Amino Acid Restriction Triggers Angiogenesis via GCN2/ATF4 Regulation of VEGF and H2S Production.

Angiogenesis, the formation of new blood vessels by endothelial cells (ECs), is an adaptive response to oxygen/nutrient deprivation orchestrated by vascular endothelial growth factor (VEGF) upon ischemia or exercise. Hypoxia is the best-understood trigger of VEGF expression via the transcription factor HIF1α. Nutrient deprivation is inseparable from hypoxia during ischemia, yet its role in angiogenesis is poorly characterized. Here, we identified sulfur amino acid restriction as a proangiogenic trigger, promoting increased VEGF expression, migration and sprouting in ECs in vitro, and increased capillary density in mouse skeletal muscle in vivo via the GCN2/ATF4 amino acid starvation response pathway independent of hypoxia or HIF1α. We also identified a requirement for cystathionine-γ-lyase in VEGF-dependent angiogenesis via increased hydrogen sulfide (H2S) production. H2S mediated its proangiogenic effects in part by inhibiting mitochondrial electron transport and oxidative phosphorylation, resulting in increased glucose uptake and glycolytic ATP production.

Three-dimensional biomimetic vascular model reveals a RhoA, Rac1, and N-cadherin balance in mural cell-endothelial cell-regulated barrier function.

The integrity of the endothelial barrier between circulating blood and tissue is important for blood vessel function and, ultimately, for organ homeostasis. Here, we developed a vessel-on-a-chip with perfused endothelialized channels lined with human bone marrow stromal cells, which adopt a mural cell-like phenotype that recapitulates barrier function of the vasculature. In this model, barrier function is compromised upon exposure to inflammatory factors such as LPS, thrombin, and TNFα, as has been observed in vivo. Interestingly, we observed a rapid physical withdrawal of mural cells from the endothelium that was accompanied by an inhibition of endogenous Rac1 activity and increase in RhoA activity in the mural cells themselves upon inflammation. Using a system to chemically induce activity in exogenously expressed Rac1 or RhoA within minutes of stimulation, we demonstrated RhoA activation induced loss of mural cell coverage on the endothelium and reduced endothelial barrier function, and this effect was abrogated when Rac1 was simultaneously activated. We further showed that N-cadherin expression in mural cells plays a key role in barrier function, as CRISPR-mediated knockout of N-cadherin in the mural cells led to loss of barrier function, and overexpression of N-cadherin in CHO cells promoted barrier function. In summary, this bicellular model demonstrates the continuous and rapid modulation of adhesive interactions between endothelial and mural cells and its impact on vascular barrier function and highlights an in vitro platform to study the biology of perivascular-endothelial interactions.

HPV16 E6/E7 -based mRNA vaccine is therapeutic in mice bearing aggressive HPV-positive lesions.

HPV (Human papillomavirus) affects 600,000 people worldwide each year. Almost all cervical cancers are associated with a past HPV infection. In particular, the positivity to the high-risk type HPV16 is detected in most of the invasive cervical cancers. FDA has approved prophylactic vaccines that protect against new HPV16 infections, but do not induce immunity in those patients with established infections or neoplasms. To date, no therapeutic vaccine targeting HPV16-associated lesions has been authorized. We have developed an mRNA-based vaccine against the HPV16 late oncoproteins E6 and E7, which are abundantly and exclusively expressed in high-grade squamous intraepithelial lesions (HSILs), a stage of the cervical disease that precedes the progression to carcinoma. Our in vitro and in vivo studies demonstrated that the translated mRNA is functional and elicits an antigen-specific adaptive immune response. Upon immunization with the vaccine, mice with HPV16+ lesions exhibited tumor growth inhibition, extension of lifespan, and development of a protective immune memory. In light of these results and the remarkable clinical success of mRNA vaccines against SARS-CoV2, we believe that our mRNA-based therapeutic vaccine has the potential to offer a non-invasive treatment alternative to the current standard of care for HPV16+ HSILs.

Improvement of a synthetic live bacterial therapeutic for phenylketonuria with biosensor-enabled enzyme engineering.

In phenylketonuria (PKU) patients, a genetic defect in the enzyme phenylalanine hydroxylase (PAH) leads to elevated systemic phenylalanine (Phe), which can result in severe neurological impairment. As a treatment for PKU, Escherichia coli Nissle (EcN) strain SYNB1618 was developed under Synlogic's Synthetic Biotic™ platform to degrade Phe from within the gastrointestinal (GI) tract. This clinical-stage engineered strain expresses the Phe-metabolizing enzyme phenylalanine ammonia lyase (PAL), catalyzing the deamination of Phe to the non-toxic product trans-cinnamate (TCA). In the present work, we generate a more potent EcN-based PKU strain through optimization of whole cell PAL activity, using biosensor-based high-throughput screening of mutant PAL libraries. A lead enzyme candidate from this screen is used in the construction of SYNB1934, a chromosomally integrated strain containing the additional Phe-metabolizing and biosafety features found in SYNB1618. Head-to-head, SYNB1934 demonstrates an approximate two-fold increase in in vivo PAL activity compared to SYNB1618.

