Cyclin dependent kinase 5, cdk5, is a positive regulator of myogenesis in mouse C2 cells

Transient induction of cyclin A in loaded chicken skeletal muscle

Cell proliferation is believed to contribute to the increased synthesis rate during load-induced growth of avian anterior latissimus dorsi (ALD) skeletal muscle, but the relative contribution of different cell types to this proliferative response and the time course of cell activation are not well documented. The present investigation measured the abundance and localization of cyclin A protein, which is uniquely present in proliferating cells and required for the entry of vertebrate cells into the DNA synthesis phase during the time course of chicken ALD loading. Total protein content in 1.5-, 7-, and 13-day loaded ALD increased by 60, 191, and 294%, respectively. Immunoblotting analysis identified that cyclin A protein per total protein was dramatically increased in ALD muscle after 1.5 days of loading but returned to control level at 7 days. In vitro kinase assays demonstrated a corresponding massive activation of the cyclin A-regulated, cyclin-dependent kinase 2 but not of cyclin-dependent kinase 2 protein level in muscle homogenates after 1.5 days of muscle loading. Immunofluorescence experiments demonstrated that the increase of cyclin A in 1.5 days of loaded ALD was primarily confined to nuclei of interstitial cells (92%) but was also found in fiber-associated cells (8%). In situ hybridization demonstrated an increased number of nuclei of interstitial cells expressing collagen I transcripts after 1.5 days of loading. These data show that the cell cycle protein cyclin A is induced in fiber-associated cells during the early growth response in loaded ALD but also implicate an activation of interstitial cells as playing an early role in this model for muscle growth.

Cdk5 regulates the organization of Nestin and its association with p35

The intermediate filament protein nestin is characterized by its specific expression during the development of neuronal and myogenic tissues. We identify nestin as a novel in vivo target for cdk5 and p35 kinase, a critical signaling determinant in development. Two cdk5-specific phosphorylation sites on nestin, Thr-1495 and Thr-316, were established, the latter of which was used as a marker for cdk5-specific phosphorylation in vivo. Ectopic expression of cdk5 and p35 in central nervous system progenitor cells and in myogenic precursor cells induced elevated phosphorylation and reorganization of nestin. The kinetics of nestin expression corresponded to elevated expression and activation of cdk5 during differentiation of myoblast cell cultures and during regeneration of skeletal muscle. In the myoblasts, a disassembly-linked phosphorylation of Thr-316 indicated active phosphorylation of nestin by cdk5. Moreover, cdk5 occurred in physical association with nestin. Inhibition of cdk5 activity-either by transfection with dominant-negative cdk5 or by using a specific cdk5 inhibitor-blocked myoblast differentiation and phosphorylation of nestin at Thr-316, and this inhibition markedly disturbed the organization of nestin. Interestingly, the interaction between p35, the cdk5 activator, and nestin appeared to be regulated by cdk5. In differentiating myoblasts, p35 was not complexed with nestin phosphorylated at Thr-316, and inhibition of cdk5 activity during differentiation induced a marked association of p35 with nestin. These results demonstrate that there is a continuous turnover of cdk5 and p35 activity on a scaffold formed by nestin. This association is likely to affect the organization and operation of both cdk5 and nestin during development.

Phosphorylation motifs regulating the stability and function of myocyte enhancer factor 2A

The phosphorylation status of the myocyte enhancer factor 2 (MEF2) transcriptional regulator is a critical determinant of its tissue-specific functions. However, due to the complexity of its phosphorylation pattern in vivo, a systematic inventory of MEF2A phosphorylation sites in mammalian cells has been difficult to obtain. We employed modern affinity purification techniques, combined with mass spectrometry, to identify several novel MEF2 phosphoacceptor sites. These include an evolutionarily conserved KSP motif, which we show is important in regulating the stability and function of MEF2A. Also, an indirect pathway in which a protein kinase casein kinase 2 phosphoacceptor site is phosphorylated by activation of p38 MAPK signaling was documented. Together, these findings identify several novel aspects of MEF2 regulation that may prove important in the control of gene expression in neuronal and muscle cells.

