Conditional gene targeting: dissecting the cellular mechanisms of retinal degenerations

Noninvasive Diagnosis of Regional Alteration of Retinal Morphology and Structure with Optical Coherence Tomography in Rodents

Spectral-domain optical coherence tomography (SD-OCT) produces high-resolution images of retinal cross sections and is becoming a method of choice for in vivo analyses of retinal morphology in rodents. We have adopted this technology to identify and analyze alterations of retinal structure, particularly those with regional and subtle changes. In this technical brief, we will demonstrate the use of SD-OCT in identifying subtle changes in retinal structure and morphology due to the effect of mosaic gene deletion in conditional knockout mice and of uneven distribution of intravitreally delivered compounds, review the application of SD-OCT in measuring pathological lesion volumes, and discuss the major benefits of SD-OCT technology over the traditional histological methods.

VEGF production and signaling in Müller glia are critical to modulating vascular function and neuronal integrity in diabetic retinopathy and hypoxic retinal vascular diseases

Müller glia (MG) are major retinal supporting cells that participate in retinal metabolism, function, maintenance, and protection. During the pathogenesis of diabetic retinopathy (DR), a neurovascular disease and a leading cause of blindness, MG modulate vascular function and neuronal integrity by regulating the production of angiogenic and trophic factors. In this article, I will (1) briefly summarize our work on delineating the role and mechanism of MG-modulated vascular function through the production of vascular endothelial growth factor (VEGF) and on investigating VEGF signaling-mediated MG viability and neural protection in diabetic animal models, (2) explore the relationship among VEGF and neurotrophins in protecting Müller cells in in vitro models of diabetes and hypoxia and its potential implication to neuroprotection in DR and hypoxic retinal diseases, and (3) discuss the relevance of our work to the effectiveness and safety of long-term anti-VEGF therapies, a widely used strategy to combat DR, diabetic macular edema, neovascular age-related macular degeneration, retinopathy of prematurity, and other hypoxic retinal vascular disorders.

Efficient induction of productive Cre-mediated recombination in retinal pigment epithelium

PURPOSE

To dissect gene functions in the retinal pigment epithelium (RPE), we previously generated a tetracycline-inducible RPE-specific Cre mouse line. Although this Cre mouse line was useful for several conditional gene targeting studies that were conducted by different laboratories, its potential has not been fully exploited, presumably due to a lack of knowledge or procedure for inducing Cre expression appropriately in this mouse line. The goal of the current study is to establish a procedure that will improve the reproducibility of Cre-mediated recombination in this mouse line.

METHODS

Analysis of Cre expression and function was performed in double transgenic mice derived from inducible RPE-specific Cre and Cre-activatable ROSA26 lacZ reporter mice. A tetracycline derivative, doxycycline, was supplied to mice intravitreally to induce Cre expression. Cre expression and function were examined with reverse transcription-PCR, immunoblotting, immunostaining, and in situ enzymatic assay for β-galactosidase. Retinal integrity was examined with electroretinography and morphometry.

RESULTS

Intravitreal Dox injection elevated Cre expression significantly and resulted in productive Cre-mediated recombination in approximately 60% of the RPE cells in this mouse line with no apparent change in retinal integrity.

CONCLUSIONS

Our results suggest that productive Cre-mediated recombination in this mouse line can be induced efficiently with intravitreal Dox delivery, with no apparent Dox or Cre toxicity. Therefore, our inducible RPE-specific Cre mice are suitable for Cre/lox-based gene activation and inactivation in adult RPE, which is critical to the effectiveness and suitability of this Cre mouse line in long-term studies requiring conditional gene targeting.

