Abnormal long-lasting synaptic plasticity and cognition in mice lacking the mental retardation gene Pak3

Loss of the polarity protein Par3 promotes dendritic spine neoteny and enhances learning and memory

The Par3 polarity protein is critical for subcellular compartmentalization in different developmental processes. Variants of PARD3, encoding PAR3, are associated with intelligence and neurodevelopmental disorders. However, the role of Par3 in glutamatergic synapse formation and cognitive functions in vivo remains unknown. Here, we show that forebrain-specific Par3 conditional knockout leads to increased long, thin dendritic spines in vivo. In addition, we observed a decrease in the amplitude of miniature excitatory postsynaptic currents. Surprisingly, loss of Par3 enhances hippocampal-dependent spatial learning and memory and repetitive behavior. Phosphoproteomic analysis revealed proteins regulating cytoskeletal dynamics are significantly dysregulated downstream of Par3. Mechanistically, we found Par3 deletion causes increased Rac1 activation and dysregulated microtubule dynamics through CAMSAP2. Together, our data reveal an unexpected role for Par3 as a molecular gatekeeper in regulating the pool of immature dendritic spines, a rate-limiting step of learning and memory, through modulating Rac1 activation and microtubule dynamics in vivo.

FRAX486, a PAK inhibitor, overcomes ABCB1-mediated multidrug resistance in breast cancer cells

The overexpression of P-glycoprotein (P-gp/ABCB1) is a leading cause of multidrug resistance (MDR). Hence, it is crucial to discover effective pharmaceuticals that counteract ABCB1-mediated multidrug resistance. FRAX486 is a p21-activated kinase (PAK) inhibitor. The objective of this study was to investigate whether FRAX486 can reverse ABCB1-mediated multidrug resistance, while also exploring its mechanism of action. The CCK8 assay demonstrated that FRAX486 significantly reversed ABCB1-mediated multidrug resistance. Furthermore, western blotting and immunofluorescence experiments revealed that FRAX486 had no impact on expression level and intracellular localization of ABCB1. Notably, FRAX486 was found to enhance intracellular drug accumulation and reduce efflux, resulting in the reversal of multidrug resistance. Docking analysis also indicated a strong affinity between FRAX486 and ABCB1. This study highlights the ability of FRAX486 to reverse ABCB1-mediated multidrug resistance and provides valuable insights for its clinical application.

PAK2 is necessary for myelination in the peripheral nervous system

Myelination enables electrical impulses to propagate on axons at the highest speed, encoding essential life functions. The Rho family GTPases, RAC1 and CDC42, have been shown to critically regulate Schwann cell myelination. P21-activated kinase 2 (PAK2) is an effector of RAC1/CDC42, but its specific role in myelination remains undetermined. We produced a Schwann cell-specific knockout mouse of Pak2 (scPak2-/-) to evaluate PAK2's role in myelination. Deletion of Pak2, specifically in mouse Schwann cells, resulted in severe hypomyelination, slowed nerve conduction velocity and behaviour dysfunctions in the scPak2-/- peripheral nerve. Many Schwann cells in scPak2-/- sciatic nerves were arrested at the stage of axonal sorting. These abnormalities were rescued by reintroducing Pak2, but not the kinase-dead mutation of Pak2, via lentivirus delivery to scPak2-/- Schwann cells in vivo. Moreover, ablation of Pak2 in Schwann cells blocked the promyelinating effect driven by neuregulin-1, prion protein and inactivated RAC1/CDC42. Conversely, the ablation of Pak2 in neurons exhibited no phenotype. Such PAK2 activity can also be either enhanced or inhibited by different myelin lipids. We have identified a novel promyelinating factor, PAK2, that acts as a critical convergence point for multiple promyelinating signalling pathways. The promyelination by PAK2 is Schwann cell-autonomous. Myelin lipids, identified as inhibitors or activators of PAK2, may be utilized to develop therapies for repairing abnormal myelin in peripheral neuropathies.

PAK3 activation promotes the tangential to radial migration switch of cortical interneurons by increasing leading process dynamics and disrupting cell polarity

Mutations of PAK3, a p21-activated kinase, are associated in humans with cognitive deficits suggestive of defective cortical circuits and with frequent brain structural abnormalities. Most human variants no longer exhibit kinase activity. Since GABAergic interneurons express PAK3 as they migrate within the cortex, we here examined the role of PAK3 kinase activity in the regulation of cortical interneuron migration. During the embryonic development, cortical interneurons migrate a long distance tangentially and then re-orient radially to settle in the cortical plate, where they contribute to cortical circuits. We showed that interneurons expressing a constitutively kinase active PAK3 variant (PAK3-ca) extended shorter leading processes and exhibited unstable polarity. In the upper cortical layers, they entered the cortical plate and extended radially oriented processes. In the deep cortical layers, they exhibited erratic non-processive migration movements and accumulated in the deep pathway. Pharmacological inhibition of PAK3 kinase inhibited the radial migration switch of interneurons to the cortical plate and reduced their accumulation in the deep cortical layers. Interneurons expressing a kinase dead PAK3 variant (PAK3-kd) developed branched leading processes, maintained the same polarity during migration and exhibited processive and tangentially oriented movements in the cortex. These results reveal that PAK3 kinase activity, by promoting leading process shortening and cell polarity changes, inhibits the tangential processive migration of interneurons and favors their radial re- orientation and targeting to the cortical plate. They suggest that patients expressing PAK3 variants with impaired kinase activity likely present alterations in the cortical targeting of their GABAergic interneurons.

PAK3 downregulation induces cognitive impairment following cranial irradiation

Cranial irradiation is used for prophylactic brain radiotherapy as well as the treatment of primary brain tumors. Despite its high efficiency, it often induces unexpected side effects, including cognitive dysfunction. Herein, we observed that mice exposed to cranial irradiation exhibited cognitive dysfunction, including altered spontaneous behavior, decreased spatial memory, and reduced novel object recognition. Analysis of the actin cytoskeleton revealed that ionizing radiation (IR) disrupted the filamentous/globular actin (F/G-actin) ratio and downregulated the actin turnover signaling pathway p21-activated kinase 3 (PAK3)-LIM kinase 1 (LIMK1)-cofilin. Furthermore, we found that IR could upregulate microRNA-206-3 p (miR-206-3 p) targeting PAK3. As the inhibition of miR-206-3 p through antagonist (antagomiR), IR-induced disruption of PAK3 signaling is restored. In addition, intranasal administration of antagomiR-206-3 p recovered IR-induced cognitive impairment in mice. Our results suggest that cranial irradiation-induced cognitive impairment could be ameliorated by regulating PAK3 through antagomiR-206-3 p, thereby affording a promising strategy for protecting cognitive function during cranial irradiation, and promoting quality of life in patients with radiation therapy.

PAK1 and Therapy Resistance in Melanoma

Malignant melanoma claims more lives than any other skin malignancy. While primary melanomas are usually cured via surgical excision, the metastatic form of the disease portents a poor prognosis. Decades of intense research has yielded an extensive armamentarium of anti-melanoma therapies, ranging from genotoxic chemo- and radiotherapies to targeted interventions in specific signaling pathways and immune functions. Unfortunately, even the most up-to-date embodiments of these therapies are not curative for the majority of metastatic melanoma patients, and the need to improve their efficacy is widely recognized. Here, we review the reports that implicate p21-regulated kinase 1 (PAK1) and PAK1-related pathways in the response of melanoma to various therapeutic modalities. Ample data suggest that PAK1 may decrease cell sensitivity to programmed cell death, provide additional stimulation to growth-promoting molecular pathways, and contribute to the creation of an immunosuppressive tumor microenvironment. Accordingly, there is mounting evidence that the concomitant inhibition of PAK1 enhances the potency of various anti-melanoma regimens. Overall, the available information suggests that a safe and effective inhibition of PAK1-dependent molecular processes would enhance the potency of the currently available anti-melanoma treatments, although considerable challenges in implementing such strategies still exist.

