Loss of the cone-enriched caspase-7 does not affect secondary cone death in retinitis pigmentosa.

Purpose: The apoptotic mechanisms responsible for secondary cone death in retinitis pigmentosa (RP) remain largely unknown. The cone-enriched apoptotic protease caspase-7 (Casp7) is thought to be triggered by endoplasmic reticulum (ER) stress and plays a pivotal role in mice deficient in the cone cyclic nucleotide-gated channels, a deficiency that causes achromatopsia in humans and in mice with autosomal dominant rhodopsin mutations, in particular the T17M mutation. Thus, we tested in two mouse models of RP whether the cone-enriched Casp7 plays a role during secondary cone death. Methods: Casp7 knockout mice were crossed to two different RP mouse models with significantly different rod and cone death kinetics: the rd1 mouse model, which carries a mutation in the Pde6b gene, and the rhodopsin knockout mouse model (Rho-KO or Rho-/- ). In both models, cone survival was assessed on retinal flat mounts by quantifying the percentage of cone arrestin staining over the retinal surface area. The analyses were performed at two different time points for each model. Results: Loss of Casp7 did not alter cone survival in either of the two mouse models tested regardless of the time point analyzed. Rod survival was also not affected in either model nor did loss of Casp7 affect rod or cone function in a wild-type background as assessed with electroretinogram analyses. Conclusions: Secondary cone death in retinitis pigmentosa is unlikely to be triggered by ER stress and is likely independent of Casp7 activity.

Loss of mTOR signaling affects cone function, cone structure and expression of cone specific proteins without affecting cone survival.

Cones are the primary photoreceptor (PR) cells responsible for vision in humans. They are metabolically highly active requiring phosphoinositide 3-kinase (PI3K) activity for long-term survival. One of the downstream targets of PI3K is the kinase mammalian target of rapamycin (mTOR), which is a key regulator of cell metabolism and growth, integrating nutrient availability and growth factor signals. Both PI3K and mTOR are part of the insulin/mTOR signaling pathway, however if mTOR is required for long-term PR survival remains unknown. This is of particular interest since deregulation of this pathway in diabetes results in reduced PR function before the onset of any clinical signs of diabetic retinopathy. mTOR is found in two distinct complexes (mTORC1 & mTORC2) that are characterized by their unique accessory proteins RAPTOR and RICTOR respectively. mTORC1 regulates mainly cell metabolism in response to nutrient availability and growth factor signals, while mTORC2 regulates pro-survival mechanisms in response to growth factors. Here we analyze the effect on cones of loss of mTORC1, mTORC2 and simultaneous loss of mTORC1 & mTORC2. Interestingly, neither loss of mTORC1 nor mTORC2 affects cone function or survival at one year of age. However, outer and inner segment morphology is affected upon loss of either complex. In contrast, concurrent loss of mTORC1 and mTORC2 leads to a reduction in cone function without affecting cone viability. The data indicates that PI3K mediated pro-survival signals diverge upstream of both mTOR complexes in cones, suggesting that they are independent of mTOR activity. Furthermore, the data may help explain why PR function is reduced in diabetes, which can lead to deregulation of both mTOR complexes simultaneously. Finally, although mTOR is a key regulator of cell metabolism, and PRs are metabolically highly active, the data suggests that the role of mTOR in regulating the metabolic transcriptome in healthy cones is minimal.

FANCJ/BACH1 acetylation at lysine 1249 regulates the DNA damage response.

BRCA1 promotes DNA repair through interactions with multiple proteins, including CtIP and FANCJ (also known as BRIP1/BACH1). While CtIP facilitates DNA end resection when de-acetylated, the function of FANCJ in repair processing is less well defined. Here, we report that FANCJ is also acetylated. Preventing FANCJ acetylation at lysine 1249 does not interfere with the ability of cells to survive DNA interstrand crosslinks (ICLs). However, resistance is achieved with reduced reliance on recombination. Mechanistically, FANCJ acetylation facilitates DNA end processing required for repair and checkpoint signaling. This conclusion was based on the finding that FANCJ and its acetylation were required for robust RPA foci formation, RPA phosphorylation, and Rad51 foci formation in response to camptothecin (CPT). Furthermore, both preventing and mimicking FANCJ acetylation at lysine 1249 disrupts FANCJ function in checkpoint maintenance. Thus, we propose that the dynamic regulation of FANCJ acetylation is critical for robust DNA damage response, recombination-based processing, and ultimately checkpoint maintenance.

