Aster-B coordinates with Arf1 to regulate mitochondrial cholesterol transport

Abstract

Objective Cholesterol plays a pivotal role in mitochondrial steroidogenesis, membrane structure, and respiration. Mitochondrial membranes are intrinsically low in cholesterol content and therefore must be replenished with cholesterol from other subcellular membranes. However, the molecular mechanisms underlying mitochondrial cholesterol transport remains poorly understood. The Aster-B gene encodes a cholesterol binding protein recently implicated in cholesterol trafficking from the plasma membrane to the endoplasmic reticulum (ER). In this study, we investigated the function and underlying mechanism of Aster-B in mediating mitochondrial cholesterol transport. Methods CRISPR/Cas9 gene editing was carried out to generate cell lines deficient in Aster-B expression. The effect of Aster-B deficiency on mitochondrial cholesterol transport was examined by both confocal imaging analysis and biochemical assays. Deletion mutational analysis was also carried out to identify the function of a putative mitochondrial targeting sequence (MTS) at the N-terminus of Aster-B for its role in targeting Aster-B to mitochondria and in mediating mitochondrial cholesterol trafficking. Results Ablation of Aster-B impaired cholesterol transport from the ER to mitochondria, leading to a significant decrease in mitochondrial cholesterol content. Aster-B is also required for mitochondrial transport of fatty acids derived from hydrolysis of cholesterol esters. A putative MTS at the N-terminus of Aster-B mediates the mitochondrial cholesterol uptake. Deletion of the MTS or ablation of Arf1 GTPase which is required for mitochondrial translocation of ER proteins prevented mitochondrial cholesterol transport, leading to mitochondrial dysfunction. Conclusions We identified Aster-B as a key regulator of cholesterol transport from the ER to mitochondria. Aster-B also coordinates mitochondrial cholesterol trafficking with uptake of fatty acids derived from cholesterol esters, implicating the Aster-B protein as a novel regulator of steroidogenesis.

Knock-Out of ACBD3 Leads to Dispersed Golgi Structure, but Unaffected Mitochondrial Functions in HEK293 and HeLa Cells

“…Although most of the research focusing on cholesterol transport into mitochondria has been carried out in the context of steroidogenesis, recently, a new mechanism of mitochondrial cholesterol transport in non-steroidogenic cells has been described [ 37 ]. The authors characterized a new protein, Aster-B, which contains a StAR-related transfer domain and a mitochondrial target sequence.…”

“…Aster-B, together with the Arf1 GTPase, is indispensable for cholesterol transport from ER to mitochondria in C2C12 cells. Depletion of Aster-B or Arf1 leads to a significant decrease in mitochondrial cholesterol content, resulting in mitochondrial dysfunction [ 37 ].…”

“…We speculate about the existence of an alternative mechanism for mitochondrial cholesterol transport, independent of ACBD3. One such mechanism could be the recently described novel pathway of cholesterol transport [ 37 ]. Altogether, the exact mechanism of cholesterol transport into mitochondria remains largely unknown, and further investigation is required to gain proper insight.…”

GRAMD1C regulates autophagy initiation and mitochondrial bioenergetics through ER-mitochondria cholesterol transport

“…Supplementing these findings, we show that GRAMD1C interacts with the mitochondria through its GRAM domain and co-precipitates with mitochondrial proteins. Interestingly, instead of its GRAM domain, the interaction of GRAMD1B to the mitochondria was reported to be dependent on a mitochondrion targeting sequence located upstream of its GRAM domain 55 , indicating that the GRAMs may be differently recruited to target organelles. It remains to be shown whether GRAMD1C interacts directly with any mitochondrial proteins or whether mitochondria fragments were indirectly pulled down with GRAMD1C.…”

Mitochondria‐associated endoplasmic reticulum membranes: At the crossroad between familiar and sporadic Alzheimer's disease

“…In FAD, in turn, accumulated APP at MAM will lure excessive cholesterol, which disrupts the MAM function. However, the inhibition of ADP Ribosylation factor 1 (Arf1), which resulted in significantly increased cholesterol content in MAM (Andersen et al., 2020), led to decreased Aβ secretion from N2a cells admitted with coconut oil (Bansal et al., 2019). Such controversy may be a consequence of disturbed role of ARF1 in other cellular process, including APP expression (Bansal et al., 2019) and secretory pathways (Ooi et al., 1998; Stamnes & Rothman, 1993).…”