The bone morphogenetic protein (BMP) pathway is a major conserved signalling pathway with diverse roles in development and homeostasis. Given that cells exist in three-dimensional environments, one important area is to understand how the BMP pathway operates within such complex cellular environments. The extracellular matrix contains information regarding tissue architecture and its mechanical properties that is transmitted to the cell via integrin receptors. In this review, I describe various examples of modulation of the BMP pathway by integrins. In the case of the Drosophila embryo and some cell line-based studies, integrins have been found to enhance BMP responses through different mechanisms, such as enhancement of BMP ligand–receptor binding and effects on Smad phosphorylation or stability. In these contexts, BMP-dependent activation of integrins is a common theme. However, I also discuss examples where integrins inhibit the BMP pathway, highlighting the context-dependent nature of integrin–BMP cross-talk.

Introduction

Within a tissue environment, a cell will typically be receiving and responding to different types of signalling molecules, while at the same time processing information relating to tissue architecture via integrin receptors. Integrins are obligate αβ heterodimers that mediate adhesion to extracellular matrix proteins, such as fibronectin and collagen IV, and can recruit different cytoplasmic signalling proteins, triggered by integrin intracellular domain clustering, to illicit diverse cellular responses [1]. Given this mixture of signals received simultaneously by a cell, there are multiple opportunities for cross-regulation during their transduction. Modulation of bone morphogenetic protein (BMP) outputs by the components of other signalling pathways, such as the epidermal growth factor/mitogen-activated protein kinase (MAPK), Wnt and Hippo pathways, has been well documented [26]. However, regulation of BMP signalling by integrins has received less attention, despite extensive literature documenting cross-talk between integrins and other classes of signal, particularly MAPK signalling [7]. In this review, I summarize data describing how integrins can influence the BMP pathway output and high-light common themes that are emerging from these studies.

Integrin–BMP synergy in Drosophila

The low redundancy and tractability make Drosophila a powerful model for studying the interplay between integrin and BMP signalling in vivo. Our recent study exploited these advantages to show how integrins modify BMP signalling in the early Drosophila embryo [8], where a heterodimer of Decapentaplegic (Dpp) and Screw is the most potent BMP signalling molecule. In Drosophila, BMP signalling leads to phosphorylation and activation of the Smad transcription factor mothers against Dpp (Mad), which together with Medea regulates BMP target gene expression (Figure 1A). In the early embryo, a gradient of BMP activity exists that patterns different cell fates in the embryonic dorsal ectoderm (Figure 1B), through the graded activation of phosphorylated Mad (pMad) and differential regulation of target gene expression [9]. At the stage in early Drosophila embryogenesis, when BMP signalling patterns cell fates in the dorsal ectoderm, only a single β integrin is expressed, βPS encoded by the myospheroid (mys) gene [8]. Therefore, given that integrins function as αβ dimers, the removal of maternal and zygotic mys eliminates integrin function from the early embryo. Analysis of BMP signalling in these maternal and zygotic mys null mutants (hereafter referred to as mys mutant embryos) revealed disrupted BMP signalling. In particular, these mutant embryos show a loss of peak target genes, such as Race (official name Ance), in the presumptive amnioserosa (Figure 1C), while the expression patterns of lower threshold BMP target genes are narrower. Consistent with this, there is a thinner pMad gradient in mys mutant embryos compared with wild-type (Figure 1C). Together, these data show that peak BMP responses in the early embryo require integrin function. BST-2016-0111TB1 

Integrins are required for peak BMP signalling in the Drosophila embryo.

Figure 1.
Integrins are required for peak BMP signalling in the Drosophila embryo.

(A) Schematic of the BMP pathway. For simplicity, only the vertebrate and fly BMP receptors discussed here are listed. (B) Cartoon showing BMP (Dpp-Scw) distribution (green) in the dorsal ectoderm of the embryo, as a lateral/side view (top embryo) or rotated 90° to show the dorsal/top view (bottom embryo). There is a peak of BMP protein (dark green) at the dorsal-most region that will specify amnioserosa cell fate, whereas dorsal epidermis forms in response to lower BMP levels. (C) Wild-type and mysXG43 null mutant (lacking maternal and zygotic mys) embryos showing expression of Race, a marker of presumptive amnioserosa, as detected by RNA in situ hybridization. pMad protein is also visualized by immunostaining. Embryos are stage 6, dorsal views. Reproduced from ref. [8].

