Biphasic regulation of myosin light chain phosphorylation by p21-activated kinase modulates intestinal smooth muscle contractility

A unified approach to dissecting biphasic responses in cell signaling

Biphasic responses are encountered at all levels in biological systems. At the cellular level, biphasic dose-responses are widely encountered in cell signaling and post-translational modification systems and represent safeguards against overactivation or overexpression of species. In this paper, we provide a unified theoretical synthesis of biphasic responses in cell signaling systems, by assessing signaling systems ranging from basic biochemical building blocks to canonical network structures to well-characterized exemplars on one hand, and examining different types of doses on the other. By using analytical and computational approaches applied to a range of systems across levels (described by broadly employed models), we reveal (i) design principles enabling the presence of biphasic responses, including in almost all instances, an explicit characterization of the parameter space (ii) structural factors which preclude the possibility of biphasic responses (iii) different combinations of the presence or absence of enzyme-biphasic and substrate-biphasic responses, representing safeguards against overactivation and overexpression, respectively (iv) the possibility of broadly robust biphasic responses (v) the complete alteration of signaling behavior in a network due to biphasic interactions between species (biphasic regulation) (vi) the propensity of different co-existing biphasic responses in the Erk signaling network. These results both individually and in totality have a number of important consequences for systems and synthetic biology.

Stretch-Induced Down-Regulation of HCN2 Suppresses Contractile Activity

Although hyperpolarization-activated and cyclic nucleotide-gated 2 channels (HCN2) are expressed in multiple cell types in the gut, the role of HCN2 in intestinal motility is poorly understood. HCN2 is down-regulated in intestinal smooth muscle in a rodent model of ileus. Thus, the purpose of this study was to determine the effects of HCN inhibition on intestinal motility. HCN inhibition with ZD7288 or zatebradine significantly suppressed both spontaneous and agonist-induced contractile activity in the small intestine in a dose-dependent and tetrodotoxin-independent manner. HCN inhibition significantly suppressed intestinal tone but not contractile amplitude. The calcium sensitivity of contractile activity was significantly suppressed by HCN inhibition. Inflammatory mediators did not affect the suppression of intestinal contractile activity by HCN inhibition but increased stretch of the intestinal tissue partially attenuated the effects of HCN inhibition on agonist-induced intestinal contractile activity. HCN2 protein and mRNA levels in intestinal smooth muscle tissue were significantly down-regulated by increased mechanical stretch compared to unstretched tissue. Increased cyclical stretch down-regulated HCN2 protein and mRNA levels in primary human intestinal smooth muscle cells and macrophages. Overall, our results suggest that decreased HCN2 expression induced by mechanical signals, such as intestinal wall distension or edema development, may contribute to the development of ileus.

Mechanosensing in the Physiology and Pathology of the Gastrointestinal Tract

Normal gastrointestinal function relies on sensing and transducing mechanical signals into changes in intracellular signaling pathways. Both specialized mechanosensing cells, such as certain enterochromaffin cells and enteric neurons, and non-specialized cells, such as smooth muscle cells, interstitial cells of Cajal, and resident macrophages, participate in physiological and pathological responses to mechanical signals in the gastrointestinal tract. We review the role of mechanosensors in the different cell types of the gastrointestinal tract. Then, we provide several examples of the role of mechanotransduction in normal physiology. These examples highlight the fact that, although these responses to mechanical signals have been known for decades, the mechanosensors involved in these responses to mechanical signals are largely unknown. Finally, we discuss several diseases involving the overstimulation or dysregulation of mechanotransductive pathways. Understanding these pathways and identifying the mechanosensors involved in these diseases may facilitate the identification of new drug targets to effectively treat these diseases.

The Role of Inflammatory Mediators in the Development of Gastrointestinal Motility Disorders

Feeding intolerance and the development of ileus is a common complication affecting critically ill, surgical, and trauma patients, resulting in prolonged intensive care unit and hospital stays, increased infectious complications, a higher rate of hospital readmission, and higher medical care costs. Medical treatment for ileus is ineffective and many of the available prokinetic drugs have serious side effects that limit their use. Despite the large number of patients affected and the consequences of ileus, little progress has been made in identifying new drug targets for the treatment of ileus. Inflammatory mediators play a critical role in the development of ileus, but surprisingly little is known about the direct effects of inflammatory mediators on cells of the gastrointestinal tract, and many of the studies are conflicting. Understanding the effects of inflammatory cytokines/chemokines on the development of ileus will facilitate the early identification of patients who will develop ileus and the identification of new drug targets to treat ileus. Thus, herein, we review the published literature concerning the effects of inflammatory mediators on gastrointestinal motility.

