Inhibition of STAT-mediated cytokine responses to chemically-induced colitis prevents inflammation-associated neurobehavioral impairments

Depression can be associated with chronic systemic inflammation, and production of peripheral proinflammatory cytokines and upregulation of the kynurenine pathway have been implicated in pathogenesis of depression. However, the mechanistic bases for these comorbidities are not yet well understood. As tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), which convert tryptophan to kynurenine, are rate-limiting enzymes of the kynurenine pathway, we screened TDO or IDO inhibitors for effects on the production of proinflammatory cytokines in a mouse macrophage cell line. The TDO inhibitor 680C91 attenuated LPS-induced pro-inflammatory cytokines including IL-1β and IL-6. Surprisingly, this effect was TDO-independent, as it occurred even in peritoneal macrophages from TDO knockout mice. Instead, the anti-inflammatory effects of 680C91 were mediated through the suppression of signal transducer and activator of transcription(STAT) signaling. Furthermore, 680C91 suppressed production of proinflammatory cytokines and STAT signaling in an animal model of inflammatory bowel disease. Specifically, 680C91 effectively attenuated acute phase colon cytokine responses in male mice subjected to dextran sulfate sodium (DSS)-induced colitis. Interestingly, this treatment also prevented the development of anxiodepressive-like neurobehaviors in DSS-treated mice during the recovery phase. The ability of 680C91 to prevent anxiodepressive-like behavior in response to chemically-induced colitis appeared to be due to rescue of attenuated dopamine responses in the nucleus accumbens. Thus, inhibition of STAT-mediated, but TDO-independent proinflammatory cytokines in macrophages can prevent inflammation-associated anxiety and depression. Identification of molecular mechanisms involved may facilitate the development of new treatments for gastrointestinal-neuropsychiatric comorbidity.

Faulty Metabolism: A Potential Instigator of an Aggressive Phenotype in Cdk5-dependent Medullary Thyroid Carcinoma

Mechanistic modeling of cancers such as Medullary Thyroid Carcinoma (MTC) to emulate patient-specific phenotypes is challenging. The discovery of potential diagnostic markers and druggable targets in MTC urgently requires clinically relevant animal models. Here we established orthotopic mouse models of MTC driven by aberrantly active Cdk5 using cell-specific promoters. Each of the two models elicits distinct growth differences that recapitulate the less or more aggressive forms of human tumors. The comparative mutational and transcriptomic landscape of tumors revealed significant alterations in mitotic cell cycle processes coupled with the slow-growing tumor phenotype. Conversely, perturbation in metabolic pathways emerged as critical for aggressive tumor growth. Moreover, an overlapping mutational profile was identified between mouse and human tumors. Gene prioritization revealed putative downstream effectors of Cdk5 which may contribute to the slow and aggressive growth in the mouse MTC models. In addition, Cdk5/p25 phosphorylation sites identified as biomarkers for Cdk5-driven neuroendocrine tumors (NETs) were detected in both slow and rapid onset models and were also histologically present in human MTC. Thus, this study directly relates mouse and human MTC models and uncovers vulnerable pathways potentially responsible for differential tumor growth rates. Functional validation of our findings may lead to better prediction of patient-specific personalized combinational therapies.

Cdk5 mediates rotational force-induced brain injury

Millions of traumatic brain injuries (TBIs) occur annually. TBIs commonly result from falls, traffic accidents, and sports-related injuries, all of which involve rotational acceleration/deceleration of the brain. During these injuries, the brain endures a multitude of primary insults including compression of brain tissue, damaged vasculature, and diffuse axonal injury. All of these deleterious effects can contribute to secondary brain ischemia, cellular death, and neuroinflammation that progress for weeks, months, and lifetime after injury. While the linear effects of head trauma have been extensively modeled, less is known about how rotational injuries mediate neuronal damage following injury. Here, we developed a new model of repetitive rotational head trauma in rodents and demonstrated acute and prolonged pathological, behavioral, and electrophysiological effects of rotational TBI (rTBI). We identify aberrant Cyclin-dependent kinase 5 (Cdk5) activity as a principal mediator of rTBI. We utilized Cdk5-enriched phosphoproteomics to uncover potential downstream mediators of rTBI and show pharmacological inhibition of Cdk5 reduces the cognitive and pathological consequences of injury. These studies contribute meaningfully to our understanding of the mechanisms of rTBI and how they may be effectively treated.

A modified standard American diet induces physiological parameters associated with metabolic syndrome in C57BL/6J mice

Investigations into the causative role that western dietary patterns have on obesity and disease pathogenesis have speculated that quality and quantity of dietary fats and/or carbohydrates have a predictive role in the development of these disorders. Standard reference diets such as the AIN-93 rodent diet have historically been used to promote animal health and reduce variation of results across experiments, rather than model modern human dietary habits or nutrition-related pathologies. In rodents high-fat diets (HFDs) became a classic tool to investigate diet-induced obesity (DIO). These murine diets often relied on a single fat source with the most DIO consistent HFDs containing levels of fat up to 45-60% (kcal), higher than the reported human intake of 33-35% (kcal). More recently, researchers are formulating experimental animal (pre-clinical) diets that reflect mean human macro- and micronutrient consumption levels described by the National Health and Nutrition Examination Survey (NHANES). These diets attempt to integrate relevant ingredient sources and levels of nutrients; however, they most often fail to include high-fructose corn syrup (HFCS) as a source of dietary carbohydrate. We have formulated a modified Standard American Diet (mSAD) that incorporates relevant levels and sources of nutrient classes, including dietary HFCS, to assess the basal physiologies associated with mSAD consumption. Mice proffered the mSAD for 15 weeks displayed a phenotype consistent with metabolic syndrome, exhibiting increased adiposity, fasting hyperglycemia with impaired glucose and insulin tolerance. Metabolic alterations were evidenced at the tissue level as crown-like structures (CLS) in adipose tissue and fatty acid deposition in the liver, and targeted 16S rRNA metagenomics revealed microbial compositional shifts between dietary groups. This study suggests diet quality significantly affects metabolic homeostasis, emphasizing the importance of developing relevant pre-clinical diets to investigate chronic diseases highly impacted by western dietary consumption patterns.

Genetic impairment of succinate metabolism disrupts bioenergetic sensing in adrenal neuroendocrine cancer

Metabolic dysfunction mutations can impair energy sensing and cause cancer. Loss of function of the mitochondrial tricarboxylic acid (TCA) cycle enzyme subunit succinate dehydrogenase B (SDHB) results in various forms of cancer typified by pheochromocytoma (PC). Here we delineate a signaling cascade where the loss of SDHB induces the Warburg effect, triggers dysregulation of [Ca²⁺]_i, and aberrantly activates calpain and protein kinase Cdk5, through conversion of its cofactor from p35 to p25. Consequently, aberrant Cdk5 initiates a phospho-signaling cascade where GSK3 inhibition inactivates energy sensing by AMP kinase through dephosphorylation of the AMP kinase γ subunit, PRKAG2. Overexpression of p25-GFP in mouse adrenal chromaffin cells also elicits this phosphorylation signaling and causes PC. A potent Cdk5 inhibitor, MRT3-007, reverses this phospho-cascade, invoking a senescence-like phenotype. This therapeutic approach halted tumor progression in vivo. Thus, we reveal an important mechanistic feature of metabolic sensing and demonstrate that its dysregulation underlies tumor progression in PC and likely other cancers.

Integrated regulation of PKA by fast and slow neurotransmission in the nucleus accumbens controls plasticity and stress responses

Cortical glutamate and midbrain dopamine neurotransmission converge to mediate striatum-dependent behaviors, while maladaptations in striatal circuitry contribute to mental disorders. However, the crosstalk between glutamate and dopamine signaling has not been entirely elucidated. Here we uncover a molecular mechanism by which glutamatergic and dopaminergic signaling integrate to regulate cAMP-dependent protein kinase (PKA) via phosphorylation of the PKA regulatory subunit, RIIβ. Using a combination of biochemical, pharmacological, neurophysiological, and behavioral approaches, we find that glutamate-dependent reduction in cyclin-dependent kinase 5 (Cdk5)-dependent RIIβ phosphorylation alters the PKA holoenzyme autoinhibitory state to increase PKA signaling in response to dopamine. Furthermore, we show that disruption of RIIβ phosphorylation by Cdk5 enhances cortico-ventral striatal synaptic plasticity. In addition, we demonstrate that acute and chronic stress in rats inversely modulate RIIβ phosphorylation and ventral striatal infusion of a small interfering peptide that selectively targets RIIβ regulation by Cdk5 improves behavioral response to stress. We propose this new signaling mechanism integrating ventral striatal glutamate and dopamine neurotransmission is important to brain function, may contribute to neuropsychiatric conditions, and serves as a possible target for the development of novel therapeutics for stress-related disorders.

