PI3K-mediated PDGFRα signaling regulates survival and proliferation in skeletal development through p53-dependent intracellular pathways

Effects of pulsed electrical stimulation on α-smooth muscle actin and type I collagen expression in human dermal fibroblasts

Pulsed electrical stimulation (PES) is known to affect cellular activities. We previously found PES to human dermal fibroblasts (HFs) promoted platelet-derived growth factor subunit A (PDGFA) gene expression, which enhanced proliferation. In this study, we investigated PES effects on fibroblast collagen production and differentiation into myofibroblasts. HFs were electrically stimulated at 4800 Hz and 5 V for 60 min. Imatinib, a specific inhibitor of PDGF receptors, was treated before PES. After 6 h of PES, PDGFA, α-smooth muscle actin (α-SMA), and collagen type I α1 chain gene expressions were upregulated in PES group. Imatinib suppressed the promoted expression except for PDGFA. Immunofluorescence staining and enzyme-linked immunosorbent assay showed the production of α-SMA and collagen I was enhanced in PES group but suppressed in PES + imatinib group at 48 h after PES. Therefore, PES promotes the production of α-SMA and collagen I in fibroblasts, which is triggered by PDGFA that is upregulated early after PES.

PDGFRα signaling regulates Srsf3 transcript binding to affect PI3K signaling and endosomal trafficking

Signaling through the platelet-derived growth factor receptor alpha (PDGFRa) plays a critical role in craniofacial development, as mutations in PDGFRA are associated with cleft lip/palate in humans and Pdgfra mutant mouse models display varying degrees of facial clefting. Phosphatidylinositol 3-kinase (PI3K)/Akt is the primary effector of PDGFRα signaling during skeletal development in the mouse. We previously demonstrated that Akt phosphorylates the RNA-binding protein serine/arginine-rich splicing factor 3 (Srsf3) downstream of PI3K-mediated PDGFRa signaling in mouse embryonic palatal mesenchyme (MEPM) cells, leading to its nuclear translocation. We further showed that ablation of Srsf3 in the murine neural crest lineage results in severe midline facial clefting, due to defects in proliferation and survival of cranial neural crest cells, and widespread alternative RNA splicing (AS) changes. Here, we sought to determine the molecular mechanisms by which Srsf3 activity is regulated downstream of PDGFRa signaling to control AS of transcripts necessary for craniofacial development. We demonstrated via enhanced UV-crosslinking and immunoprecipitation (eCLIP) of MEPM cells that PDGF-AA stimulation leads to preferential binding of Srsf3 to exons and loss of binding to canonical Srsf3 CA-rich motifs. Through the analysis of complementary RNA-seq data, we showed that Srsf3 activity results in the preferential inclusion of exons with increased GC content and lower intron to exon length ratio. Moreover, we found that the subset of transcripts that are bound by Srsf3 and undergo AS upon PDGFRα signaling commonly encode regulators of PI3K signaling and early endosomal trafficking. Functional validation studies further confirmed that Srsf3 activity downstream of PDGFRα signaling leads to retention of the receptor in early endosomes and increases in downstream PI3K-mediated Akt signaling. Taken together, our findings reveal that growth factor-mediated phosphorylation of an RNA-binding protein underlies gene expression regulation necessary for mammalian craniofacial development.

PDGFRα/β heterodimer activation negatively affects downstream ERK1/2 signaling and cellular proliferation

The platelet-derived growth factor receptor (PDGFR) family of receptor tyrosine kinases allows cells to communicate with one another by binding to growth factors at the plasma membrane and activating intracellular signaling pathways to elicit responses such as migration, proliferation, survival and differentiation. The PDGFR family consists of two receptors, PDGFRα and PDGFRβ, that dimerize to form PDGFRα homodimers, PDGFRα/β heterodimers and PDGFRβ homodimers. Here, we overcame prior technical limitations in visualizing and purifying PDGFRα/β heterodimers by generating a cell line stably expressing C-terminal fusions of PDGFRα and PDGFRβ with bimolecular fluorescence complementation fragments corresponding to the N-terminal and C-terminal regions of the Venus fluorescent protein, respectively. We found that these receptors heterodimerize relatively quickly in response to PDGF-BB ligand treatment, with a peak of receptor autophosphorylation following 5 minutes of ligand stimulation. Moreover, we demonstrated that PDGFRα/β heterodimers are rapidly internalized into early endosomes, particularly signaling endosomes, where they dwell for extended lengths of time. We showed that PDGFRα/β heterodimer activation does not induce downstream phosphorylation of ERK1/2 and significantly inhibits cell proliferation. Further, we characterized the PDGFR dimer-specific interactome and identified MYO1D as a novel protein that preferentially binds PDGFRα/β heterodimers. We demonstrated that knockdown of MYO1D leads to retention of PDGFRα/β heterodimers at the plasma membrane, resulting in increased phosphorylation of ERK1/2 and increased cell proliferation. Collectively, our findings impart valuable insight into the molecular mechanisms by which specificity is introduced downstream of PDGFR activation to differentially propagate signaling and generate distinct cellular responses.

Targeting phosphatidylinositol-3-kinase for inhibiting maxillary bone resorption

Previous studies have suggested a role of phosphatidylinositol-3-kinase gamma (PI3Kγ) in bone remodeling, but the mechanism remains undefined. Here, we explored the contribution of PI3Kγ in the resorption of maxillary bone and dental roots using models of orthodontic tooth movement (OTM), orthodontic-induced inflammatory root resorption, and rapid maxillary expansion (RME). PI3Kγ-deficient mice (PI3Kγ-/- ), mice with loss of PI3Kγ kinase activity (PI3KγKD/KD ) and C57BL/6 mice treated with a PI3Kγ inhibitor (AS605240) and respective controls were used. The maxillary bones of PI3Kγ-/- , PI3KγKD/KD , and C57BL/6 mice treated with AS605240 showed an improvement of bone quality compared to their controls, resulting in reduction of the OTM and RME in all experimental groups. PI3Kγ-/- mice exhibited increased root volume and decreased odontoclasts counts. Consistently, the pharmacological blockade or genetic deletion of PI3K resulted in increased numbers of osteoblasts and reduction in osteoclasts during OTM. There was an augmented expression of Runt-related transcription factor 2 (Runx2) and alkaline phosphatase (Alp), a reduction of interleukin-6 (Il-6), as well as a lack of responsiveness of receptor activator of nuclear factor kappa-Β (Rank) in PI3Kγ-/- and PI3KγKD/KD mice compared to control mice. The maxillary bones of PI3Kγ-/- animals showed reduced p-Akt expression. In vitro, bone marrow cells treated with AS605240 and cells from PI3Kγ-/- mice exhibited significant augment of osteoblast mineralization and less osteoclast differentiation. The PI3Kγ/Akt axis is pivotal for bone remodeling by providing negative and positive signals for the differentiation of osteoclasts and osteoblasts, respectively.

[The Role of Platelet-Derived Growth Factor-AA in the Pathogenesis and Development of Osteoarthritis]

Osteoarthritis (OA) is a chronic degenerative disease involving the entire joint. The pathogenesis and progression of OA bear close connection to the destruction and the abnormal metabolism of cartilage, subchondral bones and synovium. Platelet derived growth factor-AA (PDGF-AA) is a critical mitogenic and chemotactic factor for a variety of cells, including chondrocytes, mesenchymal stem cells, osteoclasts and osteoblasts, and PDGF-AA promotes effective wound repair. This paper reviewed the pathological changes of cartilage, subchondral bones and synovium in the process of OA development, and summarized research progress regarding the effect of PDGF-AA on the tissues and related cells mentioned above. Current studies have basically clarified the pathological changes of cartilage, subchondral bones and synovium in OA patients, and have shown that PDGF-AA serves critical regulatory function in the tissues or cells involved in OA, the internal mechanism of which remains unclear, though. More studies should be done to find ways to apply PDGF-AA for clinic purpose and to diagnose and treat OA on the cellular basis.

