Cdk5 phosphorylates and stabilizes p27kip1 contributing to actin organization and cortical neuronal migration

Triaging between post-translational modification of cell cycle regulators and their therapeutics in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, present challenges in healthcare because of their complicated etiologies and absence of healing remedies. Lately, the emerging role of post-translational modifications (PTMs), in the context of cell cycle regulators, has garnered big interest as a potential avenue for therapeutic intervention. The review explores the problematic panorama of PTMs on cell cycle regulators and their implications in neurodegenerative diseases. We delve into the dynamic phosphorylation, acetylation, ubiquitination, SUMOylation, Glycation, and Neddylation that modulate the key cell cycle regulators, consisting of cyclins, cyclin-dependent kinases (CDKs), and their inhibitors. The dysregulation of these PTMs is related to aberrant cell cycle in neurons, which is one of the factors involved in neurodegenerative pathologies. Moreover, the effect of exogenous activation of CDKs and CDK inhibitors through PTMs on the signaling cascade was studied in postmitotic conditions of NDDs. Furthermore, the therapeutic implications of CDK inhibitors and associated alteration in PTMs were discussed. Lastly, we explored the putative mechanism of PTMs to restore normal neuronal function that might reverse NDDs.

Primary Culture of Dissociated Neurons from the Embryonic Cerebral Cortex

Primary neuronal culture is a valuable in vitro model for analyzing the molecular mechanisms underlying the development and function of neural circuits. In contrast to neurons in vivo, primary cultured neurons can easily be transfected with genes of interest or treated with chemicals such as agonists and inhibitors of a specific target molecule. Furthermore, time-dependent morphological changes, such as the acquisition of neuronal polarity, axon elongation, and dendrite branch formation, can be analyzed by using primary neuronal cultures. Here, we describe a method for preparing a primary culture of neurons from the developing cerebral cortex, together with a method for gene transfer to primary cultured cortical neurons.

Rab21 regulates caveolin-1-mediated endocytic trafficking to promote immature neurite pruning

Transmembrane proteins are internalized by clathrin- and caveolin-dependent endocytosis. Both pathways converge on early endosomes and are thought to share the small GTPase Rab5 as common regulator. In contrast to this notion, we show here that the clathrin- and caveolin-mediated endocytic pathways are differentially regulated. Rab5 and Rab21 localize to distinct populations of early endosomes in cortical neurons and preferentially regulate clathrin- and caveolin-mediated pathways, respectively, suggesting heterogeneity in the early endosomes, rather than a converging point. Suppression of Rab21, but not Rab5, results in decreased plasma membrane localization and total protein levels of caveolin-1, which perturbs immature neurite pruning of cortical neurons, an in vivo-specific step of neuronal maturation. Taken together, our data indicate that clathrin- and caveolin-mediated endocytic pathways run in parallel in early endosomes, which show different molecular regulation and physiological function.

Diversin upregulates the proliferative ability of colorectal cancer by inducing cell cycle proteins

Colorectal cancer (CRC) is a common gastrointestinal tumour with increasing incidence worldwide. However, the underlying molecular mechanism of CRC proliferation is not completely clear. Diversin，as an ankyrin repeat-containing protein, is upregulated in various solid tumours and accelerates cancer progression by promoting cell proliferation and increasing S phase fraction of cells. In this study, 71 CRC samples and corresponding adjacent tissue samples were included. The expression of diversin in tissues was verified via immunohistochemical analysis. The MTS assay and flow cytometry (FCM) was used to measure cell proliferation and cell cycle. Results of immunohistochemical analysis revealed that diversin was highly expressed in human CRC tissues and was significantly associated with tumour differentiation, clinical stage and lymph node metastasis. The analysis based on the CRC data from The Cancer Genome Atlas (TCGA) database showed that a high expression of diversin correlated with the poor prognosis of CRC. Results of the MTS assay indicated that the overexpression of diversin promoted the proliferation of CRC cells, while its downregulation had an inhibitory effect on CRC cell proliferation. FCM analysises presented that diversin increased the flux of the CRC cell cycle from G1 to S and regulated cycle-related proteins, namely, P21, P27, cyclin E, CDK2, cyclin D and CDK4. The results suggest that diversin contributes to CRC proliferation that involves the distribution of the cell cycle. In CRC tissues, the expression of diversin has closely related to the prognosis. The higher the expression levels of diversin, the worse the prognosis. In vitro, diversin could increase the proliferative ability of CRC cells through the G1-S checkpoint and JNK signalling pathway, confirming that diversin contributes to CRC development.

Focusing on cyclin-dependent kinases 5: A potential target for neurological disorders

Cyclin-dependent kinases 5 (Cdk5) is a special member of proline-directed serine threonine kinase family. Unlike other Cdks, Cdk5 is not directly involved in cell cycle regulation but plays important roles in nervous system functions. Under physiological conditions, the activity of Cdk5 is tightly controlled by p35 or p39, which are specific activators of Cdk5 and highly expressed in post-mitotic neurons. However, they will be cleaved into the corresponding truncated forms namely p25 and p29 under pathological conditions, such as neurodegenerative diseases and neurotoxic insults. The binding to truncated co-activators results in aberrant Cdk5 activity and contributes to the initiation and progression of multiple neurological disorders through affecting the down-stream targets. Although Cdk5 kinase activity is mainly regulated through combining with co-activators, it is not the only way. Post-translational modifications of Cdk5 including phosphorylation, S-nitrosylation, sumoylation, and acetylation can also affect its kinase activity and then participate in physiological and pathological processes of nervous system. In this review, we focus on the regulatory mechanisms of Cdk5 and its roles in a series of common neurological disorders such as neurodegenerative diseases, stroke, anxiety/depression, pathological pain and epilepsy.

p27kip1 Modulates the Morphology and Phagocytic Activity of Microglia

p27^kip1 is a multifunctional protein that promotes cell cycle exit by blocking the activity of cyclin/cyclin-dependent kinase complexes as well as migration and motility via signaling pathways that converge on the actin and microtubule cytoskeleton. Despite the broad characterization of p27^kip1 function in neural cells, little is known about its relevance in microglia. Here, we studied the role of p27^kip1 in microglia using a combination of in vitro and in situ approaches. While the loss of p27^kip1 did not affect microglial density in the cerebral cortex, it altered their morphological complexity in situ. However, despite the presence of p27^kip1 in microglial processes, as shown by immunofluorescence in cultured cells, loss of p27^kip1 did not change microglial process motility and extension after applying laser-induced brain damage in cortical brain slices. Primary microglia lacking p27^kip1 showed increased phagocytic uptake of synaptosomes, while a cell cycle dead variant negatively affected phagocytosis. These findings indicate that p27^kip1 plays specific roles in microglia.

