Introduction

Stem cell therapy has become one of the fastest growing fields of research in the world following the first isolation of human embryonic stem cells (hESCs) in 1998. Several adult stem cells, progenitor cells, and induced pluripotent stem cells (iPSCs) have since been isolated and characterized with respect to their potential clinical benefit. Due to the unique characteristics of stem cells, namely self-renewal and differentiation potential, stem cell therapy has the potential to treat cardiac diseases, superficial wounds, neurologic diseases, and type I diabetes [1–3]. Transplanted stem cells can rebuild or replace dead tissues and recover existing cells through paracrine effects. However, stem cell therapy has several limitations that must be resolved prior to clinical use. The stem cell differentiation process in most cases is still necessarily heterogeneous, and ensuring uniformity is critical for preventing tumorigenic potential. The activation of an immune response along with an otherwise inhospitable host environment results in a low viability for the majority of transplanted cells [4]. Many research groups have approached these problems through the development of molecular delivery systems, composed of particles and scaffolds, for inserting useful proteins and genes into stem cells.

Through the use of double-stranded DNA that is integrating or not into the host genome as well as double or single-stranded RNA techniques, biological states of stem cells such as differentiation, self-renewal, and growth can be controlled [5, 6]. Numerous stem cell genetic engineering strategies have been employed to increase cell survival rate, control differentiation, and produce therapeutic factors with exciting results. Despite positive reported outcomes, the genetic manipulation of stem cells faces several problems for ultimate clinical translation, some of which are common across many applications and some of which are unique. Not only are there challenges with increasing the efficiency of transfection, controlling targeting, limiting mutagenic potential, and reducing cytotoxicity, but also whatever delivery system is employed must maintain stem cell differentiation status and viability. Research has therefore largely been on the materials and methods for delivering DNA and RNA in order to improve the therapeutic potential of stem cell therapy.

Gene transfection agents are generally categorized as either viral or non-viral. Viral vectors dominate because of high efficiency and long-term maintenance of expression, but concerns including immunogenicity, carcinogenicity, restricted DNA loading capacity, and high cost for mass production impede their commercialization and clinical use [7–9]. In contrast, non-viral vectors avoid the shortcomings of viral vectors, but the efficiency of transfection is compromised. In the case of stem cell transfection, the rate of successful modification (40 % by electroporation, ∼20–35 % by cationic polymer and liposome, 80 % by efficient viral vectors) is lower than that for differentiated cells [10]. To optimize the transfection efficiency of stem cells, research into making an ideal non-viral vector which has high efficiency and low cytotoxicity is currently underway.

Herein, we discuss several DNA and RNA agents for stem cell therapy and various non-viral methods for stem cell transfection.

Genes for Stem Cell Therapy

In order to increase the utility of stem cells as therapeutic agents, various genes encoding transcriptional factors for cellular reprogramming, control of differentiation, or the production of therapeutic proteins can be transfected into stem cells. Genes for increasing the survival rate of transplanted stem cell or to control stem cell homing to sites of therapeutic interest have also been studied (Fig. 1).

Fig. 1
figure 1

Application of genetically engineered stem cell for cell therapy. The top panel demonstrates transfection of stem cells to silence inhibitors of mineralization. This was accomplished via a CPP complexation strategy with miRNA (adapted from Suh et al. [64•], with permission from Elsevier). The middle panel shows a proposed scheme for cancer therapy using MSCs as carriers. Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) transfected into MSCs by PEI600-Cyd was injected to C57BL/6 mice for estimating the efficiency of cancer gene therapy (adapted from Hu et al. [28], with permission from the American Chemical Society). The bottom panel indicates a schematic flow for synthesis of magnetic nanoparticles which have multiple roles in transfection and magnetic resonance imaging (MRI). Catechol-functionalized polypeptide (CFP) was used for functionalizing iron oxide nanoparticles, and PEI was attached for plasmid condensation (adapted from Park et al. [76•], with permission from Elsevier)

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Genes for Reprogramming and Directing Differentiation of Stem Cells

