**3.2 Adult stem cell niche and aging**

It has been well accepted that the activities of adult stem cells, whether they stay quiescent or undergo activation, are largely instructed by the microenvironment they reside in, i.e., stem cell niche. Extensive studies have been performed to characterize the stem cell niche in different organ systems such as the skeletal muscle, bone marrow, skin, intestines, and brain (reviewed in [78]). There are common features of the adult stem cell niche. In general, the stem cell niche or microenvironment is composed of stem cells themselves and progeny, surrounding MSCs or stromal cells, extracellular matrix, and adhesion molecules, as well as the external cues from distant sources (long-range signaling factors) [78]. Collectively, both cellular and acellular components of the stem cell niche create a complex microenvironment maintaining stem cell fate and ensuring robust regenerative responses to external stimuli. Although other signaling pathways have been described, TGFβ

superfamily, Wnt pathway, and Notch signaling have been identified in different model systems as key regulators of stem cell quiescence, survival, maintenance, and activation [78]. For example, the TGFβ superfamily not only contributes to stem cell quiescence maintenance [79–81], it also plays an important role for stem cell activation and aging. TGFβ has been found to be upregulated in both the satellite cells (skeletal muscle stem cells) and serum of aged mice, which induces high levels of pSmad3 in satellite cells and interferes with their regenerative activities [82]. Importantly, Notch signaling antagonizes pSmad3 and controls satellite cell proliferation by blocking TGFβ-dependent upregulation of cyclin-dependent kinase inhibitors [82]. Meanwhile, increased Wnt signaling in the aged satellite cells has been demonstrated to contribute to cell fate conversion of satellite cells from a myogenic to a fibrogenic state [83].

Recently, Tikhonova et al. mapped the transcriptional landscape of mouse bone marrow microenvironment (HSC niche) at a single-cell resolution and reported previously unappreciated levels of cellular heterogeneity within the niche and with defined distribution of pro-hematopoietic factors [84]. Furthermore, bone marrow niche underwent transcriptional remodeling under stress conditions, leading to a significant upregulation of adipogenesis-related pathways and a global reduction in osteo-lineage-related gene expression [84]. There was also a downregulation of vascular-endothelial-expressed Notch ligand DLL4, which skewed bone marrow hematopoiesis toward a myeloid transcriptional program [84]. These results indeed provided an explanation to the observation that the HSC populations in the elderly exhibit myeloid skewing and lymphoid lineage deficiency [85].

The notion of aging has been raised as the ratio of tissue attrition to tissue regeneration [86]. This process is accompanied by reduced regenerative capacity of adult stem cells at levels of both self-renewal and differentiation (reviewed in [87]). The decline in regenerative function of adult stem cells also contributes to pathophysiological alterations in age-related diseases. Meanwhile, current studies also indicate that age-imposed biochemical changes in the stem cell niche are responsible for such regenerative declines of tissue maintenance and repair. Indeed, several studies have revealed that regenerative potential of stem cells was not controlled by the age of the stem cells themselves, but by the age of the niche they stay in [86]. In a classic experiment, minced skeletal muscle tissue containing satellite cells from young rodents was transplanted into the muscle of aged hosts, and conversely muscle tissue from old rodents was placed into the muscle of young hosts [88]. Such heterochronic transplantation demonstrated that the age of the host was more important than the age of the transplanted stem cells in muscle regeneration [88]. Serial transplantation of spermatogonial stem cells to the testes of young male mice demonstrated that spermatogenesis from that stem cell could continue for more than 3 years, long past the normal life span of the animal, when the stem cell is continually maintained in a young niche [89]. The importance of stem cell niche is further revealed by the heterochronic parabiotic studies [83, 90, 91]. In such experiments, two rodents, young-to-old (heterochronic) and young-to-young and old-to-old (isochronic), are surgically connected through a large flap of the skin, allowing vascular circulation between the two connected animals. In these studies, the isochronic parabionts were not significantly different in tissue regeneration than their respective non-parabiotic age-matched controls. However, in the heterochronic parabionts, regeneration in the muscle, liver, and brain was all significantly improved for the old animal and was decreased for the young animal [83, 90, 91]. As there was no evidence of blood cell exchange between the two connected animals, the studies suggested that tissue regeneration in the stem cells of an old animal could be promoted by "young" systemic factors (long-range factors), while

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*Innovations in Human Stem Cell Research: A Holy Grail for Regenerative Medicine*

"young" factors are sirtuin 6 (SIRT6) and Delta/Notch [82, 91–94].

the aged circulation might have "negative" regulators that suppress the regenerative activities of "young" adult stem cells [86]. Examples of identified "negative" factors include p16INK4a, C-C motif chemokine 11 (CCL11), TGFβ, and TNFα, and

