Perinatal abrogation of Cdk5 expression in brain results in neuronal migration defects

BMAL1 loss in oligodendroglia contributes to abnormal myelination and sleep

Myelination depends on the maintenance of oligodendrocytes that arise from oligodendrocyte precursor cells (OPCs). We show that OPC-specific proliferation, morphology, and BMAL1 are time-of-day dependent. Knockout of Bmal1 in mouse OPCs during development disrupts the expression of genes associated with circadian rhythms, proliferation, density, morphology, and migration, leading to changes in OPC dynamics in a spatiotemporal manner. Furthermore, these deficits translate into thinner myelin, dysregulated cognitive and motor functions, and sleep fragmentation. OPC-specific Bmal1 loss in adulthood does not alter OPC density at baseline but impairs the remyelination of a demyelinated lesion driven by changes in OPC morphology and migration. Lastly, we show that sleep fragmentation is associated with increased prevalence of the demyelinating disorder multiple sclerosis (MS), suggesting a link between MS and sleep that requires further investigation. These findings have broad mechanistic and therapeutic implications for brain disorders that include both myelin and sleep phenotypes.

Sirt2 promotes white matter oligodendrogenesis during development and in models of neonatal hypoxia

Delayed oligodendrocyte (OL) maturation caused by hypoxia (Hx)-induced neonatal brain injury results in hypomyelination and leads to neurological disabilities. Previously, we characterized Sirt1 as a crucial regulator of OL progenitor cell (OPC) proliferation in response to Hx. We now identify Sirt2 as a critical promoter of OL differentiation during both normal white matter development and in a mouse model of Hx. Importantly, we find that Hx reduces Sirt2 expression in mature OLs and that Sirt2 overexpression in OPCs restores mature OL populations. Reduced numbers of Sirt2+ OLs were also observed in the white matter of preterm human infants. We show that Sirt2 interacts with p27Kip1/FoxO1, p21Cip1/Cdk4, and Cdk5 pathways, and that these interactions are altered by Hx. Furthermore, Hx induces nuclear translocation of Sirt2 in OPCs where it binds several genomic targets. Overall, these results indicate that a balance of Sirt1 and Sirt2 activity is required for developmental oligodendrogenesis, and that these proteins represent potential targets for promoting repair following white matter injury.

Protocols for Characterization of Cdk5 Kinase Activity

Cyclin-dependent kinases (Cdks) are generally known to be involved in controlling the cell cycle, but Cdk5 is a unique member of this protein family for being most active in post-mitotic neurons. Cdk5 is developmentally important in regulating neuronal migration, neurite outgrowth, and axon guidance. Cdk5 is enriched in synaptic membranes and is known to modulate synaptic activity. Postnatally, Cdk5 can also affect neuronal processes such as dopaminergic signaling and pain sensitivity. Dysregulated Cdk5, in contrast, has been linked to neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Despite primarily being implicated in neuronal development and activity, Cdk5 has lately been linked to non-neuronal functions including cancer cell growth, immune responses, and diabetes. Since Cdk5 activity is tightly regulated, a method for measuring its kinase activity is needed to fully understand the precise role of Cdk5 in developmental and disease processes. This article includes methods for detecting Cdk5 kinase activity in cultured cells or tissues, identifying new substrates, and screening for new kinase inhibitors. Furthermore, since Cdk5 shares homology and substrate specificity with Cdk1 and Cdk2, the Cdk5 kinase assay can be used, with modification, to measure the activity of other Cdks as well. © 2021 Wiley Periodicals LLC. This article has been contributed to by US Government employees and their work is in the public domain in the USA. Basic Protocol 1: Measuring Cdk5 activity from protein lysates Support Protocol 1: Immunoprecipitation of Cdk5 using Dynabeads Alternate Protocol: Non-radioactive protocols to measure Cdk5 kinase activity Support Protocol 2: Western blot analysis for the detection of Cdk5, p35, and p39 Support Protocol 3: Immunodetection analysis for Cdk5, p35, and p39 Support Protocol 4: Genetically engineered mice (+ and - controls) Basic Protocol 2: Identifying new Cdk5 substrates and kinase inhibitors.

The Atypical Cyclin-Dependent Kinase 5 (Cdk5) Guards Podocytes from Apoptosis in Glomerular Disease While Being Dispensable for Podocyte Development

Cyclin-dependent kinase 5 (Cdk5) is expressed in terminally differentiated cells, where it drives development, morphogenesis, and survival. Temporal and spatial kinase activity is regulated by specific activators of Cdk5, dependent on the cell type and environmental factors. In the kidney, Cdk5 is exclusively expressed in terminally differentiated glomerular epithelial cells called podocytes. In glomerular disease, signaling mechanisms via Cdk5 have been addressed by single or combined conventional knockout of known specific activators of Cdk5. A protective, anti-apoptotic role has been ascribed to Cdk5 but not a developmental phenotype, as in terminally differentiated neurons. The effector kinase itself has never been addressed in animal models of glomerular disease. In the present study, conditional and inducible knockout models of Cdk5 were analyzed to investigate the role of Cdk5 in podocyte development and glomerular disease. While mice with podocyte-specific knockout of Cdk5 had no developmental defects and regular lifespan, loss of Cdk5 in podocytes increased susceptibility to glomerular damage in the nephrotoxic nephritis model. Glomerular damage was associated with reduced anti-apoptotic signals in Cdk5-deficient mice. In summary, Cdk5 acts primarily as master regulator of podocyte survival during glomerular disease and—in contrast to neurons—does not impact on glomerular development or maintenance.

