Lysosomes in retinal health and disease

Abstract

Lysosomes play crucial roles in various cellular processes - including endocytosis, phagocytosis, and autophagy - which are vital for maintaining retinal health. Moreover, these organelles serve as environmental sensors and act as central hubs for multiple signaling pathways. Through communication with other cellular components, such as mitochondria, lysosomes orchestrate the cytoprotective response essential for preserving cellular homeostasis. This coordination is particularly critical in the retina, given its high metabolic rate and susceptibility to photo-oxidative stress. Consequently, impaired lysosomal function and dysregulated communication between lysosomes and other organelles contribute significantly to the pathobiology of major retinal degenerative diseases. This review explores the pivotal role of lysosomes in retinal cells and their involvement in retinal degenerative diseases.

Keywords: age-related macular degeneration; glaucoma; lysosome membrane permeabilization; lysosome–mitochondria crosstalk; mTOR signaling; retinitis pigmentosa.

Figure 1:. Pathways in retinal physiology that converge at the lysosome.

A. Lysosomes degrade photoreceptor outer segments (phagocytosis) as well as extracellular material (heterophagy), such as shed photoreceptor outer segments during LC3-associated phagocytosis, also known as LAP. This process also allows the recycling of visual pigments. B. Lysosomes degrade and recycle intracellular components in macroautophagy. In addition, lysosomes participate in selective autophagy of mitochondria during mitophagy and in chaperone mediated autophagy. In this pathway, proteins that harbor a KFERQ aminoacidic domain are recognized by chaperones, including HSC70, which delivers the protein to the lysosomal membrane receptor LAMP2A. The oligomerization of this receptor allows the unfolding, translocation to the lysosomal lumen, and degradation of the protein. C. Lysosomes are important for secretion and exocytosis, participating in plasma membrane repair and the release of intraluminal vesicles and exosomes. D. Lysosomes participate in cell death occurring after lysosomal membrane permeabilization (LMP). After damage to the lysosomal membrane, cathepsins and other proteases are released to the cytoplasm and trigger the degradation of intracellular components. In addition, reactive oxygen species (ROS) are produced, which trigger mitochondrial membrane permeabilization (MMP) and cell death. Moreover, cathepsin activity in the cytoplasm induces MMP, which magnifies ROS levels and exacerbates cell death. Cathepsin inhibitors, as well as expression of endogenous protease inhibitors, such as cystatins, can inhibit lysosomal cell death. Abbreviations: LAMP2A, lysosomal associated membrane protein 2 type A; LAP, LC3-associated phagocytosis; LMP, lysosomal membrane permeabilization; MMP, mitochondrial membrane permeabilization; ROS, reactive oxygen species. Note that organelles are not depicted at their true relative size.

Figure 2:. Oxidative stress evokes misfolding of proteins and mitochondrial damage in RPE cells.

Aggregated, misfolded proteins and damaged mitochondria should be degraded via autophagy regulated by ubiquitin (Ub), sequestosome 1 (SQTM1/p62) and microtubule associated protein 1 light chain (MAP1LC3/LC3). In AMD, LC3-associated phagocytosis (LAP) and autophagy are disturbed due to lipofuscin accumulation. This may lead to cargo delivery via secretory autophagy regulated by interleukin 1-beta (IL-1β), tripartite motif containing 16 (TRIM16), soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), SEC22 homolog B, vesicle trafficking protein (SEC22B), synaptosome associated proteins (SNAPs) and syntaxins (STXs).

Figure 3:. βA3/A1-crystallin coordinates lysosomal signaling: impact on mTORC1 and mTORC2 activation and cellular functions.

