increased expression of Bcl-2 family proteins. Treatment with the CDK5 inhibitor roscovitine sensitizes cells to heat-induced apoptosis and its phosphorylation, which results in prevention of the apoptotic protein functions. Here, we highlight the regulatory mechanisms of CDK5 and its roles in cellular processes such as gene regulation, cell survival, and apoptosis.

Unexpected Neuroprotective Effects of Loganin on 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Induced Neurotoxicity and Cell Death in Zebrafish

1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP), which induces the pathological characteristics of Parkinson's disease in rodents, also specifically targets dopaminergic neurons in zebrafish embryos and larvae. Loganin, a traditional Chinese drug, was reported to regulate immune function and possess anti-inflammatory and anti-shock effects. Here, we investigate the role of loganin in MPTP-induced Parkinson-like abnormalities in zebrafish. MPTP treatment-induced abnormal development, in larvae, such as pericardium edema, increased yolk color, yolk sac edema, and retarded yolk sac resorption, as well as defects in brain development. Loganin could block MPTP-induced defects, with little toxicity to the eggs. Results of whole mount in situ hybridization showed loganin prevented the loss of both dopaminergic neurons and locomotor activity, exhibited by larvae treated with MPTP. In addition, loganin significantly rescued MPTP-induced neurotoxicity on PC12 cells, possibly through the suppression of PI3K/Akt/mTOR axis and JNK signaling pathways. In conclusion, loganin blocks MPTP-induced neurotoxicity and abnormal development in zebrafish. J. Cell. Biochem. 118: 615-628, 2017. © 2016 Wiley Periodicals, Inc.

In silico analyses and global transcriptional profiling reveal novel putative targets for Pea3 transcription factor related to its function in neurons

Pea3 transcription factor belongs to the PEA3 subfamily within the ETS domain transcription factor superfamily, and has been largely studied in relation to its role in breast cancer metastasis. Nonetheless, Pea3 plays a role not only in breast tumor, but also in other tissues with branching morphogenesis, including kidneys, blood vasculature, bronchi and the developing nervous system. Identification of Pea3 target promoters in these systems are important for a thorough understanding of how Pea3 functions. Present study particularly focuses on the identification of novel neuronal targets of Pea3 in a combinatorial approach, through curation, computational analysis and microarray studies in a neuronal model system, SH-SY5Y neuroblastoma cells. We not only show that quite a number of genes in cancer, immune system and cell cycle pathways, among many others, are either up- or down-regulated by Pea3, but also identify novel targets including ephrins and ephrin receptors, semaphorins, cell adhesion molecules, as well as metalloproteases such as kallikreins, to be among potential target promoters in neuronal systems. Our overall results indicate that rather than early stages of neurite extension and axonal guidance, Pea3 is more involved in target identification and synaptic maturation.

Cdk5/p25 specific inhibitory peptide TFP5 rescues the loss of dopaminergic neurons in a sub-acute MPTP induced PD mouse model

Parkinson's disease (PD) is pathologically characterized by progressively loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and the formation of Lewy bodies. In 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) induced PD mice models, the calpain- cyclin-dependent kinase 5 (Cdk5)-myocyte enhancer factor 2 (MEF2) signaling has been proven in governing dopaminergic neuronal death. Under MPTP insult, p35 is cleaved by calpain into p25, which binds to Cdk5 and exhibits hyperactivity of Cdk5/p25. Cdk5/p25 inactivates MEF2, a survivor factor, which is critical for DA neuronal death. In this study, neuroprotective effect of the Cdk5/p25 specific peptide, TFP5, was evaluated in sub-acute MPTP induced PD mouse model by intraperitoneal (i.p.) injection of MPTP for five consecutive days. The results indicated that the levels of p35 and p25, and p25/p35 ratio increased in the sub-acute MPTP mice. TFP5 broadly reached cortex neuron, hippocampus and SNpc areas after i.p. injections. Pretreatment with 45mg/kg/day TFP5, as well as 10mgkg/day Cdk5 inhibitor roscovitine, for three days significantly rescued DA neuronal loss up to 9.8% or 9.7% respectively compared to the saline treated group. Treatment of TFP5 and roscovitine reduced the levels of inactive form of MEF2 and cleaved caspase 3, thus protected apoptosis of DA neurons against MPTP insult. Our results propose that TFP5 might be a potential therapeutic candidate for PD.

