Contact sites between endoplasmic reticulum sheets and mitochondria regulate mitochondrial DNA replication and segregation

Redox regulation of UPR signalling and mitochondrial ER contact sites

Mitochondria and the endoplasmic reticulum (ER) have a synergistic relationship and are key regulatory hubs in maintaining cell homeostasis. Communication between these organelles is mediated by mitochondria ER contact sites (MERCS), allowing the exchange of material and information, modulating calcium homeostasis, redox signalling, lipid transfer and the regulation of mitochondrial dynamics. MERCS are dynamic structures that allow cells to respond to changes in the intracellular environment under normal homeostatic conditions, while their assembly/disassembly are affected by pathophysiological conditions such as ageing and disease. Disruption of protein folding in the ER lumen can activate the Unfolded Protein Response (UPR), promoting the remodelling of ER membranes and MERCS formation. The UPR stress receptor kinases PERK and IRE1, are located at or close to MERCS. UPR signalling can be adaptive or maladaptive, depending on whether the disruption in protein folding or ER stress is transient or sustained. Adaptive UPR signalling via MERCS can increase mitochondrial calcium import, metabolism and dynamics, while maladaptive UPR signalling can result in excessive calcium import and activation of apoptotic pathways. Targeting UPR signalling and the assembly of MERCS is an attractive therapeutic approach for a range of age-related conditions such as neurodegeneration and sarcopenia. This review highlights the emerging evidence related to the role of redox mediated UPR activation in orchestrating inter-organelle communication between the ER and mitochondria, and ultimately the determination of cell function and fate.

Connection Between HIV and Mitochondria in Cardiovascular Disease and Implications for Treatments

HIV infection and antiretroviral therapy alter mitochondrial function, which can progressively lead to mitochondrial damage and accelerated aging. The interaction between persistent HIV reservoirs and mitochondria may provide insight into the relatively high rates of cardiovascular disease and mortality in persons living with HIV. In this review, we explore the intricate relationship between HIV and mitochondrial function, highlighting the potential for novel therapeutic strategies in the context of cardiovascular diseases. We reflect on mitochondrial dynamics, mitochondrial DNA, and mitochondrial antiviral signaling protein in the context of HIV. Furthermore, we summarize how toxicities related to early antiretroviral therapy and current highly active antiretroviral therapy can contribute to mitochondrial dysregulation, chronic inflammation, and poor clinical outcomes. There is a need to understand the mechanisms and develop new targeted therapies. We further consider current and potential future therapies for HIV and their interplay with mitochondria. We reflect on the next-generation antiretroviral therapies and HIV cure due to the direct and indirect effects of HIV persistence, associated comorbidities, coinfections, and the advancement of interdisciplinary research fields. This includes exploring novel and creative approaches to target mitochondria for therapeutic intervention.

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.

Protocol for measuring interorganelle contact sites in primary cells using a modified proximity ligation assay

Interorganelle contact sites regulate lipid metabolism, organelle dynamics and positioning, as well as apoptosis and autophagy. Here, we present a proximity ligation assay (PLA) protocol for measuring the association of two organelles in fixed cells. We describe steps for primary cell culture, primary cell transfection, and the assay itself. We then detail procedures for manual and image J-based analysis of PLA foci. This protocol optimizes the use of assay products and improves the identification of PLA foci labeling actual contact sites. For complete details on the use and execution of this protocol, please refer to Ilamathi et al. (2023).1.

SYNJ2BP ameliorates intervertebral disc degeneration by facilitating mitochondria-associated endoplasmic reticulum membrane formation and mitochondrial Zn2+ homeostasis

Nucleus pulposus (NP) cell function-loss is one main contributor during intervertebral disc degeneration (IDD) progression. Both mitochondria and endoplasmic reticulum (ER) play vital roles in sustaining NP cell homeostasis, while the precise function of ER-mitochondria tethering and cross talk in IDD remain to be clarified. Here, we demonstrated that a notable disruption of mitochondria-associated ER membrane (MAM) was identified in degenerated discs and TBHP-induced NP cells, accompanied by mitochondrial Zn2+ overload and NP cell senescence. Importantly, experimental coupling of MAM contacts by MFN2, a critical regulator of MAM formation, could enhance NLRX1-SLC39A7 complex formation and mitochondrial Zn2+ homeostasis. Further using the sequencing data from TBHP-induced degenerative model of NP cells, combining the reported MAM proteomes, we demonstrated that SYNJ2BP loss was one critical pathological characteristic of NP cell senescence and IDD progression, which showed close relationship with MAM disruption. Overexpression of SYNJ2BP could facilitate MAM contact organization and NLRX1-SLC39A7 complex formation, thus promoted mitochondrial Zn2+ homeostasis, NP cell proliferation and intervertebral disc rejuvenation. Collectively, our present study revealed a critical role of SYNJ2BP in maintaining mitochondrial Zn2+ homeostasis in NP cells during IDD progression, partially via sustaining MAM contact and NLRX1-SLC39A7 complex formation.

ER-mitochondria contact sites in mitochondrial DNA dynamics, maintenance, and distribution

Mitochondria are central cellular metabolic hubs. Their function requires proteins encoded by nuclear DNA, but also mitochondrial DNA (mtDNA) whose maintenance is essential for the proper function of the organelle. Defective mtDNA maintenance and distribution are associated with mitochondrial diseases. mtDNA is organized into nucleo-protein complexes called nucleoids that dynamically move along the mitochondrial network and interact with each other. mtDNA replication and nucleoid distribution is an active process regulated by the complex interplay of mitochondrial dynamics, endoplasmic reticulum (ER)-mitochondria contact sites, and cytoskeletal networks. For example, defects in mitochondrial fusion and fission or ER-mitochondria contact sites affect nucleoid maintenance and distribution. In this review, we discuss the process of nucleoid dynamics and the factors regulating nucleoid maintenance and distribution.