Mapping the dynamics of insulin-responsive pathways in the blood-brain barrier endothelium using time-series transcriptomics data

Critical functions of the blood-brain barrier (BBB), including cerebral blood flow, energy metabolism, and immunomodulation, are regulated by insulin signaling pathways. Therefore, endothelial insulin resistance could lead to BBB dysfunction, which is associated with neurodegenerative diseases such as Alzheimer's disease (AD). The current study aims to map the dynamics of insulin-responsive pathways in polarized human cerebral microvascular endothelial cell (hCMEC/D3) monolayers. RNA-Sequencing was performed on hCMEC/D3 monolayers with and without insulin treatment at various time points. The Short Time-series Expression Miner (STEM) method was used to identify gene clusters with distinct and representative expression patterns. Functional annotation and pathway analysis of genes from selected clusters were conducted using Webgestalt and Ingenuity Pathway Analysis (IPA) software. Quantitative expression differences of 16,570 genes between insulin-treated and control monolayers were determined at five-time points. The STEM software identified 12 significant clusters with 6880 genes that displayed distinct temporal patterns upon insulin exposure, and the clusters were further divided into three groups. Gene ontology (GO) enrichment analysis demonstrated that biological processes protecting BBB functions such as regulation of vascular development and actin cytoskeleton reorganization were upregulated after insulin treatment (Group 1 and 2). In contrast, GO pathways related to inflammation, such as response to interferon-gamma, were downregulated (Group 3). The IPA analyses further identified insulin-responsive cellular and molecular pathways that are associated with AD pathology. These findings unravel the dynamics of insulin action on the BBB endothelium and inform about downstream signaling cascades that are potentially disrupted due to brain insulin resistance prevalent in AD.

Developmental cell death of cortical projection neurons is controlled by a Bcl11a/Bcl6‐dependent pathway

Developmental neuron death plays a pivotal role in refining organization and wiring during neocortex formation. Aberrant regulation of this process results in neurodevelopmental disorders including impaired learning and memory. Underlying molecular pathways are incompletely determined. Loss of Bcl11a in cortical projection neurons induces pronounced cell death in upper‐layer cortical projection neurons during postnatal corticogenesis. We use this genetic model to explore genetic mechanisms by which developmental neuron death is controlled. Unexpectedly, we find Bcl6, previously shown to be involved in the transition of cortical neurons from progenitor to postmitotic differentiation state to provide a major checkpoint regulating neuron survival during late cortical development. We show that Bcl11a is a direct transcriptional regulator of Bcl6. Deletion of Bcl6 exerts death of cortical projection neurons. In turn, reintroduction of Bcl6 into Bcl11a mutants prevents induction of cell death in these neurons. Together, our data identify a novel Bcl11a/Bcl6‐dependent molecular pathway in regulation of developmental cell death during corticogenesis.

p27, The Cell Cycle and Alzheimer´s Disease

The cell cycle consists of successive events that lead to the generation of new cells. The cell cycle is regulated by different cyclins, cyclin-dependent kinases (CDKs) and their inhibitors, such as p27^Kip1. At the nuclear level, p27^Kip1 has the ability to control the evolution of different phases of the cell cycle and oppose cell cycle progression by binding to CDKs. In the cytoplasm, diverse functions have been described for p27^Kip1, including microtubule remodeling, axonal transport and phagocytosis. In Alzheimer’s disease (AD), alterations to cycle events and a purported increase in neurogenesis have been described in the early disease process before significant pathological changes could be detected. However, most neurons cannot progress to complete their cell division and undergo apoptotic cell death. Increased levels of both the p27^Kip1 levels and phosphorylation status have been described in AD. Increased levels of Aβ42, tau hyperphosphorylation or even altered insulin signals could lead to alterations in p27^Kip1 post-transcriptional modifications, causing a disbalance between the levels and functions of p27^Kip1 in the cytoplasm and nucleus, thus inducing an aberrant cell cycle re-entry and alteration of extra cell cycle functions. Further studies are needed to completely understand the role of p27^Kip1 in AD and the therapeutic opportunities associated with the modulation of this target.

Prenatal Octamethylcyclotetrasiloxane Exposure Impaired Proliferation of Neuronal Progenitor, Leading to Motor, Cognition, Social and Behavioral Functions

Cyclic siloxane octamethylcyclotetrasiloxane (D4) has raised concerns as an endocrine-disrupting chemical (EDC). D4 is widely used in detergent products, cosmetics, and personal care products. Recently, robust toxicological data for D4 has been reported, but the adverse effects of D4 on brain development are unknown. Here, pregnant mice on gestational day 9.5 were treated daily with D4 to postnatal day 28, and the offspring mice were studied. The prenatal D4-treated mice exhibited cognitive dysfunction, limited memory, and motor learning defect. Moreover, prenatal D4 exposure reduced the proliferation of neuronal progenitors in the offspring mouse brain. Next, the mechanisms through which D4 regulated the cell cycle were investigated. Aberrant gene expression, such as cyclin-dependent kinases CDK6 and cyclin-dependent kinase inhibitor p27, were found in the prenatal D4-treated mice. Furthermore, the estrogen receptors ERa and ERb were increased in the brain of prenatal D4-treated mice. Overall, these findings suggest that D4 exerts estrogen activity that affects the cell cycle progression of neuronal progenitor cells during neurodevelopment, which may be associated with cognitive deficits in offspring.

Alternative Functions of Cell Cycle-Related and DNA Repair Proteins in Post-mitotic Neurons

Proper regulation of neuronal morphological changes is essential for neuronal migration, maturation, synapse formation, and high-order function. Many cytoplasmic proteins involved in the regulation of neuronal microtubules and the actin cytoskeleton have been identified. In addition, some nuclear proteins have alternative functions in neurons. While cell cycle-related proteins basically control the progression of the cell cycle in the nucleus, some of them have an extra-cell cycle-regulatory function (EXCERF), such as regulating cytoskeletal organization, after exit from the cell cycle. Our expression analyses showed that not only cell cycle regulators, including cyclin A1, cyclin D2, Cdk4/6, p21^cip1, p27^kip1, Ink4 family, and RAD21, but also DNA repair proteins, including BRCA2, p53, ATM, ATR, RAD17, MRE11, RAD9, and Hus1, were expressed after neurogenesis, suggesting that these proteins have alternative functions in post-mitotic neurons. In this perspective paper, we discuss the alternative functions of the nuclear proteins in neuronal development, focusing on possible cytoplasmic roles.

ATP-citrate lyase promotes axonal transport across species

Microtubule (MT)-based transport is an evolutionary conserved process finely tuned by posttranslational modifications. Among them, α-tubulin acetylation, primarily catalyzed by a vesicular pool of α-tubulin N-acetyltransferase 1 (Atat1), promotes the recruitment and processivity of molecular motors along MT tracks. However, the mechanism that controls Atat1 activity remains poorly understood. Here, we show that ATP-citrate lyase (Acly) is enriched in vesicles and provide Acetyl-Coenzyme-A (Acetyl-CoA) to Atat1. In addition, we showed that Acly expression is reduced upon loss of Elongator activity, further connecting Elongator to Atat1 in a pathway regulating α-tubulin acetylation and MT-dependent transport in projection neurons, across species. Remarkably, comparable defects occur in fibroblasts from Familial Dysautonomia (FD) patients bearing an autosomal recessive mutation in the gene coding for the Elongator subunit ELP1. Our data may thus shine light on the pathophysiological mechanisms underlying FD.

Microtubule tracks are important for the transport of molecules within axons. Here, the authors show that ATAT1, the enzyme responsible for acetylating a-tubulin, receives acetyl groups from ATP citrate lyase whose stability is regulated by Elongator, a protein mutated in the neuronal disease Familial dysautonomia.

