Retinal Pigment Epithelium and Neural Retinal Progenitors Interact via Semaphorin 6D to Facilitate Optic Cup Morphogenesis

Abstract

Cell movement propels embryonic tissues to acquire shapes required for mature function.The movements are driven both by acto-myosin signaling, and by cells interacting with the extracellular matrix. Unknown is whether cell-cell interactions within a tissue are also required, and the molecular mechanisms by which such communication might occur. Here we use the developing visual system of zebrafish as a model to understand the role cell-cell communication plays in tissue morphogenesis in the embryonic nervous system. We identify that cell-cell mediated contact between two distinct cell populations, progenitors of the neural retina and retinal pigment epithelium, facilitates epithelial flow to produce the mature cupped retina. We identify for the first time the need in eye morphogenesis for distinct populations of progenitors to interact, and suggest a novel role for a member of a key developmental signaling family, the transmembrane Semaphorin6d, as mediating communication between distinct cell types to control tissue morphogenesis. Significance Statement:We identify that cell-cell communication between neural progenitors in the embryonic eye and their non-neural, retinal pigmented epithelial cell neighbours promotes the cell movements which are required to form a cupped retina that mediates vision. We also reveal the repulsive guidance molecule Semaphorin6d as the molecular mechanism that allows these two distinct cell populations to work together to orchestrate optic cup morphogenesis.

Plexina4 and cell survival in the developing zebrafish hindbrain

“…The foxa2 probe was a gift from Dr. Peng Huang (University of Calgary). The fgf8a , pax6a , foxg1a and sema3fa probes were made as described previously 8,30,31 …”

“…The fgf8a, pax6a, foxg1a and sema3fa probes were made as described previously. 8,30,31 The in situ hybridization protocol was adapted from existing methods. 32 Minor modifications included incubating the embryos in anti-DIG-AP Fab fragments (Roche) in blocking buffer at room temperature (RT) for 2 h and then leaving the embryos in phosphate buffered saline/0.1% tween (PBST) overnight at 4 C. Embryos were washed in alkaline buffer, transferred to 24-well culture dishes (Thermo Scientific), and stained with NBT/BCIP (Roche) in a 4.5/3.5 μl/ml ratio, respectively.…”

“…6,7 In addition, the behavior of neural progenitors can be regulated by SEMA/PLXN signaling. For instance, Sema6s control the movements of zebrafish eye and retinal pigment epithelium progenitors, 8,9 and SEMA3B regulates the divisions of murine spinal cord progenitors. 10 Thus, PLXNs may have additional, unappreciated roles in nervous system development.…”

Optic cup morphogenesis across species and related inborn human eye defects

“…An unresolved issue is whether and how these morphogenetic forces are coordinated and sensed. Part of the coordination may rely on inner-outer layer interactions, mediated by molecules such as semaphorins and their plexin receptors, as abrogation of their expression results in incomplete ventral OC formation ( Cechmanek et al, 2021 ). It is tempting to speculate that mechano-sensor and -transducer proteins, such as Yes1-associated transcriptional regulator 1 (Yap1), may instead be involved in sensing RPE tension, a function that Yap1 exerts in many different tissues ( Totaro et al, 2018 ).…”