Membrane targeting of p21-activated kinase 1 (PAK1) induces neurite outgrowth from PC12 cells

The molecular basis of p21-activated kinase-associated neurodevelopmental disorders: From genotype to phenotype

Although the identification of numerous genes involved in neurodevelopmental disorders (NDDs) has reshaped our understanding of their etiology, there are still major obstacles in the way of developing therapeutic solutions for intellectual disability (ID) and other NDDs. These include extensive clinical and genetic heterogeneity, rarity of recurrent pathogenic variants, and comorbidity with other psychiatric traits. Moreover, a large intragenic mutational landscape is at play in some NDDs, leading to a broad range of clinical symptoms. Such diversity of symptoms is due to the different effects DNA variations have on protein functions and their impacts on downstream biological processes. The type of functional alterations, such as loss or gain of function, and interference with signaling pathways, has yet to be correlated with clinical symptoms for most genes. This review aims at discussing our current understanding of how the molecular changes of group I p21-activated kinases (PAK1, 2 and 3), which are essential actors of brain development and function; contribute to a broad clinical spectrum of NDDs. Identifying differences in PAK structure, regulation and spatio-temporal expression may help understanding the specific functions of each group I PAK. Deciphering how each variation type affects these parameters will help uncover the mechanisms underlying mutation pathogenicity. This is a prerequisite for the development of personalized therapeutic approaches.

Liver Kinase B1 Functions as a Regulator for Neural Development and a Therapeutic Target for Neural Repair

The liver kinase B1 (LKB1), also known as serine/threonine kinase 11 (STK11) and Par-4 in C. elegans, has been identified as a master kinase of AMPKs and AMPK-related kinases. LKB1 plays a crucial role in cell growth, metabolism, polarity, and tumor suppression. By interacting with the downstream signals of SAD, NUAK, MARK, and other kinases, LKB1 is critical to regulating neuronal polarization and axon branching during development. It also regulates Schwann cell function and the myelination of peripheral axons. Regulating LKB1 activity has become an attractive strategy for repairing an injured nervous system. LKB1 upregulation enhances the regenerative capacity of adult CNS neurons and the recovery of locomotor function in adult rodents with CNS axon injury. Here, we update the major cellular and molecular mechanisms of LKB1 in regulating neuronal polarization and neural development, and the implications thereof for promoting neural repair, axon regeneration, and functional recovery in adult mammals.

Membrane interaction to intercellular spread of pathology in Alzheimer’s disease

Progressive development of pathology is one of the major characteristic features of neurodegenerative diseases. Alzheimer’s disease (AD) is the most prevalent among them. Extracellular amyloid-β (Aβ) plaques and intracellular tau neurofibrillary tangles are the pathological phenotypes of AD. However, cellular and animal studies implicate tau as a secondary pathology in developing AD while Aβ aggregates is considered as a trigger point. Interaction of Aβ peptides with plasma membrane (PM) seems to be a promising site of involvement in the events that lead to AD. Aβ binding to the lipid membranes initiates formation of oligomers of Aβ species, and these oligomers are known as primary toxic agents for neuronal toxicities. Once initiated, neuropathological toxicities spread in a “prion-like” fashion probably through the mechanism of intercellular transfer of pathogenic aggregates. In the last two decades, several studies have demonstrated neuron-to-neuron transfer of neurodegenerative proteins including Aβ and tau via exosomes and tunneling nanotubes (TNTs), the two modes of long-range intercellular transfer. Emerging pieces of evidence indicate that molecular pathways related to the biogenesis of exosomes and TNTs interface with endo-lysosomal pathways and cellular signaling in connection to vesicle recycling-imposed PM and actin remodulation. In this review, we discuss interactions of Aβ aggregates at the membrane level and its implications in intercellular spread of pathogenic aggregates. Furthermore, we hypothesize how spread of pathogenic aggregates contributes to complex molecular events that could regulate pathological and synaptic changes related to AD.

Amyloid-β induced membrane damage instigates tunneling nanotube-like conduits by p21-activated kinase dependent actin remodulation

Alzheimer's disease (AD) pathology progresses gradually via anatomically connected brain regions. Direct transfer of amyloid-β_1-42 oligomers (oAβ) between connected neurons has been shown, however, the mechanism is not fully revealed. We observed formation of oAβ induced tunneling nanotubes (TNTs)-like nanoscaled f-actin containing membrane conduits, in differentially differentiated SH-SY5Y neuronal models. Time-lapse images showed that oAβ propagate from one cell to another via TNT-like structures. Preceding the formation of TNT-like conduits, we detected oAβ_-induced plasma membrane (PM) damage and calcium-dependent repair through lysosomal-exocytosis, followed by massive endocytosis to re-establish the PM. Massive endocytosis was monitored by an influx of the membrane-staining dye TMA-DPH and PM damage was quantified by propidium iodide influx in the absence of Ca²⁺. The massive endocytosis eventually caused accumulation of internalized oAβ in Lamp1 positive multivesicular bodies/lysosomes via the actin cytoskeleton remodulating p21-activated kinase1 (PAK1) dependent endocytic pathway. Three-dimensional quantitative confocal imaging, structured illumination superresolution microscopy, and flowcytometry quantifications revealed that oAβ induces activation of phospho-PAK1, which modulates the formation of long stretched f-actin extensions between cells. Moreover, the formation of TNT-like conduits was inhibited by preventing PAK1-dependent internalization of oAβ using the small-molecule inhibitor IPA-3, a highly selective cell-permeable auto-regulatory inhibitor of PAK1. The present study reveals that the TNT-like conduits are probably instigated as a consequence of oAβ induced PM damage and repair process, followed by PAK1 dependent endocytosis and actin remodeling, probably to maintain cell surface expansion and/or membrane tension in equilibrium.

Y-27632 Induces Neurite Outgrowth by Activating the NOX1-Mediated AKT and PAK1 Phosphorylation Cascades in PC12 Cells

Y-27632 is known as a selective Rho-associated coiled coil-forming kinase (ROCK) inhibitor. Y-27632 has been shown to induce neurite outgrowth in several neuronal cells. However, the precise molecular mechanisms linking neurite outgrowth to Y-27632 are not completely understood. In this study, we examined the ability of Y-27632 to induce neurite outgrowth in PC12 cells and evaluated the signaling cascade. The effect of Y-27632 on the neurite outgrowth was inhibited by reactive oxygen species (ROS) scavengers such as N-acetyl cysteine (NAC) and trolox. Furthermore, Y-27632-induced neurite outgrowth was not triggered by NADPH oxidase 1 (NOX1) knockdown or diphenyleneiodonium (DPI), a NOX inhibitor. Suppression of the Rho-family GTPase Rac1, which is under the negative control of ROCK, with expression of the dominant negative Rac1 mutant (Rac1N17) prevented Y-27632-induced neurite outgrowth. Moreover, the Rac1 inhibitor NSC23766 prevented Y-27632-induced AKT and p21-activated kinase 1 (PAK1) activation. AKT inhibition with MK2206 suppressed Y-27632-induced PAK1 phosphorylation and neurite outgrowth. In conclusion, our results suggest that Rac1/NOX1-dependent ROS generation and subsequent activation of the AKT/PAK1 cascade contribute to Y-27632-induced neurite outgrowth in PC12 cells.

