The microdomain-organizing protein MPP1 is required for insulin-stimulated activation of H-Ras

EFR3A: a new raft domain organizing protein?

BACKGROUND

Membrane rafts play a crucial role in the regulation of many important biological processes. Our previous data suggest that specific interactions of flotillins with MPP1 are responsible for membrane raft domain organization and regulation in erythroid cells. Interaction of the flotillin-based protein network with specific membrane components underlies the mechanism of raft domain formation and regulation, including in cells with low expression of MPP1.

METHODS

We sought to identify other flotillin partners via the immobilized recombinant flotillin-2-based affinity approach and mass spectrometry technique. The results were further confirmed via immunoblotting and via co-immunoprecipitation. In order to study the effect of the candidate protein on the physicochemical properties of the plasma membrane, the gene was knocked down via siRNA, and fluorescence lifetime imaging microscopy and spot-variation fluorescence correlation spectroscopy was employed.

RESULTS

EFR3A was identified as a candidate protein that interacts with flotillin-2. Moreover, this newly discovered interaction was demonstrated via overlay assay using recombinant EFR3A and flotillin-2. EFR3A is a stable component of the detergent-resistant membrane fraction of HeLa cells, and its presence was sensitive to the removal of cholesterol. While silencing the EFR3A gene, we observed decreased order of the plasma membrane of living cells or giant plasma membrane vesicles derived from knocked down cells and altered mobility of the raft probe, as indicated via fluorescence lifetime imaging microscopy and spot-variation fluorescence correlation spectroscopy. Moreover, silencing of EFR3A expression was found to disturb epidermal growth factor receptor and phospholipase C gamma phosphorylation and affect epidermal growth factor-dependent cytosolic Ca2+ concentration.

CONCLUSIONS

Altogether, our results suggest hitherto unreported flotillin-2-EFR3A interaction, which might be responsible for membrane raft organization and regulation. This implies participation of this interaction in the regulation of multiple cellular processes, including those connected with cell signaling which points to the possible role in human health, in particular human cancer biology.

Single-cell transcriptome analysis of regenerating RGCs reveals potent glaucoma neural repair genes

Axon regeneration holds great promise for neural repair of CNS axonopathies, including glaucoma. Pten deletion in retinal ganglion cells (RGCs) promotes potent optic nerve regeneration, but only a small population of Pten-null RGCs are actually regenerating RGCs (regRGCs); most surviving RGCs (surRGCs) remain non-regenerative. Here, we developed a strategy to specifically label and purify regRGCs and surRGCs, respectively, from the same Pten-deletion mice after optic nerve crush, in which they differ only in their regeneration capability. Smart-Seq2 single-cell transcriptome analysis revealed novel regeneration-associated genes that significantly promote axon regeneration. The most potent of these, Anxa2, acts synergistically with its ligand tPA in Pten-deletion-induced axon regeneration. Anxa2, its downstream effector ILK, and Mpp1 dramatically protect RGC somata and axons and preserve visual function in a clinically relevant model of glaucoma, demonstrating the exciting potential of this innovative strategy to identify novel effective neural repair candidates.

MPP1 Determines the Mobility of Flotillins and Controls the Confinement of Raft-Associated Molecules

MPP1 (membrane palmitoylated protein 1) belongs to the MAGUK (membrane-associated guanylate kinase homologs) scaffolding protein family. These proteins organize molecules into complexes, thereby maintaining the structural heterogeneity of the plasma membrane (PM). Our previous results indicated that direct, high-affinity interactions between MPP1 and flotillins (raft marker proteins) display dominant PM-modulating capacity in erythroid cells. In this study, with high-resolution structured illuminated imaging, we investigated how these complexes are organized within erythroid cells on the nanometer scale. Furthermore, using other spectroscopic techniques, namely fluorescence recovery after photobleaching (FRAP) and spot-variation fluorescence correlation spectroscopy (svFCS), we revealed that MPP1 acts as a key raft-capturing molecule, regulating temporal immobilization of flotillin-based nanoclusters, and controls local concentration and confinement of sphingomyelin and Thy-1 in raft nanodomains. Our data enabled us to uncover molecular principles governing the key involvement of MPP1-flotillin complexes in the dynamic nanoscale organization of PM of erythroid cells.

High-Level Expression of Palmitoylated MPP1 Recombinant Protein in Mammalian Cells

Our recent studies have pointed to an important role of the MAGUK family member, MPP1, as a crucial molecule interacting with flotillins and involved in the lateral organization of the erythroid plasma membrane. The palmitoylation of MPP1 seems to be an important element in this process; however, studies on the direct effect of palmitoylation on protein–protein or protein–membrane interactions in vitro are still challenging due to the difficulties in obtaining functional post-translationally modified recombinant proteins and the lack of comprehensive protocols for the purification of palmitoylated proteins. In this work, we present an optimized approach for the high-yield overexpression and purification of palmitoylated recombinant MPP1 protein in mammalian HEK-293F cells. The presented approach facilitates further studies on the molecular mechanism of lateral membrane organization and the functional impact of the palmitoylation of MPP1, which could also be carried out for other palmitoylated proteins.

