Tej K. Pareek

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.

Cdk5 disruption attenuates tumor PD-L1 expression and promotes antitumor immunity

Cyclin suppresses antitumor immunity Despite the dramatic success of cancer immunotherapy, many types of cancer do not respond. Understanding why could help us to find ways to enhance the overall responsiveness of tumors to immunotherapies. Dorand et al. report that cyclin-dependent kinase 5 (Cdk5), an enzyme that is highly expressed by neurons in many brain cancers, may dampen the ability of T cells to reject tumors. In a mouse model of medulloblastoma, if tumors were Cdk5 deficient, T cells were able to remove them. This heightened antitumor immunity correlated with reduced expression of the inhibitory molecule programmed cell death ligand 1 (PD-L1), a target of current cancer immunotherapies. Science, this issue p. 399 Cyclin-dependent kinase 5 expression by tumors inhibits antitumor immunity in human and mouse medulloblastoma. Cancers often evade immune surveillance by adopting peripheral tissue– tolerance mechanisms, such as the expression of programmed cell death ligand 1 (PD-L1), the inhibition of which results in potent antitumor immunity. Here, we show that cyclin-dependent kinase 5 (Cdk5), a serine-threonine kinase that is highly active in postmitotic neurons and in many cancers, allows medulloblastoma (MB) to evade immune elimination. Interferon-γ (IFN-γ)–induced PD-L1 up-regulation on MB requires Cdk5, and disruption of Cdk5 expression in a mouse model of MB results in potent CD4+ T cell–mediated tumor rejection. Loss of Cdk5 results in persistent expression of the PD-L1 transcriptional repressors, the interferon regulatory factors IRF2 and IRF2BP2, which likely leads to reduced PD-L1 expression on tumors. Our finding highlights a central role for Cdk5 in immune checkpoint regulation by tumor cells.

Cdk5/p25-Induced Cytosolic PLA2-Mediated Lysophosphatidylcholine Production Regulates Neuroinflammation and Triggers Neurodegeneration

The deregulation of cyclin-dependent kinase 5 (Cdk5) by p25 has been shown to contribute to the pathogenesis in a number of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinsons disease (PD) and Alzheimers disease (AD). In particular, p25/Cdk5 has been shown to produce hyperphosphorylated tau, neurofibrillary tangles as well as aberrant amyloid precursor protein processing found in AD. Neuroinflammation has been observed alongside the pathogenic process in these neurodegenerative diseases, however the precise mechanism behind the induction of neuroinflammation and the significance in the AD pathogenesis has not been fully elucidated. In this report, we uncover a novel pathway for p25-induced neuroinflammation where p25 expression induces an early trigger of neuroinflammation in vivo in mice. Lipidomic mass spectrometry, in vitro coculture and conditioned media transfer experiments show that the soluble lipid mediator lysophosphatidylcholine (LPC) is released by p25 overexpressing neurons to initiate astrogliosis, neuroinflammation and subsequent neurodegeneration. Reverse transcriptase PCR and gene silencing experiments show that cytosolic phospholipase 2 (cPLA2) is the key enzyme mediating the p25-induced LPC production and cPLA2 upregulation is critical in triggering the p25-mediated inflammatory and neurodegenerative process. Together, our findings delineate a potential therapeutic target for the reduction of neuroinflammation in neurodegenerative diseases including AD.

Triterpenoid modulation of IL-17 and Nrf-2 expression ameliorates neuroinflammation and promotes remyelination in autoimmune encephalomyelitis

