Site-specific phosphorylation of Lys-Ser-Pro repeat peptides from neurofilament H by cyclin-dependent kinase 5: structural basis for substrate recognition

TFP5/TP5 peptide provides neuroprotection in the MPTP model of Parkinson's disease

Cyclin-dependent kinase 5 (Cdk5) is a member of the serine-threonine kinase family of cyclin-dependent kinases. Cdk5 is critical to normal mammalian nervous system development and plays important regulatory roles in multiple cellular functions. Recent evidence indicates that Cdk5 is inappropriately activated in several neurodegenerative conditions, including Parkinson's disease (PD). PD is a chronic neurodegenerative disorder characterized by the loss of dopamine neurons in the substantia nigra, decreased striatal dopamine levels, and consequent extrapyramidal motor dysfunction. During neurotoxicity, p35 is cleaved to form p25. Binding of p25 with Cdk5 leads deregulation of Cdk5 resulting in number of neurodegenerative pathologies. To date, strategies to specifically inhibit Cdk5 hyperactivity have not been successful without affecting normal Cdk5 activity. Here we show that inhibition of p25/Cdk5 hyperactivation through TFP5/TP5, truncated 24-aa peptide derived from the Cdk5 activator p35 rescues nigrostriatal dopaminergic neurodegeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP/MPP⁺) in a mouse model of PD. TP5 peptide treatment also blocked dopamine depletion in the striatum and improved gait dysfunction after MPTP administration. The neuroprotective effect of TFP5/TP5 peptide is also associated with marked reduction in neuroinflammation and apoptosis. Here we show inhibition of Cdk5/p25-hyperactivation by TFP5/TP5 peptide, which identifies Cdk5/p25 as a potential therapeutic target to reduce neurodegeneration in PD.

Cdk5 interacts with Hif-1α in neurons: a new hypoxic signalling mechanism?

The cyclin dependent kinase 5 (Cdk5)/p35 complex is essential for regulation of cell survival during development and in models of neuronal excitotoxicity. Dysregulation of Cdk5, by cleavage of its neuronal specific activators p35 and p39, has been implicated in various neurodegenerative disorders such as Alzheimer's disease, however targets of the complex that regulate neuronal survival physiologically and/or during pathogenesis are largely unknown. Since hypoxia is a key feature in the pathogenesis of several neuronal disorders we investigated a role for Cdk5/p35 in the neuronal hypoxic response. Our data show that hypoxia modulates the p35/Cdk5 complex in primary cortical neurons at the transcriptional and protein level. Furthermore hypoxic induction of Cdk5 activity correlates with Hif-1α stabilisation, and direct interaction between these proteins can occur. Importantly, we demonstrate that Cdk5-mediated signaling is involved in Hif-1α stabilisation since inhibition of Cdk5 by roscovitine abrogates Hif-1α accumulation and induces cell death. Taken together our results show that the Cdk5/p35 complex may significantly contribute to modulation of Hif-1α stabilisation and impact neuronal survival during oxygen deprivation. Thus this study highlights a new hypoxia-mediated signaling pathway and implicates the cytoskeleton as a potential regulator of Hif-1α.

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.

Stable Tubule Only Polypeptides (STOP) Proteins Co-Aggregate with Spheroid Neurofilaments in Amyotrophic Lateral Sclerosis

A major cytopathological hallmark of amyotrophic lateral sclerosis (ALS) is the presence of axonal spheroids containing abnormally accumulated neurofilaments. The mechanism of their formation, their contribution to the disease, and the possibility of other co-aggregated components are still enigmatic. Here we analyze the composition of such lesions with special reference to stable tubule only polypeptide (STOP), a protein responsible for microtubule cold stabilization. In normal human brain and spinal cord, the distribution of STOP proteins is uniform between the cytoplasm and neurites of neurons. However, all the neurofilament-rich spheroids present in the tissues of affected patients are intensely labeled with 3 different anti-STOP antibodies. Moreover, when neurofilaments and microtubules are isolated from spinal cord and brain, STOP proteins are systematically co-purified with neurofilaments. By SDS-PAGE analysis, no alteration of the migration profile of STOP proteins is observed in pathological samples. Other microtubular proteins, like tubulin or kinesin, are inconstantly present in spheroids, suggesting that a microtubule destabilizing process may be involved in the pathogenesis of ALS. These results indicate that the selective co-aggregation of neurofilament and STOP proteins represent a new cytopathological marker for spheroids.

