The Role of Bone Morphogenetic Protein 7 (BMP-7) in Inflammation in Heart Diseases

Bone morphogenetic protein-7 is (BMP-7) is a potent anti-inflammatory growth factor belonging to the Transforming Growth Factor Beta (TGF-β) superfamily. It plays an important role in various biological processes, including embryogenesis, hematopoiesis, neurogenesis and skeletal morphogenesis. BMP-7 stimulates the target cells by binding to specific membrane-bound receptor BMPR 2 and transduces signals through mothers against decapentaplegic (Smads) and mitogen activated protein kinase (MAPK) pathways. To date, rhBMP-7 has been used clinically to induce the differentiation of mesenchymal stem cells bordering the bone fracture site into chondrocytes, osteoclasts, the formation of new bone via calcium deposition and to stimulate the repair of bone fracture. However, its use in cardiovascular diseases, such as atherosclerosis, myocardial infarction, and diabetic cardiomyopathy is currently being explored. More importantly, these cardiovascular diseases are associated with inflammation and infiltrated monocytes where BMP-7 has been demonstrated to be a key player in the differentiation of pro-inflammatory monocytes, or M1 macrophages, into anti-inflammatory M2 macrophages, which reduces developed cardiac dysfunction. Therefore, this review focuses on the molecular mechanisms of BMP-7 treatment in cardiovascular disease and its role as an anti-fibrotic, anti-apoptotic and anti-inflammatory growth factor, which emphasizes its potential therapeutic significance in heart diseases.

Pattering mesodermal and neural tissues, dentin formation ALK-3, 4, 5, 7 BMPR-II; ActR-IIA, ActR-IIB In addition to BMP receptors, BMP-7 also exerts its biological effects through the type 1 and type 2 receptors of activin [35,36]. It has been reported that BMP-7 deletion leads to death and its deficiency induces different diseases such as osteoporosis. Therefore, BMP-7 was used for the treatment of osteoporosis [37][38][39], a widespread condition affecting several millions of people worldwide. This disease is characterized by the loss of bone mineral density, resulting in an increased susceptibility to osteoporosis induced bone fracture [40][41][42]. However, further studies are required to understand the role of BMP-7 in tissue-specific disease development and therapeutic applications. In recent years, the use of BMP-7 has been extended to several other inflammatory diseases, including cardiovascular diseases (CVD) and cellular plasticity to neurological disorders. Therefore, the focus of this review article was to provide an overall structure of BMP-7, mechanistic pathways and its potential therapeutic significance in CVD.

Structure of BMP-7
BMP-7 is expressed by several tissues, including, sensory organs (eye and skin), major end organs (heart, lung, liver, pancreas, kidney, and brain), lymphoid organs (bone marrow, thymus and lymph nodes), the reproductive system (testis, ovary, uterus and placenta), exocrine glands (prostate and mammary gland), and organ protectors (muscle and bone) [22,[43][44][45][46][47][48][49]. It is synthesized in the cells as pro-protein form of 431 amino acid residues, including N-terminal signal peptide of 29 amino acid residues, a pro-peptide of 263 amino acids, and a mature peptide of 139 amino acid residues [50] ( Figure 1). During processing, pro-BMP-7 is hydrolyzed in the cell by furin-like proteinase on its carboxy terminal, where it is converted into mature BMP-7 of 139 amino acid residues and secreted into the extracellular matrix [51]. BMP-7 is approximately a 35 kDa glycoprotein with three N-glycosylation sites and seven cysteine residues involved in three intramolecular disulfide bonds Cys38-104, Cys67-136 and Cys71-138 [52]. More importantly the intermolecular disulfide bond formed via the 103rd cysteine form dimers in two mature BMP-7 monomers with enhanced biological activity. BMP-7 has the ability to form homodimers as well as heterodimers to induce bone formation. It has been reported that BMP-7 can form heterogenous dimers with other BMPs, specifically, BMP-2 and BMP-4 [53][54][55]. However, heterodimers are more potent than homodimers in osteogenic differentiation assays [56][57][58]. Moreover, it has been demonstrated that the biological activity of these heterogenous dimers is almost 20 times higher than that of homodimers [39,58,59]. These heterodimers also showed enhanced activity in embryonic assays of Xenopus and Zebrafish [60,61]. According to these studies, co-injection of RNA encoding BMP-7 with BMP-2 or BMP-4 into embryonic blastomere enhanced embryo ventralization and patterning compared with individual injection. Additionally, combined injection of purified recombinant proteins of BMP4/7 or BMP2/7 increased BMP signaling (SMAD pathway) in Xenopus and Zebrafish, whereas varied concentrations of individual injections of homodimers did not have that level of BMP signaling alterations, suggesting that heterodimes are more potent in BMP cell signaling [55,61,62].

