Photoreceptor phagosome processing defects and disturbed autophagy in retinal pigment epithelium ofCln3^Δex1-6mice modelling juvenile neuronal ceroid lipofuscinosis (Batten disease)

Abstract

Retinal degeneration and visual impairment are the first signs of juvenile neuronal ceroid lipofuscinosis caused by CLN3 mutations, followed by inevitable progression to blindness. We investigated retinal degeneration in Cln3Δex1-6 null mice, revealing classic ‘fingerprint’ lysosomal storage in the retinal pigment epithelium (RPE), replicating the human disease. The lysosomes contain mitochondrial F0-ATP synthase subunit c along with undigested membranes, indicating a reduced degradative capacity. Mature autophagosomes and basal phagolysosomes, the terminal degradative compartments of autophagy and phagocytosis, are also increased in Cln3Δex1-6 RPE, reflecting disruption to these key pathways that underpin the daily phagocytic turnover of photoreceptor outer segments (POS) required for maintenance of vision. The accumulated autophagosomes have post-lysosome fusion morphology, with undigested internal contents visible, while accumulated phagosomes are frequently docked to cathepsin D-positive lysosomes, without mixing of phagosomal and lysosomal contents. This suggests lysosome-processing defects affect both autophagy and phagocytosis, supported by evidence that phagosomes induced in Cln3Δex1-6-derived mouse embryonic fibroblasts have visibly disorganized membranes, unprocessed internal vesicles and membrane contents, in addition to reduced LAMP1 membrane recruitment. We propose that defective lysosomes in Cln3Δex1-6 RPE have a reduced degradative capacity that impairs the final steps of the intimately connected autophagic and phagocytic pathways that are responsible for degradation of POS. A build-up of degradative organellar by-products and decreased recycling of cellular materials is likely to disrupt processes vital to maintenance of vision by the RPE.

Caspase 1 activity influences juvenile Batten disease (CLN3) pathogenesis

“…One of the most striking consequences of CLN3 mutation is lysosomal accumulation, primarily containing autofluorescent lipofuscin and ATP synthase subunit c (Wavre‐Shapton et al . ; Brenneman et al . ).…”

“…The composition of lysosomal inclusions in CLN3 disease is heterogeneous and includes autofluorescent lipofuscin, ATP synthase subunit c, cathepsin D, and tripeptidyl peptidase‐1 among others (Wavre‐Shapton et al . ; Brenneman et al . ).…”

Mitochondria, lysosomes, and dysfunction: their meaning in neurodegeneration

“…Recent findings clearly suggest a mitochondrial dysfunction also in neuronal ceroid lipofuscinosis pre‐clinical models and patients (Wavre‐Shapton et al . ; Uusi‐Rauva et al . ).…”

Cellular Characterization of OCT and Outer Retinal Bands Using Specific Immunohistochemistry Markers and Clinical Implications

“…The lack of an adequate phagocytosis function of the outer segments can lead to photoreceptor degeneration, as occurs with the MERTK mutation. The loss of phagosome activity, as described in MERTK, in the βA3/A1-crystallin gene, or in the CLN3 mutation among others, leads to the photoreceptors death or malfunction [49][50][51] . Disappearance of this 3 rd line has also been described after retinal detachment.…”