A review of advances in regenerative medicine and stem cells

Yu Donglin

(Southern Medical University, Guangdong 510000, China)

AbstractStem cells are playing an increasingly important role in regenerative medicine, and clinical research is increasing day by day. However, due to the limitations of various conditions, the clinical application of stem cells is still hindered by many obstacles. In this paper, we take skin repair as the starting point to make a general review of stem cell-related research, hoping to provide some help for further research.

Keywords: Regenerative Medicine, Stem Cells, Skin Repair

As the body's ability to renew decreases, the probability of degenerative diseases increases. Because of the balanced movement of stem cell maintenance and differentiation, the body cannot be replenished in time, and there will be various pathological effects at the statistical level. The new idea of disease, disease is the result of cells not being replenished, such as osteoarthritis is due to the increasing susceptibility to disease caused by the continuous decline of backbone cells with age, so it can be promoted by supplementing stem cells, that is, stem cell transplantation; using the biological hierarchy model, the higher dimensional objects can play a greater role, and organ transplantation can play a better role, so artificial organs are the future development path, and the key ideas of artificial organs can be more dynamic, such as exercise can reshape bones (continuous application of mechanical force can help to mimic the growth pattern of bones in the human body, so as to approximate the performance of the original bones) We can simulate by applying various stimuli, not simply providing a scaffold to let the cells grow, but need more guidance to make them fall into place, which may require a high-dimensional object to continuously expand, because there are too many specific instructions and too complex to be added by people one by one, so it is more efficient to build an environment that can provide instructions. It is assumed that there is a multi-level system, with low dimensions achieving high dimensions, that is, molecules form cells, cells form tissues, tissues form organs, organ-forming systems, and systems form organisms. After that, the higher dimension directs the lower dimension, such as small changes at the cellular level, which can lead to large changes at the molecular level.

Artificial organs and organ transplantation can play an important role when tissues or organs suffer defects and severe damage, but there are problems such as postoperative rejection, infection, adverse reactions of immunosuppressants, high cost of immunosuppressants, and difficulty in sourcing donors. With the development of stem cell technology, traumatized or failed tissues and organs can be replaced by artificial regeneration and reconstruction: stem cells can be cultured in vitro, expanded in large quantities, and targeted induced differentiation into specific tissue cells to meet clinical needs, on the basis of which tissues and organs can be further constructed.

1 stem cell

Stem cells can be divided into two categories, such as neural stem cells, adipose stem cells, epidermal stem cells, and mesenchymal stem cells, derived from embryonic and fetal tissues [1], and adult stem cells derived from postnatal organs or adult human tissues [2].

Regenerative medicine strategies have been used to treat human diseases, such as ankle osteochondral injury [3], psoriasis [4], limbal stem cell deficiency, age-related macular degeneration (AMD), and glaucoma [5]. Bone marrow stromal stem cells have been explored for clinical use in the treatment of a variety of different diseases, including bone defects, graft-versus-host disease, cardiovascular disease, autoimmune disease, diabetes, neurological disease, liver and kidney disease [6]. Later in this article, we will look at transplanting epidermal stem cells into cultured epidermal cell sheets for the treatment of burns, chronic wounds, and more.

1.1 Reverse differentiation:

An important advance in the field of stem cells and regenerative medicine is the emergence of induced pluripotent stem cell technology, which can solve the problem of immune rejection of adult cells by establishing embryonic stem cells of any individual: Shinya Yamanaka can reprogram somatic cells into pluripotent stem cells, i.e., induced pluripotent stem cells (iPSCs), using four transcription factors Oct4, Sox2, Klf4, and c-Myc [7]. The further establishment of human iPS cells [8] makes it possible for stem cells to be applied to the human body with greater clinical value.

However, due to the use of viral vectors in IPS technology, there are potential safety risks such as low induction efficiency, foreign gene insertion and tumorigenicity. The use of small molecule compounds to induce cell fate transitions is a better option. For example, Deng Hongkui et al. [9] induced the formation of pluripotent stem cells in mouse somatic cells by replacing transcription factors with a combination of 7 small molecules.

Specific mechanisms and chemical small molecules can regulate cell fate transitions by influencing a variety of pathways, such as epigenetic modifications and signaling pathways [10, 11]. Through the exploration of the mechanism, it can lay a foundation for effectively improving the efficiency of cell reprogramming. The use of histone deacetylases (HDACs) to modulate the target through small chemical molecules has improved the efficiency of mouse and human fibroblast reprogramming to some extent [12]. DNA methylase inhibitors can also promote somatic cell reprogramming [13].

The modulation of signaling pathways through small chemical molecules can also play a role in influencing the process of somatic cell reprogramming, such as the Wnt pathway, which can promote cell reprogramming [14]. The use of chemical small molecule inhibitors, such as TGF-beta signaling pathway inhibitors, can also improve the efficiency of cell reprogramming [15, 16]. Deng et al. [9] used a combination of small molecules to induce the formation of pluripotent stem cells in somatic cells of mice, without the involvement of transcription factors.

