Defects in Neuromuscular Transmission May Underlie Motor Dysfunction in Spinal and Bulbar Muscular Atrophy

Abstract

Spinal and bulbar muscular atrophy (SBMA) in men is an androgen-dependent neuromuscular disease caused by expanded CAG repeats in the androgen receptor (AR). Whether muscle or motor neuron dysfunction or both underlies motor impairment in SBMA is unknown. Muscles of SBMA mice show significant contractile dysfunction, implicating them as a likely source of motor dysfunction, but whether disease also impairs neuromuscular transmission is an open question. Thus, we examined synaptic function in three well-studied SBMA mouse models-the AR97Q, knock-in (KI), and myogenic 141 models-by recording in vitro miniature and evoked end-plate potentials (MEPPs and EPPs, respectively) intracellularly from adult muscle fibers. We found striking defects in neuromuscular transmission suggesting that toxic AR in SBMA impairs both presynaptic and postsynaptic mechanisms. Notably, SBMA causes neuromuscular synapses to become weak and muscles to become hyperexcitable in all three models. Presynaptic defects included deficits in quantal content, reduced size of the readily releasable pool, and impaired short-term facilitation. Postsynaptic defects included prolonged decay times for both MEPPs and EPPs, marked resistance to -conotoxin (a sodium channel blocker), and enhanced membrane excitability. Quantitative PCR revealed robust upregulation of mRNAs encoding neonatal isoforms of the AChR (␥-subunit) and the voltage-gated sodium channel (Na V 1.5) in diseased adult muscles of all three models, consistent with the observed slowing of synaptic potentials and resistance to -conotoxin. These findings suggest that muscles of SBMA patients regress to an immature state that impairs neuromuscular function.

Unveiling synapse pathology in spinal bulbar muscular atrophy by genome-wide transcriptome analysis of purified motor neurons derived from disease specific iPSCs

“…Recently, the skeletal muscle specific overexpression of IGF-1, or the silencing of mutant AR, was shown to rescue the phenotypes of SBMA model mice, suggesting the involvement of skeletal muscles in the neurodegeneration in SBMA [12,13,38]. Moreover, by analyzing NMJs in the SBMA model mice, pathological fragmentation of NMJs and abnormal synaptophysin distribution, as well as defects in the neuromuscular synaptic transmission, were demonstrated, which is consistent with the previously reported insufficient synaptic functions in SBMA [14,48]. Interestingly, our transcriptome analysis of iPSC-MNs revealed upregulation of synapse-related gene sets in the SBMA-MNs, including activation of intracellular signaling via calcium ions, formation of synaptic vesicles, and the release of neurotransmitters such as acetylcholine.…”

“…Interestingly, recent analyses have shown that phenotypes and MN pathology of transgenic mice models were unexpectedly rescued by muscle-specific silencing of mutant AR, suggesting the involvement of muscle pathology in motor neuronal degeneration [12,13]. In addition, impaired transmission of neuromuscular synapses was observed in SBMA mice models [14]. These results strongly indicate muscular pathology, which causes neurodegeneration in SBMA.…”

Deterioration of muscle force and contractile characteristics are early pathological events in spinal and bulbar muscular atrophy mice

“…Although motor neuron degeneration, occurs well after the first signs of muscle force deficits, it is important to acknowledge the possibility that motor neuron dysfunction may occur without degeneration (Katsuno et al, 2004) and in conjunction there may also be there also may be NMJ abnormalities (Xu et al, 2016). Late motor neuron degeneration or dysfunction may well be specific to the AR100 mice and the possibility exist that it may not reflect the scenario in SBMA patients.…”

“…There was no evidence of morphological denervation at the NMJ at 6 months of age in AR100 mice, the age where muscle force dysfunction first becomes evident. However, there may be functional denervation without morphological changes occurs at the NMJ (Poort et al, 2016;Xu et al, 2016). Although we have shown early dysfunction in motor neurons in AR100 mice, such as ER stress (Montague et al, 2014) and transcriptional dysregulation (Malik et al, 2019), we found that plasma levels of phosphorylated neurofilament heavy chain (pNfH), a marker of neuronal damage found to be altered in motor neuron disease such as ALS, was unchanged in SBMA patients and AR100 mice (Lombardi et al, 2019).…”

The effect of NAMPT deletion in projection neurons on the function and structure of neuromuscular junction (NMJ) in mice

“…Loss of NAMPT in projection neurons alters skeletal muscle contractile responses. Disabled synaptic transmission in the motor terminal at the NMJ has been suggested to affect muscle function 35,36 . To NAD + levels in semitendinosus and gastrocnemius muscles of control mice after NMN administration.…”