Apolipoprotein E and Reelin ligands modulate tau phosphorylation through an apolipoprotein E receptor/disabled-1/glycogen synthase kinase-3beta cascade

Neurofibrillary tangles comprised of highly phosphorylated tau proteins are a key component of Alzheimer's disease pathology. Mice lacking Reelin (Reln), double-knockouts lacking the VLDL receptor (VLDLR) and ApoE receptor2 (ApoER2), and mice lacking disabled-1 (Dab1) display increased levels of phosphorylated tau. Because Reln binds to recombinant ApoE receptors, assembly of a Reln/ApoE-receptor/Dab1 (RAD) complex may initiate a signal transduction cascade that controls tau phosphorylation. Conversely, disruption of this RAD complex may increase tau phosphorylation and lead to neurodegeneration. To substantiate this concept, we mated Reln-deficient mice to ApoE-deficient mice and found that in the absence of Reln, tau phosphorylation increased as the amount of ApoE decreased. Paralleling the change in tau phosphorylation levels, we found that GSK-3beta activity increased in Reln-deficient mice and further increased in mice lacking both Reln and ApoE. CDK-5 activity was similar in mice lacking Reln, ApoE, or both. GSK-3beta and CDK-5 activity increased in Dab1-deficient mice, independent of ApoE levels. Further supporting the idea that increased tau phosphorylation results primarily from increased kinase activity, the activity of two phosphatases was similar in all conditions tested. These data support a novel, ligand-mediated signal transduction cascade--initiated by the assembly of a RAD complex that suppresses kinase activity and controls tau phosphorylation.

The neuronal cyclin-dependent kinase 5 activator p35Nck5a and Cdk5 activity in monocytic cells

Cyclin-dependent kinase 5 (Cdk5) and its activator p35Nck5a are an odd but interesting couple. Recent work has established that in spite of their names, Cdk5 is clearly not cyclin-dependent, while p35Nck5a is not exclusively neuronal. Indeed, p35Nck5a and/or Cdk5 activity has been demonstrated to be present in human and rat promyelocytic cell lines, and in normal human monocytes. Further, monocytes from mice in which p35Nck5a gene was deleted do not express non-specific esterase, an enzyme present in normal monocytes, while transfection of Cdk5 together with p35Nck5a into myeloblastic cells leads to the expression of this marker of the monocytic phenotype. Thus, the Cdk5/p35Nck5a complex appears to play an important role in monocytic differentiation.

The cdk5 homologue, crp, regulates endocytosis and secretion in dictyostelium and is necessary for optimum growth and differentiation

Dictyostelium Crp is a member of the cyclin-dependent kinase (Cdk) family of proteins. It is most related in sequence to mammalian Cdk5, which unlike other members of the family, has functions that are unrelated to the cell cycle. In order to better understand the function of Crp in Dictyostelium, we overexpressed a dominant negative form, Crp-D144N, under the control of the actin 15 promoter. Cells overexpressing Crp-D144N exhibit a reduced growth rate in suspension culture and reduced rates of fluid-phase endocytosis and phagocytosis. There is no reduction in Cdc2 kinase activity in extracts from cells overexpressing Crp-D144N, suggesting that the growth defect is not due to inhibition of Cdc2. In addition to the growth defect, the act15::crp-D144N transformants aggregate at a slower rate than wild-type cells and form large aggregation streams. These eventually break up to form small aggregates and most of these do not produce mature fruiting bodies. The aggregation defect is fully reversed in the presence of wild-type cells but terminal differentiation is only partially rescued. In act15::crp-D144N transformants, the countin component of the counting factor, a secreted protein complex that regulates the breakup of streams, mostly appears outside the cell as degradation products and the reduced level of the intact protein may at least partially account for the initial formation of the large aggregation streams. Our observations indicate that Crp is important for both endocytosis and efflux and that defects in these functions lead to reduced growth and aberrant development.

