Sialic acid (Sia) is expressed as terminal
sugar in many
glycoconjugates and plays an important role during development and regeneration. Addition of homopolymers of Sia (
polysialic acid;
polySia/PSA) is a unique and highly regulated post-translational modification of the
neural cell adhesion molecule (
NCAM). The presence of
polySia affects
NCAM-dependent cell adhesion and plays an important role during brain development, neural regeneration, and
plastic processes including learning and memory.
PolySia-
NCAM is expressed on several
neuroendocrine tumors of high
malignancy and correlates with poor prognosis. Two closely related
enzymes, the polysialyltransferases ST8SiaII and ST8SiaIV, catalyze the biosynthesis of
polySia. This review summarizes recent knowledge on Sia biosynthesis and the correlation between Sia biosynthesis and polysialylation of
NCAM and report on approaches to modify the degree of
polySia on
NCAM in vitro and in vivo. First, we describe the inhibition of polysialylation of
NCAM in ST8SiaII-expressing cells using synthetic Sia precursors. Second, we demonstrate that the key
enzyme of the Sia biosynthesis (
UDP-
N-acetylglucosamine 2-
epimerase/
N-acetylmannosamine kinase) regulates and limits the synthesis of
polySia by controlling the cellular Sia concentration.