Phosphatase Regulator NIPP1 Restrains Chemokine-Driven Skin Inflammation.

Nuclear inhibitor of protein phosphatase 1 (NIPP1) is a ubiquitously expressed nuclear protein that regulates functions of protein serine/threonine phosphatase-1 in cell proliferation and lineage specification. The role of NIPP1 in tissue homeostasis is not fully understood. This study shows that the selective deletion of NIPP1 in mouse epidermis resulted in epidermal hyperproliferation, a reduced adherence of basal keratinocytes, and a gradual decrease in the stemness of hair follicle stem cells, culminating in hair loss. This complex phenotype was associated with chronic sterile skin inflammation and could be partially rescued by dexamethasone treatment. NIPP1-deficient keratinocytes massively expressed proinflammatory chemokines and immunomodulatory proteins in a cell-autonomous manner. Chemokines subsequently induced the recruitment and activation of immune cells, in particular conventional dendritic cells and Langerhans cells, accounting for the chronic inflammation phenotype. The data identifies NIPP1 as a key regulator of epidermal homeostasis and as a potential target for the treatment of inflammatory skin diseases.

Cdk5 phosphorylation of ErbB4 is required for tangential migration of cortical interneurons.

Interneuron dysfunction in humans is often associated with neurological and psychiatric disorders, such as epilepsy, schizophrenia, and autism. Some of these disorders are believed to emerge during brain formation, at the time of interneuron specification, migration, and synapse formation. Here, using a mouse model and a host of histological and molecular biological techniques, we report that the signaling molecule cyclin-dependent kinase 5 (Cdk5), and its activator p35, control the tangential migration of interneurons toward and within the cerebral cortex by modulating the critical neurodevelopmental signaling pathway, ErbB4/phosphatidylinositol 3-kinase, that has been repeatedly linked to schizophrenia. This finding identifies Cdk5 as a crucial signaling factor in cortical interneuron development in mammals.

Fukutin-related protein alters the deposition of laminin in the eye and brain.

Mutations in fukutin-related protein (FKRP) are responsible for a common group of muscular dystrophies ranging from adult onset limb girdle muscular dystrophies to severe congenital forms with associated structural brain involvement. The defining feature of this group of disorders is the hypoglycosylation of α-dystroglycan and its inability to effectively bind extracellular matrix ligands such as laminin α2. However, α-dystroglycan has the potential to interact with a number of laminin isoforms many of which are basement membrane/tissue specific and developmentally regulated. To further investigate this we evaluated laminin α-chain expression in the cerebral cortex and eye of our FKRP knock-down mouse (FKRP(KD)). These mice showed a marked disturbance in the deposition of laminin α-chains including α1, α2, α4, and α5, although only laminin α1- and γ1-chain mRNA expression was significantly upregulated relative to controls. Moreover, there was a diffuse pattern of laminin deposition below the pial surface which correlated with an abrupt termination of many of the radial glial cells. This along with the pial basement membrane defects, contributed to the abnormal positioning of both early- and late-born neurons. Defects in the inner limiting membrane of the eye were associated with a reduction of laminin α1 demonstrating the involvement of the α-dystroglycan:laminin α1 axis in the disease process. These observations demonstrate for the first time that a reduction in Fkrp influences the ability of tissue-specific forms of α-dystroglycan to direct the deposition of several laminin isoforms in the formation of different basement membranes.

Schizophrenia is associated with dysregulation of a Cdk5 activator that regulates synaptic protein expression and cognition.

Cyclin-dependent kinase 5 is activated by small subunits, of which p35 is the most abundant. The functions of cyclin-dependent kinase 5 signalling in cognition and cognitive disorders remains unclear. Here, we show that in schizophrenia, a disorder associated with impaired cognition, p35 expression is reduced in relevant brain regions. Additionally, the expression of septin 7 and OPA1, proteins downstream of truncated p35, is decreased in schizophrenia. Mimicking a reduction of p35 in heterozygous knockout mice is associated with cognitive endophenotypes. Furthermore, a reduction of p35 in mice results in protein changes similar to schizophrenia post-mortem brain. Hence, heterozygous p35 knockout mice model both cognitive endophenotypes and molecular changes reminiscent of schizophrenia. These changes correlate with reduced acetylation of the histone deacetylase 1 target site H3K18 in mice. This site has previously been shown to be affected by truncated p35. By restoring H3K18 acetylation with the clinically used specific histone deacetylase 1 inhibitor MS-275 both cognitive and molecular endophenotypes of schizophrenia can be rescued in p35 heterozygous knockout mice. In summary, we suggest that reduced p35 expression in schizophrenia has an impact on synaptic protein expression and cognition and that these deficits can be rescued, at least in part, by the inhibition of histone deacetylase 1.

