Cdk5 promotes synaptogenesis by regulating the subcellular distribution of the MAGUK family member CASK

Synaptogenesis is a highly regulated process that underlies formation of neural circuitry. Considerable work has demonstrated the capability of some adhesion molecules, such as SynCAM and Neurexins/Neuroligins, to induce synapse formation in vitro. Furthermore, Cdk5 gain of function results in an increased number of synapses in vivo. To gain a better understanding of how Cdk5 might promote synaptogenesis, we investigated potential crosstalk between Cdk5 and the cascade of events mediated by synapse-inducing proteins. One protein recruited to developing terminals by SynCAM and Neurexins/Neuroligins is the MAGUK family member CASK. We found that Cdk5 phosphorylates and regulates CASK distribution to membranes. In the absence of Cdk5-dependent phosphorylation, CASK is not recruited to developing synapses and thus fails to interact with essential presynaptic components. Functional consequences include alterations in calcium influx. Mechanistically, Cdk5 regulates the interaction between CASK and liprin-alpha. These results provide a molecular explanation of how Cdk5 can promote synaptogenesis.     

Cdk5 is required for memory function and hippocampal plasticity via the cAMP signaling pathway

Memory formation is modulated by pre- and post-synaptic signaling events in neurons. The neuronal protein kinase Cyclin-Dependent Kinase 5 (Cdk5) phosphorylates a variety of synaptic substrates and is implicated in memory formation. It has also been shown to play a role in homeostatic regulation of synaptic plasticity in cultured neurons. Surprisingly, we found that Cdk5 loss of function in hippocampal circuits results in severe impairments in memory formation and retrieval. Moreover, Cdk5 loss of function in the hippocampus disrupts cAMP signaling due to an aberrant increase in phosphodiesterase (PDE) proteins. Dysregulation of cAMP is associated with defective CREB phosphorylation and disrupted composition of synaptic proteins in Cdk5-deficient mice. Rolipram, a PDE4 inhibitor that prevents cAMP depletion, restores synaptic plasticity and memory formation in Cdk5-deficient mice. Collectively, our results demonstrate a critical role for Cdk5 in the regulation of cAMP-mediated hippocampal functions essential for synaptic plasticity and memory formation.     

CDK5: The “pathfinder” for new born neurons in adult hippocampus?

Neurogenesis takes place in the mammalian hippocampus throughout the whole life and deficient adult hippocampal neurogenesis has been related to neurological conditions like Alzheimer disease (AD), Parkinson disease (PD) and epilepsy. The molecular mechanisms by which immature neurons and their extending neurites find their appropriate position and target area remain largely unknown. Recent work by Jessberger et al.¹ examines the role of Cdk5 in normal adult neurogenesis by a retroviral knock-down approach. Cdk5 is shown to be implicated in the migration of newborn neurons into the granule cell layer (GCL), as well as, in correct targeting of dendrites from newborn granule cells (GC) into the molecular layer (ML) of the dentate gyrus (DG). The study also shows that aberrant dendrites still seem to become synaptically integrated into the existing circuitry thereby suggesting a mechanistic dissociation between accurate dendritic targeting and subsequent synapse formation. The finding of Cdk5 guiding this integration of new born neurons at the physiologically appropriate place is an important step towards understanding adult neurogenesis that may help to overcome problems with the restorative use of neural stem cells in present grafting approaches in neurological diseases.Key words: cyclin-dependent kinase 5 (cdk5), adult neurogenesis, dentate gyrus     

Reelin signaling and Cdk5 in the control of neuronal positioning

Neuronal positioning is important for higher brain function because it is the architectural basis of the formation of precise synaptic circuits. Analysis of neurological mutant mice has led to dramatic progress in the identification and characterization of molecules important for neuronal positioning in the developing mammalian brain. Among these molecules, identification of signal pathways mediated by Reelin and Cdk5 kinase has provided a conceptual framework for exploring the molecular mechanisms underlying proper neuronal positioning in the developing mammalian brain. Recent evidence has implicated synergism between Reelin signaling and Cdk5 in contributing to the proper positioning of selective neuronal populations.     

