CERKL, a retinal dystrophy gene, regulates mitochondrial function and dynamics in the mammalian retina

Protocol for real-time assessment of energy metabolism in dissociated mouse retinal photoreceptors using a SeahorseXFe24 analyzer

Defects in retinal metabolism have been linked to the onset and progression of various retinal diseases. Herein, we provide a protocol for measuring bioenergetics in dissociated mouse retinal photoreceptors. We outline detailed instructions for obtaining morphologically intact and viable photoreceptor cells from adult mice and preparing the cells for metabolic analysis using a SeahorseXFe24 analyzer. This protocol allows a real-time assessment of mitochondrial respiration and glycolysis in retinal photoreceptors in response to genetic modifications or pathological insults in mouse models.

Effect of chloroquine on autophagy and the severity of caerulein-induced acute pancreatitis in mice

Impaired autophagy is implicated in the pathogenesis of caerulein-induced model of acute pancreatitis (AP). Chloroquine blocks the fusion of autophagosome and lysosome and affects completion of the cellular autophagic flux. Adult, male, Swiss albino mice (20-25 g) were divided into four groups- 1, 2, 3 and 4 of 6 mice each. Mice in Group1 were given 8, hourly intraperitoneal injections of normal saline. Group 2 was also given intraperitoneal injections of chloroquine (60 mg/Kg) at 14 h and 30-min prior to first injection of normal saline. Mice in Groups 3 and 4 given 8, hourly intraperitoneal injections of caerulein (50 µg /Kg/dose). Group 4 also received chloroquine as Group 2. After sacrifice at the 9th hour in CO2-chamber, blood was drawn for amylase activity and cytokines estimation (IL-6, TNF-α, GM-CSF, IL-1β and IL-10) and pancreas was harvested for histopathology, transmission electron microscopy (TEM) and immunoblotting (LC3II, Beclin 1, SQSTM1, RIPK1, P65, Caspase-3, RIPK3, HMGB1). The relative expression of SQSTM1 and the autophagic vacuole area was higher in groups 2, 3 and 4 (p < 0.05), suggestive of increased impairment of autophagic flux. Autolysosome count was significantly increased in group 3 in comparison to group 1 (p = 0.0049). Autolysosome area was also increased in group 4 in comparison to group 3 (p = 0.031), which suggested impairment of autophagy. Total histopathological score and amylase activity were equivalent in groups 3 and 4. RIPK1 in pancreas and TNF-α level in plasma were more in group 4 than 3 (p = 0.014, 0.02, respectively). Expression of Caspase-3, was lesser in group 4 than 3 (p < 0.001). Expression of HMGB1was more in group 4 than 3 (p = 0.046). Chloroquine enhances necrosis and inflammation in caerulein-induced pancreatitis.

UNILATERAL MACULAR HOLE IN A PATIENT WITH RETINITIS PIGMENTOSA TREATED WITH COVER FLAP TECHNIQUE WITH THE USE OF PLATELET-RICH PLASMA UNDER AIR TAMPONADE

PURPOSE

The purpose of the study is to show the excellent anatomical result of the inverted flap-assisted technique with platelet-rich plasma under air for retinitis pigmentosa complicated with macular hole.

METHODS

A 32-year-old woman, previously diagnosed with retinitis pigmentosa, was referred to our department complaining of decreased central vision in her left eye for 4 weeks. At the time of presentation, the optical coherence tomography and fundoscopy examination showed the presence of a macular hole of 620 μ m in diameter. The closure of the macular hole was observed after the surgical procedure, but endophthalmitis occurred 5 days, postoperatively.

RESULTS

In our presented case, the cover flap-assisted technique with platelet-rich plasma under air for retinitis pigmentosa complicated with macular hole resulted in excellent anatomical outcomes with unremarkable visual recovery.

CONCLUSION

The physiopathology of full-thickness macular holes in retinitis pigmentosa patients remains still not fully elucidated. Pars plana vitrectomy with the adjunct of highly concentrated pure platelet-rich plasma have shown successful results.

Specific photoreceptor cell fate pathways are differentially altered in NR2E3-associated diseases

Mutations in NR2E3, a gene encoding an orphan nuclear transcription factor, cause two retinal dystrophies with a distinct phenotype, but the precise role of NR2E3 in rod and cone transcriptional networks remains unclear. To dissect NR2E3 function, we performed scRNA-seq in the retinas of wildtype and two different Nr2e3 mouse models that show phenotypes similar to patients carrying NR2E3 mutations. Our results reveal that rod and cone populations are not homogeneous and can be separated into different sub-classes. We identify a previously unreported cone pathway that generates hybrid cones co-expressing both cone- and rod-related genes. In mutant retinas, this hybrid cone subpopulation is more abundant and includes a subpopulation of rods transitioning towards a cone cell fate. Hybrid photoreceptors with high misexpression of cone- and rod-related genes are prone to regulated necrosis. Overall, our results shed light on the role of NR2E3 in modulating photoreceptor differentiation towards cone and rod fates and explain how different mutations in NR2E3 lead to distinct visual disorders in humans.