Loss of the polarity protein Par3 promotes dendritic spine neoteny and enhances learning and memory

The Par3 polarity protein is critical for subcellular compartmentalization in different developmental processes. Variants of PARD3 , which encodes PAR3, are associated with intelligence and neurodevelopmental disorders. However, the role of Par3 in glutamatergic synapse formation and cognitive functions in vivo remains unknown. Here, we show that forebrain conditional knockout of Par3 leads to an increase in long, thin dendritic spines without significantly impacting mushroom spines in vivo . In addition, we observed a decrease in the amplitude of miniature excitatory postsynaptic currents. Surprisingly, loss of Par3 in vivo enhances hippocampal- dependent spatial learning. Phosphoproteomic analysis revealed proteins regulating cytoskeletal dynamics are significantly dysregulated downstream of Par3. Mechanistically, we found Par3 deletion causes increased activation of the Rac1 pathway. Together, our data reveal an unexpected role for Par3 as a molecular gatekeeper in regulating the pool of immature dendritic spines, a rate-limiting step of learning and memory, through modulating Rac1 activation in vivo .

PAK1 contributes to cerebral ischemia/reperfusion injury by regulating the blood-brain barrier integrity

Globally, stroke is one of the leading causes of death and significant contributors to disability. Gaining a thorough comprehension of the underlying pathogenic processes is essential for stroke treatment and prevention. In this study, we investigated the role of p21-activated kinase 1 (PAK1) in stroke by using oxygen-glucose deprivation (OGD) and transient middle cerebral artery occlusion and reperfusion (tMCAO/R) models. We reported that focal ischemia and reperfusion affect the PAK1 expression and activity levels. We further demonstrated that PAK1 is responsible for the endothelial hyperpermeability that occurs in the early stages of ischemia and reperfusion. Additionally, inhibition of PAK1 was discovered to alleviate blood-brain barrier disruption and protect against brain injury induced by tMCAO/R. Mechanistically, we provide the evidence that PAK1 regulates the formation of stress fibers and expression of surface junctional proteins. Together, our findings reveal a pathogenic function of PAK1 in stroke.

Restored autophagy is protective against PAK3-induced cardiac dysfunction

Despite the development of clinical treatments, heart failure remains the leading cause of mortality. We observed that p21-activated kinase 3 (PAK3) was augmented in failing human and mouse hearts. Furthermore, mice with cardiac-specific PAK3 overexpression exhibited exacerbated pathological remodeling and deteriorated cardiac function. Myocardium with PAK3 overexpression displayed hypertrophic growth, excessive fibrosis, and aggravated apoptosis following isoprenaline stimulation as early as two days. Mechanistically, using cultured cardiomyocytes and human-relevant samples under distinct stimulations, we, for the first time, demonstrated that PAK3 acts as a suppressor of autophagy through hyper-activation of the mechanistic target of rapamycin complex 1 (mTORC1). Defective autophagy in the myocardium contributes to the progression of heart failure. More importantly, PAK3-provoked cardiac dysfunction was mitigated by administering an autophagic inducer. Our study illustrates a unique role of PAK3 in autophagy regulation and the therapeutic potential of targeting this axis for heart failure.

NCK1 Modulates Neuronal Actin Dynamics and Promotes Dendritic Spine, Synapse, and Memory Formation

Memory formation and maintenance is a dynamic process involving the modulation of the actin cytoskeleton at synapses. Understanding the signaling pathways that contribute to actin modulation is important for our understanding of synapse formation and function, as well as learning and memory. Here, we focused on the importance of the actin regulator, noncatalytic region of tyrosine kinase adaptor protein 1 (NCK1), in hippocampal dependent behaviors and development. We report that male mice lacking NCK1 have impairments in both short-term and working memory, as well as spatial learning. Additionally, we report sex differences in memory impairment showing that female mice deficient in NCK1 fail at reversal learning in a spatial learning task. We find that NCK1 is expressed in postmitotic neurons but is dispensable for neuronal proliferation and migration in the developing hippocampus. Morphologically, NCK1 is not necessary for overall neuronal dendrite development. However, neurons lacking NCK1 have lower dendritic spine and synapse densities in vitro and in vivo EM analysis reveal increased postsynaptic density (PSD) thickness in the hippocampal CA1 region of NCK1-deficient mice. Mechanistically, we find the turnover of actin-filaments in dendritic spines is accelerated in neurons that lack NCK1. Together, these findings suggest that NCK1 contributes to hippocampal-dependent memory by stabilizing actin dynamics and dendritic spine formation.SIGNIFICANCE STATEMENT Understanding the molecular signaling pathways that contribute to memory formation, maintenance, and elimination will lead to a better understanding of the genetic influences on cognition and cognitive disorders and will direct future therapeutics. Here, we report that the noncatalytic region of tyrosine kinase adaptor protein 1 (NCK1) adaptor protein modulates actin-filament turnover in hippocampal dendritic spines. Mice lacking NCK1 show sex-dependent deficits in hippocampal memory formation tasks, have altered postsynaptic densities, and reduced synaptic density. Together, our work implicates NCK1 in the regulation of actin cytoskeleton dynamics and normal synapse development which is essential for memory formation.

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.

Brain-specific deletion of GIT1 impairs cognition and alters phosphorylation of synaptic protein networks implicated in schizophrenia susceptibility

Despite tremendous effort, the molecular and cellular basis of cognitive deficits in schizophrenia remain poorly understood. Recent progress in elucidating the genetic architecture of schizophrenia has highlighted the association of multiple loci and rare variants that may impact susceptibility. One key example, given their potential etiopathogenic and therapeutic relevance, is a set of genes that encode proteins that regulate excitatory glutamatergic synapses in brain. A critical next step is to delineate specifically how such genetic variation impacts synaptic plasticity and to determine if and how the encoded proteins interact biochemically with one another to control cognitive function in a convergent manner. Towards this goal, here we study the roles of GPCR-kinase interacting protein 1 (GIT1), a synaptic scaffolding and signaling protein with damaging coding variants found in schizophrenia patients, as well as copy number variants found in patients with neurodevelopmental disorders. We generated conditional neural-selective GIT1 knockout mice and found that these mice have deficits in fear conditioning memory recall and spatial memory, as well as reduced cortical neuron dendritic spine density. Using global quantitative phospho-proteomics, we revealed that GIT1 deletion in brain perturbs specific networks of GIT1-interacting synaptic proteins. Importantly, several schizophrenia and neurodevelopmental disorder risk genes are present within these networks. We propose that GIT1 regulates the phosphorylation of a network of synaptic proteins and other critical regulators of neuroplasticity, and that perturbation of these networks may contribute specifically to cognitive deficits observed in schizophrenia and neurodevelopmental disorders.

