FERMT3 alternative splicing enhances kindlin-3 membrane recruitment for neutrophil adhesion during stress myelopoiesis.

Leukocyte Adhesion Deficiency syndrome type III (LAD-III) is characterized by recurrent infections and is caused by FERMT3 gene mutations. FERMT3 encodes two isoforms, a standard kindlin-3 and a longer splicing variant, which differs by the addition of four residues Ile-Pro-Arg-Arg (IPRR) in the pleckstrin homology (PH) domain (kindlin-3-IPRR). Previous studies suggested that kindlin-3-IPRR was dysfunctional in inducing neutrophil adhesion, attributed to altered phospholipid binding of the IPRR-containing PH domain. Here we show that kindlin-3-IPRR is fully functional in activating β2 integrins and promotes neutrophil adhesion. The IPRR insert enhances phospholipid binding in vitro and promotes association of kindlin-3 with the plasma membrane. By analyzing RNA sequencing datasets of neutropoiesis, we show that the kindlin-3 short isoform lacking IPRR is highly expressed in neutrophil precursors but downregulated in mature neutrophils. In contrast, a higher proportion of the long isoform was detected in hematopoietic stem cells (HSCs) and mature neutrophils. Kindlin-3-IPRR supports robust adhesion of HSCs. We propose that kindlin-3-IPRR may be crucial in conditions requiring rapid neutrophil mobilization. Indeed, kindlin-3 splicing is altered in neutrophils from patients undergoing stress myelopoiesis after exposure to granulocyte colony-stimulating factor (G-CSF) or HSC transplantation (HSC-T) compared with healthy donors. During stress myelopoiesis induced by G-CSF treatment, the kindlin-3 long isoform is selectively upregulated in mature neutrophils, coinciding with enhanced β2 integrin activation. These findings support a model in which alternative splicing of FERMT3 dynamically fine-tunes integrin activation and leukocyte adhesion during hematopoietic stress, with potential implications for monitoring hematopoietic recovery after HSC transplantation.