Synthesis and transport of cerebrosides and sulfatides in rat brain during development

A Lipidomics Atlas of Selected Sphingolipids in Multiple Mouse Nervous System Regions

Many lipids, including sphingolipids, are essential components of the nervous system. Sphingolipids play critical roles in maintaining the membrane structure and integrity and in cell signaling. We used a multi-dimensional mass spectrometry-based shotgun lipidomics platform to selectively analyze the lipid species profiles of ceramide, sphingomyelin, cerebroside, and sulfatide; these four classes of sphingolipids are found in the central nervous system (CNS) (the cerebrum, brain stem, and spinal cord) and peripheral nervous system (PNS) (the sciatic nerve) tissues of young adult wild-type mice. Our results revealed that the lipid species profiles of the four sphingolipid classes in the different nervous tissues were highly distinct. In addition, the mRNA expression of sphingolipid metabolism genes-including the ceramidase synthases that specifically acylate the N-acyl chain of ceramide species and sphingomyelinases that cleave sphingomyelins generating ceramides-were analyzed in the mouse cerebrum and spinal cord tissue in order to better understand the sphingolipid profile differences observed between these nervous tissues. We found that the distinct profiles of the determined sphingolipids were consistent with the high selectivity of ceramide synthases and provided a potential mechanism to explain region-specific CNS ceramide and sphingomyelin levels. In conclusion, we portray for the first time a lipidomics atlas of select sphingolipids in multiple nervous system regions and believe that this type of knowledge could be very useful for better understanding the role of this lipid category in the nervous system.

Temporal and quantitative expression of the myelin-associated lipids, ethanolamine plasmalogen, galactocerebroside, and sulfatide, in the differentiating CG-4 glial cell line

We determined the expression of three myelin-typical lipids in the continuous CG-4 glial cell line of oligodendrocyte progenitor cells, as the cells differentiated into oligodendrocytes. On 6 different days during the first 9 days of oligodendrocyte development, cells were labeled for 24 h with [3H]ethanolamine to label ethanolamine plasmalogens or with [3H]galactose to label the galactocerebroside and sulfogalactocerebroside; and the amount of labeled lipid expressed on each day was determined. Each labeled lipid was expressed with its own specific time course and in a defined amount on each day of differentiation. Increased labeling of plasmalogens and sulfogalactocerebroside started at early developmental stages, and increased labeling of galactocerebroside started at later stages. The results indicate that the differentiating CG-4 cell line provides a valuable system to investigate factors affecting the early time course of myelin-lipid expression and the amounts expressed.

UDP-galactose:ceramide galactosyltransferase is a class I integral membrane protein of the endoplasmic reticulum

UDP-galactose:ceramide galactosyltransferase (CGalT) transfers UDP-galactose to ceramide to form the glycosphingolipid galactosylceramide. Galactosylceramide is the major constituent of myelin and is also highly enriched in many epithelial cells, where it is thought to play an important role in lipid and protein sorting. Although the biochemical pathways of glycosphingolipid biosynthesis are relatively well understood, the localization of the enzymes involved in these processes has remained controversial. We here have raised antibodies against CGalT and shown by immunocytochemistry on ultrathin cryosections that the enzyme is localized to the endoplasmic reticulum and nuclear envelope but not to the Golgi apparatus or the plasma membrane. In pulse-chase experiments, we have observed that newly synthesized CGalT remains sensitive to endoglycosidase H, confirming the results of the morphological localization experiments. In protease protection assays, we show that the largest part of the protein, including the amino terminus, is oriented toward the lumen of the endoplasmic reticulum. CGalT enzyme activity required import of UDP-galactose into the lumen of the endoplasmic reticulum by a UDP-galactose translocator that is present in the Golgi apparatus of CHO cells but absent in CHOlec8 cells. Finally, we show that CGalT activity previously observed in Golgi membrane fractions in vitro, in the absence of UDP-glucose, is caused by UDP-glucose:ceramide glucosyltransferase. Therefore all galactosylceramide synthesis occurs by CGalT in vivo in the lumen of the endoplasmic reticulum.

Synthesis of non-hydroxy-galactosylceramides and galactosyldiglycerides by hydroxy-ceramide galactosyltransferase

Galactosylceramide (GalCer) is the major glycolipid in brain. In order to characterize the activity of brain UDPgalactose: ceramide galactosyltransferase (CGalT), it has been stably expressed in CGalT-negative Chinese hamster ovary (CHO) cells. After fractionation of transfected cells, CHO-CGT, on sucrose gradients, the activity resides at the density of endoplasmic reticulum and not of Golgi. A lipid chromatogram from CHO-CGT cells revealed two new iodine-staining spots identified as GalCer, since they comigrate with GalCer standards, can be metabolically labelled with [3H]galactose, are recognized by anti-GalCer antibodies, and are resistant to alkaline hydrolysis. A third [3H]galactose lipid was identified as galactosyldiglyceride. In the homogenate CGalT displays a 25-fold preference for hydroxy fatty acid-containing ceramides. Remarkably, endogenous GalCer of transfected cells contains exclusively non-hydroxy fatty acids: fast atom bombardment and collision-induced dissociation mass spectrometric analysis revealed mainly C16:0 in the lower GalCer band on TLC and mainly C22:0 and C24:0 in the upper band. Our results suggest that CGalT galactosylates both hydroxy- and non-hydroxy fatty acid-containing ceramides and diglycerides, depending on their local availability. Thus, CGalT alone may be responsible for the synthesis of hydroxy- and non-hydroxy-GalCer, and galactosyldiglyceride in myelin.

