Xenopus cadherin-6 regulates growth and epithelial development of the retina

The Role of Wnt/β-Catenin Signaling and K-Cadherin in the Regulation of Intraocular Pressure

Purpose

Wnt/β-catenin signaling in the trabecular meshwork (TM) is required for maintaining normal intraocular pressure (IOP), although the mechanism(s) behind this are unknown. We hypothesize that Wnt/β-catenin signaling regulates IOP via β-catenin's effects on cadherin junctions.

Methods

Nonglaucomatous primary human TM (NTM) cells were treated with or without 100 ng/ml Wnt3a, 1 μg/ml sFRP1, or both for 4 to 48 hours. Cells were immunostained for β-catenin, total cadherins, or cadherin isoforms. Membrane proteins or whole-cell lysates were isolated for Western immunoblotting and probed for cadherin isoforms. RNA was extracted for cDNA synthesis and qPCR analysis of cadherin expression. Some NTM cells were cultured on electric plates for cell impedance assays. Ad5.CMV recombinant adenoviruses encoding K-cadherin, and/or sFRP1 were injected into eyes of 4- to 6-month-old female BALB/cJ mice (n = 8–10). Conscious IOPs were assessed for 35 days.

Results

Upon Wnt3a treatment, total cadherin expression increased and β-catenin accumulated at the TM cell membrane and on processes formed between TM cells. qPCR showed that Wnt3a significantly increased K-cadherin expression in NTM cells (P < 0.01, n = 3), and Western immunoblotting showed that Wnt3a increased K-cadherin in NTM cells, which was inhibited by the addition of sFRP1. Cell impedance assays showed that Wnt3a treatment increased transcellular resistance and anti-K-cadherin siRNA decreased transcellular resistance (P < 0.001, n = 4–6). Our in vivo study showed that K-cadherin significantly decreased sFRP1-induced ocular hypertension (P < 0.05, n = 6). Western immunoblotting also showed that K-cadherin alleviated sFRP1-induced β-catenin decrease in mouse anterior segments.

Conclusions

Our results suggest that cadherins play important roles in the regulation of TM homeostasis and IOP via the Wnt/β-catenin pathway.

Differential expression of protocadherin-19, protocadherin-17, and cadherin-6 in adult zebrafish brain

Cell adhesion molecule cadherins play important roles in both development and maintenance of adult structures. Most studies on cadherin expression have been carried out in developing organisms, but information on cadherin distribution in adult vertebrate brains is limited. In this study we used in situ hybridization to examine mRNA expression of three cadherins, protocadherin-19, protocadherin-17, and cadherin-6 in adult zebrafish brain. Each cadherin exhibits a distinct expression pattern in the fish brain, with protocadherin-19 and protocadherin-17 showing much wider and stronger expression than that of cadherin-6. Both protocadherin-19 and protocadherin-17-expressing cells occur throughout the brain, with strong expression in the ventromedial telencephalon, periventricular regions of the thalamus and anterior hypothalamus, stratum periventriculare of the optic tectum, dorsal tegmental nucleus, granular regions of the cerebellar body and valvula, and superficial layers of the facial and vagal lobes. Numerous sensory structures (e.g., auditory, gustatory, lateral line, olfactory, and visual nuclei) and motor nuclei (e.g., oculomotor, trochlear, trigeminal motor, abducens, and vagal motor nuclei) contain protocadherin-19 and/or protocadherin-17-expressing cell. Expression of these two protocadherins is similar in the ventromedial telencephalon, thalamus, hypothalamus, facial, and vagal lobes, but substantially different in the dorsolateral telencephalon, intermediate layers of the optic tectum, and cerebellar valvula. In contrast to the two protocadherins, cadherin-6 expression is much weaker and limited in the adult fish brain.

Protocadherin-9 involvement in retinal development in Xenopus laevis

Biological roles of most protocadherins (Pcdhs) are a largely unsolved problem. Therefore, we cloned cDNA for Xenopus laevis protocadherin-9 and characterized its properties to elucidate the role. The deduced amino acid sequence was highly homologous to those of mammalian protocadherin-9 s. X. laevis protocadherin-9 expressed from the cDNA in L cells showed basic properties similar to those of mammalian Pcdhs. Expression of X. laevis protocadherin-9 was first detected in stage-31 embryos and increased as the development proceeded. In the later stage embryos and the adults, the retina strongly expressed protocadherin-9, which was mainly localized at the plexiform layers. Injection of morpholino anti-sense oligonucleotide against protocadherin-9 into the fertilized eggs inhibited eye development; and eye growth and formation of the retinal laminar structure were hindered. Moreover, affected retina showed abnormal extension of neurites into the ganglion cell layer. Co-injection of protocadherin-9 mRNA with the morpholino anti-sense oligonucleotide rescued the embryos from the defects. These results suggest that X. laevis protocadherin-9 was involved in the development of retina structure possibly through survival of neurons, formation of the lamina structure and neurite localization.

Protocadherin-17 function in Zebrafish retinal development

Cadherin cell adhesion molecules play crucial roles in vertebrate development including the development of the retina. Most studies have focused on examining functions of classic cadherins (e.g. N-cadherin) in retinal development. There is little information on the function of protocadherins in the development of the vertebrate visual system. We previously showed that protocadherin-17 mRNA was expressed in developing zebrafish retina during critical stages of the retinal development. To gain insight into protocadherin-17 function in the formation of the retina, we analyzed eye development and differentiation of retinal cells in zebrafish embryos injected with protocadherin-17 specific antisense morpholino oligonucleotides (MOs). Protocadherin-17 knockdown embryos (pcdh17 morphants) had significantly reduced eyes due mainly to decreased cell proliferation. Differentiation of several retinal cell types (e.g. retinal ganglion cells) was also disrupted in the pcdh17 morphants. Phenotypic rescue was achieved by injection of protocadherin-17 mRNA. Injection of a vivo-protocadherin-17 MO into one eye of embryonic zebrafish resulted in similar eye defects. Our results suggest that protocadherin-17 plays an important role in the normal formation of the zebrafish retina.

