Stage-specific reference genes significant for quantitative PCR during mouse retinal development

Morphine pharmacokinetics and opioid transporter expression at the blood-retina barrier of male and female mice

Opioids are effective analgesics for treating moderate to severe pain, however, their use must be weighed against their dangerous side effects. Investigations into opioid pharmacokinetics provide crucial information regarding both on- and off-target drug effects. Our recent work showed that morphine deposits and accumulates in the mouse retina at higher concentrations than in the brain upon chronic systemic exposure. We also found reduced retinal expression of P-glycoprotein (P-gp), a major opioid extruder at the blood-brain barrier (BBB). Here, we systematically interrogated the expression of three putative opioid transporters at the blood-retina barrier (BRB): P-gp, breast cancer resistance protein (Bcrp) and multidrug resistance protein 2 (Mrp2). Using immunohistochemistry, we found robust expression of P-gp and Bcrp, but not Mrp2, at the inner BRB of the mouse retina. Previous studies have suggested that P-gp expression may be regulated by sex hormones. However, upon acute morphine treatment we found no sex differences in morphine deposition levels in the retina or brain, nor on transporter expression in the retinas of males and females with a high or low estrogen:progesterone ratio. Importantly, we found that P-gp, but not Bcrp, expression significantly correlated with morphine concentration in the retina, suggesting P-gp is the predominant opioid transporter at the BRB. In addition, fluorescence extravasation studies revealed that chronic morphine treatment did not alter the permeability of either the BBB or BRB. Together, these data suggest that reduced P-gp expression mediates retinal morphine accumulation upon systemic delivery, and in turn, potential effects on circadian photoentrainment.

Identification of reference genes for the normalization of retinal mRNA expression by RT-qPCR in oxygen induced retinopathy, anemia, and erythropoietin administration

BACKGROUND

Anemia and retinopathy of prematurity (ROP) are common comorbidities experienced by preterm infants, yet the role of anemia on the pathogenesis of ROP remains unclear. Reverse-transcriptase quantitative polymerase chain reaction (RT-qPCR) is a sensitive technique for estimating the gene expression changes at the transcript level but requires identification of stably expressed reference genes for accurate data interpretation. This is particularly important for oxygen induced retinopathy studies given that some commonly used reference genes are sensitive to oxygen. This study aimed to identify stably expressed reference genes among eight commonly used reference genes in the neonatal rat pups' retina upon exposure to cyclic hyperoxia-hypoxia, anemia, and erythropoietin administration at two age groups (P14.5 and P20) using Bestkeeper, geNorm, and Normfinder, three publicly available, free algorithms, and comparing their results to the in-silico prediction program, RefFinder.

RESULTS

The most stable reference gene across both developmental stages was Rpp30, as predicted by Genorm, Bestkeeper, and Normfinder. RefFinder predicted Tbp to be the most stable across both developmental stages. At P14.5, stability varied by prediction program; at P20, RPP30 and MAPK1 were the most stable reference genes. Gapdh, 18S, Rplp0, and HPRT were predicted as the least stable reference genes by at least one of the prediction algorithms.

CONCLUSION

Expression of Rpp30 is the least affected by experimental conditions of oxygen induced retinopathy, phlebotomy induced anemia and erythropoietin administration at both timepoints of P14.5 and P20.

Morphine Accumulates in the Retina Following Chronic Systemic Administration

Opioid transport into the central nervous system is crucial for the analgesic efficacy of opioid drugs. Thus, the pharmacokinetics of opioid analgesics such as morphine have been extensively studied in systemic circulation and the brain. While opioid metabolites are routinely detected in the vitreous fluid of the eye during postmortem toxicological analyses, the pharmacokinetics of morphine within the retina of the eye remains largely unexplored. In this study, we measured morphine in mouse retina following systemic exposure. We showed that morphine deposits and persists in the retina long after levels have dropped in the serum. Moreover, we found that morphine concentrations (ng/mg tissue) in the retina exceeded brain morphine concentrations at all time points tested. Perhaps most intriguingly, these data indicate that following chronic systemic exposure, morphine accumulates in the retina, but not in the brain or serum. These results suggest that morphine can accumulate in the retina following chronic use, which could contribute to the deleterious effects of chronic opioid use on both image-forming and non-image-forming visual functions.