Structure of BMP-7
BMP-7 is expressed by several tissues, including, sensory organs (eye and skin), major end organs (heart, lung, liver, pancreas, kidney, and brain), lymphoid organs (bone marrow, thymus and lymph nodes), the reproductive system (testis, ovary, uterus and placenta), exocrine glands (prostate and mammary gland), and organ protectors (muscle and bone) [22,[43][44][45][46][47][48][49]. It is synthesized in the cells as pro-protein form of 431 amino acid residues, including N-terminal signal peptide of 29 amino acid residues, a pro-peptide of 263 amino acids, and a mature peptide of 139 amino acid residues [50] ( Figure 1). During processing, pro-BMP-7 is hydrolyzed in the cell by furin-like proteinase on its carboxy terminal, where it is converted into mature BMP-7 of 139 amino acid residues and secreted into the extracellular matrix [51]. BMP-7 is approximately a 35 kDa glycoprotein with three Nglycosylation sites and seven cysteine residues involved in three intramolecular disulfide bonds Cys38-104, Cys67-136 and Cys71-138 [52]. More importantly the intermolecular disulfide bond formed via the 103rd cysteine form dimers in two mature BMP-7 monomers with enhanced biological activity. BMP-7 has the ability to form homodimers as well as heterodimers to induce bone formation. It has been reported that BMP-7 can form heterogenous dimers with other BMPs, specifically, BMP-2 and BMP-4 [53][54][55]. However, heterodimers are more potent than homodimers in osteogenic differentiation assays [56][57][58]. Moreover, it has been demonstrated that the biological activity of these heterogenous dimers is almost 20 times higher than that of homodimers [39,58,59]. These heterodimers also showed enhanced activity in embryonic assays of Xenopus and Zebrafish [60,61]. According to these studies, co-injection of RNA encoding BMP-7 with BMP-2 or BMP-4 into embryonic blastomere enhanced embryo ventralization and patterning compared with individual injection. Additionally, combined injection of purified recombinant proteins of BMP4/7 or BMP2/7 increased BMP signaling (SMAD pathway) in Xenopus and Zebrafish, whereas varied concentrations of individual injections of homodimers did not have that level of BMP signaling alterations, suggesting that heterodimes are more potent in BMP cell signaling [55,61,62].
Recently, to evaluate the heterodimer presence in vivo, Kim et al. generated knock in mice carrying a mutation (Bmp7R-GFlag) that prevents proteolytic activation of the dimerized BMP-7 precursor protein [63]. This mutation abolishes the ability of BMP-7 homo and heterodimer formation. Further, the presence of endogenous BMP4/7 heterodimer was confirmed with coimmunoprecipitation assays. These studies suggested that BMP-7 predominantly forms heterodimers with BMP-2 or BMP-4 and plays a major role during mammalian development.  Recently, to evaluate the heterodimer presence in vivo, Kim et al. generated knock in mice carrying a mutation (Bmp7R-GFlag) that prevents proteolytic activation of the dimerized BMP-7 precursor protein [63]. This mutation abolishes the ability of BMP-7 homo and heterodimer formation. Further, the presence of endogenous BMP4/7 heterodimer was confirmed with coimmunoprecipitation assays.
These studies suggested that BMP-7 predominantly forms heterodimers with BMP-2 or BMP-4 and plays a major role during mammalian development. BMP-7 is a pleiotropic growth factor and plays a crucial role in the development of various tissues and organs as represented in Table 1. It maintains multiple physiological processes such as bone development, fracture healing, and differentiation of brown adipose tissue in the body. Reduction in BMP-7 expression is associated with various diseases including osteoporosis, CVD and diabetes. In 1980, the recombinant human BMP-7 (rhBMP-7) expressed in Chinese hamster ovary cells was approved to use as a therapeutic agent in the repair of bone fractures and has been successfully implemented in clinical trials [64][65][66][67]. Moreover, BMP-7containing osteogenic implants have been used widely for the treatment of long bone non-unions, spinal fusions, and acute fractures [68]. In addition, earlier reports from our laboratory have demonstrated the potential protective role of BMP-7 in inhibiting plaque formation, monocyte infiltration and in the inhibition of pro-inflammatory cytokine secretion [69,70]. Further, we also observed reduced circulatory BMP-7 levels as atherosclerosis progressed and that the exogenous supplementation of BMP-7 significantly attenuated disease progression [71]. Recent studies revealed that BMP-7 not only reduces body fat, but also strengthens insulin signaling, further improves glucose uptake and insulin resistance [72]. Considering the beneficial effects of BMP-7 in metabolism, this review focuses on the molecular aspects of BMP-7 and its regulation in inflammation in CVD. The current literature has suggested the therapeutic efficacy of BMP-7 mediated through canonical and non-canonical mechanistic pathways in various animal disease models of CVD, diabetes and obesity [65,66].
In the canonical or Smad dependent pathway ( Figure 2), BMP-7 activates regulatory Smads (Smad-1, 5, and 8) for subsequent phosphorylation in the cytoplasm. Thereafter, phosphorylated regulatory Smad proteins form a complex with the co-stimulatory molecule Smad-4. This complex is then transduced to the nuclei to recruit cofactors and Run-related transcription factor 2 (Runx2) to regulate osteogenic gene expression and consequently influences osteoblast differentiation [65,73,74]. Mesenchymal stem cell differentiation into osteoblasts is a pre-requisite for embryonic skeletal formation, homeostatic skeletal remodeling and bone fracture repair. BMP-7 plays a major role in upregulating the transcription factor osterix (Osx) or SP7 which has the ability to stimulate differentiation of osteoblasts both in vitro and in vivo [65,75,76]. These studies suggested the involvement of canonical cell signaling pathway in osteoblast differentiation and embryo skeletal formation induced by BMP-7 [76][77][78][79][80][81][82]. BMP-7 induced activation of Smad-1/5 leads to the activation of osterix resulting in increased osteogenic markers alkaline phosphatase (ALP) activity and mineralization [83]. Lavery et al. demonstrated the BMP-7 mediated osteoblastic differentiation of primary human mesenchymal stem cells with strongly enriched established osteogenic marker genes including osteocalcin (OCN), osteopontin (OPN) and ALP along with several other osteogenic markers of unknown function [84]. It has been reported that BMP-7 differentiates murine C2C12 myoblasts into osteoblasts by suppressing myoblast determination protein 1 (MyoD) expression, and enhancing the ALP activity and the osteogenic specific gene expressions ALP, Runx2, and OCN via P38 mitogen-activated protein kinase (MAPK) dependent Smad-1/5/8 signaling pathways [85]. Alongside, a recent study from our laboratory demonstrated monocyte differentiation into anti-inflammatory M2 macrophages through the Smad-1/5/8 pathway [67]. in various animal disease models of CVD, diabetes and obesity [65,66].
