Titolo della tesi: Characterization of the molecular mechanisms underlying neuromuscular junction defects and cell death in FUS and sporadic ALS
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by motor neurons (MNs) death in the spinal cord and brain, leading to the loss of skeletal muscle mass. Previous data collected in our lab highlighted an interesting aberrant crosstalk between the ALS-linked FUS protein and the RNA-binding protein (RBP) HuD, leading to upregulation of HuD levels. As a consequence, some HuD targets such as the axonal proteins GAP43 and NRN1 are upregulated in FUS mutant MNs. Moreover, we have found NRN1-dependent aberrant increase in neurite branching and axonal outgrowth in these cells. Based on these findings, we aimed to assess whether such altered molecular circuitry, besides neurite alteration, can lead to neuromuscular junction (NMJ) disruption. To recapitulate the NMJ circuit in vitro we took advantage of human induced pluripotent stem cells (hiPSCs) to obtain a neural-muscle model system by 2D co-cultures. We found that FUS-mutant MNs were less able to establish NMJs with FUSWT muscle despite the presence of a significant increase of the neurite network. Moreover, mutant MNs co-cultures show degeneration of both cellular components suggesting that this extensive but unproductive neurite sprouting, could have a detrimental effect on neuromuscular endplate formation, leading to MNs and muscle degeneration. We observed similar phenotypes when HuD overexpressing MNs in a FUSWT genetic background were co-culture with FUSWT muscle, with implications for sporadic ALS. Interestingly, both apoptosis and NMJ impairment were rescued following HuD dampening with siRNA in FUS-mutant MNs, confirming HuD primary role in mutant aberrant phenotype. Moreover, recent studies revealed an increase of HuD expression in sporadic ALS patients devoid of mutations in known ALS-linked genes. Notably, FUSWT MNs exposed to oxidative stress showed upregulation of HuD levels, suggesting that a stressful condition, similar to that observed in ALS sporadic patients, is sufficient to trigger HuD dysregulation. Moreover, further investigations revealed that the mechanism underlying HuD upregulation levels is led by transcriptional upregulation upon stress. Finally, we found that HuD and FUS can synergistically induce GAP43 increased expression by direct mRNA binding. Taken together, these observations suggest that a gain of function of HuD, due to mutant FUS or oxidative stress, might lead to altered NMJ and neurodegeneration by apoptosis in ALS.