Cooperative recruitment of Arl4A and Pak1 to the plasma membrane contributes to sustained Pak1 activation for cell migration

Cell migration requires the coordination of multiple signaling pathways involved in membrane dynamics and cytoskeletal rearrangement. The Arf-like small GTPase Arl4A has been shown to modulate actin cytoskeleton remodeling. However, evidence of the function of Arl4A in cell migration is insufficient. Here, we report that Arl4A acts with the serine/threonine protein kinase Pak1 to modulate cell migration through their cooperative recruitment to the plasma membrane. We first observed that Arl4A and its isoform Arl4D interact with Pak1 and Pak2 and showed that Arl4A recruits Pak1 and Pak2 to the plasma membrane. The fibronectin-induced Pak1 localization at the plasma membrane is reduced in Arl4A-depleted cells. Unexpectedly, we found that Pak1, but not Arl4A-binding-defective Pak1, can recruit a cytoplasmic myristoylation-deficient Arl4A-G2A mutant to the plasma membrane. Furthermore, we found that the Arl4A-Pak1 interaction, which is independent of Rac1 binding to Pak1, is required for Arl4A-induced cell migration. Thus, we infer that there is feedback regulation between Arl4A and Pak1, in which they mutually recruit each other to the plasma membrane for Pak1 activation, thereby modulating cell migration through direct interaction.

Polyaniline Functionalized Graphene Nanoelectrodes for the Regeneration of PC12 Cells via Electrical Stimulation

The regeneration of neurons is an important goal of neuroscience and clinical medicine. The electrical stimulation of cells is a promising technique to meet this goal. However, its efficiency highly depends on the electrochemical properties of the stimulation electrodes used. This work reports on the preparation and use of a highly electroactive and biocompatible nanoelectrode made from a novel polyaniline functionalized graphene composite. This nanocomposite was prepared using a facile and efficient polymerization-enhanced ball-milling method. It was used to stimulate the growth of PC12 cells under various electrical fields. The enhanced growth of axons and improved wound regeneration of PC12 cells were observed after this treatment, suggesting a promising strategy for neuro traumatology.

Phosphatidic Acid: From Pleiotropic Functions to Neuronal Pathology

Among the cellular lipids, phosphatidic acid (PA) is a peculiar one as it is at the same time a key building block of phospholipid synthesis and a major lipid second messenger conveying signaling information. The latter is thought to largely occur through the ability of PA to recruit and/or activate specific proteins in restricted compartments and within those only at defined submembrane areas. Furthermore, with its cone-shaped geometry PA locally changes membrane topology and may thus be a key player in membrane trafficking events, especially in membrane fusion and fission steps, where lipid remodeling is believed to be crucial. These pleiotropic cellular functions of PA, including phospholipid synthesis and homeostasis together with important signaling activity, imply that perturbations of PA metabolism could lead to serious pathological conditions. In this mini-review article, after outlining the main cellular functions of PA, we highlight the different neurological diseases that could, at least in part, be attributed to an alteration in PA synthesis and/or catabolism.

WAVE2 is associated with poor prognosis in pancreatic cancers and promotes cell motility and invasiveness via binding to ACTN4

WAVE2 is a member of the WASP/WAVE family of actin cytoskeletal regulatory proteins; unfortunately, little is known about its function in pancreatic cancers. In this study, we report the role of WAVE2 in the motility and invasiveness of pancreatic cancer cells. High WAVE2 expression in human pancreatic cancer tissues was correlated with overall survival. WAVE2 accumulated in the cell protrusions of pancreatic cancer cell lines. Downregulation of WAVE2 by small interfering RNA decreased the cell protrusions and inhibited the motility and invasiveness of pancreatic cancer cells. WAVE2 promoted pancreatic cancer cell motility and invasion by forming a complex with the actin cytoskeletal protein alpha‐actinin 4 (ACTN4). Downregulation of ACTN4 by small interfering RNA also inhibited the motility and invasiveness of the cells through a decrease in cell protrusions. Further investigation showed that WAVE2/ACTN4 signaling selectively stimulated p27 phosphorylation and thereby increased the motility and invasiveness of the cells. These results suggest that WAVE2 and ACTN4 stimulate p27 phosphorylation and provide evidence that WAVE2 promotes the motility and invasiveness of pancreatic cancer cells.

Melanoma-associated mutants within the serine-rich domain of PAK5 direct kinase activity to mitogenic pathways

The overexpression and hyperactivity of p21-activated serine/threonine kinases (PAKs) is known to facilitate tumorigenesis; however, the contribution of cancer-associated PAK mutations to tumor initiation and progression remains unclear. Here, we identify p21-activated serine/threonine kinase 5 (PAK5) as the most frequently altered PAK family member in human melanoma. More than 60% of melanoma-associated PAK5 gene alterations are missense mutations, and distribution of these variants throughout the protein coding sequence make it difficult to distinguish oncogenic drivers from passengers. To address this issue, we stably introduced the five most common melanoma-associated PAK5 missense mutations into human immortalized primary melanocytes (hMELTs). While expression of these mutants did not promote single-cell migration or induce temozolomide resistance, a subset of variants drove aberrant melanocyte proliferation. These mitogenic mutants, PAK5 S364L and D421N, clustered within an unstructured, serine-rich domain of the protein and inappropriately activated ERK and PKA through kinase-independent and -dependent mechanisms, respectively. Together, our findings establish the ability of mutant PAK5 to enhance PKA and MAPK signaling in melanocytes and localize the engagement of mitogenic pathways to a serine-rich region of PAK5.

Phospholipase D1 Signaling: Essential Roles in Neural Stem Cell Differentiation

Phospholipase D1 (PLD1) is generally accepted as playing an important role in the regulation of multiple cell functions, such as cell growth, survival, differentiation, membrane trafficking, and cytoskeletal organization. Recent findings suggest that PLD1 also plays an important role in the regulation of neuronal differentiation of neuronal cells. Moreover, PLD1-mediated signaling molecules dynamically regulate the neuronal differentiation of neural stem cells (NSCs). Rho family GTPases and Ca²⁺-dependent signaling, in particular, are closely involved in PLD1-mediated neuronal differentiation of NSCs. Moreover, PLD1 has a significant effect on the neurogenesis of NSCs via the regulation of SHP-1/STAT3 activation. Therefore, PLD1 has now attracted significant attention as an essential neuronal signaling molecule in the nervous system. In the current review, we summarize recent findings on the regulation of PLD1 in neuronal differentiation and discuss the potential role of PLD1 in the neurogenesis of NSCs.

PAKs in the brain: Function and dysfunction

p21-Activated kinases (PAKs) comprise a family of proteins covering a central role in signal transduction. They are downstream effectors of Rho GTPases and can affect a variety of processes in different cell types and tissues by remodeling the cytoskeleton and by promoting gene transcription and cell survival. Given the relevance of cytoskeletal organization in neuronal development as well as synaptic function and the importance of pro-survival signals in controlling neuronal cell fate, accumulating studies investigated the role of PAKs in the nervous system. In this review, we provide a critical overview of the role of PAKs in the nervous system, both in neuronal and non-neuronal cells, and discuss their potential link with neurodegenerative diseases.