Chemical small molecules can also play a key role in the process of cell fate transition, as well as transcription factors, and then mediate cell fate transformation, and the underlying mechanism lies in the construction of some equivalence, such as the synthesis of the influence of specific signaling pathways, and the topological structure of biological networks.

1.2 Transdifferentiation:

Under the action of certain tissue-specific transcription factors, it is possible to achieve conversion between different types of cells, i.e., transdifferentiation, which can directly produce functional cells without the need to first reverse differentiate into pluripotent stem cells and then induce differentiation into specific mature cells [17, 18]. Similar to the construction process of pluripotent stem cells, nine transcription factors can directly transform sertoli cells into pluripotent neural stem cells/progenitor cells, which have similar gene expression profiles to normal neural stem cells and function in vitro and in vivo [19].

Similarly, a combination of small molecules can be used to replace the role of transcription factors to achieve transdifferentiation. For example, mouse and human fibroblasts are transdifferentiated into neurons using a combination of small molecule compounds [20], and human fibroblasts are transdifferentiated into functional cardiomyocytes [21].

Small molecule compounds can not only completely replace lineage-related transcription factors, but also can transform cell fate to a specific state alone, we can be regarded as the structure of mapping between different cells, theoretically able to construct specific transitions between any cells, the specific function construction can be decomposed into a series of features by using the idea of function expansion into series, cell A * measure B = cell C corresponds to specific treatment measures can be different signaling pathways or selective expression of genes and proteins. According to the median value theorem, theoretically there will be a specific fixed point that can achieve this specific mapping, which plays a role in shifting thousands of pounds. The construction process of this function is actually equivalent to the weight convergence of the neural network algorithm. Therefore, we can achieve this mapping by constructing neural networks. Omics-level data may allow us to train neural networks that can reprogram somatic cells, target differentiation or transdifferentiation, and maintain different pluripotency states of stem cells, select specific combinations of molecules, and achieve the conversion of any cell type we want. One of the ways in which cell reprogramming mechanisms are explored is lineage reprogramming, in which transgene expression of multiple lineage-specific transcription factors is directly transformed into another adult cell. Another strategy uses generic transcription factors to generate epigenetically labile intermediates that are capable of differentiating into somatic cells based on specific differentiation factors [22]. In fact, it can be mathematically equivalent to the transformation of matrices, so using the relevant algorithms, we may be able to obtain a large number of cells for regenerative medicine applications. Deep learning is currently being used in many fields, and we believe that it may also play a role in the study of cell reprogramming mechanisms, especially in computational biology [23].

1.3 Challenges:

One of the many challenges in the introduction of regenerative medicine into the clinic is the selection of the optimal cell type and reliable method for expanding the number of cells [24], but the effect of donor-related parameters such as age on stem cell titer is important, and the age of the donor and the number of generations of stem cell culture can affect the efficacy of autologous stem cell therapy [25]. At the same time, security needs to be further ensured [26].

1.4 Development Strategy:

Finding ways to promote cell differentiation, increase cell yield, ensure product standardization, overcome the risk of teratogenicity and immune response, and enable direct transplantation of transplanted cells or tissues without involving legal issues stipulated by the state [27].

2. Application of stem cells

We focus on the use of skin stem cells, especially after mechanical damage or burns of the skin.

2.1 Mechanical skin injury:

Wound repair is essentially about restoring the distribution, type, number, and location of different cells, so as to restore the original homeostasis: different types of cells are maintained in a certain number but can complement each other under certain conditions [28]. We first consider cell-free technologies based on bioactive factors, such as injectable gels, or growth factors, or other biomaterials, that are easy to utilize, have good usability, low relative cost, and high patient satisfaction [29]. However, a number of drawbacks remain: the duration is short, and frequent repetition of treatments is required [30].

Essentially, these treatments are all about inducing the reconstitution of the host tissue to achieve wound healing, and the injection of these bioactive molecules can modulate the function of local cells, while accumulating cells to specific areas through chemotaxis, which can partially replace the function of the original tissue. Because growth factors and cytokines can affect the signaling of cell gene expression, proliferation, migration, and differentiation, thereby regulating cell function. Growth factors have been found to help speed up the healing rate and time of diabetic foot ulcers [31].

Platelet-rich plasma (PRP), a platelet concentrate containing a variety of bioactive molecules, is an alternative to growth factors [32] and may also play a role in the treatment of diabetic ulcers [33].

We also need to consider the role of the extracellular matrix: the extracellular matrix can influence cell behavior by exposing previously hidden cell signaling sites, promoting cell migration [34], and influencing signaling by slowly releasing and delaying growth factor degradation [35].

We also need to consider the role of lower-level molecules as well as signaling pathways, and the repair of trauma focuses on the migration and adhesion of specific types of cells, where adhesion is maintained by integrin-β1, thereby activating the undifferentiated signaling of EpiSCs [36]. That is, if you want EpiSCs to remain in the stem cell nest niche, you want to activate enough integrin [37]. In integrin-β1 deletion mice, cell proliferation is sustained, but migration is impaired, and cells accumulate at the edge of trauma [38]. Therefore, the matrix used for cell patches needs to be able to promote integrin expression to promote cell adhesion. Of course, we also need to maintain this relatively stable position after the cells have migrated to a specific site.

