Lumbar degenerative disease caused by lumbar disc degeneration has seriously affected the life of patients and has become a serious social problem, but both surgery and conservative treatment did not fundamentally reverse disc degeneration [5, 6]. However, with the development of molecular mechanisms and genetic engineering of disc degeneration, it is possible to treat or reverse disc degeneration at the gene level.
Mesenchymal stem cells (MSCs) are pluripotent stem cells originated from the mesoderm, which can differentiate into bone, cartilage, fat, muscle, ligament, tendon and other tissues [7, 8]. In 2006, the international association for cell therapy proposed a common standard for the definition of mesenchymal stem cells [9]: A. in vitro culture, such cells can grow adherently to the wall; B. Some specific antigens (markers) are expressed on the cell surface; C. The ability to differentiate into adipocytes, osteoblasts and chondrocytes. Several studies have shown that there is a kind of cell in the intervertebral disc tissue, and this kind of cell has the ability to differentiate into osteoblasts, chondroblasts and lipid cells. Meanwhile, this kind of cell also characteristically expresses the surface protein molecules of mesenchymal stem cells, which can complete the three-line induced differentiation. Therefore, such cells are named as nucleus pulposus mesenchymal stem cells (NPMSCs) [10–12].
Li XC et al successfully isolated human NPMSCs by fluorescence activated cell sorting and which expressed tyrosine kinase receptor 2 (Tie-2) and ganglioside 2 (GD-2). The cells had significant proliferation and differentiation potential and could differentiate into osteoblasts, adipocytes, and chondrocytes [13]. Erwin WM et al proved that NPMSCs has a powerful ability to divide and proliferate by animal experiments, and played an important role in IVD repair, nerve repair and other renewable medicine [14]. Consequently, NPMSCs is the precursor of NPCs, which exists in normal and degenerative IVD tissues and has the potential to proliferate and differentiate into NPCs. NPMSCs are the precursor cells of nucleus pulposus cells, which exist in normal and degenerated nucleus pulposus tissues and have a strong potential to proliferate and differentiate into nucleus pulposus cells, and can be used in regenerative and repair medicine [3, 4].
Lin et al. cultured NPMSCs of rats in vitro and induced differentiation, and found that NPMSCs could express stem cell genes, such as SOX2, Oct4 and Nanog [15]. Hui Zhang et al. compared rat NPMSCs and bone marrow mesenchymal stem cells by in vitro culture, and found that both can differentiate into bone, cartilage and fat, and can express stem cell genes Nanog, oct-4 and sox-2, with no obvious difference [16]. In this study, FCM was used to detect surface immunophenotype CD molecular of NPMSCs. The immunophenotypes CD90 and CD105 expressed positive, and the immunophenotypes CD34 and CD45expressed negative. qRT-PCR was used to detect positive expressions of SOX-2 and Nanog, NPMSCs could express stem cell gene SOX2, Nanog.
Some scholars have found that human notochord cells gradually reduce with the growth of age and disc degeneration, this notochord cells are NPMSCs [17]. Rodrigues-pinto R et al. found that keratin 8, 18, 19 (KRT8, KRT18, KRT19) are specific markers of human notochord cells, and are expressed in all stages of notochord cells [18]. Minogue et al. detected the mRNA levels of KRT8, KRT18 and KRT19 in bovine nucleus pulposus cells and notochord cells by rt-pcr, and found that both expressed KRT8, KRT18 and KRT19 genes, and notochord cells expressed slightly more than nucleus pulposus cells [19]. Meanwhile, some scholars have found that nucleus pulposus cells can also express KRT8, KRT18, KRT19 and other gene phenotypes, and the expression level of nucleus pulposus cells is higher than that of articular chondrocytes and ring fibroblasts [20–22].
GDF5, also known as BMP14, is a member of the bone morphogenetic protein (BMP) family. BMP was originally thought to be a component of the mineralized bone matrix, and when a fracture or ectopic ossification occurred in the body, it can induce the formation of new bone tissue [23, 24]. Current studies have shown that GDF5 can repair degenerative intervertebral discs and promote the proteoglycan and collagen type II protein expression [25, 26]. Animal studies have found that the central region of intervertebral disc of GDF5 knockout mice showed low signal in MRI T2 weighted, the fibrous ring lost normal lamellar structure, NP atrophy and disorder, and the content of proteoglycan decreased significantly. The expression of proteoglycan and type II collagen mRNA decreased, which confirmed that the deletion of GDF5 gene was closely related to IVD degeneration. Meanwhile, GDF5 also promoted the differentiation of stem cells [27]. In this study, after GDF5 gene transfection of NPMSCs by lentivirus, mRNA and protein expression levels of KRT8, KRT18 and KRT19 in NPMSCs were significantly increased compared with other two words, suggesting that GDF5 gene can promote the differentiation of NPMSCs.
Regarding the mechanism of GDF5 acting on NPMSCs, Liu W found that GDF5 could inhibit the transcription and expression of RNA fragment microrna-34a, reduced the generate of IL-1 β, and increase the expression of proteoglycan and collagen II type. It indicates that GDF5 could delay or stop the degeneration of the disc [28]. Of course, the mechanism of GDF5 acting on NPMSCs still needs further study in follow-up experiments.