Figure 1.
Integrins are required for peak BMP signalling in the Drosophila embryo.

(A) Schematic of the BMP pathway. For simplicity, only the vertebrate and fly BMP receptors discussed here are listed. (B) Cartoon showing BMP (Dpp-Scw) distribution (green) in the dorsal ectoderm of the embryo, as a lateral/side view (top embryo) or rotated 90° to show the dorsal/top view (bottom embryo). There is a peak of BMP protein (dark green) at the dorsal-most region that will specify amnioserosa cell fate, whereas dorsal epidermis forms in response to lower BMP levels. (C) Wild-type and mysXG43 null mutant (lacking maternal and zygotic mys) embryos showing expression of Race, a marker of presumptive amnioserosa, as detected by RNA in situ hybridization. pMad protein is also visualized by immunostaining. Embryos are stage 6, dorsal views. Reproduced from ref. [8].

Table 1
Summary of BMP-integrin cross-talk
Context Integrin(s) involved Association with BMPR Effect on BMP signalling Mechanism References 
Drosophila dorsal–ventral patterning αPS1βPS, αPS3βPS Yes, Tkv (type I receptor) Positive Increases Smad activation downstream of receptor activation [8
Human osteoblasts αvβ3, αvβ5, αvβ6, αvβ8, α1β1, α2β1 Yes, BMPR1 and BMPR2 Positive Increases Smad activation downstream of receptor activation [2931
C2C12 cells, matrix-bound BMP-2 on soft substrate β3 integrins Not reported Positive (1) Facilitates Smad phosphorylation through an unknown priming role; (2) Increases Smad stability by inhibiting GSK3 [32
Murine embryonic endothelial cells α5β1 Yes, ACVRL1 and endoglin Positive Enhances ACVRL1–endoglin complex formation, which may increase BMP binding [34
Vascular endothelial cells, OSS αvβ3 Yes, BMPR1B Positive Activates Smad phosphorylation via a Shc/FAK/ERK cascade [35
EZCs and NSCs β1 Yes, BMPR1A and BMPR1B Negative Reduces the ability of BMPR to move into lipid rafts and signal [37
BMMSCs on soft ECM β1 Co-localization with BMPR1A Negative Reduces pSmad by promoting BMPR internalization via caveolae [38
BMMSCs and CHO cells α1β1 Yes, BMPR1A Negative Reduces pSmad, mechanism unknown but reduced BMP receptor binding suggested [39
Context Integrin(s) involved Association with BMPR Effect on BMP signalling Mechanism References 
Drosophila dorsal–ventral patterning αPS1βPS, αPS3βPS Yes, Tkv (type I receptor) Positive Increases Smad activation downstream of receptor activation [8
Human osteoblasts αvβ3, αvβ5, αvβ6, αvβ8, α1β1, α2β1 Yes, BMPR1 and BMPR2 Positive Increases Smad activation downstream of receptor activation [2931
C2C12 cells, matrix-bound BMP-2 on soft substrate β3 integrins Not reported Positive (1) Facilitates Smad phosphorylation through an unknown priming role; (2) Increases Smad stability by inhibiting GSK3 [32
Murine embryonic endothelial cells α5β1 Yes, ACVRL1 and endoglin Positive Enhances ACVRL1–endoglin complex formation, which may increase BMP binding [34
Vascular endothelial cells, OSS αvβ3 Yes, BMPR1B Positive Activates Smad phosphorylation via a Shc/FAK/ERK cascade [35
EZCs and NSCs β1 Yes, BMPR1A and BMPR1B Negative Reduces the ability of BMPR to move into lipid rafts and signal [37
BMMSCs on soft ECM β1 Co-localization with BMPR1A Negative Reduces pSmad by promoting BMPR internalization via caveolae [38
BMMSCs and CHO cells α1β1 Yes, BMPR1A Negative Reduces pSmad, mechanism unknown but reduced BMP receptor binding suggested [39

