Identification of the cleavage sites in the alpha6A integrin subunit: structural requirements for cleavage and functional analysis of the uncleaved alpha6Abeta1 integrin

Gain of glycosylation in integrin α3 causes lung disease and nephrotic syndrome

Integrins are transmembrane αβ glycoproteins that connect the extracellular matrix to the cytoskeleton. The laminin-binding integrin α3β1 is expressed at high levels in lung epithelium and in kidney podocytes. In podocytes, α3β1 associates with the tetraspanin CD151 to maintain a functional filtration barrier. Here, we report on a patient homozygous for a novel missense mutation in the human ITGA3 gene, causing fatal interstitial lung disease and congenital nephrotic syndrome. The mutation caused an alanine-to-serine substitution in the integrin α3 subunit, thereby introducing an N-glycosylation motif at amino acid position 349. We expressed this mutant form of ITGA3 in murine podocytes and found that hyperglycosylation of the α3 precursor prevented its heterodimerization with β1, whereas CD151 association with the α3 subunit occurred normally. Consequently, the β1 precursor accumulated in the ER, and the mutant α3 precursor was degraded by the ubiquitin-proteasome system. Thus, these findings uncover a gain-of-glycosylation mutation in ITGA3 that prevents the biosynthesis of functional α3β1, causing a fatal multiorgan disorder.

Short form of α9 promotes α9β1 integrin-dependent cell adhesion by modulating the function of the full-length α9 subunit

The α9β1 integrin is a multifunctional receptor that interacts with a variety of ligands including vascular cell adhesion molecule 1, tenascin-C, and osteopontin. A 2.3-kb truncated form of α9 integrin subunit cDNA was identified by searching the Medline database. This splice variant, which we called the short form of α9 integrin (SFα9), encodes a 632-aa isoform lacking transmembrane and cytoplasmic domains, and its authentic expression was verified by PCR and Western blotting. SFα9 is expressed on the cell surface but cannot bind ligand in the absence of the full-length α9 subunit. Over-expression of SFα9 in cells expressing full-length α9 promotes α9-dependent cell adhesion. This promoting effect of SFα9 requires the authentic cytoplasmic domain of the co-expressed full-length α9 subunit. Thus, SFα9 is a novel functional modulator of α9β1 integrin by inside-out signaling.

Spatially and temporally regulated alpha6 integrin cleavage during Xenopus laevis development

The alpha6 integrin is essential for early nervous system development in Xenopus laevis. We have previously reported a uPA cleaved form of integrin alpha6 (alpha6p), in invasive human prostate cancer tissue, whose presence correlates with increased migration and invasive capacity. We now report that alpha6 is cleaved during the normal development of Xenopus in a spatially and temporally controlled manner. In addition, unlike normal mammalian tissues, which lack alpha6p, the major form of the alpha6 integrin present in adult Xenopus is alpha6p. The protease responsible for the cleavage in mammals, uPA, is not involved in the cleavage of Xenopus alpha6. Finally, overexpression of a mammalian alpha6 mutant which cannot be cleaved leads to developmental abnormalities suggesting a potential role for the cleavage in development.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

Immunohistochemical localization of subtilisin/kexin-like proprotein convertases in human atherosclerosis

Integrins are heterodimeric alpha/beta receptors that link the cytoskeleton with the extracellular matrix, thereby regulating several cell functions important in atherosclerosis. In vitro, the subtilisin/kexin-like proprotein convertases (PCs), namely PC5 and furin, have been shown to be responsible for the endoproteolytic activation of the alpha(v) integrin subunit. Based on their cleavage activity, these PCs are potential targets in atherosclerosis. In the present study, we investigated the localization of furin and PC5 in different stages of human atherosclerosis. Immunohistochemical analysis of furin and PC5 revealed their presence in vascular smooth-muscle cells and endothelial cells in atherosclerotic and non-atherosclerotic lesions. However, in the more advanced lesions, furin and PC5 staining was significantly expressed in macrophages/foam cells. In vitro, THP-1 derived macrophages contained furin and PC5, and maturation of monocytes to macrophages was accompanied by enhanced alpha(v)beta3 cell-surface expression. Inhibition of furin/PC5 with the specific pharmacological furin-like PC-inhibitor dec-CMK inhibited alpha(v) endoproteolytic activation but did not abolish alpha(v)beta3 cell-surface expression. This indicates that furin/PC5 is required for alpha(v) endoproteolytic activation but not for alpha(v) routing and sorting to the cell surface. In conclusion, our study demonstrates that furin and PC5 are significantly expressed in mononuclear cells in advanced human atherosclerotic lesions, where they regulate alpha(v) endoproteolytic activation.