The Role of RNA-Binding Proteins in Vertebrate Neural Crest and Craniofacial Development

Cranial neural crest (NC) cells delaminate from the neural folds in the forebrain to the hindbrain during mammalian embryogenesis and migrate into the frontonasal prominence and pharyngeal arches. These cells generate the bone and cartilage of the frontonasal skeleton, among other diverse derivatives. RNA-binding proteins (RBPs) have emerged as critical regulators of NC and craniofacial development in mammals. Conventional RBPs bind to specific sequence and/or structural motifs in a target RNA via one or more RNA-binding domains to regulate multiple aspects of RNA metabolism and ultimately affect gene expression. In this review, we discuss the roles of RBPs other than core spliceosome components during human and mouse NC and craniofacial development. Where applicable, we review data on these same RBPs from additional vertebrate species, including chicken, Xenopus and zebrafish models. Knockdown or ablation of several RBPs discussed here results in altered expression of transcripts encoding components of developmental signaling pathways, as well as reduced cell proliferation and/or increased cell death, indicating that these are common mechanisms contributing to the observed phenotypes. The study of these proteins offers a relatively untapped opportunity to provide significant insight into the mechanisms underlying gene expression regulation during craniofacial morphogenesis.

Srsf3 mediates alternative RNA splicing downstream of PDGFRα signaling in the facial mesenchyme

Signaling through the platelet-derived growth factor receptor alpha (PDGFRα) is crucial for mammalian craniofacial development, although the mechanisms by which the activity of downstream intracellular effectors is regulated to mediate gene expression changes have not been defined. We find that the RNA-binding protein Srsf3 is phosphorylated at Akt consensus sites downstream of PI3K-mediated PDGFRα signaling in mouse palatal mesenchyme cells, leading to its nuclear translocation. We further demonstrate that ablation of Srsf3 in the mouse neural crest lineage leads to facial clefting due to defective cranial neural crest cell proliferation and survival. Finally, we show that Srsf3 regulates the alternative RNA splicing of transcripts encoding protein kinases in the mouse facial process mesenchyme to regulate PDGFRα-dependent intracellular signaling. Collectively, our findings reveal that alternative RNA splicing is an important mechanism of gene expression regulation downstream of PI3K/Akt-mediated PDGFRα signaling in the facial mesenchyme and identify Srsf3 as a critical regulator of craniofacial development.

PDGF-PDGFR network differentially regulates the fate, migration, proliferation, and cell cycle progression of myogenic cells

Platelet-derived growth factors (PDGFs) regulate embryonic development, tissue regeneration, and wound healing through their binding to PDGF receptors, PDGFRα and PDGFRβ. However, the role of PDGF signaling in regulating muscle development and regeneration remains elusive, and the cellular and molecular responses of myogenic cells are understudied. Here, we explore the PDGF-PDGFR gene expression changes and their involvement in skeletal muscle myogenesis and myogenic fate. By surveying bulk RNA sequencing and single-cell profiling data of skeletal muscle stem cells, we show that myogenic progenitors and muscle stem cells differentially express PDGF ligands and PDGF receptors during myogenesis. Quiescent adult muscle stem cells and myoblasts preferentially express PDGFRβ over PDGFRα. Remarkably, cell culture- and injury-induced muscle stem cell activation altered PDGF family gene expression. In myoblasts, PDGF-AB and PDGF-BB treatments activate two pro-chemotactic and pro-mitogenic downstream transducers, RAS-ERK1/2 and PI3K-AKT. PDGFRs inhibitor AG1296 inhibited ERK1/2 and AKT activation, myoblast migration, proliferation, and cell cycle progression induced by PDGF-AB and PDGF-BB. We also found that AG1296 causes myoblast G0/G1 cell cycle arrest. Remarkably, PDGF-AA did not promote a noticeable ERK1/2 or AKT activation, myoblast migration, or expansion. Also, myogenic differentiation reduced the expression of both PDGFRα and PDGFRβ, whereas forced PDGFRα expression impaired myogenesis. Thus, our data highlight PDGF signaling pathway to stimulate satellite cell proliferation aiming to enhance skeletal muscle regeneration and provide a deeper understanding of the role of PDGF signaling in non-fibroblastic cells.

PDGFRα: Expression and Function during Mitral Valve Morphogenesis

Mitral valve prolapse (MVP) is a common form of valve disease and can lead to serious secondary complications. The recent identification of MVP causal mutations in primary cilia-related genes has prompted the investigation of cilia-mediated mechanisms of disease inception. Here, we investigate the role of platelet-derived growth factor receptor-alpha (PDGFRα), a receptor known to be present on the primary cilium, during valve development using genetically modified mice, biochemical assays, and high-resolution microscopy. While PDGFRα is expressed throughout the ciliated valve interstitium early in development, its expression becomes restricted on the valve endocardium by birth and through adulthood. Conditional ablation of Pdgfra with Nfatc1-enhancer Cre led to significantly enlarged and hypercellular anterior leaflets with disrupted endothelial adhesions, activated ERK1/2, and a dysregulated extracellular matrix. In vitro culture experiments confirmed a role in suppressing ERK1/2 activation while promoting AKT phosphorylation. These data suggest that PDGFRα functions to suppress mesenchymal transformation and disease phenotypes by stabilizing the valve endocardium through an AKT/ERK pathway.

Isolation and Time-Lapse Imaging of Primary Mouse Embryonic Palatal Mesenchyme Cells to Analyze Collective Movement Attributes

Development of the palate is a dynamic process, which involves vertical growth of bilateral palatal shelves next to the tongue followed by elevation and fusion above the tongue. Defects in this process lead to cleft palate, a common birth defect. Recent studies have shown that palatal shelf elevation involves a remodeling process that transforms the orientation of the shelf from a vertical to a horizontal one. The role of the palatal shelf mesenchymal cells in this dynamic remodeling has been difficult to study. Time-lapse-imaging-based quantitative analysis has been recently used to show that primary mouse embryonic palatal mesenchymal (MEPM) cells can self-organize into a collective movement. Quantitative analyses could identify differences in mutant MEPM cells from a mouse model with palate elevation defects. This paper describes methods to isolate and culture MEPM cells from E13.5 embryos-specifically for time-lapse imaging-and to determine various cellular attributes of collective movement, including measures for stream formation, shape alignment, and persistence of direction. It posits that MEPM cells can serve as a proxy model for studying the role of palatal shelf mesenchyme during the dynamic process of elevation. These quantitative methods will allow investigators in the craniofacial field to assess and compare collective movement attributes in control and mutant cells, which will augment the understanding of mesenchymal remodeling during palatal shelf elevation. Furthermore, MEPM cells provide a rare mesenchymal cell model for investigation of collective cell movement in general.

SPECC1L-deficient primary mouse embryonic palatal mesenchyme cells show speed and directionality defects

Cleft lip and/or palate (CL/P) are common anomalies occurring in 1/800 live-births. Pathogenic SPECC1L variants have been identified in patients with CL/P, which signifies a primary role for SPECC1L in craniofacial development. Specc1l mutant mouse embryos exhibit delayed palatal shelf elevation accompanied by epithelial defects. We now posit that the process of palate elevation is itself abnormal in Specc1l mutants, due to defective remodeling of palatal mesenchyme. To characterize the underlying cellular defect, we studied the movement of primary mouse embryonic palatal mesenchyme (MEPM) cells using live-imaging of wound-repair assays. SPECC1L-deficient MEPM cells exhibited delayed wound-repair, however, reduced cell speed only partially accounted for this delay. Interestingly, mutant MEPM cells were also defective in coordinated cell movement. Therefore, we used open-field 2D cultures of wildtype MEPM cells to show that they indeed formed cell streams at high density, which is an important attribute of collective movement. Furthermore, activation of the PI3K-AKT pathway rescued both cell speed and guidance defects in Specc1l mutant MEPM cells. Thus, we show that live-imaging of primary MEPM cells can be used to assess mesenchymal remodeling defects during palatal shelf elevation, and identify a novel role for SPECC1L in collective movement through modulation of PI3K-AKT signaling.

Pdgfra and Pdgfrb Genetically Interact in the Murine Neural Crest Cell Lineage to Regulate Migration and Proliferation

Cranial neural crest cells (cNCCs) are migratory, multipotent cells that originate from the forebrain to the hindbrain and eventually give rise to the cartilage and bone of the frontonasal skeleton, among other derivatives. Signaling through the two members of the platelet-derived growth factor receptor (PDGFR) family of receptor tyrosine kinases, alpha and beta, plays critical roles in the cNCC lineage to regulate craniofacial development during murine embryogenesis. Further, the PDGFRs have been shown to genetically interact during murine craniofacial development at mid-to-late gestation. Here, we examined the effect of ablating both Pdgfra and Pdgfrb in the murine NCC lineage on earlier craniofacial development and determined the cellular mechanisms by which the observed phenotypes arose. Our results confirm a genetic interaction between the two receptors in this lineage, as phenotypes observed in an allelic series of mutant embryos often worsened with the addition of conditional alleles. The defects observed here appear to stem from aberrant cNCC migration, as well as decreased proliferation of the facial mesenchyme upon combined decreases in PDGFRα and PDGFRβ signaling. Importantly, we found that PDGFRα plays a predominant role in cNCC migration whereas PDGFRβ primarily contributes to proliferation of the facial mesenchyme past mid-gestation. Our findings provide insight into the distinct mechanisms by which PDGFRα and PDGFRβ signaling regulate cNCC activity and subsequent craniofacial development in the mouse embryo.

