Development of Functionalised Hydrogels to Enhance Stem Cell Delivery and Integration into the Ischaemic Myocardium
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Heart failure is a progressive, debilitating disease commonly caused by the irreversible loss of functional cardiac tissue after myocardial infarction. Aside from heart transplantation, current treatment options are considered palliative as they may prolong survival but fail to address the underlying damage to cardiac tissue. The delivery of multipotent stem cells to the damaged heart has been under extensive investigation for almost two decades. This treatment strategy offers a reparative approach, with the potential to salvage damaged heart muscle in the diseased organs of patients. Although safety and feasibility has been demonstrated using multiple cell types, only a minimal recovery of left ventricular function has been achieved. The curative potential of stem cells is hindered by poor retention and survival of transplanted cells in the harsh microenvironment of the infarcted heart. The overall aim of this thesis was to develop functionalised hydrogels to enhance stem cell delivery and integration into the ischaemic myocardium.
Natural biomaterials such as hyaluronic acid (HyA) are commonly used as cell delivery vehicles due to their intrinsic ability to generate hydrogels that are analogous to native tissue. However, single-factor extracellular matrix (ECM) scaffolds over-simplify the multicomponent ECM of the stem cell niche. The aim of this research was to determine if functionalisation of HyA hydrogel could improve stem cell viability and function under ischaemic culture conditions, compared to non-functionalised HyA hydrogel. As the optimal cell source for cardiac repair is unknown, two of the leading candidates, cardiac stem cells (CSCs) and mesenchymal stem cells (hMSCs), were examined.
HyA is known to resist cellular attachment, therefore the adhesive peptide RGD was incorporated to facilitate cell adhesion. In chapter 2, the mechanical properties of HyA and HyA-RGD hydrogel were compared. Both hydrogels were reported to gelate rapidly, exhibit minimal swelling and achieve appropriate mechanical properties. Next, the effect of RGD on hMSC viability, morphology and function in HyA hydrogel was investigated under standard culture conditions. Our results show that while hMSC viability was maintained in both HyA and HyA-RGD hydrogel, RGD significantly increased hMSC spreading and paracrine factor release, compared to HyA hydrogel.
In order to assess the effect of RGD on hMSC survival under ischaemic culture conditions, we sought to design a controlled experimental system using the well-known anti-apoptotic factor IGF-1 as a positive control. IGF-1 is known to enhance the growth and survival of multiple cell types, however, its effect on hMSCs is poorly understood. In Chapter 3, IGF-1 treatment was found to have no effect on hMSC proliferation or survival. The bioactivity of our IGF-1 protein was confirmed using CSCs, which demonstrated significant proliferation in response to IGF-1 stimulation.
Having demonstrated the beneficial effects of RGD under standard culture conditions, Chapter 4 investigated effect of RGD on hMSCs survival, morphology and function under ischaemic culture conditions. RGD was reported to improve encapsulated hMSC survival, but only if cells were allowed to adhere to the RGD before exposure to ischaemic culture conditions. In addition, the ECM protein nidogen-1 was successfully incorporated into HyA hydrogel and its effect on hMSC viability, morphology and function was investigated. Our results show that while nidogen-1 promoted hMSC viability and function under standard culture conditions, it was unable to enhance hMSC survival under ischaemic culture conditions. In Chapter 5, HyA-RGD hydrogel was found to promote CSC viability under both standard and ischaemic culture conditions, compared to unmodified HyA. IGF-1 was also shown to enhance rCSC viability in HyA hydrogel under ischaemic culture conditions.
Collectively, the results presented in this thesis provide evidence that the functionalisation of HyA hydrogel has the potential to enhance stem cell delivery under ischaemic conditions and to improve the efficacy of cell products already under clinical investigation.