Therapeutic potential of mesenchymal stem cells in treating both types of diabetes mellitus and associated diseases

Abstract

Diabetes mellitus is a common lifestyle disease which can be classified into type 1 diabetes mellitus and type 2 diabetes mellitus. While both result in hyperglycemia due to lack of insulin action and further associated chronic ailments, there is a marked distinction in the cause for each type due to which both require a different prophylaxis. As observed, type 1 diabetes is caused due to the autoimmune action of the body resulting in the destruction of pancreatic islet cells. On the other hand, type 2 diabetes is caused either due to insulin resistance of target cells or lack of insulin production as per physiological requirements. Attempts to cure the disease have been made by bringing drastic changes in the patients’ lifestyle; parenteral administration of insulin; prescription of drugs such as biguanides, meglitinides, and amylin; pancreatic transplantation; and immunotherapy. While these attempts cause a certain degree of relief to the patient, none of these can cure diabetes mellitus. However, a new treatment strategy led by the discovery of mesenchymal stem cells and their unique immunomodulatory and multipotent properties has inspired therapies to treat diabetes by essentially reversing the conditions causing the disease. The current review aims to enumerate the role of various mesenchymal stem cells and the different approaches to treat both types of diabetes and its associated diseases as well.

Introduction

Diabetes mellitus (DM) is the most prevalent disease across the globe since the late twentieth century. An estimated 9.3% of the global population (493 million) suffers from some form of diabetes as of 2019 [1], which has already surpassed all predictions made two decades ago. India alone has a population of at least 11.8% (62 million) diabetic individuals, and predictions only suggest a rise [2]. Further, it has been reported that diabetes alone had caused 4 million deaths globally in 2017. It is estimated that out of the globally affected people suffering from diabetes, 425 million suffer from either type 1 diabetes mellitus (T1DM) or type 2 diabetes mellitus (T2DM) [3], of which, approximately 90% of this population suffer from T2DM. However, the incidence rates of T1DM are also rising, but the cause is unclear [4, 5]. The WHO had reported that the global prevalence of diabetes in young adults has increased from 4.7% in 1980 to 8.5% in 2014. Additionally, according to the WHO, diabetes is a major cause for blindness, kidney failure, heart attack, stroke, and limb amputation. Hence, it is vital to understand what current treatments are lacking to prevent this rapid increase in the diabetes pandemic.

Diabetes is caused due to poor lifestyle which not only causes hyperglycemia but is complemented by a host of clinical conditions including retinopathy, nephropathy, liver malfunction, ketonuria, and diabetes foot. Being a disease caused due to lifestyle, the most common way of treating diabetes is by weight loss, regular exercise, and a strict healthy diet. However, these lifestyle changes are often not followed by the patients. Thus, clinicians need to resort to pharmaceutic drugs such as insulin, biguanides, sulfonylureas, meglitinides, thiazolidinediones, amylin, sodium-glucose co-transport 2 (SGLT-2) inhibitors, glucagon like-peptide 1 (GLP-1) receptor agonists, and dipeptidyl peptidase 4 (DPP-4) inhibitors [6, 7]. In severe cases of T2DM, the patient may need to undergo bariatric surgery to reduce weight which at times may prove fatal [8]. These available treatments, however, do not actually cure the patient and are not a permanent solution to the disease [9]. Therefore, the current treatments lack the rigor to prevent diabetes. Due to this, the focus to treat diabetes has shifted from pharmaceutics to stem cell–based therapy [6, 9].