Cortical interneurons require p35/Cdk5 for their migration and laminar organization.

Projection neurons and interneurons populate the cerebral cortex in a layer-specific manner. Here, we studied the role of Cyclin-dependent kinase 5 (Cdk5) and its activator p35 in cortical interneuron migration and disposition in the cortex. We found that mice lacking p35 (p35(-/-)) show accumulation of interneurons in the upper part of the cortex. We also observed an inverted distribution of both early- and late-born interneurons, with the former showing a preference for the upper and the latter for the lower aspects of the cortex. We investigated the causes of the altered laminar organization of interneurons in p35(-/-) mice and found a cell-autonomous delay in their tangential migration that may prevent them from reaching correct positions. Incomplete splitting of the preplate in p35(-/-) mice, which causes accumulation of cells in the superficial layer and defects in the "inward" and "outward" components of their radial movement, may also account for the altered final arrangement of interneurons. We, therefore, propose that p35/Cdk5 plays a key role in guiding cortical interneurons to their final positions in the cortex.

The p21-activated kinase is required for neuronal migration in the cerebral cortex.

The normal formation and function of the mammalian cerebral cortex depend on the positioning of its neurones, which occurs in a highly organized, layer-specific manner. The correct morphology and movement of neurones rely on synchronized regulation of their actin filaments and microtubules. The p21-activated kinase (Pak1), a key cytoskeletal regulator, controls neuronal polarization, elaboration of axons and dendrites, and the formation of dendritic spines. However, its in vivo role in the developing nervous system is unclear. We have utilized in utero electroporation into mouse embryo cortices to reveal that both loss and gain of Pak1 function affect radial migration of projection neurones. Overexpression of hyperactivated Pak1 predominantly caused neurones to arrest in the intermediate zone (IZ) with apparently misoriented and disorganized leading projections. Loss of Pak1 disrupted the morphology of migrating neurones, which accumulated in the IZ and deep cortical layers. Unexpectedly, a significant number of neurones with reduced Pak1 expression aberrantly entered into the normally cell-sparse marginal zone, suggesting their inability to cease migrating that may be due to their impaired dissociation from radial glia. Our findings reveal the in vivo importance of temporal and spatial regulation of the Pak1 kinase during key stages of cortical development.

The Pak1 kinase: an important regulator of neuronal morphology and function in the developing forebrain.

The mammalian central nervous system (CNS) represents a highly complex unit, the correct function of which relies on the appropriate differentiation and survival of its neurones. It is becoming apparent that the Rho family of small GTPases and their downstream targets have a major function in regulating CNS development. Among the effectors, the role of the Pak family of kinases, especially Pak1, is becoming increasingly evident. Although highest levels of Pak1 expression and activation are detected in the developing nervous system, much remains undiscovered concerning its function in neurones. This review summarises what is currently known regarding the biological and molecular role of Pak1 in the mammalian forebrain. It emphasises the importance of Pak1 in regulating neuronal polarity, morphology, migration and synaptic function. Consequently, there are also strong indications that Pak1 is required for normal cognitive function. Furthermore, loss of Pak1 has been associated with the progression of neurodegenerative disorders, particularly Alzheimer's disease, while up-regulation and de-regulation may be responsible for oncogenic transformation of support cells within the CNS, especially astrocyte progenitors. Together, these new and exciting findings encourage the future exploration into the function of Pak1 in the nervous system, thus, paving the way for novel strategies towards improved diagnosis and therapeutic treatment of diseases that affect the CNS.

The potential role of cyclin-dependent kinase 5 in focal cortical dysplasia.