Cell-specific cre recombinase expression allows selective ablation of glutamate receptors from mouse horizontal cells

In the mouse retina, horizontal cells form an electrically coupled network and provide feedback signals to photoreceptors and feedforward signals to bipolar cells. Thereby, horizontal cells contribute to gain control at the first visual synapse and to the antagonistic organization of bipolar and ganglion cell receptive fields. However, the nature of horizontal cell output remains a matter of debate, just as the exact contribution of horizontal cells to center-surround antagonism. To facilitate studying horizontal cell function, we developed a knockin mouse line which allows ablating genes exclusively in horizontal cells. This knockin line expresses a Cre recombinase under the promoter of connexin57 (Cx57), a gap junction protein only expressed in horizontal cells. Consistently, in Cx57+/Cre mice, Cre recombinase is expressed in almost all horizontal cells (>99%) and no other retinal neurons. To test Cre activity, we crossbred Cx57+/Cre mice with a mouse line in which exon 11 of the coding sequence for the ionotropic glutamate receptor subunit GluA4 was flanked by two loxP sites (GluA4fl/fl). In GluA4fl/fl:Cx57+/Cre mice, GluA4 immunoreactivity was significantly reduced (∼50%) in the outer retina where horizontal cells receive photoreceptor inputs, confirming the functionality of the Cre/loxP system. Whole-cell patch-clamp recordings from isolated horizontal cell somata showed a reduction of glutamate-induced inward currents by ∼75%, suggesting that the GluA4 subunit plays a major role in mediating photoreceptor inputs. The persistent current in GluA4-deficient cells is mostly driven by AMPA and to a very small extent by kainate receptors as revealed by application of the AMPA receptor antagonist GYKI52466 and concanavalin A, a potentiator of kainate receptor-mediated currents. In summary, the Cx57+/Cre mouse line provides a versatile tool for studying horizontal cell function. GluA4fl/fl:Cx57+/Cre mice, in which horizontal cells receive less excitatory input, can thus be used to analyze the contribution of horizontal cells to retinal processing.

Role of hypoxia-inducible factor-1α in preconditioning-induced protection of retinal ganglion cells in glaucoma

PURPOSE

We recently demonstrated in a mouse model of glaucoma that endogenous epigenetic mechanisms can be activated by a repetitive hypoxic preconditioning (RHP) stimulus to provide robust retinal ganglion cell (RGC) protection. Although we also provided evidence that RHP prevents or delays the apoptotic demise of the RGC soma, the mechanisms responsible for signaling this epigenetic response, as well as the effectors of the glaucoma-tolerant phenotype at the somatic and axonal levels, remain unidentified. In the present study, we used conditional mutant mice lacking hypoxia-inducible factor-1α (HIF-1α) in RGCs (HIF-1α RGC-knockout [KO] mice) to test the hypothesis that RHP-mediated activation of this transcription factor in these cells protects them from glaucomatous injury.

METHODS

Adult HIF-1α RGC-KO mice, generated by mating floxed HIF-1α mice with math5-Cre mice, were used. Experimental glaucoma was induced unilaterally in the HIF-1α RGC-KO mice and matched wild-types by elevating the intraocular pressure to 16-20 mmHg for 3 consecutive weeks, secondary to episcleral vein ligation. Mice of each genotype were randomized to either an RHP protocol (six total exposures to systemic hypoxia [11% oxygen], interspersed over a 2-week period, completed 3 days before ligation surgery) or to an untreated group. RGC soma and axon injury was quantified with Neuronal Nuclei (NeuN) immunohistochemistry in retinal flat mounts and SMI32 immunohistochemistry in cross sections of the post-laminar optic nerve, respectively.

RESULTS

HIF-1α RGC-KO mice exhibited normal retinal function and morphology, and crosses of math5-Cre mice with floxed ROSA26 reporter mice confirmed Cre recombinase activity was confined to the RGC axons and soma. Untreated wild-type mice exhibited a 30±2% loss of RGC soma and a 31±3% loss of RGC axons after 3 weeks of intraocular hypertension (both p<0.05 versus fellow eye). The 90% and 81% improvement in soma and axon survival, respectively, observed in the wild-type mice treated with RHP (both p<0.05 versus the glaucoma eye in the untreated mice) was still observed to a near identical extent in the RHP-treated HIF-1α RGC-KO mice. RHP had no effect on the magnitude of intraocular pressure elevation in either the KO or wild-type groups, indicating that protection was realized in both genotypes in the face of ongoing intraocular hypertension.