Inhibition of Rac1 in ventral hippocampal excitatory neurons improves social recognition memory and synaptic plasticity

Rac1 is critically involved in the regulation of the actin cytoskeleton, neuronal structure, synaptic plasticity, and memory. Rac1 overactivation is reported in human patients and animal models of Alzheimer’s disease (AD) and contributes to their spatial memory deficits, but whether Rac1 dysregulation is also important in other forms of memory deficits is unknown. In addition, the cell types and synaptic mechanisms involved remain unclear. In this study, we used local injections of AAV virus containing a dominant-negative (DN) Rac1 under the control of CaMKIIα promoter and found that the reduction of Rac1 hyperactivity in ventral hippocampal excitatory neurons improves social recognition memory in APP/PS1 mice. Expression of DN Rac1 also improves long-term potentiation, a key synaptic mechanism for memory formation. Our results suggest that overactivation of Rac1 in hippocampal excitatory neurons contributes to social memory deficits in APP/PS1 mice and that manipulating Rac1 activity may provide a potential therapeutic strategy to treat social deficits in AD.

p21-Activated kinases as promising therapeutic targets in hematological malignancies

The p21-Activated Kinases (PAKs) are a family of six serine/threonine kinases that were originally identified as downstream effectors of the Rho GTPases Cdc42 and Rac. Since the first PAK was discovered in 1994, studies have revealed their fundamental and biological importance in the development of physiological systems. Within the cell, PAKs also play significant roles in regulating essential cellular processes such as cytoskeletal dynamics, gene expression, cell survival, and cell cycle progression. These processes are often deregulated in numerous cancers when different PAKs are overexpressed or amplified at the chromosomal level. Furthermore, PAKs modulate multiple oncogenic signaling pathways which facilitate apoptosis escape, uncontrolled proliferation, and drug resistance. There is growing insight into the critical roles of PAKs in regulating steady-state hematopoiesis, including the properties of hematopoietic stem cells (HSC), and the initiation and progression of hematological malignancies. This review will focus on the most recent studies that provide experimental evidence showing how specific PAKs regulate the properties of leukemic stem cells (LSCs) and drug-resistant cells to initiate and maintain hematological malignancies. The current understanding of the molecular and cellular mechanisms by which the PAKs operate in specific human leukemia or lymphomas will be discussed. From a translational point of view, PAKs have been suggested to be critical therapeutic targets and potential prognosis markers; thus, this review will also discuss current therapeutic strategies against hematological malignancies using existing small-molecule PAK inhibitors, as well as promising combination treatments, to sensitize drug-resistant cells to conventional therapies. The challenges of toxicity and non-specific targeting associated with some PAK inhibitors, as well as how future approaches for PAK inhibition to overcome these limitations, will also be addressed.

Group I PAKs in myelin formation and repair of the central nervous system: what, when, and how

p21-activated kinases (PAKs) are a family of cell division control protein 42/ras-related C3 botulinum toxin substrate 1 (Cdc42/Rac1)-activated serine/threonine kinases. Group I PAKs (PAK1-3) have distinct activation mechanisms from group II PAKs (PAK4-6) and are the focus of this review. In transformed cancer cells, PAKs regulate a variety of cellular processes and molecular pathways which are also important for myelin formation and repair in the central nervous system (CNS). De novo mutations in group I PAKs are frequently seen in children with neurodevelopmental defects and white matter anomalies. Group I PAKs regulate virtually every aspect of neuronal development and function. Yet their functions in CNS myelination and remyelination remain incompletely defined. Herein, we highlight the current understanding of PAKs in regulating cellular and molecular pathways and discuss the status of PAK-regulated pathways in oligodendrocyte development. We point out outstanding questions and future directions in the research field of group I PAKs and oligodendrocyte development.

Energy matters: presynaptic metabolism and the maintenance of synaptic transmission

Synaptic activity imposes large energy demands that are met by local adenosine triphosphate (ATP) synthesis through glycolysis and mitochondrial oxidative phosphorylation. ATP drives action potentials, supports synapse assembly and remodelling, and fuels synaptic vesicle filling and recycling, thus sustaining synaptic transmission. Given their polarized morphological features - including long axons and extensive branching in their terminal regions - neurons face exceptional challenges in maintaining presynaptic energy homeostasis, particularly during intensive synaptic activity. Recent studies have started to uncover the mechanisms and signalling pathways involved in activity-dependent and energy-sensitive regulation of presynaptic energetics, or 'synaptoenergetics'. These conceptual advances have established the energetic regulation of synaptic efficacy and plasticity as an exciting research field that is relevant to a range of neurological disorders associated with bioenergetic failure and synaptic dysfunction.

Disruption of PAK3 Signaling in Social Interaction Induced cFos Positive Cells Impairs Social Recognition Memory

P21-activated kinase 3 (PAK3) gene mutations are linked to several neurodevelopmental disorders, but the underlying mechanisms remain unclear. In this study, we used a tetracycline-inducible system to control the expression of a mutant PAK3 (mPAK3) protein in immediate early gene, namely cFos, positive cells to disrupt PAK signaling, specifically in cells activated by social interaction in transgenic mice. We show that the expression of mPAK3-GFP proteins was in cFos-expressing excitatory and inhibitory neurons in various brain regions, such as the cortex and hippocampus, commonly activated during learning and memory. Basal expression of mPAK3-GFP proteins in cFos-positive cells resulted in social recognition memory deficits in the three-chamber social interaction test, without affecting locomotor activity or other forms of memory. The social memory deficit was rescued by doxycycline to halt the mPAK3-GFP transgene expression. In addition, we show that the expression of mPAK3-GFP proteins in a subset of cFos-positive cells, induced by an antecedent short social interaction, termed social pairing, was sufficient to impair social recognition memory. These results indicate that normal PAK signaling in cFos-positive cells activated during social interaction is critical for social memory.

WaterMap and Molecular Dynamic Simulation-Guided Discovery of Potential PAK1 Inhibitors Using Repurposing Approaches

p21-Activated kinase 1 (PAK1) is positioned at the nexus of several oncogenic signaling pathways. Currently, there are no approved inhibitors for disabling the transfer of phosphate in the active site directly, as they are limited by lower affinity, and poor kinase selectivity. In this work, a repurposing study utilizing FDA-approved drugs from the DrugBank database was pursued with an initial selection of 27 molecules out of ∼2162 drug molecules, based on their docking energies and molecular interaction patterns. From the molecules that were considered for WaterMap analysis, seven molecules, namely, Mitoxantrone, Labetalol, Acalabrutinib, Sacubitril, Flubendazole, Trazodone, and Niraparib, ascertained the ability to overlap with high-energy hydration sites. Considering many other displaced unfavorable water molecules, only Acalabrutinib, Flubendazole, and Trazodone molecules highlighted their prominence in terms of binding affinity gains through ΔΔG that ranges between 6.44 and 2.59 kcal/mol. Even if Mitoxantrone exhibited the highest docking score and greater interaction strength, it did not comply with the WaterMap and molecular dynamics simulation results. Moreover, detailed MD simulation trajectory analyses suggested that the drug molecules Flubendazole, Niraparib, and Acalabrutinib were highly stable, observed from their RMSD values and consistent interaction pattern with Glu315, Glu345, Leu347, and Asp407 including the hydrophobic interactions maintained in the three replicates. However, the drug molecule Trazodone displayed a loss of crucial interaction with Leu347, which was essential to inhibit the kinase activity of PAK1. The molecular orbital and electrostatic potential analyses elucidated the reactivity and strong complementarity potentials of the drug molecules in the binding pocket of PAK1. Therefore, the CADD-based reposition efforts, reported in this work, helped in the successful identification of new PAK1 inhibitors that requires further investigation by in vitro analysis.

