LAUSR

© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Nuclear loss and cytoplasmic aggregation of TDP-43 is the most common shared pathological feature of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), a motor neuron disease also known as Lou Gehrig’s disease [1]. TDP-43 pathology is found in 97% of ALS cases, approximately 45% of FTD cases, and in some other neurodegenerative diseases such as Alzheimer’s disease [2]. However, the direct mechanisms by which TDP-43 dysregulation contributes to neurodegeneration remain largely elusive. TDP-43 is an RNA-binding protein with multiple roles in RNA metabolism, including transcription, splicing, transport, and the localization and stability of its target mRNAs, as well as in microRNA biogenesis [3]. TDP-43 has thousands of RNA targets, but it is largely unknown which targets, if any, directly contribute to disease. Nor is it known whether any of them can be either manipulated as therapeutic targets to influence disease progression or, if not, could serve as biomarkers of TDP-43 pathology. A key nuclear function of TDP-43 is to regulate alternative splicing, most notably to suppress the inclusion of cryptic exons, intronic sequences that are normally not spliced into mature mRNAs [4]. This dysregulation may result in the inclusion of a new stretch of amino acids in the protein, the production of a truncated protein, or a loss of the full-length protein through nonsense-mediated decay (NMD) of the mRNAs in which premature termination codons (PTCs) are introduced by cryptic exons (Fig. 1). RNA-seq analysis of human neurons with reduced TDP-43 expression revealed that one of the most downregulated genes is stathmin 2 (STMN2), encoding an axonal growth-associated microtubule-stabilizing protein expressed only in neurons [5, 6]. This downregulation is specific to human STMN2 and is not conserved in rodents [5, 6]. The extent to which the loss of full-length STMN2 function contributes to ALS/FTD pathogenesis and clinical phenotypes is still not known. Nonetheless, restoring expression of full-length STMN2 represents a novel potential therapeutic approach. In studies to identify other key functional targets of TDP-43 that may directly contribute to disease pathogenesis, Ma et al. [7] and Brown et al. [8] identified novel cryptic exons regulated by TDP-43, including one in UNC13A (Fig. 1). Ma et al. [7] reanalyzed a published RNA-seq dataset for abnormal splicing events. This dataset was obtained after separating TDP-43-positive and TDP-43-negative neuronal nuclei in ALS/FTD patient postmortem brains to identify transcriptomic changes caused by nuclear TDP-43 depletion. Similarly, Brown et al. did an RNA-seq analysis of human neurons derived from induced pluripotent stem cells (iPSCs) after depletion of TDP-43 by CRISPR inhibition [8]. Among dozens of novel cryptic exons, both groups identified UNC13A as one of the most robust mis-spliced genes. Brown et al. also detected a mis-splicing event in another UNC13 family member, UNC13B [8]. To confirm their findings, Ma et al. and Brown et al. used human neuronal cell lines and iPSC-derived neurons to reduce TDP-43 levels. They found that TDP-43 depletion caused inclusion of the cryptic exon in the UNC13A transcript, confirming a role for TDP-43 in suppressing cryptic exons. This effect seems to be direct, as TDP-43 has a binding site within the intron containing the cryptic exon in UNC13A mRNA [7, 8]. Moreover, Open Access