Stem cells perspective in cure and management of diabetic retinopathy

Diabetic retinopathy is the trending issue of the vision loss worldwide that occurs due to mismanagement of glycemic level of the diabetic patients. In diabetic retinopathy neurons and capillaries, which are associated with retina, start to damage that result in alteration in the vision function of retina. Long term effective strategy for diabetic retinopathy has not been proposed yet but the stem cells can make it possible to prevent or cure diabetic retinopathy. Stem cells have the potential of repairing and replacement of damaged or defected retinal cells. Different types of stem cells like induced pluripotent stem cells, mesenchymal stem cells, some other factors like CD34+ cells, HLA factors etc have a significant role in the treatment of diabetic retinopathy. In this review, other than the role of stem cells, we have discussed the management and screening of diabetic retinopathy. Management and screening, are the parts of the strategy to reduce the risk of diabetic retinopathy that act like the tools to treat diabetic retinopathy. As stem cells are the most feasible source of treatment of diabetic retinopathy. Management and screening of diabetic retinopathy at early stage could be helpful in treatment even when the patient doesn’t show symptoms. The stem cell therapy in diabetic retinopathy offers a long lasting treatment option.


Introduction
Stem cells are the non-differentiated cells that have the potential to divide and give birth to a number of new stem cells. These cells might be of same type or different from each other and they are categorized according to their functions. Stem cells can be obtained from different sources, in a human body there are main sources to obtain stem cells; bone marrow, adipose tissues and blood [1]. Stem cells are used in bone marrow transplantation if there is need to avoid the risk of rejection of cells then the stem cells can also be obtained from umbilical cord blood.
Many medical issues like osteoarthritis, blood cell formation, blindness and vision impairment, wound healing, infertility, HIV/AIDS, cartilage repair, nervous system repair, development of pancreatic beta cells and regrowth of teeth are being treated with the help of stem cells [2]. Diabetic retinopathy (DR) is a common cause of blindness all over the world. Bone marrow mesenchymal stem cells (BMSCs) have been considered as a promising strategy for retinal regeneration in the treatment of DR [3]. Stem cells have been confirmed to be involved in the occurrence and development of diabetic retinopathy. Globally, the current authoritative studies have focused on basic research towards the stem cell treatment of diabetic retinopathy [4]. One of the trending issues which are being treated with the help of stem cells is diabetic retinopathy. Diabetic retinopathy is an eye disease in which the retina gets affected due to diabetes. The end result of this disease is blindness. Arteries and neurons linked with retina get affected badly. But the narrowing of retinal arteries and damage of neurons are the initial changes. Later on outer retina starts to get damaged which also disturbs the visual functioning and dysfunctioning of blood retinal barrier. This blood retinal barrier protects the retina from any toxic element to get in contact with it. The mammalian retina displays incomplete intrinsic regenerative capacities; therefore, retina degeneration is a major cause of irreversible blindness such as glaucoma, agerelated macular degeneration and diabetic retinopathy. These diseases lead to the loss of retinal cells and serious vision loss in the late stage. Stem cell transplantation is a great promising novel treatment for these incurable retinal degenerative diseases and represents an exciting area of regenerative neurotherapy There are different stages of disease; nonproliferative diabetic retinopathy and proliferative diabetic retinopathy (Fig. 1).

Non-proliferative diabetic retinopathy
It is the early stage of diabetic retinopathy. At this stage vessel swell due to leakage of a fluid. This fluid causes swelling in surrounding tissues although the said fluid clears up on its own but sometimes it remains as such and disturbs the vision that also leads towards bleeding in retina.

