Neuroprotective and Axon Growth-Promoting Effects following Inflammatory Stimulation on Mature Retinal Ganglion Cells in Mice Depend on Ciliary Neurotrophic Factor and Leukemia Inhibitory Factor

Abstract

After optic nerve injury retinal ganglion cells (RGCs) normally fail to regenerate axons in the optic nerve and undergo apoptosis. However, lens injury (LI) or intravitreal application of zymosan switch RGCs into an active regenerative state, enabling these neurons to survive axotomy and to regenerate axons into the injured optic nerve. Several factors have been proposed to mediate the beneficial effects of LI. Here, we investigated the contribution of glial-derived ciliary neurotrophic factor (CNTF) to LI-mediated regeneration and neuroprotection using wild-type and CNTF-deficient mice. In wild-type mice, CNTF expression was strongly upregulated in retinal astrocytes, the JAK/STAT3 pathway was activated in RGCs, and RGCs were transformed into an active regenerative state after LI. Interestingly, retinal LIF expression was correlated with CNTF expression after LI. In CNTF-deficient mice, the neuroprotective and axon growth-promoting effects of LI were significantly reduced compared with wild-type animals, despite an observed compensatory upregulation of LIF expression in CNTF-deficient mice. The positive effects of LI and also zymosan were completely abolished in CNTF/LIF double knock-out mice, whereas LI-induced glial and macrophage activation was not compromised. In culture CNTF and LIF markedly stimulated neurite outgrowth of mature RGCs. These data confirm a key role for CNTF in directly mediating the neuroprotective and axon regenerative effects of inflammatory stimulation in the eye and identify LIF as an additional contributing factor.

Neuronal Nogo-A upregulation does not contribute to ER stress-associated apoptosis but participates in the regenerative response in the axotomized adult retina

“…Mü ller cells react massively to RGC axotomy and have a key role in the inflammatory mechanisms, activating axonal regeneration in the optic nerve after injury, presumably by releasing trophic factors such as CNTF. 8,29 However, Nogo-A levels remained unchanged after optic nerve lesion in the Mü ller glia and the loss of Nogo-A in the Mü ller cells in the KO mice did not modify the axotomy-induced gliosis.…”

Hyper-IL-6: a potent and efficacious stimulator of RGC regeneration

“…12,[19][20][21][22][23][24] Besides several cell types of the innate immune response, IS activates retinal astrocytes and Müller cells to induce retinal expression of IL-6-type cytokines such as CNTF, LIF, and IL-6. 16,25,26 The essential contributions of each cytokine in mediating the neuroprotective and growth promoting effects of IS were verified by their depletion in respective genetic knockout mice, which compromised or even completely abolished the beneficial effects of IS. 16,27 Similarly, application of exogenous CNTF, LIF, and IL-6 mediates neuroprotection and stimulates axon regeneration in RGC cultures as well as the injured optic nerve in vivo, particularly upon continuous release from virally transduced cells.…”

“…16,25,26 The essential contributions of each cytokine in mediating the neuroprotective and growth promoting effects of IS were verified by their depletion in respective genetic knockout mice, which compromised or even completely abolished the beneficial effects of IS. 16,27 Similarly, application of exogenous CNTF, LIF, and IL-6 mediates neuroprotection and stimulates axon regeneration in RGC cultures as well as the injured optic nerve in vivo, particularly upon continuous release from virally transduced cells. 26,[28][29][30][31] CNTF, LIF, and IL-6 belong to the family of interleukin-6 (IL-6)-type cytokines activating the signal transducing receptor glycoprotein 130 (gp130) (Figure 1), 32 which in the adult retina is mainly expressed in RGCs.…”

“…[12][13][14] As a result of this regenerative failure together with a lack of neurotrophic support, 480% of mouse and 490% of rat RGCs undergo apoptotic cell death 21 days after an intraorbiltal optic nerve injury. [15][16][17][18] Although no clinical treatments are yet available to substantially stimulate axon regeneration or to functionally repair axonal connections in the visual pathway, experimental research has made significant progress over the last two decades. Significant neuroprotection and even moderate axonal regeneration into the injured optic nerve has been achieved for example by inflammatory stimulation (IS).…”

“…Comparably, IL-6 binds to the IL-6 receptor to form a complex with two gp130 receptors. 27,32 Although all these receptor components are expressed by mature RGCs 16,26 and cytokines directly interact with RGCs in cell cultures, 16,27,31,34 expression of the α-receptors (LIFR, CNTFR, IL6R) is seemingly limited in RGCs. In fact, CNTFRα is reportedly downregulated in RGCs upon axotomy, potentially even decreasing their responsiveness towards this respective cytokine.…”