To distinguish between an adhesion versus signalling role for integrins, a chimeric TorD-βPScyt protein was used that has the βPS integrin cytoplasmic tail fused to the extracellular and transmembrane domains of a gain-of-function Torso receptor and clusters in the absence of ligand [10] (Figure 2A). This signalling-activated form of integrin had previously been shown to rescue the integrin loss-of-function phenotype in the Drosophila midgut [10]. Expression of this TorD-βPScyt fusion protein in mys mutant embryos rescued the BMP signalling defect (Figure 2B), indicating that integrin signalling rather than adhesion mediates enhancement of BMP responses [8].

Integrin signalling enhances BMP responses.

Figure 2.
Integrin signalling enhances BMP responses.

(A) Cartoon showing ligand-dependent clustering of αβ integrin dimers, which can be mimicked by the TorD-βPScyt fusion protein. (B) RNA in situ hybridization showing loss of Race expression in the presumptive amnioserosa in a mysXG43 mutant embryo (top) that is rescued by expression of the TorD-βPScyt transgene (bottom). Embryos are stage 6, dorsal views. (C) Cartoon depicting that integrin signalling enhances BMP pathway activation (left). A loss of integrin signalling may contribute to the reduced BMP signalling observed in collagen IV mutants (middle) and be rescued by TorD-βPScyt-mediated integrin signalling (right, red arrow). (D) As in (C) except the embryos are from vkgk00236/+ females. Reproduced from ref. [8].

Figure 2.
Integrin signalling enhances BMP responses.

(A) Cartoon showing ligand-dependent clustering of αβ integrin dimers, which can be mimicked by the TorD-βPScyt fusion protein. (B) RNA in situ hybridization showing loss of Race expression in the presumptive amnioserosa in a mysXG43 mutant embryo (top) that is rescued by expression of the TorD-βPScyt transgene (bottom). Embryos are stage 6, dorsal views. (C) Cartoon depicting that integrin signalling enhances BMP pathway activation (left). A loss of integrin signalling may contribute to the reduced BMP signalling observed in collagen IV mutants (middle) and be rescued by TorD-βPScyt-mediated integrin signalling (right, red arrow). (D) As in (C) except the embryos are from vkgk00236/+ females. Reproduced from ref. [8].

Enhancement of pMad levels

A Drosophila tissue culture model that mimics the in vivo situation was also established, in which integrins increased pMad levels in response to pathway activation by either BMPs or an activated form of the Thickveins (Tkv) type I BMP receptor, TkvQD [8]. The ability of integrins to enhance TkvQD-activated pMad levels supports a role for integrins in enhancing the BMP pathway downstream of receptor activation. Moreover, the TorD-βPScyt fusion protein enhanced signalling activated by TkvQD, with this activity requiring the NPXY motifs in the βPS tail [8] that recruit various integrin binding proteins [11,12]. Co-immunoprecipitation studies revealed an association between Tkv and βPS and two α-integrins, αPS1 and αPS3, which, in the case of βPS, is dependent on sequences within its intracellular tail [8]. We focused on αPS1 and αPS3, as although three α-integrins are expressed in the early embryo, only two — multiple edematous wings (mew) and scab (scb) encoding αPS1 and αPS3, respectively — have expression domains that overlap with the dorsal region of the embryo, where BMP signalling is active. In the tissue culture model, knock-down of both mew and scb together is necessary to reduce the pMad level similar to that observed upon βPS knock-down.

These observations suggest a redundant function of αPS1βPS and αPS3βPS receptors in mediating pMad enhancement through integrin signalling in cells. Analysis of mutant embryos revealed that although mutations in either mew or scb result in a loss of Race expression, these single mutants show different phenotypes with respect to the expression patterns of other BMP target genes. In particular, loss of mew, but not scb, gives rise to phenotypes similar to that of a short gastrulation (sog) mutant, albeit at low penetrance [8]. Sog, the Drosophila ortholog of vertebrate Chordin, is an extracellular BMP-binding protein with a key role in BMP gradient formation [9]. Sog distribution was previously shown to be modulated by αPS1βPS in both the pupal wing and ovarian follicle cells [1315], likely through an interaction between αPS1 and full-length Sog or its cleavage fragments [13,14]. Therefore, we suggest that while both αPS1βPS and αPS3βPS can enhance pMad levels via intracellular signalling, αPS1 plays an additional role in BMP gradient formation through the regulation of Sog distribution or activity [8].

