Incorporation of squalene into rod outer segments

Enzyme blockade: a nonradioactive method to determine the absolute rate of cholesterol synthesis in the brain

The standard in vivo method to determine rates of brain cholesterol synthesis involves systemic injection of (3)H(2)O and measurement of incorporated radioactivity in sterols. Herein, we describe an alternative method ("enzyme blockade") that obviates the use of radioactivity. The method relies on the ability of AY9944, a potent and relatively selective inhibitor of cholesterol synthesis, to cause the time-dependent accumulation of 7-dehydrocholesterol (DHC), a cholesterol precursor detected with sensitivity and specificity by reverse-phase HPLC-coupled spectrophotometry at 282 nm. To validate the method, adult AY9944-treated and control mice were injected with [(3)H]acetate. After 24 h, most of the radioactivity in brain sterols from treated mice accumulated in DHC, without significantly perturbing overall sterol pathway activity, compared with controls (where cholesterol was the dominant radiolabeled sterol, with no label found in DHC). When adult mice were treated continuously with AY9944, the time-dependent accumulation of DHC in brain was linear (after approximately 8 h) for 3 days. The rate of brain cholesterol synthesis determined by this method ( approximately 30 microg/g/day) closely agrees with that determined by the radioactive method. We also determined the cholesterol synthesis rate in different regions of adult mouse brain, with frontal cortex having the highest rate and cerebellum having the lowest rate.

Isoprenoid metabolism in the vertebrate retina

Herein, studies concerning the biosynthesis, intracellular transport and utilization of isoprenoid lipids in vertebrate retinas are reviewed, with particular regard to rod photoreceptor cells and the assembly of rod outer segment (ROS) disk membranes. Initial in vitro studies with bovine retinas showed that [3H]mevalonate is metabolized primarily to squalene and 'methylated' sterols, rather than to cholesterol. Subsequently, similar results were obtained with frog retinas using [3H]acetate as a precursor, and the absolute rate of the sterol pathway was determined in vitro with 3H2O. With the aid of vesicular transport inhibitors, energy poisons, and reduced temperature, it was demonstrated that lipid and protein trafficking mechanisms in the rod cell are separate and independent from one another. In vivo, the majority of newly synthesized squalene in the frog retina is not metabolized to sterols; rather, it is transported to the ROS, where it turns over in parallel with the disk membranes. The remaining squalene is converted slowly to cholesterol, much of which becomes incorporated into the ROS. In contrast, the in vivo metabolism of [3H]acetate to cholesterol in the rat retina is relatively efficient and rapid. However, in both frog and rat, retinal cholesterol turnover is slow (> 60 days), suggesting the existence of a retention mechanism that minimizes the need for de novo biosynthesis. The use of pharmacological approaches to assess the biological roles of isoprenoid lipids and protein prenylation in the retina and the mechanism of retinal cholesterol homeostasis are discussed.

Isoprenoid lipid metabolism in the retina: dynamics of squalene and cholesterol incorporation and turnover in frog rod outer segment membranes

Frogs were injected intravitreally with [3H]acetate, and the formation of [3H]-labeled squalene and cholesterol in the retina and their incorporation into rod outer segment (ROS) membranes were evaluated biochemically over a 60-day time course. ROS [3H]squalene specific activity was maximal by 1-3 days, then declined with a half-time of approximately 20-30 days. In contrast, the specific activity of ROS [3H]cholesterol initially increased to a level substantially less than that of [3H]squalene, and then remained constant. Thus, ROS squalene appears to turn over without obligatory conversion to, or coturnover with, ROS cholesterol. When [3H]acetate was injected into one eye, radiolabel in non-saponifiable lipids of the contralateral retina represented < 1% of those recovered from the ipsilateral retina; hence, systemic contributions to de novo synthesis were obviated. Long-term (> or = 8 hr) in vitro incubations of isolated retinas with [3H]acetate resulted in incorporation of [3H]-labeled sterols and squalene into ROS, at levels comparable to those observed in ROS from companion incubated eyecup preparations and from retinas 8 hr after intravitreal injection of [3H]acetate. These results demonstrate that the in vitro system faithfully reflects the in vivo biosynthetic capacity with respect to isoprenoid lipid metabolism, and suggest that de novo synthesis within the neural retina is responsible for generating most, if not all, of the [3H]squalene and [3H]cholesterol formed under the given conditions. Treatment of retinas in vitro with brefeldin A or energy poisons blocked transport of newly synthesized opsin, but not squalene, to the ROS. Furthermore, frogs maintained at 8 degrees C exhibited marked suppression of incorporation of newly synthesized protein into the ROS, while [3H]squalene incorporation was only minimally reduced, compared with frogs maintained at 22 degrees C. These results are consistent with prior findings that suggest that lipids are transported to the ROS by a mechanism distinct and independent from that employed for intracellular trafficking of opsin and other ROS-destined membrane proteins.