Two Degradation Pathways of the p35 Cdk5 (Cyclin-dependent Kinase) Activation Subunit, Dependent and Independent of Ubiquitination

Cdk5 is a versatile protein kinase that is involved in various neuronal activities, such as the migration of newborn neurons, neurite outgrowth, synaptic regulation, and neurodegenerative diseases. Cdk5 requires the p35 regulatory subunit for activation. Because Cdk5 is more abundantly expressed in neurons compared with p35, the p35 protein levels determine the kinase activity of Cdk5. p35 is a protein with a short half-life that is degraded by proteasomes. Although ubiquitination of p35 has been previously reported, the degradation mechanism of p35 is not yet known. Here, we intended to identify the ubiquitination site(s) in p35. Because p35 is myristoylated at the N-terminal glycine, the possible ubiquitination sites are the lysine residues in p35. We mutated all 23 Lys residues to Arg (p35 23R), but p35 23R was still rapidly degraded by proteasomes at a rate similar to wild-type p35. The degradation of p35 23R in primary neurons and the Cdk5 activation ability of p35 23R suggested the occurrence of ubiquitin-independent degradation of p35 in physiological conditions. We found that p35 has the amino acid sequence similar to the ubiquitin-independent degron in the NKX3.1 homeodomain transcription factor. An Ala mutation at Pro-247 in the degron-like sequence made p35 stable. These results suggest that p35 can be degraded by two degradation pathways: ubiquitin-dependent and ubiquitin-independent. The rapid degradation of p35 by two different methods would be a mechanism to suppress the production of p25, which overactivates Cdk5 to induce neuronal cell death.

Cdk5 at crossroads of protein oligomerization in neurodegenerative diseases: facts and hypotheses

Cyclin-dependent kinase 5 (Cdk5) is involved in proper neurodevelopment and brain function and serves as a switch between neuronal survival and death. Overactivation of Cdk5 is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important link between disease-initiating factors and cell death effectors. A common hallmark of neurodegenerative disorders is incorrect folding of specific proteins, thus leading to their intra- and extracellular accumulation in the nervous system. Abnormal Cdk5 signaling contributes to dysfunction of individual proteins and has a substantial role in either direct or indirect interactions of proteins common to, and critical in, different neurodegenerative diseases. While the roles of Cdk5 in α-synuclein (ASN) - tau or β-amyloid peptide (Aβ) - tau interactions are well documented, its contribution to many other pertinent interactions, such as that of ASN with Aβ, or interactions of the Aβ - ASN - tau triad with prion proteins, did not get beyond plausible hypotheses and remains to be proven. Understanding of the exact position of Cdk5 in the deleterious feed-forward loop critical for development and progression of neurodegenerative diseases may help designing successful therapeutic strategies of several fatal neurodegenerative diseases. Cyclin-dependent kinase 5 (Cdk5) is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important factor involved in protein misfolding, toxicity and interaction. We suggest that Cdk5 may contribute to the vicious circle of neurotoxic events involved in the pathogenesis of different neurodegenerative diseases.

