Critical Decrease in the Level of Axon Guidance Receptor ROBO1 in Rod Synaptic Terminals Is Followed by Axon Retraction

Glial Cell Responses and Gene Expression Dynamics in Retinas of Treated and Untreated RPE65 Mutant Dogs

Purpose

The long-term evaluation of RPE65 gene augmentation initiated in middle-aged RPE65 mutant dogs previously uncovered notable inter-animal and intra-retinal variations in treatment efficacy. The study aims to gain deeper insights into the status of mutant retinas and assess the treatment impact.

Methods

Immunohistochemistry utilizing cell-specific markers and reverse transcription-quantitative PCR (RT-qPCR) analysis were conducted on archival retinal sections from normal and RPE65 mutant dogs.

Results

Untreated middle-aged mutant retinas exhibited marked downregulation in the majority of 20 examined genes associated with key retinal pathways. These changes were accompanied by a moderate increase in microglia numbers, altered expression patterns of glial-neuronal transmitter recycling proteins, and gliotic responses in Müller glia. Analysis of advanced-aged mutant dogs revealed mild outer nuclear layer loss in the treated eye compared to moderate loss in the corresponding retinal regions of the untreated control eye. However, persistent Müller glial stress response along with photoreceptor synapse loss were evident in both treated and untreated eyes. Photoreceptor synaptic remodeling, infrequent in treated regions, was observed in all untreated advanced-aged retinas, accompanied by a progressive increase in microglial cells indicative of ongoing inflammation. Interestingly, about half of the examined genes showed similar expression levels between treated and untreated advanced-aged mutant retinas, with some reaching normal levels.

Conclusions

Gene expression data suggest a shift from pro-degenerative mechanisms in middle-aged mutant retinas to more compensatory mechanisms in preserved retinal regions at advanced stages, despite ongoing degeneration. Such shift, potentially attributed to a number of surviving resilient cells, may influence disease patterns and treatment outcomes.

Coming of Age for the Photoreceptor Synapse

Purpose

To discuss the potential contribution of rod and cone synapses to the loss of visual function in retinal injury and disease.

Methods

The published literature and the authors' own work were reviewed.

Results

Retinal detachment is used as a case study of rod spherule and cone pedicle plasticity after injury. Both rod and cone photoreceptors terminals are damaged after detachment although the structural changes observed are only partially overlapping. For second-order neurons, only those associated with rod spherules respond consistently to injury by remodeling. Examination of signaling pathways involved in plasticity of conventional synapses and in neural development has been and may continue to be productive in discovering novel therapeutic targets. Rho kinase (ROCK) inhibition is an example of therapy that may reduce synaptic damage by preserving normal synaptic structure of rod and cone cells.

Conclusions

We hypothesize that synaptic damage contributes to poor visual restoration after otherwise successful anatomical repair of retinal detachment. A similar situation may exist for patients with degenerative retinal disease. Thus, synaptic structure and function should be routinely studied, as this information may disclose therapeutic strategies to mitigate visual loss.

Candidate Genetic Modifiers for RPGR Retinal Degeneration

Purpose

To define genetic variants associated with variable severity of X-linked progressive retinal atrophy 1 (XLPRA1) caused by a five-nucleotide deletion in canine RPGR exon ORF15.

Methods

A genome-wide association study (GWAS) was performed in XLPRA1 phenotype informative pedigree. Whole genome sequencing (WGS) was used for mutational analysis of genes within the candidate genomic region. Retinas of normal and mutant dogs were used for gene expression, gene structure, and RNA duplex analyses.

Results

GWAS followed by haplotype phasing identified an approximately 4.6 Mb candidate genomic interval on CFA31 containing seven protein-coding genes expressed in retina (ROBO1, ROBO2, RBM11, NRIP1, HSPA13, SAMSN1, and USP25). Furthermore, we identified and characterized two novel lncRNAs, ROBO1-AS and ROBO2-AS, that display overlapping gene organization with axon guidance pathway genes ROBO1 and ROBO2, respectively, producing sense-antisense gene pairs. Notably, ROBO1-AS and ROBO2-AS act in cis to form lncRNA/mRNA duplexes with ROBO1 and ROBO2, respectively, suggesting important roles for these lncRNAs in the ROBO regulatory network. A subsequent WGS identified candidate genes within the genomic region on CFA31 that might be implicated in modifying severity of XLPRA1. This approach led to discovery of genetic variants in ROBO1, ROBO1-AS, ROBO2-AS, and USP25 that are strongly associated with the XLPRA1 moderate phenotype.

Conclusions

The study provides new insights into the genetic basis of phenotypic variation in severity of RPGR^orf15-associated retinal degeneration. Our findings suggest an important role for ROBO pathways in disease progression further expanding on our previously reported changes of ROBO1 expression in XLPRA1 retinas.

The entangled relationship between cilia and actin

Primary cilia are microtubule-based sensory cell organelles that are vital for tissue and organ development. They act as an antenna, receiving and transducing signals, enabling communication between cells. Defects in ciliogenesis result in severe genetic disorders collectively termed ciliopathies. In recent years, the importance of the direct and indirect involvement of actin regulators in ciliogenesis came into focus as it was shown that F-actin polymerisation impacts ciliation. The ciliary basal body was further identified as both a microtubule and actin organising centre. In the current review, we summarize recent studies on F-actin in and around primary cilia, focusing on different actin regulators and their effect on ciliogenesis, from the initial steps of basal body positioning and regulation of ciliary assembly and disassembly. Since primary cilia are also involved in several intracellular signalling pathways such as planar cell polarity (PCP), subsequently affecting actin rearrangements, the multiple effectors of this pathway are highlighted in more detail with a focus on the feedback loops connecting actin networks and cilia proteins. Finally, we elucidate the role of actin regulators in the development of ciliopathy symptoms and cancer.