Absence of ephrin-A2/A3 increases retinal regenerative potential for Müller cells in Rhodopsin knockout mice

Müller cells (MC) are considered dormant retinal progenitor cells in mammals. Previous studies demonstrated ephrin-As act as negative regulators of neural progenitor cells in the retina and brain. It remains unclear whether the lack of ephrin-A2/A3 is sufficient to promote the neurogenic potential of MC. Here we investigated whether the MC is the primary retinal cell type expressing ephrin-A2/A3 and their role on the neurogenic potential of Müller cells. In this study, we showed that ephrin-A2/A3 and their receptor EphA4 were expressed in retina and especially enriched in MC. The level of ephrinAs/EphA4 expression increased as the retina matured that is correlated with the reduced proliferative and progenitor cell potential of MC. Next, we investigated the proliferation in primary MC cultures isolated from wild-type and A2-/- A3-/- mice by 5-ethynyl-2'-deoxyuridine (EdU) incorporation. We detected a significant increase of EdU+ cells in MC derived from A2-/- A3-/- mice. Next, we investigated the role of ephrin-A2/A3 in mice undergoing photoreceptor degeneration such as Rhodopsin knockout (Rho-/-) mice. To further evaluate the role of ephrin-A2/A3 in MC proliferation in vivo, EdU was injected intraperitoneally to adult wild-type, A2-/- A3-/- , Rho-/- and Rho-/- A2-/- A3-/- mice and the numbers of EdU+ cells distributed among different layers of the retina. EphrinAs/EphA4 expression was upregulated in the retina of Rho-/- mice compared to the wild-type mice. In addition, cultured MC derived from ephrin-A2-/- A3-/- mice also expressed higher levels of progenitor cell markers and exhibited higher proliferation potential than those from wild-type mice. Interestingly, we detected a significant increase of EdU+ cells in the retinas of adult ephrin-A2-/- A3-/- mice mainly in the inner nuclear layer; and these EdU+ cells were co-localized with MC marker, cellular retinaldehyde-binding protein, suggesting some proliferating cells are from MC. In Rhodopsin knockout mice (Rho-/- A2-/- A3-/- mice), a significantly greater amount of EdU+ cells were located in the ciliary body, retina and RPE than that of Rho-/- mice. Comparing between 6 and 12 weeks old Rho-/- A2-/- A3-/- mice, we recorded more EdU+ cells in the outer nuclear layer in the 12-week-old mice undergoing severe retinal degeneration. Taken together, Ephrin-A2/A3 are negative regulators of the proliferative and neurogenic potentials of MC. Absence of ephrin-A2/A3 promotes the migration of proliferating cells into the outer nuclear layer and may lead to retinal cell regeneration. All experimental procedures were approved by the Animal Care and Use Committee at Schepens Eye Research Institute, USA (approval No. S-353-0715) on October 24, 2012.

Stress Signal Regulation by Na/K-ATPase As a New Approach to Promote Physiological Revascularization in a Mouse Model of Ischemic Retinopathy

Purpose

The identification of target pathways to block excessive angiogenesis while simultaneously restoring physiological vasculature is an unmet goal in the therapeutic management of ischemic retinopathies. pNaKtide, a cell-permeable peptide that we have designed by mapping the site of α1 Na/K-ATPase (NKA)/Src binding, blocks the formation of α1 NKA/Src/reactive oxygen species (ROS) amplification loops and restores physiological ROS signaling in a number of oxidative disease models. The aim of this study was to evaluate the importance of the NKA/Src/ROS amplification loop and the effect of pNaKtide in experimental ischemic retinopathy.

Methods

Human retinal microvascular endothelial cells (HRMECs) and retinal pigment epithelium (ARPE-19) cells were used to evaluate the effect of pNaKtide on viability, proliferation, and angiogenesis. Retinal toxicity and distribution were assessed in those cells and in the mouse. Subsequently, the role and molecular mechanism of NKA/Src in ROS stress signaling were evaluated biochemically in the retinas of mice exposed to the well-established protocol of oxygen-induced retinopathy (OIR). Finally, pNaKtide efficacy was assessed in this model.