Moonlighting of mitotic regulators in cilium disassembly

Correct timing of cellular processes is essential during embryological development and to maintain the balance between healthy proliferation and tumour formation. Assembly and disassembly of the primary cilium, the cell’s sensory signalling organelle, are linked to cell cycle timing in the same manner as spindle pole assembly and chromosome segregation. Mitotic processes, ciliary assembly, and ciliary disassembly depend on the centrioles as microtubule-organizing centres (MTOC) to regulate polymerizing and depolymerizing microtubules. Subsequently, other functional protein modules are gathered to potentiate specific protein–protein interactions. In this review, we show that a significant subset of key mitotic regulator proteins is moonlighting at the cilium, among which PLK1, AURKA, CDC20, and their regulators. Although ciliary assembly defects are linked to a variety of ciliopathies, ciliary disassembly defects are more often linked to brain development and tumour formation. Acquiring a better understanding of the overlap in regulators of ciliary disassembly and mitosis is essential in finding therapeutic targets for the different diseases and types of tumours associated with these regulators.

Nestin Selectively Facilitates the Phosphorylation of the Lissencephaly-Linked Protein Doublecortin (DCX) by cdk5/p35 to Regulate Growth Cone Morphology and Sema3a Sensitivity in Developing Neurons

Nestin, an intermediate filament protein widely used as a marker of neural progenitors, was recently found to be expressed transiently in developing cortical neurons in culture and in developing mouse cortex. In young cortical cultures, nestin regulates axonal growth cone morphology. In addition, nestin, which is known to bind the neuronal cdk5/p35 kinase, affects responses to axon guidance cues upstream of cdk5, specifically, to Sema3a. Changes in growth cone morphology require rearrangements of cytoskeletal networks, and changes in microtubules and actin filaments are well studied. In contrast, the roles of intermediate filament proteins in this process are poorly understood, even in cultured neurons. Here, we investigate the molecular mechanism by which nestin affects growth cone morphology and Sema3a sensitivity. We find that nestin selectively facilitates the phosphorylation of the lissencephaly-linked protein doublecortin (DCX) by cdk5/p35, but the phosphorylation of other cdk5 substrates is not affected by nestin. We uncover that this substrate selectivity is based on the ability of nestin to interact with DCX, but not with other cdk5 substrates. Nestin thus creates a selective scaffold for DCX with activated cdk5/p35. Last, we use cortical cultures derived from Dcx KO mice to show that the effects of nestin on growth cone morphology and on Sema3a sensitivity are DCX-dependent, thus suggesting a functional role for the DCX-nestin complex in neurons. We propose that nestin changes growth cone behavior by regulating the intracellular kinase signaling environment in developing neurons. The sex of animal subjects is unknown.SIGNIFICANCE STATEMENT Nestin, an intermediate filament protein highly expressed in neural progenitors, was recently identified in developing neurons where it regulates growth cone morphology and responsiveness to the guidance cue Sema3a. Changes in growth cone morphology require rearrangements of cytoskeletal networks, but the roles of intermediate filaments in this process are poorly understood. We now report that nestin selectively facilitates phosphorylation of the lissencephaly-linked doublecortin (DCX) by cdk5/p35, but the phosphorylation of other cdk5 substrates is not affected. This substrate selectivity is based on preferential scaffolding of DCX, cdk5, and p35 by nestin. Additionally, we demonstrate a functional role for the DCX-nestin complex in neurons. We propose that nestin changes growth cone behavior by regulating intracellular kinase signaling in developing neurons.

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.

Overlapping migratory mechanisms between neural progenitor cells and brain tumor stem cells

Neural stem cells present in the subventricular zone (SVZ), the largest neurogenic niche of the mammalian brain, are able to self-renew as well as generate neural progenitor cells (NPCs). NPCs are highly migratory and traverse the rostral migratory stream (RMS) to the olfactory bulb, where they terminally differentiate into mature interneurons. NPCs from the SVZ are some of the few cells in the CNS that migrate long distances during adulthood. The migratory process of NPCs is highly regulated by intracellular pathway activation and signaling from the surrounding microenvironment. It involves modulation of cell volume, cytoskeletal rearrangement, and isolation from compact extracellular matrix. In malignant brain tumors including high-grade gliomas, there are cells called brain tumor stem cells (BTSCs) with similar stem cell characteristics to NPCs but with uncontrolled cell proliferation and contribute to tumor initiation capacity, tumor progression, invasion, and tumor maintenance. These BTSCs are resistant to chemotherapy and radiotherapy, and their presence is believed to lead to tumor recurrence at distal sites from the original tumor location, principally due to their high migratory capacity. BTSCs are able to invade the brain parenchyma by utilizing many of the migratory mechanisms used by NPCs. However, they have an increased ability to infiltrate the tight brain parenchyma and utilize brain structures such as myelin tracts and blood vessels as migratory paths. In this article, we summarize recent findings on the mechanisms of cellular migration that overlap between NPCs and BTSCs. A better understanding of the intersection between NPCs and BTSCs will to provide a better comprehension of the BTSCs' invasive capacity and the molecular mechanisms that govern their migration and eventually lead to the development of new therapies to improve the prognosis of patients with malignant gliomas.

