Human bone marrow mesenchymal stem cells (hBM-MSCs) have been widely used for tissue engineering applications because of their abundance in adults, relatively easy isolation, ability to differentiate into multiple mesodermal lineages, and capacity to secrete a spectrum of bioactive/therapeutic/trophic factors with chemoattractive, immunomodulatory, angiogenic, antiscarring, and antiapoptotic functions.1–6 Despite the notable advances in the field, their ex vivo expansion remains challenging, often associated with phenotypic drift and senescence, imposing the need for engineering more appropriate/bioinspired in vitro microenvironments to maintain their function and therapeutic potential.7,8 Among the various in vitro microenvironment modulators, physiologically relevant (to the cells) low oxygen tension (frequently termed hypoxia) has been characterized as a critical component of the stem cell niche.9–11 It has even been suggested that hypoxia preconditioning of BM-MSCs should become a clinical standard before their implantation.12 This can be rationalized considering that in vivo tissues and organs undergo a broad range of oxygen tensions, depending on their distance from the capillaries, which is markedly lower than the inhalation oxygen tension (∼20%). After entering the lungs, oxygen travels through the bloodstream, and when it reaches the different organs, oxygen tension ranges from 2% to 9%.13 In the bone marrow, for example, BM-MSCs reside in regions of low oxygen tension, ranging from 1% to 6%.10,11 Herein, the influence of low oxygen tension in hBM-MSC in vitro and in vivo fate, with emphasis to orthopaedic clinical indications, is briefly discussed.
Several studies have advocated the beneficial effects of low oxygen tension in hBM-MSCs fate. Hypoxia has been shown to maintain hBM-MSCs undifferentiated state and multipotency,14 and to suppress senescence, typically observed at 20% oxygen tension, via the downregulation of the expression of p16 and extracellular signal regulated kinase.15 In addition, 2% oxygen tension in combination with macromolecular crowding, a biophysical phenomenon known to enhance and accelerate (over 80-fold) extracellular matrix deposition,16–18 resulted in a microenvironment capable of maintaining the phenotype of hBM-MSCs and their multilineage potential.19 It has been reported that the hypoxia-inducible factor-1α is activated under low oxygen conditions, which in turn activates the Twist-related protein and downregulates the cyclin-dependent kinase inhibitor 1 (p21), which stimulates cell proliferation.20 Moreover, hypoxia-activated hypoxia-inducible factor-1α significantly increased hBM-MSCs migration through downregulation of integrin α4 and upregulation of Rho-associated kinase ROCK1 and serine/threonine kinase (Rac1/2/3) pathways.21 Furthermore, hBM-MSCs cultured at 1% oxygen and supplemented with fibroblast growth factor-2 exhibited enhanced cell proliferation, chondrogenesis, osteogenesis, migration, collagen formation and regenerative potential.22
Regarding chondrogenic induction, a wealth of studies has demonstrated the beneficial effect of low oxygen tension. For example, isolation and expansion of hBM-MSCs at 2%23 and 3%24 oxygen tension have been shown to promote chondrogenesis, compared with cells cultured at 20% to 21% oxygen tension, as revealed by the increased expression of chondrogenic-related genes and higher glycosaminoglycan content. Notably, in vivo implantation of hypoxia preconditioning hBM-MSCs embedded in alginate beads enhanced the chondrocyte phenotype of cells, producing a cartilage-like tissue in athymic mice.25 Hypoxia preconditioning of BM-MSCs has also been shown to promote osteogenesis in both in vitro and in vivo settings. Cells cultured at 1% oxygen tension in Arg-Gly-Asp-alginate hydrogels were shown to promote osteogenic and angiogenic responses.26 Two percent oxygen tension has been shown to increase the proliferation and the osteogenic differentiation of stem cells from donors with osteonecrosis of the femoral head, suggesting hypoxia preconditioning as an effective treatment to stimulate bone healing.27 In immunocompromised mice, preconditioned at 1% oxygen tension, as opposed to 20 % oxygen tension, hBM-MSCs that were seeded on hydroxyapatite/tricalcium phosphate-based scaffolds produced soluble and insoluble collagen that brought about a stable extracellular matrix and resulted in improved regenerative potential of the bone.28
Physiologically, for the hBM-MSCs, low oxygen tension has been shown to be beneficial to their proliferation, migration, plasticity, and differentiation in vitro and to augment their therapeutic potential in vivo. Yet again, only a handful of studies have assessed the therapeutic potential of hypoxia preconditioned stem cells in clinic. For example, intracoronary administration of hypoxia preconditioned of hBM-MSCs after acute myocardial infarction has been shown to be safe and feasible.29 It is imperative to conduct more clinical trials to assess the efficacy and efficiency of low oxygen tension-treated hBM-MSCs in orthopaedic clinical indications.
References printed in bold type are those published within the past 5 years.
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