The Atypical Cyclin-Dependent Kinase 5 (Cdk5) Guards Podocytes from Apoptosis in Glomerular Disease While Being Dispensable for Podocyte Development

Cyclin-dependent kinase 5 (Cdk5) is expressed in terminally differentiated cells, where it drives development, morphogenesis, and survival. Temporal and spatial kinase activity is regulated by specific activators of Cdk5, dependent on the cell type and environmental factors. In the kidney, Cdk5 is exclusively expressed in terminally differentiated glomerular epithelial cells called podocytes. In glomerular disease, signaling mechanisms via Cdk5 have been addressed by single or combined conventional knockout of known specific activators of Cdk5. A protective, anti-apoptotic role has been ascribed to Cdk5 but not a developmental phenotype, as in terminally differentiated neurons. The effector kinase itself has never been addressed in animal models of glomerular disease. In the present study, conditional and inducible knockout models of Cdk5 were analyzed to investigate the role of Cdk5 in podocyte development and glomerular disease. While mice with podocyte-specific knockout of Cdk5 had no developmental defects and regular lifespan, loss of Cdk5 in podocytes increased susceptibility to glomerular damage in the nephrotoxic nephritis model. Glomerular damage was associated with reduced anti-apoptotic signals in Cdk5-deficient mice. In summary, Cdk5 acts primarily as master regulator of podocyte survival during glomerular disease and—in contrast to neurons—does not impact on glomerular development or maintenance.

p27Kip1, an Intrinsically Unstructured Protein with Scaffold Properties

The Cyclin-dependent kinase (CDK) regulator p27^Kip1 is a gatekeeper of G1/S transition. It also regulates G2/M progression and cytokinesis completion, via CDK-dependent or -independent mechanisms. Recently, other important p27^Kip1 functions have been described, including the regulation of cell motility and migration, the control of cell differentiation program and the activation of apoptosis/autophagy. Several factors modulate p27^Kip1 activities, including its level, cellular localization and post-translational modifications. As a matter of fact, the protein is phosphorylated, ubiquitinated, SUMOylated, O-linked N-acetylglicosylated and acetylated on different residues. p27^Kip1 belongs to the family of the intrinsically unstructured proteins and thus it is endowed with a large flexibility and numerous interactors, only partially identified. In this review, we look at p27^Kip1 properties and ascribe part of its heterogeneous functions to the ability to act as an anchor or scaffold capable to participate in the construction of different platforms for modulating cell response to extracellular signals and allowing adaptation to environmental changes.

The AMPK/p27Kip1 Pathway as a Novel Target to Promote Autophagy and Resilience in Aged Cells

Once believed to solely function as a cyclin-dependent kinase inhibitor, p27^Kip1 is now emerging as a critical mediator of autophagy, cytoskeletal dynamics, cell migration and apoptosis. During periods of metabolic stress, the subcellular location of p27^Kip1 largely dictates its function. Cytoplasmic p27^Kip1 has been found to be promote cellular resilience through autophagy and anti-apoptotic mechanisms. Nuclear p27^Kip1, however, inhibits cell cycle progression and makes the cell susceptible to quiescence, apoptosis, and/or senescence. Cellular location of p27^Kip1 is regulated, in part, by phosphorylation by various kinases, including Akt and AMPK. Aging promotes nuclear localization of p27^Kip1 and a predisposition to senescence or apoptosis. Here, we will review the role of p27^Kip1 in healthy and aging cells with a particular emphasis on the interplay between autophagy and apoptosis.

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

Notch Signaling between Cerebellar Granule Cell Progenitors

Cerebellar granule cells (GCs) are cells which comprise over 50% of the neurons in the entire nervous system. GCs enable the cerebellum to properly regulate motor coordination, learning, and consolidation, in addition to cognition, emotion and language. During GC development, maternal GC progenitors (GCPs) divide to produce not only postmitotic GCs but also sister GCPs. However, the molecular machinery for regulating the proportional production of distinct sister cell types from seemingly uniform GCPs is not yet fully understood. Here we report that Notch signaling creates a distinction between GCPs and leads to their proportional differentiation in mice. Among Notch-related molecules, Notch1, Notch2, Jag1, and Hes1 are prominently expressed in GCPs. In vivo monitoring of Hes1-promoter activities showed the presence of two types of GCPs, Notch-signaling ON and OFF, in the external granule layer (EGL). Single-cell RNA sequencing (scRNA-seq) and in silico analyses indicate that ON-GCPs have more proliferative and immature properties, while OFF-GCPs have opposite characteristics. Overexpression as well as knock-down (KD) experiments using in vivo electroporation showed that NOTCH2 and HES1 are involved cell-autonomously to suppress GCP differentiation by inhibiting NEUROD1 expression. In contrast, JAG1-expressing cells non-autonomously upregulated Notch signaling activities via NOTCH2-HES1 in surrounding GCPs, eventually suppressing their differentiation. These findings suggest that Notch signaling results in the proportional generation of two types of cells, immature and differentiating GCPs, which contributes to the well-organized differentiation of GCs.

Translational Research in Vitiligo

Vitiligo is a disease of the skin characterized by the appearance of white spots. Significant progress has been made in understanding vitiligo pathogenesis over the past 30 years, but only through perseverance, collaboration, and open-minded discussion. Early hypotheses considered roles for innervation, microvascular anomalies, oxidative stress, defects in melanocyte adhesion, autoimmunity, somatic mosaicism, and genetics. Because theories about pathogenesis drive experimental design, focus, and even therapeutic approach, it is important to consider their impact on our current understanding about vitiligo. Animal models allow researchers to perform mechanistic studies, and the development of improved patient sample collection methods provides a platform for translational studies in vitiligo that can also be applied to understand other autoimmune diseases that are more difficult to study in human samples. Here we discuss the history of vitiligo translational research, recent advances, and their implications for new treatment approaches.

The Role of CDK5 in Tumours and Tumour Microenvironments

Cyclin-dependent kinase 5 (CDK5), which belongs to the protein kinase family, regulates neuronal function but is also associated with cancer development and has been proposed as a target for cancer treatment. Indeed, CDK5 has roles in cell proliferation, apoptosis, angiogenesis, inflammation, and immune response. Aberrant CDK5 activation triggers tumour progression in numerous types of cancer. In this review, we summarise the role of CDK5 in cancer and neurons and CDK5 inhibitors. We expect that our review helps researchers to develop CDK5 inhibitors as treatments for refractory cancer.

The role of SCFSkp2 and SCFβ-TrCP1/2 in the cerebellar granule cell precursors

A proper balance between proliferation and differentiation of cerebellar granule cell precursors (GCPs) is required for appropriate cerebellar morphogenesis. The Skp1-Cullin1-F-box (SCF) complex, an E3 ubiquitin ligase complex, is involved in polyubiquitination and subsequent degradation of various cell cycle regulators and transcription factors. However, it remains unknown how the SCF complex affects proliferation and differentiation of GCPs. In this study, we found that the scaffold protein Cullin1, and F-box proteins Skp2, β-TrCP1 and β-TrCP2 are expressed in the external granule layer (EGL). Knockdown of these molecules in the EGL showed that Cullin1, Skp2 and β-TrCP2 enhanced differentiation of GCPs. We also observed accumulation of cyclin-dependent kinase inhibitor p27 in GCPs when treated with a Cullin1 inhibitor or proteasome inhibitor. Furthermore, knockdown of p27 rescued enhancement of differentiation by Cullin1 knockdown. These results suggest that the SCF complex is involved in the maintenance of the proliferative state of GCPs through p27 degradation. In addition, inhibition of Cullin1 activity also prevented cell proliferation and enhanced accumulation of p27 in Daoy cells, a cell line derived from the sonic hedgehog subtype of medulloblastoma. This suggested that excess degradation of p27 through the SCF complex causes overproliferation of medulloblastoma cells.

