Heterologous expression of WT and mutant photoreceptor peripherin/rds in Madin Darby canine kidney cells: an assessment of fusogenic function

PRPH2/RDS and ROM-1: Historical context, current views and future considerations

Peripherin 2 (PRPH2), also known as RDS (retinal degeneration slow) is a photoreceptor specific glycoprotein which is essential for normal photoreceptor health and vision. PRPH2/RDS is necessary for the proper formation of both rod and cone photoreceptor outer segments, the organelle specialized for visual transduction. When PRPH2/RDS is defective or absent, outer segments become disorganized or fail to form entirely and the photoreceptors subsequently degenerate. Multiple PRPH2/RDS disease-causing mutations have been found in humans, and they are associated with various blinding diseases of the retina such as macular degeneration and retinitis pigmentosa, the vast majority of which are inherited dominantly, though recessive LCA and digenic RP have also been associated with RDS mutations. Since its initial discovery, the scientific community has dedicated a considerable amount of effort to understanding the molecular function and disease mechanisms of PRPH2/RDS. This work has led to an understanding of how the PRPH2/RDS molecule assembles into complexes and functions as a necessary part of the machinery that forms new outer segment discs, as well as leading to fundamental discoveries about the mechanisms that underlie OS biogenesis. Here we discuss PRPH2/RDS-associated research and how experimental results have driven the understanding of the PRPH2/RDS protein and its role in human disease.

An unconventional secretory pathway mediates the cilia targeting of peripherin/rds

It is unclear how unconventional secretion interplays with conventional secretion for the normal maintenance and renewal of membrane structures. The photoreceptor sensory cilium is recognized for fast membrane renewal, for which rhodopsin and peripherin/rds (P/rds) play critical roles. Here, we provide evidence that P/rds is targeted to the cilia by an unconventional secretion pathway. When expressed in ciliated hTERT-RPE1 human cell line, P/rd is localized to cilia. Cilium trafficking of P/rds was sustained even when the Golgi functions, including trans-Golgi-mediated conventional secretion, were inhibited by the small molecules brefeldin A, 30N12, and monensin. The unconventional cilia targeting of P/rds is dependent on COPII-mediated exit from the ER, but appears to be independent of GRASP55-mediated secretion. The regions in the C-terminal tail of P/rds are essential for this unconventional trafficking. In the absence of the region required for cilia targeting, P/rds was prohibited from entering the secretory pathways and was retained in the Golgi apparatus. A region essential for this Golgi retention was also found in the C-terminal tail of P/rds and supported the cilia targeting of P/rds mediated by unconventional secretion. In ciliated cells, including bovine and Xenopus laevis rod photoreceptors, P/rds was robustly sensitive to endoglycosidase H, which is consistent with its bypassing the medial Golgi and traversing the unconventional secretory pathway. Because rhodopsin is known to traffic through conventional secretion, this study of P/rds suggests that both conventional secretion and unconventional secretion need to cooperate for the renewal of the photoreceptor sensory cilium.

The tetraspanin protein peripherin-2 forms a complex with melanoregulin, a putative membrane fusion regulator

Peripherin-2, the product of the rds gene, is a tetraspanin protein. In this study, we show that peripherin-2 forms a complex with melanoregulin (MREG), the product of the Mreg locus. Genetic studies suggest that MREG is involved in organelle biogenesis. In this study, we explore the role of this protein in processes associated with the formation of disk membranes, specialized organelles of photoreceptor rod cells. MREG antibodies were generated and found to be immunoreactive with a 28 kDa protein in retinal extracts, bovine OS, ARPE-19 cells, and rat RPE. MREG colocalized with peripherin-2 in WT (CB6F1/J) and in rds+/- retinas. Western blots of serial tangential sections confirmed the close association of these two proteins within the IS and basal outer segment of rods. Immunoprecipitation (IP) of OS extracts showed formation of a complex between MREG and peripherin-2-ROM-1 hetero-oligomers. This interaction was confirmed with pulldown analyses in which the GST-PerCter protein selectively pulled down His-MREG and His-MREG selectively pulled down PerCter. Biacore analysis using peptide inhibitors and per-2 truncation mutant studies allowed us to map the MREG binding site on per-2 to the last five residues of the C-terminus (Gln341-Gly346), and kinetic data predicted a KD of 80 nM for PerCter-MREG binding. Finally, the effect of MREG on photoreceptor specific membrane fusion was assayed using a disk-plasma membrane cell free assay. Preincubation of target membranes with MREG resulted in a dose-dependent inhibition of fusion with an IC50 in the submicromolar range. Collectively, these results suggest that this newly identified protein regulates peripherin-2 function.

