The Cdc42 guanine nucleotide exchange factor FGD1 regulates osteogenesis in human mesenchymal stem cells

Targeted Proteomic Profiling Revealed Roles of Small GTPases during Osteogenic Differentiation

The small GTPase superfamily of proteins are crucial for numerous cellular processes, including early development. The roles of these proteins in osteogenic differentiation, however, remained poorly explored. In this study, we employed a high-throughput targeted proteomic method, relying on scheduled liquid chromatography-multiple-reaction monitoring (LC-MRM) coupled with synthetic stable isotope-labeled peptides, to interrogate systematically the temporal responses of the entire small GTPase proteome during the course of osteogenic differentiation of H9 human embryonic stem cells. Our results demonstrated that the method offers high quantification accuracy, reproducibility, and throughput. In addition, the quantification results revealed altered expression of a large number of small GTPases accompanied with osteogenic differentiation, especially those involved with autophagy. We also documented a previously unrecognized role of KRAS in osteogenesis, where it regulates the accumulation of extracellular matrix for mineralization through attenuating the activity of secreted matrix metalloproteinase 9 (MMP9). Together, this study represents a novel application of a state-of-the-art analytical method, i.e., targeted quantitative proteomics, for revealing the progressive reprogramming of the small GTPase proteome during osteogenic differentiation of human embryonic stem cells, and our results revealed KRAS as a new regulator for osteogenesis.

FGD1 Variant Associated With Aarskog-Scott Syndrome

BACKGROUND

Aarskog-Scott syndrome, a rare X-linked genetic disorder, is identified by combined clinical manifestations of short stature, facial, skeletal, and genital anomalies. Annually, two or three new cases are diagnosed with Aarskog-Scott syndrome, which is associated with FGD1 variants. However, there is no specific treatment for Aarskog-Scott syndrome due to its unclear mechanism.

METHODS

Clinical data were collected when the patient first visited the hospital. Trio whole-exome sequencing and Sanger sequencing were performed for the genetic cause of disease. To evaluate the pathogenicity of the variants in vitro, stable cell lines were constructed using lentivirus infection in 143B cell. Furthermore, Western blot was used to verify the expression of signaling pathway-related proteins, and the transcription levels of osteogenic-related genes were verified by luciferase reporter gene assay.

RESULTS

A 7-year-old boy was manifested with facial abnormalities, intellectual disability, and short stature (-3.98 SDS) while the growth hormone level of stimulation test was normal. Trio whole-exome sequencing and Sanger sequencing identified a variant (c.1270A>G, p.Asn424Asp) in FGD1 gene. The Asn424 residue was highly conserved and the hydrogen bond in the FGD1 variant protein has changed, which led to decrease in the interaction with CDC42 protein. In vitro study showed that the Asn424Asp variant significantly decreased the transcription levels of OCN, COL1A1, and ALP activity, and it activated the phosphorylation of JNK1.

CONCLUSION

Molecular biological mechanisms between abnormal expression of FGD1and Aarskog-Scott syndrome remain poorly understood. In our study, c.1270A>G variant of FGD1 resulted in Aarskog-Scott syndrome, and the analysis of pathogenicity supports the deleterious effect of the variant. Furthermore, we demonstrated the weakened affinity of the mutant FGD1 and CDC42. Decreased expression of osteogenic-related gene and abnormal activation of JNK1 were also shown in this work.

Arhgap21 Deficiency Results in Increase of Osteoblastic Lineage Cells in the Murine Bone Marrow Microenvironment

