Cholesterol-mediated activation of acid sphingomyelinase disrupts autophagy in the retinal pigment epithelium

Non-polarized RPE respond to stereotypical autophagy inducers such as serum starvation, whereas well-polarized terminally differentiated primary RPE cultures do not.

“First, whereas non-polarized RPE respond to “stereotypical” autophagy inducers such as serum starvation, well-polarized terminally differentiated primary RPE cultures do not (Toops et al., 2015).”

In highly differentiated primary RPE monolayers, basal, constitutive autophagy is crucial for the clearance of long-lived proteins such as p62/sequestosome-1 (SQSTM1).

“In highly differentiated primary RPE monolayers, basal, constitutive autophagy is crucial for the clearance of long-lived proteins such as p62/sequestosome-1 (SQSTM1), a specific autophagy substrate (Toops et al., 2015).”

Polarized primary RPE do not upregulate autophagy in response to the non-specific mTOR inhibitor rapamycin, but respond to the specific mTORC1 inhibitor Torin-1.

“Second, polarized primary RPE do not upregulate autophagy in response to the non-specific mTOR inhibitor rapamycin, but respond to the specific mTORC1 inhibitor Torin-1 (Toops et al., 2015).”

Dynamic microtubules (marked by tyrosination) are required for efficient transport of autophagosomes, whereas stable microtubules (marked by acetylation) hinder autophagosome trafficking.

“Dynamic microtubules (marked by tyrosination) are required for efficient transport of autophagosomes, whereas stable microtubules (marked by acetylation) hinder autophagosome trafficking (Toops et al., 2015; Mohan et al., 2019).”

In Abca4−/− RPE, both autophagosome biogenesis and autophagic flux are inhibited.

“In Abca4−/− RPE, both autophagosome biogenesis and autophagic flux are inhibited (Toops et al., 2015); however, in Batten, Bietti’s, and SLO diseases, autophagic defects manifest as an accumulation of LC3-positive autophagosomes and increased p62, suggestive of a block in autophagosome-lysosome fusion.”

Autophagic abnormalities in the RPE have been documented in several rodent and iPS-RPE models of inherited retinal degenerations, including Abca4−/− Stargardt disease, Cln3Δex1−6 Batten disease, iPS-RPE Bietti's crystalline dystrophy, and Smith-Lemli-Opitz syndrome.

“Autophagic abnormalities in the RPE have also been documented in several rodent and iPS-RPE models of inherited retinal degenerations. These include the pigmented Abca4−/− mouse model of Stargardt disease (Toops et al., 2015), the Cln3Δex1−6 mouse model of Batten disease or juvenile neuronal ceroid lipofuscinosis (Wavre-Shapton et al., 2015), and iPS-RPE models of Bietti’s crystalline dystrophy (H”

In Abca4−/− mice, lipofuscin bisretinoids such as A2E trap cholesterol and the anionic lipid bis-monoacylglycerophosphate (BMP) within RPE lysosomes.

“In Abca4−/− mice, lipofuscin bisretinoids such as A2E trap cholesterol and the anionic lipid bis-monoacylglycerophosphate (BMP) within RPE lysosomes (Lakkaraju et al., 2007; Toops et al., 2015).”

In Abca4−/− RPE cultures, defective microtubule-based autophagosome transport is a causal mechanism for impaired autophagy in the RPE.

“In Abca4−/− RPE cultures, highspeed live-cell imaging has helped identify defective microtubule-based autophagosome transport as a causal mechanism for impaired autophagy in the RPE (Toops et al., 2015) (Fig. 13).”

Sequential interactions with early and late endosomes and lysosomes are essential for the maturation and degradation of phagosomes and autophagosomes.

“Sequential interactions with early and late endosomes, and lysosomes are essential for the maturation and degradation of phagosomes and autophagosomes (Wavre-Shapton et al., 2014; Toops et al., 2015).”

Abca4−/− Stargardt disease RPE exhibit a block in autophagosome biogenesis as a result of chronic mTOR activation driven by intracellular C3 cleavage.

“In contrast to these three diseases, which are characterized by an accumulation of autophagosomes, Abca4−/− Stargardt disease RPE exhibit a block in autophagosome biogenesis (Toops et al., 2015) as a result of chronic mTOR activation driven by intracellular C3 cleavage (Kaur et al., 2018) (Fig. 13).”

Cholesterol-induced BMP accumulation is a characteristic feature of Abca4−/− RPE.

“This cholesterol-induced BMP accumulation is a characteristic feature of Abca4−/− RPE, first reported by immunostaining (Toops et al., 2015) and later confirmed by imaging mass spectrometry (Anderson et al., 2017).”

Photooxidative processes initiated by PR-derived bisretinoids likely contribute to HNE-modifications and trapping of cholesterol within RPE, potentially altering autophagic processes.

“The photooxidative processes initiated by PR derived bisretinoids likely also contribute to HNE-modifications (Zhou et al., 2005) and trapping of cholesterol within RPE, potentially altering autophagic processes (Toops et al., 2015).”