T lymphocytes negatively regulate lymph node lymphatic vessel formation.

Lymph node lymphatic vessels (LNLVs) serve as a conduit to drain antigens from peripheral tissues to within the lymph nodes. LNLV density is known to be positively regulated by vascular endothelial growth factors secreted by B cells, macrophages, and dendritic cells (DCs). Here, we show that LNLV formation was negatively regulated by T cells. In both steady and inflammatory states, the density of LNLVs was increased in the absence of T cells but decreased when T cells were restored. Interferon-γ secretion by T cells suppressed lymphatic-specific genes in lymphatic endothelial cells and consequently caused marked reduction in LNLV formation. When T cells were depleted, recruitment of antigen-carrying DCs to LNs was augmented, reflecting a compensatory mechanism for antigen presentation to T cells through increased LNLVs. Thus, T cells maintain the homeostatic balance of LNLV density through a negative paracrine action of interferon-γ.

SoxF Transcription Factors Are Positive Feedback Regulators of VEGF Signaling.

RATIONALE: Vascular endothelial growth factor (VEGF) signaling is a key pathway for angiogenesis and requires highly coordinated regulation. Although the Notch pathway-mediated suppression of excessive VEGF activity via negative feedback is well known, the positive feedback control for augmenting VEGF signaling remains poorly understood. Transcription factor Sox17 is indispensable for angiogenesis, but its association with VEGF signaling is largely unknown. The contribution of other Sox members to angiogenesis also remains to be determined. OBJECTIVE: To reveal the genetic interaction of Sox7, another Sox member, with Sox17 in developmental angiogenesis and their functional relationship with VEGF signaling. METHODS AND RESULTS: Sox7 is expressed specifically in endothelial cells and its global and endothelial-specific deletion resulted in embryonic lethality with severely impaired angiogenesis in mice, substantially overlapping with Sox17 in both expression and function. Interestingly, compound heterozygosity for Sox7 and Sox17 phenocopied vascular defects of Sox7 or Sox17 homozygous knockout, indicating that the genetic cooperation of Sox7 and Sox17 is sensitive to their combined gene dosage. VEGF signaling upregulated both Sox7 and Sox17 expression in angiogenesis via mTOR pathway. Furthermore, Sox7 and Sox17 promoted VEGFR2 (VEGF receptor 2) expression in angiogenic vessels, suggesting a positive feedback loop between VEGF signaling and SoxF. CONCLUSIONS: Our findings demonstrate that SoxF transcription factors are indispensable players in developmental angiogenesis by acting as positive feedback regulators of VEGF signaling.

The pro-metastatic role of bone marrow-derived cells: a focus on MSCs and regulatory T cells.

Several bone marrow-derived cells have been shown to promote tumour growth and progression. These cells can home to the primary tumour and become active components of the tumour microenvironment. Recent studies have also identified bone marrow-derived cells­such as mesenchymal stem cells and regulatory T cells­as contributors to cancer metastasis. The innate versatility of these cells provides diverse functional aid to promote malignancy, ranging from structural support to signal-mediated suppression of the host immune response. Here, we review the role of mesenchymal stem cells and regulatory T cells in cancer metastasis. A better understanding of the bipolar nature of these bone marrow-derived cells in physiological and malignant contexts could pave the way for new therapeutics against metastatic disease.

Angiogenesis is uncoupled from osteogenesis during calvarial bone regeneration.

Bone regeneration requires a well-orchestrated cellular and molecular response including robust vascularization and recruitment of mesenchymal and osteogenic cells. In femoral fractures, angiogenesis and osteogenesis are closely coupled during the complex healing process. Here, we show with advanced longitudinal intravital multiphoton microscopy that early vascular sprouting is not directly coupled to osteoprogenitor invasion during calvarial bone regeneration. Early osteoprogenitors emerging from the periosteum give rise to bone-forming osteoblasts at the injured calvarial bone edge. Microvessels growing inside the lesions are not associated with osteoprogenitors. Subsequently, osteogenic cells collectively invade the vascularized and perfused lesion as a multicellular layer, thereby advancing regenerative ossification. Vascular sprouting and remodeling result in dynamic blood flow alterations to accommodate the growing bone. Single cell profiling of injured calvarial bones demonstrates mesenchymal stromal cell heterogeneity comparable to femoral fractures with increase in cell types promoting bone regeneration. Expression of angiogenesis and hypoxia-related genes are slightly elevated reflecting ossification of a vascularized lesion site. Endothelial Notch and VEGF signaling alter vascular growth in calvarial bone repair without affecting the ossification progress. Our findings may have clinical implications for bone regeneration and bioengineering approaches.