Stem cells for therapeutic purposes can include pluripotent stem cells such as iPSCs and embryonic stem cells (ESCs) as well as multipotent stem cells such as mesenchymal stem cells (MSCs) and tissue-specific progenitor cells. The groundbreaking discovery of iPSCs raised the hope for personalized medicine [11]. IPSCs are a particularly attractive therapeutic cell source because they possess the desirable properties of ESCs including unlimited self-renewal and pluripotency while potentially circumventing immune rejection and ethical issues that are roadblocks to clinical translation. The original reprogramming cocktail contained four genes encoding transcriptional factors: Oct3/4, Sox2, c-Myc, and Klf4 [11]. Later studies revealed that some of these factors are not absolutely required. Yamanaka’s group showed that reprogramming can be achieved without c-Myc, and the resulting iPSCs were of high quality and minimally tumorigenic [12]. Another study reported that a single gene, Oct4, is sufficient to achieve reprogramming when supplemented with certain small molecules [13]. Generally, the choice of reprogramming genes needs to be carefully considered since the genes not only determine the reprogramming efficiency and the subsequent quality of iPSCs, but also have an effect on the safety of the cells.

While direct injection of adult stem cells like MSCs into circulation or damaged tissue has achieved positive results [14] and, in some therapeutic applications, has reached advanced clinical trials, pluripotent stem cells (ESCs and iPSCs) require appropriate differentiation, specifically to eliminate the risk of tumorigenesis. Generating relevant cell types can increase the specificity and outcome of both adult and pluripotent stem cell-based therapies. To guide differentiation, various transcription factors for inducing cardiogenesis (Gata4, Mef2c, and Tbx5), osteogenesis (bone morphogenetic proteins, bone morphogenetic protein-2 (BMP2) and BMP7), chondrogenesis (SOX-5, SOX-6, SOX-9), and neurogenesis (SOX-1, SOX-2, SOX-3, miR-124, miR-137, miR-184, and MBD1) are used for stem cell transfection to the respective tissue of interest [15–23]. Differentiated stem cells can then be injected to target tissues and replace damaged cells. Yet, since the differentiation process usually requires sequential and temporal expression of specific gene groups (as was shown for iPSCs generation), the clinical applicability of genetic engineering for directing stem cell differentiation remains an open question, unless the role and specificity of additional master key gene switches can be identified (like MyoD for skeletal muscle).

An alternative approach to stem cell differentiation based on overexpression of transcriptional factors is through gene silencing. Toward this aim, microRNAs are prime candidates for genetic engineering considering their role in controlling the expression of multiple transcription factors and in regulating key biological process including cell differentiation. For example, ESCs, genetically engineered to overexpress microRNA-1 (a cardiac/muscle-specific miRNA), efficiently differentiated in vivo to cardiomyocytes (100 % more than control ESCs) and contributed to repair of the damaged myocardium tissue postmyocardial infarct [24]. We have shown that the heterogeneity of early differentiating hESCs can be significantly reduced through efficient RNA delivery. Specifically, the mesoderm layer formation of embryonic stem cells can be selectively enhanced by as much as 90-fold through the transfection of siRNA which silences the KDR receptor gene [25].

In the more near term, genetic engineering can greatly benefit stem cell therapy by introducing fluorescent reporter genes under the control of cell type-specific promoters. Such fluorescent cell engineering allows, via fluorescent activated cell sorting, for the identification and enrichment of specific cell populations or the generation of pure cell populations for transplantation by isolating pluripotent stem cells from heterogeneous differentiating cells [26, 27].

Genes for Specific Therapeutic Purposes

Depending on the purposes of the treatment, therapeutic genes can confer specific properties to stem cells for diverse applications including HIV resistance, angiogenesis, and tumor suppression. Transfected adult stem cells can be particularly well suited for use as a drug delivery vehicle due to their immune privilege [28]. For anti-tumor applications, genetically modified MSCs expressing interleukin-2 (IL-2) augmented the anti-tumor effects of these cells compared to unmodified cells in a mouse glioma model [29]; another study using interferon-β (INF-β)-expressing MSCs eradicated the tumors in 70 % of treated mice pre-implanted with human ovarian cancer cells [30]. In the area of angiogenic therapy, MSCs transfected by vascular endothelial growth factor (VEGF) plasmid exhibited high angiogenesis potential, increasing vessel densities by 2–4-fold compared to control groups in a mouse hindlimb ischemia model [31]. Likewise, overexpression of fibroblast growth factor-2 (FGF-2) in MSCs significantly improved cell survival by 3-fold under hypoxic conditions in vitro and expressed cardiac specific markers [32]. More recently, a genetic engineering approach showed promise in treating patients with HIV. IPSCs derived from HIV patients were edited with clustered regularly interspaced short palindromic repeats-Cas9 (CRISPR-Cas9) to derive a mutation in the C-C chemokine receptor type 5 (CCR5) gene, which ultimately granted HIV resistance to the iPSCs and to any cell type differentiated from them [33••].