Cell adhesion molecule cadherin-6 function in zebrafish cranial and lateral line ganglia development

Cadherins regulate the vertebrate nervous system development. We previously showed that cadherin-6 message (cdh6) was strongly expressed in the majority of the embryonic zebrafish cranial and lateral line ganglia during their development. Here, we present evidence that cdh6 has specific functions during cranial and lateral line ganglia and nerve development. We analyzed the consequences of cdh6 loss-of-function on cranial ganglion and nerve differentiation in zebrafish embryos. Embryos injected with zebrafish cdh6 specific antisense morpholino oligonucleotides (MOs, which suppress gene expression during development; cdh6 morphant embryos) displayed a specific phenotype, including (i) altered shape and reduced development of a subset of the cranial and lateral line ganglia (e.g., the statoacoustic ganglion and vagal ganglion) and (ii) cranial nerves were abnormally formed. These data illustrate an important role for cdh6 in the formation of cranial ganglia and their nerves.

microRNA-24a is required to repress apoptosis in the developing neural retina

Programmed cell death is important for the proper development of the retina, and microRNAs (miRNAs) may be critical for its regulation. Here, we report that miR-24a is expressed in the neural retina and is required for correct eye morphogenesis in Xenopus. Inhibition of miR-24a during development causes a reduction in eye size due to a significant increase in apoptosis in the retina, whereas overexpression of miR-24a is sufficient to prevent apoptosis. We show that miR-24a negatively regulates the proapoptotic factors caspase9 and apaf1, demonstrating a role for miRNAs in the regulation of apoptosis during normal development.

Cadherin-6 function in zebrafish retinal development

Cadherin cell-adhesion molecules play crucial roles in vertebrate development including the development of the visual system. Most studies have focused on examining functions of classical type I cadherins (e.g., cadherin-2) in visual system development. There is little information on the function of classical type II cadherins (e.g., cadherin-6) in the development of the vertebrate visual system. To gain insight into cadherin-6 role in the formation of the retina, we analyzed differentiation of retinal ganglion cells (RGCs), amacrine cells, and photoreceptors in zebrafish embryos injected with cadherin-6 specific antisense morpholino oligonucleotides. Differentiation of the retinal neurons in cadherin-6 knockdown embryos (cdh6 morphants) was analyzed using multiple markers. We found that expression of transcription factors important for retinal development was greatly reduced, and expression of Notch-Delta genes and proneural gene ath5 was altered in the cdh6 morphant retina. The retinal lamination was present in the morphants, although the morphant eyes were significantly smaller than control embryos due mainly to decreased cell proliferation. Differentiation of the RGCs, amacrine cells, and photoreceptors was severely disrupted in the cdh6 morphants due to a significant delay in neural differentiation. Our results suggest that cadherin-6 plays an important role in the normal formation of the zebrafish retina. (c) 2008 Wiley Periodicals, Inc. Develop Neurobiol, 2008.

Layer-specific expression of multiple cadherins in the developing visual cortex (V1) of the ferret

Cadherins are superfamily of Ca2+-dependent transmembrane glycoproteins with more than 100 members. They play a role in a wide variety of developmental mechanisms, including cell proliferation, cell differentiation, cell-cell recognition, neurite outgrowth and synaptogenesis. We cloned 16 novel members of the classic cadherin and delta-protocadherin subgroups from ferret brain. Their expression patterns were investigated by in situ hybridization in the developing primary visual cortex (V1) of the ferret. Fifteen out of the 16 cadherins are expressed in a spatiotemporally restricted fashion throughout development. Each layer of V1 can be characterized by the combinatorial expression of a subset of cadherins at any given developmental stage. A few cadherins are expressed by subsets of neurons in specific layers or by neurons dispersed throughout all cortical layers. Generally, the expression of protocadherins is more widespread, whereas that of classic cadherins is more restricted to specific layers. At the V1/V2 boundary, changes in layer-specific cadherin expression are observed. In conclusion, our results suggest that cadherins provide a code of potentially adhesive cues for layer formation in ferret V1. The persistence of expression in the adult suggests a functional role also in the mature cortex.

The genesis of retinal architecture: an emerging role for mechanical interactions?

Patterns in nature have always fascinated human beings. They convey the idea of order, organization and optimization, and, to the enquiring mind, the alluring promise that understanding their building rules may uncover the forces that shaped them. In the retina, two patterns are outstanding: the stacking of cells in layers and, within the layers, the prevalent arrangement of neurons of the same type in orderly arrays, often referred to as mosaics for the crystalline-like order that some can display. Layers and mosaics have been essential keys to our present understanding of retinal circuital organization and function. Now, they may also be a precious guide in our exploration of how the retina is built. Here, we will review studies addressing the mechanisms controlling the formation of retinal mosaics and layers, illustrating common themes and unsolved problems. Among the intricacies of the building process, a world of physical forces is making its appearance. Cells are extremely complex to model as "physical entities", and many aspects of cell mechanotransduction are still obscure. Yet, recent experiments, focusing on the mechanical aspects of growth and differentiation, suggest that adopting this viewpoint will open new ways of understanding retinal formation and novel possibilities to approach retinal pathologies and repair.