Rho Signaling in Synaptic Plasticity, Memory, and Brain Disorders

Memory impairments are associated with many brain disorders such as autism, Alzheimer's disease, and depression. Forming memories involves modifications of synaptic transmission and spine morphology. The Rho family small GTPases are key regulators of synaptic plasticity by affecting various downstream molecules to remodel the actin cytoskeleton. In this paper, we will review recent studies on the roles of Rho proteins in the regulation of hippocampal long-term potentiation (LTP) and long-term depression (LTD), the most extensively studied forms of synaptic plasticity widely regarded as cellular mechanisms for learning and memory. We will also discuss the involvement of Rho signaling in spine morphology, the structural basis of synaptic plasticity and memory formation. Finally, we will review the association between brain disorders and abnormalities of Rho function. It is expected that studying Rho signaling at the synapse will contribute to the understanding of how memory is formed and disrupted in diseases.

The intellectual disability PAK3 R67C mutation impacts cognitive functions and adult hippocampal neurogenesis

The link between mutations associated with intellectual disability (ID) and the mechanisms underlying cognitive dysfunctions remains largely unknown. Here, we focused on PAK3, a serine/threonine kinase whose gene mutations cause X-linked ID. We generated a new mutant mouse model bearing the missense R67C mutation of the Pak3 gene (Pak3-R67C), known to cause moderate to severe ID in humans without other clinical signs and investigated hippocampal-dependent memory and adult hippocampal neurogenesis. Adult male Pak3-R67C mice exhibited selective impairments in long-term spatial memory and pattern separation function, suggestive of altered hippocampal neurogenesis. A delayed non-matching to place paradigm testing memory flexibility and proactive interference, reported here as being adult neurogenesis-dependent, revealed a hypersensitivity to high interference in Pak3-R67C mice. Analyzing adult hippocampal neurogenesis in Pak3-R67C mice reveals no alteration in the first steps of adult neurogenesis, but an accelerated death of a population of adult-born neurons during the critical period of 18-28 days after their birth. We then investigated the recruitment of hippocampal adult-born neurons after spatial memory recall. Post-recall activation of mature dentate granule cells in Pak3-R67C mice was unaffected, but a complete failure of activation of young DCX + newborn neurons was found, suggesting they were not recruited during the memory task. Decreased expression of the KCC2b chloride cotransporter and altered dendritic development indicate that young adult-born neurons are not fully functional in Pak3-R67C mice. We suggest that these defects in the dynamics and learning-associated recruitment of newborn hippocampal neurons may contribute to the selective cognitive deficits observed in this mouse model of ID.

LIM-Kinases in Synaptic Plasticity, Memory, and Brain Diseases

Learning and memory require structural and functional modifications of synaptic connections, and synaptic deficits are believed to underlie many brain disorders. The LIM-domain-containing protein kinases (LIMK1 and LIMK2) are key regulators of the actin cytoskeleton by affecting the actin-binding protein, cofilin. In addition, LIMK1 is implicated in the regulation of gene expression by interacting with the cAMP-response element-binding protein. Accumulating evidence indicates that LIMKs are critically involved in brain function and dysfunction. In this paper, we will review studies on the roles and underlying mechanisms of LIMKs in the regulation of long-term potentiation (LTP) and depression (LTD), the most extensively studied forms of long-lasting synaptic plasticity widely regarded as cellular mechanisms underlying learning and memory. We will also discuss the involvement of LIMKs in the regulation of the dendritic spine, the structural basis of synaptic plasticity, and memory formation. Finally, we will discuss recent progress on investigations of LIMKs in neurological and mental disorders, including Alzheimer’s, Parkinson’s, Williams–Beuren syndrome, schizophrenia, and autism spectrum disorders.

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

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

Dysregulation of Multiple Signaling Neurodevelopmental Pathways during Embryogenesis: A Possible Cause of Autism Spectrum Disorder

Understanding the autistic brain and the involvement of genetic, non-genetic, and numerous signaling pathways in the etiology and pathophysiology of autism spectrum disorder (ASD) is complex, as is evident from various studies. Apart from multiple developmental disorders of the brain, autistic subjects show a few characteristics like impairment in social communications related to repetitive, restricted, or stereotypical behavior, which suggests alterations in neuronal circuits caused by defects in various signaling pathways during embryogenesis. Most of the research studies on ASD subjects and genetic models revealed the involvement of mutated genes with alterations of numerous signaling pathways like Wnt, hedgehog, and Retinoic Acid (RA). Despite significant improvement in understanding the pathogenesis and etiology of ASD, there is an increasing awareness related to it as well as a need for more in-depth research because no effective therapy has been developed to address ASD symptoms. Therefore, identifying better therapeutic interventions like "novel drugs for ASD" and biomarkers for early detection and disease condition determination are required. This review article investigated various etiological factors as well as the signaling mechanisms and their alterations to understand ASD pathophysiology. It summarizes the mechanism of signaling pathways, their significance, and implications for ASD.

The Role of p21-Activated Kinases in Cancer and Beyond: Where Are We Heading?

The p21-activated kinases (PAKs), downstream effectors of Ras-related Rho GTPase Cdc42 and Rac, are serine/threonine kinases. Biologically, PAKs participate in various cellular processes, including growth, apoptosis, mitosis, immune response, motility, inflammation, and gene expression, making PAKs the nexus of several pathogenic and oncogenic signaling pathways. PAKs were proved to play critical roles in human diseases, including cancer, infectious diseases, neurological disorders, diabetes, pancreatic acinar diseases, and cardiac disorders. In this review, we systematically discuss the structure, function, alteration, and molecular mechanisms of PAKs that are involved in the pathogenic and oncogenic effects, as well as PAK inhibitors, which may be developed and deployed in cancer therapy, anti-viral infection, and other diseases. Furthermore, we highlight the critical questions of PAKs in future research, which provide an opportunity to offer input and guidance on new directions for PAKs in pathogenic, oncogenic, and drug discovery research.

The p21-activated kinases in neural cytoskeletal remodeling and related neurological disorders

The serine/threonine p21-activated kinases (PAKs), as main effectors of the Rho GTPases Cdc42 and Rac, represent a group of important molecular switches linking the complex cytoskeletal networks to broad neural activity. PAKs show wide expression in the brain, but they differ in specific cell types, brain regions, and developmental stages. PAKs play an essential and differential role in controlling neural cytoskeletal remodeling and are related to the development and fate of neurons as well as the structural and functional plasticity of dendritic spines. PAK-mediated actin signaling and interacting functional networks represent a common pathway frequently affected in multiple neurodevelopmental and neurodegenerative disorders. Considering specific small-molecule agonists and inhibitors for PAKs have been developed in cancer treatment, comprehensive knowledge about the role of PAKs in neural cytoskeletal remodeling will promote our understanding of the complex mechanisms underlying neurological diseases, which may also represent potential therapeutic targets of these diseases.

The Role of ADF/Cofilin in Synaptic Physiology and Alzheimer's Disease

Actin-depolymerization factor (ADF)/cofilin, a family of actin-binding proteins, are critical for the regulation of actin reorganization in response to various signals. Accumulating evidence indicates that ADF/cofilin also play important roles in neuronal structure and function, including long-term potentiation and depression. These are the most extensively studied forms of long-lasting synaptic plasticity and are widely regarded as cellular mechanisms underlying learning and memory. ADF/cofilin regulate synaptic function through their effects on dendritic spines and the trafficking of glutamate receptors, the principal mediator of excitatory synaptic transmission in vertebrates. Regulation of ADF/cofilin involves various signaling pathways converging on LIM domain kinases and slingshot phosphatases, which phosphorylate/inactivate and dephosphorylate/activate ADF/cofilin, respectively. Actin-depolymerization factor/cofilin activity is also regulated by other actin-binding proteins, activity-dependent subcellular distribution and protein translation. Abnormalities in ADF/cofilin have been associated with several neurodegenerative disorders such as Alzheimer’s disease. Therefore, investigating the roles of ADF/cofilin in the brain is not only important for understanding the fundamental processes governing neuronal structure and function, but also may provide potential therapeutic strategies to treat brain disorders.

