Integrins as cellular receptors for fibril-forming and transmembrane collagens

Abstract

Nykvist tutki väitöskirjatyössään säikeitä muodostavia kollageeneja I, II, III ja V, verkkomaisia rakenteita muodostavaa kollageenityyppiä IV ja solun pintaan ankkuroituja kollageeneja XIII ja XVII.Integriinit ovat joukko solun pinnan reseptiivisiä valkuaisaineita, jotka välittävät solujen välisiä ja/tai solujen ja soluväliaineen välisiä vuorovaikutuksia. Integriinit rakentuvat toisiinsa sitoutuneesta alfa- ja beeta-alayksiköstä, joiden yhdistelmät osallistuvat elimistön normaaliin toimintaan säädellen solujen kasvua, vaeltamista, erilaistumista ja ilmiasua. Ne yhdistävät solun sisäiseen viestintäjärjestelmään osallistuvat valkuaisaineet solun ympäristöön. Tämä vuorovaikutus on kaksisuuntaista viestien kulkiessa ympäristöstä solun sisään ja toisaalta solusta ympäristöön integriinien aktiivisuuden säätelyn muodossa.

The two integrin-type collagen receptors alfa1beeta1 and alfa2beeta1 integrins are structurally very similar. However, cells can concomitantly express both receptors and it has been shown that these collagen receptor integrins have distinct signaling functions, and their binding to collagen may lead to opposite cellular responses.In this study, fibrillar collagen types, I, II, III, and V, and network like structure forming collagen type IV tested were recognized by both integrins at least at the aI domain level. The aI domain recognition does not always lead for cell spreading behavior. In addition transmembrane collagen type XIII was studied. CHO-alfa1beeta1 cells could spread on recombinant human collagen type XIII, unlike CHO-alfa2beeta1 cells. This finding was supported by alfaI domain binding studies. The results indicate, that alfa1beeta1 and alfa2beeta1 integrins do have different ligand binding specificities and distinct collagen recognition mechanisms.A common structural feature in the collagen binding alfaI domains is the presence of an extra helix, named helix alfaC. A alfaC helix deletion reduced affinity for collagen type I when compared to wild-type alfa2I domain, which indicated the importance of helix alfaC in collagen type I binding. Further, point mutations in amino acids Asp219, Asp259, Asp292 and Glu299 resulted in weakened affinity for collagen type I. Cells expressing double mutated alfa2Asp219/Asp292 integrin subunit showed remarkably slower spreading on collagen type I, while spreading on collagen type IV was not affected. The data indicated that alfa2I domain binds to collagen type I with a different mechanism than binding to collagen type IV.In tissues collagen monomers form large fibrils immediately after they have been released from cells. The alfa2I domain binding data indicate that fibril formation affects the binding constant and at the same time reduces the number of putative binding sites available to the integrins. However, integrin alfa2beeta1 could still mediate spreading on fibrillar collagen and the contraction of floating collagen gels. The alfa1beeta1 cells could neither adequately spread on collagen fibrils nor mediate the contraction of collagen gels. These findings indicate that alfa2beeta1 integrin is a functional cellular receptor for collagen fibrils, while alfa1beeta1 integrin may only bind collagen monomers effectively.