Intervertebral disc degeneration (IVDD) occurs as a result of an imbalance of the anabolic and catabolic processes in the intervertebral disc, leading to an alteration in the composition of the extracellular matrix (ECM), loss of nucleus pulposus (NP) cells, excessive oxidative stress and inflammation. Degeneration of the IVD occurs naturally with age, but mechanical trauma, lifestyle factors and certain genetic abnormalities can increase the likelihood of symptomatic disease progression. IVDD, often referred to as degenerative disc disease (DDD), poses an increasingly substantial financial burden due to the aging population and increasing incidence of obesity in the United States. Current treatments for IVDD include pharmacological and surgical interventions, but these lack the ability to stop the progression of disease and restore the functionality of the IVD. Biological therapies have been evaluated but show varying degrees of efficacy in reversing disc degeneration long-term. Stem cell-based therapies have shown promising results in the regeneration of the IVD, but face both biological and ethical limitations. Exosomes play an important role in intercellular communication, and stem cell-derived exosomes have been shown to maintain the therapeutic benefit of their origin cells without the associated risks. This review highlights the current state of research on the use of stem-cell derived exosomes in the treatment of IVDD.

Mesenchymal stem cells (MSCs) are currently the most established cells for skeletal tissue engineering and regeneration; however, their availability and capability of self-renewal are limited. Recent discoveries of somatic cell reprogramming may be used to overcome these challenges. We hypothesized that induced pluripotent stem cells (iPSCs) that were differentiated into MSCs could be used for bone regeneration. Short-term exposure of embryoid bodies to transforming growth factor-β was used to direct iPSCs toward MSC differentiation. During this process, two types of iPSC-derived MSCs (iMSCs) were identified: early (aiMSCs) and late (tiMSCs) outgrowing cells. The transition of iPSCs toward MSCs was documented using MSC marker flow cytometry. Both types of iMSCs differentiated in vitro in response to osteogenic or adipogenic supplements. The results of quantitative assays showed that both cell types retained their multidifferentiation potential, although aiMSCs demonstrated higher osteogenic potential than tiMSCs and bone marrow-derived MSCs (BM-MSCs). Ectopic injections of BMP6-overexpressing tiMSCs produced no or limited bone formation, whereas similar injections of BMP6-overexpressing aiMSCs resulted in substantial bone formation. Upon orthotopic injection into radial defects, all three cell types regenerated bone and contributed to defect repair. In conclusion, MSCs can be derived from iPSCs and exhibit self-renewal without tumorigenic ability. Compared with BM-MSCs, aiMSCs acquire more of a stem cell phenotype, whereas tiMSCs acquire more of a differentiated osteoblast phenotype, which aids bone regeneration but does not allow the cells to induce ectopic bone formation (even when triggered by bone morphogenetic proteins), unless in an orthotopic site of bone fracture. Significance: Mesenchymal stem cells (MSCs) are currently the most established cells for skeletal tissue engineering and regeneration of various skeletal conditions; however, availability of autologous MSCs is very limited. This study demonstrates a new method to differentiate human fibroblast-derived induced pluripotent stem cells (iPSCs) to cells with MSC properties, which we comprehensively characterized including differentiation potential and transcriptomic analysis. We showed that these iPS-derived MSCs are able to regenerate nonunion bone defects in mice more efficiently than bone marrow-derived human MSCs when overexpressing BMP6 using a nonviral transfection method.

Multiple factors alter intervertebral disc volume, structure, shape, composition, and biomechanical properties, often leading to low back pain. Spinal fusion is frequently performed to treat this problem. We recently published results of our investigation of a novel system of in vivo bone formation, in which we used nonvirally nucleofected human mesenchymal stem cells that overexpress a bone morphogenetic protein gene. We hypothesized that primary porcine adipose tissue-derived stem cells (ASCs) nucleofected with plasmid containing recombinant human bone morphogenetic protein-6 (rhBMP-6) could induce bone formation and achieve spinal fusion in vivo. Primary ASCs were isolated from freshly harvested porcine adipose tissue. Overexpression of rhBMP-6 was achieved ex vivo by using a nucleofection technique. Transfection efficiency was monitored by assessing a parallel transfection involving an enhanced green fluorescent protein reporter gene and flow cytometry analysis. rhBMP-6 protein secreted by the cells was measured by performing an enzyme-linked immunosorbent assay. Genetically engineered cells were injected into the lumbar paravertebral muscle in immunodeficient mice. In vivo bone formation was monitored by a quantitative microcomputed tomography (μCT). The animals were euthanized 5 weeks postinjection, and spinal fusion was evaluated using in vitro μCT and histological analysis. We found formation of a large bone mass adjacent to the lumbar area, which produced posterior spinal fusion of two to four vertebrae. Our data demonstrate that efficient bone formation and spinal fusion can be achieved using ex vivo, nonvirally transfected primary ASCs. These results could pave the way to a novel biological solution for spine treatment. Disclosure of potential conflicts of interest is found at the end of this article.