Microtubule Affinity-regulating Kinase 2 Research Articles

SARS-CoV-2-derived protein Orf9b enhances MARK2 activity via interaction with the autoinhibitory KA1 domain.

Microtubule affinity-regulating kinase 2 (MARK2) is a Ser/Thr protein kinase that regulates cell polarity and immune responses. Here, we report that Orf9b, one of the accessory proteins encoded in the SARS-CoV-2 genome, increases MARK2 activity via interaction with the autoinhibitory KAI domain. We found that co-expression of Orf9b enhances the kinase activity of MARK2 in HEK293 cells. Orf9b does not bind to or enhance the activity of the mutant form of MARK2 lacking the KA1 domain. Orf9b lowers inhibitory phosphorylation of MARK2 at T595 while mutation experiments indicate that this site is dispensable for Orf9b-mediated enhancement of MARK2 activity. Our results suggest that Orf9b enhances MARK2 activity by binding the autoinhibitory KA1 domain, which closely interacts with the kinase domain.

Role of MARK2 in the nervous system and cancer.

Microtubule-Affinity Regulating Kinase 2 (MARK2), a member of the serine/threonine protein kinase family, phosphorylates microtubule-associated proteins, playing a crucial role in cancer and neurodegenerative diseases. This kinase regulates multiple signaling pathways, including the WNT, PI3K/AKT/mTOR (PAM), and NF-κB pathways, potentially linking it to cancer and the nervous system. As a crucial regulator of the PI3K/AKT/mTOR pathway, the loss of MARK2 inhibits the growth and metastasis of cancer cells. MARK2 is involved in the excessive phosphorylation of tau, thus influencing neurodegeneration. Therefore, MARK2 emerges as a promising drug target for the treatment of cancer and neurodegenerative diseases. Despite its significance, the development of inhibitors for MARK2 remains limited. In this review, we aim to present detailed information on the structural features of MARK2 and its role in various signaling pathways associated with cancer and neurodegenerative diseases. Additionally, we further characterize the therapeutic potential of MARK2 in neurodegenerative diseases and cancer, and hope to facilitate basic research on MARK2 and the development of inhibitors targeting MARK2.

Specific binding of Hsp27 and phosphorylated Tau mitigates abnormal Tau aggregation-induced pathology.

Amyloid aggregation of phosphorylated Tau (pTau) into neurofibrillary tangles is closely associated with Alzheimer's disease (AD). Several molecular chaperones have been reported to bind Tau and impede its pathological aggregation. Recent findings of elevated levels of Hsp27 in the brains of patients with AD suggested its important role in pTau pathology. However, the molecular mechanism of Hsp27 in pTau aggregation remains poorly understood. Here, we show that Hsp27 partially co-localizes with pTau tangles in the brains of patients with AD. Notably, phosphorylation of Tau by microtubule affinity regulating kinase 2 (MARK2), dramatically enhances the binding affinity of Hsp27 to Tau. Moreover, Hsp27 efficiently prevents pTau fibrillation in vitro and mitigates neuropathology of pTau aggregation in a Drosophila tauopathy model. Further mechanistic study reveals that Hsp27 employs its N-terminal domain to directly interact with multiple phosphorylation sites of pTau for specific binding. Our work provides the structural basis for the specific recognition of Hsp27 to pathogenic pTau, and highlights the important role of Hsp27 in preventing abnormal aggregation and pathology of pTau in AD.

