Migration defects of cdk5(-/-) neurons in the developing cerebellum is cell autonomous

The complex p25/Cdk5 kinase in neurofibrillary degeneration and neuronal death: the missing link to cell cycle

Emergence of the cell cycle hypothesis in neurodegenerative disease comes from the numerous lines of evidence showing a tight link between "cell cycle-like reactivation" and neuronal death. Terminally differentiated neurons remain in G0 phase and display, compared to proliferating cells, an opposite regulation pattern of cell cycle markers in that most of the key activators and inhibitors are respectively down- and up-regulated. It has been clearly established that any experimental attempt to force terminally differentiated neurons to divide ultimately leads to their death. Conversely, cell cycle blockade in experimental models of neuronal death is able to rescue neurons. Hence, cell cycle deregulation is certainly among mechanisms governing neuronal death. However, many questions remain unresolved, especially those related to which molecular mechanisms trigger cell cycle deregulation and how this deregulation leads to cell death. In the present review, we focus on neurodegeneration in Alzheimer's disease and discuss the cell cycle deregulation related to this neurodegenerative pathology. Finally, we emphasize the role of p25/Cdk5 kinase complex in this pathological process through retinoblastoma protein phosphorylation and derepression of E2F-responsive genes and other actors such as cdc2, cyclins, and MCM proteins.

Cell cycle regulation in the postmitotic neuron: oxymoron or new biology?

Adult CNS neurons are typically described as permanently postmitotic but there is probably nothing permanent about the neuronal cell cycle arrest. Rather, it appears that these highly differentiated cells must constantly keep their cell cycle in check. Relaxation of this vigilance leads to the initiation of a cell cycle and entrance into an altered and vulnerable state, often leading to death. There is evidence that neurons which are at risk of neurodegeneration are also at risk of re-initiating a cell cycle process that involves the expression of cell cycle proteins and DNA replication. Failure of cell cycle regulation might be a root cause of several neurodegenerative disorders and a final common pathway for others.

Major Cdk5-dependent phosphorylation sites of amphiphysin 1 are implicated in the regulation of the membrane binding and endocytosis

Amphiphysin 1 (amph 1) is an endocytic protein enriched in the nerve terminals that functions in the clathrin-mediated endocytosis. It acts as membrane curvature sensor, a linker of clathrin coat proteins, and an enhancer of dynamin Guanosine Triphosphatase (GTPase) activity. Amph 1 undergoes phosphorylation by cyclin-dependent kinase 5 (Cdk5), at five phosphorylation sites, serine 262, 272, 276, 285, and threonine 310, as determined by mass spectrometry (MS). We show here that Cdk5-dependent phosphorylation of amph 1 is enhanced in the presence of lipid membranes. Analysis by tandem liquid chromatograph MS revealed that the phosphorylation occurs at two phosphorylation sites. The phosphorylation was markedly decreased by mutation either Ser276 or Ser285 of amph 1 to alanine (S276A and S285A). Furthermore, mutation of both sites (S276, 285A) completely eliminated the phosphorylation. Functional studies indicated that binding of amph 1 to lipid membrane was attenuated by Cdk5-dependent phosphorylation of wild type amph 1, but not of the S276, 285A form. Interestingly, endocytosis was increased in rat pheochromocytoma cells expressing amph 1 S276, 285A in comparison with wild type. These results suggest that Ser276 and Ser285 are regulatory Cdk5 phosphorylation sites of amph 1 in the lipid-bound state. Phosphorylation at these sites alters binding of amph 1 to lipid membranes, and may be an important regulatory aspect in the regulation of synaptic vesicle endocytosis.

cdk5 modulates beta- and delta-catenin/Pin1 interactions in neuronal cells

The cdk5/p35 complex has been implicated in a variety of functions related to brain development, including axonal outgrown and neuronal migration. In this study, by co-immunoprecipitation and pull-down experiments, we have shown that the cdk5/p35 complex associates with and phosphorylates the neuronal delta-catenin. Immunocytochemical studies of delta-catenin and the cdk5-activator p35 in primary cortical neurons indicated that these proteins co-localize in the cell body of neuronal cells. In addition, cdk5 co-localized with beta-catenin in the cell-cell contacts and plasma membrane of undifferentiated and differentiated N2A cells. In this context, we identified Ser(191) and Ser(246) on beta-catenin structure as specific phosphorylation sites for cdk5/p35 complex. Moreover, Pin1, a peptidyl-prolyl isomerase (PPIase) directly bound to both, beta- and delta-catenin, once they have been phosphorylated by the cdk5/p35 complex. Studies indicate that the cdk5/p35 protein kinase system is directly involved in the regulatory mechanisms of neuronal beta- and delta-catenin.

Chemoarchitecture of the middle temporal visual area in the marmoset monkey (Callithrix jacchus): laminar distribution of calcium-binding proteins (calbindin, parvalbumin) and nonphosphorylated neurofilament

We studied the distributions of interneurons containing the calcium-binding proteins parvalbumin and calbindin D-28k, as well as that of pyramidal neurons containing nonphosphorylated neurofilament (NNF), in the middle temporal visual area (MT) of marmoset monkeys. The distributions of these classes of cells in MT are distinct from those found in adjacent areas. Similar to the primary visual area (V1), in MT, calbindin-immunopositive neurons can be objectively classified into "dark" and "light" subtypes based on optical density of stained cell bodies. Calbindin-positive dark neurons are particularly concentrated in layers 2 and 3, whereas light neurons have a more widespread distribution. In addition, a subcategory of calbindin-positive dark neuron, characterized by a "halo" of stained processes surrounding the cell body, is found within and around layer 4 of MT and V1. These cells are rare in most other visual areas. In comparison, parvalbumin-immunopositive cells in area MT have a relatively homogeneous distribution, although with a trend toward higher spatial density in lower layer 3, and are relatively uniform in terms of density of staining. Finally, MT shows a characteristic trilaminar distribution of NNF-immunopositive pyramidal cells, with stained cell bodies evident in layers 3, 5, and 6. Although the laminar distribution of cells stained for the three markers overlap to some extent, these subcategories can be readily distinguished in terms of morphology, including cell body size. Chemoarchitectural parallels observed between MT and V1 suggest comparable physiological requirements and neuronal circuitry.

Differential growth factor regulation of aspartyl-(asparaginyl)-beta-hydroxylase family genes in SH-Sy5y human neuroblastoma cells

Background

Aspartyl (asparaginyl)-β-hydroxylase (AAH) hydroxylates Asp and Asn residues within EGF-like domains of Notch and Jagged, which mediate cell motility and differentiation. This study examines the expression, regulation and function of AAH, and its related transcripts, Humbug and Junctin, which lack catalytic domains, using SH-Sy5y neuroblastoma cells.

