Expanding the Toolbox: Novel Modulators of Endolysosomal Cation Channels

A novel BRET-based assay to investigate binding and residence times of unmodified ligands to the human lysosomal ion channel TRPML1 in intact cells

Here we report a Bioluminescence Resonance Energy Transfer (BRET) assay as a novel way to investigate the binding of unlabeled ligands to the human Transient Receptor Potential Mucolipin 1 (hTRPML1), a lysosomal ion channel involved in several genetic diseases and cancer progression. This novel BRET assay can be used to determine equilibrium and kinetic binding parameters of unlabeled compounds to hTRPML1 using intact human-derived cells, thus complementing the information obtained using functional assays based on ion channel activation. We expect this new BRET assay to expedite the identification and optimization of cell-permeable ligands that interact with hTRPML1 within the physiologically-relevant environment of lysosomes.

The synthetic TRPML1 agonist ML-SA1 rescues Alzheimer-related alterations of the endosomal-autophagic-lysosomal system

Abnormalities in the endosomal-autophagic-lysosomal (EAL) system are an early event in Alzheimer's disease (AD) pathogenesis. However, the mechanisms underlying these abnormalities are unclear. The transient receptor potential channel mucolipin 1(TRPML1, also known as MCOLN1), a vital endosomal-lysosomal Ca2+ channel whose loss of function leads to neurodegeneration, has not been investigated with respect to EAL pathogenesis in late-onset AD (LOAD). Here, we identify pathological hallmarks of TRPML1 dysregulation in LOAD neurons, including increased perinuclear clustering and vacuolation of endolysosomes. We reveal that induced pluripotent stem cell (iPSC)-derived human cortical neurons expressing APOE ε4, the strongest genetic risk factor for LOAD, have significantly diminished TRPML1-induced endolysosomal Ca2+ release. Furthermore, we found that blocking TRPML1 function in primary neurons by depleting the TRPML1 agonist PI(3,5)P2 via PIKfyve inhibition, recreated multiple features of EAL neuropathology evident in LOAD. This included increased endolysosomal Ca2+ content, enlargement and perinuclear clustering of endolysosomes, autophagic vesicle accumulation and early endosomal enlargement. Strikingly, these AD-like neuronal EAL defects were rescued by TRPML1 reactivation using its synthetic agonist ML-SA1. These findings implicate defects in TRPML1 in LOAD EAL pathogenesis and present TRPML1 as a potential therapeutic target.

NAADP-Evoked Ca2+ Signaling: The DUOX2-HN1L/JPT2-Ryanodine Receptor 1 Axis

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca²⁺ mobilizing second messenger known to date. Major steps elucidating metabolism and Ca²⁺ mobilizing activity of NAADP are reviewed, with emphasis on a novel redox cycle between the inactive reduced form, NAADPH, and the active oxidized form, NAADP. Oxidation from NAADPH to NAADP is catalyzed in cell free system by (dual) NADPH oxidases NOX5, DUOX1, and DUOX2, whereas reduction from NAADP to NAADPH is catalyzed by glucose 6-phosphate dehydrogenase. Using different knockout models for NOX and DUOX isozymes, DUOX2 was identified as NAADP forming enzyme in early T-cell activation.Recently, receptors or binding proteins for NAADP were identified: hematological and neurological expressed 1-like protein (HN1L)/Jupiter microtubule associated homolog 2 (JPT2) and Lsm12 are small cytosolic proteins that bind NAADP. In addition, they interact with NAADP-sensitive Ca²⁺ channels, such as ryanodine receptor type 1 (RYR1) or two-pore channels (TPC).Due to its role as Ca²⁺ mobilizing second messenger in T cells, NAADP's involvement in inflammation is also reviewed. In the central nervous system (CNS), NAADP regulates autoimmunity because NAADP antagonism affects a couple of T-cell migration and re-activation events, e.g. secretion of the pro-inflammatory cytokine interleukin-17. Further, the role of NAADP in transdifferentiation of IL-17-producing Th17 cells into T regulatory type 1 cells in vitro and in vivo is discussed.