Identification of a reciprocal negative feedback loop between tau-modifying proteins MARK2 kinase and CBP acetyltransferase

The posttranslational regulation of the neuronal proteome is critical for brain homeostasis but becomes dysregulated in the aged or diseased brain, in which abnormal posttranslational modifications (PTMs) are frequently observed. While the full extent of modified substrates that comprise the “PTM-ome” are slowly emerging, how the upstream enzymes catalyzing these processes are regulated themselves is not well understood, particularly in the context of neurodegeneration. Here, we describe the reciprocal regulation of a kinase, the microtubule affinity-regulating kinase 2 (MARK2), and an acetyltransferase, CREB-binding protein (CBP), two enzymes known to extensively modify tau proteins in the progression of Alzheimer’s disease. We found that MARK2 negatively regulates CBP and, conversely, CBP directly acetylates and inhibits MARK2 kinase activity. These findings highlight a reciprocal negative feedback loop between a kinase and an acetyltransferase, which has implications for how PTM interplay is coordinated on substrates including tau. Our study suggests that PTM profiles occur through the posttranslational control of the master PTM remodeling enzymes themselves.

Long non-coding RNA ABHD11-AS1 facilitates the progression of cervical cancer by competitively binding to miR-330–5p and upregulating MARK2

Cervical cancer (CC) is among the most prevalent gynecological malignancies. Participation of long non-coding RNA (lncRNA) in modulating biological behaviors of CC cells has been confirmed. However, the function of lncRNA ABHD11 antisense RNA 1 (ABHD11-AS1) in CC is still unclear. RT-qPCR and Western blot were performed for measuring RNA and protein levels. Functional assays were done to evaluate ABHD11-AS1 influences on cell proliferation, apoptosis, invasion and migration. After the verification of ABHD11-AS1 distribution in CC cells, mechanism assays were conducted to study the interaction of relative RNAs. ABHD11-AS1 expression was abnormally high in CC cells. In vitro experiments showed ABHD11-AS1 downregulation restrained CC cell malignant phenotypes. In vivo experiments proved ABHD11-AS1 knockdown impeded tumor growth. Moreover, miR-330–5p was corroborated to bind with ABHD11-AS1 in CC cells and microtubule affinity regulating kinase 2 (MARK2) was confirmed to be targeted by miR-330–5p. MiR-330–5p inhibition or MARK2 overexpression could countervail the suppressive effect of ABHD11-AS1 knockdown on CC cell malignant behaviors. We found that ABHD11-AS1 facilitated CC tumorigenesis through competitively sequestering miR-330–5p to upregulate MARK2, indicating ABHD11-AS1 as a potential biomarker in CC.

MARK2 phosphorylates eIF2α in response to proteotoxic stress.

The regulation of protein synthesis is essential for maintaining cellular homeostasis, especially during stress responses, and its dysregulation could underlie the development of human diseases. The critical step during translation regulation is the phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α). Here we report the identification of a direct kinase of eIF2α, microtubule affinity-regulating kinase 2 (MARK2), which phosphorylates eIF2α in response to proteotoxic stress. The activity of MARK2 was confirmed in the cells lacking the 4 previously known eIF2α kinases. MARK2 itself was found to be a substrate of protein kinase C delta (PKCδ), which serves as a sensor for protein misfolding stress through a dynamic interaction with heat shock protein 90 (HSP90). Both MARK2 and PKCδ are activated via phosphorylation in proteotoxicity-associated neurodegenerative mouse models and in human patients with amyotrophic lateral sclerosis (ALS). These results reveal a PKCδ-MARK2-eIF2α cascade that may play a critical role in cellular proteotoxic stress responses and human diseases.

Establishment of nerve regneration animal model following spinal cord injury labed with biotinylated dextran amine

Objective To study the role of microtubule affinity- regulating kinase 2 (MARK2) in nerve regeneration in an animal model after spinal cord injury (SCI) labeled with biotinylated dextran amine (BDA) using stereotactic injection of. Methods Sixteen SD rats were randomly divided into control group (4 rats) and SCI groups (12 rats). Spinal cord was cut off by scissors in spinal cord injury group. After SCI model was established in 12 rats, normal saline subgroup (4 rats), EGFP-adenovirus subgroup (4 rats), and MARK2-adenovirus subgroup (4 rats) were given the injection of normal saline, EGFP-adenovirus and MARK2-adenovirus into cortical motor area respectively by stereotactic injection technique. One week later, 16 rats were injected with BDA in cortical motor area. After 2 weeks, the animals were executed and the whole brain and spinal cord were removed. Results In SCI groups, rats had lower limb paraplegia (100% success rate). After stereotactic injection of BDA into the motor cortex, the nerve conduction tracts labeled by BDA were confirmed by confocal laser scanning (94% success rate). Conclusion The animal model established by this method is safe, reliable and reproducible. Key words: Spinal cord injury; Model, animal; Nerve regeneration; Biotinylated dextran amine tracing

