Retrograde and Wallerian Axonal Degeneration Occur Synchronously after Retinal Ganglion Cell Axotomy

Abstract

Axonal injury and degeneration are pivotal pathological events in diseases of the nervous system. In the past decade, it has been recognized that the process of axonal degeneration is distinct from somal degeneration and that axoprotective strategies may be distinct from those that protect the soma. Preserving the cell body via neuroprotection cannot improve function if the axon is damaged, because the soma is still disconnected from its target. Therefore, understanding the mechanisms of axonal degeneration is critical for developing new therapeutic interventions for axonal disease treatment. We combined in vivo imaging with a multilaser confocal scanning laser ophthalmoscope and in vivo axotomy with a diode-pumped solid-state laser to assess the time course of Wallerian and retrograde degeneration of unmyelinated retinal ganglion cell axons in living rats for 4 weeks after intraretinal axotomy. Laser injury resulted in reproducible axon loss both distal and proximal to the site of injury. Longitudinal polarization-sensitive imaging of axons demonstrated that Wallerian and retrograde degeneration occurred synchronously. Neurofilament immunostaining of retinal whole-mounts confirmed axonal loss and demonstrated sparing of adjacent axons to the axotomy site. In vivo fluorescent imaging of axonal transport and photobleaching of labeled axons demonstrated that the laser axotomy model did not affect adjacent axon function. These results are consistent with a shared mechanism for Wallerian and retrograde degeneration.

Evidence for an enduring ischaemic penumbra following central retinal artery occlusion, with implications for fibrinolytic therapy

“…The massive disc swelling from orthograde transport block contrasts with the limited volume of axoplasm accumulating in the form of a peripapillary ring after isolated CRAO (and reflecting retrograde transport block). These volumes might be anticipated to be similar in each instance as the effects of orthograde and retrograde transport obstruction are much the same on each side of axonal injury sites generally (Banks et al, 1969;Griffin et al, 1977;Kanamori et al, 2012). The disparity at the optic disc suggests that mitochondrial transportation is very limited within the myelinated portion of an RGC axon, consistent with the much reduced concentration of cytoskeletal components within the retrolaminar optic nerve (Barron et al, 2004;Balaratnasingam et al, 2009Balaratnasingam et al, , 2010; Yu-Wai-Man et al, 2011).…”

“…First, in the majority of eyes undergoing prolonged CRA clamping (i.e, in 17 of the 22 eyes with temporary CRAO of 150e300 min duration), no optic nerve pathology was detected at final follow-up despite evidence of irreversible damage in the retina (Hayreh et al, 2004). This suggests that too insensitive a methodology was utilized in assessing orthograde (Wallerian) degeneration in the myelinated portions of RGC axons; this process could be expected to commence 7 days or so after ischaemic infarction of the unmyelinated portions of the axons (Kanamori et al, 2012). Second, the tissue sampling and histology reporting protocols failed to take into account (i) the likelihood that peri-arterial sparing, courtesy of the limited residual circulation documented in these animals, might influence the findings; (ii) the need to distinguish surviving neurons from reparative glia in the GCL and INL, and (iii) the development of retrograde RGC axonal degeneration as a secondary cause of tissue loss.…”

“…Here, 3 oxygenation-based tissue compartments (i.e. normoxic, hypoxic or anoxic) have been demonstrated within the inner retina after CRAO onset and prior to CRA reopening, and we argue that the hypoxic cells are not in a "pre-infarction" state albeit some of them (such as RGCs) are presumably at risk of retrograde axonal degeneration and apoptosis induced by the suspension of retrograde axoplasmic transport of neurotrophins (Berkelaar et al, 1994;Kanamori et al, 2012;Vidal-Sanz et al, 2012).…”

“…However, the surviving RGCs will become irreversibly disconnected from the optic nerve after z2 h as the distal intraocular segments of their axons are engulfed within the infarct developing in the posterior pole. These neurons will presumably go on to suffer retrograde degeneration and RGC apoptosis some days after the axotomy-like insult (Berkelaar et al, 1994;Kanamori et al, 2012;Vidal-Sanz et al, 2012).…”

In vivo imaging methods to assess glaucomatous optic neuropathy

“…In a laboratory setting, the visibility of RNFL axons for in vivo imaging can be further enhanced by introduction of exogenous fluorescent contrast agents(Abbott et al, 2013; Kanamori et al, 2012; Kanamori et al, 2010) or genetically-expressed fluorescent reporters. (Leung et al, 2011; Walsh and Quigley, 2008) The addition of AO to compensate for optical aberrations enables imaging of RNFL structure in exquisite detail, with or without fluorescent contrast agents by SLO, or by OCT, including in the shorter eyes of small rodents.…”