Trp53 regulates platelets in bone marrow via the PI3K pathway

The p53 gene is well known as a key tumor suppressor gene; it is vital for hematopoietic stem cell differentiation and growth. In the present study, the change of platelets (PLTs) in p53 knockout mice (p53^-/- mice) was investigated. The peripheral blood cell subsets and PLT parameters in p53^-/-mice were compared with those in age-matched p53^+/+ mice. Bleeding time as well as the alteration of PLT levels, were analyzed with the PLT marker CD41 antibody using flow cytometry. The results revealed that the number of PLTs in p53^-/- mice was significantly lower than that in p53^+/+ mice. Bleeding time was prolonged in the peripheral blood of p53^-/- mice compared with that of p53^+/+ mice. Furthermore, the related gene expression of the PI3K signaling pathway in the bone marrow of p53^-/- mice was shown to be associated with plateletogenesis. PI3K inhibitor (LY294002) was also used to treat p53^-/- mice, and the results demonstrated that LY294002 revert the change of PLTs in these mice. In summary, PLTs were altered in p53^-/- mice, and the PI3K signaling pathway was involved in that process, suggesting that the p53-dependent PI3K signaling pathway is involved in thrombocytopenia or PLT diseases. PLT number is reduced in p53 deficiency; however, this reduction could be reverted by inhibiting the PI3K pathway.

GCN5 acetylation is required for craniofacial chondrocyte maturation

Development of the craniofacial structures requires the precise differentiation of cranial neural crest cells into osteoblasts or chondrocytes. Here, we explore the epigenetic and non-epigenetic mechanisms that are required for the development of craniofacial chondrocytes. We previously demonstrated that the acetyltransferase activity of the highly conserved acetyltransferase GCN5, or KAT2A, is required for murine craniofacial development. We show that Gcn5 is required cell autonomously in the cranial neural crest. Moreover, GCN5 is required for chondrocyte development following the arrival of the cranial neural crest within the pharyngeal arches. Using a combination of in vivo and in vitro inhibition of GCN5 acetyltransferase activity, we demonstrate that GCN5 is a potent activator of chondrocyte maturation, acting to control chondrocyte maturation and size increase during pre-hypertrophic maturation to hypertrophic chondrocytes. Rather than acting as an epigenetic regulator of histone H3K9 acetylation, our findings suggest GCN5 primarily acts as a non-histone acetyltransferase to regulate chondrocyte development. Here, we investigate the contribution of GCN5 acetylation to the activity of the mTORC1 pathway. Our findings indicate that GCN5 acetylation is required for activation of this pathway, either via direct activation of mTORC1 or through indirect mechanisms. We also investigate one possibility of how mTORC1 activity is regulated through RAPTOR acetylation, which is hypothesized to enhance mTORC1 downstream phosphorylation. This study contributes to our understanding of the specificity of acetyltransferases, and the cell type specific roles in which these enzymes function.

The paradox of dipeptidyl peptidase IV inhibition in enterocytic differentiation and epithelial-mesenchymal transition in rat cholestatic sepsis

Proper enterocytic proliferation/differentiation, besides providing adequate adherens junctions (AJ) integrity, are responsible for strengthening of the gut barrier that acts as a first line defense against endotoxemia. However, the preferential role of the underlying PI3K/Akt (PKB) axis in triggering enterocytic proliferation/differentiation signaling and AJ assembly is still obscure in sepsis. Additionally, the potential involvement of dipeptidyl peptidase (DPP)-IV in cholestatic sepsis has not yet been reported. Common bile duct ligation (CBDL) insult was performed in adult male Sprague-Dawley rats except for sham operated animals; three doses of vildagliptin (VLD3, 10 and 30 mg/kg/d; p.o) were administered for 10 consecutive days post CBDL. VLD3/10/30 dose-dependently decreased DPP-IV and elevated GLP-1, IGF-1, PI3K, pS473-Akt (PKB), pS9-GSK-3β, pS133-CREB and cyclin-D1. VLD3/10 reduced fever, portal/aortic endotoxin and IgG, body weight loss as well as ileal NF-κB, TNF-α, MPO, TBARS, subepithelial/pericryptal and submucosal collagen deposition, vimentin immunoreactivity, N-cadherin, Zeb1 and pY654-β-catenin but increased E-cadherin, NPSH and colon/spleen indices - effects that were quite the opposite of VLD30. Accordingly, maintaining proper enterocytic proliferation/differentiation and phosphorylation inputs consequent to adequate DPP-IV inhibition is integral to AJ assembly in cholestatic sepsis; however, perturbed signals by excessive suppression of the enzyme activity induce toxic effects manifested as AJ disassembly and EMT, hence gut leakage and overt endotoxemia.

The TFAP2A-IRF6-GRHL3 genetic pathway is conserved in neurulation

Mutations in IRF6, TFAP2A and GRHL3 cause orofacial clefting syndromes in humans. However, Tfap2a and Grhl3 are also required for neurulation in mice. Here, we found that homeostasis of Irf6 is also required for development of the neural tube and associated structures. Over-expression of Irf6 caused exencephaly, a rostral neural tube defect, through suppression of Tfap2a and Grhl3 expression. Conversely, loss of Irf6 function caused a curly tail and coincided with a reduction of Tfap2a and Grhl3 expression in tail tissues. To test whether Irf6 function in neurulation was conserved, we sequenced samples obtained from human cases of spina bifida and anencephaly. We found two likely disease-causing variants in two samples from patients with spina bifida. Overall, these data suggest that the Tfap2a-Irf6-Grhl3 genetic pathway is shared by two embryologically distinct morphogenetic events that previously were considered independent during mammalian development. In addition, these data suggest new candidates to delineate the genetic architecture of neural tube defects and new therapeutic targets to prevent this common birth defect.

Oligodendrocytes upregulate blood-brain barrier function through mechanisms other than the PDGF-BB/PDGFRα pathway in the barrier-tightening effect of oligodendrocyte progenitor cells

White matter lesions are associated with impairment of the blood-brain barrier (BBB), an essential component of the cerebrovasculature. The BBB allows the brain to maintain its highly specialized microenvironment by restricting entry of blood-borne substances including molecules that induce myelin damage. Accumulating evidence suggests that interactions between brain endothelial cells and neighboring cells, including oligodendrocyte progenitor cells (OPCs), are required for the induction and maintenance of BBB function. Here, we compared the ability of OPCs and oligodendrocytes to modulate BBB integrity using co-cultures of rat brain endothelial cells with OPCs or oligodendrocytes. We found that OPCs lowered the brain endothelial permeability to sodium fluorescein, and this enhancement of BBB function was prevented by treatment with AG1296 (a PDGFRα inhibitor). Oligodendrocytes also enhanced BBB integrity. Pharmacological inhibition of PDGFRα did not affect the oligodendrocyte-induced BBB facilitation. These data indicate that oligodendrocytes enhance BBB integrity through pathways other than PDGF-BB/PDGFRα signaling triggered by the brain endothelial cell-derived PDGF-BB. Therefore, our findings suggest that oligodendrocytes constitutively support BBB integrity through soluble factors. Crosstalk between brain endothelial cells and oligodendrocytes could play a facilitatory role in maintaining BBB integrity in the white matter.

