Elucidating the Role of DLC1 Isoform 1 in Human Motor Neuron Development and Spinal Muscular Atrophy

Abstract

Spinal muscular atrophy (SMA) is a motor neuron (MN) disease caused by loss of the ubiquitously expressed Survival Motor Neuron (SMN) spliceosome protein1, resulting in selective degeneration of spinal MNs but the mechanisms underlying the specific loss of MNs remain unknown. A previous report showed that Deleted in Liver Cancer 1 (DLC1) is the most down-regulated gene in MNs derived from a SMA patient2 but its roles in MN development and SMA pathogenesis remain to be elucidated. Here, we found a decreased expression level of DLC1 isoform 1 (DLC1-i1) in motor neurons differentiated from urine-derived induced pluripotent stem cells (iPSCs) from SMA patients compared to healthy individuals. Similarly, Dlc1-i1 expression was reduced in Smn1-/- mice lumber spinal cord compared to wild-type. Besides, there is a gradual increase in DLC1-i1 expression level as human embryonic stem cells/iPSCs differentiated into MN lineage and DLC1-i1 is highly expressed in soma and axonal granules. Knockdown (KD) of DLC1-i1 led to a reduction in MN formation, axonal outgrowth, synapse number, and cell survival, whereas overexpression of DLC1-i1 promoted MN differentiation with extensive axonal outgrowth. Importantly, SMN KD not only caused MN loss but also intron retention of DLC1-i1 pre-mRNA, resulting in downregulation of DLC1-i1 expression. Ongoing study is to perform gene therapy by injecting AAV9 carrying DLC1-i1 into intracerebroventricular of SMN-/- mice and evaluate the impact on locomotor ability and lifespan. Altogether, the results obtained so far indicate that MN-specific DLC1-i1 plays an important role in MN differentiation and deficiency of SMN causes selective loss of MN partly through disruption of DLC1-i1 splicing.