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

Abstract

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 differentially modulates pre- and postsynaptic structural and functional central connectivity during visual system wiring

“…These effects were opposite to those of DSCAM downregulation in tectal neurons, where dendritic arbors overgrow, and neurons extend multiple axon terminals. While no obvious targeting errors were observed in individual RGC axons with DSCAM knockdown, errors in axons being able to exit the eye were observed when overexpressing DSCAM in developing RGCs of young tadpoles (data not shown), consistent with observations of misdirected RGC axons within the retina of adult DSCAM mutant mice [ 38 ]. An instructive role for DSCAM on presynaptic arbor growth that is independent of its effects on dendrites has been demonstrated for Drosophila sensory neurons, where Dscam expression levels and homophilic interactions correlate with patterned presynaptic arbor branching and size [ 39 , 40 ].…”

“…An emerging concept is that molecules that participate in neuronal wiring and that are aberrantly expressed in Down syndrome may differentially impact multiple cell-types, may affect each cell type at different times in development, and may continue to affect neuronal function even in the adult CNS [ 4 , 38 ]. A leading cause of abnormal cognitive and sensory disabilities in individuals with Down syndrome has been attributed to aberrant changes in neuronal wiring during human embryonic development.…”

Mkk4 and Mkk7 are important for retinal development and axonal injury-induced retinal ganglion cell death

“…DSCAM is a netrin-1 receptor necessary for neurite arborization and prevention of abnormal neural fasciculations 55 , 56 . Axonal fasciculations and ectopic photoreceptor phenotypes similar to those in Mkk4/Mkk7 -deficient retinas were observed in Dscam -deficient retinas 57 . Proper fasciculation is necessary for RGC axon pathfinding; however, as axons approach deeper targets in the brain, they must disassociate from their neighbors 58 .…”

Mkk4 and Mkk7 are important for retinal development and axonal injury-induced retinal ganglion cell death

“…Down syndrome cell adhesion molecule (DSCAM) is a netrin-1 receptor necessary for neurite arborization and prevention of abnormal neural fasciculations 55,56 . Axonal fasciculations and ectopic photoreceptor phenotypes similar to those in Mkk4/Mkk7 deficient retinas were similarly observed in dscam deficient retinas 57 . Proper fasciculation is necessary for RGC axon pathfinding, however, as axons approach deeper targets in the brain, they must disassociate from their neighbors 58 .…”