Genes to Enhance Stem Cell Homing and Viability

One vital challenge to be resolved for stem cell therapy is the homing of cells to the damaged tissue. The homing capacity of MSCs has been the subject of intense research. Studies have revealed numerous chemokines and growth factors responsible for stem cell homing to damaged myocardium (as reviewed in [34]), including stroma-derived factor-1α (SDF-1α), chemokine (C-X-C motif) receptor 4 (CXCR4), hepatocyte growth factor (HGF), and FGF-2. Thus, to increase the homing of stem cells, one can genetically overexpress corresponding receptors for these homing signals. These strategies can also be applied to increase homing of ESC- and iPSC-derived cells.

In addition to homing, another challenge for cell therapy is that transplanted stem cells frequently have poor survival and incomplete engraftment into injured or diseased tissue because of inflammation, hypoxic stress, and insufficient perfusion. Apoptosis and autophagy may contribute to their low survival rate, but the exact mechanisms initiating these pathways are still unclear. Various proteins, growth factors, and nucleic acids that are related to ischemia, apoptosis, and autophagy have been studied in an attempt to improve stem cell viability. Anti-apoptotic genes such as B cell lymphoma 2 (Bcl-2) and Akt [35, 36] and growth factors including VEGF, angiopoietin (Ang-1), and transforming growth factor beta 1 (TGF-β1) [37, 38] have been transfected into stem cells in order to prevent apoptosis and increase survival. In the case of hypoxic stress, methods have been described for overexpressing heme oxygenase (HO-1), which is an anti-oxidant and anti-inflammatory protein, as well as silencing prolyl hydroxylase, a cellular oxygen sensor that controls hypoxia-inducible factor and nuclear factor-κB [36, 39•].

Transfection Methods for Stem Cell Therapy

The ideal transfection agent must prevent degradation of the delivered gene, penetrate the target cell membrane, and allow for the insertion of the gene into the nucleus. Furthermore, the agent would ideally be harmless to cells [40, 41]. As base materials for non-viral vectors, liposomes, micelles, polymers, dendrimers, peptides, and inorganic nanoparticles have been studied and used for delivery to stem cells. Other techniques such as electroporation, sonoporation, and microfluidic devices for transfection have been developed recently. Gene vectors may traffic via direct penetration into the cell or by endocytosis with the route of entry based on the physical and chemical characteristics of both the cargo and transfection agent. However, the precise relationships between transfection agents and mechanisms are still unclear.

Lipids, Liposomes, and Micelles

Liposomes, which fulfill several of the criteria for the ideal transfection agent, can be fabricated with various natural and/or synthetic lipids in order facilitate DNA or RNA encapsulation. Unlike cationic polymers, charged lipids associate via hydrophobic interactions among their aliphatic tails forming liposomes and micelles. Their size (∼100 nm) and the cationic character of the head group or modifications thereof facilitate carrying anionic oligonucleotides. Chemical modification techniques such as PEGylation and RGD attachment may limit their clearance from circulation and enhance their targeting ability, respectively [42]. However, since liposomes are usually responsive to environmental conditions such as pH, temperature, salt, and other proteins such as serum, they are unstable in diverse biological systems [9]. While this instability can be used to tune nucleic acid release inside the cell, care must be taken to maintain stability prior to cellular uptake. The positive charge of the majority of cationic liposomes also has a detrimental effect on cell viability; however, surface charge can be altered. Poly(ethyleneimine) (PEI), and metal nanoparticles are commonly used in conjunction with liposomes, the former to avoid cytotoxicity of free PEI and the latter to increase conjugate stability [43, 44]. The advantage of using liposomes as gene vectors is their chemical versatility. Liposomes can be fabricated from a host of different lipids with varying chemistries and tailored to a specific response. Cationic lipids have been designed with hydrophilic head groups such as ammonium groups, polyamines, and guanidinium groups and hydrophobic tail groups including aliphatic chains, steroids, and fluorinated domains. Hydrophilic head groups primarily control the interaction with oligonucleotides and cellular membrane based on their basicity and hydrogen bonding, while hydrophobic tail groups regulate the fluidity and stability of liposomes [9]. These two molecular regions of the lipid influence the liposomes’ ultimate transfection efficiency, interaction with oligonucleotides, and cytotoxicity. The versatility of liposome fabrication has led to the development of high-throughput screening techniques. For example, screening of a synthetic lipid library generated by thiol-yne click chemistry revealed that the length of hydrophobic tail (C11 or C12), size of lipoplex (complex of liposome with nucleic acid, between 100 and 200 nm), addition of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and stable positive charge of lipoplex (above +50 mV) all affect the resulting transfection efficiency [45•].

