Many factors have been shown to promote myelination, but few have been shown to be inhibitory. Here, we show that polysialylated-neural cell adhesion molecule (PSA-NCAM) can negatively regulate myelin formation. During development, PSA-NCAM is first expressed on all growing fibers; then, axonal expression is downregulated and myelin deposition occurs only on PSA-NCAMnegative axons. Similarly, in cocultures of oligodendrocytes and neurons, PSA-NCAM expression on axons is initially high, but decreases as myelination proceeds. Importantly, if expression of PSA-NCAM is prematurely decreased in cultures, by either antibody-mediated internalization or enzymatic removal of the PSA moieties with endoneuraminidase N (endo-N), myelination increases 4-to 5-fold. In the optic nerve, premature cleavage of PSA moieties by intravitreous injection of endo-N also induces a transient increase in the number of myelinated internodes, but does not interfere with the onset of myelination. Previously, we showed that axonal electrical activity strongly induced myelination, which could be prevented by tetrodotoxin (TTX), an action potential blocker. Interestingly, removal of PSA moieties does not reverse the inhibition of myelination by TTX. Together, this suggests that myelination is tightly controlled by both positive (electrical activity) and negative (PSA-NCAM expression) regulatory signals.

The immunosuppressive calcineurin inhibitors cyclosporine A and tacrolimus alter T cell subsets and can cause hypertension, vascular dysfunction, and renal toxicity. We and others have reported that cyclosporine A and tacrolimus decrease anti-inflammatory regulatory T cells and increase pro-inflammatory interleukin-17-producing T cells, therefore we hypothesized that inhibition of these effects using non-cellular therapies would prevent the hypertension, endothelial dysfunction, and renal glomerular injury induced by calcineurin inhibitor therapy. Daily treatment of mice with cyclosporine A or tacrolimus for 1 week significantly decreased CD4+/FoxP3+ regulatory T cells in the spleen and lymph nodes, as well as induced hypertension, vascular injury and dysfunction, and glomerular mesangial expansion in mice. Daily co-treatment with all-trans retinoic acid, reported to increase regulatory T cells and decrease interleukin-17-producing T cells, prevented all of the detrimental effects of cyclosporine A and tacrolimus. All-trans retinoic acid also increased regulatory T cells and prevented the hypertension, endothelial dysfunction, and glomerular injury in genetically modified mice that phenocopy calcineurin inhibitor-treated mice (FKBP12-Tie2 KO). Treatment with an interleukin-17 neutralizing antibody also increased regulatory T cell levels and prevented the hypertension, endothelial dysfunction, and glomerular injury in cyclosporine A-treated and tacrolimus-treated mice as well as FKBP12-Tie2 KO mice, while an isotype control had no effect. Augmenting regulatory T cells and/or inhibiting interleukin-17 signaling using non-cellular therapies prevents the cardiovascular and renal toxicity of calcineurin inhibitors in mice.