Neurofilaments are part of the high molecular weight complex containing neuronal cdc2-like kinase (nclk)

p35/cyclin-dependent kinase 5 phosphorylation of ras guanine nucleotide releasing factor 2 (RasGRF2) mediates Rac-dependent Extracellular Signal-regulated kinase 1/2 activity, altering RasGRF2 and microtubule-associated protein 1b distribution in neurons

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed kinase the activity of which is dependent on association with its neuron-specific activators, p35 and p39. Cdk5 activity is critical for the proper formation of cortical structures and lamination during development. In the adult nervous system, Cdk5 function is implicated in cellular adhesion, dopamine signaling, neurotransmitter release, and synaptic activity. In addition, Cdk5 is also involved in "cross-talk" with other signal transduction pathways. To further examine its involvement in cross-talk with other pathways, we identified proteins that interacted with p35 using the yeast two-hybrid system. We report here that p35 associates with Ras guanine nucleotide releasing factor 2 (RasGRF2) in coimmunoprecipitation and colocalization studies using transfected cell lines as well as primary cortical neurons. Additionally, Cdk5 phosphorylates RasGRF2 both in vitro and in vivo, leading to a decrease in Rac-guanidine exchange factor activity and a subsequent reduction in extracellular signal-regulated kinase 1/2 activity. We show that p35/Cdk5 phosphorylates RasGRF2 on serine737, which leads to an accumulation of RasGRF2 in the neuronal cell bodies coinciding with an accumulation of microtubule-associated protein 1b. The membrane association of p35 and subsequent localization of Cdk5 activity toward RasGRF2 and Rac provide insights into important cellular signaling processes that occur at the membrane, resulting in downstream effects on signal transduction cascades.

Topographic regulation of phosphorylation in giant neurons of the squid, Loligo pealei: role of phosphatases

In previous studies of phosphorylation in squid stellate ganglion neurons, we demonstrated that a specific multimeric phosphorylation complex characterized each cellular compartment. Although the endogenous protein profile of cell body extracts (giant fiber lobe, GFL), as determined by Coomassie staining, was similar to that of axoplasm from the giant axon, in this study we show that the protein phosphorylation profiles are qualitatively different. Whereas many axoplasm proteins were phosphorylated, including most cytoskeletal proteins, virtually all phosphorylation in perikarya was confined to low molecular weight compounds (<6 kDa). Because phosphorylation of exogenous substrates, histone and casein, was equally active in extracts from both compartments, failure to detect endogenous protein phosphorylation in cell bodies was attributed to the presence of more active phosphatases. To further explore the role of phosphatases in these neurons, we studied phosphorylation in the presence of serine/threonine and protein tyrosine phosphatase (PTP) inhibitors. We found that phosphorylation of axonal cytoskeletal proteins was modulated by okadaic acid-sensitive ser/thr phosphatases, whereas cell body phosphorylation was more sensitive to an inhibitor of protein tyrosine phosphatases, such as vanadate. Inhibition of PTPs by vanadate stimulated endogenous phosphorylation of GFL proteins, including cytoskeletal proteins. Protein tyrosine kinase activity was equally stimulated by vanadate in cell body and axonal whole homogenates and Triton X-100 free soluble extracts, but only the Triton X soluble fraction (membrane bound proteins) of the GFL exhibited significant activation in the presence of vanadate, suggesting higher PTP activities in this fraction than in the axon. The data are consistent with the hypothesis that neuronal protein phosphorylation in axons and cell bodies is modulated by different phosphatases associated with compartment-specific multimeric complexes.

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.

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.

