Abstract
Sarcopenia: The most conspicuous intractable feature of advancing age is the gradual loss of skeletal muscle mass and the resultant loss of strength and mobility, and this is a major driver of morbidity and mortality in our ageing population. Loss of muscle is accompanied by loss of motoneurons, however what causes the death of motoneurons with advancing age remains unknown. Astrocytes are the most numerous and diverse cell type in the central nervous system, and play a critical role in neuronal support. Recent research has revealed that in-vitro, ageing astrocytes become senescent and express a senescence-associated secretory phenotype (SASP) that confers a reduced neuroprotective capacity. However, culturing senescent astrocytes in Glial-Derived Neurotrophic Factor (GDNF), a trophic factor critical for survival and proliferation of neurons, reversed these effects. Whether any similar changes occur in vivo is unknown, which provides a need for further investigation.
This study aimed to investigate whether astrocytes in ageing mouse spinal cord become senescent, and whether the senescence phenotype can be reversed or attenuated by exercise - a known stimulus of GDNF production. The second aim was to investigate whether GDNF levels in the lumbar spinal cord reduce with age, and whether any decline is reversed by exercise. To inform the aims of this experiment, Semi-Quantitative Immunohistochemistry (SQI) was performed on sections of spinal cord from young, elderly, and elderly exercised mice. The levels of three proteins of interest were measured: Glial-Fibrillary Acidic Protein (GFAP) an intermediate filament protein and marker of astrogliosis; p16, a marker of senescence; and the trophic factor GDNF.
Here we report that levels of GFAP within astrocytes of the lumbar lateral motor column showed a trend of increasing with age, although this was not statistically significant (p=0.052). Exercise had no effect on GFAP levels. P16-positive cell nuclei were observed in sections of both elderly sedentary and elderly exercised, but these did not co-localise with GFAP immunostaining of astrocytes. Instead, p16-positive nuclei appeared to be that of motoneurons, a novel finding. GDNF levels showed no change with age, but were increased significantly in exercised animals compared to sedentary (p<0.0001), indicating that exercise exerts neuroprotective effects by skeletal muscle-derived GDNF production.
These results indicate that astrocytes become reactive with age and as a result may show reduced neuroprotection of motoneurons, contributing to their demise associated with ageing and sarcopenia. Although exercise increased GDNF levels within spinal motoneurons, this did not correlate with a reduction in astrocyte reactivity or a reduction in the presence of p16-positive nuclei as hypothesized. Instead, GDNF may exert protective effects for motoneurons directly, attenuating their age-associated decline, and slowing the progression of sarcopenia.