Targeting Netrin-1 and -4 as a Novel Diagnostic Parameter and Treatment Option for Diabetic Retinopathy

Introduction

Diabetic retinopathy (DR) is a retinal vascular disease¹ and the most prevalent microvascular complication of both type I and type II diabetes mellitus (DM).² The prevalence is strongly correlated to the duration of DM and level of glycemic control.³ Globally, nearly 90 million people have DR, of which 21 million have diabetic macular edema (DME), 28 million have sight-threatening retinopathy, and 17 million have proliferative retinopathy.^3,4 In the United States, retinopathy is the major cause of loss of sight in productive age groups.¹

In patients with type II DM, retinopathy may not be detected for years until significant damage occurs. Therefore, patients are diagnosed with DR after it has caused serious damage (visual problems). Even though type I DM is mostly diagnosed in early age, it still needs early detection for prevention of related complications, including retinopathy and other complications.⁵

Different biological mechanisms have been associated with the pathology of hyperglycemia-induced retinopathy. Among them, polyol and hexosamine pathway activity, activation of protein kinase C isoforms, oxidative stress, and formation of advanced glycation end-product are mostly reported.^3,6 DR is characterized by specific loss of pericytes, which leads to an augmented blood vessel permeability, and the development of new blood vessels (retinal neovascularization).⁷ In addition, stiffening of the membrane in the eye and apoptosis of endothelial cells are detected during the pathogenesis, which collectively contributes to the damage of the blood–retinal barrier. Moreover, marked loss of endothelial cells and pericytes leads to upregulation of vascular endothelial growth factor (VEGF) by activating hypoxia-inducible factor 1 (HIF-1). Likewise, phospholipase A2’s (PLA2) elevation due to hyperglycemia also prompts upregulation of VEGF.⁶ DME can occur at any stage of DR and cause distortion of visual images and a decrease in visual acuity.⁸ Individuals with DR has been a higher chance to develop other vascular complications, including diabetic nephropathy, diabetic neuropathy, and cardiovascular diseases.⁹

Different conventional and newer diagnostic tools including glycated hemoglobin (HbA1c), thioredoxin-interacting protein, fructosamine, and glycated albumin have been used for detection of retinopathy in diabetic patients. However, most of them are used after it causes significant damage to patients.^9,10 In addition, they have limited specificity, sensitivity, and are imprecise in certain clinical conditions.¹¹ Moreover, increased cost related to newer diagnostic tools were also a substantial challenge.¹² Hence, the present review was intended to assess the potential role of Netrin-1 and -4 as a novel biomarker and therapeutic option for DR.

Netrin-1 and -4 and Their Receptors

Netrin is among axon guidance molecules with Semaphorins and Ephrins. It has been known to regulate axonal growth in the development of the nervous system, regulation of immune and inflammatory responses (inflammation of the nervous system).^13,14 Netrin-1 and -4 are the most known ligands of the family and critical axonal guidance protein during morphogenesis, embryonic development, and angiogenic processes.^15,16 Netrin-1 related axonal functions have been linked to two classes of receptors: the deleted in colorectal cancer (DCC) family, including DCC and its orthologue Neogenin-1, and the Unc5s family, including Unc5A^17,18 while Netrin-4 performs its activity majorly through binding to the laminin γ1 chain and disrupts laminin networks and basement membranes^15,16,19 (Figure 1).

Figure 1 Netrin and its Receptors.

The Role of Netrin in Retina Development

Netrin receptors have been expressed in developing retina and Netrin-neogenin signaling, which might stimulate retinal ganglion cell axon growth into and along the optic nerve.^20,21 Netrin-1/UNC5B may activate retinal vessel development and they can stimulate pro-angiogenesis.²² The guidance of retinal ganglion cell axons through the optic disc is dependent on the DCC/Netrin-1 axonal guidance system.²¹ Furthermore, netrin-1 contributes to steering axons out of the retina.²³ Under hypoxic conditions, the Netrin-1 level increases and it might be a key factor in inducing retinal neovascularization.^25,26 Retinal neovascularization is the major pathological condition of several diseases including diabetic retinopathy, retinal vein occlusion, and age-related macular degeneration.^8,24,27 Netrin-1 promotes retinal angiogenesis in oxygen-induced retinopathy.^28,29

Netrin-4 is highly expressed in the retina and its role is as an angiogenesis modulating factor in oxygen-dependent vascular homeostasis.²⁴ In an in vivo study, Netrin-4 is found to be a negative regulator of corneal epithelial cell proliferation and retinal vascular branching. It is also expressed in the retinal vascular basement membrane.¹⁶

Preclinical and Clinical Studies on the Level of Netrin in Diabetic Retinopathy

DR has both neural and vascular components; proteins used for both neural and vascular retinal tissues are a potentially important role. Through time the early diagnosis of retinopathy is increasing by different biomarkers related with the neural component.^30,31 Among them, Netrin was studied by various experimental and clinical models.^{7,19,24,32,33} The level of Netrin protein in the plasma has been positively correlated with DR. As compared with the normal controls, the level of Netrin was markedly increased in the early phase of DR. As shown in Table 1, different experimental studies revealed that the level of Netrin in the cornea increased substantially at the early phase of DR (Table 1).

