Identification of gelsolin as an actin regulatory component in a triton insoluble low density fraction (raft) of newborn bovine brain

Gelsolin traps ribosomal protein SA (RPSA) within lipid nanodomains of the plasma membrane and modulates the level of protein synthesis in the submembranous region of human skin melanoma cells

The connection between the F-actin and ribosome docking to the PM has been reported, but the exact mechanism has remained unclear. Previously, we discovered that gelsolin (GSN) forms complexes with numerous ribosomal proteins, including ribosomal protein SA (RPSA). Now, we have unraveled the mechanism of ribosome recruitment to the lipid nanodomains of the PM, with GSN playing a pivotal role in this process. We demonstrate that GSN directly interacts with RPSA, and microscopic analyses reveal their colocalization in the cell's submembranous region. Through spot variation fluorescence correlation spectroscopy, we confirm that GSN is responsible for trapping RPSA within PM's lipid nanodomains, a process dependent on F-actin. Importantly, we establish a correlation between the GSN level and the level of protein synthesis in melanoma cells. Furthermore, we present compelling evidence that high levels of GSN and RPSA are associated with the progression of cutaneous melanoma and a poorer prognosis for patients.

Dictyostelium discoideum cells retain nutrients when the cells are about to overgrow their food source

Dictyostelium discoideum is a unicellular eukaryote that eats bacteria, and eventually outgrows the bacteria. D. discoideum cells accumulate extracellular polyphosphate (polyP), and the polyP concentration increases as the local cell density increases. At high cell densities, the correspondingly high extracellular polyP concentrations allow cells to sense that they are about to outgrow their food supply and starve, causing the D. discoideum cells to inhibit their proliferation. In this report, we show that high extracellular polyP inhibits exocytosis of undigested or partially digested nutrients. PolyP decreases plasma membrane recycling and apparent cell membrane fluidity, and this requires the G protein-coupled polyP receptor GrlD, the polyphosphate kinase Ppk1 and the inositol hexakisphosphate kinase I6kA. PolyP alters protein contents in detergent-insoluble crude cytoskeletons, but does not significantly affect random cell motility, cell speed or F-actin levels. Together, these data suggest that D. discoideum cells use polyP as a signal to sense their local cell density and reduce cell membrane fluidity and membrane recycling, perhaps as a mechanism to retain ingested food when the cells are about to starve.

This article has an associated First Person interview with the first author of the paper.

Lipid rafts in mast cell biology

Mast cells have long been recognized to have a direct and critical role in allergic and inflammatory reactions. In allergic diseases, these cells exert both local and systemic responses, including allergic rhinitis and anaphylaxis. Mast cell mediators are also related to many chronic inflammatory conditions. Besides the roles in pathological conditions, the biological functions of mast cells include roles in innate immunity, involvement in host defense mechanisms against parasites, immunomodulation of the immune system, tissue repair, and angiogenesis. Despite their growing significance in physiological and pathological conditions, much still remains to be learned about mast cell biology. This paper presents evidence that lipid rafts or raft components modulate many of the biological processes in mast cells, such as degranulation and endocytosis, play a role in mast cell development and recruitment, and contribute to the overall preservation of mast cell structure and organization.

The Localization and Activity of Sphingosine Kinase 1 Are Coordinately Regulated with Actin Cytoskeletal Dynamics in Macrophages

The physiologic and pathologic functions of sphingosine kinase (SK) require translocation to specific membrane compartments. We tested the hypothesis that interactions with actin filaments regulate the localization of SK1 to membrane surfaces, including the plasma membrane and phagosome. Macrophage activation is accompanied by a marked increase in association of SK1 with actin filaments. Catalytically-inactive (CI)- and phosphorylation-defective (PD)-SK1 mutants exhibited reductions in plasma membrane translocation, colocalization with cortical actin filaments, membrane ruffling, and lamellipodia formation, compared with wild-type (WT)-SK1. However, translocation of CI- and PD-SK1 to phagosomes were equivalent to WT-SK1. SK1 exhibited constitutive- and stimulus-enhanced association with actin filaments and F-actin-enriched membrane fractions in both intact macrophages and a novel in vitro assay. In contrast, SK1 bound G-actin only under stimulated conditions. Actin inhibitors disrupted SK1 localization and modulated its activity. Conversely, reduction of SK1 levels or activity via RNA interference or specific chemical inhibition resulted in dysregulation of actin filaments. Thus, the localization and activity of SK1 are coordinately regulated with actin dynamics during macrophage activation.

