The Xenopus laevis homologue to the neuronal cyclin-dependent kinase (cdk5) is expressed in embryos by gastrulation

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.

Zebrafish Rohon-Beard neuron development: cdk5 in the midst

Cyclin-dependent kinase 5 (cdk5) is a proline-directed serine/threonine kinase that is activated mostly by association with its activators, p35 and p39. Initially projected as a neuron-specific kinase, cdk5 is expressed ubiquitously and its kinase activity solely depends on the presence of its activators, which are also found in some non-neuronal tissues. As a multifunctional protein, cdk5 has been linked to axonogenesis, cell migration, exocytosis, neuronal differentiation and apoptosis. Cdk5 plays a critical role in functions other than normal physiology, especially in neurodegeneration. Its contribution to both normal physiological as well as pathological processes is mediated by its specific substrates. Cdk5-null mice are embryonically lethal, therefore making it difficult to study precisely what cdk5 does to the nervous system at early stages of development, be it neuron development or programmed cell death. Zebrafish model system bypasses the impediment, as it is amenable to reverse genetics studies. One of the functions that we have followed for the cdk5 ortholog in zebrafish in vivo is its effect on the Rohon-Beard (RB) neurons. RB neurons are the primary sensory spinal neurons that die during the first two days of zebrafish development eventually to be replaced by the dorsal root ganglia (DRG). Based on ours studies and others', here we discuss possible mechanisms that may be involved in cdk5's role in RB neuron development and survival.

Cloning and characterization of zebrafish (Danio rerio) cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (cdk5) is a ubiquitous protein activated by neuron-specific activators, p35 and p39. Cdk5 regulates neuronal migration, differentiation, axonogenesis, synaptic transmission and apoptosis. However, its role in primary neurogenesis remains unexplored. Here, we have cloned and characterized the zebrafish cdk5 ortholog. Zebrafish cdk5 is 96% identical to its human counterpart. In situ hybridization analyses demonstrated that zebrafish cdk5 transcripts are ubiquitously expressed as early as the blastula stage. At 11.5h of development, cdk5 transcripts were present in the neural plate at the domains where primary neurons begin to be specified. RT-PCR analyses showed equal levels of cdk5 transcripts up to 72 h of development. SiRNA-mediated cdk5 knockdown resulted in a reduction in primary sensory neurons of the trigeminal ganglia of the peripheral nervous system, suggesting that cdk5 plays a crucial role in the development of the peripheral nervous system.

Cyclin-dependent kinase 5 influences Rohon-Beard neuron survival in zebrafish

Cyclin-dependent kinase 5 (cdk5), a member of the cyclin-dependent kinase family, is expressed predominantly in post-mitotic cell populations. Unlike the other cdks, cdk5 is abundant and most active in differentiated neurons. Here, we describe the function of a cdk5 ortholog in zebrafish. Cdk5 catalytic activity is meager but present in early stages of development. However, at 24 h post-fertilization (hpf), the activity is remarkably higher and continues to be high through 48 and 72 hpf. Knocking down cdk5 by micro-injection of a specific siRNA resulted in decreased cdk5 protein level accompanied by reduced kinase activity. In the cdk5 siRNA-injected embryos, the number of primary sensory Rohon-Beard (RB) neurons was significantly reduced and there were more apoptotic cells in the brain. These phenotypes were rescued by co-injection of cdk5 mRNA. Within the first two days of development, RB neurons undergo apoptosis in zebrafish. To examine whether cdk5 has a role in RB neuron survival, cdk5 mRNA was injected into the one- to two-cell embryos. In these embryos, RB neuron apoptosis was inhibited compared with the uninjected control embryos. These results suggest that in zebrafish, cdk5 influences RB neuron survival and potentially regulates early neuronal development.

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

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

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.