Mechanotransduction at the Plasma Membrane-Cytoskeleton Interface

Mechanical cues are crucial for survival, adaptation, and normal homeostasis in virtually every cell type. The transduction of mechanical messages into intracellular biochemical messages is termed mechanotransduction. While significant advances in biochemical signaling have been made in the last few decades, the role of mechanotransduction in physiological and pathological processes has been largely overlooked until recently. In this review, the role of interactions between the cytoskeleton and cell-cell/cell-matrix adhesions in transducing mechanical signals is discussed. In addition, mechanosensors that reside in the cell membrane and the transduction of mechanical signals to the nucleus are discussed. Finally, we describe two examples in which mechanotransduction plays a significant role in normal physiology and disease development. The first example is the role of mechanotransduction in the proliferation and metastasis of cancerous cells. In this system, the role of mechanotransduction in cellular processes, including proliferation, differentiation, and motility, is described. In the second example, the role of mechanotransduction in a mechanically active organ, the gastrointestinal tract, is described. In the gut, mechanotransduction contributes to normal physiology and the development of motility disorders.

Microelectromechanical System Measurement of Platelet Contraction: Direct Interrogation of Myosin Light Chain Phosphorylation

Myosin Light Chain (MLC) regulates platelet contraction through its phosphorylation by Myosin Light Chain Kinase (MLCK) or dephosphorylation by Myosin Light Chain Phosphatase (MLCP). The correlation between platelet contraction force and levels of MLC phosphorylation is unknown. We investigate the relationship between platelet contraction force and MLC phosphorylation using a novel microelectromechanical (MEMS) based clot contraction sensor (CCS). The MLCK and MLCP pair were interrogated by inhibitors and activators of platelet function. The CCS was fabricated from silicon using photolithography techniques and force was validated over a range of deflection for different chip spring constants. The force of platelet contraction measured by the clot contraction sensor (CCS) was compared to the degree of MLC phosphorylation by Western Blotting (WB) and ELISA. Stimulators of MLC phosphorylation produced higher contraction force, higher phosphorylated MLC signal in ELISA and higher intensity bands in WB. Inhibitors of MLC phosphorylation produced the opposite. Contraction force is linearly related to levels of phosphorylated MLC. Direct measurements of clot contractile force are possible using a MEMS sensor platform and correlate linearly with the degree of MLC phosphorylation during coagulation. Measured force represents the mechanical output of the actin/myosin motor in platelets regulated by myosin light chain phosphorylation.

Intestinal changes associated with fluoride exposure in rats: Integrative morphological, proteomic and microbiome analyses

Gastrointestinal signs and symptoms are the first signs of toxicity due to exposure to fluoride (F). This suggests the possibility that lower levels of subchronic F exposure may affect the gut. The aim of this study was to evaluate changes in the morphology, proteome and microbiome of the ileum of rats, after subchronic exposure to F. Male rats ingested water with 0, 10, or 50 mgF/L for thirty days. Treatment with F, regardless of the dose, significantly decreased the density of HuC/D-IR neurons, whereas CGRP-IR and SP-IR varicosities were significantly increased compared to the control group. Increased VIP-IR varicosities were significantly increased only in the group treated with 50 mgF/L. A significant increase in thickness of the tunica muscularis, as well as in the total thickness of the ileum wall was observed at both F doses when compared to controls. In proteomics analysis, myosin isoforms were increased, and Gastrotopin was decreased in F-exposed mice. In the microbiome metagenomics analysis, Class Clostridia was significantly reduced upon exposure to 10 mgF/L. At the higher F dose of 50 mg/L, genus Ureaplasma was significantly reduced in comparison with controls. Morphological and proteomics alterations induced by F were marked by changes associated with inflammation, and alterations in the gut microbiome. Further studies are needed to determine whether F exposure increases inflammation with secondary effects of the gut microbiome, and/or whether primary effects of F on the gut microbiome enhance changes associated with inflammation.