Inhibiting Phosphorylation of Tau (τ) Proteins at Ser262 Using Peptide-Based R1 Domain Mimetics

The microtubule-associated protein tau (τ) is a phosphoprotein that is crucial for regulating microtubule dynamics. Tau is highly enriched in neurons, where it functions by binding tubulin and stabilizing axonal microtubules. Phosphorylation of tau within its microtubule-binding repeat (R) domains significantly reduces its affinity for tubulin, leading to a loss in microtubule stability. In neurons, dysregulated kinase activity often results in the formation of hyper-phosphorylated tau isoforms that remain permanently detached from microtubules. If left untreated, hyper-phosphorylated tau can aggregate into insoluble, prion-like oligomers that contribute to the pathogenesis of neurodegenerative disease. Consequently, there is considerable interest in developing inhibitors that reduce levels of hyper-phosphorylated tau in neurons. In this study, we have generated a synthetic peptide mimetic (tR1) of the tau R1 domain as an inhibitor of microtubule-affinity regulating kinase 2 (MARK2). In vitro assays showed that tR1 inhibits the MARK2-mediated phosphorylation of tau within its R1 domain at Ser262, a residue critical for favorable tau-tubulin interactions. We also demonstrate that tR1 peptides are > 90% stable up to 24 h in neurobasal medium and RPMI media supplemented with human serum. Uptake experiments in cultured rat primary cortical neurons indicate that tR1 is internalized through an energy-dependent mechanism and can be delivered to the cytoplasm when co-treated with bafilomycin A1 or chloroquine. Furthermore, we show tR1 inhibits phosphorylation of endogenous tau at Ser262 in cultured neurons following activation of intracellular kinases. This inhibitory effect was selective for kinases that phosphorylate tau at Ser262, as tR1 did not inhibit tau phosphorylation at Thr231. Collectively, these results establish tR1 as a highly-stable, peptide-based kinase inhibitor that reduces the level of phosphorylated tau proteins in neurons.

Vangl2 regulates spermatid planar cell polarity through microtubule (MT)-based cytoskeleton in the rat testis

During spermatogenesis, developing elongating/elongated spermatids are highly polarized cells, displaying unique apico-basal polarity. For instance, the heads of spermatids align perpendicular to the basement membrane with their tails pointing to the tubule lumen. Thus, the maximal number of spermatids are packed within the limited space of the seminiferous epithelium to support spermatogenesis. Herein, we reported findings that elongating/elongated spermatids displayed planar cell polarity (PCP) in adult rat testes in which the proximal end of polarized spermatid heads were aligned uniformly across the plane of the seminiferous epithelium based on studies using confocal microscopy and 3-dimensional (D) reconstruction of the seminiferous tubules. We also discovered that spermatid PCP was regulated by PCP protein Vangl2 (Van Gogh-like protein 2) since Vangl2 knockdown by RNAi was found to perturb spermatid PCP. More important, Vangl2 exerted its regulatory effects through changes in the organization of the microtubule (MT)-based cytoskeleton in the seminiferous epithelium. These changes were mediated via the downstream signaling proteins atypical protein kinase C ξ (PKCζ) and MT-associated protein (MAP)/microtubule affinity-regulating kinase 2 (MARK2). These findings thus provide new insights regarding the biology of spermatid PCP during spermiogenesis.