Hu et al. showed that BMP-7 stimulates renal epithelial cell morphogenesis via p38 MAPK and that its action is counteracted by Smad-1. Further, these studies also revealed that responses to low doses of BMP-7 lead to increased cell proliferation, which are regulated by the p38 MAPK pathway while responses to high doses of BMP-7 suppress cell proliferation, and are controlled by the Smad pathway. In addition, suppression of the p38 MAPK activity by high doses of BMP-7 might integrate the dose-dependent cellular response to BMP-7 [93]. BMP-7 promotes proliferation of nephron progenitor cells through TAK1-mediated JNK activation as well as further activation of transcription factor Jun and activating transcription factor 2 (ATF2) [94]. BMP-7 also plays a major role in the induction of tissue factor in human mononuclear cells (MNCs) through NF-KB activity, leading to increased F3 (tissue factor gene) transcription [95] and resulting in an increased procoagulant activity.
Additionally, it has been noticed that BMP-7 binding to its receptor BMPR-II can also activate the Smad dependent and independent PI3K pathways. In this process, activation of PI3K subunit p85 occurs either by Smad-1/5/8 or BMP-7 binding to BMPR II and its subsequent phosphorylation leads to down-stream phosphorylation of phosphotidylinositol biphosphate (PIP2) to phosphatidylinositol triphosphate (PIP3) [96,97] which, in turn, leads to the phosphorylation of RAC-alpha serine/threonine-protein kinase (Akt) and downstream activation of mammalian target of rapamycin (mTOR) [98]. In immune regulation, the PI3K pathway plays an important role in maintaining the anti-inflammatory environment [97]. Furthermore, studies from our laboratory demonstrated that the Smad-PI3K-Akt-mTOR pathway specifically inhibits pro-inflammatory cytokine secretion (TNF-α, IL-6 and MCP-1), enhances anti-inflammatory cytokines (IL-10 and IL-1ra) and plays a key role in M2 macrophage polarization [67,70].

Inhibitors of BMP-7
Several extra-and intra-cellular regulators, which play a major role in BMP signaling pathways via binding receptors and blocking pathways have been identified. Almost 15 BMP antagonists have been identified and classified into four major groups based on the size and cysteine knot as represented in Table 2 [3,[99][100][101][102][103][104]. Similarly, intracellular BMP signaling is inhibited by micro-RNAs, I-Smads (Smad-6 and 7) and phosphatases (PP1 and PP2A) which play a role in dephosphorylation of both phosphorylated R-Smads and type I receptors [105][106][107][108]. Noggin, chordin and follistatin have been considered as major antagonists for BMP-7 [99,[109][110][111]. Noggin blocks the effects of BMP-7 in osteoblast differentiation and inhibits membrane ossification and further limb development [99,109]. Similarly, Chordin stops binding of BMP-7 to the receptor and further the phosphorylation of down-stream proteins, resulting in inhibition of several biological functions [110]. Follistatin inhibits the binding of the BMP-7 to BMPR2 and prevents the activation of the Smad-1/5/8 pathway [111].
BMP antagonists also play a crucial role in embryonic development. To elaborate, embryogenesis is mediated by the activity of extracellular proteins such as chordin, noggin, cerberus, and dan family protein gremlin2. Amongst these antagonists, gremlin2 acts as the strongest BMP ligand inhibitor [112]. Alongside, chordin is involved in neural induction and mesoderm dorsalization during embryonic development. Deficiency of chordin leads to abnormalities in the skull, cardiovascular defects, malfunction in cervical and thoracic vertebrae, and also the absence of parathyroid and thymus [113]. Similarly, Noggin plays an important role in bone formation, and neural tissue formation during embryogenesis. Lack of Noggin leads to abnormalities in the skeleton and is lethal [114]. Animal studies have revealed that chordin deficiency results in stillborn mice [115] while noggin deficiency results in fetal death [114].

Inhibitors Name Role
Kielin enhances BMP signaling in a paracrine way; inhibits both the activin-A and TGFB1-mediated signaling pathways Nell promotes the osteogenic differentiation of adipose-derived stromal/stem cells and inhibits adipogenic differentiation. Binding of NELL1 to Integrin beta 1 was shown to be critical for its role in promoting osteogenic differentiation and adhesion to the extracellular matrix.
CHL/Neuralin (CHRDL1): Chordin-like (CHL/CHL1, CHRDL1) is a secreted molecule with three cysteine-rich repeat (CR) modules and is known as neuralin in the mouse [116][117][118]. CHRDL1 enhances BMP-4 and BMP-7 signaling in several cell lines when expressed alone. However, it switches into a selective BMP-7 antagonist when it complexes with Twsg1 and plays a role in inhibition of injury repair and homeostasis of the mammalian kidney [109].
