StarD7 Protein Deficiency Adversely Affects the Phosphatidylcholine Composition, Respiratory Activity, and Cristae Structure of Mitochondria

Subcellular imaging of lipids and sugars using genetically encoded proximity sensors

Live cell imaging of lipids and other metabolites is a long-standing challenge in cell biology. Bioorthogonal labeling tools allow for the conjugation of fluorophores to several phospholipid classes, but cannot discern their trafficking between adjacent organelles or asymmetry across individual membrane leaflets. Here we present fluorogen-activating coincidence sensing (FACES), a chemogenetic tool capable of quantitatively imaging subcellular lipid pools and reporting their transbilayer orientation in living cells. FACES combines bioorthogonal chemistry with genetically encoded fluorogen-activating proteins (FAPs) for reversible proximity sensing of conjugated molecules. We first validate this approach for quantifying discrete phosphatidylcholine pools in the ER and mitochondria that are trafficked by lipid transfer proteins. We then show that transmembrane domain-containing FAPs can be used to reveal the membrane asymmetry of multiple lipid classes that are generated in the trans-Golgi network. Lastly, we demonstrate that FACES is a generalizable tool for subcellular bioorthogonal imaging by measuring changes in mitochondrial N -acetylhexosamine levels. These results demonstrate the use of fluorogenic tags for spatially-defined molecular imaging.

Mitochondrial phospholipid transport: Role of contact sites and lipid transport proteins

One of the major constituents of mitochondrial membranes is the phospholipids, which play a key role in maintaining the structure and the functions of the mitochondria. However, mitochondria do not synthesize most of the phospholipids in situ, necessitating the presence of phospholipid import pathways. Even for the phospholipids, which are synthesized within the inner mitochondrial membrane (IMM), the phospholipid precursors must be imported from outside the mitochondria. Therefore, the mitochondria heavily rely on the phospholipid transport pathways for its proper functioning. Since, mitochondria are not part of a vesicular trafficking network, the molecular mechanisms of how mitochondria receive its phospholipids remain a relevant question. One of the major ways that hydrophobic phospholipids can cross the aqueous barrier of inter or intraorganellar spaces is by apposing membranes, thereby decreasing the distance of transport, or by being sequestered by lipid transport proteins (LTPs). Therefore, with the discovery of LTPs and membrane contact sites (MCSs), we are beginning to understand the molecular mechanisms of phospholipid transport pathways in the mitochondria. In this review, we will present a brief overview of the recent findings on the molecular architecture and the importance of the MCSs, both the intraorganellar and interorganellar contact sites, in facilitating the mitochondrial phospholipid transport. In addition, we will also discuss the role of LTPs for trafficking phospholipids through the intermembrane space (IMS) of the mitochondria. Mechanistic insights into different phospholipid transport pathways of mitochondria could be exploited to vary the composition of membrane phospholipids and gain a better understanding of their precise role in membrane homeostasis and mitochondrial bioenergetics.

StarD7 deficiency switches on glycolysis and promotes mitophagy flux in C2C12 myoblasts

StarD7 is a member of the START protein family required for phosphatidylcholine delivery to the mitochondria, thus key to maintain mitochondrial structure. Its deficiency has been associated with an impairment of cellular processes, such as proliferation and migration, and it has also been reported that it is needed in myogenic differentiation. Here, we show that StarD7 deficiency in C2C12 muscle cells results in the accumulation of abnormal mitochondria, a reduced number of mitochondria per cell area and increased glycolysis. In addition, StarD7-deficient cells undergo an increase in mitochondria-ER contact sites, reduced connexin 43 expression, and disturbances in lipid handling, evidenced by lipid droplet accumulation and decreased levels in phosphatidylserine synthase 1 and 2 expression. Interestingly, StarD7-deficient cells showed alterations in mitophagy markers. We observed accumulation of LC3B-II and BNIP3 proteins in mitochondria-enriched fractions and accumulation of autophagolysosomal and lysosomal vesicles in StarD7-deficient cells. Furthermore, live-cell imaging experiments of StarD7 knockdown cells expressing mitochondria-targeted mKeima indicated an enhanced mitochondria delivery into lysosomes. Importantly, StarD7 reconstitution in StarD7-deficient cells restores LC3B-II expression in mitochondria-enriched fractions at similar levels to those observed in control cells. Collectively, these findings suggest that StarD7-deficient C2C12 myoblasts are associated with altered cristae structure, disturbances in neutral lipid accumulation, glucose metabolism, and increased mitophagy flux. The alterations mentioned above allow for the maintenance of mitochondrial function.

