Nerve Growth Factor Promotes Retinal Neurovascular Unit Repair: A Review

Abstract

Purpose: The purpose of this paper is to investigate how the imbalance of neurogenic factor (NGF) and its precursor (pro-NGF) mediates structural and functional impairment of retinal neurovascular unit (RNVU) that plays a role in retinal degenerative diseases.

Methods: A literature search of electronic databases was performed.

Results: The pro-apoptotic effect of pro-NGF and the pro-growth effect of NGF are essential for the pathological and physiological activities of RNVU. Studies show that NGF-based treatment of retinal degenerative diseases, including glaucoma, age-related macular degeneration, retinitis pigmentosa, and diabetic retinopathy, has achieved remarkable efficacy.

Conclusions: RNVU plays a complex and multifaceted role in retinal degenerative diseases. The exploration of the differential signaling expression of proNGF-NGF homeostasis under physiological and pathological conditions, and the corresponding pathological processes induced by its regulation, has prompted us to focus on earlier retinal neuroprotective therapeutic strategies to prevent retinal degenerative diseases.

Keywords:

• Nerve growth factor
• retinal neurovascular unit
• coupling mechanism
• retinal degenerative diseases

Introduction

Retinal degenerative diseases, including age-related macular degeneration (AMD), retinitis pigmentosa (RP), diabetic retinopathy (DR) and glaucoma, are the leading causes of impaired vision and blindness.¹ Treatments of retinal degenerative diseases have mostly focused on advanced complications or anatomical correlates, such as lowering IOP, improving fundus microcirculation, and nutritional support at late stages when vision is already profoundly compromised.^2–4 Therefore, a better understanding of the pathogenesis of retinal degeneration in its early stages is warranted to provide timely and effective prevention and intervention strategies.

In addition to neurons in the neuronal nuclear layer which consists of multiple layers from retinal pigment epithelium (RPE) to ganglion cell layer (GCL), the retinal neurovascular unit (RNVU) also includes glial cells and retinal microvessels.⁵ As a multicellular structure, RNVU plays an important role in the physiological function and homeostasis of the retina through interactions between the cells of the RNVU.⁶ Ischemia, hyperglycemia, inflammation, oxidative stress, aging, genetics, and other factors have been implicated in the development of retinal degenerative diseases by disrupting the structural and/or functional coupling of the RNVU.

Neurotrophin nerve growth factor (NGF) was firstly described in 1950s as a key mediator in neuronal survival and differentiation.⁷ More recently, it was reported that the balanced expression of NGF, its own precursors and its receptors are highly involved in the physiology and pathology of the retinal microvasculature through the regulation of axonal growth, synapse formation, and the synthesis and release of neurotransmitters and neuropeptides.⁸ The impact of NGF on the structural and functional changes in the RNVU suggests a potential role in the pathogenesis of retinal degeneration and sheds light on the development of novel NGF-based therapies for treating retinal degenerative diseases.

This review summarizes the current understanding of the involvement of NGF in the pathophysiology of the RNVU, as well as perspectives on implementing NGF-based therapeutics for treating retinal degenerative diseases at their onset.

Structure of RNVU and its necessity in the physiological retinal function

Structural and functional damage to the neuro-glial-vascular unit underlies many neurodegenerative and neuroinflammatory diseases. This concept was first applied to studies of several diseases in which the blood-brain barrier is disrupted, such as stroke, cerebral infarction, Parkinson’s disease, and Alzheimer’s disease.^9,10 NVU refers to the functional coupling and interdependence between neurons, glial cells and vascular system.¹¹ From an anatomical and physiological perspective, the eye is considered an extension of the brain. In analogy to the NVU of the brain, the structure of the RNVU consists of retinal neurons, glial cells, and retinal microvasculature, which are distributed in the corresponding 10 layers of the retina.^12,13 The structure of the retina and RNVU is shown in Figure 1.

Figure 1. Structure of retina with magnified view of retinal neurovascular unit (NVU). RPE, retinal pigment epithelium; R and C: rod and cone; OLM: outer limiting membrane; ONL: outer nuclear layer; OPL: outer plexus layer; INL: inner nucleus; IPL: inner plexus layer; GCL: ganglion cell layer; NFL: nerve fiber layer; ILM: internal limiting membrane.

