# Involvement of Vasoactive Intestinal Peptide Family Members in Diabetic Keratopathy
## 2 Overview of Diabetic Keratopathy  
Chronic exposure to hyperglycemia triggers pathophysiological changes in cells, tissues, and organ systems, due to the promotion of oxidative stress, the activation of polyol pathway and protein kinase C (PKC), the formation of advanced glycation end-products (AGEs), and alteration of gene expressions [22]. The cornea is an avascular structure containing no blood vessels, receiving glucose via trans-corneal transport from the aqueous humor. Glucose is also present in tears, but its levels are lower than in the aqueous humor and serum [23]. Given that the cornea receives glucose from the aqueous and not the adjacent tear film, it is not surprising that in patients with diabetes, the cornea is exposed to high levels of oxidative stress and inflammation representing distinct features of diabetes in all other tissues [24; 25]. Corneal complications range from mild to severe manifestations and comprise epithelial defects, corneal thickness, erosions, and corneal nerve abnormalities [26] (Figure 1). Corneal epithelial alterations observed in patients with DM include epithelial fragility, non-healing corneal ulcers, and superficial punctate keratitis (SPK). The latter is characterized by scattered areas of punctate corneal epithelial loss causing photophobia, foreign body sensation, tearing, redness, irritation, and reduced visual acuity [27]. The reduction of corneal epithelial density and thickness is due to the imbalance between cell proliferation, differentiation, migration, and death. Moreover, the accumulation of AGEs counteracts the effective migration of epithelial cells essential for wound healing, leading to recurrent erosions [28]. The impairment of corneal epithelium is closely linked to the increase in glycosylated hemoglobin levels [29], and corneal epithelium barrier alterations expose patients to a higher risk of developing ocular infections than healthy people [30]. Moreover, cataract and laser-assisted in situ keratomileus (LASIK) surgeries are some of the high-risk  
Figure 1: Schematic showing pathogenesis of DK. Effects triggered by hyperglycemia in different parts of the cornea result in three main types of dysfunctions characterizing DK: epithelliopathy, neuropathy, and endothelium. interventions for patients with DM, since corneal damage after or during surgery may lead to slow healing and thus frequent corneal erosion damage [31; 32]. DK is also characterized by a loss of corneal sensitivity, which could be used by clinicians for the early diagnosis of diabetic peripheral neuropathy and/or DK [33]. In fact, corneal epithelium represents the most innervated and sensible epithelial surface of the human body. In particular, it is innervated by free nerve endings of the ophthalmic division of the trigeminal nerve (cranial nerve V) [34], and corneal nerves are responsible for the sensations of pain from mechanical, thermal, and chemical stimulation [35]. Furthermore, corneal nerves regulate tear secretion and via the regulation of neurotrophic factors maintain ocular surface homeostasis, corneal sensitivity, epithelial health, and wound healing [36; 37]. Recent studies showed that, in patients with DM, the density of corneal nerve fiber and branch and the corneal nerve fiber length are significantly reduced. Furthermore, 17% of these patients undergo the loss of 6% or more of corneal nerve fibers per year [38; 39; 40]. The stroma also shows structural alterations in patients with diabetes, due to the accumulation of AGEs, which provokes non-enzymatic cross-linking between collagen molecules and proteoglycans, thus causing the cornea to stiffen and thicken [41]. Changes in diabetic corneas were also found in corneal endothelial cells, whose density was decreased in patients with diabetes as compared to healthy subjects [32; 42]. Accordingly, a recent study involving 120 patients with diabetes and 120 healthy patients demonstrated that hyperglycemia altered corneal endothelium (counts, morphology, and structure) as well as corneal thickness.