ER Control of Hepatic Neutral Lipid Flux [Mar. 3, 2026]

CLCC1, an ER membrane protein linked to ER ion homeostasis and ER stress control, has recently attracted attention as a factor that supports hepatic lipid homeostasis. Here, we highlight two recent papers that provide new insights into CLCC1 function. Mathiowetz et al. showed that CLCC1 directs hepatic neutral lipids toward either cytosolic lipid droplets or ER luminal lipoprotein-like particles, and reported that CLCC1 deficiency causes accumulation of giant luminal particles together with decreased nuclear pore numbers, implicating CLCC1 in ER and nuclear envelope control. Wu et al. emphasized that CLCC1 senses ER bilayer imbalance and cooperates with TMEM41B-dependent scrambling, and demonstrated that disruption of this control collapses apoB lipidation and VLDL secretion, leading, as in Mathiowetz et al., to lipid accumulation within the ER. Together, these findings provide a new mechanism for CLCC1-mediated control of neutral lipid flux.

CLCC1 promotes hepatic neutral lipid flux and nuclear pore complex assembly (Nature, 2026)
Summary: CLCC1 was identified as a factor that directs hepatic neutral lipids either toward cytosolic lipid-droplet storage or toward export as ER-luminal lipoprotein particles. In CLCC1 deficiency, giant ER-luminal lipoprotein-like particles accumulate and nuclear pore assembly defects occur (nuclear envelope bulges accumulate and pore numbers decrease), supporting CLCC1 as a central membrane-shaping/fusion factor (hemifusion→extrusion) acting at the ER and nuclear envelope.

Highlighted technique: In this study, Lipi-Blue or BODIPY 493/503 staining was used to visualize and quantify neutral lipid/lipid droplet phenotypes after perturbing target genes. To assess whether the giant structures formed upon CLCC1 loss were canonical cytosolic lipid droplets, Lipi-Blue staining was combined with PLIN2 as a lipid-droplet coat marker (PLIN2-positive LDs) for analysis.

CLCC1 governs ER bilayer equilibration to maintain lipid homeostasis (Nature, 2026)
Summary: Most ER phospholipids are synthesized on the cytosolic leaflet, so TMEM41B-dependent scrambling equilibrates lipids across the bilayer; the authors show CLCC1 senses ER bilayer imbalance and cooperates with TMEM41B to promote scrambling and maintain ER homeostasis. When this control fails, hepatic apoB lipidation and VLDL secretion collapse, lipids accumulate as ER-enclosed lumenal giant droplets, and MASH rapidly develops.

Highlighted technique: They quantified ER phospholipid scrambling in intact cells by adding alkyne-choline to the medium for cellular uptake and synthesis of alkyne-containing phosphatidylcholine, alkyne-PC, thereby introducing a “handle” on phospholipids. Next, they selectively permeabilized the cholesterol-rich plasma membrane with digitonin, labeled only alkyne-PC remaining on the cytosolic ER leaflet via a fluorophore–azide click reaction, and used fluorescence intensity, with higher signal indicating reduced scrambling, to assess scrambling activity.

Previous Science Note