bims-mecosi 2023-07-16 papers

Issue of 2023–07–16
five papers selected by
Verena Kohler, University of Graz

Contact (Thousand Oaks). 2023 Jan-Dec;6:6 25152564231181020

IP3R at ER-Mitochondrial Contact Sites: Beyond the IP3R-GRP75-VDAC1 Ca2+ Funnel.

Membrane contact sites (MCS) circumvent the topological constraints of functional coupling between different membrane-bound organelles by providing a means of communication and exchange of materials. One of the most characterised contact sites in the cell is that between the endoplasmic reticulum and the mitochondrial (ERMCS) whose function is to couple cellular Ca2+ homeostasis and mitochondrial function. Inositol 1,4,5-trisphosphate receptors (IP3Rs) on the ER, glucose-regulated protein 75 (GRP 75) and voltage-dependent anion channel 1 (VDAC1) on the outer mitochondrial membrane are the canonical component of the Ca2+ transfer unit at ERMCS. These are often reported to form a Ca2+ funnel that fuels the mitochondrial low-affinity Ca2+ uptake system. We assess the available evidence on the IP3R subtype selectivity at the ERMCS and consider if IP3Rs have other roles at the ERMCS beyond providing Ca2+. Growing evidence suggests that all three IP3R subtypes can localise and regulate Ca2+ signalling at ERMCS. Furthermore, IP3Rs may be structurally important for assembly of the ERMCS in addition to their role in providing Ca2+ at these sites. Evidence that various binding partners regulate the assembly and Ca2+ transfer at ERMCS populated by IP3R-GRP75-VDAC1, suggesting that cells have evolved mechanisms that stabilise these junctions forming a Ca2+ microdomain that is required to fuel mitochondrial Ca2+ uptake.

Keywords: Ca2+; GRP75; IP3 receptor; VDAC1; endoplasmic reticulum; membrane contact sites; mitochondria

DOI: https://doi.org/10.1177/25152564231181020

Methods Cell Biol. 2023 ;pii: S0091-679X(22)00209-6. [Epub ahead of print]177 101-123

Liver inter-organelle membrane contact sites revealed by serial section electron tomography.

Andrea Raimondi, Nicolò Ilacqua, Luca Pellegrini.

Inter-organelle membrane contact sites (MCSs) are defined as areas of close proximity between the membranes of two organelles (10-80nm). They have been implicated in many physiological processes such as Ca++, lipids or small molecules transfer, organelles biogenesis or dynamic and have an important role in many cellular processes such as apoptosis, autophagy, and signaling. Since the distance and the extent of these contacts are in the nanometer range, high resolution techniques are ideal for imaging these structures. It is for this reason that transmission electron microscopy (TEM) has been considered the gold standard for MCSs visualization and the first technique that described them. However, often TEM analysis is limited to 2D lacking information on the 3D association between the organelles involved in MCSs. To fully describe the complex architecture of MSCs and to unveil their role in cellular physiology a 3D analysis is required. This chapter provides a method for the analysis of MCSs using serial section electron tomography (ssET), a technique able to reconstruct in 3D at nanometer resolution cellular and subcellular structures. By applying this procedure, it was possible to elucidate the role of the contacts between Endoplasmic Reticulum (ER) and other organelles in liver lipid metabolism.

Keywords: High pressure freezing; Image analysis; Liver physiology; Membrane contact sites; Serial section electron tomography; Volume electron microscopy

DOI: https://doi.org/10.1016/bs.mcb.2022.12.021

Mov Disord. 2023 Jul 14.

Mitochondria-Endoplasmic Reticulum Contact Sites Dynamics and Calcium Homeostasis Are Differentially Disrupted in PINK1-PD or PRKN-PD Neurons.

Dajana Grossmann, Nina Malburg, Hannes Glaß, Veronika Weeren, Verena Sondermann, Julia F Pfeiffer, Janine Petters, Jan Lukas, Philip Seibler, Christine Klein, Anne Grünewald, Andreas Hermann.