A novel tetraspanin fusion protein, peripherin-2, requires a region upstream of the fusion domain for activity

Peripherin-2 (also known as peripherin/rds), a photoreceptor specific tetraspanin protein, is required to maintain normal cell structure through its role in renewal processes requiring membrane fusion. It is the first tetraspanin fusogen and has been shown to directly mediate fusion between disk membranes and opposing membranes to maintain the highly ordered structure of rod outer segments. Localized to the C terminus of human, bovine, and murine peripherin-2 is an amphiphilic fusion peptide domain (residues 312-326) and a highly conserved region upstream of this domain that we hypothesize is essential for fusogenic function. Our previous studies indicated that substitution of a threonine for a proline at position 296 within this highly conserved region enhanced fusion activity. In this study we wanted to determine whether this proline is essential with the introduction of three additional substitutions of proline with alanine, leucine, and glutamic acid. Wild type, P296T, P296A, P296L, and P296E mutants of peripherin-2 were expressed as His6-tagged full-length proteins in Madin-Darby canine kidney (MDCK) cells. All of the proteins were localized to intracellular membranes and detected as 42-kDa monomers by Western blot analysis. The wild type, P296A, and P296L assembled into core tetramers; in contrast the P296T and P296E formed higher order oligomers. Fusogenic activity of full-length protein expressed in MDCK membranes and purified protein reconstituted in model membrane liposomes was determined using fluorescence quenching techniques. Fusion activity was decreased in the P296L, P296A, and P296E mutants both in endogenous MDCK membranes and in model liposomes. Collectively, these results suggest that the proline at position 296 is necessary for optimal function.

The C terminus of peripherin/rds participates in rod outer segment targeting and alignment of disk incisures

Protein targeting is essential for domain specialization in polarized cells. In photoreceptors, three distinct membrane domains exist in the outer segment: plasma membrane, disk lamella, and disk rim. Peripherin/retinal degeneration slow (rds) and rom-1 are photoreceptor-specific members of the transmembrane 4 superfamily of transmembrane proteins, which participate in disk morphogenesis and localize to rod outer segment (ROS) disk rims. We examined the role of their C termini in targeting by generating transgenic Xenopus laevis expressing green fluorescent protein (GFP) fusion proteins. A GFP fusion containing residues 317-336 of peripherin/rds localized uniformly to disk membranes. A longer fusion (residues 307-346) also localized to the ROS but exhibited higher affinity for disk rims than disk lamella. In contrast, the rom-1 C terminus did not promote ROS localization. The GFP-peripherin/rds fusion proteins did not immunoprecipitate with peripherin/rds or rom-1, suggesting this region does not form intermolecular interactions and is not involved in subunit assembly. Presence of GFP-peripherin/rds fusions correlated with disrupted incisures, disordered ROS tips, and membrane whorls. These abnormalities may reflect competition of the fusion proteins for other proteins that interact with peripherin/rds. This work describes novel roles for the C terminus of peripherin/rds in targeting and maintaining ROS structure and its potential involvement in inherited retinal degenerations.

A soluble peripherin/Rds C-terminal polypeptide promotes membrane fusion and changes conformation upon membrane association