In vivo biosynthesis of cholesterol in the rat retina

Previous reports have suggested that the rate of de novo cholesterol synthesis in the adult vertebrate retina is extremely slow. We investigated cholesterol biosynthesis in the adult rat retina in vivo, following intravitreal injection of [3H]acetate. HPLC analysis of retinal non-saponifiable lipid extracts revealed co-elution of radioactivity with endogenous cholesterol mass within 4.5 h post-injection. Incorporation of [3H]acetate into cholesterol was markedly reduced by co-injection of known inhibitors of the cholesterol pathway. In contrast to previous results with retinas from other species, no radiolabel or mass corresponded to squalene, except in lipid extracts from retinas treated with NB-598, a squalene epoxidase inhibitor. These results demonstrate, for the first time, the capacity of the adult vertebrate retina to rapidly synthesize cholesterol de novo.

Regulation of isoprenoid metabolism in rat liver: near constant chain lengths of dolichyl phosphate and ubiquinone are maintained during greatly altered rates of cholesterogenesis

When rat liver slices were incubated with varying concentrations of [3H]mevalonolactone, the chain lengths of radiolabeled dolichyl phosphate and ubiquinone varied according to the initial mevalonolactone concentration, indicating that product chain length is dependent on the level of isoprenoid diphosphate intermediates. However, when livers were analyzed from rats which had been maintained on diets of either colestipol (which induces cholesterogenesis 3-fold), or normal chow, or cholesterol (which suppresses cholesterogenesis to 5% of normal) there were only minor changes in the isoprene distribution of either dolichyl phosphate or ubiquinone. In contrast, when rats were maintained on 2% cholesterol plus mevalonolactone (conditions prone to increase the levels of intermediates), the isoprene distributions of both of these compounds were greatly shifted to the higher homologs. However, under none of these conditions were the hepatic levels of these compounds changed significantly. It is concluded that under conditions of greatly altered cholesterogenesis, regulatory mechanisms exist which stabilize the levels of isoprenoid diphosphate intermediates, and that even when levels are increased (e.g., by dietary manipulation), the effect is only to alter isoprene distribution and not the rate of synthesis of dolichyl phosphate and ubiquinone.

Frog cones as well as Müller cells have peroxisomes

We have localized D-amino acid oxidase in peroxisomes of frog retina using cerium procedures on tissue fixed in mixtures containing lower concentrations of glutaraldehyde than we had previously used in our cytochemical studies of this enzyme. We find the Müller cells of these preparations contain a more striking population of peroxisomes than had previously been thought: the D-amino acid oxidase-containing bodies are especially concentrated near the outer limiting membrane, but appreciable numbers are also found in the outer plexiform layer and near the inner limiting membrane. In addition, we find peroxisomes to be present in frog cone photoreceptors, particularly in zones near the ellipsoid. To our knowledge peroxisomes have not been described hitherto in vertebrate photoreceptors. Possible roles for the peroxisomes of the neural retina include participation in the metabolism of lipids (e.g. those of the cones' oil droplets, or of the outer segment) and involvement in oxidation of transmitter-related amino acids and of other small molecules.