Results

Real time quantitative RT-PCR demonstrated 8- or 9-fold higher levels of Humbug than AAH and Junctin, and lower levels of all 3 transcripts in normal human brains compared with neuroblastic tumor cells. AAH and Humbug expression were significantly increased in response to insulin and IGF-I stimulation, and these effects were associated with increased directional motility. However, over-expression of AAH and not Humbug significantly increased motility. Treatment with chemical inhibitors of Akt, Erk MAPK, or cyclin-dependent kinase 5 (Cdk-5) significantly reduced IGF-I stimulated AAH and Humbug expression and motility relative to vehicle-treated control cells. In addition, significantly increased AAH and Humbug expression and directional motility were observed in cells co-transfected with Cdk-5 plus its p35 or p25 regulatory partner. Further studies demonstrated that activated Cdk-5 mediated its stimulatory effects on AAH through Erk MAPK and PI3 kinase.

Conclusion

AAH and Humbug are over-expressed in SH-Sy5y neuroblastoma cells, and their mRNAs are regulated by insulin/IGF-1 signaling through Erk MAPK, PI3 kinase-Akt, and Cdk-5, which are known mediators of cell migration. Although AAH and Humbug share regulatory signaling pathways, AAH and not Humbug mediates directional motility in SH-Sy5y neuroblastoma cells.

Roscovitine regulates invasive breast cancer cell (MDA-MB231) proliferation and survival through cell cycle regulatory protein cdk5

Roscovitine, a purine analogue, has been considered for the treatment of cancer. Anti-cancer therapeutic efficacy is being evaluated in clinical trials. However, the mechanisms remain unclear. In the present study, cyclic-dependent kinase 5 (cdk5) proved to be a molecular target for roscovitine-triggered apoptosis for highly invasive breast cancer cell death. Because our previous studies have shown a potential role of cdk5 in endothelial cell proliferation/apoptosis [Sharma, M.R., Tuszynski, G.P., Sharma, M.C. (2004). Angiostatin-induced inhibition of endothelial cell proliferation/apoptosis is associated with the down-regulation of cell cycle regulatory protein cdk5. J. Cell Biochem. 91, 398-409], here we not only demonstrate first that Cdk5, p35, and p25 proteins were all expressed in invasive breast cancer cells MDA-MB231 but also showed that cdk5 expression regulates MDA-MB231 cell proliferation. In addition, potent mitogen bFGF up-regulates cdk5 expression. Roscovitine specifically inhibits cdk5 expression/activity in a dose-dependent manner with concomitant inhibition of MDA-MB231 cell proliferation and induction of apoptosis. By contrast, the roscovitine analog olomoucine, a specific inhibitor of cdk4, failed to affect MDA-MB231 cell proliferation and apoptosis which implies the specific involvement of cdk5 in roscovitine-triggered cell death/proliferation. Additionally, roscovitine-mediated inhibition of proliferation is irreversible. These data suggest that cdk5 may have a significant role in the regulation of breast cancer cell proliferation and apoptosis and extend beyond its role in neurogenesis. These results suggest that Cdk5 is a novel player in roscovitine-triggered breast cancer cell apoptosis and inhibition of proliferation, therefore, may be a potential therapeutic target.

CDK5 regulates cell-cell and cell-matrix adhesion in human keratinocytes

BACKGROUND

CDK5 is a member of proline-directed serine/threonine kinases. Although its cDNA was originally cloned as a homologue to those for the other members of the cyclin-dependent kinase (CDK) family, CDK5 has been shown to function differently from other CDKs. CDK5 is activated by non-cyclin partners, p35 and p39, and important during brain development by influencing adhesion, migration and differentiation of neurones.

OBJECTIVES

We sought to investigate the expression and functions of CDK5 in human keratinocytes.

METHODS

Expression of CDK5/p35, interaction of CDK5/p35 with adhesion molecules, and its roles in cell-cell and cell-matrix adhesion were studied by reverse transcriptase-polymerase chain reaction, immunoblotting and aggregation/adhesion assays in primary cultured normal human keratinocytes from infant foreskins and a human keratinocyte HaCaT cell line. Localization of CDK5 and p35 in normal human epidermis and psoriatic epidermis was studied by immunohistochemistry.

RESULTS

Both CDK5 and p35 were expressed in primary cultured keratinocytes, HaCaT cells and normal human epidermis. Roscovitine, an inhibitor of CDK5, enhanced Ca2+-dependent (cadherin-dependent) aggregation of HaCaT cells whereas it inhibited adhesion of HaCaT cells to fibronectin associated with reduced active states of beta1 integrin. Interestingly, psoriatic skin showed reduced CDK5 and p35 expression in the lower half of the epidermis, which might be associated with decreased amount of activated beta1 integrin in the epidermis of psoriatic skin.

CONCLUSIONS

CDK5/p35 may be involved in cell-cell and cell-matrix adhesion in human keratinocytes by differently regulating cadherins and integrins. Furthermore, reduced expression of CDK5/p35 in the epidermis might be involved in the pathogenesis of psoriasis.

Cdk5 phosphorylation of doublecortin ser297 regulates its effect on neuronal migration

Mutations in the doublecortin (DCX) gene in human or targeted disruption of the cdk5 gene in mouse lead to similar cortical lamination defects in the developing brain. Here we show that Dcx is phosphorylated by Cdk5. Dcx phosphorylation is developmentally regulated and corresponds to the timing of expression of p35, the major activating subunit for Cdk5. Mass spectrometry and Western blot analysis indicate phosphorylation at Dcx residue Ser297. Phosphorylation of Dcx lowers its affinity to microtubules in vitro, reduces its effect on polymerization, and displaces it from microtubules in cultured neurons. Mutation of Ser297 blocks the effect of Dcx on migration in a fashion similar to pharmacological inhibition of Cdk5 activity. These results suggest that Dcx phosphorylation by Cdk5 regulates its actions on migration through an effect on microtubules.

Cdk5 phosphorylation of huntingtin reduces its cleavage by caspases: implications for mutant huntingtin toxicity

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded polyglutamine (polyQ) tract in the huntingtin (htt) protein. Mutant htt toxicity is exposed after htt cleavage by caspases and other proteases release NH(2)-terminal fragments containing the polyQ expansion. Here, we show htt interacts and colocalizes with cdk5 in cellular membrane fractions. Cdk5 phosphorylates htt at Ser434, and this phosphorylation reduces caspase-mediated htt cleavage at residue 513. Reduced mutant htt cleavage resulting from cdk5 phosphorylation attenuated aggregate formation and toxicity in cells expressing the NH(2)-terminal 588 amino acids (htt588) of mutant htt. Cdk5 activity is reduced in the brains of HD transgenic mice compared with controls. This result can be accounted for by the polyQ-expanded htt fragments reducing the interaction between cdk5 and its activator p35. These data predict that the ability of cdk5 phosphorylation to protect against htt cleavage, aggregation, and toxicity is compromised in cells expressing toxic fragments of htt.

p53, Apaf-1, caspase-3, and -9 are dispensable for Cdk5 activation during cell death

Cyclin-dependent kinase 5 (Cdk5) is a member of the cyclin-dependent kinase family that is mostly seen in neurons, does not vary with cell cycle, and is activated in many neurodegenerative disorders and other non-neuronal pathologies, but its relationship to non-neuronal apoptosis is not understood, nor is the control of the activation of Cdk5 by its activators. The most widely studied activator of Cdk5, p35, is cleaved to p25 by calpain, an event that has been linked with activation of Cdk5 and neuronal death. Here we report that calpain-mediated Cdk5/p25 activation accompanies non-neuronal as well as neuronal cell death, suggesting that the p35/calpain/p25/Cdk5 activation sequence is a general feature of cell death. We further demonstrate that Cdk5 can be activated in the absence of p53, Apaf-1, caspase-9, and -3 during cell death, indicating that its activation relates more to cell death than to a specific pathway of apoptosis.

