Mesenchymal stem cell-based gene therapy: A promising therapeutic strategy

Abstract

Mesenchymal stem cells (MSCs) are multipotent stromal cells that exist in bone marrow, fat, and so many other tissues, and can differentiate into a variety of cell types including osteoblasts, chondrocytes, and adipocytes, as well as myocytes and neurons. Moreover, they have great capacity for self-renewal while maintaining their multipotency. Their capacity for proliferation and differentiation, in addition to their immunomodulatory activity, makes them very promising candidates for cell-based regenerative medicine. Moreover, MSCs have the ability of mobilization to the site of damage; therefore, they can automatically migrate to the site of injury via their chemokine receptors following intravenous transplantation. In this respect, they can be applied for MSC-based gene therapy. In this new therapeutic method, genes of interest are introduced into MSCs via viral and non-viral-based methods that lead to transgene expression in them. Although stem cell-based gene therapy is a relatively new strategy, it lights a new hope for the treatment of a variety of genetic disorders. In the near future, MSCs can be of use in a vast number of clinical applications, because of their uncomplicated isolation, culture, and genetic manipulation. However, full consideration is still crucial before they are utilized for clinical trials, because the number of studies that signify the advantageous effects of MSC-based gene therapy are still limited.

Keywords::

• gene therapy
• homing
• mesenchymal stromal cell
• mobilization
• regenerative medicine

Introduction

Mesenchymal stem cells (MSCs) are a population of multipotent progenitors which reside in bone marrow, fat, and some other tissues. The potential for self-renewal and multipotency are hallmarks of stem cells in common, and MSCs in particular (Minayi et al. 2014). They can be easily isolated from bone marrow and fat for therapeutic goals (Mohammadian et al. 2013).

MSCs are considered to be the perfect candidates for applications in regenerative medicine, because of their significant potential in proliferation and differentiation. Increasing knowledge on the biology of MSCs has provided new insights into their possible clinical uses. The capacity of MSCs to differentiate into various cell lineages like adipocytes, chondrocytes, and osteocytes has led to a variety of investigational strategies to study whether they can be used for tissue engineering approaches. MSCs may also influence the course of chronic degenerative disorders through their trophic and regenerative potential (Maumus et al. 2011). MSCs are also able to be recruited into the location of tumor. The ability of MSCs to migrate to and engraft into the site of solid tumors brought about the idea of MSC-based gene therapy for cancers (Ji et al. 2004, Tögel et al. 2005, Herrera et al. 2007, Klopp et al. 2011).

As MSCs have received significant attention in the field of regenerative medicine, recent studies focus on the genetic modifying feature of MSCs with the aim of reinforcement of their regenerative capacity (Will et al. 2008, García-Castro et al. 2008). MSCs are used as the sources for delivering transgenes into patients (Uccelli et al. 2007, Jones and McTaggart 2008). They have newly emerged as a possible therapeutic vehicle for gene therapy and drug delivery, and their therapeutic usefulness is currently being assessed in a variety of disease models including inflammatory and autoimmune diseases, and cancer (Galderisi et al. 2010). This is probably due to their intrinsic ability to migrate toward tumors (Wang et al. 2009). The unique properties of MSCs in migrating to the site of damage or repair make them one of the most versatile stem cell populations for use in gene therapy studies and trials (Nauta and Fibbe 2007).

Stem cells

Stem cells are undifferentiated pluripotent cells which are found in multicellular organisms. They can differentiate into specialized cells, and are capable of self-renewal to produce more stem cells (Tuch 2006). Stem cells can be classified as embryonic or adult, depending on their tissue of origin. The role of adult stem cells is to sustain an established repertoire of mature cell types in essentially steady-state numbers over the lifetime of the organism. Adult tissues with a high turnover rate, such as blood, skin, and intestinal epithelium are maintained by tissue-specific stem cells (Drosse et al. 2008). Adult stem cells from the bone marrow are the most broadly used and characterized stem cells. Adult bone marrow includes a heterogeneous population of cells, including hematopoietic stem cells and their progeny, MSCs, adipocytes, fibroblasts, endothelial cells, etc. (Salem and Thiemermann 2010).

Stem cells are directed to differentiate into particular cell types indefinitely, which suggests the presence of a renewable source of alternative cells to treat various disorders (Abdi et al. 2008).

