Gerbil, Psammomys Obesus, a Human-like Rodent Model of Eye Research

Abstract

The purpose of this chapter is to illustrate the use of rodents other than mice and rats as relevant models of nutritionally human eye diseases. The sand rat or Psammomys obesus (P. obesus), is a desert rodent from the subfamily Gerbillinae, which has been widely used as an excellent animal model of diet-induced diabetes and metabolic syndrome. In previous studies, we showed that P. obesus develops type II diabetes when exposed to a high-calorie diet under laboratory conditions, resulting in diabetic retinopathy with similar visual disorders to that observed in humans. In this chapter, we will explore the notable similarities and differences between the human and rodent visual systems and the pertinence of using P. obesus as animal model of eye research. Retinal function, particularly that mediated by cone, will also be illustrated.

1. Introduction

Research using animal models in ophthalmology has enabled significant progress to be made in understanding the mechanisms of ocular diseases and in evaluating the efficacy of new therapeutic approaches [1]. However, commonly used models, such as mice, rats, and most of rodents, are not equivalent in terms of similarity to human vision and present major anatomical and functional differences [2]. Indeed, rodents differ in their visual adaptation according to their predominant activity during the day or night. The majority of these models have a visual system adapted to a nocturnal lifestyle, whereas the human visual system has evolved for a daytime habitat illuminated by light. In this chapter, we will explore the notable similarities and differences between the human and rodent visual systems to allow us to understand human eye disease.

2. Comparison of night and day vision adaptions in rodent and human

The most of rodent eyes are specifically adapted to their nocturnal lifestyle. Their eyes are generally larger than their heads [3]. In addition, their pupils can dilate more than those of humans. Nocturnal rodents have relatively larger crystalline lenses than diurnal rodents, completely filling the eyeball. This particular lens enables the transmission of ultraviolet light, which is required for UV-sensitive photoreceptors present in the mouse retina. This unique configuration allows them to capture and focus more light [4]. On the other hand, humans eyes are adapted to daytime vision, with a retina dominated by cones for color detection and accurate daylight vision. However, humans also have rods in their retina, enabling them to see in low light conditions, although less efficiently than rodents [5]. Nevertheless, it has been well established that rodents possess superior night vision compared to humans, primarily due to their greater density of rods in the retina.

In humans, the retinal photoreceptors encompass two distinct sensory types: cones and rods. These specialized cells contain the visual pigment [6]. Cones, located primarily in the fovea, enable us to perceive intricate details and exhibit sensitivity to colors. They are less light-sensitive compared to rods, thereby primarily contributing to daytime or photopic vision. The trichromatic model of human vision elucidates this phenomenon, where color sensations are attained by modulating the excitations of three cone types, each exhibiting distinct spectral sensitivities to various regions of the color spectrum. These cone types include those more responsive to blue light (S cones), green light (M cones), and red light (L cones) [7]. Consequently, a combination of light wavelengths corresponding to red, green, and blue can generate all perceivable color sensations. In contrast to humans, rodents possess dichromatic color vision, relying solely on cones sensitive to blue and green light. This distinction arises from the presence of two distinct types of opsin: S opsin cones (short or blue) and M opsin cones (medium or green) [8]. In the mouse retina, there exists a notable arrangement where cones predominantly expressing opsin S exhibit heightened contrast sensitivity and are situated closer to the ventral retina, preferentially surveying the upper portion of the visual field [9]. Conversely, the expression of M opsin progressively increases toward the dorsal retina, encoding the lower visual fields. This distinctive organization of the retina is believed to be a product of selective evolutionary adaptation aimed at efficiently recognizing images and natural scenes.

Anatomically, the organization of the rodent retina is similar to that of humans, with well-defined typical cellular and plexiform layers, comprising the outer retina containing the photoreceptor cell bodies and outer segments, and the inner retina, consisting of bipolar cells, amacrine cells, and the retinal ganglion cells (RGCs) [10], as recognized by Ramon and Cajal over 100 years ago [11]. However, there are notable differences in the arrangement and number of photoreceptor cells [6]. Rodents also have specific morphological features, such as more numerous ganglion cells and amacrine cells, which are involved in the processing of visual information [12, 13]. The topographic distribution of retinal ganglion cells throughout the retina has been studied in different animals, and cells are found to be densely distributed in areas of the retina where images are in focus [10].

