Harish C. Pant

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.

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.

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

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

Identification of Endogenously Phosphorylated KSP Sites in the High-Molecular-Weight Rat Neurofilament Protein

Abstract: The high‐molecular‐weight neurofilament protein (NF‐H) is highly phosphorylated in vivo, with estimates as high as 16–51 mol of Pi/mol of protein. Most of the phosphorylation sites are thought to be located on Ser residues in multiple KSP repeats, in the carboxy‐terminal tail region of the molecule. Because the extent and site‐specific patterns of tail domain phosphorylation are believed to modulate neurofilament structure and function, it becomes essential to identify the endogenous sites of phosphorylation. In this study, we have used selective proteolytic cleavage procedures, Pi determinations, microsequencing, and mass‐spectral analysis to determine the endogenously phosphorylated sites in the NF‐H tail isolated from rat spinal cord. Twenty Ser residues in NF‐H carboxy‐terminal tail were analyzed; nine of these, all located in KSP repeats, were phosphorylated. No detectable phosphorylation could be identified in any of the 11 “non‐KSP” Ser residues that were examined. KSPXKX, KSPXXX, and KSPXXK motifs were found to be phosphorylated. In addition, a 27‐kDa KSP‐rich domain, containing 43 virtually uninterrupted KSPXXX repeats, was isolated from the tail domain and found to contain between 30 and 35 mol of Pi/mol of protein. This domain appeared to be highly resistant to endoproteinase Glu‐C digestion, although it contains a large number of glutamate residues. It could be proteolyzed, however, after dephosphorylation. This suggests that phosphorylation of the tail domain may contribute to neurofilament stability in vivo. A neuronal‐derived protein kinase that specifically phosphorylates only KSPXKX motifs in neurofilaments has been reported. The presence of extensively phosphorylated KSPXXX repeats in NF‐H in vivo suggests the existence of yet another, unidentified kinase(s) with specificity for KSPXXX motifs.

Tumor Necrosis Factor-α Regulates Cyclin-dependent Kinase 5 Activity during Pain Signaling through Transcriptional Activation of p35

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase. We have previously reported that Cdk5 participates in the regulation of nociceptive signaling, and the expression of Cdk5 and its activator, p35, are up-regulated in nociceptive neurons during peripheral inflammation. The aim of our current study was to identify the proinflammatory molecules that regulate Cdk5/p35 activity in response to inflammation. We constructed a vector that contains the mouse p35 promoter driving luciferase expression. We transiently transfected this vector in PC12 cells to test the effect of several cytokines on p35 transcriptional activity and Cdk5 activity. Our results indicate that tumor necrosis factor-α (TNF-α) activates p35 promoter activity in a dose- and time-dependent manner and concomitantly up-regulates Cdk5 activity. Because TNF-α is known to activate ERK1/2, p38 MAPK, JNK, and NF-κB signaling pathways, we examined their involvement in the activation of p35 promoter activity. MEK inhibitor, which inhibits ERK activation, decreased p35 promoter activity, whereas the inhibitors of p38 MAPK, JNK, and NF-κB increased p35 promoter activity, indicating that these pathways regulate p35 expression differently. The mRNA and protein levels of Egr-1, a transcription factor, were increased by TNF-α treatment, and this increase was dependent on ERK signaling. In a mouse model of inflammation-induced pain in which carrageenan injection into the hind paw causes hypersensitivity to heat stimuli, TNF-α mRNA was increased at the site of injection. These findings suggest that TNF-α-mediated regulation of Cdk5 activity plays an important role in inflammation-induced pain signaling.

A calcium activated protease in squid axoplasm.

Evidence for a protease in squid axoplasm which is selectively activated by Ca2+ and blocked by SH‐inhibitors is presented. This protease appears to be particularly effective in degrading squid neurofilament proteins, but also extensively degrades various other major protein components in axoplasm.

Phosphorylation of human high molecular weight neurofilament protein (hNF-H) by neuronal cyclin-dependent kinase 5 (cdk5).

Neurofilaments (NFs), the neuron-specific intermediate (i.e. approximately 10-nm diameter) filaments are major cytoskeletal components of most neurons. In a mature mammalian neuron, NFs are co-assembled from three subunits, NF-L (low), NF-M (medium), and NF-H (high), with molecular masses of 68, 95, and 115 kDa, respectively. Neurofilament proteins (NF-Ps), particularly, NF-H, are most extensively phosphorylated in large myelinated axons under normal conditions. This phosphorylation occurs on the serine residues of the lysine (Lys)-serine (Ser)-proline (Pro) (KSP) multiple amino acid repeats of the carboxy-terminal tail domain. Phosphorylation of KSP motifs affects physical, biochemical, and immunological properties of NF-H. For example, phosphorylation is thought to play a pivotal role in the maintenance of the neuronal cytoskeletal structure which influences the conduction velocity of the nerve fiber. The key components responsible for phosphorylation are not known. In this study, an identified cyclin-dependent kinase 5 (cdk5), isolated from nervous tissue, has been shown to phosphorylate the human NF-H (hNF-H) and affects its electrophoretic mobility. On the basis of the following observations, it is suggested that neuronal cdk5 (cdk5) phosphorylates KSPXK motifs in the human high molecular weight neurofilament (hNF-H) and affects its electrophoretic mobility. (1) A 14-mer synthetic peptide (KSPEKAKSPVKEEA) derived from hNF-H; (2) a bacterially expressed protein containing 14 KSPXK multiple repeats of hNF-H in C-terminal tail domain; and (3) a dephosphorylated hNF-H in neurofilament preparation are phosphorylated by cdk5. The decrease in molecular mass of hNF-H caused by dephosphorylated was completely recovered upon cdk5 phosphorylation. It is proposed that neuronal cdk5 regulates phosphorylation of the KSPXK motif in hNF-H and other cytoskeletal proteins with similar motifs in the nervous system.