Proliferative diabetic retinopathy
With the progress of disease nonproliferative retinopathy turns into a proliferative stage. Vasodilation of new vessels takes place and the vessels lack oxygen. These new vessels arise along with the retina and without any proper treatment these vessels start to bleed. Fill-up of a gelatinous material called vitreous humor takes place which makes the vision cloudy. Sometimes the severe end results may be retinal detachment. With the passage of time lower membrane of retina thickens that results the degeneration of capillaries. At this stage capillary lose their cells called pericytes and vascular smooth muscles. This all result in loss of blood flow [6]. Macular edema develops in a number of patients. In this condition a fluid leaks from the damage cells onto the macula. It is the part of the retina that helps us to see the details. Macular edema is an important disclosure of diabetic retinopathy and also a most common cause of modest loss of vision [7]. Many patients with type 1 diabetes loss their vision. The range of this vision loss is up to 3.5 % [8]. Diabetic retinopathy is one of the leading cause of blindness in diabetic patients. But to get rid of this situation no particular treatment has been discovered yet for long term beneficial results. For treatment of diabetic retinopathy bone marrow derived stem cells are the most possible prospects [9]. Diabetic retinopathy degenerates the retina and the mesenchymal stem cells (MSCs) that have the ability to restore and to delay the degenerative process in retinopathy [10]. In addition, MSCs have the potential to treat diabetes although the after effects of these cells are still controversial. The treatment of diabetes through MSCs might be very helpful to delay the effect of diabetic retinopathy. During early diabetes loss of pericytes and neuroretinal damage takes place due to the oxidative stress on retina [11]. MSCs have the ability to treat with the species that cause the oxidative stress. MSCs sought out this stress through the expression of sulfoxide reductase A. MSCs can protect from degenerative process of neuroretina and prevents apoptosis. MSCs also promotes the regeneration of neuroretinal tissues [12]. The mechanism of working of ESCs in terms to treat diabetic retinopathy is not clear yet.

Figure 1. Stages of diabetic retinopathy
Non-proliferative diabetic retinopathy and proliferative diabetic retinopathy

Pluripotent stem cells (PSCs)
According to the recent developments pluripotent stem cells PSC (PSC) are used to resolve diabetic retinopathy issues. These cells are derived from CD34+ cord blood cells [13]. When we are concerned with the treatment of retinal capillaries PSCs are helpful to develop a feasible strategy. The cells which are derived from CD34+ cord blood cells give convenient strategy for the regeneration of cellular capillaries. Along with the help of stem cells many other developments are emerging to overcome this common issue of diabetic retinopathy. As the diabetic retinopathy is now a day's most common reason of vision loss in many people around the world. This issue needs to be managed properly. Most of the recent developments are focusing on the management of diabetic retinopathy.
Including early diagnosis and immediate treatment. Quick treatment at initial stage can reduce the chances of macular edema and vision loss. According to the recent managements diabetic patients need a proper checkup according to the type of the diabetes. The patients with type 1 diabetes patients need a checkup after 3-5 years. Patients with type 2 diabetes need a treatment immediately from the diagnosis of type 2 diabetes. Mostly patients are unaware about the risk factors of the diabetes like diabetic retinopathy/vision loss. Education must be given to patients and doctors about the risk factors. But this education needs to be concise in perspective to give awareness about the results. After many researches it has been revealed that genetic factors are also responsible for the severity of diabetic retinopathy. Some histocompatibility antigens are responsible for the effects of DR. HLA-DR4 are that HLA factor. Patients that have high level of HLA-DR4 have more severe diabetic retinopathy than the other patients. Another factor which is responsible for seriousness of diabetic retinopathy is glycemic control. Better glycemic control can manage the severe results of diabetic retinopathy. Bad glycemic control can lead to many problems [14]. Severity of diabetic retinopathy can be changed during pregnancies. Association of high level of fasting blood glucose level and a higher level of serum progesterone concentration in body are responsible in progression of diabetic retinopathy. Non proliferative diabetic retinopathy can turns into proliferative diabetic retinopathy. There is no written data about this factor. That's why this is still a controversial that either pregnancy is an important risk factor or not. Blood pressure is also a factor of concern in association with diabetic retinopathy. The fluctuated level of blood pressure being noticed with PDR [15]. High blood pressure itself is a risk factor. Proliferative retinopathy and high blood pressure occurs simultaneously. Above given factors are of concern in perspective of seriousness of diabetic retinopathy. These factors need to be addressed if DR is concerned to be managed. Stem cells have the ability to renew the damaged cells. Stem cells are used to replace photoreceptors to overcome the risk of vision loss through retinal diseases. Stem cells are easy to phase out the damage beneath the retina. Stem cells repair the capillaries and neurons associated with retina [16]. To facilitate the human therapies, the concentration has moved towards the cells transplantation through stem cells. Stem cells are also used for the induction of photoreceptors [17]. Now days the focus of the recent work is the generation of photoreceptors from the induced pluripotent stem cells [18]. For the sake of repair of neurons of retina, it may be more convenient to use adult stem cells which are already segregated into neurons. It is impossible yet to propose any long term prevention but the stem cells are the source to overcome the dangerous risks. Adult stem cells propose a way to repair the damaged neurons. The neurons network which is not yet damage could be replaced with the normal cells [19]. Bone marrow derived stem cells are also used for diabetic retinopathy. CD34+ cells are supposed to be more rapid in action for regeneration of cells [20]. According to recent researches another type of bone marrow derived cells has been explored. This type is CD14+ cells. These cells unlike the CD34+ cells are more efficient especially in perspective of diabetic retinopathy [21]. Many techniques have been explored but diabetic retinopathy is still the issue to focus on. This is the trending cause of vision loss. In the sense of future prospect many factors along with stem cells need to be considered for long term results or preventions. According to the American diabetes association the factor of elevated blood pressure level need to be notice because fluctuation in blood pressure level affects the blood barrier associated with retina [22]. Currently long term effective treatments are not available for diabetic retinopathy but the aid of stem cells could be good source to overcome this problem.