A PDGFRβ-PI3K signaling axis mediates periosteal cell activation during fracture healing

Insufficient and delayed fracture healing remain significant public health problems with limited therapeutic options. Phosphoinositide 3-kinase (PI3K) signaling, a major pathway involved in regulation of fracture healing, promotes proliferation, migration, and differentiation of osteoprogenitors. We have recently reported that knock-in mice with a global increase in PI3K signaling (gCbl^YF) show enhanced femoral fracture healing characterized by an extraordinary periosteal response to injury. Interestingly, of all growth factor receptors involved in fracture healing, PI3K directly binds only to PDGFR. Given these findings, we hypothesized a PDGFR-PI3K interaction is necessary for mediating robust periosteal cell activation following fracture. In this study, we isolated primary periosteal cells from gCbl^YF mice to analyze cross-talk between the PDGFRβ and PI3K signaling pathways. We found PDGFRβ signaling contributes to robust Akt phosphorylation in periosteal cells in comparison with other growth factor signaling pathways. Additionally, we performed femoral fractures on gCbl^YF mice with a conditional removal of PDGFRβ in mesenchymal progenitors using inducible alpha smooth muscle actin (αSMA) CreER^T2 mice. Our studies showed that depletion of PDGFRβ signaling within these progenitors in the early phase of fracture healing significantly abrogates PI3K-mediated periosteal activation and proliferation three days after fracture. Combined, these results suggest that PDGFRβ signaling through PI3K is necessary for robust periosteal activation in the earliest phases of fracture healing.

Endothelial and Smooth Muscle Cell Interaction via FoxM1 Signaling Mediates Vascular Remodeling and Pulmonary Hypertension

RATIONALE

Angioproliferative vasculopathy is a hallmark of pulmonary arterial hypertension (PAH). However, little is known about how endothelial cell (EC) and smooth muscle cell (SMC) crosstalk regulates the angioproliferative vascular remodeling.

OBJECTIVES

To investigate the role of EC and SMC interaction and underlying signaling pathways in pulmonary hypertension (PH) development.

METHODS

SMC-specific Foxm1 (forkhead box M1) or Cxcr4 knockout mice, EC-specific Foxm1 or Egln1 knockout mice, and EC-specific Egln1/Cxcl12 double knockout mice were used to assess the role of FoxM1 on SMC proliferation and PH. Lung tissues and cells from patients with PAH were used to validate clinical relevance. FoxM1 inhibitor thiostrepton was used in Sugen 5416/hypoxia- and monocrotaline-challenged rats.

MEASUREMENTS AND MAIN RESULTS

FoxM1 expression was markedly upregulated in lungs and pulmonary arterial SMCs of patients with idiopathic PAH and four discrete PH rodent models. Mice with SMC- (but not EC-) specific deletion of Foxm1 were protected from hypoxia- or Sugen 5416/hypoxia-induced PH. The upregulation of FoxM1 in SMCs induced by multiple EC-derived factors (PDGF-B, CXCL12, ET-1, and MIF) mediated SMC proliferation. Genetic deletion of endothelial Cxcl12 in Egln1^Tie2Cre mice or loss of its cognate receptor Cxcr4 in SMCs in hypoxia-treated mice inhibited FoxM1 expression, SMC proliferation, and PH. Accordingly, pharmacologic inhibition of FoxM1 inhibited severe PH in both Sugen 5416/hypoxia and monocrotaline-challenged rats.

CONCLUSIONS

Multiple factors derived from dysfunctional ECs induced FoxM1 expression in SMCs and activated FoxM1-dependent SMC proliferation, which contributes to pulmonary vascular remodeling and PH. Thus, targeting FoxM1 signaling represents a novel strategy for treatment of idiopathic PAH.

Platelet-derived growth factor receptor-α is essential for cardiac fibroblast survival

Platelet-derived growth factor receptor α (PDGFRα), a receptor tyrosine kinase required for cardiac fibroblast development, is uniquely expressed by fibroblasts in the adult heart. Despite the consensus that PDGFRα is expressed in adult cardiac fibroblasts, we know little about its function when these cells are at rest. Here, we demonstrate that loss of PDGFRα in cardiac fibroblasts resulted in a rapid reduction of resident fibroblasts. Furthermore, we observe that phosphatidylinositol 3-kinase signaling was required for PDGFRα-dependent fibroblast maintenance. Interestingly, this reduced number of fibroblasts was maintained long-term, suggesting that there is no homeostatic mechanism to monitor fibroblast numbers and restore hearts to wild-type levels. Although we did not observe any systolic functional changes in hearts with depleted fibroblasts, the basement membrane and microvasculature of these hearts were perturbed. Through in vitro analyses, we showed that PDGFRα signaling inhibition resulted in an increase in fibroblast cell death, and PDGFRα stimulation led to increased levels of the cell survival factor activating transcription factor 3. Our data reveal a unique role for PDGFRα signaling in fibroblast maintenance and illustrate that a 50% loss in cardiac fibroblasts does not result in lethality.NEW & NOTEWORTHY Platelet-derived growth factor receptor α (PDGFRα) is required in developing cardiac fibroblasts, but a functional role in adult, quiescent fibroblasts has not been identified. Here, we demonstrate that PDGFRα signaling is essential for cardiac fibroblast maintenance and that there are no homeostatic mechanisms to regulate fibroblast numbers in the heart. PDGFR signaling is generally considered mitogenic in fibroblasts, but these data suggest that this receptor may direct different cellular processes depending on the cell's maturation and activation status.

PDGF Signaling in Primitive Endoderm Cell Survival Is Mediated by PI3K-mTOR Through p53-Independent Mechanism

Receptor tyrosine kinase signaling pathways are key regulators for the formation of the primitive endoderm (PrE) and the epiblast (Epi) from the inner cell mass (ICM) of the mouse preimplantation embryo. Among them, FGF signaling is critical for PrE cell specification, whereas PDGF signaling is critical for the survival of committed PrE cells. Here, we investigated possible functional redundancies among FGF, PDGF, and KIT signaling and showed that only PDGF signaling is involved in PrE cell survival. In addition, we analyzed the effectors downstream of PDGFRα. Our results suggest that the role of PDGF signaling in PrE cell survival is mediated through PI3K-mTOR and independently from p53. Lastly, we uncovered a role for PI3K-mTOR signaling in the survival of Epi cells. Taken together, we propose that survival of ICM cell lineages relies on the regulation of PI3K-mTOR signaling through the regulation of multiple signaling pathways. Stem Cells 2019;37:888-898.

MAPK and PI3K signaling: At the crossroads of neural crest development

Receptor tyrosine kinase-mediated growth factor signaling is essential for proper formation and development of the neural crest. The many ligands and receptors implicated in these processes signal through relatively few downstream pathways, frequently converging on the MAPK and PI3K pathways. Despite decades of study, there is still considerable uncertainty about where and when these signaling pathways are required and how they elicit particular responses. This review summarizes our current understanding of growth factor-induced MAPK and PI3K signaling in the neural crest.

Kat2a and Kat2b Acetyltransferase Activity Regulates Craniofacial Cartilage and Bone Differentiation in Zebrafish and Mice

Cranial neural crest cells undergo cellular growth, patterning, and differentiation within the branchial arches to form cartilage and bone, resulting in a precise pattern of skeletal elements forming the craniofacial skeleton. However, it is unclear how cranial neural crest cells are regulated to give rise to the different shapes and sizes of the bone and cartilage. Epigenetic regulators are good candidates to be involved in this regulation, since they can exert both broad as well as precise control on pattern formation. Here, we investigated the role of the histone acetyltransferases Kat2a and Kat2b in craniofacial development using TALEN/CRISPR/Cas9 mutagenesis in zebrafish and the Kat2a^hat/hat (also called Gcn5) allele in mice. kat2a and kat2b are broadly expressed during embryogenesis within the central nervous system and craniofacial region. Single and double kat2a and kat2b zebrafish mutants have an overall shortening and hypoplastic nature of the cartilage elements and disruption of the posterior ceratobranchial cartilages, likely due to smaller domains of expression of both cartilage- and bone-specific markers, including sox9a and col2a1, and runx2a and runx2b, respectively. Similarly, in mice we observe defects in the craniofacial skeleton, including hypoplastic bone and cartilage and altered expression of Runx2 and cartilage markers (Sox9, Col2a1). In addition, we determined that following the loss of Kat2a activity, overall histone 3 lysine 9 (H3K9) acetylation, the main epigenetic target of Kat2a/Kat2b, was decreased. These results suggest that Kat2a and Kat2b are required for growth and differentiation of craniofacial cartilage and bone in both zebrafish and mice by regulating H3K9 acetylation.