Acetazolamide Challenge Changes Outer Retina Bioenergy-Linked and Anatomical OCT Biomarkers Depending on Mouse Strain

Purpose

The purpose of this study was to test the hypothesis that optical coherence tomography (OCT) bioenergy-linked and anatomical biomarkers are responsive to an acetazolamide (ACZ) provocation.

Methods

C57BL/6J mice (B6J, a strain with relatively inefficient mitochondria) and 129S6/ev mice (S6, a strain with relatively efficient mitochondria) were given a single IP injection of ACZ (carbonic anhydrase inhibitor) or vehicle. In each mouse, the Mitochondrial Configuration within Photoreceptors based on the profile shape Aspect Ratio (MCP/AR) index was determined from the hyper-reflective band immediately posterior to the external limiting membrane (ELM). In addition, we tested for ACZ-induced acidification by measuring contraction of the external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness; the hyporeflective band (HB) signal intensity at the photoreceptor tips was also examined. Finally, the nuclear layer thickness was measured.

Results

In response to ACZ, MCP/AR was greater-than-vehicle in B6J mice and lower-than-vehicle in S6 mice. ACZ-treated B6J and S6 mice both showed ELM-RPE contraction compared to vehicle-treated mice, consistent with dehydration in response to subretinal space acidification. The HB intensity at the photoreceptor tips and the outer nuclear layer thickness (B6J and S6), as well as the inner nuclear layer thickness of B6J mice, were all lower than vehicle following ACZ.

Conclusions

Photoreceptor respiratory efficacy can be evaluated in vivo based on distinct rod mitochondria responses to subretinal space acidification measured with OCT biomarkers and an ACZ challenge, supporting and extending our previous findings measured with light-dark conditions.

Exacerbated response to oxidative stress in the Retinitis Pigmentosa CerklKD/KO mouse model triggers retinal degeneration pathways upon acute light stress

The retina is particularly vulnerable to genetic and environmental alterations that generate oxidative stress and cause cellular damage in photoreceptors and other retinal neurons, eventually leading to cell death. CERKL (CERamide Kinase-Like) mutations cause Retinitis Pigmentosa and Cone-Rod Dystrophy in humans, two disorders characterized by photoreceptor degeneration and progressive vision loss. CERKL is a resilience gene against oxidative stress, and its overexpression protects cells from oxidative stress-induced apoptosis. Besides, CERKL contributes to stress granule-formation and regulates mitochondrial dynamics in the retina. Using the CerklKD/KO albino mouse model, which recapitulates the human disease, we aimed to study the impact of Cerkl knockdown on stress response and activation of photoreceptor death mechanisms upon light/oxidative stress. After acute light injury, we assessed immediate or late retinal stress response, by combining both omic and non-omic approaches. Our results show that Cerkl knockdown increases ROS levels and causes a basal exacerbated stress state in the retina, through alterations in glutathione metabolism and stress granule production, overall compromising an adequate response to additional oxidative damage. As a consequence, several cell death mechanisms are triggered in CerklKD/KO retinas after acute light stress. Our studies indicate that Cerkl gene is a pivotal player in regulating light-challenged retinal homeostasis and shed light on how mutations in CERKL lead to blindness by dysregulation of the basal oxidative stress response in the retina.

Primary versus Secondary Elevations in Fundus Autofluorescence

The method of quantitative fundus autofluorescence (qAF) can be used to assess the levels of bisretinoids in retinal pigment epithelium (RPE) cells so as to aid the interpretation and management of a variety of retinal conditions. In this review, we focused on seven retinal diseases to highlight the possible pathways to increased fundus autofluorescence. ABCA4- and RDH12-associated diseases benefit from known mechanisms whereby gene malfunctioning leads to elevated bisretinoid levels in RPE cells. On the other hand, peripherin2/RDS-associated disease (PRPH2/RDS), retinitis pigmentosa (RP), central serous chorioretinopathy (CSC), acute zonal occult outer retinopathy (AZOOR), and ceramide kinase like (CERKL)-associated retinal degeneration all express abnormally high fundus autofluorescence levels without a demonstrated pathophysiological pathway for bisretinoid elevation. We suggest that, while a known link from gene mutation to increased production of bisretinoids (as in ABCA4- and RDH12-associated diseases) causes primary elevation in fundus autofluorescence, a secondary autofluorescence elevation also exists, where an impairment and degeneration of photoreceptor cells by various causes leads to an increase in bisretinoid levels in RPE cells.