βA3/A1-crystallin, an essential lysosomal luminal protein, plays a pivotal role in regulating cellular processes by interacting with key lysosomal components, including vacuolar-type ATPase (vATPase) and Solute Carrier Family 36 Member 4 (SLC36A4/PAT4). vATPase maintains the acidic pH within lysosomes, while PAT4, positioned on the lysosomal membrane, modulates amino acid flux into and out of the lysosomes. Loss of Cryba1, the gene encoding βA3/A1-crystallin, specifically in the retinal pigment epithelium (RPE), triggers a cascade of events [12,24,72]. This loss leads to dysregulation of vATPase activity and PAT4 function, resulting in an increase in lysosomal pH and elevated cytosolic amino acid levels. These physiological changes lead to the hyperactivation of mTOR complex 1 (mTORC1), which subsequently induces the activation of mTOR complex 2 (mTORC2), possibly through upregulation of mammalian lethal with SEC13 protein 8 (mLST8), a component shared by both mTORC1 and mTORC2. Activation of mTORC1 and mTORC2 has well-established consequences on various cellular functions. mTORC1 hyperactivity influences TFEB-mediated lysosomal function and autophagy, protein synthesis, and metabolism. In contrast, mTORC2 activation impacts cytoskeletal organization and cellular differentiation.

Figure 4:. Regulation of lysosomal function by mTOR signaling in RPE cells.

The intricate regulatory network governing lysosomal function in the RPE is regulated by mTOR signaling. mTOR exists in two major complexes, mTORC1 and mTORC2, and is activated by diverse stimuli such as increased amino acid levels, v-ATPase activity, growth factors, and kinases like AKT1/2 and GSK3β. The schematic highlights how mTOR tightly controls TFEB nuclear activity; activated mTORC1 phosphorylates TFEB, keeping it in the cytosol, while under conditions like starvation, inactivated mTORC1 allows TFEB to translocate into the nucleus. Nuclear TFEB, in turn, activates CLEAR genes critical for lysosomal function and autophagy, and vital for RPE cell function. In the RPE, mTORC1 is negatively regulated by TSC1, SOCS2, and βA3/A1-crystallin. Loss of these proteins leads to mTORC1 hyperactivation and the development of atrophic AMD-like phenotypes. βA3/A1-crystallin localizes to the apical RPE, where it binds to PITP-β to mediate PIP metabolism and thereby maintain RPE polarity. The figure also presents the downstream consequences of AKT2 activation in Cryba1 cKO RPE cells, triggering NFκB-dependent inflammation, elevated LCN-2 and IFNλ, microglial and neutrophil infiltration into the subretinal space (SRS) and inflammasome activation. Figure created using BioRender (https://biorender.com/).

Figure 5:. Lysosome-mitochondria interaction.

Cellular stress induces a protective response by both lysosomes and mitochondria that is coordinated by cross-talk. With mild and localized mitochondrial injury, multivesicular vesicles (MDVs) are formed in damaged regions through PINK1, PARKIN, and oxidized cardiolipin. These fuse with lysosomes for degradation or enter the endosomal system for packaging into extracellular vesicles (EVs). With more severe mitochondrial injury and mitochondrial membrane potential (ΔΨm) decline, fission and then PINK1 mediated mitophagy is initiated to remove damaged regions of the mitochondrial network. Mitochondrial stress through AMPK, signals a protective response to lysosomes through MCOLN1 and Ca²⁺ to induce the dephosphorylation of TFEB/TFE3 that enables nuclear translocation, CLEAR gene expression, and lysosomal biogenesis. Lysosomes also influence mitochondria. Healthy lysosomes can induce mitochondrial biogenesis through sphingosine-1-phosphate (S1P) signaling that activates the transcription factors KLF2 and ETV1 to induce the production of nuclear encoded electron transport chain mitochondrial proteins (ETC). With lysosomal injury, AMPK can induce the dephosphorylation of TFEB/TFE3 to enable nuclear translocation, CLEAR network gene expression and lysosomal biogenesis. In addition, AMPK can induce TFEB/TFE3 or PGC1α to induce mitochondrial biogenesis. Figure created using BioRender (https://biorender.com/).