Cone outer segment extracellular matrix as binding domain for interphotoreceptor retinoid-binding protein

Area of Science:

• Ophthalmology
• Cell Biology
• Biochemistry

Background:

• Cones require interphotoreceptor retinoid-binding protein (IRBP) for retinoid delivery essential to the visual cycle.
• Cone-dominant vertebrates provide a model to study IRBP's function in photoreceptor matrix interactions.

Purpose of the Study:

• To investigate the association of IRBP with the interphotoreceptor matrix (IPM) in cone-dominant retinas.
• To determine if IRBP localization differs between cone and rod photoreceptor matrices.

Main Methods:

• Immunohistochemistry and confocal microscopy were used to visualize IRBP and specific glycans (PNA, WGA) in retinas from chicken, turkey, and pig.
• Retinas were processed with and without saline washes, and isolated photoreceptors with adherent matrix were prepared.
• Matrix components were analyzed using CaCl(2) delamination and colocalization studies.

Main Results:

• IRBP was initially observed in a diffuse, lattice-like pattern throughout the entire IPM.
• Saline washing resulted in IRBP forming distinct fluorescent annuli around cone outer segments.
• IRBP colocalized with peanut agglutinin (PNA)-binding glycans, indicating association with the cone-specific matrix.

Conclusions:

• IRBP exhibits a wash-resistant association with a matrix sheath surrounding cone outer segments.
• This cone matrix sheath is likely involved in the targeted delivery of 11-cis retinoids to cones via IRBP.