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 Ca²⁺ release from intracellular Ca²⁺ stores by targeting several Ca²⁺ channels, including two-pore channels (TPC1/2), transient receptor potential mucolipin-1 (TRPML1), or ryanodine receptor type 1 (RYR1). For activation of Ca²⁺ channels, NAADP requires binding proteins, such as JPT2/HN1L and LSM12. So far, their function has been analyzed in several cell lines and only a very limited number of primary cells; however, their physiological relevance in cell types known to utilize NAADP signaling remains unclear. Here, we generated Jpt2/Hn1L^-/- mice to evaluate the contribution of JPT2/HN1L proteins to platelet aggregation and Ca²⁺ signaling in cardiomyocytes, mast cells, and T cells. NAADP is known to contribute to collagen-related peptide (CRP-XL)-evoked platelet aggregation, but this was not altered by JPT2/HN1L deletion. Functional Ca²⁺ imaging revealed that JPT2/HN1L plays a strikingly cell-type specific role in NAADP-mediated Ca²⁺ release. In electrically paced ventricular cardiomyocytes, β-adrenergic stimulation is known to evoke arrhythmogenic spontaneous diastolic Ca²⁺ transients, which were not altered in their frequency in Jpt2/Hn1L^-/- myocytes. Further, antigen-evoked Ca²⁺ 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/CD3 stimulation, global Ca²⁺ elevations and early NAADP-driven Ca²⁺ microdomains, which occur within tens of milliseconds of TCR/CD3 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 Ca²⁺ release in T cells, but dispensable in cardiomyocytes, platelets, and mast cells, at least for the agonists employed. Accordingly, LSM12 might compensate for the loss of JPT2/HN1L. Together, JPT2/HN1L is not universally required as an NAADP-binding protein but exhibits cell-type specificity, with an essential function in T cell Ca²⁺ signaling.