bims-mitpro 2025-04-20 papers

Issue of 2025–04–20
seven papers selected by
Andreas Kohler, Umeå University

Trends Biochem Sci. 2025 Apr 15. pii: S0968-0004(25)00060-X. [Epub ahead of print]

Lipids: emerging actors in mitochondrial protein import.

Mayra A Borrero-Landazabal, Vanessa Linke, Agnieszka Chacinska.

Lipids are emerging as functional players in mitochondrial protein import beyond constituting membranes. Cryo-electron microscopy structures of protein translocases such as translocase of the outer membrane (TOM) and insertases such as translocase of the inner membrane (TIM22) link lipids to protein import by suggesting structural and functional roles for lipids in protein translocation and insertion, and for protein insertases in lipid scrambling.

Keywords: membrane complexes; mitochondrial biology; mitochondrial protein import; protein–lipid interactions

DOI: https://doi.org/10.1016/j.tibs.2025.03.011

Biol Chem. 2025 Apr 17.

The mitochondrial unfolded protein response: acting near and far.

Nikolaos Charmpilas, Qiaochu Li, Thorsten Hoppe.

Mitochondria are central hubs of cellular metabolism and their dysfunction has been implicated in a variety of human pathologies and the onset of aging. To ensure proper mitochondrial function under misfolding stress, a retrograde mitochondrial signaling pathway known as UPRmt is activated. The UPRmt ensures that mitochondrial stress is communicated to the nucleus, where gene expression for several mitochondrial proteases and chaperones is induced, forming a protective mechanism to restore mitochondrial proteostasis and function. Importantly, the UPRmt not only acts within cells, but also exhibits a conserved cell-nonautonomous activation across species, where mitochondrial stress in a defined tissue triggers a systemic response that affects distant organs. Here, we summarize the molecular basis of the UPRmt in the invertebrate model organism Caenorhabditis elegans and in mammals. We also describe recent findings on cell-nonautonomous activation of the UPRmt in worms, flies and mice, and how UPRmt activation in specific tissues affects organismal metabolism and longevity.

Keywords: cell-nonautonomous regulation; integrated stress response; mitochondria; mitochondrial unfolded protein response; stress signaling

DOI: https://doi.org/10.1515/hsz-2025-0107

Bone Res. 2025 Apr 14. 13(1): 47

SPI1 activates mitochondrial unfolded response signaling to inhibit chondrocyte senescence and relieves osteoarthritis.

Xiangyu Zu, Shenghong Chen, Zhengyuan Li, Lin Hao, Wenhan Fu, Hui Zhang, Zongsheng Yin, Yin Wang, Jun Wang.

Chondrocyte senescence is a critical pathological hallmark of osteoarthritis (OA). Aberrant mechanical stress is considered a pivotal determinant in chondrocyte aging; however, the precise underlying mechanism remains elusive. Our findings demonstrate that SPI1 plays a significant role in counteracting chondrocyte senescence and inhibiting OA progression. SPI1 binds to the PERK promoter, thereby promoting its transcriptional activity. Importantly, PERK, rather than GCN2, facilitates eIF2α phosphorylation, activating the mitochondrial unfolded protein response (UPRmt) and impeding chondrocyte senescence. Deficiency of SPI1 in mechanical overload-induced mice leads to diminished UPRmt activation and accelerated OA progression. Intra-articular injection of adenovirus vectors overexpressing SPI1 and PERK effectively mitigates cartilage degeneration. In summary, our study elucidates the crucial regulatory role of SPI1 in the pathogenesis of chondrocyte senescence by activating UPRmt signaling through PERK, which may present a novel therapeutic target for treating OA. SPI1 alleviates the progression of OA by inhibiting mechanical stress-induced chondrocyte senescence through mitochondrial UPR signaling.

DOI: https://doi.org/10.1038/s41413-025-00421-4

bioRxiv. 2025 Apr 01. pii: 2025.03.31.646376. [Epub ahead of print]

An Msp1-Protease Chimera Captures Transient AAA+ Interactions and Unveils Ost4 Mislocalization Errors.

Deepika Gaur, Brian Acquaviva, Matthew L Wohlever.

Membrane protein homeostasis (proteostasis) is essential for maintaining the integrity of eukaryotic organelles. Msp1 is a membrane anchored AAA+ (ATPase Associated with cellular Activities) protein that maintains mitochondrial proteostasis by extracting aberrant proteins from the outer mitochondrial membrane. A comprehensive understanding of the physiological roles of Msp1 has been hindered because AAA+ proteins interact with substrates transiently and common strategies to stabilize this interaction lead to undesirable mitochondrial phenotypes. To circumvent these drawbacks, we fused catalytically active Msp1 to the inactivated protease domain of the AAA+ protease Yme1. The resulting chimera sequesters substrates in the catalytically inactive degradation chamber formed by the protease domain. We performed mass spectrometry analysis with the Msp1-protease chimera and identified the signal anchored protein Ost4 as a novel Msp1 substrate. Topology experiments show that Ost4 adopts mixed orientations when mislocalized to mitochondria and that Msp1 extracts mislocalized Ost4 regardless of orientation. Together, this work develops new tools for capturing transient interactions with AAA+ proteins, identifies new Msp1 substrates, and shows a surprising error in targeting of Ost4.

DOI: https://doi.org/10.1101/2025.03.31.646376

bioRxiv. 2025 Apr 10. pii: 2025.04.03.647084. [Epub ahead of print]

STAR/STARD1: a mitochondrial intermembrane space cholesterol shuttle degraded through mitophagy.

