Ocular manifestation of storage diseases

Lysosomes as coordinators of cellular catabolism, metabolic signalling and organ physiology

Every cell must satisfy basic requirements for nutrient sensing, utilization and recycling through macromolecular breakdown to coordinate programmes for growth, repair and stress adaptation. The lysosome orchestrates these key functions through the synchronised interplay between hydrolytic enzymes, nutrient transporters and signalling factors, which together enable metabolic coordination with other organelles and regulation of specific gene expression programmes. In this Review, we discuss recent findings on lysosome-dependent signalling pathways, focusing on how the lysosome senses nutrient availability through its physical and functional association with mechanistic target of rapamycin complex 1 (mTORC1) and how, in response, the microphthalmia/transcription factor E (MiT/TFE) transcription factors exert feedback regulation on lysosome biogenesis. We also highlight the emerging interactions of lysosomes with other organelles, which contribute to cellular homeostasis. Lastly, we discuss how lysosome dysfunction contributes to diverse disease pathologies and how inherited mutations that compromise lysosomal hydrolysis, transport or signalling components lead to multi-organ disorders with severe metabolic and neurological impact. A deeper comprehension of lysosomal composition and function, at both the cellular and organismal level, may uncover fundamental insights into human physiology and disease.

Through the Smoke: An In-Depth Review on Cigarette Smoking and Its Impact on Ocular Health

Smoking is a widespread and pervasive habit, impacting health across various care settings, including acute care, subacute care, home-based care, and long-term care. Smoking is a serious global public health concern that has been related to many chronic diseases. However, the effect of smoking on eye disorders has been less studied. Cigarette smoke contains a complex mixture of harmful constituents, including nicotine and toxic chemicals, which permeate the bloodstream, affecting ocular tissues. The oxidative stress and inflammation induced by smoking are central to its detrimental effects on ocular health. Age-related macular degeneration (AMD), a leading cause of vision loss, exhibits a strong association with smoking. Research consistently demonstrates that smokers face a heightened risk of both early and advanced AMD. Cataracts, another prevalent ocular condition, develop earlier and progress more rapidly in smokers. The oxidative stress on the lens and reduced antioxidants among smokers contribute to the increased severity of cataracts. Moreover, the health of the eyes may be compromised by smoking-related chemicals that reduce blood flow and/or hasten thrombus formation in ocular capillaries thus increasing the chance of acquiring glaucoma, cataracts, AMD, and Graves' eye disease. Beyond individual health concerns, the societal implications of smoking on ocular health are substantial, including increased healthcare costs and diminished quality of life for affected individuals. Understanding the underlying mechanisms can provide insights into potential therapeutic interventions for preventing and managing smoking-related ocular damage. Given the global prevalence of smoking, raising awareness about the ocular risks associated with smoking is crucial for promoting eye health. The review underscores the urgent need for comprehensive anti-smoking initiatives and smoking cessation programs to alleviate the burden of ocular diseases associated with smoking.

Autophagy in the retinal pigment epithelium: a new vision and future challenges

The retinal pigment epithelium (RPE) is a highly specialized monolayer of polarized, pigmented epithelial cells that resides between the vessels of the choriocapillaris and the neural retina. The RPE is essential for the maintenance and survival of overlying light-sensitive photoreceptors, as it participates in the formation of the outer blood-retinal barrier, phagocytosis, degradation of photoreceptor outer segment (POS) tips, maintenance of the retinoid cycle, and protection against light and oxidative stress. Autophagy is an evolutionarily conserved 'self-eating' process, designed to maintain cellular homeostasis. The daily autophagy demands in the RPE require precise gene regulation for the digestion and recycling of intracellular and POS components in lysosomes in response to light and stress conditions. In this review, we discuss selective autophagy and focus on the recent advances in our understanding of the mechanism of cell clearance in the RPE for visual function. Understanding how this catabolic process is regulated by both transcriptional and post-transcriptional mechanisms in the RPE will promote the recognition of pathological pathways in genetic disease and shed light on potential therapeutic strategies to treat visual impairments in patients with retinal disorders associated with lysosomal dysfunction.