Cdk5 regulates cell-matrix and cell-cell adhesion in lens epithelial cells

Cdk5 is a member of the cyclin-dependent kinase family, which is expressed predominantly in terminally differentiated neurons. Lower levels of Cdk5 are also found in a wide variety of cell types, including the lens. Although Cdk5 has been shown to play an important role in neuronal migration and neurite outgrowth, its function in non-neuronal cells is not known. Therefore, this study was undertaken to explore the role of Cdk5 in the lens. Results showed that, within the adult mouse lens, Cdk5 was localized to the cytoplasm,especially along the lateral membranes of differentiating primary fiber cells,which suggests a role in cell-cell adhesion. Staining at the tips of elongating fiber cells was also particularly strong, suggesting a role in cell-matrix adhesion. To examine the possible role of Cdk5 in lens epithelial cell adhesion, we stably transfected N/N1003A rabbit lens epithelial cells with cDNAs for Cdk5 or a dominant-negative mutation, Cdk5-T33. Attachment to a fibronectin matrix, as measured with substrate-coated cell adhesion strips,was increased by Cdk5 overexpression, while an equivalent overexpression of Cdk5-T33 had no effect. Cdk5 also increased the rate of cell attachment and spreading as measured by electric cell-substrate impedance sensing (ECIS). In addition, Cdk5 overexpression decreased cell-cell adhesion as measured by a cell aggregation assay. These findings suggest that Cdk5 plays a role in regulating both cell-matrix and cell-cell interactions in the lens.

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.

Cyclin dependent kinase 5 and its interacting proteins in cell death induced in vivo by cyclophosphamide in developing mouse embryos

Activation or inactivation of members of the cyclin-dependent kinase family is important during cell cycle progression. However, Cdk5, a member of this family that was originally identified because of its high structural homology to Cdc2, is activated during cell differentiation and cell death but not during cell cycle progression. We previously demonstrated a correlation between the up-regulation of Cdk5 protein and kinase activity and cell death during development and pathogenesis. We report here that cyclophosphamide (CP) induces massive apoptotic cell death in mouse embryos and that Cdk5 is expressed in apoptotic cells displaying fragmented DNA. During CP-induced cell death, Cdk5 protein expression is substantially increased as detected by immunohistochemistry but not by Western blot, while its mRNA level remains the same as control, and its kinase activity is markedly elevated. The up-regulation of Cdk5 during CP-induced cell death is not due to de novo protein synthesis. We also examined p35, a regulatory protein of Cdk5 in neuronal differentiation. Using a yeast two-hybrid system, we isolated p35, a neuronal differentiation specific protein, as a protein that interacts with Cdk5 in CP-treated embryos. p35 mRNA level does not change, but the protein expression of p25, a truncated form of p35, is elevated during cell death in vivo, as established here, as well as during cell death in vitro. Our results suggest a role for Cdk5 and its regulatory proteins during CP induced cell death. These results further support the view that Cdk5 and its regulation may be key players in the execution of cell death regardless of how the cell dies, whether through biological mechanisms, disease states such as Alzheimer's disease, or induction by CP.

Induction of Cdk5 activity in rat skeletal muscle after nerve injury

Cyclin-dependent kinase 5 (Cdk5) was originally identified as a serine/threonine kinase and subsequently demonstrated to play a critical role in the development of CNS. We recently reported the novel function of Cdk5 in the neuregulin signaling pathway during the development of neuromuscular junction (NMJ). Here, we report the regulation of Cdk5 and p35 in rat skeletal muscle after nerve injury. Northern blot analysis revealed that Cdk5 and p35 transcripts were up-regulated in muscle after nerve denervation. The temporal profiles for the regulation of Cdk5 and p35 transcripts were different, suggesting that these changes in gene transcription might be regulated by different mechanism. Our finding on the ability of tetrodotoxin to induce p35 transcript in muscle suggested that electrical activity could regulate p35 expression. In addition to the induction of mRNA expression, the total Cdk5 and p35-associated kinase activity in muscle increased prominently after nerve denervation. Taken together, our findings suggest that Cdk5 and p35 may play important physiological roles in muscle regeneration following nerve injury.

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.

Phosphorylation of protein phosphatase inhibitor-1 by Cdk5

Protein phosphatase inhibitor-1 is a prototypical mediator of cross-talk between protein kinases and protein phosphatases. Activation of cAMP-dependent protein kinase results in phosphorylation of inhibitor-1 at Thr-35, converting it into a potent inhibitor of protein phosphatase-1. Here we report that inhibitor-1 is phosphorylated in vitro at Ser-67 by the proline-directed kinases, Cdk1, Cdk5, and mitogen-activated protein kinase. By using phosphorylation state-specific antibodies and selective protein kinase inhibitors, Cdk5 was found to be the only kinase that phosphorylates inhibitor-1 at Ser-67 in intact striatal brain tissue. In vitro and in vivo studies indicated that phospho-Ser-67 inhibitor-1 was dephosphorylated by protein phosphatases-2A and -2B. The state of phosphorylation of inhibitor-1 at Ser-67 was dynamically regulated in striatal tissue by glutamate-dependent regulation of N-methyl-d-aspartic acid-type channels. Phosphorylation of Ser-67 did not convert inhibitor-1 into an inhibitor of protein phosphatase-1. However, inhibitor-1 phosphorylated at Ser-67 was a less efficient substrate for cAMP-dependent protein kinase. These results demonstrate regulation of a Cdk5-dependent phosphorylation site in inhibitor-1 and suggest a role for this site in modulating the amplitude of signal transduction events that involve cAMP-dependent protein kinase activation.