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.

Domain unfolding in neurofilament sidearms: effects of phosphorylation and ATP

Lateral projections of neurofilaments (NF) called sidearms (SA) affect axon stability and caliber. SA phosphorylation is thought to modulate inter-NF distance and interactions between NF and other subcellular organelles. SA were probed by atomic force microscopy (AFM) and dynamic light scattering (DLS) as a function of phosphorylation and ATP content. DLS shows SA are larger when phosphorylated, and AFM shows four unfoldable domains in SA regardless of phosphorylation state or the presence of ATP. However, the native phosphorylated SA requires three-fold higher force to unfold by AFM than dephosphorylated SA, suggesting a less pliant as well as larger structure when phosphorylated.

Binding of cGMP to GAF domains in amphibian rod photoreceptor cGMP phosphodiesterase (PDE). Identification of GAF domains in PDE alphabeta subunits and distinct domains in the PDE gamma subunit involved in stimulation of cGMP binding to GAF domains

Retinal cGMP phosphodiesterase (PDE6) is a key enzyme in vertebrate phototransduction. Rod PDE contains two homologous catalytic subunits (Palphabeta) and two identical regulatory subunits (Pgamma). Biochemical studies have shown that amphibian Palphabeta has high affinity, cGMP-specific, non-catalytic binding sites and that Pgamma stimulates cGMP binding to these sites. Here we show by molecular cloning that each catalytic subunit in amphibian PDE, as in its mammalian counterpart, contains two homologous tandem GAF domains in its N-terminal region. In Pgamma-depleted membrane-bound PDE (20-40% Pgamma still present), a single type of cGMP-binding site with a relatively low affinity (K(d) approximately 100 nm) was observed, and addition of Pgamma increased both the affinity for cGMP and the level of cGMP binding. We also show that mutations of amino acid residues in four different sites in Pgamma reduced its ability to stimulate cGMP binding. Among these, the site involved in Pgamma phosphorylation by Cdk5 (positions 20-23) had the largest effect on cGMP binding. However, except for the C terminus, these sites were not involved in Pgamma inhibition of the cGMP hydrolytic activity of Palphabeta. In addition, the Pgamma concentration required for 50% stimulation of cGMP binding was much greater than that required for 50% inhibition of cGMP hydrolysis. These results suggest that the Palphabeta heterodimer contains two spatially and functionally distinct types of Pgamma-binding sites: one for inhibition of cGMP hydrolytic activity and the second for activation of cGMP binding to GAF domains. We propose a model for the Palphabeta-Pgamma interaction in which Pgamma, by binding to one of the two sites in Palphabeta, may preferentially act either as an inhibitor of catalytic activity or as an activator of cGMP binding to GAF domains in frog PDE.

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.

Structure and regulation of the CDK5-p25(nck5a) complex

CDK5 plays an indispensable role in the central nervous system, and its deregulation is involved in neurodegeneration. We report the crystal structure of a complex between CDK5 and p25, a fragment of the p35 activator. Despite its partial structural similarity with the cyclins, p25 displays an unprecedented mechanism for the regulation of a cyclin-dependent kinase. p25 tethers the unphosphorylated T loop of CDK5 in the active conformation. Residue Ser159, equivalent to Thr160 on CDK2, contributes to the specificity of the CDK5-p35 interaction. Its substitution with threonine prevents p35 binding, while the presence of alanine affects neither binding nor kinase activity. Finally, we provide evidence that the CDK5-p25 complex employs a distinct mechanism from the phospho-CDK2-cyclin A complex to establish substrate specificity.

