Molecular Mechanism Of Rhodopsin Homoeostasis In Drosophila

Rhodopsins(Rhs)are a type of G-protein-coupled receptors that function as light-sensors in photoreceptors and are essential for the visual transduction.They are directly activated through photon absorption,and initiate subsequent phototransduction cascades,transferring light signals to electronic signals.Homoeostatic regulation of Rh proteins is critical for the maintenance of light sensitivity and survival of photoreceptors.In humans,mutations or abnormal metabolism of Rhs cause retinitis pigmentosa,a degenerative disease that ultimately results in blindness.However,the molecular mechanism of Rh homoeostatic regulation is not fully understood.Rhl,the major rhodopsin in Drosophila is evolutionarily conserved with human Rh.Here,we use Drosophila visual system as a model system to investigate the molecular mechanism of Rh1 homoeostasis and physiological consequence of Rhl dysregulation,employing combinational approaches of genetics,biochemistry,molecular biology and electrophysiology.Rhl in Drosophila is encoded by ninaE gene.We find that under normal conditions,the mRNA level of ninaE maintains high in adult photoreceptors.Considering the turnover rate of Rhl protein is slow and the protein level of Rhl remains constant under conditions of 24h constant darkness or 12 h light/dark cycles,we suspect that Rh1 mRNA translation is normally repressed in adult flies.Our investigation reveal that the fragile X mental retardation protein(Fmr1)associates with ninaE mRNAs in adult flies,and loss of Fmrl up-regulates the protein level of Rhl.However specific overexpress of Fmrl in adult photoreceptors significantly reduces Rhl translation.Therefore Fmrl suppresses ninaE mRNA translation.We further demonstrate that this suppression is dependent on the phosphorylation statuts of Fmrl.Upon activation of the phototransduction cascade by light exposure,Ca2+ influx triggers light-induced Fmr1 dephosphorylation.The regulatory subunit B of PP2A(CKa)sensors the increase of Ca2+ and activate the catalytic subunit C of PP2A,which directly dephosphorylates Fmrl and initiates Rhl translation.Blocking light-induced Rhl translation results in reduced Rhl proteins and impaired light sensitivity,while excessive light exposure lead to an accumulation of Rhl in the cytoplasm of photoreceptors and triggers Rhl degradation,which eventually leads to the downregulation of Rh1 and retinal degeneration.Taken together,we provide strong evidence to support that the light-induced phosphorylation/dephosphorylation cycle of Fmrl bidirectionally controls n.inaE mRNA translation,and present a new mechanism for rhodopsin homoeostatic regulation.Our studies will shad a light on the pathogenesis of human retinitis pigmentosa.