IFI27 Integrates Succinate and Fatty Acid Oxidation to Promote Adipocyte Thermogenic Adaption

Mitochondria are the pivot organelles to control metabolism and energy homeostasis. The capacity of mitochondrial metabolic adaptions to cold stress is essential for adipocyte thermogenesis. How brown adipocytes keep mitochondrial fitness upon a challenge of cold-induced oxidative stress has not been well characterized. This manuscript shows that IFI27 plays an important role in cristae morphogenesis, keeping intact succinate dehydrogenase (SDH) function and active fatty acid oxidation to sustain thermogenesis in brown adipocytes. IFI27 protein interaction map identifies SDHB and HADHA as its binding partners. IFI27 physically links SDHB to chaperone TNF receptor associated protein 1 (TRAP1), which shields SDHB from oxidative damage-triggered degradation. Moreover, IFI27 increases hydroxyacyl-CoA dehydrogenase trifunctional multienzyme complex subunit alpha (HADHA) catalytic activity in β-oxidation pathway. The reduced SDH level and fatty acid oxidation in Ifi27-knockout brown fat results in impaired oxygen consumption and defective thermogenesis. Thus, IFI27 is a novel regulator of mitochondrial metabolism and thermogenesis.

SUMO1 degrader induces ER stress and ROS accumulation through deSUMOylation of TCF4 and inhibition of its transcription of StarD7 in colon cancer

Small molecule degraders of small ubiquitin-related modifier 1 (SUMO1) induce SUMO1 degradation in colon cancer cells and inhibits the cancer cell growth; however, it is unclear how SUMO1 degradation leads to the anticancer activity of the degraders. Genome-wide CRISPR-Cas9 knockout screen has identified StAR-related lipid transfer domain containing 7 (StarD7) as a critical gene for the degrader's anticancer activity. Here, we show that both StarD7 mRNA and protein are overexpressed in human colon cancer and its knockout significantly reduces colon cancer cell growth and xenograft progression. The treatment with the SUMO1 degrader lead compound HB007 reduces StarD7 mRNA and protein levels and increases endoplasmic reticulum (ER) stress and reactive oxygen species (ROS) production in colon cancer cells and three-dimensional (3D) organoids. The study further provides a novel mechanism of the compound anticancer activity that SUMO1 degrader-induced decrease of StarD7 occur through degradation of SUMO1, deSUMOylation and degradation of T cell-specific transcription 4 (TCF4) and thereby inhibition of its transcription of StarD7 in colon cancer cells, 3D organoids and patient-derived xenografts (PDX).

Phospholipids can regulate complex I assembly independent of their role in maintaining mitochondrial membrane integrity

Several phospholipid (PL) molecules are intertwined with some mitochondrial complex I (CI) subunits in the membrane domain of CI, but their function is unclear. We report that when the Drosophila melanogaster ortholog of the intramitochondrial PL transporter, STARD7, is severely disrupted, assembly of the oxidative phosphorylation (OXPHOS) system is impaired, and the biogenesis of several CI subcomplexes is hampered. However, intriguingly, a restrained knockdown of STARD7 impairs the incorporation of NDUFS5 and NDUFA1 into the proximal part of the CI membrane domain without directly affecting the incorporation of subunits in the distal part of the membrane domain, OXPHOS complexes already assembled, or mitochondrial cristae integrity. Importantly, the restrained knockdown of STARD7 appears to induce a modest amount of cardiolipin remodeling, indicating that there could be some alteration in the composition of the mitochondrial phospholipidome. We conclude that PLs can regulate CI biogenesis independent of their role in maintaining mitochondrial membrane integrity.

