ALS and FTLD associated FUS in zebrafish investigating disease mechanisms in vivo : investigating disease mechanisms in vivo

Amyotrophic lateral sclerosis (ALS) and Frontotemporal lobar degeneration (FTLD) are neurodegenerative diseases, characterized by selective and progressive loss of neurons. Several gene mutations were found to co-segregate with the diseases. Mutations in the FUS gene were found to cause about 5% of all inherited forms of ALS and 1% of sporadic cases with no family history. Moreover, FUS positive inclusions in the cytosol of neurons and glial cells are another hallmark of ALS cases with FUS mutations besides the specific degeneration of motor neurons. Additionally, FUS positive inclusion were also found in a subset of FTLD cases, subsequently termed FTLD-FUS. However, exact molecular pathomechanisms leading to insoluble FUS inclusions and death of neurons are elusive. To clarify the physiological function of FUS and to test whether loss of FUS is necessary and sufficient to elicit ALS and/or FTLD related pathology, I studied FUS loss of function consequences in an in vivo approach using the zebrafish as a small vertebrate model. Additionally, ZFN mediated genomic editing the endogenous zebrafish fus locus in a way that resembeled an ALS patients mutation allowed to recapitulate pathomechanisms on molecular and cellular levels in vivo, devoid of unspecific toxic side effects often generated by transgenic overexpression. Interestingly, complete loss of function mutants were not identified with the ZFN set used in this study, reflecting putative crucial functions of zebrafish fus during germ cell development, whereas embryonic depletion of fus via knockdown has no obvious phenotypic consequences. However, I generated a zebrafish model carrying an ALS patient like mutation, the Fus^mde1500 premature stop allele, resulting in a C-terminally truncated Fus protein lacking the entire nuclear localization signal (NLS) and parts of the arginine rich (RGG3) domain. Strikingly, the Fus^mde1500 mutant protein recapitulates some features of the pathologic FUS protein in ALS and FTLD patients including a tendency to become insoluble and partial cytosolic redistribution upon transgenic expression in zebrafish and primary cortical neurons. Remarkably, Fus^mde1500 mutant zebrafish exhibit no obvious phenotypes, indicating that pathogenicity of the Fus^mde1500 mutant protein is not sufficient to elicit ALS/FTLD reminiscent symptoms and pathology in zebrafish. Thus, besides the Fus^mde1500 mutation additional challenges such as cellular and/or environmental stress are necessary to induce pathogenesis in zebrafish. Taken together, I generated Fus^mde1500 mutant zebrafish reflecting a biochemical and cell biological model suitable to analyze influences of aging and other risk factors on pathogenesis of FUSopathies in a preconditioned whole organisms approach.