Early exercise regimen improves insulin sensitivity in the intrauterine growth-restricted adult female rat offspring.

We examined the effect of early exercise training (Ex) on glucose kinetics, basal, and insulin-stimulated skeletal muscle (SKM) plasma membrane (PM) GLUT4 in pre- and/or postnatal nutrient-restricted adult rat offspring compared with sedentary (Sed) state. Pregestational control female (Ex CON vs. Sed CON) and offspring exposed to prenatal (Ex IUGR vs. Sed IUGR), postnatal (Ex PNGR vs. Sed PNGR), or pre- and postnatal (Ex IUGR + PNGR vs. Sed IUGR + PNGR) nutrient restriction were studied. The combined effect of exercise and pre/postnatal nutrition in the Ex IUGR demonstrated positive effects on basal and glucose-stimulated plasma insulin response (GSIR) with suppression of endogenous hepatic glucose production (HGP) compared with sedentary state. Ex PNGR was hyperglycemic after glucose challenge with no change in glucose-stimulated insulin production or HGP compared with sedentary state. Ex IUGR + PNGR remained glucose tolerant with unchanged glucose-stimulated insulin production but increased endogenous HGP compared with sedentary state. Basal SKM PM-associated GLUT4 was unchanged by exercise in all four groups. Whereas Ex PNGR and Ex IUGR + PNGR insulin responsiveness was similar to that of Ex CON, Ex IUGR remained nonresponsive to insulin. Early introduction of regular Ex in the pregestational female offspring had a positive effect on hepatic adaptation to GSIR and HGP in IUGR and IUGR + PNGR, with no effect in PNGR. Change in insulin responsiveness of SKM GLUT4 translocation was observed in exercised IUGR + PNGR and PNGR but not in exercised IUGR.

Perturbed skeletal muscle insulin signaling in the adult female intrauterine growth-restricted rat.

To determine the molecular mechanism(s) linking fetal adaptations in intrauterine growth restriction (IUGR) to adult maladaptations of type 2 diabetes mellitus, we investigated the effect of prenatal seminutrient restriction, modified by early postnatal ad libitum access to nutrients (CM/SP) or seminutrient restriction (SM/SP), vs. early postnatal seminutrient restriction alone (SM/CP) or control nutrition (CM/CP) on the skeletal muscle postreceptor insulin-signaling pathway in the adult offspring. The altered in utero hormonal/metabolic milieu was associated with no change in basal total IRS-1, p85, and p110beta subunits of PI 3-kinase, PKCtheta, and PKCzeta concentrations but an increase in basal IRS-2 (P < 0.05) only in the CM/SP group and an increase in basal phospho (p)-PDK-1 (P < 0.05), p-Akt (P < 0.05), and p-PKCzeta (P < 0.05) concentrations in the CM/SP and SM/SP groups. Insulin-stimulated increases in p-PDK-1 (P < 0.05) and p-Akt (P < 0.0007), with no increase in p-PKCzeta, were seen in both CM/SP and SM/SP groups. SHP2 (P < 0.03) and PTP1B (P < 0.03) increased only in SM/SP with no change in PTEN in CM/SP and SM/SP groups. Aberrations in kinase and phosphatase moieties in the adult IUGR offspring were initiated in utero but further sculpted by the early postnatal nutritional state. Although the CM/SP group demonstrated enhanced kinase activation, the SM/SP group revealed an added increase in phosphatase concentrations with the net result of heightened basal insulin sensitivity in both groups. The inability to further respond to exogenous insulin was due to the key molecular distal roadblock consisting of resistance to phosphorylate and activate PKCzeta necessary for GLUT4 translocation. This protective adaptation may become maladaptive and serve as a forerunner for gestational and type 2 diabetes mellitus.

Laminin-alpha1 globular domains 3 and 4 induce heterotrimeric G protein binding to alpha-syntrophin's PDZ domain and alter intracellular Ca2+ in muscle.