CONCLUSIONS

These findings indicate that the robust, glaucomatous protection of the RGC soma and axons induced by RHP does not require HIF-1α-mediated transcription of survival genes and other adaptive responses within the RGCs themselves. Rather, we infer that RGC survival is augmented secondary to the activation of other hypoxia-sensitive transcription factors in RGCs and/or the action of diffusible HIF-1α target gene proteins released from neighboring retinal cells. Ideally, the involvement of such autocrine- and/or paracrine-based mechanisms would be confirmed in future studies, but distinct components of the integrated, pleiotropic, and multicellular basis of this endogenous epigenetic response may prove difficult to demonstrate experimentally, as we found in the present study.

Cre-mediated recombination efficiency and transgene expression patterns of three retinal bipolar cell-expressing Cre transgenic mouse lines

PURPOSE

Retinal bipolar cells, comprising multiple types, play an essential role in segregating visual information into multiple parallel pathways in the retina. The ability to manipulate gene expression in specific bipolar cell type(s) in the retina is important for understanding the molecular basis of their normal physiological functions and diseases/disorders. The Cre/LoxP recombination system has become an important tool for allowing gene manipulation in vivo, especially with the increasing availability of cell- and tissue-specific Cre transgenic mouse lines. Detailed in vivo examination of the Cre/LoxP recombination efficiency and the transgene expression patterns for cell- and tissue-specific Cre transgenic mouse lines is essential for evaluating their utility. In this study, we investigated the Cre-mediated recombination efficiency and transgene expression patterns of retinal bipolar cell-expressing Cre transgenic lines by crossing with a Cre reporter mouse line and through Cre-dependent recombinant adeno-associated virus (rAAV) vector-mediated transgene delivery.

METHODS

Three retinal bipolar cell-expressing Cre-transgenic mouse lines, 5-HTR2a-cre, Pcp2-cre, and Chx10-cre, were crossed with a strong Cre reporter mouse line that expresses a red fluorescent protein variant, tdTomato. rAAV2 vectors carrying a double-floxed inverted open-reading frame sequence encoding channelrhodopsin-2-mCherry (ChR2-mCherry) driven by a ubiquitous neuronal EF1α or a ubiquitous CMV promoter with a rAAV2 capsid mutation (Y444F) were injected into the intravitreal space of the eyes. Immunohistochemistry using retinal bipolar cell type-specific markers was performed to examine Cre-mediated recombination efficiency and the transgene expression patterns in bipolar cells in retinal whole mounts and vertical sections.

RESULTS

For the 5-HTR2a-cre and Pcp2-cre mouse lines, the expression pattern of the Cre-mediated recombination by crossing the reporter line largely resembled the expression pattern of Cre. The bipolar cells showing Cre-mediated recombination in the 5-HTR2a-cre line and the Pcp2-cre line were predominantly type 4 cone bipolar cells and rod bipolar cells, respectively. For the Chx10-cre mouse line, the expression pattern of the Cre-mediated recombination by crossing the reporter line was different from that of Cre. The Cre-mediated transgene expression in retinal bipolar cells in the Chx10-cre line was not observed by crossing with the reporter mouse line but through Cre-dependent rAAV vector delivery. A rAAV2 vector with the combination of a CMV promoter and the Y444F capsid mutation achieved Cre-dependent transgene expression in retinal bipolar cells.

CONCLUSIONS

The retinal bipolar cell-expressing Cre-transgenic lines and the Cre-dependent rAAV vector reported in this study could be valuable tools for gene targeting and manipulation in retinal bipolar cells in mice.

RPE barrier breakdown in diabetic retinopathy: seeing is believing

Diabetic retinopathy (DR) is a major complication of diabetes and a leading cause of blindness in working-age Americans. DR is traditionally regarded as a disorder of blood-retina barriers, and the leakage of blood content is a major pathological characteristic of the disease. While the breakdown of the endothelial barrier in DR has been investigated extensively, the vascular leakage through the retinal pigment epithelium (RPE) barrier in the disease has not been widely acknowledged. As the blood content leaked through the RPE barrier causes excessive water influx to the retina, the breakdown of the RPE barrier is likely to play a causative role in the development of some forms of diabetic macular edema, a major cause of vision loss in DR. In this article, we will discuss the clinical evidences of the diabetes-induced RPE barrier breakdown, the alteration of the RPE in diabetes, the molecular and cellular mechanism of RPE barrier breakdown, and the research tools for the analysis of RPE barrier leakage. Finally, we will discuss the methodology and potential applications of our recently developed fluorescent microscopic imaging for the diabetes- or ischemia-induced RPE barrier breakdown in rodents.