Cdk5 is involved in neuregulin-induced AChR expression at the neuromuscular junction

Here we describe an important involvement of Cdk5/p35 in regulating the gene expression of acetylcholine receptor (AChR) at the neuromuscular synapse. Cdk5 and p35 were prominently expressed in embryonic muscle, and concentrated at the neuromuscular junction in adulthood. Neuregulin increased the p35-associated Cdk5 kinase activity in the membrane fraction of cultured C2C12 myotubes. Co-immunoprecipitation studies revealed the association between Cdk5, p35 and ErbB receptors in muscle and cultured myotubes. Inhibition of Cdk5 activity not only blocked the NRG-induced AChR transcription, but also attenuated ErbB activation in cultured myotubes. In light of our finding that overexpression of p35 alone led to an increase in AChR promoter activity in muscle, Cdk5 activation is sufficient to mediate the up-regulation of AChR gene expression. Taken together, these results reveal the unexpected involvement of Cdk5/p35 in neuregulin signaling at the neuromuscular synapse.

Cdk5 mediates changes in morphology and promotes apoptosis of astrocytoma cells in response to heat shock

The cyclin-dependent kinase member, Cdk5, is expressed in a variety of cell types, but neuron-specific expression of its activator, p35, is thought to limit its activity to neurons. Here we demonstrate that both Cdk5 and p35 are expressed in the human astrocytoma cell line, U373. Cdk5 and p35 are present in the detergent-insoluble cytoskeletal fraction of this cell line and Cdk5 localizes to filopodia and vinculin-rich regions of cell-matrix contact in lamellopodia. When exposed to a 46(o)C heat shock, U373 cells change shape, lose cell-matrix contacts and show increased levels of apoptosis. To test whether Cdk5 activation might play a role in these events, U373 cells were stably transfected with histidine-tagged or green fluorescent protein-tagged constructs of Cdk5 or a dominant negative mutation, Cdk5T33. Under normal growth conditions, growth characteristics of the stably transfected lines were indistinguishable from untransfected U373 cells and Cdk5 localization was not changed. However, when subjected to heat shock, cells stably transfected with Cdk5-T33 remained flattened, showed little loss of cell-matrix adhesion, and exhibited significantly lower levels of apoptosis. In contrast, cells that overexpressed wild-type Cdk5 showed morphological changes similar to those seen in untransfected U373 cells in response to heat shock and had significantly higher levels of apoptosis. Heat-shocked cells showed changes in p35 mobility and stability of the Cdk5/p35 complex consistent with endogenous Cdk5 activity. Together these findings suggest that endogenous Cdk5 activity may play a key role in regulating morphology, attachment, and apoptosis in U373 cells, and raise the possibility that Cdk5 may be a general regulator of cytoskeletal organization and cell adhesion in both neuronal and non-neuronal cells.

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.

Regulation of muscle regulatory factors by DNA-binding, interacting proteins, and post-transcriptional modifications

Skeletal muscle differentiation is influenced by multiple pathways, which regulate the activity of myogenic regulatory factors (MRFs)-the myogenic basic helix-loop-helix proteins and the MEF2-family members-in positive or negative ways. Here we will review and discuss the network of signals that regulate MRF function during myocyte proliferation, differentiation, and post-mitotic growth. Elucidating the mechanisms governing muscle-specific transcription will provide important insight in better understanding the embryonic development of muscle at the molecular level and will have important implications in setting out strategies aimed at muscle regeneration. Since the activity of MRFs are compromised in tumors of myogenic derivation-the rhabdomyosarcomas-the studies summarized in this review can provide a useful tool to uncover the molecular basis underlying the formation of these tumors.