Inactivation of Cyclin-Dependent Kinase 5 in Hair Cells Causes Hearing Loss in Mice

Cyclin-dependent kinase 5 (CDK5) is abundantly expressed in post-mitotic cells including neurons. It is involved in multiple cellular events, such as cytoskeletal dynamics, signaling cascades, gene expression, and cell survival, et al. Dysfunction of CDK5 has been associated with a number of neurological disorders. Here we show that CDK5 is expressed in mouse cochlear hair cells, and CDK5 inactivation in hair cells causes hearing loss in mice. CDK5 inactivation has no effect on stereocilia development in the cochlear hair cells. However, it affects stereocilia maintenance, resulting in stereocilia disorganization and eventually stereocilia loss. Consistently, hair cell loss was significantly elevated by CDK5 inactivation. Despite that CDK5 has been shown to play important roles in synapse development and/or function, CDK5 inactivation does not affect the formation of ribbon synapses of cochlear hair cells. Further investigation showed that CDK5 inactivation causes reduced phosphorylation of ERM (ezrin, radixin, and moesin) proteins, which might contribute to the stereocilia deficits. Taken together, our data suggest that CDK5 plays pivotal roles in auditory hair cells, and CDK5 inactivation causes hearing loss in mice.

Cyclin-dependent kinase 5 is involved in the phosphorylation of gephyrin and clustering of GABAA receptors at inhibitory synapses of hippocampal neurons

CDK5 has been implicated in neural functions including growth, neuronal migration, synaptic transmission and plasticity of excitatory chemical synapses. Here we report robust effects of CDK5 on phosphorylation of the postsynaptic scaffold protein gephyrin and clustering of inhibitory GABA_A receptors in hippocampal neurons. shRNA-mediated knockdown of CDK5 and pharmacological inhibition of cyclin-dependent kinases reduced phosphorylated gephyrin clusters and postsynaptic γ2-containing GABA_A receptors. Phosphorylation of S270 is antagonized by PP1/PP2a phosphatase and site-directed mutagenesis and in vitro phosphorylation experiments indicate that S270 is a putative CDK5 phosphorylation site of gephyrin. Our data suggest that CDK5 plays an essential role for the stability of gephyrin-dependent GABA_A receptor clusters in hippocampal neurons.

Ccm3, a gene associated with cerebral cavernous malformations, is required for neuronal migration

Loss of function of cerebral cavernous malformation 3 (CCM3) results in an autosomal dominant cerebrovascular disorder. Here, we uncover a developmental role for CCM3 in regulating neuronal migration in the neocortex. Using cell type-specific gene inactivation in mice, we show that CCM3 has both cell autonomous and cell non-autonomous functions in neural progenitors and is specifically required in radial glia and newly born pyramidal neurons migrating through the subventricular zone, but not in those migrating through the cortical plate. Loss of CCM3 function leads to RhoA activation, alterations in the actin and microtubule cytoskeleton affecting neuronal morphology, and abnormalities in laminar positioning of primarily late-born neurons, indicating CCM3 involvement in radial glia-dependent locomotion and possible interaction with the Cdk5/RhoA pathway. Thus, we identify a novel cytoplasmic regulator of neuronal migration and demonstrate that its inactivation in radial glia progenitors and nascent neurons produces severe malformations of cortical development.

Regulation of Sox6 by cyclin dependent kinase 5 in brain

Cyclin dependent kinase 5 (Cdk5) is a proline-directed Ser/Thr kinase involved in various biological functions during normal brain development and neurodegeneration. In brain, Cdk5 activity is specific to post-mitotic neurons, due to neuronal specific expression of its activator p35. The biological functions of Cdk5 have been ascribed to its cytoplasmic substrates, however not much is known in nucleus. Here, we show that nuclear transcription factor Sox6 is a direct nuclear target of Cdk5. Sox6 is expressed in Tuj1 positive neurons, suggesting that Sox6 is expressed in differentiating neurons. The expression of Sox6 is high in mitotic nuclei during embryonic day 12 (E12) and gradually decreases during development into adult. On the other hand, Cdk5 expression gradually increases during its development. We show that Sox6 is expressed in mitotic nuclei in embryonic day 12 (E12) and in migrating neurons of E16. Sox6 is phosphorylated in vivo. Sox6 was detected by phospho-Ser/Thr and phospho-Ser/Thr-Pro and MPM-2 (Mitotic protein #2) antibodies in brain. Furthermore, calf intestinal alkaline phosphatase (CIAP) digestion resulted in faster migration of Sox6 band. The GST-Sox6 was phosphorylated by Cdk5/p35. The mass spectrometry analysis revealed that Sox6 is phosphorylated at T119PER motif. We show that Sox6 steady state levels are regulated by Cdk5. Cdk5 knockout mice die in utero and Sox6 protein expression is remarkably high in Cdk5-/- brain, however, there is no change in mRNA expression, suggesting a post-translational regulation of Sox6 by Cdk5. Transfection of primary cortical neurons with WT Cdk5 reduced Sox6 levels, while dominant negative (DN) Cdk5 and p35 increased Sox6 levels. Thus, our results indicate that Cdk5 regulates Sox6 steady state protein level that has an important role in brain development and function.