Cellular and Molecular Mechanisms of Pathogenesis Underlying Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are congenital retinal degenerative diseases that have various inheritance patterns, including dominant, recessive, X-linked, and mitochondrial. These diseases are most often the result of defects in rod and/or cone photoreceptor and retinal pigment epithelium function, development, or both. The genes associated with these diseases, when mutated, produce altered protein products that have downstream effects in pathways critical to vision, including phototransduction, the visual cycle, photoreceptor development, cellular respiration, and retinal homeostasis. The aim of this manuscript is to provide a comprehensive review of the underlying molecular mechanisms of pathogenesis of IRDs by delving into many of the genes associated with IRD development, their protein products, and the pathways interrupted by genetic mutation.

Effect of calorie-restriction and rapamycin on autophagy and the severity of caerulein-induced experimental acute pancreatitis in mice

Background

Impaired autophagy contributes to development of acute pancreatitis (AP). We studied the effect of inducing autophagy by calorie-restriction and rapamycin, separately, in the caerulein-induced model of severe AP.

Methods

Adult, male, Swiss albino mice were given eight, hourly, intraperitoneal injections of caerulein (Ce) (50µg/Kg/dose). The interventions were calorie restriction (CR) and rapamycin (2mg/Kg). Mice were sacrificed at the 9th hour. Pancreas was harvested for histopathology and immunoblotting. Amylase activity and the levels of cytokines were measured in plasma.

Results

The histopathological score and amylase activity were significantly lower in calorie-restricted caerulein-induced AP (CRCeAP) in comparison to animals that had unrestricted access to chow. In the CRCeAP group, levels of IL-6 and GM-CSF in plasma were lower and the expression of LC3II and Beclin-1 were higher. On transmission electron-microscopy, the area occupied by autophagic vacuoles was higher in CRCeAP. The expression of caspase-8 and caspase-9 was also higher in CRCeAP. In rapamycin with caerulein-induced AP (Rapa+CeAP), the histopathological score and amylase activity were significantly lower than caerulein-induced AP (CeAP). In Rapa+CeAP, the expression of LC3II and Beclin-1 were higher, whereas; SQSTM1 was decreased. The number of autophagic vacuoles in Rapa+CeAP group was fewer. Interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and monocyte chemoattractant protein-1 (MCP-1) were lower in Rapa+CeAP. Caspase-3 increased and high mobility group box 1 (HMGB1) decreased in Rapa+CeAP.

Conclusion

Calorie-restriction and rapamycin can individually decrease the severity of injury in the caerulein-induced model of severe AP.

miR-181a/b downregulation: a mutation-independent therapeutic approach for inherited retinal diseases

Inherited retinal diseases (IRDs) are a group of diseases whose common landmark is progressive photoreceptor loss. The development of gene-specific therapies for IRDs is hampered by their wide genetic heterogeneity. Mitochondrial dysfunction is proving to constitute one of the key pathogenic events in IRDs; hence, approaches that enhance mitochondrial activities have a promising therapeutic potential for these conditions. We previously reported that miR-181a/b downregulation boosts mitochondrial turnover in models of primary retinal mitochondrial diseases. Here, we show that miR-181a/b silencing has a beneficial effect also in IRDs. In particular, the injection in the subretinal space of an adeno-associated viral vector (AAV) that harbors a miR-181a/b inhibitor (sponge) sequence (AAV2/8-GFP-Sponge-miR-181a/b) improves retinal morphology and visual function both in models of autosomal dominant (RHO-P347S) and of autosomal recessive (rd10) retinitis pigmentosa. Moreover, we demonstrate that miR-181a/b downregulation modulates the level of the mitochondrial fission-related protein Drp1 and rescues the mitochondrial fragmentation in RHO-P347S photoreceptors. Overall, these data support the potential use of miR-181a/b downregulation as an innovative mutation-independent therapeutic strategy for IRDs, which can be effective both to delay disease progression and to aid gene-specific therapeutic approaches.

