Peichl, L. | Search bioRxiv

bioRxiv2024

Eye features and retinal photoreceptors of the nocturnal aardvark (Orycteropus afer, Tubulidentata)

The nocturnal aardvark Orycteropus afer is the only extant species in the mammalian order Tubulidentata. Previous studies have claimed that it has an all-rod retina. In the retina of one aardvark, we found rod densities ranging from 124,000/mm2 in peripheral retina to 214,000/mm2 in central retina; the retina of another aardvark had 182,000 - 245,000 rods/mm2. This is moderate in comparison to other nocturnal mammals. With opsin immunolabelling we found that the aardvark also has a small population of cone photoreceptors. Cone densities ranged from 300 to 1,300/mm2 in one animal, and from 1,100 to 1,600/mm2 in the other animal, with large local variations and no large central-peripheral density gradient. Overall, cones comprised 0.25-0.9% of the photoreceptors. Both typical mammalian cone opsins, longwave-sensitive (L) and shortwave-sensitive (S), were present. However, there was colocalization of the two opsins in many cones across the retina (35 - 96% dual pigment cones). Pure L cones and S cones formed smaller populations. This probably results in poor colour discrimination. Thyroid hormones, important regulators of cone opsin expression, showed normal blood serum levels. The relatively low rod density and hence a relatively thin retina may be related to the fact that the aardvark retina is avascular and its oxygen and nutrient supply have to come from the choriocapillaris by diffusion. In contrast to some previous studies, we found that the aardvark eye has a reflective tapetum lucidum with features of a choroidal tapetum fibrosum, in front of which the retinal pigment epithelium is unpigmented. The discussion considers these findings from a comparative perspective.

zoology↗

bioRxiv2024

Full-length Cryptochrome 1 in the outer segments of the retinal blue cone photoreceptors in humans and great apes suggests a role beyond transcriptional repression

Mammalian cryptochrome 1 (CRY1) is a central player in the circadian transcription-translation feedback loop, crucial for maintaining a roughly 24-hour rhythm. CRY1 was suggested to also function as blue-light photoreceptor in humans and has been found to be expressed at the mRNA level in various cell types of the inner retina. However, attempts to detect CRY1 at the protein level in the human retina have remained unsuccessful so far. Using various C-terminal specific antibodies recognizing full-length CRY1 protein, we consistently detected selective labelling in the outer segments of short wavelength-sensitive (SWS1, "blue") cone photoreceptor cells across human, bonobo, and gorilla retinae. No other retinal cell types were stained, which is in contrast to what would be expected of a ubiquitous clock protein. Subcellular fractionation experiments in transfected HEK cells using a C-terminal specific antibody located full-length CRY1 in the cytosol and membrane fractions. Our findings indicate that human CRY1 has several different functions including at least one non-clock function. Our results also raise the likely possibility that several different versions of CRY1 exists in humans. We suggest that truncation of the C-terminal tail, maybe to different degrees, may affect the localization and function of human CRY1.

cell biology↗