TDP-43 is an RNA-binding protein that forms toxic aggregates in the neuronal cytoplasm that are commonly detected in multiple neurodegenerative diseases. Mislocalization of TDP-43 to the cytoplasm is thought to disrupt RNA processing, which may also contribute to its pathological effects. Loss of nuclear TDP-43 causes alternative splicing of the gene STMN2, which encodes stathmin-2, a key protein that regulates axon growth and regeneration; this splicing leads to a nonfunctional truncated variant. However, it was unclear how TDP-43 directly alters STMN2 splicing.
Baughn et al. used iCLIP (individual-nucleotide crosslinking and immunoprecipitation) to reveal that TDP-43 bound to a GU-rich motif in the first intron of STMN2 precursor mRNA. Replacement of the GU-rich motif with an MS2 aptamer resulted in reduced expression of full-length STMN2 mRNA and increased expression of the truncated STMN2 mRNA. The authors proposed that TDP-43 uses a direct steric blocking mechanism to suppress recruitment of splicing factors to a cryptic splice site. On the basis of this mechanism, they designed a CRISPR effector (dCasRx) and repair antisense oligonucleotides that targeted the proximity of the TDP-43-binding site to restore the expression of full-length stathmin-2 in TDP-43-mutant cells. Administration of the repair antisense oligonucleotides restored axonal growth and regeneration ability in TDP-43-deficient motor neurons derived from pluripotent stem cells, as well as in humanized mouse models. This study provides a potential therapeutic strategy for neurodegenerative diseases associated with TDP-43 proteinopathies.
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