Mechanisms underlying TDP-43 dependent structural deficits in ALS muscles

ABSTRACT Amyotrophic lateral sclerosis (ALS) is fatal neurodegenerative disease that affects 2/100,000 individuals worldwide. The disease is caused by motor neuron dysfunction and death, which in turn causes paralysis within 2-5 years of diagnosis. A pathological hallmark of ALS is the presence of cytoplasmic aggregates containing the DNA/RNA binding protein TDP-43 in 97% of ALS cases. Furthermore, TDP-43 is associated with cytoplasmic aggregates across a broad range of neuromuscular and neurodegenerative disorders including ALS, inclusion body myositis (IBM) and fronto-temporal dementia (FTD). While a plethora of studies have focused on TDP-43 pathophysiology in neurons and glia, much remains unknown about TDP-43 proteinopathy in muscle, the cell type most visibly affected by ALS as it undergoes rapid atrophy. TDP-43 loss of function has been shown to cause muscle weakness and degeneration in mice, zebrafish and flies. Recent studies have shown that TDP-43 associates with myogranules and plays a role in muscle formation, likely by regulating cytoplasmic mRNAs, including several that encode sarcomeric proteins such as titin and myosin heavy chain. In addition to their physiological role in muscle differentiation and regeneration, TDP-43 containing myogranules are also present in mouse models of IBM, consistent with also having a role in disease pathomechanisms. Furthermore, these findings suggest that TDP-43 containing myogranules provide an opportunity to uncover differences between protective and toxic aggregates.To address the gap in knowledge regarding TDP-43 pathophysiology in muscles, we set out to study TDP-43 dependent, muscle specific alteration in newly synthesized proteins. Preliminary results using Non-Canonical Aminoacid Tagging (NCAT) in vivo, in the context of Drosophila models of TDP-43 proteinopathy show that TDP-43 overexpression in muscles causes a marked reduction in newly synthesized proteins involved in translation itself, myofibril assembly, mitochondria and nuclear pores among others. We hypothesize that similar to its neuronal role, TDP-43 alters the translation of specific mRNA targets in muscles, and this in turn may impact the morphology and/or function of muscles, and contribute to the muscle atrophy observed in patients. We will test this hypothesis by first identifying TDP-43 translation targets in vivo, in Drosophila muscles using both overexpression and loss of function approaches (Aim 1). Next, we will validate candidate targets in fly muscles and patient tissues (Aim 2). These studies are poised to identify novel, muscle specific therapeutic targets of TDP-43 proteinopathies and related neuromuscular disorders.