There are five types of retinal neurons: photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells. Photoreceptors predominantly comprise cones and rods and are located in the outer nuclear layer (ONL). The cell bodies of horizontal, bipolar, and anaplastic cells are primarily located in the internal nuclear layer (INL), whereas retinal ganglion cells (RGCs) are distributed in the ganglion cell layer (GCL).¹⁴ The outer plexiform layer (OPL) and the inner plexiform layer (IPL) are the sites where these three levels of neurogenic synapses are connected.¹⁵ The internal limited membrane (ILM) brings together the synapses of ganglion cells, which are sent by the optic nerve to the visual cortex to complete the transmission of visual signals.

The glial component of the RNVU consists of macroglia (astrocytes and Müller cells), microglia, and oligodendrocytes, which surround the neurons, form retinal defense structures, and help maintain retinal homeostasis. Glial cells are the interface between neurons and the vascular system and are therefore key regulators of functional communication between them.¹⁶ Müller cells, the only cells that span the entire width of the retina, reside between the inner boundary membrane (ILM) and outer membrane (OLM) and are in contact with almost every cell type in the retina. The unique position of Müller cells allows them to perform various functions necessary for retinal homeostasis, including converting glycogen to lactate, prompting the synthesis of retinoid from retinol, modulating the concentration of extracellular ions to regulate plasma membrane polarization, maintaining extracellular pH, controlling neurotransmission, recycling transmitters, and transporting metabolites.^17,18 Astrocytes are present only in the nerve fiber layer (NFL) and GCL, and provide metabolic and mechanical support for neurons.^19,20 Most nutrients, waste products, ions, water, and other molecules are transported between vessels and neurons via macroglia, and disruption of their functional imbalance can result in inflammation and apoptosis. Microglia are highly specialized phagocytes that constantly monitor local synaptic activity and remove dead cells and metabolic debris.²⁰ In chronic retinal disease, they secrete large amounts of proinflammatory cytokines that affect the normal function of the neurovascular units.²¹

It is now well recognized that retinal microvasculature is distributed from the NFL to the INL but absent in the ONL.^22,23 Telopods of endothelial cells, pericytes, and a small number of macroglia aggregate to form the retinal microvasculature. The physiological demands of retinal neurons determine the filling of vessels and changes in their luminal diameter, vasodilation, and vascular narrowing.^24,25 The retinal microvasculature reciprocally provides nutrients and oxygen to the RNVU and excretes waste products from the tissue to meet the high demand for oxygen consumption by the retina, a metabolically active neurovascular tissue.^26,27

The normal function of RNVU relies on the delicate coordination of various cell types inside RNVU, which supports the nutrition and metabolism of the retina and provide a suitable environment for neural signaling.²⁸ Neurons are the most critical cells for neurological function, acting as “pacemakers”. When subtle changes occur in the retinal environment, neurons receive signals and establish synaptic connections with neighboring glial cells to complete communication with the vasculature and regulate the stability of the intraretinal environment. Therefore, the neuroglial-vascular coupling of the RNVU is essential for retinal homeostasis.

RNVU changes in retinal degenerative diseases

Structural disruption of the RNVU and abnormal signal transmission are the main pathological mechanisms of retinal degenerative diseases.²⁹ Oxidative stress, apoptosis, inflammation, and endoplasmic reticulum stress activate the NF-κB proinflammatory pathway and caspase apoptotic pathway,^30,31 leading to pathological changes in the retinal microvasculature, neurodegeneration and disruption of blood retinal barrier function.^32,33 Changes in the retinal microvasculature are mainly manifested by thickening of the basement membrane, loss of pericytes and endothelial cells, formation of microaneurysms, and retinal leakage.³³ Recent studies have demonstrated that neurodegeneration occurs prior to the appearance of visible microvascular lesions.³⁴ Neuronal apoptosis is an early pathological feature of retinal degenerative disease. Activation of the NF-κB proinflammatory pathway is a major cause of RGC death.³⁵ Damage to any component of the RNVU can lead to impairment of the entire coupling mechanism, resulting in local inflammation or apoptosis, paralysis of the entire neurovascular unit, progressive retrograde axonal degeneration, and impaired messaging.