Molecular characterization of direct interactions between MPP1 and flotillins

Flotillins are the major structural proteins in erythroid raft domains. We have shown previously that the dynamic nanoscale organization of raft domains in erythroid cells may depend on flotillin-MPP1 interactions. Here, by using molecular dynamic simulations and a surface plasmon resonance-based approach we determined that high-affinity complexes of MPP1 and flotillins are formed via a so far unidentified region within the D5 domain of MPP1. Significantly, this particular “flotillin binding motif” is of key physiological importance, as overexpression of peptides containing this motif inhibited endogenous MPP1-flotillin interaction in erythroid precursor cells, thereby causing lateral disorganization of raft domains. This was reflected by both reduction in the plasma membrane order and markedly decreased activation of signal transduction via the raft-dependent insulin receptor pathway. Our data highlight new molecular details concerning the mechanism whereby MPP1 functionally links flotillins to exert their physiological role in raft domain formation.

Not Just Another Scaffolding Protein Family: The Multifaceted MPPs

Membrane palmitoylated proteins (MPPs) are a subfamily of a larger group of multidomain proteins, namely, membrane-associated guanylate kinases (MAGUKs). The ubiquitous expression and multidomain structure of MPPs provide the ability to form diverse protein complexes at the cell membranes, which are involved in a wide range of cellular processes, including establishing the proper cell structure, polarity and cell adhesion. The formation of MPP-dependent complexes in various cell types seems to be based on similar principles, but involves members of different protein groups, such as 4.1-ezrin-radixin-moesin (FERM) domain-containing proteins, polarity proteins or other MAGUKs, showing their multifaceted nature. In this review, we discuss the function of the MPP family in the formation of multiple protein complexes. Notably, we depict their significant role for cell physiology, as the loss of interactions between proteins involved in the complex has a variety of negative consequences. Moreover, based on recent studies concerning the mechanism of membrane raft formation, we shed new light on a possible role played by MPPs in lateral membrane organization.

eQTL analysis from co-localization of 2739 GWAS loci detects associated genes across 14 human cancers

Genetic variants can predict other "linked" diseases because alterations in one or more genes in vivo may affect relevant phenotype properties. Our study systematically explored the pan-cancer common gene and cancer type-specific genes based on GWAS loci and TCGA data of multiple cancers. It was found that there were 17 SNPs were significantly associated with the expression of 18 genes. Associations between the 18 cis-regulatory genes and the pathologic stage of each cancer showed that MYL2 and PTGFR in HNSC, 4 genes (F8, SATB2, G6PD and UGT1A6) in KIRP, 3 genes (CHMP4C, MAP3K1 and MECP2) in LUAD were all strongly associated with cancer stage levels. Additionally, the survival association analysis showed that SATB2 was correlated with HNSC survival, and MPP1 was strongly associated with the survival of SARC. This study will shed light on the biological pathways involved in cancer-genetic associations, and has the potential to be applied to the predictions of the risk of cancers developing in healthy individuals.

Immunoliposomes with Simvastatin as a Potential Therapeutic in Treatment of Breast Cancer Cells Overexpressing HER2-An In Vitro Study

Lipophilic statins are promising candidates for breast cancer treatment. However, anticancer therapy requires much higher doses of statins than can be delivered orally, and such high doses are known to exert more adverse effects. The main objective of our study was to design a targeted, therapeutic liposomal carrier of simvastatin characterised by high stability and specificity towards breast cancer cells. We chose SKBR3, the cell line that showed the highest sensitivity for simvastatin and liposomal simvastatin treatment. Additionally, SKBR3 has a notably high expression level of human epidermal growth factor receptor 2 (HER2), which we used as a target for our immunoliposomes. To do so we attached humanized anti-HER2 antibody to the envelope of liposomes. We tested the stability and selectivity of the proposed formulation along with the toxicity, ability to induce apoptosis and the effect on signalling pathways involving Akt and Erk kinases. The immunoliposomal formulation of simvastatin is characterized by long-term stability, high selectivity towards HER2-overexpressing breast cancer cells, low non-specific cytotoxicity and effective inhibition of the growth of target cells, presumably by inhibition of signalling pathways and induction of apoptosis. Hence, for the first time, we propose the use of immunoliposomes with simvastatin, targeted directly towards breast cancer cells overexpressing HER2. The prepared immunoliposomes may become a proof of concept in developing new anticancer therapy.