Biology subjects

Ma, C.-I. J.

Publications and source records attributed to Ma, C.-I. J..

3 recordsLinked to original sources

bioRxiv2025

The Na V1.5 auxiliary subunit FGF13 modulates channels by regulating membrane cholesterol independent of channel binding

Fibroblast growth factor homologous factors (FHFs) bind to the cytoplasmic carboxy terminus of voltage-gated sodium channels (VGSCs) and modulate channel function. Variants in FHFs or VGSCs perturbing that bimolecular interaction are associated with arrhythmias. Like some channel auxiliary subunits, FHFs exert additional cellular regulatory roles, but whether these alternative roles affect VGSC regulation is unknown. Using a separation-of-function strategy, we show that a structurally guided, binding incompetent mutant FGF13 (the major FHF in mouse heart), confers complete regulation of VGSC steady-state inactivation (SSI), the canonical effect of FHFs. In cardiomyocytes isolated from Fgf13 knockout mice, expression of the mutant FGF13 completely restores wild-type regulation of SSI. FGF13 regulation of SSI derives from effects on local accessible membrane cholesterol, which is unexpectedly polarized and concentrated in cardiomyocytes at the intercalated disc (ID) where most VGSCs localize. Fgf13 knockout eliminates the polarized cholesterol distribution and causes loss of VGSCs from the ID. Moreover, we show that the previously described FGF13-dependent stabilization of VGSC currents at elevated temperatures depends on the cholesterol mechanism. These results provide new insights into how FHFs affect VGSCs and alter the canonical model by which channel auxiliary exert influence.

biophysics↗

bioRxiv2024

A high-content microscopy drug screening platform for regulators of the extracellular digestion of lipoprotein aggregates by macrophages

The recruitment of macrophages to the intima of arteries is a critical event in atherosclerotic progression. These macrophages accumulate excessive lipid droplets and become "foam cells", a hallmark of atherosclerosis. Most studies focus on lipid accumulation through macrophage interaction with modified monomeric low-density lipoprotein (LDL). However, in the intima, macrophages predominantly encounter aggregated LDL (agLDL), an interaction that has been studied far less. Macrophages digest agLDL and generate free cholesterol in an extracellular, acidic, hydrolytic compartment. They form a tight seal around agLDL through actin polymerization and deliver lysosomal contents into this space in a process termed digestive exophagy. There is some evidence that inhibiting digestive exophagy to slow cholesterol accumulation in macrophages protects them from becoming foam cells and slows the progression of atherosclerotic lesions. Thus, understanding the mechanisms of digestive exophagy is critical. Here, we describe a high-content microscopy screen on a library of repurposed drugs for compounds that inhibit lysosome exocytosis during digestive exophagy. We identified many hit compounds and further characterized the effects that five of these compounds have on various aspects of digestive exophagy. In addition, three of the five compounds do not inhibit oxidized LDL-induced foam cell formation, indicating the two pathways to foam cell formation can be targeted independently. We demonstrate that this high-content screening platform has great potential as a drug discovery tool with the ability to effectively and efficiently screen for modulators of digestive exophagy.

cell biology↗

bioRxiv2023

Mutual dependence of Osbp and PI4KII in the maturation of regulated secretory granules

Secretory granules (SGs) are crucial for normal animal physiology due to their role in regulated exocytosis of biologically active molecules. SG membranes are enriched in phosphatidylinositol 4-phosphate (PI4P) and cholesterol, and previous studies suggest lipid composition is important for SG biogenesis and function. Nonetheless, the molecular details of how lipids are regulated during SG biogenesis remain poorly understood. Here, we identify Oxysterol binding protein (Osbp) as a novel regulator of SG biogenesis in a Drosophila model. We show Osbp expression level positively correlates with SG size and that Osbp requires type II phosphatidylinositol 4-kinase (PI4KII) to increase SG size. Moreover, Osbp is needed for proper PI4KII and PI4P distribution, autophagic resolution and formation of cholesterol-rich endosomal tubules that are positive for PI4KII. Feeding larvae food supplemented with sterol leads to partial suppression of SG size and PI4P distribution defects in Osbp mutants. Our results indicate that reciprocal regulation of Osbp and PI4KII drives formation of membrane tubules that mediate SG maturation through elevating PI4P levels on SG membranes. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=200 SRC="FIGDIR/small/551178v1_ufig1.gif" ALT="Figure 1"> View larger version (67K): org.highwire.dtl.DTLVardef@19566fborg.highwire.dtl.DTLVardef@19beeadorg.highwire.dtl.DTLVardef@52e048org.highwire.dtl.DTLVardef@1029b3c_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIOsbp is needed for formation of PI4KII-positive endosomal tubules that enrich secretory granule membranes with PI4P and facilitate granule maturation. C_LIO_LIReciprocally, PI4KII is needed for Osbp-mediated secretory granule growth in salivary gland cells. C_LIO_LIChanges in Osbp expression levels alter autophagy initiation and autophagosome resolution in salivary gland cells. C_LIO_LIPI4KII-positive endosomal tubules are enriched in sterols, and sterol feeding suppresses defects caused by loss of Osbp. C_LI

cell biology↗