Inhibition of Protein Ubiquitination by Paraquat and 1-Methyl-4-Phenylpyridinium Impairs Ubiquitin-Dependent Protein Degradation Pathways

Intracytoplasmic inclusions of protein aggregates in dopaminergic cells (Lewy bodies) are the pathological hallmark of Parkinson's disease (PD). Ubiquitin (Ub), alpha (α)-synuclein, p62/sequestosome 1, and oxidized proteins are the major components of Lewy bodies. However, the mechanisms involved in the impairment of misfolded/oxidized protein degradation pathways in PD are still unclear. PD is linked to mitochondrial dysfunction and environmental pesticide exposure. In this work, we evaluated the effects of the pesticide paraquat (PQ) and the mitochondrial toxin 1-methyl-4-phenylpyridinium (MPP(+)) on Ub-dependent protein degradation pathways. No increase in the accumulation of Ub-bound proteins or aggregates was observed in dopaminergic cells (SK-N-SH) treated with PQ or MPP(+), or in mice chronically exposed to PQ. PQ decreased Ub protein content, but not its mRNA transcription. Protein synthesis inhibition with cycloheximide depleted Ub levels and potentiated PQ-induced cell death. The inhibition of proteasomal activity by PQ was found to be a late event in cell death progression and had neither effect on the toxicity of either MPP(+) or PQ, nor on the accumulation of oxidized sulfenylated, sulfonylated (DJ-1/PARK7 and peroxiredoxins), and carbonylated proteins induced by PQ. PQ- and MPP(+)-induced Ub protein depletion prompted the dimerization/inactivation of the Ub-binding protein p62 that regulates the clearance of ubiquitinated proteins by autophagy. We confirmed that PQ and MPP(+) impaired autophagy flux and that the blockage of autophagy by the overexpression of a dominant-negative form of the autophagy protein 5 (dnAtg5) stimulated their toxicity, but there was no additional effect upon inhibition of the proteasome. PQ induced an increase in the accumulation of α-synuclein in dopaminergic cells and membrane-associated foci in yeast cells. Our results demonstrate that the inhibition of protein ubiquitination by PQ and MPP(+) is involved in the dysfunction of Ub-dependent protein degradation pathways.

The Impact of Mitochondrial Fusion and Fission Modulation in Sporadic Parkinson's Disease

Accumulating data suggests that mitochondrial deficits may underline both sporadic and familial Parkinson's disease (PD) neurodegenerative process. Impairment of mitochondrial dynamics results in reactive oxygen species (ROS) production, decreases mitochondrial membrane potential, and could potentiate the accumulation of dysfunctional mitochondria. Excessive mitochondrial fragmentation is associated with the pathology of sporadic PD. Therefore, we modulated mitochondria fusion and fission in different sporadic PD cellular models. We found alterations in two proteins known to regulate mitochondrial fusion and fission events (OPA1 and Drp1, respectively). OPA1 long isoform cleavage seems to be, at least in part, responsible for mitochondrial fragmented pattern observed in sporadic PD cellular models. Moreover, mitochondrial fragmentation can also occur due to an increase in Drp1 that is translocated into the mitochondria by phosphorylation. To disclose the relevance of these alterations to the fragmentation of the mitochondrial network, we overexpressed OPA1 and knock down Drp1. OPA1 overexpression did not rescue MPP(+)-induced increase in ROS. Nevertheless, Drp1 knockdown due to an increase in mitochondrial elongation and interconnectivity rescued mitochondrial membrane potential and decreased ROS production in sporadic PD cells. Overall, our findings suggest that Drp1-dependent mitochondrial fragmentation plays a crucial role in mediating mitochondrial DNA induced mitochondria abnormalities and cellular dysfunction in sporadic PD.

Regulation of the VHL/HIF-1 pathway by DJ-1

DJ-1 (PARK7) is a gene linked to autosomal recessive Parkinson disease (PD). We showed previously that DJ-1 loss sensitizes neurons in models of PD and stroke. However, the biochemical mechanisms underlying this protective role are not completely clear. Here, we identify Von Hippel Lindau (VHL) protein as a critical DJ-1-interacting protein. We provide evidence that DJ-1 negatively regulates VHL ubiquitination activity of the α-subunit of hypoxia-inducible factor-1 (HIF-1α) by inhibiting HIF-VHL interaction. Consistent with this observation, DJ-1 deficiency leads to lowered HIF-1α levels in models of both hypoxia and oxidative stress, two stresses known to stabilize HIF-1α. We also demonstrate that HIF-1α accumulation rescues DJ-1-deficient neurons against 1-methyl-4-phenylpyridinium-induced toxicity. Interestingly, lymphoblast cells extracted from DJ-1-related PD patients show impaired HIF-1α stabilization when compared with normal individuals, indicating that the DJ-1-VHL link may also be relevant to a human context. Together, our findings delineate a model by which DJ-1 mediates neuronal survival by regulation of the VHL-HIF-1α pathway.