All these studies underscore the importance of understanding how the stem cell microenvironment controls stem cell dynamics and function. Identifying specific molecular and cellular mechanisms that either rejuvenate or compromise the regenerative capacity of adult stem cells will be instructive in developing therapies antagonizing aging and age-related pathological changes. In animal studies, it has been demonstrated that forced activation of Notch signaling and inhibition of TGFβ and Wnt pathways could restore the regenerative capacities of

Understanding the impact of microenvironment on stem cell activities is also essential for the development of an effective stem cell therapy. Whether the cells are derived from pluripotent stem cells or isolated from adult tissue, ex vivo culture or manipulation in a microenvironment that maintains cell identity and potency is critical for the efficacy of cell transplantation. Furthermore, a substantial challenge in the regenerative therapy is acute cell death after transplantation of the cells into a degenerative/pathological microenvironment. A recent clinical islet transplantation study quantitated the level of circulating cell-free (cf) DNA as a biomarker for the dead beta cells after transplantation [96]. The authors reported that a distinctive peak of cfDNA was observed 1 hour after transplantation in 83.8% of patients. The cfDNA was also detected 24 hours posttransplant, and that signal was correlated with overall poor clinical outcome (higher insulin requirement, lower stimulated C-peptide level, and decreased 3-month engraftment) [96]. Instant blood-mediated inflammatory response has been speculated to contribute to the significant graft attrition at this early stage of transplant [97]. Therefore, modulation on microenvironment that ensures the survival, integration, and function of the therapeutic cells after transplantation would greatly

MSCs were initially identified as rare non-hematopoietic colony-forming units following plastic adherence of bone marrow cells (reviewed in [98]). Although the original notion of MSCs specifically referred to cells in the bone marrow (also called bone marrow stromal cells, BMSCs), MSCs have been derived from other sources such as cord blood, adipose tissue, and dental pulp. MSCs can be identified by the expression of cell surface markers including CD90, CD73, and CD105 and lack of expression of hematopoietic and endothelial cell markers. MSCs have the capacity to differentiate along mesoderm lineage into osteoblasts, chondrocytes, adipocytes, and fibroblasts. MSCs were also reported to give rise to other mesodermal cell types such as cardiomyocytes and endothelial cells, as well as the cells of other lineages such as neurons and hepatocytes. However, as these claims were mainly based on the expression of the markers and not functional studies, whether MSCs truly have

There are also controversial results on the age-related changes in MSCs. Some studies reported an age-dependent reduction in the number of MSCs isolated from human bone marrow, while others demonstrated no correlation between MSC numbers and age, even in patients with osteoarthritis [99–102]. Functionally, Sun et al. demonstrated using young and aged mice that although the frequency of MSCs was not significantly different between the young and old bone marrow, the

such differentiation capacities require more extensive validation.

*DOI: http://dx.doi.org/10.5772/intechopen.88790*

aged muscle [83, 95].

improve the efficacy of cell therapies.

**3.3 Mesenchymal stem cells**

### *Innovations in Human Stem Cell Research: A Holy Grail for Regenerative Medicine DOI: http://dx.doi.org/10.5772/intechopen.88790*

the aged circulation might have "negative" regulators that suppress the regenerative activities of "young" adult stem cells [86]. Examples of identified "negative" factors include p16INK4a, C-C motif chemokine 11 (CCL11), TGFβ, and TNFα, and "young" factors are sirtuin 6 (SIRT6) and Delta/Notch [82, 91–94].

All these studies underscore the importance of understanding how the stem cell microenvironment controls stem cell dynamics and function. Identifying specific molecular and cellular mechanisms that either rejuvenate or compromise the regenerative capacity of adult stem cells will be instructive in developing therapies antagonizing aging and age-related pathological changes. In animal studies, it has been demonstrated that forced activation of Notch signaling and inhibition of TGFβ and Wnt pathways could restore the regenerative capacities of aged muscle [83, 95].

Understanding the impact of microenvironment on stem cell activities is also essential for the development of an effective stem cell therapy. Whether the cells are derived from pluripotent stem cells or isolated from adult tissue, ex vivo culture or manipulation in a microenvironment that maintains cell identity and potency is critical for the efficacy of cell transplantation. Furthermore, a substantial challenge in the regenerative therapy is acute cell death after transplantation of the cells into a degenerative/pathological microenvironment. A recent clinical islet transplantation study quantitated the level of circulating cell-free (cf) DNA as a biomarker for the dead beta cells after transplantation [96]. The authors reported that a distinctive peak of cfDNA was observed 1 hour after transplantation in 83.8% of patients. The cfDNA was also detected 24 hours posttransplant, and that signal was correlated with overall poor clinical outcome (higher insulin requirement, lower stimulated C-peptide level, and decreased 3-month engraftment) [96]. Instant blood-mediated inflammatory response has been speculated to contribute to the significant graft attrition at this early stage of transplant [97]. Therefore, modulation on microenvironment that ensures the survival, integration, and function of the therapeutic cells after transplantation would greatly improve the efficacy of cell therapies.