CERKL, a Retinal Dystrophy Gene, Regulates Mitochondrial Transport and Dynamics in Hippocampal Neurons

Mutations in the Ceramide Kinase-like (CERKL) gene cause retinal dystrophies, characterized by progressive degeneration of retinal neurons, which eventually lead to vision loss. Among other functions, CERKL is involved in the regulation of autophagy, mitochondrial dynamics, and metabolism in the retina. However, CERKL is nearly ubiquitously expressed, and it has been recently described to play a protective role against brain injury. Here we show that Cerkl is expressed in the hippocampus, and we use mouse hippocampal neurons to explore the impact of either overexpression or depletion of CERKL on mitochondrial trafficking and dynamics along axons. We describe that a pool of CERKL localizes at mitochondria in hippocampal axons. Importantly, the depletion of CERKL in the Cerkl^KD/KO mouse model is associated with changes in the expression of fusion/fission molecular regulators, induces mitochondrial fragmentation, and impairs axonal mitochondrial trafficking. Our findings highlight the role of CERKL, a retinal dystrophy gene, in the regulation of mitochondrial health and homeostasis in central nervous system anatomic structures other than the retina.

Specialization of the photoreceptor transcriptome by Srrm3-dependent microexons is required for outer segment maintenance and vision

Retinal photoreceptors have a distinct transcriptomic profile compared to other neuronal subtypes, likely reflecting their unique cellular morphology and function in the detection of light stimuli by way of the ciliary outer segment. We discovered a layer of this molecular specialization by revealing that the vertebrate retina expresses the largest number of tissue-enriched microexons of all tissue types. A subset of these microexons is included exclusively in photoreceptor transcripts, particularly in genes involved in cilia biogenesis and vesicle-mediated transport. This microexon program is regulated by Srrm3, a paralog of the neural microexon regulator Srrm4. Despite the fact that both proteins positively regulate retina microexons in vitro, only Srrm3 is highly expressed in mature photoreceptors. Its deletion in zebrafish results in widespread down-regulation of microexon inclusion from early developmental stages, followed by other transcriptomic alterations, severe photoreceptor defects, and blindness. These results shed light on the transcriptomic specialization and functionality of photoreceptors, uncovering unique cell type-specific roles for Srrm3 and microexons with implications for retinal diseases.

Treatment and prevention of pathological mitochondrial dysfunction in retinal degeneration and in photoreceptor injury

Pathological deterioration of mitochondrial function is increasingly linked with multiple degenerative illnesses as a mediator of a wide range of neurologic and age-related chronic diseases, including those of genetic origin. Several of these diseases are rare, typically defined in the United States as an illness affecting fewer than 200,000 people in the U.S. population, or about one in 1600 individuals. Vision impairment due to mitochondrial dysfunction in the eye is a prominent feature evident in numerous primary mitochondrial diseases and is common to the pathophysiology of many of the familiar ophthalmic disorders, including age-related macular degeneration, diabetic retinopathy, glaucoma and retinopathy of prematurity - a collection of syndromes, diseases and disorders with significant unmet medical needs. Focusing on metabolic mitochondrial pathway mechanisms, including the possible roles of cuproptosis and ferroptosis in retinal mitochondrial dysfunction, we shed light on the potential of α-lipoyl-L-carnitine in treating eye diseases. α-Lipoyl-L-carnitine is a bioavailable mitochondria-targeting lipoic acid prodrug that has shown potential in protecting against retinal degeneration and photoreceptor cell loss in ophthalmic indications.

Dynamic lipid turnover in photoreceptors and retinal pigment epithelium throughout life

The retinal pigment epithelium-photoreceptor interphase is renewed each day in a stunning display of cellular interdependence. While photoreceptors use photosensitive pigments to convert light into electrical signals, the RPE supports photoreceptors in their function by phagocytizing shed photoreceptor tips, regulating the blood retina barrier, and modulating inflammatory responses, as well as regenerating the 11-cis-retinal chromophore via the classical visual cycle. These processes involve multiple protein complexes, tightly regulated ligand-receptors interactions, and a plethora of lipids and protein-lipids interactions. The role of lipids in maintaining a healthy interplay between the RPE and photoreceptors has not been fully delineated. In recent years, novel technologies have resulted in major advancements in understanding several facets of this interplay, including the involvement of lipids in phagocytosis and phagolysosome function, nutrient recycling, and the metabolic dependence between the two cell types. In this review, we aim to integrate the complex role of lipids in photoreceptor and RPE function, emphasizing the dynamic exchange between the cells as well as discuss how these processes are affected in aging and retinal diseases.

