Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 103
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How are the logic circuits constructed thatallow cross-talk between diverse signaling pathways? Howdo these circuits integrate the information from these pathways? How is the combination of outside-in and inside-outsignaling mediated by CAMs and adhesion receptors mergedinto such circuits?The importance of specialized GAG sequences in controlling cellular activities, especially interactions betweensome growth factors and their receptors, is now clear. Withthe identification of the biosynthetic mechanisms by whichthese complex structures are generated and the developmentof tools to manipulate GAG structures and test their functions in cultured systems and intact animals, we can expecta dramatic increase in our understanding of the cell biologyof GAGs in the next several years.A structural hallmark of CAMs, adhesion receptors,and ECM proteins is the presence of multiple domains thatimpart diverse functions to a single polypeptide chain.
Itis generally agreed that such multidomain proteins aroseevolutionarily by the assembly of distinct DNA sequencesencoding the distinct domains. Genes encoding multipledomains provide opportunities to generate enormous sequence and functional diversity by alternative splicing andthe use of alternate promoters within a gene. Thus, evenAnalyze the Datathough the number of independent genes in the humangenome seems surprisingly small in comparison with otherorganisms, far more distinct protein molecules can be produced than predicted from the number of genes. Such diversity seems especially well suited to the generation ofproteins that take part in specifying adhesive connectionsin the nervous system, especially the brain.
Indeed, severalgroups of proteins expressed by neurons appear to havejust such combinatorial diversity of structure. They includethe protocadherins, a family of cadherins with as many as70 proteins encoded per gene; the neurexins, which comprise more than 1000 proteins encoded by three genes; andthe Dscams, members of the IgCAM superfamily encodedby a Drosophila gene that has the potential to express38,016 distinct proteins owing to alternative splicing. Acontinuing goal for future work will be to describe and understand the molecular basis of functional cell–cell andcell–matrix attachments—the “wiring”—in the nervoussystem and how that wiring ultimately permits complexneuronal control and, indeed, the intellect required to understand molecular cell biology.2.
Cadherins are known to mediate homophilic interactions between cells. What is a homophilic interaction, andhow can it be demonstrated experimentally for E-cadherins?3. What is the normal function of tight junctions? Whatcan happen to tissues when tight junctions do not functionproperly?4.
What is collagen, and how is it synthesized? How do weknow that collagen is required for tissue integrity?5. You have synthesized an oligopeptide containing anRGD sequence surrounded by other amino acids. What is theeffect of this peptide when added to a fibroblast cell culturegrown on a layer of fibronectin absorbed to the tissue culturedish? Why does this happen?6. Blood clotting is a crucial function for mammalian survival. How do the multiadhesive properties of fibronectinlead to the recruitment of platelets to blood clots?7. Using structural models, explain how integrins mediateoutside-in and inside-out signaling.8. How do changes in molecular connections between theextracellular matrix (ECM) and cytoskeleton give rise toDuchenne muscular dystrophy?KEY TERMSadhesion receptor 199anchoring junction 202basal lamina 202cadherin 199cell-adhesionmolecule (CAM) 198cell line 236cell strain 236cell wall 231connexin 231dystrophin glycoproteincomplex (DGC) 227epithelium 201extracellular matrix(ECM) 199fibril-associatedcollagen 217fibrillar collagen 217fibronectin 220glycosaminoglycan(GAG) 213241HAT medium 238hyaluronan 217hybridoma 238immunoglobulincell-adhesion molecule(IgCAM) 227integrin 199laminin 211monoclonal antibody 237multiadhesive matrixprotein 209paracellular pathway 208plasmodesma 233proteoglycan 209RGD sequence 221selectin 199syndecan 214tight junction 202REVIEW THE CONCEPTS1.
Using specific examples, describe the two phenomenathat give rise to the diversity of adhesive molecules.9. What is the difference between a cell strain, a cell line,and a clone?10. Explain why the process of cell fusion is necessary toproduce monoclonal antibodies used for research.A N A LY Z E T H E DATAResearchers have isolated two E-cadherin mutant isoformsthat are hypothesized to function differently from that of thewild-type E-cadherin.
An E-cadherin negative mammary carcinoma cell line was transfected with the mutant E-cadheringenes A (part a in the figure) and B (part b) (diamonds) andthe wild-type E-cadherin gene (black circles) and comparedto untransfected cells (open circles) in an aggregation assay.In this assay, cells are first dissociated by trypsin treatmentand then allowed to aggregate in solution over a period ofminutes. Aggregating cells from mutants A and B arepresented in panels a and b respectively. To demonstrate thatthe observed adhesion was cadherin-mediated, the cellswere pretreated with a nonspecific antibody (left panel) or afunction-blocking anti-E-cadherin monoclonal antibody(right panel).a. Why do cells transfected with the wild-type E-cadheringene have greater aggregation than control, nontransfectedcells?b.
From these data, what can be said about the function ofmutants A and B?242CHAPTER 6 • Integrating Cells into TissuesNonspecificAggregation (%)(a)Anti-E-cadherin808060604040202000−200204060−200Time (min)204060Time (min)Aggregation (%)(b)808060604040202000−200204060Time (min)c. Why does the addition of the anti-E-cadherin monoclonal antibody, but not the nonspecific antibody, blockaggregation?d. What would happen to the aggregation ability of the cellstransfected with the wild-type E-cadherin gene if the assaywere performed in media low in Ca2?REFERENCESCell–Cell and Cell–Matrix Adhesion:An OverviewGumbiner, B. M. 1996. Cell adhesion: the molecular basis of tissue architecture and morphogenesis.
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