In the past decades, various stem cells and their sources have been identified. Among the plethora of stem cells available, researchers have pinned their hopes on mesenchymal stem cells (MSC) as a potential therapeutic agent. MSC have been a favorite among researchers owing to their hypoimmunogenic nature, multipotent nature, and relative abundance in human tissue. MSCs are derived from various sources like bone marrow (BM), adipose tissue (AT), cord blood (CB), dermal papilla (DP), and placenta (P). Certain characteristic properties are common to MSC such as plastic adherence, expression of specific cell surface markers, and differential capability [10, 11]. MSC derived from placenta, CB, and Wharton Jelly (WJ) provides a readily accessible source of it. While BM-MSC is being used as a gold standard in applying MSC for treatment of diabetes, recent studies have shown that AT-MSC, CB-MSC, and P-MSC produce a similar effect [12–14]. The level of immunomodulation exhibited by AT-MSC, CB-MSC, and P-MSC was reported to be similar to that of BM-MSC [15–22]. Nevertheless, MSC derived from different sources varies slightly from each other based on the cytokines and genes expressed [11, 23–26]. Studies have further revealed that MSCs derived from different adult tissues and neonatal tissues have different doubling times [27–30], thus affecting their differentiation capacity differently [31]. A study reported that mean doubling time of WJ-MSC was approximately 40 h. AT-MSC had a mean doubling time of nearly 46 h; P-MSC had a mean doubling time of 57 h, while BM-MSC had a mean doubling time of 71 h [32]. However, the proliferative rates vary depending on the media used and culture condition. Hence, by ascertaining the doubling time, one can induce differentiation at an optimal passage number [31].

Another desirable trait of MSC is the variations in cytokines expressed by them. Furthermore, MSCs derived from different sources differ in the cytokines expressed and aid in understanding the MSC-dependent immuno-modulation and homing property of MSC. Cytokine variations help determine cell contact interactions as well. MSCs are considered to be hypoimmunogenic as they do not exhibit Fas ligand, CD14, CD40, CD80, and CD86 surface markers [33, 34]. They lack expression of CD14, CD45, CD11A/lymphocyte function associated antgen-1, glycophorin A, and CD31 (platelet and endothelial cell) surface markers [35, 36]. The immunomodulatory response of MSC aids in reducing allergies, immune rejection, and graft vs host disease [37] by changing secretion pattern of dendritic cells (DC) leading to increase in concentration of anti-inflammatory cytokine interleukin (IL)-10 and decrease in concentration of proinflammatory interferon-γ (IFN-γ) [38]. Thus, MSCs are a viable cell therapy option for treating diabetes mellitus and its associated diseases due to the characteristics exhibited by them.

MSC therapy for diabetes-associated diseases

MSCs can act as therapeutic agents for diabetes-associated diseases: diabetic cardiomyopathy (DCM), diabetic nephropathy, diabetic neuropathy, diabetic retinopathy, diabetes foot, and diabetic wounds [39]. For instance, DCM occurs due to prolonged hyperglycemia resulting in ventricular dysfunction of the heart [40]. DCM is characterized by hypertrophy and apoptosis of the cardiomyocytes and degradation of the extracellular matrix leading to increased collagen deposition. There is also a drastic increase in functioning of the matrix metalloproteinase (MMP) which contributes to apoptosis and collagen deposition [41, 42]. Thus, pathological signs such as defective microcirculation, necrosis, and fibrosis are observed [43]. Studies demonstrate that MSCs can help in angiogenesis and myogenesis by secreting several growth factors like vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF-1), and hepatocyte growth factor (HGF) [43, 44], hence aiding in regeneration of vascular tissue.

While in case of diabetic nephropathy (DN), a condition of the kidney where an excess supply of glucose to the nephron results in angiopathy of the blood vessels and capillaries supplying the glomeruli and release of proteins into urine. In an experiment conducted on NOD/SCID mice, human MSCs (hMSCs) were transplanted and differentiated into renal cells; this regenerated the necrotic segments of the kidney and did not induce any immune response [45, 46] as well. A few MSCs also differentiated into the endothelial cells, replenishing the blood capillaries in the glomeruli which was further proven by CD31 marker [46]. Since MSCs also developed into insulin-producing cells (IPCs), glycosuria was also reduced [13]. In a recent study, NSG mice with induced DN when treated with MSC displayed inhibition in fibrosis. Secretion of anti-fibrotic extracellular vesicles derived from MSC reversed the action of streptozotocin, thus preventing the progress of chronic kidney injury [47, 48].

However, diabetic polyneuropathy (DPN) is the most common and most evident form of complication that is observed in DM [40]. Its symptoms include neuron damage, hyperalgesia, decreased sensation, and lowered blood supply to the nerves [40, 49]. Even though MSCs are capable of differentiating into neural cells, astrocytes, oligodendrocytes, and Schwann cells, they did not show a notable effect in the therapy of DPN [46]. Studies have proved that MSCs lead to the re-establishment of muscles and blood capillaries by production of FGF and VEGF, improved hyperalgesia, better functioning of nerve fibers, but delayed the nerve conduction velocity [50].

