Exploring the roles of splicing factor, SFPQ, in neuronal development and neurodegeneration

Abstract

The structure and function of a cell is heavily dictated by the complement of proteins it expresses. Neurons display particularly sophisticated morphologies, with functionally distinct cellular compartments residing huge distances apart. One such compartment, the synapse, experiences rapid protein turnover during development, presenting a unique challenge for neurons: ensuring efficient protein dynamics far from the nucleus. Increasingly, local RNA regulation and local translation are implicated in coping with this demand. An army of RNA-binding proteins (RBPs) including splicing factor, SFPQ, regulate RNA processing events to ensure proteins are produced correctly in form, space and time. In addition to their traditional nuclear expression, many splicing factors, including SFPQ, have recently been observed in the axons and dendrites of some neuron sub-types. Compared to their nuclear functions, cytoplasmic roles remain enigmatic. Recent findings suggest that cytoplasmic SFPQ is vitally important for motor neuron development, and neurodegeneration (Thomas-Jinu et al., 2017). Indeed, sfpq null zebrafish embryos exhibit failed motor axon extension and no motility, however, these phenotypes are rescued upon expression of an exclusively cytoplasmic SFPQ variant. Nuclear depletion and cytoplasmic accumulation of SFPQ are ALS hallmarks (Luisier et al., 2018). Intriguingly, specific missense mutations in sfpq, identified in ALS patients, disrupt the normal axonal localisation and function of the protein (Thomas-Jinu et al., 2017). Here, I show that sfpq null motor neurons are able to extend axons in a wild-type background. Appearing a day later than those from normal motor neurons, they fail to branch or form synapses with muscle fibers. Some show classical signs of degeneration. These observations identify specific processes requiring SFPQ cell autonomously. They also show that axons, although morphologically abnormal, are capable of extension in absence of SFPQ, providing that the environmental conditions are permissive/instructive. Embarking upon a molecular exploration, RNAseq was performed on neurites isolated from primary cultures of sfpq sibling and null neurons, and transcriptomes were compared. This identified key mature transcripts whose correct neurite localisation and/or regulation require SFPQ. 5 Unexpected patterns of intron retention were also observed, with a collection of intronretaining transcripts enriched in sibling neurites, and sfpq null neurons exhibiting neuritespecific increases in intron retention. Together, these results support the notion that SFPQ underpins local RNA regulation in neurites, necessary for their normal development & homeostasis.

Date of Award	1 Feb 2021
Original language	English
Awarding Institution	King's College London
Supervisor	Corinne Houart (Supervisor)