Stromal vascular fraction from adipose tissue forms profound vascular network through the dynamic reassembly of blood endothelial cells.

OBJECTIVE: Tremendous efforts have been made to establish effective therapeutic neovascularization using adipose tissue-derived stromal vascular fraction (SVF), but the efficiency is low, and underlying mechanisms and their interaction with the host in a new microenvironment are poorly understood. METHODS AND RESULTS: Here we demonstrate that direct implantation of SVF derived from donor adipose tissue can create a profound vascular network through the disassembly and reassembly of blood endothelial cells at the site of implantation. This neovasculature successfully established connection with recipient blood vessels to form a functionally perfused circuit. Addition of vascular growth factors to the SVF implant improved the efficiency of functional neovasculature formation. In contrast, spheroid culture of SVF before implantation reduced the capacity of vasculature formation, possibly because of cellular alteration. Implanting SVF into the mouse ischemic hindlimb induced the robust formation of a local neovascular network and salvaged the limb. Moreover, the coimplantation of SVF prevented fat absorption in the subcutaneous adipose tissue graft model. CONCLUSIONS: Freshly isolated SVF can effectively induce new vessel formation through the dynamic reassembly of blood endothelial cells and could be applied to achieve therapeutic neovascularization for relieving ischemia and preventing fat absorption in an autologous manner.

In vivo observation of multi-phase spatiotemporal cellular dynamics of transplanted HSPCs during early engraftment.

Hematopoietic stem cell transplantation (HSCT) is commonly used to treat patients with various blood disorders, genetic and immunological diseases, and solid tumors. Several systemic complications following HSCT are critical limiting factors for achieving a successful outcome. These systemic complications are mainly due to the lack of initial engraftment after transplantation. However, the detailed underlying cellular dynamics of early engraftment have not been fully characterized yet. We performed in vivo longitudinal visualization of early engraftment characteristics of transplanted hematopoietic stem and progenitor cells (HSPCs) in the mouse calvarial bone marrow (BM). To achieve this, we utilized an in vivo laser-scanning confocal microscopy imaging system with a cranial BM imaging window and stereotaxic device. We observed two distinct cellular behaviors of HSPCs in vivo, cluster formation and cluster dissociation, early after transplantation. Furthermore, we successfully identified three cellular phases of engraftment with distinct cellular distances which are coordinated with cell proliferation and cell migration dynamics during initial engraftment.

A specialized bone marrow microenvironment for fetal haematopoiesis.

In adult mammalian bone marrow (BM), vascular endothelial cells and perivascular reticular cells control the function of haematopoietic stem and progenitor cells (HSPCs). During fetal development, the mechanisms regulating the de novo haematopoietic cell colonization of BM remain largely unknown. Here, we show that fetal and adult BM exhibit fundamental differences in cellular composition and molecular interactions by single cell RNA sequencing. While fetal femur is largely devoid of leptin receptor-expressing cells, arterial endothelial cells (AECs) provide Wnt ligand to control the initial HSPC expansion. Haematopoietic stem cells and c-Kit+ HSPCs are reduced when Wnt secretion by AECs is genetically blocked. We identify Wnt2 as AEC-derived signal that activates ß-catenin-dependent proliferation of fetal HSPCs. Treatment of HSPCs with Wnt2 promotes their proliferation and improves engraftment after transplantation. Our work reveals a fundamental switch in the cellular organization and molecular regulation of BM niches in the embryonic and adult organism.

VEGFR2 signaling drives meningeal vascular regeneration upon head injury.

Upon severe head injury (HI), blood vessels of the meninges and brain parenchyma are inevitably damaged. While limited vascular regeneration of the injured brain has been studied extensively, our understanding of meningeal vascular regeneration following head injury is quite limited. Here, we identify key pathways governing meningeal vascular regeneration following HI. Rapid and complete vascular regeneration in the meninges is predominantly driven by VEGFR2 signaling. Substantial increase of VEGFR2 is observed in both human patients and mouse models of HI, and endothelial cell-specific deletion of Vegfr2 in the latter inhibits meningeal vascular regeneration. We further identify the facilitating, stabilizing and arresting roles of Tie2, PDGFRß and Dll4 signaling, respectively, in meningeal vascular regeneration. Prolonged inhibition of this angiogenic process following HI compromises immunological and stromal integrity of the injured meninges. These findings establish a molecular framework for meningeal vascular regeneration after HI, and may guide development of wound healing therapeutics.

Generation of PDGFRα+ Cardioblasts from Pluripotent Stem Cells.