Life is a journey: a genetic look at neocortical development

Although the basic principles of neocortical development have been known for quite some time, it is only recently that our understanding of the molecular mechanisms that are involved has improved. Such understanding has been facilitated by genetic approaches that have identified key proteins involved in neocortical development, which have been placed into signalling pathways by molecular and cell-biological studies. The challenge of current research is to understand the manner in which these various signalling pathways are interconnected to gain a more comprehensive picture of the molecular intricacies that govern neocortical development.     

Kinetic mechanism of activation of the Cdk2/cyclin A complex. Key role of the C-lobe of the Cdk

Eukaryotic cell cycle progression is controlled by the ordered action of cyclin-dependent kinases, activation of which occurs through the binding of the cyclin to the Cdk followed by phosphorylation of a conserved threonine in the T-loop of the Cdk by Cdk-activating kinase (CAK). Despite our understanding of the structural changes, which occur upon Cdk/cyclin formation and activation, little is known about the dynamics of the molecular events involved. We have characterized the mechanism of Cdk2/cyclin A complex formation and activation at the molecular and dynamic level by rapid kinetics and demonstrate here that it is a two-step process. The first step involves the rapid association between the PSTAIRE helix of Cdk2 and helices 3 and 5 of the cyclin to yield an intermediate complex in which the threonine in the T-loop is not accessible for phosphorylation. Additional contacts between the C-lobe of the Cdk and the N-terminal helix of the cyclin then induce the isomerization of the Cdk into a fully mature form by promoting the exposure of the T-loop for phosphorylation by CAK and the formation of the substrate binding site. This conformational change is selective for the cyclin partner.     

Cyclin-dependent kinase-5 is involved in neuregulin-dependent activation of phosphatidylinositol 3-kinase and Akt activity mediating neuronal survival

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway plays an important role in mediating survival signals in wide variety of neurons and cells. Recent studies show that Akt also regulates metabolic pathways to regulate cell survival. In this study, we reported that cyclin-dependent kinase-5 (Cdk5) regulates Akt activity and cell survival through the neuregulin-mediated PI 3-kinase signaling pathway. We found that brain extracts of Cdk5-/-mice display a lower PI 3-kinase activity and phosphorylation of Akt compared with that in wild type mice. Moreover, we demonstrated that Cdk5 phosphorylated Ser-1176 in the neuregulin receptor ErbB2 and phosphorylated Thr-871 and Ser-1120 in the ErbB3 receptor. We identified the Ser-1120 sequence RSRSPR in ErbB3 as a novel phosphorylation consensus sequence of Cdk5. Finally, we found that Cdk5 activity is involved in neuregulin-induced Akt activity and neuregulin-mediated neuronal survival. These findings suggest that Cdk5 may exert a key role in promoting neuronal survival by regulating Akt activity through the neuregulin/PI 3-kinase signaling pathway.     

Cdk5 Contributes to Huntington’s Disease Learning and Memory Deficits via Modulation of Brain Region-Specific Substrates

Cognitive deficits are a major hallmark of Huntington’s disease (HD) with a great impact on the quality of patient’s life. Gaining a better understanding of the molecular mechanisms underlying learning and memory impairments in HD is, therefore, of critical importance. Cdk5 is a proline-directed Ser/Thr kinase involved in the regulation of synaptic plasticity and memory processes that has been associated with several neurodegenerative disorders. In this study, we aim to investigate the role of Cdk5 in learning and memory impairments in HD using a novel animal model that expresses mutant huntingtin (mHtt) and has genetically reduced Cdk5 levels. Genetic reduction of Cdk5 in mHtt knock-in mice attenuated both corticostriatal learning deficits as well as hippocampal-dependent memory decline. Moreover, the molecular mechanisms by which Cdk5 counteracts the mHtt-induced learning and memory impairments appeared to be differentially regulated in a brain region-specific manner. While the corticostriatal learning deficits are attenuated through compensatory regulation of NR2B surface levels, the rescue of hippocampal-dependent memory was likely due to restoration of hippocampal dendritic spine density along with an increase in Rac1 activity. This work identifies Cdk5 as a critical contributor to mHtt-induced learning and memory deficits. Furthermore, we show that the Cdk5 downstream targets involved in memory and learning decline differ depending on the brain region analyzed suggesting that distinct Cdk5 effectors could be involved in cognitive impairments in HD.     