Suppression of neuroinflammation in forebrain-specific Cdk5 conditional knockout mice by PPARγ agonist improves neuronal loss and early lethality

Background

Cyclin-dependent kinase 5 (Cdk5) is essential for brain development and function, and its deregulated expression is implicated in some of neurodegenerative diseases. We reported earlier that the forebrain-specific Cdk5 conditional knockout (cKO) mice displayed an early lethality associated with neuroinflammation, increased expression of the neuronal tissue-type plasminogen activator (tPA), and neuronal migration defects.

Methods

In order to suppress neuroinflammation in the cKO mice, we first treated these mice with pioglitazone, a PPARγ agonist, and analyzed its effects on neuronal loss and longevity. In a second approach, to delineate the precise role of tPA in neuroinflammation in these mice, we generated Cdk5 cKO; tPA double knockout (dKO) mice.

Results

We found that pioglitazone treatment significantly reduced astrogliosis, microgliosis, neuronal loss and behavioral deficit in Cdk5 cKO mice. Interestingly, the dKO mice displayed a partial reversal in astrogliosis, but they still died at early age, suggesting that the increased expression of tPA in the cKO mice does not contribute significantly to the pathological process leading to neuroinflammation, neuronal loss and early lethality.

Conclusion

The suppression of neuroinflammation in Cdk5 cKO mice ameliorates gliosis and neuronal loss, thus suggesting the potential beneficial effects of the PPARγ agonist pioglitazone for the treatment for neurodegenerative diseases.

Skeletal muscle calpain acts through nitric oxide and neural miRNAs to regulate acetylcholine release in motor nerve terminals

Cholinergic overactivity in diseases of neuromuscular transmission elicits a retrograde signal resembling homeostatic synaptic plasticity that downregulates transmitter release. Understanding this compensatory pathway could provide insights into novel therapeutic avenues and molecular mechanisms underlying learning and memory. Here we identify nitric oxide as a possible source of this signal in pathological human and mouse muscle samples and link this signaling pathway to changes in synaptic function in the neuromuscular junction. We further show that neuronal nitric oxide synthase is regulated by cholinergic excess through activation of skeletal muscle calpain and its effect on Cdk5 and CaMKII, leading to direct modulation of presynaptic function. Finally, we show that this signaling pathway acts through specific miRNA control of presynaptic vesicle protein expression. The control of presynaptic miRNA levels by postsynaptic activity represents a novel mechanism for the modulation of synaptic activity in normal or pathological conditions.

Dual origins of the mammalian accessory olfactory bulb revealed by an evolutionarily conserved migratory stream

The accessory olfactory bulb (AOB) is a critical olfactory structure that has been implicated in mediating social behavior. It receives input from the vomeronasal organ and projects to targets in the amygdaloid complex. Its anterior and posterior components (aAOB and pAOB) display molecular, connectional and functional segregation in processing reproductive and defensive and aggressive behaviors, respectively. We observed a dichotomy in the development of the projection neurons of the aAOB and pAOB in mice. We found that they had distinct sites of origin and that different regulatory molecules were required for their specification and migration. aAOB neurons arose locally in the rostral telencephalon, similar to main olfactory bulb neurons. In contrast, pAOB neurons arose caudally, from the neuroepithelium of the diencephalic-telencephalic boundary, from which they migrated rostrally to reach their destination. This unusual origin and migration is conserved in Xenopus, providing an insight into the origin of a key component of this system in evolution.

Cyclin-dependent kinase 5 is required for normal cerebellar development

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase, and its kinase activity is dependent upon its association with either of the activating subunits p35 or p39, which are mainly expressed in neurons. We previously reported that Cdk5 knockout (KO) mice exhibit perinatal lethality, defective neuronal migration, and abnormal positioning of neurons in the facial motor nucleus and inferior olive in the hindbrain and Purkinje cells (PCs) in the cerebellum. In this study, we focused on the analysis of the role of Cdk5 in cerebellar development. For this purpose we generated midbrain-hindbrain-specific Cdk5 conditional knockout (MHB-Cdk5 KO) mice because the cerebellum develops postnatally, whereas Cdk5 KO mice die perinatally. Histological analysis of the MHB-Cdk5 KO mice revealed a significant size reduction of the cerebellum. In addition, profound disturbance of inward migration of granule cells (GC) was observed in the developing cerebellum. A normal dendritic development of the Purkinje cells (PCs) was disturbed in MHB-Cdk5 KO mice. Cultured Cdk5-null PCs showed similar dendritic abnormalities. These results indicate that Cdk5/p35 plays an important role in neuronal migration of PCs and GCs and dendrite formation of PCs in cerebellar development.