Isolating actively proliferating cardioblasts is the first crucial step for cardiac regeneration through cell implantation. However, the origin and identity of putative cardioblasts are still unclear. Here, we uncover a novel class of cardiac lineage cells, PDGFRα+Flk1- cardioblasts (PCBs), from mouse and human pluripotent stem cells induced using CsAYTE, a combination of the small molecules Cyclosporin A, the rho-associated coiled-coil kinase inhibitor Y27632, the antioxidant Trolox, and the ALK5 inhibitor EW7197. This novel population of actively proliferating cells is cardiac lineage-committed but in a morphologically and functionally immature state compared to mature cardiomyocytes. Most important, most of CsAYTE-induced PCBs spontaneously differentiated into functional αMHC+ cardiomyocytes (M+CMs) and could be a potential cellular resource for cardiac regeneration.

Normal and cancerous mammary stem cells evade interferon-induced constraint through the miR-199a-LCOR axis.

Tumour-initiating cells, or cancer stem cells (CSCs), possess stem-cell-like properties observed in normal adult tissue stem cells. Normal and cancerous stem cells may therefore share regulatory mechanisms for maintaining self-renewing capacity and resisting differentiation elicited by cell-intrinsic or microenvironmental cues. Here, we show that miR-199a promotes stem cell properties in mammary stem cells and breast CSCs by directly repressing nuclear receptor corepressor LCOR, which primes interferon (IFN) responses. Elevated miR-199a expression in stem-cell-enriched populations protects normal and malignant stem-like cells from differentiation and senescence induced by IFNs that are produced by epithelial and immune cells in the mammary gland. Importantly, the miR-199a-LCOR-IFN axis is activated in poorly differentiated ER- breast tumours, functionally promotes tumour initiation and metastasis, and is associated with poor clinical outcome. Our study therefore reveals a common mechanism shared by normal and malignant stem cells to protect them from suppressive immune cytokine signalling.

Endogenous bone marrow MSCs are dynamic, fate-restricted participants in bone maintenance and regeneration.

Mesenchymal stem cells (MSCs) commonly defined by in vitro functions have entered clinical application despite little definition of their function in residence. Here, we report genetic pulse-chase experiments that define osteoblastic cells as short-lived and nonreplicative, requiring replenishment from bone-marrow-derived, Mx1(+) stromal cells with "MSC" features. These cells respond to tissue stress and migrate to sites of injury, supplying new osteoblasts during fracture healing. Single cell transplantation yielded progeny that both preserve progenitor function and differentiate into osteoblasts, producing new bone. They are capable of local and systemic translocation and serial transplantation. While these cells meet current definitions of MSCs in vitro, they are osteolineage restricted in vivo in growing and adult animals. Therefore, bone-marrow-derived MSCs may be a heterogeneous population with the Mx1(+) population, representing a highly dynamic and stress responsive stem/progenitor cell population of fate-restricted potential that feeds the high cell replacement demands of the adult skeleton.

CXCR4 signaling regulates metastasis of chemoresistant melanoma cells by a lymphatic metastatic niche.

Highly metastatic and chemotherapy-resistant properties of malignant melanomas stand as challenging barriers to successful treatment; yet, the mechanisms responsible for their aggressive characteristics are not fully defined. We show that a distinct population expressing CD133 (Prominin-1), which is highly enriched after administration of a chemotherapeutic drug, dacarbazine, has enhanced metastatic potential in vivo. CD133(+) tumor cells are located close to tumor-associated lymphatic vessels in metastatic organs such as the regional lymph nodes and lung. Lymphatic endothelial cells promote the migratory activity of a CD133(+) subset to target organs and regulation of lymphatic growth efficiently modulates the metastasis of CD133(+) tumor cells. We found that lymphatic vessels in metastatic tissues stimulate chemokine receptor 4 (CXCR4)(+)/CD133(+) cell metastasis to target organs by secretion of stromal cell-derived factor-1 (SDF-1). The CXCR4(+)/CD133(+) cells exhibited higher metastatic activity compared with CXCR4(-)/CD133(+) cells and, importantly, blockade of CXCR4 coupled with dacarbazine efficiently inhibited both tumor growth and metastasis; dacarbazine alone could not attenuate tumor metastasis. The current study demonstrates a previously unidentified role of the lymphatic microenvironment in facilitating metastasis of chemoresistant melanoma cells via a specific chemotactic axis, SDF-1/CXCR4. Our findings suggest that targeting the SDF-1/CXCR4 axis in addition to dacarbazine treatment could therapeutically block chemoresistant CD133(+) cell metastasis toward a lymphatic metastatic niche.