Another complication due to diabetes mellitus is diabetic retinopathy (DR). A vascular disorder of the eye caused by prolonged DM. It is signified by the formation of macular edema, pericyte loss in the blood vessel, thickening of basement membrane, and dysfunction of endothelial cells. This leads to ischemia in the retina and excessive secretion of growth factors [51, 52]. Thus, stem cells promote regeneration of the retinal tissue by differentiating into photoreceptors, neurons, glial cells, and retinal pigment cells. In case of vascular damage, HSCs promoted angiogenesis [53]. Release of paracrine factors lead to improvement in blood-retinal barrier and photoreceptor cells in retina [50, 54]. A therapeutic study conducted in Ins2^Akita mice suffering from diabetic retinopathy, using CD140⁺ AT-MSC, reported amelioration of all visual defects caused due to diabetes [55].

While delayed wound healing followed by necrosis of the tissue is another common characteristic of DM, this is due to the lack of growth factors, angiogenic factors production, and unstable formation of collagen matrix [56]. The diabetic wound undergoes severe pathogenesis such as neovascularization, foci of necrotic tissue entrapped with neutrophils because of lowered functionality and production of factors like TGFB, epidermal growth factors (EGF), platelet-derived growth factors (PDGF), and keratinocyte derived growth factors (KGF) [56]. Transplantation of MSCs by circulation led to better quality production of collagen with improved strength and integrity and also production of TGFB, KGF, PDGF, and VEGF, leading to wound healing [57] . Alternatively, MSCs pretreated with salidroside have been used in therapeutic treatment [58]. Studies have revealed that salidroside promotes paracrine function and neoangiogenesis while reducing hyperglycemia-induced reactive oxygen species (ROS) generation [59, 60]. Salidroside-pretreated MSCs have shown to successfully reverse hyperglycemia-induced suppression of wound healing factors: heme oxygenase-1 (HO-1) [59]; fibroblast growth factor 2 (FGF-2); and hepatocyte growth factor (HGF) [61, 62].

MSCs have also showed their efficiency in treatment of diabetic foot ulceration (DFU) by combination of autologous biograft of skin fibroblast and MSCs [63]. A recently approved commercial drug based on hydrogel sheet containing allogenic AT-MSC (ALLO-ASC-Sheet; Anterogen, Seoul, South Korea) was studied for a period of 12 weeks in 59 patients [64, 65]. The drug displayed satisfactory regeneration capability in 82% patients by the 12th week. Notably, 16 subjects treated with this drug had elevated anti-HLA antibodies; however, no clinical signs of rejection were observed. Further, those 16 subjects were under observation for 2 years during which no adverse reactions were recorded due to the drug [66, 67]. Furthermore, MSCs derived from placenta, bone marrow, and human umbilical cord have been used for treating DFU [68].

Although MSCs have shown to have immense effect in the treatment of DM-associated diseases, this is accompanied by several limitations. The ability and guarantee of MSCs differentiation are rare in the host tissue [40]. There could be malignancy and cytogenic aberrations [69]. There is loss of the graft because of toxicity caused by immunosuppressants [70, 71]. Chance of immune rejection exists. The immediate response after transplantation is inflammation [71, 72]. Sometimes, there is lack of supply of islet precursor cells [73]. Further, these approaches do not provide a permanent cure to DM, rather a highly symptom-directed treatment.

Therapeutic approaches to treat T1DM using MSC

T1DM is a chronic autoimmune disease which results in diabetes mellitus in young adults [74]. Autoimmune response is initiated by infiltration of the pancreatic islets by macrophages and dendritic cells (DC) [75, 76]. Studies have demonstrated that Th1 type CD4 T cells are activated by IL-12 secreted by macrophages. Activated CD4 T cells secrete IL-2 and proinflammatory cytokines such as TNF-α, IFN-γ, and IL-1β. Cytotoxic CD8 T cells are hence activated under the influence of these factors [77]. As a result, T cell-mediated destruction of pancreatic islet β cells occurs, producing a lack of insulin production and secretion [78] as its aftermath (Fig. 1). Common therapy to treat T1DM includes taking insulin injections or wearing patches which can deliver insulin at an automated programmed time. However, such delivery methods do not address complications arising due to hyperglycemic condition such as nephropathy, foot diseases, and ketoacidosis [6]. Furthermore, the dosage and time of action of insulin delivered by injections or patches are not identical to the physiological level of insulin action [6]. Treatment of T1DM by transplantation is also challenging due to acute shortage of pancreatic tissue donors and complications of immune rejection [79]. Thus, research is being carried out to utilize the immunomodulatory effects and differentiability of MSC for treating T1DM [80–82]. As the multipotent nature of MSC is used to derive IPC as a potent therapy for both types of DM, thus, it shall be discussed in a separate section.