Dab1-mediated colocalization of multi-adaptor protein CIN85 with Reelin receptors, ApoER2 and VLDLR, in neurons

Reelin-Dab1 signaling is indispensable for proper positioning of neurons in mammalian brain. Reelin is a glycoprotein secreted from Cajal-Reztuis cells in marginal zone of cerebral cortex, and its receptors are Apolipoprotein E receptor 2 (ApoER2) or very low density lipoprotein receptor (VLDLR) expressed on migrating neurons. When Reelin binds to ApoER2 or VLDLR, an adaptor protein Dab1 bound to the receptors undergoes Tyr phosphorylation that is essential for Reelin signaling. We reported previously that Cdk5-p35 phosphorylates Dab1 at Ser400 and Ser491 and the phosphorylation regulates its binding to CIN85, which is an SH3-containing multiadaptor protein involved in endocytic downregulation of receptor-tyrosine kinases. However, the interaction of CIN85 with Dab1 has not been addressed in neurons. We examined here a possibility that CIN85 has a role in Reelin signaling. We found nonpho-sphorylated Dab1-mediated colocalization of CIN85 with ApoER2. The colocalization of CIN85 with ApoER2 was increased in neurons stimulated with Reelin repeats 3-6, an active Reelin fragment. The stimulation recruited CIN85 to domains in plasma membrane where it colocalized with ApoER2 and Dab1 and then to EEA1-labeled early endosomes in the cytoplasm. In addition, Tyr phosphorylation of Dab1 strengthened the binding to CIN85. These results suggest that CIN85 participates in Reelin signaling through the binding to Dab1.

Bacopa monnieri Phytochemicals Mediated Synthesis of Platinum Nanoparticles and Its Neurorescue Effect on 1-Methyl 4-Phenyl 1,2,3,6 Tetrahydropyridine-Induced Experimental Parkinsonism in Zebrafish

Current discovery demonstrates the rapid formation of platinum nanoparticles using leaf extract of a neurobeneficial plant, Bacopa monnieri (BmE). The nanoparticles (BmE-PtNPs) were stabilized and then coated with varied phytochemicals present within the leaf extract. These nanoparticles demonstrated the same activity of Complex I, as that of oxidizing NADH to NAD⁺ using a spectrophotometric method. This suggests that BmE-PtNPs are a potential medicinal substance for oxidative stress mediated disease with suppressed mitochondrial complex I, namely, Parkinson's disease (PD). Hence, the neuroprotective potentials of the phytochemical coated nanoparticle were explored in 1-methyl 4-phenyl 1,2,3,6 tetrahydropyridine- (MPTP-)induced experimental Parkinsonism in zebrafish model. BmE-PtNPs pretreatment significantly reversed toxic effects of MPTP by increasing the levels of dopamine, its metabolites, GSH and activities of GPx, catalase, SOD and complex I, and reducing levels of MDA along with enhanced locomotor activity. Taken together, these findings suggest that BmE-PtNPs have protective effect in MPTP-induced neurotoxicity in this model of Parkinson's disease via their dual functions as mitochondrial complex I and antioxidant activity.

