Expression and function of PDGF-C in development and stem cells

Platelet-derived growth factor C (PDGF-C) is a relatively new member of the PDGF family, discovered nearly 20 years after the finding of platelet-derived growth factor A (PDGF-A) and platelet-derived growth factor B (PDGF-B). PDGF-C is generally expressed in most organs and cell types. Studies from the past 20 years have demonstrated critical roles of PDGF-C in numerous biological, physiological and pathological processes, such as development, angiogenesis, tumour growth, tissue remodelling, wound healing, atherosclerosis, fibrosis, stem/progenitor cell regulation and metabolism. Understanding PDGF-C expression and activities thus will be of great importance to various research disciplines. In this review, however, we mainly discuss the expression and functions of PDGF-C and its receptors in development and stem cells.

PDGF-C was discovered in 2000 [8], about 20 years after the finding of PDGF-A and PDGF-B [1,3]. PDGF-C mainly binds to PDGFR-α [8]. When PDGFR-β is co-expressed with PDGFR-α, it can be engaged by PDGF-C as well [9]. Studies from the past 20 years or so have demonstrated important roles of PDGF-C in diverse biological processes, such as development, tumour growth, angiogenesis, wound healing, tissue remodelling, fibrosis, atherosclerosis, metabolism and stem/progenitor cell regulation [5,[10][11][12][13][14][15][16]. In this review, however, we mainly discuss the roles and expressions of PDGF-C and its receptors in human and murine development and stem cells.

PDGF-C and its receptors are highly expressed in pre-implantation embryos
In the early stage of human embryonic development, such as in eight-cell stage embryos, PDGF-C and its receptors  PDGFR-α, the major receptor used by PDGF-C, is detected as early as in four-cell stage embryos and blastocyst. PDGFR-β, which can be engaged by PDGF-C when it is coexpressed with PDGFR-α, is also detected in four-cell stage embryos [24]. In mice, like in humans, abundant Pdgf-c expression is found in early embryonic development. Pdgf-c is detected in mouse zygotes and in embryos of two-cell, eight-cell and blastocyst stages as shown by single-cell RNA sequencing [25]. Pdgf-c is upregulated in EPI at E4.5 [26,27]. Both PDGFR-α and PDGFR-β are abundantly expressed in mouse zygotes and blastocyst [25,28]. In summary, both PDGF-C and its receptors are abundantly expressed in human and mouse pre-implantation embryos, suggesting the possible effects of PDGF-C on early embryogenesis.

PDGF-C and its receptors are expressed in all the three germ layers in post-implantation embryos
In post-implantation embryos, PDGF-C is highly expressed in all the three germ layers and their derivatives (   receptors in the three germ layers in post-implantation embryos suggest possible functions of PDGF-C during organogenesis.
PDGF-C expression can be downregulated by various factors. It has been shown that sulfatase 2 (Sulf2) downregulates PDGF-C expression in breast cancer [112]. In cultured mouse embryos, retinoic acid administration markedly downregulated PDGF-C and PDGFR-α expression, leading to branchial arch malformation and impaired proliferation of mouse embryonic palatal mesenchymal cells (MEPMC) [113,114]. In human retinal pigment epithelial cells, interleukin 1 beta (IL-1β) downregulates PDGF-C and inhibits RPE proliferation and migration [115]. In human hepatic stellate cells, micro-RNA-29a downregulates PDGF-C to suppress cell migration and proliferation [77]. Thus, modulating these factors may be of usage to regulate PDGF-C expression levels.

PDGF-C is critical for the development of multiple organs and tissues
It has been shown that PDGF-C has a vital role in embryonic development. Genetic deletion of Pdgf-c leads to embryonic lethality in mice on a 129/S background [116]. PDGF-C deficiency results in multiple defects in various organs and

Vascular development
PDGF-C is essential for the proper development of the vascular network (table 5). Genetic deletion of Pdgf-c in both 129/S and C57BL/6 mice caused vascular defects, such as extracranial vessel haemorrhage [116], and abnormal morphology, density and poor SMC coverage of cerebral blood vessels [138]. Genetic deletion of the major receptor for PDGF-C, Pdgfr-α, also results in various vascular defects, such as abnormal yolk sac vasculature and extensive bleeding in various organs [139]. Moreover, Pdgfr-α mutation in mice impairs the proper development of aortic and the pulmonary vessels [44] (table 5). These data thus demonstrate an essential role of PDGF-C and its receptor during the development of the vascular system.

Neural system
The neural tube is the primitive central structure of the nervous system during embryonic development, from which the brain and spinal cord develop [140]. PDGF-C is required for the development of the neural tube, notochord and the mesenchyme tissues surrounding them [116,138] (table 5). PDGF-C is critical for the migration and survival of neural crest cells, which are vital for CNS development [56]. Pdgf-c genetic deletion in 129/S mice leads to multiple developmental defects in the brain, such as oedema and haemorrhage [116]. Loss of Pdgf-c in C57BL/6 mice also results in various defects in the neural system, such as wavy neural tube blisters and ventricular malformations with distorted ependymal linings [138] (table 5). Moreover, Pdgf-c/Pdgfr-α double knockout mice also display severe CNS defects, such as irregularly shaped cerebral hemispheres, unusually small cerebella and abnormal interhemispheric fissures [120]. Furthermore, PDGF-C has important functions for the formation of meninges and assembly of the glia limitans basement membrane [120]. In addition, PDGFR-α has been shown to be crucial for oligodendrocyte development and production, and promotes their proliferation and migration [28,39,141,142]. PDGFR-β is expressed in the neural system, and Pdgfr-β deficient mice are more vulnerable to brain injury [143]. Thus, plenty of data have shown that PDGF-C and its receptors are of particular importance for the development of the neural system.