Mechanisms of Modulation of Mitochondrial Architecture

Mitochondrial network architecture plays a critical role in cellular physiology. Indeed, alterations in the shape of mitochondria upon exposure to cellular stress can cause the dysfunction of these organelles. In this scenario, mitochondrial dynamics proteins and the phospholipid composition of the mitochondrial membrane are key for fine-tuning the modulation of mitochondrial architecture. In addition, several factors including post-translational modifications such as the phosphorylation, acetylation, SUMOylation, and o-GlcNAcylation of mitochondrial dynamics proteins contribute to shaping the plasticity of this architecture. In this regard, several studies have evidenced that, upon metabolic stress, mitochondrial dynamics proteins are post-translationally modified, leading to the alteration of mitochondrial architecture. Interestingly, several proteins that sustain the mitochondrial lipid composition also modulate mitochondrial morphology and organelle communication. In this context, pharmacological studies have revealed that the modulation of mitochondrial shape and function emerges as a potential therapeutic strategy for metabolic diseases. Here, we review the factors that modulate mitochondrial architecture.

StarD7 deficiency hinders cell motility through p-ERK1/2/Cx43 reduction

StarD7 belongs to START protein family involved in lipid traffic, metabolism, and signaling events. Its precursor, StarD7.I which is important for mitochondrial homeostasis, is processed to the StarD7.II isoform that lacks the mitochondrial targeting sequence and is mainly released to the cytosol. StarD7 knockdown interferes with cell migration by an unknown mechanism. Here, we demonstrate that StarD7 silencing decreased connexin 43 (Cx43), integrin β1, and p-ERK1/2 expression in the non-tumoral migratory HTR-8/SVneo cells. StarD7-deficient cells exhibited Golgi disruption and reduced competence to reorient the microtubule-organizing center. The migratory capacity of StarD7-silenced cells was reestablished when Cx43 level was resettled, while p-ERK1/2 expression remained low. Importantly, ectopic expression of the StarD7.II isoform not only restored cell migration but also ERK1/2, Cx43, and integrin β1 expression. Thus, StarD7 is implicated in cell migration through an ERK1/2/Cx43 dependent mechanism but independent of the StarD7.I function in the mitochondria.

Mitochondrial cristae architecture protects against mtDNA release and inflammation

Mitochondrial damage causes mitochondrial DNA (mtDNA) release to activate the type I interferon (IFN-I) response via the cGAS-STING pathway. mtDNA-induced inflammation promotes autoimmune- and aging-related degenerative disorders. However, the global picture of inflammation-inducing mitochondrial damages remains obscure. Here, we have performed a mitochondria-targeted CRISPR knockout screen for regulators of the IFN-I response. Strikingly, our screen reveals dozens of hits enriched with key regulators of cristae architecture, including phospholipid cardiolipin and protein complexes such as OPA1, mitochondrial contact site and cristae organization (MICOS), sorting and assembly machinery (SAM), mitochondrial intermembrane space bridging (MIB), prohibitin (PHB), and the F₁F_o-ATP synthase. Disrupting these cristae organizers consistently induces mtDNA release and the STING-dependent IFN-I response. Furthermore, knocking out MTX2, a subunit of the SAM complex whose null mutations cause progeria in humans, induces a robust STING-dependent IFN-I response in mouse liver. Taken together, beyond revealing the central role of cristae architecture to prevent mtDNA release and inflammation, our results mechanistically link mitochondrial cristae disorganization and inflammation, two emerging hallmarks of aging and aging-related degenerative diseases.