Activated-PAK4 predicts worse prognosis in breast cancer and promotes tumorigenesis through activation of PI3K/AKT signaling

The p21-activated kinase 4 (PAK4) is sufficient to transform noncancerous mammary epithelial cells and to form tumors in the mammary glands of mice. The accumulated information suggests that PAK4 might be an oncogenic protein in breast cancer. In this study, we sought to identify the role for PAK4 in breast cancer progression. Immunohistochemical study revealed that high PAK4 expression is associated with larger tumor size, lymph node metastasis, and advanced stage cancer in 93 invasive breast carcinoma patients. Moreover, high PAK4 expression was significantly associated with poor overall and disease-free survival. PAK4 remained an independent adverse prognosticator after univariate and multivariate analysis. Ectopic expression of wild-type PAK4 in MDA-MB-231 cells activated PI3K/AKT signaling and resulted in the enhancement of the cell proliferation, migration, and invasion, whereas PAK4-induced effects were blocked by the PAK4 kinase inhibitor PF- 3758309, PAK4 siRNAs or the PI3K inhibitor LY294002. Furthermore, a kinase-active PAK4 (S474E) strongly induced PI3K/AKT activation, and promoted proliferation, migration and invasion in breast cancer cells. A kinase-inactive PAK4 KD (K350A/K351A) did partially upregulate PI3K/AKT, and promoted invasive phenotype. Taken together, these findings suggest that PAK4-activated PI3K/AKT signaling is both kinase-dependent and -independent, which contributes to breast cancer progression. Thus, our results imply that dual inhibition of PAK4 and PI3K/AKT signaling might be a potential therapeutic approach for breast cancer therapy.

A novel orally bioavailable compound KPT-9274 inhibits PAK4, and blocks triple negative breast cancer tumor growth

Breast cancer is a heterogeneous disease consisting of several subtypes. Among these subtypes, triple negative breast cancer is particularly difficult to treat. This is due to a lack of understanding of the mechanisms behind the disease, and consequently a lack of druggable targets. PAK4 plays critical roles in cell survival, proliferation, and morphology. PAK4 protein levels are high in breast cancer cells and breast tumors, and the gene is often amplified in basal like breast cancers, which are frequently triple negative. PAK4 is also overexpressed in other types of cancer, making it a promising drug target. However, its inhibition is complicated by the fact that PAK4 has both kinase-dependent and -independent functions. Here we investigate a new clinical compound KPT-9274, which has been shown to inhibit PAK4 and NAMPT. We find that KPT-9274 (and its analog, KPT-8752) can reduce the steady state level of PAK4 protein in triple negative breast cancer cells. These compounds also block the growth of the breast cancer cells in vitro, and stimulate apoptosis. Most importantly, oral administration of KPT-9274 reduces tumorigenesis in mouse models of human triple negative breast cancer. Our results indicate that KPT-9274 is a novel therapeutic option for triple negative breast cancer therapy.

Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth

Filamin is a scaffolding protein that functions in many cells as an actin-crosslinker. FLN90, an isoform of the Drosophila ortholog Filamin/cheerio that lacks the actin-binding domain, is here shown to govern the growth of postsynaptic membrane folds and the composition of glutamate receptor clusters at the larval neuromuscular junction. Genetic and biochemical analyses revealed that FLN90 is present surrounding synaptic boutons. FLN90 is required in the muscle for localization of the kinase dPak and, downstream of dPak, for localization of the GTPase Ral and the exocyst complex to this region. Consequently, Filamin is needed for growth of the subsynaptic reticulum. In addition, in the absence of filamin, type-A glutamate receptor subunits are lacking at the postsynapse, while type-B subunits cluster correctly. Receptor composition is dependent on dPak, but independent of the Ral pathway. Thus two major aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering, require postsynaptic Filamin.

DOI:http://dx.doi.org/10.7554/eLife.19991.001

Regulation of pulmonary endothelial barrier function by kinases

The pulmonary endothelium is the target of continuous physiological and pathological stimuli that affect its crucial barrier function. The regulation, defense, and repair of endothelial barrier function require complex biochemical processes. This review examines the role of endothelial phosphorylating enzymes, kinases, a class with profound, interdigitating influences on endothelial permeability and lung function.

Expression of p21-activated kinases 1 and 3 is altered in the brain of subjects with depression

The p21-activated kinases (PAKs) of group I are the main effectors for the small Rho GTPases, critically involved in neurodevelopment, plasticity and maturation of the nervous system. Moreover, the neuronal complexity controlled by PAK1/PAK3 signaling determines the postnatal brain size and synaptic properties. Stress induces alterations at the level of structural and functional synaptic plasticity accompanied by reductions in size and activity of the hippocampus and the prefrontal cortex (PFC). These abnormalities are likely to contribute to the pathology of depression and, in part, reflect impaired cytoskeleton remodeling pointing to the role of Rho GTPase signaling. Thus, the present study assessed the expression of the group I PAKs and their activators in the brain of depressed subjects. Using quantitative polymerase chain reaction (qPCR), mRNA levels and coexpression of the group I PAKs: PAK1, PAK2, and PAK3 as well as of their activators: RAC1, CDC42 and ARHGEF7 were examined in postmortem samples from the PFC (n=25) and the hippocampus (n=23) of subjects with depression and compared to control subjects (PFC n=24; hippocampus n=21). Results demonstrated that mRNA levels of PAK1 and PAK3, are significantly reduced in the brain of depressed subjects, with PAK1 being reduced in the PFC and PAK3 in the hippocampus. No differences were observed for the ubiquitously expressed PAK2. Following analysis of gene coexpression demonstrated disruption of coordinated gene expression in the brain of subjects with depression. Abnormalities in mRNA expression of PAK1 and PAK3 as well as their altered coexpression patterns were detected in the brain of subjects with depression.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Inhibition of Nischarin Expression Promotes Neurite Outgrowth through Regulation of PAK Activity

Nischarin is a cytoplasmic protein expressed in various organs that plays an inhibitory role in cell migration and invasion and the carcinogenesis of breast cancer cells. We previously reported that Nischarin is highly expressed in neuronal cell lines and is differentially expressed in the brain tissue of adult rats. However, the physiological function of Nischarin in neural cells remains unknown. Here, we show that Nischarin is expressed in rat primary cortical neurons but not in astrocytes. Nischarin is localized around the nucleus and dendrites. Using shRNA to knockdown the expression of endogenous Nischarin significantly increases the percentage of neurite-bearing cells, remarkably increases neurite length, and accelerates neurite extension in neuronal cells. Silencing Nischarin expression also promotes dendrite elongation in rat cortical neurons where Nischarin interacts with p21-activated kinase 1/2 (PAK1/2) and negatively regulates phosphorylation of both PAK1 and PAK2. The stimulation of neurite growth observed in cells with decreased levels of Nischarin is partially abolished by IPA3-mediated inhibition of PAK1 activity. Our findings indicate that endogenous Nischarin inhibits neurite outgrowth by blocking PAK1 activation in neurons.