Lung
PDGF-C plays a critical role in lung development (table 5). PDGF-C overexpression resulted in various defects in the lung and embryonic lethality, including the excessive proliferation of mesenchymal cells, mesenchymal-epithelial disruption and enlarged and immature lungs [144]. Consistently, genetic deletion of Pdgf-c in mice caused emphysema [120]. In addition, PDGF-C has been shown to promote proliferation and inhibits apoptosis and differentiation of lung mesenchymal cells [80]. Moreover, PDGF-C prevents the differentiation of distal airway and airspace epithelial cells into type I alveolar epithelial cells, which constitute the structure of the alveoli and mediate gas exchange [145]. Of the two receptors for PDGF-C, PDGFR-β seems to be more important for lung development since PDGFR-β activity is critically required for embryonic lung growth [57], and inhibition of PDGFR-β signalling with antisense oligodeoxynucleotides significantly reduced embryonic lung epithelial growth and lung size [57,146]. In addition, PDGFR-α signalling has been shown to be vital for lung alveolarization [147]. Thus, both PDGF-C and PDGFRs are critical for lung development.

Palate and kidney
The mouse palate forms at E11.5 from the maxillary processes and mainly comprises epithelial and mesenchymal cells [113,116]. Genetic deletion of Pdgf-c or blocking PDGF-C with neutralization antibody leads to palate branchial arch abnormalities, complete cleft palate and embryonic lethality [113,116] (table 5). Consistently, genetic deletion of the major royalsocietypublishing.org/journal/rsob Open Biol. 11: 210268 receptor for PDGF-C, Pdgfr-α, also results in cleft palate [116]. By contrast, loss of Pdgfr-β does not cause cleft palate, suggesting a unique role of the PDGF-C-PDGFR-α axis in palate development. In addition, PDGF-C plays important roles in kidney development by promoting the formation of ureteric buds and mesangial cells in the glomerulus as well as the maturation of kidney arteries and arterioles [8,64]. PDGFR-α is highly expressed in kidney interstitial cells and arterial and venous vessels, suggesting a role of PDGFR-α in kidney development [148]. Genetic deletion of Pdgfr-β leads to glomerular mesangial cell failure in mice, demonstrating critical roles of PDGFR-β in glomerular morphogenesis [148].

Embryonic stem cell (ESC)
PDGF-C is highly expressed in the very early stage of embryonic development [22][23][24], and genetic deletion of Pdgf-c leads to embryonic lethality [116]. Moreover, both receptors for PDGF-C, PDGFR-α and PDGFR-β, are highly expressed in ESCs [23,25, [154][155][156], further suggesting potential effects of PDGF-C on ESCs. It has been shown that PDGFRs are critical for the undifferentiated state of human ESCs, since inhibition of the PDGFRs downregulated the master pluripotency factors NANOG and OCT4 and led to ESC differentiation [173] (table 6), suggesting a potential role of PDGF-C in maintaining ESC pluripotency. Moreover, it is reported that PDGFRinduces ERK activation inhibits ESC apoptosis [187]. On the other hand, other studies reported that the PDGFR signalling induces differentiation of ESCs into various cell types. For example, inhibition of PDGFR-α by microRNA-218 (miR-218) suppressed ESC migration and differentiation [188], while upregulation of PDGFR-α by mix-like protein 1 (Mixl1) induced ESC differentiation into mesendoderm cells [189]. Moreover, it has been shown that PDGFR-α induces ESC differentiation into blood cells [190], and PDGR-β activation by cyclic strain induces ESC differentiation into vascular SMCs [191]. Furthermore, PDGFR-β is reported to activate the STAT5 and phosphatidylinositol-3 kinase (PI3 K) pathways and induce ESC differentiation into bone marrow cells [192]. These observations thus suggest possible effects of PDGF-C on ESCs and warrant further studies to look into it.

Concluding remarks
Since the discovery of PDGF-C about two decades ago, studies have demonstrated its critical roles in embryonic development. Loss or overexpression of PDGF-C lead to various developmental defects in multiple organs and tissues, such as in the neural system, palate, lung, kidney and the vasculature. In addition, PDGF-C and its receptors are abundantly expressed in various types of stem cells, such as ESCs, ASCs and iPSCs. PDGFRs have been amply demonstrated to regulate stem cell pluripotency or differentiation, thus suggesting a possible role of PDGF-C in these processes. Future studies are warranted to verify whether and how PDGF-C plays a role in stem cell regulation, particularly, in neural, lung, palate or kidney progenitor/stem cells. It is also critical to identify the regulatory factors governing PDGF-C expression, the discovery of which might lead to new possibilities of therapeutic interventions for developmental defects or stem cell therapy.
Data accessibility. This article has no additional data. Table 6. Effects of PDGF-C and PDGFRs on stem cells.