These cells have a great therapeutic tendency to repopulate damaged tissues, as well as the potential of being genetically manipulated and used in cell-based gene therapy (Hodgkinson et al. 2010, Huang et al. 2010).

Mesenchymal stem cells

More than 30 years ago, Friedenstein pioneered the concept that the marrow microenvironment resided within the so-called stromal cells of the marrow that were capable of abandoning the hematopoietic microenvironment to ectopic sites (Friedenstein 1991, Friedenstein et al. 1974). Since then, scientists began to explore the full potential of these cells, leading to the realization that this population of cells has the properties of true stem cells. These cells were officially termed MSCs (Caplan 1991). MSCs are non-hematopoietic progenitor cells that can proliferate and actively differentiate into various cell lineages such as osteoblasts, adipocytes, and chondrocytes. MSCs can be simply isolated from bone marrow aspirates, adipose tissue, and many other sources by their special feature of plastic adherence. They can be identified by expression of some surface molecules like CD73, CD90, and CD105, and the lack of specific hematopoietic markers including CD34, CD45, and HLA-DR. Subsequently, they can be expanded and genetically altered in vitro and then used for cancer therapeutic strategies in vivo (Mohammadian et al. 2013). The wide distribution of MSCs throughout the body leads to the assumption that these cells may play a critical role in organ homeostasis (Gotherstrom et al. 2005). They are promising tools for regenerative medicine, in the repair of injured tissues. Many studies have focused on two significant features of MSC therapy: (I) systemically administered MSCs home to sites of ischemia or injury, and (II) MSCs can modulate immunological responses (Khakoo et al. 2006, Qiao et al. 2008, Secchiero et al. 2010, Li et al. 2010, Li et al. 2011, Otsu et al. 2009, Tian et al. 2011, Dvorak 1986, Kucerova et al. 2007, Kidd et al. 2008) (Figure 1).

Figure 1. Benefits of MSC therapy. The cellular characteristics of MSCs are essential for their therapeutic uses.

MSC application in regenerative medicine

MSCs have attracted a lot of attention in clinical situations, mostly due to their potential in regenerative medicine and tissue engineering. They can be applied for tissue reconstruction, and repair occurs both at the bench and by the bedside. MSCs isolated from bone marrow are presently being examined for their therapeutic features in a diversity of disorders including kidney failure, cardiovascular injury, hematological and non-hematological malignancies, and neurological and bone disorders (Hodgkinson et al. 2010, Huang et al. 2010).

MSCs have exclusive therapeutic capacity because of their immunosuppressive and anti-inflammatory properties, via both innate and adaptive immune systems (English et al. 2009). The immunomodulatory features of MSCs allow the sustained release of therapeutic molecules, which elicits no undesirable immune responses even in the allogeneic or xenogeneic transplantations (Seo et al. 2011). Furthermore, the possible role of MSCs in angiogenesis is under consideration. Improved neovascular genesis has been related with regeneration of infracted myocardium by bone marrow-derived MSCs (Fuchs et al. 2001).

Homing of MSCs

Homing is the process by which cells migrate to, and engraft within the sites of ischemia or damage. It is a multistep process that begins with interactions between flowing MSCs and the vascular endothelium at the target tissue, mediated via some homing receptors expressed on MSCs that connect to the related endothelial co-receptors (Yagi et al. 2010).

An astonishing attribute of MSCs is their noticeable ability to migrate to the site of injury and engraft there, in vivo. They express chemokine receptors and ligands involved in the processes of cell migration and homing (Marlow et al. 2007, Kim et al. 2013). In this regard, MSCs are very promising candidates for clinical applications in cell-based replacement therapies (Mohammadian et al. 2013). For example, numerous investigations have shown that transplanted MSCs can home to injured tissues in animal models, including myocardial injury (Penna et al. 2008) and acute renal failure (Nauta and Fibbe 2007). For purposes of cell-based regenerative therapy, MSCs can be injected in close proximity to injured tissue, which may not be clinically possible in many cases due to potential invasiveness, for example, into the heart or brain. Moreover, locally injected cells often die because of diffusion limitations of nutrients and oxygen, before considerably exerting their therapeutic effects (Karp and Teo 2009, Muschler et al. 2004). The key to the therapeutic possibility and success of systemically injected MSCs is that MSCs have been shown to selectively migrate to the damaged or inflamed tissues (Karp and Teo 2009, Spaeth et al. 2008).