RabGEF1/Rabex-5 Regulates TrkA-Mediated Neurite Outgrowth and NMDA-Induced Signaling Activation in NGF-Differentiated PC12 Cells

Nerve growth factor (NGF) binds to its cognate receptor TrkA and induces neuronal differentiation by activating distinct downstream signal transduction events. RabGEF1 (also known as Rabex-5) is a guanine nucleotide exchange factor for Rab5, which regulates early endosome fusion and vesicular trafficking in endocytic pathways. Here, we used the antisense (AS) expression approach to induce an NGF-dependent sustained knockdown of RabGEF1 protein expression in stable PC12 transfectants. We show that RabGEF1 is a negative regulator of NGF-induced neurite outgrowth and modulates other cellular and signaling processes that are activated by the interaction of NGF with TrkA receptors, such as cell cycle progression, cessation of proliferation, and activation of NGF-mediated downstream signaling responses. Moreover, RabGEF1 can bind to Rac1, and the activation of Rac1 upon NGF treatment is significantly enhanced in AS transfectants, suggesting that RabGEF1 is a negative regulator of NGF-induced Rac1 activation in PC12 cells. Furthermore, we show that RabGEF1 can also interact with NMDA receptors by binding to the NR2B subunit and its associated binding partner SynGAP, and negatively regulates activation of nitric oxide synthase activity induced by NMDA receptor stimulation in NGF-differentiated PC12 cells. Our data suggest that RabGEF1 is a negative regulator of TrkA-dependent neuronal differentiation and of NMDA receptor-mediated signaling activation in NGF-differentiated PC12 cells.

The Role of the Pleckstrin Homology Domain-containing Protein CKIP-1 in Activation of p21-activated Kinase 1 (PAK1)

Upon growth factor stimulation, PAK1 is recruited to the plasma membrane and activated by a mechanism that requires its phosphorylation at Ser-223 by the protein kinase CK2. However, the upstream signaling molecules that regulate this phosphorylation event are not clearly defined. Here, we demonstrate a major role of the CK2α-interacting protein CKIP-1 in activation of PAK1. CK2α, CKIP-1, and PAK1 are translocated to membrane ruffles in response to the epidermal growth factor (EGF), where CKIP-1 mediates the interaction between CK2α and PAK1 in a PI3K-dependent manner. Consistently, PAK1 mediates phosphorylation and modulation of the activity of p41-Arc, one of its plasma membrane substrate, in a fashion that requires PI3K and CKIP-1. Moreover, CKIP-1 knockdown or PI3K inhibition suppresses PAK1-mediated cell migration and invasion, demonstrating the physiological significance of the PI3K-CKIP-1-CK2-PAK1 signaling pathway. Taken together, these findings identify a novel mechanism for the activation of PAK1 at the plasma membrane, which is critical for cell migration and invasion.

PAK1 regulates cortical development via promoting neuronal migration and progenitor cell proliferation

Background

p21-activated kinase 1 (PAK1) is a serine/threonine kinase known to be activated by the Rho family small GTPases and to play a key role in cytoskeletal reorganization, spine morphology and synaptic plasticity. PAK1 is also implicated in a number of neurodevelopmental and neurodegenerative diseases, including autism, intellectual disability and Alzheimer’s disease. However, the role of PAK1 in early brain development remains unknown.

Results

In this study, we employed genetic manipulations to investigate the role of PAK1 in the cerebral cortical development in mice. We showed that compared to the wild type littermates, PAK1 knockout mice have a reduction in the number of pyramidal neurons in several layers of the cerebral cortex, which is associated with a smaller pool of neural progenitor cells and impaired neuronal migration.

Conclusion

These results suggest that PAK1 regulates cortical development by promoting the proliferation of neural progenitor cells and facilitating the migration of these neurons to specific regions of the cortex.

Tyrosyl phosphorylated serine-threonine kinase PAK1 is a novel regulator of prolactin-dependent breast cancer cell motility and invasion

Despite efforts to discover the cellular pathways regulating breast cancer metastasis, little is known as to how prolactin (PRL) cooperates with extracellular environment and cytoskeletal proteins to regulate breast cancer cell motility and invasion. We implicated serine-threonine kinase p21-activated kinase 1 (PAK1) as a novel target for PRL-activated Janus-kinase 2 (JAK2). JAK2-dependent PAK1 tyrosyl phosphorylation plays a critical role in regulation of both PAK1 kinase activity and scaffolding properties of PAK1. Tyrosyl phosphorylated PAK1 facilitates PRL-dependent motility via at least two mechanisms: formation of paxillin/GIT1/βPIX/pTyr-PAK1 complexes resulting in increased adhesion turnover and phosphorylation of actin-binding protein filamin A. Increased adhesion turnover is the basis for cell migration and phosphorylated filamin A stimulates the kinase activity of PAK1 and increases actin-regulating activity to facilitate cell motility. Tyrosyl phosphorylated PAK1 also stimulates invasion of breast cancer cells in response to PRL and three-dimensional (3D) collagen IV via transcription and secretion of MMP-1 and MMP-3 in a MAPK-dependent manner. These data illustrate the complex interaction between PRL and the cell microenvironment in breast cancer cells and suggest a pivotal role for PRL/PAK1 signaling in breast cancer metastasis.

PAK4-6 in cancer and neuronal development

PAKs 4, 5 and 6 are members of the group B family of p21-activated kinases. Among this group, PAK4 has been most extensively studied. While it has essential roles in embryonic development, in adults high levels of PAK4 are frequently associated with cancer. PAK4 is overexpressed in a variety of cancers, and the Pak4 gene is amplified in some cancers. PAK4 overexpression is sufficient to cause oncogenic transformation in cells and in mouse models. The tight connection between PAK4 and cancer make it a promising diagnostic tool as well as a potential drug target. The group B PAKs also have important developmental functions. PAK4 is important for many early developmental processes, while PAK5 and PAK6 play roles in learning and memory in mice. This chapter provides an overview of the roles of the group B PAKs in cancer as well as development, and includes a discussion of PAK mediated signaling pathways and cellular functions.

Expression of PKC iota affects neuronal differentiation of PC12 cells at least partly independent of kinase function

Atypical PKC (aPKC) plays a role in establishing cell polarity and has been indicated in neuronal differentiation and polarization, including neurite formation in rat pheochromocytoma PC12 cells, albeit by unclear mechanisms. Here, the role of the aPKC isoform, PKC iota (PKCι), in the early neuronal differentiation of PC12 cells was investigated. NGF-treated PC12 cells with stably expressed exogenous wild-type PKCι showed decreased expression of a neuroendocrine marker, increased expression of a neuronal marker, and increased neurite formation. Stable expression of a kinase- inactive PKCι, but not constitutively active PKCι lacking a regulatory domain, had similar although less potent effects. Pharmacological inhibition of endogenous aPKC kinase activity in parental PC12 cells did not inhibit neurite formation, suggesting that some of the observed effects of PKCι expression on neuronal differentiation are kinase- independent. Interestingly, exogenous expression of wild-type and kinase-inactive PKCι had little effect on overall PKCι activity, but caused a decrease in PKC zeta (PKCζ) kinase activity, suggesting an interplay between the two isoforms that may underlie the observed results. Overall, these findings suggest that in PC12 and perhaps other neuroendocrine precursor cells, PKCι influences an early differentiation decision between the neuroendocrine (chromaffin) and sympathetic neuron cell lineages, potentially by affecting PKCζ function.

An oncogenic kinase: putting PAK5 forward

INTRODUCTION

Overexpression of p21-activated kinase 5 (PAK5) is discovered in many tumors, probably due to its regulation in cytoskeleton, antiapoptosis and proliferation. A better understanding of the modulation mechanisms of PAK5 is needed for the development of tumor treatment where current therapeutics is inadequate.

