Different roles of Foxo1 and Foxo3 in the control of endothelial cell morphology

FOXO1 promotes endothelial cell elongation and angiogenesis by up-regulating the phosphorylation of myosin light chain 2

The forkhead box O1 (FOXO1) is an important transcription factor related to proliferation, metabolism, and homeostasis, while the major phenotype of FOXO1-null mice is abnormal vascular morphology, such as vessel enlargement and dilation. In in vitro mouse embryonic stem cell (ESC)-differentiation system, Foxo1-/- vascular endothelial cells (ECs) fail to elongate, and mimic the abnormalities of FOXO1-deficiency in vivo. Here, we identified the PPP1R14C gene as the FOXO1 target genes responsible for elongating using transcriptome analyses in ESC-derived ECs (ESC-ECs), and found that the FOXO1-PPP1R14C-myosin light chain 2 (MLC2) axis is required for EC elongation during angiogenesis. MLC2 is phosphorylated by MLC kinase (MLCK) and dephosphorylated by MLC phosphatase (MLCP). PPP1R14C is an inhibitor of PP1, the catalytic subunit of MLCP. The abnormal morphology of Foxo1-/- ESC-ECs was associated with low level of PPP1R14C and loss of MLC2 phosphorylation, which were reversed by PPP1R14C-introduction. Knockdown of either FOXO1 or PPP1R14C suppressed vascular cord formation and reduced MLC2 phosphorylation in human ECs (HUVECs). The mouse and human PPP1R14C locus possesses an enhancer element containing conserved FOXO1-binding motifs. In vivo chemical inhibition of MLC2 phosphorylation caused dilated vascular structures in mouse embryos. Furthermore, foxo1 or ppp1r14c-knockdown zebrafish exhibited vascular malformations, which were also restored by PPP1R14C-introduction. Mechanistically, FOXO1 suppressed MLCP activity by up-regulating PPP1R14C expression, thereby promoting MLC2 phosphorylation and EC elongation, which are necessary for vascular development. Given the importance of MLC2 phosphorylation in cell morphogenesis, this study may provide novel insights into the role of FOXO1 in control of angiogenesis.

The role of forkhead box class O1 during implant osseointegration

FOXO1, a member of the forkhead family of transcription factors, plays a vital role in the osteogenic lineage commitment of mesenchymal stem cells, and affects multiple cellular functions of osteogenic cells. However, prior studies have focused on mesenchymal stem cells but not on differentiated osteoblasts. In addition, studies about the role of FOXO1 during osseointegration are lacking. In this present study, we constructed osteoblast conditional FOXO1 knock-out mice and lentivirus-mediated FoxO1 overexpression to investigate maxillary titanium implant osseointegration. After 4 wk post implant placement, micro-computed tomography, histomorphometric analyses, and RT-qPCR assays were performed. Results showed that compared with the control group, overexpression of FOXO1 significantly enhanced bone formation around implant and bone-implant contact ratio, while loss of FOXO1 impaired peri-implant osteogenesis and osseointegration. Moreover, overexpression of FoxO1 enhanced expression of osteogenesis-related genes, such as Runx2, Alp1, Col1a1, and Bglap. Whereas, knock-out of Foxo1 reduced the expression of osteogenesis-related genes. Taken together, our results suggested that FOXO1 in osteoblasts could enhance osteogenesis-related gene expression to improve osseointegration.

The canonical smooth muscle cell marker TAGLN is present in endothelial cells and is involved in angiogenesis

Elongation of vascular endothelial cells (ECs) is an important process in angiogenesis; however, the molecular mechanisms remain unknown. The actin-crosslinking protein TAGLN (transgelin, also known as SM22 or SM22α) is abundantly expressed in smooth muscle cells (SMCs) and is widely used as a canonical marker for this cell type. In the course of studies using mouse embryonic stem cells (ESCs) carrying an Tagln promoter-driven fluorescence marker, we noticed activation of the Tagln promoter during EC elongation. Tagln promoter activation co-occurred with EC elongation in response to vascular endothelial growth factor A (VEGF-A). Inhibition of phosphoinositide 3-kinase (PI3K)-Akt signaling and mTORC1 also induced EC elongation and Tagln promoter activation. Human umbilical vein endothelial cells (HUVECs) elongated, activated the TAGLN promoter and increased TAGLN transcripts in an angiogenesis model. Genetic disruption of TAGLN augmented angiogenic behaviors of HUVECs, as did the disruption of TAGLN2 and TAGLN3 genes. Tagln expression was found in ECs in mouse embryos. Our results identify TAGLN as a putative regulator of angiogenesis whose expression is activated in elongating ECs. This finding provides insight into the cytoskeletal regulation of EC elongation and an improved understanding of the molecular mechanisms underlying the regulation of angiogenesis.

