The palmitoylation state of PMP22 modulates epithelial cell morphology and migration

The systemic inhibition of the terminal complement system reduces neuroinflammation but does not improve motor function in mouse models of CMT1A with overexpressed PMP22

Charcot-Marie-Tooth disease type 1A (CMT1A) is the most prevalent hereditary demyelinating neuropathy. This autosomal, dominantly inherited disease is caused by a duplication on chromosome 17p which includes the peripheral myelin protein 22 (PMP22) gene. There is clinical evidence that the disability in CMT1A is to a large extend due to axonal damage rather than demyelination. Over-expression of PMP22 is recently thought to impede cholesterol trafficking causing a total shutdown of local cholesterol and lipid synthesis in the Schwann cells, thus disturbing their ability to remyelinate. But there is a large variety in disease burden between CMT1A patients with the same genetic defect, indicating the presence of modifying factors that affect disease severity. One of these potential factors is the immune system. Several reports have described patients with co-occurrence of CMT1A with chronic inflammatory demyelinating disease or Guillain-Barré syndrome. We have previously shown in multiple animal models that the innate immune system and specifically the terminal complement system is a driver of inflammatory demyelination. To test the contribution of the terminal complement system to neuroinflammation and disease progression in CMT1A, we inhibited systemic complement C6 in two transgenic mouse models for CMT1A, the C3-PMP22 and C3-PMP22 c-JunP0Cre models. Both models over-express human PMP22, and one (C3-PMP22 c-JunP0Cre) also has a Schwann cell-specific knockout of c-Jun, a crucial regulator of myelination controlling autophagy. We found that systemic inhibition of C6 using antisense oligonucleotides affects the neuroinflammation, Rho GTPase and ERK/MAPK signalling pathways in the CMT1A mouse models. The cholesterol synthesis pathway remained unaffected. Analysis of motor function during treatment with C6 antisense oligonucleotides did not reveal any significant improvement in the CMT1A mouse models. This study shows that the contribution of the terminal complement system to progressive loss of motor function in the CMT1A mouse models tested is limited.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

Peripheral myelin protein 22 preferentially partitions into ordered phase membrane domains

The preferential partitioning of single-span membrane proteins for ordered phase domains in phase-separated membranes is now reasonably well understood, but little is known about this phase preference for multispan helical membrane proteins. Here, it is shown that the disease-linked tetraspan membrane protein, PMP22, displays a pronounced preference to partition into the ordered phase, a preference that is reversed by disease mutations. This phase preference may be related to the role of PMP22 in cholesterol homeostasis in myelinating Schwann cells, a role that is also known to be disrupted under conditions of Charcot–Marie–Tooth disease (CMTD) peripheral neuropathy caused by pmp22 mutations.

The ordered environment of cholesterol-rich membrane nanodomains is thought to exclude many transmembrane (TM) proteins. Nevertheless, some multispan helical transmembrane proteins have been proposed to partition into these environments. Here, giant plasma membrane vesicles (GPMVs) were employed to quantitatively show that the helical tetraspan peripheral myelin protein 22 (PMP22) exhibits a pronounced preference for, promotes the formation of, and stabilizes ordered membrane domains. Neither S-palmitoylation of PMP22 nor its putative cholesterol binding motifs are required for this preference. In contrast, Charcot–Marie–Tooth disease-causing mutations that disrupt the stability of PMP22 tertiary structure reduce or eliminate this preference in favor of the disordered phase. These studies demonstrate that the ordered phase preference of PMP22 derives from global structural features associated with the folded form of this protein, providing a glimpse at the structural factors that promote raft partitioning for multispan helical membrane proteins.