ARHGAP21 is a member of the RhoGAP family of proteins involved in cell growth, differentiation, and adhesion. We have previously shown that the heterozygous Arhgap21 knockout mouse model (Arhgap21^+/-) presents several alterations in the hematopoietic compartment, including increased frequency of hematopoietic stem and progenitor cells (HSPC) with impaired adhesion in vitro, increased mobilization to peripheral blood, and decreased engraftment after bone marrow transplantation. Although these HSPC functions strongly depend on their interactions with the components of the bone marrow (BM) niche, the role of ARHGAP21 in the marrow microenvironment has not yet been explored. In this study, we investigated the composition and function of the BM microenvironment in Arhgap21^+/- mice. The BM of Arhgap21^+/- mice presented a significant increase in the frequency of phenotypic osteoblastic lineage cells, with no differences in the frequencies of multipotent stromal cells or endothelial cells when compared to the BM of wild type mice. Arhgap21^+/- BM cells had increased capacity of generating osteogenic colony-forming units (CFU-OB) in vitro and higher levels of osteocalcin were detected in the Arhgap21^+/- BM supernatant. Increased expression of Col1a1, Ocn and decreased expression of Trap1 were observed after osteogenic differentiation of Arhgap21^+/- BM cells. In addition, Arhgap21^+/- mice recipients of normal BM cells showed decreased leucocyte numbers during transplantation recovery. Our data suggest participation of ARHGAP21 in the balanced composition of the BM microenvironment through the regulation of osteogenic differentiation.

The MEK5/ERK5 Pathway in Health and Disease

The MEK5/ERK5 mitogen-activated protein kinases (MAPK) cascade is a unique signaling module activated by both mitogens and stress stimuli, including cytokines, fluid shear stress, high osmolarity, and oxidative stress. Physiologically, it is mainly known as a mechanoreceptive pathway in the endothelium, where it transduces the various vasoprotective effects of laminar blood flow. However, it also maintains integrity in other tissues exposed to mechanical stress, including bone, cartilage, and muscle, where it exerts a key function as a survival and differentiation pathway. Beyond its diverse physiological roles, the MEK5/ERK5 pathway has also been implicated in various diseases, including cancer, where it has recently emerged as a major escape route, sustaining tumor cell survival and proliferation under drug stress. In addition, MEK5/ERK5 dysfunction may foster cardiovascular diseases such as atherosclerosis. Here, we highlight the importance of the MEK5/ERK5 pathway in health and disease, focusing on its role as a protective cascade in mechanical stress-exposed healthy tissues and its function as a therapy resistance pathway in cancers. We discuss the perspective of targeting this cascade for cancer treatment and weigh its chances and potential risks when considering its emerging role as a protective stress response pathway.

Aarskog-Scott syndrome: phenotypic and genetic heterogeneity

Aarskog-Scott syndrome (AAS) is a rare developmental disorder which primarily affects males and has a relative prevalence of 1 in 25,000 in the general population. AAS patients usually present with developmental complications including short stature and facial, skeletal and urogenital anomalies. The spectrum of genotype-phenotype correlations in AAS is unclear and mutations of the FGD1 gene on the proximal short arm of chromosome X account for only 20% of the incidence of the disorder. Failure to identify pathogenic variants in patients referred for FGD1 screening suggests heterogeneity underlying pathophysiology of the condition. Furthermore, overlapping features of AAS with several other developmental disorders increase the complexity of diagnosis. Cytoskeletal signaling may be involved in the pathophysiology of AAS. The FGD1 protein family has a role in activation of CDC42 (Cell Division Control protein 42 homolog) which has a core function in remodeling of extracellular matrix and the transcriptional activation of many modulators of development. Therefore, mutations in components in the EGFR1 (Epidermal Growth Factor Receptor 1) signaling pathway, to which CDC42 belongs, may contribute to pathophysiology. Parallel sequencing strategies (so-called next generation sequencing or high throughput sequencing) enables simultaneous production of millions of sequencing reads that enormously facilitate cost-effective identification of cryptic mutations in heterogeneous monogenic disorders. Here we review the source of phenotypic and genetic heterogeneity in the context of AAS and discuss the applicability of next generation sequencing for identification of novel mutations underlying AAS.

