Distinct mechanisms controlling rough and smooth endoplasmic reticulum-mitochondria contacts

Abstract

Gp78 (also known as AMFR), an endoplasmic-reticulum (ER)-associated protein degradation (ERAD) E3 ubiquitin ligase, localizes to mitochondria-associated ER and targets the mitofusin (Mfn1 and Mfn2) mitochondrial fusion proteins for degradation. Gp78 is also the cell surface receptor for autocrine motility factor (AMF), which prevents Gp78-dependent mitofusin degradation. Gp78 ubiquitin ligase activity promotes ER-mitochondria association and ER-mitochondria Ca 2+ coupling, processes that are reversed by AMF. Electron microscopy of HT-1080 fibrosarcoma cancer cells identified both smooth ER (SER; ∼8 nm) and wider (∼50-60 nm) rough ER (RER)-mitochondria contacts. Both short hairpin RNA (shRNA)-mediated knockdown of Gp78 (shGp78) and AMF treatment selectively reduced the extent of RER-mitochondria contacts without impacting on SER-mitochondria contacts. Concomitant small interfering RNA (siRNA)-mediated knockdown of Mfn1 increased SER-mitochondria contacts in both control and shGp78 cells, whereas knockdown of Mfn2 increased RER-mitochondria contacts selectively in shGp78 HT-1080 cells. The mitofusins therefore inhibit ER-mitochondria interaction. Regulation of close SER-mitochondria contacts by Mfn1 and of RER-mitochondria contacts by AMFsensitive Gp78-mediated degradation of Mfn2 define new mechanisms that regulate ER-mitochondria interactions.

Ubiquitin mediated regulation of the E3 ligase GP78 by Mahogunin in trans affects mitochondrial homeostasis

“…Furthermore, an involvement of Ca 2+ -dependent regulation in the regulation of ER-mitochondrial contacts by GP78 has been previous reported (Wang et al, 2000(Wang et al, , 2015Goetz et al, 2007;Li et al, 2015). Presence of autocrine motility factor (AMF, also known as GPI) or the RING mutant GP78 has been shown to decrease ER-mitochondria contacts and increase ER-mitochondria Ca 2+ coupling times upon ATP stimulation (Wang et al, 2015). Depletion of mitofusins leads to enhanced ER-mitochondria Ca 2+ crosstalk, and increased sensitivity to mitochondrial-Ca 2+ -mediated cell death (Filadi et al, 2015;Wang et al, 2015).…”

“…Further insight into the mechanism might come from a better understanding of the regulation of GP78 under physiological conditions. Furthermore, an involvement of Ca 2+ -dependent regulation in the regulation of ER-mitochondrial contacts by GP78 has been previous reported (Wang et al, 2000(Wang et al, , 2015Goetz et al, 2007;Li et al, 2015). Presence of autocrine motility factor (AMF, also known as GPI) or the RING mutant GP78 has been shown to decrease ER-mitochondria contacts and increase ER-mitochondria Ca 2+ coupling times upon ATP stimulation (Wang et al, 2015).…”

Structure, function, and regulation of mitofusin‐2 in health and disease

“…In line with this, higher ER-mitochondrial tethering was associated with lower levels of Mfn1 and Mfn2 expression (Li et al, 2015;Wang et al, 2015). Interestingly, different roles for Mfn1 and Mfn2 in the modulation of mitochondria contacts with smooth and rough ER has been proposed (Wang et al, 2015). Despite the controversies concerning the involvement Biological Reviews 93 (2018) of Mfn2 in the formation of ER-mitochondria contact sites, it is clear that a close apposition between the two organelles is necessary for cellular regulation.…”

“…By contrast, cells with reduced Mfn2 expression have increased numbers of ER-mitochondria contact sites, high calcium transfer between the organelles, and are sensitive to apoptotic stimuli (Filadi et al, 2015). In line with this, higher ER-mitochondrial tethering was associated with lower levels of Mfn1 and Mfn2 expression (Li et al, 2015;Wang et al, 2015). Interestingly, different roles for Mfn1 and Mfn2 in the modulation of mitochondria contacts with smooth and rough ER has been proposed (Wang et al, 2015).…”

The coming of age of the mitochondria–ER contact: a matter of thickness

“…Recent studies suggest that mitochondria are tethered to the smooth and rough ER through distinct mechanisms, with the Glycoprotein 78 E3 ubiquitin ligase protein, an ER membrane-anchored ubiquitin ligase (E3), being implicated in the formation of the ribo-MERCs. 69 However, much remains unknown about these structure, which are often found to be in continuity with the 'normal', ribosome-free MERCs (Figure 1 and Figure 3). 4 Future studies will need to focus on the function of these structures and on whether or not they remodel their length and thickness during ER stress or metabolic transitions as it is the case for other types of MERCs.…”