ORP5 Transfers Phosphatidylserine To Mitochondria And Regulates Mitochondrial Calcium Uptake At Endoplasmic Reticulum - Mitochondria Contact Sites

Background:

Cellular functions are compartmentalized mostly by membrane bound organelles. Like a well-structured company, all these compartmentalized parts need to remain in constant communication. A mechanism that ensures proper communication between organelles is maintained by protein tethers between two membranes at membrane contact sites (MCSs). One of the major functions of these tethers is to allow exchange of signaling factors and lipids to maintain cellular homeostasis. For example, in yeast, the ER-mitochondria encounter structure (ERMES) bridges ER to mitochondria and enables exchange of phosphatidylcholine (PC) between the two organelles (Kornmann et al., 2009). However, the ERMES complex is not conserved in mammals. Although some of the tethers that function as lipid transport proteins (LTPs) at MCSs are known in mammals, our understanding of the mechanism and function of how lipids are exchanged between ER and mitochondria is still rudimentary.

A class of conserved LTPs that are localized to ER belongs to the Oxysterol binding protein (OSBP)-related protein family (ORP). Among this family, ORP5 and ORP8 are two similar ER membrane proteins that transfer phosphatidylserine (PS) between ER and plasma membrane (Chung et al., 2015). These two proteins are also localized to ER-mitochondria contact sites where they regulate mitochondrial morphology (Galmes et al., 2016). However, the role of ORP5/ORP8 in regulation of ER-mitochondria lipid transport is not clear.

Key Findings:

Previous studies had established a role for ORP5/ORP8 in ER-plasma membrane contact sites (Chung et al., 2015, Sohn et al., 2018). In this preprint, Rochin and Sauvanet et al., further characterize the role of ORP5 and ORP8. By using immuno-EM, the authors show that ORP5 and ORP8 are also localized to ER-Mitochondria contact sites in close proximity to structures that bridge inner and outer mitochondrial membranes. It was shown that the OSBP-related lipid binding/transfer domain (ORD) of ORP8 is able to transfer PS in vitro (Chung et al., 2015). The authors confirm this finding and also show that although ORD5 and ORD8 can transfer PS in vitro they cannot transfer PC and phosphatidylethanolamine (PE). Because ER-derived PS is a precursor for mitochondrial PE, the authors ask if ORP5/ORP8 Knock-down (KD) could affect PE levels in mitochondria. To address this question, they perform mass spectrometry (MS)-based lipidomics and show that mitochondrial PE levels are highly reduced in ORP5 KD cells whereas total PE levels remain unchanged. Consistent with PE’s importance for mitochondrial morphology, ORP5/ORP8 KD leads to an aberrant mitochondrial morphology. The authors also show that ORP5 forms a complex with ORP8 and binds to several members of the mitochondrial inner membrane bridging (MIB) complex. This interaction between ORP5/ORP8 and MIB complex has a cooperative role in PS transfer as the double KDs with ORP5 and members of MIB lead to an additive effect on PE decrease. Together, these findings indicate that ORP5/ORP8 transfer PS from ER to mitochondria and function near the outer-inner mitochondrial membrane bridge to facilitate PS to PE conversion at inner mitochondrial membrane.

Although ORP5 and ORP8 seem to function as a complex in PS transfer, the authors show that ORP5 diverges from ORP8 in certain functions. First, they observe that the KD of ORP5 but not ORP8 leads to an increase in certain members of MIB complex. Previous studies have shown that the increase in MIB complex members leads to accumulation of voltage-dependent Ca2+ channel, VDAC (Colombini 2012). In line with this previous study, ORP5 KD (but not ORP8 KD) also leads to an increase in VDAC. Finally, the authors show that KD of ORP5 but not ORP8 leads to an increase in mitochondrial Ca2+ influx consistent with an accumulation of VDAC. Overall, these observations indicate that although ORP5 and ORP8 are involved in PS transfer, ORP5 governs an additional function in controlling mitochondrial Ca2+ uptake. Further studies will tell the if ORP5 related changes in mitochondrial Ca2+ uptake are tied to its function in ER-plasma membrane or its role in ER-mitochondria contacts.

What did I like about this study:

It is fascinating to see how different organelles interact and communicate with each other. This study reveals how ER and mitochondria stay in touch and share a key ingredient for their health. The authors indicate a physiological role for phosphatidylserine (PS) transfer from ER to mitochondria by measuring PS derived mitochondrial PE that is shown to control mitochondrial morphology and health. Moreover, this study suggests a tripartite contact between ER and inner and outer mitochondrial membranes that may directly link physiological state of the ER to that of mitochondria. I am looking forward to seeing the new biology that this tripartite connection will unravel.

Open Questions:

1. It seems like although both ORD5 and ORD8 transfer PS (Fig1C), only KD of ORP5 leads to a significant decrease in mitochondrial PE levels (Fig 1C). Do you think that ORP5 compensates for PS transfer in the absence of ORP8?

2. The authors have previously shown that ORP5 KD leads to a decrease in mitochondrial oxygen consumption rate (OCR). Considering that ORP5/ORP8 double KD leads to a severely affected cristae formation, it would be interesting to see if these cells have a lower OCR when compared to single ORP5 KD. Also, I am wondering if total ATP levels are affected by ORP5/ORP8 double KD or if ATP levels are somehow compensated by a different mechanism? This question is especially important as the authors did not observe a change in mitochondrial membrane potential.

3. Considering that ORP5/8 seem to have similar functions, do you have any speculations as to how the functional differences between ORP5 KD and ORP8 KD are regulated?

References:

Chung J, Torta F, Masai K, Lucast L, Czapla H, Tanner LB, Narayanaswamy P, Wenk MR, Nakatsu F, De Camilli P (2015) PI4P/phosphatidylserine counter transport at ORP5- and ORP8-mediated ER-plasma membrane contacts. Science 349: 428-32

Colombini M (2012) VDAC structure, selectivity, and dynamics. Biochimica et biophysica acta 1818: 1457-65

Galmes R, Houcine A, van Vliet AR, Agostinis P, Jackson CL, Giordano F (2016) ORP5/ORP8 localize to endoplasmic reticulum-mitochondria contacts and are involved in mitochondrial function. EMBO reports 17: 800-10

Kornmann B, Currie E, Collins SR, Schuldiner M, Nunnari J, Weissman JS, Walter P (2009) An ER-mitochondria tethering complex revealed by a synthetic biology screen. Science 325: 477-81

Sohn M, Korzeniowski M, Zewe JP, Wills RC, Hammond GRV, Humpolickova J, Vrzal
L, Chalupska D, Veverka V, Fairn GD, Boura E, Balla T (2018) PI(4,5)P(2) controls
plasma membrane PI4P and PS levels via ORP5/8 recruitment to ER-PM contact sites.
J Cell Biol. 217(5):1797-1813Close encounters with the other kind: How membrane contact sites regulate lipid and ion transfer from ER to mitochondria

Tags: er, lipid transfer, membrane contact sites, mitochondria, mitochondrial ca2+ uptake, orp

doi: https://doi.org/10.1242/prelights.13157

Read preprint (No Ratings Yet)