Maintenance of adult tissues depends on stem cell self-renewal in local niches. Spermatogonial stem cells (SSC) are germline adult stem cells necessary for spermatogenesis and fertility. We show that testicular endothelial cells (TECs) are part of the SSC niche producing glial cell line-derived neurotrophic factor (GDNF) and other factors to support human and mouse SSCs in long-term culture. We demonstrate that FGF-2 binding to FGFR1 on TECs activates the calcineurin pathway to produce GDNF. Comparison of the TEC secretome to lung and liver endothelial cells identified 5 factors sufficient for long-term maintenance of human and mouse SSC colonies in feeder-free cultures. Male cancer survivors after chemotherapy are often infertile since SSCs are highly susceptible to cytotoxic injury. Transplantation of TECs alone restores spermatogenesis in mice after chemotherapy-induced depletion of SSCs. Identifying TECs as a niche population necessary for SSC self-renewal may facilitate fertility preservation for prepubertal boys diagnosed with cancer.

Tumor-associated endothelial cells (TECs) regulate tumor cell aggressiveness. However, the core mechanism by which TECs confer stem cell-like activity to indolent tumors is unknown. Here, we used in vivo murine and human tumor models to identify the tumor-suppressive checkpoint role of TEC-expressed insulin growth factor (IGF) binding protein-7 (IGFBP7/angiomodulin). During tumorigenesis, IGFBP7 blocks IGF1 and inhibits expansion and aggresiveness of tumor stem-like cells (TSCs) expressing IGF1 receptor (IGF1R). However, chemotherapy triggers TECs to suppress IGFBP7, and this stimulates IGF1R+ TSCs to express FGF4, inducing a feedforward FGFR1-ETS2 angiocrine cascade that obviates TEC IGFBP7. Thus, loss of IGFBP7 and upregulation of IGF1 activates the FGF4-FGFR1-ETS2 pathway in TECs and converts naive tumor cells to chemoresistant TSCs, thereby facilitating their invasiveness and progression.

Chemical or traumatic damage to the liver is frequently associated with aberrant healing (fibrosis) that overrides liver regeneration. The mechanism by which hepatic niche cells differentially modulate regeneration and fibrosis during liver repair remains to be defined. Hepatic vascular niche predominantly represented by liver sinusoidal endothelial cells deploys paracrine trophogens, known as angiocrine factors, to stimulate regeneration. Nevertheless, it is not known how pro-regenerative angiocrine signals from liver sinusoidal endothelial cells is subverted to promote fibrosis. Here, by combining an inducible endothelial-cell-specific mouse gene deletion strategy and complementary models of acute and chronic liver injury, we show that divergent angiocrine signals from liver sinusoidal endothelial cells stimulate regeneration after immediate injury and provoke fibrosis after chronic insult. The pro-fibrotic transition of vascular niche results from differential expression of stromal-derived factor-1 receptors, CXCR7 and CXCR4 (refs 18, 19, 20, 21), in liver sinusoidal endothelial cells. After acute injury, CXCR7 upregulation in liver sinusoidal endothelial cells acts with CXCR4 to induce transcription factor Id1, deploying pro-regenerative angiocrine factors and triggering regeneration. Inducible deletion of Cxcr7 in sinusoidal endothelial cells (Cxcr7(iΔEC/iΔEC)) from the adult mouse liver impaired liver regeneration by diminishing Id1-mediated production of angiocrine factors. By contrast, after chronic injury inflicted by iterative hepatotoxin (carbon tetrachloride) injection and bile duct ligation, constitutive FGFR1 signalling in liver sinusoidal endothelial cells counterbalanced CXCR7-dependent pro-regenerative response and augmented CXCR4 expression. This predominance of CXCR4 over CXCR7 expression shifted angiocrine response of liver sinusoidal endothelial cells, stimulating proliferation of desmin(+) hepatic stellate-like cells and enforcing a pro-fibrotic vascular niche. Endothelial-cell-specific ablation of either Fgfr1 (Fgfr1(iΔEC/iΔEC)) or Cxcr4 (Cxcr4(iΔEC/iΔEC)) in mice restored the pro-regenerative pathway and prevented FGFR1-mediated maladaptive subversion of angiocrine factors. Similarly, selective CXCR7 activation in liver sinusoidal endothelial cells abrogated fibrogenesis. Thus, we demonstrate that in response to liver injury, differential recruitment of pro-regenerative CXCR7-Id1 versus pro-fibrotic FGFR1-CXCR4 angiocrine pathways in vascular niche balances regeneration and fibrosis. These results provide a therapeutic roadmap to achieve hepatic regeneration without provoking fibrosis.