PAK1 Regulates MEC-17 Acetyltransferase Activity and Microtubule Acetylation during Proplatelet Extension

Mature megakaryocytes extend long processes called proplatelets from which platelets are released in the blood stream. The Rho GTPases Cdc42 and Rac as well as their downstream target, p21-activated kinase 2 (PAK2), have been demonstrated to be important for platelet formation. Here we address the role, during platelet formation, of PAK1, another target of the Rho GTPases. PAK1 decorates the bundled microtubules (MTs) of megakaryocyte proplatelets. Using a validated cell model which recapitulates proplatelet formation, elongation and platelet release, we show that lack of PAK1 activity increases the number of proplatelets but restrains their elongation. Moreover, in the absence of PAK1 activity, cells have hyperacetylated MTs and lose their MT network integrity. Using inhibitors of the tubulin deacetylase HDAC6, we demonstrate that abnormally high levels of MT acetylation are not sufficient to increase the number of proplatelets but cause loss of MT integrity. Taken together with our previous demonstration that MT acetylation is required for proplatelet formation, our data reveal that MT acetylation levels need to be tightly regulated during proplatelet formation. We identify PAK1 as a direct regulator of the MT acetylation levels during this process as we found that PAK1 phosphorylates the MT acetyltransferase MEC-17 and inhibits its activity.

The cross-talk of energy sensing and mitochondrial anchoring sustains synaptic efficacy by maintaining presynaptic metabolism

Mitochondria supply ATP essential for synaptic transmission. Neurons face exceptional challenges in maintaining energy homoeostasis at synapses. Regulation of mitochondrial trafficking and anchoring is critical for neurons to meet increased energy consumption during sustained synaptic activity. However, mechanisms recruiting and retaining presynaptic mitochondria in sensing synaptic ATP levels remain elusive. Here we reveal an energy signalling axis that controls presynaptic mitochondrial maintenance. Activity-induced presynaptic energy deficits can be rescued by recruiting mitochondria through the AMP-activated protein kinase (AMPK)-p21-activated kinase (PAK) energy signalling pathway. Synaptic activity induces AMPK activation within axonal compartments and AMPK-PAK signalling triggers phosphorylation of myosin VI, which drives mitochondrial recruitment and syntaphilin-mediated anchoring on presynaptic filamentous actin. This pathway maintains presynaptic energy supply and calcium clearance during intensive synaptic activity. Disrupting this signalling cross-talk triggers local energy deficits and intracellular calcium build-up, leading to impaired synaptic efficacy during trains of stimulation and reduced recovery from synaptic depression after prolonged synaptic activity. Our study reveals a mechanistic cross-talk between energy sensing and mitochondria anchoring to maintain presynaptic metabolism, thus fine-tuning short-term synaptic plasticity and prolonged synaptic efficacy.

Novel PAK3 gene missense variant associated with two Chinese siblings with intellectual disability: a case report

BACKGROUND

Intellectual disability (ID) constitutes the most common group of neurodevelopmental disorders. Exome sequencing has enabled the discovery of genetic mutations responsible for a wide range of ID disorders.

CASE PRESENTATION

In this study, we reported on two male siblings, aged 4 and 2 years, with motor and mental developmental delays and mild dysmorphic facial features. To identify the genetic causes of these symptoms, we employed trio-whole exome sequencing for the proband. We found a novel hemizygous missense variant in the PAK3 gene (c.1112G > A, p.Cys371Tyr), which encodes the p21-activated kinase 3, in the proband, which inherited from mother. The younger brother also has the hemizygous variant, which was confirmed by Sanger sequencing. The variant is located in the kinase domain and was regarded as a likely pathogenic variant in this family.

CONCLUSION

We diagnosed two male siblings with developmental delays as having a PAK3 likely pathogenic variant. This finding expands the list of PAK3 gene mutations associated with neurodevelopmental disorders and provides further details on its clinical features.

Altered Hippocampal Epigenetic Regulation Underlying Reduced Cognitive Development in Response to Early Life Environmental Insults

The hippocampus is involved in learning and memory and undergoes significant growth and maturation during the neonatal period. Environmental insults during this developmental timeframe can have lasting effects on brain structure and function. This study assessed hippocampal DNA methylation and gene transcription from two independent studies reporting reduced cognitive development stemming from early life environmental insults (iron deficiency and porcine reproductive and respiratory syndrome virus (PRRSv) infection) using porcine biomedical models. In total, 420 differentially expressed genes (DEGs) were identified between the reduced cognition and control groups, including genes involved in neurodevelopment and function. Gene ontology (GO) terms enriched for DEGs were associated with immune responses, angiogenesis, and cellular development. In addition, 116 differentially methylated regions (DMRs) were identified, which overlapped 125 genes. While no GO terms were enriched for genes overlapping DMRs, many of these genes are known to be involved in neurodevelopment and function, angiogenesis, and immunity. The observed altered methylation and expression of genes involved in neurological function suggest reduced cognition in response to early life environmental insults is due to altered cholinergic signaling and calcium regulation. Finally, two DMRs overlapped with two DEGs, VWF and LRRC32, which are associated with blood brain barrier permeability and regulatory T-cell activation, respectively. These results support the role of altered hippocampal DNA methylation and gene expression in early life environmentally-induced reductions in cognitive development across independent studies.

PAK2 Haploinsufficiency Results in Synaptic Cytoskeleton Impairment and Autism-Related Behavior

Synaptic cytoskeleton dysfunction represents a common pathogenesis in neurodevelopmental disorders, such as autism spectrum disorder (ASD). The serine/threonine kinase PAK2 is a critical regulator of cytoskeleton dynamics. However, its function within the central nervous system and its role in ASD pathogenesis remain undefined. Here, we found that Pak2 haploinsufficiency resulted in markedly decreased synapse densities, defective long-term potentiation, and autism-related behaviors in mice. Phosphorylation levels of key actin regulators LIMK1 and cofilin, together with their mediated actin polymerization, were reduced in Pak2^+/-mice. We identified one de novo PAK2 nonsense mutation that impaired PAK2 function in vitro and in vivo and four de novo copy-number deletions containing PAK2 in large cohorts of patients with ASD. PAK2 deficiency extensively perturbed functional networks associated with ASD by regulating actin cytoskeleton dynamics. Our genetic and functional results demonstrate a critical role of PAK2 in brain development and autism pathogenesis.