Expression of nerve growth factors and their signaling pathways

Nerve Growth Factor-β (NGF) was first isolated by Nobel Prize winners Srita Levi Montalcini and Stanley Cohen in 1956 and is the earliest discovered member of the neurotrophic factor family.³⁶ In the early 1980s, three NGF homologous growth factors were identified, namely brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5).³⁷ NGF is considered a powerful and highly selective growth factor for sympathetic and sensory neurons and cells from the neural crest. It is involved in the survival, differentiation, and function of peripheral and central nervous cells through interaction with specific cell receptors, regulating the gene expression of neurons. The biological effects of NGF also extend to the immune and endocrine systems.^38–42 NGF is produced from its precursor pro-NGF by the hydrolysis of matrix metalloproteinase-7 (MMP-7) and fibrinolytic protein.⁴³ Studies have shown that pro-NGF is not only hydrolyzed into mature NGF but is also secreted out of the cell to exert its unique biological activities.⁴⁴ P75 neurotrophin receptor (p75NTR) is a member of the tumor necrosis factor receptor superfamily (TNFRSF), also known as TNFRSF16, which plays a pro-apoptotic role by binding to its high-affinity ligand pro-NGF. This process requires the participation of its co-receptor sortilin, which specifically binds pro-NGF and increases the affinity of p75NTR to pro-NGF.^45–47 The formed Pro-NGF/sortilin/p75NTR apoptotic signal complex is involved in the regulation of synaptic inhibition, cell death, and neurodegeneration.⁴⁸

As an upstream signal initiator, the Pro-NGF/sortilin/p75NTR signal axis induces the activation of a series of downstream signaling pathways. For example, the complex-mediated activation of RhoA kinase and paracrine inflammatory response further activate the p38 MAPK and JNK pathways and participate in cell apoptosis.⁴⁹ Activated MAPK contributes to NF-κB activation and promotes the release of proinflammatory cytokines, including IL-1β, IL-6, IL-8, and TNF-α.^50–54 This complex can also interact with apoptotic pathways, resulting in changes in Bax, Bcl-2 and Caspase-3 levels.⁵⁵

Mature NGF mediates its growth-promoting biological function via the tropomyosin kinase receptor A (TrkA).⁵⁶ NGF binds to high-affinity TrkA receptors and low-affinity p75NTR to activate the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT), phospholipase C-γ (PLCγ), and mitogen-activated protein kinase/extracellular signal-associated kinase (MAPK/ERK) pathways.^57–60 Activation of PI3K/AKT further activates the cAMP response element binding protein (CREB) pathway and inhibits the apoptosis signal-regulating kinase 1/ Forkhead box O1 (ASK1/FoxO1) pathway.^61,62 All these factors contribute to neuronal survival, synaptic growth, ganglion cell survival, and other neurotrophic effects. The expression of nerve growth factors and their signaling pathways are shown in Figure 2.

Figure 2. Expression map of nerve growth factor and its signaling pathway. ProNGF is hydrolysed by MMP-7 and fibronectin to produce mature NGF. p75NTR binds to its high-affinity ligand proNGF to exert a pro-apoptotic effect, a process that requires the involvement of its co-receptor sortilin and formation of the proNGF/sortilin/p75NTR apoptotic signaling complex. The sortilin/p75NTR signaling axis acts as an upstream signal initiator RhoA kinase and NF-κB, which are involved in apoptosis and inflammatory responses. proNGF binds to the high-affinity TrkA receptor and the low-affinity receptor p75NTR to activate the PI3K/AKT, PLCγ and MAPK/ERK pathways, which are involved in the proliferation, growth, and survival of retinal neurons and microvascular cells.

Nerve growth factor is involved in the pathology and physiology of RNVU

Neurotrophic factors have been shown to be locally produced by retinal neurons and glial cells, or paracrine and recycled from the superior colliculus back to the retina.^63,64 However, a similar superior colliculus transport mechanism has not been found for NGF, which raises the possibility that NGF is most likely synthesized and secreted locally in retinal neurons and glial cells.^65,66 NGF plays a crucial role in the embryonic development of the vertebrate visual system.⁶⁷ As early as 1976, American medical scientist J.E. Turner found that a single 200 BU intraocular injection of NGF at optic nerve dissection significantly accelerated the response of goldfish RGCs to axon dissection.⁶⁸ Jian Q. et al. found that the levels of NGF and TrkA were augmented in parallel with the NGF/TrkA signaling downstream components, such as PI3K and Akt, in rat retinal Müller cells transplanted from the subretinal cavity of rat bone marrow mesenchymal stem cells (rBMSCs), suggesting that activation of this pathway plays an important role in the protection of photoreceptors and RGCs.⁶⁹