Postoperative Ileus and Postoperative Gastrointestinal Tract Dysfunction: Pathogenic Mechanisms and Novel Treatment Strategies Beyond Colorectal Enhanced Recovery After Surgery Protocols

Postoperative ileus (POI) and postoperative gastrointestinal tract dysfunction (POGD) are well-known complications affecting patients undergoing intestinal surgery. GI symptoms include nausea, vomiting, pain, abdominal distention, bloating, and constipation. These iatrogenic disorders are associated with extended hospitalizations, increased morbidity, and health care costs into the billions and current therapeutic strategies are limited. This is a narrative review focused on recent concepts in the pathogenesis of POI and POGD, pipeline drugs or approaches to treatment. Mechanisms, cellular targets and pathways implicated in the pathogenesis include gut surgical manipulation and surgical trauma, neuroinflammation, reactive enteric glia, macrophages, mast cells, monocytes, neutrophils and ICC's. The precise interactions between immune, inflammatory, neural and glial cells are not well understood. Reactive enteric glial cells are an emerging therapeutic target that is under intense investigation for enteric neuropathies, GI dysmotility and POI. Our review emphasizes current therapeutic strategies, starting with the implementation of colorectal enhanced recovery after surgery protocols to protect against POI and POGD. However, despite colorectal enhanced recovery after surgery, it remains a significant medical problem and burden on the healthcare system. Over 100 pipeline drugs or treatments are listed in Clin.Trials.gov. These include 5HT4R agonists (Prucalopride and TAK 954), vagus nerve stimulation of the ENS-macrophage nAChR cholinergic pathway, acupuncture, herbal medications, peripheral acting opioid antagonists (Alvimopen, Methlnaltexone, Naldemedine), anti-bloating/flatulence drugs (Simethiocone), a ghreline prokinetic agonist (Ulimovelin), drinking coffee, and nicotine chewing gum. A better understanding of the pathogenic mechanisms for short and long-term outcomes is necessary before we can develop better prophylactic and treatment strategies.

CXCL1 is upregulated during the development of ileus resulting in decreased intestinal contractile activity

BACKGROUND

Although the development of ileus is widespread and negatively impacts patient outcomes, the mechanism by which ileus develops remains unclear. The purpose of our study was to examine the contribution of myogenic mechanisms to postoperative ileus development and the involvement of inflammation in mediating intestinal smooth muscle dysfunction.

METHODS

Contractile activity and the effects of CXCL1 were studied in a gut manipulation model.

KEY RESULTS

Contraction amplitude in the ileum decreased significantly, while tone increased significantly in response to gut manipulation. Differences in contraction amplitude were affected by tetrodotoxin at earlier time points, but not at later time points. Agonist-induced contractions in the small intestine decreased significantly with ileus development. Intestinal transit slowed significantly after the induction of ileus. Myosin light chain phosphorylation was significantly decreased and edema increased significantly in the intestinal wall. Conditioned media from mechanically activated macrophages depressed intestinal contractile activity. CXCL1 (GroA) was significantly increased in the mechanically activated macrophages and intestinal smooth muscle within 1 hour after induction of ileus compared with control cells and sham animals, respectively. Treatment with CXCL1 significantly decreased contraction amplitude and agonist-induced contractile activity and increased tone in the small intestine. In the gut manipulation model, treatment with a CXCR2 antagonist prevented the decrease in agonist-induced contractile activity but not contraction amplitude.

CONCLUSIONS & INFERENCES

These data suggest that CXCL1, released from macrophages during intestinal wall stress, can suppress intestinal contractile activity. CXCL1 is a potential target for preventing or treating ileus in trauma patients.

Incidence and Effects of Feeding Intolerance in Trauma Patients

BACKGROUND

Although feeding intolerance is a common complication in trauma patients, the incidence, development, and effects are poorly understood.

METHODS

We performed a retrospective study in which trauma patients were classified as having feeding intolerance based on time to reach feeding goal. Subsequently, we sorted patients by gastric residual volumes (GRVs) or symptoms of slowed gastrointestinal motility.

RESULTS

One-third of trauma patients experienced delayed time to reach feeding goal after diet initiation. Delayed feeding was associated with prolonged intensive care unit (ICU) stays, increased readmission rates, and increased incidence of sepsis. Patients with elevated GRV (>500 mL) had significantly prolonged ICU and hospital stays and increase incidence of sepsis. Patients with >2 symptoms of slowed gastrointestinal motility had prolonged ICU and hospital stays, delayed time to reach feeding goals, significantly increased readmission rates, increased incidence of infectious and thromboembolic complications and sepsis, decreased serum prealbumin levels, and increased CRP levels.

CONCLUSION

Decreased gastrointestinal motility in trauma patients is associated with worse outcomes and increased systemic inflammation.