2.2 Burns:

Cell patches have been shown to improve survival in burn patients [39], but they also have a number of drawbacks: adhesion, inefficient integration, inadequate appearance and function, and low persistence [40]. And because the components are too simple, it is not possible to achieve specific cell signals [41].

Fat grafting may be a way to transfer fat cells to their natural matrix, such as extracting fat cells from fat-rich areas such as the abdomen and buttocks and transferring them to damaged areas. Due to its abundance and absence of the risk of immune rejection, it can be used to explore skin damage repair, such as the improvement of dark circles [42].

Our ideal situation is to restore a series of signals from the original cells, and through the matrix components and cell components of the cell patch to simulate the original environment, and continue to approximate. Of course, it is impossible to achieve exactly the same, but considering this goal as a function, we can construct a high-dimensional proto-function to achieve it, that is, epidermal stem cell transplantation. We do not need to directly construct the structure of mature fibroblasts, adipocytes, etc., the use of stem cells can be differentiated into a variety of types of cells, easy to expand in vitro, and have the property of self-renewal ability in vivo, so that they can automatically proliferate and differentiate according to environmental signals, and the result of their adaptive growth may be to restore the function of the original tissue. We agree with the saying that nature does not do useless work, everything moves in the direction of the lowest energy, and it may be that healthy tissues are in this state of the lowest energy, so stem cells can develop in this direction and differentiate into specific types of cells in specific parts to play a certain function.

Adipose grafting may be effective because adipose tissue contains pluripotent adipose tissue-derived stem cells (ADSCs), which can become adipocytes, osteoblasts, chondrocytes, and other mesenchymal cells [43]. and may be associated with the secretion of ADSCs, which have paracrine effects on neighboring ones, inducing upregulation of type I and III collagen and fibronectin and downregulation of matrix metalloproteinase-1. It may also stimulate angiogenesis and suppress inflammation [44].

On the one hand, we can directly extract normal epidermal stem cells from patients, proliferate them through in vitro culture, and finally use them to make cell patches, on the other hand, in some special cases such as severe burns, we can use other somatic cells to reverse differentiate into induced pluripotent stem cells and then differentiate under specific conditions, or transdifferentiate into epidermal stem cells. The use of connective tissue growth factor (CTGF) and TGF-β1 allows stem cells to gradually differentiate into fibroblasts and subsequently into myofibroblasts [45].

The abundance of skin stem cells in the cell patch promotes improved performance, reduces scarring, and enables long-term self-renewal. However, in the end, stem cell depletion can lead to transplant failure [46], and we can observe that low epidermal stem cell content is associated with cell patch failure [47].

3 Epilogue and outlook

The regenerative capacity of mammals has been demonstrated. Specific molecular mechanisms have also been explored, such as the inability of embryos to form scars, low expression of TGF-β1 and high expression of TGF-β3 [48]. This can be instructive to make the expression pattern of the damaged site similar to that of embryos through various measures, such as the application of ginsenoside Rb1 (a potential TGF-β1 inhibitor) to achieve low expression of TGF-β1, which can reduce the scarring of α-SMA and type I collagen in rabbit models [49]. However, simple antagonism to these molecular mechanisms does not lead to good clinical efficacy, such as the use of TGF-β3 (Avotermin) in phase III clinical trials that did not have a good effect on scarring [50].

Because the body system is a complex system that often affects the whole body, we may need to target more molecules to have a better correlation, as Shinya Yamanaka transferred four transcription factors to cause fibroblasts to differentiate backward into pluripotent stem cells [7]. It is also understandable why freshman cocktail therapy [51]. Of course, there is a small chance that there are drug targets that can play a large role, such as Keytruda, a monoclonal antibody for melanoma, which blocks the PD-1/PD-L1 signaling pathway and causes cancer cell death by binding to the PD-1 protein used on the surface of tumor cells for immune evasion [52]. We can think of this as a more critical central node that can play a role in four or two shifts, and according to the power-law distribution of biological networks (few nodes with high connectivity and many nodes with low connectivity), we know that there are few such targets, and more nodes with small correlations [53].

Stem cells have a wide range of applications, such as epidermal stem cells, which exhibit more plasticity than normal cells, and are able to differentiate into all three germ layers after blastocyst injection in mice [54]. Even epithelial injuries at other sites, such as the cornea and urethra, may play a role [55, 56]. Slow vascularization and high sensitivity of cells to ischemia are obstacles to graft viability due to slow vascularization and cell transplantation. Safety concerns, such as tumorigenesis, also need to be explored in animal models [29].

Conclusion:

This article reviews the important role that stem cells can play in regenerative medicine today. In order to address the source of stem cells, there are methods to induce pluripotent stem cells and cell transdifferentiation, both of which can be induced by viral transduction of transcription factors or a combination of small molecules. In addition, the clinical potential of stem cells was demonstrated by combining the cell patch method for skin trauma to demonstrate the superiority of stem cell therapy. It also lays the foundation for the establishment of further artificial organs.

References