EhLimA, a novel LIM protein, localizes to the plasma membrane in Entamoeba histolytica

The parasitic protozoan Entamoeba histolytica relies on a very dynamic cytoskeleton in order to invade and survive in host tissues. Identification of cytoskeletal elements is key to understanding these processes. Here we present the characterization of EhLimA, the first LIM protein of E. histolytica. EhLimA consists of a single LIM domain at its N terminus and exhibits the highest degree of homology with DdLimE from Dictyostelium discoideum. Immunofluorescence localization of EhLimA using anti-EhLimA antibodies revealed that EhLimA is highly concentrated at the plasma membrane of cells. Silencing or overexpression of the EhLimA gene did not have a significant effect on the growth or morphology of the parasite. EhLimA associates with the cytoskeleton as demonstrated by the enrichment of the protein in cytoskeleton fractions as well as in pull-down assays that revealed that cytoskeleton association involves interaction with actin. EhLimA binding to actin was shown to be dependent on the N-terminal LIM domain of EhLimA, as removal of even half of the LIM domain resulted in almost complete inhibition of the binding to actin. We also found that a portion of EhLimA floats to the lower-density regions of a sucrose gradient together with portions of the Gal-lectin light subunit and actin. Treatment of cells with the cholesterol-sequestering agent digitonin resulted in increased solubility of EhLimA. These results indicate that in addition to cytoskeletal association, EhLimA may also associate with lipid rafts in the parasite plasma membrane and suggest that EhLimA may be part of the molecular system connecting the actin cytoskeleton to membrane rafts.

The schistosome in the mammalian host: understanding the mechanisms of adaptation

In this review, we envisage the host environment, not as a hostile one, since the schistosome thrives there, but as one in which the relationship between the two organisms consists of constant communication, through signalling mechanisms involving sense organs, surface glycocalyx, surface membrane and internal organs of the parasite, with host fluids and cells. The surface and secretions of the schistosome egg have very different properties from those of other parasite stages, but adapted for the dispersal of the eggs and for the preservation of host liver function. We draw from studies of mammalian cells and other organisms to indicate how further work might be carried out on the signalling function of the surface glycocalyx, the raft structure of the surface and existence of pores in the surface membrane, the repair of the surface membrane, the role of the membrane structure in ion channel function (including recent work on the actin cytoskeleton and calcium channels) and the possible role of P-glycoproteins in the adaptation of the parasite to its environment. We are speculative in some areas, such as the suggestions that variability in surface properties of schistosomes may relate to the existence of membrane rafts and that parasite communities may exhibit quorum sensing. This speculative approach is adopted with the hope that future work on the whole organisms and their interactions will be encouraged.

Characterization of plasma gelsolin as a substrate for matrix metalloproteinases

We previously showed that plasma gelsolin, a major component of the extracellular actin scavenging system, is an matrix metalloproteinase (MMP)-14 substrate. Here we confirmed that plasma gelsolin is cleaved by MMP-14 at the plasma level, and found that it was most efficiently digested by MMP-3 followed by MMP-2, MMP-1, MMP-14, and MMP-9, in that order. Plasma gelsolin (90 kDa) was cut into several fragments of 43-48 kDa by MMP-3. The MMP-3 cleavage sites in plasma gelsolin were determined by labeling the C termini generated by in-gel digestion with 50% H2 18O combined with peptide mass mapping, and sequencing of the N-terminal amino acids. Plasma gelsolin was cleaved at Asn416-Val417, Ser51-Met52, and Ala435-Gln436. Proteolytic cleavage by MMP-3 resulted in considerable loss of its actin filament-depolymerizing activity. This suggests that MMPs weaken the extracellular actin-scavenging system by cleaving plasma gelsolin and may, therefore, be involved in pathological conditions induced by extracellular actin, such as endothelial injury, respiratory distress syndrome, hepatic necrosis, and septic shock.

