Down-regulation of p21-activated serine/threonine kinase 1 is involved in loss of mesencephalic dopamine neurons

Rac is required for the survival of cortical neurons

Rac1, a member of small Rho GTPases, is involved in diverse cellular processes in neuronal cells. Rac1 plays especially important roles during development, and its roles have been extensively studied using Rac1-deficient mice. Rac3, a close homolog of Rac1, is ubiquitously expressed in the nervous system and may therefore compensate for Rac1 in Rac1-deficient cells. Exploration of the roles of Rac in neurons may therefore be difficult. We thus deleted both Rac1 and Rac3 in cortical neurons. Rac-deficient cerebral cortices formed slightly hypoplastic but almost normally layered structures at birth, but cortical neurons underwent apoptosis soon after birth. Rac-deficient cortical neurons had poor survivability and there was reduction in the length and the number of neurites in vitro. Activation of Pak1, a downstream effector of Rac, in Rac-deficient cortical neurons rescued the survivability in vitro. Pak1-activated Rac-deficient neurons had numerous dendrites, but no axons. Restoration of p35, a regulator of Cdk5, partly rescued the survivability of Rac-deficient neurons both in vitro and in vivo. Expression of p35 also partly rescued the length and the number of neurites in Rac-deficient neurons in vitro. Rac was shown to be indispensable for the survival of cortical neurons, and Pak1 and Cdk5/p35 work as downstream effectors of Rac to promote neuronal survival.

Treatment with FRAX486 rescues neurobehavioral and metabolic alterations in a female mouse model of CDKL5 deficiency disorder

Introduction

CDKL5 deficiency disorder (CDD) is a rare neurodevelopmental condition, primarily affecting girls for which no cure currently exists. Neuronal morphogenesis and plasticity impairments as well as metabolic dysfunctions occur in CDD patients. The present study explored the potential therapeutic value for CDD of FRAX486, a brain‐penetrant molecule that was reported to selectively inhibit group I p21‐activated kinases (PAKs), serine/threonine kinases critically involved in the regulation of neuronal morphology and glucose homeostasis.

Methods

The effects of treatment with FRAX486 on CDD‐related alterations were assessed in vitro (100 nM for 48 h) on primary hippocampal cultures from Cdkl5‐knockout male mice (Cdkl5‐KO) and in vivo (20 mg/Kg, s.c. for 5 days) on Cdkl5‐KO heterozygous females (Cdkl5‐Het).

Results

The in vitro treatment with FRAX486 completely rescued the abnormal neuronal maturation and the number of PSD95‐positive puncta in Cdkl5‐KO mouse neurons. In vivo, FRAX486 normalized the general health status, the hyperactive profile and the fear learning defects of fully symptomatic Cdkl5‐Het mice. Systemically, FRAX486 treatment normalized the levels of reactive oxidizing species in the whole blood and the fasting‐induced hypoglycemia displayed by Cdkl5‐Het mice. In the hippocampus of Cdkl5‐Het mice, treatment with FRAX486 rescued spine maturation and PSD95 expression and restored the abnormal PAKs phosphorylation at sites which are critical for their activation (P‐PAK‐Ser144/141/139) or for the control cytoskeleton remodeling (P‐PAK1‐Thr212).

Conclusions

Present results provide evidence that PAKs may represent innovative therapeutic targets for CDD.

p21-Activated kinase 1 (PAK1) in aging and longevity: An overview

The p21-activated kinases (PAKs) belong to serine/threonine kinases family, regulated by ∼21 kDa small signaling G proteins RAC1 and CDC42. The mammalian PAK family comprises six members (PAK1-6) that are classified into two groups (I and II) based on their domain architecture and regulatory mechanisms. PAKs are implicated in a wide range of cellular functions. PAK1 has recently attracted increasing attention owing to its involvement in oncogenesis, tumor progression, and metastasis as well as several life-limiting diseases and pathological conditions. In Caenorhabditis elegans, PAK1 functions limit the lifespan under basal conditions by inhibiting forkhead transcription factor DAF-16. Interestingly, PAK depletion extended longevity and attenuated the onset of age-related phenotypes in a premature-aging mouse model and delayed senescence in mammalian fibroblasts. These observations implicate PAKs as not only oncogenic but also aging kinases. Therefore, PAK-targeting genetic and/or pharmacological interventions, particularly PAK1-targeting, could be a viable strategy for developing cancer therapies with relatively no side effects and promoting healthy longevity. This review describes PAK family proteins, their biological functions, and their role in regulating aging and longevity using C. elegans. Moreover, we discuss the effect of small-molecule PAK1 inhibitors on the lifespan and healthspan of C. elegans.

