Biological approaches to ischemic tissue repair: gene- and cell-based strategies

Oroxylin a Attenuates Limb Ischemia by Promoting Angiogenesis via Modulation of Endothelial Cell Migration

Oroxylin A (OA) has been shown to simultaneously increase coronary flow and provide a strong anti-inflammatory effect. In this study, we described the angiogenic properties of OA. OA treatment accelerated perfusion recovery, reduced tissue injury, and promoted angiogenesis after hindlimb ischemia (HLI). In addition, OA regulated the secretion of multiple cytokines, including vascular endothelial growth factor A (VEGFA), angiopoietin-2 (ANG-2), fibroblast growth factor-basic (FGF-2), and platelet derived growth factor BB (PDGF-BB). Specifically, those multiple cytokines were involved in cell migration, cell population proliferation, and angiogenesis. These effects were observed at 3, 7, and 14 days after HLI. In skeletal muscle cells, OA promoted the release of VEGFA and ANG-2. After OA treatment, the conditioned medium derived from skeletal muscle cells was found to significantly induce endothelial cell (EC) proliferation. OA also induced EC migration by activating the Ras homolog gene family member A (RhoA)/Rho-associated coiled-coil kinase 2 (ROCK-II) signaling pathway and the T-box20 (TBX20)/prokineticin 2 (PROK2) signaling pathway. In addition, OA was able to downregulate the number of macrophages and neutrophils, along with the secretion of interleukin-1β, at 3 days after HLI. These results expanded current knowledge about the beneficial effects of OA in angiogenesis and blood flow recovery. This research could open new directions for the development of novel therapeutic intervention for patients with peripheral artery disease (PAD).

Transplantation of CD34+ cells for myocardial ischemia

CD34+ cells are multipotent hematopoietic stem cells also known as endothelial progenitor cells and are useful in regenerative medicine. Naturally, these cells are mobilized from the bone marrow into peripheral circulation in response to ischemic tissue injury. CD34+ cells are known for their high proliferative and differentiation capacities that play a crucial role in the repair process of myocardial damage. They have an important paracrine activity in secreting factors to stimulate vasculogenesis, reduce endothelial cells and cardiomyocytes apoptosis, remodel extracellular matrix and activate additional progenitor cells. Once they migrate to the target site, they enhance angiogenesis, neovascularization and tissue regeneration. Several trials have demonstrated the safety and efficacy of CD34+ cell therapy in different settings, such as peripheral limb ischemia, stroke and cardiovascular disease. Herein, we review the potential utility of CD34+ cell transplantation in acute myocardial infarction, refractory angina and ischemic heart failure.

Mechanisms Underlying Cell Therapy in Liver Fibrosis: An Overview

Fibrosis is a common feature in most pathogenetic processes in the liver, and usually results from a chronic insult that depletes the regenerative capacity of hepatocytes and activates multiple inflammatory pathways, recruiting resident and circulating immune cells, endothelial cells, non-parenchymal hepatic stellate cells, and fibroblasts, which become activated and lead to excessive extracellular matrix accumulation. The ongoing development of liver fibrosis results in a clinically silent and progressive loss of hepatocyte function, demanding the constant need for liver transplantation in clinical practice, and motivating the search for other treatments as the chances of obtaining compatible viable livers become scarcer. Although initially cell therapy has emerged as a plausible alternative to organ transplantation, many factors still challenge the establishment of this technique as a main or even additional therapeutic tool. Herein, the authors discuss the most recent advances and point out the corners and some controversies over several protocols and models that have shown promising results as potential candidates for cell therapy for liver fibrosis, presenting the respective mechanisms proposed for liver regeneration in each case.

Endogenous Sonic Hedgehog limits inflammation and angiogenesis in the ischaemic skeletal muscle of mice

Aims

Hedgehog (Hh) signalling has been shown to be re-activated in ischaemic tissues and participate in ischaemia-induced angiogenesis. Sonic Hedgehog (Shh) is upregulated by more than 80-fold in the ischaemic skeletal muscle, however its specific role in ischaemia-induced angiogenesis has not yet been fully investigated. The purpose of the present study was to investigate the role of endogenous Shh in ischaemia-induced angiogenesis.

