| dc.description.abstract |
<p>Acetyl-CoA holds a prominent position as the common metabolic intermediate of glucose,
amino acid and fatty acid oxidation. Because acetyl-CoA fuels the tricarboxylic acid
(TCA) cycle, the primary source of reducing equivalents that drives mitochondrial
oxidative phosphorylation, understanding acetyl-CoA pool regulation becomes imperative
to understanding mitochondrial energetics. Carnitine acetyltransferase (CrAT), a muscle-enriched
mitochondrial enzyme, catalyzes the freely reversible conversion of acetyl-CoA to
its membrane permeant carnitine ester, acetylcarnitine. Because CrAT has long been
thought to regulate the acetyl-CoA metabolite pool, we investigated the role of CrAT
in acetyl-CoA regulation. Although the biochemistry and enzymology of the CrAT reaction
has been well studied, its physiological role remains unknown. Investigations herein
suggest that CrAT-mediated maintenance of the mitochondrial acetyl-CoA pool is imperative
for preservation of energy homeostasis. We provide compelling evidence that CrAT is
critical for fine-tuning acetyl-CoA balance during the fasted to fed transition and
during exercise. These studies suggest that compromised CrAT activity results in derangements
in mitochondrial homeostasis.</p><p>In chapter 3, we examined the effects of obesity
and lipid exposure on CrAT activity. Recent studies have shown that acetyl-CoA-mediated
inhibition of pyruvate dehydrogenase (PDH), the committed step in glucose oxidation,
is modulated by the CrAT enzyme. Because PDH and glucose oxidation are negatively
regulated by high fat feeding and obesity, we reasoned that nutritional conditions
that promote lipid availability and fat oxidation might likewise compromise CrAT activity.
We report an accumulation of long chain acylcarnitines and acyl-CoAs but a decline
in the acetylcarnitine/acetyl-CoA ratio in obese and diabetic rodents. This reduction
in the skeletal muscle acetylcarnitine/acetyl-CoA ratio was accompanied by a decrease
in CrAT specific activity, despite increased protein abundance. Exposure to long chain
acyl-CoAs in vitro demonstrated that palmitoyl-CoA acts as a mixed model inhibitor
of CrAT. Furthermore, primary human skeletal muscle myocytes exposed to fatty acid
and or CPT1b overexpression had elevated long chain acylcarnitines but decreased production
and efflux of CrAT-derived short chain acylcarnitines. These data suggest that exposure
to fatty acids in obesity and diabetes can counter-regulate the CrAT enzyme leading
to decreased activity. </p><p>Alternatively, chapter 4 addresses the importance of
acetyl-CoA buffering during exercise and suggests that a deficit in CrAT activity
leads to fatigue. Because CrAT is highly expressed in tissues specifically designed
for work and because acetylcarnitine, the primary product of the CrAT reaction, is
increased during contraction, we reasoned that CrAT could play an important role in
exercise. To investigate this possibility, we employed exercise intervention and ex-vivo
analysis on a genetically novel mouse model of skeletal muscle CrAT deficiency (CrATSM-/-).
Though resting acetyl-CoA levels were elevated in CrATSM-/- mice, these levels dropped
significantly after intense exercise while acetylcarnitine content followed the opposite
pattern. This contraction-induced acetyl-CoA deficit in CrATSM-/- mice was coupled
with compromised performance and diminished whole body glucose oxidation during high
intensity exercise. These results imply that working muscles clear and consume acetylcarnitine
in order to maintain acetyl-CoA buffering during exercise. Importantly, provision
of acetylcarnitine enhanced force generation, delayed fatigue and improved mitochondrial
energetics in muscles from CrATfl/fl controls but not CrATSM-/- littermates, emphasizing
the importance of acetyl-CoA maintenance. In aggregate, these data demonstrate a critical
role for CrAT-mediated acetyl-CoA buffering in exercise tolerance and suggest its
involvement in energy metabolism during skeletal muscle contraction and fatigue. These
findings could have important clinical implications for individuals with muscle weakness
and fatigue due to multiple conditions, such as peripheral vascular or cardiometabolic
disease. </p><p>In summary, data herein emphasize the role of CrAT in regulation of
mitochondrial acetyl-CoA pool. We demonstrate that CrAT is critical for fine-tuning
acetyl-CoA balance both during the fasted to fed transition and during exercise. These
data suggest that a deficit in CrAT activity leads to glucose intolerance and exercise
fatigue. We examine these studies and suggest future areas of study.</p>
|
|
