{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,17]],"date-time":"2026-08-17T12:24:38Z","timestamp":1786969478973,"version":"build-2736575974"},"reference-count":25,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2026,8,17]],"date-time":"2026-08-17T00:00:00Z","timestamp":1786924800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"ALS Finding a Cure Foundation","award":["2024-BSA"],"award-info":[{"award-number":["2024-BSA"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Biosensors"],"abstract":"<jats:p>TDP-43 dysfunction is a defining feature of amyotrophic lateral sclerosis (ALS), yet no biofluid biomarker directly measures its functional activity. We developed a serum-based homogeneous time-resolved FRET (hTR-FRET) assay that quantifies TDP-43 RNA binding activity using synthetic UU-rich RNA probes. We analyzed 1080 serum samples from controls, sporadic ALS, and genetic subgroups (C9orf72, SOD1) across multiple biorepositories. Cross-sectionally, TDP-43 functional activity was elevated in ALS (mean 390 a.u.) versus controls (302 a.u.), yielding AUC = 0.79. Genotype means were 392 a.u. (sporadic), 382 a.u. (C9orf72), and 323 a.u. (SOD1); a 366 a.u. threshold achieved 95% specificity against controls. Longitudinally, Target ALS showed a modest but significant inverse correlation between TDP-43 activity and ALSFRS-R, while other cohorts exhibited similar non-significant trends. Elevated signal in serum likely reflects increased extracellular release of probe-competent TDP-43 species during cell death and exosomal shedding, rather than restored intracellular nuclear splicing function. This assay provides a proof-of-concept platform for the direct functional measurement of probe-competent TDP-43 species in serum. While it demonstrates moderate group-level discrimination, individual diagnostic performance requires prospective validation. The assay may support exploratory applications in genotype stratification and progression monitoring in future clinical studies.<\/jats:p>","DOI":"10.3390\/bios16080446","type":"journal-article","created":{"date-parts":[[2026,8,17]],"date-time":"2026-08-17T11:00:05Z","timestamp":1786964405000},"page":"446","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Functional Activity of TDP-43: A Direct Biomarker for ALS"],"prefix":"10.3390","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-8382-1140","authenticated-orcid":false,"given":"Kirti Shila","family":"Sonkar","sequence":"first","affiliation":[{"name":"Nemdx, Auburndale, MA 02466, USA"},{"name":"Qbiotix Developments Ltd., Discovery Park, Sandwich, Kent CT13 9FE, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0007-2594-5800","authenticated-orcid":false,"given":"Vito","family":"Levi 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UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6737-7978","authenticated-orcid":false,"given":"Ellie","family":"Giles","sequence":"additional","affiliation":[{"name":"Nemdx, Auburndale, MA 02466, USA"},{"name":"Qbiotix Developments Ltd., Discovery Park, Sandwich, Kent CT13 9FE, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2263-2418","authenticated-orcid":false,"given":"Tyler","family":"Howell-Bray","sequence":"additional","affiliation":[{"name":"Nemdx, Auburndale, MA 02466, USA"},{"name":"Qbiotix Developments Ltd., Discovery Park, Sandwich, Kent CT13 9FE, UK"},{"name":"School of Environmental and Life Science, Faculty of Science and Engineering, University of Hull, Hull HU6 7RX, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-5258-6300","authenticated-orcid":false,"given":"Becky","family":"Fillingham","sequence":"additional","affiliation":[{"name":"Healey & AMG Centre for ALS, Mass General Brigham, Boston, MA 02114, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Merit E.","family":"Cudkowicz","sequence":"additional","affiliation":[{"name":"Healey & AMG Centre for ALS, Mass General Brigham, Boston, MA 02114, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0927-5855","authenticated-orcid":false,"given":"Avindra","family":"Nath","sequence":"additional","affiliation":[{"name":"National Institute of Neurological Disorders and Stroke (NINDS), NIH, Bethesda, MD 20892, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jeffrey D.","family":"Rothstein","sequence":"additional","affiliation":[{"name":"School of Medicine, Johns Hopkins University, Baltimore, MD 21287, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Robert","family":"Bowser","sequence":"additional","affiliation":[{"name":"Barrow Neurological Institute, Dignity Health, Phoenix, AZ 85013, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Barbara","family":"Borroni","sequence":"additional","affiliation":[{"name":"Department of Clinical and Experimental Sciences, University of Brescia, 25123 Brescia, Italy"},{"name":"IRCCS Centro Istituto Fatebenefratelli San Giovanni di Dio, 25125 Brescia, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0898-6081","authenticated-orcid":false,"given":"James D.","family":"Berry","sequence":"additional","affiliation":[{"name":"Healey & AMG Centre for ALS, Mass General Brigham, Boston, MA 02114, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ghazaleh","family":"Sadri-Vakili","sequence":"additional","affiliation":[{"name":"Healey & AMG Centre for ALS, Mass General Brigham, Boston, MA 02114, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1356-9074","authenticated-orcid":false,"given":"Emanuele","family":"Buratti","sequence":"additional","affiliation":[{"name":"Molecular Pathology Group, International Centre for Genetic Engineering and Biotechnology (ICGEB), 34149 Trieste, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ian P.","family":"Thrippleton","sequence":"additional","affiliation":[{"name":"Nemdx, Auburndale, MA 02466, USA"},{"name":"Qbiotix Developments Ltd., Discovery Park, Sandwich, Kent CT13 9FE, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,8,17]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"53","DOI":"10.1186\/s13024-025-00839-8","article-title":"Molecular mechanisms and consequences of TDP-43 phosphorylation in neurodegeneration","volume":"20","author":"Kellett","year":"2025","journal-title":"Mol. 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