Cdc42 regulates cranial suture morphogenesis and ossification

Cdc42 (cell division cycle 42) is ubiquitously expressed small GTPases belonging to the Rho family of proteins. Previously, we generated limb bud mesenchyme-specific Cdc42 inactivated mice (Cdc42 conditional knockout mice; Cdc42^fl/fl; Prx1-Cre), which showed short limbs and cranial bone deformities, though the mechanism related to the cranium phenotype was unclear. In the present study, we investigated the role of Cdc42 in cranial bone development. Our results showed that loss of Cdc42 caused a defect of intramembranous ossification in cranial bone tissues which is related to decreased expressions of cranial suture morphogenesis genes, including Indian hedgehog (Ihh) and bone morphogenetic proteins (BMPs). These findings demonstrate that Cdc42 plays a crucial role in cranial osteogenesis, and is controlled by Ihh- and BMP-mediated signaling during cranium development.

Identification of CDC42 Effectors Operating in FGD1-Dependent Trafficking at the Golgi

Loss of function mutations in the FGD1 gene cause a rare X-linked disease, faciogenital dysplasia (FGDY, also known as Aarskog-Skott syndrome), which is associated with bone and urogenital abnormalities. The FGD1 gene encodes à CDC42-specific guanine nucleotide exchange factor. The mutations are frequently located in the DH module of FGD1 preventing its transformation to the active form. We previously reported that Golgi-associated FGD1 regulates post-Golgi transport of some conventional and bone-specific proteins in a CDC42-dependent manner. However, the downstream targets of FGD1/CDC42 signaling that operate to support transport from the Golgi remain elusive. Here, we demonstrate that Golgi-localized CDC42 effectors might be involved in FGD1-mediated post-Golgi transport, probably through coordination of Golgi membrane and cytoskeleton dynamics. Overexpression of effector-specific CDC42 mutants (exhibiting preferential affinities for PAK1, IQGAP1, N-WASP, or PAR6) only partially rescue membrane trafficking in FGD1-deficient cells, indicating that the orchestrated activities of several downstream targets of CDC42 are required to support FGD1-mediated export from the Golgi. Our findings provide new insights into understanding the molecular mechanisms behind FGD1/CDC42-dependent transport events and uncover new targets whose potential might be explored for correction of membrane trafficking in FGDY.

Mechanisms of Cdc42-mediated rat MSC differentiation on micro/nano-textured topography

Micro/nano-textured titanium surface topography promotes osteoblast differentiation and the Wnt/β-catenin signaling pathway. However, the response of rat bone mesenchymal stem cells (MSCs) to micro/nano-textured topography, and the underlying mechanisms of its effects, are not well understood. We hypothesized that cell division cycle 42 protein (Cdc42), a key member of the Rho GTPases family, may regulate rat MSCs morphology and osteogenic differentiation by micro/nano-textured topography, and that crosstalk between Cdc42 and Wnt/β-catenin is the underlying mechanism. To confirm the hypothesis, we first tested rat MSCs' morphology, cytoskeleton, and osteogenic differentiation on micro/nano-textured topography. We then examined the cells' Wnt pathway and Cdc42 signaling activity. The results show that micro/nano-textured topography enhances MSCs' osteogenic differentiation. In addition, the cells' morphology and cytoskeletal reorganization were dramatically different on smooth surfaces and micropitted/nanotubular topography. Ligands of the canonical Wnt pathway, as well as accumulation of β-catenin in the nucleus, were up-regulated by micro/nano-textured topography. Cdc42 protein expression was markedly increased under these conditions; conversely, Cdc42 silencing significantly depressed the enhancement of MSCs osteogenic differentiation by micro/nano-textured topography. Moreover, Cdc42si attenuated p-GSK3β activation and resulted in β-catenin cytoplasmic degradation on the micro/nano-textured topography. Our results indicate that Cdc42 is a key modulator of rat MSCs morphology and cytoskeletal reorganization, and that crosstalk between Cdc42 and Wnt/β-catenin signaling though GSK3β regulates MSCs osteogenic differentiation by implant topographical cues.