Conditional deletion of platelet derived growth factor receptor alpha (Pdgfra) in urorectal mesenchyme causes mesenchyme apoptosis and urorectal developmental anomalies in mice

In mammals, urorectal development starts at early embryonic stage, defective urorectal development results in anorectal malformations, which are common congenital developmental defects of the anus and the urethra in newborns. The etiology and embryology of the defects are still largely unknown. Platelet-derived growth factor receptor alpha (Pdgfra) is a cell surface receptor tyrosine kinase, upon binding to its ligands (Pdgfa-d), mediates intracellular signaling and regulates embryonic development. The expression of Pdgfra is tightly regulated in the developing urorectal mesenchyme, and its dysregulation is associated with urorectal defects in animals with urorectal defects. Knockout of Pdgfra induces early embryo lethality which precludes investigation of Pdgfra in urorectal development. To address the temporal requirement of Pdgfra in urorectal development, we conditionally deleted Pdgfra in Pdgfra-expressing tissues using a tamoxifen inducible Cre-loxP approach in mice, examined the urorectal development in Pdgfra conditional knockout (Pdgfra-cKO) embryos. We showed that conditional deletion of Pdgfra in Pdgfra-expressing tissues at E10-E11 caused cloaca septation defect, anteriorly displaced anus, defective urogenital folds development and abnormal urethra tubularization in both male and female mice. Furthermore, we showed that Pdgfra was required for the survival of urorectal mesenchyme, deletion of Pdgfra caused apoptosis in the peri-cloacal, the peri-urethra and the urorectal septum mesenchyme of Pdgfra-cKO mutants, associated with an induction of p53, Ndrg1 and activation of caspase-3 in Pdgfra-cKO embryos. In conclusion, Pdgfra is required for the development and survival of the urorectal mesenchyme in embryo, dysregulated Pdgfra signaling induced urorectal defects in mice resembling human congenital diseases of anorectal malformations and hypospadias. Perturbation of PDGFRA signaling may contribute to anorectal malformations and hypospadias in human.

Distinct mechanisms for PDGF and FGF signaling in primitive endoderm development

FGF signaling is known to play a critical role in the specification of primitive endoderm (PrE) and epiblast (Epi) from the inner cell mass (ICM) during mouse preimplantation development, but how FGFs synergize with other growth factor signaling pathways is unknown. Because PDGFRα signaling has also been implicated in the PrE, we investigated the coordinate functions of PDGFRα together with FGFR1 or FGFR2 in PrE development. PrE development was abrogated in Pdgfra; Fgfr1 compound mutants, or significantly reduced in Pdgfra; Fgfr2 or PdgfraPI3K; Fgfr2 compound mutants. We provide evidence that both Fgfr2 and Pdgfra play roles in PrE cell survival while Fgfr1 controls PrE cell specification. Our results suggest a model where FGFR1-engaged ERK1/2 signaling governs PrE specification while PDGFRα- and by analogy possibly FGFR2- engaged PI3K signaling regulates PrE survival and positioning in the embryo. Together, these studies indicate how multiple growth factors and signaling pathways can cooperate in preimplantation development.

Identification of Skt11-regulated genes in chondrocytes by integrated bioinformatics analysis

SKT11, an important tumor suppressor, is a member of the serine/threonine kinase family and plays a crucial role in tumor invasion and metastasis by activated adenine monophosphate-activated protein kinase (AMPK) and AMPK-related kinase proteins. However, few studies have elaborated its regulations of development and metabolism of cartilage, as well as skeleton. This study was aimed to investigate the role of Stk11-knockout in chondrocyte by bioinformatics analysis. The gene expression profiles for Stk11-knockout and wild-type mice were downloaded from the Gene Expression Omnibus (GEO) database. A total of 1104 differentially expressed genes (DEGs) were identified by Affymetrix Expression Console and Transcriptome Analysis Console (TAC) software, including 560 up-regulated and 544 down-regulated genes. The protein-protein interaction (PPI) networks were built by mapping DEGs into STRING, in which hub genes such as Fos, Pdgfrb, Pdgfra, Flt1/Vegfr1, Smad3, Mapk14, Twist and Aurkb were further identified. For the up-regulated genes, PI3K-AKT signaling pathway and Wnt signaling pathway were two main pathways in the KEGG analysis, and ossification and extracellular matrix organization were involved in the Gene Ontology (GO) analysis. On the other hand, the down-regulated genes were mainly involved in systemic lupus erythematosus and alcoholism pathways, and B cell receptor signaling pathway and immune system process biological processes. MiRNA-9, miRNA-134, miRNA-492, miRNA-224 and miRNA-142-5p were identified as key regulators in the miRNAs-DEG regulatory network. Additionally, OSF2/RUNX2, and NFAT regulated DEGs collectively in the transcription factor regulatory network. The results of RT-PCR verified that the expression of hub genes, transcription factors and miRNAs in our experiment were basically consistent with the microarray hybridization. In this study, we provide an insight into the role of Stk11 in chondrocyte and identify novel genes related to Stk11.

mTOR acts as a pivotal signaling hub for neural crest cells during craniofacial development

mTOR is a highly conserved serine/threonine protein kinase that is critical for diverse cellular processes in both developmental and physiological settings. mTOR interacts with a set of molecules including Raptor and Rictor to form two distinct functional complexes, namely the mTORC1 and mTORC2. Here, we used novel genetic models to investigate functions of the mTOR pathway for cranial neural crest cells (NCCs), which are a temporary type of cells arising from the ectoderm layer and migrate to the pharyngeal arches participating craniofacial development. mTOR deletion elicited a proliferation deficit and excessive apoptosis of post-migratory NCCs, leading to growth arrest of the facial primordia along with midline orofacial clefts. Furthermore, NCC differentiation was impaired. Thus, NCC derivatives, such as skeletons, vasculatures and neural tissues were either rudimentary or malformed. We further demonstrate that disruption of mTOR caused P53 hyperactivity and cell cycle arrest in cranial NCCs, and lowering P53 activity by one copy reduction attenuated the severity of craniofacial phenotype in NCC-mTOR knockout mice. Remarkably, NCC-Rptor disruption caused a spectrum of defects mirroring that of the NCC-mTOR deletion, whereas NCC-Rictor disruption only caused a mild craniofacial phenotype compared to the mTOR and Rptor conditional knockout models. Altogether, our data demonstrate that mTOR functions mediated by mTORC1 are indispensable for multiple processes of NCC development including proliferation, survival, and differentiation during craniofacial morphogenesis and organogenesis, and P53 hyperactivity in part accounts for the defective craniofacial development in NCC-mTOR knockout mice.

Neuropilin-1 and platelet-derived growth factor receptors cooperatively regulate intermediate filaments and mesenchymal cell migration during alveolar septation

Generation of secondary alveolar septa occurs primarily after birth in humans and is complete in mice postnatally, when mechanical stresses vary as air space pressure oscillates. Alveolar mesenchymal cells deposit elastic fibers, which limit cell strain; although when the elastic fiber network is incomplete, this function is also served by the intracellular cytoskeleton. Intermediate filament proteins support deformation during cell division and migration, which occur during septal elongation. Because platelet-derived growth factor receptor-α (PDGFRα) signaling is essential for alveolar septation, we hypothesized that neuropilin-1 (NRP1) may link PDGFRα to cytoskeletal deformation. During cell migration, NRP1 links receptor tyrosine kinase signaling to cytoskeletal and focal adhesion remodeling. Therefore, we examined the consequences of nrp1 gene deletion in alveolar mesenchymal cells (myofibroblasts and pericytes). NRP1 depletion reduced the proportion of mesenchymal cells that contain nestin and desmin within the subpopulation that lacked PDGFRα but contained PDGFRβ. Desmin was reduced at alveolar entry rings, air spaces were enlarged, and surface area was reduced after NRP1 depletion. PDGFRα and NRP1 colocalized to membrane lipid rafts, which are known to contain Src kinase. NRP1 depletion reduced alveolar mesenchymal cell migration and PDGF-A-mediated activation of Src kinase, which may limit accumulation of desmin at septal tips (alveolar entry rings). Cooperation between NRP1 and PDGF signaling is required for secondary septation, and manipulation of NRP1 could promote alveolar regeneration without producing fibrosis.