Utilizing an Animal Model to Identify Brain Neurodegeneration-Related Biomarkers in Aging

Glycine N-methyltransferase (GNMT) regulates S-adenosylmethionine (SAMe), a methyl donor in methylation. Over-expressed SAMe may cause neurogenic capacity reduction and memory impairment. GNMT knockout mice (GNMT-KO) was applied as an experimental model to evaluate its effect on neurons. In this study, proteins from brain tissues were studied using proteomic approaches, Haemotoxylin and Eosin staining, immunohistochemistry, Western blotting, and ingenuity pathway analysis. The expression of Receptor-interacting protein 1(RIPK1) and Caspase 3 were up-regulated and activity-dependent neuroprotective protein (ADNP) was down-regulated in GNMT-KO mice regardless of the age. Besides, proteins related to neuropathology, such as excitatory amino acid transporter 2, calcium/calmodulin-dependent protein kinase type II subunit alpha, and Cu-Zn superoxide dismutase were found only in the group of aged wild-type mice; 4-aminobutyrate amino transferase, limbic system-associated membrane protein, sodium- and chloride-dependent GABA transporter 3 and ProSAAS were found only in the group of young GNMT-KO mice and are related to function of neurons; serum albumin and Rho GDP dissociation inhibitor 1 were found only in the group of aged GNMT-KO mice and are connected to neurodegenerative disorders. With proteomic analyses, a pathway involving Gonadotropin-releasing hormone (GnRH) signal was found to be associated with aging. The GnRH pathway could provide additional information on the mechanism of aging and non-aging related neurodegeneration, and these protein markers may be served in developing future therapeutic treatments to ameliorate aging and prevent diseases.

Interrogation of kinase genetic interactions provides a global view of PAK1-mediated signal transduction pathways

Kinases are critical components of intracellular signaling pathways and have been extensively investigated with regard to their roles in cancer. p21-activated kinase-1 (PAK1) is a serine/threonine kinase that has been previously implicated in numerous biological processes, such as cell migration, cell cycle progression, cell motility, invasion, and angiogenesis, in glioma and other cancers. However, the signaling network linked to PAK1 is not fully defined. We previously reported a large-scale yeast genetic interaction screen using toxicity as a readout to identify candidate PAK1 genetic interactions. En masse transformation of the PAK1 gene into 4,653 homozygous diploid Saccharomyces cerevisiae yeast deletion mutants identified ∼400 candidates that suppressed yeast toxicity. Here we selected 19 candidate PAK1 genetic interactions that had human orthologs and were expressed in glioma for further examination in mammalian cells, brain slice cultures, and orthotopic glioma models. RNAi and pharmacological inhibition of potential PAK1 interactors confirmed that DPP4, KIF11, mTOR, PKM2, SGPP1, TTK, and YWHAE regulate PAK1-induced cell migration and revealed the importance of genes related to the mitotic spindle, proteolysis, autophagy, and metabolism in PAK1-mediated glioma cell migration, drug resistance, and proliferation. AKT1 was further identified as a downstream mediator of the PAK1-TTK genetic interaction. Taken together, these data provide a global view of PAK1-mediated signal transduction pathways and point to potential new drug targets for glioma therapy.