Connective tissue growth factor (CTGF): CTGF binds BMP-2,-4,-7 via its CR domain. Disruption of CTGF gene in mice revealed its requirement for coordination of chondrogenesis and angiogenesis during skeletal development [127], which depends on the CTGF activity to modulate BMP signaling during chondrocyte differentiation [128,129]. BMP signaling is controlled by different types of regulators, including extracellular matrix proteins (ECM), I-smads, ubiquitin proteasome complex, corepressors and miRNA. Based on the availability of ligands, ECM controls BMP signaling whereas I-smads antagonize the steps involved in smad signaling. Similarly, ubiquitin proteasome controls different types of inhibitors and signal transducers involved. Corepressors regulate BMP signaling at the transcriptional level and miRNAs regulate at the translational level [7].

BMP-7 as an Anti-Inflammatory Agent in Atherosclerosis
Atherosclerosis is a serious cardiovascular condition that involves the constriction of the arterial wall leading to the development of myocardial infarction. Atherogenesis is regulated by cholesteryl ester (CE) accumulation, foam cell formation, smooth muscle cell migration, necrotic core formation, and increased calcification [66,[130][131][132]. Moreover, the developed atherogenesis creates turbulence in blood flow leading to plaque rupture and thrombosis. Although these atherogenic factors are well-established, recent data suggests the involvement of modified LDL, extracellular components in the plaque activation and rupture [133]. Therefore, atherosclerosis was considered to be the product of lipoprotein accumulation, particularly LDL in the arterial wall [134,135].
Recently, it is speculated that atherosclerosis is a complex process that involves the participation of both immune systems, oxidative stress, various cell types, receptors, lipids, enzymes, signaling pathways, trace elements, and other products [136][137][138]. Inflammation and oxidative stress are considered to be major players in the progression of the disease [139][140][141][142]. Altered vessel wall structure and disturbed blood flow patterns include inflammation and varied stress levels in developed atherosclerosis [143]. Despite the abundance of research literature on the topic, the role of lipids, especially fatty acids and their oxidation products like peroxidized linoleic acid (HPODE), 4-hydroxynonenal (HNE), oxo-nonanoic acid (ONA), and their interaction with inflammatory molecules such as oxidized LDL, phospholipids, TNF-α, vascular cell adhesion molecule (VCAM1) in many of these processes are poorly understood.
Monocytes, which are precursors of macrophages as well as dendritic cells (DCs) and migrate into the areas of "injury" as a result of a chemotactic stimuli such as monocyte chemotactic protein 1 and 3 (MCP-1&3). Migration of monocytes into the arterial wall has been considered as one of the initial events in atherogenesis which persists in different stages of disease progression [140][141][142]. In tissues, based on the environmental growth factors and pro-inflammatory cytokines, monocytes differentiate into either M1 macrophages or DCs. Monocyte adherence, their differentiation into pro-inflammatory macrophages/dendritic cells that release pro-inflammatory cytokines which are involved in the generation of complex pathophysiology of atherosclerosis [140] (Figure 3). Macrophages were initially viewed as a mere scavenger of altered lipoproteins. However, the presence of macrophages along with lymphocytes in atherosclerotic plaques showed enhanced inflammatory immune response and release of pro-inflammatory molecules. The specific roles of different stages of atherosclerosis and presence of these inflammatory macrophages, foam cells, lymphocytes, and vascular smooth muscle cells are not yet completely understood. For example, M2 macrophages, are known for high endocytic clearance capacity due to their higher expression of scavenger receptors (SR) during wound healing and repair processes [144]. Van Tits et al. demonstrated that M2 macrophages are susceptible in forming foam cells in presence of oxidized LDL and shift towards the M1 phenotype with enhanced secretion of the pro-inflammatory cytokines IL-6, IL-8 and MCP-1 [145]. Furthermore, this increased production of pro-inflammatory cytokines by polarized M1 macrophages from M2 macrophages which are residing in subendothelial space of the vessel wall might lead to the initiation of the inflammatory cascade that mediates disease progression [145]. Similarly, in human atherosclerotic lesions different macrophage phenotypes exist in different plaque locations. M2 (CD68 + CD206 + ) macrophages were located in plaque stable zones far from the lipid core, whereas M1 (CD68 + CCL2 + ) macrophages exhibited a distinct tissue localization pattern [146] suggesting that the tissue microenvironment decides the fate of macrophage polarization. Subsequent research studies confirmed this finding by demonstrating the presence of lipid droplets in CD68 + CD206 + macrophages in comparison with CD68 + CD206 − macrophages [147]. This discovery suggests that despite the anti-inflammatory nature of M2 macrophages they tend to form foam cells, a significant contributor of atherogenesis. initial events in atherogenesis which persists in different stages of disease progression [140][141][142]. In tissues, based on the environmental growth factors and pro-inflammatory cytokines, monocytes differentiate into either M1 macrophages or DCs. Monocyte adherence, their differentiation into proinflammatory macrophages/dendritic cells that release pro-inflammatory cytokines which are involved in the generation of complex pathophysiology of atherosclerosis [140] (Figure 3). Macrophages were initially viewed as a mere scavenger of altered lipoproteins. However, the presence of macrophages along with lymphocytes in atherosclerotic plaques showed enhanced inflammatory immune response and release of pro-inflammatory molecules. The specific roles of different stages of atherosclerosis and presence of these inflammatory macrophages, foam cells, lymphocytes, and vascular smooth muscle cells are not yet completely