Genetic dissection of non-syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) belongs to a group of pigmentary retinopathies. It is the most common form of inherited retinal dystrophy, characterized by progressive degradation of photoreceptors that leads to nyctalopia, and ultimately, complete vision loss. RP is distinguished by the continuous retinal degeneration that progresses from the mid-periphery to the central and peripheral retina. RP was first described and named by Franciscus Cornelius Donders in the year 1857. It is one of the leading causes of bilateral blindness in adults, with an incidence of 1 in 3000 people worldwide. In this review, we are going to focus on the genetic heterogeneity of this disease, which is provided by various inheritance patterns, numerosity of variations and inter-/intra-familial variations based upon penetrance and expressivity. Although over 90 genes have been identified in RP patients, the genetic cause of approximately 50% of RP cases remains unknown. Heterogeneity of RP makes it an extremely complicated ocular impairment. It is so complicated that it is known as “fever of unknown origin”. For prognosis and proper management of the disease, it is necessary to understand its genetic heterogeneity so that each phenotype related to the various genetic variations could be treated.

Clinical and biochemical footprints of inherited metabolic disorders. VII. Ocular phenotypes

Ocular manifestations are observed in approximately one third of all inherited metabolic disorders (IMDs). Although ocular involvement is not life-threatening, it can result in severe vision loss, thereby leading to an additional burden for the patient. Retinal degeneration with or without optic atrophy is the most frequent phenotype, followed by oculomotor problems, involvement of the cornea and lens, and refractive errors. These phenotypes can provide valuable clues that contribute to its diagnosis. In this issue we found 577 relevant IMDs leading to ophthalmologic manifestations. This article is the seventh of a series attempting to create and maintain a comprehensive list of clinical and metabolic differential diagnoses according to system involvement.

CERKL alleviates ischemia reperfusion-induced nervous system injury through modulating the SIRT1/PINK1/Parkin pathway and mitophagy induction

Recent studies showed that Ceramide Kinase-Like Protein (CERKL)was expressed in the nerve cells and could regulate autophagy. Sirtuin-1 (SIRT1) is the regulator of the mitophagy, which can be stabilized by CERKL. Furthermore, the study also revealed that the SIRT1 induced mitophagy by activating PINK1/Parkin signaling. Therefore, we speculated that CERKL has potential to activate the SIRT1/PINK1/Parkin pathway to induce mitophagy. In this study, cerebral ischemia reperfusion mouse model was established. CERKL was overexpressed in those mice and human neuroblastoma cells. Tunel staining and flow cytometry were applied for the detection of cell apoptosis. The ratios of LC3Ⅱ to LC3Ⅰ and the expression of LC3Ⅱ in mitochondria were determined by gel electrophoresis. Overexpression of CERKL alleviated the cerebral ischemia reperfusion injury and damage to OGD/R human neuroblastoma cells. Overexpression of CERKL enhanced the expression of LC3 Ⅱ in mitochondria and induced occurrence of mitophagy. Overexpression of CERKL promoted the stability of SIRT1 and facilitated the expression of PINK1 and Parkin in those cells. Knockdown of PINK1 impeded the mitophagy and suppressed the expression of LC3 Ⅱ in mitochondria. It can be concluded that CERKL alleviated the ischemia reperfusion induced nervous system injury through inducing mitophagy in a SIRT1/PINK1/Parkin dependent pathway.

Overexpression of CERKL Protects Retinal Pigment Epithelium Mitochondria from Oxidative Stress Effects

The precise function of CERKL, a Retinitis Pigmentosa (RP) causative gene, is not yet fully understood. There is evidence that CERKL is involved in the regulation of autophagy, stress granules, and mitochondrial metabolism, and it is considered a gene that is resilient against oxidative stress in the retina. Mutations in most RP genes affect photoreceptors, but retinal pigment epithelium (RPE) cells may be also altered. Here, we aimed to analyze the effect of CERKL overexpression and depletion in vivo and in vitro, focusing on the state of the mitochondrial network under oxidative stress conditions. Our work indicates that the depletion of CERKL increases the vulnerability of RPE mitochondria, which show a shorter size and altered shape, particularly upon sodium arsenite treatment. CERKL-depleted cells have dysfunctional mitochondrial respiration particularly upon oxidative stress conditions. The overexpression of two human CERKL isoforms (558 aa and 419 aa), which display different protein domains, shows that a pool of CERKL localizes at mitochondria in RPE cells and that CERKL protects the mitochondrial network—both in size and shape—against oxidative stress. Our results support CERKL being a resilient gene that regulates the mitochondrial network in RPE as in retinal neurons and suggest that RPE cell alteration contributes to particular phenotypic traits in patients carrying CERKL mutations.