Methods and results

To this aim, we used inducible Shh knock-out (KO) mice and unexpectedly found that capillary density was significantly increased in re-generating muscle of Shh deficient mice 5 days after hind limb ischaemia was induced, demonstrating that endogenous Shh does not promote angiogenesis but more likely limits it. Myosin and MyoD expression were equivalent in Shh deficient mice and control mice, indicating that endogenous Shh is not required for ischaemia-induced myogenesis. Additionally, we observed a significant increase in macrophage infiltration in the ischaemic muscle of Shh deficient mice. Our data indicate that this was due to an increase in chemokine expression by myoblasts in the setting of impaired Hh signalling, using tissue specific Smoothened conditional KO mice. The increased macrophage infiltration in mice deficient for Hh signalling in myocytes was associated with increased VEGFA expression and a transiently increased angiogenesis, demonstrating that Shh limits inflammation and angiogenesis indirectly by signalling to myocytes.

Conclusion

Although ectopic administration of Shh has previously been shown to promote ischaemia-induced angiogenesis, the present study reveals that endogenous Shh does not promote ischaemia-induced angiogenesis. On the contrary, the absence of Shh leads to aberrant ischaemic tissue inflammation and a transiently increased angiogenesis.

Hedgehog-dependent regulation of angiogenesis and myogenesis is impaired in aged mice

OBJECTIVE

The purpose of this study is to further document alteration of signal transduction pathways, more particularly of hedgehog (Hh) signaling, causing impaired ischemic muscle repair in old mice.

APPROACH AND RESULTS

We used 12-week-old (young mice) and 20- to 24-month-old C57BL/6 mice (old mice) to investigate the activity of Hh signaling in the setting of hindlimb ischemia-induced angiogenesis and skeletal muscle repair. In this model, delayed ischemic muscle repair observed in old mice was associated with an impaired upregulation of Gli1. Sonic Hh expression was not different in old mice compared with young mice, whereas desert Hh (Dhh) expression was downregulated in the skeletal muscle of old mice both in healthy and ischemic conditions. The rescue of Dhh expression by gene therapy in old mice promoted ischemia-induced angiogenesis and increased nerve density; nevertheless, it failed to promote myogenesis or to increase Gli1 mRNA expression. After further investigation, we found that, in addition to Dhh, smoothened expression was significantly downregulated in old mice. We used smoothened haploinsufficient mice to demonstrate that smoothened knockdown by 50% is sufficient to impair activation of Hh signaling and ischemia-induced muscle repair.

CONCLUSIONS

The present study demonstrates that Hh signaling is impaired in aged mice because of Dhh and smoothened downregulation. Moreover, it shows that hegdehog-dependent regulation of angiogenesis and myogenesis involves distinct mechanisms.

Gli3 regulation of myogenesis is necessary for ischemia-induced angiogenesis

RATIONALE

A better understanding of the mechanism underlying skeletal muscle repair is required to develop therapies that promote tissue regeneration in adults. Hedgehog signaling has been shown previously to be involved in myogenesis and angiogenesis: 2 crucial processes for muscle development and regeneration.

OBJECTIVE

The objective of this study was to identify the role of the hedgehog transcription factor Gli3 in the cross-talk between angiogenesis and myogenesis in adults.

METHODS AND RESULTS

Using conditional knockout mice, we found that Gli3 deficiency in endothelial cells did not affect ischemic muscle repair, whereas in myocytes, Gli3 deficiency resulted in severely delayed ischemia-induced myogenesis. Moreover, angiogenesis was also significantly impaired in HSA-Cre(ERT2); Gli3(Flox/Flox) mice, demonstrating that impaired myogenesis indirectly affects ischemia-induced angiogenesis. The role of Gli3 in myocytes was then further investigated. We found that Gli3 promotes myoblast differentiation through myogenic factor 5 regulation. In addition, we found that Gli3 regulates several proangiogenic factors, including thymidine phosphorylase and angiopoietin-1 both in vitro and in vivo, which indirectly promote endothelial cell proliferation and arteriole formation. In addition, we found that Gli3 is upregulated in proliferating myoblasts by the cell cycle-associated transcription factor E2F1.