Sending mixed signals: Cilia-dependent signaling during development and disease

Molecular signals are the guiding force of development, imparting direction upon cells to divide, migrate, differentiate, etc. The mechanisms by which a cell can receive and transduce these signals into measurable actions remains a 'black box' in developmental biology. Primary cilia are ubiquitous, microtubule-based organelles that dynamically extend from a cell to receive and process molecular and mechanical signaling cues. In the last decade, this organelle has become increasingly intriguing to the research community due to its ability to act as a cellular antenna, receive and transduce molecular stimuli, and initiate a cellular response. In this review, we discuss the structure of primary cilia, emphasizing how the ciliary components contribute to the transduction of signaling pathways. Furthermore, we address how the cilium integrates these signals and conveys them into cellular processes such as proliferation, migration and tissue patterning. Gaining a deeper understanding of the mechanisms used by primary cilia to receive and integrate molecular signals is essential, as it opens the door for the identification of therapeutic targets within the cilium that could alleviate pathological conditions brought on by aberrant molecular signaling.

Pten regulates neural crest proliferation and differentiation during mouse craniofacial development

BACKGROUND

The phosphatase and tensin homolog deleted on chromosome TEN (Pten) is implicated in a broad range of developmental events and diseases. However, its role in neural crest and craniofacial development has not been well illustrated.

RESULTS

Using genetically engineered mouse models, we showed that inactivating Pten specifically in neural crest cells causes malformation of craniofacial structures. Pten conditional knockout mice exhibit perinatal lethality with overgrowth of craniofacial structures. At the cellular level, Pten deficiency increases cell proliferation rate and enhances osteoblast differentiation. Our data further revealed that inactivating Pten elevates PI3K/Akt signaling activity in neural crest derivatives, and confirmed that attenuation of PI3K/Akt activity led to decreased neural crest cell proliferation and differentiation both in vitro and in vivo.

CONCLUSIONS

Our study revealed that Pten is essential for craniofacial morphogenesis in mice. Inactivating Pten in neural crest cells increases proliferation rate and promotes their differentiation toward osteoblasts. Our data further indicate that Pten acts via modulating PI3K/Akt activity during these processes. Developmental Dynamics 247:304-314, 2018. © 2017 Wiley Periodicals, Inc.

Ehrlichia Activation of Wnt-PI3K-mTOR Signaling Inhibits Autolysosome Generation and Autophagic Destruction by the Mononuclear Phagocyte

In multicellular organisms, autophagy is induced as an innate defense mechanism. Notably, the obligate intracellular bacterium Ehrlichia chaffeensis resides in early endosome-like vacuoles and circumvents lysosomal fusion through an unknown mechanism, thereby avoiding destruction in the autophagolysosome. In this report, we reveal that Wnt signaling plays a crucial role in inhibition of lysosomal fusion and autolysosomal destruction of ehrlichiae. During early infection, autophagosomes fuse with ehrlichial vacuoles to form an amphisome indicated by the presence of autophagy markers such as LC3 (microtubule-associated protein 1 light chain 3), Beclin-1, and p62. LC3 colocalized with ehrlichial morulae on days 1, 2, and 3 postinfection, and increased LC3II levels were detected during infection, reaching a maximal level on day 3. Ehrlichial vacuoles did not colocalize with the lysosomal marker LAMP2, and lysosomes were redistributed and dramatically reduced in level in the infected cells. An inhibitor specific for the Wnt receptor signaling component Dishevelled induced lysosomal fusion with ehrlichial inclusions corresponding to p62 degradation and promoted transcription factor EB (TFEB) nuclear localization. E. chaffeensis infection activated the phosphatidylinositol 3-kinase (PI3K)-Akt-mTOR (mechanistic target of rapamycin) pathway, and activation was induced by three ehrlichial tandem repeat protein (TRP) effectors, with TRP120 inducing the strongest activation. Moreover, induction of glycogen synthase kinase-3 (GSK3) performed using a Wnt inhibitor and small interfering RNA (siRNA) knockdown of critical components of PI3K-GSK3-mTOR signaling decreased ehrlichial survival. This report reveals Ehrlichia exploitation of the evolutionarily conserved Wnt pathway to inhibit autolysosome generation, thereby leading to evasion of this important innate immune defense mechanism.

Dysregulated PDGFRα signaling alters coronal suture morphogenesis and leads to craniosynostosis through endochondral ossification

Craniosynostosis is a prevalent human birth defect characterized by premature fusion of calvarial bones. In this study, we show that tight regulation of endogenous PDGFRα activity is required for normal calvarium development in the mouse and that dysregulated PDGFRα activity causes craniosynostosis. Constitutive activation of PDGFRα leads to expansion of cartilage underlying the coronal sutures, which contribute to suture closure through endochondral ossification, in a process regulated in part by PI3K/AKT signaling. Our results thus identify a novel mechanism underlying calvarial development in craniosynostosis.

STAT1 modulates tissue wasting or overgrowth downstream from PDGFRβ

In this study, He et al. investigated how gain-of-function PDGFRβ mutations cause human disease, specifically whether PDGFRB mutations alone are responsible for genetic diseases characterized by musculoskeletal wasting or overgrowth. Using a genetic approach, their findings suggest a molecular mechanism by which STAT1 suppresses PDGFRβ-driven fibrosis and bone growth.

Platelet-derived growth factor (PDGF) acts through two conserved receptor tyrosine kinases: PDGFRα and PDGFRβ. Gain-of-function mutations in human PDGFRB have been linked recently to genetic diseases characterized by connective tissue wasting (Penttinen syndrome) or overgrowth (Kosaki overgrowth syndrome), but it is unclear whether PDGFRB mutations alone are responsible. Mice with constitutive PDGFRβ signaling caused by a kinase domain mutation (D849V) develop lethal autoinflammation. Here we used a genetic approach to investigate the mechanism of autoinflammation in Pdgfrb^+/D849V mice and test the hypothesis that signal transducer and activator of transcription 1 (STAT1) mediates this phenotype. We show that Pdgfrb^+/D849V mice with Stat1 knockout (Stat1^−/−Pdgfrb^+/D849V) are rescued from autoinflammation and have improved life span compared with Stat1^+/−Pdgfrb^+/D849V mice. Furthermore, PDGFRβ–STAT1 signaling suppresses PDGFRβ itself. Thus, Stat1^−/−Pdgfrb^+/D849V fibroblasts exhibit increased PDGFRβ signaling, and mice develop progressive overgrowth, a distinct phenotype from the wasting seen in Stat1^+/−Pdgfrb^+/D849V mice. Deletion of interferon receptors (Ifnar1 or Ifngr1) does not rescue wasting in Pdgfrb^+/D849V mice, indicating that interferons are not required for autoinflammation. These results provide functional evidence that elevated PDGFRβ signaling causes tissue wasting or overgrowth reminiscent of human genetic syndromes and that the STAT1 pathway is a crucial modulator of this phenotypic spectrum.

Generation of an immortalized mouse embryonic palatal mesenchyme cell line

Palatogenesis is a complex morphogenetic process, disruptions in which result in highly prevalent birth defects in humans. In recent decades, the use of model systems such as genetically-modified mice, mouse palatal organ cultures and primary mouse embryonic palatal mesenchyme (MEPM) cultures has provided significant insight into the molecular and cellular defects underlying cleft palate. However, drawbacks in each of these systems have prevented high-throughput, large-scale studies of palatogenesis in vitro. Here, we report the generation of an immortalized MEPM cell line that maintains the morphology, migration ability, transcript expression and responsiveness to exogenous growth factors of primary MEPM cells, with increased proliferative potential over primary cultures. The immortalization method described in this study will facilitate the generation of palatal mesenchyme cells with an unlimited capacity for expansion from a single genetically-modified mouse embryo and enable mechanistic studies of palatogenesis that have not been possible using primary culture.

Pulsed electromagnetic fields stimulate osteogenic differentiation and maturation of osteoblasts by upregulating the expression of BMPRII localized at the base of primary cilium

Pulsed electromagnetic fields (PEMFs) have been considered as a potential candidate for the prevention and treatment of osteoporosis, however, the mechanism of its action is still elusive. We have previously reported that 50Hz 0.6mT PEMFs stimulate osteoblastic differentiation and mineralization in a primary cilium- dependent manner, but did not know the reason. In the current study, we found that the PEMFs promoted osteogenic differentiation and maturation of rat calvarial osteoblasts (ROBs) by activating bone morphogenetic protein BMP-Smad1/5/8 signaling on the condition that primary cilia were normal. Further studies revealed that BMPRII, the primary binding receptor of BMP ligand, was readily and strongly upregulated by PEMF treatment and localized at the bases of primary cilia. Abrogation of primary cilia with small interfering RNA sequence targeting IFT88 abolished the PEMF-induced upregulation of BMPRII and its ciliary localization. Knockdown of BMPRII expression level with RNA interference had no effects on primary cilia but significantly decreased the promoting effect of PEMFs on osteoblastic differentiation and maturation. These results indicated that PEMFs stimulate osteogenic differentiation and maturation of osteoblast by primary cilium-mediated upregulation of BMPRII expression and subsequently activation of BMP-Smad1/5/8 signaling, and that BMPRII is the key component linking primary cilium and BMP-Smad1/5/8 pathway. This study has thus revealed the molecular mechanism for the osteogenic effect of PEMFs.

