PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux

Abstract

The absence of functional peripheral myelin protein 22 (PMP22) is associated with shortened lifespan in rodents and severe peripheral nerve myelin abnormalities in several species including humans. Schwann cells and nerves from PMP22 knock-out (KO) mice show deranged cholesterol distribution and aberrant lipid raft morphology, supporting an unrecognized role for PMP22 in cellular lipid metabolism. To examine the mechanisms underlying these abnormalities, we studied Schwann cells and nerves from male and female PMP22 KO mice. Whole-cell current-clamp recordings in cultured Schwann cells revealed increased membrane capacitance and decreased membrane resistance in the absence of PMP22, which was consistent with a reduction in membrane cholesterol. Nerves from PMP22-deficient mice contained abnormal lipid droplets, with both mRNA and protein levels of apolipoprotein E (apoE) and ATPbinding cassette transporter A1 (ABCA1) being highly upregulated. Despite the upregulation of ABCA1 and apoE, the absence of PMP22 resulted in reduced localization of the transporter to the cell membrane and diminished secretion of apoE. The absence of PMP22 also impaired ABCA1-mediated cholesterol efflux capacity. In nerves from ABCA1 KO mice, the expression of PMP22 was significantly elevated and the subcellular processing of the overproduced protein was aberrant. In wild-type samples, double immunolabeling identified overlapping distribution of PMP22 and ABCA1 at the Schwann cell plasma membrane and the two proteins were coimmunoprecipitated from Schwann cell and nerve lysates. Together, these results reveal a novel role for PMP22 in regulating lipid metabolism and cholesterol trafficking through functional interaction with the cholesterol efflux regulatory protein ABCA1. Significance StatementUnderstanding the subcellular events that underlie abnormal myelin formation in hereditary neuropathies is critical for advancing therapy development. Peripheral myelin protein 22 (PMP22) is an essential peripheral myelin protein because its genetic abnormalities account for ϳ80% of hereditary neuropathies. Here, we demonstrate that in the absence of PMP22, the cellular and electrophysiological properties of the Schwann cells' plasma membrane are altered and cholesterol trafficking and lipid homeostasis are perturbed. The molecular mechanisms for these abnormalities involve a functional interplay among PMP22, cholesterol, apolipoprotein E, and the major cholesterol-efflux transporter protein ATP-binding cassette transporter A1 (ABCA1). These findings establish a critical role for PMP22 in the maintenance of cholesterol homeostasis in Schwann cells.

Peripheral Myelin Protein 22 Preferentially Partitions into Ordered Phase Membrane Domains

“…We next tested whether either or both of the predicted cholesterol binding sites in PMP22 play a role in its ordered phase preference. It has been shown that pmp22 -/-Schwann cells exhibit reduced plasma membrane levels and abnormal localization of cholesterol (30,32). These cells also show reduced migration, adhesion, and lamellipodia extension, all of which can be restored through external supplementation of cholesterol in the culture media.…”

“…5C). This may be due to an increased concentration in cholesterol in PMP22-containing GPMVs since it has recently been shown that PMP22 is regulates cholesterol PM trafficking (32). It seems likely that the mechanisms underpinning this regulatory function of PMP22, as well as its ability to promote ordered phase formation, is closely related to its preference to partition into ordered membrane phase domains.…”

“…1A). PMP22 appears to play multiple roles in myelinating Schwann cells and peripheral myelin (27)(28)(29)(30)(31) including a critical role in cholesterol homeostasis, particularly in trafficking of cholesterol through the secretory pathway to the membrane surface (32). This is especially important in Schwann cells given their specialized function as the factory for myelin production in the peripheral nervous system (PNS).…”

Subcellular diversion of cholesterol by gain‐ and loss‐of‐function mutations in PMP22

“…Images were acquired on a Nikon A1RMPsi laser‐scanning confocal microscope and were analyzed using Nikon NIS Elements Software 4.40 or 4.51 (Zhou, Miles, et al, 2019). Background‐reduced images after 2D deconvolution were quantified for subcellular filipin intensity.…”

“…However, in CMT1A rats, which overexpress PMP22, genes related to sterol metabolism are significantly downregulated (Fledrich et al, 2018), while nerves from Trembler (Tr) mice carrying a glycine (G) 150 to aspartic acid (D) mutation in PMP22 exhibited reduced cholesterol levels and synthesis (Bourre, Morand, Dumont, Boutry, & Hauw, 1981; Clouet & Bourre, 1988; Heape, Juguelin, Fabre, Boiron, & Cassagne, 1986; Yao & Bourre, 1985). Recently, through the study of cells and tissue from PMP22 knockout (KO) mice, we discovered the requirement for PMP22 in cholesterol efflux from Schwann cells (Zhou, Miles, et al, 2019).…”