Axonal transport of phospholipids in rabbit optic pathway

In vivo chromium-enhanced MRI of the retina

Chromium (Cr) has been used histologically to stabilize lipid fractions in the retina and is suggested to enhance oxidizable lipids in brain MRI. This study explored the feasibility, sensitivity, and specificity of in vivo chromium-enhanced MRI of retinal lipids by determining its spatiotemporal profiles and toxic effect after intravitreal Cr(VI) injection to normal adult rats. One day after 3 μL Cr(VI) administration at 1-100 mM, the retina exhibited a dose-dependent increase in T1-weighted hyperintensity until 50 mM. Time-dependently, significant T1-weighted hyperintensity persisted up to 2 weeks after 10 mM Cr(VI) administration. Three-dimensional chromium-enhanced MRI of ex vivo normal eyes at isotropic 50-μm resolution showed at least five alternating bands across retinal layers, with the outermost layer being the brightest. This agreed with histology indicating alternating lipid contents with the highest level in the photoreceptor layer of the outer retina. Although Cr(VI) reduction may induce oxidative stress and depolymerize microtubules, manganese-enhanced MRI after chromium-enhanced MRI showed a dose-dependent effect of Cr toxicity on manganese uptake and axonal transport along the visual pathway. These results potentiated future longitudinal chromium-enhanced MRI studies on retinal lipid metabolism upon further optimization of Cr doses with visual cell viability.

Remodeling and sorting process of ethanolamine and choline glycerophospholipids during their axonal transport in the rabbit optic pathway

The existence of a mechanism by which the ester- and ether-linked aliphatic chains of the major phospholipids are retailored during their axonal transport and sorted to specific membrane systems along the optic nerve and tract was investigated. A mixture of [1-14C]hexadecanol and [3H]arachidonic acid was injected into the vitreous body of albino rabbits. At 24 h and 8 days later, the distribution (as measured by the 3H/14C ratio) and the positioning (as monitored by hydrolytic procedures) of radioactivity in the various phospholipid classes of retina, purified axons, and myelin of the optic nerve and tract were determined. At the two intervals after labeling, the 3H/14C ratios of each diradyl type of phosphatidylethanolamine and phosphatidylcholine were (a) substantially unchanged all along the axons within the optic nerve and tract and (b) markedly modified in comparison with those found in the retina and axons for molecular species selectively restricted to myelin sheath. Evidence is thus available that intraxonally moving ethanolamine and choline glycerophospholipids, among others, are added to axonal membranes most likely without extensive modifications. In contrast, they are transferred into myelin after retailoring. Through these two processes, the sorting and targeting of newly synthesized phospholipids to their correct membrane domains, such as axoplasmic organelles, axolemma, or periaxonal myelin, could be controlled.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in rapid transport of phospholipids in the rat sciatic nerve during axonal regeneration

Axonal transport of phospholipids in normal and regenerating sciatic nerve of the rat was studied. At various intervals after axotomy of the right sciatic nerve in the midthigh region and subsequent perineurial sutures of the transected fascicles, a mixture of 60 mu Ci [Me-14C]choline and 15 muCi [2-3H]glycerol in the region of the spinal motor neurons of the L5 and L6 segments was injected bilaterally. The amount of radioactive lipid (and in certain cases its distribution in various lipid classes) along the nerve was determined as a function of time. Three days after fascicular suture and 6 h after spinal cord injection of precursors, there was an accumulation of labeled phospholipids and sphingolipids in the transected sciatic nerve in the region immediately proximal to the site of suture. Nine days after, there was a marked increase in the accumulation of radioactivity in the distal segments of the injured nerve, which increased up to 14 days after cutting and disappeared as regeneration proceeded (21-45 days). In all segments of both normal and regenerating nerve fibers, as well as in L5 and L6 spinal cord segments, only phosphatidylcholine and sphingomyelin were labeled with [14C]choline. These results suggest that the regeneration process in a distal segment of a peripheral neuron, following cutting and fascicular repairing by surgical sutures, is sustained in the first 3 weeks by changes in the amount of phospholipids rapidly transported along the axon towards the site of nerve fiber outgrowth.