Membrane type-1 matrix metalloproteinase (MT1-MMP) processing of pro-αv integrin regulates cross-talk between αvβ3 and α2β1 integrinsin breast carcinoma cells

We have recently demonstrated that in breast carcinoma MCF7 cells MT1-MMP processes the alphav, alpha3, and alpha5 integrin precursors generating the respective mature S-S-linked heavy and light alpha-chains. The precursor of alpha2 integrin subunit was found resistant to MT1-MMP proteolysis. The processing of the alphav subunit by MT1-MMP facilitated alphavbeta3-dependent adhesion, activation of FAK signaling pathway, and migration of MCF7 cells on vitronectin. To elucidate further the effects of MT1-MMP on cellular integrins, we examined the functional activity of alpha5beta1 and alpha2beta1 integrins in MCF7 cells expressing MT1-MMP. Either expression of MT1-MMP alone or its coexpression with alphavbeta3 failed to affect the functionality of alpha5beta1 integrin, and adhesion of cells to fibronectin. MT1-MMP, however, profoundly affected the cross-talk involving alphavbeta3 and alpha2beta1 integrins. In MT1-MMP-deficient cells, integrin alphavbeta3 suppressed the functional activity of the collagen-binding alpha2beta1 integrin receptor and diminished cell adhesion to type I collagen. Coexpression of MT1-MMP with integrin alphavbeta3 restored the functionality of alpha2beta1 integrin and, consequently, the ability of MCF7 cells to adhere efficiently to collagen. We conclude that the MT1-MMP-controlled cross-talk between alphavbeta3 and alpha2beta1 integrins supports binding of aggressive, MT1-MMP-, and alphavbeta3 integrin-expressing malignant cells on type I collagen, the most common substratum of the extracellular matrix.

Processing of alpha4 integrin by the proprotein convertases: histidine at position P6 regulates cleavage

The proprotein convertases (PCs) participate in the limited proteolysis of integrin alpha4 subunit at the H(592)VISKR(597) downward arrow ST site (where underlined residues indicate positively charged amino acids important for PC-mediated cleavage and downward arrow indicates the cleavage site), since this cleavage is inhibited by the serpin alpha1-PDX (alpha1-antitrypsin Portland). Co-expression of alpha4 with each convertase in LoVo (furin-deficient human colon carcinoma) cells revealed that furin and proprotein convertase 5A (PC5A) are the best pro-alpha4 convertases. In agreement, processing of endogenous pro-alpha4 in human lymphoblastoid CEM-T4 cells was enhanced greatly in stable transfectants overexpressing either enzyme. In many leucocyte cell lines, the expression of furin closely correlated with the endogenous processing efficacy, suggesting that furin is a candidate pro-alpha4 convertase. Mutational analysis showed that replacement of P1 Arg(597) with alanine (R597A) abrogated cleavage, whereas the P6 mutant H592R is even better processed by the endogenous convertases of Chinese-hamster ovary CHO-K1 cells. In vitro kinetic studies using synthetic peptides confirmed the importance of a positively charged residue at P6 and showed that wild-type alpha4 processing is performed best by furin and PC5A at acidic and neutral pHs, respectively. Biosynthetic analysis of pro-alpha4 and its H592R and H592K mutants in the presence or absence of the weak base, NH(4)Cl, revealed that the P6 histidine residue renders its processing by furin sensitive to cellular pH. This suggests that pro-alpha4 cleavage occurs preferentially in acidic compartments. In conclusion, although the accepted furin processing motif is Arg-Xaa-(Lys/Arg)-Arg downward arrow, our data further extend it to include a regulatory histidine residue at P6 in precursors that lack a basic residue at P4.