Systemic Administration of a Brain Permeable Cdk5 Inhibitor Alters Neurobehavior

Cyclin-dependent kinase 5 (Cdk5) is a crucial regulator of neuronal signal transduction. Cdk5 activity is implicated in various neuropsychiatric and neurodegenerative conditions such as stress, anxiety, depression, addiction, Alzheimer's disease, and Parkinson's disease. While constitutive Cdk5 knockout is perinatally lethal, conditional knockout mice display resilience to stress-induction, enhanced cognition, neuroprotection from stroke and head trauma, and ameliorated neurodegeneration. Thus, Cdk5 represents a prime target for treatment in a spectrum of neurological and neuropsychiatric conditions. While intracranial infusions or treatment of acutely dissected brain tissue with compounds that inhibit Cdk5 have allowed the study of kinase function and corroborated conditional knockout findings, potent brain-penetrant systemically deliverable Cdk5 inhibitors are extremely limited, and no Cdk5 inhibitor has been approved to treat any neuropsychiatric or degenerative diseases to date. Here, we screened aminopyrazole-based analogs as potential Cdk5 inhibitors and identified a novel analog, 25-106, as a uniquely brain-penetrant anti-Cdk5 drug. We characterize the pharmacokinetic and dynamic responses of 25-106 in mice and functionally validate the effects of Cdk5 inhibition on open field and tail-suspension behaviors. Altogether, 25-106 represents a promising preclinical Cdk5 inhibitor that can be systemically administered with significant potential as a neurological/neuropsychiatric therapeutic.

RORγt-Expressing Pathogenic CD4+ T Cells Cause Brain Inflammation during Chronic Colitis

Neurobehavioral disorders and brain abnormalities have been extensively reported in both Crohn's disease and ulcerative colitis patients. However, the mechanism causing neuropathological disorders in inflammatory bowel disease patients remains unknown. Studies have linked the Th17 subset of CD4⁺ T cells to brain diseases associated with neuroinflammation and cognitive impairment, including multiple sclerosis, ischemic brain injury, and Alzheimer's disease. To better understand how CD4⁺ T lymphocytes contribute to brain pathology in chronic intestinal inflammation, we investigated the development of brain inflammation in the T cell transfer model of chronic colitis. Our findings demonstrate that CD4⁺ T cells infiltrate the brain of colitic Rag1 ^-/- mice in proportional levels to colitis severity. Colitic mice developed hypothalamic astrogliosis that correlated with neurobehavioral disorders. Moreover, the brain-infiltrating CD4⁺ T cells expressed Th17 cell transcription factor retinoic acid-related orphan receptor γt (RORγt) and displayed a pathogenic Th17 cellular phenotype similar to colonic Th17 cells. Adoptive transfer of RORγt-deficient naive CD4⁺ T cells failed to cause brain inflammation and neurobehavioral disorders in Rag1 ^-/- recipients, with significantly less brain infiltration of CD4⁺ T cells. The finding is mirrored in chronic dextran sulfate sodium-induced colitis in Rorc^fl/fl Cd4-Cre mice that showed lower frequency of brain-infiltrating CD4⁺ T cells and astrogliosis despite onset of significantly more severe colitis compared with wild-type mice. These findings suggest that pathogenic RORγt⁺CD4⁺ T cells that aggravate colitis migrate preferentially into the brain, contributing to brain inflammation and neurobehavioral disorders, thereby linking colitis severity to neuroinflammation.

Neuropathological Effects of Chemotherapeutic Drugs

Novel treatments, screening, and detection methods have prolonged the lives of numerous cancer patients worldwide. Unfortunately, existing and many promising new chemotherapeutics can cause deleterious, off-target side effects in normal tissue and organ systems. The central and peripheral nervous systems are widely recognized as frequent off-target effectors of anticancer drugs which can produce persistent neurological and neuropsychiatric symptoms collectively termed "chemobrain". Following chemotherapy, patients report several forms of cognitive impairment occurring acutely and sometimes persisting years after treatment. There are no effective treatments for cognitive decline induced by chemotherapeutics, and the underlying molecular mechanisms are poorly characterized and understood. In this study, we find that chronic treatment with two common chemotherapeutic agents, cisplatin and gemcitabine, impairs brain region-specific metabolism, hippocampus-dependent memory formation, and stress response behavior. This corresponds to reduced hippocampal synaptic excitability, altered neuronal signal transduction, and neuroinflammation. These findings underline that a better understanding of the basic pathological consequences of chemotherapy-induced cognitive impairment is the first step toward improving cancer treatment survivorship.

Anti-SSTR2 antibody-drug conjugate for neuroendocrine tumor therapy

Neuroendocrine (NE) tumors include a diverse spectrum of hormone-secreting neoplasms that arise from the endocrine and nervous systems. Current chemo- and radio-therapies have marginal curative benefits. The goal of this study was to develop an innovative antibody-drug conjugate (ADC) to effectively treat NE tumors (NETs). First, we confirmed that somatostatin receptor 2 (SSTR2) is an ideal cancer cell surface target by analyzing 38 patient-derived NET tissues, 33 normal organs, and three NET cell lines. Then, we developed a new monoclonal antibody (mAb, IgG1, and kappa) to target two extracellular domains of SSTR2, which showed strong and specific surface binding to NETs. The ADC was constructed by conjugating the anti-SSTR2 mAb and antimitotic monomethyl auristatin E. In vitro evaluations indicated that the ADC can effectively bind, internalize, release payload, and kill NET cells. Finally, the ADC was evaluated in vivo using a NET xenograft mouse model to assess cancer-specific targeting, tolerated dosage, pharmacokinetics, and antitumor efficacy. The anti-SSTR2 ADC exclusively targeted and killed NET cells with minimal toxicity and high stability in vivo. This study demonstrates that the anti-SSTR2 ADC has a high-therapeutic potential for NET therapy.

Gamma Secretase Inhibitors in Cancer: A Current Perspective on Clinical Performance

Gamma secretase inhibitors (GSIs), initially developed as Alzheimer's therapies, have been repurposed as anticancer agents given their inhibition of Notch receptor cleavage. The success of GSIs in preclinical models has been ascribed to induction of cancer stem-like cell differentiation and apoptosis, while also impairing epithelial-to-mesenchymal transition and sensitizing cells to traditional chemoradiotherapies. The promise of these agents has yet to be realized in the clinic, however, as GSIs have failed to demonstrate clinical benefit in most solid tumors with the notable exceptions of CNS malignancies and desmoid tumors. Disappointing clinical performance to date reflects important questions that remain to be answered. For example, what is the net impact of these agents on antitumor immune responses, and will they require concurrent targeting of tumor-intrinsic compensatory pathways? Addressing these limitations in our current understanding of GSI mechanisms will undoubtedly facilitate their rational incorporation into combinatorial strategies and provide a valuable tool with which to combat Notch-dependent cancers. In the present review, we provide a current understanding of GSI mechanisms, discuss clinical performance to date, and suggest areas for future investigation that might maximize the utility of these agents. IMPLICATIONS FOR PRACTICE: The performance of gamma secretase inhibitors (GSIs) in clinical trials generally has not reflected their encouraging performance in preclinical studies. This review provides a current perspective on the clinical performance of GSIs across various solid tumor types alongside putative mechanisms of antitumor activity. Through exploration of outstanding gaps in knowledge as well as reasons for success in certain cancer types, the authors identify areas for future investigation that will likely enable incorporation of GSIs into rational combinatorial strategies for superior tumor control and patient outcomes.

Management of Gastrointestinal Neuroendocrine Tumors

Neuroendocrine neoplasms (NENs) are derived from neuroendocrine cell system and can have benign or malignant characteristics. They are rare tumors, but have been increasing in incidence over the past 40 years. Patients with NENs may develop symptoms due to primary tumor invasion, metastasis, or from secretion of hormonally active tumor substances. Multiple imaging modalities are used for diagnosis and staging, including specialty scans such as ¹¹¹In pentetreotide (Octreoscan) and ⁶⁸Gallium-DOTATATE, along with endoscopy, endoscopic ultrasound, and biochemical marker testing. Treatment involves both surgical approach, for both primary and metastatic lesions, as well as medical management for symptom management and disease progression. This article will review the current clinical knowledge regarding the diagnosis, treatment, and prognosis of these fascinating neoplasms and the associated hormonal syndromes.

