Heart failure is associated with a high mortality rate and is one of the most prevalent diseases worldwide whereby susceptibility increases with age. The development of heart failure occurs over an extensive period of time in which arrhythmias and hypertrophy are both very prevalent manifestations throughout this progression. Arrhythmias are defined as an irregular rhythm originating from intracellular calcium dysregulation, which can be fatal. Cardiac hypertrophy is a compensatory condition induced by increased workload involving augmented cardiomyocyte growth accompanied by myocardial remodelling. However, under prolonged periods of increased stress this compensatory mechanism can lead to cardiac dysfunction. The current treatments for heart failure are mainly aimed at relieving symptoms or itself possess proarrhythmic ability. Therefore it is fundamental to elucidate the pathways involved in arrhythmias and hypertrophy for the development of more effective treatment. p21 activated protein kinase (Pak1) is a novel gene involved in the regulation of cardiac function, however, the mechanisms involved remain inconclusive. This study has demonstrated Pak1 to be both antiarrhythmic and antihypertrophic, emphasizing Pak1 as a credible therapeutic target for simultaneously treating both manifestations. The antiarrhythmic properties of Pak1 were demonstrated through cardiomyocyte-specific Pak1 knockout (Pak1cko) mouse model which underwent Isoproterenol (ISO) stimulation for 2 weeks. Compared with ISO treated control group, the Pak1cko group had increased calcium irregularities and particularly a prolongation in sarcoplasmic reticulum (SR) calcium refill. The absence of Pak1 abrogated the transcriptional up-regulation of sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) under stressed conditions. Further analysis in neonatal rat cardiomyocytes (NRCMs) revealed this regulation to be through activation of the transcription factor, SRF. The antihypertrophic effects of Pak1 were further illustrated through cardiomyocyte-specific overexpressed constitutively-active Pak1 (Pak1cTG) mice which were subjected to transverse aortic constriction (TAC) for 3 weeks. Compared to TAC control group, Pak1cTG mice had improved cardiac performance accompanied with diminished fibrosis. Further analysis led to the discovery of a novel antihypertrophic pathway of Pak1 involving positive regulation of the E3ligase, Fbxo32 through activation of Smad3. This pathway is vital in the prevention of calcineurin (PP2B) accretion. Berberine administration in TAC treated mice corroborated that Fbxo32 up-regulation is sufficient in the prevention of hypertrophy. In conclusion, my study has demonstrated that Pak1 conveys antiarrhythmic influence through the up-regulation of SERCA2a. In the prevention of pathological hypertrophy, Pak1 inhibits PP2B through positive regulation of Fbxo32. Overall, my thesis has advanced the knowledge about cardioprotective pathways initiated by Pak1 under stressed conditions, presenting Pak1 as a promising therapeutic target.

Our laboratory has demonstrated previously that the proglucagon gene (gcg), which encodes the incretin hormone GLP-1, is among the downstream targets of the Wnt signaling pathway; and that Pak1 mediates the stimulatory effect of insulin on Wnt target gene expression in mouse gut non- endocrine cells. Here, I asked whether Pak1 controls gut gcg expression and GLP-1 production, and whether Pak1 deletion leads to impaired metabolic homeostasis in mice. I detected the expression of Pak1 and two other group I Paks in the gut endocrine L cell line GLUTag, and co-localized Pak1 and GLP-1 in the mouse gut. Insulin was shown to stimulate Pak1 Thr423 and β-cat Ser675 phosphorylation. The stimulation of insulin on β-cat Ser675 phosphorylation, gcg promoter activity and gcg mRNA expression could be attenuated by the Pak inhibitor IPA3. Male Pak1-/- mice showed significant reduction in both gut and brain gcg expression levels, and attenuated elevation of plasma GLP-1 levels in response to oral glucose challenge. Notably, the Pak1-/- mice were intolerant to both intraperitoneal and oral glucose administration. Aged Pak1-/- mice showed a severe defect in response to intraperitoneal pyruvate challenge (IPPTT). In primary hepatocytes, however, IPA3 reduced basal glucose production, attenuated glucagon-stimulated glucose production, and inhibited the expression of Pck1 and G6pc. This implicates that the direct effect of group I Paks in hepatocytes is the stimulation of gluconeogenesis, and that the impairment in IPPTT in aged Pak1-/- mice is due to the lack of Pak1 elsewhere. The defect in IPPTT in aged Pak1-/- mice could be rescued by stimulating gcg expression with forskolin injection or by enhancing the incretin effect via sitagliptin administration. In summary, my study demonstrates that: 1) Pak1 positively regulates GLP-1 production, 2) Pak1/β-cat signaling plays a role in gut/liver axis or gut/pancreas/liver axis governing glucose homeostasis, and 3) Pak1-/- mice can be utilized as a novel model for metabolic research.

