ALS implicated Protein protein TDP-43 sustains levels of STMN2, a mediator of motor neuron growth and repair
Joseph R. Klim1, Luis A. Williams1, Francesco Limone1, Irune Guerra San Jaun1, Brandi N. Davis-Dusenbury1, Daniel A. Mordes1,5, Aaron Burberry1, Rob Moccia1, Seth Cassel1, Brian J. Wainger6, and Kevin Eggan1,2,3,4
1Department of Stem Cell and Regenerative Biology, Harvard University, Cambridge, Massachusetts 02138, USA; 2Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, USA; 3Stanley Center for Psychiatric Research, Broad Institute of MIT and Harvard, Cambridge, Massachusetts 02142, USA; 4Harvard Stem Cell Institute, Cambridge, Massachusetts 02138, USA; 5Department of Pathology, Massachusetts General Hospital, Boston, MA 02114, USA; 6FM Kirby Neurobiology Center, Boston Children's Hospital and Department of Neurobiology, Harvard Medical School, Boston, MA 02115, USA
Aggregation of the RNA-binding protein TDP-43 in neuronal cytoplasm is a diagnostic pathology found in most patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Furthermore, mutations in the gene TARDP, which encodes TDP-43, cause familial ALS and FTD. Although it has been proposed that these genetic and pathological perturbations to the normal function of TDP-43 in regulating RNA splicing or transport contribute to neural degeneration, the identity of the RNAs regulated by TDP-43 in human neurons remains poorly understood. Here, we used RNA sequencing to identify transcripts whose abundances in purified human motor neurons (hMNs) were sensitive to a reduction in TDP-43 levels. We found that transcript levels of Stathmin 2 (STMN2), a regulator of microtubule stability and neurite extension normally highly expressed in motor neurons, were reproducibly and sharply reduced. This reduction was also the case in hMNs differentiated from patient-derived induced pluripotent stem cell lines with pathogenic TDP-43 mutations. Notably, TDP-43 associates directly with STMN2 mRNA, and under conditions of proteasome inhibition, TDP-43 exits the nucleus concomitant with a decrease in STMN2 levels. Taken together, our disease modeling results link protein homeostasis, RNA metabolism, and cytoskeletal dynamics downstream of TDP-43. Although hMNs generated in vitro share key molecular and functional properties with bona fide hMNs, the in vivo validation of discoveries from stem cell-based models of ALS is a critical test of their relevance to disease mechanisms and therapeutic strategies. To this end, we used human adult spinal cord tissues to provide in vivo evidence corroborating the finding that TDP-43 dysregulation alters the expression of STMN2. We further demonstrate the ability to leverage this information to develop a potential ALS biomarker assay. In conclusion, findings from human stem cell-based models can be used to make discoveries about underlying pathomechanisms of ALS and can lead to the illumination of potential therapeutic targets and novel biomarkers.
J. R. K. is supported by the Tom Kirchhoff Family Postdoctoral Fellowship from Project ALS.