Latest Therapies for Type 1 Diabetes: Exsisting Setup and “Post-Insulin” Standpoints

Type 1 diabetes affects millions of people globally and requires careful management to avoid serious long-term complications, including heart and kidney disease, stroke, and loss of sight. The present standard-of-care for type 1 diabetes is exogenic insulin substitutional therapy. The most advanced stretegies in this area is the development of hybrid-closed loop system and the producing of long-acting insulins. Progresses in stem cell therapies have started to revolutionize the care of patients with type 1 diabetes; however, significant challenges remain including the limited islets availability, difficulties in maintaining the viability, the heterogeneity within a complex pathology and in patients’ responses to treatment. On the way, a considerable amount of efforts in maximizing the islet transplantation effectiveness by controlling the advantageous of different stem cell approaches. With the availability and the use of big data, the concept of precision medicine is gaining wide attention worldwide and could bring the dream of “presonlaized” therapies as a reality in the near future. Here we review the current range of treatments available as well as recent pre-clinical breakthroughs in the field of personlaized medicine for type 1 diabetes.


Introduction
Type 1 Diabetes (T1D) is a devastating, multifactorial autoimmune disorder characterized by T-cell-mediated destruction of pancreatic β cells, resulting in a deficiency of insulin synthesis and secretion [1]. The incidence of type 1 diabetes has been increasing globally Despite these notable advances in understanding, there is no effective prevention or curative treatment for T1D. Achieving good glycemic control is necessary to prevent the long-term morbidity that is associated with poorly-controlled T1D; however the heterogeneity of factors initiating/exacerbating the disease in individual patients, patients' diverse responses to these factors, and the variable efficacy of traditional therapies, render this simple aim complex to achieve in practice. An emerging move towards precision medicine may be the answer: precision medicine generates tailored diagnostic, prognostic, and therapeutic options for each patient based on their genetic background. By exploiting established clinical, phenotypic, physiological and pathological indexes this approach has the potential to provide the best available care for each patient, but is yet to be employed on a wide scale. In this review we summarize the current range of treatments available for T1D, as well as recent pre-clinical breakthroughs, and discuss how precision medicine might be implemented for the treatment of T1D in the future.

Therapeutic Approaches
Before the discovery of insulin in 1921, it was remarkable for T1D patients to live more than one or two years after disease onset: one of the twentieth century's utmost medical breakthroughs, insulin replacement, is still the only successful treatment for T1D patients today. That said, innovative ways of achieving improved insulin-mediated glycemic control are becoming accessible to patients, while tissue transplants, genetic modification and stemcell therapies are showing promise in pre-clinical models and human trials. and improve metabolic variables without increasing adverse events (reviewed in [41]). Of these, promising candidates include metformin [42] and pramlintide, which have a role in glycemic control in both T1D and T2D and can modestly reduce triglyceride levels in T1D patients accompanied by HBA1c lowering and weight loss [43]. In addition, glucagon-like peptide-1 receptor agonists (GLP-RAs) combined with insulin can reduce the daily-weight required bolus insulin dose and improve glucose control and weight loss [44]. Dipeptidyl peptidase-4 inhibitors (DPP-4) are an adjunct therapy in a closed loop-system that can reduce postprandial blood glucose levels [45] and can significantly reduce the daily insulin dose but not the HBA1c level or the risk of hypoglycemia [46]. Finally, sodium-glucose cotransporter inhibitors (SGLTi) are associated with improved glycemic control and a reduced insulin dosage leading to lower rate of hypoglycemic episodes [47].
In summary, traditional and combined approaches to insulin therapy remain important tools in the treatment of T1D, but they do not represent a cure and may not be able to achieve the level of glucose control necessary to avoid long-term complications arising from diabetes. Thus increasing efforts are being made towards uncovering novel ways of restoring physiological regulation of glucose metabolism.