Cyclin-Dependent Kinase 5 Phosphorylation of Human Septin SEPT5 (hCDCrel-1) Modulates Exocytosis

Cyclin-dependent kinase 5 (Cdk5) is predominantly expressed in the nervous system, where it is involved in neuronal migration, synaptic transmission, and survival. The role of Cdk5 in synaptic transmission is mediated by regulating the cellular functions of presynaptic proteins such as synapsin, Munc18, and dynamin 1. Its multifunctional role at the synapse is complex and probably involves other novel substrates. To explore this possibility, we used a yeast two-hybrid screen of a human cDNA library with p35 as bait and isolated human septin 5 (SEPT5), known also as hCDCrel-1, as an interacting clone. Here we report that p35 associates with SEPT5 in GST (glutathione S-transferase)-pull-down and coimmunoprecipitation assays. We confirmed that Cdk5/p35 phosphorylates SEPT5 in vitro and in vivo and identified S327 of SEPT5 as a major phosphorylation site. A serine (S)-to-alanine (A) 327 mutant of SEPT5 bound syntaxin more efficiently than SEPT5 wild type. Additionally, coimmunoprecipitation from synaptic vesicle fractions and Cdk5 wild-type and knock-out lysates showed that phosphorylation of septin 5 by Cdk5/p35 decreases its binding to syntaxin-1. Moreover, mutant nonphosphorylated SEPT5 potentiated regulated exocytosis more than the wild type when each was expressed in PC12 cells. These data suggest that Cdk5 phosphorylation of human septin SEPT5 at S327 plays a role in modulating exocytotic secretion.

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.

Direct evidence of phosphorylated neuronal intermediate filament proteins in neurofibrillary tangles (NFTs): phosphoproteomics of Alzheimer's NFTs.

Alzheimers disease (AD) is a neurodegenerative disorder characterized by brain pathology of intracellular neurofibrillary tangles (NFTs) and extracellular amyloid plaques. NFTs contain aberrantly hyperphosphorylated Tau as paired helical filaments (PHFs). Although NFs have been shown immunohistologically to be part of NFTs, there has been debate that the identity of NF proteins in NFTs is due to the cross‐reactivity of phosphorylated NF antibodies with phospho‐Tau. Here, we provide direct evidence on the identity of NFs in NFTs by immunochemical and mass spectrometric analysis. We have purified sarkosyl‐insoluble NFTs and performed liquid chromatography/tandem mass spectrometry of NFT tryptic digests. The phosphoproteomics of NFTs clearly identified NF‐M phosphopeptides SPVPKS∗PVEEAK, corresponding to Ser685, and KAES∗PVKEEAVAEVVTITK, corresponding to Ser736, and an NF‐H phosphopeptide, EPDDAKAKEPS∗KP, corresponding to Ser942. Western blotting of purified tangles with SMI31 showed a 150‐kDa band corresponding to phospho‐NF‐M, while RT97 antibodies detected phospho‐NF‐H. The proteomics analysis also identified an NF‐L peptide (ALYEQEIR, EAEEEKKVEGAGEEQAAAK) and another intermediate filament protein, vimentin (FADLSEAANR). Mass spectrometry revealed Tau phosphopeptides corresponding to Thr231, Ser235, Thr181, Ser184, Ser185, Thr212, Thr217, Ser396, and Ser403. And finally, phosphopeptides corresponding to MAP1B (corresponding to Ser1270, Ser1274, and Ser1779) and MAP2 (corresponding to Thr350, Ser1702, and Ser1706) were identified. In corresponding matched control preparations of PHF/NFTs, none of these phosphorylated neuronal cytoskeletal proteins were found. These studies independently demonstrate that NF proteins are an integral part of NFTs in AD brains.—Rudrabhatla, P., Jaffe, H., Pant, H. C. Direct evidence of phosphorylated neuronal intermediate filament proteins in neurofibrillary tangles (NFTs): phosphoproteomics of Alzheimers NFTs. FASEB J. 25, 3896–3905 (2011). www.fasebj.org