The biosynthesis of cholesterol and other sterols by brain tissue. II. A comparison of in vitro and in vivo methods

Biomolecular Markers of Brain Aging

Characterized by the gradual loss of physiological integrity, impaired function, and increased susceptibility to death, aging is considered the primary risk factor for major human diseases, such as cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. The time-dependent accumulation of cellular damage is widely considered the general cause of aging. While the mechanism of normal aging is still unresolved, researchers have identified different markers of aging, including genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Theories of aging can be divided into two categories: (1) aging is a genetically programmed process, and (2) aging is a random process caused by gradual damage to the organism over time as a result of its vital activities. Aging affects the entire human body, and aging of the brain is undoubtedly different from all other organs, as neurons are highly differentiated postmitotic cells, and the lifespan of most neurons in the postnatal period is equal to the lifespan of the brain. In this chapter, we discuss the conserved mechanisms of aging that may underlie the changes observed in the aging brain, with a focus on mitochondrial function and oxidative stress, autophagy and protein turnover, insulin/IGF signaling, target of rapamycin (TOR) signaling, and sirtuin function.

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.

Utilization of carbon from dietary polyunsaturates for brain cholesterol synthesis during early postnatal development in the rat: a 13C NMR study

Incorporation of 13C from a dietary precursor into cholesterol was studied in neonatal rats. Rats were given uniformly 13C-enriched polyunsaturated fatty acids intragastrically and total lipid extracts of liver and brain were analyzed by 13C-NMR 1, 4, 8, and 15 days later. 13C-enrichment was detected in brain but not in liver cholesterol. Maximal 13C-labeling was observed 4 days after injection of the label. Spectra revealed that 70% of newly incorporated 13C had 13C as an adjacent neighbor, the other 30% had 12C as the neighbor. Double quantum NMR revealed the arrangement in the cholesterol skeleton of the 13C-13C pairs transferred from precursors to cholesterol. Desmosterol, an intermediate of cholesterol synthesis, was identified in the spectra of brain lipids. Comparison of 13C-13C unit arrangements in both cholesterol and desmosterol allowed carbons 26 and 27 of desmosterol to be unambiguously assigned.

Metabolism of intracerebrally injected mevalonate in brain of suckling and young adult rats

We have investigated the in vivo metabolism via sterol and nonsterol pathways of intracerebrally injected mevalonate (MVA) in brains from suckling (10-day-old) and young adult (60-day-old) rats. Results of our study indicated that increasing the amounts of MVA injected increased MVA incorporation into all the lipid fractions examined. The incorporation of MVA into nonsaponiable lipids (NSF) and digitonin precipitable sterols (DPS) was similar in brains from adult and suckling rats. In brain tissue from both suckling and young adult rats the synthesis of dolichol from MVA varied with the amounts of MVA injected. Significant amounts of MVA were recovered in phosphorylated and free polyprenols (farnesol and geraniol) in brain tissue from rats of both ages. Also in both groups of animals, the amounts of MVA incorporated in phosphorylated and free farnesol were higher than the amounts recovered in either, phosphorylated or free geraniol. The amounts of MVA incorporated into the prenoic/fatty acid fraction by brain tissue from both suckling and young adult rats were less than 1% of the total MVA incorporated (nonsaponifiable and saponifiable lipids). Incorporation of MVA into the prenoic/fatty acid fraction by brain tissue was higher in suckling than in young adult rats. These data indicate that the brain tissue from suckling and young adult rats do not differ in their capacity to metabolize MVA into squalene and sterols and that in brain, metabolism of MVA by a shunt pathway is minimal. This suggests that in vivo regulation of cholesterol synthesis during brain development must occur at a step(s) in the sterol synthetic pathway prior to mevalonate, and that metabolism of mevalonate by shunt pathway did not play a role in the developmental regulation of brain sterol synthesis. The data also suggest that in both groups of animals the synthesis of squalene by synthetase may in part control brain sterol synthesis and the synthesis of dolichol is regulated by MVA concentration in the tissue.

Turnover of phosphatidyl choline and changes in enzymatic activity in cell membranes of the CNS during early myelination

Subfractions of fraction A, which floats on top on 0.8 M sucrose in the classical density gradient used for the isolation of brain subcellular fractions (A1 corresponding in adult rats to myelin and A2 which corresponds to the myelin-like fraction), were studied in comparison to other brain subcellular fractions in 5 day old rats and at different stages of development, up to 60 days of age. Variations in the enzymatic activity of acetylcholinesterase, non-specific cholinesterase and 2',3'-Cyclic nucleotide phosphohydrolase, changes in lipid and protein composition and turnover of phosphatidyl choline were investigated. Results indicate that fractions A1 and A2 obtained prior to the beginning of myelination could be composed of fragments of the oligodendroglial cell plasma membranes, and that both fractions undergo substantial changes in chemical composition, enzymatic activity and in turnover of phosphatidyl choline during maturation. In vivo experiments at short times, using radioactive choline as a precursor of phosphatidylcholine suggest that membrane fragments isolated in fraction A2 are precursors of those isolated in fraction A1 at all ages.

Urinary cholesterol. VIII. Its excretion in women with ovarian neoplasms

The urinary excretion of nonesterified cholesterol (NEC) has been investigated in 57 women with ovarian neoplasms and/or related nonneoplastic diseases. Twelve patients had benign tumors or lesions and 45 had malignant neoplasms of their ovaries. All patients with nonmalignant ovarian tumors or lesions had normal NEC excretion irrespective of the type of tumor or lesion or its degree of extension. In contrast, urinary NEC hyperexcretion occurred with the following frequencies in patients with active malignant ovarian neoplasms: 18 of 19 cystadenocarcinomas of the serous and/or mucinous types; one of one endometrioid carcinoma; four of four malignant granulosa cell tumors; two of two mixed malignant germ cell tumors; and one of one malignant mixed müllerian tumor. Single cases of clear cell carcinoma and of rhabdomyosarcoma had a normal NEC excretion. Urinary hyperexcretion of NEC was also found after surgery in two of seven surviving patients with apparently localized resectable disease according to their staging. It is possible that in these two patients NEC hyperexcretion was due to undetected foci of cancer (wrong staging), since neither omental and peritoneal biopsies, nor cytologic examination of peritoneal washings or free fluid were performed. A normal excretion of urinary NEC has been characteristic of 19 of 21 surviving patients treated by surgery and adjunctive therapy in whom we have performed follow-up NEC determinations. They were 16 of 18 cystadenocarcinomas malignant germ cell tumor. The 94% correlation between the presence of proven active ovarian carcinomas and urinary NEC hyperexcretion is significant. The clinical significance of this investigation is even greater when one considers that cystadenocarcinomas constitute more than 75% of all primary malignant ovarian tumors.