PDGFRβ regulates craniofacial development through homodimers and functional heterodimers with PDGFRα

Craniofacial development is a complex morphogenetic process, disruptions in which result in highly prevalent human birth defects. While platelet-derived growth factor (PDGF) receptor α (PDGFRα) has well-documented functions in this process, the role of PDGFRβ in murine craniofacial development is not well established. We demonstrate that PDGFRα and PDGFRβ are coexpressed in the craniofacial mesenchyme of mid-gestation mouse embryos and that ablation of Pdgfrb in the neural crest lineage results in increased nasal septum width, delayed palatal shelf development, and subepidermal blebbing. Furthermore, we show that the two receptors genetically interact in this lineage, as double-homozygous mutant embryos exhibit an overt facial clefting phenotype more severe than that observed in either single-mutant embryo. We reveal a physical interaction between PDGFRα and PDGFRβ in the craniofacial mesenchyme and demonstrate that the receptors form functional heterodimers with distinct signaling properties. Our studies thus uncover a novel mode of signaling for the PDGF family during vertebrate development.

Skin Adipocyte Stem Cell Self-Renewal Is Regulated by a PDGFA/AKT-Signaling Axis

Tissue growth and maintenance requires stem cell populations that self-renew, proliferate, and differentiate. Maintenance of white adipose tissue (WAT) requires the proliferation and differentiation of adipocyte stem cells (ASCs) to form postmitotic, lipid-filled mature adipocytes. Here we use the dynamic adipogenic program that occurs during hair growth to uncover an unrecognized regulator of ASC self-renewal and proliferation, PDGFA, which activates AKT signaling to drive and maintain the adipogenic program in the skin. Pdgfa expression is reduced in aged ASCs and is required for ASC proliferation and maintenance in the dermis, but not in other WATs. Our molecular and genetic studies uncover PI3K/AKT2 as a direct PDGFA target that is activated in ASCs during WAT hyperplasia and is functionally required for dermal ASC proliferation. Our data therefore reveal active mechanisms that regulate ASC self-renewal in the skin and show that distinct regulatory mechanisms operate in different WAT depots.

Canonical and noncanonical intraflagellar transport regulates craniofacial skeletal development

The primary cilium is a cellular organelle that coordinates signaling pathways critical for cell proliferation, differentiation, survival, and homeostasis. Intraflagellar transport (IFT) plays a pivotal role in assembling primary cilia. Disruption and/or dysfunction of IFT components can cause multiple diseases, including skeletal dysplasia. However, the mechanism by which IFT regulates skeletogenesis remains elusive. Here, we show that a neural crest-specific deletion of intraflagellar transport 20 (Ift20) in mice compromises ciliogenesis and intracellular transport of collagen, which leads to osteopenia in the facial region. Whereas platelet-derived growth factor receptor alpha (PDGFRα) was present on the surface of primary cilia in wild-type osteoblasts, disruption of Ift20 down-regulated PDGFRα production, which caused suppression of PDGF-Akt signaling, resulting in decreased osteogenic proliferation and increased cell death. Although osteogenic differentiation in cranial neural crest (CNC)-derived cells occurred normally in Ift20-mutant cells, the process of mineralization was severely attenuated due to delayed secretion of type I collagen. In control osteoblasts, procollagen was easily transported from the endoplasmic reticulum (ER) to the Golgi apparatus. By contrast, despite having similar levels of collagen type 1 alpha 1 (Col1a1) expression, Ift20 mutants did not secrete procollagen because of dysfunctional ER-to-Golgi trafficking. These data suggest that in the multipotent stem cells of CNCs, IFT20 is indispensable for regulating not only ciliogenesis but also collagen intracellular trafficking. Our study introduces a unique perspective on the canonical and noncanonical functions of IFT20 in craniofacial skeletal development.

SPECC1L deficiency results in increased adherens junction stability and reduced cranial neural crest cell delamination

Cranial neural crest cells (CNCCs) delaminate from embryonic neural folds and migrate to pharyngeal arches, which give rise to most mid-facial structures. CNCC dysfunction plays a prominent role in the etiology of orofacial clefts, a frequent birth malformation. Heterozygous mutations in SPECC1L have been identified in patients with atypical and syndromic clefts. Here, we report that in SPECC1L-knockdown cultured cells, staining of canonical adherens junction (AJ) components, β-catenin and E-cadherin, was increased, and electron micrographs revealed an apico-basal diffusion of AJs. To understand the role of SPECC1L in craniofacial morphogenesis, we generated a mouse model of Specc1l deficiency. Homozygous mutants were embryonic lethal and showed impaired neural tube closure and CNCC delamination. Staining of AJ proteins was increased in the mutant neural folds. This AJ defect is consistent with impaired CNCC delamination, which requires AJ dissolution. Further, PI3K-AKT signaling was reduced and apoptosis was increased in Specc1l mutants. In vitro, moderate inhibition of PI3K-AKT signaling in wildtype cells was sufficient to cause AJ alterations. Importantly, AJ changes induced by SPECC1L-knockdown were rescued by activating the PI3K-AKT pathway. Together, these data indicate SPECC1L as a novel modulator of PI3K-AKT signaling and AJ biology, required for neural tube closure and CNCC delamination.

Extracellular signaling molecules to promote fracture healing and bone regeneration

To date, the delivery of signaling molecules for bone regeneration has focused primarily on factors that directly affect the bone formation pathways (osteoinduction) or that serve to increase the number of bone forming progenitor cells. The first commercialized growth factors approved for bone regeneration, Bone Morphogenetic Protein 2 and 7 (BMP2 and BMP7), are direct inducers of osteoblast differentiation. As well, newer generations of potential therapeutics that target the Wnt signaling pathway are also direct osteoinducers. On the other hand, some signaling molecules may play a role as mitogens and serve to increase the number of bone producing cells or may increase vascularization. This is true for factors such as Platelet Derived Growth Factor (PDGF) or Fibroblast Growth Factor (FGF). Vascular Endothelial Growth Factor (VEGF) likely has a special role. Not only does it induce new blood vessel formation, it also has direct effects on osteoblasts through endothelial cell-based BMP production. In addition to these pathways that classically have targeted bone production, there are also opportunities to target other aspects of the bone healing process such as inflammation, vascularization, and cell ingress to the fracture site. Bone regeneration is highly complex with defined, yet overlapping stages of healing. We will review established and novel extracellular signaling factors associated with various stages of fracture healing that could be targeted to promote enhanced bone regeneration. Importantly, multiple potential cell and tissues could be targeted to enhance healing in addition to focusing solely on osteoinductive therapeutics.

Sox10ER(T2) CreER(T2) mice enable tracing of distinct neural crest cell populations

BACKGROUND

Neural crest cells play an important role in craniofacial morphogenesis and many other developmental processes. The formation of neural crest cells (NCCs) in vivo is a highly dynamic process and remains to be fully understood.

RESULTS

To investigate the spatiotemporal patterning of NCCs in vivo, we have generated Sox10ER(T2) CreER(T2) (SECE) mice, a transgenic line driving inducible Cre expression in NCCs. Inducing Cre activity at different stages triggered reporter expression in distinct NCC populations in SECE; R26R mice. By optimizing the timing and dosage of tamoxifen administration, we controlled Cre expression specifically in cranial NCCs. Using this approach, we demonstrate an important role for PDGFRα in cranial NCCs mitosis within the mandibular processes. Further reducing Cre activity within the cranial NCCs of SECE; R26R embryos revealed that SECE labels preferentially progenitors of medial nasal process (MNP) rather than the lateral nasal process (LNP), before their formation from the frontonasal prominence (FNP).

CONCLUSIONS

Our results indicate that NCCs are formed sequentially from rostral to caudal regions along the neural tube. These findings also suggest that NCCs within the FNP become specified regionally and genetically before they divide into MNP and LNP.