The physiological and pathophysiological roles of the autophagy lysosomal system in the conventional aqueous humor outflow pathway: More than cellular clean up

During the last few years, the autophagy lysosomal system is emerging as a central cellular pathway with roles in survival, acting as a housekeeper and stress response mechanism. Studies by our and other labs suggest that autophagy might play an essential role in maintaining aqueous humor outflow homeostasis, and that malfunction of autophagy in outflow pathway cells might predispose to ocular hypertension and glaucoma pathogenesis. In this review, we will collect the current knowledge and discuss the molecular mechanisms by which autophagy does or might regulate normal outflow pathway tissue function, and its response to different types of stressors (oxidative stress and mechanical stress). We will also discuss novel roles of autophagy and lysosomal enzymes in modulation of TGFβ signaling and ECM remodeling, and the link between dysregulated autophagy and cellular senescence. We will examine what we have learnt, using pre-clinical animal models about how dysregulated autophagy can contribute to disease and apply that to the current status of autophagy in human glaucoma. Finally, we will consider and discuss the challenges and the potential of autophagy as a therapeutic target for the treatment of ocular hypertension and glaucoma.

Eye involvement in inherited metabolic disorders

Inherited metabolic disorders are a large group of rare disorders affecting normal biochemical pathways. Many metabolic disorders can present with symptoms affecting the eye, and eye disorders can evolve later in the natural history of an already diagnosed metabolic disorder. The ophthalmic involvement can be very varied affecting any part of the eye, including abnormalities of cornea, lens dislocation and cataracts, retina and the distal optic pathway, and extraocular muscles. Awareness of inherited metabolic disorders is important to facilitate early diagnosis and in some cases instigate early treatment if a patient presents with eye involvement suggestive of a metabolic disorder. Ophthalmological interventions are also an important component of the multisystem holistic approach to treating patients with metabolic disorders.

Vision loss in juvenile neuronal ceroid lipofuscinosis (CLN3 disease)

Juvenile neuronal ceroid lipofuscinosis (JNCL; also known as CLN3 disease) is a devastating neurodegenerative lysosomal storage disorder and the most common form of Batten disease. Progressive visual and neurological symptoms lead to mortality in patients by the third decade. Although ceroid-lipofuscinosis, neuronal 3 (CLN3) has been identified as the sole disease gene, the biochemical and cellular bases of JNCL and the functions of CLN3 are yet to be fully understood. As severe ocular pathologies manifest early in disease progression, the retina is an ideal tissue to study in the efforts to unravel disease etiology and design therapeutics. There are significant discrepancies in the ocular phenotypes between human JNCL and existing murine models, impeding investigations on the sequence of events occurring during the progression of vision impairment. This review focuses on current understanding of vision loss in JNCL and discusses future research directions toward molecular dissection of the pathogenesis of the disease and associated vision problems in order to ultimately improve the quality of patient life and cure the disease.

The autophagic lysosomal system in outflow pathway physiology and pathophysiology

Malfunction of the trabecular meshwork (TM)/schlemm's canal (SC) conventional outflow pathway is associated with elevated intraocular pressure (IOP) and, therefore, increased risk of developing glaucoma, a potentially blinding disease affecting more than 70 million people worldwide. This TM/SC tissue is subjected to different types of stress, including mechanical, oxidative, and phagocytic stress. Long-term exposure to these stresses is believed to lead to a progressive accumulation of damaged cellular and tissue structures causing permanent alterations in the tissue physiology, and contribute to the pathologic increase in aqueous humor (AH) outflow resistance. Autophagy is emerging as an essential cellular survival mechanism against a variety of stressors. In addition to performing basal functions, autophagy acts as a cellular survival pathway and represents an essential mechanism by which organisms can adapt to acute stress conditions and repair stress-induced damage. A decline in autophagy has been observed in most tissues with aging and has been considered responsible, at least in part, for the accumulation of damaged cellular components in almost all tissues of aging organisms. Dysfunction in the autophagy pathway is associated with several human diseases, from infectious diseases to cancer and neurodegeneration. In this review, we will summarize our current knowledge of the emerging roles of autophagy in outflow tissue physiology and pathophysiology, including novel evidence suggesting compromised autophagy in the glaucomatous outflow pathway.

