Lpaatδ/Agpat4 deficiency impairs maximal force contractility in soleus and alters fibre type in extensor digitorum longus muscle.

Lysophosphatidic acid acyltransferase (LPAAT) δ/acylglycerophosphate acyltransferase 4 is a mitochondrial enzyme and one of five homologues that catalyze the acyl-CoA-dependent synthesis of phosphatidic acid (PA) from lysophosphatidic acid. We studied skeletal muscle LPAATδ and found highest levels in soleus, a red oxidative fibre-type that is rich in mitochondria, and lower levels in extensor digitorum longus (EDL) (white glycolytic) and gastrocnemius (mixed fibre-type). Using Lpaatδ-deficient mice, we found no change in soleus or EDL mass, or in treadmill time-to-exhaustion compared to wildtype littermates. There was, however, a significant reduction in the proportion of type I and type IIA fibres in EDL but, surprisingly, not soleus, where these fibre-types predominate. Also unexpectedly, there was no impairment in force generation by EDL, but a significant reduction by soleus. Oxidative phosphorylation and activity of complexes I, I + II, III, and IV in soleus mitochondria was unchanged and therefore could not explain this effect. However, pyruvate dehydrogenase activity was significantly reduced in Lpaatδ-/- soleus and EDL. Analysis of cellular lipids indicated no difference in soleus triacylglycerol, but specific elevations in soleus PA and phosphatidylethanolamine levels, likely due to a compensatory upregulation of Lpaatß and Lpaatε in Lpaatδ-/- mice. An anabolic effect for PA as an activator of skeletal muscle mTOR has been reported, but we found no change in serine 2448 phosphorylation, indicating reduced soleus force generation is unlikely due to the loss of mTOR activation by a specific pool of LPAATδ-derived PA. Our results identify an important role for LPAATδ in soleus and EDL.

Changes in insulin, glucagon and ER stress precede immune activation in type 1 diabetes.

It is unknown whether there is a gene signature in pancreas which is associated with type 1 diabetes (T1D). We performed partial pancreatectomies on 30-day preinsulitic, diabetes-prone BioBreeding (BBdp) rats to prospectively identify factors involved in early prediabetes. Microarrays of the biopsies revealed downregulation of endoplasmic reticulum (ER) stress, metabolism and apoptosis. Based on these results, additional investigations compared gene expression in control (BBc) and BBdp rats age ~8, 30 and 60 days using RT-qPCR. Neonates had increased ER stress gene expression in pancreas. This was associated with decreased insulin, cleaved caspase-3 and Ins1 whereas Gcg and Pcsk2 were increased. The increase in ER stress was not sustained at 30 days and decreased by 60 days. In parallel, the liver gene profile showed a similar signature in neonates but with an early decrease of the unfolded protein response (UPR) at 30 days. This suggested that changes in the liver precede those in the pancreas. Tnf and Il1b expression was increased in BBdp pancreas in association with increased caspase-1, cleaved caspase-3 and decreased proinsulin area. Glucagon area was increased in both 30-day and 60-day BBdp rats. Increased colocalization of BIP and proinsulin was observed at 60 days in the pancreas, suggesting insulin-related ER dysfunction. We propose that dysregulated metabolism leads to ER stress in neonatal rats long before insulitis, creating a microenvironment in both pancreas and liver that promotes autoimmunity.

Chronic 5-Aminoimidazole-4-Carboxamide-1-ß-d-Ribofuranoside Treatment Induces Phenotypic Changes in Skeletal Muscle, but Does Not Improve Disease Outcomes in the R6/2 Mouse Model of Huntington's Disease.

Huntington's disease (HD) is an autosomal dominant neurodegenerative genetic disorder characterized by motor, cognitive, and psychiatric symptoms. It is well established that regular physical activity supports brain health, benefiting cognitive function, mental health as well as brain structure and plasticity. Exercise mimetics (EMs) are a group of drugs and small molecules that target signaling pathways in skeletal muscle known to be activated by endurance exercise. The EM 5-aminoimidazole-4-carboxamide-1-ß-d-ribofuranoside (AICAR) has been shown to induce cognitive benefits in healthy mice. Since AICAR does not readily cross the blood-brain barrier, its beneficial effect on the brain has been ascribed to its impact on skeletal muscle. Our objective, therefore, was to examine the effect of chronic AICAR treatment on the muscular and neurological pathology in a mouse model of HD. To this end, R6/2 mice were treated with AICAR for 8 weeks and underwent regular neurobehavioral testing. Under our conditions, AICAR increased expression of PGC-1α, a powerful phenotypic modifier of muscle, and induced the expected shift toward a more oxidative muscle phenotype in R6/2 mice. However, this treatment failed to induce benefits on HD progression. Indeed, neurobehavioral deficits, striatal, and muscle mutant huntingtin aggregate density, as well as muscle atrophy were not mitigated by the chronic administration of AICAR. Although the muscle adaptations seen in HD mice following AICAR treatment may still provide therapeutically relevant benefits to patients with limited mobility, our findings indicate that under our experimental conditions, AICAR had no effect on several hallmarks of HD.

Autophagic signaling and proteolytic enzyme activity in cardiac and skeletal muscle of spontaneously hypertensive rats following chronic aerobic exercise.

Hypertension is a cardiovascular disease associated with deleterious effects in skeletal and cardiac muscle. Autophagy is a degradative process essential to muscle health. Acute exercise can alter autophagic signaling. Therefore, we aimed to characterize the effects of chronic endurance exercise on autophagy in skeletal and cardiac muscle of normotensive and hypertensive rats. Male Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) were assigned to a sedentary condition or 6 weeks of treadmill running. White gastrocnemius (WG) of hypertensive rats had higher (p<0.05) caspase-3 and proteasome activity, as well as elevated calpain activity. In addition, skeletal muscle of hypertensive animals had elevated (p<0.05) ATG7 and LC3I protein, LAMP2 mRNA, and cathepsin activity, indicative of enhanced autophagic signaling. Interestingly, chronic exercise training increased (p<0.05) Beclin-1, LC3, and p62 mRNA as well as proteasome activity, but reduced (p<0.05) Beclin-1 and ATG7 protein, as well as decreased (p<0.05) caspase-3, calpain, and cathepsin activity. Left ventricle (LV) of hypertensive rats had reduced (p<0.05) AMPKα and LC3II protein, as well as elevated (p<0.05) p-AKT, p-p70S6K, LC3I and p62 protein, which collectively suggest reduced autophagic signaling. Exercise training had little effect on autophagy-related signaling factors in LV; however, exercise training increased (p<0.05) proteasome activity but reduced (p<0.05) caspase-3 and calpain activity. Our results suggest that autophagic signaling is altered in skeletal and cardiac muscle of hypertensive animals. Regular aerobic exercise can effectively alter the proteolytic environment in both cardiac and skeletal muscle, as well as influence several autophagy-related factors in skeletal muscle of normotensive and hypertensive rats.