Disruption of sphingomyelin synthase 2 gene alleviates cognitive impairment in a mouse model of Alzheimer's disease

The membrane raft accommodates the key enzymes synthesizing amyloid β (Aβ). One of the two characteristic components of the membrane raft, cholesterol, is well known to promote the key enzymes that produce amyloid-β (Aβ) and exacerbate Alzheimer's disease (AD) pathogenesis. Given that the raft is a physicochemical platform for the sound functioning of embedded bioactive proteins, the other major lipid component sphingomyelin may also be involved in AD. Here we knocked out the sphingomyelin synthase 2 gene (SMS2) in 3xTg AD model mice by hybridization, yielding SMS2KO mice (4S mice). The novel object recognition test in 9/10-month-old 4S mice showed that cognitive impairment in 3xTg mice was alleviated by SMS2KO, though performance in the Morris water maze (MWM) was not improved. The tail suspension test detected a depressive trait in 4S mice, which may have hindered the manifestation of performance in the wet, stressful environment of MWM. In the hippocampal CA1, hyperexcitability in 3xTg was also found alleviated by SMS2KO. In the hippocampal dentate gyrus of 4S mice, the number of neurons positive with intracellular Aβ or its precursor proteins, the hallmark of young 3xTg mice, is reduced to one-third, suggesting an SMS2KO-led suppression of syntheses of those peptides in the dentate gyrus. Although we previously reported that large-conductance calcium-activated potassium (BK) channels are suppressed in 3xTg mice and their recovery relates to cognitive amelioration, no changes occurred by hybridization. Sphingomyelin in the membrane raft may serve as a novel target for AD drugs.

Activated EGFR and PDGFR internalize in separate vesicles and downstream AKT and ERK1/2 signaling are differentially impacted by cholesterol depletion

The interplay between membrane subregions and receptor tyrosine kinases (RTK) will influence signaling in both normal and pathological RTK conditions. In this study, epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor β (PDGFR-β) internalizations were investigated by immunofluorescent microscopy following simultaneous treatment with EGF and PDGF-BB. We found that the two receptors utilize separate routes of internalization, which merges in a common perinuclear endosomal compartment after 45 min of stimulation. This is further strengthened when contrasting the recruitment of either EGFR or PDGFR-β to either clathrin or caveolin-1: PDGFR-β dissociates from caveolin-1 upon stimulation, and engages clathrin, whilst an increased recruitment of EGFR, to both clathrin and caveolin-1, was observed upon EGF stimulation. The association between EGFR and caveolin-1 is supported by the observation that EGFR was localized in lipid raft associated fractions, whereas PDGFR-β was not. We also found that disruption of lipid rafts using MβCD led to an increased EGFR dimerization and phosphorylation in response to ligand, as well as a dramatic decrease in AKT- and a smaller but robust decrease in ERK1/2 phosphorylation. This suggest that lipid rafts may be important to effectively connect the EGFR with downstream proteins to facilitate signaling. Our data implies that cholesterol depletion of the plasma membrane affect the signaling of EGFR and PDGFRβ differently.

Adaptive threshold-stochastic resonance (AT-SR) in MHC clusters on the cell surface