Phosphoregulation of the RNA-binding protein Hu antigen R (HuR) by Cdk5 affects centrosome function

Hu antigen R (HuR) is an mRNA-binding protein belonging to the ELAV family. It is highly expressed in cancer and involved in cell survival and proliferation. The impact of post-translational regulation of HuR and resulting cellular effects are poorly understood. In the current report, we describe a direct interaction between HuR and Cdk5 in glioma. We determined that Cdk5 specifically phosphorylates HuR at the serine 202 residue in the unique hinge region. The molecular consequences of this interaction are an altered HuR ability to bind, stabilize, and promote translation of mRNAs. At the cellular level, the anomalous HuR phosphorylation at this site evokes robust defects in centrosome duplication and cohesion as well as arrest of cell cycle progression. Subcellular fractionation and immunofluorescence technique confirm a direct integration of HuR and Cdk5 with centrosomes. We propose that HuR stores mRNA in the centrosome and that HuR phosphorylation by Cdk5 controls de novo protein synthesis in near proximity to centrosomes and, thus, impacts centrosome function.

Extraneuronal activities and regulatory mechanisms of the atypical cyclin-dependent kinase Cdk5

Cyclin-dependent kinase, Cdk5, is an atypical but essential member of the Cdk family of proline-directed serine/threonine kinases with no evident role in cell cycle progression. Cdk5 is present in post-mitotic and terminally differentiated neuronal/glial cells and is also known to arrest cell cycle. Also atypical is the activation of Cdk5 by binding of a non-cyclin activator protein, namely, the Cdk5 regulatory proteins Cdk5R1 (p35), truncated Cdk5R1 (p25), or Cdk5R2 (p39). Despite its ubiquitous presence in all cells and tissues, Cdk5 is often referred to as a neuron-specific kinase largely due to the abundant presence of the activator proteins in neuronal cells. Recently, this concept of a canonical neuronal function of Cdk5 has been extended, if not challenged, by the observation of p35 and p39 expression, as well as Cdk5 activity, in multiple non-neuronal cells. Extraneuronal Cdk5 regulates critical biological processes including transcript-selective translation control for regulation of macrophage gene expression, glucose-inducible insulin secretion, hematopoietic cell differentiation, vascular angiogenesis, cell migration, senescence, and wound-healing, among others. Recent advances in the extraneuronal functions of Cdk5 are reviewed and discussed here in the context of their physiological activities and pathophysiological implications with some speculative comments on the endogenous control mechanisms that might "turn on" Cdk5 activity. The potential importance of targeted inhibition of Cdk5 as therapeutic agents against glucotoxicity, diabetes, cardiovascular diseases, and cancer is also discussed.

Cyclin-dependent kinases in brain development and disease

Cyclin-dependent kinase 5 (Cdk5) is a multifaceted serine/threonine kinase protein with important roles in the nervous system. Two related proteins, p35 and p39, activate Cdk5 upon direct binding. Over the past decade, Cdk5 activity has been demonstrated to regulate many events during brain development, including neuronal migration as well as axon and dendrite development. Recent evidence also suggests a pivotal role for Cdk5 in synaptic plasticity, behavior, and cognition. Dysfunction of Cdk5 has been implicated in a number of neurological disorders and neurodegenerative diseases including Alzheimer's disease, amyotrophic lateral sclerosis, Niemann-Pick type C disease, and ischemia. Hyperactivation of Cdk5 due to the conversion of p35 to p25 by the calcium-dependent protease calpain during neurotoxicity also contributes to the pathological state. This review surveys recent literature surrounding Cdk5 in synaptic plasticity and homeostasis, with particular emphasis on Cdk5 kinase activity under neurodegenerative conditions.

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

Hypomyelination phenotype caused by impaired differentiation of oligodendrocytes in Emx1-cre mediated Cdk5 conditional knockout mice

Cyclin-dependent kinase 5 (Cdk5) plays a pivotal role in neuronal migration and differentiation, and in axonal elongation. Although many studies have been conducted to analyze neuronal functions of Cdk5, its kinase activity has also been reported during oligodendrocyte differentiation, which suggests Cdk5 may play an important role in oligodendrocytes. Here, we describe a hypomyelination phenotype observed in Emx1-cre mediated Cdk5 conditional knockout (cKO) mice (Emx1-cKO), in which the Cdk5 gene was deleted in neurons, astrocytes and oligodendrocyte -lineage cells. In contrast, the Cdk5 gene in CaMKII cKO mice was deleted only in neurons. Because the development of mature oligodendrocytes from oligodendrocyte precursor cells is a complex process, we performed in situ hybridization using markers for the oligodendrocyte precursor cell and for the differentiated oligodendrocyte. Our results indicate that hypomyelination in Emx1-cKO is due to the impaired differentiation of oligodendrocytes, rather than to the proliferation or migration of their precursors. The present study confirmed the in vivo role of Cdk5 in oligodendrocyte differentiation.