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.

enok encodes a Drosophila putative histone acetyltransferase required for mushroom body neuroblast proliferation

Mushroom bodies in the Drosophila brain are centers for olfactory learning and memory. We have previously shown that the mushroom bodies comprise three types of neurons with distinct axonal projections. These three types of neurons are generated sequentially from common neuroblasts. We report here the identification of a gene that we have named enoki mushroom (enok), which when it is mutated gives rise to mushroom bodies with reduced axonal structures. enok encodes a putative histone acetyltransferase (HAT) of the MYST family, members of which have been implicated as important modulators of transcriptional activity. A single amino acid change in the zinc finger motif of the putative catalytic HAT domain gives the same phenotype as a null allele, and this finding indicates the importance of HAT activity to Enok's function. Further phenotypic analysis demonstrates that the mushroom body defect is due to an arrest in neuroblast proliferation rather than a failure of either cell fate switching or axon branching. Clonal analyses in the wing discs and the ovaries suggest that enok is essential for normal cell proliferation in some, but not all, tissues. Our results provide in vivo evidence for essential functions of a histone acetyltransferase in the construction of the Drosophila brain.

A LIS1/NUDEL/cytoplasmic dynein heavy chain complex in the developing and adult nervous system

Mutations in mammalian Lis1 (Pafah1b1) result in neuronal migration defects. Several lines of evidence suggest that LIS1 participates in pathways regulating microtubule function, but the molecular mechanisms are unknown. Here, we demonstrate that LIS1 directly interacts with the cytoplasmic dynein heavy chain (CDHC) and NUDEL, a murine homolog of the Aspergillus nidulans nuclear migration mutant NudE. LIS1 and NUDEL colocalize predominantly at the centrosome in early neuroblasts but redistribute to axons in association with retrograde dynein motor proteins. NUDEL is phosphorylated by Cdk5/p35, a complex essential for neuronal migration. NUDEL and LIS1 regulate the distribution of CDHC along microtubules, and establish a direct functional link between LIS1, NUDEL, and microtubule motors. These results suggest that LIS1 and NUDEL regulate CDHC activity during neuronal migration and axonal retrograde transport in a Cdk5/p35-dependent fashion.

Effect of oxidative stress on stability and structure of neurofilament proteins

Neurofilament proteins are highly phosphorylated molecules in the axonal compartment of the adult nervous system. We report the structural analysis of neurofilament proteins after oxidative damage. SDS-PAGE, immunoblotting, circular dichroism, and Fourier transform infrared spectroscopy were used to investigate the relative sensitivity of neurofilaments to oxidative stress and to identify changes in their molecular organization. An ascorbate-Fe+3-O2 buffer system as well as catechols were used to generate free radicals on a substrate of phosphorylated and dephosphorylated neurofilaments. By Fourier Transform Infrared spectroscopy and circular dichroism, we established that the neurofilament secondary structure is mainly composed of α-helices and that after free radical damage of the peptide backbone of neurofilaments, those helices are partly modified into β-sheet and random coil structures. These characteristic reorganizations of the neurofilament structure after oxidative exposure suggest that free radical activity might play an important role in the biogenesis of the cytoplasmic inclusions found in several neurodegenerative diseases.Key words: neurofilaments, oxidative stress, neurodegeneration, phosphorylation, infrared spectroscopy, circular dichroism.

cAMP-dependent protein kinase phosphorylation of EVL, a Mena/VASP relative, regulates its interaction with actin and SH3 domains