Localized Phosphorylation of a Kinesin-1 Adaptor by a Capsid-Associated Kinase Regulates HIV-1 Motility and Uncoating

Although microtubule motors mediate intracellular virus transport, the underlying interactions and control mechanisms remain poorly defined. This is particularly true for HIV-1 cores, which undergo complex, interconnected processes of cytosolic transport, reverse transcription, and uncoating of the capsid shell. Although kinesins have been implicated in regulating these events, curiously, there are no direct kinesin-core interactions. We recently showed that the capsid-associated kinesin-1 adaptor protein, fasciculation and elongation protein zeta-1 (FEZ1), regulates HIV-1 trafficking. Here, we show that FEZ1 and kinesin-1 heavy, but not light, chains regulate not only HIV-1 transport but also uncoating. This required FEZ1 phosphorylation, which controls its interaction with kinesin-1. HIV-1 did not stimulate widespread FEZ1 phosphorylation but, instead, bound microtubule (MT) affinity-regulating kinase 2 (MARK2) to stimulate FEZ1 phosphorylation on viral cores. Our findings reveal that HIV-1 binds a regulatory kinase to locally control kinesin-1 adaptor function on viral cores, thereby regulating both particle motility and uncoating.

MARK2 Rescues Nogo-66-Induced Inhibition of Neurite Outgrowth via Regulating Microtubule-Associated Proteins in Neurons In Vitro.

The ability of neurons in the adult mammalian central nervous system (CNS) to regenerate after injury is limited by inhibitors in CNS myelin. Nogo-66 is the most important myelin inhibitor but the mechanisms of Nogo-66 inhibition of neurite outgrowth remain poorly understood. Particularly, the relationship between Nogo-66 and microtubule-affinity regulating kinase 2 (MARK2) has not been examined. This study investigated the role of MARK2 in Nogo-66 inhibition and the function of MARK2 in neurite elongation in neurons in vitro. MARK2 and phosphorylated MARK2 at Ser212 (p-Ser212) alterations in Neuro 2a cells were assessed at different Nogo-66 exposure times; the relationships between MARK2 and microtubule-associated proteins (MAPs) were determined via the overexpression or interference of MARK2. Our study reports that Nogo-66 inhibited the expression of total MARK2 but also reduced Ser212 phosphorylation of MARK2, whereas levels of MAP1-b and tau varied depending on MARK2 overexpression or reduced expression. Furthermore, MARK2 increased the proportion of tyrosinated α-tubulin, thereby disrupting the stability of tubulin, most likely affecting axonal growth. In line with these results, overexpression of MARK2 promoted neurite elongation and therefore is able to rescue the inhibitory effect of Nogo-66 on neurite growth. In conclusion, the intracellular PKB/MARK2/MAPs/α-tubulin pathway appears to be essential for neurite elongation in neurons in vitro. These results suggest a critical role for MARK2 in overcoming Nogo-66-induced inhibition of axon outgrowth in neurons. Pharmacological activators of MARK2 may be applicable to promote successful axonal outgrowth following many types of CNS injuries.

Impact of structural polymorphism for the Helicobacter pylori CagA oncoprotein on binding to polarity-regulating kinase PAR1b.

Chronic infection with cagA-positive Helicobacter pylori is the strongest risk factor for atrophic gastritis, peptic ulcers, and gastric cancer. CagA, the product of the cagA gene, is a bacterial oncoprotein, which, upon delivery into gastric epithelial cells, binds to and inhibits the polarity-regulating kinase, partitioning-defective 1b (PAR1b) [also known as microtubule affinity-regulating kinase 2 (MARK2)], via its CagA multimerization (CM) motif. The inhibition of PAR1b elicits junctional and polarity defects, rendering cells susceptible to oncogenesis. Notably, the polymorphism in the CM motif has been identified among geographic variants of CagA, differing in either the copy number or the sequence composition. In this study, through quantitative analysis of the complex formation between CagA and PAR1b, we found that several CagA species have acquired elevated PAR1b-binding activity via duplication of the CM motifs, while others have lost their PAR1b-binding activity. We also found that strength of CagA-PAR1b interaction was proportional to the degrees of stress fiber formation and tight junctional disruption by CagA in gastric epithelial cells. These results indicate that the CM polymorphism is a determinant for the magnitude of CagA-mediated deregulation of the cytoskeletal system and thereby possibly affects disease outcome of cagA-positive H. pylori infection, including gastric cancer.

