A Novel Mouse Dscam Mutation Inhibits Localization and Shedding of DSCAM

R Dee Schramm,F Marty Ytreberg,Shuai Li,Peter G Fuerst,Robert W Burgess,Belinda S Harris,Ryan P Rounds,Cheng-Ting Chien

doi:10.1371/journal.pone.0052652

R Dee Schramm, F Marty Ytreberg + Show 6 more

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https://doi.org/10.1371/journal.pone.0052652

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Abstract

The differential adhesion hypothesis of development states that patterning of organisms, organs and tissues is mediated in large part by expression of cell adhesion molecules. The cues provided by cell adhesion molecules are also hypothesized to facilitate specific connectivity within the nervous system. In this study we characterize a novel mouse mutation in the gene Dscam (Down Syndrome Cell Adhesion Molecule). Vertebrate DSCAM is required for normal development of the central nervous system and has been best characterized in the visual system. In the visual system DSCAM is required for regulation of cell number, mosaic formation, laminar specificity, and refinement of retinal-tectal projections. We have identified a novel mutation in Dscam that results in a single amino acid substitution, R1018P, in the extracellular domain of the DSCAM protein. Mice homozygous for the R1018P mutation develop a subset of defects observed in Dscam null mice. In vitro analysis identified defects in DSCAMR1018P localization to filopodia. We also find that wild type DSCAM protein is constitutively cleaved and shed from transfected cells. This secretion is inhibited by the R1018P mutation. We also characterized a novel splice isoform of Dscam and identified defects in lamination of type 2 and type 6 cone bipolar cells in Dscam mutant mice. The identification and characterization of partial loss of function mutations in genes such as Dscam will be helpful in predicting signs and symptoms that may be observed in human patients with partial loss of DSCAM function.

Highlights

Identifying the mechanisms by which cells differentiate into complex tissues is a central goal of developmental biology
Based on the presentation of these phenotypes, and their initial mapping to the distal end of Chromosome 16, a complementation test was established between nm2122 and Dscam2J, a mutant mouse line that carries a loss of function allele of Dscam [15,16]
The nm2122 Dscam open reading frame was sequenced from whole brain cDNA and a single nucleotide substitution, guanine to cytosine at nucleotide 3052 of the Dscam transcript, was identified (Figure 1D)

Summary

Introduction

Identifying the mechanisms by which cells differentiate into complex tissues is a central goal of developmental biology. Semaphorins and plexins prevent fasciculation of the neurites of some cell types in which they are expressed [7,8,9] Proteins such as MEGF10 and MEGF11 mediate avoidance and are required in order to facilitate horizontal spacing of cholinergic and horizontal cells within the retina, while the gamma-protocadherin complex mediates isoneuronal avoidance between the processes of a single cell [10,11]. DSCAM is produced in a large number of spatially overlapping cell types that all independently require DSCAM to prevent adhesion, but maintain their homotypic identity in its absence, in that they adhere to like cells This suggests that the simple and elegant model proposed for MEGF protein function, in which a homophilic adhesion molecule mediates avoidance of cells within a cell type from like cells, is insufficient to explain DSCAM function. Our findings suggest a model in which wild type DSCAM can act as a diffusible ligand

Results

Discussion

Materials and Methods