Isoflurane mediated neuropathological and cognitive impairments in the triple transgenic Alzheimer's mouse model are associated with hippocampal synaptic deficits in an age-dependent manner

Many in vivo studies suggest that inhalational anesthetics can accelerate or prevent the progression of neuropathology and cognitive impairments in Alzheimer Disease (AD), but the synaptic mechanisms mediating these ambiguous effects are unclear. Here, we show that repeated exposures of neonatal and old triple transgenic AD (3xTg) and non-transgenic (NonTg) mice to isoflurane (Iso) distinctly increased neurodegeneration as measured by S100β levels, intracellular Aβ, Tau oligomerization, and apoptotic markers. Spatial cognition measured by reference and working memory testing in the Morris Water Maze (MWM) were altered in young NonTg and 3xTg. Field recordings in the cornu ammonis 1 (CA1) hippocampus showed that neonatal control 3xTg mice exhibited hypo-excitable synaptic transmission, reduced paired-pulse facilitation (PPF), and normal long-term potentiation (LTP) compared to NonTg controls. By contrast, the old control 3xTg mice exhibited hyper-excitable synaptic transmission, enhanced PPF, and unstable LTP compared to NonTg controls. Repeated Iso exposures reduced synaptic transmission and PPF in neonatal NonTg and old 3xTg mice. LTP was normalized in old 3xTg mice, but reduced in neonates. By contrast, LTP was reduced in old but not neonatal NonTg mice. Our results indicate that Iso-mediated neuropathologic and cognitive defects in AD mice are associated with synaptic pathologies in an age-dependent manner. Based on these findings, the extent of this association with age and, possibly, treatment paradigms warrant further study.

Synaptic actin stabilization protein loss in Down syndrome and Alzheimer disease

Reduced spine densities and age-dependent accumulation of amyloid β and tau pathology are shared features of Down syndrome (DS) and Alzheimer's disease (AD). Both spine morphology and the synaptic plasticity that supports learning depend upon the actin cytoskeleton, suggesting that disturbances in actin regulatory signaling might underlie spine defects in both disorders. The present study evaluated the synaptic levels of two proteins that promote filamentous actin stabilization, the Rho GTPase effector p21-activated kinase 3 (PAK3) and Arp2, in DS vs. AD. Fluorescent deconvolution tomography was used to determine postsynaptic PAK3 and Arp2 levels for large numbers of excitatory synapses in the parietal cortex of individuals with DS plus AD pathology (DS + AD) or AD alone relative to age-matched controls. Though numbers of excitatory synapses were not different between groups, synaptic PAK3 levels were greatly reduced in DS + AD and AD individuals vs. controls. Synaptic Arp2 levels also were reduced in both disorders, but to a greater degree in AD. Western blotting detected reduced Arp2 levels in the AD group, but there was no correlation with phosphorylated tau levels suggesting that the Arp2 loss does not contribute to mechanisms that drive tau pathology progression. Overall, the results demonstrate marked synaptic disturbances in two actin regulatory proteins in adult DS and AD brains, with greater effects in individuals with AD alone. As both PAK and the Arp2/3 complex play roles in the actin stabilization that supports synaptic plasticity, reductions in these proteins at synapses may be early events in spine dysfunction that contribute to cognitive impairment in these disorders.

Identification of Pak1 inhibitors using water thermodynamic analysis

p21-activated kinases (Paks) play an integral component in various cellular diverse processes. The full activation of Pak is dependent upon several serine residues present in the N-terminal region, a threonine present at the activation loop, and finally the phosphorylation of these residues ensure the complete activation of Pak1. The present study deals with the identification of novel potent candidates of Pak1 using computational methods as anti-cancer compounds. A diverse energy based pharmacophore (e-pharmacophore) was developed using four co-crystal inhibitors of Pak1 having pharmacophore features of 5 (DRDRR), 6 (DRHADR), and 7 (RRARDRP and DRRDADH) hypotheses. These models were used for rigorous screening against e-molecule database. The obtained hits were filtered using ADME/T and molecular docking to identify the high affinity binders. These hits were subjected to hierarchical clustering using dendritic fingerprint inorder to identify structurally diverse molecules. The diverse hits were scored against generated water maps to obtain WM/MM ΔG binding energy. Furthermore, molecular dynamics simulation and density functional theory calculations were performed on the final hits to understand the stability of the complexes. Five structurally diverse novel Pak1 inhibitors (4835785, 32198676, 32407813, 76038049, and 32945545) were obtained from virtual screening, water thermodynamics and WM/MM ΔG binding energy. All hits revealed similar mode of binding pattern with the hinge region residues replacing the unstable water molecules in the binding site. The obtained novel hits could be used as a platform to design potent drugs that could be experimentally tested against cancer patients having increased Pak1 expression.

Activating Mutations in PAK1, Encoding p21-Activated Kinase 1, Cause a Neurodevelopmental Disorder

p21-activated kinases (PAKs) are serine/threonine protein kinases acting as effectors of CDC42 and RAC, which are members of the RHO family of small GTPases. PAK1's kinase activity is autoinhibited by homodimerization, whereas CDC42 or RAC1 binding causes PAK1 activation by dimer dissociation. Major functions of the PAKs include actin cytoskeleton reorganization, for example regulation of the cellular protruding activity during cell spreading. We report the de novo PAK1 mutations c.392A>G (p.Tyr131Cys) and c.1286A>G (p.Tyr429Cys) in two unrelated subjects with developmental delay, secondary macrocephaly, seizures, and ataxic gait. We identified enhanced phosphorylation of the PAK1 targets JNK and AKT in fibroblasts of one subject and of c-JUN in those of both subjects compared with control subjects. In fibroblasts of the two affected individuals, we observed a trend toward enhanced PAK1 kinase activity. By using co-immunoprecipitation and size-exclusion chromatography, we observed a significantly reduced dimerization for both PAK1 mutants compared with wild-type PAK1. These data demonstrate that the two PAK1 variants function as activating alleles. In a cell spreading assay, subject-derived fibroblasts showed significant enrichment in cells occupied by filopodia. Interestingly, application of the PAK1 inhibitor FRAX486 completely reversed this cellular phenotype. Together, our data reveal that dominantly acting, gain-of-function PAK1 mutations cause a neurodevelopmental phenotype with increased head circumference, possibly by a combined effect of defective homodimerization and enhanced kinase activity of PAK1. This condition, along with the developmental disorders associated with RAC1 and CDC42 missense mutations, highlight the importance of RHO GTPase members and effectors in neuronal development.

Rho GTPases in Intellectual Disability: From Genetics to Therapeutic Opportunities

Rho-class small GTPases are implicated in basic cellular processes at nearly all brain developmental steps, from neurogenesis and migration to axon guidance and synaptic plasticity. GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Rho GTPases are highly regulated by a complex set of activating (GEFs) and inactivating (GAPs) partners, via protein:protein interactions (PPI). Misregulated RhoA, Rac1/Rac3 and cdc42 activity has been linked with intellectual disability (ID) and other neurodevelopmental conditions that comprise ID. All genetic evidences indicate that in these disorders the RhoA pathway is hyperactive while the Rac1 and cdc42 pathways are consistently hypoactive. Adopting cultured neurons for in vitro testing and specific animal models of ID for in vivo examination, the endophenotypes associated with these conditions are emerging and include altered neuronal networking, unbalanced excitation/inhibition and altered synaptic activity and plasticity. As we approach a clearer definition of these phenotype(s) and the role of hyper- and hypo-active GTPases in the construction of neuronal networks, there is an increasing possibility that selective inhibitors and activators might be designed via PPI, or identified by screening, that counteract the misregulation of small GTPases and result in alleviation of the cognitive condition. Here we review all knowledge in support of this possibility.

