Down syndrome cell adhesion molecule is important for early development in Xenopus tropicalis

The regulatory proteins DSCR6 and Ezh2 oppositely regulate Stat3 transcriptional activity in mesoderm patterning during Xenopus development

Embryonic cell fate specification and axis patterning requires integration of several signaling pathways that orchestrate region-specific gene expression. The transcription factor signal transducer and activator of transcription 3 (Stat3) plays important roles during early development, but it is unclear how Stat3 is activated. Here, using Xenopus as a model, we analyzed the post-translational regulation and functional consequences of Stat3 activation in dorsoventral axis patterning. We show that Stat3 phosphorylation, lysine methylation, and transcriptional activity increase before gastrulation and induce ventral mesoderm formation. Down syndrome critical region gene 6 (DSCR6), a RIPPLY family member that induces dorsal mesoderm by releasing repressive polycomb group proteins from chromatin, bound to the Stat3 C-terminal region and antagonized its transcriptional and ventralizing activities by interfering with its lysine methylation. Enhancer of zeste 2 polycomb-repressive complex 2 subunit (Ezh2) also bound to this region; however, its methyltransferase activity was required for Stat3 methylation and activation. Loss of Ezh2 resulted in dorsalization of ventral mesoderm and formation of a secondary axis. Furthermore, interference with Ezh2 phosphorylation also prevented Stat3 lysine methylation and transcriptional activity. Thus, inhibition of either Ezh2 phosphorylation or Stat3 lysine methylation compensated for the absence of DSCR6 function. These results reveal that DSCR6 and Ezh2 critically and post-translationally regulate Stat3 transcriptional activity. Ezh2 promotes Stat3 activation in ventral mesoderm formation independently of epigenetic regulation, whereas DSCR6 specifies dorsal fate by counteracting this ventralizing activity. This antagonism helps pattern the mesoderm along the dorsoventral axis, representing a critical facet of cell identity regulation during development.

Zebrafish expression reporters and mutants reveal that the IgSF cell adhesion molecule Dscamb is required for feeding and survival

Down syndrome cell adhesion molecules (DSCAMs) are broadly expressed in nervous systems and play conserved roles in programmed cell death, neuronal migration, axon guidance, neurite branching and spacing, and synaptic targeting. However, DSCAMs appear to have distinct functions in different vertebrate animals, and little is known about their functions outside the retina. We leveraged the genetic tractability and optical accessibility of larval zebrafish to investigate the expression and function of a DSCAM family member, dscamb. Using targeted genome editing to create transgenic reporters and loss-of-function mutant alleles, we discovered that dscamb is expressed broadly throughout the brain, spinal cord, and peripheral nervous system, but is not required for overall structural organization of the brain. Despite the absence of obvious anatomical defects, homozygous dscamb mutants were deficient in their ability to ingest food and rarely survived to adulthood. Thus, we have discovered a novel function for dscamb in feeding behavior. The mutant and transgenic lines generated in these studies will provide valuable tools for identifying the molecular and cellular bases of these behaviors.

Novel axon projection after stress and degeneration in the Dscam mutant retina

The Down syndrome cell adhesion molecule gene (Dscam) is required for normal dendrite patterning and promotes developmental cell death in the mouse retina. Loss-of-function studies indicate that Dscam is required for refinement of retinal ganglion cell (RGC) axons in the lateral geniculate nucleus, and in this study we report and describe a requirement for Dscam in the maintenance of RGC axon projections within the retina. Mouse Dscam loss of function phenotypes related to retinal ganglion cell axon outgrowth and targeting have not been previously reported, despite the abundance of axon phenotypes reported in Drosophila Dscam1 loss and gain of function models. Analysis of the Dscam deficient retina was performed by immunohistochemistry and Western blot analysis during postnatal development of the retina. Conditional targeting of Dscam and Jun was performed to identify factors underlying axon-remodeling phenotypes. A subset of RGC axons were observed to project and branch extensively within the Dscam mutant retina after eye opening. Axon remodeling was preceded by histological signs of RGC stress. These included neurofilament accumulation, axon swelling, axon blebbing and activation of JUN, JNK and AKT. Novel and extensive projection of RGC axons within the retina was observed after upregulation of these markers, and novel axon projections were maintained to at least one year of age. Further analysis of retinas in which Dscam was conditionally targeted with Brn3b or Pax6α Cre indicated that axon stress and remodeling could occur in the absence of hydrocephalus, which frequently occurs in Dscam mutant mice. Analysis of mice mutant for the cell death gene Bax, which executes much of Dscam dependent cell death, identified a similar axon misprojection phenotype. Deleting Jun and Dscam resulted in increased axon remodeling compared to Dscam or Bax mutants. Retinal ganglion cells have a very limited capacity to regenerate after damage in the adult retina, compared to the extensive projections made in the embryo. In this study we find that DSCAM and JUN limit ectopic growth of RGC axons, thereby identifying these proteins as targets for promoting axon regeneration and reconnection.

DSCAM promotes refinement in the mouse retina through cell death and restriction of exploring dendrites

In this study we develop and use a gain-of-function mouse allele of the Down syndrome cell adhesion molecule (Dscam) to complement loss-of-function models. We assay the role of Dscam in promoting cell death, spacing, and laminar targeting of neurons in the developing mouse retina. We find that ectopic or overexpression of Dscam is sufficient to drive cell death. Gain-of-function studies indicate that Dscam is not sufficient to increase spatial organization, prevent cell-to-cell pairing, or promote active avoidance in the mouse retina, despite the similarity of the Dscam loss-of-function phenotype in the mouse retina to phenotypes observed in Drosophila Dscam1 mutants. Both gain- and loss-of-function studies support a role for Dscam in targeting neurites; DSCAM is necessary for precise dendrite lamination, and is sufficient to retarget neurites of outer retinal cells after ectopic expression. We further demonstrate that DSCAM guides dendrite targeting in type 2 dopaminergic amacrine cells, by restricting the stratum in which exploring retinal dendrites stabilize, in a Dscam dosage-dependent manner. Based on these results we propose a single model to account for the numerous Dscam gain- and loss-of-function phenotypes reported in the mouse retina whereby DSCAM eliminates inappropriately placed cells and connections.