Glycogen Content Regulates Peroxisome Proliferator Activated Receptor-∂ (PPAR-∂) Activity in Rat Skeletal Muscle

Performing exercise in a glycogen depleted state increases skeletal muscle lipid utilization and the transcription of genes regulating mitochondrial β-oxidation. Potential candidates for glycogen-mediated metabolic adaptation are the peroxisome proliferator activated receptor (PPAR) coactivator-1α (PGC-1α) and the transcription factor/nuclear receptor PPAR-∂. It was therefore the aim of the present study to examine whether acute exercise with or without glycogen manipulation affects PGC-1α and PPAR-∂ function in rodent skeletal muscle. Twenty female Wistar rats were randomly assigned to 5 experimental groups (n = 4): control [CON]; normal glycogen control [NG-C]; normal glycogen exercise [NG-E]; low glycogen control [LG-C]; and low glycogen exercise [LG-E]). Gastrocnemius (GTN) muscles were collected immediately following exercise and analyzed for glycogen content, PPAR-∂ activity via chromatin immunoprecipitation (ChIP) assays, AMPK α1/α2 kinase activity, and the localization of AMPK and PGC-1α. Exercise reduced muscle glycogen by 47 and 75% relative to CON in the NG-E and LG-E groups, respectively. Exercise that started with low glycogen (LG-E) finished with higher AMPK-α2 activity (147%, p<0.05), nuclear AMPK-α2 and PGC-1α, but no difference in AMPK-α1 activity compared to CON. In addition, PPAR-∂ binding to the CPT1 promoter was significantly increased only in the LG-E group. Finally, cell reporter studies in contracting C2C12 myotubes indicated that PPAR-∂ activity following contraction is sensitive to glucose availability, providing mechanistic insight into the association between PPAR-∂ and glycogen content/substrate availability. The present study is the first to examine PPAR-∂ activity in skeletal muscle in response to an acute bout of endurance exercise. Our data would suggest that a factor associated with muscle contraction and/or glycogen depletion activates PPAR-∂ and initiates AMPK translocation in skeletal muscle in response to exercise

The Genetics of Functional Axon Regeneration Using C. Elegans

How do organisms attain the capacity to regenerate a structure, entire body, or not to regenerate? These are fundamental questions in biology for understanding how replicative systems are evolved to renew, age, and/or die. One outstanding question in regenerative biology that attracts attention is how and why the human central nervous system fails to regenerate after injury. Nervous system injuries are characterized by axonal damage and loss of synaptic function that contribute to debilitating neuronal dysfunctions. Although the molecular underpinnings of axon regeneration are well characterized, very little is known about how and what molecular pathways modulate reformation of synapses within regenerating axons to restore function. Thus, understanding the fundamental molecular and genetic mechanisms of functional axon regeneration (FAR), restoration of both axon and synapse, for the functional recovery of the nervous system remains elusive. In Chapter I, I outline the biology of regeneration and provide evolutionary perspectives of this phenomenon. Then, I provide clinical perspectives of central nervous system regeneration and therapeutic innovations. I next introduce the regulators of axon regeneration and how C. elegans as a genetic system allows detailed characterization of axon regeneration. In Chapter II, using C. elegans as a platform, I show how axon regeneration and synaptic reformation are controlled by distinct genetic pathways. I show how Poly-ADP ribose polymerase (PARP) pathway modulates functional restoration by regulating divergent genetic pathways leading to axon regeneration and synapse restoration. Finally, in Chapter III, I summarize the model of axon regeneration, evolutionary perspectives, and epistemic limitations of C. elegans axon regeneration

AMPK activity following exercise in a normal or low glycogen environment.

<p>The activity of (A) α1-AMPK and (B) α2-AMPK was determined in each of the 5 groups by IP kinase activity assay. * indicates significantly different than control (p<0.05; n = 4).</p

Glycogen levels in the 5 experimental groups.

<p>Glycogen was determined after hydrolysis of 30* indicates significantly different than CON and † indicates significantly different than NG-E and LG-C (p<0.05; n = 4).</p

Schematic of the experimental design.

<p>Skeletal muscle glycogen content was manipulated using pre-exercise and varying periods of recovery, prior to a main 45 min exercise trial. Five groups performed 3-days acclimatization to treadmill running. Following which, the CON group was sacrificed 24 h after the final acclimatization trial, the normal glycogen control (NG-C) ran for 1 hour and then rested 23 h before sacrifice, the normal glycogen exercise (NG-E) performed a subsequent 45 min run 23 h after the initial 1 hour run, the low glycogen control (LG-C) was collected 1 h after the initial 1 hour run, and the low glycogen exercise group (LG-E) was collected immediately following a 45 min run commenced 1 hour after the initial 1 hour trial. All experiments took place in the morning to avoid significant alterations in diet and diurnal variation in metabolic responses.</p

PPAR-∂ activity in response to exercise in a normal or low glycogen environment.

<p>Post-exercise skeletal muscle activity of PPAR-∂ was determined by chromatin immunoprecipitation (ChIP) assay followed by qRT-PCR using primers spanning the PPAR-∂ binding region in the CPT-1 promoter. * indicates significantly different than control (p<0.05; n = 4).</p