Role of the lipid transport protein StarD7 in mitochondrial dynamics

Mitochondria are dynamic organelles crucial for cell function and survival implicated in oxidative energy production whose central functions are tightly controlled by lipids. StarD7 is a lipid transport protein involved in the phosphatidylcholine (PC) delivery to mitochondria. Previous studies have shown that StarD7 knockdown induces alterations in mitochondria and endoplasmic reticulum (ER) with a reduction in PC content, however whether StarD7 modulates mitochondrial dynamics remains unexplored. Here, we generated HTR-8/SVneo stable cells expressing the precursor StarD7.I and the mature processed StarD7.II isoforms. We demonstrated that StarD7.I overexpression altered mitochondrial morphology increasing its fragmentation, whereas no changes were observed in StarD7.II-overexpressing cells compared to the control (Ct) stable cells. StarD7.I (D7.I) stable cells were able to transport higher fluorescent PC analog to mitochondria than Ct cells, yield mitochondrial fusions, maintained the membrane potential, and produced lower levels of reactive oxygen species (ROS). Additionally, the expression of Dynamin Related Protein 1 (Drp1) and Mitofusin (Mfn2) proteins were increased, whereas the amount of Mitofusin 1 (Mfn1) decreased. Moreover, transfections with plasmids encoding Drp1-K38A, Drp1-S637D or Drp1-S637A mutants indicated that mitochondrial fragmentation in D7.I cells occurs in a fission-dependent manner via Drp1. In contrast, StarD7 silencing decreased Mfn1 and Mfn2 fusion proteins without modification of Drp1 protein level. These cells increased ROS levels and presented donut-shape mitochondria, indicative of metabolic stress. Altogether our findings provide novel evidence indicating that alterations in StarD7.I expression produce significant changes in mitochondrial morphology and dynamics.

Phospholipid ebb and flow makes mitochondria go

Mitochondria, so much more than just being energy factories, also have the capacity to synthesize macromolecules including phospholipids, particularly cardiolipin (CL) and phosphatidylethanolamine (PE). Phospholipids are vital constituents of mitochondrial membranes, impacting the plethora of functions performed by this organelle. Hence, the orchestrated movement of phospholipids to and from the mitochondrion is essential for cellular integrity. In this review, we capture recent advances in the field of mitochondrial phospholipid biosynthesis and trafficking, highlighting the significance of interorganellar communication, intramitochondrial contact sites, and lipid transfer proteins in maintaining membrane homeostasis. We then discuss the physiological functions of CL and PE, specifically how they associate with protein complexes in mitochondrial membranes to support bioenergetics and maintain mitochondrial architecture.

Characterization of glycerophosphodiesterase 4-interacting molecules Gαq/11 and Gβ, which mediate cellular lysophospholipase D activity

We previously purified lysophospholipase D (lysoPLD), which hydrolyzes lysophosphatidylcholine (lysoPC) to lysophosphatidic acid (LPA), from rat brain and identified the heterotrimeric G protein subunits Gαq and Gβ1 in the lysoPLD active fractions. Tag-affinity purified Gαq exhibits lysoPLD activity but a mutant that affected cellular localization or interaction with the Gβ subunit reduced lysoPLD activity. Size exclusion chromatography revealed that active lysoPLD is a much higher molecular mass complex than is heterotrimeric G protein, suggesting the presence of other components. Liquid chromatography-tandem mass spectrometry of lysoPLD purified from rat brain identified glycerophosphodiesterase 4 (GDE4), recently reported as lysoPLD, in the same fraction as G proteins. The overexpressed and tag-purified Gαq fractions, which exhibit lysoPLD activity, contained GDE4. Exogenously expressed GDE4 was co-immunoprecipitated with endogenous Gαq and Gβ and exhibited high lysoPLD activity. The results of confocal microscopy and cell fractionation experiments indicated that exogenously expressed GDE4 in cells mainly localized at the endoplasmic reticulum and partially co-localized with Gαq protein at the plasma membrane. Proteinase K protection assay results suggested that the catalytic domain of GDE4 faces the lumen/extracellular space. Mutations at the conserved amino acids in the C-terminus cytoplasmic regions amongst GDE1, 4 and 7, dramatically suppressed GDE4 enzyme activities. When both the Gαq and Gα11 genes in Neuro2A cells were disrupted using the CRISPR-Cas9 system, endogenous lysoPLD activity was partially reduced but rescued by overexpression of Gαq. These results suggest that GDE4 is a new effector of G protein signaling that produces bioactive phospholipid LPA and/or modulates membrane homeostasis.