AREAS COVERED

This review discusses the current understanding of PAK5 functions as an oncogenic kinase in tumor cellular regulation. Mechanisms of action and molecular pathways involved in cytoskeleton regulation, antiapoptosis and proliferation of tumors are discussed.

EXPERT OPINION

PAKs are serine/threonine kinases and downstream effectors for Cdc42 and Rac, the subfamilies of Rho small GTPases. PAK5 shares sequence identities in p21-GTPase-binding domain and kinase domain and is completely different in other regions compared with other PAKs. Overexpression of PAK5 has been found in several tumors, probably due to its contribution to proliferation, cytoskeleton and anti-apoptosis. Additional regulation mechanisms which are independent of Rho GTPases also indicate that PAK5 functions as a special signal molecule in cellular signaling pathways of tumor progression.

P21 activated kinases: structure, regulation, and functions

The p21 activated kinases (Paks) are well known effector proteins for the Rho GTPases Cdc42 and Rac. The Paks contain 6 members, which fall into 2 families of proteins. The first family consists of Paks 1, 2, and 3, and the second consists of Paks 4, 5, and 6. While some of the Paks are ubiquitously expressed, others have more restrictive tissue specificity. All of them are found in the nervous system. Studies using cell culture, transgenic mice, and knockout mice, have revealed important roles for the Paks in cytoskeletal organization and in many aspects of cell growth and development. This review discusses the basic structures of the Paks, and their roles in cell growth, development, and in cancer.

The Beta-amyloid protein of Alzheimer's disease: communication breakdown by modifying the neuronal cytoskeleton

Alzheimer's disease (AD) is one of the most prevalent severe neurological disorders afflicting our aged population. Cognitive decline, a major symptom exhibited by AD patients, is associated with neuritic dystrophy, a degenerative growth state of neurites. The molecular mechanisms governing neuritic dystrophy remain unclear. Mounting evidence indicates that the AD-causative agent, β-amyloid protein (Aβ), induces neuritic dystrophy. Indeed, neuritic dystrophy is commonly found decorating Aβ-rich amyloid plaques (APs) in the AD brain. Furthermore, disruption and degeneration of the neuronal microtubule system in neurons forming dystrophic neurites may occur as a consequence of Aβ-mediated downstream signaling. This review defines potential molecular pathways, which may be modulated subsequent to Aβ-dependent interactions with the neuronal membrane as a consequence of increasing amyloid burden in the brain.

p21-Activated kinase 5: a pleiotropic kinase

The PAKs (p21-activated kinases) are highly conserved serine/threonine protein kinases which comprise six mammalian PAKs. PAK5 (p21-activated kinase 5) is the least understood member of PAKs that regulate many intracellular processes when they are stimulated by activated forms of the small GTPases Cdc42 and Rac. PAK5 takes an important part in multiple signal pathways in mammalian cells and controls a variety of cellular functions including cytoskeleton organization, cell motility and apoptosis. The main goal of this review is to describe the structure, mechanisms underlying its activity regulation, its role in apoptosis and the likely directions of further research.

p21-Activated kinase (PAK) is required for Bone Morphogenetic Protein (BMP)-induced dendritogenesis in cortical neurons

Bone Morphogenetic Proteins (BMPs) are crucial for many aspects of the development and differentiation of the nervous system and are important in controlling cytoskeletal remodeling during neuronal morphogenesis. BMPs are TGFβ superfamily members that signal through a heteromeric complex of type I and type II BMP receptors. The BMPRII receptor is particularly important in mediating remodeling of the neuronal cytoskeleton through the activation of BMPRII-bound cytoskeletal regulators, such as LIM Kinase (LIMK). Here, we show that PAK1, a key regulator of diverse neuronal processes and an upstream activator of LIMK, binds to the BMP type I receptor, ALK2. Although, PAK1 is dispensable for activation of the Smad transcriptional mediators, abrogation of PAK1 expression or inhibition of PAK1 activity prevents BMP-induced neurite outgrowth in cultured neuroblastoma cell lines. Moreover, in primary murine embryonic cortical neurons, inhibition of PAK activity blocks BMP7-induced cofilin phosphorylation, prevents remodeling of the actin cytoskeleton and thereby blocks BMP7-induced dendrite formation. Thus, we propose a model in which BMP7 signaling leads to the recruitment of ALK2-bound PAK1 to BMPRII, which binds a downstream regulator of the actin cytoskeleton, LIMK1, and that the BMP receptor complex thereby acts as a scaffold to localize and coordinate actin cytoskeletal remodeling. We propose that this scaffold plays a key role in mediating BMP7-dependent dendritogenesis in primary cortical neurons.

Unique catalytic activities and scaffolding of p21 activated kinase-1 in cardiovascular signaling

P21 activated kinase-1 (Pak1) has diverse functions in mammalian cells. Although a large number of phosphoproteins have been designated as Pak1 substrates from in vitro studies, emerging evidence has indicated that Pak1 may function as a signaling molecule through a unique molecular mechanism – scaffolding. By scaffolding, Pak1 delivers signals through an auto-phosphorylation-induced conformational change without transfer of a phosphate group to its immediate downstream effector(s). Here we review evidence for this regulatory mechanism based on structural and functional studies of Pak1 in different cell types and research models as well as in vitro biochemical assays. We also discuss the implications of Pak1 scaffolding in disease-related signaling processes and the potential in cardiovascular drug development.

Evidence for a novel mechanism of the PAK1 interaction with the Rho-GTPases Cdc42 and Rac

P21-activated kinase 1 (PAK1) is activated by binding to GTP-bound Rho GTPases Cdc42 and Rac via its CRIB domain. Here, we provide evidence that S79 in the CRIB domain of PAK1 is not directly involved in this binding but is crucial for PAK1 activation. S79A mutation reduces the binding affinity of PAK1 for the GTPases and inhibits autophosphorylation and kinase activity of PAK1. Thus, this mutation abrogates the ability of PAK1 to induce changes in cell morphology and motility and to promote malignant transformation of prostate epithelial cells. We also show that growth of the prostate cancer cell line PC3 is inhibited by the treatment of a PAK1-inhibiting peptide comprising 19 amino acids centered on S79, but not by the PAK1 peptide containing the S79A mutation, and that this growth inhibition is correlated with reduced autophosphorylation activity of PAK1. Together, these findings demonstrate a significant role of S79 in PAK1 activation and provide evidence for a novel mechanism of the CRIB-mediated interaction of PAK1 with Cdc42 and Rac.

Distinct roles for Ste20-like kinase SLK in muscle function and regeneration

BACKGROUND

Cell growth and terminal differentiation are controlled by complex signaling systems that regulate the tissue-specific expression of genes controlling cell fate and morphogenesis. We have previously reported that the Ste20-like kinase SLK is expressed in muscle tissue and is required for cell motility. However, the specific function of SLK in muscle tissue is still poorly understood.

METHODS

To gain further insights into the role of SLK in differentiated muscles, we expressed a kinase-inactive SLK from the human skeletal muscle actin promoter. Transgenic muscles were surveyed for potential defects. Standard histological procedures and cardiotoxin-induced regeneration assays we used to investigate the role of SLK in myogenesis and muscle repair.