Cyclin-dependent kinase 5 colocalizes with phosphorylated tau in human inclusion-body myositis paired-helical filaments and may play a role in tau phosphorylation

To investigate the possible role of cyclin-dependent kinase 5 (cdk5) in the formation of paired helical filaments (PHFs) in muscle of patients with inclusion-body myositis (IBM), we immunolocalized cdk5, by light- and electron- microscopy, in muscle biopsies of six IBM patients. Approximately 80-90% of IBM vacuolated muscle fibers, and 10-15% of nonvacuolated fibers, contained well defined cdk5-immunoreactive inclusions that colocalized with phosphorylated tau in 70-80% of those fibers. Immunoelectronmicroscopy revealed the association of cdk5 with tau-immunoreactive PHFs. In all biopsies that contained them, regenerating muscle fibers had diffuse, moderate to strong cdk5 immunoreactivity. At all neuromuscular junctions, there was strong cdk5 immunoreactivity postsynaptically. Our study suggests that cdk5: (1) plays a role in IBM pathogenesis, possibly mediating phosphorylation of PHF-related tau; (2) is involved in muscle regeneration; and (3) has a novel function at normal neuromuscular junctions.

Calpain-dependent proteolytic cleavage of the p35 cyclin-dependent kinase 5 activator to p25

Cyclin-dependent kinase 5 (CDK5) is a unique CDK, the activity of which can be detected in postmitotic neurons. To date, CDK5 purified from mammalian brains has always been associated with a truncated form of the 35-kDa major brain specific activator (p35, also known as nck5a) of CDK5, known as p25. In this study, we report that p35 can be cleaved to p25 both in vitro and in vivo by calpain. In a rat brain extract, p35 was cleaved to p25 by incubation with Ca(2+). This cleavage was inhibited by a calpain inhibitor peptide derived from calpastatin and was ablated by separating the p35.CDK5 from calpain by centrifugation. The p35 recovered in the pellet after centrifugation could then be cleaved to p25 by purified calpain. Cleavage of p35 was also induced in primary cultured neurons by treatment with a Ca(2+) ionophore and Ca(2+) and inhibited by calpain inhibitor I. The cleavage changed the solubility of the CDK5 active complex from the particulate fraction to the soluble fraction but did not affect the histone H1 kinase activity. Increased cleavage was detected in cultured neurons undergoing cell death, suggesting a role of the cleavage in neuronal cell death.

Ser67-phosphorylated inhibitor 1 is a potent protein phosphatase 1 inhibitor

Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), a major eukaryotic Ser/Thr phosphatase. Nonphosphorylated I-1 is inactive, whereas phosphorylated I-1 is a potent PP1 inhibitor. I-1 is phosphorylated in vivo on Thr(35) and Ser(67). Thr(35) is phosphorylated by cAMP-dependent protein kinase (A kinase), and Thr(35)-phosphorylated I-1 inhibits PP1. Until now the kinase that phosphorylates Ser(67) had not been identified and the physiological role of Ser(67) phosphorylation was unknown. In this study we detected a high level of kinase activity in brain extract when a glutathione S-transferase (GST) fusion I-1 mutant containing an Ala substituted for Thr(35) [GST-I-1(T35A)] was used as the substrate. GST-I-1(T35A) kinase and neuronal cdc2-like protein kinase (NCLK) in the brain extract could not be separated from each other by a series of sequential chromatographies. GST-I-1(T35A) kinase immunoprecipitated with anti-NCLK antibody from kinase-active column fractions. Purified NCLK-phosphorylated GST-I-1(T35A) and I-1 (0.7 mole of phosphate per mole of I-1). HPLC phosphopeptide mapping, amino acid sequencing, and site-directed mutagenesis determined that NCLK phosphorylates Ser(67) of I-1. NCLK-phosphorylated I-1 and I-1(T35A) inhibited PP1 with IC(50) values approximately 9.5 and 13. 8 nM, respectively. When compared, A kinase-phosphorylated I-1 was only approximately 1.2 times more inhibitory than NCLK-phosphorylated I-1. Our data indicate that NCLK is a potential in vivo I-1 kinase and that Thr(35) and Ser(67) phosphorylation independently activate I-1.