Neuronal Cdc2-like protein kinase (Cdk5/p25) is associated with protein phosphatase 1 and phosphorylates inhibitor-2

Protein phosphatase 1 (PP1) is complexed with inhibitor 2 (I-2) in the cytosol. In rabbit muscle extract PP1.I-2 is activated upon preincubation with ATP/Mg. This activation is caused by phosphorylation of I-2 on Thr(72) by glycogen synthase kinase 3 (GSK3). We have found that PP1.I-2 in bovine brain extract is also activated upon preincubation with ATP/Mg. However, blocking GSK3 action by LiCl inhibited only approximately 29% of PP1 activity and indicated that GSK3 is not the sole PP1.I-2 activator in the brain. When bovine brain extract was analyzed by gel filtration PP1.I-2 and neuronal Cdc2-like protein kinase (NCLK), a heterodimer of Cdk5 and the regulatory p25 subunit, co-eluted as a approximately 450-kDa size species. The NCLK from the eluted column fractions bound to PP1-specific microcystin-Sepharose and glutathione S-transferase (GST)-I-2-coated glutathione-agarose beads. Similarly, PP1 from the eluted column fractions was pulled down with GST-Cdk5-coated glutathione-agarose beads. In vitro, NCLK phosphorylated I-2 on Thr(72) and activated PP1.I-2 in an ATP/Mg-dependent manner. NCLK bound to PP1 through its Cdk5 subunit and the PP1 binding region was localized to Cdk5 residues 28-41. Our data demonstrate that in brain extract PP1.I-2 and NCLK are associated within a complex of approximately 450 kDa and suggest that NCLK is one of the PP1.I-2-activating kinases in the mammalian brain.

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.

Coexpression of human cdk5 and its activator p35 with human protein tau in neurons in brain of triple transgenic mice

The potential contribution of cyclin-dependent protein kinase 5 (cdk5) to hyperphosphorylate protein tau, as claimed in Alzheimer's disease, was investigated in vivo. We generated single, double, and triple transgenic mice that coexpress human cdk5 and its activator p35 as well as human protein tau in cerebral neurons. Whereas expression and increased cdk5-kinase activity was obtained, as measured in vitro and demonstrated in vivo, neither murine nor human protein tau was appreciably phosphorylated in the brain of double and triple transgenic mice. These mice behaved and reproduced normally. Silver impregnation and immunohistochemistry of brain sections demonstrated that neurofilament proteins became redistributed in apical dendrites of cortical neurons. This suggested a cytoskeletal effect, but no other relevant brain pathology became apparent. These observations indicate that cdk5/p35 is not a major protein tau kinase and that cdk5/p35 did not cause neurodegeneration in mouse brain, as opposed to cdk5/p25.

Interaction of neuronal Cdc2-like protein kinase with microtubule-associated protein tau

Neuronal Cdc2-like protein kinase (NCLK), a approximately 58-kDa heterodimer, was isolated from neuronal microtubules (Ishiguro, K., Takamatsu, M., Tomizawa, K., Omori, A., Takahashi, M., Arioka, M., Uchida, T. and Imahori, K. (1992) J. Biol. Chem. 267, 10897-10901). The biochemical nature of NCLK-microtubule association is not known. In this study we found that NCLK is released from microtubules upon microtubule disassembly as a 450-kDa species. The 450-kDa species is an NCLK.tau complex, and NCLK-bound tau is in a nonphosphorylated state. Tau phosphorylation causes NCLK.tau complex dissociation, and phosphorylated tau does not bind to NCLK. In vitro, the Cdk5 subunit of NCLK binds to the microtubule-binding region of tau and NCLK associates with microtubules only in the presence of tau. Our data indicate that in brain extract NCLK is complexed with tau in a tau phosphorylation-dependent manner and that tau anchors NCLK to microtubules. Recently NCLK has been suggested to be aberrantly activated and to hyperphosphorylate tau in Alzheimer's disease brain (Patrick, G. N., Zukerberg, L., Nikolic, M., de la Monte, S., Dikkes, P, and Tsai, L.-H. (1999) Nature 402, 615-622). Our findings may explain why in Alzheimer's disease NCLK specifically hyperphosphorylates tau, although this kinase has a number of protein substrates in the brain.