Urinary Sediment Transcriptomic and Longitudinal Data to Investigate Renal Function Decline in Type 1 Diabetes

Introduction: Using a discovery/validation approach we investigated associations between a panel of genes selected from a transcriptomic study and the estimated glomerular filtration rate (eGFR) decline across time in a cohort of type 1 diabetes (T1D) patients. Experimental: Urinary sediment transcriptomic was performed to select highly modulated genes in T1D patients with rapid eGFR decline (decliners) vs. patients with stable eGFR (non-decliners). The selected genes were validated in samples from a T1D cohort (n = 54, mean diabetes duration of 21 years, 61% women) followed longitudinally for a median of 12 years in a Diabetes Outpatient Clinic. Results: In the discovery phase, the transcriptomic study revealed 158 genes significantly different between decliners and non-decliners. Ten genes increasingly up or down-regulated according to renal function worsening were selected for validation by qRT-PCR; the genes CYP4F22, and PMP22 were confirmed as differentially expressed comparing decliners vs. non-decliners after adjustment for potential confounders. CYP4F22, LYPD3, PMP22, MAP1LC3C, HS3ST2, GPNMB, CDH6, and PKD2L1 significantly modified the slope of eGFR in T1D patients across time. Conclusions: Eight genes identified as differentially expressed in the urinary sediment of T1D patients presenting different eGFR decline rates significantly increased the accuracy of predicted renal function across time in the studied cohort. These genes may be a promising way of unveiling novel mechanisms associated with diabetic kidney disease progression.

PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux

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 ATP-binding 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 STATEMENT Understanding 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 ∼80% 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.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Topologically Diverse Human Membrane Proteins Partition to Liquid-Disordered Domains in Phase-Separated Lipid Vesicles

The integration of membrane proteins into “lipid raft” membrane domains influences many biochemical processes. The intrinsic structural properties of membrane proteins are thought to mediate their partitioning between membrane domains. However, whether membrane topology influences the targeting of proteins to rafts remains unclear. To address this question, we examined the domain preference of three putative raft-associated membrane proteins with widely different topologies: human caveolin-3, C99 (the 99 residue C-terminal domain of the amyloid precursor protein), and peripheral myelin protein 22. We find that each of these proteins are excluded from the ordered domains of giant unilamellar vesicles containing coexisting liquid-ordered and liquid-disordered phases. Thus, the intrinsic structural properties of these three topologically distinct disease-linked proteins are insufficient to confer affinity for synthetic raft-like domains.

Sparse feature selection methods identify unexpected global cellular response to strontium-containing materials

Despite the increasing sophistication of biomaterials design and functional characterization studies, little is known regarding cells' global response to biomaterials. Here, we combined nontargeted holistic biological and physical science techniques to evaluate how simple strontium ion incorporation within the well-described biomaterial 45S5 bioactive glass (BG) influences the global response of human mesenchymal stem cells. Our objective analyses of whole gene-expression profiles, confirmed by standard molecular biology techniques, revealed that strontium-substituted BG up-regulated the isoprenoid pathway, suggesting an influence on both sterol metabolite synthesis and protein prenylation processes. This up-regulation was accompanied by increases in cellular and membrane cholesterol and lipid raft contents as determined by Raman spectroscopy mapping and total internal reflection fluorescence microscopy analyses and by an increase in cellular content of phosphorylated myosin II light chain. Our unexpected findings of this strong metabolic pathway regulation as a response to biomaterial composition highlight the benefits of discovery-driven nonreductionist approaches to gain a deeper understanding of global cell-material interactions and suggest alternative research routes for evaluating biomaterials to improve their design.

PMP22 is critical for actin-mediated cellular functions and for establishing lipid rafts