Altered Levels of Proteins and Phosphoproteins, in the Absence of Early Causative Transcriptional Changes, Shape the Molecular Pathogenesis in the Brain of Young Presymptomatic Ki91 SCA3/MJD Mouse

Spinocerebellar ataxia type 3 (SCA3/MJD) is a polyQ neurodegenerative disease where the presymptomatic phase of pathogenesis is unknown. Therefore, we investigated the molecular network of transcriptomic and proteomic triggers in young presymptomatic SCA3/MJD brain from Ki91 knock-in mouse. We found that transcriptional dysregulations resulting from mutant ataxin-3 are not occurring in young Ki91 mice, while old Ki91 mice and also postmitotic patient SCA3 neurons demonstrate the late transcriptomic changes. Unlike the lack of early mRNA changes, we have identified numerous early changes of total proteins and phosphoproteins in 2-month-old Ki91 mouse cortex and cerebellum. We discovered the network of processes in presymptomatic SCA3 with three main groups of disturbed processes comprising altered proteins: (I) modulation of protein levels and DNA damage (Pabpc1, Ddb1, Nedd8), (II) formation of neuronal cellular structures (Tubb3, Nefh, p-Tau), and (III) neuronal function affected by processes following perturbed cytoskeletal formation (Mt-Co3, Stx1b, p-Syn1). Phosphoproteins downregulate in the young Ki91 mouse brain and their phosphosites are associated with kinases that interact with ATXN3 such as casein kinase, Camk2, and kinases controlled by another Atxn3 interactor p21 such as Gsk3, Pka, and Cdk kinases. We conclude that the onset of SCA3 pathology occurs without altered transcript level and is characterized by changed levels of proteins responsible for termination of translation, DNA damage, spliceosome, and protein phosphorylation. This disturbs global cellular processes such as cytoskeleton and transport of vesicles and mitochondria along axons causing energy deficit and neurodegeneration also manifesting in an altered level of transcripts at later ages.

Solution structures and biophysical analysis of full-length group A PAKs reveal they are monomeric and auto-inhibited in cis

The group A p21-activated kinases (PAKs) exist in an auto-inhibited form until activated by GTPase binding and auto-phosphorylation. In the auto-inhibited form, a regulatory domain binds to the kinase domain (KD) blocking the binding of substrates, and CDC42 or Rac binding to the regulatory domain relieves this auto-inhibition allowing auto-phosphorylation on the KD activation loop. We have determined the crystal structure of the PAK3 catalytic domain and by small angle X-ray scattering, the solution-phase structures of full-length inactive PAK1 and PAK3. The structures reveal a compact but elongated molecular shape that demonstrates that, together with multiple independent biophysical measurements and in contrast with previous assumptions, group A PAKs are monomeric both before and after activation, consistent with an activation mechanism of cis-auto-inhibition and initial cis-auto-phosphorylation, followed by transient dimerisation to allow trans-auto-phosphorylation for full activation, yielding a monomeric active PAK protein.

PAK4 signaling in health and disease: defining the PAK4-CREB axis

p21-Activated kinase 4 (PAK4), a member of the PAK family, regulates a wide range of cellular functions, including cell adhesion, migration, proliferation, and survival. Dysregulation of its expression and activity thus contributes to the development of diverse pathological conditions. PAK4 plays a pivotal role in cancer progression by accelerating the epithelial–mesenchymal transition, invasion, and metastasis. Therefore, PAK4 is regarded as an attractive therapeutic target in diverse types of cancers, prompting the development of PAK4-specific inhibitors as anticancer drugs; however, these drugs have not yet been successful. PAK4 is essential for embryonic brain development and has a neuroprotective function. A long list of PAK4 effectors has been reported. Recently, the transcription factor CREB has emerged as a novel effector of PAK4. This finding has broad implications for the role of PAK4 in health and disease because CREB-mediated transcriptional reprogramming involves a wide range of genes. In this article, we review the PAK4 signaling pathways involved in prostate cancer, Parkinson’s disease, and melanogenesis, focusing in particular on the PAK4-CREB axis.

An enzyme that regulates an important controller of gene expression may offer a therapeutic target for cancer and other diseases. cAMP response element-binding protein (CREB) interacts with various other proteins to switch a myriad of target genes on and off in different cells. A review by Eung-Gook Kim, Eun-Young Shin and colleagues at Chungbuk National University, Cheongju, South Korea, explores the interplay between CREB and an enzyme called p21-activated kinase 4 (PAK4) in human health and disease. PAK4, for example, has been shown to promote CREB’s gene-activating function in prostate cancer, and PAK4 overexpression is a feature of numerous other tumor types. Disruptions in PAK4-mediated regulation of CREB activity have also been observed in neurons affected by Parkinson’s disease. The authors see strong clinical promise in further exploring the biology of the PAK4-CREB pathway.

