Muscle-specificPikfyvegene disruption causes glucose intolerance, insulin resistance, adiposity, and hyperinsulinemia but not muscle fiber-type switching

Abstract

The evolutionarily conserved kinase PIKfyve that synthesizes PtdIns5P and PtdIns(3,5)P₂ has been implicated in insulin-regulated GLUT4 translocation/glucose entry in 3T3-L1 adipocytes. To decipher PIKfyve's role in muscle and systemic glucose metabolism, here we have developed a novel mouse model with Pikfyve gene disruption in striated muscle (MPIfKO). These mice exhibited systemic glucose intolerance and insulin resistance at an early age but had unaltered muscle mass or proportion of slow/fast-twitch muscle fibers. Insulin stimulation of in vivo or ex vivo glucose uptake and GLUT4 surface translocation was severely blunted in skeletal muscle. These changes were associated with premature attenuation of Akt phosphorylation in response to in vivo insulin, as tested in young mice. Starting at 10-11 wk of age, MPIfKO mice progressively accumulated greater body weight and fat mass. Despite increased adiposity, serum free fatty acid and triglyceride levels were normal until adulthood. Together with the undetectable lipid accumulation in liver, these data suggest that lipotoxicity and muscle fiber switching do not contribute to muscle insulin resistance in MPIfKO mice. Furthermore, the 80% increase in total fat mass resulted from increased fat cell size rather than altered fat cell number. The observed profound hyperinsulinemia combined with the documented increases in constitutive Akt activation, in vivo glucose uptake, and gene expression of key enzymes for fatty acid biosynthesis in MPIfKO fat tissue suggest that the latter is being sensitized for de novo lipid anabolism. Our data provide the first in vivo evidence that PIKfyve is essential for systemic glucose homeostasis and insulin-regulated glucose uptake/GLUT4 translocation in skeletal muscle.

Distinct Requirements for Vacuolar Protein Sorting 34 Downstream Effector Phosphatidylinositol 3-Phosphate 5-Kinase in Podocytes Versus Proximal Tubular Cells

“…32,[61][62][63] In fact, depending on the cell type and status (cycling or differentiated), #60%-88% diminution of PtdIns(3,5)P2 may not produce aberrant endomembrane vacuolation as revealed in differentiated adipocytes in culture, yeast cells, muscle, fat, or other tissues. 27,32,[61][62][63][64][65] Although our data with the mTmG reporter and Western blotting are consistent with a complete gene/protein loss ( Figure 2), a potential insufficiency of the antibody sensitivity and/or discrepancies occurring by the mTmG reporter 35 could not rule out residual amounts of the PIKfyve protein. Concordantly, lack of a complete loss of PtdIns(3,5)P2 in PIKfyve-deleted podocytes is suggested by detecting approximately 15% residual levels of the PIKfyve lipid products PtdIns(3,5)P2 and PtdIns(5)P by HPLC ( Figure 3B).…”

Class III PI 3-kinase is the main source of PtdIns3P substrate and membrane recruitment signal for PIKfyve constitutive function in podocyte endomembrane homeostasis

“…The two models share remarkably similar metabolic abnormalities, including systemic glucose intolerance, muscle insulin resistance and increased adiposity, suggesting that the metabolic defects in these mice might be related, in part, to ablation of a class I PI3K-controlled PtdIns3P pool that serves as a precursor for PIKfyve function. These data become more prominent in light of findings for distinct phenotypes in mice with muscle-specific Vps34 KO (30,60) and MPIfKO mice (59) (also controlled by the same MCK promoter-driven Cre recombinase and on Bl6 background). Indeed, MPIfKO mice have normal life span and size of heart or striated muscles (59), whereas muscle-specific Vps34 KO causes premature death at 2–4 months of age, enlarged heart and striated muscular dystrophy (30,60).…”

“…These data become more prominent in light of findings for distinct phenotypes in mice with muscle-specific Vps34 KO (30,60) and MPIfKO mice (59) (also controlled by the same MCK promoter-driven Cre recombinase and on Bl6 background). Indeed, MPIfKO mice have normal life span and size of heart or striated muscles (59), whereas muscle-specific Vps34 KO causes premature death at 2–4 months of age, enlarged heart and striated muscular dystrophy (30,60). The more severe phenotypes upon muscle-specific Vps34 KO suggests that if the Vps34-controlled PtdIns3P pool is also a main substrate supplier for PIKfyve functionality in muscle cells, as observed herein for podocytes, only a subfraction of this pool might undergo the PAS complex-dependent PtdIns3P-PtdIns(3,5)P 2 cycling.…”

“…S5), corroborating the notion of a class I PI3K-related endosomal PtdIns3P pool being used for PtdIns(3,5)P 2 production. That this pathway could be physiologically significant is inferred by the comparable phenotypes between mice with muscle-specific KO of Pikfyve (MPIfKO) and those with muscle-specific inactivation of Pik3r1 combined with global Pik3r2 (58,59) (note that both models are generated by the same muscle-creatine kinase (MCK) promoter-driven Cre recombinase and on C57Bl6 background). The two models share remarkably similar metabolic abnormalities, including systemic glucose intolerance, muscle insulin resistance and increased adiposity, suggesting that the metabolic defects in these mice might be related, in part, to ablation of a class I PI3K-controlled PtdIns3P pool that serves as a precursor for PIKfyve function.…”

PtdIns5P: News and views of its appearance, disappearance and deeds

“…However, more work is required to shed light onto the molecular events responsible for the acute rise and fall of PtdIns5P by insulin and how these modulations are linked with glucose uptake activation. Whatever the molecular mechanism, it is significant to emphasize recent metabolic data from the newly developed mouse model with Pikfyve specific disruption in skeletal muscle [117], the tissue responsible for the majority of postprandial insulin-stimulated glucose disposal. The observations for whole-body glucose intolerance and insulin resistance due to a primary defect of muscle insulin resistance indicate the physiological role of PtdIns5P and/or PtdIns(3,5)P 2 in glucose homeostasis [117].…”