Cyclin-dependent kinase 5 in synaptic plasticity, learning and memory

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase with a multitude of functions. Although Cdk5 is widely expressed, it has been studied most extensively in neurons. Since its initial characterization, the fundamental contribution of Cdk5 to an impressive range of neuronal processes has become clear. These phenomena include neural development, dopaminergic function and neurodegeneration. Data from different fields have recently converged to provide evidence for the participation of Cdk5 in synaptic plasticity, learning and memory. In this review, we consider recent data implicating Cdk5 in molecular and cellular mechanisms underlying synaptic plasticity. We relate these findings to its emerging role in learning and memory. Particular attention is paid to the activation of Cdk5 by p25, which enhances hippocampal synaptic plasticity and memory, and suggests formation of p25 as a physiological process regulating synaptic plasticity and memory.     

S-nitrosylation of Cdk5

Aberrant activation of Cdk5 has been implicated in the process of neurodegenerative diseases such as Alzheimer's disease (AD). We recently reported that S-nitrosylation of Cdk5 (forming SNO-Cdk5) at specific cysteine residues results in excessive activation of Cdk5, contributing to mitochondrial dysfunction, synaptic damage, and neuronal cell death in models of AD. Furthermore, SNO-Cdk5 acts as a nascent S-nitrosylase, transnitrosylating the mitochondrial fission protein Drp1 and enhancing excessive mitochondrial fission in dendritic spines. However, a molecular mechanism that leads to the formation of SNO-Cdk5 in neuronal cells remained obscure. Here, we demonstrate that neuronal nitric oxide synthase (NOS1) interacts with Cdk5 and that the close proximity of the two proteins facilitates the formation of SNO-Cdk5. Interestingly, as a negative feedback mechanism, Cdk5 phosphorylates and suppresses NOS1 activity. Thus, together with our previous report, these findings delineate an S-nitrosylation pathway wherein Cdk5/NOS1 interaction enhances SNO-Cdk5 formation, mediating mitochondrial dysfunction and synaptic loss during the etiology of AD.     

S-nitrosylation of Cdk5: Potential implications in amyloid-β-related neurotoxicity in Alzheimer disease

Aberrant activation of Cdk5 has been implicated in the process of neurodegenerative diseases such as Alzheimer's disease (AD). We recently reported that S-nitrosylation of Cdk5 (forming SNO-Cdk5) at specific cysteine residues results in excessive activation of Cdk5, contributing to mitochondrial dysfunction, synaptic damage, and neuronal cell death in models of AD. Furthermore, SNO-Cdk5 acts as a nascent S-nitrosylase, transnitrosylating the mitochondrial fission protein Drp1 and enhancing excessive mitochondrial fission in dendritic spines. However, a molecular mechanism that leads to the formation of SNO-Cdk5 in neuronal cells remained obscure. Here, we demonstrate that neuronal nitric oxide synthase (NOS1) interacts with Cdk5 and that the close proximity of the two proteins facilitates the formation of SNO-Cdk5. Interestingly, as a negative feedback mechanism, Cdk5 phosphorylates and suppresses NOS1 activity. Thus, together with our previous report, these findings delineate an S-nitrosylation pathway wherein Cdk5/NOS1 interaction enhances SNO-Cdk5 formation, mediating mitochondrial dysfunction and synaptic loss during the etiology of AD.     