A signaling role for the uncleaved form of alpha 6 integrin in differentiating lens fiber cells

Many alpha integrin subunits are cleaved during their processing to yield heavy and light chains, which remain associated by disulfide bonds. While uncleaved alpha integrin subunits can form functional receptors that sometimes have distinct signaling roles from their better-characterized endoproteolytically cleaved counterparts, their expression at the cell surface and their association with signaling complexes have yet to be determined in vivo. In this study, we demonstrate that, in differentiating lens fiber cells, the uncleaved form of alpha 6 integrin was expressed at the cell surface. This form of alpha 6 integrin coimmunoprecipitated with both the signaling adaptor molecule Shc and its downstream effector Grb2, suggesting that, in lens fiber cells, uncleaved alpha 6 integrin was associated with a Shc-mediated signaling complex. We show that expression of the cleaved form of alpha 6 integrin progressively decreased relative to uncleaved alpha 6 integrin as the state of lens cell differentiation increased, resulting in the predominance of uncleaved alpha 6 integrin in the lens fiber cell zones. Interestingly, we previously have shown that alpha 6 integrin is localized principally along the extensive cell-cell interfaces of these lens fiber cells, in the absence of its extracellular matrix ligand laminin. While we found that the cleaved form of alpha 6 integrin contained both high mannose and complex sugars, the uncleaved form of alpha 6 integrin contained only high mannose sugars. These properties suggest that the uncleaved form of alpha 6 integrin may have a unique role in the embryonic lens. Its high association with Shc and Grb2 in the differentiating cortical fiber cell zone indicates that alpha 6 integrin may provide a cell survival signal in the presence of the apoptotic-like processes that are initiated in this region of the embryonic lens to clear the lens cells of their organelles.

Evidence that integrins contribute to multiple stages in the consolidation of long term potentiation in rat hippocampus

Three structurally distinct groups of antagonists were used to test the hypothesis that integrin adhesion receptors play an essential role in consolidating (stabilizing) long term potentiation of the Schaffer collaterals in rat hippocampus. Comparisons were made of percent potentiation at antagonist-treated versus control sites within CA1 stratum radiatum of the same hippocampal slice. Function blocking antibodies against the alpha5 subunit of the fibronectin receptor had no effect on baseline responses or initial potentiation but resulted in a >30% reduction, relative to within-slice control long term potentiation, 45 min later. Larger reductions were recorded in separate experiments continued for 4 h after the induction of potentiation. Alpha(v) and alpha2 subunit antibodies did not reliably affect the stabilization of potentiation. An antagonist peptide with preference for beta1 integrins produced a slowly developing decline of the type seen with alpha5 antibodies. A cyclic peptide antagonist reduced potentiation within 10 min of induction and caused an almost 40% decrease over 45 min. Two disintegrins (snake toxins that potently block integrins) were very effective in preventing the consolidation of long term potentiation: echistatin reduced potentiation by >70%, while triflavin caused approximately 50% decrease. The suppressing effects of echistatin were concentration-dependent, obtained with treatment after induction, and much more rapid than the effects of antibodies. Rapid declines in potentiation were particularly evident when the two disintegrins were applied together. These results indicate that hippocampal fibronectin receptors (alpha5/beta1 integrin) contribute importantly to a slowly developing phase of long term potentiation consolidation. They also suggest that other integrins are critical to aspects of consolidation occurring in the first few minutes after induction.

Identification of a novel structural variant of the alpha 6 integrin

The alpha(6) integrin is a 140-kDa (nonreduced) laminin receptor. We have identified a novel 70-kDa (nonreduced) form of the alpha(6) integrin called alpha(6)p for the latin word parvus, meaning small. The variant was immunoprecipitated from human cells using four different alpha(6)-specific monoclonal antibodies but not with alpha(3) or alpha(5) antibodies. The alpha(6)p integrin contained identical amino acid sequences within exons 13--25, corresponding to the extracellular "stalk region" and the cytoplasmic tail of the alpha(6) integrin. The light chains of alpha(6) and alpha(6)p were identical as judged by alpha(6)A-specific antibodies and electrophoretic properties. The alpha(6)p variant paired with either beta(1) or beta(4) subunits and was retained on the cell surface three times longer than alpha(6). Reverse transcription/polymerase chain reaction analysis revealed a single polymerase chain reaction product. The alpha(6)p variant was found in human prostate (DU145H, LnCaP, PC3) and colon (SW480) cancer cell lines but not in normal prostate (PrEC), breast cancer (MCF-7), or lung cancer (H69) cell lines or a variant of a prostate carcinoma cell line (PC3-N). Protein levels of alpha(6)p increased 3-fold during calcium-induced terminal differentiation in a normal mouse keratinocyte model system. A novel form of the alpha(6) integrin exists on cell surfaces that contains a dramatically altered extracellular domain.