understood. For example, M2 macrophages, are known for high endocytic clearance capacity due to their higher expression of scavenger receptors (SR) during wound healing and repair processes [144]. Van Tits et al. demonstrated that M2 macrophages are susceptible in forming foam cells in presence of oxidized LDL and shift towards the M1 phenotype with enhanced secretion of the pro-inflammatory cytokines IL-6, IL-8 and MCP-1 [145]. Furthermore, this increased production of pro-inflammatory cytokines We demonstrated from our laboratory that rhBMP-7 is able to inhibit the atherosclerosis associated inflammation at both acute (Day-14) and mid-stage (Day-28) time points of atherosclerosis by promoting monocyte differentiation into the anti-inflammatory M2 phenotype via reducing phosphorylated kinases p-38 and JNK while increasing p-Smad and ERK pathways [69,71]. Additionally, a recent study from our laboratory demonstrated the significantly increased BMPR2 expression on monocytes following BMP-7 treatment, and further polarization into M2 macrophages [67]. BMP-7 treatment showed increased M2 macrophages [approximately 25% at Day-14 and 60% at Day-28] than M1 macrophages [15% at Day-14 and 30% at Day-28] leading to decrease in pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), IL-6 and MCP-1 associated with atherosclerotic lesion development and to an increase in anti-inflammatory cytokines such as IL-10 and IL-1ra levels. Further, BMP-7 improved blood flow in the artery after post ligation, reduced the inflammatory kinases, and completely slowed down disease progression (Figure 3). In addition, we also demonstrated that, upon macrophage depletion by liposomal clodronate, BMP-7 fails to significantly reduce plaque progression and inflammation suggesting the direct role of BMP-7 on macrophages [71]. The literature on BMP-7 in macrophage polarization is new and growing; however, there are certain unanswered questions such as whether BMP-7 can inhibit the formation of foam cells; and if BMP-7 can inhibit the conversion of M2 macrophage into foam cell formation in atherosclerosis?

BMP-7 as an Anti-Calcifying Agent
Calcification is an important step in atherosclerosis as a result of inflammation and is classified into two main types; (1) intimal and (2) medial calcification [148][149][150]. Intimal calcification occurs during the progression of atherosclerotic lesions, whereas medial calcification occurs in between the layers of smooth muscle cells. Existing reports suggest that vascular calcification is a cell mediated process in which vascular smooth muscle cells (VSMCs) and pericytes, differentiate and mineralize the vascular matrix through abnormal deposition of calcium phosphate [148][149][150][151]. Recently, Riad et al. demonstrated the role of lipid peroxidation derived dicarboxylic acid, azelaic acid in calcium sequestration and subsequent calcification [152]. Evidence suggests that BMP-2 plays a major role in vascular calcification by inhibiting VSMC proliferation through p21 cyclin dependent kinases inhibition and subsequent cell cycle arrest [153][154][155][156][157]. In addition, it also causes the loss of smooth muscle cell markers while promoting the osteoblastic gene expression markers including ALP, OPN etc. by stimulating the osteogenic transcription factor Msx2 and inducing apoptosis, which is a critical step in calcification initiation. In contrast to BMP-2, BMP-7 counteracts atherosclerotic calcification by increasing SMC proliferation via upregulating p21 cyclin dependent kinases and regulating skeletal remodeling and maintaining SMC phenotype [158,159]. Several factors, including reactive oxygen species (ROS), reactive nitrogen species (RNS), vitamin D, phosphate, azeloate and parathyroid hormone increase the calcification process [152,160,161]. Various studies showed that BMP-7 inhibits vascular calcification by preserving the SMC phenotype and the process towards osteoblastic phenotype [156,157,162,163]. Kang et al. demonstrated that rhBMP-7 inhibited vitamin D and phosphate induced vascular calcification in vivo (mice) and in vitro (human aortic smooth muscle cells) [164]. In this study, C57BL/6J mice were treated with high concentrations of vitamin D in the presence and absence of rhBMP-7 and calcification markers were analyzed by IHC and western blotting. Vitamin D significantly increased osteoblastic markers (OPN and OCN) and calcium staining of aortas and hearts; whereas, pre-treatment with rhBMP-7 completely abolished the Vit-D mediated effects on osteoblastic markers and calcium staining.
Further, these studies also demonstrated the efficacy of BMP-7 in attenuation of beta-glycerophosphate promoted osteogenic markers and calcium staining in vascular smooth muscle cells. Therefore, these studies also suggested the potential beneficial role of BMP-7 in reducing CVD related to vascular calcification [164].

BMP-7 Inhibits Inflammation and Adverse Remodeling in the Infarcted Heart
Myocardial infarction (MI) (Figure 3) is a condition due to the formation of lesions in the arteries, resulting in reduced blood flow of nutrients and oxygen supply, which leads to myocardial injury. Cardiac myocyte cell loss in the infarcted region happens via apoptosis, pyroptosis and necrosis leading to end stage heart failure [130,[165][166][167][168]. Furthermore, cardiac hypertrophy and fibrosis has been considered as a major remodeling mechanism to compensate for the requirements under pathophysiological conditions in which increased cardiac cell size (hypertrophy) and expression ofECM proteins (collagens types I and III) have been observed [169,170]. In the injured myocardium, fibrosis stiffens the heart muscle and affects the systolic and diastolic function. Moreover, these ECM proteins can be degraded by endopeptidases such as matrix metalloproteinases (MMPs) leading to alterations in ventricular structure and function post-MI. Moreover, following myocardial injury, the heart undergoes a sequence of molecular events including cell death, cytokine release, and infiltration/recruitment of immune cells, which play a major role in cardiac wound healing and stabilization of cardiac remodeling [171].