Results

The results suggest a key role of α1 NKA in the regulation of ROS stress and the Nrf2 pathway in mouse OIR retinas. Inhibition of α1 NKA/Src by pNaKtide reduced pathologic ROS signaling and restored normal expression of hypoxia-inducible factor 1-α/vascular endothelial growth factor (VEGF). Unlike anti-VEGF agents, pNaKtide did promote retinal revascularization while inhibiting neovascularization and inflammation.

Conclusions

Targeting α1 NKA represents a novel strategy to develop therapeutics that not only inhibit neovascularization but also promote physiological revascularization in ischemic eye diseases.

LKB1 and AMPK instruct cone nuclear position to modify visual function

Cone photoreceptors detect light and are responsible for color vision. These cells display a distinct polarized morphology where nuclei are precisely aligned in the apical retina. However, little is known about the mechanisms involved in cone nuclear positioning or the impact of this organization on retina function. We show that the serine/threonine kinase LKB1 and one of its substrates, AMPK, regulate cone nuclear positioning. In the absence of either molecule, cone nuclei are misplaced along the axon, resulting in altered nuclear lamination. LKB1 is required specifically in cones to mediate this process, and disruptions in nuclear alignment result in reduced cone function. Together, these results identify molecular determinants of cone nuclear position and indicate that cone nuclear position alignment enables proper visual function.

Core-clock genes Period 1 and 2 regulate visual cascade and cell cycle components during mouse eye development

The retinas from Period 1 (Per1) and Period 2 (Per2) double-mutant mice (Per1^-/-Per2^Brdm1) display abnormal blue-cone distribution associated with a reduction in cone opsin mRNA and protein levels, up to 1 year of age. To reveal the molecular mechanisms by which Per1 and Per2 control retina development, we analyzed genome-wide gene expression differences between wild-type (WT) and Per1^-/-Per2^Brdm1 mice across ocular developmental stages (E15, E18 and P3). All clock genes displayed changes in transcript levels along with normal eye development. RNA-Seq data show major gene expression changes between WT and mutant eyes, with the number of differentially expressed genes (DEG) increasing with developmental age. Functional annotation of the genes showed that the most significant changes in expression levels in mutant mice involve molecular pathways relating to circadian rhythm signaling at E15 and E18. At P3, the visual cascade and the cell cycle were respectively higher and lower expressed compared to WT eyes. Overall, our study provides new insights into signaling pathways -phototransduction and cell cycle- controlled by the circadian clock in the eye during development.

A metabolic perspective on CSF-mediated neurodegeneration in multiple sclerosis

Multiple sclerosis is an autoimmune demyelinating disorder of the CNS, characterized by inflammatory lesions and an underlying neurodegenerative process, which is more prominent in patients with progressive disease course. It has been proposed that mitochondrial dysfunction underlies neuronal damage, the precise mechanism by which this occurs remains uncertain. To investigate potential mechanisms of neurodegeneration, we conducted a functional screening of mitochondria in neurons exposed to the CSF of multiple sclerosis patients with a relapsing remitting (n = 15) or a progressive (secondary, n = 15 or primary, n = 14) disease course. Live-imaging of CSF-treated neurons, using a fluorescent mitochondrial tracer, identified mitochondrial elongation as a unique effect induced by the CSF from progressive patients. These morphological changes were associated with decreased activity of mitochondrial complexes I, III and IV and correlated with axonal damage. The effect of CSF treatment on the morphology of mitochondria was characterized by phosphorylation of serine 637 on the dynamin-related protein DRP1, a post-translational modification responsible for unopposed mitochondrial fusion in response to low glucose conditions. The effect of neuronal treatment with CSF from progressive patients was heat stable, thereby prompting us to conduct an unbiased exploratory lipidomic study that identified specific ceramide species as differentially abundant in the CSF of progressive patients compared to relapsing remitting multiple sclerosis. Treatment of neurons with medium supplemented with ceramides, induced a time-dependent increase of the transcripts levels of specific glucose and lactate transporters, which functionally resulted in progressively increased glucose uptake from the medium. Thus ceramide levels in the CSF of patients with progressive multiple sclerosis not only impaired mitochondrial respiration but also decreased the bioavailability of glucose by increasing its uptake. Importantly the neurotoxic effect of CSF treatment could be rescued by exogenous supplementation with glucose or lactate, presumably to compensate the inefficient fuel utilization. Together these data suggest a condition of 'virtual hypoglycosis' induced by the CSF of progressive patients in cultured neurons and suggest a critical temporal window of intervention for the rescue of the metabolic impairment of neuronal bioenergetics underlying neurodegeneration in multiple sclerosis patients.