Immunohistochemical staining of the rat retina have shown universal expression of NGF across normal retinal neural tissues from the RPE to the GCL, with the highest abundance in RPEs, RGCs, and Müller cells.^70,71 NGF receptors, including high-affinity TrkA and low-affinity receptor p75NTR, are present in NGF-responsive neurons and glial cells, respectively.^72–74 In addition to its neuroprotective effects, NGF may prevent pericyte loss and the formation of acellular occluded capillaries in rats developing diabetic retinopathy.^75,76 The downstream PI3K/Akt pathway activated by NGF/TrkA is an important intracellular signaling pathway involved in a range of cellular responses of retinal microvascular endothelial cells and pericytes, such as cell proliferation, survival, apoptosis, cytoskeleton construction, vesicle transport, glucose transport, and platelet function.^77,78 NGF is not only an important regulator of neuronal and glial survival and death in the eye, but also mediates the development, survival, and regeneration of microvascular cells. NGF-mediated effects are achieved through neuro-glial-vascular coupling.⁷⁹

In animal models, pro-NGF labelling was found to be widely distributed across retinal tissues, including RPE, photoreceptor outer segments, ONL, INL, IPL, GCL, microglia, and Müller cells.^71,80,81 In addition, an increase in the release of pro-NGF and p75NTR and a reduction in the production of NGF were observed in retinal tissues of animal models of retinal degenerative diseases.⁸² For example, in a diabetic retinopathy (DR) mouse model, upregulation of the proNGF-p75NTR axis induces apoptosis of pericytes, mediates ligand-dependent production of inflammatory cytokines, and destroys vascular units, resulting in damage to the blood-retinal barrier.⁸³ In contrast, elevated NGF levels attenuated pericyte damage and reduced neointimal formation by decreasing VEGF expression.⁸⁴ NGF and inflammatory mediators are released into the vitreous, leading to local activation of retinal RGCs and Müller cells in a Reelin-deficient mouse model.⁸⁵ The degeneration of retinal photoreceptor cells also results from reduced NGF.⁸⁶ The formation of peroxynitrite induced by ischemia, hyperglycemia, inflammation, oxidative stress, and aging has been shown to impairs the production and activity of MMP-7. This further leads to the accumulation of pro-NGF in RNVU cells and a decrease in NGF levels. Pro-NGF activation and reduced bioavailability of NGF can activate downstream signaling pathways and induce inflammation, Müller cell and astrocyte gliosis, microglial phagocytosis impairment, microvascular degeneration, in RNVU tissues.

Potential NGF-based therapies in the treatment of retinal degenerative diseases

Retinitis pigmentosa

Retinitis pigmentosa (RP) is caused by hereditary retinal photoreceptor dystrophy, with photoreceptor apoptosis as the final event in the disease process.⁸⁷ In recent years, RP has been implicated in neuronal cell death, neuronal morphological changes and migration, retinal nerve fiber atrophy, altered glial metabolism and morphology,^17,88 and reduction in the caliber of both small retinal arteries and veins.⁸⁹ RP is a disease accompanied by RNVU damage.

NGF is produced by retinal nerve cells and stored to maintain photoreceptor survival and prevent photoreceptor degeneration.⁹⁰ Studies have demonstrated that short-term topical application of NGF improves best-corrected visual acuity (BCVA), macular focal electroretinogram recordings (fERG), and Goldmann visual field tests in a subset of RP patients.⁹¹ Further studies revealed that intraocular injection of NGF prevented photoreceptor degeneration and regulated retinoid expression in rats with RP.⁹² Platon-Corchado et al.⁹³ investigated the interaction between pro-NGF/p75NTR signaling in RP-related photoreceptor degeneration processes and demonstrated that p75NTR antagonists have potential therapeutic value in RP treatment. Furthermore, NGF and anti-vascular endothelial growth factor (αVEGF) in combination significantly increased retinal cell survival, retinoid expression, and neuropil growth in the photoreceptors. In the same study, NGF/α-VEGF also upregulated Bcl-2 mRNA, downregulated Bax mRNA, and upregulated Trk-A mRNA.⁹⁴

Glaucoma

Glaucoma is a neurodegenerative disease associated with abnormal sensitivity to intraocular pressure (IOP). Elevated IOP suppresses retinal output neurons, leading to progressive degeneration of RGCs and axons, as well as damage to the optic nerve.^95,96 Damaged RGCs or astrocytes can release damage-associated molecular patterns (DAMPs) that trigger retinal inflammatory responses.⁹⁷ Narrowing of the fundus and disturbances in blood flow have been recognized as the major causes of progressive visual field loss in patients with glaucoma.^98–100 A variety of mechanisms have been proposed to explain the pathogenesis of glaucoma, such as neuroinflammation, loss of neurotrophic factors, destruction of the axonal cytoskeleton, and disruption of the neurovascular unit, including vascular disorders and coupling imbalances.^101,102