Preclinical characterization of tyrosine kinase inhibitor-based targeted therapies for neuroendocrine thyroid cancer

Medullary thyroid carcinoma (MTC) is a slow growing neuroendocrine (NE) tumor for which few treatment options are available. Its incidence is rising and mortality rates have remained unchanged for decades. Increasing the repertoire of available treatments is thus crucial to manage MTC progression. Scarcity of patient samples and of relevant animal models are two challenges that have limited the development of effective non-surgical treatments. Here we use a clinically accurate mouse model of MTC to assess the effects and mode of action of the tyrosine kinase inhibitor (TKI) Vandetanib, one of only two drugs currently available to treat MTC. Effects on tumor progression, histopathology, and tumorigenic signaling were evaluated. Vandetanib blocked MTC growth through an anti-angiogenic mechanism. Furthermore, Vandetanib had an apparent anti-angiogenic effect in a patient MTC sample. Vandetanib displayed minimal anti-proliferative effects in vivo and in human and mouse MTC tumor-derived cells. Based on these results, we evaluated the second-generation TKI, Nintedanib, alone and in combination with the histone deacetylase (HDAC) inhibitor, Romidepsin, as potential alternative treatments to Vandetanib. Nintedanib showed an anti-angiogenic effect while Romidepsin decreased proliferation. Mechanistically, TKIs attenuated RET-, VEGFR2- and PI3K/AKT/FOXO signaling cascades. Nintedanib alone or in combination with Romidepsin, but not Vandetanib, inhibited mTOR signaling suggesting Nintedanib may have broader anti-cancer applicability. These findings validate the MTC mouse model as a clinically relevant platform for preclinical drug testing and reveal the modes of action and limitations of TKI therapies.

Exposure to mild blast forces induces neuropathological effects, neurophysiological deficits and biochemical changes

Direct or indirect exposure to an explosion can induce traumatic brain injury (TBI) of various severity levels. Primary TBI from blast exposure is commonly characterized by internal injuries, such as vascular damage, neuronal injury, and contusion, without external injuries. Current animal models of blast-induced TBI (bTBI) have helped to understand the deleterious effects of moderate to severe blast forces. However, the neurological effects of mild blast forces remain poorly characterized. Here, we investigated the effects caused by mild blast forces combining neuropathological, histological, biochemical and neurophysiological analysis. For this purpose, we employed a rodent blast TBI model with blast forces below the level that causes macroscopic neuropathological changes. We found that mild blast forces induced neuroinflammation in cerebral cortex, striatum and hippocampus. Moreover, mild blast triggered microvascular damage and axonal injury. Furthermore, mild blast caused deficits in hippocampal short-term plasticity and synaptic excitability, but no impairments in long-term potentiation. Finally, mild blast exposure induced proteolytic cleavage of spectrin and the cyclin-dependent kinase 5 activator, p35 in hippocampus. Together, these findings show that mild blast forces can cause aberrant neurological changes that critically impact neuronal functions. These results are consistent with the idea that mild blast forces may induce subclinical pathophysiological changes that may contribute to neurological and psychiatric disorders.

Biomimetic nanoparticles with enhanced affinity towards activated endothelium as versatile tools for theranostic drug delivery

Activation of the vascular endothelium is characterized by increased expression of vascular adhesion molecules and chemokines. This activation occurs early in the progression of several diseases and triggers the recruitment of leukocytes. Inspired by the tropism of leukocytes, we investigated leukocyte-based biomimetic nanoparticles (i.e., leukosomes) as a novel theranostic platform for inflammatory diseases.

Methods: Leukosomes were assembled by combining phospholipids and membrane proteins from leukocytes. For imaging applications, phospholipids modified with rhodamine and gadolinium were used. Leukosomes incubated with antibodies blocking lymphocyte function-associated antigen 1 (LFA-1) and CD45 were administered to explore their roles in targeting inflammation. In addition, relaxometric assessment of NPs was evaluated.

Results: Liposomes and leukosomes were both spherical in shape with sizes ranging from 140-170 nm. Both NPs successfully integrated 8 and 13 µg of rhodamine and gadolinium, respectively, and demonstrated less than 4% variation in physicochemical features. Leukosomes demonstrated a 16-fold increase in breast tumor accumulation relative to liposomes. Furthermore, quantification of leukosomes in tumor vessels demonstrated a 4.5-fold increase in vessel lumens and a 14-fold increase in vessel walls. Investigating the targeting mechanism of action revealed that blockage of LFA-1 on leukosomes resulted in a 95% decrease in tumor accumulation. Whereas blockage of CD45 yielded a 60% decrease in targeting and significant increases in liver and spleen accumulation. In addition, when administered in mice with atherosclerotic plaques, leukosomes exhibited a 4-fold increase in the targeting of inflammatory vascular lesions. Lastly, relaxometric assessment of NPs demonstrated that the incorporation of membrane proteins into leukosomes did not impact the r₁ and r₂ relaxivities of the NPs, demonstrating 6 and 30 mM^-1s^-1, respectively.

Conclusion: Our study demonstrates the ability of leukosomes to target activated vasculature and exhibit superior accumulation in tumors and vascular lesions. The versatility of the phospholipid backbone within leukosomes permits the incorporation of various contrast agents. Furthermore, leukosomes can potentially be loaded with therapeutics possessing diverse physical properties and thus warrant further investigation toward the development of powerful theranostic agents.

Inhibition of endothelial Cdk5 reduces tumor growth by promoting non-productive angiogenesis

Therapeutic success of VEGF-based anti-angiogenic tumor therapy is limited due to resistance. Thus, new strategies for anti-angiogenic cancer therapy based on novel targets are urgently required. Our previous in vitro work suggested that small molecule Cdk5 inhibitors affect angiogenic processes such as endothelial migration and proliferation. Moreover, we recently uncovered a substantial role of Cdk5 in the development of lymphatic vessels. Here we pin down the in vivo impact of endothelial Cdk5 inhibition in angiogenesis and elucidate the underlying mechanism in order to judge the potential of Cdk5 as a novel anti-angiogenic and anti-cancer target. By the use of endothelial-specific Cdk5 knockout mouse models and various endothelial and tumor cell based assays including human tumor xenograft models, we show that endothelial-specific knockdown of Cdk5 results in excessive but non-productive angiogenesis during development but also in tumors, which subsequently leads to inhibition of tumor growth. As Cdk5 inhibition disrupted Notch function by reducing the generation of the active Notch intracellular domain (NICD) and Cdk5 modulates Notch-dependent endothelial cell proliferation and sprouting, we propose that the Dll4/Notch driven angiogenic signaling hub is an important and promising mechanistic target of Cdk5. In fact, Cdk5 inhibition can sensitize tumors to conventional anti-angiogenic treatment as shown in tumor xenograft models. In summary our data set the stage for Cdk5 as a drugable target to inhibit Notch-driven angiogenesis condensing the view that Cdk5 is a promising target for cancer therapy.

Involvement of aberrant cyclin-dependent kinase 5/p25 activity in experimental traumatic brain injury

Traumatic brain injury (TBI) is associated with adverse effects on brain functions, including sensation, language, emotions and/or cognition. Therapies for improving outcomes following TBI are limited. A better understanding of the pathophysiological mechanisms of TBI may suggest novel treatment strategies to facilitate recovery and improve treatment outcome. Aberrant activation of cyclin-dependent kinase 5 (Cdk5) has been implicated in neuronal injury and neurodegeneration. Cdk5 is a neuronal protein kinase activated via interaction with its cofactor p35 that regulates numerous neuronal functions, including synaptic remodeling and cognition. However, conversion of p35 to p25 via Ca(2+) -dependent activation of calpain results in an aberrantly active Cdk5/p25 complex that is associated with neuronal damage and cell death. Here, we show that mice subjected to controlled cortical impact (CCI), a well-established experimental TBI model, exhibit increased p25 levels and consistently elevated Cdk5-dependent phosphorylation of microtubule-associated protein tau and retinoblastoma (Rb) protein in hippocampal lysates. Moreover, CCI-induced neuroinflammation as indicated by increased astrocytic activation and number of reactive microglia. Brain-wide conditional Cdk5 knockout mice (Cdk5 cKO) subjected to CCI exhibited significantly reduced edema, ventricular dilation, and injury area. Finally, neurophysiological recordings revealed that CCI attenuated excitatory post-synaptic potential field responses in the hippocampal CA3-CA1 pathway 24 h after injury. This neurophysiological deficit was attenuated in Cdk5 cKO mice. Thus, TBI induces increased levels of p25 generation and aberrant Cdk5 activity, which contributes to pathophysiological processes underlying TBI progression. Hence, selectively preventing aberrant Cdk5 activity may be an effective acute strategy to improve recovery from TBI. Traumatic brain injury (TBI) increases astrogliosis and microglial activation. Moreover, TBI deregulates Ca(2+) -homeostasis triggering p25 production. The protein kinase Cdk5 is aberrantly activated by p25 leading to phosphorylation of substrates including tau and Rb protein. Loss of Cdk5 attenuates TBI lesion size, indicating that Cdk5 is a critical player in TBI pathogenesis and thus may be a suitable therapeutic target for TBI.

