compounds reduced protein aggregation and inclusion incidence which correlated with protection against cell death and caspase activation (Mendes and Cheetham 2008). However, these Hsp90 inhibitors did not promote the processing of P23H rod opsin beyond the ER or affect the dominant-negative effect of the mutant opsin (Mendes and Cheetham 2008). As with kosmotropes, the reduction of inclusion incidence and increased degradation of aggregate prone species, without the promotion of protein folding appeared to be sufficient to reduce the gain of function mechanisms induced by the mutant protein.

Another chaperone inducer celastrol, a quinone methide triterpene, which is an active component from Chinese herbal medicine with potent anti-inflammatory and anti-oxidative effects, in addition to activating HSF1, was less successful in the RP cell model, as it was toxic at concentrations that were starting to be effective at reducing protein aggregation and inclusion formation (Mendes and Cheetham 2008).

36.2.4 Autophagy Inducers

In addition to the proteasome, autophagy plays a major role in protein degradation in eukaryotic cells. Autophagy is upregulated when the proteasome is inhibited and P23H rod opsin aggregation inhibits the proteasome (Illing et al., 2002). Furthermore, autophagy may play a role in the degradation of mutant rhododpsin, as the autophagic marker proteins, Atg7, Atg8 (LC3), and LAMP-1 co-localised with P23H rod opsin in cells (Kaushal 2006).

Autophagy can be induced by inhibiting the mammalian target of rapamycin (mTOR) with rapamycin and related drugs, as mTOR negatively regulates autophagy. Induction of autophagy with rapamycin and analogues protected against neurodegeneration and decreased the levels of inclusions in a mouse model of Huntington’s disease (Ravikumar et al., 2004). Furthermore, Sarkar and colleagues (2007) demonstrated that the effects of rapamycin on autophagy substrates such as mutant huntingtin and α-synuclein were enhanced by the presence of trehalose.

Rapamycin protected against the toxic gain of function effects (reducing aggregation, inclusion formation and cell death) associated with P23H rod opsin expression and, importantly, this effect was enhanced by the presence of trehalose (Mendes and Cheetham 2008). Similar to kosmotropes and chaperone inducers, however, rapamycin did not promote mutant rod opsin processing or inhibit the dominantnegative effects.

36.3 Conclusion

These in vitro studies suggest that a pharmacological approach could be used to alleviate the gain of function and dominant-negative effects of misfolded mutant rod opsin (Fig. 36.1). This could be achieved by promoting the folding of the mutant rod opsin (retinoids; Fig. 36.1a), reducing protein aggregation (Fig. 36.1b)

Fig. 36.1 Schematic representation of a rod photoreceptor inner segment and potential pharmacological intervention in rhodopsin retinitis pigmentosa. Gain of function and dominant-negative effects of misfolded mutant rod opsin could be alleviated by pharmacological chaperones, such as 9-cis-retinal and 11-cis-retinal, which assist the folding of mutant protein (a). Kosmotropes, molecular chaperone inducers could alleviate the gain of function effects by reducing aggregation (b) and/or promoting proteasomal degradation (c). Rapamycin could reduce the toxic gain of function effects of mutant rod opsin by increasing autophagy (d)

and promoting degradation of aggregate prone species (Fig. 36.1c), or stimulating autophagy (Fig. 36.1d). It is now necessary to establish the effects of these compounds, or potential combination therapies based on targeting multiple pathways, in photoreceptors in vivo prior to the translation to the clinic.

Acknowledgments This work is supported by the British Retinitis Pigmentosa Society (BRPS), Fight for Sight, the Daphne Jackson Trust and National Institute for Health Research (NIHR).

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