Double labeling studies of the pattern of
protein synthesis in goldfish and mouse brain identified a class of
glycoproteins (the
ependymins) whose turnover rate was enhanced after training. A variety of control experiments indicated that these macromolecules have an important role in the molecular and cell biology of learning.
Antisera to the
ependymins when injected into the brains of trained goldfish cause
amnesia of a newly acquired behavior. Isolation and localization studies by immunocytochemical methods indicate that the
ependymins are released into the brain extracellular fluid by a class of neurosecretory cells. In mammalian brain ependymin-containing cells are highly concentrated in the limibic system. The
ependymins are constituted from two
disulfide-linked acidic
polypeptide chains (M.W.37K and 31K). They contain at least 5% covalently bound
carbohydrate per chain with
mannose,
galactose,
N-acetylglucosamine and
N-acetylneuraminic acid as the predominant components. The highly soluble
ependymins can rapidly polymerize to form an insoluble fibrous matrix if
calcium is removed from
solution by the addition of a Ca2+-
chelating agent or dialysis. The self-aggregation property of the
ependymins can be triggered by the depletion of Ca2+ from the extracellular space. Studies of the kinetics of the aggregation phenomenon by measurements of turbidity changes indicate that the process can be terminated but not reversed by restoring Ca2+ to its normal CSF level. Immunohistochemical studies of the brains of trained goldfish show the presence of punctate statining sites in the perimeter of certain cells located in specific brain regions. This suggests that ependymin aggregation might occur in vivo during learning. A molecular hypothesis relating the aggregation properties of the
ependymins to neuroplasticity and learning is proposed.