Cell cycle arrest induced by ectopic expression of p27 is not sufficient to promote oligodendrocyte differentiation

Oligodendrocyte differentiation is accompanied by dramatic changes in gene expression as well as cell cycle arrest. To determine whether cell cycle arrest is sufficient to induce the changes in cell phenotype associated with differentiation, we inhibited oligodendrocyte precursor proliferation in vitro by overexpressing p27, a cyclin kinase inhibitor, using a recombinant adenovirus. Ectopic expression of p27 efficiently inhibited oligodendrocyte precursor cell division, even in the presence of exogenous mitogens, by blocking the activity of the cyclin-dependent kinase, cdk2. Although the cells had stopped dividing, they did not express galactocerebroside (GalC) or myelin basic protein (MBP), changes associated with oligodendrocyte differentiation, suggesting that they had not differentiated. After removal of exogenous mitogens, however, adenovirus-expressing oligodendrocyte precursors differentiated with a temporal profile similar to that of control, uninfected oligodendrocytes, as indicated by expression of GalC and MBP. We conclude that cell cycle arrest is not sufficient to induce differentiation of dividing oligodendrocyte precursors, and that modulation of additional, as yet unknown, signaling pathways is required for this to occur.

Time course of changes in markers of myogenesis in overloaded rat skeletal muscles

During the process of compensatory muscle hypertrophy, satellite cells are thought to proliferate, differentiate, and then fuse with existing myofibers. We hypothesized that early in this process changes occur in the expression of cellular markers indicative of the onset of myogenic processes. The plantaris muscles of rats were overloaded via the unilateral ablation of synergists. Groups of rats were killed at time points from 6 h to 12 days. Changes in muscle gene expression (mRNA) of cyclin D1, p21, myogenin, MyoD, and insulin-like growth factor I (IGF-I, mRNA and peptide) were measured. Cyclin D1 (a cell cycle marker) was increased after 24 h of overloading and corresponded with changes in muscle DNA content. In contrast, p21 and myogenin, markers of cellular differentiation, were increased after just 12 h. Muscle IGF-I peptide levels were also increased at early time points. The results of this study indicate that myogenic processes are activated in response to increased loading at very early time points (e.g., 12 h) and that IGF-I may be modulating this response. Furthermore, these findings suggest that some cells may have been differentiating very early in the adaptation process before events leading to cellular proliferation have been initiated.

Substrate targeting of the yeast cyclin-dependent kinase Pho85p by the cyclin Pcl10p

In Saccharomyces cerevisiae, PHO85 encodes a cyclin-dependent protein kinase (Cdk) catalytic subunit with multiple regulatory roles thought to be specified by association with different cyclin partners (Pcls). Pcl10p is one of four Pcls with little sequence similarity to cyclins involved in cell cycle control. It has been implicated in specifying the phosphorylation of glycogen synthase (Gsy2p). We report that recombinant Pho85p and Pcl10p produced in Escherichia coli reconstitute an active Gsy2p kinase in vitro. Gsy2p phosphorylation required Pcl10p, occurred at physiologically relevant sites, and resulted in inactivation of Gsy2p. The activity of the reconstituted enzyme was even greater than Pho85p-Pcl10p isolated from yeast, and we conclude that, unlike many Cdks, Pho85p does not require phosphorylation for activity. Pcl10p formed complexes with Gsy2p, as judged by (i) gel filtration of recombinant Pcl10p and Gsy2p, (ii) coimmunoprecipitation from yeast cell lysates, and (iii) enzyme kinetic behavior consistent with Pcl10p binding the substrate. Synthetic peptides modeled on the sequences of known Pho85p sites were poor substrates with high K(m) values, and we propose that Pcl10p-Gsy2p interaction is important for substrate selection. Gel filtration of yeast cell lysates demonstrated that most Pho85p was present as a monomer, although a portion coeluted in high-molecular-weight fractions with Pcl10p and Gsy2p. Overexpression of Pcl10p sequestered most of the Pho85p into association with Pcl10p. We suggest a model for Pho85p function in the cell whereby cyclins like Pcl10p recruit Pho85p from a pool of monomers, both activating the kinase and targeting it to substrate.