Non-dependence of cyclin E/Cdk2 kinase activity on the initiation of oocyte maturation in goldfish

Cdk2 kinase activity increases during oocyte maturation but neither cyclin A nor B is associated with Cdk2 in mature oocytes in goldfish. As a potential Cdk2 partner in meiosis, a cyclin E homolog was isolated from a goldfish oocyte cDNA library. A monoclonal antibody was raised against bacterially produced full-length goldfish cyclin E. Both cyclin E and Cdk2 were already present in immature oocytes and their protein levels did not change remarkably during oocyte maturation. Cyclin E formed a complex mainly with Cdk2 just at the time of germinal vesicle breakdown (GVBD) in association with the increase in Cdk2 kinase activity, although a fraction of cyclin E bound to Cdk(s) other than Cdk2 and Cdc2. Ectopic activation of cyclin E/Cdk2 by the injection of cyclin E messenger RNA (mRNA) into immature oocytes did not induce maturation-promoting factor (MPF) activation and GVBD. Furthermore, inhibition of cyclin E/Cdk2 kinase activity by the injection of p21SDI1 into the oocytes treated with 17alpha,20beta-dihydroxy-4-pregnen-3-one had no effect on MPF activation and GVBD. These results indicate that cyclin E/Cdk2 kinase activity is insufficient and unnecessary for initiating goldfish oocyte maturation.

Neuronal differentiation and patterning in Xenopus: the role of cdk5 and a novel activator xp35.2

Cdk5, a member of the cyclin-dependent kinase family, has been shown to play an important role in development of the central nervous system in mammals when partnered by its activator p35. Here we describe the cloning and characterization of a novel activator of cdk5 in Xenopus, Xp35.2. Xp35.2 is expressed during development initially in the earliest differentiating primary neurons in the neural plate and then later in differentiating neural tissue of the brain. This is in contrast to the previously described Xenopus cdk5 activator Xp35.1 which is expressed over the entire expanse of the neural plate in both proliferating and differentiating cells. Expression of both Xp35.1 and Xp35.2 and activation of cdk5 kinase occur when terminal neural differentiation is induced by neurogenin and neuro D overexpression but not when only early stages of neural differentiation are induced by noggin. Moreover, blocking cdk5 kinase activity specifically results in disruption and reduction of the embryonic eye where cdk5 and its Xp35 activators are expressed. Thus, cdk5/p35 complexes function in aspects of neural differentiation and patterning in the early embryo and particularly in formation of the eye.

Sequence and expression patterns of two forms of the middle molecular weight neurofilament protein (NF-M) of Xenopus laevis

The middle molecular weight neurofilament protein (NF-M) is relevant to our understanding of vertebrate neurofilaments in growing axons, both because it exists in all vertebrates and because it undergoes characteristic changes in its phosphorylation state during axonal development. Indeed, all vertebrate neurofilament proteins are believed to have originated by gene duplication from an ancestral, NF-M-like protein. The role of NF-M in axonal development has been studied extensively in the frog, Xenopus laevis, through the use of monoclonal antibodies. To acquire a better understanding of the relationship of X. laevis NF-M to that of other vertebrates and to obtain additional reagents to study and perturb neurofilaments in developing axons, we isolated cDNA clones from the nervous system. These clones encoded two forms of NF-M, which exhibited 93% amino acid identity overall and 94%, 96% and 90% identity over their head, rod, and tail domains, respectively. Synonymous nucleotide substitution rates between the two forms tied their origin to an ancestral duplication of the Xenopus genome, which occurred approximately 30 million years ago. Non-synonymous substitution rates indicated that the tail domain is evolving more rapidly than the rod domain. Both forms shared structural features in common with other vertebrate NF-Ms but had only a single example of the KSP phosphorylation motif that is repeated multiple times in the NF-Ms of bird, goldfish and mammal. In post-metamorphic frogs, the NF-M(1) transcript was consistently expressed at higher levels than that of NF-M(2), although their anatomical patterns of expression were qualitatively similar. During development, however, only NF-M(2) was detectable in retinal ganglion cells through stage 42. We speculate that the differences observed between these two forms may represent early stages of protein diversification akin to what occurred after the gene duplications that gave rise to other vertebrate neurofilament proteins.

