Senataxin is encoded by the SETX gene and is mainly involved in two different
neurodegenerative diseases, the dominant juvenile form of
amyotrophic lateral sclerosis type 4 and a recessive form of
ataxia with oculomotor apraxia type 2. Based on
protein homology, senataxin is predicted to be a putative
DNA/
RNA helicase, while senataxin interactors from patients' lymphoblast cell lines suggest a possible involvement of the
protein in different aspects of
RNA metabolism. Except for an increased sensitivity to oxidative
DNA damaging agents shown by some
ataxia with neuropathy patients' cell lines, no data are available about possible functional consequences of dominant SETX mutations and no studies address the function of senataxin in neurons. To start elucidating the physiological role of senataxin in neurons and how disease-causing mutations in this
protein lead to neurodegeneration, we analysed the effect of senataxin on neuronal differentiation in primary hippocampal neurons and
retinoic acid-treated P19 cells by modulating the expression levels of wild-type senataxin and three different dominant mutant forms of the
protein. Wild-type senataxin overexpression was required and sufficient to trigger neuritogenesis and protect cells from apoptosis during differentiation. These actions were reversed by silencing of senataxin. In contrast, overexpression of the dominant mutant forms did not affect the regular differentiation process in primary hippocampal neurons. Analysis of the cellular pathways leading to neuritogenesis and cytoprotection revealed a role of senataxin in modulating the expression levels and signalling activity of
fibroblast growth factor 8. Silencing of senataxin reduced, while overexpression enhanced,
fibroblast growth factor 8 expression levels and the phosphorylation of related target
kinases and effector
proteins. The effects of senataxin overexpression were prevented when
fibroblast growth factor 8 signalling was inhibited, while exogenous
fibroblast growth factor 8 reversed the effects of senataxin silencing. Overall, these results reveal a key role of senataxin in neuronal differentiation through the
fibroblast growth factor 8 signalling and provide initial molecular bases to explain the neurodegeneration associated with loss-of-function mutations in senataxin found in recessive
ataxia. The lack of effect on neuritogenesis observed with the overexpression of the dominant mutant forms of senataxin apparently excludes a dominant negative effect of these mutants while favouring haploinsufficiency as the pathogenic mechanism implicated in the
amyotrophic lateral sclerosis 4-related degenerative condition. Alternatively, a different
protein function, other than the one involved in neuritogenesis, may be implicated in these dominant degenerative processes.