Mitochondrial Uncoupling Protein 2 Knock-out Promotes Mitophagy to Decrease Retinal Ganglion Cell Death in a Mouse Model of Glaucoma

Glaucoma is a neurodegenerative disorder characterized by mitochondrial dysfunction and an increase in oxidative damage, leading to retinal ganglion cell (RGC) death. The oxidative status of RGCs is regulated intrinsically and also extrinsically by retinal glia. The mitochondrial uncoupling protein 2 (UCP2) relieves oxidative and neuronal damage in a variety of neurodegenerative disease models. We hypothesized that deletion of Ucp2 in either RGCs or retinal glia would increase retinal damage and RGC death in a mouse model of glaucoma. Paradoxically, we found the reverse, and deletion of mitochondrial Ucp2 decreased oxidative protein modification and reduced RGC death in male and female mice. This paradox was resolved after finding that Ucp2 deletion also increased levels of mitophagy in cell culture and retinal tissue. Our data suggest that Ucp2 deletion facilitates increased mitochondrial function by improving quality control. An increase in mitochondrial function explains the resistance of Ucp2-deleted retinas to glaucoma and may provide a therapeutic avenue for other chronic neurodegenerative conditions.SIGNIFICANCE STATEMENT Many unsolved neurodegenerative conditions result from defects in mitochondrial function. Molecular tools that can manipulate mitochondria will therefore be central to developing neuroprotective therapies. Among the most potent regulators of mitochondrial function are the uncoupling proteins, particularly UCP2. In this manuscript, we show that, while loss of Ucp2 does increase mitochondrial membrane potential and the production of reactive oxygen species, it also initiates an increase in mitophagy that is ultimately neuroprotective. This novel protective consequence of uncoupling protein inhibition may lead to new therapeutic approaches to combat neurodegenerative disease, particularly because pharmacological compounds do exist that can selectively inhibit UCP2.

Tbr1 instructs laminar patterning of retinal ganglion cell dendrites

Visual information is delivered to the brain by >40 types of retinal ganglion cells (RGCs). Diversity in this representation arises within the inner plexiform layer (IPL), where dendrites of each RGC type are restricted to specific sublaminae, limiting the interneuronal types that can innervate them. How such dendritic restriction arises is unclear. We show that the transcription factor Tbr1 is expressed by four mouse RGC types with dendrites in the outer IPL and is required for their laminar specification. Loss of Tbr1 results in elaboration of dendrites within the inner IPL, while misexpression in other cells retargets their neurites to the outer IPL. Two transmembrane molecules, Sorcs3 and Cdh8, act as effectors of the Tbr1-controlled lamination program. However, they are expressed in just one Tbr1⁺ RGC type, supporting a model in which a single transcription factor implements similar laminar choices in distinct cell types by recruiting partially non-overlapping effectors.

Retinal S-opsin dominance in Ansell's mole-rats (Fukomys anselli) is a consequence of naturally low serum thyroxine

Mammals usually possess a majority of medium-wavelength sensitive (M-) and a minority of short-wavelength sensitive (S-) opsins in the retina, enabling dichromatic vision. Unexpectedly, subterranean rodents from the genus Fukomys exhibit an S-opsin majority, which is exceptional among mammals, albeit with no apparent adaptive value. Because thyroid hormones (THs) are pivotal for M-opsin expression and metabolic rate regulation, we have, for the first time, manipulated TH levels in the Ansell's mole-rat (Fukomys anselli) using osmotic pumps. In Ansell's mole-rats, the TH thyroxine (T4) is naturally low, likely as an adaptation to the harsh subterranean ecological conditions by keeping resting metabolic rate (RMR) low. We measured gene expression levels in the eye, RMR, and body mass (BM) in TH-treated animals. T4 treatment increased both, S- and M-opsin expression, albeit M-opsin expression at a higher degree. However, this plasticity was only given in animals up to approximately 2.5 years. Mass-specific RMR was not affected following T4 treatment, although BM decreased. Furthermore, the T4 inactivation rate is naturally higher in F. anselli compared to laboratory rodents. This is the first experimental evidence that the S-opsin majority in Ansell's mole-rats is a side effect of low T4, which is downregulated to keep RMR low.

