Lipid transporter TMEM24/C2CD2L is a Ca ²⁺ -regulated component of ER–plasma membrane contacts in mammalian neurons

Abstract

Close appositions between the endoplasmic reticulum (ER) and the plasma membrane (PM) are a general feature of all cells and are abundant in neurons. A function of these appositions is lipid transport between the two adjacent bilayers via tethering proteins that also contain lipid transport modules. However, little is known about the properties and dynamics of these proteins in neurons.Here we focused on TMEM24/C2CD2L, an ER-localized SMP domain containing phospholipid transporter expressed at high levels in the brain, previously shown to be a component of ER-PM contacts in pancreatic β-cells. TMEM24 is enriched in neurons versus glial cells and its levels increase in parallel with neuronal differentiation. It populates ER-PM contacts in resting neurons, but elevations of cytosolic Ca 2+ mediated by experimental manipulations or spontaneous activity induce its transient redistribution throughout the entire ER. Dissociation of TMEM24 from the plasma membrane is mediated by phosphorylation of an array of sites in the C-terminal region of the protein. These sites are only partially conserved in C2CD2, the paralogue of TMEM24 primarily expressed in nonneuronal tissues, which correspondingly display a much lower sensitivity to Ca 2+ elevations. ER-PM contacts in neurons are also sites where Kv2 (the major delayed rectifier K + channels in brain) and other PM and ER ion channels are concentrated, raising the possibility of a regulatory feedback mechanism between neuronal excitability and lipid exchange between the ER and the PM.SMP domain | lipid-transfer protein | potassium channel | extended synaptotagmin | VAP

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

“…However, at least in the presynaptic region, E-Syt may not be required for lipid homeostasis, since synaptic balance of phospholipids PI(4,5)P 2 and PI(3)P appears normal in E-Syt mutants (Kikuma et al, 2017). Transmembrane protein 24 (TMEM24), also regulated by Ca 2+ and with SMP domains, is enriched at ER-PM MCSs, where it acts as a tether and transfers phospholipids in mammalian neurons, including in axons (Sun et al, 2019).…”

Integrated regulation of the phosphatidylinositol cycle and phosphoinositide‐driven lipid transport at ER‐PM contact sites

“…In addition to the E‐Syts, another TULIP superfamily and SMP domain‐containing LTP, transmembrane protein 24 (TMEM24 or C2CD2L), has recently been shown to regulate PM lipid composition and excitable cell functions . The domain organization of TMEM24 shows some similarities to E‐Syts, with an N‐terminal transmembrane segment, anchoring the protein in the ER, which is flanked by an extended cytosolic region containing the SMP domain along with a single C2 module .…”

“…Recruitment of TMEM24 to the PM is thought to involve anionic lipids other than PtdIns(4,5)P 2 , including a likely role for PtdSer, as the PM‐binding of TMEM24 is not sensitive to acute depletion of PtdIns(4,5)P 2 within the PM . TMEM24 localization to the PM is also reversibly regulated by phosphorylation events within the unstructured C‐terminus that are sensitive to cytosolic Ca 2+ elevations, involving the Ca 2+ ‐sensitive isoforms of PKC as triggers for the dissociation of TMEM24 from ER‐PM contact sites . Furthermore, the Ca 2+ ‐ and calmodulin‐regulated protein phosphatase, calcineurin, appears to facilitate the dephosphorylation of TMEM24 to allow for the re‐association of the C‐terminus with the PM .…”

“…Furthermore, the Ca 2+ ‐ and calmodulin‐regulated protein phosphatase, calcineurin, appears to facilitate the dephosphorylation of TMEM24 to allow for the re‐association of the C‐terminus with the PM . Ca 2+ ‐dependent coordination of these phosphorylation events allows TMEM24 to dynamically cycle in and out of ER‐PM contacts; a striking feature which has been used to associate TMEM24 with lipid transfer events during pulsatile insulin secretion from pancreatic β cells and neurotransmission . More recently, a paralog of TMEM24 has also been described, C2CD2, which lacks the consensus PKC phosphorylation sites within the C‐terminus and, as a result, its localization to ER‐PM contacts does not appear to be regulated by Ca 2+ .…”

“…Ca 2+ ‐dependent coordination of these phosphorylation events allows TMEM24 to dynamically cycle in and out of ER‐PM contacts; a striking feature which has been used to associate TMEM24 with lipid transfer events during pulsatile insulin secretion from pancreatic β cells and neurotransmission . More recently, a paralog of TMEM24 has also been described, C2CD2, which lacks the consensus PKC phosphorylation sites within the C‐terminus and, as a result, its localization to ER‐PM contacts does not appear to be regulated by Ca 2+ . Importantly, the SMP domain of TMEM24 shows a preference for PtdIns, which has direct implications for an involvement in the PtdIns cycle.…”