Focal cortical dysplasia (FCD) is the most common malformation of cortical development found in epilepsy surgical series. Characterised by cortical mislamination, dysplastic neurons and, in a subgroup of cases, balloon cells, FCD is potently epileptogenic. Despite decades of study, the underlying aetiology of FCD remains uncertain and research has been hampered by the lack of a good animal model in which to simulate the condition. In this article we review some of the potential molecular mechanisms that might underpin human FCD. In particular we examine the potential role of cyclin-dependent kinase 5 and its principal activator p35 in FCD and estimate the contribution that deregulation of cyclin-dependent kinase 5 might make to the pathogenesis of this condition.

Neurabin-I is phosphorylated by Cdk5: implications for neuronal morphogenesis and cortical migration.

The correct morphology and migration of neurons, which is essential for the normal development of the nervous system, is enabled by the regulation of their cytoskeletal elements. We reveal that Neurabin-I, a neuronal-specific F-actin-binding protein, has an essential function in the developing forebrain. We show that gain and loss of Neurabin-I expression affect neuronal morphology, neurite outgrowth, and radial migration of differentiating cortical and hippocampal neurons, suggesting that tight regulation of Neurabin-I function is required for normal forebrain development. Importantly, loss of Neurabin-I prevents pyramidal neurons from migrating into the cerebral cortex, indicating its essential role during early stages of corticogenesis. We demonstrate that in neurons Rac1 activation is affected by the expression levels of Neurabin-I. Furthermore, the Cdk5 kinase, a key regulator of neuronal migration and morphology, directly phosphorylates Neurabin-I and controls its association with F-actin. Mutation of the Cdk5 phosphorylation site reduces the phenotypic consequences of Neurabin-I overexpression both in vitro and in vivo, suggesting that Neurabin-I function depends, at least in part, on its phosphorylation status. Together our findings provide new insight into the signaling pathways responsible for controlled changes of the F-actin cytoskeleton that are required for normal development of the forebrain.

Localized activation of p21-activated kinase controls neuronal polarity and morphology.

In the developing forebrain, neuronal polarization is a stepwise and initially reversible process that underlies correct migration and axon specification. Many aspects of cytoskeletal changes that accompany polarization are currently molecularly undefined and thus poorly understood. Here we reveal that the p21-activated kinase (Pak1) is essential for the specification of an axon and dendrites. In hippocampal neurons, activation of Pak1 is spatially restricted to the immature axon despite its uniform presence in all neurites. Hyperactivation of Pak1 at the membrane of all neurites or loss of Pak1 expression disrupts both neuronal morphology and the distinction between an axon and dendrites. We reveal that Pak1 acts on polarity in a kinase-dependent manner, by affecting the F-actin and microtubule cytoskeleton at least in part through Rac1 and cofilin. Our data are the first to demonstrate the importance of localized Pak1 kinase activation for neuronal polarization and differentiation.

Increased immunoreactivity of cdk5 activators in hippocampal sclerosis.

Cyclin-dependent kinase 5 is important in several in-vitro neurodegeneration paradigms. Whether cyclin-dependent kinase 5 contributes to cell death in human neurodegenerative diseases remains uncertain, particularly because post-mortem delay and other extrinsic factors might influence cyclin-dependent kinase 5 activity. Here we demonstrate increased immunoreactivity for the activators of cyclin-dependent kinase 5 in post-mortem human hippocampi affected by the neurodegenerative condition hippocampal sclerosis, but not in histologically normal hippocampi. Moreover, in post-mortem brain tissue from patients with unilateral hippocampal sclerosis, increased immunoreactivity for cyclin-dependent kinase 5 activators was detected in the hippocampus with sclerosis, but not in the contralateral hippocampus, suggesting that extrinsic factors are unlikely to account for the differential staining observed. Our findings suggest that deregulation of cyclin-dependent kinase 5 might contribute to the pathogenesis of hippocampal sclerosis.

Pathological tau tangles localize to focal cortical dysplasia in older patients.