Core cell cycle machinery is crucially involved in both life and death of post-mitotic neurons

A persistent dogma in neuroscience supported the idea that terminally differentiated neurons permanently withdraw from the cell cycle. However, since the late 1990s, several studies have shown that cell cycle proteins are expressed in post-mitotic neurons under physiological conditions, indicating that the cell cycle machinery is not restricted to proliferating cells. Moreover, many studies have highlighted a clear link between cell cycle-related proteins and neurological disorders, particularly relating to apoptosis-induced neuronal death. Indeed, cell cycle-related proteins can be upregulated or overactivated in post-mitotic neurons in case of acute or degenerative central nervous system disease. Given the considerable lack of effective treatments for age-related neurological disorders, new therapeutic approaches targeting the cell cycle machinery might thus be considered. This review aims at summarizing current knowledge about the role of the cell cycle machinery in post-mitotic neurons in healthy and pathological conditions.

Regulation of p27Kip1 and p57Kip2 Functions by Natural Polyphenols

In numerous instances, the fate of a single cell not only represents its peculiar outcome but also contributes to the overall status of an organism. In turn, the cell division cycle and its control strongly influence cell destiny, playing a critical role in targeting it towards a specific phenotype. Several factors participate in the control of growth, and among them, p27^Kip1 and p57^Kip2, two proteins modulating various transitions of the cell cycle, appear to play key functions. In this review, the major features of p27 and p57 will be described, focusing, in particular, on their recently identified roles not directly correlated with cell cycle modulation. Then, their possible roles as molecular effectors of polyphenols’ activities will be discussed. Polyphenols represent a large family of natural bioactive molecules that have been demonstrated to exhibit promising protective activities against several human diseases. Their use has also been proposed in association with classical therapies for improving their clinical effects and for diminishing their negative side activities. The importance of p27^Kip1 and p57^Kip2 in polyphenols’ cellular effects will be discussed with the aim of identifying novel therapeutic strategies for the treatment of important human diseases, such as cancers, characterized by an altered control of growth.

Physiological and Pathological Roles of Cdk5: Potential Directions for Therapeutic Targeting in Neurodegenerative Disease

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine (ser)/threonine (Thr) kinase that has been demonstrated to be one of the most functionally diverse kinases within neurons. Cdk5 is regulated via binding with its neuron-specific regulatory subunits, p35 or p39. Cdk5-p35 activity is critical for a variety of developmental and cellular processes in the brain, including neuron migration, memory formation, microtubule regulation, and cell cycle suppression. Aberrant activation of Cdk5 via the truncated p35 byproduct, p25, is implicated in the pathogenesis of several neurodegenerative diseases. The present review highlights the importance of Cdk5 activity and function in the brain and demonstrates how deregulation of Cdk5 can contribute to the development of neurodegenerative conditions such as Alzheimer's and Parkinson's disease. Additionally, we cover past drug discovery attempts at inhibiting Cdk5-p25 activity and discuss which types of targeting strategies may prove to be the most successful moving forward.

MAST1 modulates neuronal differentiation and cell cycle exit via P27 in neuroblastoma cells

Although 19p13.13 microdeletion syndrome has been consistently associated with intellectual disability, overgrowth, and macrocephaly, the underlying mechanisms remain unclear. MAST1, a member of the microtubule‐associated serine/threonine kinase family, has been suggested as a potential candidate gene responsible for neurologic abnormalities in 19p13.13 microdeletion syndrome, but its role in nervous system development remains to be elucidated. Here, we investigated how MAST1 contributes to neuronal development. We report that MAST1 is upregulated during neuronal differentiation of the human neuroblastoma cell line, SH‐SY5Y. Inhibition of MAST1 expression by RNA interference attenuated neuronal differentiation of SH‐SY5Y cells. Cell cycle analyses revealed that MAST1‐depleted cells did not undergo cell cycle arrest after RA treatment. Consistent with this observation, the number of EdU‐positive cells significantly increased in MAST1 knockdown cells. Intriguingly, levels of P27, a cyclin‐dependent kinase inhibitor, were also increased during neuronal differentiation, and MAST1 knockdown reduced the expression of P27. Moreover, reduced neuronal differentiation caused by MAST1 depletion was rescued partially by P27 overexpression in SH‐SY5Y cells. Collectively, these results suggest that MAST1 influences nervous system development by affecting neuronal differentiation through P27.

Human antigen R-regulated mRNA metabolism promotes the cell motility of migrating mouse neurons

Neocortex development during embryonic stages requires the precise control of mRNA metabolism. Human antigen R (HuR) is a well-studied mRNA-binding protein that regulates mRNA metabolism, and it is highly expressed in the neocortex during developmental stages. Deletion of HuR does not impair neural progenitor cell proliferation or differentiation, but it disturbs the laminar structure of the neocortex. We report that HuR is expressed in postmitotic projection neurons during mouse brain development. Specifically, depletion of HuR in these neurons led to a mislocalization of CDP⁺ neurons in deeper layers of the cortex. Time-lapse microscopy showed that HuR was required for the promotion of cell motility in migrating neurons. PCR array identified profilin 1 (Pfn1) mRNA as a major binding partner of HuR in neurons. HuR positively mediated the stability of Pfn1 mRNA and influenced actin polymerization. Overexpression of Pfn1 successfully rescued the migration defects of HuR-deleted neurons. Our data reveal a post-transcriptional mechanism that maintains actin dynamics during neuronal migration.

Summary: Maintaining actin dynamics is crucial for cell motility. Post-transcriptional regulation plays a pivotal role in supporting actin dynamics during neuronal migration.

Dynamic Phosphorylation and Dephosphorylation of Cyclase-Associated Protein 1 by Antagonistic Signaling through Cyclin-Dependent Kinase 5 and cAMP Are Critical for the Protein Functions in Actin Filament Disassembly and Cell Adhesion

Cyclase-associated protein 1 (CAP1) is a conserved actin-regulating protein that enhances actin filament dynamics and also regulates adhesion in mammalian cells. We previously found that phosphorylation at the Ser307/Ser309 tandem site controls its association with cofilin and actin and is important for CAP1 to regulate the actin cytoskeleton. Here, we report that transient Ser307/Ser309 phosphorylation is required for CAP1 function in both actin filament disassembly and cell adhesion. Both the phosphomimetic and the nonphosphorylatable CAP1 mutant, which resist transition between phosphorylated and dephosphorylated forms, had defects in rescuing the reduced rate of actin filament disassembly in the CAP1 knockdown HeLa cells. The phosphorylation mutants also had defects in alleviating the elevated focal adhesion kinase (FAK) activity and the enhanced focal adhesions in the knockdown cells. In dissecting further phosphoregulatory cell signals for CAP1, we found that cyclin-dependent kinase 5 (CDK5) phosphorylates both Ser307 and Ser309 residues, whereas cAMP signaling induces dephosphorylation at the tandem site, through its effectors protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac). No evidence supports an involvement of activated protein phosphatase in executing the dephosphorylation downstream from cAMP, whereas preventing CAP1 from accessing its kinase CDK5 appears to underlie CAP1 dephosphorylation induced by cAMP. Therefore, this study provides direct cellular evidence that transient phosphorylation is required for CAP1 functions in both actin filament turnover and adhesion, and the novel mechanistic insights substantially extend our knowledge of the cell signals that function in concert to regulate CAP1 by facilitating its transient phosphorylation.

