Retinal Degeneration Protein 3 (RD3) in normal human tissues: Novel insights

In vivo CRISPR screening directly targeting testicular cells

CRISPR-Cas9 short guide RNA (sgRNA) library screening is a powerful approach to understand the molecular mechanisms of biological phenomena. However, its in vivo application is currently limited. Here, we developed our previously established in vitro revival screening method into an in vivo one to identify factors involved in spermatogenesis integrity by utilizing sperm capacitation as an indicator. By introducing an sgRNA library into testicular cells, we successfully pinpointed the retinal degeneration 3 (Rd3) gene as a significant factor in spermatogenesis. Single-cell RNA sequencing (scRNA-seq) analysis highlighted the high expression of Rd3 in round spermatids, and proteomics analysis indicated that Rd3 interacts with mitochondria. To search for cell-type-specific signaling pathways based on scRNA-seq and proteomics analyses, we developed a computational tool, Hub-Explorer. Through this, we discovered that Rd3 modulates oxidative stress by regulating mitochondrial distribution upon ciliogenesis induction. Collectively, our screening system provides a valuable in vivo approach to decipher molecular mechanisms in biological processes.

Retinal degeneration protein 3 controls membrane guanylate cyclase activities in brain tissue

The retinal degeneration protein RD3 is involved in regulatory processes of photoreceptor cells. Among its main functions is the inhibition of photoreceptor specific membrane guanylate cyclases during trafficking from the inner segment to their final destination in the outer segment. However, any physiological role of RD3 in non-retinal tissue is unsolved at present and specific protein targets outside of retinal tissue have not been identified so far. The family of membrane bound guanylate cyclases share a high homology of their amino acid sequences in their cytoplasmic domains. Therefore, we reasoned that membrane guanylate cyclases that are activated by natriuretic peptides are also regulated by RD3. We analyzed transcript levels of the rd3 gene and natriuretic peptide receptor genes Npr1 and Npr2 in the mouse retina, cerebellum, hippocampus, neocortex, and the olfactory bulb during development from the embryonic to the postnatal stage at P60. The rd3 gene showed a lower expression level than Npr1 and Npr2 (encoding for GC-A and GC-B, respectively) in all tested brain tissues, but was at least one order of magnitude higher in the retina. RD3 and natriuretic peptide receptor GCs co-express in the retina and brain tissue leading to functional tests. We expressed GC-A and GC-B in HEK293T cells and measured the inhibition of GCs by RD3 after activation by natriuretic peptides yielding inhibitory constants around 25 nM. Furthermore, endogenous GCs in astrocytes were inhibited by RD3 to a similar extent. We here show for the first time that RD3 can inhibit two hormone-stimulated GCs, namely GC-A and GC-B indicating a new regulatory feature of these hormone receptors.

Transcriptome changes in undifferentiated Caco-2 cells exposed to food-grade titanium dioxide (E171): contribution of the nano- and micro- sized particles

The food additive titanium dioxide (TiO2), or E171, is a white food colorant. Recent studies showed after E171 ingestion a significantly increased number of colorectal tumours in a colorectal cancer mouse model as well as inflammatory responses and dysregulation of the immune system in the intestine of rats. In the mouse colon, E171 induced gene expression changes related to oxidative stress, impairment of the immune system, activation of signalling and cancer-related processes. E171 comprises nanoparticles (NPs) and microparticles (MPs). Previous in vitro studies showed that E171, NPs and MPs induced oxidative stress responses, DNA damage and micronuclei formation. This study aimed to investigate the relative contribution of the NPs and MPs to effects of E171 at the transcriptome level in undifferentiated Caco-2 cells by genome wide microarray analysis. The results showed that E171, NPs, and MPs induce gene expression changes related to signalling, inflammation, immune system, transport and cancer. At the pathway level, metabolism of proteins with the insulin processing pathway and haemostasis were specific to E171 exposure. The gene expression changes associated with the immune system and inflammation induced by E171, MPs, and NPs suggest the creation of a favourable environment for colon cancer development.

De novo regulation of RD3 synthesis in residual neuroblastoma cells after intensive multi-modal clinical therapy harmonizes disease evolution

Most high-risk neuroblastomas that initially respond to therapy will ultimately relapse. Currently, no curative treatment is available. Acquired genetic/molecular rearrangement in therapy-resistant cells contributes to tumor relapse. Recently, we identified significant RD3 loss in progressive disease (PD) and defined its association with advanced disease-stage and poor clinical outcomes. Here, we investigated whether RD3 loss is an acquired process in cells that survive intensive multi-modal clinical therapy (IMCT) and its significance in disease evolution. RD3 status (mRNA, protein) during diagnosis (Dx) and PD after IMCT was investigated in NB patient cohort (n = 106), stage-4 NB cell lines (n = 15) with known treatment status and validated with independent data from another set of 15 cell-lines. Loss of RD3 in metastatic disease was examined using a mouse model of PD and metastatic-site-derived aggressive cells (MSDACs) ex vivo. RD3 silencing/expression assessed changes in metastatic state. Influence of RD3 loss in therapy resistance was examined through independent in vitro and in vivo studies. A significant loss of RD3 mRNA and protein was observed in resistant cells derived from patients with PD after IMCT. This is true to the effect within and between patients. Results from the mouse model identified significant transcriptional/translational loss of RD3 in metastatic tumors and MSDACs. RD3 re-expression in MSDACs and silencing RD3 in parental cells defined the functional relevance of RD3-loss in PD pathogenesis. Analysis of independent studies with salvage therapeutic agents affirmed RD3 loss in surviving resistant cells and residual tumors. The profound reductions in RD3 transcription indicate the de novo regulation of RD3 synthesis in resistant cells after IMCT. Defining RD3 loss in PD and the benefit of targeted reinforcement could improve salvage therapy for progressive neuroblastoma.

Control of the Nucleotide Cycle in Photoreceptor Cell Extracts by Retinal Degeneration Protein 3

Retinal degeneration protein 3 (RD3) is crucial for photoreceptor cell survival and linked to Leber Congenital Amaurosis type 12 (LCA12), a hereditary retinal disease in humans. RD3 inhibits photoreceptor guanylate cyclases GC-E and GC-F and is involved in transport of GCs from the inner to the outer segments. Otherwise, its role in photoreceptor physiology is poorly understood. Here, we describe a new function of RD3. Purified RD3 evoked an increase in guanylate kinase activity, an enzyme that is involved in the nucleotide cycle in photoreceptors. We demonstrate a direct interaction between guanylate kinase and RD3 using back-scattering interferometry and show by immunohistochemistry of mouse retina sections that RD3 and guanylate kinase co-localize in photoreceptor inner segments and to a lesser extent in the outer plexiform layer. Our findings point toward a more complex function of RD3 in photoreceptors. The RD3 – guanylate kinase interaction may also play a role in other cellular systems, while the GC – RD3 interaction is exclusive to photoreceptors.