Abstract
Purpose: :
Retinal ganglion cells undergo an early phase of atrophy before death in both acute (crush) and chronic (elevated IOP) models of damage to the optic nerve. A principal feature of the atrophy is soma shrinkage, which has been termed the apoptotic volume decrease (AVD) in other cell-types. As a prelude to studying the underlying biology of the AVD in ganglion cells, we performed a comprehensive evaluation of the kinetics of nuclear shrinkage of these cells.
Methods: :
Apoptosis of ganglion cells was induced by optic nerve crush. Metrics of ganglion cell AVD were measured in Nissl-stained wholemounts of retinas from wild-type, and Bax-deficient mice. Blockage and/or reversal of the AVD were probed in experiments using growth factors including CNTF. The involvement of K+ channels in AVD was examined using 1.5 mM BaCl2 to block K+ efflux, and immunofluorescence was used to localize the Kv2.1 voltage-gated channel in ganglion cells.
Results: :
Somas and nuclei proportionately shrink (range between 15-40%) in ganglion cells within 2 hours after optic nerve crush. Nuclear shrinkage was typified by the formation of heterochromatin. Nuclear condensation also occurred rapidly in Bax-deficient cells and persisted for up to 18 months after nerve damage, suggesting that the AVD preceded the committed stage of apoptosis in ganglion cells. CNTF (100 ng) applied intravitreally either before or after AVD changes were unable to either block or reverse the changes in Bax-deficient retinas, respectively. Application of BaCl2 at the time of crush, however, was able to attenuate the AVD, but only in cells resistant to apoptosis. The Kv2.1 channel, which regulates the AVD in other neurons, was highly expressed in somas of the ganglion cell layer and axons in the optic nerve.
Conclusions: :
Ganglion cells rapidly undergo the AVD after optic nerve crush. This event occurs prior to the committed stage of apoptosis as defined by Bax function. K+ efflux likely regulates this event. Electrophysiological examination of Kv channels in ganglion cells, and in vivo measurements of K+ efflux after crush are planned.
Keywords: apoptosis/cell death • ganglion cells • ion channels