Senataxin modulates neurite growth through fibroblast growth factor 8 signalling

Abstract

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.

Human Senataxin Modulates Structural Plasticity of the Neuromuscular Junction in <b><i>Drosophila</i></b> through a Neuronally Conserved TGFβ Signalling Pathway

“…Recent studies in primary hippocampal neurons also support the observation that SETX localizes to the nucleus as well as cytoplasm, consistent with its dual role as helicase and a regulator of RNA metabolism [6,11]. Interestingly, expression of wild-type (WT) but not the mutant forms of SETX in neuronal cells causes axonal overgrowth.…”

“…Individuals with a mutation in SETX exhibit selective motoneuron death and show clinical symptoms such as distal muscle weakness, spasticity, motoneuron damage and elevated α-fetoprotein levels, suggesting a common mechanism of selective cell death [4]. Although the key mutations implicated in AOA2 and ALS4 have been identified, their contribution towards neuropathy still remains elusive [3,5,6]. …”

“…Interestingly, expression of wild-type (WT) but not the mutant forms of SETX in neuronal cells causes axonal overgrowth. SETX-mediated neurite growth depends on fibroblast growth factor (FGF)-8 signalling [6]. The neurite overgrowth caused by SETX expression in neurons was found to be mediated through Akt/GSK3β and ERK1/2 MAP kinase pathways and had a positive influence on neuronal elongation, branching and plasticity [6].…”

“…SETX-mediated neurite growth depends on fibroblast growth factor (FGF)-8 signalling [6]. The neurite overgrowth caused by SETX expression in neurons was found to be mediated through Akt/GSK3β and ERK1/2 MAP kinase pathways and had a positive influence on neuronal elongation, branching and plasticity [6]. While several mutations in the SETX gene have been identified that contribute to the motor neuron pathogenesis, the exact cellular pathways and the molecular mechanisms remain largely unknown.…”

“…While several mutations in the SETX gene have been identified that contribute to the motor neuron pathogenesis, the exact cellular pathways and the molecular mechanisms remain largely unknown. Notably, the expression of dominant mutants of SETX did not affect the differentiation process in the primary hippocampal neurons suggesting a pathogenic model of haploinsufficiency [6]. Since SETX primarily affects RNA processing under diverse cellular contexts, it is likely that several signalling and/or secondary pathways, such as endosomal trafficking, may be affected leading to neuronal death as in the case of other neurodegenerative disorders [12].…”