Neuroglobin is an endogenous neuroprotectant for retinal ganglion cells against glaucomatous damage.

Neuroglobin (NGB), a newly discovered member of the globin superfamily, may regulate neuronal survival under hypoxia or oxidative stress. Although NGB is greatly expressed in retinal neurons, the biological functions of NGB in retinal diseases remain largely unknown. We investigated the role of NGB in an experimental model of glaucoma, a neurodegenerative disorder that usually involves elevation of intraocular pressure (IOP). Elevated IOP is thought to induce oxidative stress in retinal ganglion cells (RGCs), thereby causing RGC death and, eventually, blindness. We found that NGB plays a critical role in increasing RGC resistance to ocular hypertension and glaucomatous damage. Elevation of IOP stimulated a transient up-regulation of endogenous NGB in RGCs. Constitutive overexpression of NGB in transgenic mice prevented RGC damage induced by glutamate cytotoxicity in vitro and/or by chronic IOP elevation in vivo. Moreover, overexpression of NGB attenuated ocular hypertension-induced superoxide production and the associated decrease in ATP levels in mice, suggesting that NGB acts as an endogenous neuroprotectant to reduce oxidative stress and improve mitochondrial function, thereby promoting RGC survival. Thus, NGB may modulate RGC susceptibility to glaucomatous neural damage. Manipulating the expression and bioactivity of NGB may represent a novel therapeutic strategy for glaucoma.

IGFBPL1 Regulates Axon Growth through IGF-1-mediated Signaling Cascades.

Activation of axonal growth program is a critical step in successful optic nerve regeneration following injury. Yet the molecular mechanisms that orchestrate this developmental transition are not fully understood. Here we identified a novel regulator, insulin-like growth factor binding protein-like 1 (IGFBPL1), for the growth of retinal ganglion cell (RGC) axons. Expression of IGFBPL1 correlates with RGC axon growth in development, and acute knockdown of IGFBPL1 with shRNA or IGFBPL1 knockout in vivo impaired RGC axon growth. In contrast, administration of IGFBPL1 promoted axon growth. Moreover, IGFBPL1 bound to insulin-like growth factor 1 (IGF-1) and subsequently induced calcium signaling and mammalian target of rapamycin (mTOR) phosphorylation to stimulate axon elongation. Blockage of IGF-1 signaling abolished IGFBPL1-mediated axon growth, and vice versa, IGF-1 required the presence of IGFBPL1 to promote RGC axon growth. These data reveal a novel element in the control of RGC axon growth and suggest an unknown signaling loop in the regulation of the pleiotropic functions of IGF-1. They suggest new therapeutic target for promoting optic nerve and axon regeneration and repair of the central nervous system.

Postnatal onset of retinal degeneration by loss of embryonic Ezh2 repression of Six1.

Some adult-onset disorders may be linked to dysregulated embryonic development, yet the mechanisms underlying this association remain poorly understood. Congenital retinal degenerative diseases are blinding disorders characterized by postnatal degeneration of photoreceptors, and affect nearly 2 million individuals worldwide, but ∼50% do not have a known mutation, implicating contributions of epigenetic factors. We found that embryonic deletion of the histone methyltransferase (HMT) Ezh2 from all retinal progenitors resulted in progressive photoreceptor degeneration throughout postnatal life, via derepression of fetal expression of Six1 and its targets. Forced expression of Six1 in the postnatal retina was sufficient to induce photoreceptor degeneration. Ezh2, although enriched in the embryonic retina, was not present in the mature retina; these data reveal an Ezh2-mediated feed-forward pathway that is required for maintaining photoreceptor homeostasis in the adult and suggest novel targets for retinal degeneration therapy.

Dynamic patterns of histone lysine methylation in the developing retina.

PURPOSE: Histone lysine methylation (HKM) is an important epigenetic mechanism that establishes cell-specific gene expression and functions in development. However, epigenetic control of retinal development is poorly understood. To study the roles of HKM in retinogenesis, the authors examined the dynamic changes of three HKM modifications and of two of their regulators, the histone methyltransferases (HMTases) Ezh2 and G9a, in the mouse retina. METHODS: Retinal sections and lysates from embryonic day 16 through adult were processed for immunohistochemistry and immunoblotting using antibodies against various marks and HMTases. To further analyze the biological functions of HKM, the effects of small molecule inhibitors of HMTases were examined in vitro. RESULTS: Methylation marks of trimethyl lysine 4 and 27 on histone H3 (H3K4me3 and H3K27me3) were detected primarily in differentiated retinal neurons in the embryonic and adult retina. In contrast, dimethyl lysine 9 on histone H3 (H3K9me2) was noted in early differentiating retinal ganglion cells but was lost after birth. The HMTases controlling H3K27me3, H3K9me2, Ezh2, and G9a were enriched in the inner embryonic retina during the period of active retinogenesis. Using the chemical inhibitors of Ezh2 and G9a, the authors reveal a role for HKM in regulating retinal neuron survival. CONCLUSIONS: HKM is a dynamic and spatiotemporally regulated process in the developing retina. Epigenetic regulation of gene transcription by Ezh2- and G9a-mediated HKM plays crucial roles in retinal neuron survival and may represent novel epigenetic targets to enhance viability in retinal neurodegenerative diseases such as glaucoma.