Biosynthesis and compartmentalization of Po, apolipoprotein A-I, and lipids in the myelinating chick sciatic nerve

Myelin deposition in developing chick sciatic nerve is associated with rapid synthesis of lipids, the major myelin protein Po and apo A-I, a major constituent of plasma lipoproteins. In order to understand possible roles of apo A-I in myelin assembly the synthesis and appearance of Po, apo A-I and lipids was studied in an intracellular fraction, an intralamellar fraction thought to be related to, or derived from, myelin and compact myelin from rapidly myelinating sciatic nerve of 1 day chicks. Incorporation with methionine or pulse-chase experiments indicated that initial synthesis of Po occurs in the intracellular fraction followed by movement to the intralamellar fraction and myelin. Incorporation of labelled oleate into phospholipids suggested that initial synthesis occurs in the intracellular and intralamellar fractions with slow movement to myelin. Incorporation of labelled galactose into cerebrosides suggested that initial synthesis occurs partially in myelin with slow loss from this fraction to the intralamellar fraction. However, incorporation of methionine into apo A-I indicated that initial synthesis occurred in the intracellular fraction with some transfer to the intralamellar fraction and secretion of a major portion into the incubation medium. It is concluded that the subcellular distribution of nascent apo A-I is not well coordinated with the distribution of other nascent constituents of the myelin membrane. The accumulation of nascent Po, phospholipids and cerebrosides in the intralamellar fraction compared to compact myelin suggests that this fraction may play a role as a precursor membrane or as a storage site for assembly of myelin constituents into compact myelin.

Hydroxy- and non-hydroxy-galactolipids in developing rat CNS

Rat spinal cord (1-24 weeks postnatal) was analysed by HPLC for various species of galactolipids that accumulate in mammalian myelin during development. Cerebral tissue of the same animals was taken as reference. The levels of the major galactolipids, galactosylceramide (GalCer) and its sulfated analog (SGalCer), increased linearly during the first 2 months after birth. At 3 months, constant levels were reached that were approx. 4-fold (GalCer) and 2.5-fold (SGalCer) higher than in cerebral tissue of corresponding age. The accumulation of galactoglycerolipids slightly preceded that of galactosphingolipids. Levels of galacto-glycerolipids were much lower (4% of galactosphingolipids in 3-and 2.5% in 6-month-old spinal cord on weight basis) and decreased upon CNS maturation. During the first postnatal month, the ratio of non-hydroxy- over hydroxy-species (NFA/HFA) of cerebral GalCer declined from 2.2 to 0.5 whereas the NFA/HFA ratio for cerebral SGalCer increased from 1.0 to 1.8 in the same period. Through development the hydroxy-species contributed 56-60% to GalCer and 28-41% to SGalCer in spinal cord, whereas in cerebrum of 24-week-old rats 73% of GalCer and 48% of SGalCer was alpha-hydroxylated in the ceramide moiety. These data point to different developmental programs with respect to galactolipid metabolism of oligodendrocytes in high- (spinal cord) as compared to low-myelinated (cerebral) areas of rat CNS.

Peripheral nerve sphingomyelin and cerebroside are both formed via two metabolically and kinetically distinct pathways in vivo

We have studied the labeling kinetics of peripheral nerve sphingolipids in vivo. The kinetic analysis of the labeling profiles observed for the various sphingolipids demonstrated that 90% of cerebrosides, but only 30% of sphingomyelin, were synthesized via a de novo synthesized ceramide intermediate following the injection of 1-4 pmol [3H]palmitate into mouse sciatic nerves. The remaining sphingolipid labeling (30% of the total) was due to direct acylation events, using free fatty acids originating from a pool different from those implicated in the de novo ceramide pathway. Direct acylation events ceased within 1 h following substrate administration, while labeling via the ceramide pathway continued through 5 h. The results provide the first in vivo demonstration that the formation of cerebrosides and sphingomyelin in peripheral nerves in situ can be simultaneously assured via two metabolically and kinetically distinct pathways that employ different fatty acid pools.

Regulation of UDP-galactose:ceramide galactosyltransferase and UDP-glucose:ceramide glucosyltransferase after crush and transection nerve injury

The enzyme activities of ceramide galactosyltransferase and ceramide glucosyltransferase were assayed as a function of time (0, 1, 2, 4, 7, 14, 21, 28, and 35 days) after crush injury or permanent transection of the adult rat sciatic nerve. These experimental models of neuropathy are characterized by the presence and absence of axonal regeneration and subsequent myelin assembly. Within the first 4 days after both injuries, a 50% reduction of ceramide galactosyltransferase-specific activity was observed compared to values found in the normal adult nerve. This activity remained unchanged at 7 days after injury; however, by 14 days the ceramide galactosyltransferase activity diverged in the two models. The activity increased in the crushed nerve and reached control values by 21 days, whereas a further decrease was observed in the transected nerve such that the activity was nearly immeasurable by 35 days. In contrast, the ceramide glucosyltransferase activity showed a rapid increase between 1 and 4 days, followed by a plateau that was 3.4-fold greater than that in the normal adult nerve, which persisted throughout the observation period in both the crush and transection models. [3H]Galactose precursor incorporation studies at 7, 14, 21, and 35 days after injury confirmed the previously observed shift in biosynthesis from the galactocerebrosides during myelin assembly in the crush model to the glucocerebrosides and oligohexosylceramide homologues in the absence of myelin assembly in the transection model. The transected nerves were characterized by a peak of biosynthesis of the glucocerebrosides at 14 days. Of particular interest is the biosynthesis of the glucocerebrosides and the oligohexosylceramides at 7 and 14 days after crush injury.(ABSTRACT TRUNCATED AT 250 WORDS)