PMEL is an amyloidogenic protein that appears to be exclusively expressed in pigment cells and forms intralumenal fibrils within early stage melanosomes upon which eumelanins deposit in later stages. PMEL is well conserved among vertebrates, and allelic variants in several species are associated with reduced levels of eumelanin in epidermal tissues. However, in most of these cases it is not clear whether the allelic variants reflect gain-of-function or loss-of-function, and no complete PMEL loss-of-function has been reported in a mammal. Here, we have created a mouse line in which the Pmel gene has been inactivated (Pmel −/−). These mice are fully viable, fertile, and display no obvious developmental defects. Melanosomes within Pmel −/− melanocytes are spherical in contrast to the oblong shape present in wild-type animals. This feature was documented in primary cultures of skin-derived melanocytes as well as in retinal pigment epithelium cells and in uveal melanocytes. Inactivation of Pmel has only a mild effect on the coat color phenotype in four different genetic backgrounds, with the clearest effect in mice also carrying the brown/Tyrp1 mutation. This phenotype, which is similar to that observed with the spontaneous silver mutation in mice, strongly suggests that other previously described alleles in vertebrates with more striking effects on pigmentation are dominant-negative mutations. Despite a mild effect on visible pigmentation, inactivation of Pmel led to a substantial reduction in eumelanin content in hair, which demonstrates that PMEL has a critical role for maintaining efficient epidermal pigmentation.

This study demonstrates the positive association between PLD and glaucoma, between narrowing of the iridocorneal angle and glaucoma and the effect of age on the iridocorneal angle. Mating of dogs with normal iridocorneal angles appears to reduce the presence and degree of abnormal appearance of the iridocorneal angle in the offspring. However, breeding only dogs with normal iridocorneal angles without consideration of their relationship to dogs with glaucoma is not a guarantee for preventing glaucoma in the offspring.

The full-field, flash electroretinogram (ERG) is now a widely used test of canine retinal function for the clinical diagnosis of hereditary retinal dystrophies and other causes of retinal degeneration, assessment of retinal function in patients with opaque media, ruling out of generalized retinal diseases in patients with sudden loss of vision and in ophthalmological research, as well as in pharmaceutical and toxicological screening for deleterious side effects of drugs and other chemical compounds. In 2002, the first guidelines for clinical ERGs in this species adopted by the European College of Veterinary Ophthalmologists were published. This work provides an update of these guidelines.

BackgroundMultiple congenital ocular anomalies (MCOA) syndrome is a hereditary congenital eye defect that was first described in Silver colored Rocky Mountain horses. The mutation causing this disease is located within a defined chromosomal interval, which also contains the gene and mutation that is associated with the Silver coat color (PMEL17, exon 11). Horses that are homozygous for the disease-causing allele have multiple defects (MCOA-phenotype), whilst the heterozygous horses predominantly have cysts of the iris, ciliary body or retina (Cyst-phenotype). It has been argued that these ocular defects are caused by a recent mutation that is restricted to horses that are related to the Rocky Mountain Horse breed. For that reason we have examined another horse breed, the Icelandic horse, which is historically quite divergent from Rocky Mountain horses.ResultsWe examined 24 Icelandic horses and established that the MCOA syndrome is present in this breed. Four of these horses were categorised as having the MCOA-phenotype and were genotyped as being homozygous for the PMEL17 mutation. The most common clinical signs included megaloglobus, iris stromal hypoplasia, abnormal pectinate ligaments, iridociliary cysts occasionally extending into the peripheral retina and cataracts. The cysts and pectinate ligament abnormalities were observed in the temporal quadrant of the eyes. Fourteen horses were heterozygous for the PMEL17 mutation and were characterized as having the Cyst-phenotype with cysts and occasionally curvilinear streaks in the peripheral retina. Three additional horses were genotyped as PMEL17 heterozygotes, but in these horses we were unable to detect cysts or other forms of anomalies.One eye of a severely vision-impaired 18 month-old stallion, homozygous for the PMEL17 mutation was examined by light microscopy. Redundant duplication of non-pigmented ciliary body epithelium, sometimes forming cysts bulging into the posterior chamber and localized areas of atrophy in the peripheral retina were seen.ConclusionsThe MCOA syndrome is segregating with the PMEL17 mutation in the Icelandic Horse population. This needs to be taken into consideration in breeding decisions and highlights the fact that MCOA syndrome is present in a breed that are more ancient and not closely related to the Rocky Mountain Horse breed.

BackgroundCorneal ulcers are one of the most common eye problems in the horse and can cause varying degrees of visual impairment. Secondary infection and protease activity causing melting of the corneal stroma are always concerns in patients with corneal ulcers. Corneal collagen cross-linking (CXL), induced by illumination of the corneal stroma with ultraviolet light (UVA) after instillation of riboflavin (vitamin B2) eye drops, introduces crosslinks which stabilize melting corneas, and has been used to successfully treat infectious ulcerative keratitis in human patients. Therefore we decided to study if CXL can be performed in sedated, standing horses with ulcerative keratitis with or without stromal melting.ResultsNine horses, aged 1 month to 16 years (median 5 years) were treated with a combination of CXL and medical therapy. Two horses were diagnosed with mycotic, 5 with bacterial and 2 with aseptic ulcerative keratitis. A modified Dresden-protocol for CXL could readily be performed in all 9 horses after sedation. Stromal melting, diagnosed in 4 horses, stopped within 24 h. Eight of nine eyes became fluorescein negative in 13.5 days (median time; range 4–26 days) days after CXL. One horse developed a bacterial conjunctivitis the day after CXL, which was successfully treated with topical antibiotics. One horse with fungal ulcerative keratitis and severe uveitis was enucleated 4 days after treatment due to panophthalmitis.ConclusionsCXL can be performed in standing, sedated horses. We did not observe any deleterious effects attributed to riboflavin or UVA irradiation per se during the follow-up, neither in horses with infectious nor aseptic ulcerative keratitis. These data support that CXL can be performed in the standing horse, but further studies are required to compare CXL to conventional medical treatment in equine keratitis and to optimize the CXL protocol in this species.

