An N-terminal peptide of Tar DNA binding Protein 43 lacking nuclear localization signal translocates to the nucleus of GC-1 spermatogonial cells

TAR DNA-binding protein of 43 kDa (TDP-43) is an RNA/DNA binding protein expressed in the brain and the testis. Mutations in TDP-43 lead to mislocalization and cytoplasmic aggregation of this protein causing neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal dementia. TDP-43 has also been implicated in maintaining spermatogenesis. While homodimerization of TDP-43 is critical for its physiological functions, higher-order aggregation of this protein impairs its functions. This study was aimed to map the critical amino acids of the N-terminus of this protein in mediating its homodimerization. We generated deletion constructs of Tdp-43 containing NRRM1 domain alone (TDP-43∆3-183) and N-terminal peptide of TDP-43 which lacks the nuclear localization signal (NLS) (TDP-43∆1-50) with fluorescent reporters having non-overlapping emission properties. These constructs were co-transfected into a mouse spermatogonial cell line to examine their dimerization and nuclear translocation capabilities in vitro. We found that TDP-43∆3-183 alone was not capable of homodimerization. On the other hand, TDP-43∆1-50 when co-transfected into GC1-spg cells along with full length TDP-43 translocated to the nucleus oligomerized with the latter and translocated to the nucleus, indicating the importance of amino acids 1-50 of TDP-43 in dimerization. The N-terminal segment of TDP-43 spanning amino acids 1-50 is responsible for dimerization, while that spanning amino acids 51-183 directs it to the nucleus.The physiological and pathological implications of this finding need to be examined.