Beyond the Matrix: The Many Non-ECM Ligands for Integrins.

The traditional view of integrins portrays these highly conserved cell surface receptors as mediators of cellular attachment to the extracellular matrix (ECM), and to a lesser degree, as coordinators of leukocyte adhesion to the endothelium. These canonical activities are indispensable; however, there is also a wide variety of integrin functions mediated by non-ECM ligands that transcend the traditional roles of integrins. Some of these unorthodox roles involve cell-cell interactions and are engaged to support immune functions such as leukocyte transmigration, recognition of opsonization factors, and stimulation of neutrophil extracellular traps. Other cell-cell interactions mediated by integrins include hematopoietic stem cell and tumor cell homing to target tissues. Integrins also serve as cell-surface receptors for various growth factors, hormones, and small molecules. Interestingly, integrins have also been exploited by a wide variety of organisms including viruses and bacteria to support infectious activities such as cellular adhesion and/or cellular internalization. Additionally, the disruption of integrin function through the use of soluble integrin ligands is a common strategy adopted by several parasites in order to inhibit blood clotting during hematophagy, or by venomous snakes to kill prey. In this review, we strive to go beyond the matrix and summarize non-ECM ligands that interact with integrins in order to highlight these non-traditional functions of integrins.

Binding of MAGP2 to microfibrils is regulated by proprotein convertase cleavage.

MAGP2 is a small extracellular protein with both tumor angiogenesis and cell signaling activity. MAGP2 was originally isolated biochemically from microfibril-rich connective tissue. The localization of MAGP2 to microfibrils has been confirmed by both immunohistochemistry and immunogold electron microscopy. Whether MAGP2 binding to microfibrils is regulated post-translationally is still unclear, however, and a better understanding of this process would be instructive to understanding the angiogenesis and signaling functions ascribed to MAGP2. Here we show via immunofluorescence studies that the T3 cell line, derived from ovarian mouse tumor cells, produces abundant fibrillin-2 microfibrils to which MAGP2 can bind. Co-localization of MAGP2 and fibrillin-2 can be detected either when MAGP2 is overexpressed in, or exogenously introduced to, the cells. As expected, matrix association of MAGP2 required its conserved Matrix Binding Domain. Matrix association was positively regulated by proprotein convertase (PC) cleavage of MAGP2; mutation of the MAGP2 PC consensus site reduced the amount of matrix-associated MAGP2. Deletion analysis of the C-terminal 20-amino acid domain that is defined by the PC cleavage site suggests that this domain also positively modulates matrix localization of MAGP2, in a manner that requires the amino-terminal half of the protein. Together, our data indicate that matrix localization of MAGP2 by its Matrix Binding Domain is promoted by PC cleavage and the presence of its C-terminal 20 amino acids.

Identification of a functional proprotein convertase cleavage site in microfibril-associated glycoprotein 2.

Microfibril-associated glycoprotein 2 (MAGP2) is a secreted protein associated with multiple cellular activities including the organization of elastic fibers in the extracellular matrix (ECM), angiogenesis, as well as regulating Notch and integrin signaling. Importantly, increases in MAGP2 positively correlate with poor prognosis for some ovarian cancers. It has been assumed that full-length MAGP2 is responsible for all reported effects; however, here we show MAGP2 is a substrate for the proprotein convertase (PC) family of endoproteases. Proteolytic processing of MAGP2 by PC cleavage could serve to regulate secretion and thus, activity and function as reported for other extracellular and cell-surface proteins. In support of this idea, MAGP2 contains an evolutionarily conserved PC consensus cleavage site, and amino acid sequencing of a newly identified MAGP2 C-terminal cleavage product confirmed functional PC cleavage. Additionally, mutagenesis of the MAGP2 PC consensus cleavage site or treatment with PC inhibitors prevented MAGP2 proteolytic processing. Finally, both cleaved and uncleaved MAGP2 were detected extracellularly and MAGP2 secretion appeared independent of PC cleavage, suggesting that PC processing occurs mainly outside the cell. Our characterization of alternative forms of MAGP2 present in the extracellular space not only enhances diversity of this ECM protein but also provides a previously unrecognized molecular mechanism for regulation of MAGP2 biological activity.