Differential expression of cell cycle regulators in CDK5-dependent medullary thyroid carcinoma tumorigenesis

Medullary thyroid carcinoma (MTC) is a neuroendocrine cancer of thyroid C-cells, for which few treatment options are available. We have recently reported a role for cyclin-dependent kinase 5 (CDK5) in MTC pathogenesis. We have generated a mouse model, in which MTC proliferation is induced upon conditional overexpression of the CDK5 activator, p25, in C-cells, and arrested by interrupting p25 overexpression. Here, we identify genes and proteins that are differentially expressed in proliferating versus arrested benign mouse MTC. We find that downstream target genes of the tumor suppressor, retinoblastoma protein, including genes encoding cell cycle regulators such as CDKs, cyclins and CDK inhibitors, are significantly upregulated in malignant mouse tumors in a CDK5-dependent manner. Reducing CDK5 activity in human MTC cells down-regulated these cell cycle regulators suggesting that CDK5 activity is critical for cell cycle progression and MTC proliferation. Finally, the same set of cell cycle proteins was consistently overexpressed in human sporadic MTC but not in hereditary MTC. Together these findings suggest that aberrant CDK5 activity precedes cell cycle initiation and thus may function as a tumor-promoting factor facilitating cell cycle protein expression in MTC. Targeting aberrant CDK5 or its downstream effectors may be a strategy to halt MTC tumorigenesis.

The role of ventral striatal cAMP signaling in stress-induced behaviors

The cAMP and cAMP-dependent protein kinase A (PKA) signaling cascade is a ubiquitous pathway acting downstream of multiple neuromodulators. We found that the phosphorylation of phosphodiesterase-4 (PDE4) by cyclin-dependent protein kinase 5 (Cdk5) facilitated cAMP degradation and homeostasis of cAMP/PKA signaling. In mice, loss of Cdk5 throughout the forebrain elevated cAMP levels and increased PKA activity in striatal neurons, and altered behavioral responses to acute or chronic stressors. Ventral striatum- or D1 dopamine receptor-specific conditional knockout of Cdk5, or ventral striatum infusion of a small interfering peptide that selectively targeted the regulation of PDE4 by Cdk5, produced analogous effects on stress-induced behavioral responses. Together, our results demonstrate that altering cAMP signaling in medium spiny neurons of the ventral striatum can effectively modulate stress-induced behavioral states. We propose that targeting the Cdk5 regulation of PDE4 could be a new therapeutic approach for clinical conditions associated with stress, such as depression.

Regulation of ERK Kinase by MEK1 Kinase Inhibition in the Brain

Metabotropic (slow) and ionotropic (fast) neurotransmission are integrated by intracellular signal transduction mechanisms involving protein phosphorylation/dephosphorylation to achieve experience-dependent alterations in brain circuitry. ERK is an important effector of both slow and fast forms of neurotransmission and has been implicated in normal brain function and CNS diseases. Here we characterize phosphorylation of the ERK-activating protein kinase MEK1 by Cdk5, ERK, and Cdk1 in vitro in intact mouse brain tissue and in the context of an animal behavioral paradigm of stress. Cdk5 only phosphorylates Thr-292, whereas ERK and Cdk1 phosphorylate both Thr-292 and Thr-286 MEK1. These sites interact in a kinase-specific manner and inhibit the ability of MEK1 to activate ERK. Thr-292 and Thr-286 MEK1 are phosphorylated in most mouse brain regions to stoichiometries of ~5% or less. Phosphorylation of Thr-292 MEK1 is regulated by cAMP-dependent signaling in mouse striatum in a manner consistent with negative feedback inhibition in response to ERK activation. Protein phosphatase 1 and 2A contribute to the maintenance of the basal phosphorylation state of both Thr-292 and Thr-286 MEK1 and that of ERK. Activation of the NMDA class of ionotropic glutamate receptors reduces inhibitory MEK1 phosphorylation, whereas forced swim, a paradigm of acute stress, attenuates Thr-292 MEK1 phosphorylation. Together, the data indicate that these inhibitory MEK1 sites phosphorylated by Cdk5 and ERK1 serve as mechanistic points of convergence for the regulation of ERK signaling by both slow and fast neurotransmission.

Abstract 3923: Molecular characterization of an animal model for neuroendocrine thyroid cancer

Neuroendocrine tumors are rare forms of cancers originating from hormone-producing cells that are scattered throughout the body. Medullary thyroid carcinoma (MTC) develops from calcitonin-secreting, C cells, in the thyroid gland. Surgical removal of the thyroid is the best treatment currently available for early stage cancer patients but advanced forms of MTC are often incurable and fatal. Thus, better therapies are needed. A lack of relevant animal models for this disease has slowed the development of new treatments. We have recently reported a novel conditional mouse model for MTC. In these mice, overexpression of the cyclin-dependent kinase 5 (Cdk 5), activator, p25 in C cells induces MTC tumors. Furthermore, interruption of p25 overexpression results in growth arrest of these tumors. Here we take advantage of our conditional mouse model to investigate the molecular pathways underlying MTC tumorigenesis. We used Illumina Mouse WG-6 v2.0 Expression BeadChip to compare the gene expression profile of proliferating versus arrested mouse MTC tumors. We found that a total of 920 genes encoding transcription factors, cell cycle proteins, motor proteins and angiogenesis factors were differentially regulated in growing versus arrested tumors. Particularly the genes encoding cell cycle proteins that are involved in the regulation of Rb pathway were significantly up-regulated in the proliferating tumors. Selected genes with higher degrees of change were evaluated by quantitative PCR and immunoblotting analyses. Taken together these findings suggests that the Rb signaling pathways is involved in MTC tumorigenesis and that Cdk5 may act as a critical regulator of the Rb signaling pathway in this cancer. Thus, targeting Cdk5 and/or the Rb pathway may be a valid strategy to stop the progression of neuroendocrine thyroid cancer.

Citation Format: Karine Pozo, Antje Hillmann, Alexander Augustyn, Tanvir Singh, Florian Plattner, John Minna, Herbert Chen, Gilbert J. Cote, James A. Bibb, Fiemu E. Nwariaku. Molecular characterization of an animal model for neuroendocrine thyroid cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3923. doi:10.1158/1538-7445.AM2014-3923

Memory enhancement by targeting Cdk5 regulation of NR2B

Many psychiatric and neurological disorders are characterized by learning and memory deficits, for which cognitive enhancement is considered a valid treatment strategy. The N-methyl-D-aspartate receptor (NMDAR) is a prime target for the development of cognitive enhancers because of its fundamental role in learning and memory. In particular, the NMDAR subunit NR2B improves synaptic plasticity and memory when overexpressed in neurons. However, NR2B regulation is not well understood and no therapies potentiating NMDAR function have been developed. Here, we show that serine 1116 of NR2B is phosphorylated by cyclin-dependent kinase 5 (Cdk5). Cdk5-dependent NR2B phosphorylation is regulated by neuronal activity and controls the receptor's cell surface expression. Disrupting NR2B-Cdk5 interaction via a small interfering peptide (siP) increases NR2B surface levels, facilitates synaptic transmission, and improves memory formation in vivo. Our results reveal a regulatory mechanism critical to NR2B function that can be targeted for the development of cognitive enhancers.

VIDEO ABSTRACT

p38α regulates SERCA2a function

cAMP-dependent protein kinase (PKA) regulates the L-type calcium channel, the ryanodine receptor, and phospholamban (PLB) thereby increasing inotropy. Cardiac contractility is also regulated by p38 MAPK, which is a negative regulator of cardiac contractile function. The aim of this study was to identify the mechanism mediating the positive inotropic effect of p38 inhibition. Isolated adult and neonatal cardiomyocytes and perfused rat hearts were utilized to investigate the molecular mechanisms regulated by p38. PLB phosphorylation was enhanced in cardiomyocytes by chemical p38 inhibition, by overexpression of dominant negative p38α and by p38α RNAi, but not with dominant negative p38β. Treatment of cardiomyocytes with dominant negative p38α significantly decreased Ca(2+)-transient decay time indicating enhanced sarco/endoplasmic reticulum Ca(2+)-ATPase function and increased cardiomyocyte contractility. Analysis of signaling mechanisms involved showed that inhibition of p38 decreased the activity of protein phosphatase 2A, which renders protein phosphatase inhibitor-1 phosphorylated and thereby inhibits PP1. In conclusion, inhibition of p38α enhances PLB phosphorylation and diastolic Ca(2+) uptake. Our findings provide evidence for novel mechanism regulating cardiac contractility upon p38 inhibition.