Myosin IIA-mediated forces regulate multicellular integrity during vascular sprouting.

Angiogenic sprouting is a critical process involved in vascular network formation within tissues. During sprouting, tip cells and ensuing stalk cells migrate collectively into the extracellular matrix while preserving cell-cell junctions, forming patent structures that support blood flow. Although several signaling pathways have been identified as controlling sprouting, it remains unclear to what extent this process is mechanoregulated. To address this question, we investigated the role of cellular contractility in sprout morphogenesis, using a biomimetic model of angiogenesis. Three-dimensional maps of mechanical deformations generated by sprouts revealed that mainly leader cells, not stalk cells, exert contractile forces on the surrounding matrix. Surprisingly, inhibiting cellular contractility with blebbistatin did not affect the extent of cellular invasion but resulted in cell-cell dissociation primarily between tip and stalk cells. Closer examination of cell-cell junctions revealed that blebbistatin impaired adherens-junction organization, particularly between tip and stalk cells. Using CRISPR/Cas9-mediated gene editing, we further identified NMIIA as the major isoform responsible for regulating multicellularity and cell contractility during sprouting. Together, these studies reveal a critical role for NMIIA-mediated contractile forces in maintaining multicellularity during sprouting and highlight the central role of forces in regulating cell-cell adhesions during collective motility.

Pericyte ALK5/TIMP3 Axis Contributes to Endothelial Morphogenesis in the Developing Brain.

The murine embryonic blood-brain barrier (BBB) consists of endothelial cells (ECs), pericytes (PCs), and basement membrane. Although PCs are critical for inducing vascular stability, signaling pathways in PCs that regulate EC morphogenesis during BBB development remain unexplored. Herein, we find that murine embryos lacking the transforming growth factor ß (TGF-ß) receptor activin receptor-like kinase 5 (Alk5) in brain PCs (mutants) develop gross germinal matrix hemorrhage-intraventricular hemorrhage (GMH-IVH). The germinal matrix (GM) is a highly vascularized structure rich in neuronal and glial precursors. We show that GM microvessels of mutants display abnormal dilation, reduced PC coverage, EC hyperproliferation, reduced basement membrane collagen, and enhanced perivascular matrix metalloproteinase activity. Furthermore, ALK5-depleted PCs downregulate tissue inhibitor of matrix metalloproteinase 3 (TIMP3), and TIMP3 administration to mutants improves endothelial morphogenesis and attenuates GMH-IVH. Overall, our findings reveal a key role for PC ALK5 in regulating brain endothelial morphogenesis and a substantial therapeutic potential for TIMP3 during GMH-IVH.

In situ expansion of engineered human liver tissue in a mouse model of chronic liver disease.

Control of both tissue architecture and scale is a fundamental translational roadblock in tissue engineering. An experimental framework that enables investigation into how architecture and scaling may be coupled is needed. We fabricated a structurally organized engineered tissue unit that expanded in response to regenerative cues after implantation into mice with liver injury. Specifically, we found that tissues containing patterned human primary hepatocytes, endothelial cells, and stromal cells in a degradable hydrogel expanded more than 50-fold over the course of 11 weeks in mice with injured livers. There was a concomitant increase in graft function as indicated by the production of multiple human liver proteins. Histologically, we observed the emergence of characteristic liver stereotypical microstructures mediated by coordinated growth of hepatocytes in close juxtaposition with a perfused vasculature. We demonstrated the utility of this system for probing the impact of multicellular geometric architecture on tissue expansion in response to liver injury. This approach is a hybrid strategy that harnesses both biology and engineering to more efficiently deploy a limited cell mass after implantation.

Decellularized matrices for cardiovascular tissue engineering.

Cardiovascular disease (CVD) is one of the leading causes of death in the Western world. The replacement of damaged vessels and valves has been practiced since the 1950's. Synthetic grafts, usually made of bio-inert materials, are long-lasting and mechanically relevant, but fail when it comes to "biointegration". Decellularized matrices, instead, can be considered biological grafts capable of stimulating in vivo migration and proliferation of endothelial cells (ECs), recruitment and differentiation of mural cells, finally, culminating in the formation of a biointegrated tissue. Decellularization protocols employ osmotic shock, ionic and non-ionic detergents, proteolitic digestions and DNase/RNase treatments; most of them effectively eliminate the cellular component, but show limitations in preserving the native structure of the extracellular matrix (ECM). In this review, we examine the current state of the art relative to decellularization techniques and biological performance of decellularized heart, valves and big vessels. Furthermore, we focus on the relevance of ECM components, native and resulting from decellularization, in mediating in vivo host response and determining repair and regeneration, as opposed to graft corruption.

Amniotic fluid stem cells in a bone microenvironment: driving host angiogenic response.