In pathological conditions such as glaucoma, elevated IOP disrupts the balanced expression of pro-NGF, NGF, and its receptors and induces apoptosis in RGCs.⁶² Serum NGF levels were found to be significantly lower in patients with glaucoma than in healthy controls.^103,104 In a mouse model of glaucoma, NGF eye drops plus ultrasound contrast agent (UCA) intervention significantly reduced IOP, elevated transient visually evoked potentials, enhanced activity of RGCs, and diminished apoptosis and autophagy.¹⁰⁵ In clinics, administration of NGF drops for glaucoma significantly reduced RGC apoptosis and showed long-term improvements in visual field, optic nerve function, contrast sensitivity, and visual acuity, suggesting neuroprotective effects of NGF in clinics.¹⁰⁶ In another clinical trial of patients with open-angle glaucoma, application of 80 μg/mL recombinant human nerve growth factor (rhNGF) eye drops significantly improved visual function and retinal structural measurements compared to the placebo group.¹⁰⁷ Lin et al. demonstrated that the protective effect of NGF on glaucomatous retinal RGCs was achieved through inhibition of endoplasmic retinal stress.¹⁰⁸

Diabetic retinopathy

Diabetic retinopathy (DR) destroys all the components of the neurovascular unit.¹⁰⁹ Traditionally, DR is considered to be primarily a microvascular disease characterized by morphological changes in the microvasculature, thickening of the basement membrane, absence of tight junctions between the endothelium, early and selective loss of pericytes, increased vascular permeability, capillary occlusion, and microaneurysms.¹¹⁰ Recent studies have pointed out that disruption of the neuroglial-vascular unit and imbalance in its coupling mechanism (coupling) play a key role in the early onset of DR.³³ A high-glucose environment leads to dopaminergic neuronal dysfunction¹¹¹ and progressive structural and functional alterations in both macro- and microglia.¹¹² In contrast, inflammatory factors that are currently thought to play a key role in the development of DR are more closely related to microglial activation.¹¹³

A growing body of evidence supports the relationship between pro-NGF/NFG imbalance, inflammation, and neurological and microvascular degeneration in early diabetic retinopathy.¹¹⁴ Diabetes-induced formation of peroxynitrite was observed in parallel with impaired MMP-7 production and activity, pro-NGF accumulation, and reduced NGF levels in the analysis of samples from DR patients and experimental models.⁸⁰ Inhibition of p75 (NTR) blunted pro-NGF-induced retinal inflammation, abrogated diabetes-induced glial fibrillary acidic protein expression, reduced ganglion cell loss, and increased vascular permeability.¹¹⁵ Elsherbiny et al. found that carbamazepine, a neuroprotective agent, attenuates diabetes-induced neuronal loss and reduces diabetes-induced inflammation by activating NGF/PI3K/Akt.¹¹⁶ Troullinaki et al. revealed the anti-apoptotic effect of NGF in the hypoxic retinal endothelium, suggesting that NGF could be a potential target for the treatment of proliferative retinal diseases.¹¹⁷ Postball administration of NGF attenuates visual damage in DR rats by reducing the loss of RGCs but does not affect the retinal microvasculature.¹¹⁸ Recently, studies on the repair of neurovascular units by restoring the balance between pro-NGF and NGF have been recognized as a promising approach for the treatment of DR.

Age-related macular degeneration

Age-related macular degeneration (AMD) is characterized by neuroinflammation of retinal neurons, gliosis of Müller cells and astrocytes, and impairment of microglial phagocytosis, which reduces the bioavailability of nerve growth factors and ultimately leads to pathological changes in the retinal pigment epithelium and choroid.^119,120 AMD is primarily a result of structural and functional alterations in the RPE.¹²¹ The macula is divided into four regions: the central concave avascular zone, central concave zone, paracentral concave zone, and pericentral concave zone. The macula accounts for only 3% of the retinal surface area but has the highest oxygen consumption, with extensive capillary distribution in all regions except the central concave avascular zone.¹²² There are primarily two types of AMD, wet and dry, which are categorized based on the presence and absence of neovascularization, respectively.¹²³ Therefore, it is unlikely that nerve damage does not involve the microvasculature, particularly in wet AMD. In addition, a growing body of optical coherence tomography angiography (OCTA) evidence suggests that altered choroidal capillary (choriocapillaris) blood flow plays a dominant role in the pathogenesis of dry AMD.¹²⁰