A non-canonical role for p27Kip1 in restricting proliferation of corneal endothelial cells during development

The cell cycle regulator p27^Kip1 is a critical factor controlling cell number in many lineages. While its anti-proliferative effects are well-established, the extent to which this is a result of its function as a cyclin-dependent kinase (CDK) inhibitor or through other known molecular interactions is not clear. To genetically dissect its role in the developing corneal endothelium, we examined mice harboring two loss-of-function alleles, a null allele (p27⁻) that abrogates all protein function and a knockin allele (p27^CK−) that targets only its interaction with cyclins and CDKs. Whole-animal mutants, in which all cells are either homozygous knockout or knockin, exhibit identical proliferative increases (~0.6-fold) compared with wild-type tissues. On the other hand, use of mosaic analysis with double markers (MADM) to produce infrequently-occurring clones of wild-type and mutant cells within the same tissue environment uncovers a roughly three- and six-fold expansion of individual p27^CK−/CK− and p27^−/− cells, respectively. Mosaicism also reveals distinct migration phenotypes, with p27^−/− cells being highly restricted to their site of production and p27^CK−/CK− cells more widely scattered within the endothelium. Using a density-based clustering algorithm to quantify dispersal of MADM-generated clones, a four-fold difference in aggregation is seen between the two types of mutant cells. Overall, our analysis reveals that, in developing mouse corneal endothelium, p27 regulates cell number by acting cell autonomously, both through its interactions with cyclins and CDKs and through a cyclin-CDK-independent mechanism(s). Combined with its parallel influence on cell motility, it constitutes a potent multi-functional effector mechanism with major impact on tissue organization.

Multiple Functions of KBP in Neural Development Underlie Brain Anomalies in Goldberg-Shprintzen Syndrome

Kinesin-binding protein (KBP; KIF1BP; KIAA1279) functions as a regulator for a subset of kinesins, many of which play important roles in neural development. Previous studies have shown that KBP is expressed in nearly all tissue with cytoplasmic localization. Autosomal recessive mutations in KIAA1279 cause a rare neurological disorder, Goldberg-Shprintzen syndrome (GOSHS), characterized by microcephaly, polymicrogyria, intellectual disability, axonal neuropathy, thin corpus callosum and peripheral neuropathy. Most KIAA1279 mutations found in GOSHS patients are homozygous nonsense mutations that result in KBP loss-of-function. However, it is not fully understood how KBP dysfunction causes these defects. Here, we used in utero electroporation (IUE) to express KBP short hairpin RNA (shRNA) with green fluorescent protein (GFP) in neural progenitor cells of embryonic day (E) 14 mice, and collected brain slices at different developmental stages. By immunostaining of neuronal lineage markers, we found that KBP knockdown does not affect the neural differentiation process. However, at 4 days post IUE, many cells were located in the intermediate zone (IZ). Moreover, at postnatal day (P) 6, about one third of the cells, which have become mature neurons, remained ectopically in the white matter (WM), while cells that have reached Layer II/III of the cortex showed impaired dendritic outgrowth and axonal projection. We also found that KBP knockdown induces apoptosis during the postnatal period. Our findings indicate that loss of KBP function leads to defects in neuronal migration, morphogenesis, maturation, and survival, which may be responsible for brain phenotypes observed in GOSHS.

Systemic Administration of the Cyclin-Dependent Kinase Inhibitor (S)-CR8 Selectively Reduces Escalated Ethanol Intake in Dependent Rats

BACKGROUND

Chronic exposure to ethanol (EtOH) and other drugs of abuse can alter the expression and activity of cyclin-dependent kinase 5 (CDK5) and its cofactor p35, but the functional implication of CDK5 signaling in the regulation of EtOH-related behaviors remains unknown. In the present study, we sought to determine whether CDK5 activity plays a role in the escalation of EtOH self-administration triggered by dependence.

METHODS

We tested the effect of systemically administered (S)-CR8, a nonselective CDK inhibitor, on operant responding for EtOH or saccharin, a highly palatable reinforcer, in adult male Wistar rats. Half of the rats were made EtOH-dependent via chronic intermittent EtOH inhalation (CIE). We then sought to identify a possible neuroanatomical locus for the behavioral effect of (S)-CR8 by quantifying protein levels of CDK5 and p35 in subregions of the extended amygdala and prefrontal cortex from EtOH-naïve, nondependent, and dependent rats at the expected time of EtOH self-administration. We also analyzed the phosphorylation of 4 CDK5 substrates and of the CDK substrate consensus motif.

RESULTS

(S)-CR8 dose-dependently reduced EtOH self-administration in dependent rats. It had no effect on water or saccharin self-administration, nor in nondependent rats. The abundance of CDK5 or p35 was not altered in any of the brain regions analyzed. In the bed nucleus of the stria terminalis, CDK5 abundance was negatively correlated with intoxication levels during EtOH vapor exposure but there was no effect of dependence on the phosphorylation ratio of CDK5 substrates. In contrast, EtOH dependence increased the phosphorylation of low-molecular-weight CDK substrates in the basolateral amygdala (BLA).

CONCLUSIONS

The selective effect of (S)-CR8 on excessive EtOH intake has potential therapeutic value for the treatment of alcohol use disorders. Our data do not support the hypothesis that this effect would be mediated by the inhibition of up-regulated CDK5 activity in the extended amygdala nor prefrontal cortex. However, increased activity of CDKs other than CDK5 in the BLA may contribute to excessive EtOH consumption in alcohol dependence. Other (S)-CR8 targets may also be implicated.

The cell cycle in stem cell proliferation, pluripotency and differentiation

Cyclins, cyclin-dependent kinases and other components of the core cell cycle machinery drive cell division. Growing evidence indicates that this machinery operates in a distinct fashion in some mammalian stem cell types, such as pluripotent embryonic stem cells. In this Review, we discuss our current knowledge of how cell cycle proteins mechanistically link cell proliferation, pluripotency and cell fate specification. We focus on embryonic stem cells, induced pluripotent stem cells and embryonic neural stem/progenitor cells.

Spatiotemporal Regulation of Rho GTPases in Neuronal Migration

Neuronal migration is essential for the orchestration of brain development and involves several contiguous steps: interkinetic nuclear movement (INM), multipolar–bipolar transition, locomotion, and translocation. Growing evidence suggests that Rho GTPases, including RhoA, Rac, Cdc42, and the atypical Rnd members, play critical roles in neuronal migration by regulating both actin and microtubule cytoskeletal components. This review focuses on the spatiotemporal-specific regulation of Rho GTPases as well as their regulators and effectors in distinct steps during the neuronal migration process. Their roles in bridging extracellular signals and cytoskeletal dynamics to provide optimal structural support to the migrating neurons will also be discussed.

Linking Cell Polarity to Cortical Development and Malformations

Cell polarity refers to the asymmetric distribution of signaling molecules, cellular organelles, and cytoskeleton in a cell. Neural progenitors and neurons are highly polarized cells in which the cell membrane and cytoplasmic components are compartmentalized into distinct functional domains in response to internal and external cues that coordinate polarity and behavior during development and disease. In neural progenitor cells, polarity has a prominent impact on cell shape and coordinate several processes such as adhesion, division, and fate determination. Polarity also accompanies a neuron from the beginning until the end of its life. It is essential for development and later functionality of neuronal circuitries. During development, polarity governs transitions between multipolar and bipolar during migration of postmitotic neurons, and directs the specification and directional growth of axons. Once reaching final positions in cortical layers, neurons form dendrites which become compartmentalized to ensure proper establishment of neuronal connections and signaling. Changes in neuronal polarity induce signaling cascades that regulate cytoskeletal changes, as well as mRNA, protein, and vesicle trafficking, required for synapses to form and function. Hence, defects in establishing and maintaining cell polarity are associated with several neural disorders such as microcephaly, lissencephaly, schizophrenia, autism, and epilepsy. In this review we summarize the role of polarity genes in cortical development and emphasize the relationship between polarity dysfunctions and cortical malformations.

