Complementary research models and methods to study axonal regeneration in the vertebrate retinofugal system

Abstract

Due to the lack of axonal regeneration, age-related deterioration in the central nervous system (CNS) poses a significant burden on the wellbeing of a growing number of elderly. To overcome this regenerative failure and to improve the patient's life quality, the search for novel regenerative treatment strategies requires valuable (animal) models and techniques. As an extension of the CNS, the retinofugal system, consisting of retinal ganglion cells that send their axons along the optic nerve to the visual brain areas, has importantly contributed to the current knowledge on mechanisms underlying the restricted regenerative capacities and to the development of novel strategies to enhance axonal regeneration. It provides an extensively used research tool, not only in amniote vertebrates including rodents, but also in anamniote vertebrates, such as zebrafish. Indeed, the latter show robust regeneration capacities, thereby providing insights into the factors that contribute to axonal regrowth and proper guidance, complementing studies in mammals. This review provides an integrative and critical overview of the classical and state-of-the-art models and methods that have been employed in the retinofugal system to advance our knowledge on the signaling pathways underlying the restricted versus robust axonal regeneration in rodents and zebrafish, respectively. In vitro, ex vivo and in vivo models and techniques to improve the visualization and analysis of regenerating axons are summarized. As such, the retinofugal system is presented as a valuable model to further facilitate research on axonal regeneration and to open novel therapeutic avenues for CNS pathologies.

Neuroinflammation and Optic Nerve Regeneration: Where Do We Stand in Elucidating Underlying Cellular and Molecular Players?

“…The retinofugal system is a powerful model system to investigate the mechanisms underlying (restricted) regenerative capacity, to study the molecules that affect axonal regeneration and to develop new strategies to stimulate axonal regeneration in the adult mammalian CNS. 2,3 Indeed, (1) the optic nerve and retina are the most accessible regions of the CNS; (2) their morphology and function are well characterized; (3) the axons of the optic nerve originate from a single population of projection neurons, the retinal ganglion cells (RGCs); (4) the typical organization of the glial cells facilitates the study of glial reactivity; (5) RGCs can be manipulated with low-invasive techniques; and (6) the vitreous body can be used as a reservoir for the administration of drugs, molecules and recombinant genes. 3,4 The optic nerve crush and optic nerve transection paradigms are widely used in rodents to induce local axonal damage and investigate RGC survival and axonal regeneration 4 , as well as glial scar formation and inflammation.…”

“…3,4 The optic nerve crush and optic nerve transection paradigms are widely used in rodents to induce local axonal damage and investigate RGC survival and axonal regeneration 4 , as well as glial scar formation and inflammation. 2 Importantly, optic nerve lesions not only induce acute inflammatory processes at the site of injury (e.g. formation of a glial scar) but also in the retina, where the RGC somata reside.…”

“…8,9 In contrast, in zebrafish subjected to optic nerve crush, spontaneous regeneration with full functional recovery takes place: almost all RGCs survive 10 and, after an initial injury response of less than 1 day, RGCs prepare for axonal outgrowth (between 1 and 7 days after injury), elongate their axons towards their brain targets (day 5-18) and reestablish synaptic contacts in the optic tectum (day 7-25). 2,[11][12][13] Hurdles for axonal regeneration in the adult mammalian central nervous system…”

“…45,47,48,63,75 Moreover, inflammatory stimulation in the spontaneously regenerating zebrafish accelerates axonal regeneration after optic nerve injury. 2,76,77 Notably, an intravitreal injection (injection through the sclera immediately posterior to the limbus, without damaging the lens) has different effects on RGC apoptosis and axonal regeneration compared to an intra-optic nerve injection (injection piercing the optic nerve pia midway between the lamina cribrosa and the site of optic nerve injury). Stimulation of inflammation both in the optic nerve and the vitreous protects RGCs from cell death, yet only vitreal inflammation promotes axonal regeneration, suggesting that this local inflammatory response stimulates axonal regeneration by activating certain signaling pathways in the RGC somata.…”

Glial cell ecology in zebrafish development and regeneration

“…Glia are crucial in the context of development, disease progression, and injury response [4,5]. In addition, axonal regeneration is an important step during spinal cord regeneration, and there are many excellent review articles on this important topic [6,7,8,9,10,11,12,13]. In our review article, we wish to focus more attention on how extracellular and intracellular molecular signaling are integrated to control glial bridge formation after spinal cord injury [1].…”