Molecular signatures of selective motor neuron vulnerability in old age

Motor neuron (MN) death and impairment of fast-fatigable motor units is widely recognized in elderly humans and rats. However, the molecular signatures identifying vulnerable MNs remains underexplored. In murine models of amyotrophic lateral sclerosis, matrix metalloproteinase 9 (MMP-9) and chondrolectin (Chodl) decline with disease progression and have been considered molecular markers of fast-fatigable MNs. However, their expression in brainstem and lumbar MNs, in accordance with the 'size principle' (in which motor units are recruited in a size-dependent manner, with MNs of lower capacitance activated before those of greater capacitance) and their expression throughout the lifespan remain unclear. Here, we evaluated MMP-9 and Chodl in lumbar and hypoglossal MNs in young (6 months) and old (24 months) Fischer 344 (F344) rats. Longitudinal lumbar spinal cord transverse brainstem sections (the orientations providing maximal MN somal sizes for each pool) were immunolabelled for choline acetyltransferase, MMP-9 and Chodl. Expression was quantified using confocal microscopy in young and old rats (n = 5 females, n = 5 males per age). Overall, MMP-9 demonstrated a positive association with lumbar MN somal surface area in young rats. Paired within-animal analysis showed that in larger MNs, MMP-9 positivity was ∼70% greater than smaller MNs. The size-dependence of MMP-9 expression was absent in old lumbar MNs, suggesting either selective loss or downregulation of MMP-9 MN expression in ageing. No size-dependent change in Chodl was observed in lumbar or hypoglossal MNs, nor was MMP-9 associated with MN size in hypoglossal MNs. Collectively, these findings identify MMP-9 as a marker enriched within large lumbar MNs in young rats and suggest that age-related alterations in MMP-9 expression may reflect the selective vulnerability of the lumbar MNs.