Materials and Methods
Diabetic retinopathy is treated through mesenchymal stem cells and endothelial progenitor cells (Fig. 2).

Mesenchymal stem cells Modulation of ROS production through mesenchymal stem cells
Reactive oxygen species are the oxygen derived molecules. These molecules react with a number of molecules such as proteins, lipids and nucleic acid. Increased number of reactive oxygen species inhibits the mesenchymal stem cells proliferation. High RPS level cause cellular damage and also dysfunctioning. Whereas mesenchymal stem cells have a low level of ROS and a high level of an antioxidant called glutathione [23]. Oxidative stress induces by ROS introduces many changes. These changes include; decrease in proliferation, decrease in certain surface antigens for example, CD13, CD29 and CD44. This stress also diminishes the ability of suppressing T cells [24]. Mesenchymal stem cells to target swelling Low grade chronic swelling appears as a result of pathogenesis of diabetic retinopathy Helper T cells are the part of adaptive immune system and are responsible for all the adaptive immune responses. Helper T cells activate the cytotoxic T cells to kill the targeted infected cell. T helper cells are derived from local inflammation, acts as a conductor in chronic and acute responses. CD4+ and CD8+ T cells infiltrate the vitreous of proliferative diabetic retinopathy. Helper T cells are responsible for the determination of the cytokinine; that what cytokinine will allow the immune system to be more beneficial for the host. Different helper T cells acts against different attack of different organism. For example, Th1 cells give a cell mediated response. The response is against intracellular bacteria and protozoa. Th2 cells give a humoral immune response and the response is against extracellular parasites. Another type is Th17 helper T cells. These cells are the subset of helper T cells. These cells are different from Th1 and Th2 cells. Th17 helper T cells give response against extracellular pathogen and fungi. CD34+ cells and endothelial progenitor cells CD34+ cells circulate as endothelial progenitor cells. This circulation has great importance in regeneration and angiogenesis. CD 34 is a transmembrane cell surface protein. These cells are in higher concentration in bone marrow but according to a few studies CD34 cells can also be obtained from umbilical cord blood, fetal liver and adipose tissues. CD34 cells which are derived from bone marrow are considered to be more effective because adult CD34 cells can differentiate in retinal cells [32]. Patients with proliferative diabetic retinopathy PDR have a higher level of circulation of CD34+. High level of glucose in diabetic patients is the main cause of vision loss through diabetic retinopathy. In Proliferative diabetic retinopathy ischemia is caused due to the vascular damage of retina. EPCs have the ability of repair at the sites of ischemia [33]. The level of circulation of EPCs in both the patients with type1 and type 2 diabetes is different [34]. Poor glycemic control is responsible for the variation in the numbers of circulating EPCs. Poor control of glucose decreases the numbers of circulating EPCs whereas normal control of glucose level increases the number of EPCs. EPCs are the potential biomarkers in diabetic retinopathy [35]. As the result of diabetic retinopathy both the endothelial cells and pericytes are affected. EPCs are used for the treatment of DR. First of all, in the treatment of DR through EPCs determination of cell type to be used and cell number should be done. If a cell is single, then endothelial colony forming cells ECFCs need to be considered to use. Second is the estimation of the number of cells to be used. If the cells going to be introduced through injections, then the route of action must be chosen along with the estimation of number of cells [36]. Thirdly that population should be used that are more likely to benefit from treatment with EPCs. Fourthly, the most appropriate outcome measures that would allow the arrangements for proper treatment. At last the compilations of data from the clinical trials of action of EPCs in diabetic retinopathy. HLA factors in diabetic retinopathy Human leukocytes antigen system HLA factors plays important role in immune responses In DR. Studies have shown the relationship between HLA factors and destruction of pancreatic islets in the patients who are insulin dependent [37].The presence of HLA factor determines the severity of diabetic retinopathy. The patients with negative HLA factors have less severity of DR either they are insulin dependent. HLA-A24 is the most considerable factor. HLA factors sometimes considered as the markers to examine the severity of DR [38]. HLA factors consideration is the strongest factor in development of proliferative diabetic retinopathy. According to many findings HLA-A24 has an indirect association with the development of diabetic retinopathy. This association was conciliated by the long term glycemic control and function of residual B function; another indirect association has been reported [39].