PTEN deletion in Bergmann glia leads to premature differentiation and affects laminar organization

Development of the central nervous system is controlled by both intrinsic and extrinsic signals that guide neuronal migration to form laminae. Although defects in neuronal mobility have been well documented as a mechanism for abnormal laminar formation, the role of radial glia, which provide the environmental cues, in modulating neuronal migration is less clear. We provide evidence that loss of PTEN in Bergmann glia leads to premature differentiation of this crucial cell population and subsequently to extensive layering defects. Accordingly, severe granule neuron migration defects and abnormal laminar formation are observed. These results uncover an unexpected role for PTEN in regulating Bergmann glia differentiation, as well as the importance of time-dependent Bergmann glia differentiation during cerebellar development.

Control of cyclin-dependent kinase 5 (Cdk5) activity by glutamatergic regulation of p35 stability

Although the roles of cyclin-dependent kinase 5 (Cdk5) in neurodevelopment and neurodegeneration have been studied extensively, regulation of Cdk5 activity has remained largely unexplored. We report here that glutamate, acting via NMDA or kainate receptors, can induce a transient Ca(2+)/calmodulin-dependent activation of Cdk5 that results in enhanced autophosphorylation and proteasome-dependent degradation of a Cdk5 activator p35, and thus ultimately down-regulation of Cdk5 activity. The relevance of this regulation to synaptic plasticity was examined in hippocampal slices using theta burst stimulation. p35(-/-) mice exhibited a lower threshold for induction of long-term potentiation. Thus excitatory glutamatergic neurotransmission regulates Cdk5 activity through p35 degradation, and this pathway may contribute to plasticity.

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.

A Cdk5 inhibitory peptide reduces tau hyperphosphorylation and apoptosis in neurons

The extracellular aggregation of amyloid beta (Abeta) peptides and the intracellular hyperphosphorylation of tau at specific epitopes are pathological hallmarks of neurodegenerative diseases such as Alzheimer's disease (AD). Cdk5 phosphorylates tau at AD-specific phospho-epitopes when it associates with p25. p25 is a truncated activator, which is produced from the physiological Cdk5 activator p35 upon exposure to Abeta peptides. We show that neuronal infections with Cdk5 inhibitory peptide (CIP) selectively inhibit p25/Cdk5 activity and suppress the aberrant tau phosphorylation in cortical neurons. Furthermore, Abeta(1-42)-induced apoptosis of these cortical neurons was also reduced by coinfection with CIP. Of particular importance is our finding that CIP did not inhibit endogenous or transfected p35/Cdk5 activity, nor did it inhibit the other cyclin-dependent kinases such as Cdc2, Cdk2, Cdk4 and Cdk6. These results, therefore, provide a strategy to address, and possibly ameliorate, the pathology of neurodegenerative diseases that may be a consequence of aberrant p25 activation of Cdk5, without affecting 'normal' Cdk5 activity.

Angiostatin-induced inhibition of endothelial cell proliferation/apoptosis is associated with the down-regulation of cell cycle regulatory protein cdk5

Endothelial cells (ECs) are quiescent in normal blood vessels, but undergo rapid bursts of proliferation after vascular injury, hypoxia or induced by powerful angiogenic cytokines like fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). Deregulated proliferation of ECs facilitates angiogenic processes and promotes tumor growth. In dividing cells, cell cycle-associated protein kinases, which are referred as cyclin-dependent kinases (cdks), regulate proliferation, differentiation, senescence, and apoptosis. Cyclin-dependent kinase-5 (cdk5) is expressed in neuronal cells and plays an important role in neurite outgrowth, of neuronal migration and neurogenesis, its functions in non-neuronal cells are unclear. Here, we show for the first time that the cdk5 is expressed at high levels in proliferating bovine aortic endothelial (BAE) cells, by contrast insignificant low levels of cdk5 expression in quiescent BAE cells. In addition, bFGF up-regulates cdk5 expression in a dose-dependent fashion. Interestingly, temporal expression data suggests that cdk5 expression is very low between 24-48 h, but high level of cdk5 expression was detected during 60-72 h. This later time corresponds to the time of completion of one cell cycle (doubling of cell population) of BAE cell culture. Angiostatin (AS), a powerful inhibitor of angiogenesis inhibits ECs proliferation in dose-dependent manner with concomitant down-regulation of cdk5 expression. The role of cdk5 in ECs, proliferation and apoptosis was confirmed by selective inhibition of cdk5 expression by the purine derivative roscovitine, which inhibits bFGF-stimulated BAE cells proliferation and induces apoptosis in dose-specific manner. By contrast, the roscovitine analog olomoucine, which is a specific inhibitor of cdk4, but not of cdk5 failed to affect ECs proliferation and apoptosis. These data suggest for the first time that neuron specific protein cdk5 may have significant role in the regulation of ECs proliferation, apoptosis, and angiogenesis and extends beyond its role in neurogenesis.

Dual phases of migration of cerebellar granule cells guided by axonal and dendritic leading processes

During lamination of the cerebellar cortex, granule cells initially migrate tangentially along the external granule layer, and then make a vertical turn and migrate radially to the internal granule layer. We comparatively analyzed the properties of biphasic migration of granule cells in a microexplant culture in which quantitation of morphology and subcellular localization of molecules were readily accomplished. Tangential migration was guided by a leading process that later formed a parallel fiber axon. Translocation of the soma within the axonal process occurred independently of the rapid displacement of the large growth cone. On the other hand, radial migration was guided by a leading process that differentiated into a dendrite after completion of migration. Displacement of the soma and the tiny growth cone were linked so that the radial leading process adopted locomotion and kept a constant length. We propose that the dual phases of granule cell migration are achieved by distinct cellular mechanisms guided by the leading processes forming an axon and a dendrite, respectively.

