Часть 2 (1121000), страница 61
Текст из файла (страница 61)
Syntheticpeptides corresponding to different segments of the cell-binding domain havebeen used to identify a specific tripeptide sequence (Arg-Gly-Asp,or RGD),which is found in one of the type III repeats (seeFigure l9-72C) , as a central feature of the cell-binding site. Even very short peptides containing this RGDsequence can compete with fibronectin for the binding site on cells, therebyinhibiting the attachment of the cells to a fibronectin matrix.
If these peptidesare coupled to a solid surface,they cause cells to adhere to it.Several extracellular proteins besides fibronectin also have an RGDsequence that mediates cell-surface binding. Some of these are involved inblood clotting, and peptides containing the RGD sequence have been useful inthe development of anti-clotting drugs. Some snakes use a similar strategy tocause their victims to bleed: they secreteRGD-containing anti-clotting proteinscalled disintegrlns into their venom.The cell-surface receptors that bind RGD-containing proteins are membersof the integrin family.
Each integrin, however, specifically recognizes its ownsmall set of matrix molecules, indicating that tight binding requires more thanjust the RGD sequence. Moreover, RGD sequences are not the only sequencemotifs used for binding to integrins: many integrins recognize and bind to othermotifs instead.CellsHaveto BeAbleto DegradeMatrix,asWellas MakeitThe ability of cells to degrade and destroy extracellular matrix is as importantas their ability to make it and bind to it.
Rapid matrix degradation is requiredin processessuch as tissue repair, and even in the seemingly static extracellular matrix of adult animals there is a slow continuous turnover, with matrixmacromolecules being degraded and resynthesized. This allows bone, forexample, to be remodelled so as to adapt to the stresseson it, as discussed inChapter 23.From the point of view of individual cells, the ability to cut through matrix iscrucial in two ways: it enables them to divide while embedded in matrix, and itenables them to travel through it. As we have already mentioned, cells in connective tissues generally need to be able to stretch out in order to divide.
If a celllacks the enzymes needed to cut through the surrounding matrix or is embedded in a matrix that resists their action, it remains rounded, unable to extendprocessesbecause the matrix is impenetrable; as a result, the cell is stronglyinhibited from dividing, as well as being hindered from migrating.Localized degradation of matrix components is also required wherever cellshave to escapefrom confinement by a basal lamina. It is needed during normalbranching growth of epithelial structures such as glands, for example, to allowthe population of epithelial cells to expand, and needed also when white bloodcells migrate acrossthe basal lamina of a blood vesselinto tissuesin responsetoinfection or injury.
Less benignly, matrix degradation is important both for thespread of cancer cells through the body and for the ability of the cancer cells toproliferate in the tissues that they invade (discussedin Chapter 20).Figure19-74Organizationoffibronectininto fibrilsat the cellsurface,The pictureshowsthe front endTheof a migratingmousefibroblast.adhesionproteinfibronectinextracellularis stainedgreen,andthe intracellularfilamentsof actin,red.Thefibronectinisinitiallypresentassmalldot-likenearthe leadingedgeof theaggregatesat focaladhesionscell.lt accumulates(sitesof anchorageof actinfilaments)and becomesorganizedinto fibrilsparallelto the actinfilaments.Integrinspanningthe cellmembranemoleculeslinkthe fibronectinoutsidethe cellto theactinfilamentsinsideit.Tensionexertedthroughthison the fibronectinmoleculeslinkageis thoughtto stretchthem,exposingbindingsitesthat promotefibrilformation.(Courtesyof RoumenPankovand KennethYamada.)1"194 chapter19:cell Junctions,cell Adhesion,and the ExtracellularMatrixMatrixDegradationls Localizedto the Vicinityof CellsIn general, matrix components are degraded by extracellular proteolyticenzymes (proteases)that act close to the cells that produce them.
Thus, antibodies that recognize the products of proteolltic cleavage stain matrix onlyaround cells. Many of these proteasesbelong to one of two general classes.Mostare matrix metalloproteases, which depend on bound caz* or znz* for activity;the others are serine proteases, which have a highly reactive serine in theiractive site.
Together, metalloproteases and serine proteases cooperate todegrade matrix proteins such as collagen, laminin, and fibronectin. some metalloproteases,such as the collagenases,are highly specific, cleaving particularproteins at a small number of sites. In this way, the structural integrity of thematrix is largely retained, while the limited amount of proteolysis that occurs issufficient for cell migration. other metalloproteases may be less specific, but,because they are anchored to the plasma membrane, they can act just wherethey are needed; it is this type of matrix metalloprotease that is crucial for a cell'sability to divide when embedded in matrix.clearly, the activity of the proteasesthat degrade the matrix components hasto be tightly controlled, if the fabric of the body is not to collapse in a heap. Threebasic control mechanisms operate:Local actiuation:Many proteasesare secretedas inactive precursors that canbe activated locally when needed.