Cdk5/p25(nck5a) interaction with synaptic proteins in bovine brain

Cyclin-dependent kinase 5 (Cdk5) exists in large multimeric complexes, but its function and binding partners in these complexes are unclear. We explored these issues by chromatographic and immunochemical analyses of Cdk5 and p25(nck5a) (a neuronal Cdk5 activator) and their associated proteins from bovine brain. Mono-S column enzyme eluates were divided into three fractions and analyzed by gel filtration. The majority of p25(nck5a) from Mono-S fractions I, II, and III eluted from the gel filtration column at approximately 60, 200, and 400 kDa, respectively, and Cdk5 was abundant in fractions >400 kDa. We characterized these macromolecular structures by immunoprecipitating p25(nck5a), followed by a second immunoprecipitation of remaining unbound proteins using a Cdk5 antibody. The p25(nck5a) immunoprecipitates showed association with Cdk5. Amphiphysin was detected in the 400-kDa complex and synapsin I in the >400 kDa structure. The Cdk5 immunoprecipitates, however, revealed abundant retained Cdk5 but no remaining p25(nck5a), indicating that Cdk5 in macromolecular structures is mostly unassociated with p25(nck5a). Thus, we demonstrate: an amphiphysin-associated 400-kDa Cdk5/p25(nck5a) complex, a synapsin I-associated >400-kDa Cdk5/p25(nck5a) complex, and nck5a-free Cdk5 complexes (200 to >400 kDa). Amphiphysin acts as a Cdk5/p25(nck5a) substrate in the 400-kDa complex and we speculate that Cdk5/p25(nck5a) participates in amphiphysin-mediated endocytosis.

Topographic regulation of cytoskeletal protein phosphorylation by multimeric complexes in the squid giant fiber system

In mammalian and squid nervous systems, the phosphorylation of neurofilament proteins (NFs) seems to be topographically regulated. Although NFs and relevant kinases are synthesized in cell bodies, phosphorylation of NFs, particularly in the lys-ser-pro (KSP) repeats in NF-M and NF-H tail domains, seem to be restricted to axons. To explore the factors regulating the cellular compartmentalization of NF phosphorylation, we separated cell bodies (GFL) from axons in the squid stellate ganglion and compared the kinase activity in the respective lysates. Although total kinase activity was similar in each lysate, the profile of endogenous phosphorylated substrates was strikingly different. Neurofilament protein 220 (NF220), high-molecular-weight NF protein (HMW), and tubulin were the principal phosphorylated substrates in axoplasm, while tubulin was the principal GFL phosphorylated substrate, in addition to highly phosphorylated low-molecular-weight proteins. Western blot analysis showed that whereas both lysates contained similar kinases and cytoskeletal proteins, phosphorylated NF220 and HMW were completely absent from the GFL lysate. These differences were highlighted by P13(suc1) affinity chromatography, which revealed in axoplasm an active multimeric phosphorylation complex(es), enriched in cytoskeletal proteins and kinases; the equivalent P13 GFL complex exhibited six to 20 times less endogenous and exogenous phosphorylation activity, respectively, contained fewer cytoskeletal proteins and kinases, and expressed a qualitatively different cdc2-like kinase epitope, 34 kDa rather than 49 kDa. Cell bodies and axons share a similar repertoire of molecular consitutents; however, the data suggest that the cytoskeletal/kinase phosphorylation complexes extracted from each cellular compartment by P13 are fundamentally different.

Elevated neuronal Cdc2-like kinase activity in the Alzheimer disease brain

Neurofibrillary tangles (NFT) in Alzheimer's disease (AD) consist largely of hyperphosphorylated tau protein. Many of the phosphorylation sites on tau are serine/threonine-proline sequences, several of which are phosphorylated in vitro by neuronal Cdc2-like kinase (Nclk), a kinase composed of Cdk5 and its activator(s). Thus, tau hyperphosphorylation in AD may result in part from deregulation of Nclk. To test this hypothesis, we examined Nclk activity in prefrontal and cerebellar cortex from 15 postmortem AD and 16 age-matched control subjects, and corrected either for Cdk5 level or for neuronal loss. The ratio of Nclk activity in prefrontal versus cerebellar cortex was then compared. When corrections were made for neuronal loss, the ratios of kinase activity in prefrontal versus cerebellar cortex were significantly higher in AD (6.45+/-0.86) than the controls (3.13+/-0.46; P = 0.003). This finding is consistent with a role for Nclk in the pathogenesis of NFT in AD.