In mammals, the livers regenerate after chemical injury or resection of hepatic lobe by hepatectomy. How liver regeneration is initiated after mass loss remains to be defined. Here, we report that following liver injury, activated platelets deploy SDF-1 and VEGF-A to stimulate CXCR7+ liver sinusoidal endothelial cell (LSEC) and VEGFR1+ myeloid cell, orchestrating hepatic regeneration. After carbon tetrachloride (CCl4) injection or hepatectomy, platelets and CD11b+VEGFR1+ myeloid cells were recruited LSEC, and liver regeneration in both models was impaired in thrombopoietin-deficient (Thpo-/-) mice lacking circulating platelets. This impeded regeneration phenotype was recapitulated in mice with either conditional ablation of Cxcr7 in LSEC (Cxcr7iΔ/iΔ) or Vegfr1 in myeloid cell (Vegfr1lysM/lysM). Both Vegfr1lysM/lysM and Cxcr7iΔ/iΔ mice exhibited suppressed expression of hepatocyte growth factor and Wnt2, two crucial trophogenic angiocrine factors instigating hepatocyte propagation. Of note, administration of recombinant thrombopoietin restored the prohibited liver regeneration in the tested genetic models. As such, our data suggest that platelets and myeloid cells jointly activate the vascular niche to produce pro-regenerative endothelial paracrine/angiocrine factors. Modulating this "hematopoietic-vascular niche" might help to develop regenerative therapy strategy for hepatic disorders.

The lung alveoli regenerate after surgical removal of the left lobe by pneumonectomy (PNX). How this alveolar regrowth/regeneration is initiated remains unknown. We found that platelets trigger lung regeneration by supplying stromal-cell-derived factor-1 (SDF-1, also known as CXCL12). After PNX, activated platelets stimulate SDF-1 receptors CXCR4 and CXCR7 on pulmonary capillary endothelial cells (PCECs) to deploy the angiocrine membrane-type metalloproteinase MMP14, stimulating alveolar epithelial cell (AEC) expansion and neo-alveolarization. In mice lacking platelets or platelet Sdf1, PNX-induced alveologenesis was diminished. Reciprocally, infusion of Sdf1(+/+) but not Sdf1-deficient platelets rescued lung regeneration in platelet-depleted mice. Endothelial-specific ablation of Cxcr4 and Cxcr7 in adult mice similarly impeded lung regeneration. Notably, mice with endothelial-specific Mmp14 deletion exhibited impaired expansion of AECs but not PCECs after PNX, which was not rescued by platelet infusion. Therefore, platelets prime PCECs to initiate lung regeneration, extending beyond their haemostatic contribution. Therapeutic targeting of this haemo-vascular niche could enable regenerative therapy for lung diseases.

Angiocrine factors, such as Notch ligands, supplied by the specialized endothelial cells (ECs) within the bone marrow and splenic vascular niche play an essential role in modulating the physiology of adult hematopoietic stem and progenitor cells (HSPCs). However, the relative contribution of various Notch ligands, specifically jagged-2, to the homeostasis of HSPCs is unknown. Here, we show that under steady state, jagged-2 is differentially expressed in tissue-specific vascular beds, but its expression is induced in hematopoietic vascular niches after myelosuppressive injury. We used mice with EC-specific deletion of the gene encoding jagged-2 (Jag2) to demonstrate that while EC-derived jagged-2 was dispensable for maintaining the capacity of HSPCs to repopulate under steady-state conditions, by activating Notch2 it did contribute to the recovery of HSPCs in response to myelosuppressive conditions. Engraftment and/or expansion of HSPCs was dependent on the expression of endothelial-derived jagged-2 following myeloablation. Additionally, jagged-2 expressed in bone marrow ECs regulated HSPC cell cycle and quiescence during regeneration. Endothelial-deployed jagged-2 triggered Notch2/Hey1, while tempering Notch2/Hes1 signaling in HSPCs. Collectively, these data demonstrate that EC-derived jagged-2 activates Notch2 signaling in HSPCs to promote hematopoietic recovery and has potential as a therapeutic target to accelerate balanced hematopoietic reconstitution after myelosuppression.