Idiopathic gastroparesis is associated with specific transcriptional changes in the gastric muscularis externa

BACKGROUND

The molecular changes that occur in the stomach that are associated with idiopathic gastroparesis are poorly described. The aim of this study was to use quantitative analysis of mRNA expression to identify changes in mRNAs encoding proteins required for the normal motility functions of the stomach.

METHODS

Full-thickness stomach biopsy samples were collected from non-diabetic control subjects who exhibited no symptoms of gastroparesis and from patients with idiopathic gastroparesis. mRNA was isolated from the muscularis externa and mRNA expression levels were determined by quantitative reverse transcriptase (RT)-PCR.

KEY RESULTS

Smooth muscle tissue from idiopathic gastroparesis patients had decreased expression of mRNAs encoding several contractile proteins, such as MYH11 and MYLK1. Conversely, there was no significant change in mRNAs characteristic of interstitial cells of Cajal (ICCs) such as KIT or ANO1. There was also a significant decrease in mRNA-encoding platelet-derived growth factor receptor α (PDGFRα) and its ligand PDGFB and in Heme oxygenase 1 in idiopathic gastroparesis subjects. In contrast, there was a small increase in mRNA characteristic of neurons. Although there was not an overall change in KIT expression in gastroparesis patients, KIT expression showed a significant correlation with gastric emptying whereas changes in MYLK1, ANO1 and PDGFRα showed weak correlations to the fullness/satiety subscore of patient assessment of upper gastrointestinal disorder-symptom severity index scores.

CONCLUSIONS AND INFERENCES

Our findings suggest that idiopathic gastroparesis is associated with altered smooth muscle cell contractile protein expression and loss of PDGFRα⁺ cells without a significant change in ICCs.

p21-Activated kinase (Pak) regulates airway smooth muscle contraction by regulating paxillin complexes that mediate actin polymerization

KEY POINTS

In airway smooth muscle, tension development caused by a contractile stimulus requires phosphorylation of the 20 kDa myosin light chain (MLC), which activates crossbridge cycling and the polymerization of a pool of submembraneous actin. The p21-activated kinases (Paks) can regulate the contractility of smooth muscle and non-muscle cells, and there is evidence that this occurs through the regulation of MLC phosphorylation. We show that Pak has no effect on MLC phosphorylation during the contraction of airway smooth muscle, and that it regulates contraction by mediating actin polymerization. We find that Pak phosphorylates the adhesion junction protein, paxillin, on Ser273, which promotes the formation of a signalling complex that activates the small GTPase, cdc42, and the actin polymerization catalyst, neuronal Wiskott-Aldrich syndrome protein (N-WASP). These studies demonstrate a novel role for Pak in regulating the contractility of smooth muscle by regulating actin polymerization.

ABSTRACT

The p21-activated kinases (Pak) can regulate contractility in smooth muscle and other cell and tissue types, but the mechanisms by which Paks regulate cell contractility are unclear. In airway smooth muscle, stimulus-induced contraction requires phosphorylation of the 20 kDa light chain of myosin, which activates crossbridge cycling, as well as the polymerization of a small pool of actin. The role of Pak in airway smooth muscle contraction was evaluated by inhibiting acetylcholine (ACh)-induced Pak activation through the expression of a kinase inactive mutant, Pak1 K299R, or by treating tissues with the Pak inhibitor, IPA3. Pak inhibition suppressed actin polymerization and contraction in response to ACh, but it did not affect myosin light chain phosphorylation. Pak activation induced paxillin phosphorylation on Ser273; the paxillin mutant, paxillin S273A, inhibited paxillin Ser273 phosphorylation and inhibited actin polymerization and contraction. Immunoprecipitation analysis of tissue extracts and proximity ligation assays in dissociated cells showed that Pak activation and paxillin Ser273 phosphorylation triggered the formation of an adhesion junction signalling complex with paxillin that included G-protein-coupled receptor kinase-interacting protein (GIT1) and the cdc42 guanine exchange factor, βPIX (Pak interactive exchange factor). Assembly of the Pak-GIT1-βPIX-paxillin complex was necessary for cdc42 and neuronal Wiskott-Aldrich syndrome protein (N-WASP) activation, actin polymerization and contraction in response to ACh. RhoA activation was also required for the recruitment of Pak to adhesion junctions, Pak activation, paxillin Ser273 phosphorylation and paxillin complex assembly. These studies demonstrate a novel role for Pak in the regulation of N-WASP activation, actin dynamics and cell contractility.