To identify ultraviolet (UV) and middle- (M) wavelength-sensitive cone and rod signals in murine retinal ganglion cells, single ganglion cell responses were studied in anesthetized, light-adapted C57/BL6 mice with tungsten microelectrodes driven through the sclera and vitreous to the neural retina. One hundred fifty-four ganglion cells were examined in 43 retinas of 34 mice. The retina was stimulated with diffuse flashes and/or pulses of ultraviolet (360 nm) or green (520 nm) light in the presence and absence of a strong steady orange adapting light. Twelve ganglion cells were studied in the dark-adapted retina in order to identify the signals of rods. Three functionally different types of ganglion cells were found: (1) phasic responding cells (31%) with no spontaneous activity and large impulse amplitudes; (2) tonic responding cells (60%) with irregular, low frequency (5-10 Hz) spontaneous activity and smaller impulse amplitudes; and (3) metronome-like cells (9%) with regular, relatively high-frequency (20-40 Hz) spontaneous activity. A few cells (1%) had habituating responses. Every cell encountered was affected by diffuse stimulation. The more common two types were excited at either the ON or OFF or at both the ON and OFF phases of stimulation. Type III cells had weaker responses, sometimes only inhibited by turning off a light. In the light-adapted state, most cells received signals of the same polarity from UV- and M-cones but UV-cone inputs were usually more dominant, especially in ventral retina. A fraction of cells received signals from only UV- (18%) or only M- (3%) cones. In rare cases (2%) these cone inputs had an opposite polarity on the same cell. In the dark-adapted state, all cells were at least four or five logarithmic units more sensitive and more to green than ultraviolet light. The results indicate that co-expression of both UV-and M-cone opsins cannot be ubiquitous in murine retina. Some cones, especially UV cones, exist without the presence of any functional M-cone opsin. This must be the case to explain the presence of ganglion cells that receive inputs only from UV-cones and others that receive inputs of opposite polarity from UV- and M-cones. The results support the hypothesis that murine retina has the physiological capacity to relay signals to the brain that allow the sensing of chromatic contrast and color vision.

The assessment of flicker fusion frequency (FFF), the stimulus frequency at which a flickering light stimulus can no longer be resolved and appears continuous, and critical flicker fusion frequency (CFF; the highest frequency at any light intensity that an observer can resolve flicker) are useful methods for comparing temporal resolution capabilities between animals. Behavioural experiments have found that average CFFs in domestic chickens (Gallus gallus domesticus) are in the range of ca. 75-87 Hz, measured in response to full spectrum (i.e. white light plus UV) stimuli. In order to examine whether the chicken retina is able to detect flicker at higher frequencies, we used electroretinograms (ERGs) to assess FFF/CFF in adult hens from two commercial genotypes, Lohmann Selected Leghorns (LSLs) and Lohmann Browns (LBs). ERGs were recorded in response to flickering light at ten full spectrum light intensities ranging from 0.7 to 2740 cd m(-2). Two methods were used to determine FFF/CFF from the ERG recordings and these methods yielded very similar results, with average FFF ranging from ca. 20Hz at 0.7 cd m(-2) to an average CFF of ca. 105 Hz at 2740 cd m(-2). In some individuals, CFFs of 118-119 Hz were recorded. The Intensity/FFF (I/FFF) curves are double-branched with a break point representing the rod-cone transition occurring between 2.5 and 5.9 cd m(-2). No significant differences in the I/FFF curves were found between the two genotypes. At stimulus light intensities >250 cd m(-2), the ERG-derived FFF and CFF values are all higher than those from behavioural studies using the same stimuli. Although hens do not appear to be able to consciously perceive flicker above approximately 90 Hz, the finding that the ERG responses are able to remain in phase with light flickering at frequencies >100 Hz means that the retinae of domestic poultry housed in artificial light conditions may be able to resolve flicker from fluorescent lamps. As range of detrimental effects have been reported in humans as a result of exposure to such "invisible flicker", the possibility exists that flicker from fluorescent lamps also acts as stressor in domesticated birds.

Equine Multiple Congenital Ocular Anomalies (MCOA) syndrome is a heritable eye disorder mainly affecting silver colored horses. Clinically, the disease manifests in two distinct classes depending on the horse genotype. Horses homozygous for the mutant allele present with a wide range of ocular defects, such as iris stromal hypoplasia, abnormal pectinate ligaments, megaloglobus, iridociliary cysts and cataracts. The phenotype of heterozygous horses is less severe and predominantly includes iridociliary cysts, which occasionally extend into the temporal retina. In order to determine the genetic cause of MCOA syndrome we sequenced the entire previously characterized 208 kilobase region on chromosome 6 in ten individuals; five MCOA affected horses from three different breeds, one horse with the intermediate Cyst phenotype and four unaffected controls from two different breeds. This was performed using Illumina TruSeq technology with paired-end reads. Through the systematic exclusion of all polymorphisms barring two SNPs in PMEL, a missense mutation previously reported to be associated with the silver coat colour and a non-conserved intronic SNP, we establish that this gene is responsible for MCOA syndrome. Our finding, together with recent advances that show aberrant protein function due to the coding mutation, suggests that the missense mutation is causative and has pleiotrophic effect, causing both the horse silver coat color and MCOA syndrome.