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H Uri Saragovi, Yulia Sidorova, Mart Saarma, Sean Jmaeff; Retinal neuroprotection using small molecule GDNF mimetics. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5754.
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Glial cell-line derived neurotrophic factor (GDNF) and its receptors (GFRa1 and the tyrosine kinase RET) are expressed by retinal neurons. GDNF yields pro-survival signals by activating AKT and related pathways in cells expressing GDNF receptors. GDNF receptor dysfunction has been implicated in degenerative diseases including Retinitis Pigmentosa (RP). Indeed, GDNF has been tested in human clinical trials. While the rationale for using GDNF as a therapeutic agent is very strong, its success in clinical trials has been poor. GDNF protein has significant drawbacks that rule out its use as a pharmaceutical (poor delivery and penetration, stability, selectivity, and kinetics). As an alternative, we designed small molecule mimetics which selectively activate the RET signaling pathway. Using genetic models of RP, we tested RET-agonists to quantify trophic support to photoreceptors, and studied their mechanism of action.
Compounds were screened ex vivo in biochemical assays by Western Blotting of p-RET and its effectors p-Akt and p-Erk. Cell survival experiments were performed using MTT assays on cultures stressed by H2O2. Immunostaining was used on retinal sections to determine RET expression and p-Akt localization following intravitreal injection of test compounds. Retinal lysates were prepared at various timepoints after treatment and analyzed by Western blot. Retinal explants form RD10 or RHOP347S mouse models of RP were treated with compound 48p and TUNEL was quantified. Optical Coherence Tomography (OCT) was employed as the in vivo endpoint to measure the thickness of the outer nuclear layer in RP models.
Biochemical screening ex vivo validated 48p as a selective agonist of RET, and dose and time-dependent activation of RET and its effectors were observed. 48p promoted pro-survival signals in cell cultures stressed by H2O2. In vivo, RET expression in the retina was localized to the photoreceptor and ganglion cell layers, and 48p administration led to increased p-Akt staining in those layers 1hr after treatment. Kinetic assays showed that 48p activated RET in a more sustained fashion in vivo compared to GDNF. Retinal explants form RD10 or RHOP347S mouse models of RP treated with compound 48p had reduced TUNEL counts.
These results validate activating RET in vivo as a potential therapeutic target for RP. Using small molecules with drug-like properties provides a pharmacological advantage over proteins.
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