Differential sensitivity to detergents of actin cytoskeleton from nerve endings

Do ABC transporters regulate plasma membrane organization?

The plasma membrane (PM) spatiotemporal organization is one of the major factors controlling cell signaling and whole-cell homeostasis. The PM lipids, including cholesterol, determine the physicochemical properties of the membrane bilayer and thus play a crucial role in all membrane-dependent cellular processes. It is known that lipid content and distribution in the PM are not random, and their transversal and lateral organization is highly controlled. Mainly sphingolipid- and cholesterol-rich lipid nanodomains, historically referred to as rafts, are extremely dynamic “hot spots” of the PM controlling the function of many cell surface proteins and receptors. In the first part of this review, we will focus on the recent advances of PM investigation and the current PM concept. In the second part, we will discuss the importance of several classes of ABC transporters whose substrates are lipids for the PM organization and dynamics. Finally, we will briefly present the significance of lipid ABC transporters for immune responses.

Calcium enhances binding of Clostridium perfringens epsilon toxin to sulfatide

Epsilon toxin (Etx) from Clostridium perfringens is synthesized as a very low-active prototoxin form (proEtx) that becomes active upon proteolytic activation and has the capacity to cross the blood-brain barrier (BBB), thereby producing severe neurological effects. The identity and requirements of host receptors of Etx remain a matter of controversy. In the present study, we analysed the binding of proEtx or Etx to liposomes containing distearoylphosphatidylcholine (DSPC), cholesterol and sulfatide, or alternatively to detergent-solubilized lipids, using surface plasmon resonance (SPR). We also tested the influence of calcium on Etx or proEtx binding. Our findings show that the presence of sulfatide in liposomes increases both Etx and proEtx binding, and Etx binding is enhanced by calcium. These results were corroborated when SPR was conducted with immobilized toxin, since detergent-solubilized sulfatide increases its binding to Etx in the presence of calcium, but not to proEtx. Moreover, binding affinity is also affected, since the treatment of liposomes with sulfatase causes the dissociation rate constants (K_D) in both proEtx and Etx to increase, especially in the case of proEtx in the presence of calcium. In addition, protein-lipid overlay assays corroborated the calcium-induced enhancement of Etx binding to sulfatide, and to lipids extracted from sulfatide-enriched rat brain lipid rafts. In conclusion, the present work highlights the role of sulfatide as an important element in the pathophysiology of Etx and reveals the influence of calcium in the interaction of Etx, but not of proEtx, with the target membrane.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Membrane dynamics of resting and internalin B-bound MET receptor tyrosine kinase studied by single-molecule tracking

The human MET receptor tyrosine kinase contributes to vertebrate development and cell proliferation. As a proto-oncogene, it is a target in cancer therapies. MET is also relevant for bacterial infection by Listeria monocytogenes and is activated by the bacterial protein internalin B. The processes of ligand binding, receptor activation, and the diffusion behavior of MET within the plasma membrane as well as its interconnections with various cell components are not fully understood. We investigated the receptor diffusion dynamics using single-particle tracking and imaging fluorescence correlation spectroscopy and elucidated mobility states of resting and internalin B-bound MET. We show that internalin B-bound MET exhibits lower diffusion coefficients and diffuses in a more confined area in the membrane. We report that the fraction of immobile receptors is larger for internalin B-bound receptors than for resting MET. Results of single-particle tracking in cells treated with various cytotoxins depleting cholesterol from the membrane and disrupting the actin cytoskeleton and microtubules suggest that cholesterol and actin influence MET diffusion dynamics, while microtubules do not have any effect.

Clostridium Perfringens Epsilon Toxin Binds to Membrane Lipids and Its Cytotoxic Action Depends on Sulfatide

Epsilon toxin (Etx) is one of the major lethal toxins produced by Clostridium perfringens types B and D, being the causal agent of fatal enterotoxemia in animals, mainly sheep and goats. Etx is synthesized as a non-active prototoxin form (proEtx) that becomes active upon proteolytic activation. Etx exhibits a cytotoxic effect through the formation of a pore in the plasma membrane of selected cell targets where Etx specifically binds due to the presence of specific receptors. However, the identity and nature of host receptors of Etx remain a matter of controversy. In the present study, the interactions between Etx and membrane lipids from the synaptosome-enriched fraction from rat brain (P2 fraction) and MDCK cell plasma membrane preparations were analyzed. Our findings show that both Etx and proEtx bind to lipids extracted from lipid rafts from the two different models as assessed by protein-lipid overlay assay. Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids. Binding of proEtx to sulfatide, phosphatidylserine, phosphatidylinositol (3)-phosphate and phosphatidylinositol (5)-phosphate was detected. Removal of the sulphate groups via sulfatase treatment led to a dramatic decrease in Etx-induced cytotoxicity, but not in proEtx-GFP binding to MDCK cells or a significant shift in oligomer formation, pointing to a role of sulfatide in pore formation in rafts but not in toxin binding to the target cell membrane. These results show for the first time the interaction between Etx and membrane lipids from host tissue and point to a major role for sulfatides in C. perfringens epsilon toxin pathophysiology.