Conditional deletion of neuronal cyclin-dependent kinase 5 in developing forebrain results in microglial activation and neurodegeneration

Neuronal migration disorders are often identified in patients with epilepsy refractory to medical treatment. The prolonged or repeated seizures are known to cause neuronal death; however, the mechanism underlying seizure-induced neuronal death remains to be elucidated. An essential role of cyclin-dependent kinase 5 (Cdk5) in brain development has been demonstrated in Cdk5(-/-) mice, which show neuronal migration defects and perinatal lethality. Here, we show the consequences of Cdk5 deficiency in the postnatal brain by generating Cdk5 conditional knockout mice, in which Cdk5is selectively eliminated from neurons in the developing forebrain. The conditional mutant mice were viable, but exhibited complex neurological deficits including seizures, tremors, and growth retardation. The forebrain not only showed disruption of layering, but also neurodegenerative changes accompanied by neuronal loss and microglial activation. The neurodegenerative changes progressed with age and were accompanied by up-regulation of the neuronal tissue-type plasminogen activator, a serine protease known to mediate microglial activation. Thus age-dependent neurodegeneration in the Cdk5 conditional knockout mouse brain invoked a massive inflammatory reaction. These findings indicate an important role of Cdk5 in inflammation, and also provide a mouse model to examine the possible involvement of inflammation in the pathogenesis of progressive cognitive decline in patients with neuronal migration disorders.

Making a neuron: Cdk5 in embryonic and adult neurogenesis

Cyclin-dependent kinase 5 (Cdk5) has been implicated in the migration, maturation and survival of neurons born during embryonic development. New evidence suggests that Cdk5 has comparable but also distinct functions in adult neurogenesis. Here we summarize accumulating evidence on the role of Cdk5 in regulation of the cell cycle, migration, survival, maturation and neuronal integration. We specifically highlight the many similarities and few tantalizing differences in the roles of Cdk5 in the embryonic and adult brain. We discuss the signaling pathways that might contribute to Cdk5 action in regulating embryonic and adult neurogenesis, highlighting future research directions that will help to clarify the mechanisms underlying lifelong neurogenesis in the mammalian brain.

Cdk5 is essential for adult hippocampal neurogenesis

The molecular factors regulating adult neurogenesis must be understood to harness the therapeutic potential of neuronal stem cells. Although cyclin-dependent kinase 5 (Cdk5) plays a critical role in embryonic corticogenesis, its function in adult neurogenesis is unknown. Here, we assessed the role of Cdk5 in the generation of dentate gyrus (DG) granule cell neurons in adult mice. Cre recombinase-mediated conditional knockout (KO) of Cdk5 from stem cells and their progeny in the DG subgranular zone (SGZ) prevented maturation of new neurons. In addition, selective KO of Cdk5 from mature neurons throughout the hippocampus reduced the number of immature neurons. Furthermore, Cdk5 gene deletion specifically from DG granule neurons via viral-mediated gene transfer also resulted in fewer immature neurons. In each case, the total number of proliferating cells was unaffected, indicating that Cdk5 is necessary for progression of adult-generated neurons to maturity. This role for Cdk5 in neurogenesis was activating-cofactor specific, as p35 KO but not p39 KO mice also had fewer immature neurons. Thus, Cdk5 has an essential role in the survival, but not proliferation, of adult-generated hippocampal neurons through both cell-intrinsic and cell-extrinsic mechanisms.

Neurodegeneration and neuroinflammation in cdk5/p25-inducible mice: a model for hippocampal sclerosis and neocortical degeneration

The cyclin-dependent kinase cdk5 is atypically active in postmitotic neurons and enigmatic among the kinases proposed as molecular actors in neurodegeneration. We generated transgenic mice to express p25, the N-terminally truncated p35 activator of cdk5, in forebrain under tetracycline control (TET-off). Neuronal expression of p25 (p25(ON)) caused high mortality postnatally and early in life. Mortality was completely prevented by administration of doxycycline in the drinking water of pregnant dams and litters until P42, allowing us to study the action of p25 in adult mouse forebrain. Neuronal p25 triggered neurodegeneration and also microgliosis, rapidly and intensely in hippocampus and cortex. Progressive neurodegeneration was severe with marked neuron loss, causing brain atrophy (40% loss at age 5 months) with nearly complete elimination of the hippocampus. Neurodegeneration did not involve phosphorylation of protein tau or generation of amyloid peptide. Degenerating neurons did not stain for terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling or activated caspase-3 but were marked by FluoroJadeB in early stages. Diseased neurons were always closely associated with activated microglia already very early in the disease process. Primary neurons derived from p25 embryos were more prone to apoptosis than wild-type neurons, and they activated microglial cells in co-culture. The inducible p25 mice present as a model for neurodegeneration in hippocampal sclerosis and neocortical degeneration, with important contributions of activated microglia.