Photoreceptor rod cells contain a unique tetraspanin fusion protein known as peripherin/rds. This protein is important in membrane fusion events hypothesized to be essential to disk membrane morphogenesis and disk shedding. In vivo and in vitro fusogenic activity has been mapped to the C-terminal domain of peripherin/rds. Moreover, a fusion peptide domain localized to a 15 amino acid long region (residues 311-325) is essential for mediating lipid bilayer fusion of model membranes. To address the functional and structural properties required for peripherin/rds dependent membrane fusion, constructs of the entire C-terminal domain (residues 284-346) were generated and polypeptides expressed. A wild type-peripherin/rds C-terminal GST fusion construct that included the entire C-terminus (PERCTER) or a C-terminal truncation mutant (PERCTN) were engineered with a thrombin cleavage site. Protein expression was induced in E. coli with IPTG, expressed proteins cleaved from the GST with thrombin and purified to homogeneity on a Superdex 75 column. Purity was confirmed by SDS-PAGE and Western blot analysis. The purified wt C-terminal protein resolved as a monomer under reducing conditions on SDS-PAGE (15%) and was immunoreactive with anti peripherin/rds antibody 2B6 (gift from Dr R. Molday). The purified polypeptide promoted the requisite steps of fusion, membrane destabilization, lipid mixing and aqueous contents mixing. Conversely, the truncation mutant lacking a portion of the fusion domain was unable to promote these steps. A common feature of most membrane fusion proteins is a change in conformation upon membrane association. Structural changes in the C-terminal polypeptide were investigated using far UV CD. The far UV CD spectra of the purified C-terminal polypeptide indicated substantial alpha-helical content in the wt peptide in isotonic aqueous buffer. An increase in intensity of 208 and 222 nm CD bands upon addition of DPC vesicles indicated an increase in alpha-helical content of the polypeptide. These results demonstrate that a purified soluble form of the C-terminus of peripherin/rds can interact with biological phospholipids; moreover, this interaction promotes a conformational change that is most consistent with an increase in alpha-helical content.

Deletional analysis of the rod photoreceptor cell peripherin/RDS carboxy-terminal region

The C-terminal region of peripherin/rds contains three predicted alpha-helical domains. One of these domains, corresponding to amino acids 311-322, form an amphiphilic alpha-helix previously shown to promote membrane fusion. The present studies were conducted to determine how the additional alpha-helical regions of the peripherin/rds C-terminus affect complex formation with rom-1, glycosylation, intracellular localization and membrane fusion properties. Bovine peripherin/rds and rom-1 were epitope tagged with an amino-terminal FLAG-tag or amino-terminal hemagglutinin (HA)-tag, respectively, and cloned into the pCI-neo expression vector for transient transfection into COS cells. Similarly, four C-terminal peripherin/rds truncation mutants (Delta1, Delta2, Delta3 and Delta4), corresponding to deletions of -19, -29, -39 and -59 amino acids were designed to disrupt the alpha-helical domains. Immunofluorescence microscopy and enzymatic digestions demonstrated that full-length peripherin/rds and the four C-terminal deletion mutants were localized to intracellular membranes and were all Endo-H sensitive. Western blotting and immunoprecipitation studies showed that the FLAG-tagged bovine peripherin/rds (full-length) was expressed as a 76kDa dimer, which associates with HA-tagged rom-1 to form a higher order complex. The deletion mutants were also able to associate with rom-1. However, when analyzed using non-denaturing tricine electrophoresis, full-length peripherin/rds and the Delta1, Delta2 and Delta3 mutants formed homo-oligomeric complexes, while the Delta4 mutant appeared to form only homodimers suggesting a region upstream of amino acid 300 may be involved in C-terminal interactions. Membrane fusion was then evaluated using fluorescence resonance energy transfer (RET) techniques. Intracellular COS cell membranes containing full-length peripherin/rds fused with rod outer segment plasma membrane vesicles. This fusion was inhibited with the addition of a synthetic peptide (PP-5) corresponding to the fusion domain of peripherin/rds. In contrast, fusion was negligible with any of the C-terminal truncation mutants. Collectively, these results suggest that in addition to the fusion domain, other regions of the peripherin/rds C-terminus are required for fusion. Most interesting is the observation that the last 19amino acids, a region downstream of the fusion peptide that is deleted in the Delta1 mutant, appear to be necessary for fusion. This region corresponds to the epitope for anti-peripherin/rds monoclonal antibody 2B6, which is shown to partially inhibit peripherin/rds mediated membrane fusion.

Photoreceptor renewal: a role for peripherin/rds

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transported, and incorporated into newly forming outer segment membranes, a compensatory shedding of older membranous material occurs, thereby maintaining the segment at a constant length. These processes are collectively referred to as ROS (rod outer segment) or COS (cone outer segment) renewal. We review the cellular and molecular events responsible for these renewal processes and present the recent but compelling evidence, drawn from molecular genetic, biochemical, and biophysical approaches, pointing to an essential role for a unique tetraspanning membrane protein, called peripherin/rds, in the processes of disk morphogenesis.