Hypoglossal Motoneuron (XII MN) Survival Over the Lifespan in Pompe Disease

Abstract

Glycogen Storage Disease II, more commonly known as Pompe disease, is a lysosomal storage disease characterized by a partial or complete loss of acid α‐glucosidase (GAA) activity. Lingual impairment and sleep disordered breathing are common symptoms of Pompe disease with a potential role of XII MN dysfunction in both cases. Through innervation of the tongue muscles, XII MNs contribute to swallowing, speech, and maintenance of upper airway patency. Studies of Pompe (Gaa‐/‐) mice have found molecular and histological markers of neurodegeneration, and a recent case study of a patient with late‐onset Pompe disease revealed atrophied and possible degenerating XII MNs. However, whether the absence of GAA activity in Pompe disease directly leads to death of XII MNs is not definitively established. Because the preponderance of evidence indicates progressive neurodegeneration in Pompe disease, here we tested the hypothesis that when XII MNs do not express GAA activity, there will be a progressive loss of these cells over the lifespan. A novel transgenic mouse model (ChAT‐cre+/+●Gaa+/‐●eYFP+/+●Flox‐Gaa+/+) was generated in which GAA activity is absent in ChAT positive neurons, which also carry a yellow fluorescent protein (YFP) marker. Thus MNs in this mouse model do not express GAA, but skeletal myofibers have normal GAA expression. Brainstem tissue was harvested in young (1‐3 months of age) and advanced age mice (21‐24 months). Tissues were cryosectioned at 20 μm increments across the entire XII motor nucleus. Sections were slide mounted and visualized under fluorescence microscopy to detect YFP. The XII motor nucleus was identified based on anatomical landmarks and MNs were identified based on location, size, morphology, and YFP fluorescence. Preliminary MN counts across the rostral‐caudal extent of the XII nucleus (n=3 per group) indicate an approximately 7% decline in detectable YFP‐positive XII MNs over the age span (young: 658±19; old 617±28). Further, the histological appearance of XII MNs showed striking differences between the two age groups. The MNs from older mice showed the prototypical hallmarks of neuronal lysosomal storage disease including a markedly enlarged soma and extensive vacuolization. These preliminary findings are consistent with the hypothesis that absence of GAA in XII MNs leads to a histopathological appearance and neurodegeneration. Ongoing studies are evaluating XII MNs in age matched wild type mice.