A uniform expression library for the exploration of FOX transcription factor biology

Forkhead box (FOX) family transcription factors play essential roles in development, tissue homeostasis, and disease. Although the biology of several FOX proteins has been studied in depth, it is unclear to what extent these findings apply to even closely related family members, which frequently exert overlapping but non-redundant functions. To help address this question, we have generated a uniform, ready-to-use expression library of all 44 human FOX transcription factors with a convenient peptide tag for parallel screening assays. In addition, we have generated multiple universal forkhead box reporter plasmids, which can be used to monitor the transcriptional activity of most FOX proteins with high fidelity. As a proof-of-principle, we use our plasmid library to identify the DNA repair protein XRCC6/Ku70 as a selective FOX interaction partner and regulator of FOX transcriptional activity. We believe that these tools, which we make available via the Addgene plasmid repository, will considerably expedite the investigation of FOX protein biology.

Sirtuin-1 and Its Relevance in Vascular Calcification

Vascular calcification (VC) is highly associated with cardiovascular disease and all-cause mortality in patients with chronic kidney disease. Dysregulation of endothelial cells and vascular smooth muscle cells (VSMCs) is related to VC. Sirtuin-1 (Sirt1) deacetylase encompasses a broad range of transcription factors that are linked to an extended lifespan. Sirt1 enhances endothelial NO synthase and upregulates FoxOs to activate its antioxidant properties and delay cell senescence. Sirt1 reverses osteogenic phenotypic transdifferentiation by influencing RUNX2 expression in VSMCs. Low Sirt1 hardly prevents acetylation by p300 and phosphorylation of β-catenin that, following the facilitation of β-catenin translocation, drives osteogenic phenotypic transdifferentiation. Hyperphosphatemia induces VC by osteogenic conversion, apoptosis, and senescence of VSMCs through the Pit-1 cotransporter, which can be retarded by the sirt1 activator resveratrol. Proinflammatory adipocytokines released from dysfunctional perivascular adipose tissue (PVAT) mediate medial calcification and arterial stiffness. Sirt1 ameliorates release of PVAT adipokines and increases adiponectin secretion, which interact with FoxO 1 against oxidative stress and inflammatory arterial insult. Conclusively, Sirt1 decelerates VC by means of influencing endothelial NO bioavailability, senescence of ECs and VSMCs, osteogenic phenotypic transdifferentiation, apoptosis of VSMCs, ECM deposition, and the inflammatory response of PVAT. Factors that aggravate VC include vitamin D deficiency-related macrophage recruitment and further inflammation responses. Supplementation with vitamin D to adequate levels is beneficial in improving PVAT macrophage infiltration and local inflammation, which further prevents VC.

Morphology regulation in vascular endothelial cells

Morphological change in endothelial cells is an initial and crucial step in the process of establishing a functional vascular network. Following or associated with differentiation and proliferation, endothelial cells elongate and assemble into linear cord-like vessels, subsequently forming a perfusable vascular tube. In vivo and in vitro studies have begun to outline the underlying genetic and signaling mechanisms behind endothelial cell morphology regulation. This review focuses on the transcription factors and signaling pathways regulating endothelial cell behavior, involved in morphology, during vascular development.

Inhibition of the PI3K-Akt and mTORC1 signaling pathways promotes the elongation of vascular endothelial cells

Endothelial cell morphology needs to be properly regulated during angiogenesis. Vascular endothelial growth factor (VEGF) induces endothelial cell elongation, which promotes sprouting of pre-existing vessels. However, therapeutic angiogenesis using VEGF has been hampered by side effects such as elevated vascular permeability. Here, we attempted to induce endothelial cell elongation without an overdose of VEGF. By screening a library of chemical inhibitors, we identified phosphatidylinositol 3-kinase (PI3K)-Akt pathway inhibitors and mammalian target of rapamycin complex 1 (mTORC1) inhibitors as potent inducers of endothelial cell elongation. The elongation required VEGF at a low concentration, which was insufficient to elicit the same effect by itself. The elongation also depended on Foxo1, a transcription factor indispensable for angiogenesis. Interestingly, the Foxo1 dependency of the elongation was overridden by inhibition of mTORC1, but not by PI3K-Akt, under stimulation by a high concentration of VEGF. Dual inhibition of mTORC1 and mTORC2 failed to induce cell elongation, revealing mTORC2 as a positive regulator of elongation. Our findings suggest that the PI3K-Akt-Foxo1 and mTORC1-mTORC2 pathways differentially regulate endothelial cell elongation, depending on the microenvironmental levels of VEGF.