Notch signaling--constantly on the move.

The Notch pathway is a highly conserved and ubiquitous signaling system that functions in determining a diverse array of cell fates and regulates many cellular processes during embryonic development and throughout adulthood. Links to cancer, stroke and Alzheimer's disease underscore the need to define the molecular basis of Notch activation. Notch signaling is induced through direct cell-cell interactions that promote receptor activation following engagement with a membrane-bound Delta, Serrate, Lag-2 (DSL) ligand on adjacent cells. Cells take on distinct fates because Notch signaling is consistently activated in only one of the two interacting cells, highlighting the importance of establishing and maintaining signaling polarity. Studies in flies and worms have identified positive and negative transcriptional feedback mechanisms that amplify small differences in Notch and DSL ligand expression to bias which cells send or receive signals. However, endocytosis by signal-sending and signal-receiving cells also appears critical for directing and regulating Notch activation. In particular, endocytosis and membrane trafficking of DSL ligands, Notch and modulators can determine the competence of cells to send or receive signals that ensure reproducibility in generating cell types regulated by Notch signaling.

DSL ligand endocytosis physically dissociates Notch1 heterodimers before activating proteolysis can occur.

Cleavage of Notch by furin is required to generate a mature, cell surface heterodimeric receptor that can be proteolytically activated to release its intracellular domain, which functions in signal transduction. Current models propose that ligand binding to heterodimeric Notch (hNotch) induces a disintegrin and metalloprotease (ADAM) proteolytic release of the Notch extracellular domain (NECD), which is subsequently shed and/or endocytosed by DSL ligand cells. We provide evidence for NECD release and internalization by DSL ligand cells, which, surprisingly, did not require ADAM activity. However, losses in either hNotch formation or ligand endocytosis significantly decreased NECD transfer to DSL ligand cells, as well as signaling in Notch cells. Because endocytosis-defective ligands bind hNotch, but do not dissociate it, additional forces beyond those produced through ligand binding must function to disrupt the intramolecular interactions that keep hNotch intact and inactive. Based on our findings, we propose that mechanical forces generated during DSL ligand endocytosis function to physically dissociate hNotch, and that dissociation is a necessary step in Notch activation.

Microfibrillar proteins MAGP-1 and MAGP-2 induce Notch1 extracellular domain dissociation and receptor activation.

Unlike most receptors, Notch serves as both the receiver and direct transducer of signaling events. Activation can be mediated by one of five membrane-bound ligands of either the Delta-like (-1, -2, -4) or Jagged/Serrate (-1, -2) families. Alternatively, dissociation of the Notch heterodimer with consequent activation can also be mediated experimentally by calcium chelators or by mutations that destabilize the Notch1 heterodimer, such as in the human disease T cell acute lymphoblastic leukemia. Here we show that MAGP-2, a protein present on microfibrils, can also interact with the EGF-like repeats of Notch1. Co-expression of MAGP-2 with Notch1 leads to both cell surface release of the Notch1 extracellular domain and subsequent activation of Notch signaling. Moreover, we demonstrate that the C-terminal domain of MAGP-2 is required for binding and activation of Notch1. Based on the high level of homology, we predicted and further showed that MAGP-1 can also bind to Notch1, cause the release of the extracellular domain, and activate signaling. Notch1 extracellular domain release induced by MAGP-2 is dependent on formation of the Notch1 heterodimer by a furin-like cleavage, but does not require the subsequent ADAM metalloprotease cleavage necessary for production of the Notch signaling fragment. Together these results demonstrate for the first time that the microfibrillar proteins MAGP-1 and MAGP-2 can function outside of their role in elastic fibers to activate a cellular signaling pathway.

