Selective agonist of TRPML2 reveals direct role in chemokine release from innate immune cells

Abstract

Cytokines and chemokines are produced and secreted by a broad range of immune cells including macrophages. Remarkably, little is known about how these inflammatory mediators are released from the various immune cells. Here, the endolysosomal cation channel TRPML2 is shown to play a direct role in chemokine trafficking and secretion from murine macrophages. To demonstrate acute and direct involvement of TRPML2 in these processes, the first isoform-selective TRPML2 channel agonist was generated, ML2-SA1. ML2-SA1 was not only found to directly stimulate release of the chemokine CCL2 from macrophages but also to stimulate macrophage migration, thus mimicking CCL2 function. Endogenous TRPML2 is expressed in early/recycling endosomes as demonstrated by endolysosomal patch-clamp experimentation and ML2-SA1 promotes trafficking through early/recycling endosomes, suggesting CCL2 being transported and secreted via this pathway. These data provide a direct link between TRPML2 activation, CCL2 release and stimulation of macrophage migration in the innate immune response.

TRPML Cation Channels in Inflammation and Immunity

“…The release is most likely mediated via the early/recycling endosomal pathway, since activation of TRPML2 by ML2-SA1 promotes trafficking through this pathway. In addition, Plesch et al found that direct activation of TRPML2 leads to enhanced recruitment of macrophages (5). These findings by Sun et al and Plesch et al strongly suggest that TRPML2 plays a crucial role in the release of CCL2 and likely other chemokines as well as in the stimulation of macrophage migration.…”

“…All three channels are regulated by the phosphoinositide PI(3,5)P 2 , a major component of endolysosomal membranes and by luminal pH (1,(3)(4)(5). Thus, TRPML1 is most active at highly acidic pH as found in lysosomes while TRPML2 and TRPML3 are more active at higher pH as it occurs in early and recycling endosomes (1,(3)(4)(5). TRPML1 activity already decreases under mild acidic conditions (pH 5.5; Figure 1) and is lowest at neutral conditions (4).…”

“…One subfamily of the TRP channels, the mucolipin or TRPML/MCOLN subfamily comprises three members in mammalian genomes, TRPML1, 2, and 3 which are all found to be expressed in the endolysosomal system, i.e., in early and late endosomes, in recycling endosomes and in lysosomes to various degrees. All three channels are regulated by the phosphoinositide PI(3,5)P 2 , a major component of endolysosomal membranes and by luminal pH (1,(3)(4)(5). Thus, TRPML1 is most active at highly acidic pH as found in lysosomes while TRPML2 and TRPML3 are more active at higher pH as it occurs in early and recycling endosomes (1,(3)(4)(5).…”

“…In contrast to the ubiquitously expressed TRPML1, the expression of TRPML2 and TRPML3 is restricted to certain cell types including a number of immune cells. Recently, several roles of immune cell function could be linked to TRPML channels, e.g., a role of TRPML1 for the phagocytosis of large particles (17), the migration of dendritic cells (18), or for the tuning of the functional potential in self-KIR+ natural killer (NK) cells (19) while TRPML2 was found to play a role in chemokine and cytokine secretion by macrophages (5,20). In the following, the current state of knowledge on TRPML channel expression and function in different immune cells shall be discussed (Figure 2).…”

Pathophysiological Role of Transient Receptor Potential Mucolipin Channel 1 in Calcium-Mediated Stress-Induced Neurodegenerative Diseases

“…There are two other members in the TRPML subfamily, TRPML2 and TRPML3, encoded by MCOLN2 and MCOLN3 genes. Like TRPML1, they are active in late endosomes/lysosomes; in addition, TRPML2 and TRPML3 are active in early endosomes, and TRPML2 also in recycling (Chen et al, 2017;Plesch et al, 2018). They have not been correlated with neurodegeneration in humans so far.…”