An unusual member of the Cdk family: Cdk5

The proline-directed serine threonine kinase, Cdk5, is an unusual molecule that belongs to the well-known large family of proteins, cyclin-dependent kinases (Cdks). While it has significant homology with the mammalian Cdk2 and yeast cdc2, unlike the other Cdks, it has little role to play in cell cycle regulation and is activated by non-cyclin proteins, p35 and p39. It phosphorylates a spectrum of proteins, most of them associated with cell morphology and motility. A majority of known substrates of Cdk5 are cytoskeletal elements, signalling molecules or regulatory proteins. It also appears to be an important player in cell-cell communication. Highly conserved, Cdk5 is most abundant in the nervous system and is of special interest to neuroscientists as it appears to be indispensable for normal neural development and function. In normal cells, transcription and activity of Cdk5 is tightly regulated. Present essentially in post-mitotic neurons, its normal activity is obligatory for migration and differentiation of neurons in developing brain. Deregulation of Cdk5 has been implicated in Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease and acute neuronal injury. Regulators of Cdk5 activity are considered as potential therapeutic molecules for degenerative diseases. This review focuses on the role of Cdk5 in neural cells as regulator of cytoskeletal elements, axonal guidance, membrane transport, synaptogenesis and cell survival in normal and pathological conditions.

Cyclin-dependent kinase 5 is required for control of neuroblast migration in the postnatal subventricular zone

At the lateral wall of the lateral ventricles in the adult rodent brain, neuroblasts form an extensive network of elongated cell aggregates called chains in the subventricular zone and migrate toward the olfactory bulb. The molecular mechanisms regulating this migration of neuroblasts are essentially unknown. Here, we report a novel role for cyclin-dependent kinase 5 (Cdk5), a neuronal protein kinase, in this process. Using in vitro and in vivo conditional knock-out experiments, we found that Cdk5 deletion impaired the chain formation, speed, directionality, and leading process extension of the neuroblasts in a cell-autonomous manner. These findings suggest that Cdk5 plays an important role in neuroblast migration in the postnatal subventricular zone.

Gene expression profile analysis of epilepsy-associated gangliogliomas

Gangliogliomas (GG) constitute the most frequent tumor entity in young patients undergoing surgery for intractable epilepsy. The histological composition of GG, with the presence of dysplastic neurons, corroborates their maldevelopmental origin. However, their histogenesis, the pathogenetic relationship with other developmental lesions, and the molecular alterations underlying the epileptogenicity of these tumors remain largely unknown. We performed gene expression analysis using the Affymetrix Gene Chip System (U133 plus 2.0 array). We used GENMAPP and the Gene Ontology database to identify global trends in gene expression data. Our analysis has identified various interesting genes and processes that are differentially expressed in GG when compared with normal tissue. The immune and inflammatory responses were the most prominent processes expressed in GG. Several genes involved in the complement pathway displayed a high level of expression compared with control expression levels. Higher expression was also observed for genes involved in cell adhesion, extracellular matrix and proliferation processes. We observed differential expression of genes as cyclin D1 and cyclin-dependent kinases, essential for neuronal cell cycle regulation and differentiation. Synaptic transmission, including GABA receptor signaling was an under-expressed process compared with control tissue. These data provide some suggestions for the molecular pathogenesis of GG. Furthermore, they indicate possible targets that may be investigated in order to dissect the mechanisms of epileptogenesis and possibly counteract the epileptogenic process in these developmental lesions.

Cdk5 is required for multipolar-to-bipolar transition during radial neuronal migration and proper dendrite development of pyramidal neurons in the cerebral cortex

The mammalian cerebral cortex consists of six layers that are generated via coordinated neuronal migration during the embryonic period. Recent studies identified specific phases of radial migration of cortical neurons. After the final division, neurons transform from a multipolar to a bipolar shape within the subventricular zone-intermediate zone (SVZ-IZ) and then migrate along radial glial fibres. Mice lacking Cdk5 exhibit abnormal corticogenesis owing to neuronal migration defects. When we introduced GFP into migrating neurons at E14.5 by in utero electroporation, we observed migrating neurons in wild-type but not in Cdk5(-/-) embryos after 3-4 days. Introduction of the dominant-negative form of Cdk5 into the wild-type migrating neurons confirmed specific impairment of the multipolar-to-bipolar transition within the SVZ-IZ in a cell-autonomous manner. Cortex-specific Cdk5 conditional knockout mice showed inverted layering of the cerebral cortex and the layer V and callosal neurons, but not layer VI neurons, had severely impaired dendritic morphology. The amount of the dendritic protein Map2 was decreased in the cerebral cortex of Cdk5-deficient mice, and the axonal trajectory of cortical neurons within the cortex was also abnormal. These results indicate that Cdk5 is required for proper multipolar-to-bipolar transition, and a deficiency of Cdk5 results in abnormal morphology of pyramidal neurons. In addition, proper radial neuronal migration generates an inside-out pattern of cerebral cortex formation and normal axonal trajectories of cortical pyramidal neurons.

