Chaperones and retinal disorders.

Defects in protein folding and trafficking are a common cause of photoreceptor degeneration, causing blindness. Photoreceptor cells present an unusual challenge to the protein folding and transport machinery due to the high rate of protein synthesis, trafficking and the renewal of the outer segment, a primary cilium that has been modified into a specialized light-sensing compartment. Phototransduction components, such as rhodopsin and cGMP-phosphodiesterase, and multimeric ciliary transport complexes, such as the BBSome, are hotspots for mutations that disrupt proteostasis and lead to the death of photoreceptors. In this chapter, we review recent studies that advance our understanding of the chaperone and transport machinery of phototransduction proteins.

Archaeal Unfoldase Counteracts Protein Misfolding Retinopathy in Mice.

Deregulation of cellular proteostasis due to the failure of the ubiquitin proteasome system to dispose of misfolded aggregation-prone proteins is a hallmark of various neurodegenerative diseases in humans. Microorganisms have evolved to survive massive protein misfolding and aggregation triggered by heat shock using their protein-unfolding ATPases (unfoldases) from the Hsp100 family. Because the Hsp100 chaperones are absent in homoeothermic mammals, we hypothesized that the vulnerability of mammalian neurons to misfolded proteins could be mitigated by expressing a xenogeneic unfoldase. To test this idea, we expressed proteasome-activating nucleotidase (PAN), a protein-unfolding ATPase from thermophilic Archaea, which is homologous to the 19S eukaryotic proteasome and similar to the Hsp100 family chaperones in rod photoreceptors of mice. We found that PAN had no obvious effect in healthy rods; however, it effectively counteracted protein-misfolding retinopathy in Gγ1 knock-out mice. We conclude that archaeal PAN can rescue a protein-misfolding neurodegenerative disease, likely by recognizing misfolded mammalian proteins.SIGNIFICANCE STATEMENT This study demonstrates successful therapeutic application of an archaeal molecular chaperone in an animal model of neurodegenerative disease. Introducing the archaeal protein-unfolding ATPase proteasome-activating nucleotidase (PAN) into the retinal photoreceptors of mice protected these neurons from the cytotoxic effect of misfolded proteins. We propose that xenogeneic protein-unfolding chaperones could be equally effective against other types of neurodegenerative diseases of protein-misfolding etiology.

Farnesylation of the Transducin G Protein Gamma Subunit Is a Prerequisite for Its Ciliary Targeting in Rod Photoreceptors.

Primary cilia are microtubule-based organelles, which protrude from the plasma membrane and receive a wide range of extracellular signals. Various cilia use G protein-coupled receptors (GPCRs) for the detection of these signals. For instance, vertebrate rod photoreceptors use their cilia (also called outer segments) as antennae detecting photons by GPCR rhodopsin. Rhodopsin recognizes incoming light and activates its G protein, transducin, which is composed of three subunits α, ß, and γ. Similar to all G protein γ subunits, the transducin Gγ1 subunit undergoes C-terminal prenylation resulting in the addition of an isoprenoid farnesyl; however, the significance of this posttranslational modification is unclear. To study the role of the farnesyl group, we genetically introduced a mutant Gγ1 that lacked the prenylation site into the retinal photoreceptors of mice. The biochemical and physiological analyses of these mice revealed that mutant Gγ1 dimerizes with the endogenous transducin Gß1 subunit and that the resulting Gßγ dimers display reduced hydrophobicity. Although mutant Gßγ dimers could form a heterotrimeric G protein, they could not mediate phototransduction. This deficiency was due to a strong exclusion of non-farnesylated Gßγ complexes from the cilia (rod outer segments). Our results provide the first evidence that farnesylation is required for trafficking of G-protein ßγ subunits to the cilium of rod photoreceptors.

Dephosphorylation by protein phosphatase 2A regulates visual pigment regeneration and the dark adaptation of mammalian photoreceptors.

Resetting of G-protein-coupled receptors (GPCRs) from their active state back to their biologically inert ground state is an integral part of GPCR signaling. This "on-off" GPCR cycle is regulated by reversible phosphorylation. Retinal rod and cone photoreceptors arguably represent the best-understood example of such GPCR signaling. Their visual pigments (opsins) are activated by light, transduce the signal, and are then inactivated by a GPCR kinase and arrestin. Although pigment inactivation by phosphorylation is well understood, the enzyme(s) responsible for pigment dephosphorylation and the functional significance of this reaction remain unknown. Here, we show that protein phosphatase 2A (PP2A) acts as opsin phosphatase in both rods and cones. Elimination of PP2A substantially slows pigment dephosphorylation, visual chromophore recycling, and ultimately photoreceptor dark adaptation. These findings demonstrate that visual pigment dephosphorylation regulates the dark adaptation of photoreceptors and provide insights into the role of this reaction in GPCR signaling.