Ischaemia-induced retinal neovascularisation and diabetic retinopathy in mice with conditional knockout of hypoxia-inducible factor-1 in retinal Müller cells

AIMS/HYPOTHESIS

Retinal Müller cells are known to produce inflammatory and angiogenic cytokines, which play important roles in diabetic retinopathy. Hypoxia-inducible factor (HIF)-1 has been shown to play a crucial role in retinal inflammation and neovascularisation. We sought to determine the role of Müller cell-derived HIF-1 in oxygen-induced retinopathy (OIR) and diabetic retinopathy using conditional Hif-1α (also known as Hif1a) knockout (KO) mice.

METHODS

Conditional Hif-1α KO mice were generated by crossing mice expressing cyclisation recombinase (cre, also known as P1_gp003) in Müller cells with floxed Hif-1α mice and used for OIR and streptozotocin-induced diabetes to induce retinal neovascularisation and inflammation, respectively. Abundance of HIF-1α and pro-angiogenic and pro-inflammatory factors was measured by immunoblotting and immunohistochemistry. Retinal neovascularisation was visualised by angiography and quantified by counting pre-retinal nuclei. Retinal inflammation was evaluated by leucostasis and vascular leakage.

RESULTS

While the Hif-1α KO mice showed significantly decreased HIF-1α levels in the retina, they exhibited no apparent histological or visual functional abnormalities under normal conditions. Compared with wild-type counterparts, Hif-1α KO mice with OIR demonstrated attenuated overproduction of vascular endothelial growth factor (VEGF) and intercellular adhesion molecule (ICAM)-1, reduced vascular leakage and alleviated neovascularisation in the retina. Under diabetes conditions, disruption of Hif-1α in Müller cells attenuated the increases of retinal vascular leakage and adherent leucocytes, as well as the overproduction of VEGF and ICAM-1.

CONCLUSIONS/INTERPRETATION

Müller cell-derived HIF-1α is a key mediator of retinal neovascularisation, vascular leakage and inflammation, the major pathological changes in diabetic retinopathy. Müller cell-derived HIF-1α is therefore a promising therapeutic target for diabetic retinopathy.

Significance of outer blood-retina barrier breakdown in diabetes and ischemia

PURPOSE

The outer blood-retina barrier (BRB) separates the neural retina from the choroidal vasculature, which is responsible for approximately 80% of blood supplies in the eye. To determine the significance of outer BRB breakdown in diabetic retinopathy, the outer BRB-specific leakage of macromolecules in diabetic and ischemic rodents was investigated.

METHODS

Diabetes and ischemia were induced in rodents by streptozotocin and oxygen-induced retinopathy, respectively. Diabetic and ischemic rodents were injected intravenously with fluorescein isothiocyanate (FITC)-dextran. The outer BRB-specific leakage in diabetic and ischemic rodents was visualized by fluorescent microscopy.

RESULTS

A microscopic imaging assay was developed to examine outer BRB breakdown. The outer BRB-specific leakage of fluorescent macromolecules was visualized in diabetic and ischemic rodents. Substantial leakages of macromolecules through the outer BRB in diabetic and ischemic rodents were detected with this assay. The number of severe outer BRB leakage sites is inversely proportional to the size of macromolecules. Significant depletion of occludin in the RPE of ischemic and diabetic rodents was also observed.

CONCLUSIONS

For the first time, a microscopic imaging assay for directly visualizing macromolecules leaked through the outer BRB in rodents was developed. Using this assay, the authors demonstrated the significance of outer BRB breakdown in diabetes and ischemia, which will have implications to the understanding, diagnosis, and treatment of diabetic macular edema and other ocular diseases with outer BRB defects. The microscopic imaging assay established in this study will likely be very useful to the development of drugs for macular edema.