Neuronal Cdc2-like kinases: neuron-specific forms of Cdk5

Neuronal Cdc2-like kinase, Nclk, is a heterodimer of a Cdk5 catalytic subunit and a 25 kDa regulatory subunit derived proteolytically from a neuron- and central nervous system-specific 35 kDa protein. The regulatory subunit is mandatory for kinase activity, hence it is designated the neuronal Cdk5 activator, p25/p35nck5a. Nclk has been suggested to play a regulatory role in neuro-cytoskeleton dynamics and in neuronal differentiation. In addition to the activation by Nck5a, Cdk5 is regulated by other mechanisms including additional activator proteins and inhibition by phosphorylation of specific amino acid residues. While Nclk shares common catalytic and regulatory properties with other members of the cdc2-like kinase family, it also displays unique characteristics that may be important for its neuronal functions.

The role of cyclin-dependent kinase 5 and a novel regulatory subunit in regulating muscle differentiation and patterning

Cyclin-dependent kinase 5, coupled with its activator p35, is required for normal neuronal differentiation and patterning. We have isolated a novel member of the p35 family, Xp35.1, from Xenopus embryos which can activate cdk5. Xp35.1 is expressed in both proliferating and differentiated neural and mesodermal cells and is particularly high in developing somites where cdk5 is also expressed. Using dominant-negative cdk5 (cdk5 DN), we show that cdk5 kinase activity is required for normal somitic muscle development; expression of cdk5 DN results in disruption of somitic muscle patterning, accompanied by stunting of the embryos. Using explants of animal pole tissue from blastula embryos, which will differentiate into mesoderm in response to activin, we show that blocking cdk5 kinase activity down-regulates the expression of the muscle marker muscle actin in response to activin, whereas the pan-mesodermal marker Xbra is unaffected. Expression of MyoD and MRF4 (master regulators of myogenesis) is suppressed in the presence of cdk5 DN, indicating that these myogenic genes may be a target for cdk5 regulation, whereas the related factor Myf5 is largely unaffected. In addition, overexpression of Xp35.1 disrupts muscle organization. Thus, we have demonstrated a novel role for cdk5 in regulating myogenesis in the early embryo.

Novel members of the cdc2-related kinase family in Drosophila: cdk4/6, cdk5, PFTAIRE, and PITSLRE kinase

In addition to the previously identified Drosophila cdc2 and cdc2c genes, we have identified four additional cdc2-related genes with low stringency and polymerase chain reaction approaches. Sequence comparisons suggest that the four putative kinases represent the Drosophila homologues of vertebrate cdk4/6, cdk5, PCTAIRE, and PITSLRE kinases. Although the similarity between human and Drosophila homologues is extensive in the case of cdk5, PCTAIRE, and PITSLRE kinases (78%, 58%, and 65% identity in the kinase domain), only limited conservation is observed for Drosophila cdk4/6 (47% identity). However, like vertebrate cdk4 and cdk6, Drosophila cdk4/6 binds also to a D-type cyclin according to the results of two-hybrid experiments in yeast. Northern blot analysis indicated that the four Drosophila kinases are expressed throughout embryogenesis. Expression in early embryogenesis appeared to be ubiquitous according to in situ hybridization. Abundant expression already at the start of embryogenesis and long before neuron differentiation was also observed in the case of cdk5 protein, which has been described as predominantly neuron specific in mice. Sequence conservation and expression pattern, therefore, suggest that all of these kinases perform important cellular functions.