Curcio and his colleagues [5] measured the density of photoreceptors in retinas from human eyes taken postmortem and analyzed histologically. They showed that the human retina contains a relatively high number of cones photoreceptors with an average of 4.6 million cones (4.08–5.29 million). However, they represent only 5–10% of all photoreceptors and are mainly concentrated in the fovea, in the center of the retina, along the optical axis. The peak cone density in the fovea is around 199,000 cones per mm² with significant inter-individual variability. The fovea is devoid of rods over a diameter of 600 microns. Beyond this, the rods appear progressively and their density becomes equal to that of the cones at a distance of 400 to 500 microns from the center of the fovea. The retina contains 95% of total photoreceptors with an average of 92 million rods (77.9 to 107.3 million). The fovea has long been recognized as the site of maximal visual acuity [14]. However, it contains 0.3% of the total cones and 25% of the ganglion cells, illustrating its importance in primate vision [5, 15]. Rod density is the highest along an elliptical perimeter located at the same distance as the optic nerve from the fovea. Rod density appears to be the highest in the superior retina. The cell density per unit in nocturnal rodents in retinas, such as rats and mice, is ~3–4 times higher in the central retina than in the human macula, and therefore, the phagocytic load per RPE cell is higher in mice than in humans. Rods are very sensitive to light, but their density and their connections with other retinal cells (several rods for a bipolar cell) mean that they are unable to discern fine details in an image. Similar to other mammals, the retina of nocturnal rodents is predominantly composed of rod photoreceptors. In fact, they have approximately 6.4 million rods, which make up around 97% of the total photoreceptor population [16]. In addition, nocturnal rodents such as rats and mice have a retina that is low in the cones, responsible for daytime vision, which differs considerably from the foveolar region in humans, which is composed exclusively of cones. There are only around 200,000 cones, accounting for just 3% of the photoreceptors [6, 13]. As a result, these rodents are poorly suited to studying the morpho-functional changes in this population of cells. Interestingly, diurnal rodents have higher proportions of cone photoreceptors than nocturnal rodents, which facilitates the study of cone pathophysiology [2].

The most striking adaptation was mentioned by German researchers, in 2009 17] when they observed on histological sections of mouse retina that rod nuclei have an inverted structure compared with any other cell, with a dark center and a bright periphery corresponding to euchromatin toward the outside while heterochromatin is compact in the middle [17]. This distribution is found only in nocturnal species such as the mouse. In contrast, the rod nuclei of diurnal mammals such as horses, squirrels, and humans have a typical, universal organization (euchromatin in the center, heterochromatin at the periphery). Nocturnal rodents, like the 40 species of nocturnal mammals studied, have a unique architecture.

It is, therefore, essential to have a suitable animal model that faithfully reproduces the anatomical, physiological, and pathological characteristics specific to eye diseases in humans. In our studies, we proposed, the Tunisian gerbil P. obesus as a promising model for eye research, offering notable anatomical and functional similarities with the human eye [18, 19]. In this chapter, we explore the use of the gerbil P. obesus as a rodent model for eye research, highlighting its similarities to the human eye and its potential in the study of eye diseases.

3. Importance of P. obesus in vision research

3.1 General characteristics

P. obesus, commonly known as the Sand Rat, is a species of gerbil first described by P. Cretzschmar in 1828 (Figure 1). Belonging to the subfamily Gerbillinae, this rodent species is found in the arid desert regions of North Africa and the Middle East, extending from Morocco to the eastern parts of Arabia [20]. Studies carried out since the 1960s showed that when sand rats were maintained under laboratory conditions and fed a standard rodent diet, they developed type 2 diabetes (T2D) [21, 22]. Diabetic features observed in Psammomys cover persistent hyperglycemia accompanied by hyperinsulinemia, dyslipidemia, to more advanced stages characterized by hypoinsulinemia, ketoacidosis, and diabetic retinopathy [18, 23, 24, 25]. These unique features have aroused great interest among researchers, making Psammomys a valuable and novel animal model for studying the mechanisms underlying (T2D) induced by nutritional manipulation. Therefore, P. obesus is well recognized as an interesting model for studying the relationships between diet, metabolism, and diabetes.

Figure 1.

Psammomys obesus. Obesus, or the sand rat Cretzschmar in (1828).

In 2017, Hargreaves and his colleagues conducted genome sequencing of the sand rat, leading to the discovery of a remarkable unusual chromosomal region, characterized by an abundance of G and C nucleotides [26]. Within this region, the Pdx1 homeobox gene, a transcriptional activator of insulin, exhibited significant sequence alterations, which has undergone massive sequence changes, probably contributing to diabetes and adaptation to low caloric intake. The main findings of this study imply that mutation rates vary within the same genome and that parts of the genome with high mutation rates may influence adaptation and ecological constraints. This unique genomic structure contributes to substantial divergence in the Pdx1 sequence. In addition, the authors warn that divergent regions may be missed by conventional short-read sequencing approaches, a consideration for current and future genome sequencing projects.