During embryogenesis, endothelial cells induce organogenesis before the development of circulation. These findings suggest that endothelial cells not only form passive conduits to deliver nutrients and oxygen, but also establish an instructive vascular niche, which through elaboration of paracrine trophogens stimulates organ regeneration, in a manner similar to endothelial-cell-derived angiocrine factors that support haematopoiesis. However, the precise mechanism by which tissue-specific subsets of endothelial cells promote organogenesis in adults is unknown. Here we demonstrate that liver sinusoidal endothelial cells (LSECs) constitute a unique population of phenotypically and functionally defined VEGFR3(+)CD34(-)VEGFR2(+)VE-cadherin(+)FactorVIII(+)CD45(-) endothelial cells, which through the release of angiocrine trophogens initiate and sustain liver regeneration induced by 70% partial hepatectomy. After partial hepatectomy, residual liver vasculature remains intact without experiencing hypoxia or structural damage, which allows study of physiological liver regeneration. Using this model, we show that inducible genetic ablation of vascular endothelial growth factor (VEGF)-A receptor-2 (VEGFR2) in the LSECs impairs the initial burst of hepatocyte proliferation (days 1-3 after partial hepatectomy) and subsequent reconstitution of the hepatovascular mass (days 4-8 after partial hepatectomy) by inhibiting upregulation of the endothelial-cell-specific transcription factor Id1. Accordingly, Id1-deficient mice also manifest defects throughout liver regeneration, owing to diminished expression of LSEC-derived angiocrine factors, including hepatocyte growth factor (HGF) and Wnt2. Notably, in in vitro co-cultures, VEGFR2-Id1 activation in LSECs stimulates hepatocyte proliferation. Indeed, intrasplenic transplantation of Id1(+/+) or Id1(-/-) LSECs transduced with Wnt2 and HGF (Id1(-/-)Wnt2(+)HGF(+) LSECs) re-establishes an inductive vascular niche in the liver sinusoids of the Id1(-/-) mice, initiating and restoring hepatovascular regeneration. Therefore, in the early phases of physiological liver regeneration, VEGFR2-Id1-mediated inductive angiogenesis in LSECs through release of angiocrine factors Wnt2 and HGF provokes hepatic proliferation. Subsequently, VEGFR2-Id1-dependent proliferative angiogenesis reconstitutes liver mass. Therapeutic co-transplantation of inductive VEGFR2(+)Id1(+)Wnt2(+)HGF(+) LSECs with hepatocytes provides an effective strategy to achieve durable liver regeneration.

Here, we show that the liver-derived apolipoprotein M (ApoM) protects the lung and kidney from pro-fibrotic insults and that this circulating factor is attenuated in aged mice. Aged mouse hepatocytes exhibit transcriptional suppression of ApoM. This leads to reduced sphingosine-1-phosphate (S1P) signaling via the S1P receptor 1 (S1PR1) in the vascular endothelial cells of lung and kidney. Suboptimal S1PR1 angiocrine signaling causes reduced resistance to injury-induced vascular leak and leads to organ fibrosis. Plasma transfusion from Apom transgenic mice but not Apom knockout mice blocked fibrosis in the lung. Similarly, infusion of recombinant therapeutics, ApoM-Fc fusion protein enhanced kidney and lung regeneration and attenuated fibrosis in aged mouse after injury. Furthermore, we identified that aging alters Sirtuin-1-hepatic nuclear factor 4α circuit in hepatocytes to downregulate ApoM. These data reveal an integrative organ adaptation that involves circulating S1P chaperone ApoM+ high density lipoprotein (HDL), which signals via endothelial niche S1PR1 to spur regeneration over fibrosis.

The lung's unique extracellular matrix (ECM), while providing structural support for cells, is critical in the regulation of developmental organogenesis, homeostasis and injury-repair responses. The ECM, via biochemical or biomechanical cues, regulates diverse cell functions, fate and phenotype. The composition and function of lung ECM become markedly deranged in pathological tissue remodeling. ECM-based therapeutics and bioengineering approaches represent promising novel strategies for regeneration/repair of the lung and treatment of chronic lung diseases. In this review, we assess the current state of lung ECM biology, including fundamental advances in ECM composition, dynamics, topography, and biomechanics; the role of the ECM in normal and aberrant lung development, adult lung diseases and autoimmunity; and ECM in the regulation of the stem cell niche. We identify opportunities to advance the field of lung ECM biology and provide a set recommendations for research priorities to advance knowledge that would inform novel approaches to the pathogenesis, diagnosis, and treatment of chronic lung diseases.