Location and function of TDP-43 in platelets: Alterations in neurodegenerative diseases and arising considerations for current plasma biobank protocols

The TAR DNA Binding Protein (TDP-43) has been implicated in the pathogenesis of human neurodegenerative diseases and exhibits hallmark neuropathology in amyotrophic lateral sclerosis (ALS). Here, we explore its tractability as a plasma biomarker of disease and describe its localization and possible functions in the cytosol of platelets. Novel TDP-43 immunoassays were developed on three different technical platforms and qualified for specificity, signal-noise ratio, detection range, variation, spike recovery and dilution linearity in human plasma samples. Fractionation studies revealed that >95% of plasma TDP-43 protein [RL1] was located within the platelet cytosol, together with numerous RNAs. Platelet-derived TDP-43 exhibits TDP-43 proteoforms detected in neurodegenerative diseases, TARDBP RNA splice variants and TDP-43 RNA targets found in the central nervous system (CNS). We propose that TDP-43 serves similar functional roles in platelets and synapses, suggesting that the study of platelet TDP-43 might provide a window into TDP-43 proteinopathies within the CNS. The restricted compartmentalization of plasma TDP-43 in platelets provides a highly concentrated substrate for further biochemical analyses. Moreover, our results suggest that current plasma biobanking protocols are subject to considerable heterogeneity in platelet recovery and measurements of TDP-43 in plasma.