Specific silencing of the REST target genes in insulin-secreting cells uncovers their participation in beta cell survival

The absence of the transcriptional repressor RE-1 Silencing Transcription Factor (REST) in insulin-secreting beta cells is a major cue for the specific expression of a large number of genes. These REST target genes were largely ascribed to a function of neurotransmission in a neuronal context, whereas their role in pancreatic beta cells has been poorly explored. To identify their functional significance, we have generated transgenic mice expressing REST in beta cells (RIP-REST mice), and previously discovered that REST target genes are essential to insulin exocytosis. Herein we characterized a novel line of RIP-REST mice featuring diabetes. In diabetic RIP-REST mice, high levels of REST were associated with postnatal beta cell apoptosis, which resulted in gradual beta cell loss and sustained hyperglycemia in adults. Moreover, adenoviral REST transduction in INS-1E cells led to increased cell death under control conditions, and sensitized cells to death induced by cytokines. Screening for REST target genes identified several anti-apoptotic genes bearing the binding motif RE-1 that were downregulated upon REST expression in INS-1E cells, including Gjd2, Mapk8ip1, Irs2, Ptprn, and Cdk5r2. Decreased levels of Cdk5r2 in beta cells of RIP-REST mice further confirmed that it is controlled by REST, in vivo. Using siRNA-mediated knock-down in INS-1E cells, we showed that Cdk5r2 protects beta cells against cytokines and palmitate-induced apoptosis. Together, these data document that a set of REST target genes, including Cdk5r2, is important for beta cell survival.

Involvement and interplay of Parkin, PINK1, and DJ1 in neurodegenerative and neuroinflammatory disorders

The involvement of parkin, PINK1, and DJ1 in mitochondrial dysfunction, oxidative injury, and impaired functioning of the ubiquitin-proteasome system (UPS) has been intensively investigated in light of Parkinson's disease (PD) pathogenesis. However, these pathological mechanisms are not restricted to PD, but are common denominators of various neurodegenerative and neuroinflammatory disorders. It is therefore conceivable that parkin, PINK1, and DJ1 are also linked to the pathogenesis of other neurological diseases, including Alzheimer's disease (AD) and multiple sclerosis (MS). The importance of these proteins in mechanisms underlying neurodegeneration is reflected by the neuroprotective properties of parkin, DJ1, and PINK1 in counteracting oxidative stress and improvement of mitochondrial and UPS functioning. This review provides a concise overview on the cellular functions of the E3 ubiquitin ligase parkin, the mitochondrial kinase PINK1, and the cytoprotective protein DJ1 and their involvement and interplay in processes underlying neurodegeneration in common neurological disorders.

LMTK1/AATYK1 is a novel regulator of axonal outgrowth that acts via Rab11 in a Cdk5-dependent manner

Axonal outgrowth is a coordinated process of cytoskeletal dynamics and membrane trafficking; however, little is known about proteins responsible for regulating the membrane supply. LMTK1 (lemur kinase 1)/AATYK1 (apoptosis-associated tyrosine kinase 1) is a serine/threonine kinase that is highly expressed in neurons. We recently reported that LMTK1 plays a role in recycling endosomal trafficking in CHO-K1 cells. Here we explore the role of LMTK1 in axonal outgrowth and its regulation by Cdk5 using mouse brain cortical neurons. LMTK1 was expressed and was phosphorylated at Ser34, the Cdk5 phosphorylation site, at the time of axonal outgrowth in culture and colocalized with Rab11A, the small GTPase that regulates recycling endosome traffic, at the perinuclear region and in the axon. Overexpression of the unphosphorylated mutant LMTK1-S34A dramatically promoted axonal outgrowth in cultured neurons. Enhanced axonal outgrowth was diminished by the inactivation of Rab11A, placing LMTK1 upstream of Rab11A. Unexpectedly, the downregulation of LMTK1 by knockdown or gene targeting also significantly enhanced axonal elongation. Rab11A-positive vesicles were transported anterogradely more quickly in the axons of LMTK1-deficient neurons than in those of wild-type neurons. The enhanced axonal outgrowth was reversed by LMTK1-WT or the LMTK1-S34D mutant, which mimics the phosphorylated state, but not by LMTK1-S34A. Thus, LMTK1 can negatively control axonal outgrowth by regulating Rab11A activity in a Cdk5-dependent manner, and Cdk5-LMTK1-Rab11 is a novel signaling pathway involved in axonal outgrowth.

