Short TDP-43 Isoforms as Therapeutic Targets and Biomarkers for Amyotrophic Lateral Sclerosis

The need to identify new therapies and reliable biomarkers for amyotrophic lateral sclerosis (ALS) is urgent. There are currently no effective strategies to prevent, slow, or stop this disease. This may be because most ALS patients lack a specific mutation that can be identified and targeted, and because of the large variability in age of onset, site of onset, disease duration, and disease symptoms. These factors, along with the absence of ALS biomarkers, make it challenging to diagnose ALS in the clinic, predict outcomes, and determine if newly developed therapies are acting as intended. This proposal focuses on a key event observed in almost all ALS patients--the abnormal localization and aggregation of transactive response DNA binding protein 43 (TDP-43), a pivotal protein required for normal RNA processing. Because of the widespread nature of TDP-43 mislocalization and aggregation, any advances made in the course of this work may be relevant to the large majority (>95%) of ALS patients, rather than subsets of patients carrying inherited mutations. While the exact mechanism responsible for TDP-43 mislocalization/aggregation in ALS remains unknown, we recently uncovered a potential explanation in investigating the phenomenon of neuronal hyperexcitability, which was initially described in ALS patients over 20 years ago. We observed an increase in unusual, shortened (s)TDP-43 proteins in hyperexcitable neurons, mirroring the changes in TDP-43 mislocalization and aggregation found in ALS patients. Additional evidence suggests that sTDP-43 proteins are highly expressed in motor neurons, the cell type most heavily affected in ALS, and that sTDP-43 accumulation contributes to cell death. As such, we believe that therapeutic strategies capable of selectively reducing sTDP-43 will prevent the characteristic TDP-43 mislocalization and aggregation witnessed in ALS, promote neuronal survival, and block disease progression. In this application, we propose to assess the biological significance and therapeutic potential of sTDP-43 proteins in cells derived from ALS patients and in mice through targeted gene therapy. We also propose to determine the sensitivity and specificity of methods to detect sTDP-43 proteins in biological fluids, and assess sTDP-43 levels as a promising biomarker for ALS. The seamless integration of the proposed studies, led by an interdisciplinary team of investigators at the Mayo Clinic, University of Washington, and University of Michigan, will ultimately enable therapeutic innovations that would otherwise be impossible.