Cc2d1a Loss of Function Disrupts Functional and Morphological Development in Forebrain Neurons Leading to Cognitive and Social Deficits

Loss-of-function (LOF) mutations in CC2D1A cause a spectrum of neurodevelopmental disorders, including intellectual disability, autism spectrum disorder, and seizures, identifying a critical role for this gene in cognitive and social development. CC2D1A regulates intracellular signaling processes that are critical for neuronal function, but previous attempts to model the human LOF phenotypes have been prevented by perinatal lethality in Cc2d1a-deficient mice. To overcome this challenge, we generated a floxed Cc2d1a allele for conditional removal of Cc2d1a in the brain using Cre recombinase. While removal of Cc2d1a in neuronal progenitors using Cre expressed from the Nestin promoter still causes death at birth, conditional postnatal removal of Cc2d1a in the forebrain via calcium/calmodulin-dependent protein kinase II-alpha (CamKIIa) promoter-driven Cre generates animals that are viable and fertile with grossly normal anatomy. Analysis of neuronal morphology identified abnormal cortical dendrite organization and a reduction in dendritic spine density. These animals display deficits in neuronal plasticity and in spatial learning and memory that are accompanied by reduced sociability, hyperactivity, anxiety, and excessive grooming. Cc2d1a conditional knockout mice therefore recapitulate features of both cognitive and social impairment caused by human CC2D1A mutation, and represent a model that could provide much needed insights into the developmental mechanisms underlying nonsyndromic neurodevelopmental disorders.

Next-Generation Sequencing Reveals Novel Mutations in X-linked Intellectual Disability

Robust diagnostics for many human genetic disorders are much needed in the pursuit of global personalized medicine. Next-generation sequencing now offers new promise for biomarker and diagnostic discovery, in developed as well as resource-limited countries. In this broader global health context, X-linked intellectual disability (XLID) is an inherited genetic disorder that is associated with a range of phenotypes impacting societies in both developed and developing countries. Although intellectual disability arises due to diverse causes, a substantial proportion is caused by genomic alterations. Studies have identified causal XLID genomic alterations in more than 100 protein-coding genes located on the X-chromosome. However, the causes for a substantial number of intellectual disability and associated phenotypes still remain unknown. Identification of causative genes and novel mutations will help in early diagnosis as well as genetic counseling of families. Advent of next-generation sequencing methods has accelerated the discovery of new genes involved in mental health disorders. In this study, we analyzed the exomes of three families from India with nonsyndromic XLID comprising seven affected individuals. The affected individuals had varying degrees of intellectual disability, microcephaly, and delayed motor and language milestones. We identified potential causal variants in three XLID genes, including PAK3 (V294M), CASK (complex structural variant), and MECP2 (P354T). Our findings reported in this study extend the spectrum of mutations and phenotypes associated with XLID, and calls for further studies of intellectual disability and mental health disorders with use of next-generation sequencing technologies.

Improvement of Self-Injury With Dopamine and Serotonin Replacement Therapy in a Patient With a Hemizygous PAK3 Mutation: A New Therapeutic Strategy for Neuropsychiatric Features of an Intellectual Disability Syndrome

PAK3-related intellectual disability is caused by mutations in the gene encoding the p21-activated kinase (PAK) protein. It is characterized by mild to moderate cognitive impairment, micro/normocephaly, and a neurobehavioral phenotype characterized by short attention span, anxiety, restlessness, aggression, and self-abusive behaviors. The authors report a patient with a novel PAK3 mutation, who presented with intellectual disability, severe automutilation, and epilepsy. His magnetic resonance imaging changes were most likely secondary to lacerations from parenchymal contusions. His behavior was difficult to manage with behavior interventions or multiple medications. After finding low levels of dopamine and borderline low serotonin metabolites in the spinal fluid, treatment with low dose L-dopa/carbidopa and 5-hydroxytryptophan significantly improved his self-injurious behavior. This is the first case of PAK3-related intellectual disability presenting with severe self-injury with improvement following treatment. The patient's response to neurotransmitter replacement therapy raises the question if this treatment intervention might help other individuals suffering genetic syndromes and self-injurious behaviors.

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.

Pdx1-Cre-driven conditional gene depletion suggests PAK4 as dispensable for mouse pancreas development

Constitutive depletion of p21-activated kinase 4 (PAK4) in the mouse causes embryonic lethality associated with heart and brain defects. Given that conventional gene depletion of PAK1 or PAK3 caused functional deficits in the mouse pancreas, while gene depletion of PAK5 or PAK6 did not, we asked if PAK4 might have a functional role in pancreas development. We therefore introduced conditional, Pdx1-Cre-mediated, pancreatic PAK4 gene depletion in the mouse, verified by loss of PAK4 protein expression in the pancreas. PAK4 knock-out (KO) mice were born at Mendelian ratios in both genders. Further, morphological and immunohistochemical examinations and quantifications indicated that exocrine, endocrine and ductal compartments retained the normal proportions and distributions upon PAK4 gene depletion. In addition, body weight records and a glucose tolerance test revealed no differences between WT and PAK4 KO mice. Together, this suggests that PAK4 is dispensable for mouse pancreas development. This will facilitate future use of our Pdx1-Cre-driven conditional PAK4 KO mouse model for testing in vivo potential functions of PAK4 in pancreatic disease models such as for pancreatitis and different pancreatic cancer forms.

Dynamic Arc SUMOylation and Selective Interaction with F-Actin-Binding Protein Drebrin A in LTP Consolidation In Vivo

Activity-regulatedcytoskeleton-associated protein (Arc) protein is implicated as a master regulator of long-term forms of synaptic plasticity and memory formation, but the mechanisms controlling Arc protein function are little known. Post-translation modification by small ubiquitin-like modifier (SUMO) proteins has emerged as a major mechanism for regulating protein-protein interactions and function. We first show in cell lines that ectopically expressed Arc undergoes mono-SUMOylation. The covalent addition of a single SUMO1 protein was confirmed by in vitro SUMOylation of immunoprecipitated Arc. To explore regulation of endogenous Arc during synaptic plasticity, we induced long-term potentiation (LTP) in the dentate gyrus of live anesthetized rats. Using coimmunoprecipitation of native proteins, we show that Arc synthesized during the maintenance phase of LTP undergoes dynamic mono-SUMO1-ylation. Levels of unmodified Arc increase in multiple subcellular fractions (cytosol, membrane, nuclear and cytoskeletal), whereas enhanced Arc SUMOylation was specific to the synaptoneurosomal and the cytoskeletal fractions. Dentate gyrus LTP consolidation requires a period of sustained Arc synthesis driven by brain-derived neurotrophic factor (BDNF) signaling. Local infusion of the BDNF scavenger, TrkB-Fc, during LTP maintenance resulted in rapid reversion of LTP, inhibition of Arc synthesis and loss of enhanced Arc SUMO1ylation. Furthermore, coimmunoprecipitation analysis showed that SUMO1-ylated Arc forms a complex with the F-actin-binding protein drebrin A, a major regulator of cytoskeletal dynamics in dendritic spines. Although Arc also interacted with dynamin 2, calcium/calmodulindependentprotein kinase II-beta (CaMKIIβ), and postsynaptic density protein-95 (PSD-95), these complexes lacked SUMOylated Arc. The results support a model in which newly synthesized Arc is SUMOylated and targeted for actin cytoskeletal regulation during in vivo LTP.