Prasanthi P Koganti, Amy H Zhao, Michael C Kern, Auriole C R Fassinou, Vimal Selvaraj.

The import of cholesterol to the inner mitochondrial membrane by the steroidogenic acute regulatory protein (STAR/STARD1) is essential for de novo steroid hormone biosynthesis and the acidic pathway of bile acid synthesis. This robust system, evolved to start and stop colossal cholesterol movement, ensures pulsatile yet swift mitochondrial steroid metabolism in cells. Nonetheless, the proposed mechanism and components involved in this process has remained a topic of ongoing debate. In this study, we elucidate the mitochondrial import machinery and structural aspects of STAR, revealing its role as an intermembrane space cholesterol shuttle that subsequently undergoes rapid degradation by mitophagy. This newfound mechanism illuminates a fundamental process in cell biology and provides precise interpretations for the full range of human STAR mutation-driven lipoid congenital adrenal hyperplasia in patients.

DOI: https://doi.org/10.1101/2025.04.03.647084

Proc Natl Acad Sci U S A. 2025 Apr 22. 122(16): e2419881122

Mechanism of allosteric activation in human mitochondrial ClpP protease.

Monica M Goncalves, Adwaith B Uday, Taylor J B Forrester, S Quinn W Currie, Angelina S Kim, Yue Feng, Yulia Jitkova, Algirdas Velyvis, Robert W Harkness, Matthew S Kimber, Aaron D Schimmer, Natalie Zeytuni, Siavash Vahidi.

Human ClpP protease contributes to mitochondrial protein quality control by degrading misfolded proteins. ClpP is overexpressed in cancers such as acute myeloid leukemia (AML), where its inhibition leads to the accumulation of damaged respiratory chain subunits and cell death. Conversely, hyperactivating ClpP with small-molecule activators, such as the recently discovered ONC201, disrupts mitochondrial protein degradation and impairs respiration in cancer cells. Despite its critical role in human health, the mechanism underlying the structural and functional properties of human ClpP remains elusive. Notably, human ClpP is paradoxically activated by active-site inhibitors. All available structures of human ClpP published to date are in the inactive compact or compressed states, surprisingly even when ClpP is bound to an activator molecule such as ONC201. Here, we present structures of human mitochondrial ClpP in the active extended state, including a pair of structures where ClpP is bound to an active-site inhibitor. We demonstrate that amino acid substitutions in the handle region (A192E and E196R) recreate a conserved salt bridge found in bacterial ClpP, stabilizing the extended active state and significantly enhancing ClpP activity. We elucidate the ClpP activation mechanism, highlighting a hormetic effect where substoichiometric inhibitor binding triggers an allosteric transition that drives ClpP into its active extended state. Our findings link the conformational dynamics of ClpP to its catalytic function and provide high-resolution structures for the rational design of potent and specific ClpP inhibitors, with implications for targeting AML and other disorders with ClpP involvement.

Keywords: ClpP protease; HDX–MS; allostery; cryo-EM; intracellular protein degradation

DOI: https://doi.org/10.1073/pnas.2419881122

Front Pharmacol. 2025 ;16 1575733

BAM8-22 targets spinal MrgC receptors to modulate UPRmt activity in the mechanism of bone cancer pain.

Mingming Xie, Dan Li, Haohao Zeng, Yulin Huang, Rui Xu, Zhen Wang, Jiacheng Yu, Yu'e Sun.

Background: Bone cancer pain (BCP) significantly impacts patients' overall quality of life. Cellular energy metabolism homeostasis is critically dependent on mitochondrial integrity, and emerging evidence suggests that mitochondrial dysfunction in chronic BCP exacerbates pain progression by disrupting nociceptive signaling pathways. Notably, G protein-coupled receptors (GPCRs), a major class of membrane receptors, modulate mitochondrial function through diverse molecular mechanisms. In this study, we investigated the role of Mas-related G protein-coupled receptor C (MrgC) in BCP pathogenesis and its regulatory effects on mitochondrial function.
Methods: Male C3H/HeN mice were utilized to establish a BCP model. Transmission electron microscopy and flow cytometry were employed to assess changes in mitochondrial ultrastructure, as well as levels of mtROS, ATP, and MMP in mice experiencing BCP. Following intrathecal injection of BAM8-22, we analyzed the effects of activated MrgC on mitochondrial unfolded protein response (UPRmt)-related molecules (ATF5, HSP60, LONP1, CLPP) and pain-related behaviors in BCP mice. The regulatory mechanism of MrgC on UPRmt was further explored in N2a and 293T cells.
Results: Mice with bone cancer pain showed improved mRNA and protein levels of UPRmt-related molecules, increased MMP and ATP, decreased mitochondrial ROS levels in the spinal cord after receiving an intrathecal injection of BAM8-22. Additionally, the paw withdrawal mechanical threshold in BCP mice increased, while the number of spontaneous foot lifts decreased. In complementary cellular studies, transfection-mediated overexpression of MrgC in N2a cells enhanced UPRmt biomarker expression, whereas RNA interference-mediated MrgC knockdown produced the opposite effect.
Conclusion: By activating spinal MrgC to mediate UPRmt activity and protect mitochondrial function, BAM8-22 contributes to the molecular development of BCP. This discovery suggests a new therapeutic target for BCP and offers a possible research avenue.

Keywords: BAM8-22; Mas-related G protein-coupled receptor C; bone cancer pain; mitochondrial dysfunction; mitochondrial unfolded protein response

DOI: https://doi.org/10.3389/fphar.2025.1575733