Endoproteolytic processing of integrin pro-alpha subunits involves the redundant function of furin and proprotein convertase (PC) 5A, but not paired basic amino acid converting enzyme (PACE) 4, PC5B or PC7

Several integrin alpha subunits undergo post-translational endoproteolytic processing at pairs of basic amino acids that is mediated by the proprotein convertase furin. Here we ask whether other convertase family members can participate in these processing events. We therefore examined the endoproteolysis rate of the integrin subunits pro-alpha5, alpha6 and alphav by recombinant furin, proprotein convertase (PC)5A, paired basic amino acid converting enzyme (PACE)4, PC1, PC2 and PC7 in vitro and/or ex vivo after overexpression in LoVo cells that were deficient in furin activity. We found that 60-fold more PC1 than furin was needed to produce 50% cleavage of pro-alpha subunit substrates in vitro; the defective pro-alpha chain endoproteolysis in LoVo cells was not rescued by overexpression of PC1 or PC2. No endoproteolysis occurred with PC7 either in vitro or ex vivo, although similar primary sequences of the cleavage site are found in integrins and in proteins efficiently processed by PC7, which suggests that a particular conformation of the cleavage site is required for optimal convertase-substrate interactions. In vitro, 50% cleavage of pro-alpha subunits was obtained with one-third of the amount of PC5A and PACE4 than of furin. In LoVo cells, PC5A remained more active than furin, PACE4 activity was quite low, and PC5B, which differs from PC5A by a C-terminal extension containing a transmembrane domain, was very inefficient in processing integrin alpha-subunit precursors. In conclusion, these results indicate that integrin alpha-subunit endoproteolytic processing involves the redundant function of furin and PC5A and to a smaller extent PACE4, but not of PC1, PC2, PC5B or PC7.

Recombinant soluble human alpha 3 beta 1 integrin: purification, processing, regulation, and specific binding to laminin-5 and invasin in a mutually exclusive manner

Using insect cells, we expressed large quantities of soluble human integrin alpha 3 beta 1 ectodomain heterodimers, in which cytoplasmic and transmembrane domains were replaced by Fos and Jun dimerization motifs. In direct ligand binding assays, soluble alpha 3 beta 1 specifically bound to laminin-5 and laminin-10, but not to laminin-1, laminin-2, fibronectin, various collagens, nidogen, thrombospondin, or complement factors C3 and C3b. Soluble alpha 3 beta1 integrin also bound to invasin, a bacterial surface protein, that mediates entry of Yersinia species into the eukaryotic host cell. Invasin completely displaced laminin-5 from the alpha 3 beta 1 integrin, suggesting sterically overlapping or identical binding sites. In the presence of 2 mM Mg2+, alpha 3 beta 1's binding affinity for invasin (Kd = 3.1 nM) was substantially greater than its affinity for laminin-5 (Kd > 600 nM). Upon addition of 1 mM Mn2+, or activating antibody 9EG7, binding affinity for both laminin-5 and invasin increased by about 10-fold, whereas the affinity decreased upon addition of 2 mM Ca2+. Thus, functional regulation of the purified soluble integrin alpha 3 beta 1 ectodomain heterodimer resembles that of wild-type membrane-anchored beta 1 integrins. The integrin alpha 3 subunit was entirely cleaved into disulfide-linked heavy and light chains, at a newly defined cleavage site located C-terminal of a tetrabasic RRRR motif. Within the alpha 3 light chain, all potential N-glycosylation sites bear N-linked mannose-rich carbohydrate chains, suggesting an important structural role of these sugar residues in the stalk-like region of the integrin heterodimer. In conclusion, studies of our recombinant alpha 3 beta 1 integrin have provided new insights into alpha 3 beta1 structure, ligand binding function, specificity, and regulation.