Cationic Polymers and Dendrimers

Many cationic polymers including poly-l-lysine (PLL) and PEI have been studied for condensing and delivering negatively charged oligonucleotides. The quest to find the ideal gene transfection agent has generated diverse cationic polymers, varying in molecular weight, number of branches, and characteristics of side groups, such as primary, secondary, or tertiary amines. Yet, the high cytotoxicity of these agents caused by positive localized charge has led to the development of several chemical and/or physical modification strategies. These included rendering the polymer backbone biodegradable, generating heterocyclic amine-derivatized polymers and PEGylated polymers [46, 47], crosslinking low-molecular-weight polymers with small molecules or lipid grafting [48, 49], and combining cationic polymers with other molecules including lipids [50]. The majority of these modifications are attempted to improve the toxicity of the polymers by making them degradable or rendering the relative size of the agent larger. Cationic polysaccharides have inherent characteristics that are well suited for use as a transfection agent. In general, they are biocompatible, biodegradable, and non-toxic. Some cationic polysaccharides have built in specific cell targeting capabilities. For example, pullulan and curdlan have binding sites for Dectin-1 receptors uniquely found on dendritic cells, macrophages, and B cells [38, 51, 52]. Alternatively, researchers have generated libraries of synthetic biodegradable cationic polymers such as poly beta-amino esters (PBAE) and utilized high-throughput screening strategies to identify candidates that will efficiently transfect ESCs and MSCs [21, 53, 54, 55••]. A major research thrust involving cationic polymers has been devoted to enhancing the escape of polymer/oligonucleotide polyplexes from the endosome, one of the leading hurdles for gene transfection. Toward this aim, environmentally sensitive polymers have been designed with improved transfection efficiency and cytotoxicity. During endocytosis, the pH of the endosome is reduced; such a change can trigger a proton sponge effect in these polymers, which then disrupts the endosomal membrane and leads to release of gene cargo into the cytoplasm. Various amine-modified PEIs having pKa values from 5 to 6 can cause endosomal escape via osmotic burst [46]. Poly ethylacrylic acids (PEAA) which exhibit a transition in their hydrophilicity at pH 5–6 can also disrupt the endosomal membrane [47, 56].

Dendrimers are large, monodisperse, highly branched molecules. They usually have a globular structure and can exhibit specific functional groups on their surface. Since dendrimers are synthesized through recurrent chemical reactions, their size is predictable, typically 1 to 10 nm [57]. These strengths make dendrimers advantageous for use as designing gene delivery agents, and many amine group-containing dendrimers such as poly(amidoamine) (PAMAM), melamine-based dendrimers, and polypropylenimine (PPI) dendrimers have been studied [9, 58]. Notably, PAMAM dendrimers have been shown to efficiently deliver interfering RNA to ESCs compared to PEI-based lipopolymer controls. In order to enhance their transfection efficiency and specificity and reduce cytotoxicity, modifications such as PEGylation, RGD attachment, and charge modification have been applied [59, 60].