Collagen IV as an integrin ligand in Drosophila

In terms of identifying the ECM protein that activates integrin receptor signalling in the Drosophila embryo, collagen IV functions as an integrin ligand in vertebrates. While the major collagen IV-binding integrins in vertebrates are α1β1 and α2β1, flies lack an ortholog of the vertebrate α1 and α2 integrins [16]. However, there is evidence for a binding site for α3β1 integrin in the collagenous domain of collagen IV [17], with Drosophila αPS1 most closely related to vertebrate α3, α6 and α7 integrins [16]. In addition, both α3β1 and α6β1 integrins, amongst others, can bind the collagen IV C-terminal NC1 domain, which is released during matrix turnover [18]. Analysis of embryos with reduced levels of either of the two collagen IV proteins, called Viking (Vkg) and Cg25C, has shown that collagen IV augments BMP signalling by playing a key role in BMP gradient formation in Drosophila [19,20]. We hypothesized that another contributor to the reduced BMP signalling phenotype in collagen IV mutant embryos could be a failure to activate integrin signalling [8] (Figure 2C). In support of this hypothesis, the TorD-βPScyt signalling-activated integrin fusion protein, but not wild-type βPS, rescued the BMP phenotype in around half of the vkg mutant embryos (Figure 2D). Additional supporting data were obtained from tissue culture cells, in which integrins enhanced pMad levels when cells were plated on collagen IV but not plastic, whereas the TorD-βPScyt protein does increase pMad levels when cells are plated on plastic, consistent with it being constitutively active.

There is also evidence that collagen IV acts as an αPS1βPS integrin ligand in Drosophila during rotation of the egg chambers of ovarian follicles around their anterior–posterior axis that elongates the egg [21]. This includes a critical requirement for both collagen IV fibres and regulation of αPS1βPS levels and adhesion in follicle cells for the cell migration, and the similarity in the vkg and mys phenotypes with both mutations giving round eggs [21,22]. Although our data support collagen IV as an integrin ligand, other data support laminin activating αPS1βPS integrins in Drosophila [23], which fits with the specificity of orthologous vertebrate integrins [16,24]. Similarly, evidence exists for laminin also activating αPS3βPS, including common mutant phenotypes, genetic interactions, and the ability of αPS3βPS-expressing tissue culture cells to spread on a laminin fragment [25,26]. Collectively, these findings suggest that the Drosophila αPS1βPS and αPS3βPS integrins can be activated by both laminin and collagen IV. A precedent for this exists for some of the vertebrate integrins, including α1β1, α2β1 and α3β1, with data supporting their activation by both laminin and collagen IV [17,27,28].

Analysis of embryos carrying a mutation in the β-laminin subunit revealed an increase in the expression of BMP target genes, including the peak target, Race [8]. If, as discussed, αPS1βPS and αPS3βPS can be activated by laminin and collagen IV, this raises the possibility that these ligands compete for integrins in the early embryo, with only collagen IV-activated integrin signalling capable of augmenting the BMP pathway. In support of such competition, we found that adding laminin to BMP-treated tissue culture cells decreased pMad levels when cells are plated on collagen IV, an effect that can be rescued by mys overexpression. In contrast, no reduction in pMad levels was observed upon the addition of laminin to BMP-treated cells plated on plastic. Further experiments are required to fully address a role for ligand competition in vivo but, if true, this would suggest that it is the intricate balance of matrix protein abundance, rather than absolute levels, which determines BMP pathway activity.