Inflammatory cytokines and endogenous anti-oxidants are variables affecting disease progression in multiple sclerosis (MS). Here we demonstrate the dual capacity of triterpenoids to simultaneously repress production of IL-17 and other pro-inflammatory mediators while exerting neuroprotective effects directly through Nrf2-dependent induction of anti-oxidant genes. Derivatives of the natural triterpene oleanolic acid, namely CDDO-trifluoroethyl-amide (CDDO-TFEA), completely suppressed disease in a murine model of MS, experimental autoimmune encephalomyelitis (EAE), by inhibiting Th1 and Th17 mRNA and cytokine production. Encephalitogenic T cells recovered from treated mice were hypo-responsive to myelin antigen and failed to adoptively transfer the disease. Microarray analyses showed significant suppression of pro-inflammatory transcripts with concomitant induction of anti-inflammatory genes including Ptgds and Hsd11b1. Finally, triterpenoids induced oligodendrocyte maturation in vitro and enhanced myelin repair in an LPC-induced non-inflammatory model of demyelination in vivo. These results demonstrate the unique potential of triterpenoid derivatives for the treatment of neuroinflammatory disorders such as MS.

Specific Inhibition of p25/Cdk5 Activity by the Cdk5 Inhibitory Peptide Reduces Neurodegeneration In Vivo

The aberrant hyperactivation of Cyclin-dependent kinase 5 (Cdk5), by the production of its truncated activator p25, results in the formation of hyperphosphorylated tau, neuroinflammation, amyloid deposition, and neuronal death in vitro and in vivo. Mechanistically, this occurs as a result of a neurotoxic insult that invokes the intracellular elevation of calcium to activate calpain, which cleaves the Cdk5 activator p35 into p25. It has been shown previously that the p25 transgenic mouse as a model to investigate the mechanistic implications of p25 production in the brain, which recapitulates deregulated Cdk5-mediated neuropathological changes, such as hyperphosphorylated tau and neuronal death. To date, strategies to inhibit Cdk5 activity have not been successful in targeting selectively aberrant activity without affecting normal Cdk5 activity. Here we show that the selective inhibition of p25/Cdk5 hyperactivation in vivo, through overexpression of the Cdk5 inhibitory peptide (CIP), rescues against the neurodegenerative pathologies caused by p25/Cdk5 hyperactivation without affecting normal neurodevelopment afforded by normal p35/Cdk5 activity. Tau and amyloid pathologies as well as neuroinflammation are significantly reduced in the CIP–p25 tetra transgenic mice, whereas brain atrophy and subsequent cognitive decline are reversed in these mice. The findings reported here represent an important breakthrough in elucidating approaches to selectively inhibit the p25/Cdk5 hyperactivation as a potential therapeutic target to reduce neurodegeneration.

Cdk5: A New Player in Pain Signaling

Cyclin-dependent kinase 5 (Cdk5) is predominantly active in postmitotic neurons.Despite its structural homology with other cyclin-dependent kinases, Cdk5 is apparentlynot involved in the cell cycle process. The monomeric form of Cdk5 is inactive andrequires the association of p35 or p39 in order to perform its kinase activity. This kinaseis essential for normal brain development and function, but uncontrolled activity of Cdk5may lead to numerous neurodegenerative processes. Although Cdk5 activity has beenimplicated in several neuronal functions, but its precise role in the peripheral nervoussystem has not been determined. Recently we reported for the first time the essential rolefor Cdk5 in pain signaling (Pareek et al., PNAS 2006). Altered nociceptive responses tobasal thermal noxious stimuli in p35 knockout (p35-/-) and p35-overexpresing transgenicmice (Tgp35) have established the important role of this gene in the nociceptive process.Here, we report that Cdk5 regulates mitogen-activated protein kinase kinase1/2(MEK1/2) activity through a negative feedback loop during the peripheral inflammatoryresponse. Moreover a differential nociceptive response after chronic morphine exposurein p35-/- and Tgp35 mice suggests that Cdk5 activity is important for opioid tolerance.In conclusion, our data indicate important molecular roles for Cdk5 in pain signaling andopioid tolerance, which makes it a potential target for analgesic drug development.

