Abstract
In chick development, ciliary ganglion (CG) neurons go through a period of axon extension from approximately embryonic day (E)4 to E8, followed by a period of synaptogenesis and neuronal cell death. By examining the immunohistochemical localization of laminin, in conjunction with Dil labeling of the ciliary nerve projection, we have determined that the pathway taken by these neurons is rich in laminin expression. Therefore, laminins are good candidate molecules for mediating outgrowth of these neurons in vivo. In vitro, the ability of CG neurons to extend neurites on laminin-1 (EHS laminin, alpha 1 beta 1 gamma 1) is maximal up to E8, then declines dramatically. CG neuron outgrowth on laminin-1 requires the activity of beta 1-class integrins. We have used subunit-specific antibodies to determine which of the five beta 1- containing heterodimers known to be laminin receptors (alpha 1 beta 1, alpha 2 beta 1, alpha 6 beta 1, alpha 7 beta 1) are expressed, and which mediate neurite outgrowth. While we could not detect expression of alpha 2 or alpha 7, we have found that alpha 1, alpha 3 beta 1, and alpha 6 beta 1 are expressed on the surface of ciliary ganglion neuron cell bodies and axons, both in vitro and in vivo. Furthermore, antibodies against alpha 3 and alpha 6, but not alpha 1, interfered with CG neurite outgrowth on laminin-1 in vitro. Taken together, these data suggest that interactions of cell surface alpha 3 beta 1 and alpha 6 beta 1 integrins with laminin-1 are likely to mediate growth of CG neurons during pathfinding in vivo.