Gene expression analysis of microtubule affinity-regulating kinase 2 in non-small cell lung cancer.

Lung cancer is the leading cause of cancer death worldwide, and has a five-year survival rate of 18% [1]. MARK2 is a serine/threonine-protein kinase, and is a key component in the phosphorylation of microtubule-associated proteins [2], [3]. A recent study published by Hubaux et al. found that microtubule affinity-regulating kinase 2 (MARK2) showed highly frequent DNA and RNA level disruption in lung cancer cell lines and independent non-small cell lung cancer (NSCLC) cohorts [4]. These alterations result in the acquisition of oncogenic properties in cell lines, such as increased viability and anchorage-independent growth. Furthermore, a microarray-based transcriptome analysis of three short hairpin RNA (shRNA)-mediated MARK2 knockdown lung adenocarcinoma cell lines (GEO#: GSE57966) revealed an association between MARK2 gene expression and cell cycle activation and DNA damage response. Here, we present a detailed description of transcriptome analysis to support the described role of MARK2 in promoting a malignant phenotype.

Protein Kinase A Rescues Microtubule Affinity-regulating Kinase 2-induced Microtubule Instability and Neurite Disruption by Phosphorylating Serine 409

Microtubule affinity-regulating kinase 2 (MARK2)/PAR-1b and protein kinase A (PKA) are both involved in the regulation of microtubule stability and neurite outgrowth, but whether a direct cross-talk exists between them remains unclear. Here, we found the disruption of microtubule and neurite outgrowth induced by MARK2 overexpression was blocked by active PKA. The interaction between PKA and MARK2 was confirmed by coimmunoprecipitation and immunocytochemistry both in vitro and in vivo. PKA was found to inhibit MARK2 kinase activity by phosphorylating a novel site, serine 409. PKA could not reverse the microtubule disruption effect induced by a serine 409 to alanine (Ala) mutant of MARK2 (MARK2 S409A). In contrast, mutation of MARK2 serine 409 to glutamic acid (Glu) (MARK2 S409E) did not affect microtubule stability and neurite outgrowth. We propose that PKA functions as an upstream inhibitor of MARK2 in regulating microtubule stability and neurite outgrowth by directly interacting and phosphorylating MARK2.

Effects of T208E activating mutation on MARK2 protein structure and dynamics: Modeling and simulation.

Microtubule Affinity-Regulating Kinase 2 (MARK2) protein has a substantial role in regulation of vital cellular processes like induction of polarity, regulation of cell junctions, cytoskeleton structure and cell differentiation. The abnormal function of this protein has been associated with a number of pathological conditions like Alzheimer disease, autism, several carcinomas and development of virulent effects of Helicobacter pyloriββαβββ.

In VivoImaging of Disease-Related Mitochondrial Dynamics in a Vertebrate Model System

Mitochondria provide ATP, maintain calcium homeostasis, and regulate apoptosis. Neurons, due to their size and complex geometry, are particularly dependent on the proper functioning and distribution of mitochondria. Thus disruptions of these organelles and their transport play a central role in a broad range of neurodegenerative diseases. While in vitro studies have greatly expanded our knowledge of mitochondrial dynamics, our understanding in vivo remains limited. To address this shortcoming, we developed tools to study mitochondrial dynamics in vivo in optically accessible zebrafish. We demonstrate here that our newly generated tools, including transgenic "MitoFish," can be used to study the in vivo "life cycle" of mitochondria and allows identifying pharmacological and genetic modulators of mitochondrial dynamics. Furthermore we observed profound mitochondrial transport deficits in real time in a zebrafish tauopathy model. By rescuing this phenotype using MARK2 (microtubule-affinity regulating kinase 2), we provide direct in vivo evidence that this kinase regulates axonal transport in a Tau-dependent manner. Thus, our approach allows detailed studies of the dynamics of mitochondria in their natural environment under normal and disease conditions.

