Dysregulated balance of D- and L-amino acids modulating glutamatergic neurotransmission in severe spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disorder caused by reduced expression of the survival motor neuron (SMN) protein. In addition to motor neuron survival, SMN deficiency affects the integrity and function of afferent synapses that provide glutamatergic excitatory drive essential for motor neuron firing and muscle contraction. However, it is unknown whether deficits in the metabolism of excitatory amino acids and their precursors contribute to neuronal dysfunction in SMA. To address this issue, we measured the levels of the main neuroactive D- and L-amino acids acting on glutamatergic receptors in the central nervous system of SMN∆7 mice as well as the cerebrospinal fluid (CSF) of SMA patients of varying severity before and after treatment with the SMN-inducing drug Nusinersen. Our findings reveal that SMN deficiency is associated with disruption of glutamate and serine metabolism in the CSF of severe SMA patients, including decreased concentration of L-glutamate, which is partially corrected by Nusinersen therapy. Moreover, we identify dysregulated L-glutamine/L-glutamate ratio as a shared neurochemical signature of altered glutamatergic synapse metabolism that implicates neuron-astrocyte dysfunction in both severe SMA patients and mouse models. Lastly, consistent with hypo-glutamatergic neurotransmission in SMA, we show that daily supplementation with the NMDA receptor co-agonist D-serine improves neurological deficits in SMN∆7 mice. Altogether, these findings provide direct evidence for central dysregulation of D- and L-amino acid metabolism linked to glutamatergic neurotransmission in severe SMA and have potential implications for treating this neurological disorder.