Cdk5-Mediated Phosphorylation of δ-Catenin Regulates Its Localization and GluR2-Mediated Synaptic Activity

Cyclin-dependent kinase 5 (Cdk5)-mediated phosphorylation plays an important role in proper synaptic function and transmission. Loss of Cdk5 activity results in abnormal development of the nervous system accompanied by massive disruptions in cortical migration and lamination, therefore impacting synaptic activity. The Cdk5 activator p35 associates with δ-catenin, the synaptic adherens junction protein that serves as part of the anchorage complex of AMPA receptor at the postsynaptic membrane. However, the implications of Cdk5-mediated phosphorylation of δ-catenin have not been fully elucidated. Here we show that Cdk5-mediated phosphorylation of δ-catenin regulates its subcellular localization accompanied by changes in dendritic morphogenesis and synaptic activity. We identified two Cdk5 phosphorylation sites in mouse δ-catenin, serines 300 and 357, and report that loss of Cdk5 phosphorylation of δ-catenin increased its localization to the membrane. Furthermore, mutations of the serines 300 and 357 to alanines to mimic nonphosphorylated δ-catenin resulted in increased dendritic protrusions accompanied by increased AMPA receptor subunit GluR2 localization at the membrane. Consistent with these observations, loss of Cdk5 phosphorylation of δ-catenin increased the AMPA/NMDA ratio. This study reveals how Cdk5 phosphorylation of the synaptic mediator protein δ-catenin can alter its localization at the synapse to impact neuronal synaptic activity.

Cyclin-dependent kinase 5 represses Foxp3 gene expression and Treg development through specific phosphorylation of Stat3 at Serine 727

Cyclin-dependent kinase 5 (Cdk5) is known as a unique member of the cyclin-dependent family of serine/threonine kinases. Previously, we demonstrated Cdk5 to be an important regulator of T cell function and that disruption of Cdk5 expression ameliorates T cell mediated neuroinflammation. Here, we show a novel role of Cdk5 in the regulation of Foxp3 expression in murine CD4(+) T cells. Our data indicate that disruption of Cdk5 activity in T cells abrogates the IL-6 suppression of Foxp3 expression. This effect is achieved through Cdk5 phosphorylation of the signal transducer and activator of transcription 3 (Stat3) specifically at Serine 727 in T cells, and we show this post-translational modification is required for proper Stat3 DNA binding to the Foxp3 gene on the enhancer II region. Taken together, our data point to an essential role for Cdk5 in the differentiation of T cells as it regulates Foxp3 gene expression through phosphorylation of Stat3.

Cdk5 controls IL-2 gene expression via repression of the mSin3a-HDAC complex

Cyclin-dependent kinase 5 (Cdk5) is a unique member of a family of serine/threonine cyclin-dependent protein kinases. We previously demonstrated disruption of Cdk5 gene expression in mice impairs T-cell function and ameliorates T-cell-mediated neuroinflammation. Here, we show Cdk5 modulates gene expression during T-cell activation by impairing the repression of gene transcription by histone deacetylase 1 (HDAC1) through specific phosphorylation of the mSin3a protein at serine residue 861. Disruption of Cdk5 activity in T-cells enhances HDAC activity and binding of the HDAC1/mSin3a complex to the IL-2 promoter, leading to suppression of IL-2 gene expression. These data point to essential roles for Cdk5 in regulating gene expression in T-cells and transcriptional regulation by the co-repressor mSin3a.

Conditional deletion of cyclin-dependent kinase 5 in primary sensory neurons leads to atypical skin lesions.

The key role of cyclin-dependent kinase 5 (Cdk5) in neuronal function has been well established but understanding of its importance in sensory pathways is in its infancy. Recently we described the important role of Cdk5 in pain signaling. Our studies indicated that conditional deletion of Cdk5 in small sensory neurons causes hypoalgesia. In current study, we identified development of atypical non-healing skin lesions in these mutant mice during the general colony maintenance. Detailed examination of these lesions clearly distinguishes them from ulcerative dermatitis. Here we hypothesize that these skin lesions are due to general sensation loss in these mice as evident from deep skin scratches that turn into unhealed wounds.