In addition, MSCs can recruit to the site of inflammation and tissue damage/repair, as well as toward the tumor environment. The tumor-trophic capacity of MSCs allows them to search and wipe out undetectable tumor cells (Ji et al. 2004, Tögel et al. 2005, Herrera et al. 2007, Klopp et al. 2011). The differences in the effects of MSCs on tumor growth depend on the tumor model, and on the dose and time of cell-based therapy (Aiuti et al. 2002). In vitro studies have expressed that cell contact between MSCs and tumor cells is not necessary for MSC antitumorigenic activity, as the antiproliferative effect was also observed with the MSC-conditioned medium (Hacein-Bey-Abina et al. 2002, Aboody et al. 2000, Benedetti et al. 2000, Herrlinger et al. 2000). In addition to the timing of delivery and dose, the delivery route is recognized to be a key factor determining the success of MSC therapy (Karp and Teo 2009).

Detailed research regarding MSC migration and the factors influencing this tropism, has made MSC-based gene therapies probable (Dao et al. 1997, Nolta et al. 1994, Pereira et al. 1998, Ringden et al. 2006, Brooke et al. 2007).

Stem cell-based gene therapy

Gene therapy is a new therapeutic branch of modern medicine which is still exceedingly experimental, but is progressively going forward toward becoming an important treatment strategy. Modifying the properties of stem cells may be required to fully utilize their potential. Genetic engineering, with plenty of methodologies to induce gene expression in a precise and well- controlled manner, is especially attractive for this goal (Nowakowski et al. 2013). Therapeutic genes should be introduced into the cell used for therapy. There are viral and non-viral-based methods of genetic manipulation (Gracey Maniar et al. 2013). Genes may be introduced into cells by transfection or transduction. Transfection utilizes chemical or physical methods for entry of DNA encoding the gene of interest into the target cells (Ma et al. 2003). Transduction is the stable integration of the gene of interest into the cell using viruses, which leads to stable transgene expression at high efficiencies and preserves the transgene over several passages during cell division and differentiation (Zare et al. 2014).

Stem cell-based gene therapy is a relatively new and highly experimental approach in the treatment of human disease. However, it has recently lit new hope for the treatment of various disorders. Researchers have of late applied MSCs as delivery vehicles for gene therapy, in part due to their accessibility for genetic modification in vitro and mostly due to the simplicity of their culture and expansion in vitro (Aboody et al. 2000, Bao et al. 2010, Maumus et al. 2011). These cells are appropriate for gene transfer, and as such, are excellent candidates for genetic manipulation to promote proliferation, enhance differentiation to the specific cell lineage, and avoid detrimental cellular dedifferentiation and senescence (Li et al. 2004).

There is hope to bring the results of research to clinical use. For instance, genetic manipulation of MSCs to express angiogenic factors is a promising approach for further improving the efficacy of stem cells for therapeutic angiogenesis. Virally modified VEGF-overexpressing MSCs were reported to increase angiogenesis in vivo and improve myocardial function (Jabbarzadeh et al. 2008, Matsumoto et al. 2005). Furthermore, genetically engineered MSCs have been used for expansion of hematopoietic stem cells in stem cell transplantation following myeloablative chemotherapy (Dominici et al. 2006, Dimmeler et al. 2005, Gussoni et al. 1999, Zhu et al. 2006, Yu et al. 2008, Djouad et al. 2006, Ramasamy et al. 2007, Wong 2011) (Figure 2).

Figure 2. Stem cells can be manipulated to express the enzymes that metabolize non-toxic prodrugs locally, therefore allowing production of the active form of the drug or the expression of cytokines that perform directly on the tumor or activate immune cells, which, in order, attack the tumor.