BACKGROUND: It is generally believed that the pathogenesis of PINK1/parkin-related Parkinson's disease (PD) is due to a disturbance in mitochondrial quality control. However, recent studies have found that PINK1 and Parkin play a significant role in mitochondrial calcium homeostasis and are involved in the regulation of mitochondria-endoplasmic reticulum contact sites (MERCSs).
OBJECTIVE: The aim of our study was to perform an in-depth analysis of the role of MERCSs and impaired calcium homeostasis in PINK1/Parkin-linked PD.
METHODS: In our study, we used induced pluripotent stem cell-derived dopaminergic neurons from patients with PD with loss-of-function mutations in PINK1 or PRKN. We employed a split-GFP-based contact site sensor in combination with the calcium-sensitive dye Rhod-2 AM and applied Airyscan live-cell super-resolution microscopy to determine how MERCSs are involved in the regulation of mitochondrial calcium homeostasis.
RESULTS: Our results showed that thapsigargin-induced calcium stress leads to an increase of the abundance of narrow MERCSs in wild-type neurons. Intriguingly, calcium levels at the MERCSs remained stable, whereas the increased net calcium influx resulted in elevated mitochondrial calcium levels. However, PINK1-PD or PRKN-PD neurons showed an increased abundance of MERCSs at baseline, accompanied by an inability to further increase MERCSs upon thapsigargin-induced calcium stress. Consequently, calcium distribution at MERCSs and within mitochondria was disrupted.
CONCLUSIONS: Our results demonstrated how the endoplasmic reticulum and mitochondria work together to cope with calcium stress in wild-type neurons. In addition, our results suggests that PRKN deficiency affects the dynamics and composition of MERCSs differently from PINK1 deficiency, resulting in differentially affected calcium homeostasis. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Keywords: PINK1; Parkin; Parkinson´s disease; calcium; mitochondria-ER contact sites

DOI: https://doi.org/10.1002/mds.29525

J Periodontol. 2023 Jul 12.

Evaluation of the role of mitofusin-1 and mitofusin-2 in periodontal disease.

Ömer Alperen Kırmızıgül, Arife Sabanci, Faruk Dişli, Sedat Yıldız, Michael R Milward, Kübra Aral.

BACKGROUND: Mitochondria and endoplasmic reticulum are key cellular organelles and create contact sites (MERC), which plays a major role in calcium metabolism, apoptotic processes and inflammation. Previously, proteins that have been associated with these MERC contact sites mitofusin-1 (MFN1) and mitofusin-2 (MFN2) have been found to be downregulated in periodontal disease in vitro. Therefore, the aim of the current study was to evaluate MFN1 and MFN2 in gingival crevicular fluid (GCF) of patients with periodontal disease compared to healthy controls clinically.
METHODS: A total of 48 participants were divided into three groups including periodontally healthy (group H)(n = 16), patients with gingivitis (group G)(n = 16) and patients with stage 3 grade B periodontitis (group P)(n = 16). GCF levels of MFN1, MFN2, calcium (Ca), caspase-1 and tumour necrosis factor alpha (TNF-α) were determined via enzyme-linked immunosorbent assay (ELISA). Results were calculated as total amount and concentration.
RESULTS: MFN1 levels (total amount) were significantly higher in periodontitis and gingivitis patients when compared to healthy controls (p<0.05). However concentration levels of MFN1, MFN2, Ca, caspase-1, TNF-α significantly decreased in periodontal disease groups compared to healthy controls (p<0.05). A positive correlation was detected among all evaluated markers (p<0.05).
CONCLUSION: The mitochondria and endoplasmic reticulum contact protein MFN1 may have a role in the pathogenesis of periodontal disease due to its increase in GCF of periodontitis and gingivitis patients. This article is protected by copyright. All rights reserved.

Keywords: Calcium; Caspase 1; Mfn1; Tumor Necrosis Factor-alpha; mitochondria; periodontitis

DOI: https://doi.org/10.1002/JPER.23-0072