Mechanisms of Protein Mislocalization in Neurodegenerative Disease

Fronto Temporal Lobar Degeneration (FTLD) and Amyotrophic Lateral Sclerosis (ALS) are two fatal and rapidly progressing neurodegenerative diseases. A unifying characteristic of these diseases is the mislocalization of an RNA-binding protein, TDP-43. In unstressed cells, TDP-43 is predominantly nuclear and constantly shuttling to the cytosol; in ALS/FTLD, TDP-43 is aggregated in the cytosol. Two lines of evidence suggest this shift is a cause, rather than an effect, of disease. First, point mutations in the C-terminus of TDP-43, which enhance its aggregation, are a rare cause of familial ALS. Second, animal models which replicate the disease-linked redistribution of TDP-43 in motor neurons demonstrate the progressive muscle weakness and loss of spinal cord mass seen in patients. However, little is known about the cellular insults that promote TDP-43 mislocalization. My graduate work makes two contributions to this understanding. First, I elucidated a major determinant of normal TDP-43 trafficking. TDP-43 localization is governed by the balance between nuclear import and nuclear export. While a model for TDP-43 nuclear export had been proposed, there was no direct experimental evidence supporting it. I have shown that the proposed model of TDP-43 nuclear export is incorrect; the putative nuclear export signal (NES) does not mediate nuclear export, and TDP-43 nuclear export is XPO1 independent. Additionally, my data suggest no discrete trafficking signal within TDP-43. Rather, I propose that TDP-43 nuclear export is primarily driven by diffusion through the nuclear pore. Second, I focused on an upstream event known to affect TDP-43 localization: progranulin secretion. One genetic cause of FTLD is a single loss-of-function mutation in GRN, which causes progranulin haploinsufficiency. For reasons that are still unclear, a lifetime of progranulin haploinsufficiency in a patient causes FTLD with TDP-43 mislocalization. I focused on how a subset of GRN mutations- the signal sequence mutations- prevent progranulin secretion. I found that the W7R and A9D mutations disrupt co-translational recruitment of the targeting factor SRP (Signal Recognition Particle). This triggers a quality control pathway called RAPP (Regulation of Aberrant Protein Production), which results in degradation of both mutant protein and mutant mRNA. Thus, RAPP mediates progranulin haploinsufficiency in these patients.