Functional analysis of rare variants found in schizophrenia implicates a critical role for GIT1-PAK3 signaling in neuroplasticity

Although the pathogenesis of schizophrenia (SCZ) is proposed to involve alterations of neural circuits via synaptic dysfunction, the underlying molecular mechanisms remain poorly understood. Recent exome sequencing studies of SCZ have uncovered numerous single-nucleotide variants (SNVs); however, the majority of these SNVs have unknown functional consequences, leaving their disease relevance uncertain. Filling this knowledge gap requires systematic application of quantitative and scalable assays to assess known and novel biological functions of genes. Here we demonstrate loss-of-function effects of multiple rare coding SNVs found in SCZ subjects in the GIT1 (G protein-coupled receptor kinase interacting ArfGAP 1) gene using functional cell-based assays involving coexpression of GIT1 and PAK3 (p21 protein (Cdc42/Rac)-activated kinase 3). Most notably, a GIT1-R283W variant reported in four independent SCZ cases was defective in activating PAK3 as well as MAPK (mitogen-activated protein kinase). Similar functional deficits were found for a de novo SCZ variant GIT1-S601N. Additional assays revealed deficits in the capacity of GIT1-R283W to stimulate PAK phosphorylation in cultured hippocampal neurons. In addition, GIT1-R283W showed deficits in the induction of GAD1 (glutamate decarboxylase 1) protein expression. Extending these functional assays to 10 additional rare GIT1 variants revealed the existence of an allelic series with the majority of the SCZ case variants exhibiting loss of function toward MAPK activation in a manner correlated with loss of PAK3 activation. Taken together, we propose that rare variants in GIT1, along with other genetic and environmental factors, cause dysregulation of PAK3 leading to synaptic deficits in SCZ.

Identification of a Protein Network Driving Neuritogenesis of MGE-Derived GABAergic Interneurons

Interneurons are essential modulators of brain activity and their abnormal maturation may lead to neural and intellectual disabilities. Here we show that cultures derived from murine medial ganglionic eminences (MGEs) produce virtually pure, polarized γ-aminobutyric acid (GABA)-ergic interneurons that can form morphologically identifiable inhibitory synapses. We show that Rac GTPases and a protein complex including the GIT family scaffold proteins are expressed during maturation in vitro, and are required for the normal development of neurites. GIT1 promotes neurite extension in a conformation-dependent manner, while affecting its interaction with specific partners reduces neurite branching. Proteins of the GIT network are concentrated at growth cones, and interaction mutants may affect growth cone behavior. Our findings identify the PIX/GIT1/liprin-α1/ERC1 network as critical for the regulation of interneuron neurite differentiation in vitro, and show that these cultures represent a valuable system to identify the molecular mechanisms driving the maturation of cortical/hippocampal interneurons.

Structure, biochemistry, and biology of PAK kinases

PAKs, p21-activated kinases, play central roles and act as converging junctions for discrete signals elicited on the cell surface and for a number of intracellular signaling cascades. PAKs phosphorylate a vast number of substrates and act by remodeling cytoskeleton, employing scaffolding, and relocating to distinct subcellular compartments. PAKs affect wide range of processes that are crucial to the cell from regulation of cell motility, survival, redox, metabolism, cell cycle, proliferation, transformation, stress, inflammation, to gene expression. Understandably, their dysregulation disrupts cellular homeostasis and severely impacts key cell functions, and many of those are implicated in a number of human diseases including cancers, neurological disorders, and cardiac disorders. Here we provide an overview of the members of the PAK family and their current status. We give special emphasis to PAK1 and PAK4, the prototypes of groups I and II, for their profound roles in cancer, the nervous system, and the heart. We also highlight other family members. We provide our perspective on the current advancements, their growing importance as strategic therapeutic targets, and our vision on the future of PAKs.

The intellectual disability protein PAK3 regulates oligodendrocyte precursor cell differentiation

Oligodendrocyte and myelin deficits have been reported in mental/psychiatric diseases. The p21-activated kinase 3 (PAK3), a serine/threonine kinase, whose activity is stimulated by the binding of active Rac and Cdc42 GTPases is affected in these pathologies. Indeed, many mutations of Pak3 gene have been described in non-syndromic intellectual disability diseases. Pak3 is expressed mainly in the brain where its role has been investigated in neurons but not in glial cells. Here, we showed that PAK3 is highly expressed in oligodendrocyte precursors (OPCs) and its expression decreases in mature oligodendrocytes. In the developing white matter of the Pak3 knockout mice, we found defects of oligodendrocyte differentiation in the corpus callosum and to a lesser extent in the anterior commissure, which were compensated at the adult stage. In vitro experiments in OPC cultures, derived from Pak3 knockout and wild type brains, support a developmental and cell-autonomous role for PAK3 in regulating OPC differentiation into mature oligodendrocytes. Moreover, we did not detect any obvious alterations of the proliferation or migration of Pak3 null OPCs compared to wild type. Overall, our data highlight PAK3 as a new regulator of OPC differentiation.

Impact of neonatal iron deficiency on hippocampal DNA methylation and gene transcription in a porcine biomedical model of cognitive development

BACKGROUND

Iron deficiency is a common childhood micronutrient deficiency that results in altered hippocampal function and cognitive disorders. However, little is known about the mechanisms through which neonatal iron deficiency results in long lasting alterations in hippocampal gene expression and function. DNA methylation is an epigenetic mark involved in gene regulation and altered by environmental factors. In this study, hippocampal DNA methylation and gene expression were assessed via reduced representation bisulfite sequencing and RNA-seq on samples from a previous study reporting reduced hippocampal-based learning and memory in a porcine biomedical model of neonatal iron deficiency.

RESULTS

In total 192 differentially expressed genes (DEGs) were identified between the iron deficient and control groups. GO term and pathway enrichment analysis identified DEGs associated with hypoxia, angiogenesis, increased blood brain barrier (BBB) permeability, and altered neurodevelopment and function. Of particular interest are genes previously implicated in cognitive deficits and behavioral disorders in humans and mice, including HTR2A, HTR2C, PAK3, PRSS12, and NETO1. Altered genome-wide DNA methylation was observed across 0.5 million CpG and 2.4 million non-CpG sites. In total 853 differentially methylated (DM) CpG and 99 DM non-CpG sites were identified between groups. Samples clustered by group when comparing DM non-CpG sites, suggesting high conservation of non-CpG methylation in response to neonatal environment. In total 12 DM sites were associated with 9 DEGs, including genes involved in angiogenesis, neurodevelopment, and neuronal function.

CONCLUSIONS

Neonatal iron deficiency leads to altered hippocampal DNA methylation and gene regulation involved in hypoxia, angiogenesis, increased BBB permeability, and altered neurodevelopment and function. Together, these results provide new insights into the mechanisms through which neonatal iron deficiency results in long lasting reductions in cognitive development in humans.

PAK1 regulates inhibitory synaptic function via a novel mechanism mediated by endocannabinoids

The Rho family small GTPases and their effectors, including PAKs, are extensively studied in the context of the actin cytoskeleton, excitatory synaptic function, spine morphology and memory formation. However, their roles in inhibitory synaptic function remain poorly understood. We have recently shown that PAK1 is a potent regulator of GABAergic synaptic transmission. Thus, disruption of PAK1 leads to significant impairments in inhibitory postsynaptic currents which are manifested as reduced GABA presynaptic releases. Interestingly, this effect of PAK1 is distinct from its previously known role in spines and excitatory synaptic transmission in that it is independent of postsynaptic actin, but requires retrograde messengers produced and released from the postsynaptic neurons to suppress presynaptic GABA releases. We have further identified eCBs as the retrograde messengers and shown that PAK1 regulates the eCB signaling via restricting the tissue level of AEA by promoting synaptic expression of COX-2, a key enzyme to oxidize AEA. These results have established a novel pathway whereby PAK1, and by extension Rho proteins, regulates cellular processes, synaptic function and behaviors and have important implications in understanding and treating various diseases linked to PAKs and Rho signaling.

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.