Alpha-syntrophin is a component of the dystrophin glycoprotein complex (DGC). It is firmly attached to the dystrophin cytoskeleton via a unique COOH-terminal domain and is associated indirectly with alpha-dystroglycan, which binds to extracellular matrix laminin. Syntrophin contains two pleckstrin homology (PH) domains and one PDZ domain. Because PH domains of other proteins are known to bind the betagamma-subunits of the heterotrimeric G proteins, whether this is also a property of syntrophin was investigated. Isolated syntrophin from rabbit skeletal muscle binds bovine brain Gbetagamma-subunits in gel blot overlay experiments. Laminin-1-Sepharose or specific antibodies against syntrophin, alpha- and beta-dystroglycan, or dystrophin precipitate a complex with Gbetagamma from crude skeletal muscle microsomes. Bacterially expressed syntrophin fusion proteins and truncation mutants allowed mapping of Gbetagamma binding to syntrophin's PDZ domain; this is a novel function for PDZ domains. When laminin-1 is bound, maximal binding of Gsalpha and Gbetagamma occurs and active Gsalpha, measured as GTP-gamma35S bound, decreases. Because intracellular Ca2+ is elevated in Duchenne muscular dystrophy and Gsalpha is known to activate the dihydropyridine receptor Ca2+ channel, whether laminin also altered intracellular Ca2+ was investigated. Laminin-1 decreases active (GTP-gammaS-bound) Gsalpha, and the Ca2+ channel is inhibited by laminin-1. The laminin alpha1-chain globular domains 4 and 5 region, the region bound by DGC alpha-dystroglycan, is sufficient to cause an effect, and an antibody that specifically blocks laminin binding to alpha-dystroglycan inhibits Gbeta binding by syntrophin in C2C12 myotubes. These observations suggest that DGC is a matrix laminin, G protein-coupled receptor.

Skeletal muscle signaling pathway through the dystrophin glycoprotein complex and Rac1.

The dystrophin glycoprotein complex has been proposed to be involved in signal transduction. Here we have shown that laminin binding causes syntrophin to recruit Rac1 from the rabbit skeletal muscle. Laminin-Sepharose and syntrophin-Sepharose bind a protein complex containing Rac1 from the muscle membranes. The presence of heparin, which inhibits laminin interactions, prevents recruitment of Rac1. The dystrophin glycoprotein complex recruits Rac1 via syntrophin through a Grb2.Sos1 complex. A syntrophin antibody also prevents recruitment of Rac1, suggesting that the signaling complex requires syntrophin. PAK1 is in turn bound by Rac1. c-Jun NH2-terminal kinase-p46 is phosphorylated and activated only when laminin is present, and the p54 isoform is activated when laminin is depleted or binding is inhibited with heparin. In the presence of laminin, c-Jun is activated in both skeletal muscle microsomes and in C2C12 myoblasts, and proliferation increases in C2C12 myoblasts. We postulate that this pathway signals muscle homeostasis and hypertrophy.

Dystrophin-glycoprotein complex and Ras and Rho GTPase signaling are altered in muscle atrophy.

The dystrophin-glycoprotein complex (DGC) is a sarcolemmal complex whose defects cause muscular dystrophies. The normal function of this complex is not clear. We have proposed that this is a signal transduction complex, signaling normal interactions with matrix laminin, and that the response is normal growth and homeostasis. If so, the complex and its signaling should be altered in other physiological states such as atrophy. The amount of some of the DGC proteins, including dystrophin, beta-dystroglycan, and alpha-sarcoglycan, is reduced significantly in rat skeletal muscle atrophy induced by tenotomy. Furthermore, H-Ras, RhoA, and Cdc42 decrease in expression levels and activities in muscle atrophy. When the small GTPases were assayed after laminin or beta-dystroglycan depletion, H-Ras, Rac1, and Cdc42 activities were reduced, suggesting a physical linkage between the DGC and the GTPases. Dominant-negative Cdc42, introduced with a retroviral vector, resulted in fibers that appeared atrophic. These data support a putative role for the DGC in transduction of mechanical signals in muscle.