CONCLUSIONS

This study shows for the first time that Gli3-regulated postnatal myogenesis is necessary for muscle repair-associated angiogenesis. Most importantly, it implies that myogenesis drives angiogenesis in the setting of skeletal muscle repair and identifies Gli3 as a potential target for regenerative medicine.

Therapeutic potential of genes in cardiac repair

Cardiovascular diseases remain the primary reason of premature death and contribute to a major percentage of global patient morbidity. Recent knowledge in the molecular mechanisms of myocardial complications have identified novel therapeutic targets along with the availability of vectors that offer the chance for designing gene therapy technique for protection and revival of the diseased heart functions. Gene transfer procedure into the myocardium is demonstrated through direct injection of plasmid DNA or through the coronary vasculature using the direct or indirect delivery of viral vectors. Direct DNA injection to the myocardium is reported to be of immense value in research studies that aims at understanding the activities of various elements in myocardium. It is also deemed vital for investigating the effect of the myocardial pathophysiology on expression of the foreign genes that are transferred. Gene therapies have been reported to heal cardiac pathologies such as myocardial ischemia, heart failure and inherited myopathies in several animal models. The results obtained from these animal studies have also encouraged a flurry of early clinical trials. This translational research has been triggered by an enhanced understanding of the biological mechanisms involved in tissue repair after ischemic injury. While safety concerns take utmost priority in these trials, several combinational therapies, various routes and dose of delivery are being tested before concrete optimization and complete potential of gene therapy is convincingly understood.

The Role of Endothelial Progenitor Cells in Postnatal Vasculogenesis: Implications for Therapeutic Neovascularization and Wound Healing

SIGNIFICANCE

Postnatal vasculogenesis mediated via endothelial progenitor cells (EPCs) contributes to re-endothelialization and augments neovascularization after ischemia and tissue injury, providing a novel therapeutic application. However, controversy exists with respect to the origin, identification, and contributions of the EPCs to neovascularization, necessitating further study.

RECENT ADVANCES

Bone marrow (BM) or circulating cells expressing cd133/vascular endothelial growth factor receptor 2 include those with endothelial progenitor capacity. Increasing evidence suggests that there are additional BM-derived (myeloid; mesenchymal cells) and non-BM-derived (peripheral and cord-blood; tissue-resident) cell populations which also give rise to endothelial cells (ECs) and contribute to re-endothelialization and growth factor release after ischemia and tissue injury. Currently, EPCs are being used as diagnostic markers for the assessment of cardiovascular and tumor risk/progression. Techniques aimed at enhancing ex vivo expansion and the therapeutic potential of these cells are being optimized.

CRITICAL ISSUES

Mobilization and EPC-mediated neovascularization are critically regulated. Stimulatory (growth factors, statins, and exercise) or inhibitory factors (obesity, diabetes, and other cardiovascular diseases) modulate EPC numbers and function. Recruitment and incorporation of EPCs require a coordinated sequence of signaling events, including adhesion, migration (by integrins), and chemoattraction. Finally, EPCs differentiate into ECs and/or secrete angiogenic growth factors. These cells are highly plastic, and depending on the microenvironment and presence of other cells, EPCs transdifferentiate and/or undergo cell fusion and become cells of a different lineage. Therefore, in vitro culture conditions should be optimized to mimic the in vivo milieu to fully characterize the biological function and contribution of EPCs to postnatal vasculogenesis.

FUTURE DIRECTIONS

Advances in characterization of the EPC biology and enhancement of EPC functions are required. In addition, innovative tissue-engineered carrier matrices that permit embedding of EPCs and provide optimal conditions for EPC survival and endothelial outgrowth will further contribute to EPC-mediated therapeutic applications in wound healing and ischemia repair.