Fig. 1.

Mechanism of autoimmunity exhibited in T1DM through cascade of cytokines secreted from macrophages and dendritic cells

Autoimmunity reversal and immune suppression to treat T1DM

In addition to the differential potential of MSCs, these cells exhibit useful immunomodulatory and immunosuppressive properties as well [83]. Promising results have been observed and confirmed where MSC release cytokines like IL-6 which inhibit the differentiation of monocytes to DC [84, 85]. Consequently, secretion of IL-6 has been shown to delay apoptosis of lymphocytes and neutrophils [86, 87]. Studies have shown that MSCs promote the formation of regulatory T cells (T_reg), thus supporting β cell survival [88]. Recent studies have identified the production of HLA-G5 as the causative agent for promoting generation of T_reg [89]. Interaction of MSC with T cell-induced IL-10 has been shown to stimulate the production of HLA-G5 [90] (Fig. 2.). Additionally, it has been found that production of HLA-G5 suppresses T cell proliferation as well as natural killer cell cytotoxicity [91, 92]. In a related study, CB-MSC has shown to restore Th1/Th2 cytokine balance in blood, promote CD4⁺ CD25⁺ FoxP3⁺ T_reg proliferation, and even induce apoptosis of infiltrated leukocytes in the pancreatic islets [93]. In addition to this, MSCs arrest proliferation of B-cells [94], thus effectively reversing the autoimmune condition in T1DM (Fig. 2).

Fig. 2.

Role of MSC in inhibiting autoimmune action in T1DM by interacting with T_reg cell to produce HLA-G5 which suppresses cytotoxic T cell and promotes T_reg cell

MSC secretes soluble factors that mediate immunosuppression. In humans, indoleamine-2,3-dioxygenase (IDO) secreted by MSCs have shown immunosuppressive effects when co-stimulated by IFN-γ [95–97]. In murine models, nitric oxide produced by murine MSCs acted as immunosuppressant. NO is a bioactive gas which is produced upon activation by IFN-γ, TNF-α, IL-1α, or IL-1β [98] (Fig. 3). NO affects the functioning of macrophages and T cells [99, 100]. Prostaglandin E2 (PGE2) production by MSC is enhanced in the presence of IFN-γ and TNF-α [101]. PGE2 acts on macrophages by stimulating production of IL-10 and inhibits monocyte from differentiating into DC [102, 103] (Fig. 3). Studies have reported that MSCs exhibit immunosuppressive activity by binding to T cells [104]. Adhesion molecules such as ICAM-1, VCAM-1, and CXCR-3 present on surface of MSC aid in adhering T cells [105]. Further, CXCR-3 is the receptor for ligand chemokines expressed by leukocytes (CXCL-9, CXCL-10, and CXCL-11) [106]. Further, MSC can suppress T cell proliferation by binding the inhibitory programmed death (PD) 1 molecule to PDL-1 and PDL-2 ligands [107].

Fig. 3.

Reduction of autoimmunity effect by immunosuppressive action of MSC mediated via TNF-alpha and cell adhesion molecules

Therapeutic approaches to treat T2DM using MSC

As discussed previously, T2DM is caused either due to lack of insulin secretion or due to development of resistance toward insulin. Research groups have observed that mutation in KCNQ1 [108], TCF7L2 [109], and KCNJ11 [110] gene is related to insulin secretion deficiency [111]. While mutations in IRS-1 gene cause abnormalities in IRS-1 thus causing insulin resistance [112], studies have further revealed that excessive serine phosphorylation of IRS-1 protein reduces the binding capability of IRS-1 [113, 114]. T2DM can be counteracted by using MSCs to produce IPCs, which aid in regeneration of β islet cells, and reduce insulin resistance [115]. The approach of treating T2DM by differentiating MSCs into IPCs coincides with MSC therapy to treat T1DM; hence, it shall be discussed elaborately later.

