Laser inactivation of fasciclin I disrupts axon adhesion of grasshopper pioneer neurons.

A molecular mechanism for selective axonal adhesion is a central question of neural development. Cell adhesion molecules have been identified, but it has been difficult to ascribe functions for these proteins in vivo. Here we show that the neuronal membrane glycoprotein fasciclin I has a role in the adhesion of sister axons during the development of the grasshopper limb bud. To do this we used a new technique, chromophore-assisted laser inactivation (CALI), which causes the precisely timed thermal denaturation of specific proteins by laser light targeted through a dye-labelled antibody, without any other observable damage to living cells. This can be achieved by relaxation of the laser-excited dye which releases heat to denature the bound protein; the rapid dissipation of heat with distance insulates unbound proteins from damage. CALI is a molecular analogue of cellular laser ablation and provides an unprecedented level of spatial and temporal resolution. Using dye-labelled antibodies that recognize fasciclin I, CALI disrupts fasciculation of the pioneer neurons without affecting their growth or guidance.