RESULTS

High levels of kinase-inactive SLK in muscle tissue produced an overall decrease in SLK activity in muscle tissue, resulting in altered muscle organization, reduced litter sizes, and reduced breeding capacity. The transgenic mice did not show any differences in fiber-type distribution but displayed enhanced regeneration capacity in vivo and more robust differentiation in vitro.

CONCLUSIONS

Our results show that SLK activity is required for optimal muscle development in the embryo and muscle physiology in the adult. However, reduced kinase activity during muscle repair enhances regeneration and differentiation. Together, these results suggest complex and distinct roles for SLK in muscle development and function.

The neural cell adhesion molecule (NCAM) associates with and signals through p21-activated kinase 1 (Pak1)

The Neural cell adhesion molecule (NCAM) plays an important role in regulation of nervous system development. To expand our understanding of the molecular mechanisms via which NCAM influences differentiation of neurons, we used a yeast two-hybrid screening to search for new binding partners of NCAM and identified p21-activated kinase 1 (Pak1). We show that NCAM interacts with Pak1 in growth cones of neurons. The autophosphorylation and activity of Pak1 were enhanced when isolated growth cones were incubated with NCAM function triggering antibodies, which mimic the interaction between NCAM and its extracellular ligands. The association of Pak1 with cell membranes, the efficiency of Pak1 binding to its activators, and Pak1 activity were inhibited in brains of NCAM-deficient mice. NCAM-dependent Pak1 activation was abolished after lipid raft disruption, suggesting that NCAM promotes Pak1 activation in the lipid raft environment. Phosphorylation of the downstream Pak1 effectors LIMK1 and cofilin was reduced in growth cones from NCAM-deficient neurons, which was accompanied by decreased levels of filamentous actin and inhibited filopodium mobility in the growth cones. Dominant-negative Pak1 inhibited and constitutively active Pak1 enhanced the ability of neurons to increase neurite outgrowth in response to the extracellular ligands of NCAM. Our combined observations thus indicate that NCAM activates Pak1 to drive actin polymerization to promote neuronal differentiation.

PAK-PIX interactions regulate adhesion dynamics and membrane protrusion to control neurite outgrowth

The roles of P21-activated kinase (PAK) in the regulation of axon outgrowth downstream of extracellular matrix (ECM) proteins are poorly understood. Here we show that PAK1-3 and PIX are expressed in the developing spinal cord and differentially localize to point contacts and filopodial tips within motile growth cones. Using a specific interfering peptide called PAK18, we found that axon outgrowth is robustly stimulated on laminin by partial inhibition of PAK-PIX interactions and PAK function, whereas complete inhibition of PAK function stalls axon outgrowth. Furthermore, modest inhibition of PAK-PIX stimulates the assembly and turnover of growth cone point contacts, whereas strong inhibition over-stabilizes adhesions. Point mutations within PAK confirm the importance of PIX binding. Together our data suggest that regulation of PAK-PIX interactions in growth cones controls neurite outgrowth by influencing the activity of several important mediators of actin filament polymerization and retrograde flow, as well as integrin-dependent adhesion to laminin.

The pak4 protein kinase in breast cancer

Paks4, along with Paks5, and 6 are members of the group B family of p21-activated kinases (Paks). The Paks play multiple different roles in controlling cell morphology, cell growth, proliferation, and signaling. Pak4 has essential roles in embryonic development (Qu et al., 2003), but in adults high levels of Pak4 are frequently associated with cancer. Pak4 has been implicated in several types of cancer (Wells and Jones, 2010; Eswaran et al., 2009; Liu et al., 2008; and Liu et al., 2010) and it is strongly linked to breast cancer (Liu et al., 2008; Liu et al. 2010; Yu et al., 2009; Rafn et al., 2012; and So et al., 2012). Breast tumors and breast cancer cell lines frequently have high levels of Pak4 (Liu et al., 2008), and overexpression of Pak4 in mammary epithelial cells leads to tumorigenesis in mice (Liu et al., 2010). This paper summarizes the current work on the role of Pak4 in breast cancer.

Methods for treating neurological conditions (WO2011159945)

This patent application claims that inhibition of p21-activated kinases (PAK) reverses, partially reverses or delays clinical signs in neurological conditions (main claim for Huntington's disease (HD), substance abuse and addiction, Parkinson's disease, depression, bipolar disorder, anxiety disorder, posttraumatic stress disorder and neurofibromatosis). Several compounds with a pyrido-[2,3-d]pyrimidine-7(8H)-one core and high affinity to the catalytic domain of PAK1-4 are described in the patent. These PAK inhibitors are hypothesized to exert beneficial effects on clinical symptoms via modulation of dendritic spine morphology and/or synaptic function. Preliminary preclinical data suggest that PAK inhibition may be an interesting approach for the treatment of HD, neurofibromatosis and fragile X syndrome, while data for other neurological conditions are missing. Current limitations call for a comprehensive characterization of the role of PAK dysfunction in neurological disorders before further testing the effect of PAK inhibitors in relevant preclinical models. If ever, it will probably take many years before the most promising compounds will head to the clinic for further assessment in patients with neurological disorders.

WAVE2 Protein Complex Coupled to Membrane and Microtubules

E-cadherin is one of the key molecules in the formation of cell-cell adhesion and interacts intracellularly with a group of proteins collectively named catenins, through which the E-cadherin-catenin complex is anchored to actin-based cytoskeletal components. Although cell-cell adhesion is often disrupted in cancer cells by either genetic or epigenetic alterations in cell adhesion molecules, disruption of cell-cell adhesion alone seems to be insufficient for the induction of cancer cell migration and invasion. A small GTP-binding protein, Rac1, induces the specific cellular protrusions lamellipodia via WAVE2, a member of WASP/WAVE family of the actin cytoskeletal regulatory proteins. Biochemical and pharmacological investigations have revealed that WAVE2 interacts with many proteins that regulate microtubule growth, actin assembly, and membrane targeting of proteins, all of which are necessary for directional cell migration through lamellipodia formation. These findings might have important implications for the development of effective therapeutic agents against cancer cell migration and invasion.

The apical polarity determinant Crumbs 2 is a novel regulator of ESC-derived neural progenitors

ESCs undergoing neural differentiation in vitro display an intrinsic heterogeneity with a large subset of the cells forming polarized neural rosettes that maintain the neural progenitor microenvironment. This heterogeneity is not only necessary for normal development but also causes substantial technical challenges for practical applications. Here, we report a novel regulator of early neural progenitors, the apical polarity protein Crb2 (Crumbs homologue 2). Employing monolayer differentiation of mouse ESCs to model neurogenesis in vitro, we find that Crb2 is upregulated with Sox1 and Musashi at the onset of neuroepithelial specification and localizes to the apical side of neural rosettes. Stable Crb2-knockdown (KD) lines die at the onset of neural specification and fail to stabilize several apical polarity proteins. However, these cells are able to proliferate under self-renewing conditions and can be differentiated into mesodermal and endodermal lineages. Conversely, Crb2 overexpression during neural differentiation results in elevated levels of other apical polarity proteins and increases proliferation. Additionally, sustained overexpression of Crb2 reduces terminal differentiation into TuJ1-positive neurons. Furthermore, we demonstrate that Crb2 overexpression under self-renewing conditions increases glycogen synthase kinase (GSK)-3β inhibition, correlating with an increase in clonogenicity. To confirm the importance of GSK-3β inhibition downstream of Crb2, we show that Crb2-KD cells can be forced into neural lineages by blocking GSK-3β function and supplementing Epidermal Growth Factor (EGF) and basic Fibroblast Growth Factor (bFGF). Thus, this is the first demonstration that a member of the Crumbs family is essential for survival and differentiation of ESC-derived neural progenitors.

