Increased Homer Activity and NMJ Localization in the Vestibular Lesion het−/− Mouse soleus Muscle

TL;DR

This study explores how Homer proteins behave in specific mouse muscles to understand how a vestibular disorder affects muscle function and neuromuscular junctions.

Contribution

The study reveals muscle-specific changes in Homer protein isoforms and their localization in response to a vestibular lesion.

Findings

Het−/− mice show increased Homer dimers and multimers in the soleus muscle's soluble fraction.

Altered Homer subcellular localization at neuromuscular junctions correlates with myofiber properties in postural muscles.

No Homer dimers were detected in the gastrocnemius muscle of het−/− mice.

Abstract

We investigated the shuttling of Homer protein isoforms identified in soluble (cytosolic) vs. insoluble (membrane–cytoskeletal) fraction and Homer protein–protein interaction/activation in the deep postural calf soleus (SOL) and non-postural gastrocnemius (GAS) muscles of het−/− mice, i.e., mice with an autosomal recessive variant responsible for a vestibular disorder, in order to further elucidate a) the underlying mechanisms of disrupted vestibular system-derived modulation on skeletal muscle, and b) molecular signaling at respective neuromuscular synapses. Heterozygote mice muscles served as the control (CTR). An increase in Homer cross-linking capacity was present in the SOL muscle of het−/− mice as a compensatory mechanism for the altered vestibule system function. Indeed, in both fractions, different Homer immunoreactive bands were detectable, as were Homer monomers (~43–48 kDa), Homer dimers (~100 kDa), and several other Homer multimer bands (>150 kDA). The het−/− GAS particulate fraction showed no Homer dimers vs. SOL. The het−/− SOL soluble fraction showed a twofold increase (+117%, p ≤ 0.0004) in Homer dimers and multimers. Homer monomers were completely absent from the SOL independent of the animals studied, suggesting muscle-specific changes in Homer monomer vs. dimer expression in the postural SOL vs. the non-postural GAS muscles. A morphological assessment showed an increase (+14%, p ≤ 0.0001) in slow/type-I myofiber cross-sectional area in the SOL of het−/− vs. CTR mice. Homer subcellular immuno-localization at the neuromuscular junction (NMJ) showed an altered expression in the SOL of het−/−mice, whereas only not-significant changes were found for all Homer isoforms, as judged by RT-qPCR analysis. Thus, muscle-specific changes, myofiber properties, and neuromuscular signaling mechanisms share causal relationships, as highlighted by the variable subcellular Homer isoform expression at the instable NMJs of vestibular lesioned het−/− mice.

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