The role of Cdk5 in neuroendocrine thyroid cancer

Medullary thyroid carcinoma (MTC) is a neuroendocrine cancer that originates from calcitonin-secreting parafollicular cells, or C cells. We found that Cdk5 and its cofactors p35 and p25 are highly expressed in human MTC and that Cdk5 activity promotes MTC proliferation. A conditional MTC mouse model was generated and corroborated the role of aberrant Cdk5 activation in MTC. C cell-specific overexpression of p25 caused rapid C cell hyperplasia leading to lethal MTC, which was arrested by repressing p25 overexpression. A comparative phosphoproteomic screen between proliferating and arrested MTC identified the retinoblastoma protein (Rb) as a crucial Cdk5 downstream target. Prevention of Rb phosphorylation at Ser807/Ser811 attenuated MTC proliferation. These findings implicate Cdk5 signaling via Rb as critical to MTC tumorigenesis and progression.

Enhancement of neuromuscular dynamics and strength behavior using extremely low magnitude mechanical signals in mice

Exercise in general, and mechanical signals in particular, help ameliorate the neuromuscular symptoms of aging and possibly other neurodegenerative disorders by enhancing muscle function. To better understand the salutary mechanisms of such physical stimuli, we evaluated the potential for low intensity mechanical signals to promote enhanced muscle dynamics. The effects of daily brief periods of low intensity vibration (LIV) on neuromuscular functions and behavioral correlates were assessed in mice. Physiological analysis revealed that LIV increased isometric force production in semitendinosus skeletal muscle. This effect was evident in both young and old mice. Isometric force recordings also showed that LIV reduced the fatiguing effects of intensive synaptic muscle stimulation. Furthermore, LIV increased evoked neurotransmitter release at neuromuscular synapses but had no effect on spontaneous end plate potential amplitude or frequency. In behavioral studies, LIV increased mouse grip strength and potentiated initial motor activity in a novel environment. These results provide evidence for the efficacy of LIV in producing changes in the neuromuscular system that translate into performance gains at a behavioral scale.

Early postnatal in vivo gliogenesis from nestin-lineage progenitors requires cdk5

The early postnatal period is a unique time of brain development, as diminishing amounts of neurogenesis coexist with waves of gliogenesis. Understanding the molecular regulation of early postnatal gliogenesis may provide clues to normal and pathological embryonic brain ontogeny, particularly in regards to the development of astrocytes and oligodendrocytes. Cyclin dependent kinase 5 (Cdk5) contributes to neuronal migration and cell cycle control during embryogenesis, and to the differentiation of neurons and oligodendrocytes during adulthood. However, Cdk5's function in the postnatal period and within discrete progenitor lineages is unknown. Therefore, we selectively removed Cdk5 from nestin-expressing cells and their progeny by giving transgenic mice (nestin-CreERT2/R26R-YFP/CDK5(flox/flox) [iCdk5] and nestin-CreERT2/R26R-YFP/CDK5(wt/wt) [WT]) tamoxifen during postnatal (P) days P2-P 4 or P7-P 9, and quantified and phenotyped recombined (YFP+) cells at P14 and P21. When Cdk5 gene deletion was induced in nestin-expressing cells and their progeny during the wave of cortical and hippocampal gliogenesis (P2-P4), significantly fewer YFP+ cells were evident in the cortex, corpus callosum, and hippocampus. Phenotypic analysis revealed the cortical decrease was due to fewer YFP+ astrocytes and oligodendrocytes, with a slightly earlier influence seen in oligodendrocytes vs. astrocytes. This effect on cortical gliogenesis was accompanied by a decrease in YFP+ proliferative cells, but not increased cell death. The role of Cdk5 in gliogenesis appeared specific to the early postnatal period, as induction of recombination at a later postnatal period (P7-P9) resulted in no change YFP+ cell number in the cortex or hippocampus. Thus, glial cells that originate from nestin-expressing cells and their progeny require Cdk5 for proper development during the early postnatal period.

p39, the primary activator for cyclin-dependent kinase 5 (Cdk5) in oligodendroglia, is essential for oligodendroglia differentiation and myelin repair

Cyclin-dependent kinase 5 (Cdk5) plays key roles in normal brain development and function. Dysregulation of Cdk5 may cause neurodegeneration and cognitive impairment. Besides the well demonstrated role of Cdk5 in neurons, emerging evidence suggests the functional requirement of Cdk5 in oligodendroglia (OL) and CNS myelin development. However, whether neurons and OLs employ similar or distinct mechanisms to regulate Cdk5 activity remains elusive. We report here that in contrast to neurons that harbor high levels of two Cdk5 activators, p35 and p39, OLs express abundant p39 but negligible p35. In addition, p39 is selectively up-regulated in OLs during differentiation along with elevated Cdk5 activity, whereas p35 expression remains unaltered. Specific knockdown of p39 by siRNA significantly attenuates Cdk5 activity and OL differentiation without affecting p35. Finally, expression of p39, but not p35, is increased during myelin repair, and remyelination is impaired in p39(-/-) mice. Together, these results reveal that neurons and OLs harbor distinct preference of Cdk5 activators and demonstrate important functions of p39-dependent Cdk5 activation in OL differentiation during de novo myelin development and myelin repair.

Cyclin dependent kinase 5 is required for the normal development of oligodendrocytes and myelin formation

The development of oligodendrocytes, the myelinating cells of the vertebrate CNS, is regulated by a cohort of growth factors and transcription factors. Less is known about the signaling pathways that integrate extracellular signals with intracellular transcriptional regulators to control oligodendrocyte development. Cyclin dependent kinase 5 (Cdk5) and its co-activators play critical roles in the regulation of neuronal differentiation, cortical lamination, neuronal cell migration and axon outgrowth. Here we demonstrate a previously unrecognized function of Cdk5 in regulating oligodendrocyte maturation and myelination. During late embryonic development Cdk5 null animals displayed a reduction in the number of MBP+ cells in the spinal cord, but no difference in the number of OPCs. To determine whether the reduction of oligodendrocytes reflected a cell-intrinsic loss of Cdk5, it was selectively deleted from Olig1+ oligodendrocyte lineage cells. In Olig1(Cre/+); Cdk5(fl/fl) conditional mutants, reduced levels of expression of MBP and PLP mRNA were observed throughout the CNS and ultrastructural analyses demonstrated a significant reduction in the proportion of myelinated axons in the optic nerve and spinal cord. Pharmacological inhibition or RNAi knockdown of Cdk5 in vitro resulted in the reduction in oligodendrocyte maturation, but had no effect on OPC cell proliferation. Conversely, over-expression of Cdk5 promoted oligodendrocyte maturation and enhanced process outgrowth. Consistent with this data, Cdk5(-/-) oligodendrocytes developed significantly fewer primary processes and branches than control cells. Together, these findings suggest that Cdk5 function as a signaling integrator to regulate oligodendrocyte maturation and myelination.

A mutation in CLOCK leads to altered dopamine receptor function

Mice with a mutation in the Clock gene (ClockΔ19) have a number of behavioral phenotypes that suggest alterations in dopaminergic transmission. These include hyperactivity, increased exploratory behavior, and increased reward value for drugs of abuse. However, the complex changes in dopaminergic transmission that underlie the behavioral abnormalities in these mice remain unclear. Here we find that a loss of CLOCK function increases dopamine release and turnover in striatum as indicated by increased levels of metabolites HVA and DOPAC, and enhances sensitivity to dopamine receptor antagonists. Interestingly, this enlarged dopaminergic tone results in downstream changes in dopamine receptor (DR) levels with a surprising augmentation of both D1- and D2-type DR protein, but a significant shift in the ratio of D1 : D2 receptors in favor of D2 receptor signaling. These effects have functional consequences for both behavior and intracellular signaling, with alterations in locomotor responses to both D1-type and D2-type specific agonists and a blunted response to cAMP activation in the ClockΔ19 mutants. Taken together, these studies further elucidate the abnormalities in dopaminergic transmission that underlie mood, activity, and addictive behaviors.