Phosphorylation of the regulatory light chain of myosin in striated muscle: methodological perspectives

Phosphorylation of the regulatory light chain (RLC) of myosin modulates cellular functions such as muscle contraction, mitosis, and cytokinesis. Phosphorylation defects are implicated in a number of diseases. Here we focus on striated muscle where changes in RLC phosphorylation relate to diseases such as hypertrophic cardiomyopathy and muscular dystrophy, or age-related changes. RLC phosphorylation in smooth muscle and non-muscle cells are covered briefly where relevant. There is much scientific interest in controlling the phosphorylation levels of RLC in vivo and in vitro in order to understand its physiological function in striated muscles. A summary of available and emerging in vivo and in vitro methods is presented. The physiological role of RLC phosphorylation and novel pathways are discussed to highlight the differences between muscle types and to gain insights into disease processes.

P21-Activated Kinase Inhibitors FRAX486 and IPA3: Inhibition of Prostate Stromal Cell Growth and Effects on Smooth Muscle Contraction in the Human Prostate

Prostate smooth muscle tone and hyperplastic growth are involved in the pathophysiology and treatment of male lower urinary tract symptoms (LUTS). Available drugs are characterized by limited efficacy. Patients' adherence is particularly low to combination therapies of 5α-reductase inhibitors and α1-adrenoceptor antagonists, which are supposed to target contraction and growth simultaneously. Consequently, molecular etiology of benign prostatic hyperplasia (BPH) and new compounds interfering with smooth muscle contraction or growth in the prostate are of high interest. Here, we studied effects of p21-activated kinase (PAK) inhibitors (FRAX486, IPA3) in hyperplastic human prostate tissues, and in stromal cells (WPMY-1). In hyperplastic prostate tissues, PAK1, -2, -4, and -6 may be constitutively expressed in catecholaminergic neurons, while PAK1 was detected in smooth muscle and WPMY-1 cells. Neurogenic contractions of prostate strips by electric field stimulation were significantly inhibited by high concentrations of FRAX486 (30 μM) or IPA3 (300 μM), while noradrenaline- and phenylephrine-induced contractions were not affected. FRAX486 (30 μM) inhibited endothelin-1- and -2-induced contractions. In WPMY-1 cells, FRAX486 or IPA3 (24 h) induced concentration-dependent (1-10 μM) degeneration of actin filaments. This was paralleled by attenuation of proliferation rate, being observed from 1 to 10 μM FRAX486 or IPA3. Cytotoxicity of FRAX486 and IPA3 in WPMY-1 cells was time- and concentration-dependent. Stimulation of WPMY-1 cells with endothelin-1 or dihydrotestosterone, but not noradrenaline induced PAK phosphorylation, indicating PAK activation by endothelin-1. Thus, PAK inhibitors may inhibit neurogenic and endothelin-induced smooth muscle contractions in the hyperplastic human prostate, and growth of stromal cells. Targeting prostate smooth muscle contraction and stromal growth at once by a single compound is principally possible, at least under experimental conditions.

Myosin light-chain phosphatase regulates basal actomyosin oscillations during morphogenesis

Contractile actomyosin networks generate forces that drive tissue morphogenesis. Actomyosin contractility is controlled primarily by reversible phosphorylation of the myosin-II regulatory light chain through the action of myosin kinases and phosphatases. While the role of myosin light-chain kinase in regulating contractility during morphogenesis has been largely characterized, there is surprisingly little information on myosin light-chain phosphatase (MLCP) function in this context. Here, we use live imaging of Drosophila follicle cells combined with mathematical modelling to demonstrate that the MLCP subunit flapwing (flw) is a key regulator of basal myosin oscillations and cell contractions underlying egg chamber elongation. Flw expression decreases specifically on the basal side of follicle cells at the onset of contraction and flw controls the initiation and periodicity of basal actomyosin oscillations. Contrary to previous reports, basal F-actin pulsates similarly to myosin. Finally, we propose a quantitative model in which periodic basal actomyosin oscillations arise in a cell-autonomous fashion from intrinsic properties of motor assemblies.

Actomyosin contractility is regulated by phosphorylation of myosin regulatory light chain; much of the work in this area has focused on the kinase. Here the authors use Drosophila follicle cells and modelling to show that the phosphatase subunit Flapwing controls the initiation and dynamics of actomyosin oscillations.