Assistance in regeneration of β islet cells

MSCs have exhibited regenerative ability to repair pancreatic islet β cells by homing to the injured sites. Table 1 shows the cytokines expressed by various MSCs, of these IL-6, SDF-1, G-CSF, and GM-CSF have shown to aid in repair processes [116]. MSCs promote repair by creating a microenvironment consisting of such cytokines along with growth factors like VEGF-α, PDGF-BB, IGF-1, and angiopoientin-1 [46, 117].

Table 1.

Variation in cytokines secreted by MSCs from different sources

Cytokines	BM-MSC	AT-MSC	DP-MSC	CB-MSC	P-MSC	References
GRO-α	+	+	+	+	+	Park et al. [118] Heo et al. [31] Bastidas-Coral et al. [119] Cuerquis et al. [120]
sICAM-1	−	+	−	−	+
IL-1ra	+	+	−	−	−
IL-6	+	+	+	+	+
IL-8	+	+	+	+	+
MCP-1	+	+	+	−	+
MCP-2	−	−	+	−	−
MCP-3	−	−	+	−	−
MIF	+	+	−	+	+
Serpin E1	+	+	−	+	+
RANTES	+	+	+	−	−
SDF-1	+	+	+	−	−
TNFα	−	+	−	−	−
SCF	+	−	+	−	−

Evaluation of fasting C-peptide levels is a common and effective indicator for improvement in β islet cell function. Studies have demonstrated an increase in fasting C-peptide level after intrapancreatic MSC transfusion though it lowered after 3 months [115]. While another study has shown that there was no significant change in fasting C-peptide levels after transvenous MSC transfusion [121, 122], in a parallel study, it was observed that fasting C-peptide levels increased gradually up to 6 months but decreased post 12 months [123]. Thus, longer disease duration, poor islet function, and other complications in T2DM produced a less supportive microenvironment for transplanted MSCs. Hence, recovery of β islet cells function was short term and requires further breakthrough to achieve long-term effects. As an alternative approach, studies have inhibited apoptosis in β islet cells instead by regulating glucagon-like peptide-1 (GLP-1) with an analogue called liraglutide. Liraglutide has shown to regulate β islet cells proliferation and apoptosis via PI3K/Akt/FoxO1 and AMPK/mTOR/P70S6K signaling pathway [124, 125]. By combining liraglutide and umbilical cord MSC (UC-MSC) for treatment, an effective regenerative therapy was recently reported [126]. Such a therapy reduced apoptosis in β islet cells by the action of liraglutide as well as increased target cell proliferation through UC-MSC.

Improvement in insulin resistance condition

As stated above, T2DM is characterized by lack of insulin production and insulin resistance. MSCs can improve insulin production by the formation of IPCs as well as by assisting in regeneration of islet β cells due to their multipotent nature. However, recent studies have shown that MSCs are capable of ameliorating insulin resistance as well. The mechanisms are yet to be completely understood, but MSCs have shown to induce IRS-1 signaling pathway [127]. Activation of IRS-1 signaling pathway demonstrates an increase in translocation and expression of GLUT-4, thus ameliorating insulin resistance at target sites [128] (Fig. 4). Additionally, it has been found that insulin resistance is a product of chronic inflammation mediated by proinflammatory cytokines such as TNF-α and IL-1β [129] (Fig. 4). Discovery of M2 macrophages or anti-inflammatory macrophages has shown to effectively reduce insulin resistance by counteracting the proinflammatory cytokines [130]. Studies have revealed the ability of MSCs to promote M2 macrophage polarization both in vitro and in vivo [131, 132] and hence capable of directly influencing factors to ameliorate insulin resistance (Fig. 4).

Fig. 4.

Amelioration of insulin resistance in T2DM by induction of IRS-1 signaling pathway with the help of MSC

Common approach to treat both types of DM by producing IPCs from MSCs

To understand the differentiation of MSC for enhancing regeneration of pancreatic β cells for treatment of T1DM and T2DM, it is necessary to identify and annotate both intrinsic and extrinsic factors responsible for MSC cell fate. Nearly 30 years have been spent by various research groups on studying transcription factors, growth factors, and inhibitors critical for various MSC (Fig. 5) to differentiate into functional pancreatic β cells or pancreatic lobe [133]. Pdx1 along with Ptf1a and Hlbx9 is essential to initiate pancreatic bud development [134–137].

