Microsequencing of bovine cerebrospinal fluid apolipoproteins: identification of bovine apolipoprotein E

Sequence of horse (Equus caballus) apoA-II. Another example of a dimer forming apolipoprotein

Apolipoprotein A-II, the second major apolipoprotein of human HDL, also has been observed in a variety of mammals; however, it is either present in trace amounts or absent in other mammals. In humans and chimpanzee, and probably in other great apes, apoA-II with a cysteine at residue 6 is able to form a homodimer. In other primates as well as other mammals, apoA-II, lacking a cysteine residue, is monomeric. However, horse HDL has been reported to contain dimeric apoA-II that following reduction forms monomers. In this report, we extend these observations by reporting on the first complete sequence for a horse apolipoprotein and by demonstrating that horse apoA-II also contains a cysteine residue at position 6. Both the intact protein and its enzymatic fragments were analyzed by chemical sequence analysis and time-of-flight MALDI-MS (matrix assisted laser desorption ionization mass spectrometry). We also obtained molecular mass data on dimeric and monomeric apoA-II using electrospray-ionization mass spectrometry (ESI-MS). The data are compared with other mammalian sequences of apoA-II and are discussed in terms of resulting similarities and variations in the primary sequences.

Characterization of lipoproteins in cerebrospinal fluid of mares during pregnancy and lactation

OBJECTIVE

To measure apolipoproteins in cerebrospinal fluid (CSF) from healthy mares and to determine whether CSF concentrations of apolipoproteins change during pregnancy and lactation.

ANIMALS

5 healthy pregnant mares.

PROCEDURE

2 sets of CSF samples were obtained; initial samples were obtained 10 to 30 days before parturition (mean, 18 days; median, 17 days), and second samples were obtained 19 to 26 days after parturition (mean, 23 days; median, 23 days). Cerebrospinal fluid was collected from the lumbosacral subarachnoid space of standing horses by use of routine collection techniques. Cerebrospinal fluid cholesterol concentrations were measured by use of a sensitive enzymatic assay. Ultracentrifugal fractions of CSF lipoproteins were characterized by determining the distribution of apolipoproteins, using polyacrylamide gel electrophoresis.

RESULTS

Analyses of isolated ultracentrifugal fractions by polyacrylamide gel electrophoresis revealed 2 apolipoproteins, with the expected molecular weights for apolipoprotein E and apolipoprotein A-I. No significant differences were observed between pre- and postpartum values in mares. The concentration of cholesterol in CSF fluid of mares was comparable to values reported in other mammals.

CONCLUSIONS AND CLINICAL RELEVANCE

Apolipoprotein E in CSF of horses is a major apolipoprotein associated with high-density lipoproteins, which is similar to findings in other mammals. Additional characterization of the role of apolipoproteins in mammalian CSF may provide critical insight into various degenerative neurologic disease processes.

Phylogenetic distribution of apolipoproteins A-I and E in vertebrates as determined by Western blot analysis

A putative apolipoprotein E (apoE) has been identified in the HDL and VHDL fractions of the turtle. This observation is of particular interest considering apoE has been reported absent in the domestic hen (Hermier et al., '95; Biochim Biophys Acta: 105-118, 1995) and thus presumed absent in nonmammalian vertebrates altogether. As a result, partial amino acid sequencing of this protein was performed and revealed that one fragment shared 41% sequence identity to human apoE. Western blot analysis using antisera to apoE demonstrated cross-reactivity to a 34-kDa protein (putative apoE) in turtle plasma. Further investigation using anti-apoE antibody in Western blot analysis detected immunoreactive apoE in the plasma of lamprey, spiny dogfish, skate, and alligator, but not in flounder, newt or python; its absence in several species of birds was confirmed. Using anti-apoA-I antibody, apoA-I was detected in all vertebrate groups except a representative teleost (flounder). Apo-A-I antibody cross-reacted weakly with some putative apoE proteins (chicken, spiny dogfish and skate) and the reverse was true for anti-apoE, which cross-reacted with putative apoA-I in birds, reptiles, and elasmobranchs, confirming the molecular similarity and phylogenetic relatedness of these two proteins.

Human, but not bovine, oxidized cerebral spinal fluid lipoproteins disrupt neuronal microtubules

Cerebral spinal fluid (CSF) lipoproteins have become a focus of research since the observation that inheritance of particular alleles of the apolipoprotein E gene affects the risk of Alzheimer's disease (AD). There is evidence of increased lipid peroxidation in CSF lipoproteins from patients with AD, but the biological significance of this observation is not known. A characteristic of the AD brain is a disturbance of the neuronal microtubule organization. We have shown previously that 4-hydroxy-2(E)-nonenal, a major product of lipid peroxidation, causes disruption of neuronal microtubules and therefore tested whether oxidized CSF lipoproteins had the same effect. We exposed Neuro 2A cells to human CSF lipoproteins and analyzed the microtubule organization by immunofluorescence. In vitro oxidized human CSF lipoproteins caused disruption of the microtubule network, while their native (nonoxidized) counterparts did not. Microtubule disruption was observed after short exposures (1 h) and lipoprotein concentrations were present in CSF (20 microg/mL), conditions that did not result in loss of cell viability. Importantly, adult bovine CSF lipoproteins, oxidized under identical conditions, had no effect on the microtubule organization of Neuro 2A cells. Comparison of human and bovine CSF lipoproteins revealed similar oxidation-induced modifications of apolipoproteins E and A-I as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Fatty acid analysis revealed substantially lower amounts of unsaturated fatty acids in bovine CSF lipoproteins, when compared to their human counterparts. Our data therefore indicate that oxidized human CSF lipoproteins are detrimental to neuronal microtubules. This effect is species-specific, since equally oxidized bovine CSF lipoproteins left the neuronal microtubule organization unchanged.