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

Cyclin-dependent kinase 5 (Cdk5) is essential for the proper development of the CNS, as is evident from the perinatal lethality of conventional Cdk5 knockout (Cdk5-/-) mice. Cdk5 is also implicated in numerous complex functions of the adult CNS such as synaptic transmission, synaptic plasticity, and neuronal signaling. To elucidate the molecular roles of Cdk5 in the adult CNS, we have abrogated neuronal expression of Cdk5 in perinatal mice by using a cre-loxP system. The Cdk5-loxP flanked mice were crossed with the cre-transgenic mice in which the cre expression is driven by the murine neurofilament-heavy chain promoter, resulting in generation of viable Cdk5 conditional knockout mice with the restricted deletion of the Cdk5 gene in specific neurons beginning around embryonic day 16.5. Twenty-five percent of the Cdk5 conditional knockout mice carrying the heterozygous cre allele had neuronal migration defects confined to brain areas where neuronal migration continues through the perinatal period. These results indicate that abrogation of Cdk5 expression in mature neurons results in a viable mouse model that offers further opportunities to investigate the molecular roles of Cdk5 in the adult CNS.

Mutant superoxide dismutase 1 causes motor neuron degeneration independent of cyclin-dependent kinase 5 activation by p35 or p25

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by selective loss of motor neurons in the brain and spinal cord. Neurotoxicity mediated by glutamate is thought to play a role in the neuronal death through intracellular calcium-dependent signaling cascades. Cyclin-dependent kinase 5 (Cdk5) has been proposed as one of the calcium-dependent mediators that may cause neuronal death observed in this disease. Cdk5 is activated in neurons by the association with its activators, p35 or p39. The calcium-activated protease calpain cleaves p35 to its truncated product, p25, which eventually causes the cellular mislocalization and prolonged activation of Cdk5. This deregulated Cdk5 induces cytoskeletal disruption and apoptosis. To examine whether inhibition of the calpain-mediated conversion of p35 to p25 can delay the disease progression of ALS, we generated double transgenic mice in which ALS-linked mutant copper/zinc superoxide dismutase 1 (SOD1G93A) was expressed in a p35-null background. The absence of p35 neither affected the onset and progression of motor neuron disease in the mutant SOD1 mice nor ameliorated the pathological lesions in these mice. Our results provide direct evidence that the pathogenesis of motor neuron disease in the mutant SOD1 mice is independent of the Cdk5 activation by p35 or p25.

Decreased cyclin-dependent kinase 5 (cdk5) activity is accompanied by redistribution of cdk5 and cytoskeletal proteins and increased cytoskeletal protein phosphorylation in p35 null mice

Cdk5/p35 has been implicated in cytoskeletal protein phosphorylation in normal brain and in many human neurodegenerative disorders. Yet, mouse models of cdk5/p35 hyperactivity have not yielded corresponding changes in cytoskeletal protein phosphorylation. To elucidate the relationship between p35, cdk5, and the neuronal cytoskeleton, we deleted the p35 gene in mice having a pure C57BL/6 background. We found that p35 deficiency leads to a 38% reduction of cdk5 activity in adult brain. In addition, loss of p35 causes an anterograde redistribution of cdk5 toward peripheral neuronal processes. The unusual presence of nonphosphorylated neurofilament (NF) in aberrant axon fascicles and the relocation of tau and MAP2B from cell bodies and proximal neuronal processes to more distal sites of the neuropil in p35-/- mouse brain implicate p35 in neuronal trafficking, particularly in dynein-driven retrograde transport. In many axons of normal brain, cdk5 fails to colocalize with phosphorylated cytoskeletal protein epitopes. This observation, together with an unexpected increase of NF, tau, and MAP2B phosphoepitopes accompanying the decreased cdk5 activity in p35-/- mice, supports the idea that cdk5 does not phosphorylate cytoskeletal proteins directly. Rather, in structures where cdk5 does colocalize with phosphorylated cytoskeletal protein epitopes, it may function as a negative regulator of other proline-directed kinases that directly phosphorylate the proteins. Evidence for increased glycogen synthase kinase 3beta (GSK3beta) activity in p35-/- mice suggests that GSK3beta may be one such kinase regulated by cdk5. Our studies illustrate that p35 regulates the subcellular distribution of cdk5 and cytoskeletal proteins in neurons and that cdk5 has a hierarchical role in regulating the phosphorylation and function of cytoskeletal proteins.

Outer dense fibers serve as a functional target for Cdk5.p35 in the developing sperm tail

Cdk5 is ubiquitously expressed in all tissues, but its activators, p35 and p39, are principally found in brain, and Cdk5 activity has mostly been associated with brain development, particularly neuronal differentiation and migration. Here we show that the p35 transcript and protein are also present in the testis, and an active Cdk5.p35 complex exists in this tissue as well. Cdk5 and p35 are prominently observed in elongating spermatid tails, particularly over the tail outer dense fibers (ODF). The appearance of Cdk5.p35 proceeds from the proximal to the distal end of elongating spermatids, coinciding with the proximal to distal assembly of ODF along the length of the tail axoneme. Incidentally, increased Cdk5.p35 activity is observed in isolated elongating spermatids and at a time when elongating spermatids appear in the developing testis, suggesting a role for Cdk5.p35 in spermiogenesis. The presence of Cdk5 and p35 in ODF isolated from rat sperm tails implies a strong association among these proteins. In vitro ODF phosphorylation by Cdk5.p35 and decreased in vivo sperm tail ODF phosphorylation in p35-deficient mice indicate that Cdk5.p35 is an integral component of ODF and that ODF is a functional Cdk5.p35 target in the testis. Our results demonstrate for the first time that Cdk5.p35 may participate in the regulation of sperm tail development via a mechanism involving ODF phosphorylation. Apparently, as in brain development, Cdk5.p35 plays a part in testis development.

Disregulated RhoGTPases and actin cytoskeleton contribute to the migration defect in Lis1-deficient neurons

Lissencephaly is a severe brain malformation caused by impaired neuronal migration. Lis1, a causative gene, functions in an evolutionarily conserved nuclear translocation pathway regulating dynein motor and microtubule dynamics. Whereas microtubule contributions to neuronal motility are incompletely understood, the actin cytoskeleton is essential for crawling cell movement of all cell types investigated. Lis1 haploinsufficiency is shown here to also result in reduced filamentous actin at the leading edge of migrating neurons, associated with upregulation of RhoA and downregulation of Rac1 and Cdc42 activity. Disruption of RhoA function through pharmacological inhibition of its effector kinase, p160ROCK, restores normal Rac1 and Cdc42 activity and rescues the motility defect in Lis1+/- neurons. These data indicate a previously unrecognized role for Lis1 protein in neuronal motility by promoting actin polymerization through the regulation of Rho GTPase activity. This effect of Lis1 on GTPases does not appear to occur through direct Lis1 binding of Rho, but could involve Lis1 effects on Rho modulatory proteins or on microtubule dynamics.

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.