Polarity-regulating Kinase Partitioning-defective 1b (PAR1b) Phosphorylates Guanine Nucleotide Exchange Factor H1 (GEF-H1) to Regulate RhoA-dependent Actin Cytoskeletal Reorganization

Partitioning-defective 1b (PAR1b), also known as microtubule affinity-regulating kinase 2 (MARK2), is a member of evolutionally conserved PAR1/MARK serine/threonine kinase family, which plays a key role in the establishment and maintenance of cell polarity at least partly by phosphorylating microtubule-associated proteins (MAPs) that regulate microtubule stability. PAR1b has also been reported to influence actin cytoskeletal organization, raising the possibility that PAR1b functionally interacts with the Rho family of small GTPases, central regulators of the actin cytoskeletal system. Consistent with this notion, PAR1 was recently found to be physically associated with a RhoA-specific guanine nucleotide exchange factor H1 (GEF-H1). This observation suggests a functional link between PAR1b and GEF-H1. Here we show that PAR1b induces phosphorylation of GEF-H1 on serine 885 and serine 959. We also show that PAR1b-induced serine 885/serine 959 phosphorylation inhibits RhoA-specific GEF activity of GEF-H1. As a consequence, GEF-H1 phosphorylated on both of the serine residues loses the ability to stimulate RhoA and thereby fails to induce RhoA-dependent stress fiber formation. These findings indicate that PAR1b not only regulates microtubule stability through phosphorylation of MAPs but also influences actin stress fiber formation by inducing GEF-H1 phosphorylation. The dual function of PAR1b in the microtubule-based cytoskeletal system and the actin-based cytoskeletal system in the coordinated regulation of cell polarity, cell morphology, and cell movement.

Abstract 3080: Novel splicing activity for RNA binding protein CUGBP2 in radiation-induced mitotic catastrophe

Abstract RNA binding protein CUGBP2 encodes various functions at the posttranscriptional level including alternative splicing (e.g. tau, troponin T), RNA editing (apolipoprotein B) and mRNA translation (cyclooxygenase-2, Mcl1). CUGBP2 contains two RNA recognition motifs (RRMs) near the N terminus, a divergent domain loaded with alanine and glutamine residues, and a third RRM near the C terminus. Based on deletion analyses, the divergent domain is believed to encode activation modules necessary for splicing activity. Recently, we have demonstrated that there are three promoters that drive the expression of the CUGBP2 gene, transcription from which results in three variant forms of the protein. Translation of the transcripts driven from the upstream promoters adds additional amino acids to the N-terminus of CUGBP2. In mouse intestine, the predominant form of the protein is variant 2. However, after exposure to 6 Gy γ-irradiation (IR), the levels of CUGBP2 mRNA shift to being predominantly variant 1. In colon cancer cells, IR induced the cells to undergo mitotic catastrophe, a poorly understood form of cell death where cells escape the G2/M checkpoint, enter mitosis prematurely and concurrently induce an apoptotic pathway. Interestingly, there was an increase in the CUGBP2 variant that is expressed from the proximal promoter (var 1). Furthermore, downregulation of CUGBP2 through specific siRNA significantly reduced the IR-induced mitotic catastrophe. While var 1 predominantly localizes to the nucleus, the other isoforms primarily reside in the cytoplasm. We next sought to determine the mechanism by which CUGBP2 induces mitotic catastrophe. We have observed that it occurs in part through inhibiting the translation of transcripts encoding anti-apoptotic proteins Bcl-2 and Mcl1. CUGBP2 also affected the splicing of two transcripts that encode microtubule-binding proteins. MARK2 (microtubule affinity regulating kinase 2) is involved in the breakdown of the microtubule network, which is important for cell division and CKAP5 (cytoskeleton associated protein 5) is a protein present in the microtubule-microfilament complex. Exons 15 and 36 are alternatively spliced in MARK2 and CKAP5, respectively. Using RT-PCR analyses, we observed that following CUGBP2 var 1 overexpression, these two exons, 15 and 36 were spliced out from the MARK2 and CKAP5 mRNA, respectively. This was associated with the cells undergoing radiation-induced mitotic catastrophe. In summary, CUGBP2 has been shown to undergo alternative promoter usage and alternative use of the first exon following exposure of cells to IR. With the increase in var 1, we observe inhibition of Bcl2 and Mcl1 translation, and increased alternative splicing of MARK2 and CKAP5. These data, taken together, suggest that CUGBP2 regulates cell viability by two different posttranscriptional mechanisms of alternative splicing and translation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3080. doi:10.1158/1538-7445.AM2011-3080

