Chronically shortened rod outer segments accompany photoreceptor cell death in Choroideremia

Area of Science:

• Ophthalmology
• Cell Biology
• Genetics

Background:

• X-linked choroideremia (CHM) causes progressive retinal degeneration due to loss of Rab Escort Protein 1 (REP1).
• Defects in Rab protein prenylation, crucial for membrane traffic, are central to CHM pathogenesis.
• The eye's high membrane traffic demand, particularly in photoreceptors and retinal pigment epithelium, may explain organ specificity.

Purpose of the Study:

• To investigate membrane trafficking defects contributing to photoreceptor cell death in choroideremia.
• To analyze cellular and molecular changes in a mouse model of CHM.

Main Methods:

• Utilized a heterozygous null female mouse model (Chmnull/WT) for CHM.
• Assessed photoreceptor degeneration using TUNEL staining, mitochondrial stress markers, and microglial infiltration.
• Examined photoreceptor morphology, outer segment length, rhodopsin density, and transport vesicle accumulation.

Main Results:

• Photoreceptor layer degeneration was observed, with increased TUNEL-positive cells, mitochondrial stress, and microglial infiltration.
• Despite cell death, rod photoreceptors largely maintained normal morphology and outer segments.
• A significant finding was the shortening of rod outer segments, evident early but not progressing during cell death, with decreased rhodopsin density.

Conclusions:

• Photoreceptor cell death in CHM is not primarily triggered by defects in rhodopsin transport or outer segment renewal.
• Outer segment shortening and reduced rhodopsin density are key features, but their role as primary triggers for cell death requires further investigation.
• The study suggests that the underlying mechanisms of photoreceptor cell death in CHM may differ from previously assumed membrane trafficking defects.