Autophagic dysregulation in glaucomatous trabecular meshwork cells

Primary open angle glaucoma (POAG) is a degenerative disease commonly associated with aging and elevated intraocular pressure (IOP). Higher resistance to aqueous humor (AH) outflow through the trabecular meshwork (TM) generates the elevated IOP in POAG; unfortunately the underlying molecular mechanisms responsible for elevated resistance are unknown. It is widely accepted, however, that differences between normal and POAG TM tissues are presumably a consequence of cellular dysfunction. Here, we investigated the autophagic function and response to chronic oxidative stress in TM cells isolated from glaucomatous and age-matched donor eyes. Glaucomatous TM cells showed elevated senescence-associated-beta-galactosidase (SA-β-Gal) and cellular lipofuscin, together with decreased steady-state levels of LC3B-II, decreased levels of pRPS6K-T389 and reduced proteolysis of long-live proteins. Moreover, the glaucomatous cultures failed to activate autophagy when exposed to hyperoxic conditions. These results strongly suggest mTOR-dependent dysregulation of the autophagic pathway in cells isolated from the glaucomatous TM. Such dysregulated autophagic capacity can have a detrimental impact in outflow pathway tissue, i.e. mechanotransduction, and thus represent an important factor contributing to the progression of the disease.

Spectrum of ocular manifestations in CLN2-associated batten (Jansky-Bielschowsky) disease correlate with advancing age and deteriorating neurological function

Background

Late infantile neuronal ceroid lipofuscinosis (LINCL), one form of Batten’s disease is a progressive neurodegenerative disorder resulting from a CLN2 gene mutation. The spectrum of ophthalmic manifestations of LINCL and the relationship with neurological function has not been previously described.

Methods

Patients underwent ophthalmic evaluations, including anterior segment and dilated exams, optical coherence tomography, fluorescein and indocyanine green angiography. Patients were also assessed with the LINCL Neurological Severity Scale. Ophthalmic findings were categorized into one of five severity scores, and the association of the extent of ocular disease with neurological function was assessed.

Results

Fifty eyes of 25 patients were included. The mean age at the time of exam was 4.9 years (range 2.5 to 8.1). The mean ophthalmic severity score was 2.6 (range 1 to 5). The mean neurological severity score was 6.1 (range 2 to 11). Significantly more severe ophthalmic manifestations were observed among older patients (p<0.005) and patients with more severe neurological findings (p<0.03). A direct correlation was found between the Ophthalmic Severity Scale and the Weill Cornell Neurological Scale (p<0.002). A direct association was also found between age and the ophthalmic manifestations (p<0.0002), with older children having more severe ophthalmic manifestations.

Conclusions

Ophthalmic manifestations of LINCL correlate closely with the degree of neurological function and the age of the patient. The newly established LINCL Ophthalmic Scale may serve as an objective marker of LINCL severity and disease progression, and may be valuable in the evaluation of novel therapeutic strategies for LINCL, including gene therapy.

Lysosomal basification and decreased autophagic flux in oxidatively stressed trabecular meshwork cells: implications for glaucoma pathogenesis

Increasing evidence suggests oxidative damage as a key factor contributing to the failure of the conventional outflow pathway tissue to maintain appropriate levels of intraocular pressure, and thus increase the risk for developing glaucoma, a late-onset disease which is the second leading cause of permanent blindness worldwide. Autophagy is emerging as an essential cellular survival mechanism against a variety of stressors, including oxidative stress. Here, we have monitored, by using different methodologies (LC3-I to LC3-II turnover, tfLC3, and Cyto ID), the induction of autophagy and autophagy flux in TM cells subjected to a normobaric hyperoxic model of mild chronic oxidative stress. Our data indicate the MTOR-mediated activation of autophagy and nuclear translocation of TFEB in oxidatively stressed TM cells, as well as the role of autophagy in the occurrence of SA-GLB1/SA-β-gal. Concomitant with the activation of the autophagic pathway, TM cells grown under oxidative stress conditions displayed, however, reduced cathepsin (CTS) activities, reduced lysosomal acidification and impaired CTSB proteolytic maturation, resulting in decreased autophagic flux. We propose that diminished autophagic flux induced by oxidative stress might represent one of the factors leading to progressive failure of cellular TM function with age and contribute to the pathogenesis of primary open angle glaucoma.

Mucopolysaccharidoses and mucolipidoses

The mucopolysaccharidoses (MPS) and mucolipidoses (ML) are progressive storage disorders that share many clinical features varying from facial dysmorphism, bone dysplasia, hepatosplenomegaly, neurological abnormalities, developmental regression, and a reduced life expectancy at the severe end of the clinical spectrum to an almost normal clinical phenotype and life span in patients with more attenuated disease. MPS and ML are transmitted in an autosomal recessive manner, except for the X-linked MPS II (Hunter syndrome). Diagnosis is initially by detecting partially degraded GAG or oligosaccharide in urine and confirmed by specific enzyme assays in serum, leukocytes, or skin fibroblasts. For the majority of disorders treatment is palliative, but there have been important advances in the use of specific enzyme replacement therapy strategies for some MPS disorders and this is an area of very rapid development. In addition, hematopoietic stem cell transplantation (HSCT) can improve outcome in carefully selected patients with MPS (especially MPS IH, Hurler syndrome), but this procedure is associated with significant risk. Gene augmentation/transfer using a variety of vectors has been successful in animal models but has not yet been successfully performed in a human patient with one of these disorders. It is important to remember that prenatal diagnosis is possible for all of these disorders.