Ca2+/Calmodulin-dependent Protein Kinase IV-mediated LIM Kinase Activation Is Critical for Calcium Signal-induced Neurite Outgrowth

Actin cytoskeletal remodeling is essential for neurite outgrowth. LIM kinase 1 (LIMK1) regulates actin cytoskeletal remodeling by phosphorylating and inactivating cofilin, an actin filament-disassembling factor. In this study, we investigated the role of LIMK1 in calcium signal-induced neurite outgrowth. The calcium ionophore ionomycin induced LIMK1 activation and cofilin phosphorylation in Neuro-2a neuroblastoma cells. Knockdown of LIMK1 or expression of a kinase-dead mutant of LIMK1 suppressed ionomycin-induced cofilin phosphorylation and neurite outgrowth in Neuro-2a cells. Ionomycin-induced cofilin phosphorylation and neurite outgrowth were also blocked by KN-93, an inhibitor of Ca(2+)/calmodulin-dependent protein kinases (CaMKs), and STO-609, an inhibitor of CaMK kinase. An active form of CaMKIV but not CaMKI enhanced Thr-508 phosphorylation of LIMK1 and increased the kinase activity of LIMK1. Moreover, the active form of CaMKIV induced cofilin phosphorylation and neurite outgrowth, and a dominant negative form of CaMKIV suppressed ionomycin-induced neurite outgrowth. Taken together, our results suggest that LIMK1-mediated cofilin phosphorylation is critical for ionomycin-induced neurite outgrowth and that CaMKIV mediates ionomycin-induced LIMK1 activation.

Polarity-Regulating Kinase Partitioning-Defective 1/Microtubule Affinity-Regulating Kinase 2 Negatively Regulates Development of Dendrites on Hippocampal Neurons

Neurons are highly polarized cells that possess two morphologically and functionally different types of protrusions, axons and dendrites, that function in the transmission and reception of neural signals, respectively. A great deal of attention has been paid to the specification and guidance of axons, but the mechanism of dendrite development remains mostly unknown. We report here that a polarity-regulating kinase, partitioning-defective 1 (Par1b)/microtubule affinity-regulating kinase 2 (MARK2), specifically regulates development of dendrites in hippocampal neurons. Ectopic expression of Par1b/MARK2 shortens the length and decreases branching of dendrites without significant effects on axons. Knockdown of endogenous Par1b/MARK2 by RNA interference stimulates dendrite development. Wnt stimulation and Dishevelled expression, both of which are known to induce dendrite development, induced recruitment of Par1b/MARK2 to the membrane fraction. Expression of a Par1b/MARK2 mutant, that contains a myristoylation signal and accumulates exclusively in membranes, does not affect dendrite development. In addition, Par1b/MARK2 efficiently phosphorylated MAP2, which is localized mainly in dendrites. These results indicate that Par1b/MARK2 negatively regulates dendrite development through phosphorylation of MAP2.