Identification of a novel, membrane-associated neuronal kinase, cyclin-dependent kinase 5/p35-regulated kinase

Here we characterize a novel neuronal kinase, cyclin-dependent kinase 5 (cdk5)/p35-regulated kinase (cprk). Cprk is a member of a previously undescribed family of kinases that are predicted to contain two N-terminal membrane-spanning domains and a long C terminus, which harbors a dual-specificity serine/threonine/tyrosine kinase domain. Cprk was isolated in a yeast two-hybrid screen using the neuronal cdk5 activator p35 as "bait." Cprk interacts with p35 in the yeast-two hybrid system, binds to p35 in glutathione S-transferase fusion pull-down assays, and colocalizes with p35 in cultured neurons and transfected cells. In these cells, cprk is present with p35 in the Golgi apparatus. Cprk is expressed in a number of tissues but is enriched in brain and muscle and within the brain is found in a wide range of neuronal populations. Cprk displays catalytic activity in in vitro kinase assays and is itself phosphorylated by cdk5/p35. Cdk5/p35 inhibits cprk activity. Cdk5/p35 may therefore regulate cprk function in the brain.

Partial rescue of the p35-/- brain phenotype by low expression of a neuronal-specific enolase p25 transgene

Cyclin-dependent kinase 5 (Cdk5) is activated on binding of activator proteins p35 and p39. A N-terminally truncated p35, termed p25, is generated through cleavage by the Ca(2+)-dependent protease calpain after induction of ischemia in rat brain. p25 has been shown to accumulate in brains of patients with Alzheimer's disease and may contribute to A-beta peptide-mediated toxicity. Studies from transfected neurons as well as p35 and p25 transgenic mice have indicated that Cdk5, when activated by p25, gains some toxic function compared with p35/Cdk5. It remains unclear, however, whether p25/Cdk5 signaling additionally channels into pathways usually used by p35/Cdk5 and whether p25 is associated with a loss of p35 function. To clarify these issues, we have generated p25-transgenic mice in a p35-null background. We find that low levels of p25 during development induce a partial rescue of the p35-/- phenotype in several brain regions analyzed, including a rescue of cell positioning of a subset of neurons in the neocortex. In accordance with the partial rescue of brain anatomy, phosphorylation of the Cdk5 substrate mouse disabled 1 is partially restored during development. Besides this, p25/Cdk5 fails to phosphorylate other substrates that are normally phosphorylated by p35/Cdk5. Our results show that p25 can substitute for p35/Cdk5 under certain circumstances during development. In addition, they suggest that p25 may have lost some functions of p35.

Cyclin-dependent kinase 5 (CDK5) and neuronal cell death

Many neurological disorders like Parkinson's and Alzheimer's disease, amyotrophic lateral sclerosis (ALS) or stroke have in common a definite loss of CNS neurons due to apoptotic or necrotic neuronal cell death. Previous studies suggested that proapoptotic stimuli may trigger an abortive and, therefore, eventually fatal cell cycle reentry in postmitotic neurons. Neuroprotective effects of small molecule inhibitors of cyclin-dependent kinases (CDKs), which are key regulators of cell cycle progression, support the cell cycle theory of neuronal apoptosis. However, growing evidence suggests that deregulated CDK5, which is not involved in cell cycle control, rather than cell cycle relevant members of the CDK family, promotes neuronal cell death. Here we summarize the current knowledge about the involvement of CDK5 in neuronal cell death and discuss possible up- or downstream partners of CDK5. Moreover, we discuss potential therapeutic options that might arise from the identification of CDK5 as an important upstream element of neuronal cell death cascades.

Coordinate control of proliferation and migration by the p27Kip1/cyclin-dependent kinase/retinoblastoma pathway in vascular smooth muscle cells and fibroblasts

Previous studies have demonstrated a protective effect of the cyclin-dependent kinase (CDK) inhibitor p27Kip1 against atherosclerosis and restenosis, two disorders characterized by abundant proliferation and migration of vascular smooth muscle cells and adventitial fibroblasts. These therapeutic effects might result from p27Kip1-dependent suppression of both cell proliferation and migration. However, the interplay between cell growth and locomotion remains obscure. We show here that p27Kip1 inhibits cellular changes that normally occur during cell locomotion (eg, lamellipodia formation and reorganization of actin filaments and focal adhesions). Importantly, a p27Kip1 mutant lacking CDK inhibitory activity failed to inhibit vascular smooth muscle cell and fibroblast proliferation and migration. Moreover, a constitutively active mutant of the retinoblastoma protein (pRb) insensitive to CDK-dependent hyperphosphorylation inhibited both cell proliferation and migration. In contrast, inactivation of pRb by forced expression of the adenoviral oncogene E1A correlated with high proliferative and migratory activity. Collectively, these results suggest that cellular proliferation and migration are regulated in a coordinated manner by the p27Kip1/CDK/pRb pathway. These findings might have important implications for the development of novel therapeutic strategies targeting the fibroproliferative/migratory component of vascular occlusive disorders.

Hippocampal heterotopia with molecular and electrophysiological properties of neocortical neurons

Cortical malformations resulting from aberrant brain development can be associated with mental retardation, dyslexia, and intractable forms of epilepsy. Despite emerging interest in the pathology and etiology of cortical malformations, little is known about the phenotype of cells within these lesions. In utero exposure to the DNA methylating agent methylazoxymethanol acetate (MAM) during a critical stage in neurodevelopment results in animals with distinct clusters of displaced neurons in hippocampus, i.e. nodular heterotopia. Here we examined the molecular and electrophysiological properties of cells within hippocampal heterotopia using rats exposed to MAM during gestation. Molecular analysis revealed that heterotopic cells do not express mRNA markers normally found in hippocampal pyramidal cells or dentate granule cells (SCIP, Math-2, Prox-1, neuropilin-2). In contrast, Id-2 mRNA, normally abundant in Layer II-III supragranular neocortical neurons but not in CA1 pyramidal neurons, was prominently expressed in hippocampal heterotopia. Current-clamp analysis of the firing properties of heterotopic neurons revealed a striking similarity with supragranular cortical neurons. In particular, both cells were characterized by small hyperpolarizing 'sag' potentials, high input resistance values, slow spike-train afterhyperpolarizations, and the absence of a depolarizing afterpotential. Normotopic CA1 pyramidal neurons (e.g. pyramidal cells with normal lamination adjacent to a heterotopia) in the MAM brain exhibited molecular and electrophysiological properties that were nearly identical to those of age-matched CA1 pyramidal neurons from control rats. We conclude that neuronal heterotopiae in the hippocampus of MAM-exposed rats are comprised of neurons with a Layer II-III supragranular cortex phenotype. The MAM model, therefore, may serve as a useful tool in examination of the factors influencing aberrant brain development and epilepsy.