The START-domain proteins in intracellular lipid transport and beyond

The Steroidogenic Acute Regulatory Protein-related Lipid Transfer (START) domain is a ~210 amino acid sequence that folds into an α/β helix-grip structure forming a hydrophobic pocket for lipid binding. The helix-grip fold structure defines a large superfamily of proteins, and this review focuses on the mammalian START domain family members that include single START domain proteins with identified ligands, and larger multi-domain proteins that may have novel roles in metabolism. Much of our understanding of the mammalian START domain proteins in lipid transport and changes in metabolism has advanced through studies using knockout mouse models, although for some of these proteins the identity and/or physiological role of ligand binding remains unknown. The findings that helped define START domain lipid-binding specificity, lipid transport, and changes in metabolism are presented to highlight that fundamental questions remain regarding the biological function(s) for START domain-containing proteins.

The phosphatidylcholine transfer protein StarD7 is important for myogenic differentiation in mouse myoblast C2C12 cells and human primary skeletal myoblasts

StarD7 is a phosphatidylcholine (PC)-specific lipid transfer protein essential for the maintenance of mitochondrial PC composition, morphogenesis, and respiration. Here, we studied the role of StarD7 in skeletal myoblast differentiation using mouse myoblast C2C12 cells and human primary myoblasts. Immunofluorescence and immuno-electron microscopy revealed that StarD7 was distributed in the cytosol, inner mitochondria space, and outer leaflet of the outer mitochondrial membrane in C2C12 cells. Unlike human kidney embryonic cell line HEK293 cells, the mitochondrial proteinase PARL was not involved in the processing and maturation of StarD7 in C2C12 cells. StarD7 was constantly expressed during myogenic differentiation of C2C12 cells. The siRNA-mediated knockdown of StarD7 in C2C12 cells and human primary myoblasts significantly impaired myogenic differentiation and reduced the expression of myomaker, myomerger and PGC-1α. The reduction in mitochondrial PC levels and oxygen consumption rates, decreased expression of myomaker, myomerger and PGC-1α, as well as impaired myogenic differentiation, were completely restored when the protein was reintroduced into StarD7-knockout C2C12 cells. These results suggest that StarD7 is important for skeletal myogenesis in mammals.

Proteolytic Control of Lipid Metabolism

Synthesis and regulation of lipid levels and identities is critical for a wide variety of cellular functions, including structural and morphological properties of organelles, energy storage, signaling, and stability and function of membrane proteins. Proteolytic cleavage events regulate and/or influence some of these lipid metabolic processes and as a result help modulate their pleiotropic cellular functions. Proteins involved in lipid regulation are proteolytically cleaved for the purpose of their relocalization, processing, turnover, and quality control, among others. The scope of this review includes proteolytic events governing cellular lipid dynamics. After an initial discussion of the classic example of sterol regulatory element-binding proteins, our focus will shift to the mitochondrion, where a range of proteolytic events are critical for normal mitochondrial phospholipid metabolism and enforcing quality control therein. Recently, mitochondrial phospholipid metabolic pathways have been implicated as important for the proliferative capacity of cancers. Thus, the assorted proteases that regulate, monitor, or influence the activity of proteins that are important for phospholipid metabolism represent attractive targets to be manipulated for research purposes and clinical applications.