The divergent DSL ligand Dll3 does not activate Notch signaling but cell autonomously attenuates signaling induced by other DSL ligands.

Mutations in the DSL (Delta, Serrate, Lag2) Notch (N) ligand Delta-like (Dll) 3 cause skeletal abnormalities in spondylocostal dysostosis, which is consistent with a critical role for N signaling during somitogenesis. Understanding how Dll3 functions is complicated by reports that DSL ligands both activate and inhibit N signaling. In contrast to other DSL ligands, we show that Dll3 does not activate N signaling in multiple assays. Consistent with these findings, Dll3 does not bind to cells expressing any of the four N receptors, and N1 does not bind Dll3-expressing cells. However, in a cell-autonomous manner, Dll3 suppressed N signaling, as was found for other DSL ligands. Therefore, Dll3 functions not as an activator as previously reported but rather as a dedicated inhibitor of N signaling. As an N antagonist, Dll3 promoted Xenopus laevis neurogenesis and inhibited glial differentiation of mouse neural progenitors. Finally, together with the modulator lunatic fringe, Dll3 altered N signaling levels that were induced by other DSL ligands.

The extracellular matrix protein MAGP-2 interacts with Jagged1 and induces its shedding from the cell surface.

Elastic fibers are composed of the protein elastin and a network of 10-12-nm microfibrils, which are composed of several glycoproteins, including fibrillin-1, fibrillin-2, and MAGP1/2 (microfibril-associated glycoproteins-1 and -2). Although fibrillins and MAGPs covalently associate, we find that the DSL (Delta/Serrate/LAG2) protein Jagged1, an activating ligand for Notch receptor signaling, also interacts with MAGP-2 in both yeast two-hybrid and coimmunoprecipitation studies. Interaction between Jagged1 and MAGP-2 requires the epidermal growth factor-like repeats of Jagged1. MAGP-2 was found complexed with the Jagged1 extracellular domain shed from 293T cells and COS-7 cells coexpressing full-length Jagged1 and MAGP-2. MAGP-2 shedding of the Jagged1 extracellular domain was decreased by the metalloproteinase hydroxamate inhibitor BB3103 implicating proteolysis in its release. Although MAGP-2 also interacted with the other DSL ligands, Jagged2 and Delta1, they were not found associated with MAGP-2 in the conditioned media, identifying differential effects of MAGP-2 on DSL ligand shedding. The related microfibrillar protein MAGP-1 was also found to interact with DSL ligands but, unlike MAGP-2, was unable to facilitate the shedding of Jagged1. Our findings suggest that in addition to its role in microfibrils, MAGP-2 may also affect cellular differentiation through modulating the Notch signaling pathway either by binding to cell surface DSL ligands or by facilitating release and/or stabilization of a soluble extracellular form of Jagged1.

Notch signaling promotes astrogliogenesis via direct CSL-mediated glial gene activation.

In the developing central nervous system (CNS), Notch signaling preserves progenitor pools and inhibits neurogenesis and oligodendroglial differentiation. It has recently been postulated that Notch instructively drives astrocyte differentiation. Whether the role of Notch signaling in promoting astroglial differentiation is permissive or instructive has been debated. We report here that the astrogliogenic role of Notch is in part mediated by direct binding of the Notch intracellular domain to the CSL DNA binding protein, forming a transcriptional activation complex onto the astrocyte marker gene, glial fibrillary acidic protein (GFAP). In addition, we found that, in CSL-/- neural stem cell cultures, astrocyte differentiation was delayed but continued at a normal rate once initiated, suggesting that CSL is involved in regulating the onset of astrogliogenesis. Importantly, although the classical CSL-dependent Notch signaling pathway is intact and able to activate the Notch canonical target promoter during the neurogenic phase, it is unable to activate the GFAP promoter during neurogenesis. Therefore, the effect of Notch signaling on target genes is influenced by cellular context in regulation of neurogenesis and gliogenesis.