Cdk5 inhibitory peptide (CIP) inhibits Cdk5/p25 activity induced by high glucose in pancreatic beta cells and recovers insulin secretion from p25 damage

Cyclin-dependent Kinase 5 and Neurodegenerative Diseases

Neurodegenerative diseases are a group of diseases characterized by the progressive loss of neurons, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. These diseases have a high incidence and mortality rate globally, placing a heavy burden on patients and their families. The pathogenesis of neurodegenerative diseases is complex, and there are no effective treatments at present. Cyclin-dependent kinase 5 is a proline-directed serine/threonine protein kinase that is closely related to the development and function of the nervous system. Under physiological conditions, it is involved in regulating the process of neuronal proliferation, differentiation, migration, and synaptic plasticity. Moreover, there is increasing evidence that cyclin-dependent kinase 5 also plays an important role in the pathogenesis of neurodegenerative diseases. In this review, we address the biological characteristics of cyclin-dependent kinase 5 and its role in neurodegenerative diseases. In particular, this review highlights the underlying mechanistic linkages between cyclin-dependent kinase 5 and mitochondrial dysfunction, oxidative stress and neuroinflammation in the context of neurodegeneration. Finally, we also summarize the currently available cyclin-dependent kinase 5 inhibitors and their prospects for the treatment of neurodegenerative diseases. Taken together, a better understanding of the molecular mechanisms of cyclin-dependent kinase 5 involved in neurodegenerative diseases can lead to the development of new strategies for the prevention and treatment of these devastating diseases.

Calcium-binding protein 7 expressed in muscle negatively regulates age-related degeneration of neuromuscular junctions in mice

The neuromuscular junction (NMJ) forms centrally in myotubes and, as the only synapse between motor neuron and myotube, are indispensable for motor activity. The midmuscle formation of NMJs, including midmuscle-restricted expression of NMJ-related genes, is governed by the muscle-specific kinase (MuSK). However, mechanisms underlying MuSK-mediated signaling are unclear. Here, we find that the Calcium-binding protein 7 (Cabp7) gene shows midmuscle-restricted expression, and muscle-specific depletion of Cabp7 in mice accelerated age-related NMJ degeneration, muscle weakness/atrophy, and motor dysfunction. Surprisingly, forced expression in muscle of CIP, an inhibitory peptide of the negative regulator of NMJ formation cyclin-dependent kinase 5 (Cdk5), restored NMJ integrity and muscle strength, and healed muscle atrophy in muscle-specific Cabp7-deficient mice, which showed increased muscle expression of the Cdk5 activator p25. These findings together demonstrate that MuSK-mediated signaling induces muscle expression of Cabp7, which suppresses age-related NMJ degeneration likely by attenuating p25 expression, providing insights into prophylactic/therapeutic intervention against age-related motor dysfunction.

TFP5, a Peptide Derived from Cdk5 Activator p35, Protects Pancreatic β Cells from Glucose Toxicity

We studied the effect of TFP5 on MIN6 cells (cultured mouse islet β cells) treated with different concentrations of glucose (5 or 25 mM). The results were verified in C57BL/6J mice (control; n=12) and db/db mice with type 2 diabetes mellitus (n=12). To synthesize TFP5, peptide p5 (a derivative of p35 protein, activator of cyclin-dependent kinase 5, Cdk5) was conjugated with a FITC tag at the N-terminus and an 11-amino acid TAT protein transduction domain at the C-terminus. TFP5 was employed to inhibit Cdk5 activity and then to evaluate its efficiency in treating experimental type 2 diabetes mellitus. TFP5 effectively inhibited the pathological hyperactivity of Cdk5, enhanced insulin secretion, and protected pancreatic β cells from apoptosis in vitro and in vivo. In addition, TFP5 inhibited inflammation in pancreatic islets by reducing the expression of inflammatory cytokines TGF-β1, TNFα, and IL-1β. These novel data indicates that TFP5 is a promising candidate for treatment of type 2 diabetes mellitus.