Subcellular localization of collapsin response mediator proteins to lipid rafts

Collapsin response mediator proteins (CRMPs) are involved in signal transduction after exposure of neural cells to the axon guidance molecule Semaphorin 3A/collapsin. All five known CRMPs are expressed in the developing cerebral cortex and neocortical neurons are responsive to Semaphorin 3A. Here, we examine the expression and subcellular localization of CRMPs in neocortical neurons and in neonatal rat brain. In neocortical neurons CRMP-4 was detected in the perikaryon with a diffuse cytosolic distribution. In neurites and at growth cones punctate staining patterns were observed. Extraction of neuron cultures with methyl-beta-cyclodextrin to deplete cholesterol caused rapid redistribution of the punctate CRMP-4 staining into larger patches and abundant growth cone collapse. Western blotting of brain extracts demonstrated for all CRMPs the existence of soluble, detergent-extractable, and Triton X-100-resistant forms. Furthermore, sucrose density gradient centrifugation after solubilization of brain membranes with Triton X-100 revealed that CRMP-1, -3, -5, and to a lower extent CRMP-4 are associated with a detergent-resistant fraction with low buoyant density, but CRMP-2 was not detectable in this fraction. Thus, we propose that lipid rafts form sites for the compartmentalization of signaling events involving specific CRMPs and that the integrity of these membrane microdomains is essential for the maintenance of growth cones.

Molecular characterization of the detergent-insoluble cholesterol-rich membrane microdomain (raft) of the central nervous system

Many fundamental neurological issues such as neuronal polarity, the formation and remodeling of synapses, synaptic transmission, and the pathogenesis of the neuronal cell death are closely related to the membrane dynamics. The elucidation of functional roles of a detergent-insoluble cholesterol-rich domain (raft) could therefore provide good clues to the molecular understanding of these important phenomena, for the participation of the raft in the fundamental cell functions, such as signal transduction and selective transport of lipids and proteins, has been elucidated in nonneural cells. Interestingly, the brain is rich in raft and the brain-derived raft differs in its lipid and protein components from other tissue-derived rafts. Since many excellent reviews are written on the membrane lipid dynamics of this microdomain, signal transduction, and neuronal glycolipids, we review on the characterization of the raft proteins recovered in the detergent-insoluble low-density fraction from rat brain. Special focus is addressed on the biochemical characterization of a neuronal enriched protein, NAP-22, for the lipid organizing activity of this protein has become increasingly clear.

Biochemical and morphological analysis on the localization of Rac1 in neurons

The acquisition of cell type-specific morphologies is a central feature of neuronal differentiation. Many extra- and intracellular signals are known to cause the morphological changes of neuronal cells through the reconstruction of the microfilaments underneath the cell membrane. The membrane microdomain called "raft" has been paid much attention, for this domain contains many signal-transducing molecules including trimeric G proteins and cytoskeletal proteins. The raft domain is recovered in a low-density fraction after the treatment of the membrane with the non-ionic detergent such as Triton X-100 and the enrichment of cholesterol and sphingolipids is ascribed to be responsible for the detergent insolubility. In contrast to the well-known localization of trimeric G proteins in raft, the localization of small G proteins in the raft is poorly characterized. Since Rho family small G proteins (Rho, Rac, and Cdc42) regulate the microfilament system, we studied the localization of Rho family small G proteins in the raft of rat brain with western blotting. Specific localization of Rac1 was detected in the raft from 10-day-old and 8-week-old rat whole brain, and also in the raft prepared from the growth cone and synaptic plasma membrane fractions. Rho and Cdc42 were, in contrast, recovered in the Triton soluble fraction. Double immunostaining of cultured hippocampal neurons with antibodies to Rac1 and MAP-2, or Rac1 and tau, showed punctate distribution of Rac1 in axons as well as in dendrites.