VEGF and podocytes in diabetic nephropathy

Vascular endothelial growth factor-A (VEGF-A) is a protein secreted by podocytes that is necessary for survival of endothelial cells, podocytes, and mesangial cells. VEGF-A regulates slit-diaphragm signaling and podocyte shape via VEGF-receptor 2-nephrin-nck-actin interactions. Chronic hyperglycemia-induced excess podocyte VEGF-A and low endothelial nitric oxide drive the development and the progression of diabetic nephropathy. The abnormal cross-talk between VEGF-A and nitric oxide pathways is fueled by the diabetic milieu, resulting in increased oxidative stress. Recent findings on these pathogenic molecular mechanisms provide new potential targets for therapy for diabetic renal disease.

FOXOs and sirtuins in vascular growth, maintenance, and aging

Blood vessels form the first organ in the developing embryo and build extensive networks that supply all cells with nutrients and oxygen throughout life. As blood vessels get older, they often become abnormal in structure and function, thereby contributing to numerous age-associated diseases including ischemic heart and brain disease, neurodegeneration, or cancer. First described as regulators of the aging process in invertebrate model organisms, Forkhead box "O" (FOXO) transcription factors and sirtuin deacetylases are now emerging as key regulators of mammalian vascular development and disease. The integration of individual FOXO and sirtuin family members into various aspects of vessel growth, maintenance, and function provides new perspectives on disease mechanisms of aging, the most important risk factor for medical maladies of the vascular system.

Phosphorylation of AKT: a mutational analysis

Akt (cellular homolog of murine thymoma virus akt8 oncogene) is an essential component of the PI3K (phosphatidylinositol 3-kinase) pathway. Its activity is stimulated by receptor tyrosine kinases and G-protein coupled receptors and plays a critical role in the regulation of cell proliferation, differentiation and apoptosis. A gain of function in Akt can lead to uncontrolled cell proliferation and resistance to apoptosis, both hallmarks of oncogenic transformation. In this communication, we have investigated the phosphorylation at the Akt residues T308, S473 and T450 and their roles in oncogenic transformation and signaling. We find that T450 phosphorylation has only a minimal part in these activities. In contrast, the phosphorylation of T308 and of S473 fulfills essential, distinct, and non-overlapping functions that we define with inactivating and with phosphomimetic mutations of these sites.

Vascular endothelial growth factor receptor 2 direct interaction with nephrin links VEGF-A signals to actin in kidney podocytes

The transmembrane protein nephrin is an essential component of slit diaphragms, the specialized cell junctions that link podocyte foot processes. Podocytes are epithelial cells that surround the glomerular capillaries in the kidney and are necessary for the organ-filtering function. Nephrin signaling complex transduces extracellular cues to the podocyte cytoskeleton and regulates podocyte shape and function. Vascular endothelial growth factor A (VEGF-A) is a required growth factor produced and secreted by podocytes. Accumulating evidence suggests a cross-talk between VEGF-A and nephrin signaling pathways. We previously showed that in vivo nephrin associates with VEGF receptor-2 (VEGFR2), the signaling receptor for VEGF-A. In the present work, we characterized the interaction between nephrin and VEGFR2 in cultured cells and in vitro. We demonstrate that nephrin-VEGFR2 interaction is direct using mass spectrometry, immunoprecipitation, GST-binding assays, and blot overlay experiments. This interaction occurs through VEGFR2 and nephrin cytoplasmic domains. Nephrin-VEGFR2 interaction is modulated by tyrosine phosphorylation of both cytoplasmic domains. Furthermore, the nephrin-VEGFR2 complex involves Nck and actin. VEGF-A signaling via this complex results in decreased cell size. We provide evidence that this multiprotein interaction occurs in cultured podocytes. We propose that the nephrin-VEGFR2 complex acts as a key mediator to transduce local VEGF-A signals to the podocyte actin cytoskeleton, regulating the foot process structure and glomerular filter integrity.

Key transcriptional regulators of early vascular development

The formation of the vasculature depends on the precise spatial and temporal control of gene expression to define endothelial cell identity and to ensure the correct distribution and structure of the forming vessel network. This review provides an overview of the establishment of the vascular system, accompanied by a detailed discussion of the transcription factors involved in regulating endothelial gene expression during vasculogenesis and early vessel formation in both fish and mammalian systems. We also review the transcriptional pathways lying both upstream and downstream of key vascular transcription factors.