Organophosphates dysregulate dopamine signaling, glutamatergic neurotransmission, and induce neuronal injury markers in striatum

The neurological effects of organophosphate (OP) pesticides, commonly used on foods and in households, are an important public health concern. Furthermore, subclinical exposure to combinations of organophosphates is implicated in Gulf War illness. Here, we characterized the effects of the broadly used insecticide chlorpyrifos (CPF) on dopamine and glutamatergic neurotransmission effectors in corticostriatal motor/reward circuitry. CPF potentiated protein kinase A (PKA)-dependent phosphorylation of the striatal protein dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) and the glutamate receptor 1 (GluR1) subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in mouse brain slices. It also increased GluR1 phosphorylation by PKA when administered systemically. This correlated with enhanced glutamate release from cortical projections in rat striatum. Similar effects were induced by the sarin congener, diisopropyl fluorophosphate, alone or in combination with the putative neuroprotectant, pyridostigmine bromide and the pesticide N,N-diethyl-meta-toluamide (DEET). This combination, meant to mimic the neurotoxicant exposure encountered by veterans of the 1991 Persian Gulf War, also induced hyperphosphorylation of the neurofibrillary tangle-associated protein tau. Diisopropyl fluorophosphate and pyrodostigmine bromide, alone or in combination, also increased the aberrant activity of the protein kinase, Cdk5, as indicated by conversion of its activating cofactor p35 to p25. Thus, consistent with recent findings in humans and animals, organophosphate exposure causes dysregulation in the motor/reward circuitry and invokes mechanisms associated with neurological disorders and neurodegeneration.

Striatal signal transduction and drug addiction

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca²⁺ and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

The role of Cdk5 in cognition and neuropsychiatric and neurological pathology

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that is ubiquitous in the nervous system and interacts with a myriad of substrates. Its modulation of synaptic plasticity and associated mechanisms of learning and memory as well as neurodegeneration and cognitive disease highlights its importance in the human brain. Cdk5 is active throughout the neuron via its kinase activity, protein-protein interactions, and nuclear associations. It regulates functions thought vital to memory and plasticity, including synaptic vesicle recycling, dendritic spine formation, neurotransmitter receptor density, and neuronal excitability. Although conditional knockout of Cdk5 improves learning and plasticity, the associated deleterious effects of increased excitability cast doubts on the therapeutic efficacy of systemic inhibitors. However, through further work on the regulation of Cdk5 and its effectors, this important molecule promises to aid in elucidating key pathways involved in learning and memory and uncover innovative therapeutic targets to treat neurodegenerative and neuropsychiatric diseases.

Sustained hemodynamic stress disrupts normal circadian rhythms in calcineurin-dependent signaling and protein phosphorylation in the heart

RATIONALE

Despite overwhelming evidence of the importance of circadian rhythms in cardiovascular health and disease, little is known regarding the circadian regulation of intracellular signaling pathways controlling cardiac function and remodeling.

OBJECTIVE

To assess circadian changes in processes dependent on the protein phosphatase calcineurin, relative to changes in phosphorylation of cardiac proteins, in normal, hypertrophic, and failing hearts.

METHODS AND RESULTS

We found evidence of large circadian oscillations in calcineurin-dependent activities in the left ventricle of healthy C57BL/6 mice. Calcineurin-dependent transcript levels and nuclear occupancy of the NFAT (nuclear factor of activated T cells) regularly fluctuated as much as 20-fold over the course of a day, peaking in the morning when mice enter a period of rest. Phosphorylation of the protein phosphatase 1 inhibitor 1 (I-1), a direct calcineurin substrate, and phospholamban, an indirect target, oscillated directly out of phase with calcineurin-dependent signaling. Using a surgical model of cardiac pressure overload, we found that although calcineurin-dependent activities were markedly elevated, the circadian pattern of activation was maintained, whereas, oscillations in phospholamban and I-1 phosphorylation were lost. Changes in the expression of fetal gene markers of heart failure did not mirror the rhythm in calcineurin/NFAT activation, suggesting that these may not be direct transcriptional target genes. Cardiac function in mice subjected to pressure overload was significantly lower in the morning than in the evening when assessed by echocardiography.

CONCLUSIONS

Normal, opposing circadian oscillations in calcineurin-dependent activities and phosphorylation of proteins that regulate contractility are disrupted in heart failure.

Attention-deficit/hyperactivity phenotype in mice lacking the cyclin-dependent kinase 5 cofactor p35

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) may result from delayed establishment of corticolimbic circuitry or perturbed dopamine (DA) neurotransmission. Despite the widespread use of stimulants to treat ADHD, little is known regarding their long-term effects on neurotransmitter levels and metabolism. Cyclin-dependent kinase 5 (Cdk5) regulates DA signaling through control of synthesis, postsynaptic responses, and vesicle release. Mice lacking the Cdk5-activating cofactor p35 are deficient in cortical lamination, suggesting altered motor/reward circuitry.

METHODS

We employed mice lacking p35 to study the effect of altered circuitry in vivo. Positron emission tomography measured glucose metabolism in the cerebral cortex using 2-deoxy-2-[¹⁸F] fluoro-d-glucose as the radiotracer. Retrograde dye tracing and tyrosine hydroxylase immunostains assessed the effect of p35 knockout on the medial prefrontal cortex (PFC), especially in relation to mesolimbic circuit formation. We defined the influence of Cdk5/p35 activity on catecholaminergic neurotransmission and motor activity via examination of locomotor responses to psychostimulants, monoamine neurotransmitter levels, and DA signal transduction.

RESULTS

Here, we report that mice deficient in p35 display increased glucose uptake in the cerebral cortex, basal hyperactivity, and paradoxical decreased locomotion in response to chronic injection of cocaine or methylphenidate. Knockout mice also exhibited an increased susceptibility to changes in PFC neurotransmitter content after chronic methylphenidate exposure and altered basal DAergic activity in acute striatal and PFC slices.

CONCLUSIONS

Our findings suggest that dysregulation of Cdk5/p35 activity during development may contribute to ADHD pathology, as indicated by the behavioral phenotype, improperly established mesolimbic circuitry, and aberrations in striatal and PFC catecholaminergic signaling in p35 knockout mice.

Extrasynaptic NMDA receptors couple preferentially to excitotoxicity via calpain-mediated cleavage of STEP

NMDA receptor (NMDAR)-mediated excitotoxicity plays an important role in several CNS disorders, including epilepsy, stroke, and ischemia. Here we demonstrate the involvement of striatal-enriched protein tyrosine phosphatase (STEP) in this critical process. STEP(61) is an alternatively spliced member of the family that is present in postsynaptic terminals. In an apparent paradox, STEP(61) regulates extracellular signal-regulated kinase 1/2 (ERK1/2) and p38, two proteins with opposing functions; activated p38 promotes cell death, whereas activated ERK1/2 promotes cell survival. We found that synaptic stimulation of NMDARs promoted STEP(61) ubiquitination and degradation, concomitant with ERK1/2 activation. In contrast, extrasynaptic stimulation of NMDARs invoked calpain-mediated proteolysis of STEP(61), producing the truncated cleavage product STEP(33) and activation of p38. The calpain cleavage site on STEP was mapped to the kinase interacting motif, a domain required for substrate binding. As a result, STEP(33) neither interacts with nor dephosphorylates STEP substrates. A synthetic peptide spanning the calpain cleavage site efficiently reduced STEP(61) degradation and attenuated p38 activation and cell death in slice models. Furthermore, this peptide was neuroprotective when neurons were subjected to excitotoxicity or cortical slices were exposed to ischemic conditions. These findings suggest a novel mechanism by which differential NMDAR stimulation regulates STEP(61) to promote either ERK1/2 or p38 activation and identifies calpain cleavage of STEP(61) as a valid target for the development of neuroprotective therapy.

Regulation of hippocampal and behavioral excitability by cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that has been implicated in learning, synaptic plasticity, neurotransmission, and numerous neurological disorders. We previously showed that conditional loss of Cdk5 in adult mice enhanced hippocampal learning and plasticity via modulation of calpain-mediated N-methyl-D-aspartic acid receptor (NMDAR) degradation. In the present study, we characterize the enhanced synaptic plasticity and examine the effects of long-term Cdk5 loss on hippocampal excitability in adult mice. Field excitatory post-synaptic potentials (fEPSPs) from the Schaffer collateral CA1 subregion of the hippocampus (SC/CA1) reveal that loss of Cdk5 altered theta burst topography and enhanced post-tetanic potentiation. Since Cdk5 governs NMDAR NR2B subunit levels, we investigated the effects of long-term Cdk5 knockout on hippocampal neuronal excitability by measuring NMDAR-mediated fEPSP magnitudes and population-spike thresholds. Long-term loss of Cdk5 led to increased Mg²⁺-sensitive potentials and a lower threshold for epileptiform activity and seizures. Biochemical analyses were performed to better understand the role of Cdk5 in seizures. Induced-seizures in wild-type animals led to elevated amounts of p25, the Cdk5-activating cofactor. Long-term, but not acute, loss of Cdk5 led to decreased p25 levels, suggesting that Cdk5/p25 may be activated as a homeostatic mechanism to attenuate epileptiform activity. These findings indicate that Cdk5 regulates synaptic plasticity, controls neuronal and behavioral stimulus-induced excitability and may be a novel pharmacological target for cognitive and anticonvulsant therapies.