Growth Arrest Triggers Extra-Cell Cycle Regulatory Function in Neurons: Possible Involvement of p27kip1 in Membrane Trafficking as Well as Cytoskeletal Regulation

Cell cycle regulation is essential for the development of multicellular organisms, but many cells in adulthood, including neurons, exit from cell cycle. Although cell cycle-related proteins are suppressed after cell cycle exit in general, recent studies have revealed that growth arrest triggers extra-cell cycle regulatory function (EXCERF) in some cell cycle proteins, such as p27(kip1), p57(kip2), anaphase-promoting complex/cyclosome (APC/C), and cyclin E. While p27 is known to control G1 length and cell cycle exit via inhibition of cyclin-dependent kinase (CDK) activities, p27 acquires additional cytoplasmic functions in growth-arrested neurons. Here, we introduce the EXCERFs of p27 in post-mitotic neurons, mainly focusing on its actin and microtubule regulatory functions. We also show that a small amount of p27 is associated with the Golgi apparatus positive for Rab6, p115, and GM130, but not endosomes positive for Rab5, Rab7, Rab8, Rab11, SNX6, or LAMTOR1. p27 is also colocalized with Dcx, a microtubule-associated protein. Based on these results, we discuss here the possible role of p27 in membrane trafficking and microtubule-dependent transport in post-mitotic cortical neurons. Collectively, we propose that growth arrest leads to two different fates in cell cycle proteins; either suppressing their expression or activating their EXCERFs. The latter group of proteins, including p27, play various roles in neuronal migration, morphological changes and axonal transport, whereas the re-activation of the former group of proteins in post-mitotic neurons primes for cell death.

The Mechanism of Rap1 Regulates N-cadherin to Control Neuronal Migration

Rap1 and N-cadherin regulate glia-independent translocation of cortical neurons. It remains unclear how Rap1 regulates N-cadherin-mediated neuronal migration. Here, we overexpressed Rap1gap in mouse brains (embryonic day 16) to inactivate Rap1, and observed that neurons did not migrate to the outer layer. We confirmed that Rap1 was involved in the regulation of late neurons in vivo. Rap1gap overexpression and Rap1 suppression in CHO cells decreased the expression of cytoskeletal proteins such as tubulin. Changes in the expression of cell morphology regulators, such as N-cadherin and β-catenin, were also observed. Inhibition of N-cadherin in mouse brains prevented neuronal migration to the outer layer. The morphology of CHO cells was changed after overexpression of Rap1gap. We propose that Rap1 regulates the expression of N-cadherin during embryonic development, which affects β-catenin expression. Beta-catenin in turn regulates cytoskeletal protein expression, ultimately affecting neuronal morphology and migration.

[Pharmacological approach to cerebral cortical development]

In the developing mammalian cerebral cortex, newly generated neurons migrate toward the pial surface to form a mammalian-specific six-layered cerebral cortex. Genetic studies of human neurological diseases have suggested the involvement of several molecules in cortical neuronal migration. In vivo electroporation is another powerful tool for understanding the molecular mechanisms of neuronal migration. By using these techniques, however, it is difficult to understand molecular basis of time-dependent changes of neuronal morphologies. Here, we introduce a pharmacological approach to cerebral cortical development. Major advantages of the pharmacological approach include the transient suppression of molecules of interest and analyzing time-dependent changes of neuronal morphologies. It also allows us to search molecules regulating neuronal migration with comparative ease. We propose the complementarity between the pharmacological approach and genetics or in vivo electroporation experiments.

Vitronectin is Involved in the Morphological Transition of Neurites in Retinoic Acid-Induced Neurogenesis of Neuroblastoma Cell Line Neuro2a

Vitronectin (Vtn), one of the extracellular matrix proteins, has been reported to result in cell cycle exit, neurite formation, and polarization of neural progenitor cells during neurogenesis. The underlying mechanism, however, has not been fully understood. In this study, we investigated the roles of Vtn and its integrin receptors, during the transition of neurites from multipolar to bipolar morphology, accompanying the cell cycle exit in neural progenitor cells. We used mouse neuroblastoma cell line Neuro2a as a model of neural progenitor cells which can induce cell cycle exit and the morphological transition of neurites by retinoic acid (RA)-stimulation. Treatment with an antibody for Vtn suppressed the RA-induced cell cycle exit and multipolar-to-bipolar transition. Furthermore, immunostaining results showed that in the cells displaying multipolar morphology Vtn was partially localized at the tips of neurites and in cells displaying bipolar morphology at both tips. This Vtn localization and multipolar-to-bipolar transition was perturbed by the transfection of a dominant negative mutant of cell polarity regulator Par6. In addition, a knockdown of β5 integrin, which is a receptor candidate for Vtn, affected the multipolar-to-bipolar transition. Taken together, these results suggest that Vtn regulates the multipolar-to-bipolar morphological transition via αvβ5 integrin.

p27kip1 at the crossroad between actin and microtubule dynamics

The p27^kip1 protein, mainly known as a negative regulator of cell proliferation, has also been involved in the control of other cellular processes, including the regulation of cytoskeleton dynamics. Notably, these two functions involve distinct protein domains, residing in the N- and C-terminal halves, respectively. In the last two decades, p27^kip1 has been reported to interact with microtubule and acto-myosin cytoskeletons, both in direct and indirect ways, overall drawing a picture in which several factors play their role either in synergy or in contrast one with another. As a result, the role of p27^kip1 in cytoskeleton dynamics has been implicated in cell migration, both in physiologic and in neoplastic contexts, modulating cytokinesis, lipid raft trafficking, and neuronal development. Recently, two distinct papers have further reported a central role for p27^kip1 in the control of microtubule stability and post-translational modifications, dissecting the interaction between p27^kip1 and α-tubulin-acetyl-transferase (α-TAT), an enzyme involved in the stability of microtubules, and protein-regulator of cytokinesis 1 (PRC1), a nuclear regulator of the central spindle during mitosis. In light of these recent evidences, we will comment on the role of p27^kip1 on cytoskeleton regulation and its implication for cancer progression.

Sema6A-plexin-A2 axis stimulates RANKL-induced osteoclastogenesis through PLCγ-mediated NFATc1 activation

Recently, several plexins and semaphorins have been associated with osteoclastogenesis, a vital process for bone remodeling. Plexin-A2 is implicated in bone homeostasis, however, whether it plays a role in osteoclastogenesis and the underlying mechanism remain unknown. We show that plexin-A2 expression is upregulated during RANKL-induced osteoclastogenesis. In addition, the soluble Sema6A fused with IgG1 Fc region (Fc-Sema6A) interacts with plexin-A2 from cell lysates of osteoclasts, suggesting that plexin-A2 acts as a receptor of Sema6A in osteoclasts. Moreover, Sema6A treatment stimulates RANKL-induced osteoclastogenesis, and this effect is abolished when plexin-A2 is neutralized, which illustrates an indispensable role of plexin-A2 in mediating Sema6A effect on osteoclastogenesis. Mechanistically, Sema6A-plexin-A2 axis enhances RANKL-induced activation of PLCγ as well as downstream target NFATc1, one master transcriptional factor of osteoclastogenesis. Lastly, inhibition of PLCγ by pharmacological inhibitor U73122 abrogates Sema6A-stimulated NFATc1 activation and RANKL-induced osteoclastogenesis, thus demonstrating that the PLCγ-mediated NFATc1 activation accounts for the promotive role of Sema6A-plexin-A2 axis in RANKL-induced osteoclastogenesis. Taken together, this study uncovers a novel role of Sema6A and plexin-A2 in osteoclastogenesis, and also offers them as possible therapeutic targets in the intervention of osteolytic diseases.

