Investigations into the possible role of polysialic acid and sialyltransferase activity in neural plasticity in the domestic chick.

Abstract

Polysialic acid (PSA) is an unusual linear homopolymer of a2,8 linked N-acetyl-neuraminic acid residues that occurs in the vertebrate nervous system as a post-translational modification of the neural cell adhesion molecule, NCAM. During development NCAM tends to shift from a highly polysialylated to a lightly polysialylated form - a transformation that is believed to reflect the progress from developmental neural morphoplasticity to a more stable, mature character. Immunocytochemistry, with a specific anti-PSA antibody, is used here to map the distribution of PSA in the brain, retina and optic nerve of Ross Chunky chick embryos at embryonic days 3 (E3), 5 (E5), 9 (E9), 13 (E13), 17 (E17) and 19 (E19) and post hatching days 1 (PI) and 7 (P7). During the first two weeks of development, PSA occurred ubiquitously throughout the central nervous system, although there was some down- regulation between E9 and El3 at the levels of the third and eighth cranial nerves. Between El7 and P7, PSA was down- regulated in most of the cerebellum, pons, medulla and most fibre bundles. PSA was down regulated by the optic nerve between PI and P7. PSA down-regulation was also observed in the internal tectal nuclei, and a few discrete regions of the diencephalon and telencephalon including the nucleus rotundus and the ectostriatum. In the telencephalon, high levels of PSA were detected as late as P7 in the intermediate mediale hyperstriatum ventrale (IMHV) and the lobus parolfactorius (LPO), regions known to be important in memory formation. Multiple immunofluorescence was used to compare PSA expression with labelling by specific antibodies that detect galactocerebroside (GalC), a myelin-specific molecule; glial fibrillary acid protein (GFAP), that is specific to fibrous (type II) astrocytes; neuronal cell bodies and neurofilaments. These studies showed that there is no consistent relationship between PSA expression and the onset of myelination. It was shown that PSA is expressed by brain surface astrocytes in most, but not all, brain regions as late as P7 but that PSA is down-regulated by many white matter astrocytes as early as El3. Overall, these results point to significant PSA down-regulation in several chick brain regions close to the time of hatching. Transient increases in PSA levels have been reported in rats 12-24 hours after acquisition of a step-down avoidance task, following training in a Morris water maze. PSA levels were investigated in the brains of day-old chicks at 12 hours after training on a passive avoidance task in which birds pecked either a dry bead or a bead coated in the aversant methylanthranilate. PSA levels were estimated by using a grey level analysis of immunolabelled brain sections in the left and right IMHV and the left and right LPO. No differences between trained and control birds were detected. Since altered synthetic enzyme activity may mediate changes in PSA levels, sialyltransferase (ST) activity was assayed at 6, 12 and 24 hours after passive avoidance training. Activity was assayed, using CMP-(14c)-NeuNAc as sialic acid donor and fetuin as exogenous acceptor, in the left LPO. No differences in ST activity between groups were evident at any of the time points investigated.