CALCIUM WAVE-FRONTS THAT CROSS GAP-JUNCTIONS MAY SIGNAL NEURONAL DEATH DURING DEVELOPMENT

Embryonic anterior pagoda (AP) neurons in the leech interact with thei r segmental homologs in adjacent ganglia through transient axons that overlap during a critical period of development and then retract. Howe ver, when an AP neuron is ablated mechanically or by irradiation durin g this period, an adjacent homolog responds by reinitiating growth of its overlapped axon and thereby taking over vacated territory (Gao and Macagno, 1987b; Gao, 1989). The death of an AP cell is therefore comm unicated to its homolog, but the mechanism underlying this signaling i s presently unknown. Since it was recently found that AP homologs are electrically and dye coupled through their transient axons (Wolszon et al., 1994), we investigated the possibility that gap junctions may me diate the cell death signal that could occur between developing neuron s. Among several candidate intercellular signals, we began by studying calcium dynamics in embryonic AP cells, in situ, since calcium is kno wn to cross gap junctions and is implicated in cell death in many syst ems. We found that elements that usually increase [Ca2+], in adult neu rons, such as releasable internal stores or voltage-dependent calcium channels, were not present at the critical period. Instead, mechanisms that reduce free calcium, such as buffering and pumping, were the mos t robust. When a large, focal calcium rise was produced in an AP axon by making a lesion with a UV microbeam (leading to eventual death of t hese neurons), calcium did not rise quickly throughout the cell, but r ather moved in a stow (0.05-0.25 mu m/sec) wave front away from the le sion site, into other processes of the damaged cell. Furthermore, when a calcium wave front reached the growth cone of a transient axon, it crossed at the gap junctions into the coupled axon of the neighboring AP neuron, but went no further. Since it is known that an AP responds to a neighbor's death by reinitiating growth only in that axon that co ntacts the dying cell (Gao and Macagno, 1987b; Gao, 1989), these obser vations are consistent with playing a role in the signaling of cell de ath to homologs that are coupled to a dying cell.