The repair of skeletal defects remains a substantial economic and biomedical burden. The extra-embryonic fetal stem cells derived from amniotic fluid (AFSCs) have been used for the treatment of large bone defects, but mechanisms of repair are not clear. Here we studied the potential contribution of human AFSCs to the modeling of an ectopic bone. We found that AFSCs are not osteogenic in vivo, and, compared to bone marrow-derived stromal cells, recruit more host CD31 and VEGF-R2 positive cells. Finally, when AFSCs were co-implanted with human-bone forming cells, a normo-osteosynthesis occurred, the engineered ossicle was hyper-vascularized, but AFSCs were not retrieved in the implant within 2weeks. We concluded that AFSCs do not contribute to the deposition of new bone, but act as a powerful proinflammatory/proangiogenic boost, driving a host response, ending in AFSC clearance and vascularization of the bone environment. In our model, a source of osteocommitted cells, capable to engraft and proliferate in vivo, is needed in order to engineer bone. The angio-attractant properties of AFSCs could be exploited in strategies of endogenous cell homing to actively recruit host progenitors into a predefined anatomic location for in situ bone tissue regeneration.

Proangiogenic soluble factors from amniotic fluid stem cells mediate the recruitment of endothelial progenitors in a model of ischemic fasciocutaneous flap.

Skin flaps are routinely used in surgery for the functional and cosmetic repair of wounds or disfiguring scars. The recent concept of therapeutic angiogenesis has emerged as an attractive approach to overcome the problem of blood supply deficiency, often resulting in the flap grafting failure. In the present study, we embedded a gelatin membrane with amniotic fluid stem cells (AFSC) derived conditioned media (ACM) to topically deliver angiogenic growth factors and cytokines into a rat model of ischemic full-thickness skin flap elevated in the epigastric region. AFSC secretome triggered the endogenous repair by the recruitment of endothelial progenitor cells. We studied the vascular perfusion rate, the vessel distribution, and the survival of ACM-treated flaps. In detail, the ischemic sectors of ACM-treated flaps showed at day 7 a perfusion level 50% higher than the preoperation baseline. The ensuing necrosis development was delayed and the histology analysis showed a normal arrangement of epidermal and dermal structures and a high density of vessels in subcutaneous tissues. Further, we found that ACM recruited CD31⁺/VEGFR2⁺ and CD31⁺/CD34⁺ cells into the ischemic subcutaneous tissues and that the isolated progenitors were capable to form clusters of von Willebrand factor-positive cells in culture. We propose ACM as a cell-free cocktail of chemokines and growth factors to be adopted for clinical applications.

Amniotic liquid derived stem cells as reservoir of secreted angiogenic factors capable of stimulating neo-arteriogenesis in an ischemic model.

Most urgent health problems are related to a blood vessel formation failure. The use of stem cells from different sources or species for both in vitro and in vivo engineering of endothelium does not necessarily imply their direct commitment towards a vascular phenotype. In the present study, we used human amniotic fluid stem cells (AFSC) to evoke a strong angiogenic response in murine recipients, in terms of host guided-regeneration of new vessels, and we demonstrated that the AFSC secretome is responsible for the vascularising properties of these cells. We indentified in AFSC conditioned media (ACM) pro-angiogenic soluble factors, such as MCP-1, IL-8, SDF-1, VEGF. Our in vitro results suggest that ACM are cytoprotective, pro-differentiative and chemoattractive for endothelial cells. We also tested ACM on a pre-clinical model of hind-limb ischemic mouse, concluding that ACM contain mediators that promote the neo-arteriogenesis, as remodelling of pre-existing collateral arteries to conductance vessels, thus preventing the capillary loss and the tissue necrosis of distal muscles. In line with the current regenerative medicine trend, in the present study we assert the concept that stem cell-secreted mediators can guide the tissue repair by stimulating or recruiting host reparative cells.

Recruitment of host's progenitor cells to sites of human amniotic fluid stem cells implantation.

The amniotic fluid is a new source of multipotent stem cells with a therapeutic potential for human diseases. Cultured at low cell density, human amniotic fluid stem cells (hAFSCs) were still able to generate colony-forming unit-fibroblast (CFU-F) after 60 doublings, thus confirming their staminal nature. Moreover, after extensive in vitro cell expansion hAFSCs maintained a stable karyotype. The expression of genes, such as SSEA-4, SOX2 and OCT3/4 was confirmed at early and later culture stage. Also, hAFSCs showed bright expression of mesenchymal lineage markers and immunoregulatory properties. hAFSCs, seeded onto hydroxyapatite scaffolds and subcutaneously implanted in nude mice, played a pivotal role in mounting a response resulting in the recruitment of host's progenitor cells forming tissues of mesodermal origin such as fat, muscle, fibrous tissue and immature bone. Implanted hAFSCs migrated from the scaffold to the skin overlying implant site but not to other organs. Given their in vivo: (i) recruitment of host progenitor cells, (ii) homing towards injured sites and (iii) multipotentiality in tissue repair, hAFSCs are a very appealing reserve of stem cells potentially useful for clinical application in regenerative medicine.