Peptides

Since peptides are the molecular building blocks of proteins conferring numerous biological functions, peptides have been investigated as gene transfection agents. In order to interact with oligonucleotides, penetrate the cell membrane, and traffic genes into the nucleus, specific peptide regions are employed. Peptides can be designed to contain DNA-binding domains, cell-penetrating peptides (CPPs), also known as peptide transduction domains, and nuclear localization signal (NLS) peptides. CPPs usually have basic amino acids that possess both positive charge and amphipathic secondary structure for interacting with cell membranes and destabilizing them [61]. Various natural and synthetic CPPs including TAT, penetratin, R8 and R9, Pep family, transportan, and GALA have been studied for their ability to transfect genes, used to generate novel synthetic CPPs, and modified through lipid and peptide addition [18, 62, 63, 64•]. However, CPPs with high positive charge can also be cytotoxic and demonstrate non-specific interaction with serum proteins. Recently, novel hydrophobic CPPs from signal peptides and Bax-inhibiting peptides were found. In particular, Bax-inhibiting peptides have been conjugated to non-toxic nanoparticle carriers to improve cellular uptake of gene cargo into MSCs. This is of particular interest as MSCs typically exhibit low transfection efficiencies [10]. The reported role of NLS peptides has been to import proteins into the nucleus, another critical step in the transfection process. These amino acid sequences are also non-toxic and can penetrate cell as well as nuclear membranes, thereby increasing transfection efficiency [65]. Studies are ongoing to determine the precise biophysical mechanisms for peptide insertion into membranes [18, 66, 67].

Inorganic Nanoparticles

Inorganic solid particles with a 10–1000-nm size have many advantages as gene transfection agents. As a result of a solid core, they are more stable than liposomes and can present a high density of surface ligand. Their small size facilitates cell penetration, and inorganic nanoparticles can protect nucleic acids from degradation during cellular trafficking [68•]. A range of inorganic materials such as gold, silver, calcium phosphate, iron oxide, silica particles, quantum dots, and carbon nanotubes have all been studied as potential gene vectors. The non-toxic and readily modifiable characteristics of gold make them a leading candidate for gene transfection [10, 44]. Silver, which is known for its bactericidal effect yet can also be cytotoxic to mammalian cells, has been recently used for generating a photo-activated delivery agent. Silver has the advantage of having an approximately ten times greater localized surface plasmon field than gold [68•, 69]. Calcium phosphate, a non-toxic mineral, can interact with DNA and destabilize the endosomal membrane in response to acidic pH. It has been used as transfection agent for over 30 years. Recently, a wide variety of calcium phosphate nanoparticles and scaffolds have been studied for the dual role of increasing transfection efficiency as well as serving as a substrate for tissue regeneration [70•, 71–73]. Iron oxide, another mineral in the form of nanoparticles, has been applied to make multifunctional transfection agents because of its superparamagnetic character. Transfection agents that can be detected through magnetic resonance imaging (MRI) were recently synthesized, and it was demonstrated that applying oscillating magnetic fields improves the transfection efficiency of this agent [74, 75, 76•]. Lastly, the prospect of mesoporous silica particles (MSNs) as effective transfection agents has been studied. The nanometer size of pore channels can contain cargo effectively, and the easily controllable pore size and surface chemistry make MSNs an attractive transfection agent. By using aminated MSNs, Kim et al. showed production of BMP2 in mesenchymal stem cells (MSCs) of 66 % efficiency [15].

Electroporation, Sonoporation, and Microfluidic Devices

In an effort to develop transfection methods without cytotoxic carriers, several researchers have studied transfection with physical methods such as electroporation, sonoporation, and laser irradiation. Among them, reports on electroporation are most common; however, the high voltages used can cause cell death [77]. Instead, sonoporation and various microfluidic devices have been developed to improve physical gene transfection. In the case of sonoporation, several studies have reported that ultrasound-mediated microbubble burst increases gene transfection efficiency both in vitro and in vivo [78, 79]. Microfabricated fluidic devices have also been used in order to overcome limitations with gene transfection. Sharei et al. used a microfluidic platform that minimally deforms cells in order to make the membrane permeable when cells flow through a narrow path. This platform was then used to introduce reprogramming proteins into fibroblasts, exhibiting a 10-fold improvement in iPSC generation compared to other non-viral methods [80••]. Others have focused on constructing electroporation-based microfluidic devices that reduce the voltage necessary for transfection. Xi et al. generated a nanoelectroporation platform composed of alumina nanostraws for reducing electroporation voltage and increasing uniformity over a large area [81]. Kang et al. fabricated a two-level electroporation device composed of a cell culture chamber situated on top of a series of microchannels. The microchannels were designated for loading transfection materials and reducing the voltage through localized electroporation using a nanofountain probe. This microfluidic device showed 50 % plasmid transfection efficiency with significantly lower voltage (10 V) than traditional electroporation [77]. Lastly, Grigsby et al. designed a microfluidic system for producing polymer-nucleic acid nanoparticle that is smaller, more uniform, with less aggregation than otherwise possible. The physical characteristics of nanocomplexes enhanced the transfection efficiency in various cell lines including both stem cells and primary cells [82].

