Structural and functional aspects of tumor cell sialomucins

Sialomucins are abundant on the surfaces of certain ascites tumor cells and have been implicated in the escape of tumors from immune destruction and metastasis. They are large, highly glycosylated glycoproteins which are rich in serine and threonine and have a variety of 0-linked oligosaccharides. The sialomucin (ASGP-1) or 13762 rat mammary adenocarcinoma ascites cells represents more than 0.5% of the total cell protein and can be isolated from cell membranes by centrifugation in 4 M guanidine hydrochloride-cesium chloride. ASGP-1 can also be isolated from membranes or cells by nonionic detergent extraction as a 1:1 complex with a second glycoprotein ASGP-2. Studies with the fluorescent lectins peanut agglutinin, which binds ASGP-1, and Concanavalin A, which binds ASGP-2, indicate that the glycoproteins are present at the cell surface as a complex. ASGP-1 is shed into cell culture medium or ascites fluid, apparently by a proteolytic cleavage mechanism. 13762 ascites cells grown in culture or as solid tumors lose their ASGP-1. The sialomucin reappears with extensive passage of the tumor cells in ascites form. Studies on the biosynthesis of ASGP-1 indicate that carbohydrate is being added over nearly the entire period of transit of ASGP-1 from the site of polypeptide synthesis to the plasma membrane. The negatively charged, rod-like structure of the sialomucins suggests that they may play a role in inhibiting recognition or binding processes necessary for the immune destruction of these tumor cells.     

Structural and functional roles of ether lipids

Ether lipids, such as plasmalogens, are peroxisomederived glycerophospholipids in which the hydrocarbon chain at the sn-1 position of the glycerol backbone is attached by an ether bond, as opposed to an ester bond in the more common diacyl phospholipids. This seemingly simple biochemical change has profound structural and functional implications. Notably, the tendency of ether lipids to form non-lamellar inverted hexagonal structures in model membranes suggests that they have a role in facilitating membrane fusion processes. Ether lipids are also important for the organization and stability of lipid raft microdomains, cholesterol-rich membrane regions involved in cellular signaling. In addition to their structural roles, a subset of ether lipids are thought to function as endogenous antioxidants, and emerging studies suggest that they are involved in cell differentiation and signaling pathways. Here, we review the biology of ether lipids and their potential significance in human disorders, including neurological diseases, cancer, and metabolic disorders.     

Structural and functional alterations of lipid-depleted ascites tumor cells in culture

Ascites tumor cells growth-arrested in lipid-depleted medium were modified with respect to their lipid composition, i.e. mainly cholesterol and the phospholipid fraction. These so-called lipid-depleted cells were generally smaller, had a surface area reduced by 55% compared to the control cells and had an altered cell surface architecture with large parts being smooth, interrupted by isolated bundles of microvilli and blebs as revealed by scanning electron microscopy. This deorganization process of the cells. Lectin-induced agglutination and receptor binding capacity was reduced, and also the receptor distribution was changed resulting in a cap-like formation on the surface as shown with FITC-labelled concanavalin A. The reduction in lipid content yielding a lower C/P ratio profoundly decreased the plasma membrane fluidity which was determined by fluorescence polarization measurements. Studies on fatty acid and cholesterol de novo synthesis revealed only small increases under lipid-free conditions not sufficient to meet the requirements of the lipid-depleted cells for these substances. It is therefore concluded that ascites tumor cells need exogenous preformed lipids for adequate functioning of the cell.     

Structural and functional aspects of stomata

Differential cell wall thickening in developing guard cells of Polypodium vulgare L. has been studied with particular reference to guard cell protoplast deformation and the eventual formation of the stomatal pore. Concomitant studies on the development of guard cell chloroplasts and their starch inclusions during ontogeny of the stomatal complex have provided data which have been incorporated into a model to account for the formation of the pore. Guard cell starch inclusions reach a maximum density per unit volume at the same time as the guard cell walls achieve maximum differential thickening. These events coincide with the development of the pore. It is suggested that, whilst pore formation is initiated enzymatically, the mechanical forces required to bring about the separation of the two guard cells are of an osmotic nature derived from starch hydrolysis. The development of the mesophyll in relation to the epidermis is examined in respect of the formation of substomatal chambers.     

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.     

Structural and functional aspects of thyroid peroxidase

Thyroperoxidase (TPO) is the enzyme involved in thyroid hormone synthesis. Although many studies have been carried out on TPO since it was first identified as being the thyroid microsomal antigen involved in autoimmune thyroid disease, previous authors have focused more on the immunological than on the biochemical aspects of TPO during the last few years. Here, we review the latest contributions in the field of TPO research and provide a large reference list of original publications. Given this promising background, scientists and clinicians will certainly continue in the future to investigate the mechanisms whereby TPO contributes to hormone synthesis and constitutes an important autoantigen involved in autoimmune thyroid disease, and the circumstances under which the normal physiological function of this enzyme takes on a pathological role.     

