Human JAM2 blocking peptide

We have identified a third member of the junctional adhesion molecule (JAM) family. At the protein level JAM3 displays 36 and 32% identity to JAM2 and JAM1, respectively. The coding region is distributed over 9 exons and maps to chromosome 11q25. The gene shows widespread tissue expression with higher levels apparent in the kidney, brain, and placenta. At the cellular level we show expression of JAM3 transcript within endothelial cells. Our major finding is that JAM3 and JAM2 are binding partners. Thus, JAM3 ectodomain binds firmly to JAM2-Fc. This heterotypic interaction is maintained when JAM3-Fc is used to capture Chinese hamster ovary cells expressing full-length JAM2. In static adhesion assays we show that JAM3 is unable to bind to leukocyte cell lines. This is consistent with the lack of JAM2 expression. However, using JAM2-Fc pulldown experiments in combination with polyclonal anti-JAM3 serum, we demonstrate that JAM3 is the previously uncharacterized 43-kDa counter-receptor that mediates JAM2 adhesion to T cells. Most significantly we demonstrate up-regulation of JAM3 protein on peripheral blood lymphocytes following activation. Finally we show the utility of JAM3 ectodomain as an inhibitor of JAM2 adhesion.

The mechanism underlying selective myelination of axons versus dendrites or neuronal somata relies on the expression of somatodendritic membrane myelination inhibitors (i.e. JAM2). However, axons still present long unmyelinated segments proposed to contribute to axonal plasticity and higher order brain functions. Why these segments remain unmyelinated is still an unresolved issue. The bifunctional lectin galectin-4 (Gal-4) organizes the transport of axon glycoproteins by binding to N-acetyllactosamine (LacNac) termini of N-glycans. We have shown that Gal-4 is sorted to segmental domains (G4Ds) along the axon surface, reminiscent of these long unmyelinated axon segments in cortical neurons. We report here that oligodendrocytes (OLGs) do not deposit myelin on Gal-4 covered surfaces or myelinate axonal G4Ds. In addition, Gal-4 interacts and co-localizes in G4Ds with contactin-1, a marker of another type of non-myelinated segments, the nodes of Ranvier. Neither Gal-4 expression nor G4D dimensions are affected by myelin extracts or myelinating OLGs, but are reduced with neuron maturation. As in vitro, Gal-4 is consistently segregated from myelinated structures in the brain. Our data shape the novel concept that neurons establish axon membrane domains expressing Gal-4, the first inhibitor of myelination identified in axons, whose regulated boundaries delineate myelination-incompetent axon segments along development.