JPT2/HN1L functions as an NAADP-binding protein in a cell type-specific manner

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger evoking Ca ²⁺ release from intracellular Ca ²⁺ stores by targeting several Ca ²⁺ release channels, including two-pore channels (TPC1/2), transient receptor potential mucolipin-1 (TRPML1), or ryanodine receptor type 1 (RyR1). NAADP does not bind these channels directly but requires binding proteins, such as JPT2/HN1L and LSM12. So far, their function has been analyzed in several cell lines; however, their physiological relevance in cell types known to utilize NAADP signaling remains unclear. Here, we generated JPT2/HN1L-deficient ( Jpt2/Hn1l ^–/– ) mice to evaluate the contribution of JPT2/HN1L proteins to platelet aggregation and Ca ²⁺ signaling in cardiomyocytes, mast cells, and CD4 ⁺ T cells. NAADP is known to contribute to Collagen-related peptide (CRP)-evoked platelet aggregation, but this was not altered by JPT2/HN1L deletion. Functional Ca ²⁺ imaging revealed that JPT2/HN1L plays a strikingly cell-type specific role in NAADP-mediated Ca2+ release. In beating ventricular cardiomyocytes, β-adrenergic stimulation is known to evoke arrhythmogenic spontaneous diastolic Ca ²⁺ transients, which were not altered in their frequency in Jpt2/Hn1l ^–/– myocytes. Antigen-evoked Ca ²⁺ transients in peritoneal mast cells (PMCs) are not changed in Jpt2/Hn1l ^–/– PMCs. However, CD4 ⁺ T-cells displayed a pronounced requirement for JPT2/HN1L. Following T-cell receptor stimulation with anti-CD3, global Ca ²⁺ elevations and early NAADP-driven Ca ²⁺ microdomains, which occur within milliseconds of TCR engagement and serve as initiating signals for downstream immune activation, were significantly decreased in Jpt2/Hn1l ^–/– CD4 ⁺ cells. We conclude that JPT2/HN1L is indispensable for NAADP-mediated Ca ²⁺ release in T-cells, but dispensable in cardiomyocytes, platelets, and mast cells, where the presence of LSM12 might compensate for the loss of JPT2/HN1L. Together, JPT2/HN1L is not universally required as an NAADP-binding protein but exhibits distinct cell-type specificity, with an essential function in T-cell Ca ²⁺ signaling.