Table 1 Role of Netrin-1 and -4 in Diabetic Retinopathy as a Biomarker and Treatment Modality for DR

Association of Netrin-1 and -4 with Diabetic Retinopathy

DR is one of the major complications of DM and known to cause visual loss in many cases.⁵ It involved a neurovascular lesion that can damage blood vessels and nerves of the retina.² Due to that protein mostly found in nerve endings were used as indicators for changes which occurred in the system.⁷ Among them, Netrin has been recently considered as a novel biomarker and therapeutic agent for DR. Netrin-1 and -4 has been well studied regarding their role in DR.³⁹ Alteration of the body’s Netrin-1 and -4 level may be reflected as upcoming biological protein to identify retinopathy promptly and to decide its severity.

Angiogenesis secondary to DR causes pericytes cell injury, which is due to infiltration of neutrophils and monocytes like IFN-γ and IL-17.⁶ Generally, Netrin-1 regulates hypoxia-induced inflammation through COX-2/PGE2 and NFκB pathways during inflammation secondary to hypoxia. It has a protective role in DR by conversing new blood vessel formation and overwhelming inflammation in the cornea^32,38 (Figure 2).

Figure 2 Relation of Netrin-1 and -4 with Diabetic Retinopathy.

Different studies also proved that the level of Netrin in the cornea significantly increased at the early stage of DR. So considering this protein as a biomarker is imperative to detect the disease as timely before it progresses and causes severe damage.^26,32,34 According to Miloudi et al, Netrin-1 level has been increased in the inflamed retina. In addition, degradation of Netrin-1 by collagenases and MMPs generated fragments, such as the VI–V peptide, that exacerbate retinal edema. Moreover, these data further suggest that cells of the ganglion layer undergo diabetes-induced retinopathy.³²

Netrin-4 also has a role in vascular remodeling such as an increased retinal blood vessel tortuosity, persistence of the hyaloid artery in adult mice and spontaneous focal retinal leakage.^36,40 Studies done on knockout mice (Ntn-4-/-) confirmed that lack of Netrin-4 has a major role in the pathophysiology of neovascularization due to ischemia but it does not relate to inflammatory neovascularization²⁴ (Figure 2).

Unlike other biomarkers, various preclinical/clinical studies proved that administration of exogenous Netrin (Netrin-1and -4) has a therapeutic effect (Table 1).^19,25,36,39 According to Crespo-Garcia et al, exogenous administration of Netrin-1 and -4 into the vitreous of diabetic rats reduced retinal vascular permeability. These effects may relate to the potential role of Netrin-1 in vascular repair, inhibition of angiogenesis, and preferentially preserves the remaining normal vasculature to supply ischemic tissue during retinopathy. In addition, Netrin-4 acts as an antiangiogenic factor through the regulation of both endothelial and perivascular cells in the retina.⁴⁰ Netrin-4 decreased human umbilical vein endothelial cells (HUVECs) viability in a dose dependent manner by regulating HUVEC proliferation and apoptosis. It also prevented corneal neovascularization, enhanced the regression of corneal neovascularization, and inhibited apoptosis in alkali-burned rat corneas. Another study done by Prieto et al³⁸ revealed that Netrin aroused endothelial cell migration in vitro at minimal concentrations while at higher concentrations it tended to promote endothelial cell migration to a smaller degree.

Conclusion and Future Perspectives

The review revealed that Netrin-1 and -4 are a potential to be an alternative biomarker for DR, and exogenous supplementation of these proteins can be used as a therapeutic agent. Change of the retinal/corneal Netrin-1 and -4 levels may be reflected as upcoming biological protein to identify DR promptly, which in turn becomes eye-opening for novel drug discovery for this upsetting disease condition. Further detailed animal and human studies are required to advance the role of Netrin as a novel biomarker for DR. Additionally, further investigations are needed to determine the mechanisms by which Netrin is used as a therapeutic agent for the treatment of DR.