Gene Therapy
Gene therapy offers a promising alternative to insulin injection for T1D treatment and may aim to prevent or delay onset of T1D, correct insulin deficiency, promote β-cell proliferation and survival, modulate the immune/inflammatory response or induce insulin secretion by non-β cells (reviewed in [48]). Of particular interest is the induced overexpression of insulin-like growth factor 1 (IGF1), which regulates immune functions and enhances the survival and proliferation of β-cells. Non-obese diabetic (NOD) mice spontaneously develop diabetes from around 10 weeks-of-age; however when 4-week-old skeletal muscle of diabetic dogs was able to induce metabolic normalization and normoglycaemia lasting 8 years [52]. This study represents an important safety and efficacy step forwards for diabetes gene therapy, as although AAV vectors have been trialed in humans, their therapeutic use for gene transduction has yet to be tested clinically. There are concerns that transduced cells might be susceptible to recurring autoimmune attack, so enduring autoimmune protection must be demonstrated [53,54]. It is also possible that the viral vectors themselves might trigger an immune response that could worsen the disease condition [55], though Jaen et al. did not report any evidence of this in their study [52].
Modifications to the AAV vectors might hold some of the answers: in response to concerns that constitutive over-expression of insulin might risk hypo-glycaemia, one group has developed a Tet-Off regulatable AAV vector for insulin expression that was able to both induce the expression of human insulin in diabetic mice, and be reversibly switched off to reduce insulin levels [56]. Thus fine tuning of viral vectors combined with more long-term studies will be required to move towards vector-mediated reinstatement of insulin production in human patients.
In addition to induced insulin expression, several studies have looked at other targets implicated in T1D pathogenesis. For example, Klotho is an anti-aging gene that is expressed in pancreatic islets in mice [57] and humans [58]; a Klotho deficiency is linked with β-cell apoptosis, and reinstating its expression in mice under the control of a β-cell-specific promoter led to protection of β-cell function [57]. In human islet cells, treatment with the T1D drug gamma-aminobutyric acid in vitro significantly increased Klotho expression [59], indicating the possible clinical potential for this approach. Combining gene therapy with immune modulation may also be promising. When NOD mice were pre-treated with anti-T-cell receptor β chain monoclonal antibody followed by hepatic gene therapy with Neurogenin-3 (which determines islet lineage) and the islet growth factor betacellulin, the researchers observed sustained induction of insulin-producing cells in the liver that

Whole Pancreas and Pancreatic Islets Transplantation
The ideal T1D treatment would restore a fully functioning pancreas to patients, and one way of achieving this is by transplantation of a whole donor pancreas or pancreatic islets.
Presently, over 50,000 patients with diabetes have been transplanted in more than 200 centers worldwide [63], but this procedure is far from a panacea: despite major improvements in patient survival rate, pancreatic transplantation is a complicated and risky  There is accumulating evidence that successful human islet transplantation dramatically decreases the occurrence of hypoglycemic events and reduces the frequency of long-term complications of T1D [72]. Thus, this approach might be a particularly powerful strategy for those patients with hard-to-manage T1D, enabling them to avoid or delay potentially life-threatening complications, such as renal failure. Despite this promise, <1,000 islet transplants have been performed in Western countries in the past 10 years, largely because of a shortage of donor organs [73], and the high cost of this therapy over traditional insulin-injection approaches [74]. One possible way of overcoming the limitations of few donor organs is to develop techniques to expand pancreatic β-cells ex vivo. Normally β-cell proliferation is restricted to a brief developmental window between birth and 1 year-of-age, but several studies have now managed to induce β-cell proliferation following a variety of genetic or pharmacologic interventions: cell division protein kinase 6 (cdk6) supported human β-cell proliferation and function in vivo [75], while aminopyrazine compounds stimulated robust β-cell proliferation in healthy adult primary islets in vitro, and these cells retained functionality in vivo after transplantation into diabetic mice [76]. Similarly, treatment with the herbal medicines, harmine, which is isolated from peganum harmala seeds, reported to possesses several pharmacological activities such as stimulate the proliferation of adult primary human β-cells grafted into diabetic mice [77]. These studies show the potential for expansion of functional donor β-cells that might in future enable more widespread use of islet cell transplants for patients.
In addition to the donor selection, further improvements in islet transplantation outcomes are expected as we further refine the pancreatic islet isolation technique and improve transplantation strategies including the immune suppressive protocols, islet quality, and β-cell mass and purity [78,79]. Moreover, the development of alternative transplantation sites and new cell sources, including porcine islet cells and the expansion of donor cells, might yet unlock an era of "on-demand" unlimited cell therapy for diabetes [80].