Positive integrin–BMP cross-talk

Other examples of integrin–BMP synergy have been described through a range of different mechanisms (Table 1). For example, in human osteoblasts or osteosarcoma cells, type I and II BMP receptors associate with αv- (αvβ) and -β1 containing integrins [29]. Blocking αvβ, α1β1 α2β1 integrin function reduced BMP responses and Smad-responsive transcription [2931], without affecting BMP receptor-binding or BMP receptor–integrin co-localization [29]. Attenuation of BMP responses was also observed when the BMP pathway was stimulated by a constitutively active BMP receptor, indicating that, as described for the Drosophila embryo [8], these integrins enhance BMP responses in osteoblasts downstream of BMP receptor activation.

In C2C12 cells, β3 integrins increase signalling by matrix-bound BMP-2 on soft substrate by inhibiting GSK3, which phosphorylates Smad1 leading to its proteasomal degradation [32]. This mechanism requires the Src proto-oncogene tyrosine kinase and focal adhesion kinase (FAK), integrin-linked kinase (ILK) and the Rho GTPase Cdc42 [32], with ILK previously shown to negatively regulate glycogen synthase kinase 3 (GSK3) by phosphorylation [33]. In addition, β3 integrins also increase pSmad1/5/8 levels by an unknown mechanism that is upstream of the integrin-dependent inhibition of GSK3-induced Smad degradation [32].

In murine embryonic endothelial cells, transforming growth factor β1 (TGF-β1) and BMP-9 signalling through the activin A receptor-like type 1 (ACVRL1, also called ALK1) receptor and endoglin co-receptor leads to Smad1/5/8 phosphorylation, which is increased by fibronectin activation of α5β1 integrin [34]. α5 integrin associates with both endoglin, via its extracellular domain, and ACVRL1. Thereby, fibronectin-dependent clustering of α5β1 integrin can increase endoglin interaction with ACVRL1, which enhances Smad phosphorylation, potentially by increasing BMP binding to the clustered endoglin-ACVRL1 receptors. Co-internalization of endoglin and α5β1 integrin via a clathrin-dependent pathway was also observed. This internalization was required for activation of integrin signalling, but had no detectable effect on pSmad levels, suggesting that this is not part of the mechanism by which α5β1 integrin enhances Smad signalling.

Finally, oscillatory shear stress (OSS) in vascular endothelial cells has been proposed to promote Smad1/5 activation via a mechanism that involves αvβ3 integrin [35]. OSS induces sustained association of BMP receptor type 1B (BMPR1B, also called ALK6) with αvβ3 integrin, which is dependent on the BMP receptor type 2 (BMPR2) intracellular kinase domain, leading to activation of a Shc–FAK–extracellular signal-regulated kinase (ERK) cascade and phosphorylation of Smad1/5. Smad1/5 associates with runt-related transcription factor-2 (Runx2), resulting in activation of Runx2, mammalian target of rapamycin and p70S6 kinase signalling that drives endothelial cell proliferation. Treatment with the BMP antagonist Noggin did not inhibit endothelial cell proliferation, suggesting that either the effect is ligand independent [35] or a Noggin-insensitive BMP, such as BMP-9 [36], may be involved.

Inhibition of BMP signalling by integrins

In contrast with the enhancement of BMP signalling by integrins described thus far, other reports highlight how integrins can repress BMP signalling (Table 1). Ependymal zone stem cells (EZCs) can differentiate into astrocytes following spinal cord injury, with signalling through BMPR1B detrimental to recovery as it leads to the formation of a glial scar that inhibits axonal regeneration [37]. However, EZCs up-regulate β1 integrin in response to injury, which inhibits canonical and non-canonical BMP signalling mediated by Smad1/5/8 phosphorylation and p38 MAPK activation, respectively. A similar negative effect of β1 integrin on BMP signalling is also observed in neural stem cells (NSCs), where BMP signalling can promote astrocytic differentiation. Loss of β1 integrin increases NSC differentiation. β1 integrin associates with BMP receptor type 1A (BMPR1A, also called ALK3) and BMPR1B in NSCs, but not with BMPR2, suggesting that the effect of β1 integrin on BMP signalling is mediated at the level of BMP receptors or downstream. Moreover, in the presence of β1 integrin, there is reduced partitioning of BMPR1B into lipid rafts, with BMPR1B localization in lipid rafts favouring signalling in NSCs. Together, these data suggest that β1 integrin limits the deleterious effect of BMP signalling in EZCs by preventing the movement of BMPR1B into lipid rafts that is necessary for signalling.