Regulation of mitochondrial transport and inter-microtubule spacing by tau phosphorylation at the sites hyperphosphorylated in Alzheimer's disease

The microtubule-associated protein Tau is a major component of the neurofibrillary tangles that serve as a neuropathological hallmark of Alzheimer's disease. Tau is a substrate for protein phosphorylation at multiple sites and occurs in tangles in a hyperphosphorylated state. However, the physiological functions of Tau phosphorylation or how it may contribute mechanistically to Alzheimer's pathophysiology are not completely understood. Here, we examined the function of human Tau phosphorylation at three sites, Ser199, Ser202, and Thr205, which together comprise the AT8 sites that mark abnormal phosphorylation in Alzheimer's disease. Overexpression of wild-type Tau or mutated forms in which these sites had been changed to either unphosphorylatable alanines or phosphomimetic aspartates inhibited mitochondrial movement in the neurite processes of PC12 cells as well as the axons of mouse brain cortical neurons. However, the greatest effects on mitochondrial translocation were induced by phosphomimetic mutations. These mutations also caused expansion of the space between microtubules in cultured cells when membrane tension was reduced by disrupting actin filaments. Thus, Tau phosphorylation at the AT8 sites may have meaningful effects on mitochondrial movement, likely by controlling microtubule spacing. Hyperphosphorylation of the AT8 sites may contribute to axonal degeneration by disrupting mitochondrial transport in Alzheimer's disease.

Phosphorylation of p35 and p39 by Cdk5 determines the subcellular location of the holokinase in a phosphorylation-site-specific manner

Cdk5 is a member of the cyclin-dependent kinase (Cdk) family, which is activated by neuronal activators p35 or p39. Cdk5 regulates a variety of neuronal activities including migration, synaptic activity and neuronal death. p35 and p39 impart cytoplasmic membrane association of p35-Cdk5 and p39-Cdk5, respectively, through their myristoylation, but it is not clearly understood how the cellular localization is related to different functions. We investigated the role of Cdk5 activity in the subcellular localization of p35-Cdk5 and p39-Cdk5. Cdk5 activity affected the localization of p35-Cdk5 and p39-Cdk5 through phosphorylation of p35 or p39. Using unphosphorylated or phosphomimetic mutants of p35 and p39, we found that phosphorylation at Ser8, common to p35 and p39, by Cdk5 regulated the cytoplasmic localization and perinuclear accumulation of unphosphorylated S8A mutants, and whole cytoplasmic distribution of phosphomimetic S8E mutants. Cdk5 activity was necessary to retain Cdk5-activator complexes in the cytoplasm. Nevertheless, small but distinct amounts of p35 and p39 were detected in the nucleus. In particular, nuclear p35 and p39 were increased when the Cdk5 activity was inhibited. p39 had a greater propensity to accumulate in the nucleus than p35, and phosphorylation at Thr84, specific to p39, regulated the potential nuclear localization activity of the Lys cluster in p39. These results suggest that the subcellular localization of the Cdk5-activator complexes is determined by its kinase activity, and also implicate a role for p39-Cdk5 in the nucleus.

Cdk5: a multifaceted kinase in neurodegenerative diseases

Since the identification of cyclin-dependent kinase-5 (Cdk5) as a tau kinase and member of the Cdk family almost 20 years ago, deregulation of Cdk5 activity has been linked to an array of neurodegenerative diseases. As knowledge on the etiopathological mechanisms of these diseases evolved through the years, Cdk5 has also been implicated in additional cellular events that are affected under these pathological conditions. From the role of Cdk5 in the regulation of synaptic functions to its involvement in autophagy deregulation, significant insights have been obtained regarding the role of Cdk5 as a key regulator of neurodegeneration. Here, we summarize recent findings on the involvement of Cdk5 in the pathophysiological mechanisms underlying various neurodegenerative diseases.