PURPOSE: Reactivation of neurodevelopmental processes may contribute to neurodegeneration. For example, the proteins cyclin dependent kinase 5 (cdk5) and glycogen synthase kinase 3 beta (GSK3beta), which are essential to normal cortical development, can hyperphosphorylate tau and might contribute to the pathogenesis of Alzheimer's disease. Focal cortical dysplasia (FCD) is an important neurodevelopmental cause of refractory human epilepsy within which dysplastic neurons exhibit increased immunoreactivity for cdk5 and GSK3beta as well as neurofilamentous accumulations. We therefore hypothesized that the developmentally abnormal cortex of FCD might be more susceptible to tau-mediated neurodegeneration than adjacent histologically normal cortex. MATERIALS AND METHODS: We examined a series of 15 cases of FCD, spanning a wide age range, for beta-amyloid, pathologically phosphorylated tau and neurofibrillary tangles using silver staining, immunohistochemistry for tau, AT8, RD3, RD4 and two-dimensional cell counting. RESULTS: Beta-amyloid plaques, aberrantly phosphorylated tau and neurofibrillary tangles are only found in older patients. The hyperphosphorylated tau tangles are confined to dysplastic neurons. Immunoreactivity for 3- and 4-repeat tau was again only detected within regions of FCD in older patients. With increasing age, the dysplastic cortex became hypocellular and a higher proportion of dysplastic neurons exhibited pathological tau phosphorylation. CONCLUSIONS: In older patients, FCD appears more susceptible to formation of pathologically phosphorylated tau neurofibrillary tangles than adjacent histologically normal cortex. Our results suggest a novel convergence of pathological neurodevelopment with pathological age-related neurodegeneration.

Increased NKCC1 expression in refractory human epilepsy.

Cation-chloride co-transporters (CCTs), particularly NKCC1, may be important in epileptogenesis. We have performed a detailed histological examination of NKCC1 in large samples of patients with hippocampal sclerosis (HS) or focal cortical dysplasia (FCD), pathologies both commonly associated with pharmacoresistant epilepsy. We consistently found increased immunoreactivity for NKCC1 in HS and FCD, but not in adjacent histologically normal cortex. Our results suggest that NKCC1 might contribute to the pathogenesis or pathophysiology of HS and FCD, thereby potentially offering a new therapeutic target in the treatment of pharmacoresistant epilepsy.

Role of p35/Cdk5 in preplate splitting in the developing cerebral cortex.

The earliest generated cells of the mammalian cerebral cortex form the preplate layer (PPL). The subsequently born cortical plate (CP) cells split this layer into the superficial layer I (LI) and the deep subplate (SP). The cellular and molecular mechanisms that underlie this event are unclear. To investigate the role of the cyclin-dependent kinase 5 (Cdk5) and its activator p35 in preplate splitting, we used Nissl staining, carbocyanine dye tracing, cell birthdating, and immunohistochemistry for calretinin (CalR) in p35 and Cdk5 knockout mice. Our data demonstrated changes in early cortical lamination and aberrant thalamic axon trajectories in these mice. Specifically, LI was thicker, and cell-dense and thalamic axons did not accumulate in the SP layer before invading the CP. Instead, they grew past the SP and more superficial cortical layers and coursed obliquely toward the pial surface. This behavior has been previously observed in reeler mice and suggests a defect in PPL splitting. CalR immunohistochemistry and bromo-deoxyuridine birthdating confirmed the abnormality in position of the earliest generated cortical cells of mutants. These observations suggest that the p35/Cdk5 pathway plays a role in preplate splitting in addition to regulating layer formation.

Distinct priming kinases contribute to differential regulation of collapsin response mediator proteins by glycogen synthase kinase-3 in vivo.

Collapsin response mediator proteins (CRMPs) are a family of neuron-enriched proteins that regulate neurite outgrowth and growth cone dynamics. Here, we show that Cdk5 phosphorylates CRMP1, CRMP2, and CRMP4, priming for subsequent phosphorylation by GSK3 in vitro. In contrast, DYRK2 phosphorylates and primes CRMP4 only. The Cdk5 and DYRK2 inhibitor purvalanol decreases the phosphorylation of CRMP proteins in neurons, whereas CRMP1 and CRMP2, but not CRMP4, phosphorylation is decreased in Cdk5(-/-) cortices. Stimulation of neuroblastoma cells with IGF1 or TPA decreases GSK3 activity concomitantly with CRMP2 and CRMP4 phosphorylation. Conversely, increased GSK3 activity is not sufficient to increase CRMP phosphorylation. However, the growth cone collapse-inducing protein Sema3A increases Cdk5 activity and promotes phosphorylation of CRMP2 (but not CRMP4). Therefore, inhibition of GSK3 alters phosphorylation of all CRMP isoforms; however, individual isoforms can be differentially regulated by their respective priming kinase. This is the first GSK3 substrate found to be regulated in this manner and may explain the hyperphosphorylation of CRMP2 observed in Alzheimer's disease.