After 48-72 h of MI, monocytes were recruited to the infarct area of the heart in two phases [172]. In the first phase, a significant increase in number of Ly-6c high monocytes are observed in the MI which are chemokine receptor type 2 (CCR2) dependent [173,174]. These monocytes secrete TNF-α and IL-1β and are converted to pro-inflammatory macrophages to clear the debris of dead cells and extracellular matrix by phagocytosis. It is postulated that monocytes infiltrated in response to cardiac cellular injury to clear the dead cardiac cells, and also generate an inflammatory microenvironment, that triggers adverse cardiac remodeling. In the second phase, Ly-6c low monocytes are recruited which are C-X3-C motif chemokine receptor (CX3CR1) dependent [175]. These monocytes are less in number, but convert to macrophages, which play a role in wound healing and repair [176] by promoting collagen deposition, angiogenesis, and myofibroblast accumulation. These infiltrated monocytes interact with ECM and release fibronectin, which stabilizes/reduces the infarct [176]. In addition, early efferocytosis promotes the conversion of M1 macrophages into M2 macrophages, reduces secretion of pro-inflammatory cytokines and increases production of anti-inflammatory cytokines IL-10 and TGF-β [177][178][179].
Cell death due to apoptosis has been considered as a key step in the development and progression of post-MI remodeling which further leads to chronic heart failure [180,181]. Apoptosis is a type of programmed cell death, which occurs during aging and development as a homeostatic process as well as a defense mechanism in various diseases. Apoptosis leads to a cascade of cellular events including cytoplasmic blebbing, cell shrinkage, protein cleavage by caspases, chromatin condensation and DNA fragmentation [182]. Cardiac myocyte apoptosis is mediated through extrinsic and intrinsic pathways. TNF-α, FAS ligand, and TNF-related apoptosis-inducing ligand (TRAIL) triggers the extrinsic pathway whereas caspases triggers the intrinsic pathway [183]. Further, cardiac myocyte apoptosis provides a microenvironment to infiltrate monocytes, and initiates inflammation that activates cardiac fibroblasts, which play a major role in the cascade of inflammation, cellular infiltration, and fibrosis in both infarcted and peri-infarcted areas. These cellular alterations lead to adverse cardiac remodeling that generates organ dysfunction. Interstitial fibrosis occurs between cardiac myocytes whereas vascular fibrosis occurs in and around vessel walls [182,184]. Several pro-inflammatory as well as profibrotic cytokines released by leukocyte infiltration lead to fibroblasts activation, increased TGF-β secretion and ECM protein synthesis [185][186][187][188][189][190][191][192].
The role of TGF-β1 in inflammation and cardiac injury is reported in myocardial infarction [5,193,194]. Upregulation of TGF-β activates Smad signaling proteins 2,3,4 in the infarct area of the heart and also the peri-ischemic zone under pathological conditions [193,195], which might play a role in increased collagen type-I expression [196]. Schneiders et al. demonstrated the involvement of Smad proteins in cardiomyocyte apoptosis [197]. It has been noticed that cardiomyocyte treatment with TGF-β1 enhances cardiomyocytes apoptosis, increases caspase3/7 activity and decreases B-cell lymphoma 2 (Bcl-2) expression by upregulating Smad-7 [198]. Activation of the TGF-β/Smad pathway leads to increased ECM components, such as fibronectin, type-1 collagen, connective tissue growth factor (CTGF), and transcription genes related to the collagen production, which leads to fibrosis development [199]. It has been reported that overexpression of TGF-β1 in mice showed a significant increase in left ventricular fibrosis [200]. In addition, TGF-β1 is known to increase plasminogen activator inhibitor-1 which plays a major role in ECM degradation [200,201]. Evidence suggests that cardiac fibrosis is mediated by TGF-β/Smad signaling [202][203][204]. Smad-4 plays a role in initiation of Smad-2/3 associated TGF-β induced fibrosis whereas Smad-7 inhibits collagen, smooth muscle actin, and reduces matrix protein by inhibiting phosphorylation of Smad-2/3 [205]. BMP-7 acts as an antifibrotic factor through the Smad pathway in which it induces the phosphorylation of Smad-1/5/8 and downregulates TGF-β signaling, which is mediated by Smad-2/3 phosphorylation [206]. The downregulation of BMP-7 in pathological fibrosis of organs has been reported [207][208][209][210][211][212]. Additionally, administration of exogenous BMP-7 or overexpression of BMP-7 protects the tissues such as kidneys [207,210], liver [208], lungs [209] and heart [211] from fibrosis. Moreover, exogenous administration of BMP-7 downregulates myocardial interstitial fibrosis as well as kidney fibrosis by inhibiting TGF-β signaling pathway and protects cardiac function. Recently, Jin et al. demonstrated that exogenous BMP-7 facilitates cardiac function recovery after acute myocardial infarction by attenuating myocardial fibrosis through counteracting TGF-β1 signaling pathway [211]. In this study, the group established acute myocardial infarction by ligating the left anterior descending artery with and without BMP-7 treatment. BMP-7 treatment significantly attenuated myocardial fibrosis, reduced the infarct size, and improved cardiac function. In addition, this study also reported that BMP-7 treatment inhibited myocardial fibrosis by attenuating TGF-β signaling and its downstream effectors Smad-2 and Smad-3 [211]. Furthermore, the beneficial role of BMP-7/Smad signaling has been shown in fibrotic disease of the heart [212,213]. However, the direct role of BMP-7 in monocyte differentiation or M1 macrophage polarization into M2 macrophages in the infarcted heart is still unknown.