An integrated approach to uncover drivers of cancer

Systematic characterization of cancer genomes has revealed a staggering number of diverse aberrations that differ among individuals, such that the functional importance and physiological impact of most tumor genetic alterations remain poorly defined. We developed a computational framework that integrates chromosomal copy number and gene expression data for detecting aberrations that promote cancer progression. We demonstrate the utility of this framework using a melanoma data set. Our analysis correctly identified known drivers of melanoma and predicted multiple tumor dependencies. Two dependencies, TBC1D16 and RAB27A, confirmed empirically, suggest that abnormal regulation of protein trafficking contributes to proliferation in melanoma. Together, these results demonstrate the ability of integrative Bayesian approaches to identify candidate drivers with biological, and possibly therapeutic, importance in cancer.

Regulation and role of cyclin-dependent kinase activity in neuronal survival and death

Cyclin-dependent kinase (Cdk)5 is a proline-directed Ser/Thr protein kinase that functions mainly in neurons and is activated by binding to a regulatory subunit, p35 or p39. Kinase activity is mainly determined by the amount of p35 available, which is controlled by a balance between synthesis and degradation. Kinase activity is also regulated by Cdk5 phosphorylation, but the activity of phosphorylated Cdk5 is in contrast to that of cycling Cdks. Cdk5 is a versatile protein kinase that regulates multiple neuronal activities including neuronal migration and synaptic signaling. Further, Cdk5 plays a role in both survival and death of neurons. Long-term inactivation of Cdk5 triggers cell death, and the survival activity of Cdk5 is apparent when neurons suffer from stress. In contrast, hyper-activation of Cdk5 by p25 promotes cell death, probably by reactivating cell-cycle machinery in the nucleus. The pro-death activity is suppressed by membrane association of Cdk5 via myristoylation of p35. Appropriate activity, localization, and regulation of Cdk5 may be critical for long-term survival of neurons, which is more than 80 years in the case of humans.

Membrane association facilitates degradation and cleavage of the cyclin-dependent kinase 5 activators p35 and p39

Cyclin-dependent kinase 5 (Cdk5) is activated by binding to its activators, p35 and p39. The level of Cdk5 activity is determined by the amount of p35 and p39, which is regulated not only by transcription but also via proteasomal degradation. Alternatively, calpain-induced cleavage of p35 to p25 can induce aberrant Cdk5 activation. As the regulation of p35 and p39 proteolysis is not well understood, we have studied here the mechanisms governing their degradation and cleavage. We find that p35 and p39 undergo proteasomal degradation in neurons, with p39 showing a slower degradation rate than p35. Degradation of the activators is dependent on their respective N-terminal p10 region, as indicated by experiments in which cognate p10 regions were swapped between p35 and p39. The effect of the p10 region on degradation and cleavage could be assigned to its membrane binding properties, mediated predominantly by myristoylation. Together, these results indicate that both proteasomal degradation and calpain cleavage of p35 and p39 are stimulated by membrane association, which is in turn mediated via myristoylation of their p10 regions. However, p35 and p39 show differences in degradation and cleavage rates, which may in fact underlie the distinct physiological and pathological functions of these two Cdk5 activators.

Cross-talk between mitochondria and proteasome in Parkinson's disease pathogenesis

Parkinson's disease (PD) is the most common progressive neurodegenerative movement disorder, characterized by the selective loss of nigrostriatal dopaminergic neurons, and the presence of intracellular insoluble proteinaceous inclusions, known as Lewy Bodies. Although PD etiopathogenesis remains elusive, the leading hypothesis for the death of specific groups of neurons establishes that mitochondrial dysfunction, alterations in the ubiquitin-proteasomal system (UPS), and oxidative stress are major events that act synergistically causing this devastating disease. In this review we will focus on mitochondrial impairment and its implications on proteasomal function and alpha-synuclein aggregation. We will address the role of mitochondria and proteasome cross-talk in the neuronal loss that leads to PD and discuss how this knowledge might further improve patient therapy.