The secret life of kinases: functions beyond catalysis

Protein phosphorylation participates in the regulation of all fundamental biological processes, and protein kinases have been intensively studied. However, while the focus was on catalytic activities, accumulating evidence suggests that non-catalytic properties of protein kinases are essential, and in some cases even sufficient for their functions. These non-catalytic functions include the scaffolding of protein complexes, the competition for protein interactions, allosteric effects on other enzymes, subcellular targeting, and DNA binding. This rich repertoire often is used to coordinate phosphorylation events and enhance the specificity of substrate phosphorylation, but also can adopt functions that do not rely on kinase activity. Here, we discuss such kinase independent functions of protein and lipid kinases focussing on kinases that play a role in the regulation of cell proliferation, differentiation, apoptosis, and motility.

PAK1-Nck regulates cyclin D1 promoter activity in response to prolactin

Prolactin (PRL) is critical for alveolar proliferation and differentiation in normal mammary development and is also implicated in breast cancer. PRL influences cell proliferation and growth by altering the expression of cyclin D1. Cyclin D1 expression is directly regulated by PRL through the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5-mediated transcriptional activation of the cyclin D1 promoter. A p21-activated serine-threonine kinase (PAK)1 has also been implicated in the regulation of cyclin D1 gene expression. We have previously demonstrated that JAK2 directly phosphorylates PAK1 and extend these data here to demonstrate that PAK1 activates the cyclin D1 promoter in response to PRL. We show that mutation of PAK1 Tyr 153, 201, and 285 (sites of JAK2 phosphorylation; PAK1 Y3F) decreases both PAK1 nuclear translocation in response to PRL and PRL-induced cyclin D1 promoter activity by 55%. Mutation of the PAK1 nuclear localization signals decreases PRL-induced cyclin D1 promoter activity by 46%. A PAK1 Y3F mutant lacking functional nuclear localization signals decreases PRL-induced cyclin D1 activity by 68%, suggesting that there is another PAK1-dependent mechanism to activate the cyclin D1 promoter. We have found that adapter protein Nck sequesters PAK1 in the cytoplasm and that coexpression of both PAK1 and Nck inhibits the amplifying effect of PRL-induced PAK1 on cyclin D1 promoter activity (95% inhibition). This inhibition is partially abolished by disruption of PAK1-Nck binding. We propose two PAK1-dependent mechanisms to activate cyclin D1 promoter activity in response to PRL: via nuclear translocation of tyrosyl-phosphorylated PAK1 and via formation of a Nck-PAK1 complex that sequesters PAK1 in the cytoplasm.

WT1 and its transcriptional cofactor BASP1 redirect the differentiation pathway of an established blood cell line

The Wilms' tumour suppressor WT1 (Wilms' tumour 1) is a transcriptional regulator that plays a central role in organogenesis, and is mutated or aberrantly expressed in several childhood and adult malignancies. We previously identified BASP1 (brain acid-soluble protein 1) as a WT1 cofactor that suppresses the transcriptional activation function of WT1. In the present study we have analysed the dynamic between WT1 and BASP1 in the regulation of gene expression in myelogenous leukaemia K562 cells. Our findings reveal that BASP1 is a significant regulator of WT1 that is recruited to WT1-binding sites and suppresses WT1-mediated transcriptional activation at several WT1 target genes. We find that WT1 and BASP1 can divert the differentiation programme of K562 cells to a non-blood cell type following induction by the phorbol ester PMA. WT1 and BASP1 co-operate to induce the differentiation of K562 cells to a neuronal-like morphology that exhibits extensive arborization, and the expression of several genes involved in neurite outgrowth and synapse formation. Functional analysis revealed the relevance of the transcriptional reprogramming and morphological changes, in that the cells elicited a response to the neurotransmitter ATP. Taken together, the results of the present study reveal that WT1 and BASP1 can divert the lineage potential of an established blood cell line towards a cell with neuronal characteristics.

Kinase/phosphatase overexpression reveals pathways regulating hippocampal neuron morphology

Kinases and phosphatases that regulate neurite number versus branching versus extension are weakly correlated.

The kinase family that most strongly enhances neurite growth is a family of non-protein kinases; sugar kinases related to NADK.

Pathway analysis revealed that genes in several cancer pathways were highly active in enhancing neurite growth.

In neural development, neuronal precursors differentiate, migrate, extend long axons and dendrites, and finally establish connections with their targets. Clinical conditions such as spinal cord injury, traumatic brain injury, stroke, multiple sclerosis, Parkinson's disease, Huntington's disease, and Alzheimer's disease are often associated with a loss of axon and/or dendrite connectivity and treatment strategies would be enhanced by new therapies targeting cell intrinsic mechanisms of axon elongation and regeneration.

Phosphorylation controls most cellular processes, including the cell cycle, proliferation, metabolism, and apoptosis. Neuronal differentiation, including axon formation and elongation, is also regulated by a wide range of kinases and phosphatases. For example, the non-receptor tyrosine kinase Src is required for cell adhesion molecule-dependent neurite outgrowth. In addition to individual kinases and phosphatases, signaling pathways like the MAPK, growth factor signaling, PIP3, cytoskeletal, and calcium-dependent pathways have been shown to impinge on or control neuronal process development. Recent results have implicated GSK3 and PTEN as therapeutically relevant targets in axonal regeneration after injury. However, these and other experiments have studied only a small fraction of the total kinases and phosphatases in the genome. Because of recent advances in genomic knowledge, large-scale cDNA production, and high-throughput phenotypic analysis, it is now possible to take a more comprehensive approach to understanding the functions of kinases and phosphatases in neurons.

We performed a large, unbiased set of experiments to answer the question ‘what effect does the overexpression of genes encoding kinases, phosphatases, and related proteins have on neuronal morphology?' We used ‘high-content analysis' to obtain detailed results about the specific phenotypes of neurons. We studied embryonic rat hippocampal neurons because of their stereotypical development in vitro (Dotti et al, 1988) and their widespread use in studies of neuronal differentiation and signaling. We transfected over 700 clones encoding kinases and phosphatases into hippocampal neurons and analyzed the resulting changes in neuronal morphology.

Many known genes, including PP1a, ERK1, ErbB2, atypical PKC, Calcineurin, CaMK2, IGF1R, FGFR, GSK3, and PIK3 were observed to have significant effects on neurite outgrowth in our system, consistent with earlier findings in the literature.

We obtained quantitative data for many cellular and neuronal morphological parameters from each neuron imaged. These included nuclear morphology (nuclear area and Hoechst dye intensity), soma morphology (tubulin intensity, area, and shape), and numerous parameters of neurite morphology (e.g. tubulin intensity along the neurites, number of primary neurites, neurite length, number of branches, distance from the cell body to the branches, number of crossing points, width and area of the neurites, and longest neurite; Supplementary Figure 1). Other parameters were reported on a ‘per well' basis, including the percentage of transfected neurons in a condition, as well as the percentage of neurons initiating neurite growth. Data for each treatment were normalized to a control (pSport CAT) within the same experiment, then aggregated across replicate experiments.

