Retinal degeneration in tulp1-/- mice: vesicular accumulation in the interphotoreceptor matrix

The Tulp1 gene is a member of the tubby gene family with unknown function. Mutations in the human TULP1 gene cause autosomal recessive retinitis pigmentosa. To understand the pathogenic mechanism associated with TULP1 mutations and to explore the physiologic function of this protein, we examined tissue distribution of the Tulp1 protein in normal mice and the photoreceptor disease phenotype in Tulp1-ablated mice.

Tissue distribution of the Tulp1 protein in normal mice was examined by immunoblotting and immunocytochemistry. The disease phenotype in tulp1-/- mice was studied by light and electron microscopy, electroretinography (ERG), and immunocytochemistry. These results were compared with another mouse model of retinal degeneration carrying a rhodopsin mutation.

Tulp1 is found exclusively in photoreceptors, localizing predominantly in the inner segments. It is a soluble protein with an apparent molecular weight of approximately 70 kDa. Photoreceptor degeneration developed in tulp1-/- mice, with early involvement of both rods and cones. At the early stage of degeneration, rod and cone opsins, but not peripherin/RDS, exhibited prominent ectopic localization. Electron microscopy revealed massive accumulation of extracellular vesicles surrounding the distal inner segments.

The function of Tulp1 is required to maintain viability of rod and cone photoreceptors. Extracellular vesicular accumulation is not a common phenomenon associated with photoreceptor degeneration but appears to be a distinct ultrastructural feature shared by a small group of retinal disease models. The defect in tulp1-/- mice may be consistent with a loss of polarized transport of nascent opsin to the outer segments.