Valproic Acid Induces the Hyperacetylation of P53, Expression of P53 Target Genes, and Markers of the Intrinsic Apoptotic Pathway in Midorganogenesis Murine Limbs

In utero exposure to valproic acid (VPA), an anticonvulsant and histone deacetylase inhibitor (HDACi), increases the risk of congenital malformations. Although the mechanisms leading to the teratogenicity of VPA remain unsolved, several HDAC inhibitors increase cell death in cancer cell lines and embryonic tissues. Moreover, P53, the master regulator of apoptosis, is an established HDAC target. The purpose of this study was to investigate the effects of VPA on P53 signaling and markers of apoptosis during midorganogenesis in vitro limb development. Timed-pregnant CD1 mice (gestation day 12) were euthanized; embryonic forelimbs were excised and cultured in vitro for 3, 6, 12, or 24 hr in the presence or absence of VPA or valpromide (VPD), a non-HDACi analog of VPA. Quantitative RT-PCR and Western blots were used to assess the expression of candidate genes and proteins involved in P53 signaling and apoptosis. P53 hyperacetylation and a decrease (Survivin/Birc5 and Bcl2) or an increase (p21/Cdkn1a) in the expression of p53 target genes was observed only in VPA-exposed limbs. VPA exposure also triggered an increase in markers of apoptosis and DNA damage; the concentrations of cleaved caspase 9 and caspase 3, cleaved-poly (ADP-ribose) polymerase, and γ-H2AX were increased in VPA-exposed limbs. VPD treatment caused a small but significant increase in cleaved caspase 3. Thus, in vitro exposure to an HDACi such as VPA leads to P53 hyperacetylation, enhances the expression of P53 target genes, and triggers an increase in apoptosis that may contribute to teratogenicity.

Animal models for studying neural crest development: is the mouse different?

The neural crest is a uniquely vertebrate cell type and has been well studied in a number of model systems. Zebrafish, Xenopus and chick embryos largely show consistent requirements for specific genes in early steps of neural crest development. By contrast, knockouts of homologous genes in the mouse often do not exhibit comparable early neural crest phenotypes. In this Spotlight article, we discuss these species-specific differences, suggest possible explanations for the divergent phenotypes in mouse and urge the community to consider these issues and the need for further research in complementary systems.

Receptor tyrosine kinases modulate distinct transcriptional programs by differential usage of intracellular pathways

Receptor tyrosine kinases (RTKs) signal through shared intracellular pathways yet mediate distinct outcomes across many cell types. To investigate the mechanisms underlying RTK specificity in craniofacial development, we performed RNA-seq to delineate the transcriptional response to platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) signaling in mouse embryonic palatal mesenchyme cells. While the early gene expression profile induced by both growth factors is qualitatively similar, the late response is divergent. Comparing the effect of MEK (Mitogen/Extracellular signal-regulated kinase) and PI3K (phosphoinositide-3-kinase) inhibition, we find the FGF response is MEK dependent, while the PDGF response is PI3K dependent. Furthermore, FGF promotes proliferation but PDGF favors differentiation. Finally, we demonstrate overlapping domains of PDGF-PI3K signaling and osteoblast differentiation in the palate and increased osteogenesis in FGF mutants, indicating this differentiation circuit is conserved in vivo. Our results identify distinct responses to PDGF and FGF and provide insight into the mechanisms encoding RTK specificity.

DOI:http://dx.doi.org/10.7554/eLife.07186.001

Cells produce many different proteins that play a variety of important roles. For example, proteins called receptor tyrosine kinases can detect particular molecules and send signals to other parts of the cell to regulate the activity (or “expression”) of genes involved in cell division, movement, and other processes.

Humans have 58 receptor tyrosine kinases, and defects in these proteins have been linked to diseases such as cancer and diabetes. However, many different receptors regulate the activities of shared sets of genes, so it is not clear how an individual receptor can specifically control the genes involved in a particular process.

Two receptor tyrosine kinases called PDGFR and FGFR are crucial for the development of the face, palate, and head in humans and other animals. Vasudevan et al. used a technique called RNA-sequencing to find out which genes are regulated by these receptors in mouse palate cells. The experiments show that there is a common set of genes whose activities change quickly—within 1 hour—in response to the activation of either PDGFR or FGFR. However, several hours later, cells in which PDGFR is activated have different patterns of gene expression compared to those with active FGFR.

Vasudevan et al. also found that FGFR promotes cell division, while PDGFR promotes the changing of palate cells into different types with more specialized roles. These different outcomes arise because PDGFR and FGFR use different signaling pathways that involve distinct proteins. For example, a protein called PI3K is critical for changes in gene expression in response to PDGFR but not FGFR.

These results suggest that PGDRF and FGFR control different cellular processes in the palate by sending distinct signals into the cell. Understanding the receptor tyrosine kinases and the networks of genes they activate will help us to identify the signals that are important for other processes, such as the development of the face.

DOI:http://dx.doi.org/10.7554/eLife.07186.002

Receptor tyrosine kinase signaling: regulating neural crest development one phosphate at a time

Receptor tyrosine kinases (RTKs) bind to a subset of growth factors on the surface of cells and elicit responses with broad roles in developmental and postnatal cellular processes. Receptors in this subclass consist of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular domain harboring a catalytic tyrosine kinase and regulatory sequences that are phosphorylated either by the receptor itself or by various interacting proteins. Once activated, RTKs bind signaling molecules and recruit effector proteins to mediate downstream cellular responses through various intracellular signaling pathways. In this chapter, we highlight the role of a subset of RTK families in regulating the activity of neural crest cells (NCCs) and the development of their derivatives in mammalian systems. NCCs are migratory, multipotent cells that can be subdivided into four axial populations, cranial, cardiac, vagal, and trunk. These cells migrate throughout the vertebrate embryo along defined pathways and give rise to unique cell types and structures. Interestingly, individual RTK families often have specific functions in a subpopulation of NCCs that contribute to the diversity of these cells and their derivatives in the mammalian embryo. We additionally discuss current methods used to investigate RTK signaling, including genetic, biochemical, large-scale proteomic, and biosensor approaches, which can be applied to study intracellular signaling pathways active downstream of this receptor subclass during NCC development.

PDGF-AA promotes osteogenic differentiation and migration of mesenchymal stem cell by down-regulating PDGFRα and derepressing BMP-Smad1/5/8 signaling

Platelet-derived growth factors (PDGFs) play important roles in skeletal development and bone fracture healing, yet how PDGFs execute their functions remains incompletely understood. Here we show that PDGF-AA, but not -AB or -BB, could activate the BMP-Smad1/5/8 pathway in mesenchymal stem cells (MSCs), which requires BMPRIA as well as PDGFRα. PDGF-AA promotes MSC osteogenic differentiation through the BMP-Smad1/5/8-Runx2/Osx axis and MSC migration via the BMP-Smad1/5/8-Twist1/Atf4 axis. Mechanistic studies show that PDGF-AA activates BMP-Smad1/5/8 signaling by feedback down-regulating PDGFRα, which frees BMPRI and allows for BMPRI-BMPRII complex formation to activate smad1/5/8, using BMP molecules in the microenvironment. This study unravels a physical and functional interaction between PDGFRα and BMPRI, which plays an important role in MSC differentiation and migration, and establishes a link between PDGF-AA and BMPs pathways, two essential regulators of embryonic development and tissue homeostasis.

SRF regulates craniofacial development through selective recruitment of MRTF cofactors by PDGF signaling

Receptor tyrosine kinase signaling is critical for mammalian craniofacial development, but the key downstream transcriptional effectors remain unknown. We demonstrate that serum response factor (SRF) is induced by both platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) signaling in mouse embryonic palatal mesenchyme cells and that Srf neural crest conditional mutants exhibit facial clefting accompanied by proliferation and migration defects. Srf and Pdgfra mutants interact genetically in craniofacial development, but Srf and Fgfr1 mutants do not. This signal specificity is recapitulated at the level of cofactor activation: while both PDGF and FGF target gene promoters show enriched genome-wide overlap with SRF ChIP-seq peaks, PDGF selectively activates a network of MRTF-dependent cytoskeletal genes. Collectively, our results identify a role for SRF in proliferation and migration during craniofacial development and delineate a mechanism of receptor tyrosine kinase specificity mediated through differential cofactor usage, leading to a PDGF-responsive SRF-driven transcriptional program in the midface.