A nutritional memory effect counteracts benefits of dietary restriction in old mice

Dietary restriction (DR) during adulthood can greatly extend lifespan and improve metabolic health in diverse species. However, whether DR in mammals is still effective when applied for the first time at old age remains elusive. Here, we report results of a late-life DR switch experiment employing 800 mice, in which 24 months old female mice were switched from ad libitum (AL) to DR or vice versa. Strikingly, the switch from DR-to-AL acutely increases mortality, whereas the switch from AL-to-DR causes only a weak and gradual increase in survival, suggesting a memory of earlier nutrition. RNA-seq profiling in liver, brown (BAT) and white adipose tissue (WAT) demonstrate a largely refractory transcriptional and metabolic response to DR after AL feeding in fat tissue, particularly in WAT, and a proinflammatory signature in aged preadipocytes, which is prevented by chronic DR feeding. Our results provide evidence for a nutritional memory as a limiting factor for DR-induced longevity and metabolic remodeling of WAT in mammals.

LINC00116 codes for a mitochondrial peptide linking respiration and lipid metabolism

Short peptides are encoded in genomes of all organisms and have important functions. Due to the small size of such open reading frames, they are frequently overlooked by automatic genome annotation. We investigated the gene that was misannotated as long noncoding RNA LINC00116 and demonstrated that this gene codes for a 56-amino-acid-long peptide, Mtln, which is localized in mitochondria. Inactivation of the Mtln coding gene leads to reduction of oxygen consumption attributed to respiratory complex I activity and perturbs lipid composition of the cell. This influence is mediated by Mtln interaction with NADH-dependent cytochrome b5 reductase. Disruption of the mitochondrial localization of the latter phenocopies Mtln inactivation.

Genes coding for small peptides have been frequently misannotated as long noncoding RNA (lncRNA) genes. Here we have demonstrated that one such transcript is translated into a 56-amino-acid-long peptide conserved in chordates, corroborating the work published while this manuscript was under review. The Mtln peptide could be detected in mitochondria of mouse cell lines and tissues. In line with its mitochondrial localization, lack of the Mtln decreases the activity of mitochondrial respiratory chain complex I. Unlike the integral components and assembly factors of NADH:ubiquinone oxidoreductase, Mtln does not alter its enzymatic activity directly. Interaction of Mtln with NADH-dependent cytochrome b5 reductase stimulates complex I functioning most likely by providing a favorable lipid composition of the membrane. Study of Mtln illuminates the importance of small peptides, whose genes might frequently be misannotated as lncRNAs, for the control of vitally important cellular processes.

The relevance of the supramolecular arrangements of the respiratory chain complexes in human diseases and aging

Mitochondrial dysfunction, a common factor in several diseases is accompanied with reactive oxygen species (ROS) production. These molecules react with proteins and lipids at their site of generation, establishing a vicious cycle which might result in further mitochondrial injury. It is well established that mitochondrial respiratory complexes can be organized into supramolecular structures called supercomplexes (SCs) or respirasomes; yet, the physiological/pathological relevance of these structures remains unresolved. Changes in their stabilization and content have been documented in Barth's syndrome, degenerative diseases such as Parkinson's and Alzheimer, cardiovascular diseases including heart failure and ischemia-reperfusion damage, as well as in aging. Under pathological conditions, SCs stability could have relevant biomedical implications or might be used as a reliable marker of mitochondrial damage. The purpose of this review is to recapitulate the current state of the significance on mitochondrial bioenergetics of these structures and their possible role in pathophysiologies related with ROS increase.

