Enzymatic removal of polysialic acid from neural cell adhesion molecule interrupts gonadotropin releasing hormone (GnRH) neuron–glial remodeling

There is abundant evidence to prove that the astrocytes are highly dynamic cell type in CNS and under physiological conditions such as reproduction, these cells display a remarkable structural plasticity especially at the level of their distal processes ensheathing the gonadotropin releasing hormone (GnRH) axon terminals. The morphology of GnRH axon terminals and astrocytes in the median eminence region of hypothalamus show activity dependent structural plasticity during different phases of estrous cycle. In the current study, we have assessed the functional contribution of ∞-2,8-linked polysialic acid (PSA) on neural cell adhesion molecule (PSA-NCAM) in this neuronal–glial plasticity using both in vitro and in vivo model systems. In vivo experiments were carried out after stereotaxic injection of endoneuraminidase enzyme (endo-N) near median eminence region of hypothalamus to specifically remove PSA residues on NCAM followed by localization of GnRH, PSA-NCAM and glial fibrillary acidic protein (GFAP) by immunostaining. Using in vitro model, structural remodeling of GnV-3 cells, (a conditionally immortalized GnRH cell line) co-cultured with primary astrocytes was studied after treating the cells with endo-N. Marked morphological changes were observed in GnRH axon terminals in proestrous phase rats and control GnV-3 cells as compared to endo-N treatment i.e. after removal of PSA. The specificity of endo-N treatment was also confirmed by studying the expression of PSA-NCAM by Western blotting in cultures treated with and without endo-N. Removal of PSA from surfaces with endo-N prevented stimulation associated remodeling of GnRH axon terminals as well as their associated glial cells under both in vivo and in vitro conditions. The current data confirms the permissive role of PSA to promote dynamic remodeling of GnRH axon terminals and their associated glia during reproductive cycle in rats.

Schematic representation showing PSA-NCAM on GnRH axon terminals and astrocytes. GnRH neurons from the preoptic area extend their axon terminals towards median eminence region of hypothalamus to secrete GnRH near the perivascular space. During proestrous phase of estrous cycle, when expression of PSA-NCAM is maximum, glial cells processes are withdrawn from around the GnRH axon terminals due to anti-adhesive property of PSA and GnRH neurons show sprouting and extension to release the hormone. The enzymatic removal of PSA form NCAM prevents withdrawal of glial processes from GnRH axon terminals which in turn block their sprouting. GnRH, gonadotropin releasing hormone; POA, pre-optic area; PSA, polysialic acid; ME, median eminence; endo-N, endoneuraminidase.Figure optionsDownload high-quality image (112 K)Download as PowerPoint slideHighlights
► PSA-NCAM plays permissive role in structural remodeling of GnRH neuron.
► In vitro and in vivo model systems were used to illustrate GnRH neuron remodeling.
► Morphological changes were inhibited by endo-N treatment of GnV-3 cells.
► Enzymatic removal of PSA from ME prevented dynamic remodeling of GnRH axon terminals.
► PSA expression on NCAM facilitates release of GnRH hormone in perivascular space.