The mechanical regulation of Eph/ephrin signaling in the developing brain.

Abstract

Eph receptors and their membrane-bound ligands, ephrins, provide key signals in many developmental processes including neuronal guidance. However, despite immense progress in our understanding of Eph/ephrin signaling, discrepancies between in vitro and in vivo work remain. As axon pathfinding is regulated by chemical and mechanical signals, and the mechanical regulation of Eph/ephrin signaling is currently poorly understood, we here investigated the role of mechanical cues in this signaling pathway. Xenopus retinal neurons cultured on soft substrates mechanically resembling brain tissue had the opposite response to ephrinB1 compared to those cultured on glass. Furthermore, in vivo atomic force microscopy data showed that the Xenopus visual area of the brain, the optic tectum, becomes mechanically heterogenous as retinal neurons approach the diencephalon-tectum boundary and begin to innervate it. The stiffness gradient which develops correlates with both a cell density gradient and a concentration gradient of EphB expression detected by hybridization chain reaction. Since EphB/ephrinB signaling in Xenopus retinal neurons is affected by substrate stiffness in vitro, and a stiffness gradient develops across the optic tectum at the time of innervation, our data suggest that mechanical cues could be important in tuning retinotectal mapping through the regulation of chemical signaling. A similar regulation of chemical signaling through tissue mechanics is likely to be important across multiple aspects of neural development, as well as in other organ systems.