Pancreatic β-Cell Regeneration and Reprogramming
It was long held that the diabetic pancreas was devoid of functional β-cells, but there are now several lines of evidence that indicate the possibility to restore a degree of function to a T1D patient's own pancreas. This approach would have the major advantage of avoiding the need for cadaveric donors, surgery and immunosuppression. Altered β-cell identity, rather than β-cell apoptosis, in the setting of chronic hyperglycemia was first   hyperglycemia [115]. Subsequently, an important step forwards in the use of hESCs for T1D therapy occurred when scientists from the University of British Columbia developed a seven-stage protocol that efficiently converted hESCs into IPCs. This protocol generated endocrine cells with insulin content similar to that of human islet cells and that were capable of glucose-stimulated insulin secretion in vitro as well as rapid reversal of diabetes in vivo in mice [116]. Additional studies have highlighted the possible roles of other growth and extracellular matrix factors, including laminin, nicotinamide, insulin [117] and retinoic acid [118] in the generation of IPCs from ESCs, but these findings have yet to be integrated into a combined approach suitable for clinical use.
Human ESCs also have the potential to generate cells uniquely tailored for the recipient.
Recently, Sui et al. showed that transferring the nucleus of skin fibroblasts from T1D patients into hESCs gave rise to differentiated β-cells with comparable performance to naturallyoccurring β-cells when transplanted into mice [119].
Despite the promise of hESCs, great concern around their potential to initiate teratomas has largely limited their clinical exploration in T1D. However, Qadir  Naturally, the hPSCs are immature cells that have the capacity to become nearly any cell type originated in the body. In order to impressionist the normal pancreatic cell development, a sequence of specific stages of cellular differentiations including the involvement of of certain growth factors, and signaling pathways activating/inhibiting molecules [128,129] .
Another approach of transplantation is through in vitro differentiated β-cells. A differentiation protocol to generate β-like cells with enhanaced function from pancreatic progenitors through modeulating EGF-β signaling, ceelular cluster size controlling in addition to use an enhanced serum free medica which lead to produce stem cells-derived βcells with the ability to express β-cell markers and insulin [130,131]. Results indiacted that certain time-frame is needed to control the TGF-β signaling during the differentiation process in order to achieve vigorous function, which is the ultimate requirments that make these cells a promising cellular therapy for diabetes [ Historically, the bone marrow has been the main source of MSCs [142]. Xie et al. first trialed generating IPCs from T1D patients' bone marrow MSCs (BM-MSCs) and showed the coexpression of insulin and C-peptide in cells injected into diabetic mice, leading to attenuated hyperglycemia [143]. Alongside, genetically-modified human BM-MSCs expressing vascular endothelial growth factor and PDX1 reversed hyperglycemia in more than half of diabetic mice and enabled survival and weight maintenance in all animals [144]. These promising pre-clinical results led to human trials: when BM-MSCs were injected into the splenic artery of T1D patients, they induced an increase in C-peptide levels that was maintained for 3 years; unfortunately, this had no significant effects on glycemic control due to insufficient production of insulin by the grafted cells [145]. Since then, new methods have been developed aiming to improve in vivo outcomes. For example, Zhang et al. co-cultured BM-MSCs with pancreatic stem cells which led the MSCs to adopt a pancreatic islet morphology; when these cells were injected into diabetic rats they attenuated glycated albumin levels and significantly increased serum insulin and C-peptide [146].
The main disadvantage of BM-MSCs is the difficulty in isolating the cells and the morbidity associated with the procedure. These issues led to interest in the use of Muscle-Derived Stem/Progenitor Cells (MDSPCs), which exist in skeletal muscle and have the capacity for long-term proliferation, are resistant to oxidative and inflammatory stress, and show multilineage differentiation potential [147]. To investigate the therapeutic potential of autologous MDSPCs transplantation for T1D, Lan et al. applied a four-stage MDSPC differentiation protocol to generate IPCs in vitro and injected them into diabetic mice: these β-cell-like-cells effectively improved hyperglycemia and glucose intolerance and increased the survival rate in diabetic mice without the use of immunosuppressants [148].
Building on the promise of BM-MSCs and MDSPCs, researchers sought an equally potent but more abundant and easily-accessed source of stem cells. Adipose-Derived Stem Cells (ADSCs) have recently been explored for T1D treatment, and have the advantage over MDSPCs of being readily accessible and harvested, even in older patients [149]. IPCs differentiated from ADSCs show significant expression of β-cell markers, insulin and cpeptide following transfer into diabetic mice [150]. In 2019, IPCs derived from ADSCs using a novel 3D xeno-antigen-free protocol were shown to exhibit key features of pancreatic β cells in vitro and differentiated into IPCs in diabetic nude mice in vivo [151]. Another study showed the potential for combining ADSC treatment with gene therapy by transducing ADSC with a furin-cleavable insulin gene which led to enhanced insulin expression in the differentiated adipocytes, and alleviated hyperglycemia in diabetic mice [152].
Removing the need for adult stem cell donors completely, the umbilical cord is now used as a successful alternative stem cell source for regenerative medicine. cultured with human UCB-MSCs; this led to increased insulin secretion, reduced hyperglycemia and preservation of islet architecture [157][158][159].
Despite promising signs in rodent studies, the potential of UCB-MSC treatment for T1D in humans has yet to be fully realized. Haller  and immunosuppression [163]. Briefly, WJ-MSC collection occurs at the time of delivery and avoids the known adverse effects associated with adult stem cell collection from the bone marrow or adipose tissue. Furthermore, features including a high WJ-MSC proliferation rate, an immune privileged status, minimal associated ethical concerns, and nontumorigenic capacity render these cells an excellent option to be used in regenerative medicine applications [164].
One of the first studies to use β-cell-like cells derived from WJ-MSCs tested their effects following transplantation into patients with new-onset T1D [165]. Interestingly, a concurrent study suggested that the WJ-MSCs might restore the function of -cell in type 1