Correlations among the 19 normalized parameters were analyzed for neurons transfected with all kinase and phosphatase clones (Figure 2). On the basis of this analysis, the primary variables that define the neurite morphology are primary neurite count, neurite average length, and average branches. Interestingly, primary neurite count was not well correlated with neurite length or branching. The Pearson correlation coefficient (r²) between the number of primary neurites and the average length of the neurites was 0.3, and between the number of primary neurites and average branching was 0.2. In contrast, the correlation coefficient of average branching with neurite average length was 0.7. The most likely explanation is that signaling mechanisms underlying the neurite number determination are different than those controlling length/branching of the neurites.

Related proteins are often involved in similar neuronal functions. For example, families of receptor protein tyrosine phosphatases are involved in motor axon extension and guidance in both Drosophila and in vertebrates, and a large family of Eph receptor tyrosine kinases regulates guidance of retinotectal projections, motor axons, and axons in the corpus callosum. We therefore asked whether families of related genes produced similar phenotypes when overexpressed in hippocampal neurons. Our set of genes covered 40% of the known protein kinases, and many of the non-protein kinases and phosphatases.

Gene families commonly exhibit redundant function. Redundant gene function has often been identified when two or more knockouts are required to produce a phenotype. Our technique allowed us to measure whether different members of gene families had similar (potentially redundant) or distinct effects on neuronal phenotype.

To determine whether groups of related genes affect neuronal morphology in similar ways, we used sequence alignment information to construct gene clusters (Figure 6). Genes were clustered at nine different thresholds of similarity (called ‘tiers'). The functional effect for a particular parameter was then averaged within each cluster of a given tier, and statistics were performed to determine the significance of the effect. We analyzed the results for three key neurite parameters (average neurite length, primary neurite count, and average branching). Genes that perturbed each of these phenotypes are grouped in Figure 6. Eight families, most with only a few genes, produced significant changes for one or two parameters. A diverse family of non-protein kinases had a positive effect on neurite outgrowth in three of the four parameters analyzed. This family of kinases consisted of a variety of enzymes, mostly sugar and lipid kinases. A similar analysis was performed using pathway cluster analysis with pathways from the KEGG database, rather than sequence homology. Interestingly, pathways involved in cancer cell proliferation potentiated neurite extension and branching.

Our studies have identified a large number of kinases and phosphatases, as well as structurally and functionally defined families of these proteins, that affect neuronal process formation in specific ways. We have provided an analytical methodology and new tools to analyze functional data, and have implicated genes with novel functions in neuronal development. Our studies are an important step towards the goal of a molecular description of the intrinsic control of axodendritic growth.

Development and regeneration of the nervous system requires the precise formation of axons and dendrites. Kinases and phosphatases are pervasive regulators of cellular function and have been implicated in controlling axodendritic development and regeneration. We undertook a gain-of-function analysis to determine the functions of kinases and phosphatases in the regulation of neuron morphology. Over 300 kinases and 124 esterases and phosphatases were studied by high-content analysis of rat hippocampal neurons. Proteins previously implicated in neurite growth, such as ERK1, GSK3, EphA8, FGFR, PI3K, PKC, p38, and PP1a, were confirmed to have effects in our functional assays. We also identified novel positive and negative neurite growth regulators. These include neuronal-developmentally regulated kinases such as the activin receptor, interferon regulatory factor 6 (IRF6) and neural leucine-rich repeat 1 (LRRN1). The protein kinase N2 (PKN2) and choline kinase α (CHKA) kinases, and the phosphatases PPEF2 and SMPD1, have little or no established functions in neuronal function, but were sufficient to promote neurite growth. In addition, pathway analysis revealed that members of signaling pathways involved in cancer progression and axis formation enhanced neurite outgrowth, whereas cytokine-related pathways significantly inhibited neurite formation.

p21-Activated kinases 1 and 3 control brain size through coordinating neuronal complexity and synaptic properties

The molecular mechanisms that coordinate postnatal brain enlargement, synaptic properties, and cognition remain an enigma. Here, we demonstrate that neuronal complexity controlled by p21-activated kinases (PAKs) is a key determinant for postnatal brain enlargement and synaptic properties. We showed that double-knockout (DK) mice lacking both PAK1 and PAK3 were born healthy, with normal brain size and structure, but severely impaired in postnatal brain growth, resulting in a dramatic reduction in brain volume. Remarkably, the reduced brain size was accompanied by minimal changes in total cell count, due to a significant increase in cell density. However, the DK neurons have smaller soma, markedly simplified dendritic arbors/axons, and reduced synapse density. Surprisingly, the DK mice had elevated basal synaptic responses due to enhanced individual synaptic potency but were severely impaired in bidirectional synaptic plasticity. The actions of PAK1 and PAK3 are possibly mediated by cofilin-dependent actin regulation, because the activity of cofilin and the properties of actin filaments were altered in the DK mice. These results reveal an essential in vivo role of PAK1 and PAK3 in coordinating neuronal complexity and synaptic properties and highlight the critical importance of dendrite/axon growth in dictating postnatal brain growth and attainment of normal brain size and function.

p190RhoGAP and Rap-dependent RhoGAP (ARAP3) inactivate RhoA in response to nerve growth factor leading to neurite outgrowth from PC12 cells

Rat pheochromocytoma (PC12) cells have been used to investigate neurite outgrowth. Nerve growth factor (NGF) has been well known to induce neurite outgrowth from PC12 cells. RhoA belongs to Ras-related small GTP-binding proteins, which regulate a variety of cellular processes, including cell morphology alteration, actin dynamics, and cell migration. NGF suppressed GTP-RhoA levels after 12 h in PC12 cells and was consistently required for a long time to induce neurite outgrowth. Constitutively active (CA)-RhoA suppressed neurite outgrowth from PC12 cells in response to NGF, whereas dominant-negative (DN)-RhoA stimulated it, suggesting that RhoA inactivation is essential for neurite outgrowth. Here, we investigated the mechanism of RhoA inactivation. DN-p190RhoGAP abrogated neurite outgrowth, whereas wild-type (WT)-p190RhoGAP and WT-Src synergistically stimulated it along with accelerating RhoA inactivation, suggesting that p190RhoGAP, which can be activated by Src, is a major component in inhibiting RhoA in response to NGF in PC12 cells. Contrary to RhoA, Rap1 was activated by NGF, and DN-Rap1 suppressed neurite outgrowth, suggesting that Rap1 is also essential for neurite outgrowth. RhoA was co-immunoprecipitated with Rap1, suggesting that Rap1 interacts with RhoA. Furthermore, a DN-Rap-dependent RhoGAP (ARAP3) prevented RhoA inactivation, abolishing neurite formation from PC12 cells in response to NGF. These results suggest that NGF activates Rap1, which, in turn, up-regulates ARAP3 leading to RhoA inactivation and neurite outgrowth from PC12 cells. Taken together, p190RhoGAP and ARAP3 seem to be two main factors inhibiting RhoA activity during neurite outgrowth in PC12 cells in response to NGF.