Highly conserved 2D receptor clusters (membrane rafts) of immunological signaling molecules with MHCI and MHCII antigens as their cores have been observed in the past on the surface of T- and B-cell lines of lymphoid origin, as well as on cells from patients with colon tumor and Crohn's disease. Conservativity is related to the ever presence of MHCI molecules. Although they are suspected to play a role in maintaining these clusters and facilitating transmembrane signaling, their exact role has been left largely enigmatic. Here we are suggesting stochastic resonance (SR), or "noise-assisted signal detection", as a general organizing principle for transmembrane signaling events evoked by processes like immune recognition and cytokine binding taking place in these clusters. In the conceptual framework of SR, in immune recognition as a prototype of transmembrane signaling, the sea of self-peptide-MHC complexes around a nonself-peptide presenting MHC is conceived as a source of quickly fluctuating unspecific signal ("athermal noise") serving the extra energy for amplifying the weak sub-threshold specific signal of the nonself-peptide presenting MHC. This same noise is also utilized for a readjustment of the threshold - and also the sensitivity and specificity - of detection by a closed loop feedback control of the TcR-CD8 (CD4) proximity on the detecting T-cell. The weak sub threshold specific signal of nonself-peptide presenting MHC is amplified by the superposing unspecific signals of the neighboring self peptide-MHC complexes towards the T-cell receptor as the detector. Because in a successful detection event both self- and nonself-peptides are detected simultaneously, the principle of coincidence (or lock-in) detection is also realized. The ever presence of MHC islands gets a natural explanation as a source of extra power - in a form of "athermal noise" - needed for coincidence detection and frequency encoding the evoked downstream signals. The effect is quite general, because the actual type of molecules surrounding a chief signaling molecule - like nonself-peptide holding MHC, interleukin-2 and -15 cytokine receptors (IL-2R/15R) - as the fluctuating interaction energy sources is immaterial. The model applies also for other types of signaling, such as those evoked by cytokine binding. The phenomenon of SR can also be interpreted as sampling of a low frequency, specific signal with a high frequency unspecific signal, the "noise". Recipes for identifying other forms of SR in membrane clusters with biophysical tools are recommended.

4-cholesten-3-one decreases breast cancer cell viability and alters membrane raft-localized EGFR expression by reducing lipogenesis and enhancing LXR-dependent cholesterol transporters

Background

The alteration of lipid metabolism in cancer cells is recognized as one of the most important metabolic hallmarks of cancer. Membrane rafts defined as plasma membrane microdomains enriched in cholesterol and sphingolipids serve as platforms for signaling regulation in cancer. The main purpose of this study was to evaluate the effect of the cholesterol metabolite, 4-cholesten-3-one, on lipid metabolism and membrane raft integrity in two breast cancer cell lines, MCF-7 and MDA-MB-231. Its ability to reduce cell viability and migration has also been investigated.

Methods

RT-qPCR was performed to evaluate the expression of enzymes involved in lipogenesis and cholesterol synthesis, and ABCG1 and ABCA1 transporters involved in cholesterol efflux. Its effect on cell viability and migration was studied using the MTT assay, the wound healing assay and the Transwell migration assay, respectively. The effect of 4-cholesten-3-one on membrane rafts integrity was investigated by studying the protein expression of flotillin-2, a membrane raft marker, and raft-enriched EGFR by western blot.

Results

Interestingly, we found that 4-cholesten-3-one treatment decreased mRNA expression of different enzymes including ACC1, FASN, SCD1 and HMGCR. We further demonstrated that 4-cholesten-3-one increased the expression of ABCG1 and ABCA1. We also found that 4-cholesten-3-one decreased the viability of MCF-7 and MDA-MB-231 cells. This effect was neutralized after treatment with LXR inverse agonist or after LXRβ knockdown by siRNA. As a result, we also demonstrated that 4-cholesten-3-one disrupts membrane rafts and cell migration capacity.

Conclusion

Our results show that 4-cholesten-3-one exerts promising antitumor activity by altering LXR-dependent lipid metabolism in breast cancer cells without increasing lipogenesis.

Mechanisms of tethering and cargo transfer during epididymosome-sperm interactions

BACKGROUND

The mammalian epididymis is responsible for the provision of a highly specialized environment in which spermatozoa acquire functional maturity and are subsequently stored in preparation for ejaculation. Making important contributions to both processes are epididymosomes, small extracellular vesicles released from the epididymal soma via an apocrine secretory pathway. While considerable effort has been focused on defining the cargo transferred between epididymosomes and spermatozoa, comparatively less is known about the mechanistic basis of these interactions. To investigate this phenomenon, we have utilized an in vitro co-culture system to track the transfer of biotinylated protein cargo between mouse epididymosomes and recipient spermatozoa isolated from the caput epididymis; an epididymal segment that is of critical importance for promoting sperm maturation.