Cyclin-dependent kinase 5 activity enhances monocytic phenotype and cell cycle traverse in 1,25-dihydroxyvitamin D3-treated HL60 cells

The function of most cyclin-dependent kinases (Cdks) is to facilitate progression through the checkpoints of the cell cycle, but Cdk5 is known to be involved in differentiation of CNS, muscle, and lens cells, though not in the cell cycle traverse. Here we show an additional role for Cdk5, an enhancement of monocytic differentiation with abrogation of the G1 checkpoint. Human leukemia HL60 cells exposed to 1alpha,25-dihydroxyvitamin D3 (1,25D3) displayed monocytic phenotype and increased Cdk5 kinase activity. An analog of 1,25D3 which does not induce differentiation failed to upregulate Cdk5, and 1,25D3-resistant cells had reduced Cdk5 activity. Active or inactive Cdk5 was associated with cyclin D1, but only active Cdk5 exhibited threonine phosphorylation. Inhibition of Cdk5 expression by an antisense construct reduced the intensity of 1, 25D3-induced expression of CD14, a marker of monocytes, and increased the 1,25D3-induced G1 block. These findings demonstrate a novel aspect of Cdk5 activity-facilitation of the G1- to S-phase transition in cells which are approaching replicative quiescence and a concomitant enhancement of monocytic differentiation.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

cdk1- and cdk2-mediated phosphorylation of MyoD Ser200 in growing C2 myoblasts: role in modulating MyoD half-life and myogenic activity

We have examined the role of protein phosphorylation in the modulation of the key muscle-specific transcription factor MyoD. We show that MyoD is highly phosphorylated in growing myoblasts and undergoes substantial dephosphorylation during differentiation. MyoD can be efficiently phosphorylated in vitro by either purified cdk1-cyclin B or cdk1 and cdk2 immunoprecipitated from proliferative myoblasts. Comparative two-dimensional tryptic phosphopeptide mapping combined with site-directed mutagenesis revealed that cdk1 and cdk2 phosphorylate MyoD on serine 200 in proliferative myoblasts. In addition, when the seven proline-directed sites in MyoD were individually mutated, only substitution of serine 200 to a nonphosphorylatable alanine (MyoD-Ala200) abolished the slower-migrating hyperphosphorylated form of MyoD, seen either in vitro after phosphorylation by cdk1-cyclin B or in vivo following overexpression in 10T1/2 cells. The MyoD-Ala200 mutant displayed activity threefold higher than that of wild-type MyoD in transactivation of an E-box-dependent reporter gene and promoted markedly enhanced myogenic conversion and fusion of 10T1/2 fibroblasts into muscle cells. In addition, the half-life of MyoD-Ala200 protein was longer than that of wild-type MyoD, substantiating a role of Ser200 phosphorylation in regulating MyoD turnover in proliferative myoblasts. Taken together, our data show that direct phosphorylation of MyoD Ser200 by cdk1 and cdk2 plays an integral role in compromising MyoD activity during myoblast proliferation.

DNA binding protein dbpA binds Cdk5 and inhibits its activity

Progress in the cell cycle is governed by the activity of cyclin dependent kinases (Cdks). Unlike other Cdks, the Cdk5 catalytic subunit is found mostly in differentiated neurons. Interestingly, the only known protein that activates Cdk5 (i.e. p35) is expressed solely in the brain. It has been suggested that, besides its requirement in neuronal differentiation, Cdk5 activity is induced during myogenesis. However, it is not clear how this activity is regulated in the pathway that leads proliferative cells to differentiation. In order to find if there exists any Cdk5-interacting protein, the yeast two-hybrid system was used to screen a HeLa cDNA library. We have determined that a C-terminal 172 amino acid domain of the DNA binding protein, dbpA, binds to Cdk5. Biochemical analyses reveal that this fragment (dbpA(Cdelta)) strongly inhibits p35-activated Cdk5 kinase. The protein also interacts with Cdk4 and inhibits the Cdk4/cyclin D1 enzyme. Surprisingly, dbpA(Cdelta) does not bind Cdk2 in the two-hybrid assay nor does it inhibit Cdk2 activated by cyclin A. It could be that dbpA's ability to inhibit Cdk5 and Cdk4 reflects an apparent cross-talk between distinct signal transduction pathways controlled by dbpA on the one hand and Cdk5 or Cdk4 on the other.