Negative regulation of cyclin-dependent kinase 5 targets by protein kinase C

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed protein serine/threonine kinase essential for brain development and implicated in synaptic plasticity, dopaminergic neurotransmission, drug addiction, and neurodegenerative disorders. Relatively little is known about the molecular mechanisms that regulate the activity of Cdk5 in vivo. In order to determine whether protein kinase C (PKC) regulates Cdk5 activity in the central nervous system, the phosphorylation levels of two Cdk5 substrates were evaluated under conditions of altered PKC activity in vivo. Treatment of acute striatal slices with a PKC-activating phorbol ester caused a time- and dose-dependent decrease in the levels of phospho-Ser6 inhibitor-1, phospho-Ser67 inhibitor-1, and phospho-Thr75 dopamine- and cAMP-regulated phosphoprotein, Mr 32,000 (DARPP-32). This effect was reversed by the PKC inhibitor, Ro-32-0432. Moreover, phospho-Ser6 inhibitor-1, phospho-Ser67 inhibitor-1, and phospho-Thr75 DARPP-32 levels were elevated in brain tissue from mice lacking the gene for PKC-alpha. PKC did not phosphorylate Cdk5 or its cofactor, p25, in vitro. Striatal levels of the Cdk5 cofactor, p35, did not change in response to phorbol ester treatment. Furthermore, Cdk5 immunoprecipitated from striatal slices treated with phorbol ester had unaltered activity toward a control substrate in vitro. These results suggest that PKC exerts its effects on the phosphorylation state of Cdk5 substrates through an indirect mechanism that may involve the regulatory binding partners of Cdk5 other than its neuronal cofactors.

Differential regulation of the Cdk5-dependent phosphorylation sites of inhibitor-1 and DARPP-32 by depolarization

While cyclin-dependent kinase 5 (Cdk5) is of growing importance to neuronal signaling, its regulation remains relatively unexplored. Examination of the mechanism by which NMDA modulates the phosphorylation of protein phosphatase inhibitor-1 at Ser6 and Ser67 and dopamine- and cAMP-regulated phosphoprotein M(r) 32 000 at Thr75 revealed that generalized depolarization, rather than specific activation of NMDA receptors, was sufficient to induce decreases in these Cdk5 sites. Although no evidence for the involvement of the Cdk5 cofactors p35 or p39, or for L- and T-type voltage-gated Ca(2+) channels, was found, evaluation of the role of phosphatases and extracellular cations revealed differential regulation of the three sites. NMDA-induced decreases in the phosphorylation of Thr75 of dopamine- and cAMP-regulated phosphoprotein M(r) 32 000 required protein phosphatase 1/2A activity and extracellular Ca(2+). In contrast, the effects on Ser6 and Ser67 of inhibitor-1 were not cation specific; either Na(+) or Ca(2+) sufficed. Furthermore, while the decrease in phosphorylation of Ser6 was partially dependent on protein phosphatase 2B, that of Ser67 was independent of the major protein serine/threonine phosphatases, likely indicating the presence of a pathway by which NMDA inhibits Cdk5 activity. Thus, in the striatum the regulation of phosphorylation of Cdk5-dependent sites by NMDA occurs through multiple distinct pathways.

Alternative roles for Cdk5 in learning and synaptic plasticity

Protein kinases mediate the intracellular signal transduction pathways controlling synaptic plasticity in the central nervous system. While the majority of protein kinases achieve this function via the phosphorylation of synaptic substrates, some kinases may contribute through alternative mechanisms in addition to enzymatic activity. There is growing evidence that protein kinases may often play structural roles in plasticity as well. Cyclin-dependent kinase 5 (Cdk5) has been implicated in learning and synaptic plasticity. Initial scrutiny focused on its enzymatic activity using pharmacological inhibitors and genetic modifications of Cdk5 cofactors. Quite recently Cdk5 has been shown to govern learning and plasticity via regulation of glutamate receptor degradation, a function that may not dependent on phosphorylation of downstream effectors. From these new studies, two roles emerge for Cdk5 in plasticity: one in which it controls structural plasticity via phosphorylation of synaptic substrates, and a second where it regulates functional plasticity via protein-protein interactions.

Cyclin-dependent kinase 5 governs learning and synaptic plasticity via control of NMDAR degradation

Learning is accompanied by modulation of postsynaptic signal transduction pathways in neurons. Although the neuronal protein kinase cyclin-dependent kinase 5 (Cdk5) has been implicated in cognitive disorders, its role in learning has been obscured by the perinatal lethality of constitutive knockout mice. Here we report that conditional knockout of Cdk5 in the adult mouse brain improved performance in spatial learning tasks and enhanced hippocampal long-term potentiation and NMDA receptor (NMDAR)-mediated excitatory postsynaptic currents. Enhanced synaptic plasticity in Cdk5 knockout mice was attributed to reduced NR2B degradation, which caused elevations in total, surface and synaptic NR2B subunit levels and current through NR2B-containing NMDARs. Cdk5 facilitated the degradation of NR2B by directly interacting with both it and its protease, calpain. These findings reveal a previously unknown mechanism by which Cdk5 facilitates calpain-mediated proteolysis of NR2B and may control synaptic plasticity and learning.

Regulation of protein phosphatase inhibitor-1 by cyclin-dependent kinase 5

Inhibitor-1, the first identified endogenous inhibitor of protein phosphatase 1 (PP-1), was previously reported to be a substrate for cyclin-dependent kinase 5 (Cdk5) at Ser67. Further investigation has revealed the presence of an additional Cdk5 site identified by mass spectrometry and confirmed by site-directed mutagenesis as Ser6. Basal levels of phospho-Ser6 inhibitor-1, as detected by a phosphorylation state-specific antibody against the site, existed in specific regions of the brain and varied with age. In the striatum, basal in vivo phosphorylation and dephosphorylation of Ser6 were mediated by Cdk5, PP-2A, and PP-1, respectively. Additionally, calcineurin contributed to dephosphorylation under conditions of high Ca2+. In biochemical assays the function of Cdk5-dependent phosphorylation of inhibitor-1 at Ser6 and Ser67 was demonstrated to be an intramolecular impairment of the ability of inhibitor-1 to be dephosphorylated at Thr35; this effect was recapitulated in two systems in vivo. Dephosphorylation of inhibitor-1 at Thr35 is equivalent to inactivation of the protein, as inhibitor-1 only serves as an inhibitor of PP-1 when phosphorylated by cAMP-dependent kinase (PKA) at Thr35. Thus, inhibitor-1 serves as a critical junction between kinase- and phosphatase-signaling pathways, linking PP-1 to not only PKA and calcineurin but also Cdk5.

Inhibition of Cdk5 in the nucleus accumbens enhances the locomotor-activating and incentive-motivational effects of cocaine

Neuronal adaptations in striatal dopamine signaling have been implicated in enhanced responses to addictive drugs. Cyclin-dependent kinase 5 (Cdk5) regulates striatal dopamine signaling and is a downstream target gene of the transcription factor DeltaFosB, which accumulates in striatal neurons after chronic cocaine exposure. Here we investigated the role of Cdk5 activity in the nucleus accumbens (NAc) on cocaine-induced locomotor sensitization, responding for reward-associated stimuli (conditioned reinforcement), and cocaine self-administration under a progressive ratio schedule. Repeated infusions of the Cdk5 inhibitor roscovitine into the NAc before cocaine injections augmented both the development and expression of cocaine sensitization without having any intrinsic stimulant actions of its own. Additionally, repeated intra-NAc infusions of roscovitine to saline-injected rats enhanced locomotor responses to a subsequent cocaine challenge. Similar effects were found after infusions of another Cdk5 inhibitor, olomoucine, but not its inactive congener, iso-olomoucine. Repeated inhibition of Cdk5 within the NAc also robustly enhanced the incentive-motivational effects of cocaine, similar to the effect of prior repeated cocaine exposure. The enhanced responding with conditioned reinforcement induced by cocaine persisted at least 2 weeks after the final roscovitine infusion. NAc infusions of olomoucine also produced acute and enduring increases in "breakpoints" achieved on a progressive ratio schedule for cocaine reinforcement. These results demonstrate profound and persistent effects of NAc Cdk5 inhibition on locomotor sensitization and incentive-motivational processes and provide direct evidence for a role for striatal Cdk5-induced alterations in the brain's long-term adaptations to cocaine.