TFP5 attenuates cyclin-dependent kinase 5-mediated islet β-cell damage in diabetes

Islet β-cell damage and dysfunction represent the pathophysiological basis of diabetes. Excessive activation of cyclin-dependent kinase 5 (CDK5) is involved in the pathogenesis of type 2 diabetes mellitus (T2DM), although the exact mechanism remains unclear. Therefore, this study investigated the role of a CDK5 inhibitor (TFP5) in islet β-cell damage under diabetic conditions by regulating the expression of CDK5 in vitro and in vivo. CDK5 was upregulated under high glucose conditions in vivo and in vitro, which resulted in inflammation, oxidative stress, and apoptosis of islet β-cells, thereby decreasing insulin secretion. However, TFP5 treatment inhibited the overexpression of CDK5; reduced the inflammatory response, oxidative stress, and apoptosis of islet β cells; and restored insulin secretion. In conclusion, CDK5 is involved in islet β-cell damage under high glucose conditions, and TFP5 may represent a promising candidate for the development of treatments for T2DM.

Three decades of Cdk5

Cdk5 is a proline-directed serine/threonine protein kinase that governs a variety of cellular processes in neurons, the dysregulation of which compromises normal brain function. The mechanisms underlying the modulation of Cdk5, its modes of action, and its effects on the nervous system have been a great focus in the field for nearly three decades. In this review, we provide an overview of the discovery and regulation of Cdk5, highlighting recent findings revealing its role in neuronal/synaptic functions, circadian clocks, DNA damage, cell cycle reentry, mitochondrial dysfunction, as well as its non-neuronal functions under physiological and pathological conditions. Moreover, we discuss evidence underscoring aberrant Cdk5 activity as a common theme observed in many neurodegenerative diseases.

Post-translational modifications of CDK5 and their biological roles in cancer

Post-translational modifications (PTMs) of Cyclin-dependent kinase 5 (CDK5) have emerged as important regulatory mechanisms that modulate cancer development in patients. Though CDK5 is an atypical member of the cyclin-dependent kinase family, its aberrant expression links to cell proliferation, DNA damage response, apoptosis, migration and angiogenesis in cancer. Current studies suggested that, new PTMs on CDK5, including S-nitrosylation, sumoylation, and acetylation, serve as molecular switches to control the kinase activity of CDK5 in the cell. However, a majority of these modifications and their biological significance in cancer remain uncharacterized. In this review, we discussed the role of PTMs on CDK5-mediated signaling cascade, and their possible mechanisms of action in malignant tumors, as well as the challenges and future perspectives in this field. On the basis of the newly identified regulatory signaling pathways of CDK5 related to PTMs, researchers have investigated the cancer therapeutic potential of chemical compounds, small-molecule inhibitors, and competitive peptides by targeting CDK5 and its PTMs. Results of these preclinical studies demonstrated that targeting PTMs of CDK5 yields promising antitumor effects and that clinical translation of these therapeutic strategies is warranted.

CDK2 limits the highly energetic secretory program of mature β cells by restricting PEP cycle-dependent KATP channel closure

Hallmarks of mature β cells are restricted proliferation and a highly energetic secretory state. Paradoxically, cyclin-dependent kinase 2 (CDK2) is synthesized throughout adulthood, its cytosolic localization raising the likelihood of cell cycle-independent functions. In the absence of any changes in β cell mass, maturity, or proliferation, genetic deletion of Cdk2 in adult β cells enhanced insulin secretion from isolated islets and improved glucose tolerance in vivo. At the single β cell level, CDK2 restricts insulin secretion by increasing K_ATP conductance, raising the set point for membrane depolarization in response to activation of the phosphoenolpyruvate (PEP) cycle with mitochondrial fuels. In parallel with reduced β cell recruitment, CDK2 restricts oxidative glucose metabolism while promoting glucose-dependent amplification of insulin secretion. This study provides evidence of essential, non-canonical functions of CDK2 in the secretory pathways of quiescent β cells.

Despite loss of proliferative capacity with age, mature β cells continually synthesize CDK2. Sdao et al. demonstrate that CDK2 depletion in adult β cells improves glucose tolerance in vivo. By augmenting PEP cycle-dependent K_ATP channel closure, CDK2 inactivation lowers the set point for membrane depolarization, augmenting oxidative metabolism and insulin secretion.

A kinase of many talents: non-neuronal functions of CDK5 in development and disease

The cyclin-dependent kinase 5 (CDK5) represents an unusual member of the family of cyclin-dependent kinases, which is activated upon binding to non-cyclin p35 and p39 proteins. The role of CDK5 in the nervous system has been very well established. In addition, there is growing evidence that CDK5 is also active in non-neuronal tissues, where it has been postulated to affect a variety of functions such as the immune response, angiogenesis, myogenesis, melanogenesis and regulation of insulin levels. Moreover, high levels of CDK5 have been observed in different tumour types, and CDK5 was proposed to play various roles in the tumorigenic process. In this review, we discuss these various CDK5 functions in normal physiology and disease, and highlight the therapeutic potential of targeting CDK5.

