Changes in glucose and energy metabolism in Vivo in developing retinae from visually-competent (DBA-1J) and mutant (C3H-HeJ) mice

Creatine transporter localization in developing and adult retina: importance of creatine to retinal function

Creatine and phosphocreatine are required to maintain ATP needed for normal retinal function and development. The aim of the present study was to determine the distribution of the creatine transporter (CRT) to gain insight to how creatine is transported into the retina. An affinity-purified antibody raised against the CRT was applied to adult vertebrate retinas and to mouse retina during development. Confocal microscopy was used to identify the localization pattern as well as co-localization patterns with a range of retinal neurochemical markers. Strong labeling of the CRT was seen in the photoreceptor inner segments in all species studied and labeling of a variety of inner neuronal cells (amacrine, bipolar, and ganglion cells), the retinal nerve fibers and sites of creatine transport into the retina (retinal pigment epithelium, inner retinal blood vessels, and perivascular astrocytes). The CRT was not expressed in Müller cells of any of the species studied. The lack of labeling of Müller cells suggests that neurons are independent of this glial cell in accumulating creatine. During mouse retinal development, expression of the CRT progressively increased throughout the retina until approximately postnatal day 10, with a subsequent decrease. Comparison of the distribution patterns of the CRT in vascular and avascular vertebrate retinas and studies of the mouse retina during development indicate that creatine and phosphocreatine are important for ATP homeostasis.

Retinitis pigmentosa: understanding the clinical presentation, mechanisms and treatment options

Retinitis pigmentosa (RP) is a leading cause of human blindness due to degeneration of retinal photoreceptor cells. Causes of retinal degeneration include defects in the visual pigment, defects in the proteins important for photoreceptor function or in enzymes involved in initiating visual transduction. Despite the diversity of genetic mutations identified in inherited forms of retinal dystrophy, there is a common end result of photoreceptor death and functional blindness. In this review, pertinent anatomical and physiological pathways involved in RP and the underlying genetic mutations are outlined, including a discussion on the inheritance patterns revealed by advances in molecular biological techniques. Characteristics of progression rates of visual field loss and current management options will provide useful clinical guidelines for the management of patients with RP.

Gluconeogenesis in the amphibian retina. Lactate is preferred to glutamate as the gluconeogenic precursor

The capacity for gluconeogenesis in the isolated amphibian retina was found to be approx. 70-fold greater with lactate than with glutamate as the gluconeogenic precursor, 1426 versus 21 pmol of glucose incorporated into glycogen/h per mg of protein. It was also found that 11-15% of the glucosyl units in glycogen are derived from C3 metabolites of the glycolytic pathway, suggesting that lactate is recycled within the retina. In concert with these metabolic observations, a full complement of the gluconeogenic enzymes was detected in retinal homogenates. These included: glucose-6-phosphatase, fructose-1,6-bisphosphatase, acetyl-CoA-dependent pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Agents that regulate the rate of gluconeogenesis in hepatic tissue were tested on the retina. At concentrations of glutamate and lactate that are presumed to be relevant physiologically, it was found that vasoactive intestinal peptide, ionophore A23187 and elevated [K+] each enhanced the rate of gluconeogenesis in Ringer containing 50 microM-glutamate, whereas in Ringer containing 8.5 mM-lactate these agents inhibited the rate of gluconeogenesis. Further, it was found that the classic gluconeogenic hormone glucagon inhibited gluconeogenesis in both glutamate- and lactate-containing Ringer. Retinal energy metabolism was found to be altered in lactate-containing Ringer, in that lactate production was suppressed completely. In addition, glycogen metabolism appeared to be dependent on increased cytosolic Ca2+ and was insensitive to increased retinal cyclic AMP.

Docosahexaenoate metabolism and fatty-acid composition in developing retinas of normal and rd mutant mice

The fatty-acid composition of retinal lipids in developing control and rd mice (C57BL/6J) was determined. In addition, fatty-acid composition in brain and retina of normal and rd adult animals was compared. At 11 days of postnatal age, rd retinas contained proportionally less docosahexaenoate than controls, whereas the reverse relationship held for oleate at 20 days of age. In contrast, no differences in the fatty-acid composition of brain lipids were observed between rd and control animals. We also examined docosahexaenoate metabolism in rd and control retinas of different postnatal ages in vitro. These studies of [1-14C]docosahexaenoic-acid incorporation into retinal phospholipids and neutral lipids demonstrated significantly higher incorporation into triacylglycerols of the rd retina, beginning at 14-15 days of postnatal age. Incorporation into diacylglycerols and phospholipids was also higher in rd retinas than in controls at 14 days of age and older. Moreover, the concentration of ganglioside (a glycolipid class probably enriched within inner retinal layers) was higher in adult rd retinas. Degeneration of the rd retina can be detected histologically as early as 8-9 days. Therefore, the alterations of fatty-acid composition and docosahexaenoate metabolism described here are probably an effect, rather than a cause, of retinal degeneration in the rd mouse.

Cyclic-nucleotide phosphodiesterase: an early defect in inherited retinal degeneration of C3H mice

Cyclic nucleotides have been implicated in the differentiation and function of the vertebrate retina. In the normal retina of DBA mice, the specific activity of cyclic-nucleotide phosphodiesterase (PDE), with cyclic-AMP as the substrate (cAMP-PDE), increases eightfold between the 6th and 20th postnatal day. Kinetic analysis of retinae from newborn mice reveals a PDE with a single Michaelis constant (K(m)) value for cyclic-AMP (low K(m)-PDE). After the 6th postnatal day, a second PDE with a high K(m) for cyclic-AMP (high K(m)-PDE) can be demonstrated. The appearance and increasing activity of the high K(m)-PDE coincides with the differentiation and growth of photoreceptor outer segments. Additionally, the high K(m)-PDE is shown by microchemical techniques to be concentrated in the photoreceptor cell layer and the low K(m)-PDE within the inner layers of the normal retina. In C3H mice afflicted with an inherited degeneration of the photoreceptor layer, the postnatal increase in the specific activity of cAMP-PDE is substantially lower than in the normal retina. The postnatal increase in the specific activity of cAMP-PDE in two regions of the brain of C3H mice is the same as in the normal strain. A deficiency in high K(m)-PDE activity in the C3H retina is evident on the 7th postnatal day, when the activity of low K(m)-PDE, photoreceptor morphology, and rhodopsin content of these retina are essentially normal. In the adult C3H retina, the PDE activity with cyclic-GMP and cyclic-UMP as substrates is significantly below that of the normal retina. These data indicate that an alteration in cyclic-AMP metabolism occurs before photoreceptor cell degeneration in the retinae of C3H mice.