Future Standpoints
Despite advances in the various therapies discussed above, an ongoing challenge in T1D treatment is the extreme heterogeneity in patients' disease triggers, prognosis, pathological pathways and hence their response to treatment [103,104,172,173]; this is compounded by the recent explosion in the less-understood fulminant T1D subtype, which occurs suddenly and is associated with rapid and complete β-cell destruction [174]. This heterogeneity across the T1D patient population suggests that we are unlikely to discover a "one-size-fits-all" therapy able to cure every case: thus there is the need for more precise treatment approaches that are personalized and tailored to individual patients' situations.
This is the aim of Precision Medicine [175]. In diabetes, the precision medeicine approach has been inspired by work including that of Zhao et al., who first developed stem cell educator therapy where T1D patients' lymphocytes are briefly separated from the blood and co-cultured with UC-MSCs within a closed-loop-system, before being returned to the patient; this treatment dramatically improved metabolic control, reversed autoimmunity and promoted β-cell regeneration [176]. Al-Anazi et al. used a similar approach to try and treat multiple myeloma in 45 adults with T1D who had undergone autologous HSCs; surprisingly the patients were also cured of their diabetes and became insulin-independent [177].
The next step towards stem-cell-mediated precision medicine for T1D is likely to involve synergizing existing stem cell approaches with advances in cellular and genetic engineering techniques, such as nuclear transfer and genome editing. Moreover, an emerging understanding of the TFs and epigenetic processes that control pancreatic islet lineage-commitment [178], as well as the role of microRNAs in driving cell lineage differentiation [179] are beginning to unlock new knowledge on T1D pathogenesis [25,26] and are opening fresh possibilities in β-cell generation [180,181]. New schemes conceived to regulat the immune system in T1D including the utilization of antigen-based immunotherapies [182].
Together these factors can all be used towards designing a successful protocol for precision medicine in T1D. Alongside, the reframing of T1D as primarily a metabolic disorder (rather than an autoimmune condition) that reflects the combined genomic and environmental landscape of the patient, has facilitated the discovery of new therapeutic targets and diagnostic/prognostic biomarkers [183,184]. Finally, the ongoing discovery of new and important influences on diabetic pathology, such as the role of gut microbiota [185], xxxxxxxx and the latest perceptions into the mechanism of T1D and the accumulated recent data that being translated into prospects for tissue-specific prevention trials toward eliminating progressive β-cell loss [182], continues to add to our understanding of this important disease, and thereby our ability to rationally design and test novel interventions with the promise of the future eradication of T1D ( Figure 5).