Our lab had shown that the use of polymers derived from ECM proteins, i.e., fibrin glue, Type I Collagen and/or Matrigel, or even polymers that have been modified with ECM-derived functional moieties, can induce angiogenesis enhancing the formation of new capillaries and functional arterioles in the MI region, while at the same time induce a higher influx of myofibroblasts, which are known to help restore structural integrity to the infarcted scar tissue.17–19
However, one drawback to using these polymers is that they require direct injection or implantation to the injured site, which can result in further health complications. A less invasive procedure would be to somehow inject the polymer intravenously. Unfortunately, these polymers are too large to pass through the capillaries. One solution is either to use smaller fragments of the polymer or relevant functional groups derived from the ECM proteins, i.e., ECM-derived peptides. Although these ECM-derived peptides have been used to modify synthetic polymers to improve the materials' ability to interact with the pre-existing tissue, no one has considered using these peptides alone to promote wound healing and tissue regeneration.
We postulated that it might be possible to use fragments or even peptides derived from ECM proteins rather than the whole protein itself to influence the microenvironment to promote regeneration of lost tissue within the MI. We had investigated four ECM-derived peptides—Hep I, Hep III, RGD and FC/HV.7
Our in vitro analysis of these ECM-peptides revealed that Hep I, Hep III and RGD were all able to promote cell attachment, proliferation and migration, although not necessarily to the same extent as their whole protein counterparts. With FC/HV, we noted adherent cells only within the first 24 h, followed by subsequent loss of these cells. Interestingly, all of our ECM peptides were able to induce Erk1/2 activation, which is involved in the signaling pathway leading to angiogenesis and arteriogenesis.20
To test the in vivo effect of the ECM-peptides, we injected the peptides into rats two days after inducing a myocardial infarction. In order to target to ECM-peptides to the site of injury, we had conjugated the peptides to antibodies specific for an antigen expressed shortly after a myocardial infarction. This antibody also had the added advantage of maintaining the peptides within the injury site for longer periods of time. Nuclear distribution studies have found that the antibody remained at detectable levels for as long as two weeks. Histological assessment of the heart tissue harvested six weeks post-treatment indicated that Hep I, Hep III and RGD promoted angiogenesis and arteriogenesis within the injured heart tissue. Yet, despite their ability to promote angiogenesis, we failed to observe statistically significant improvement in both the size of the infarct scar and in heart function. At the same time, however, we did note that the Hep III-treated heart managed to prevent worsening of the heart function. Hep III peptides can interact with one another to form a polymer-like matrix. It is possible that a comparable situation is occurring in rats treated with Hep III and may explain why Hep III was the only peptide that was able to prevent further negative remodeling as indicated by the echocardiography data. Furthermore, the peptides do not necessarily have to be interacting with one another to form a matrix. They also can interact with surrounding ECM proteins in a similar manner to form a matrix, thus alter the structural properties of the infarct tissue within the LV and prevent the negative remodeling associated with a MI.18,21
The results with Hep III suggest that the formation of an in vivo scaffold changes the material properties of the LV to prevent LV aneurismal formation.
We have demonstrated that ECM-derived peptides can effectively help to produce an ECM favorable for angiogenesis. A critical barrier to tissue regeneration is the lack of an adequate vascular network. The creation of a vascular bed in the infracted myocardium may allow for greater cell engraftment and survival.22,23
The use of the ECM-derived peptides alone, without growth factors or cells, was sufficient to promote an angiogenic response in infarcted rat hearts. The induction of new vessel formation suggests that targeting active components of the ECM can influence the microenvironment and allow the body to act as its own bioreactor to regenerate vital structures of the myocardium.