Conclusions and Future Directions

Diverse genes and viral and non-viral methods have been studied for stem cell transfection. As a whole, stem cells are generally more difficult to transfect than their primary cell counterparts and care must be taken to control differentiation status, viability, and tumorigenicity. The focus of gene transfection for stem cell therapy has been the insertion of genes controlling differentiation, preventing apoptosis, enhancing angiogenesis, and encoding therapeutic proteins. Stem cells are transfected in order to increase their survival rate and to subsequently improve therapeutic efficacy.

Future advances will likely involve specific nucleases such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR/Cas in order to improve the precision of genome editing, prevent mutations and maintain stable, long-term gene expression. Since stem cell mutations, particularly with embryonic stem cells, caused by improper genome editing produces tumors, accurate genome engineering is absolutely essential. In the past decade, engineered nucleases consisting of non-specific nucleases and sequence specific DNA-binding domain have been studied. Among several systems, ZFNs and TALENs that embody a flexible editing system dominate genome engineering. The 2010 discovery of the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas system, which is a bacterial immune mechanism for removing foreign genes, offered a path toward very precise and flexible genome engineering. A system composed of CRISPR/Cas was generated and applied in 2013 [83] Most recently, the Zhang group has identified a new CRISPER enzyme, cf1, that offers a more precise and simpler gene editing than Cas [84]. It will be interesting to see its impact on genetically engineered stem cells. New gene engineering tools such as ZFNs, TALENs, and CRISPR modules offer great potential, yet they still need to be delivered into cells and therefore have been used with various viral and non-viral vectors with similar limitations related to transfection efficiencies [85, 86, 87••, 88–92].

As has been shown previously, it is possible to combine strategies for gene transfection along with other therapeutic purposes such as cell tracking (as in the case of metal nanoparticles), cell sorting (using genes encoding fluorescent proteins), and, more recently, gene delivery via scaffolds that may also allow for cell growth and tissue remodeling. Many biocompatible and functionalized scaffolds, made by electrospinning, lithography, microfabrication, and self-assembly, have been developed for tissue regeneration. Various biocompatible and biodegradable compounds such as poly(lactic-co-glycolic acid) (PLGA), poly-caprolactone (PCL), poly(amido amine), PEG, collagen, and chitosan have been used and studied [17, 93–100]. Researchers have devised gene-activated scaffolds for combination gene and cell therapy. Several characteristics of scaffolds may be particularly well suited toward use in combination with gene therapy including transfection agent preservation, sustained agent presentation via controlled release, and the enhancement of transfection through the maintenance of the proper 3D stem cell microenvironment. Gene-activated biomaterials for stem cell differentiation toward osteogenesis [17, 98, 99] and chondrogenesis [95, 97] have been intensively studied due to the requirement of a biomimetic platform. Murphy’s group has described a high-throughput 3D scaffold screening system for analyzing diverse transfection factors and optimizing mineral coating for osteogenesis of hMSC [93, 100]. This work may serve to inform future combination strategies.

Various materials containing lipids, polymers, dendrimers, peptides, nanoparticles, and physical transfection agents have been considered as transfection carriers. Of non-viral gene carriers, no one solution has emerged as optimal, and therefore, there is a need for investigation into multifunctional carriers that combine several strategies together. We speculate that such multifunctional carriers can eventually lead to designing transfection agents that are both non-toxic with high transfection efficiency. Considering the unique factors that prevent safe and efficient transfection of stem cells, several research groups have approached the problem by constructing material libraries and combine existing materials in order to have a large search space for obtaining the optimal solution. A large body of existing literature in adult stem cell transfection will serve as the base for understanding gene transfection mechanisms in stem cell transfection.