Motor transmission defects with sex differences in a new mouse model of mild spinal muscular atrophy

Abstract

Background: Mouse models of mild spinal muscular atrophy (SMA) have been extremely challenging to generate. This paucity of model systems has limited our understanding of pathophysiological events in milder forms of the disease and of the effect of SMN depletion during aging. Methods: A mild mouse model of SMA, termed Smn 2B/À ;SMN2 +/À , was generated by crossing Smn À/À ;SMN2 and Smn 2B/2B mice. This new model was characterized using behavioral testing, histology, western blot, muscle-nerve electrophysiology as well as ultrasonography to study classical SMA features and extra-neuronal involvement. Findings: Smn 2B/À ;SMN2 +/À mice have normal survival, mild but sustained motor weakness, denervation and neuronal/neuromuscular junction (NMJ) transmission defects, and neurogenic muscle atrophy that are more prominent in male mice. Increased centrally located nuclei, intrinsic contractile and relaxation muscle defects were also identified in both female and male mice, with some male predominance. There was an absence of extra-neuronal pathology. Interpretation: The Smn 2B/À ;SMN2 +/À mouse provides a model of mild SMA, displaying some hallmark features including reduced weight, sustained motor weakness, electrophysiological transmission deficit, NMJ defects, and muscle atrophy. Early and prominent increase central nucleation and intrinsic electrophysiological deficits demonstrate the potential role played by muscle in SMA disease. The use of this model will allow for the understanding of the most susceptible pathogenic molecular changes in motor neurons and muscles, investigation of the effects of SMN depletion in aging, sex differences and most importantly will provide guidance for the currently aging SMA patients treated with the recently approved genetic therapies.

Metabolic Dysfunction in Spinal Muscular Atrophy

“…Nevertheless, it is most likely that pressures from other metabolic organs such as the pancreas, skeletal muscle, adipose tissue, and GI tract contribute to the pathogenesis of NAFLD in SMA (see Section 7 on “Metabolic crosstalk and proposed mechanisms”). Interestingly, a newly described very mild model of SMA (similar to SMA type IV) called the Smn 2B/− ;SMN2 +/− mouse, does not show fatty liver phenotype throughout its lifespan [ 101 ]. This emphasizes that reaching a certain threshold of SMN can reverse the fatty liver phenotype.…”

Survival motor neuron protein deficiency alters microglia reactivity

“…Behavioral testing was performed as previously described in (Deguise et al, 2020). Blinding was not possible given the obvious emergence of the SMA phenotype.…”

Central and peripheral delivery of AAV9-SMN target different pathomechanisms in a mouse model of spinal muscular atrophy

“…Righting reflex test (Treat-NMD SOP D_M.2.2.002, treat-nmd.org) evaluated overall body strength, pen test (Treat-NMD SOP SMA_M.2.1.001) assessed motor balance and coordination, and mesh grip test (Treat-NMD SOP SMA_M.2.1.002) evaluated limb strength. Motor function tests were performed as reported previously 48 . Briefly, righting reflex test was performed from P7 to P19.…”

“…Pancreata were fixed in 4% paraformaldehyde (PFA) for 48 h at 4ºC and then transferred to 70% ethanol at 4ºC until processing. Lumbar spinal cord was fixed in 4% PFA overnight at 4ºC then prepared for cryosectioning as previously described 48 . The abdominal musculature was dissected and fixed in 4% PFA and the transversus abdominis (TVA) was separated from the abdominal musculature as per 49 .…”

“…Lumbar SC was prepared for choline acetyltransferase (ChAT) staining of motor neurons as previously desribed 48 . The number of ChAT positive motor neurons per ventral horn was recorded for ten different sections per animal, each separated by at least 100 µm to avoid counting the same motor neuron twice.…”