CHARACTERIZATION OF THE RETRACTION RESPONSE OF GOLDFISH RETINAL GANGLION-CELL AXONS INDUCED BY MONOCLONAL-ANTIBODY 8A2 INVITRO

Retraction similar to that occurring spontaneously in senescent axonal fields of goldfish regenerating ganglion cell axons is reliably induced by monoclonal antibody (mAb) 8A2. The retraction response is characterized by transformation of the growth cone into a nodular motile mass, which undergoes retrograde translocation in conjunction with the contiguous column of axoplasm, generating evacuated distal strands. The growth cone-to-motile mass transformation involves a reorganization of F-actin. In addition, the reorganization of F-actin is a necessary antecedent for retrograde bulk translocation of axoplasm. Contractile tension contributes to compaction within the motile mass, while that within the column of distal axoplasm is oriented longitudinally and appears to contribute to bulk movement. As a derivative of the growth cone, the motile mass exhibits protrusive activities and a capacity to translocate independently when microtubules are partially disrupted. Apparent compressive forces cause buckling of microtubules in the adjacent segment which appear as elbow-like protrusions. Cytochalasin D blocks mAb 8A2 induced retraction and immediately arrests retrograde translocation when it is in progress; however, neither nocodazole nor taxol blocks retraction. Phalloidin and immunofluorescence double labeling of retracted axons reveals that myosin II, MLCK, and calmodulin co-localize with dense F-actin structures within the motile mass. These results suggest that microtubules play a subordinate, passive role, and that actomyosin interactions mediate the formation of the motile mass and the retraction response. Finally, axons grown on laminin exhibit a more robust retraction response than those grown on polylysine, implicating membrane-cytoskeletal interactions as modulating factors.