Old-age muscle atrophy : cellular mechanisms and behavioral consequenses

Abstract

With advancing age, humans and rodents alike lose about one third of the skeletal muscle mass. A process referred to as old-age muscle atrophy or sarcopenia. Atrophy is a major contributor to disability and morbidity among elderly adults hence the aim of this thesis is to shed light on the molecular mechanisms underlying old-age associated muscle atrophy and behavioral changes related to age in a rat model.

In Paper I, we characterized the growth patterns, survival and behavioral alterations linked to advancing age in the rat. The median survival age was, on average, between 29 30 months for both female and male Sprague Dawley (SD) rats. There was a gradual decline in locomotor activity and explorative behavior associated with age, while disturbances in both coordination and balance did not become evident until later times points. In old age, weight carrying capacity, limb movement and temperature threshold were also impaired. While body weight continues to increase over the better part of the life span of rats, the behavioral changes in old age associated with a decrease in both total body weight and, in particular, muscle mass. Dietary restriction (DR) was found to increase median life span expectancy and impede the development of sarcopenia, and to retard the pace of behavioral aging.

In Paper II, we used two-dimensional gel electrophoresis and mass spectrometry techniques to determine changes in protein expression as well as cDNA profiling to assess transcriptional regulations in skeletal muscle of adult and aged male SD rats. Among the highly expressed proteins, thirty-five were differentially expressed in aged muscle. Proteins and mRNA transcripts involved in redox homeostasis and iron load were increased, representing novel components previously not associated with sarcopenia. Iron levels in tissue were elevated in senescence, paralleling an increase in transferrin. Proteins involved in redox homeostasis were found to display a complex pattern of changes involving increases in SOD1 and decreases in SOD2. Together these results suggest that an elevated iron load is a significant component of sarcopenia with a potential to be exploited clinically and that the mitochondria of aged striated muscle may be more vulnerable to radicals produced during cell respiration.

Muscle atrophy, in many conditions, shares a common mechanism for up-regulation of the muscle-specific ubiquitin E3-ligases Atrogin-1 and MuRF1. E3-ligases are part of the ubiquitin proteasome system (UPS) utilized for protein degradation during muscle atrophy. In Paper III, we show that Atrogin-1 and MuRF1 are down-regulated in old age-associated muscle atrophy. Our results suggest that this is mediated by AKT-induced inactivation of FOXO4. DR impeded sarcopenia as well as both FOXO4 inactivation and up-regulation of Atrogin-1 and MuRF1 transcripts. Our findings allow us to conclude that sarcopenia is mechanistically different from acute atrophies induced by disuse, disease, and denervation.

The 26S proteasome is responsible for most cytosolic proteolysis. Molecules that inhibit or specially tag proteasomes are helpful tools for analysis of the UPS. In Paper IV, we present a new class of proteasome inhibitors, considerably extended in comparison to the commonly used fluorescent substrates and peptide-based inhibitors. Modification of the most active compound, Ada-Ahx3L3VS, capable of proteasome inhibition in living cells, afforded a new set of radio- and affinity labels. N-terminal extension of peptide vinyl sulfones was found to have a profound influence on both their efficacy and selectivity as proteasome inhibitors. Results demonstrated that such extensions greatly enhanced inhibition and largely obliterated their selectivity towards individual catalytic activities.

The role of the UPS in aging-related muscle atrophy is highly controversial. In Paper V, we showed an accumulation of assembled proteasome particles with a corresponding increase in both proteasomal activity and protein degradation in old age muscle atrophy. This was accompanied by a wide range of UPS enzyme-regulation, including an increase in the activity of deubiquitylating enzymes. The accumulation of proteasomes was found to correlate well with muscle wasting. Both the accumulation of proteasome particles as well as the progression of muscle atrophy, were impeded when the normal pattern of aging was challenged by DR. In contrast to many conditions with UPS-associated muscle catabolism, the accumulation of proteasomes during senile muscle atrophy is not caused by transcriptional induction, but rather by decreases in their degradation. The lysosomal pathway is a candidate for degrading proteasomes. In Paper V, we demonstrated that impaired lysosomal function, achieved through chloroquine treatment, induced accumulation of proteasomes in adult rats. This emphasizes the existence of a functional link between the lysosomal pathway and the UPS suggesting that a decline in lysosomal function may contribute to increased proteasomal proteolysis in old-age skeletal muscle atrophy.