A study of bone marrow mesenchymal stem cells (BMMSCs) also provides evidence for integrin regulation of BMP receptor trafficking [38]. On a soft versus stiff substrate, there is higher β1 integrin activation in these cells, but lower levels of cell surface β1 integrin due to enhanced internalization via caveolae. This β1 integrin internalization is necessary for neural fate specification on soft substrate. β1 integrin promotes caveolae-mediated internalization of BMPRIA, with co-localization of BMPRIA and β1 integrin in vesicles observed, which leads to reduced pSmad1/5/8 levels on soft substrate, allowing differentiation into a neural fate.

In contrast with the data described in the previous section, which support α1β1 integrin-mediated enhancement of BMP responses downstream of receptor activation [30], a negative role for α1β1 integrin through inhibition of BMP interaction with its receptor has also been proposed [39]. Mutagenesis identified residues 85–89 in BMPR1A as important for the α1β1 integrin interaction in co-immunoprecipitation assays. As these BMPR1A residues have previously been shown to be required for BMP-2 interaction, this suggested that α1β1 integrin interaction with BMPR1A prevents BMP-2 binding. Consistent with this, α1 integrin knock-down in BMMSCs increases pSmad levels, although as yet it has not been directly shown that the presence of α1β1 integrin disrupts BMP receptor binding.

Integrins as BMP targets

Positive regulation of integrins by BMP signalling has been reported in a range of contexts, with many of the studies described above showing increases in the expression levels or surface expression of integrins in response to BMP signalling. These include αvβ3, αvβ5, αvβ6, αvβ8 and α1β1 in osteoblasts [29,30] and β1 integrin in NSCs [37]. Similarly, in endothelial cells, TGF-β1 increases α5β1 integrin levels by preventing lysosomal degradation, in addition to increasing α5β1 integrin activity by phosphorylation in an endoglin-dependent manner [34]. In the Drosophila embryo, scb expression is transcriptionally activated by BMP signalling [8]. While mew is expressed in the dorsal and neural ectoderm, scb has a restricted expression pattern in the dorsal ectoderm. Such restricted expression is the hallmark of a BMP target gene and we showed that increasing the copies of dpp or overexpression of TkvQD resulted in an expansion of scb, whereas its expression is lost in dpp mutant embryos. In addition to increasing integrin levels, BMPs can also promote integrin activation. In C2C12 cells, matrix-bound, but not soluble, BMP-2 induces cell spreading on soft matrix, by inducing clustering of fibronectin-activated β3 integrins, including αvβ3, which increases cell adhesion dynamics and leads to reorganization of the cytoskeleton and filopodia formation [32].

This stimulation of integrin levels by BMP signalling occurs in diverse contexts associated with integrins enhancing BMP responses. Such positive feedback may serve to amplify the BMP signal, generate a switch-like or bi-stable response from a graded input BMP signal and/or provide robustness to the cellular response [40,41]. It will be interesting to determine whether integrins are also BMP targets in the context of inhibitory cross-talk. If this is the case, such negative feedback could limit the duration of a signal, maintain homoeostasis by stabilising fluctuations in signalling or allow an adaptive or transient response to a constant BMP signal [40,41]. Finally, given that integrins are BMP targets in many of the synergistic contexts, it will also be useful to evaluate any BMP signalling regulatory roles for integrins in other situations, where BMPs are known to stimulate integrin levels. These include αPS2 integrin in the Drosophila eye disc [42], β1 integrin in rat pheochromocytoma-derived PC12 cells [43], αv integrin during hypertrophic differentiation of mouse chondrocytes [44] and αvβ3 in human chondrosarcoma cells [45].

Perspectives

It is clear that integrins regulate BMP signalling in a range of different cell types, with the resulting effect very context-dependent, much like the output of the BMP signalling pathway itself. One theme that has emerged though is that BMP receptor–integrin association — likely direct, although this has yet to be demonstrated — is central to the regulation [8,29,34,35,37,39]. In terms of the collagen IV–integrin–BMP synergy identified in the Drosophila embryo, this core mechanism may be relevant to angiogenesis, kidney development and stem cell fate decisions, as these are all contexts where collagen IV, integrin and BMP function have separately been implicated [4651].