Proteins of the Ena/VASP family are implicated in processes that require dynamic actin remodeling such as axon guidance and platelet activation. In this work, we explored some of the pathways that likely regulate actin dynamics in part via EVL (Ena/VASP-like protein). Two isoforms, EVL and EVL-I, were highly expressed in hematopoietic cells of thymus and spleen. In CD3-activated T-cells, EVL was found in F-actin-rich patches and at the distal tips of the microspikes that formed on the activated side of the T-cells. Like the other family members, EVL localized to focal adhesions and the leading edge of lamellipodia when expressed in fibroblasts. EVL was a substrate for the cAMP-dependent protein kinase, and this phosphorylation regulated several of the interactions between EVL and its ligands. Unlike VASP, EVL nucleated actin polymerization under physiological conditions, whereas phosphorylation of both EVL and VASP decreased their nucleating activity. EVL bound directly to the Abl, Lyn, and nSrc SH3 domains; the FE65 WW domain; and profilin, likely via its proline-rich core. Binding of Abl and nSrc SH3 domains, but not profilin or other SH3 domains, was abolished by cAMP-dependent protein kinase phosphorylation of EVL. We show strong cooperative binding of two profilin dimers on the polyproline sequence of EVL. Additionally, profilin competed with the SH3 domains for binding to partially overlapping binding sites. These data suggest that the function of EVL could be modulated in a complex manner by its interactions with multiple ligands and through phosphorylation by cyclic nucleotide dependent kinases.

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.

Bicyclic peptides as models of calcium binding sites: synthesis and conformation of a homodetic undecapeptide

A bicyclic undecapeptide of sequence cyclo-(Ala(1)-Pro(2)-Asp(3)-Glu(4)-Lys(5)-Ala(6)-Pro(7)-Asp(8)-Ser(9) -Glu(10))-cyclo-(10gamma --> 5varepsilon)-Gly(11), designed to mimic the calcium coordination site I of Calmodulin, has been synthesized and its conformation and calcium binding properties have been investigated by means of CD and nmr spectroscopy. The nmr analysis of the free peptide, carried out in DMSO and in TFE/H(2)O at different pH values, shows the presence in solution of one stable conformer, exhibiting trans configuration around both Proline residues. The nmr results in both solvents suggest for the molecule a rectangular shape constituted by two antiparallel beta-strands connected by two beta-turns. Interproton distances, evaluated by NOE contacts, have been used to obtain feasible models by means of Restrained Molecular Dynamic (RMD). The average models from RMD calculations, for both solvents, exhibit good analogies with Calmodulin site I. The model system, when compared with the reference system (Asp(20)-Glu(31) segment in CaM), shows similar dimensions and an effective superimposition of the respective sequence segments Ala(1)-Glu(4) and Thr(28)-Glu(31). The remaining segments of the model peptide exhibit a bending that is intermediate between that of the free and Ca(2+)-coordinated site I. CD spectra, recorded in TFE solutions, point to a 1:1 stoichiometry for the Ca(2+)-peptide complex, with an association constant of at least 1 x 10(5) M(-1).

Distribution of CTP:phosphocholine cytidylyltransferase (CCT) isoforms. Identification of a new CCTbeta splice variant

CTP:phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCTalpha, has been studied extensively and a second isoform, CCTbeta, was recently identified. We identify and characterize a third cDNA, CCTbeta2, that differs from CCTbeta1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCTbeta gene. Like CCTalpha, CCTbeta2 is heavily phosphorylated in vivo, in contrast to CCTbeta1. CCTbeta1 and CCTbeta2 mRNAs were differentially expressed by the human tissues examined, whereas CCTalpha was more uniformly represented. Using isoform-specific antibodies, both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCTalpha and CCTbeta1 supporting proliferation at the nonpermissive conditions. Overexpression experiments did not reveal discrete physiological functions for the three isoforms that catalyze the same biochemical reaction; however, the differential cellular localization and tissue-specific distribution suggest that CCTbeta1 and CCTbeta2 may play a role that is distinct from ubiquitously expressed CCTalpha.