Calcineurin cleavage is triggered by elevated intraocular pressure, and calcineurin inhibition blocks retinal ganglion cell death in experimental glaucoma

Abstract

Increased intraocular pressure (IOP) leads, by an unknown mechanism, to apoptotic retinal ganglion cell (RGC) death in glaucoma. We now report cleavage of the autoinhibitory domain of the protein phosphatase calcineurin (CaN) in two rodent models of increased IOP. Cleaved CaN was not detected in rat or mouse eyes with normal IOP. In in vitro systems, this constitutively active cleaved form of CaN has been reported to lead to apoptosis via dephosphorylation of the proapoptotic Bcl-2 family member, Bad. In a rat model of glaucoma, we similarly detect increased Bad dephosphorylation, increased cytoplasmic cytochrome c (cyt c), and RGC death. Oral treatment of rats with increased IOP with the CaN inhibitor FK506 led to a reduction in Bad dephosphorylation and cyt c release. In accord with these biochemical results, we observed a marked increase in both RGC survival and optic nerve preservation. These data are consistent with a CaN-mediated mechanism of increased IOP toxicity. CaN cleavage was not observed at any time after optic nerve crush, suggesting that axon damage alone is insufficient to trigger cleavage. These findings implicate this mechanism of CaN activation in a chronic neurodegenerative disease. These data demonstrate that increased IOP leads to the initiation of a CaN-mediated mitochondrial apoptotic pathway in glaucoma and support neuroprotective strategies for this blinding disease.retina ͉ optic nerve ͉ apoptosis

Efficient estimation of retinal ganglion cell number: A stereological approach

“…RGC counts in control eyes ranged from 78,000 to 93,600 with a mean (±s.d.) of 84,862 ± 5,753, as previously reported for some eyes (Huang et al, 2005a;Huang et al, 2005b). In eyes with elevated IOP for 10 days RGC counts ranged from 51,100 to 63,029 with a mean of 56,837 ± 4,272, indicating a 33% loss of RGCs (n=8; p < 0.05).…”

“…We also applied the method of cross-sectional stereological counting as a tool for assessing RGC survival and neuroprotection in experimental glaucoma as a result of drug treatment. In glaucoma eyes treated with the calcineurin inhibitor FK506, stereological counting methods detected a 50% protection of RGCs (Huang et al, 2005b).…”

Mechanisms of noise-induced hearing loss indicate multiple methods of prevention

“…Inhibiting calcineurin activation reduces BAD dephosphorylation and improves cell survival (Wang et al, 1999a;Springer et al, 2000;Uchino et al, 2002;Berthier et al, 2004;Mano et al, 2004;Shou et al, 2004;Watabe and Nakaki, 2004;Yang et al, 2004a;Biswas et al, 2005;Cardoso and Oliveira, 2005;Huang et al, 2005). Reducing calcineurin activation also reduces the downstream caspase activity (Springer et al, 2000;Nottingham et al, 2002;Agostinho and Oliveira, 2003;Cardoso and Oliveira, 2005;Grosskreutz et al, 2005).…”

Neuroprotection in Glaucoma

“…Once Bad is on the outer mitochondrial members, it will bind to anti-apoptotic Bcl-2 or Bcl-XL and promote release of cytochrome c. (Wang et al, 1999;Heckman et al, 2006) Calcineurin activation and cytochrome c release has been linked with RGC death after optic nerve crush. (Huang et al, 2005) Lastly, RGC death has been shown to be attenuated in experimental models of optic nerve crush after treatment with a calcineurin inhibitor, FK506 (Huang et al, 2005). Calpain is a Ca 2+ -dependent cysteine protease that is implicated in many neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease (Nixon, 2003;Goll et al, 2003) Calpain activates calcineurin and transforms it into its constitutively activated form.…”