Cdk5 regulates multiple cellular events in neural development,function and disease

Cyclin‐dependent kinases (CDKs) generally regulate cell proliferation in dividing cells, including neural progenitors. In contrast, an unconventional CDK, Cdk5, is predominantly activated in post‐mitotic cells, and involved in various cellular events, such as microtubule and actin cytoskeletal organization, cell–cell and cell–extracellular matrix adhesions, and membrane trafficking. Interestingly, recent studies have indicated that Cdk5 is associated with several cell cycle‐related proteins, Cyclin‐E and p27^kip1. Taking advantage of multiple functionality, Cdk5 plays important roles in neuronal migration, layer formation, axon elongation and dendrite arborization in many regions of the developing brain, including cerebral cortex and cerebellum. Cdk5 is also required for neurogenesis at least in the cerebral cortex. Furthermore, Cdk5 is reported to control neurotransmitter release at presynaptic sites, endocytosis of the NMDA receptor at postsynaptic sites and dendritic spine remodeling, and thereby regulate synaptic plasticity and memory formation and extinction. In addition to these physiological roles in brain development and function, Cdk5 is associated with many neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. In this review, I will introduce the physiological and pathological roles of Cdk5 in mammalian brains from the viewpoint of not only in vivo phenotypes but also its molecular and cellular functions.     

Cdk5 is involved in BDNF-stimulated dendritic growth in hippocampal neurons

Neurotrophins are key regulators of neuronal survival and differentiation during development. Activation of their cognate receptors, Trk receptors, a family of receptor tyrosine kinases (RTKs), is pivotal for mediating the downstream functions of neurotrophins. Recent studies reveal that cyclin-dependent kinase 5 (Cdk5), a serine/threonine kinase, may modulate RTK signaling through phosphorylation of the receptor. Given the abundant expression of both Cdk5 and Trk receptors in the nervous system, and their mutual involvement in the regulation of neuronal architecture and synaptic functions, it is of interest to investigate if Cdk5 may also modulate Trk signaling. In the current study, we report the identification of TrkB as a Cdk5 substrate. Cdk5 phosphorylates TrkB at Ser478 at the intracellular juxtamembrane region of TrkB. Interestingly, attenuation of Cdk5 activity or overexpression of a TrkB mutant lacking the Cdk5 phosphorylation site essentially abolishes brain-derived neurotrophic factor (BDNF)–triggered dendritic growth in primary hippocampal neurons. In addition, we found that Cdk5 is involved in BDNF-induced activation of Rho GTPase Cdc42, which is essential for BDNF-triggered dendritic growth. Our observations therefore reveal an unanticipated role of Cdk5 in TrkB-mediated regulation of dendritic growth through modulation of BDNF-induced Cdc42 activation.     

Glutathione-S-transferase P1 is a critical regulator of Cdk5 kinase activity

Cyclin dependent kinase-5 (Cdk5) activity is deregulated in Alzheimer's disease (AD) and contributes to all three hallmarks: neurotoxic β-amyloid formation, neurofibrillary tangles, and neuronal death. However, the mechanism leading to Cdk5 deregulation remains controversial. Cdk5 deregulation in AD is usually linked to the formation of p25, a proteolysis product of Cdk5 activator p35, which leads to Cdk5 mislocalization and hyperactivation. A few studies have indeed shown increased p25 levels in AD brains; however, others have refuted this observation. These contradictory findings suggest that additional factors contribute to Cdk5 deregulation. This study identified glutathione-S-transferase pi 1 (GSTP1) as a novel Cdk5 regulatory protein. We demonstrate that it is a critical determinant of Cdk5 activity in human AD brains and various cancer and neuronal cells. Increased GSTP1 levels were consistently associated with reduced Cdk5 activity. GSTP1 directly inhibits Cdk5 by dislodging p25/p35, and indirectly by eliminating oxidative stress. Cdk5 promotes and is activated by oxidative stress, thereby engaging a feedback loop which ultimately leads to cell death. Not surprisingly, GSTP1 transduction conferred a high degree of neuroprotection under neurotoxic conditions. Given the critical role of oxidative stress in AD pathogenesis, an increase in GSTP1 level may be an alternative way to modulate Cdk5 signaling, eliminate oxidative stress, and prevent neurodegeneration.     