Regulation of in vivo dynein force production by CDK5 and 14-3-3ε and KIAA0528

Single-molecule cytoplasmic dynein function is well understood, but there are major gaps in mechanistic understanding of cellular dynein regulation. We reported a mode of dynein regulation, force adaptation, where lipid droplets adapt to opposition to motion by increasing the duration and magnitude of force production, and found LIS1 and NudEL to be essential. Adaptation reflects increasing NudEL-LIS1 utilization; here, we hypothesize that such increasing utilization reflects CDK5-mediated NudEL phosphorylation, which increases the dynein-NudEL interaction, and makes force adaptation possible. We report that CDK5, 14-3-3ε, and CDK5 cofactor KIAA0528 together promote NudEL phosphorylation and are essential for force adaptation. By studying the process in COS-1 cells lacking Tau, we avoid confounding neuronal effects of CDK5 on microtubules. Finally, we extend this in vivo regulatory pathway to lysosomes and mitochondria. Ultimately, we show that dynein force adaptation can control the severity of lysosomal tug-of-wars among other intracellular transport functions involving high force.

Dynein plays roles in vesicular, organelle, chromosomal and nuclear transport but so far it is unclear how dynein activity in cells is regulated. Here authors study several dynein cofactors and their role in force adaptation of dynein during lipid droplet, lysosomal, and mitochondrial transport.

Mesenchymal Stem Cells Loaded with p5, Derived from CDK5 Activator p35, Inhibit Calcium-Induced CDK5 Activation in Endothelial Cells

The potential use of stem cells as therapeutics in disease has gained momentum over the last few years and recently phase-I clinical trials have shown favourable results in treatment of a small cohort of acute stroke patients. Similarly, they have been used in preclinical models drug-loaded for the effective treatment of solid tumours. Here we have characterized uptake and release of a novel p5-cyclin-dependent kinase 5 (CDK5) inhibitory peptide by mesenchymal stem cells and showed release levels capable of blocking aberrant cyclin-dependent kinase 5 (CDK5) signaling pathways, through phosphorylation of cyclin-dependent kinase 5 (CDK5) and p53. These pathways represent the major acute mechanism stimulating apoptosis after stroke and hence its modulation could benefit patient recovery. This work indicates a potential use for drug-loaded stem cells as delivery vehicles for stroke therapeutics and in addition as anticancer receptacles particularly, if a targeting and/or holding mechanism can be defined.

Knockdown of Expression of Cdk5 or p35 (a Cdk5 Activator) Results in Podocyte Apoptosis

Podocytes are terminally differentiated glomerular epithelial cells. Podocyte loss has been found in many renal diseases. Cdk5 is a cyclin-dependent protein kinase which is predominantly regulated by p35. To study the role of Cdk5/p35 in podocyte survival, we first applied western blotting (WB) analysis to confirm the time-course expression of Cdk5 and p35 during kidney development and in cultured immortalized mouse podocytes. We also demonstrated that p35 plays an important role in promoting podocyte differentiation by overexpression of p35 in podocytes. To deregulate the expression of Cdk5 or p35 in mouse podocytes, we used RNAi and analyzed cell function and apoptosis assaying for podocyte specific marker Wilms Tumor 1 (WT1) and cleaved caspase 3, respectively. We also counted viable cells using cell counting kit-8. We found that depletion of Cdk5 causes decreased expression of WT1 and apoptosis. It is noteworthy, however, that downregulation of p35 reduced Cdk5 activity, but had no effect on cleaved caspase 3 expression. It did, however, reduce expression of WT1, a transcription factor, and produced podocyte dysmorphism. On the other hand increased apoptosis could be detected in p35-deregulated podocytes using the TUNEL analysis and immunofluorescent staining with cleaved caspase3 antibody. Viability of podocytes was decreased in both Cdk5 and p35 knockdown cells. Knocking down Cdk5 or p35 gene by RNAi does not affect the cycline I expression, another Cdk5 activator in podocyes. We conclude that Cdk5 and p35 play a crucial role in maintaining podocyte differentiation and survival, and suggest these proteins as targets for therapeutic intervention in podocyte-damaged kidney diseases.