Age-Dependent Changes of Monocarboxylate Transporter 8 Availability in the Postnatal Murine Retina

The thyroid hormones (TH) triiodothyronine (T3) and its prohormone thyroxine (T4) are crucial for retinal development and function, and increasing evidence points at TH dysregulation as a cause for retinal degenerative diseases. Thus, precise regulation of retinal TH supply is required for proper retinal function, but knowledge on these mechanisms is still fragmentary. Several transmembrane transporters have been described as key regulators of TH availability in target tissues of which the monocarboxylate transporter 8 (MCT8), a high affinity transporter for T4 and T3, plays an essential role in the central nervous system. Moreover, in the embryonic chicken retina, MCT8 is highly expressed, but the postnatal availability of MCT8 in the mammalian retina was not reported to date. In the present study, spatiotemporal retinal MCT8 availability was examined in mice of different age. For this purpose, we quantified expression levels of Mct8 via Real-Time Reverse-Transcriptase PCR in mouse eyecups (C57BL/6) of juvenile and adult age groups. Additionally, age-dependent MCT8 protein levels were quantified via Western blotting and localized via immunofluorescence confocal microscopy. While no difference in Mct8 expression levels could be detected between age groups, MCT8 protein levels in juvenile animals were about two times higher than in adult animals based on Western blot analyses. Immunohistochemical analyses showed that MCT8 immunoreactivity in the eyecup was restricted to the retina and the retinal pigment epithelium. In juvenile mice, MCT8 was broadly observed along the apical membrane of the retinal pigment epithelium, tightly surrounding photoreceptor outer segments. Distinct immunopositive staining was also detected in the inner nuclear layer and the ganglion cell layer. However, in adult specimens, immunoreactivity visibly declined in all layers, which was in line with Western blot analyses. Since MCT8 was abundantly present in juvenile and about twofold lower in adult retinae, our findings suggest a pivotal role of MCT8 especially during postnatal maturation. The present study provides novel insights into age-dependent retinal TH supply, which might help to understand different aspects regarding retinal development, function, and disorders.

Stability of Reference Genes for Messenger RNA Quantification by Real-Time PCR in Mouse Dextran Sodium Sulfate Experimental Colitis

Background

Many animal models have been developed to characterize the complexity of colonic inflammation. In dextran sodium sulfate (DSS) experimental colitis in mice the choice of reference genes is critical for accurate quantification of target genes using quantitative real time PCR (RT-qPCR). No studies have addressed the performance of reference genes in mice DSS-experimental colitis. This study aimed to determine the stability of reference genes expression (RGE) in DSS-experimental murine colitis.

Methods

Colitis was induced in male C57BL/6 mice using DSS5% for 5 days, control group received water. RNA was extracted from inflamed and non-inflamed colon. Using RT-qPCR, comparative analysis of 13 RGE was performed according to predefined criteria and relative colonic TNF-α and IL-1β gene expression was determined by calculating the difference in the threshold cycle.

Results

Colitis significantly altered the stability of mucosal RGE. Commonly used glyceraldehyde-3-phosphate dehydrogenase (Gapdh), β-actin (Actb), or β2-microglobulin (β2m) showed the highest variability within the inflamed and control groups. Conversely, TATA-box-binding protein (Tbp) and eukaryotic translation elongation factor 2 (Eef2) were not affected by inflammation and were the most stable genes. Normalization of colonic TNF-α and IL-1β mRNA levels was dependent on the reference gene used. Depending on the genes used to normalize the data, statistical significance varied from significant when TBP / Eef2 were used to non-significant when Gapdh, Actb or β2m were used.

Conclusions

This study highlights the appropriate choice of RGE to ensure adequate normalization of RT-qPCR data when using this model. Suboptimal RGE may explain controversial results from published studies. We recommend using Tbp and Eef2 instead of Gapdh, Actb or β2m as reference genes.