Fig. 5.

Various sources of MSCs in an adult human

While Pdx1 does not regulate the patterning of pancreas, it regulates the quantity of different pancreatic endocrine cells [137–139]. Hlbx9 plays a crucial role in dorsal patterning of the pancreatic bud [134, 136]. Notably, Pdx1 is required for maturation and functioning of β cells [138]. Inactivation of Pdx1 in adult β cells gives rise to glucose intolerance leading to T2DM [140–147]. Consequently, studies have demonstrated the role of Pdx1 in regulation of expression of Pax4 [147], insulin [138, 148, 149], MafA [146], glut2 [150, 151], glucagon [138, 150, 152], IGRP [144], IAPP [138, 152–154], and glucokinase [155, 156] genes. These genes are vital for β cell development, maintenance of β cells, and physiological functioning of β cells [143–147]. It can be concluded that Pdx1 expression not only initiates the pancreatic development but also maintains the functionality in adult β cells. Post initiation, Ptf1a/Pdx1 +/+ cells define pancreatic anlagen [135]. Expression of Ptf1a has been shown to govern stem cell differentiation into endocrine as well as exocrine cell types. The absence of Ptf1a expression leads to hypoplasia of the pancreas [135, 157]. Studies suggest that Pdx1 and Ptf1a may regulate each other’s expression; however, Ptf1a is independent of the expression of Pdx1 [158, 159]. Transitioning from pancreatic bud cell development to endocrine cells, inhibition of Shh is required to inhibit further cell fate toward intestinal cell lineage [160]. Activation of Ngn3 ensures differentiation of the pancreatic precursor cells into endocrine cells [161]. Hence, endocrine cells arise from Ngn3-expressing progenitor cells [162, 163]. Knockdown of Ngn3 has shown severe reduction in number of insulin⁺ cells [164]. Hnf6 expression promotes the expression of Ngn3 [165, 166]. However, maintained expression of Hnf6 in endocrine cells causes disruption in islet architecture followed by defects in insulin production and secretion due to glucose stimulation [167, 168]. Thus, a dynamic expression of Hnf6 is essential for lineage specification of progenitor cells to endocrine cells as well as maturation and morphology of endocrine cells. Endocrine progenitor cells can be characterized by high expression of Prox1 and MafA [169–172]. In later stages of development, retinoic acid (RA) influences exocrine vs endocrine cell lineages [173]. It has been demonstrated that pancreatic buds cultured in the presence of RA displayed nearly 2.5-fold increase in the number of insulin-positive cells [174]. Pax4 expression is observed in secondary transition for β cell differentiation [175, 176]. Additionally, Pax4 inhibits expression of Arx to promote β cell fates. This suggests antagonistic relation between Pax4 and Arx as the presence of Arx directs cell fate to α cells [177]. Parallel to Pax4 expression, Nkx2.2 expression occurs which is essential for β cell specification [176]. Nkx2.2 regulates expression of MafA, and inactivation of Nkx2.2 results in complete loss of β cells [146, 178, 179]. Furthermore, α cells and β cells have a similar machinery to metabolize glucose [180]; however, they differ in the way they transport glucose [181, 182]. Hence, an increased expression of Nkx6.1 and Pdx1 while expressing GLUT2 in scenarios of β cell loss can transdifferentiate α cells into β cells [183].

Therefore, MSCs can be differentiated into IPCs to treat both T1DM and T2DM (Table 2). However, immune suppression should be accompanied with this approach while treating T1DM/T2DM and amelioration of insulin resistance to achieve a robust and long-term solution. Interestingly, studies aimed at using UC-MSC along with autologous bone marrow mononuclear cell (aBM-MNC) stem cell transplantation have been successful in causing insulin secretion without any immunotherapy [184, 185]. By preserving the mass of β cells by transplanting aBM-MNC stem cell, the requirement of immunosuppression was bypassed [186, 187].

Table 2.

