{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,2]],"date-time":"2026-07-02T06:22:00Z","timestamp":1782973320307,"version":"3.54.5"},"reference-count":47,"publisher":"MDPI AG","issue":"15","license":[{"start":{"date-parts":[[2023,7,31]],"date-time":"2023-07-31T00:00:00Z","timestamp":1690761600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"National Cancer Institute","award":["P30CA093373"],"award-info":[{"award-number":["P30CA093373"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Cells"],"abstract":"<jats:p>CD40L is expressed in activated T cells, and it plays a major role in immune response and is a major therapeutic target for inflammation. High IgM syndrome type 1 (HIGM1) is a congenital functional defect in CD40L\/CD40 signaling due to defective CD40L. CD40L is also stored in platelet granules and transported to the surface upon platelet activation. Platelet integrin \u03b1IIb\u03b23 is known to bind to fibrinogen and activation of \u03b1IIb\u03b23 is a key event that triggers platelet aggregation. Also, the KGD motif is critical for \u03b1IIb\u03b23 binding and the interaction stabilizes thrombus. Previous studies showed that CD40L binds to and activates integrins \u03b1v\u03b23 and \u03b15\u03b21 and that HIGM1 mutations are clustered in the integrin-binding sites. However, the specifics of CD40L binding to \u03b1IIb\u03b23 were unclear. Here, we show that CD40L binds to \u03b1IIb\u03b23 in a KGD-independent manner using CD40L that lacks the KGD motif. Two HIGM1 mutants, S128E\/E129G and L155P, reduced the binding of CD40L to the classical ligand-binding site (site 1) of \u03b1IIb\u03b23, indicating that \u03b1IIb\u03b23 binds to the outer surface of CD40L trimer. Also, CD40L bound to the allosteric site (site 2) of \u03b1IIb\u03b23 and allosterically activated \u03b1IIb\u03b23 without inside-out signaling. Two HIMG1 mutants, K143T and G144E, on the surface of trimeric CD40L suppressed CD40L-induced \u03b1IIb\u03b23 activation. These findings suggest that CD40L binds to \u03b1IIb\u03b23 in a manner different from that of \u03b1v\u03b23 and \u03b15\u03b21 and induces \u03b1IIb\u03b23 activation. HIGM1 mutations are clustered in \u03b1IIb\u03b23 binding sites in CD40L and are predicted to suppress thrombus formation and immune responses through \u03b1IIb\u03b23.<\/jats:p>","DOI":"10.3390\/cells12151977","type":"journal-article","created":{"date-parts":[[2023,8,1]],"date-time":"2023-08-01T09:14:40Z","timestamp":1690881280000},"page":"1977","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":10,"title":["CD40L Activates Platelet Integrin \u03b1IIb\u03b23 by Binding to the Allosteric Site (Site 2) in a KGD-Independent Manner and HIGM1 Mutations Are Clustered in the Integrin-Binding Sites of CD40L"],"prefix":"10.3390","volume":"12","author":[{"given":"Yoko K.","family":"Takada","sequence":"first","affiliation":[{"name":"Department of Dermatology, School of Medicine, UC Davis, Sacramento, CA 95817, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michiko","family":"Shimoda","sequence":"additional","affiliation":[{"name":"Department of Dermatology, School of Medicine, UC Davis, Sacramento, CA 95817, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5481-9589","authenticated-orcid":false,"given":"Yoshikazu","family":"Takada","sequence":"additional","affiliation":[{"name":"Department of Dermatology, School of Medicine, UC Davis, Sacramento, CA 95817, USA"},{"name":"Department of Biochemistry and Molecular Medicine, School of Medicine, UC Davis, Sacramento, CA 95817, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2023,7,31]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"215","DOI":"10.1186\/gb-2007-8-5-215","article-title":"The integrins","volume":"8","author":"Takada","year":"2007","journal-title":"Genome Biol."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"673","DOI":"10.1016\/S0092-8674(02)00971-6","article-title":"Integrins: Bidirectional, allosteric signaling machines","volume":"110","author":"Hynes","year":"2002","journal-title":"Cell"},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Hassan, G.S., Salti, S., and Mourad, W. 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