Lipids at membrane contact sites: cell signaling and ion transport

Abstract

Communication between organelles is essential to coordinate cellular functions and the cell's response to physiological and pathological stimuli. Organellar communication occurs at membrane contact sites (MCSs), where the endoplasmic reticulum (ER) membrane is tethered to cellular organelle membranes by specific tether proteins and where lipid transfer proteins and cell signaling proteins are located. MCSs have many cellular functions and are the sites of lipid and ion transfer between organelles and generation of second messengers. This review discusses several aspects of MCSs in the context of lipid transfer, formation of lipid domains, generation of Ca²⁺ and cAMP second messengers, and regulation of ion transporters by lipids.

Keywords: contact sites; ion transport; lipid transfer; signaling.

Figure 1. Organellar lipid synthesis and transport

Biosynthetic lipid pathways are localized in specific compartments, the plasma membrane (PM), the endoplasmic reticulum (ER), or the mitochondria. At the PM, phosphatidylinositol phosphate (PI) is phosphorylated to PI(4)P and then to PI(4,5)P₂ by PI4 kinase (PI4K) and PI5K, respectively, and PI(4,5)P₂ is hydrolyzed to DAG and IP₃ by phospholipase C (PLC). DAG is converted to PA by diacylglycerol kinase (DGK), the precursor for PI and phosphatidylserine (PS) in the ER, and prostaglandins (PG) in the mitochondria. Lipids are transferred between compartments at membrane contact sites (MCSs). In mammals, ER/PM MCSs are tethered by the extended synaptotagmins (E‐Syts) and likely by the lipid transfer proteins ORP5/8 and Nir2/3. The yeast ER/mitochondria are tethered by the ER–mitochondria encounter structure (ERMES) and ER membrane protein complex (EMC) complexes. In mammalian cells, the ER/mitochondria are tethered by interaction of VAMP‐associated protein‐B (VAP‐B) with protein tyrosine phosphatase‐interacting protein 51 (PTPIP51) and perhaps by ORP5/8. Extensive phospholipid transfer takes place between the PM and ER and between the ER and mitochondria, likely mediated by specific lipid transfer proteins, the identity of most of which is not known at present. However, ORP5/8 mediate PI(4)P/PS exchange between the ER/PM and ER/mitochondria and Nir2 mediates PA/PI exchange between the ER/PM.

Figure 2. Ca²⁺ transfer at the ER/mitochondria MCSs

The ER and outer mitochondrial membrane (OMM) are tethered by interaction of the ER VAP‐B and Fis1 with the OMM PTPIP51 and BAP31, respectively, while mitofusin 2 (MTF2) determines the distance between the ER and OMM to form the ER/mitochondria MCSs. The IP₃Rs‐VDAC1‐MCU complex at the ER/mitochondria MCSs mediates Ca²⁺ transfer from the ER to the mitochondria, while mitochondrial specific AKAP/PKA at the ER/mitochondria MCSs controls PKA activity at the OMM.

Figure 3. Regulation by translocation between PI(4,5)P₂ domains

The MCSs at ER/PM junctions are tethered by E‐Syt1 and E‐Syt2 and Ist2/ANO, with E‐Syt1 affecting STIM1‐Orai1 complex localization and E‐Syt2 may control plasma membrane PI(4)P levels through the PI(4) phosphatase Sac‐1 to determine PI(4,5)P₂ levels of the PI(4,5)P₂‐rich domain at the ER/PM junctions. The AC/PKA/cAMP signaling pathway is also present at the ER/PM junctions. The ER/PM junctions are dynamic and are likely formed or at least stabilized in response to cell stimulation, and the signaling proteins are recruited to the ER/PM junctions after their formation/stabilization. Prior to cell stimulation, STIM1 is in an inactive conformation and is not clustered at the ER/PM junctions. The STIM1 inhibitor SARAF is also at the ER and does not interact with STIM1. AC1, AC3, and AC8 may not be localized at the ER/PM junctions, but at a nearby domain and AC8 is not associated with Orai1. All proteins are at a PM PI(4,5)P₂‐poor domain. In response to depletion of ER Ca²⁺, STIM1 clusters at the ER/PM junctions to expand and stabilize the junctions. STIM1 clusters first at a PI(4,5)P₂‐poor domain and recruits Orai1 to this domain to activate Ca²⁺ influx. Subsequently, the STIM1‐Orai1 complexes translocate to the PI(4,5)P₂‐rich domain where SARAF interacts with STIM1 to initiate the Ca²⁺‐dependent inhibition of Ca²⁺ influx and prevent Ca²⁺ toxicity. Once at the PI(4,5)P₂‐rich domain, STIM1 and Orai1 interact with the Ca²⁺‐activated ACs to trigger generation of cAMP and activation of PKA. The ACs may also be recruited to the PI(4,5)P₂‐rich domain after its stabilization by STIM1, and thus, translocation between PI(4,5)P₂‐poor and PI(4,5)P₂‐rich domains regulates both Ca²⁺ and cAMP signaling.