Corneal alterations during combined therapy with cyclodextrin/allopregnanolone and miglustat in a knock-out mouse model of NPC1 disease

Background

Niemann Pick disease type C1 is a neurodegenerative disease caused by mutations in the NPC1 gene, which result in accumulation of unesterified cholesterol and glycosphingolipids in the endosomal-lysosomal system as well as limiting membranes. We have previously shown the corneal involvement in NPC1 pathology in form of intracellular inclusions in epithelial cells and keratocytes. The purpose of the present study was to clarify if these inclusions regress during combined substrate reduction- and by-product therapy (SRT and BPT).

Methodology/Principal Findings

Starting at postnatal day 7 (P7) and thereafter, NPC1 knock-out mice (NPC1^−/−) and wild type controls (NPC1^+/+) were injected with cyclodextrin/allopregnanolone weekly. Additionally, a daily miglustat injection started at P10 until P23. Starting at P23 the mice were fed powdered chow with daily addition of miglustat. The sham group was injected with 0.9% NaCl at P7, thereafter daily starting at P10 until P23, and fed powdered chow starting at P23. For corneal examination, in vivo confocal laser-scanning microscopy (CLSM) was performed one day before experiment was terminated. Excised corneas were harvested for lipid analysis (HPLC/MS) and electron microscopy.

In vivo CLSM demonstrated a regression of hyperreflective inclusions in all treated NPC1^−/−mice. The findings varied between individual mice, demonstrating a regression, ranging from complete absence to pronounced depositions. The reflectivity of inclusions, however, was significantly lower when compared to untreated and sham-injected NPC1^−/− mice. These confocal findings were confirmed by lipid analysis and electron microscopy. Another important CLSM finding revealed a distinct increase of mature dendritic cell number in corneas of all treated mice (NPC1^−/− and NPC1^+/+), including sham-treated ones.

Conclusions/Significance

The combined substrate reduction- and by-product therapy revealed beneficial effects on the cornea. In vivo CLSM is a non-invasive tool to monitor disease progression and treatment effects in NPC1 disorder.

Morphological alterations of the cornea in the mouse model of niemann-pick disease type c1

PURPOSE

Niemann-Pick disease type C1 (NPC1) is a genetic neurovisceral disorder characterized by abnormalities in intracellular sterol trafficking. A knockout mouse model (NPC1) is an important tool for the study of pathogenesis and treatment strategies. In the present study, NPC1 mice were examined for pathological changes in the cornea.

METHODS

Fifteen inbred homozygous NPC1 knockout mice (NPC1, 5-10 weeks old), 5 age-matched heterozygous mice (NPC1), and 14 wild-type control mice (NPC1) were examined. In vivo confocal laser scanning microscopy (CLSM) was performed on both eyes of each animal; afterward, the eyes were processed for histology, electron microscopy, and lipid analysis.

RESULTS

In vivo CLSM disclosed hyperreflective intracellular deposits in the intermediate and basal cell layers of corneal epithelium in all NPC1 mice. At the electron microscopy level, however, vacuolated cytoplasmic structures, 200-500 nm in diameter, with electron-dense material appeared in all structures investigated, including all epithelial layers and stromal keratocytes. These deposits were negative for filipin, a marker for unesterified cholesterol. Lipid analysis showed a marked increase in disialotetrahexosylganglioside 2 (GM2) level in NPC1 mice corneas, whereas no changes were detected in free cholesterol and disialotetrahexosylganglioside 3 (GM3) levels when compared with controls.

CONCLUSIONS

Morphological changes characteristic for the NPC1 mouse cornea were visualized in all epithelial layers and keratocytes. In vivo CLSM findings were confirmed by other techniques. In vivo detection of ocular manifestations and analysis of ocular tissue have the potential to aid the diagnosis of NPC1 disease and to monitor the efficacy of treatment.