Cycling at the interface between neurodevelopment and neurodegeneration

The discovery of cell cycle regulators has directed cell research into uncharted territory. In dividing cells, cell cycle-associated protein kinases, which are referred to as cyclin-dependent-kinases (Cdks), regulate proliferation, differentiation, senescence and apoptosis. In contrast, all Cdks in post-mitotic neurons, with the notable exception of Cdk5, are silenced. Surprisingly, misregulation of Cdks occurs in neurons in a wide diversity of neurological disorders, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Ectopic expression of these proteins in neurons potently induces cell death with hallmarks of apoptosis. Deregulation of the unique, cell cycle-unrelated Cdk5 by its truncated co-activator, p25 and p29, contributes to neurodegeneration by altering the phosphorylation state of non-membrane-associated proteins and possibly through the induction of cell cycle proteins. On the other hand, cycling Cdks such as Cdk2, Cdk4 and Cdk6, initiate death pathways by derepressing E2F-1/Rb-dependent transcription at the neuronal G1/S checkpoint. Thus, Cdk5 and cycling Cdks may have little in common in the healthy CNS, but they likely conspire in leading neurons to their demise.

Reelin signaling and Cdk5 in the control of neuronal positioning

Neuronal positioning is important for higher brain function because it is the architectural basis of the formation of precise synaptic circuits. Analysis of neurological mutant mice has led to dramatic progress in the identification and characterization of molecules important for neuronal positioning in the developing mammalian brain. Among these molecules, identification of signal pathways mediated by Reelin and Cdk5 kinase has provided a conceptual framework for exploring the molecular mechanisms underlying proper neuronal positioning in the developing mammalian brain. Recent evidence has implicated synergism between Reelin signaling and Cdk5 in contributing to the proper positioning of selective neuronal populations.

A peptide derived from cyclin-dependent kinase activator (p35) specifically inhibits Cdk5 activity and phosphorylation of tau protein in transfected cells

Cyclin-dependent kinase-5 (Cdk5) is a serine/threonine kinase activated by its neuron-specific activator, p35, or its truncated form, p25. It has been proposed that the deregulation of Cdk5 activity by association with p25 in human brain tissue disrupts the neuronal cytoskeleton and may be involved in neurodegenerative diseases such as Alzheimer's disease. In this study, we demonstrate that a short peptide (amino acid residues 154-279; Cdk5 inhibitory peptide; CIP), derived from p35, specifically inhibits Cdk5 activity in vitro and in HEK293 cells cotransfected with the peptide and Cdk5/p25, but had no effect on endogenous cdc2 kinase activity. Moreover, we demonstrate that the phosphorylation of tau in HEK293 cells, cotransfected with Cdk5/p25 and CIP, is effectively reduced. These results suggest that CIP specifically inhibits both Cdk5/p25 complex activity and the tau hyperphosphorylation induced by Cdk5/p25. The elucidation of the molecular basis of p25 activation and CIP inhibition of Cdk5 activity may provide insight into mechanisms underlying the pathology of Alzheimer's disease and contribute to therapeutic strategies.

Abnormal expression of cdk5 in focal cortical dysplasia in humans

Focal cortical dysplasia (FCD) is an important cause of refractory epilepsy in humans. The origin of its pathognomonic abnormal cell types and the links between abnormal cell morphology and epileptogenicity remain unknown. The developmentally-regulated kinase cdk5 and its neuronal activator p35 are known to be central to a number of key components in neuronal development, cellular morphology, cytoskeletal function, synaptic plasticity and neurodegeneration. Here we examine eight cases of human FCD for expression of cdk5. We show abnormal cdk5 immunoreactivity and aggregation of protein suggesting alterations in cdk5 may also be involved in this important epileptogenic human pathology.

The role of Rho GTPases and associated kinases in regulating neurite outgrowth

Neurones are highly specialised cells that can extend over great distances, enabling the complex networking of the nervous system. We are beginning to understand in detail the molecular mechanisms that control the shape of neurones during development. One family of proteins that are clearly essential are the Rho GTPases which have a pivotal role in regulating the actin cytoskeleton in all cell types. The Rho GTPases are responsible for the activation and downregulation of many downstream kinases. This review discusses individual kinases that are regulated by three members of the Rho GTPases, Rac, Rho and Cdc42 and their function during neurite outgrowth and remodelling.

Cyclin-dependent kinase 5 phosphorylates disabled 1 independently of Reelin signaling

Two major signaling pathways that control neuronal positioning during brain development have been uncovered as a result of genetic and biochemical studies on neurological mouse mutants. Mice deficient in Reelin, Disabled 1 (Dab1), or both the very low-density lipoprotein receptor (VLDLR) and the apolipoprotein E receptor 2 (ApoER2) exhibit identical neuroanatomic defects in laminar structures throughout the brain. These proteins function as components of the Reelin signaling pathway. Reelin is a secreted glycoprotein that binds to VLDLR and ApoER2, inducing tyrosine phosphorylation of Dab1, an intracellular adapter protein. Neuronal migration is also regulated by cyclin-dependent kinase 5 (Cdk5) and its activating subunits p35 and p39. Mice deficient in Cdk5, p35, or both p35 and p39 exhibit lamination defects that are similar but not identical to those observed in mice with a defect in the Reelin signaling pathway. Cdk5 phosphorylates proteins that maintain cytoskeletal structures and promote cell motility. To explore the possibility that Cdk5 influences the Reelin pathway, we sought to determine whether Dab1 is a substrate for Cdk5. Here we show that Cdk5 phosphorylates Dab1 on serine 491 in vitro and in vivo, independently of Reelin signaling. We also show that ectopic neurons in Cdk5-deficient mice exhibit reduced levels of Reelin signaling during later stages of cortical development, although Cdk5 is not required for Reelin-induced tyrosine phosphorylation of Dab1. Although the functional significance of Dab1 serine phosphorylation is unclear, our results suggest that there is biochemical cross-talk between two signaling pathways that control cell positioning.

Expression of Cdk5 and its activators in NT2 cells during neuronal differentiation

We have recently developed a rapid protocol involving NT2 cell aggregation and treatment with retinoic acid (RA) to produce terminally differentiated CNS neurons. As a first step to explore the functional roles of cell-cycle regulatory proteins in the process of neuronal differentiation, the expression profiles of cyclin-dependent kinases (Cdks) and their regulators were examined in NT2 cells following treatment with RA. One of the Cdks, Cdk5, has been demonstrated to affect the process of neuronal differentiation and suggested to play an important role in development of the nervous system. We found that the expression of Cdk5 was gradually increased, while its activators (p35 and p39) as well as Cdk5 kinase activity were induced in NT2 cells during the process of neuronal differentiation. Moreover, both p35 and p39 were localized along the axons and varicosity-like structures of differentiated NT2 neurons. Taken together, our results demonstrated that NT2 cells provide a good in vitro model system to examine signaling pathways involved in the regulation of Cdk5 activators and to elucidate the functional roles of Cdk5 in neuronal differentiation.