A novel role for CAMKIIβ in the regulation of cortical neuron migration: implications for neurodevelopmental disorders

Perturbation of CaMKIIβ expression has been associated with multiple neuropsychiatric diseases, highlighting CaMKIIβ as a gene of interest. Yet, in contrast to CaMKIIα, the specific functions of CaMKIIβ in the brain remain poorly explored. Here, we reveal a novel function for this CaMKII isoform in vivo during neuronal development. By using in utero electroporation, we show that CaMKIIβ is an important regulator of radial migration of projection neurons during cerebral cortex development. Knockdown of CaMKIIβ causes accelerated migration of nascent pyramidal neurons, whereas overexpression of CaMKIIβ inhibits migration, demonstrating that precise regulation of CaMKIIβ expression is required for correct neuronal migration. More precisely, CaMKIIβ controls the multipolar-bipolar transition in the intermediate zone and locomotion in the cortical plate through its actin-binding and -bundling activities. In addition, our data indicate that a fine-tuned balance between CaMKIIβ and cofilin activities is necessary to ensure proper migration of cortical neurons. Thus, our findings define a novel isoform-specific function for CaMKIIβ, demonstrating that CaMKIIβ has a major biological function in the developing brain.

The prognostic value of Cyclin-Dependent Kinase 5 and Protein Phosphatase 2A in Gastric Cancer

Purpose To discuss the relationship between the clinicopathological data, long-term survival of gastric cancer patients and different expression levels of Cyclin-Dependent Kinase 5 (CDK5) and Protein Phosphatase 2A (PP2A). Method The expression levels of CDK5 and PP2A were detected by immunohistochemistry in specimens from 124 patients with primary gastric cancer. The correlation among the expression of CDK5 and PP2A, clinicopathological factors and prognosis was investigated. Result The expression level of CDK5 was correlated with the TNM stage (p=0.030) and N stage (p=0.001), while the expression level of PP2A was correlated with the TNM stage and N stage (p=0.001 and p=0.004) as well as the degree of differentiation (p=0.046). The expression of CDK5 was positively correlated with the expression of PP2A in gastric cancer. Co-expression of CDK5 and PP2A is an independent prognostic factor that affected overall survival, and provided more accurate prognostic value for the overall survival of gastric cancer patients. Conclusion The expression of CDK5 and PP2A is positively correlated in gastric cancer. Co-expression of CDK5 and PP2A was an independent prognostic factor in patients with gastric cancer.

Identification and characterization of a novel phosphoregulatory site on cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (CDK5) is a serine/threonine kinase essential for embryonic development whose overactivation has been implicated in several pathologies including neurodegeneration, cancer cell metastasis and type II diabetes. Therefore, it is important to investigate molecular mechanism(s) that mediate regulation of CDK5 activity. Here we identify and characterize a novel phosphoregulatory site on CDK5. Our mass spectrometry analysis identified seven putative phosphorylation sites on CDK5. Using phosphomimetic and non-phosphorylatable mutants, we determined that phosphorylation of S47, one of the identified sites, renders the kinase catalytically inactive. The inactivation of the kinase due to the phosphomimetic change at S47 results from inhibition of its interaction with its cognate activator, p35. We connect the effect of this regulatory event to a cellular phenotype by showing that the S47D CDK5 mutant inhibits cell migration and promotes cell proliferation. Together, these results have uncovered a potential physiological mechanism to regulate CDK5 activity. The evolutionary placement of a phosphorylatable residue (S/T) at this position not only in CDK5 but also in the majority of other CDK family members suggests that this phosphosite may represent a shared regulatory mechanism across the CDK family.

Caveolin-1 Promotes Early Neuronal Maturation via Caveolae-Independent Trafficking of N-Cadherin and L1

Axon specification is morphologically reproducible in vitro, whereas dendrite formation differs in vitro and in vivo. Cortical neurons initially develop immature neurites, but in vivo these are eliminated concurrently with the formation of a leading process, the future dendrite. However, the molecular mechanisms underlying these neuronal maturation events remain unclear. Here we show that caveolin-1, a major component of caveolae that are never observed in neurons, regulates in vivo-specific steps of neuronal maturation. Caveolin-1 is predominantly expressed in immature cortical neurons and regulates clathrin-independent endocytosis. In vivo knockdown of caveolin-1 disturbs immature neurite pruning, leading process elongation, and subsequent neuronal migration. Importantly, N-cadherin and L1, which are required for immature neurite formation, undergo caveolin-1-mediated endocytosis to eliminate immature neurites. Collectively, our findings indicate that caveolin-1 regulates N-cadherin and L1 trafficking independent of caveolae, which contributes to spatiotemporally restricted cellular events; immature neurite pruning and leading process elongation during early neuronal maturation.

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Clathrin-independent endocytosis regulates early neuronal maturation

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Caveolin-1 promotes immature neurite pruning and leading process elongation in vivo

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Caveolin-1 controls N-cadherin and L1 trafficking independent of caveolae

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Suppression of caveolin-1 results in defects in cortical neuronal migration

Synaptic roles of cyclin-dependent kinase 5 & its implications in epilepsy

There is an urgent need to understand the molecular mechanisms underlying epilepsy to find novel prognostic/diagnostic biomarkers to prevent epilepsy patients at risk. Cyclin-dependent kinase 5 (CDK5) is involved in multiple neuronal functions and plays a crucial role in maintaining homeostatic synaptic plasticity by regulating intracellular signalling cascades at synapses. CDK5 deregulation is shown to be associated with various neurodegenerative diseases such as Alzheimer's disease. The association between chronic loss of CDK5 and seizures has been reported in animal models of epilepsy. Genetic expression of CDK5 at transcriptome level has been shown to be abnormal in intractable epilepsy. In this review various possible mechanisms by which deregulated CDK5 may alter synaptic transmission and possibly lead to epileptogenesis have been discussed. Further, CDK5 has been proposed as a potential biomarker as well as a pharmacological target for developing treatments for epilepsy.

Gβ2 Regulates the Multipolar-Bipolar Transition of Newborn Neurons in the Developing Neocortex

Proper neuronal migration is critical for the formation of the six-layered neocortex in the mammalian brain. However, the precise control of neuronal migration is not well understood. Heterotrimeric guanine nucleotide binding proteins (G proteins), composed of Gα and Gβγ, transduce signals from G protein-coupled receptors to downstream effectors and play crucial roles in brain development. However, the functions of individual subunits of G proteins in prenatal brain development remain unclear. Here, we report that Gβ2 is expressed in the embryonic neocortex, with abundant expression in the intermediate zone, and is significantly upregulated in differentiated neurons. Perturbation of Gβ2 expression impairs the morphogenetic transformation of migrating neurons from multipolar to bipolar and subsequently delays neuronal migration. Moreover, Gβ2 acts as a scaffold protein to organize the MP1-MEK1-ERK1/2 complex and mediates the phosphorylation of ERK1/2. Importantly, expression of a constitutively active variant of MEK1 rescues the migration defects that are caused by the loss of Gβ2. In conclusion, our findings reveal that Gβ2 regulates proper neuronal migration during neocortex development by activating the ERK1/2 signaling pathway.