Antisense oligonucleotide modulation of non-productive alternative splicing upregulates gene expression.

While most monogenic diseases are caused by loss or reduction of protein function, the need for technologies that can selectively increase levels of protein in native tissues remains. Here we demonstrate that antisense-mediated modulation of pre-mRNA splicing can increase endogenous expression of full-length protein by preventing naturally occurring non-productive alternative splicing and promoting generation of productive mRNA. Bioinformatics analysis of RNA sequencing data identifies non-productive splicing events in 7,757 protein-coding human genes, of which 1,246 are disease-associated. Antisense oligonucleotides targeting multiple types of non-productive splicing events lead to increases in productive mRNA and protein in a dose-dependent manner in vitro. Moreover, intracerebroventricular injection of two antisense oligonucleotides in wild-type mice leads to a dose-dependent increase in productive mRNA and protein in the brain. The targeting of natural non-productive alternative splicing to upregulate expression from wild-type or hypomorphic alleles provides a unique approach to treating genetic diseases.

Activated mTORC1 promotes long-term cone survival in retinitis pigmentosa mice.

Retinitis pigmentosa (RP) is an inherited photoreceptor degenerative disorder that results in blindness. The disease is often caused by mutations in genes that are specific to rod photoreceptors; however, blindness results from the secondary loss of cones by a still unknown mechanism. Here, we demonstrated that the mammalian target of rapamycin complex 1 (mTORC1) is required to slow the progression of cone death during disease and that constitutive activation of mTORC1 in cones is sufficient to maintain cone function and promote long-term cone survival. Activation of mTORC1 in cones enhanced glucose uptake, retention, and utilization, leading to increased levels of the key metabolite NADPH. Moreover, cone death was delayed in the absence of the NADPH-sensitive cell death protease caspase 2, supporting the contribution of reduced NADPH in promoting cone death. Constitutive activation of mTORC1 preserved cones in 2 mouse models of RP, suggesting that the secondary loss of cones is caused mainly by metabolic deficits and is independent of a specific rod-associated mutation. Together, the results of this study address a longstanding question in the field and suggest that activating mTORC1 in cones has therapeutic potential to prolong vision in RP.

Replication-dependent histone genes are actively transcribed in differentiating and aging retinal neurons.

In the mammalian genome, each histone family contains multiple replication-dependent paralogs, which are found in clusters where their transcription is thought to be coupled to the cell cycle. Here, we wanted to interrogate the transcriptional regulation of these paralogs during retinal development and aging. We employed deep sequencing, quantitative PCR, in situ hybridization (ISH), and microarray analysis, which revealed that replication-dependent histone genes were not only transcribed in progenitor cells but also in differentiating neurons. Specifically, by ISH analysis we found that different histone genes were actively transcribed in a subset of neurons between postnatal day 7 and 14. Interestingly, within a histone family, not all paralogs were transcribed at the same level during retinal development. For example, expression of Hist1h1b was higher embryonically, while that of Hist1h1c was higher postnatally. Finally, expression of replication-dependent histone genes was also observed in the aging retina. Moreover, transcription of replication-dependent histones was independent of rapamycin-mediated mTOR pathway inactivation. Overall, our data suggest the existence of variant nucleosomes produced by the differential expression of the replication-dependent histone genes across retinal development. Also, the expression of a subset of replication-dependent histone isotypes in senescent neurons warrants re-examining these genes as "replication-dependent." Thus, our findings underscore the importance of understanding the transcriptional regulation of replication-dependent histone genes in the maintenance and functioning of neurons.

Retinal gene delivery by rAAV and DNA electroporation.

Ocular gene therapy is a fast-growing area of research. The eye is an ideal organ for gene therapy since it is immune privileged and easily accessible, and direct viral delivery results primarily in local infection. Because the eye is not a vital organ, mutations in eye-specific genes tend to be more common. To date, over 40 eye-specific genes have been identified harboring mutations that lead to blindness. Gene therapy with recombinant adeno-associated virus (rAAV) holds the promise to treat patients with such mutations. However, proof-of-concept and safety evaluation for gene therapy remains to be established for most of these diseases. This unit describes the in vivo delivery of genes to the mouse eye by rAAV-mediated gene transfer and plasmid DNA electroporation. Advantages and limitations of these methods are discussed, and detailed protocols for gene delivery, required materials, and subsequent tissue processing methods are described.