Evaluation of neuronal phosphoproteins as effectors of caffeine and mediators of striatal adenosine A2A receptor signaling

Adenosine A(2A) receptors are predominantly expressed in the dendrites of enkephalin-positive gamma-aminobutyric acidergic medium spiny neurons in the striatum. Evidence indicates that these receptors modulate striatal dopaminergic neurotransmission and regulate motor control, vigilance, alertness, and arousal. Although the physiological and behavioral correlates of adenosine A(2A) receptor signaling have been extensively studied using a combination of pharmacological and genetic tools, relatively little is known about the signal transduction pathways that mediate the diverse biological functions attributed to this adenosine receptor subtype. Using a candidate approach based on the coupling of these receptors to adenylate cyclase-activating G proteins, a number of membranal, cytosolic, and nuclear phosphoproteins regulated by PKA were evaluated as potential mediators of adenosine A(2A) receptor signaling in the striatum. Specifically, the adenosine A(2A) receptor agonist, CGS 21680, was used to determine whether the phosphorylation state of each of the following PKA targets is responsive to adenosine A(2A) receptor stimulation in this tissue: Ser40 of tyrosine hydroxylase, Ser9 of synapsin, Ser897 of the NR1 subunit of the N-methyl-d-aspartate-type glutamate receptor, Ser845 of the GluR1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptor, Ser94 of spinophilin, Thr34 of the dopamine- and cAMP-regulated phosphoprotein, M(r) 32,000, Ser133 of the cAMP-response element-binding protein, Thr286 of Ca(2+)/calmodulin-dependent protein kinase II, and Thr202/Tyr204 and Thr183/Tyr185 of the p44 and p42 isoforms, respectively, of mitogen-activated protein kinase. Although the substrates studied differed considerably in their responsiveness to selective adenosine A(2A) receptor activation, the phosphorylation state of all postsynaptic PKA targets was up-regulated in a time- and dose-dependent manner by treatment with CGS 21680, whereas presynaptic PKA substrates were unresponsive to this agent, consistent with the postsynaptic localization of adenosine A(2A) receptors. Finally, the phosphorylation state of these proteins was further assessed in vivo by systemic administration of caffeine.

Suppression of Calpain-dependent Cleavage of the CDK5 Activator p35 to p25 by Site-specific Phosphorylation

Cdk5 is a proline-directed Ser/Thr protein kinase predominantly expressed in postmitotic neurons together with its activator, p35. N-terminal truncation of p35 to p25 by calpain results in deregulation of Cdk5 and contributes to neuronal cell death associated with several neurodegenerative diseases. Previously we reported that p35 occurred as a phosphoprotein, phospho-p35 levels changed with neuronal maturation, and that phosphorylation of p35 affected its vulnerability to calpain cleavage. Here, we identify the p35 residues Ser(8) and Thr(138) as the major sites of phosphorylation by Cdk5. Mutagenesis of these sites to unphosphorylatable Ala increased susceptibility to calpain in cultured cells and neurons while changing them to phosphomimetic glutamate-attenuated cleavage. Furthermore, phosphorylation state-specific antibodies to these sites revealed that Thr(138) was dephosphorylated in adult rat, although both Ser(8) and Thr(138) were phosphorylated in prenatal brains. In cultured neurons, inhibition of protein phosphatases converted phosho-Ser(8) p35 to dual phospho-Ser(8)/Thr(138) p35 and conferred resistance to calpain cleavage. These results suggest phosphorylation of Thr(138) predominantly defines the susceptibility of p35 to calpain-dependent cleavage and that dephosphorylation of this site is a critical determinant of Cdk5-p25-induced cell death associated with neurodegeneration.

Identification of pyruvate kinase as an antigen associated with Tourette syndrome

Immune responses to beta-hemolytic streptococcal infections are hypothesized to trigger tic disorders and early-onset obsessive-compulsive disorder (OCD) in some pediatric populations. Here we identify the M1 isoform of the glycolytic enzyme, pyruvate kinase (PK) as an autoimmune target in Tourette syndrome and associated disorders. Antibodies to PK reacted strongly with surface antigens of infectious strains of streptococcus, and antibodies to streptococcal M proteins reacted with PK. Moreover, immunoreactivity to PK in patients with exacerbated symptoms who had recently acquired a streptococcal infection was 7-fold higher compared to patients with exacerbated symptoms and no evidence of a streptococcal infection. These data suggest that PK can function as an autoimmune target and that this immunoreactivity may be associated with Tourette syndrome, OCD, and associated disorders.

Phosphorylation of the tubulin-binding protein, stathmin, by Cdk5 and MAP kinases in the brain

Regulation of cytoskeletal dynamics is essential to neuronal plasticity during development and adulthood. Dysregulation of these mechanisms may contribute to neuropsychiatric and neurodegenerative diseases. The neuronal protein kinase, cyclin-dependent kinase 5 (Cdk5), is involved in multiple aspects of neuronal function, including regulation of cytoskeleton. A neuroproteomic search identified the tubulin-binding protein, stathmin, as a novel Cdk5 substrate. Stathmin was phosphorylated by Cdk5 in vitro at Ser25 and Ser38, previously identified as mitogen-activated protein kinase (MAPK) and p38 MAPKdelta sites. Cdk5 predominantly phosphorylated Ser38, while MAPK and p38 MAPKdelta predominantly phosphorylated Ser25. Stathmin was phosphorylated at both sites in mouse brain, with higher levels in cortex and striatum. Cdk5 knockout mice exhibited decreased phospho-Ser38 levels. During development, phospho-Ser25 and -Ser38 levels peaked at post-natal day 7, followed by reduction in total stathmin. Inhibition of protein phosphatases in striatal slices caused an increase in phospho-Ser25 and a decrease in total stathmin. Interestingly, the prefrontal cortex of schizophrenic patients had increased phospho-Ser25 levels. In contrast, total and phospho-Ser25 stoichiometries were decreased in the hippocampus of Alzheimer's patients. Thus, microtubule regulatory mechanisms involving the phosphorylation of stathmin may contribute to developmental synaptic pruning and structural plasticity, and may be involved in neuropsychiatric and neurodegenerative disorders.

Phosphorylation of protein phosphatase inhibitor-1 by protein kinase C

Inhibitor-1 becomes a potent inhibitor of protein phosphatase 1 when phosphorylated by cAMP-dependent protein kinase at Thr(35). Moreover, Ser(67) of inhibitor-1 serves as a substrate for cyclin-dependent kinase 5 in the brain. Here, we report that dephosphoinhibitor-1 but not phospho-Ser(67) inhibitor-1 was efficiently phosphorylated by protein kinase C at Ser(65) in vitro. In contrast, Ser(67) phosphorylation by cyclin-dependent kinase 5 was unaffected by phospho-Ser(65). Protein kinase C activation in striatal tissue resulted in the concomitant phosphorylation of inhibitor-1 at Ser(65) and Ser(67), but not Ser(65) alone. Selective pharmacological inhibition of protein phosphatase activity suggested that phospho-Ser(65) inhibitor-1 is dephosphorylated by protein phosphatase 1 in the striatum. In vitro studies confirmed these findings and suggested that phospho-Ser(67) protects phospho-Ser(65) inhibitor-1 from dephosphorylation by protein phosphatase 1 in vivo. Activation of group I metabotropic glutamate receptors resulted in the up-regulation of diphospho-Ser(65)/Ser(67) inhibitor-1 in this tissue. In contrast, the activation of N-methyl-d-aspartate-type ionotropic glutamate receptors opposed increases in striatal diphospho-Ser(65)/Ser(67) inhibitor-1 levels. Phosphomimetic mutation of Ser(65) and/or Ser(67) did not convert inhibitor-1 into a protein phosphatase 1 inhibitor. On the other hand, in vitro and in vivo studies suggested that diphospho-Ser(65)/Ser(67) inhibitor-1 is a poor substrate for cAMP-dependent protein kinase. These observations extend earlier studies regarding the function of phospho-Ser(67) and underscore the possibility that phosphorylation in this region of inhibitor-1 by multiple protein kinases may serve as an integrative signaling mechanism that governs the responsiveness of inhibitor-1 to cAMP-dependent protein kinase activation.