Cdc2 phosphorylation of BAD links the cell cycle to the cell death machinery

A mechanism that triggers neuronal apoptosis has been characterized. We report that the cell cycle-regulated protein kinase Cdc2 is expressed in postmitotic granule neurons of the developing rat cerebellum and that Cdc2 mediates apoptosis of cerebellar granule neurons upon the suppression of neuronal activity. Cdc2 catalyzes the phosphorylation of the BH3-only protein BAD at a distinct site, serine 128, and thereby induces BAD-mediated apoptosis in primary neurons by opposing growth factor inhibition of the apoptotic effect of BAD. The phosphorylation of BAD serine 128 inhibits the interaction of growth factor-induced serine 136-phosphorylated BAD with 14-3-3 proteins. Our results suggest that a critical component of the cell cycle couples an apoptotic signal to the cell death machinery via a phosphorylation-dependent mechanism that may generally modulate protein-protein interactions.

Cyclin-dependent kinase 5 prevents neuronal apoptosis by negative regulation of c-Jun N-terminal kinase 3

Cyclin-dependent kinase 5 (cdk5) is a serine/threonine kinase activated by associating with its neuron-specific activators p35 and p39. Analysis of cdk5(-/-) and p35(-/-) mice has demonstrated that both cdk5 and p35 are essential for neuronal migration, axon pathfinding and the laminar configuration of the cerebral cortex, suggesting that the cdk5-p35 complex may play a role in neuron survival. However, the targets of cdk5 that regulate neuron survival are unknown. Here, we show that cdk5 directly phosphorylates c-Jun N-terminal kinase 3 (JNK3) on Thr131 and inhibits its kinase activity, leading to reduced c-Jun phosphorylation. Expression of cdk5 and p35 in HEK293T cells inhibits c-Jun phosphorylation induced by UV irradiation. These effects can be restored by expression of a catalytically inactive mutant form of cdk5. Moreover, cdk5-deficient cultured cortical neurons exhibit increased sensitivity to apoptotic stimuli, as well as elevated JNK3 activity and c-Jun phosphorylation. Taken together, these findings show that cdk5 may exert its role as a key element by negatively regulating the c-Jun N-terminal kinase/stress-activated protein kinase signaling pathway during neuronal apoptosis.

Phosphorylation of MEK1 by cdk5/p35 down-regulates the mitogen-activated protein kinase pathway

Cyclin-dependent protein kinase 5 (cdk5), a member of the cdk family, is active mainly in postmitotic cells and plays important roles in neuronal development and migration, neurite outgrowth, and synaptic transmission. In this study we investigated the relationship between cdk5 activity and regulation of the mitogen-activated protein (MAP) kinase pathway. We report that cdk5 phosphorylates the MAP kinase kinase-1 (MEK1) in vivo as well as the Ras-activated MEK1 in vitro. The phosphorylation of MEK1 by cdk5 resulted in inhibition of MEK1 catalytic activity and the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2. In p35 (cdk5 activator) -/- mice, which lack appreciable cdk5 activity, we observed an increase in the phosphorylation of NF-M subunit of neurofilament proteins that correlated with an up-regulation of MEK1 and ERK1/2 activity. The activity of a constitutively active MEK1 with threonine 286 mutated to alanine (within a TPXK cdk5 phosphorylation motif in the proline-rich domain) was not affected by cdk5 phosphorylation, suggesting that Thr286 might be the cdk5/p35 phosphorylation-dependent regulatory site. These findings support the hypothesis that cdk5 and the MAP kinase pathway cross-talk in the regulation of neuronal functions. Moreover, these data and the recent studies of Harada et al. (Harada, T., Morooka, T., Ogawa, S., and Nishida, E. (2001) Nat. Cell Biol. 3, 453-459) have prompted us to propose a model for feedback down-regulation of the MAP kinase signal cascade by cdk5 inactivation of MEK1.

The cerebellar deficient folia (cdf) gene acts intrinsically in Purkinje cell migrations

Cerebellar deficient folia (cdf) is a recently identified mouse mutation causing ataxia and cerebellar abnormalities including lobulation defects and abnormal placement of a specific subset of Purkinje cells. To understand the etiology of the cerebellar defects in cdf mutant mice, we examined postnatal development of the cdf/cdf cerebellum. Our results demonstrate that Purkinje cell ectopia and foliation defects are apparent at birth, suggesting the cdf mutation disrupts the positioning of many, but not all, Purkinje cells during development. In addition to cerebellar abnormalities, we observed lamination defects in the hippocampus of cdf mutant mice, although neocortical defects were not seen. Furthermore, ectopic Purkinje cells in cdf/cdf mice express an increased level of Dab1 protein, as previously observed in mice with mutations in genes in the reelin signaling pathway. Lastly, analysis of cdf <-->ROSA26 chimeric mice demonstrated that the cdf mutation is intrinsic to Purkinje cells. We suggest that the cdf gene product is required in a subset of Purkinje cells, possibly to respond to Reelin signals.

Phosphorylation of Pak1 by the p35/Cdk5 kinase affects neuronal morphology

The small GTPase Rac and its effectors, the Pak1 and p35/Cdk5 kinases, have been assigned important roles in regulating cytoskeletal dynamics in neurons. Our previous work revealed that the neuronal p35/Cdk5 kinase associates with Pak1 in a RacGTP-dependent manner, causing hyperphosphorylation and down-regulation of Pak1 kinase activity. We have now demonstrated direct phosphorylation of Pak1 on threonine 212 by the p35/Cdk5 kinase. In neuronal growth cones, Pak1 phosphorylated on Thr-212 localized to actin and tubulin-rich areas, suggesting a role in regulating growth cone dynamics. The expression of a non-phosphorylatable Pak1 mutant (Pak1A212) induced dramatic neurite disorganization. We also observed a strong association between p35/Cdk5 and the Pak1 C-terminal kinase domain. Overall, our data show that in neurons, membrane-associated, active Pak1 is regulated by the p35/Cdk5 kinase both by association and phosphorylation, which is essential for the proper regulation of the cytoskeleton during neurite outgrowth and remodeling.

Neocortical cell migration: GABAergic neurons and cells in layers I and VI move in a cyclin-dependent kinase 5-independent manner

The adult mammalian cerebral cortex arises from a complex series of neuronal migrations. The primitive layer known as the preplate is split into an outer marginal zone and an inner subplate by invading cortical plate neurons in an "inside-out" pattern of layering with respect to time of neuronal origin. In cyclin-dependent kinase 5 (Cdk5)-deficient mice (cdk5(-/-)), the earliest born cortical neurons split the preplate, but later born neurons arrest below the subplate, resulting in an ectopic "outside-in" layer of neurons normally destined for layers II-V. We have pursued this analysis in cdk5(-/-) <--> wild-type chimeric mice coupled with experiments in cell culture. In vitro migration assays show no difference in migrational ability between embryonic cdk5(-/-) and wild-type neurons. In cdk5(-/-) chimeras, layers I and VI are made up of both mutant and wild-type genotype neurons, whereas layers II-V contain predominantly wild-type cells. In addition, a thin layer of neurons is found below layer VI, made up of cdk5(-/-) cells; bromodeoxyuridine labeling suggests that these neurons were destined for layers II-V. Scattered cdk5(-/-) cells are found throughout layers II-V, but these neurons are always found to be GABAergic. The findings suggest that Cdk5 is not required for migration of either the deepest cortical plate neurons or the GABAergic neurons from the ganglionic eminences. The migration of layer II-V pyramidal neurons, however, is intrinsically blocked by Cdk5 deficiency, thus suggesting that different neuronal cell types use distinct mechanisms of migration.