Meis1 Coordinates Cerebellar Granule Cell Development by Regulating Pax6 Transcription, BMP Signaling and Atoh1 Degradation

Cerebellar granule cell precursors (GCPs) and granule cells (GCs) represent good models to study neuronal development. Here, we report that the transcription factor myeloid ectopic viral integration site 1 homolog (Meis1) plays pivotal roles in the regulation of mouse GC development. We found that Meis1 is expressed in GC lineage cells and astrocytes in the cerebellum during development. Targeted disruption of the Meis1 gene specifically in the GC lineage resulted in smaller cerebella with disorganized lobules. Knock-down/knock-out (KO) experiments for Meis1 and in vitro assays showed that Meis1 binds to an upstream sequence of Pax6 to enhance its transcription in GCPs/GCs and also suggested that the Meis1-Pax6 cascade regulates morphology of GCPs/GCs during development. In the conditional KO (cKO) cerebella, many Atoh1-positive GCPs were observed ectopically in the inner external granule layer (EGL) and a similar phenomenon was observed in cultured cerebellar slices treated with a bone morphogenic protein (BMP) inhibitor. Furthermore, expression of Smad proteins and Smad phosphorylation were severely reduced in the cKO cerebella and Meis1-knock-down GCPs cerebella. Reduction of phosphorylated Smad was also observed in cerebellar slices electroporated with a Pax6 knock-down vector. Because it is known that BMP signaling induces Atoh1 degradation in GCPs, these findings suggest that the Meis1-Pax6 pathway increases the expression of Smad proteins to upregulate BMP signaling, leading to degradation of Atoh1 in the inner EGL, which contributes to differentiation from GCPs to GCs. Therefore, this work reveals crucial functions of Meis1 in GC development and gives insights into the general understanding of the molecular machinery underlying neural differentiation from neural progenitors.SIGNIFICANCE STATEMENT We report that myeloid ectopic viral integration site 1 homolog (Meis1) plays pivotal roles in the regulation of mouse granule cell (GC) development. Here, we show Meis1 is expressed in GC precursors (GCPs) and GCs during development. Our knock-down and conditional knock-out (cKO) experiments and in vitro assays revealed that Meis1 is required for proper cerebellar structure formation and for Pax6 transcription in GCPs and GCs. The Meis1-Pax6 cascade regulates the morphology of GCs. In the cKO cerebella, Smad proteins and bone morphogenic protein (BMP) signaling are severely reduced and Atoh1-expressing GCPs are ectopically detected in the inner external granule layer. These findings suggest that Meis1 regulates degradation of Atoh1 via BMP signaling, contributing to GC differentiation in the inner EGL, and should provide understanding into GC development.

MUNC18-1 gene abnormalities are involved in neurodevelopmental disorders through defective cortical architecture during brain development

While Munc18–1 interacts with Syntaxin1 and controls the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) complex to regulate presynaptic vesicle fusion in developed neurons, this molecule is likely to be involved in brain development since its gene abnormalities cause early infantile epileptic encephalopathy with suppression-burst (Ohtahara syndrome), neonatal epileptic encephalopathy and other neurodevelopmental disorders. We thus analyzed physiological significance of Munc18–1 during cortical development. Munc18–1-knockdown impaired cortical neuron positioning during mouse corticogenesis. Time-lapse imaging revealed that the mispositioning was attributable to defects in radial migration in the intermediate zone and cortical plate. Notably, Syntaxin1A was critical for radial migration downstream of Munc18–1. As for the underlying mechanism, Munc18–1-knockdown in cortical neurons hampered post-Golgi vesicle trafficking and subsequent vesicle fusion at the plasma membrane in vivo and in vitro, respectively. Notably, Syntaxin1A-silencing did not affect the post-Golgi vesicle trafficking. Taken together, Munc18–1 was suggested to regulate radial migration by modulating not only vesicle fusion at the plasma membrane to distribute various proteins on the cell surface for interaction with radial fibers, but also preceding vesicle transport from Golgi to the plasma membrane. Although knockdown experiments suggested that Syntaxin1A does not participate in the vesicle trafficking, it was supposed to regulate subsequent vesicle fusion under the control of Munc18–1. These observations may shed light on the mechanism governing radial migration of cortical neurons. Disruption of Munc18–1 function may result in the abnormal corticogenesis, leading to neurodevelopmental disorders with MUNC18–1 gene abnormalities.

Cdk5 Regulation of the GRAB-Mediated Rab8-Rab11 Cascade in Axon Outgrowth

Neurons communicate with each other through their axons and dendrites. However, a full characterization of the molecular mechanisms involved in axon and dendrite formation is still incomplete. Neurite outgrowth requires the supply of membrane components for surface expansion. Two membrane sources for axon outgrowth are suggested: Golgi secretary vesicles and endocytic recycling endosomes. In non-neuronal cells, trafficking of secretary vesicles from Golgi is regulated by Rab8, a member of Rab small GTPases, and that of recycling endosomes is by Rab11, another member of Rabs. However, whether these vesicles are coordinately or independently transported in growing axons is unknown. Herein, we find that GRAB, a guanine nucleotide exchange factor for Rab8, is a novel regulator of axon outgrowth. Knockdown of GRAB suppressed axon outgrowth of cultured mouse brain cortical neurons. GRAB mediates the interaction between Rab11A and Rab8A, and this activity is regulated by phosphorylation at Ser169 and Ser180 by Cdk5-p35. The nonphosphorylatable GRAB mutant S169/180A promoted axonal outgrowth to a greater extent than did the phosphomimetic GRAB mutant S169/180D. Phosphorylation of GRAB suppressed its guanine nucleotide exchange factor activity and its ability to recruit Rab8A- to Rab11A-positive endosomes. In vivo function of GRAB and its Cdk5-phophorylation were shown in migration and process formation of developing neurons in embryonic mouse brains. These results indicate that GRAB regulates axonal outgrowth via activation and recruitment of Rab8A- to Rab11A-positive endosomes in a Cdk5-dependent manner.

SIGNIFICANCE STATEMENT

While axon outgrowth requires membrane supply for surface expansion, the molecular mechanisms regulating the membrane transport in growing axons remain unclear. Here, we demonstrate that GRAB, a guanine nucleotide exchange factor for Rab8, is a novel regulator of axon outgrowth. GRAB promotes the axonal membrane transport by mediating the interaction between Rab11 and Rab8 in neurons. The activity of GRAB is regulated by phosphorylation with Cdk5. We describe an in vivo role for GRAB and its Cdk5 phosphorylation during neuronal migration and process formation in embryonic brains. Thus, the membrane supply for axonal outgrowth is regulated by Cdk5 through the Rab11-GRAB-Rab8 cascade.

Morphological and Molecular Basis of Cytoplasmic Dilation and Swelling in Cortical Migrating Neurons

During corticogenesis, neuronal migration is an essential step for formation of a functional brain, and abnormal migration is known to cause various neurological disorders. Neuronal migration is not just a simple movement of the cell body, but a consequence of various morphological changes and coordinated subcellular events. Recent advances in in vivo and ex vivo cell biological approaches, such as in utero gene transfer, slice culture and ex vivo chemical inhibitor techniques, have revealed details of the morphological and molecular aspects of neuronal migration. Migrating neurons have been found to have a unique structure, dilation or swelling, at the proximal region of the leading process; this structure is not found in other migrating cell types. The formation of this structure is followed by nuclear deformation and forward movement, and coordination of this three-step sequential morphological change (the dilation/swelling formation, nuclear elongation and nuclear movement) is essential for proper neuronal migration and the construction of a functional brain structure. In this review, we will introduce the morphological features of this unique structure in migrating neurons and summarize what is known about the molecules regulating the dilation/swelling formation and nuclear deformation and movement.