It is also possible that disruption of BMP–integrin cross-talk contributes to different disease phenotypes. For example, pulmonary arterial hypertension (PAH) is associated with disrupted BMP signalling, most commonly due to BMPR2 mutations. PAH arises from abnormal remodelling of small vessels in the lung, due to altered proliferation, survival and migration of vascular smooth muscle cells and endothelial cells with reduced BMP signalling [52,53]. As described above, BMPR1B and αvβ3 integrin associate in vascular endothelial cells, with integrins required for the Smad-dependent endothelial cell proliferation in response to OSS [35]. Both BMP-9 and laminin-activated α3β1 integrin signalling protect pulmonary arterial endothelial cells from apoptosis [54,55], raising the possibility that BMP and integrin signalling also converge to promote cell survival. In these pulmonary arterial endothelial cells, laminin-activated α3-integrin recruits the adenomatous polyposis coli (APC) tumour suppressor protein, leading to activation of ILK1, phosphorylation of the serine/threonine kinase proto-oncogene Akt and cell survival. Reduced APC levels are detected in PAH patients' pulmonary arterial endothelial cells and it has been suggested that low APC activity may lead to a higher risk of PAH in combination with BMPR2 mutations [55]. In pulmonary arterial smooth muscle cells, α4β1 integrin is required for BMP-2 induced cell migration, through effects on ILK-1 activation and both the canonical and planar cell polarity Wnt pathways [56], although how this mechanism is disrupted leading to the abnormal migration of smooth muscle cells in PAH is currently unclear.

Disruption of BMP–integrin synergy may also contribute to the anti-angiogenic properties of fragments of collagen IV that can suppress tumour growth in vivo [48]. The effect of these collagen IV fragments is integrin-dependent, via an altered integrin signalling response [57]. As BMPs can promote angiogenesis [49], it is possible that the soluble collagen IV fragments function, at least in part, by reducing BMP signalling. One option is that while the collagen IV network could activate integrins leading to a response that potentiates BMP signalling, the collagen IV fragments would outcompete the endogenous collagen IV network for binding to integrins, resulting in a distinct response that fails to enhance BMP responses. In this way, the collagen IV fragments would dampen the BMP pro-angiogenic response.

Finally, as BMPs are widely used to influence the fates of different stem cell types in vitro [46,58], manipulation of integrin signalling, potentially by plating cells on a particular substrate, may provide an opportunity to maximize, for example, the cellular response to BMP signalling. Consistent with this, addition of an RGD peptide with BMP-2 has been found to act synergistically to enhance the osteogenic commitment of human BMMSCs [59].

Abbreviations

ACVRL1, activin A receptor-like type 1; APC, adenomatous polyposis coli; BMMSC, bone marrow mesenchymal stem cell; BMP, bone morphogenetic protein; BMPR1A, BMP receptor type 1A; BMPR1B, BMP receptor type 1B; BMPR2, BMP receptor type 2; CHO, Chinese Hamster Ovary; Dpp, Decapentaplegic; ERK, extracellular signal-regulated kinase; EZCs, ependymal zone stem cells; FAK, focal adhesion kinase; GSK3, glycogen synthase kinase 3; ILK, integrin-linked kinase; Mad, mothers against Dpp; MAPK, mitogen-activated protein kinase; mew, multiple edematous wings; mys, myospheroid; NSCs, neural stem cells; OSS, oscillatory shear stress; PAH, pulmonary arterial hypertension; pMad, phosphorylated Mad; RGD, Arg-Gly-Asp; Runx2, runt-related transcription factor-2; scb, scab; sog, short gastrulation; TGF-β1, transforming growth factor β1; Tkv, Thickveins; Vkg, Viking.

Funding

The Ashe lab research described in this review was supported by the Wellcome Trust (grant number [092005/Z/10/A]).

Acknowledgments

I thank Annick Sawala and Margherita Scarcia for helpful comments.

Competing Interests

The Author declares that there are no competing interests associated with this manuscript.

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