Deregulation of cdk5 in Hippocampal sclerosis.

Hippocampal sclerosis (HS) is the most common cause of chronic medically refractory epilepsy in adults. Histologically, HS is characterized by segmental neuronal loss and gliosis. Although neuronal loss is important to the pathophysiology of HS, the molecular mechanisms underlying the neuronal loss remain uncertain. Recently, it has been appreciated that proteins important in neurodevelopment may also have a role in neurodegeneration. Cyclin-dependent kinase 5 (cdk5), known to be crucial in development of the normal cerebral cortex, has now been shown as pivotal in several cell death paradigms, including apoptosis and necrosis. Deregulation of cdk5 by p25 causes hyperphosphorylation of tau and may contribute to pathology in several neurodegenerative conditions. Furthermore, it has been shown that after transient forebrain ischemia, cdk5 causes specific death of CA1 neurons in the rat hippocampus by direct phosphorylation of the NR2A subunit of the NMDA receptor and subsequent excitotoxicity. Because apoptosis, necrosis, and excitotoxicity are all thought to contribute to neuronal loss in HS, we hypothesized that abnormalities of the cdk5 pathway would accompany this disorder. Surgically resected cases of HS with adjacent histologically normal lateral temporal cortex were examined for cdk5 and its activator p35/p25. We consistently found increased immunoreactivity for p35/p25 in surviving neurons within areas of neuronal loss compared with areas where neurons were preserved. Western blots showed the ratio of p25 to p35 to be greater in diseased hippocampi than in the adjacent histologically normal temporal lobe. Histone-based kinase assays demonstrated increased activity of the p25-cdk5 complex in HS compared with the temporal lobe despite neuronal loss in the hippocampal samples. Our results suggest that p25 is pathologically increased in HS and that deregulation of cdk5 by p25 might contribute to neuronal death in this condition.

The molecular mystery of neuronal migration: FAK and Cdk5.

The basic building blocks of a cell are its cytoskeletal proteins, the orderly but dynamic organization of which is essential. How signalling molecules regulate the cytoskeleton in the developing nervous system is still largely unknown. A recent breakthrough sheds light on a pathway involving Cdk5 (cyclin-dependent kinase 5) and FAK (focal adhesion kinase), demonstrating their role in regulating microtubule structure and thus nuclear positioning in radially migrating cortical neurones.

Pak1 and its T212 phosphorylated form accumulate in neurones and epithelial cells of the developing rodent.

The serine/threonine kinase Pak1 is a target of the RhoGTPases Rac and Cdc42 and an important regulator of cell morphology and migration. Recent work from several laboratories has indicated that Pak1 controls microtubule dynamics as well as the organisation of F-actin microfilaments. Pak1 is phosphorylated on T212 by the p35/Cdk5 or cyclin B1/Cdc2 kinase in postmitotic neurones and mitotic cells, respectively. To understand its function during development, we have carried out a detailed temporal and spatial analysis of Pak1 expression and phosphorylation on T212. In the embryonic forebrain, Pak1 and Pak1T212(PO4) were seen to accumulate in the corpus callosum, intermediate zone, lateral olfactory tracts, and anterior commissures. Epithelial cells of the mouse embryo lung, kidney, intestine, and skin also exhibited high levels of Pak1 and Pak1T212(PO4), suggesting a previously unsuspected role in epithelial differentiation. Pak1T212(PO4) was undetectable in all adult tissues. Together, these data indicate a specific, developmentally regulated role of the Pak1 kinase.

High midsummer for small GTPases.

The recent FASEB meeting on 'Small GTPases and Cell dynamics' (Snowmass, 6-11 July, 2002) focused on the emerging connections between small GTPases and the actin and tubulin cytoskeletons, advances in understanding where and when these proteins are activated, and their role in directing intracellular traffic and cell polarity in systems ranging from fibroblasts in Petri dishes to neurons in living organisms. The variety and vigor of the presentations showed that this field of cell biology remains a fertile ground for discovery.