Delivery routes

Currently, human stem cells are being used to test new drugs. New medications are subjected to experiments on safety of differentiated cells generated from human pluripotent cell lines. Other types of cell lines have a long history of being used in this method. Cancer cell lines, for instance, are used to screen potential antitumor drugs. The viability of pluripotent stem cells would allow drug testing in a broader range of cell types. Stem cells, directed to separate into particular cell types, suggest the possibility of a renewable source of alternate cells and tissues to treat diseases including macular degeneration, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis. For instance, it may become probable to make healthy heart muscle cells in the laboratory and then to transplant those cells into patients with chronic heart disease (Abdi et al. 2008). In addition to the timing of delivery and dose, the delivery route is recognized to be a key factor determining the success of MSC therapy (Karp and Teo 2009). MSCs can be injected close by to injured tissue, which, though, may not be clinically possible in many cases due to its potential invasiveness—for example, into the heart or brain, and locally injected cells often die before considerably exerting the therapeutic effects because of diffusion limitations of nutrients and oxygen (Karp and Teo 2009, Muschler et al. 2004). The key to the therapeutic possibility and success of systemically injected MSCs is that MSCs have been shown to selectively “home” to injured or inflamed tissues (Karp and Teo 2009, Spaeth et al. 2008). For example, numerous investigations have shown that transplanted MSCs can home to injured tissues in animal models, including myocardial injury (Penna et al. 2008) and acute renal failure (Nauta and Fibbe 2007).

Rheumatologic disorders

Increasing knowledge on the biology of MSCs has provided new insights into their possible clinical uses, mainly for rheumatologic disorders. Their ability to differentiate into cells of the bone and cartilage lineages has led to a variety of investigational strategies to study whether MSCs can be used for tissue engineering approaches. Presently, an emerging field of study comes from the possibility that these cells, through their trophic/regenerative potential, may also influence the course of chronic degenerative disorders and put off cartilage degradation in osteoarthritis (Maumus et al. 2011). Bone engineering strategies are warranted in cases of large bone defects or non-union fractures, which remain a serious problem as the associated loss of function considerably impairs the quality of life of affected patients. For successful tissue engineering approaches, implantation of these cells will need the use of growth and differentiation factors that will allow the induction of the particular differentiation pathways and the preservation of the bone or chondrocyte phenotype, together with a suitable scaffold to supply a three-dimensional environment (Drosse et al. 2008, Will et al. 2008).

Immunologic characteristics

Considerably, MSCs, which, as discussed, are immunologically privileged, have extremely strong immunosuppressive effects on a diversity of immune cells via cell–cell contact or soluble factors (summarized in Figure 3) (Jones and McTaggart 2008, Nauta and Fibbe 2007).

Figure 3. Immunoregulatory effects of MSCs on immune cells. CTL, cytotoxic T cells; HO, heme oxygenase; iDCs, immature dendritic cells; INOS, inducible nitric oxide synthase; mDCs, mature dendritic cells; NK cells, natural killer cells; PG, prostaglandin; Treg, regulatory T cells.

Researchers have found that the direct contact between MSCs and T cells, which modulates the expression of cytokine receptors and transduction molecules for cytokine signaling, also contributes remarkably to the inhibition of T cell proliferation (Abdi et al. 2008, Nauta and Fibbe 2007).

Conclusion

MSCs have huge potential for regenerative medicine due to their intrinsic capacity of self-renewal and differentiation, as well as immunomodulatory activity. In addition, MSCs, because of their ability of mobilization to the site of damage, can be used to restore tissue function by providing some paracrine trophic effects to target tissue. The last feature, in addition to the simplicity of isolation and culture, make them an excellent tool for cell-based regenerative medicine to improve damaged heart tissue. Moreover, they can be utilized for gene therapy by manipulation their DNA by so many genetic engineering methods.

Although MSCs might suggest a great hope for regenerative medicine, full consideration is still essential before they are marketed for clinical trials. Yet, the number of studies that signify advantageous effects of MSC-based gene therapy is hard to dismiss.

Authors’ contributions

MM and EA conceived of the study and participated in its design and coordination. AA participated in the sequence alignment and drafted the manuscript. All authors read and approved the final manuscript.

Acknowledgments

The authors thank the Department of Medical Nanotechnology, Faculty of Advanced Medical Science of Tabriz University, for all support provided. This work is funded by a 2014 grant by the Drug Applied Research Center, Tabriz University of Medical Sciences.

Declaration of interest

The authors have no declaration of interest. The authors alone are responsible for the content and writing of the paper.