Calcium signaling in neuronal motility

Neuronal motility is a fundamental feature that underlies the development, regeneration, and plasticity of the nervous system. Two major developmental events--directed migration of neuronal precursor cells to the proper positions and guided elongation of axons to their target cells--depend on large-scale neuronal motility. At a finer scale, motility is also manifested in many aspects of neuronal structures and functions, ranging from differentiation and refinement of axonal and dendritic morphology during development to synapse remodeling associated with learning and memory in the adult brain. As a primary second messenger that conveys the cytoplasmic actions of electrical activity and many neuroactive ligands, Ca(2+) plays a central role in the regulation of neuronal motility. Recent studies have revealed common Ca(2+)-dependent signaling pathways that are deployed for regulating cytoskeletal dynamics associated with neuronal migration, axon and dendrite development and regeneration, and synaptic plasticity.

Cyclin-dependent kinase 5 modulates nociceptive signaling through direct phosphorylation of transient receptor potential vanilloid 1

Transient receptor potential vanilloid 1 (TRPV1), a ligand-gated cation channel highly expressed in small-diameter sensory neurons, is activated by heat, protons, and capsaicin. The phosphorylation of TRPV1 provides a versatile regulation of intracellular calcium levels and is critical for TRPV1 function in responding to a pain stimulus. We have previously reported that cyclin-dependent kinase 5 (Cdk5) activity regulates nociceptive signaling. In this article we report that the Cdk5-mediated phosphorylation of TRPV1 at threonine-407 can modulate agonist-induced calcium influx. Inhibition of Cdk5 activity in cultured dorsal root ganglia neurons resulted in a significant reduction of TRPV1-mediated calcium influx, and this effect could be reversed by restoring Cdk5 activity. Primary nociceptor-specific Cdk5 conditional-knockout mice showed reduced TRPV1 phosphorylation, resulting in significant hypoalgesia. Thus, the present study indicates that Cdk5-mediated TRPV1 phosphorylation is important in the regulation of pain signaling.

Brief update on animal models of hypoxic-ischemic encephalopathy and neonatal stroke

The discovery of safe and effective therapies for perinatal hypoxia ischemia (HI) and stroke remains an unmet goal of neonatal-perinatal medicine. Because of the many developmental and functional differences between the neonatal brain and the adult brain, the ability to extrapolate adult data to the neonatal condition is very limited. For this reason, it is incumbent on scientists in the field of neonatal brain injury to address the questions of therapeutic efficacy of an array of potential therapies in a developmentally appropriate model. Toward that end, a number of new models of neonatal HI and stroke have been introduced recently. Additionally, some of the established models have been adapted to different species and different ages, giving scientists a greater choice of models for the study of neonatal HI and stroke. Many of these models are now also being used for functional and behavioral testing, an absolute necessity for preclinical therapeutic trials. This review focuses primarily on the newly developed models, recent adaptations to established models, and the studies of functional outcome that have been published since 2000.

GDNF is a chemoattractant factor for neuronal precursor cells in the rostral migratory stream

Olfactory bulb (OB) interneurons are generated from neuroblast cells derived from the anterior subventricular zone (SVZa) of the forebrain. The mechanisms guiding the rostral migration of these neuronal precursors are not well understood. Here, we show that glial cell line-derived neurotrophic factor (GDNF) is produced in the olfactory bulb but distributed along the rostral migratory stream (RMS) in a pattern concordant with the expression of its GPI-anchored receptor GFRalpha1. We demonstrate that GDNF is a chemoattractant factor for RMS-derived neuronal precursors, but not for SVZa neuroblast cells. In agreement with this, GDNF increased Cyclin-dependent kinase 5 (Cdk5) activity in RMS cells, a kinase critically involved in neuronal migration and guidance. GDNF-mediated cell chemoattraction was abrogated in RMS explants treated with the Cdk5 inhibitor Roscovitine as well as in RMS explants isolated from Ncam mutant mice. Chemical cross-linking assays showed that 125I-GDNF is able to interact directly with NCAM in RMS-derived cells. Taken together, these data demonstrate that GDNF is a direct chemoattractant factor for neuroblast cells migrating along the RMS and support the participation of NCAM during this guidance process.

Deficits in motor coordination with aberrant cerebellar development in mice lacking testicular orphan nuclear receptor 4

Since testicular orphan nuclear receptor 4 (TR4) was cloned, its physiological function has remained largely unknown. Throughout postnatal development, TR4-knockout (TR4-/-) mice exhibited behavioral deficits in motor coordination, suggesting impaired cerebellar function. Histological examination of the postnatal TR4-/- cerebellum revealed gross abnormalities in foliation; specifically, lobule VII in the anterior vermis was missing. Further analyses demonstrated that the laminations of the TR4-/- cerebellar cortex were changed, including reductions in the thickness of the molecular layer and the internal granule layer, as well as delayed disappearance of the external granule cell layer (EGL). These lamination irregularities may result from interference with granule cell proliferation within the EGL, delayed inward migration of postmitotic granule cells, and a higher incidence of apoptotis. In addition, abnormal development of Purkinje cells was observed in the postnatal TR4-/- cerebellum, as evidenced by aberrant dendritic arborization and reduced calbindin staining intensity. Expression of Pax-6, Sonic Hedgehog (Shh), astrotactin (Astn), reelin, and Cdk-5, genes correlated with the morphological development of the cerebellum, is reduced in the developing TR4-/- cerebellum. Together, our findings suggest that TR4 is required for normal cerebellar development.