An exampre is plasminogen, an inactive protease precursor that is abundant in the blood. proteases called plasminogenactiuators cleaveplasminogen locally to yield the active serine protease plasmin,-type plasminogen actiuator (tpA) iswhich helps break up blood clots. rz'ssueoften given to patients who have just had a heart attack orthrombotic stroke; ithelps dissolve the arterial clot that caused the attack, thereby restoring bloodflow to the tissue.conftnement by cell-surface receptors: Many cells have receptors on theirsurface that bind proteases,thereby confining the enzyme to the siteswhere it is{ A ) c e l l sw i t h f u n c t i o n a lproteasereceptors( B ) c e l l sw i t h b l o c k e dproteasereceptorsUPAreceptorsactive protease(uPA)TUMOR GROWTHAND METASTASISi n a c t i v ep r o t e a s e( m u t a n tu P A )TUMOR GROWTHBUT NO METASTASISFigure19-75The importanceofproteasesbound to cell-surfacereceptors.(A)Humanprostatecancercellsmakeand secretethe serineproteaseuPA,which binds to cell-surfaceUPAreceptorproteins.(B)The samecellshavebeentransfectedwith DNAthatencodesan excessof an inactiveform ofuPA,which bindsto the uPAreceptorsbut has no proteaseactivity.Byoccupyingmost of the uPAreceptors,theinactiveuPApreventsthe activeproteasefrom bindingto the cellsurface.Bothtypes of cellssecreteactiveuPA,growrapidly,and producetumorswheninjectedinto experimentalanimals.Butthe cellsin (A) metastasizewidely,whereasthe cellsin (B)do not.Tometastasizevia the blood,tumor cellshaveto crawlthroughbasallaminaeandotherextracellularmatriceson the wayinto and out of the bloodstream.Thisexperimentsuggeststhat proteasesmustbe boundto the cellsurfaceto facilitatemigrationthroughthe matrix.1195THEPLANTCELLWALLSummaryCells in connectiue tissuesare embeddedin an intricate extracellular matrix that notonly binds the cells together but also influences their suruiual, deuelopment,shape,polarity,and migratory behauior The matrix containsuariousproteinftbers interwouen in a hydrated gel composedof a network of glycosaminoglycan(GAG)chains.group of negatiuelychargedpolysaccharidechains thatGAGsare q heterogeneous(except for hyaluronan) are coualently linked to protein to form proteoglycanmolecules.The negatiuechargesattrqct counterions, which haue a powerful osmoticeffect,drawing water into the matrix and keepingit swollen so as to occupya large uolume of extracellular space.Proteoglycansare alsofound on the surfaceof cells,wherethey oftenfunction as co-receptorsto help cells respondto secretedsignal proteins.Fiber-forming proteins giue the matrix strength and resilience.They also formstructures to which cells can be anchored,often uia large multidomain glycoproteinssuch as laminin and fibronectin that hauemultiple binding sitesfor integrinson thecell surface.Elasticity is prouided by elastin molecules,which form an extensiuecrosslinked network of fibers and sheetsthat can stretch and recoil.
Thefibrillar collagens(typesI, II, m, V and XI) prouide tensile strength. They are ropelike, triple-strandedhelical moleculesthat aggregateinto long fibrils in the extracellular space.Thefibrilsin turn can assembleinto uarious highly ordered arrays. Fibril-associated collagenmolecules,such as typesIX and XII, decoratethe surfaceof collagenflbrils and influence the interactions of the fibrils with one another and with other matrix components.Matrix components are degraded by extracellular proteolytic enzymes.Most oftheseare matrix metalloproteases,which depend on bound Ca2*or Zn2' for actiuiry'while others are serineproteases,which hque a reactiueserine in their actiue site. Thedegradation of matrix components is subject to complex controls, and cells can, forexample,causea localized degradation of matrix componentsto clear a path throughthe matrix.THEPLANTCELLLLEach cell in a plant deposits, and is in turn completely enclosedby, an elaborateextracellular matrix called the plant cell wall.
ft was the thick cell walls of cork,visible in a primitive microscope, that in 1663 enabled Robert Hooke to distinguish and name cells for the first time. The walls of neighboring plant cells,cemented together to form the intact plant (Figure f 9-76), are generally thicker'stronger,and, most important of all, more rigid than the extracellularmatrix produced by animal cells.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