STATEMENT OF SIGNIFICANCE

Topographical modification at micro- and nanoscale is widely applied to enhance the tissue integration properties of biomaterials. However, the response of bone mesenchymal stem cells (MSCs) to the micro/nano-textured topography and the underlying mechanisms are not well understood. This study shows that the micropitted/nanotubular hierarchical topography produced by etching and anodic oxidation treatment drives fusiform cell morphology, cytoskeletal reorganization as well as better MSCs osteogenic differentiation. The cross-talk between Cdc42 pathway and Wnt/β-catenin pathway though GSK3β modulates the osteoinductive effect of the micro/nano-textured topography on MSCs. This finding sheds light on a novel mechanism involved in micro/nano-textured surface-mediated MSCs osteogenic differentiation and is a major step in the development of new surface modifications aiming to accelerate and enhance the process of osseointegration.

Emerging role of Cdc42-specific guanine nucleotide exchange factors as regulators of membrane trafficking in health and disease

It is widely accepted that the Golgi complex operates as a main sorting station in the biosynthetic pathway. On the other hand, the Golgi complex harbors numerous signaling molecules that generate the platform for the coordination of the transduction of specific signals and of membrane transport events. A part of these processes, which require the complex integration of transport-, cytoskeleton- and polarity-associated mechanisms, is tightly regulated by molecular machineries comprising guanine nucleotide exchange factors (GEF) and their down-stream effectors, such as the small GTPase Cdc42. Dysfunction of several Cdc42-specific GEFs has been shown to cause a number of human diseases, which are associated with impaired intracellular trafficking at the level of the Golgi complex as well as in other compartments. Here we briefly overview how mutations in Cdc42-specific GEFs have an impact on the organization of intracellular trafficking fluxes and how such trafficking aberrations could be associated with a number of human disorders.

Signaling Cascades Governing Cdc42-Mediated Chondrogenic Differentiation and Mensenchymal Condensation

Endochondral ossification consists of successive steps of chondrocyte differentiation, including mesenchymal condensation, differentiation of chondrocytes, and hypertrophy followed by mineralization and ossification. Loss-of-function studies have revealed that abnormal growth plate cartilage of the Cdc42 mutant contributes to the defects in endochondral bone formation. Here, we have investigated the roles of Cdc42 in osteogenesis and signaling cascades governing Cdc42-mediated chondrogenic differentiation. Though deletion of Cdc42 in limb mesenchymal progenitors led to severe defects in endochondral ossification, either ablation of Cdc42 in limb preosteoblasts or knockdown of Cdc42 in vitro had no obvious effects on bone formation and osteoblast differentiation. However, in Cdc42 mutant limb buds, loss of Cdc42 in mesenchymal progenitors led to marked inactivation of p38 and Smad1/5, and in micromass cultures, Cdc42 lay on the upstream of p38 to activate Smad1/5 in bone morphogenetic protein-2-induced mesenchymal condensation. Finally, Cdc42 also lay on the upstream of protein kinase B to transactivate Sox9 and subsequently induced the expression of chondrocyte differential marker in transforming growth factor-β1-induced chondrogenesis. Taken together, by using biochemical and genetic approaches, we have demonstrated that Cdc42 is involved not in osteogenesis but in chondrogenesis in which the BMP2/Cdc42/Pak/p38/Smad signaling module promotes mesenchymal condensation and the TGF-β/Cdc42/Pak/Akt/Sox9 signaling module facilitates chondrogenic differentiation.

Energy Metabolism in Mesenchymal Stem Cells During Osteogenic Differentiation

There is emerging interest in stem cell energy metabolism and its effect on differentiation. Bioenergetic changes in differentiating bone marrow mesenchymal stem cells (MSCs) are poorly understood and were the focus of our study. Using bioenergetic profiling and transcriptomics, we have established that MSCs activate the mitochondrial process of oxidative phosphorylation (OxPhos) during osteogenic differentiation, but they maintain levels of glycolysis similar to undifferentiated cells. Consistent with their glycolytic phenotype, undifferentiated MSCs have high levels of hypoxia-inducible factor 1 (HIF-1). Osteogenically induced MSCs downregulate HIF-1 and this downregulation is required for activation of OxPhos. In summary, our work provides important insights on MSC bioenergetics and proposes a HIF-based mechanism of regulation of mitochondrial OxPhos in MSCs.