Thromboxane-induced actin polymerization in hypoxic neonatal pulmonary arterial myocytes involves Cdc42 signaling

In hypoxic pulmonary arterial (PA) myocytes, challenge with thromboxane mimetic U46619 induces marked actin polymerization and contraction, phenotypic features of persistent pulmonary hypertension of the newborn (PPHN). Rho GTPases regulate the actin cytoskeleton. We previously reported that U46619-induced actin polymerization in hypoxic PA myocytes occurs independently of the RhoA pathway and hypothesized involvement of the Cdc42 pathway. PA myocytes grown in normoxia or hypoxia for 72 h were stimulated with U46619, then analyzed for Rac/Cdc42 activation by affinity precipitation, phosphatidylinositide-3-kinase (PI3K) activity by phospho-Akt, phospho-p21-activated kinase (PAK) by immunoblot, and association of Cdc42 with neuronal Wiskott Aldrich Syndrome protein (N-WASp) by immunoprecipitation. The effect of Rac or PAK inhibition on filamentous actin was quantified by laser-scanning cytometry and by cytoskeletal fractionation; effects of actin-modifying agents were measured by isometric myography. Basal Cdc42 activity increased in hypoxia, whereas Rac activity decreased. U46619 challenge increased Cdc42 and Rac activity in hypoxic cells, independently of PI3K. Hypoxia increased phospho-PAK, unaltered by U46619. Association of Cdc42 with N-WASp decreased in hypoxia but increased after U46619 exposure. Hypoxia doubled filamentous-to-globular ratios of α- and γ-actin isoforms. Jasplakinolide stabilized γ-filaments, increasing force; cytochalasin D depolymerized all actin isoforms, decreasing force. Rac and PAK inhibition decreased filamentous actin in tissues although without decrease in force. Rho inhibition decreased myosin phosphorylation and force. Hypoxia induces actin polymerization in PA myocytes, particularly increasing filamentous α- and γ-actin, contributing to U46619-induced contraction. Hypoxic PA myocytes challenged with a thromboxane mimetic polymerize actin via the Cdc42 pathway, reflecting increased Cdc42 association with N-WASp. Mechanisms regulating thromboxane-mediated actin polymerization are potential targets for future PPHN pharmacotherapy.

P21-activated kinase in inflammatory and cardiovascular disease

P-21 activated kinases, or PAKs, are serine-threonine kinases that serve a role in diverse biological functions and organ system diseases. Although PAK signaling has been the focus of many investigations, still our understanding of the role of PAK in inflammation is incomplete. This review consolidates what is known about PAK1 across several cell types, highlighting the role of PAK1 and PAK2 in inflammation in relation to NADPH oxidase activation. This review explores the physiological functions of PAK during inflammation, the role of PAK in several organ diseases with an emphasis on cardiovascular disease, and the PAK signaling pathway, including activators and targets of PAK. Also, we discuss PAK1 as a pharmacological anti-inflammatory target, explore the potentials and the limitations of the current pharmacological tools to regulate PAK1 activity during inflammation, and provide indications for future research. We conclude that a vast amount of evidence supports the idea that PAK is a central molecule in inflammatory signaling, thus making PAK1 itself a promising prospective pharmacological target.

p21-Activated protein kinases and their emerging roles in glucose homeostasis

p21-Activated protein kinases (PAKs) are centrally involved in a plethora of cellular processes and functions. Their function as effectors of small GTPases Rac1 and Cdc42 has been extensively studied during the past two decades, particularly in the realms of cell proliferation, apoptosis, and hence tumorigenesis, as well as cytoskeletal remodeling and related cellular events in health and disease. In recent years, a large number of studies have shed light onto the fundamental role of group I PAKs, most notably PAK1, in metabolic homeostasis. In skeletal muscle, PAK1 was shown to mediate the function of insulin on stimulating GLUT4 translocation and glucose uptake, while in pancreatic β-cells, PAK1 participates in insulin granule localization and vesicle release. Furthermore, we demonstrated that PAK1 mediates the cross talk between insulin and Wnt/β-catenin signaling pathways and hence regulates gut proglucagon gene expression and the production of the incretin hormone glucagon-like peptide-1 (GLP-1). The utilization of chemical inhibitors of PAK and the characterization of Pak1(-/-) mice enabled us to gain mechanistic insights as well as to assess the overall contribution of PAKs in metabolic homeostasis. This review summarizes our current understanding of PAKs, with an emphasis on the emerging roles of PAK1 in glucose homeostasis.