**1. Introduction**

Diabetes is now considered a worldwide epidemic [1,2]. Recent reports indicate that over 90% of diabetic individuals have type 2 diabetes (T2D) [3,4]. The most common microvascular complication of diabetes is diabetic retinopathy (DR) [2]. Despite a growing number of different approaches to arrest DR, the incidence and prevalence of DR continues to rise [5]. The understanding of the pathogenesis of DR remains incomplete [4], and this is, in part, due to the lack of readily available models that completely recapitulate the metabolic phenotype [6]. The high-fat diet (HFD) mouse model has

been described as a robust model for investigating obesity-associated T2D and its related metabolic complications [7]. Studies have shown that HFD-fed mice develop obesity, impaired glucose tolerance, and reduced insulin sensitivity [8,9] with systemic manifestations involving adipose tissue [10], liver [8], and kidneys [11]. However, the ocular changes associated with the HFD model have not been fully investigated. Moreover, the typical Western diet (WD; 40% fat) has also been given to rodents to recapitulate obesity-driven pathology. However, to mimic the features of T2D, the administration of low-dose Streptozotocin (STZ) is also given to the WD mice [12–14].

The retinal response to high fat exposure would likely involve local changes in the expression of lipid transport proteins, such as the liver X receptors (LXRs). The LXRs are the key transcription factors that regulate lipid and cholesterol metabolism [15]. While liver X receptor alpha (LXR α) is expressed only in some tissues, the expression of liver X receptor beta (LXRβ) is ubiquitous [12]. Previously we showed that whole body LXR α/β deficiency resulted in the generation of increased numbers of acellular capillaries, while LXR agonists improved DR in Streptozotocin (STZ)-induced diabetes [12] and in diabetic Leprdb/db (db/db) mice [16]; however, it is not known if the WD modulates the expression of LXR in the retina.

Retinopathy is typically characterized by macroglia activation and gliosis identified by glial fibrillary acidic protein (GFAP) overexpression, which can be considered as a marker for retinal damage [17,18]. In the healthy mammalian retina, GFAP is expressed only in astrocytes and not in Muller cells. Following inherited or acquired retinal pathology, GFAP is expressed also in Muller cells [19,20]. GFAP expression in Muller cells has been widely used as a cellular marker for retinal pathology [21–25]. Hypoxia-inducible factor 1 alpha (HIF-1 α) is known to be a key regulator of a tissue's response to hypoxia [26] and plays a role in obesity-induced metabolic syndrome. It has been shown that HFD leads to gradual increase in HIF-1 α and associated pathological changes in the liver [27,28]. However, the role of HIF-1 α in the retina of WD-fed mice is not known.

A better understanding of DR in obesity-driven models is needed and may facilitate the optimal choice of disease models for future investigations. Thus, in the present study, we hypothesized that HFD and WD feeding would result in a distinct retinal phenotype and a time course slower than that observed in models of T2D, such as the db/db mouse [29] or the high fructose and high fat fed mouse [30]. For this purpose, we characterized not only systemic endpoints of glucose and lipid metabolism but also the function of the retina and development of retinal pathology, including retinal vascular changes and changes in expression of the critical proteins LXRβ, HIF-1 α, and GFAP.