Our laboratory has demonstrated previously that the proglucagon gene (gcg), which encodes the incretin hormone GLP-1, is among the downstream targets of the Wnt signaling pathway; and that Pak1 mediates the stimulatory effect of insulin on Wnt target gene expression in mouse gut non- endocrine cells. Here, I asked whether Pak1 controls gut gcg expression and GLP-1 production, and whether Pak1 deletion leads to impaired metabolic homeostasis in mice. I detected the expression of Pak1 and two other group I Paks in the gut endocrine L cell line GLUTag, and co-localized Pak1 and GLP-1 in the mouse gut. Insulin was shown to stimulate Pak1 Thr423 and β-cat Ser675 phosphorylation. The stimulation of insulin on β-cat Ser675 phosphorylation, gcg promoter activity and gcg mRNA expression could be attenuated by the Pak inhibitor IPA3. Male Pak1-/- mice showed significant reduction in both gut and brain gcg expression levels, and attenuated elevation of plasma GLP-1 levels in response to oral glucose challenge. Notably, the Pak1-/- mice were intolerant to both intraperitoneal and oral glucose administration. Aged Pak1-/- mice showed a severe defect in response to intraperitoneal pyruvate challenge (IPPTT). In primary hepatocytes, however, IPA3 reduced basal glucose production, attenuated glucagon-stimulated glucose production, and inhibited the expression of Pck1 and G6pc. This implicates that the direct effect of group I Paks in hepatocytes is the stimulation of gluconeogenesis, and that the impairment in IPPTT in aged Pak1-/- mice is due to the lack of Pak1 elsewhere. The defect in IPPTT in aged Pak1-/- mice could be rescued by stimulating gcg expression with forskolin injection or by enhancing the incretin effect via sitagliptin administration. In summary, my study demonstrates that: 1) Pak1 positively regulates GLP-1 production, 2) Pak1/β-cat signaling plays a role in gut/liver axis or gut/pancreas/liver axis governing glucose homeostasis, and 3) Pak1-/- mice can be utilized as a novel model for metabolic research.

Indiana University-Purdue University Indianapolis (IUPUI) / Insulin-stimulated glucose uptake into skeletal muscle cells requires translocation of the glucose transporter-4 (GLUT4) from the cell interior to the plasma membrane. Insulin-stimulated GLUT4 vesicle translocation is dysregulated in Type 2 diabetes (T2D). The Group I p21–activated kinase (PAK1) is a required element in insulin-stimulated GLUT4 vesicle translocation in mouse skeletal muscle in vivo, although its placement and function(s) in the canonical insulin signaling cascade in skeletal muscle cells, remain undetermined. Therefore, the objective of my project is to determine the molecular mechanism(s) underlying the requirement for PAK1 in the process of insulin-stimulated GLUT4 vesicle translocation and subsequent glucose uptake by skeletal muscle cells. Toward this, my studies demonstrate that the pharmacological inhibition of PAK1 activation blunts insulin-stimulated GLUT4 translocation and subsequent glucose uptake into L6-GLUT4myc skeletal myotubes. Inhibition of PAK1 activation also ablates insulin-stimulated F-actin cytoskeletal remodeling, a process known to be required for mobilizing GLUT4 vesicles to the plasma membrane. Consistent with this mechanism, PAK1 activation was also required for the activation of cofilin, another protein implicated in F-actin remodeling. Interestingly, my studies reveal a novel molecular mechanism involving PAK1 signaling to p41-ARC, a regulatory subunit of the cytoskeletal Arp2/3 complex, and its interactions with another cytoskeletal factor, N-WASP, to elicit the insulin-stimulated F-actin remodeling in skeletal muscle cells. Pharmacological inactivation of N-WASP fully abrogated insulin-stimulated GLUT4 vesicle translocation to the cell surface, coordinate with blunted F-actin remodeling. Furthermore, my studies revealed new insulin-induced interactions amongst N WASP, actin, p41-ARC and PAK1; inactivation of PAK1 signaling blocked these dynamic interactions. Taken together, the above studies demonstrate the significance of PAK1 and its downstream signaling to F-actin remodeling in insulin-stimulated GLUT4 vesicle translocation and glucose uptake, revealing new signaling elements that may prove to be promising targets for future therapeutic design.

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Gastrulation is a highly complex series of cellular rearrangements that leads to the internalization of the mesoderm and endoderm. The cellular behaviors that underlie morphogenesis are dependent upon changes in cell motility and polarity. Eph receptors belong to a family of receptor tyrosine kinases that are involved in a variety of developmental processes. This study is the first to examine the role EphA4 during Xenopus gastrulation. Morpholino oligonucleotide (MO) mediated knockdown of EphA4 resulted in attenuated mesoderm involution and reduced the expression of the posterior mesoderm marker brachyury (Xbra). Expression of EphA4 in the blastocoel roof was sufficient to promote ectopic Xbra expression. I show that EphA4 can regulate Xbra expression and involution movements by signaling through PAK1. Temporal regulation of Xbra was sufficent to rescue EphA4 induced gastrulation defects. This study has uncovered a novel EphA4/PAK1 pathway which is required for mesoderm involution and Xbra expression during Xenopus gastrulation.

Gastrulation is a highly complex series of cellular rearrangements that leads to the internalization of the mesoderm and endoderm. The cellular behaviors that underlie morphogenesis are dependent upon changes in cell motility and polarity. Eph receptors belong to a family of receptor tyrosine kinases that are involved in a variety of developmental processes. This study is the first to examine the role EphA4 during Xenopus gastrulation. Morpholino oligonucleotide (MO) mediated knockdown of EphA4 resulted in attenuated mesoderm involution and reduced the expression of the posterior mesoderm marker brachyury (Xbra). Expression of EphA4 in the blastocoel roof was sufficient to promote ectopic Xbra expression. I show that EphA4 can regulate Xbra expression and involution movements by signaling through PAK1. Temporal regulation of Xbra was sufficent to rescue EphA4 induced gastrulation defects. This study has uncovered a novel EphA4/PAK1 pathway which is required for mesoderm involution and Xbra expression during Xenopus gastrulation.

No description available.

No description available.

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