Haploinsufficiency of peripheral myelin protein 22 (PMP22) causes hereditary neuropathy with liability to pressure palsies, a peripheral nerve lesion induced by minimal trauma or compression. As PMP22 is localized to cholesterol-enriched membrane domains that are closely linked with the underlying actin network, we asked whether the myelin instability associated with PMP22 deficiency could be mediated by involvement of the protein in actin-dependent cellular functions and/or lipid raft integrity. In peripheral nerves and cells from mice with PMP22 deletion, we assessed the organization of filamentous actin (F-actin), and actin-dependent cellular functions. Using in vitro models, we discovered that, in the absence of PMP22, the migration and adhesion capacity of Schwann cells and fibroblasts are similarly impaired. Furthermore, PMP22-deficient Schwann cells produce shortened myelin internodes, and display compressed axial cell length and collapsed lamellipodia. During early postnatal development, F-actin-enriched Schmidt-Lanterman incisures do not form properly in nerves from PMP22(-/-) mice, and the expression and localization of molecules associated with uncompacted myelin domains and lipid rafts, including flotillin-1, cholesterol, and GM1 ganglioside, are altered. In addition, we identified changes in the levels and distribution of cholesterol and ApoE when PMP22 is absent. Significantly, cholesterol supplementation of the culture medium corrects the elongation and migration deficits of PMP22(-/-) Schwann cells, suggesting that the observed functional impairments are directly linked with cholesterol deficiency of the plasma membrane. Our findings support a novel role for PMP22 in the linkage of the actin cytoskeleton with the plasma membrane, likely through regulating the cholesterol content of lipid rafts.

MAL overexpression leads to disturbed expression of genes that influence cytoskeletal organization and differentiation of Schwann cells

In the developing peripheral nervous system, a coordinated reciprocal signaling between Schwann cells and axons is crucial for accurate myelination. The myelin and lymphocyte protein MAL is a component of lipid rafts that is important for targeting proteins and lipids to distinct domains. MAL overexpression impedes peripheral myelinogenesis, which is evident by a delayed onset of myelination and reduced expression of the myelin protein zero (Mpz/P0) and the low-affinity neurotrophin receptor p75(NTR). This study shows that MAL overexpression leads to a significant reduction of Mpz and p75(NTR) expression in primary mouse Schwann cell cultures, which was already evident before differentiation, implicating an effect of MAL in early Schwann cell development. Their transcription was robustly reduced, despite normal expression of essential transcription factors and receptors. Further, the cAMP response element-binding protein (CREB) and phosphoinositide 3-kinase signaling pathways important for Schwann cell differentiation were correctly induced, highlighting that other so far unknown rate limiting factors do exist. We identified novel genes expressed by Schwann cells in a MAL-dependent manner in vivo and in vitro. A number of those, including S100a4, RhoU and Krt23, are implicated in cytoskeletal organization and plasma membrane dynamics. We showed that S100a4 is predominantly expressed by nonmyelinating Schwann cells, whereas RhoU was localized within myelin membranes, and Krt23 was detected in nonmyelinating as well as in myelinating Schwann cells. Their differential expression during early peripheral nerve development further underlines their possible role in influencing Schwann cell differentiation and myelination.

Palmitoylation of plakophilin is required for desmosome assembly

Desmosomes are prominent adhesive junctions found in various epithelial tissues. The cytoplasmic domains of desmosomal cadherins interact with a host of desmosomal plaque proteins, including plakophilins, plakoglobin and desmoplakin, which, in turn, recruit the intermediate filament cytoskeleton to sites of cell-cell contact. Although the individual components of the desmosome are known, mechanisms regulating the assembly of this junction are poorly understood. Protein palmitoylation is a posttranslational lipid modification that plays an important role in protein trafficking and function. Here, we demonstrate that multiple desmosomal components are palmitoylated in vivo. Pharmacologic inhibition of palmitoylation disrupts desmosome assembly at cell-cell borders. We mapped the site of plakophilin palmitoylation to a conserved cysteine residue present in the armadillo repeat domain. Mutation of this single cysteine residue prevents palmitoylation, disrupts plakophilin incorporation into the desmosomal plaque and prevents plakophilin-dependent desmosome assembly. Finally, plakophilin mutants unable to become palmitoylated act in a dominant-negative manner to disrupt proper localization of endogenous desmosome components and decrease desmosomal adhesion. Taken together, these data demonstrate that palmitoylation of desmosomal components is important for desmosome assembly and adhesion.