Figure 2. Treatment options of diabetic retinopathy
Diabetic retinopathy is treated through mesenchymal stem cells and endothelial progenitor cells

Mesenchymal stem cells absorb reactive oxygen species (ROS)
Mesenchymal stem cells to target swelling CD34+ cells and endothelial progenitor cells

Current prospect
As the diabetic retinopathy is the severe disease and do not have any long term prevention technique yet but the stem cells have the potential to give a long term relaxation from risk of diabetic retinopathy. Retina is an easy to access tissue in the body, stem cells can be injected at the defected areas [40]. Stem cell therapies are the most feasible way to protect against diabetic retinopathy. These stem cell therapies are based on two different theories such as, neuroprotective effects and cell replacement. Cell replacement therapy for diabetic retinopathy something different from the other therapies for other retinal diseases [41]. Stem cell therapy for diabetic retinopathy is different because of dysfunctioning and death of pericytes, vascular smooth muscles and endothelial cells which are linked with retina [42]. Therefore, the therapies for DR are concerned to repair the retinal vascular system instead to replace the vascular system. Endothelial progenitor cells are considered to be the sufficient participants of stem cell therapy in DR [43]. Along with the selection of stem cells the introduction of stem cells in defected retinal areas is also a point to be noticed. The introduction of stem cells in defected area could be done by subretinal, intravitreal and intravenous injections [44]. Different types of stem cells are used to treat DR. Adipose modified stem cells (ADMSC), induced pluripotent stem cells and mesenchymal stem cells. (Fig. 3).

Adipose mesenchymal stromal cells
ADMSCs could be an effective source to treat DR that are introduced intravenously in the retina. ADMSCs are able to reduce blood glucose level and also to repair the blood retina breakdown which is the result of DR

Some future prospects
In current status and future there is a lot of work to resolve the hazardous results of vision loss due to DR. According to some recent publications a device has been developed to restore the vision loss. This device could be very helpful in future for the restoration of vision loss due to DR [56]. This finding of introducing a device in retina which restores the vision loss is very feasible. The feasibility of this strategy is because of the stimulus which is introduced at that place where already the light-sensitive cells have been used to exist. This stimulus takes advantage of the downstream process. This downstream process always occurs in retinal neurons before the signals reach the visual cortex. This finding supports the stem cell replacement strategy. The mesenchymal cells are considered to be the new players in the treatment of the DR. experimented on rats and they demonstrated good results. These injections also normalize the blood glucose level as well as with the treatment of DR. Stem cells are the most feasible source for a long term satisfying results for DR. Potential of stem cells in cell therapy is controlled through various objects. These factors enhance the potential of stem cells [58]. Now a day's new strategies are developed to enhance the potential of stem cells to cure the neural cells and vascular cells of retina.

Dr. management
Along with the stem cell therapy for DR treatment many other factors are also involved in management of diabetic retinopathy. For example, blood glucose level and blood pressure level management (Fig 4).

Glucose level
Hyperglycemia and its duration is the main cause of progression of DR [59]. Due to the mismanagement in the glucose level exerts many hazardous results. Oxidative stress exerts on the epithelial cell lining that increases the interactions between the circulating inflammatory cells and make them activated [60].

Level of lipids
High level of lipids also progress the DR. High lipid level results dysfunction of endothelial due to the reduction of availability of nitric oxide. Increase in oxidative stress takes place which results the peroxidation of lipids. This oxidative stress is associated with hyperglycemia. This increased hyperglycemia is responsible for the retinal emanation formation in Dr. Aldose reductase management Increased expression of aldose reductase is responsible for the progression of DR. In hyperglycemic condition the excess of glucose in blood is reduced by aldose reductase to sorbitol at the expense of NADPH. Increase in the level of sorbitol induces the osmotic stress. This osmotic stress reduces the level of NADPH. Glutathione cause the deterioration of antioxidant defense mechanism that results the oxidative damage of retina and capillaries associated with retina.