Only a few studies have investigated the direct correlation between NGF and AMD. Among these limited studies, Telegina et al. reported increased NGF staining in Müller cells of OXYS rats, which developed advanced AMD-like retinopathy at 18 months of age.¹²⁴ Activation and dysregulation of proinflammatory signaling pathways, such as JNK, MAPK, and NF-κB, are more frequently observed in AMD. As discussed in the previous sections, these signaling pathways are highly associated with the structural and functional degeneration of retinal components, including neuroinflammation in the retina, axonal cytoskeleton reconstruction, impaired axonal signaling, abnormal expression of electrical signals, and neurovascular unit lesions, including vascular disorders.^125–128 Are these pathways mediated by an upstream pro-NGF-NGF imbalance? There is ample evidence for this, and more studies are needed to enrich the theoretical body of the close relationship between pro-NGF, NGF and AMD.

Conclusion

RNVU plays a complex and multifaceted role in retinal degenerative diseases, and there is evolving recognition of the protective role of NGF in this pathological process. In addition to traditional pharmacological and surgical interventions that primarily target retinopathy at late stages, novel retinal neuroprotective therapeutics that focus on neurovascular coupling are a promising strategy to prevent retinal degeneration at early stages. The expression of pro-NGF/NGF has been correlated with the differential regulation of multiple signaling pathways involved in retinal degeneration. Interestingly, researchers have used a number of NGF-based therapies that have neuroprotective effects and prevent degeneration of the RNVU: (1) the use of eye drops containing NGF or the injection of NGF into or behind the eye to increase mature retinal NGF.¹²⁹ For example, intravitreal injection (ivt) or eye drop (ed) administration of rhNGF has been shown to restore retinal and optic nerve damage, reduce RGC loss, stimulate axonal regeneration in a rat model,^130,131 significantly increase retinal thickness, and reduce apoptosis of retinal photoreceptors.^132,133 Similarly, one study demonstrated that topical application of rhNGF twice daily for three weeks significantly improved the survival of retinal RGCs in an animal model of optic neuropathy, increased the number of subretinal RGCs, promoted axon survival, and inhibited astrocyte activity in the optic nerve, thus providing optic nerve protection.¹³⁴ (2) TrkA agonists that activate the NGF/TrkA signaling pathway and promote cell survival in RNVU.¹³⁵ (3) Silencing of the pro-NGF signaling pathway using p75NTR antagonists such as LM11A-31 to block the binding of p75NTR to pro-NGF with no effect on the binding of NGF to TrkA.¹³⁶ (4) Fibrin activators that convert pro-NGF into mature NGF.¹³⁷

The role of the balance between NGF and its precursors in the physiology of RNVU and the pathological processes involved in retinal degenerative disease are not fully understood.^114,138 More studies are needed to understand the differences in the distribution of genes involved in this signaling axis in different retinal tissues, as well as the different activation signals due to different pathologies. The main challenges in NGF neuroprotection are to reduce off-target effects and provide safe therapeutic levels of neuroprotective drugs. NGF therapy is mostly in its early stages, and further clinical studies are needed to prove its clinical benefits. In addition, NGF has been administered in saline or other aqueous delivery vehicles in experimental and clinical trials to date despite its low aqueous solubility. Studies are encouraged to explore other ophthalmic drug delivery tools that are more compatible with the physiochemical properties and pharmacokinetic profile of NGF and the accessibility of current ophthalmic equipment instruments. The source of NGF biologics should also focus on more natural ingredients, such as cord blood serum,^139,140 ginsenosides,¹⁴¹ Astragalus membranaceus,¹⁴² and saffron.¹⁴³ Furthermore, it should be pondered: is NVU dysfunction the end result of a degenerative retinal disease process or a causative factor? How are glial cells involved in vasoactive factor release, hemodynamics, metabolic adaptation, and microvascular constriction and dilation? Are NGFs really the most upstream signals in disease development, and what other signaling pathways are potentially involved in the event? Therefore, there is a need to fully elucidate the role of NVU dysfunction in neurodegenerative diseases and the specific mechanisms by which imbalances in NGF and pro-NGF are involved in their pathology. New knowledge in this area may open new avenues for the prevention, early diagnosis, and treatment of degenerative diseases in humans.

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Correction Statement

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