Regulation of NMDA receptors by cyclin-dependent kinase-5

Members of the N-methyl-d-aspartate (NMDA) class of glutamate receptors (NMDARs) are critical for development, synaptic transmission, learning and memory; they are targets of pathological disorders in the central nervous system. NMDARs are phosphorylated by both serine/threonine and tyrosine kinases. Here, we demonstrate that cyclin dependent kinase-5 (Cdk5) associates with and phosphorylates NR2A subunits at Ser-1232 in vitro and in intact cells. Moreover, we show that roscovitine, a selective Cdk5 inhibitor, blocks both long-term potentiation induction and NMDA-evoked currents in rat CA1 hippocampal neurons. These results suggest that Cdk5 plays a key role in synaptic transmission and plasticity through its up-regulation of NMDARs.

A decade of CDK5

Since it was identified a decade ago, cyclin-dependent kinase 5 (CDK5) has emerged as a crucial regulator of neuronal migration in the developing central nervous system. CDK5 phosphorylates a diverse list of substrates, implicating it in the regulation of a range of cellular processes - from adhesion and motility, to synaptic plasticity and drug addiction. Recent evidence indicates that deregulation of this kinase is involved in the pathology of neurodegenerative diseases.

Role of the reelin signaling pathway in central nervous system development

The neurological mutant mouse reeler has played a critical role in the evolution of our understanding of normal brain development. From the earliest neuroanatomic studies of reeler, it was anticipated that the characterization of the gene responsible would elucidate important molecular and cellular principles governing cell positioning and the formation of synaptic circuits in the developing brain. Indeed, the identification of reelin has challenged many of our previous notions and has led to a new vision of the events involved in the migration of neurons. Several neuronal populations throughout the brain secrete Reelin, which binds to transmembrane receptors located on adjacent cells triggering a tyrosine kinase cascade. This allows neurons to complete migration and adopt their ultimate positions in laminar structures in the central nervous system. Recent studies have also suggested a role for the Reelin pathway in axonal branching, synaptogenesis, and pathology underlying neurodegeneration.

p35 and p39 are essential for cyclin-dependent kinase 5 function during neurodevelopment

Cyclin-dependent kinase 5 (Cdk5) plays a pivotal role in brain development and neuronal migration. Cdk5 is abundant in postmitotic, terminally differentiated neurons. The ability of Cdk5 to phosphorylate substrates is dependent on activation by its neuronal-specific activators p35 and p39. There exist striking differences in the phenotypic severity of Cdk5-deficient mice and p35-deficient mice. Cdk5-null mutants show a more severe disruption of lamination in the cerebral cortex, hippocampus, and cerebellum. In addition, Cdk5-null mice display perinatal lethality, whereas p35-null mice are viable. These discrepancies have been attributed to the function of other Cdk5 activators, such as p39. To understand the roles of p39 and p35, we created p39-null mice and p35/p39 compound-mutant mice. Interestingly, p39-null mice show no obvious detectable abnormalities, whereas p35(-/-)p39(-/-) double-null mutants are perinatal lethal. We show here that the p35(-/-)p39(-/-) mutants exhibit phenotypes identical to those of the Cdk5-null mutant mice. Other compound-mutant mice with intermediate phenotypes allow us to determine the distinct and redundant functions between p35 and p39. Our data strongly suggest that p35 and p39 are essential for Cdk5 activity during the development of the nervous system. Thus, p35 and p39 are likely to be the principal, if not the only, activators of Cdk5.

Cyclin-dependent protein kinase 5 (Cdk5) and the regulation of neurofilament metabolism

Cyclin-dependent kinase 5 (Cdk5), a complex of Cdk5 and its activator p35 (Cdk5/p35), phosphorylates diverse substrates which have multifunctional roles in the nervous system. During development, it participates in neuronal differentiation, migration, axon outgrowth and synaptogenesis. Cdk5, acting together with other kinases, phosphorylates numerous KSPXK consensus motifs in diverse cytoskeletal protein target molecules, including neurofilaments, and microtubule associated proteins, tau and MAPs. Phosphorylation regulates the dynamic interactions of cytoskeletal proteins with one another during all aspects of neurogenesis and axon radial growth. In this review we shall focus on Cdk5 and its regulation as it modulates neurofilament metabolism in axon outgrowth, cytoskeletal stabilization and radial growth. We suggest that Cdk5/p35 forms compartmentalized macromolecular complexes of cytoskeletal substrates, other neuronal kinases, phosphatases and activators ('phosphorylation machines') which facilitate the dynamic molecular interactions that underlie these processes.

Synergistic contributions of cyclin-dependant kinase 5/p35 and Reelin/Dab1 to the positioning of cortical neurons in the developing mouse brain

Cyclin-dependent kinase (Cdk) 5 is a unique member of the Cdk family, because Cdk5 kinase activity is detected only in the nervous tissue. Two neuron-specific activating subunits of Cdk5, p35 and p39, have been identified. Overlapping expression pattern of these isoforms in the embryonic mouse brain and the significant residual Cdk5 kinase activity in brain homogenate of the p35-/- mice indicate the redundant functions of the Cdk5 activators in vivo. Severe neuronal migration defects in p35-/-Cdk5 +/- mice further support the idea that the redundant expression of the Cdk5 activators may cause a milder phenotype in p35-/- mice compared with Cdk5-/- mice. Mutant mice lacking either Cdk5 or p35 exhibit certain similarities with Reelin/Dab1-mutant mice in the disorganization of cortical laminar structure in the brain. To elucidate the relationship between Cdk5/p35 and Reelin/Dab1 signaling, we generated mouse lines that have combined defects of these genes. The addition of heterozygosity of either Dab1 or Reelin mutation to p35-/- causes the extensive migration defects of cortical neurons in the cerebellum. In the double-null mice of p35 and either Dab1 or Reelin, additional migration defects occur in the Purkinje cells in the cerebellum and in the pyramidal neurons in the hippocampus. These additional defects in neuronal migration in mice lacking both Cdk5/p35 and Reelin/Dab1 indicate that Cdk5/p35 may contribute synergistically to the positioning of the cortical neurons in the developing mouse brain.