Extracellular Matrix Scaffolds - A Tissue-Specific Bioactive Niche for Stem Cells in Regenerative Therapy

Aiming to restore the normal function of diseased or injured tissues, regenerative therapy approaches are generally based on the engineering of complex tissue-mimicking grafts, encompassing biomaterial scaffolds, stem cells, or their combinations [1-4]. Due to the major role of stem cells in physiological regenerative mechanisms, regenerative therapies normally rely on either stem cells transplantation or stem cell recruitment from the neighboring tissue into the implanted scaffold. The design of grafts that provide a supportive bioactive microenvironment for stem cells is, thus, crucial for its ability to control and direct stem cells’ fate towards the induction of reparative processes [5,6]. Such microenvironment can be provided through the careful selection of biomaterials and their engineering processes, thus considering their structural properties as well as their ability to support cell-biomaterial interactions through diverse biochemical and biophysical cues. 

The Extracellular Matrix (ECM) is the physiological structural support of all tissues, and it plays an important role in controlling the tissue’s cellular processes such as cell proliferation, migration, and differentiation [7]. The use of decellularized ECM from different sources in its native structure or processed through different technologies is, therefore, considered an excellent candidate biomaterial for regenerative therapy scaffolds [8,9]. The ECM’s source or tissue of origin, however, was not always deemed a relevant concern [10]. 

In our recent work [11], we studied porcine ECM (pECM)-based hydrogels derived from cardiac, artery, pancreas, and adipose tissues, thus exploring their common and distinct attributes and their relevance for regenerative therapy. This study demonstrated that while pECM hydrogels derived from different tissues share major componential and structural similarities, their minor differences make each hydrogel a unique scaffold that is most suitable to different applications depending on the biological, mechanical, and physical demands. For example, the adipose tissue-derived hydrogel is mechanically weaker than the cardiac one, which is expected due to their different physiological roles. Cardiac pECM hydrogel will, therefore, be a better choice for applications where mechanical support is required. Furthermore, through diverse cell-ECM interactions, each pECM hydrogel grants its residing stem cells a particular tissue-specific microenvironment that dictates their proliferation rate and morphology. Most importantly, pECM hydrogels derived from the different tissues had directed the spontaneous differentiation of human induced pluripotent stem cells (hiPSCs) towards the lineages typical of each pECM original tissue. This tissue-specific spontaneous differentiation was demonstrated through the expression of markers that are specific for cells residing in each tissue of origin, but also through a comprehensive gene expression sequencing analysis that revealed unique gene expression profiles for cells on each pECM hydrogel. 

Tissue-specific effects of ECM-based scaffolds were previously observed in scaffolds processed through different fabrication techniques such as soft lithography, electrospinning, and 3D printing [12]. Offering a rich variety of scaffold options like sheets [13], microcarriers [14,15], hydrogels [16], and 3D constructs [17], they can address numerous medical needs, thus emphasizing the advantages of using ECM-based materials derived from the target organ [18]. In a paper by Gao et al. [19], for instance, a bio-ink was developed from decellularized ECM and alginate. They succeeded in preserving the ECM attributes and constructing blood vessels using 3D coaxial printing technology. Using a mouse model, the researchers demonstrated that the ECM-based blood vessel graft promoted stem cells survival, engraftment efficiency, endothelial differentiation, and tissue repair; alas the graft had weak mechanical strength. Another example is the electrospinning of ECM, which can produce scaffolds that closely mimic the fibrous structures of native ECM, support cell functions and nutrient transport. In our earlier work [20], we have developed an electrospun scaffold composed solely of decellularized porcine cardiac ECM and demonstrated the ability of this scaffold to support the bioactivity and viability of cultured human mesenchymal stem cells (hMSCs), hiPSCs, and Neonatal rat cardiomyocytes (rCM). Furthermore, processed ECM can be functionalized with different biomolecules, such as growth factors and drugs to achieve a more specific effect on cells and tissues. This was demonstrated by Sonnenberg et al. [21], who had incorporated hepatocyte growth factor (HGF-f) within pericardial pECM hydrogel as a treatment for myocardial infarction (MI). The delivered HGF-f was shown to prevent cell death and promote angiogenesis. Moreover, In-vivo rat model experiments demonstrated that the HGF-f loaded ECM hydrogel had preserved or improved cardiac function, increasing vascularization, and reducing fibrosis, compared to ECM hydrogel alone or HGF-f in saline. 

Unveiling the preservation of ECM’s tissue-specific attributes through its processing to hydrogel and other fabrication techniques paves the path to the design of ECM-based scaffolds that are reproducible and well-defined. Such scaffolds can circumvent batch-to-batch variability, which is the major disadvantage of ECM as a natural biomaterial; while still benefit its structural advantages and tissue-specific bioactivity to support stem cells. Nevertheless, for these scaffolds to gain translational impact, further standardization approaches should be applied to ensure repeatable outcomes, particularly in terms of cell-ECM interactions. One promising approach is harnessing new technologies such as next-generation proteomics to quantify the retention of native ECM components. This relatively new technology enables in depth examination of the matrisome [22-24], which together with artificial intelligence tools such as DeepMind's AlphaFold [25] can predict protein structures. Consequently, these technologies can facilitate both componential and structural standardization. 

In summary, ECM-based scaffolds produced through diverse processing technologies hold tremendous potential as a tissue-specific supportive niche for stem cells in diverse regenerative therapies. Furthermore, the ability to direct stem cells fate through the choice of ECM source, together with prospective standardized manufacturing processes can significantly expedite the clinical translation of such ECM-based platforms.

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