Cyclin-Dependent Kinase 5 Links Extracellular Cues to Actin Cytoskeleton During Dendritic Spine Development

Emerging evidence has indicated a regulatory role of cyclin-dependent kinase 5 (Cdk5) in synaptic plasticity as well as in higher brain functions, such as learning and memory. However, the molecular and cellular mechanisms underlying the actions of Cdk5 at synapses remain unclear. Recent findings demonstrate that Cdk5 regulates dendritic spine morphogenesis through modulating actin dynamics. Ephexin1 and WAVE-1, two important regulators of the actin cytoskeleton, have both been recently identified as substrates for Cdk5. Importantly, phosphorylation of these proteins by Cdk5 leads to dendritic spine loss, revealing a potential mechanism by which Cdk5 regulates synapse remodeling. Furthermore, Cdk5-dependent phosphorylation of ephexin1 is required for the ephrin-A1 mediated spine retraction, pointing to a critical role of Cdk5 in conveying signals from extracellular cues to actin cytoskeleton at synapses. Taken together, understanding the precise regulation of Cdk5 and its downstream targets at synapses would provide important insights into the multi-regulatory roles of Cdk5 in actin remodeling during dendritic spine development.     

Protein-protein interactions in Cdk5 regulation and function

Cdk5 is a unique member of the cyclin-dependent kinase (Cdk) family of small protein kinases. In association with its neuron-specific activator p35 or p39, Cdk5 displays many regulatory properties distinct from other Cdks. A growing body of evidence has suggested that Cdk5-p35 has important implications in a variety of neuronal activities occurring in the central nervous system. In brain, Cdk5-p35 appears to exist as large molecular complexes with other proteins, and protein-protein interactions appear to be a molecular principle for Cdk5-p35 to conduct its physiological functions. Over the past decade, a number of proteins have been identified to associate with Cdk5-p35. While the majority of these proteins mediate their interaction with Cdk5 through p35, implying that p35 may act not only as an activator of Cdk5 but also as an adaptor to associate Cdk5 with its regulators and physiological targets, a small group of other proteins are found to link directly with Cdk5. In addition, Cdk5 has been found to phosphorylate a diverse list of substrates, further implicating its regulatory roles in a wide range of cellular processes. In this review, we present an updated inventory of the interacting proteins of Cdk5-p35 kinase and its substrates as well as a discussion on the implicated effects of these interactions.     

A Jekyll and Hyde kinase: roles for Cdk5 in brain development and disease

Cyclin-dependent kinase 5 (Cdk5) is a multi-faced kinase implicated in both development and disease of the mammalian central nervous system. These different faces of Cdk5 are preferentially regulated by the activation of Cdk5 by its different binding partners. The precise molecular and cellular mechanisms governing the role of Cdk5 in brain development and disease are unclear. Emerging evidence is now unraveling how Cdk5 normally orchestrates new signaling pathways that dictate the proper maturation and maintenance of the central nervous system. Under pathological conditions, however, Cdk5 activity goes awry and the malevolent face of Cdk5 surfaces. Recently developed animal models that display this deregulated Cdk5 activity reveal the intimate involvement of Cdk5 in tau pathology and neuronal cell death, and underscore the importance of phosphorylation in the progression of neurodegenerative diseases.     

Cdk5: a multifaceted kinase in neurodegenerative diseases

Since the identification of cyclin-dependent kinase-5 (Cdk5) as a tau kinase and member of the Cdk family almost 20 years ago, deregulation of Cdk5 activity has been linked to an array of neurodegenerative diseases. As knowledge on the etiopathological mechanisms of these diseases evolved through the years, Cdk5 has also been implicated in additional cellular events that are affected under these pathological conditions. From the role of Cdk5 in the regulation of synaptic functions to its involvement in autophagy deregulation, significant insights have been obtained regarding the role of Cdk5 as a key regulator of neurodegeneration. Here, we summarize recent findings on the involvement of Cdk5 in the pathophysiological mechanisms underlying various neurodegenerative diseases.