BMP-7 Ameliorates Diabetic Cardiomyopathy
Diabetes is a major metabolic disorder and an alarming epidemic affecting millions of people globally. It is considered as the seventh leading cause of death [214]. Pre-diabetes is a condition in which impaired glucose levels have been considered as markers for diagnosis which leads to type 1 and type 2 diabetes (T2DM) [215]. Type I diabetes is known as an autoimmune disease caused by insulin deficiency due to the destruction of insulin producing pancreatic beta cells of the islets of Langerhans [216]. Similarly, T2DM is characterized by insulin resistance resulting from impairment of the normal function of pancreatic β-cells which induces hyperglycemia, and eventually leads to cardiac failure and nephropathy [217][218][219]. Diabetes is usually accompanied with hyperglycemia, oxidative stress, and inflammation potentially leading to CVD, muscle atrophy, nephropathy, neuropathy, periodontal disease, retinopathy, impaired wound healing, and tissue damage [217][218][219].
Diabetic cardiomyopathy (DC) is the leading cause of death worldwide and has attracted global attention. Due to the fact that low levels of glucose can initiate microvascular complications; therefore, the impaired glucose tolerance has been considered as a major risk factor for CVD related deaths [220]. Increased inflammation and oxidative stress have been observed in both clinical and experimental diabetes mellitus which are also implicated in the etiology of chronic diabetic complications [221][222][223][224] such as diabetes-induced cardiomyopathy and muscle toxicity. However, the exact mechanisms of developed chronic diabetic complications are not yet completely understood. Diabetes results from the functional imbalance of innate and adaptive immune response [225]. The elevated blood levels of pro-inflammatory cytokines such as TNF-α and IL-6 have been noticed in diabetic subjects [226,227]. Recent studies have suggested that increased levels of IL-6 should be considered as a risk factor for diabetes [228]. Inhibition and decreased expression of TNF-α and IL-6 might play a possible role in alleviating diabetic complications.
TNF-α is known to induce cardiomyocyte apoptosis in vitro by initiating the apoptotic cascade via caspase-3 triggering in vitro [229]. Cardiac apoptosis can be induced by various mechanisms, including oxidative stress, inflammatory cytokines, loss of normal insulin signaling, hyperglycemia and advanced glycation end products (AGEs). Recent evidence suggested the upregulation of cardiomyocyte apoptosis in diabetic subjects as well as in animal models [230][231][232][233].
Izhumi et al. demonstrated the ability of BMPs in attenuating apoptosis in rat cardiomyocytes. According to this study, BMP-2 can attenuate the serum deprivation induced apoptosis in cardiac myocytes. In addition, these studies elucidated the up-regulation of B-cell lymphoma-extra-large (Bcl-xL) via the Smad-1 pathway, which has a protective effect and plays an important role in regulation of the myocardium [233]. Studies from our laboratory correlated with these in vitro studies in that BMP-7 attenuates cardiac myocyte apoptosis in diabetes-induced mice [234]. BMP-7 treated pre-diabetic group has shown significantly increased levels of anti-inflammatory IL-10 and reduction in TNF-α [234]. IL-10 has been known to decrease TNF-α induced cardiomyocyte apoptosis [72]. Further, BMP-7 reduced diabetic cardiac apoptosis is mediated through Phosphatase and tensin homolog (PTEN) and Akt pathways [234]. Elevated levels of PTEN protein were observed in pre-diabetic mice hearts as compared to control mice whereas significant downregulation of PTEN was observed in BMP-7 treated pre-diabetic mice. In addition to anti-apoptotic effects of BMP-7, this study also reported the anti-fibrotic effects that leads to improved cardiac function in pre-diabetic mice [234]. According to Kurlawalla et al., PTEN decreases insulin sensitivity, and lack of PTEN increases glucose tolerance and insulin sensitivity in adipose tissue. It has been shown that PTEN knock-out mice are resistant to streptozotocin (STZ)-induced diabetes which might suggest PTEN as promising target to aim in reversing insulin resistance [235]. Moreover, BMP-7 inhibits apoptosis via PTEN-Akt pathway and decreases hyperglycemia in pre-diabetic mice.

BMP-7 Differentiates Monocytes into M2 Macrophages in Heart Diseases
Monocyte polarization plays a key role in the progression of various inflammatory diseases such as atherosclerosis, myocardial infarction, and diabetic cardiomyopathy [236][237][238]. Infiltrated monocyte differentiation depends on the tissue microenvironment they reside in and the external stimuli they receive [239]. Monocytes will differentiate into M1 macrophages if tissues have an inflammatory microenvironment stimulated with interferon gamma (IFN-γ), macrophage colony stimulating factor (MCSF) and TNF-α [240]. Conversely, infiltrated monocytes will polarize into M2 macrophages or alternative macrophages if the tissue microenvironment is surrounded by certain specific factors, such as granulocyte macrophage colony stimulating factor (GMCSF) and anti-inflammatory cytokines, such as IL-4 and IL-13 [187]. In addition, two distinct subsets of M2 macrophages, M2a and M2c are notable in which the former participates in wound healing and are induced by IL-4 and IL-13 whereas the latter takes part in regulation of disease progression, which is induced by glucocorticoids, TGF-β and IL-10 [187,241]. Differentiated M1 macrophages are known to secrete pro-inflammatory cytokines such as inducible nitric oxide synthase (iNOS), IL-6, TNF-α, and MCP-1, while alternative M2 macrophages are known to secrete anti-inflammatory cytokines as such IL-10 and arginase-1 [67,[242][243][244].