Structural and functional aspects of cultured epididymal epithelial cells isolated from pubertal rats

Acidic epididymal glycoprotein (AEG) and androgen-binding protein (ABP) antisera were used to study functional activities of primary cell cultures of the epididymal epithelium of 20--23-day-old rats. Extensive AEG immunoreactivity was associated with almost all epithelial cells of the distal caput, corpus and cauda epididymidis. ABP immunoreactivity was solely confined to some epithelial cells of the caput epididymidis. AEG and ABP immunoreactive cells were identified as principal cells. Morphological studies of enzymically dispersed aggregates of the epididymal epithelial cells showed that stromal cells were satisfactorily removed and that cell aggregates consisted of a predominant population for cells displaying the morphological characteristics of principal cells. Scanning and transmission electron microscopic studies of cultured epididymal epithelial cells in monolayers demonstrated that microvilli and pit-like invaginations of the cell surface were preserved during the first 7--10 days of culture and then gradually disappeared. Other characteristic subcellular structures such as Golgi apparatus and rough endoplasmic reticulum cisterna were preserved. Electrophoretic analysis of [35S]methionine-labelled secretory polypeptides released by epididymal epithelial cells into the culture medium demonstrated a distinct protein band pattern which differed from that observed in the medium of cultured rat Sertoli cells. These results demonstrate that primary cultures of epididymal epithelial cells isolated from sexually immature rats maintain several differentiated characteristics of the intact organ and therefore provide a valuable system for the study of epididymal epithelial cell function.     

Structural and functional aspects of peroxisomal membranes in yeasts

Abstract: The peroxisomal membrane compartmentalizes specific metabolic functions in the intermediary metabolism of various aerobic eukarya. In yeast, peroxisomal membranes are typified by their small width (±7–8 nm) and absence of large integral membrane proteins in freeze-etch replicas. They show a unique polypeptide profile which, in contrast to their phospholipid composition, differs from that of other membranes in the cell. Part of these proteins are substrate- inducible and are probably related to specific peroxisomal function(s). In vivo, the observed proton motive force across the peroxisomal membrane may play a role in the function of the organelle in that it contributes to the driving force required for selective transport of various enzyme substrates and/or metabolic intermediates. To date only few peroxisomal membrane proteins (PMPs) have been functionally characterized. A major constitutive 31-kDa PMP present in the peroxisomal membrane of Hansenula polymorpha has been purified and was shown to display poreforming properties. In addition, a peroxisomal H⁺-ATPase has been identified which most probably is involved in the generation/maintenance of the in vivo pH gradient across the peroxisomal membrane. Other functions of peroxisomal membrane proteins remain obscure although the first genes encoding yeast PMPs are now being cloned and sequenced. Studies on peroxisome-deficient yeast mutants revealed that specific peroxisome functions are strictly dependent on the intactness of the peroxisomal membrane. In this contribution several examples are presented of metabolic disorders due to peroxisomal malfunction in yeast.     

Structural and functional aspects of salivary fluid section in Calliphora

Calliphora salivary glands are described, emphasizing correlations between structure and physiology. In vitro studies show that the distal part of each gland produces a potassium-rich primary saliva when stimulated with 5-hydroxytryptamine or cyclic AMP. The secretory cells have elaborate canaliculi opening into the lumen. Stimulation of the secretory region causes a 60-fold increase in fluid secretion rate without affecting cell structure. The proximal part of the gland reabsorbs potassium when stimulated with cyclic AMP, but 5-HT has no effect. Potassium reabsorption from the primary saliva results in formation of a dilute saliva. The structure of the secretory and reabsorptive cells is discussed with regard to the functional role of long narrow channels in transport.     

Structural,functional and genetic aspects of peroxisome biogenesis

Intracellular organelles, peroxisomes, occur in cells of most eukaryotic species. Human severe congenital disorders are associated with defective assembly and functioning of peroxisomes, which partly explains the attention of researchers paid to peroxisome biogenesis. It has been shown that peroxisomes are involved in the realization of eukaryotic developmental programs (in particular, neuroblast differentiation and postembryonic development). Cytobiochemical and electron-microscopic studies of peroxisomal mutations showed that the primary role in peroxisome biogenesis is played by synthesis of specific proteins (peroxins) and their transport and incorporation into peroxisome membranes. More than 30 peroxin-encoding genes have been examined. These proteins are synthesized on free polysomes and transported into peroxisomes by means of specific signaling peptides, PTS1, PTS2, and PTS3. The import of matrix proteins depends on at least two shuttle receptor proteins, Pex5p and Pex7p. Some proteins regulating peroxisome proliferation in cells have been identified.Translated from Genetika, Vol. 41, No. 2, 2005, pp. 149–165.Original Russian Text Copyright © 2005 by Kurbatova, Dutova, Trotsenko.     

Structural, functional and genetic aspects of peroxisome biogenesis

Intracellular organelles, peroxisomes, occur in cells of most eukaryotic species. Human severe congenital disorders are associated with defective assembly and functioning of peroxisomes, which partly explains the attention of researchers paid to peroxisome biogenesis. It has been shown that peroxisomes are involved in the realization of eukaryotic developmental programs (in particular, neuroblast differentiation and postembryonic development). Cytobiochemical and electron-microscopic studies of mutations involving peroxisomes showed that the primary role in peroxisome biogenesis is played by synthesis of proteins (peroxins) and their transport and incorporation into peroxisome membranes. More than 30 peroxin-encoding genes have been examined. These genes are synthesized on free polysomes and transported into peroxisomes by means of specific signaling peptides, PTS1, PTS2, and PTS3. The import of matrix proteins depends on at least two shuttle receptor proteins, Pex5p and Pex7p. Some proteins regulating peroxisome proliferation in cells have been identified.     

IgM and its receptors: Structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cμ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/μR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcμR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.