MN dysfunction onset correlates with proprioceptive synaptic impairment in SMA
SMA MNs exhibit selective vulnerability depending on the muscles they innervate, with proximal muscles more affected than distal muscles in mice14 and humans15. We previously reported that SMA MNs in the first lumbar segment (L1) exhibit an increase in input resistance early in the disease13. Whether vulnerable SMN-deficient MNs become dysfunctional as a result of synaptic impairment or due to MN-autonomous mechanisms is unknown. To study vulnerable MNs, we focused on those that innervate the iliopsoas (IL) and quadratus lumborum (QL) muscles13,14. These muscles are involved in posture and locomotion15 and in the righting reflex, which is impaired in SMA mouse models13. The IL/QL MNs reside within the L1 to L3 spinal segments16. Tracing experiments from muscle together with ventral root L2 dye fill13 indicated that most IL/QL MNs lie laterally within the L2 spinal segment (Supplementary Fig. 1).
The excitability of a neuron is defined by its ability to generate action potentials in response to injected currents or synaptic inputs and is regulated by its intrinsic membrane properties. To study the intrinsic passive and active membrane properties and monosynaptic sensory-induced synaptic potentials of L2 MNs, we made whole-cell current clamp recordings in an intact mouse spinal cord ex vivo preparation13 at postnatal day (P) 2 (Supplementary Fig. 2a). Analysis of the intrinsic membrane properties of SMA MNs revealed two populations (Supplementary Fig. 2b). 57% were similar to wild-type (WT) MNs (“SMA-unaffected”), while 43% exhibited signs of dysfunction (“SMA-affected”), evidenced by increased input resistance and time constant and reduced rheobase (Fig. 1a–e). To investigate whether resistant SMA MNs were also affected, we studied L5 lateral MNs, which innervate the gastrocnemius and tibialis anterior, distal hindlimb muscles, at P4. We found no difference in the intrinsic membrane properties of WT and SMA L5 MNs even at this later stage of the disease (Supplementary Fig. 2c–e). Thus, ubiquitous SMN deficiency in MNs does not cause dysfunction in all spinal MNs, further highlighting the selective vulnerability of specific MN pools.