The exact role of infiltrated monocytes and their differentiation into pro-inflammatory M1 and anti-inflammatory M2 macrophages, as well as their role in development and progression at different stages of the diseases of atherosclerosis, myocardial infarction, and diabetic cardiomyopathy is far from clear; however, we are beginning to understand that increased M2 macrophages attenuate developed cardiac pathophysiology and function. In early onset of disease, monocytes move to the injury site and polarize into M2 macrophages [65] to repair by secreting anti-inflammatory cytokines such as IL-10 and IL-1Ra as well as scavenge the apoptotic cells [245][246][247]. As the disease progresses, the infiltrated monocytes polarize into M1 macrophages due to the microenvironment resulting in increased secretion of pro-inflammatory cytokines including MCP-1, TNF-α and IL-6, further increasing the necrotic core formation and calcification [223,248]. Balancing the ratio of M1 to M2 could control the severity of disease progression.
Considering the beneficial effects of M2 macrophages in the attenuation of inflammation, wound healing, and repair processes, the factors/molecules/compounds that have the ability to convert the monocytes into M2 macrophages have attained major attention due to their potential therapeutic implications. BMP-7 is one such factor which has the ability to polarize monocytes into M2 macrophages in both normal as well as under stressed conditions. A recent study demonstrated the potential efficacy of BMP-7 in monocyte polarization to M2 macrophages by upregulating M2 macrophage marker CD206, and down regulating the monocyte marker CD14 [67]. In addition, it was also noticed that BMP-7 significantly reduced pro-inflammatory cytokines such as IL-6, TNF-α, MCP-1, but enhanced the anti-inflammatory cytokines secretion of IL-10 and IL-1Ra [67] suggests that BMP-7 has a potential to enhance M2 macrophages which are anti-inflammatory in nature. Further, this study suggests that M2 macrophage polarization decreases the activation of the inflammatory p38 and JNK pathways while increases the activation of Smad and ERK pathways at mid-stage (Day-28) time point of atherosclerosis [69]. BMP-7 upregulates and binds BMPR2, phosphorylates SMAD1/5/8, and activates PI3K, which results in downstream activation of Akt and mTOR as shown in Figure 2. Evidence demonstrated that the expression of p-PTEN, an inhibitor of the PI3K pathway was significantly upregulated in apoptotic conditions and significantly downregulated upon BMP-7 treatment, suggesting the ability of BMP-7 to not only promote PI3K signaling through upregulated mediators but also by directly blocking inhibition of the signaling cascade [70,97]. Furthermore, these studies also demonstrated how BMP-7 inhibitor follistatin inhibitied p-SMAD1/5/8 expression and decreased PI3K expression, which supports and suggests the necessity of BMP-7 to bind BMPR2 thus activating SMAD1/5/8 and subsequently PI3K [111,249]. As mentioned above, the PI3K pathway plays a key role in increased anti-inflammatory markers such as arginase-1 and IL-10 as well as inhibition of the production of pro-inflammatory markers [97,250,251]. It has been noticed that activation of the PI3K pathway resulted in increased polarization of M2 macrophages, specifically in bone marrow derived macrophages [98]. Evidence has also suggested that inhibition of either PI3K or mTOR results in M1 macrophage polarization signifying the role of these pathways in monocyte polarization into M1 macrophages [98,250]. Moreover, Rocher et al. demonstrated that BMP-7 administration along with apoptotic conditional medium to monocytes resulted in an increased expression of anti-inflammatory cytokines (IL-1ra, IL-10 and arginase-1) and inhibited expression of pro-inflammatory cytokines (iNOS, TNF-alpha, IL-6 and MCP-1) which promoted paracrine effects on monocytes and macrophages yielding increased M2 macrophage polarization [70]. According to Mantovani et al. M2 macrophages counteract inflammation by enhanced secretion of IL-10 [252]. It has also been reported that these anti-inflammatory cytokines have the ability to inhibit pro-inflammatory cytokines IL-6 and TNF-α from immune cells and can be used as therapeutic agents [253] in several inflammation-associated diseases.

Conclusions and Future Directions
In conclusion, these studies support that BMP-7 is an effective growth factor that has the potential to inhibit apoptosis, fibrosis and acts as an anti-calcifying agent, which ultimately improves cardiac function in different heart diseases, as summarized in this review. The novel and most interesting role of BMP-7 is its ability to promote the differentiation of infiltrated pro-inflammatory monocytes into anti-inflammatory M2 macrophages in different cardiac diseases. However, further studies are required to understand whether BMP-7 can acts as a direct anti-inflammatory agent to inhibit cardiac pathophysiology. Further investigation is needed to determine if BMP-7 treatment differentiates monocytes or polarizes M1 to M2 macrophages, and whether it can be reverted due to low concentration of BMP-7. It is also not yet clear whether a single dose of BMP-7 is enough to decrease diabetes and diabetic cardiomyopathy as long-term studies are not well established. Therefore, a new significant research avenue remains to be explored to understand the cell protective role of BMP-7 in treating heart diseases.

Acknowledgments:
The authors would like to thank Sarah Ashiqueali and Fatima Bianca Dessouki for their assistance in proof reading of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.