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.

Microarray analysis of an synthetic α-synuclein induced cellular model reveals the expression profile of long non-coding RNA in Parkinson's disease

Long non-coding RNAs (lncRNAs) are a new research focus that are reported to influence the pathogenetic process of neurodegenerative disorders. To uncover new disease-associated genes and their relevant mechanisms, we carried out a gene microarray analysis based on a Parkinson's disease (PD) in vitro model induced by α-synuclein oligomers. This cellular model induced by 25 μmol/L α-synuclein oligomers has been confirmed to show the stable, transmissible neurotoxicity of α-synuclein, a typical PD pathological marker. And several differentially expressed lncRNAs and mRNAs were identified in this model, such as G046036, G030771, AC009365.4, RPS14P3, CTB-11I22.1, and G007549. Subsequent ceRNA analysis determined the potential relationships between these lncRNAs and their associated mRNAs and microRNAs. The results of the present study widen our horizon of PD susceptibility genes and provide new pathways towards efficient diagnostic biomarkers and therapeutic targets for PD.

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.

Aberrant Rac1-cofilin signaling mediates defects in dendritic spines, synaptic function, and sensory perception in fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disabilities and a leading cause of autism. FXS is caused by a trinucleotide expansion in the gene FMR1 on the X chromosome. The neuroanatomical hallmark of FXS is an overabundance of immature dendritic spines, a factor thought to underlie synaptic dysfunction and impaired cognition. We showed that aberrantly increased activity of the Rho GTPase Rac1 inhibited the actin-depolymerizing factor cofilin, a major determinant of dendritic spine structure, and caused disease-associated spine abnormalities in the somatosensory cortex of FXS model mice. Increased cofilin phosphorylation and actin polymerization coincided with abnormal dendritic spines and impaired synaptic maturation. Viral delivery of a constitutively active cofilin mutant (cofilinS3A) into the somatosensory cortex of Fmr1-deficient mice rescued the immature dendritic spine phenotype and increased spine density. Inhibition of the Rac1 effector PAK1 with a small-molecule inhibitor rescued cofilin signaling in FXS mice, indicating a causal relationship between PAK1 and cofilin signaling. PAK1 inhibition rescued synaptic signaling (specifically the synaptic ratio of NMDA/AMPA in layer V pyramidal neurons) and improved sensory processing in FXS mice. These findings suggest a causal relationship between increased Rac1-cofilin signaling, synaptic defects, and impaired sensory processing in FXS and uncover a previously unappreciated role for impaired Rac1-cofilin signaling in the aberrant spine morphology and spine density associated with FXS.

Noonan syndrome-associated SHP2 mutation differentially modulates the expression of postsynaptic receptors according to developmental maturation

Glutamate is the major excitatory neurotransmitter in the central nervous system, and related signaling involves both AMPA and NMDA subtype receptors. The expression of glutamate receptors is dynamically regulated during development. Recent studies showed that the dysregulation of glutamate receptor expression and function is associated with neurodevelopmental disorders including intellectual disability. Previously, a Noonan syndrome (NS)-associated SHP2 mutation (SHP2^D61G) was shown to increase the synaptic delivery of AMPA receptor, subsequently impairing synaptic plasticity and learning in adult mice. However, how the mutant SHP2 affects glutamate receptor expression during development is not known. Here, we found that the SHP2^D61G differentially regulates the expression of AMPA and NMDA receptors depending on the stage of neuronal maturation. In cultured neurons (immature stage; DIV 6), overexpression of SHP2^D61G significantly increased the average size and the number of NMDA receptor-containing particles, but not those with AMPA receptors. In early matured neurons (DIV 12), SHP2^D61G significantly increased only the average size of AMPA receptor particles, and subsequently increased their number in matured neurons (DIV 18). Importantly, all the changes described above for SHP2^D61G neurons were reversed by inhibiting MAPK. These data demonstrate that the increased activation of MAPK signaling pathway by SHP2^D61G could deregulate the surface expression of synaptic receptors during neuronal development, which likely contributes to cognitive impairments in NS patients.