Rapid axonal transport of glycerophospholipids in regenerating hypoglossal nerve of the rabbit

The intraaxonal transport of phospholipids in regenerating hypoglossal nerve of the rabbit was investigated by administration of labeled lipid precursors into the medulla oblongata. At various time intervals after crushing the left hypoglossal nerve at the level of the digastric muscle, a mixture of 60 mu Ci of [2-3H]glycerol and 15 mu Ci of [1-14C]palmitate, dissolved in 15% bovine serum albumin, was injected into the calamus scriptorius of the fourth ventricle. The amount and the pattern of labeling of glycerophospholipids synthesized in the motor neurons were determined. Three days after nerve crush there was an accumulation of labeled glycerophospholipids immediately proximal to the injury site. Seven days after crushing, the regenerating nerve incorporated rapidly transported labeled lipids in greater amounts than the contralateral normal nerve; the incorporation was elevated along the entire length of the nerve containing both regenerating axons and the post-crush sprouting terminals. The difference between the two sides increased up to 14 days, but disappeared as regeneration proceeded (21-45 days). The "pool" of radioactive lipids remaining in the cell bodies of hypoglossal nuclei, in the segments of nerve, both proximal and distal to the crush site, and in all the segments of uncrushed nerve was similar 6-12 h after labeling. Among the phospholipids, the highest 3H and 14C radioactivity was observed in phosphatidylcholine and phosphatidylethanolamine. These results support the hypothesis than an increase in the amount of glycerophospholipids, conveyed by rapid axonal transport, takes place in the first 2 weeks during nerve regeneration. The increased transport of lipids presumably reflects an augmented demand for membrane precursors during the sprouting process.

Rapid migration of inositol phospholipids with axonally transported substances in the rabbit optic pathway

Following an intraocular injection of myo-[2-3H]inositol, the axonal transport of labelled water-soluble substances and inositol phospholipids was investigated. Evidence was obtained for a rapid axonal transport of a relatively small amount of labelled inositol phospholipids. In contrast to other axonally transported phospholipids, there was no significant accumulation of labelled, rapidly transported inositol phospholipids in the nerve terminal region at later time intervals following the isotope administration.

Transfer of axonally transported phospholipids into myelin isolated from the rabbit optic pathway

The contribution of the axonal transport to the biosynthesis of myelin phospholipids was investigated in the rabbit optic pathway. A double labeling technique was used. The same animals were injected with one isotope intravitreally and the other intraventricularly. This procedure allows double labeling of the optic nerves, optic tracts, lateral geniculate bodies (LGB), and superior colliculus (SC). The precursors simultaneously injected were: [1-14C]palmitate (15 microCi intravitreally in both eyes or 50 microCi intraventricularly) and [2-3H]glycerol (50 microCi intravitreally in both eyes of 100 microCi intraventricularly). Twenty four hours and 10 days after the injections, myelin was purified from pooled optic nerves and optic tracts as well as from pooled LGBs or SCs. The phospholipids were extracted and then separated by thin-layer chromatography; the specific radioactivity of the various classes of phospholipids was determined. Using both administration routes of C- or 3H-precursors, the distribution of label and specific radioactivity of myelin phospholipids in the retina and in all other optic structures were very similar. Phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine + phosphoinositol were preferentially labeled with both precursors. These results suggest that, in the rabbit optic pathway the phospholipids synthesized in the retinal ganglion cells and transported along the axons, could undergo transaxonal transfer into myelin.

Changes in enzyme activities of glycerolipid metabolism of guinea-pig cerebral hemispheres during experimental hypoxia

Hypoxic treatment causes changes of some enzymatic activities involved in the glycerolipid metabolism in subcellular fractions of guinea pig cerebral hemispheres. The activity of lysophosphatidylcholine acyltransferase, choline phosphotransferase, glycerol-3-phosphate acyltransferase(s), as well as the activity of triacylglycerol lipase significantly decreased in the microsomes of cerebral hemispheres of animals intermittently exposed to hypoxic treatment for eighty hours. At the same time, a marked activation of microsomal and mitochondrial phospholipase A2 occurred. The changes of the above-mentioned enzymatic activities after the hypoxic treatment correlated well with the increase in the level of brain and blood free fatty acids. The changes also correlated with the decrease of labeled lipid precursors incorporated into lipids of the cerebral hemispheres, observed during oxygen insufficiency.