| Mark4, also known as Par-1d/MarkL1, is a member of the microtubule affinity regulatory kinases (MARKs). MARKs represent the mammalian homologsof nematode Par-1. The mammalian Par-1family consists of four members (Par-lc/Markl, Par-1b/Mark2/Emk, Par-1a/Mark3/C-TAKl,and Par-ld/MARK4/K-arkLl).In particular,MARK phosphorylation of axonal MAP tau at KXGS motifs within the micro-tubule-binding domain, dissociates tau from micro-tubules, leading to destabilization of the microtubule network. Par-1is an evolutionarily conserved protein kinase required for polarity in worms, flies, frogs and mammals. It is well demonstrated that AMP activated protein kinase AMPK is implicated in the regulation of dynamic biological functions, including glucose and energy homeostasis. As the AMPK-related family of kinases, physiological functions of the MARK2and MARK3kinase have recently been studied using targeted gene knockout approaches in mice. Two independently derived mouse lines null for MARK2have implicated this protein kinase in diverse physiological processes, including fertility, immune system homeostasis, learning and memory, glucose homeostasis, and energy metabolism. In addition to MARK2, most information regarding the cell biological functions of the Par-1kinase comes from studies of M-ARK3. Loss of MARK3also leads to reduced adiposity, resistance to hepatic steatosis, and defective gluconeogenesis. However, the physiological function of MARK4in mammals remains elusive. Here we investigated a role of MARK4in regulating energy homeostasis by generating mice with targeted inactivation of the MARK4gene. We got the following results:1. Study on the role of MARK4in regulating the onset of adiposity and insuli-n sensitivity during high fat feeding. It was observed that weight gain of MARK4-/-mice on HFD was significantly lower than wild type mice, concomitant with reduced fat mass, lower levels of blood triglyceride and cholesterol. Consistent with reduced adiposity, enhanced insulin sensitivity was also observed in MARK4-/-mice characterized by improved glucose tolerance and lower level of blood insulin after glucose challenge.2. Study on the role of MARK4in regulating metabolic rate by CLAMS.It was shown that MARK4-/-mice were hyperactive during high fat feeding and fasting. Consequently, the energy expenditure (EE) rate and oxygen were significantly higher in MARK4-/-mice when normalized with lean mass. Consistent with reduced fat mass, the Respiratory quotient (RQ) was significantly lower in MARK4-/-mice relative to the WT control mice, suggesting that MARK4-/-mice consumed more fat as an energy source.3. Study on the role of MARK4in modulating thermogenesis in Brown adipose tissue (BAT). The mRNA expression of both UCP1and PGC-1α was significantly up-regulated in BAT from MARK4-/-mice, suggesting an increased brown adipose activity in the MARK4-/-mice.4. Study on the role of MARK4in regulating insulin-stimulated AMPK and ins-ulin signalling in major metabolic tissues. Consistent with improved insulin sensitivity and reduced adiposity, MARK4deficiency significantly stimulated phosphorylation of AMPK in skeletal muscle and heart and phosphorylation of AKT1in all tissues, including white adipose, skeletal muscle, liver and heart. In further support of a role of MARK4in regulation of AMPK, ablation of MARK4also significantly increased the expression of AMPK in both white adipose tissue and liver. It was also observed that MARK4overexpression suppressed the activation of AKT/mTOR signalling in3T3-L1cells, suggesting a negative role of MARK4in regulating insulin signalling.5. Study on the effect of MARK4on regulating lipid metabolic and apoptosis signalling in adipotyte. Consistent with increased energy expenditure and reduced adiposity, MARK4overexpression in3T3-L1cells significantly increased the expression of SREBP-lc and FAS and decreased the expression of PPAR-y and ATGL. It was also found that MARK4overexpression significantly stimulated phosphorylation of JNK1and activated JNK1mediated apoptosis signalling in T3-L1cells by increasing the expression of pro-apoptosis protein Bax, decreasing anti-apoptosis protein Bcl-2abundance and promoting the cleavage of Caspase3... |
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