Axon behaviour at Schwann cell – astrocyte boundaries: manipulation of axon signalling pathways and the neural adhesion molecule L1 can enable axons to cross

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Abstract
Axon regeneration in vivo is blocked at boundaries between Schwann cells and astrocytes, such as occur at the dorsal root entry zone and around peripheral nerve or Schwann cell grafts. We have created a tissue culture model of these boundaries in Schwann cell - astrocyte monolayer co-cultures. Axon behaviour resembles that in vivo, with axons showing a strong preference for Schwann cells over astrocytes. At boundaries between the two cell types, axons growing on astrocytes cross readily onto Schwann cells, but only 15% of axons growing on Schwann cells are able to cross onto astrocytes. Treatment with chondroitinase or chlorate to reduce inhibition by proteoglycans did not change this behaviour. The neural adhesion molecule L1 is present on Schwann cells and not astrocytes, and manipulation of L1 by application of an antibody, L1-Fc in solution, or adenoviral transduction of L1 into astrocytes increased the proportion of axons able to cross onto astrocytes to 40-50%. Elevating cAMP levels increased crossing from Schwann cells onto astrocytes in live and fixed cultures, and had a co-operative effect with NT-3 but not with NGF. Inactivation of Rho with a cell-permeant form of C3 exoenzyme also increased crossing from Schwann cells to astrocytes. Our experiments indicate that the preference of axons for Schwann cells is largely mediated by the presence of L1 on Schwann cells but not astrocytes, and that manipulation of growth cone signalling pathways can allow axons to disregard boundaries between the two cell types.