Hexosamine pathway regulates StarD7 expression in JEG-3 cells

StarD7 is a lipid binding protein involved in the delivery of phosphatidylcholine to the mitochondria whose promoter is activated by Wnt/β-catenin signaling. Although the majority of glucose enters glycolysis, ~ 2-5% of it can be metabolized via the hexosamine biosynthetic pathway (HBP). Considering that HBP has been implicated in the regulation of β-catenin we explored if changes in glucose levels modulate StarD7 expression by the HBP in trophoblast cells. We found an increase in StarD7 as well as in β-catenin expression following high-glucose (25 mM) treatment in JEG-3 cells; these effects were abolished in the presence of HBP inhibitors. Moreover, since HBP is able to promote unfolded protein response (UPR) the protein levels of GRP78, Ire1α, calnexin, p-eIF2α and total eIF2α as well as XBP1 mRNA was measured. Our results indicate that a diminution in glucose concentration leads to a decrease in StarD7 expression and an increase in the UPR markers: GRP78 and Ire1α. Conversely, an increase in glucose is associated to high StarD7 levels and low GRP78 expression, phospho-eIF2α and XBP1 splicing, although Ire1α remains high when cells are restored to high glucose. Taken together these findings indicate that glucose modulates StarD7 and β-catenin expression through the HBP associated to UPR, suggesting the existence of a link between UPR and HBP in trophoblast cells. This is the first study reporting the effects of glucose on StarD7 in trophoblast cells. These data highlight the importance to explore the role of StarD7 in placenta disorders related to nutrient availability.

EFFECTS OF Wnt / β-CATENIN SIGNALING PATHWAY AND STAR D7 ON TESTOSTERONE SYNTHESIS

BACKGROUND

This study aimed to assess the mechanism through which Wnt/ beta - catenin signaling pathway, and StarD7, prometes testosterone synthesis, and to explore a new pathway for the regulation of testosterone synthesis.

ANIMALS AND METHODS

Leydig cells were isolated from male Sprague-Dawley rats divided into four groups and treated with Annexin 5 in concentration of 0, 0.1, 1 and 10 nmol/L. Testosterone secretion, expression of StarD7, StarD7 mRNA, β-catenin and changes of β - catenin localization in Leydig cells of testis of rats were tested in the four groups.

RESULTS

mRNA and protein levels of StarD7 and β-catenin increased significantly, upon stimulation with 1 nmol/L annexin 5. Accumulation of β-catenin inside the cells and the nucleus, was observed by immunofluorescence staining, in cells treated with annexin 5. These findings indicate a possible role of StarD7 and β-catenin in the process of annexin5-mediated stimulation of testosterone synthesis.

CONCLUSIONS

Wnt/β-catenin signaling pathway and StarD7 are involved in the process of annexin5 stimulation of testosterone synthesis. Activation of Wnt/ β-catenin signaling pathway by Annexin5, and increase in StarD7 expression lead to elevated expression of key regulatory enzymes in testosterone synthesis, thus promoting testosterone synthesis.

A search for ceramide binding proteins using bifunctional lipid analogs yields CERT-related protein StarD7

Ceramides are central intermediates of sphingolipid metabolism with dual roles as mediators of cellular stress signaling and mitochondrial apoptosis. How ceramides exert their cytotoxic effects is unclear and their poor solubility in water hampers a search for specific protein interaction partners. Here, we report the application of a photoactivatable and clickable ceramide analog, pacCer, to identify ceramide binding proteins and unravel the structural basis by which these proteins recognize ceramide. Besides capturing ceramide transfer protein (CERT) from a complex proteome, our approach yielded CERT-related steroidogenic acute regulatory protein D7 (StarD7) as novel ceramide binding protein. Previous work revealed that StarD7 is required for efficient mitochondrial import of phosphatidylcholine (PC) and serves a critical role in mitochondrial function and morphology. Combining site-directed mutagenesis and photoaffinity labeling experiments, we demonstrate that the steroidogenic acute regulatory transfer domain of StarD7 harbors a common binding site for PC and ceramide. While StarD7 lacks robust ceramide transfer activity in vitro, we find that its ability to shuttle PC between model membranes is specifically affected by ceramides. Besides demonstrating the suitability of pacCer as a tool to hunt for ceramide binding proteins, our data point at StarD7 as a candidate effector protein by which ceramides may exert part of their mitochondria-mediated cytotoxic effects.