Level of vascular endothelial growth factor (VEGF)
The maintenance of VEGF level is also very important to overcome the progression of DR. It is an important biomarker in progression of DR. VEGF is also an important permeability-inducing inflammatory agent

Dr. management
Control of blood pressure and lipid level are included in primary pretentious control of glucose level. Diabetic retinopathy is the trending issue of vision loss in the world. The early detection might be helpful to resolve this issue. The diabetic patients are recommended to consult any specialist right after 12 months [63]. After the detection of the disease DR needs a proper treatment. Although DR does not have any proper treatment yet but above prevention are absolutely very helpful. Laser photocoagulation is one of the parts of proper treatment of diabetic retinopathy. There are 50% patients with proliferative diabetic retinopathy suffering from vision loss. Laser photocoagulation reduces the 3 years' prevalence of vision loss. Many physicians recommend laser photocoagulation to overcome the vision loss. Anti-VEGF (vascular endothelial growth factor) therapy is also feasible. This therapy is convenient to decrease the therapy sessions as per year. Anti-VEGF therapy is a monthly practice.
Aspirin was considered to be one of the treatments but it was found that aspirin therapy did not work for progression of diabetic retinopathy. Especially for the patients who need this for cardiovascular diseases or other conditions [64]. Some steroid based injections are also used for the treatment of diabetic retinopathy.

Intravitreal steroid injections
Corticosteroids are the anti-inflammatory agents used to cure diabetic retinopathy. These corticosteroids inhibit the activity and expression of the VEGF and also helpful in prevention from the breakdown of bloodretinal barrier. Among the use of corticosteroids; Triamcinolone acetonide TA is more popular. Intravitreal TA is the promising treatment for the patients of macular edemathatdoes not show response on laser treatment. But a continuous application of Intravitreal TA injections is inconvenient for some patients. To overcome this situation a biodegradable intravitreal implant called Retisert has been developed. In case of severe proliferative diabetic retinopathy there is a high risk of vision loss. At this stage photocoagulation surgery is one of the promising strategies to restore the vision loss. Scatter laser treatment is also a feasible strategy for restoration of vision loss in case of severe non-proliferative diabetic retinopathy or in case of non-severe proliferative diabetic retinopathy [65].

Screening of Dr.
Screening of DR is very important for being very helpful in reduction of risk of diabetic retinopathy. Screening and early treatment with laser can reduces the risk of DR. According to the early treatment of diabetic retinopathy Study laser photocoagulation could be helpful in prevention of vision loss even with patients' proliferative diabetic retinopathy. At this threshold level of DR patients are mostly symptom free. The progression of diabetic retinopathy is based on the duration and severity of diabetes in any patient [66]. Screening is important in terms to identify the diabetic patients suffering from DR. They still in symptomless even with proliferative diabetic retinopathy. According to current studies in DR the patients who have type 1 diabetes their DR situation should be examined after 5 years of diagnosis. The patients with type 2 diabetes should be examined right after every 1 year. The diabetic patients who become pregnant also have an increased risk of DR. They should examine their DR status in their 1 st trimester [67]. Screening is very important even you are just new diagnosed diabetic patient. Photocoagulation is feasible even for the screening of macular edema [68]. Along with the above techniques of screening risk of diabetic retinopathy could be managed. Although there is not any particular strategy for DR treatment but along with the stem cell therapy management and screening of DR are the most feasible strategies to overcome the risk of diabetic retinopathy.

Conclusion
This study is about the prospect of stem cells in diabetic retinopathy, its management and screening. Diabetic retinopathy is the trending cause of vision loss. Stem cells have the potential restore the vision loss in diabetic retinopathy. Different stem cells like mesenchymal stem cells, induced pluripotent stem cells, CD34+ cells and HLA factors have different aspects of effectiveness in diabetic retinopathy treatment. Along with the stem cells many other strategies for example; laser photocoagulation, anti-VEGF therapy, intravitreal injections and scatter laser therapy. Despite of all the proper treatment risk of diabetic retinopathy need to be manage. Management of diabetic retinopathy includes the management of glucose level, lipid level, aldose reductase level and level of VEGF. The management of all the above factors can reduce the severity of diabetic retinopathy. Screening of diabetic retinopathy is also a part of the management of this disease. Proper screening of diabetic retinopathy can reduce the risk of vision loss. Although there is not any strategy yet that can completely nullify the risk of diabetic retinopathy but stem cell therapy, proper treatment, management and screening can reduce the risk of vision loss from diabetic retinopathy.