3.2 Interest of P. obesus in ophthalmological research

The desert-dwelling rodent P. obesus exhibits a sophisticated visual system and possesses distinctive attributes associated with its adaptation to arid habitats. Notably, it demonstrates striking parallels to the human eye, particularly concerning retinal structure and the signaling cascades implicated in human ocular pathologies [18, 19]. Furthermore, P. obesus, characterized as a diurnal species, resembles the human retina in multiple aspects. The relatively larger size of its eyes (~8 mm in diameter) compared to rats (4–5 mm) potentially augments its visual capabilities and facilitates manipulations during experimental investigations [2].

The histology of the whole eye of Psammomys was done by Ref. [27]. However, the first study to determine the retinal phenotype was carried out by our team and was published in 2011 [18], as mentioned by Ref. [27]. The work highlighted the similarity in the organization of the different layers of the retina between gerbils and humans, making it easier to transpose research results to the human scale. We have shown that the P. obesus retina has a typical stratified structure (Figure 2A and B), with cones accounting for an average of 41% of total photoreceptor cells. Short and medium wavelength cones are present in typical relative proportions. Among diurnal rodents, P. obesus showed a higher percentage of cones related to rods, in comparison with the Mongolian gerbil (Meriones unguiculatus) whose retinal photoreceptors comprise 13% cones and the Arvicanthis species, Arvicanthis ansorgei (33%), and Arvicanthis niloticus (35%), which still considering pertinent models of the study of cone pathophysiology [28, 29, 30].

Figure 2.

Structural, neuronal, functional, and vascular exploration of the healthy P. obesus retina. A) Light micrograph of a vertical section through P. obesus retina (Hematoxylin and eosin staining). The basic retinal structure of P. obesus is arranged in different layers of cells, from retinal pigment epithelium (RPE), photoreceptor layer (PR), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL), ganglion cell layer (GCL), scale bar 50 μm. B) Dapi immunostaining of different neuronal retinal cells observed by fluorescence microscopy, scale bar 50 μm. C) Typical ERG response of a P. obesus healthy retina to a white flash under photopic condition using 3 cd.s/m² light intensity stimulation, a-wave mixed response elicited at 3 cd.s/m², b-wave mixed response in the dark adapted eye. D) Eye fundus vasculature (a) in the optic nerve and (b) in the visual streak.

The importance of the cone system in our proposed model was confirmed by functional electrophysiological measurements in P. obesus (Figure 2C), done with ISCEV Standard for clinical ERG testing [19, 31]. The results were compared with those obtained in human subjects and Wistar rats. ERG measurements showed that the amplitudes of scotopic responses in P. obesus were quite similar to those in human subjects. However, under photopic conditions, ERG measurements share several characteristics with human ERG responses, while being quite different from those of the rat, for more details please see [19].

A strong cone-driven retinal response is indicated by the amplitude of the photopic a-wave, very similar to that observed in humans and six times higher than in the albino rat. While the b-wave is significantly larger in P. obesus than in man and rat, the photopic b−/a ratio is lower than in rats and closer to the value in man, showing another surprising difference with nocturnal rodents.

The amplitudes of the photopic oscillatory (OP) and flicker potentials at 30 Hz were all significantly larger in P. obesus than in human and Wistar rats. Furthermore, like the human photopic ERG, the photopic ERG of P. obesus also includes prominent post-b wave components (i.e., i and d waves), whereas the ERG of Wistar rats does not show it. Specifically, another striking feature shared by P. obesus concerns the photopic ERG responses after the b-wave: the i-wave and the negative photopic response (PhNR). These response patterns are commonly observed in human photopic electroretinograms (ERGs), but does not exist in nocturnal rodents [32]. Similarly suggesting that gerbils may represent a valuable complement to mice models, particularly for investigating retinal cone function and studying ocular diseases associated with cones.

In addition, P. obesus has emerged as a novel animal model for research on diabetic retinopathy [25, 33, 34]. One remarkable feature of P. obesus is its prominent visual streak, which exhibits remarkably high densities of cones and ganglion cells (Figure 2D). Additionally, this species possesses specialized vasculature in the visual streak, characterized by the absence of major vessels in this area. The anatomical resemblance of P. obesus visual streak to the human fovea makes it a potentially valuable model for studying foveal pathologies associated with DR, such as diabetic macular edema.

4. Conclusion

In conclusion, the gerbil P. obesus provides a valuable model for studying the physiology and pathology of the human retina, complementing traditional nocturnal models. Its diurnal activity cycle and cone-rich retina make it particularly useful for studying cone and color processing cell function, and a powerful tool for research into ocular diseases and the development of new therapeutic strategies between different rodent species.

Acknowledgments

This research was funded by the Ministry of Higher Education and Scientific Research (MHESR) through the project: PAQ-Collabora (PAR&I-Tk), period 2020–2022.