PARL partitions the lipid transfer protein STARD7 between the cytosol and mitochondria

Intramembrane-cleaving peptidases of the rhomboid family regulate diverse cellular processes that are critical for development and cell survival. The function of the rhomboid protease PARL in the mitochondrial inner membrane has been linked to mitophagy and apoptosis, but other regulatory functions are likely to exist. Here, we identify the START domain-containing protein STARD7 as an intramitochondrial lipid transfer protein for phosphatidylcholine. We demonstrate that PARL-mediated cleavage during mitochondrial import partitions STARD7 to the cytosol and the mitochondrial intermembrane space. Negatively charged amino acids in STARD7 serve as a sorting signal allowing mitochondrial release of mature STARD7 upon cleavage by PARL On the other hand, membrane insertion of STARD7 mediated by the TIM23 complex promotes mitochondrial localization of mature STARD7. Mitochondrial STARD7 is necessary and sufficient for the accumulation of phosphatidylcholine in the inner membrane and for the maintenance of respiration and cristae morphogenesis. Thus, PARL preserves mitochondrial membrane homeostasis via STARD7 processing and is emerging as a critical regulator of protein localization between mitochondria and the cytosol.

Identification of the N-terminal transmembrane domain of StarD7 and its importance for mitochondrial outer membrane localization and phosphatidylcholine transfer

StarD7 facilitates phosphatidylcholine (PC) transfer to mitochondria, and is essential for mitochondrial homeostasis. However, the molecular mechanism for PC transfer by protein remains poorly understood. Herein, we describe a putative novel transmembrane (TM) domain C-terminal to the mitochondria-targeting signal (MTS) sequence at the N-terminus of StarD7. The mature form of StarD7 is integrated and/or associated onto the outer leaflet of the outer mitochondrial membrane (OMM) in HEPA-1 and HepG2 cells. A truncated form of StarD7 lacking the TM domain is distributed in the inner space of the mitochondria, and cannot reverse mitochondrial abnormalities, such as complex formation and PC content, when re-expressed in StarD7-KO HEPA-1 cells. Re-expression of wild StarD7 can compensate these mitochondrial functions of StarD7-KO HEPA-1 cells. The precursor form of StarD7 is cleaved between Met⁷⁶ and Ala⁷⁷, and Ala⁷⁷ and Ala⁷⁸ in the TM domain to produce the mature form. These results suggest that StarD7 is anchored onto the OMM through its N-terminal TM domain, and the C-terminal START domain may extend into the cytoplasm and shuttle PC between the ER and OMM at the ER-mitochondria contact sites.

The Phosphatidylcholine Transfer Protein Stard7 is Required for Mitochondrial and Epithelial Cell Homeostasis

Mitochondria synthesize select phospholipids but lack the machinery for synthesis of the most abundant mitochondrial phospholipid, phosphatidylcholine (PC). Although the phospholipid transfer protein Stard7 promotes uptake of PC by mitochondria, the importance of this pathway for mitochondrial and cellular homeostasis represents a significant knowledge gap. Haploinsufficiency for Stard7 is associated with significant exacerbation of allergic airway disease in mice, including an increase in epithelial barrier permeability. To test the hypothesis that Stard7 deficiency leads to altered barrier structure/function downstream of mitochondrial dysfunction, Stard7 expression was knocked down in a bronchiolar epithelial cell line (BEAS-2B) and specifically deleted in lung epithelial cells of mice (Stard7^epi∆/∆). Stard7 deficiency was associated with altered mitochondrial size and membrane organization both in vitro and in vivo. Altered mitochondrial structure was accompanied by disruption of mitochondrial homeostasis, including decreased aerobic respiration, increased oxidant stress, and mitochondrial DNA damage that, in turn, was linked to altered barrier integrity and function. Both mitochondrial and barrier defects were largely corrected by targeting Stard7 to mitochondria or treating epithelial cells with a mitochondrial-targeted antioxidant. These studies suggest that Stard7-mediated transfer of PC is crucial for mitochondrial homeostasis and that mitochondrial dysfunction contributes to altered barrier permeability in Stard7-deficient mice.