Role of crucial growth factors in differentiating MSC into IPC for treating diabetes mellitus

Genes involved	Function	Reference
Pdx1	Regulates Pax4, insulin, MafA, glut2, glucagon, IGRP, LAPP and glucokinase essential for development of β islet cells. Maintain the quantity of different types of pancreatic endocrine cells	Harrison K.A. et al. [134]
Hlbx9	Dorsal patterning of pancreatic bud
Ptf1a	Development of endocrine and exocrine cells	Burlison J.S. et al. [158]
Ngn3	Differentiates pancreatic precursor cells into endocrine cells	Villasenor A, Chong D.C & Cleaver O. [161]
Hnf6	Determines the specific lineage of progenitor cells	Burke Z & Oliver G. [169]
Pax4	Inhibits Arx, hence promotes β cell development	Wang Q et al. [177]
RA	Increase the number of endocrine cells	Chen Y et al. [173]
Nkx2.2	Β cell specification	Wang J et al. [176]

Recent advances and trends

With new techniques and breakthroughs being reported in the field of MSC derivation, differentiation, and expansion, there is a need to be well versed with such techniques such that they can be applied in therapy. To cope up with the demand of high cell numbers for transplantation and transfusion, a recent trend in isolating MSCs from bone marrow involves the use of a nonwoven rayon/polyethylene fabric where the isolated cells can be put directly in the cell culture [188]. A major advantage of this technique is that 2 to 3 times more MSCs can be isolated after two passages when complemented to the traditional method [189]. Mesenchymal stem cells are considered to be a major alternative which finds application in clinical studies. They are used for treating various ailments, and diabetes is one of them for their ability to exhibit high proliferative capacity and immunomodulatory properties [190]. A new method is employed which involves fabricating the cells by layer-layer assembly where bioorthogonal binding of biologically active molecules can be done. This results in biorthogonal covalent layer-layer interaction which enhances the efficiency and viability of the islet cells [191]. These islet cells will then be used for curing diabetes. Several new methods one of which is the flow focusing method is used where the islet cells are coated individually, uniformly as a result of which the function of islet cells is restored as well as the cells, remain protected in our body [192]. Label-free sorting is one more method which uses microfluidic technology that provides an alternative way to isolate and purify mesenchymal cells from the mature tissue for cell treatment. Adrenal cortical progenitor cells are enriched for using label-free size-based ordering, which eliminate their differentiated counterparts in a microfluidic chip. As a result of extensive knowledge on nuclear deformability as a marker of pluripotency, sterile fluids are included for hydrodynamic stretching for ensuring the characteristic in human and murine embryonic stem cells and their differentiated parts. In this way, mechanophenotyping of the cells from the tissues having mesenchymal stem cells exhibits criteria for multipotent and regeneration properties [193]. ROS is a finding application in mesenchymal stem cells and its use in enhancing differentiation [194]. According to a recent study conducted in mice, placenta-derived mesenchymal stem cells exhibit several advantages which the mesenchymal stem cells derived from other sources might not exhibit; for instance, noninvasive methods can be used to obtain the cells. The isolation and expansion of the cells become easier. The cells can differentiate into different type of cell lineages [195]. A lot of research is going on these days on mesenchymal stem cells to put forward a standard procedure for determining and regulating quality and safety of the cells [196]. This will not only help in replicating the experiments with high accuracy but also help in maximum efficiency in using the cells. Human albumin and muropeptide are used for cryopreservation these days instead of FBS as the latter is known to cause adverse reactions in the transplanted MSCs; also the MSCs can now be preserved for a longer time [197].

Conclusion

Mesenchymal stem cells dominate the world of stem cell–based therapy for diabetes mellitus owing to its ease of accessibility, multipotency, and unique immunogenic and immunomodulating properties. MSCs initially were used to produce IPCs for treatment of both types of diabetes, but recent advances in the past decade have enabled the use of MSC to directly treat or reverse the causative factors of diabetes. Furthermore, UC-MSC and P-MSC have become more popular in diabetic studies owing to their ease in availability. However, most of the therapeutic measures are in clinical trials and are awaiting approval for market launch. This signifies that much research is required to fine tune the longevity of the therapy, the efficacy of the differentiated and immunomodulating cells, and establish a stable pool for retrieval of MSC.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.