Identification of ribosomal protein L34 as a novel Cdk5 inhibitor

The cell cycle is regulated by sequential activation, inactivation of cyclin dependent kinases (Cdk-s). Like all other Cdk-s, the catalytic subunit of Cdk5 is present in cycling cells. However, its highest concentration is found in differentiated neurons, and the only known protein that activates Cdk5 (i.e., p35) is expressed solely in the brain. Active Cdk5 is thought to be involved in the in vivo phosphorylation of the neurofilament proteins and tau which are hyperphosphorylated in neurodegenerative diseases. Recent reports suggest that Cdk5 may also contribute to cellular differentiation. Therefore, it would not be unusual to surmise that there exist specific proteins that regulate Cdk5 activity in cycling cells. In order to find if this was true, a cDNA library prepared from HeLa cells was screened using the yeast-two-hybrid system. The 60S ribosomal protein, L34, was identified as a Cdk5-interacting protein. Biochemical analyses reveal that L34 cannot activate Cdk5 but potently inhibits the p35-activated kinase. L34 also interacts with Cdk4 and, in parallel, inhibits the Cdk4/cyclin D1 activity. Interestingly, L34 does not interact with Cdk2 in the two-hybrid assay nor does it inhibit the Cdk2/cyclin A enzyme. The fact that a ribosomal protein inhibits Cdk5 and Cdk4 may suggest that these two kinases have a cellular role in translational regulation.

Identification of a human cDNA encoding a kinase-defective cdk5 isoform

The cyclin-dependent kinase 5 (Cdk5) catalytic subunit is expressed in both cycling and noncycling cells and is present in many tissues. Neuronal and muscle cells contain the highest amount of this protein. The p35 protein, which is expressed solely in the brain, activates Cdk5. Cdk5 activity is involved in terminal differentiation of neurons and muscle cells. We attempted to clone cdk5 by PCR from a human fetal brain cDNA library. Surprisingly, we amplified two forms of the cdk5 gene, the wild type and a cdk5 variant that lacks the complete kinase domain VI. The variant is also found in SH-SY-5Y neuroblastoma cells but not in T-cells, HeLa cells, the thymus, and placental tissue. The protein encoded by the cdk5 variant, the Cdk5 isoform (Cdk5i), purifies with p35 when coexpressed in insect cells. The activity associated with the heterodimer Cdk5i/p35 is found to be appreciably weaker than the wild-type Cdk5/p35 kinase. Moreover, Cdk5i/p35 cannot autophosphorylate its two subunits as with Cdk5/p35. Interestingly, kinase-defective Cdk5i can abolish the activity of wild-type Cdk5 when both are coexpressed with p35 in insect cells, suggesting that Cdk5i may have a function in regulating Cdk5 activity in human cells too.

Regulation of differentiation by HBP1, a target of the retinoblastoma protein

Differentiation is a coordinated process of irreversible cell cycle exit and tissue-specific gene expression. To probe the functions of the retinoblastoma protein (RB) family in cell differentiation, we isolated HBP1 as a specific target of RB and p130. Our previous work showed that HBP1 was a transcriptional repressor and a cell cycle inhibitor. The induction of HBP1, RB, and p130 upon differentiation in the muscle C2C12 cells suggested a coordinated role. Here we report that the expression of HBP1 unexpectedly blocked muscle cell differentiation without interfering with cell cycle exit. Moreover, the expression of MyoD and myogenin, but not Myf5, was inhibited in HBP1-expressing cells. HBP1 inhibited transcriptional activation by the MyoD family members. The inhibition of MyoD family function by HBP1 required binding to RB and/or p130. Since Myf5 might function upstream of MyoD, our data suggested that HBP1 probably blocked differentiation by disrupting Myf5 function, thus preventing expression of MyoD and myogenin. Consistent with this, the expression of each MyoD family member could reverse the inhibition of differentiation by HBP1. Further investigation implicated the relative ratio of RB to HBP1 as a determinant of whether cell cycle exit or full differentiation occurred. At a low RB/HBP1 ratio cell cycle exit occurred but there was no tissue-specific gene expression. At elevated RB/HBP1 ratios full differentiation occurred. Similar changes in the RB/HBP1 ratio have been observed in normal C2 differentiation. Thus, we postulate that the relative ratio of RB to HBP1 may be one signal for activation of the MyoD family. We propose a model in which a checkpoint of positive and negative regulation may coordinate cell cycle exit with MyoD family activation to give fidelity and progression in differentiation.