Identification of novel mutations in Mexican patients with Aarskog-Scott syndrome

Aarskog-Scott syndrome (AAS), also known as faciogenital dysplasia (FGD, OMIM # 305400), is an X-linked disorder of recessive inheritance, characterized by short stature and facial, skeletal, and urogenital abnormalities. AAS is caused by mutations in the FGD1 gene (Xp11.22), with over 56 different mutations identified to date. We present the clinical and molecular analysis of four unrelated families of Mexican origin with an AAS phenotype, in whom FGD1 sequencing was performed. This analysis identified two stop mutations not previously reported in the literature: p.Gln664* and p.Glu380*. Phenotypically, every male patient met the clinical criteria of the syndrome, whereas discrepancies were found between phenotypes in female patients. Our results identify two novel mutations in FGD1, broadening the spectrum of reported mutations; and provide further delineation of the phenotypic variability previously described in AAS.

FGD1 as a central regulator of extracellular matrix remodelling--lessons from faciogenital dysplasia

Disabling mutations in the FGD1 gene cause faciogenital dysplasia (also known as Aarskog-Scott syndrome), a human X-linked developmental disorder that results in disproportionately short stature, facial, skeletal and urogenital anomalies, and in a number of cases, mild mental retardation. FGD1 encodes the guanine nucleotide exchange factor FGD1, which is specific for the Rho GTPase cell division cycle 42 (CDC42). CDC42 controls cytoskeleton-dependent membrane rearrangements, transcriptional activation, secretory membrane trafficking, G1 transition during the cell cycle and tumorigenic transformation. The cellular mechanisms by which FGD1 mutations lead to the hallmark skeletal deformations of faciogenital dysplasia remain unclear, but the pathology of the disease, as well as some recent discoveries, clearly show that the protein is involved in the regulation of bone development. Two recent studies unveiled new potential functions of FGD1, in particular, its involvement in the regulation of the formation and function of invadopodia and podosomes, which are cellular structures devoted to degradation of the extracellular matrix in tumour and endothelial cells. Here, we discuss the hypothesis that FGD1 might be an important regulator of events controlling extracellular matrix remodelling and possibly cell invasion in physiological and pathological settings. Additionally, we focus on how studying the cell biology of FGD1 might help us to connect the dots that link CDC42 signalling with remodelling of the extracellular matrix (ECM) in physiology and complex diseases, while, at the same time, furthering our understanding of the pathogenesis of faciogenital dysplasia.

MLK3 regulates bone development downstream of the faciogenital dysplasia protein FGD1 in mice

Mutations in human FYVE, RhoGEF, and PH domain-containing 1 (FGD1) cause faciogenital dysplasia (FGDY; also known as Aarskog syndrome), an X-linked disorder that affects multiple skeletal structures. FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase CDC42. However, the mechanisms by which mutations in FGD1 affect skeletal development are unknown. Here, we describe what we believe to be a novel signaling pathway in osteoblasts initiated by FGD1 that involves the MAP3K mixed-lineage kinase 3 (MLK3). We observed that MLK3 functions downstream of FGD1 to regulate ERK and p38 MAPK, which in turn phosphorylate and activate the master regulator of osteoblast differentiation, Runx2. Mutations in FGD1 found in individuals with FGDY ablated its ability to activate MLK3. Consistent with our description of this pathway and the phenotype of patients with FGD1 mutations, mice with a targeted deletion of Mlk3 displayed multiple skeletal defects, including dental abnormalities, deficient calvarial mineralization, and reduced bone mass. Furthermore, mice with knockin of a mutant Mlk3 allele that is resistant to activation by FGD1/CDC42 displayed similar skeletal defects, demonstrating that activation of MLK3 specifically by FGD1/CDC42 is important for skeletal mineralization. Thus, our results provide a putative biochemical mechanism for the skeletal defects in human FGDY and suggest that modulating MAPK signaling may benefit these patients.