RESULTS

Our data indicate that epididymosome-sperm interactions are initiated via tethering of the epididymosome to receptors restricted to the post-acrosomal domain of the sperm head. Thereafter, epididymosomes mediate the transfer of protein cargo to spermatozoa via a process that is dependent on dynamin, a family of mechanoenzymes that direct intercellular vesicle trafficking. Notably, upon co-culture of sperm with epididymosomes, dynamin 1 undergoes a pronounced relocation between the peri- and post-acrosomal domains of the sperm head. This repositioning of dynamin 1 is potentially mediated via its association with membrane rafts and ideally locates the enzyme to facilitate the uptake of epididymosome-borne proteins. Accordingly, disruption of membrane raft integrity or pharmacological inhibition of dynamin both potently suppress the transfer of biotinylated epididymosome proteins to spermatozoa.

CONCLUSION

Together, these data provide new mechanistic insight into epididymosome-sperm interactions with potential implications extending to the manipulation of sperm maturation for the purpose of fertility regulation.

Epidermal growth factor-nanoparticle conjugates change the activity from anti-apoptotic to pro-apoptotic at membrane rafts

The proliferation epidermal growth factor (EGF) is known to acquire contradictory apoptotic activities upon conjugation with gold nanoparticles (GNPs) through hitherto unknown mechanisms. Here, we identified an essential role of membrane rafts in the drastic activity switching of EGF-GNPs through the following intracellular signaling. (1) In contrast to the rapid diffusion of activated EGF receptor after the soluble EGF stimulation, the receptor is confined within membrane rafts upon binding to the EGF-GNPs. (2) This initial receptor confinements switch its endocytosis process from normal clathrin-mediated endocytosis to caveolin-mediated one, changing the phosphorylation dynamics of essential downstream kinases, i.e., extracellular signal-regulated kinase and AKT. Importantly, the destruction of membrane rafts by β-cyclodextrin reversed this trafficking and signaling, restoring EGF-GNPs to lost anti-apoptotic property. These results reveal the importance of GNP-mediated signal condensation at membrane rafts in conferring the unique apoptotic activity on EGF-nanoparticle conjugates. STATEMENT OF SIGNIFICANCE: Epidermal growth factor (EGF) is a small secretory protein that induces cell proliferation upon binding to its receptor existed on cellular plasma membranes. One interesting feature of the protein in the nanobiology field is, its acquisition of apoptosis-inducing (cellular suicide) activity rather than proliferative one upon conjugation to gold nanoparticles through hitherto unknown mechanisms. Here, we identified the involvement of membrane rafts, plasma membrane nanodomains enriched with cholesterol, in the apoptosis processes by changing the receptor trafficking and downstream signal transduction pathways. Moreover, the destruction of lipid rafts restored the EGF-nanoparticle conjugates with lost anti-apoptotic activity. These finding highlight potential applications of EGF-nanoparticle conjugates to cancer therapy, as the EGF receptor are highly expressed in cancer cells.

Myelin structure in unfixed, single nerve fibers: Scanning X-ray microdiffraction with a beam size of 200nm

Previous raster-scanning with a 1μm X-ray beam of individual, myelinated fibers from glutaraldehyde-fixed rat sciatic nerve revealed a spatially-dependent variation in the diffraction patterns from single fibers. Analysis indicated differences in the myelin periodicity, membrane separations, distribution of proteins, and orientation of membrane lamellae. As chemical fixation is known to produce structural artifacts, we sought to determine in the current study whether the structural heterogeneity is intrinsic to unfixed myelin. Using a 200nm-beam that was about five-fold smaller than before, we raster-scanned individual myelinated fibers from both the peripheral (PNS; mouse and rat sciatic nerves) and central (CNS; rat corpus callosum) nervous systems. As expected, the membrane stacking in the internodal region was nearly parallel to the fiber axis and in the paranodal region it was perpendicular to the axis. A myelin lattice was also frequently observed when the incident beam was injected en face to the sheath. Myelin periodicity and diffracted intensity varied with axial position along the fiber, as did the calculated membrane profiles. Raster-scanning with an X-ray beam at sub-micron resolution revealed for the first time that the individual myelin sheaths in unfixed nerve are heterogeneous in both membrane structure and packing.

Cholesterol lipids and cholesterol-containing lipid rafts in bacteria

Sterols are important components of eukaryotic membranes, but rare in bacteria. Some bacteria obtain sterols from their host or environment. In some cases, these sterols form membrane domains analogous the lipid rafts proposed to exist in eukaryotic membranes. This review describes the properties and roles of sterols in Borrelia and Helicobacter.