Reduced sensory synaptic excitation impairs motor neuron function via Kv2.1 in spinal muscular atrophy

Abstract

Behavioral deficits in neurodegenerative diseases are often attributed to the selective dysfunction of vulnerable neurons via cell-autonomous mechanisms. Although vulnerable neurons are embedded in neuronal circuits, the contribution of their synaptic partners to the disease process is largely unknown. Here, we show that in a mouse model of spinal muscular atrophy (SMA), a reduction in proprioceptive synaptic drive leads to motor neuron dysfunction and motor behavior impairments. In SMA mice or after the blockade of proprioceptive synaptic transmission we observed a decrease in the motor neuron firing which could be explained by the reduction in the expression of the potassium channel Kv2.1 at the surface of motor neurons. Increasing neuronal activity pharmacologically by chronic exposure in vivo led to a normalization of Kv2.1 expression and an improvement in motor function. Our results demonstrate a key role of excitatory synaptic drive in shaping the function of motor neurons during development and the contribution of its disruption to a neurodegenerative disease.

Converging Mechanisms of p53 Activation Drive Motor Neuron Degeneration in Spinal Muscular Atrophy

“…Our findings indicate that the overall limited behavioral benefit of preventing motor neuron death by p53 inhibition in SMA mice reflects the persistence of remaining severe functional deficits in the motor circuit and further underscores their key functional contribution to disease pathogenesis. Moreover, they reveal that reduction of the excitatory drive and loss of central synapses on motor neurons in SMA mice are independent of p53 activation and mechanistically uncoupled from the process of motor neuron death, in agreement with the non-cell autonomous nature of these defects we recently reported (Fletcher et al, 2017; Simon et al, 2016). …”

“…To do so, we employed selective restoration of SMN in motor neurons of SMA mice using Cre-dependent expression (Smn Res ) under the control of the choline acetyltransferase (ChAT) promoter as previously described (Lutz et al, 2011; Martinez et al, 2012; Fletcher et al, 2017). ChAT-driven, selective expression of SMN [SMA+SMN(ChAT Cre )] strongly reduced nuclear accumulation of p53 in motor neurons of SMA mice (Figure 2G–H) and was accompanied by rescue of motor neurons from degeneration (Figure 2H), consistent with previous studies (Fletcher et al, 2017; Martinez et al, 2012). Thus, cell autonomous activation of p53 signaling induced by SMN deficiency drives the degeneration of vulnerable motor neurons in SMA mice.…”

“…Experimental protocols for somatodendritic labeling of motor neurons (Mentis et al, 2005; Mentis et al, 2006) and for immunostaining of L1 and L5 segments of the spinal cord (Mentis et al, 2011) as well as of NMJs innervating the QL muscle (Fletcher et al, 2017) have been described before. ChAT+ motor neurons, VGluT1+ synapses and NMJs were counted off-line using the Leica LASAF software from z-stack images acquired with SP5 or SP8 Leica confocal microscopes as previously described (Fletcher et al, 2017; Mentis et al, 2011).…”

“…ChAT+ motor neurons, VGluT1+ synapses and NMJs were counted off-line using the Leica LASAF software from z-stack images acquired with SP5 or SP8 Leica confocal microscopes as previously described (Fletcher et al, 2017; Mentis et al, 2011). Further details are provided in Supplemental Experimental Procedures.…”

“…In contrast to neurodegenerative diseases such as Parkinson's, Huntington's and amyotrophic lateral sclerosis (ALS), in which both cell-autonomous and non-cell-autonomous pathways contribute to degeneration of vulnerable neurons (Boillee et al, 2006; Michel et al, 2016; Sambataro and Pennuto, 2012), the cell-autonomous origin of motor neuron death in spinal muscular atrophy (SMA) is well established (Fletcher et al, 2017; Gogliotti et al, 2012; Martinez et al, 2012; McGovern et al, 2015; Simon et al, 2016). Thus, SMA provides an ideal context to identify cellular pathways and key drivers of selective neurodegeneration.…”