摘要:

Summary1. Extracellular matrices are organized networks of diverse macromolecules, secreted and deposited in the vicinity of cells. They not only play structural roles but are also involved in dynamic processes such as cell migration and differentiation, embryo development, wound healing and cancerous transformation. They are composed, mainly, of collagens, adhesive glycoproteins and proteoglycans, which interact with each other and with cell‐surface receptors through specific binding sites.2. Collagens are a multigenic family whose proteins have triple‐helical domains which contain repeats of the Gly‐X‐Y sequence. They aggregate to form fibrils, networks or filamentous structures. Gene organization reveals that fibril‐forming collagens might have originated from an ancestral 54 bp exon encoding 6 units of the Gly‐X‐Y triplet. Non‐fibrillar collagens, on the contrary, have evolved through different pathways which are not closely related to this mechanism.3. Fibronectins are dimers made up of three types of internal repeats: I, II and III. The first two are encoded by one exon each and have homologous counterparts in other proteins. Most of the type three repeats are encoded by two exons each. Cell‐specific alternative splicing in three different regions of the primary transcript generates, in humans, up to 20 polypeptide variants and explains structural differences between cellular and plasma fibronectins. Fibronectin interacts with its cell receptors through the sequence Arg‐Gly‐Asp.4. Laminin is a cross‐shaped molecule, characteristics of basement membranes, formed by three distinct polypeptides. Primary structure of one of its subunits reveals a repetitive organization with regions homologous to other proteins like myosin and epidermal growth factor. Laminin has a cell‐binding site, different from the Arg‐Gly‐Asp tripeptide, which is constituted by the sequence Tyr‐Ile‐Gly‐Ser‐Arg.5. Von Willebrand factor is a high‐molecular‐weight glycoprotein stored in specialized structures of platelets and endothelial cells. It participates in haemostatic mechanisms favouring the formation of the platelet plug. This protein has a particularly long propeptide and four types of internal homologies. It binds to two different platelet surface receptors, one of which interacts with an Arg‐Gly‐Asp sequence present in the von Willebrand polypeptide.6. Thrombospondin is an adhesive glycoprotein formed by three identical subunits which show striking homologies withPlasmodiumproteins, epidermal growth factor and procollagen I. It also contains multiple calcium‐binding sites similar to those of calmodulin. An Arg‐Gly‐Asp sequence is also present, but its surface receptor has not yet been identified.7. Vitronectin is a glycoprotein, presumably involved in the process of blood coagulation, which is related to the extracellular matrix through binding to various of its components. It also binds to cell surfaces via an Arg‐Gly‐Asp sequence which is disrupted by a proteolytic cleavage that, concomitantly, originates somatomedin B, a peptide of unknown function.8. Proteoglycans are formed by glycosaminoglycan chains covalently bound to core proteins. They show a wide tissue distribution and structural variations. Most or all core proteins could possibly be synthesized as pre‐propolypeptides, and contain Ser‐Gly or Thr‐Gly repeats, which represent attachment sites for the glycosaminoglycans.9. A superfamily of cell‐surface receptors that recognizes RGD‐containing proteins is described. These receptors are intrinsic membrane proteins with large extracellular domains and an α/β heterodimeric structure. They are grouped in four families, each of them characterized by dimers which share a common β subunit and different α chains. Other receptors for extracellular mat

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00668.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Summary1. The paper summarizes the published evidence on the relation between mating frequency and fecundity in insects. There is experimental evidence of varying quality for 63 species and non‐experimental evidence for about 60.2. Repeated mating may be universally necessary for full fecundity and fertility in female insects (in species in which the females normally mate more than once).3. The evidence is remarkably poor. We need more properly designed experiments (and not just observations of natural variation), with sufficient sample sizes and statistics, and measurements of the fecundities and fertilities of singly and multiply mated females, when the multiple matings are separated by many days or weeks. Most of the existing experiments of this sort are defective in some way.4. In species with greater total fecundity and longevity, multiple mating may be more likely to enhance fertility than in species with small fecundity and short life span.5. Females in naturally monandrous species do not show increased fecundity or fertility with repeated mating, whereas females of polyandrous species do.6. There is no obvious connexion between paternal investment, in so far as we know about it, and the increase of fecundity by repeated mating.7. There is a small tendency for females to breed more quickly and be shorter lived if they mate repeatedl

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00669.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SummaryKnowledge of the mechanism of action of Ca2+‐mobilizing agonists in liver has progressed considerably following the discovery that their interaction with specific receptors on the plasma membrane is accompanied by the hydrolysis of PIP2and the generation of the second messengers diacylglycerol and IP3, for the activation of protein kinase C and the mobilization of intracellular Ca2+, respectively. Although the second messenger functions of diacylglycerol and IP3in these actions seem well established, it is not yet clear how the agonists are able to regulate Ca2+influx across the plasma membrane, an event which is crucial for those actions of the agonists which are dependent on the maintenance of an elevated level of cytosolic Ca2+, Whilst there is evidence for the existence of more than one pathway for Ca2+influx in liver, it appears that in each instance the Ca2+influx process is regulated differently to the Ca2+influx through the volage‐sensitive Ca2+channels that is known to occur in excitable tissues. At present it is not clear whether any of the Ca2+influx pathways in liver is regulated by direct coupling to the agonist receptor mechanism on the outer surface of the plasma membrane, or whether the regulation involves the production of some second messenger(s). However, indirect evidence from a number of tissues appears to favour the involvement of both IP3and IP4in the regulation of Ca2+influx. The mechanism by which IP3and IP4may regulate Ca2+influx remains to be established, but it has been proposed that Ca2+entry into the cell occurs through a pathway connecting the plasma membrane and the endoplasmic reticulum, following the release of intracellular Ca2+from the lumen of the endoplasmic reticulum.Although it is not yet known whether glucagon (or cyclic AMP) activates the same pathway for Ca2+influx as Ca2+‐mobilizing agonists, the marked potentiation by cyclic AMP of the Ca2+influx induced by Ca2+‐mobilizing agonists has provided a powerful system with which to study the regulation of Ca2+influx in liver. Whether this Ca2+influx process occurs through some ion exchange mechanism (such as Ca2+/Na+exchange) remains to be determined. Results from this study suggests that the Ca2+influx is inhibited by neomycin, acidic pH, and a depolarization of the plasma membrane. The observation that cyclic AMP synergistically potentiates the influx of Ca2+induced by Ca2+‐mobilizing agonists, that this influx appears to correlate with the reported ability of these agonists to induce PIP2hydrolysis and accumulation of IP3, and that cyclic AMP synergistically potentiates the production of IP4by vasopressin, are all consistent with the notion that IP3and IP4are involved in regulating Ca2+influx. Whilst little is known about the Ca2+transport process itself, these studies coupled with the recent finding that Ca2+influx into the liver cell can occur through different pathways, seem set to lead to a better understanding of this important process in the ne

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00670.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00671.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY