The rod synapse in aging wildtype and Dscaml1 mutant mice

The retina is an intricately organized neural tissue built on cone and rod pathways for color and night vision. Genetic mutations that disrupt the proper function of the rod circuit contribute to blinding diseases including retinitis pigmentosa and congenital stationary night blindness (CSNB). Down Syndrome cell adhesion molecule like 1 (Dscaml1) is expressed by rods, rod bipolar cells (RBCs), and sub-populations of amacrine cells, and has been linked to a middle age onset of CSNB in humans. However, how Dscaml1 contributes to this visual deficit remains unexplored. Here, we probed Dscaml1's role in the maintenance of the rod-to-RBC synapse using a loss of function mouse model. We used immunohistochemistry to investigate the anatomical formation and maintenance of the rod-to-RBC synapse in the young, adult, and aging retina. We generated 3D reconstructions, using serial electron micrographs, of rod spherules and RBCs to measure the number of invaginating neurites, RBC dendritic tip number, and RBC mitochondrial morphology. We find that while rod-to-RBC synapses form and are maintained, similar to wildtype, that there is an increase in the number of invaginating neurites in rod spherules, a reduction in RBC dendritic tips, and reduced mitochondrial volume and complexity in the Dscaml1 mutant retina compared to controls. We also observed precocious sprouting of RBC dendrites into the outer nuclear layer (ONL) of the Dscaml1 mutant retina compared to controls. These results contribute to our knowledge of Dscaml1's role in rod circuit development and maintenance and give additional insight into possible genetic therapy targets for blinding diseases and disorders like CSNB.

DSCAM gene triplication causes excessive GABAergic synapses in the neocortex in Down syndrome mouse models

Down syndrome (DS) is caused by the trisomy of human chromosome 21 (HSA21). A major challenge in DS research is to identify the HSA21 genes that cause specific symptoms. Down syndrome cell adhesion molecule (DSCAM) is encoded by a HSA21 gene. Previous studies have shown that the protein level of the Drosophila homolog of DSCAM determines the size of presynaptic terminals. However, whether the triplication of DSCAM contributes to presynaptic development in DS remains unknown. Here, we show that DSCAM levels regulate GABAergic synapses formed on neocortical pyramidal neurons (PyNs). In the Ts65Dn mouse model for DS, where DSCAM is overexpressed due to DSCAM triplication, GABAergic innervation of PyNs by basket and chandelier interneurons is increased. Genetic normalization of DSCAM expression rescues the excessive GABAergic innervations and the increased inhibition of PyNs. Conversely, loss of DSCAM impairs GABAergic synapse development and function. These findings demonstrate excessive GABAergic innervation and synaptic transmission in the neocortex of DS mouse models and identify DSCAM overexpression as the cause. They also implicate dysregulated DSCAM levels as a potential pathogenic driver in related neurological disorders.

Student remote and distance research in neuroanatomy: Mapping Dscaml1 expression with a LacZ gene trap in mouse brain

Undergraduate student engagement in research increases retention and degree completion, especially for students who are underrepresented in science. Several approaches have been adopted to increase research opportunities including curriculum based undergraduate research opportunities (CUREs), in which research is embedded into course content. Here we report on efforts to tackle a different challenge: providing research opportunities to students engaged in remote learning or who are learning at satellite campuses or community colleges with limited research infrastructure. In our project we engaged students learning remotely or at regional centers to map gene expression in the mouse brain. In this project we mapped expression of the Down syndrome cell adhesion molecule like 1 (Dscaml1) gene in mouse brain using a LacZ expression reporter line. Identifying where Dscaml1 is expressed in the brain is an important next step in determining if its roles in development and function in the retina are conserved in the rest of the brain. Students working remotely reconstruct brain montages and annotated Dscaml1 expression in the brain of mice carrying one or two copies of the gene trap. We built on these findings by further characterizing Dscaml1 expression in inhibitory neurons of the visual pathway. These results build on and extend previous findings and demonstrate the utility of including distance learners in an active research group for both the student learners and the research team. We conclude with best practices we have developed based on this and other distance learner focused projects.

Migrating Pyramidal Neurons Require DSCAM to Bypass the Border of the Developing Cortical Plate

During mammalian neocortex development, nascent pyramidal neurons migrate along radial glial cells and overtake earlier-born neurons to terminate at the front of the developing cortical plate (CP), leading to the outward expansion of the CP border. While much has been learned about the cellular and molecular mechanisms that underlie the migration of pyramidal neurons, how migrating neurons bypass the preceding neurons at the end of migration to reach their final positions remains poorly understood. Here, we report that Down syndrome cell adhesion molecule (DSCAM) is required for migrating neurons to bypass their postmigratory predecessors during the expansion of the upper cortical layers. DSCAM is a type I transmembrane cell adhesion molecule. It has been linked to Down syndrome through its location on Chromosome 21 trisomy and to autism spectrum disorders through loss-of-function mutations. Ex vivo time-lapse imaging demonstrates that DSCAM is required for migrating neurons to bypass their postmigratory predecessors, crossing the CP border to expand the upper cortical layers. In DSCAM-deficient cortices, migrating neurons stop prematurely under the CP border, leading to thinner upper cortical layers with higher neuronal density. We further show that DSCAM weakens cell adhesion mediated by N-cadherin in the upper cortical plate, allowing migrating neurons to traverse the CP border and expand the CP. These findings suggest that DSCAM is required for proper migratory termination and final positioning of nascent pyramidal neurons, which may provide insight into brain disorders that exhibit thinner upper layers of the cerebral cortex without neuronal loss.SIGNIFICANCE STATEMENT Newly born neurons in the developing mammalian neocortex migrate outward toward the cortical surface, bypassing earlier born neurons to expand the developing cortex. How migrating neurons bypass the preceding neurons and terminate at the front of the expanding cortex remains poorly understood. We demonstrate that Down syndrome cell adhesion molecule (DSCAM), linked to Down syndrome and autism spectrum disorder, is required by migrating neurons to bypass their postmigratory predecessors and terminate migration in the outwardly expanding cortical layer. Migrating neurons deficient in DSCAM stop prematurely, failing to expand the cortex. We further show that DSCAM likely mediates migratory termination by weakening cell adhesion mediated by N-cadherin.

OFF bipolar cell density varies by subtype, eccentricity, and along the dorsal ventral axis in the mouse retina

The neural retina is organized along central-peripheral, dorsal-ventral, and laminar planes. Cellular density and distributions vary along the central-peripheral and dorsal-ventral axis in species including primates, mice, fish, and birds. Differential distribution of cell types within the retina is associated with sensitivity to different types of damage that underpin major retinal diseases, including macular degeneration and glaucoma. Normal variation in retinal distribution remains unreported for multiple cell types in widely used research models, including mouse. Here we map the distribution of all known OFF bipolar cell (BC) populations and horizontal cells. We report significant variation in the distribution of OFF BC populations and horizontal cells along the dorsal-ventral and central-peripheral axes of the retina. Distribution patterns are much more pronounced for some populations of OFF BC cells than others and may correspond to the cell type's specialized functions.

Nicotinamide provides neuroprotection in glaucoma by protecting against mitochondrial and metabolic dysfunction

Nicotinamide adenine dinucleotide (NAD) is a REDOX cofactor and metabolite essential for neuronal survival. Glaucoma is a common neurodegenerative disease in which neuronal levels of NAD decline. We assess the effects of nicotinamide (a precursor to NAD) on retinal ganglion cells (the affected neuron in glaucoma) in normal physiological conditions and across a range of glaucoma relevant insults including mitochondrial stress and axon degenerative insults. We demonstrate retinal ganglion cell somal, axonal, and dendritic neuroprotection by nicotinamide in rodent models which represent isolated ocular hypertensive, axon degenerative, and mitochondrial degenerative insults. We performed metabolomics enriched for small molecular weight metabolites for the retina, optic nerve, and superior colliculus which demonstrates that ocular hypertension induces widespread metabolic disruption, including consistent changes to α-ketoglutaric acid, creatine/creatinine, homocysteine, and glycerophosphocholine. This metabolic disruption is prevented by nicotinamide. Nicotinamide provides further neuroprotective effects by increasing oxidative phosphorylation, buffering and preventing metabolic stress, and increasing mitochondrial size and motility whilst simultaneously dampening action potential firing frequency. These data support continued determination of the utility of long-term nicotinamide treatment as a neuroprotective therapy for human glaucoma.

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Nicotinamide is neuroprotective in cell and animal models that recapitulate isolated features of glaucoma.

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Systemic nicotinamide administration has limited molecular side-effects on visual system tissue under basal conditions.

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Nicotinamide provides a robust reversal in the disease metabolic profile of glaucomatous animals.

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Nicotinamide increases oxidative phosphorylation, buffers and prevents metabolic stress, and increases mitochondrial size.

Increased density and age-related sharing of synapses at the cone to OFF bipolar cell synapse in the mouse retina

Neural circuits in the adult nervous system are characterized by stable, cell type-specific patterns of synaptic connectivity. In many parts of the nervous system these patterns are established during development through initial over-innervation by multiple pre- or postsynaptic targets, followed by a process of refinement that takes place during development and is in many instances activity dependent. Here we report on an identified synapse in the mouse retina, the cone photoreceptor➔type 4 bipolar cell (BC4) synapse, and show that its development is distinctly different from the common motif of over-innervation followed by refinement. Indeed, the majority of cones are contacted by single BC4 throughout development, but are contacted by multiple BC4s through ongoing dendritic elaboration between 1 and 6 months of age-well into maturity. We demonstrate that cell density drives contact patterns downstream of single cones in Bax null mice and may serve to maintain constancy in both the dendritic and axonal projective field.

Expression patterns of dscam and sdk gene paralogs in developing zebrafish retina

Purpose

The differential adhesion hypothesis states that a cell adhesion code provides cues that direct the specificity of nervous system development. The Down syndrome cell adhesion molecule (DSCAM) and sidekick (SDK) proteins belong to the immunoglobulin superfamily of cell adhesion molecules (CAMs) and provide both attractive and repulsive cues that help to organize the nervous system during development, according to the differential adhesion hypothesis. The zebrafish genome is enriched in dscam and sdk genes, making the zebrafish an excellent model system to further test this hypothesis. The goal of this study is to describe the phylogenetic relationships of the paralogous CAM genes and their spatial expression and co-expression patterns in the embryonic zebrafish retina.

Methods

Exon-intron structures, karyotypic locations, genomic context, and amino acid sequences of the zebrafish CAM genes (dscama, dscamb, dscaml1, sdk1a, sdk1b, sdk2a, and sdk2b) were obtained from the Ensembl genome database. The Prosite and SMART programs were used to determine the number and identity of protein domains for each CAM gene. The randomized axelerated maximum likelihood (RaxML) program was used to perform a phylogenetic analysis of the zebrafish CAM genes and orthologs in other vertebrates. A synteny analysis of regions surrounding zebrafish CAM paralogs was performed. Digoxigenin (dig)-labeled cRNA probes for each CAM gene were generated to perform in situ hybridization of retinal cryosections from zebrafish embryos and larvae. Dual in situ hybridization of retinal cryosections from zebrafish larvae was performed with dig- and fluorescein-labeled cRNA probes.

Results

We found the studied zebrafish CAM genes encode similar protein domain structures as their corresponding orthologs in mammals and possess similar intron-exon organizations. CAM paralogs were located on different chromosomes. Phylogenetic and synteny analyses provided support for zebrafish dscam and sdk2 paralogs having originated during the teleost genome duplication. We found that dscama and dscamb are co-expressed in the ganglion cell layer (GCL) and the basal portion of the inner nuclear layer (INL), with weak expression in the photoreceptor-containing outer nuclear layer (ONL). Of the dscam genes, only dscamb was strongly expressed in ONL. Sdk1a and sdk1b were co-expressed in the GCL and the basal portion of the INL. Sdk2a and sdk2b also showed co-expression in the GCL and basal portion of the INL. All Sdk genes were expressed in the ciliary marginal zone (CMZ). Dual in situ hybridizations revealed alternating patterns of co-expression and exclusive expression for the dscam and sdk1 paralogs in cells of the GCL and the INL. The same alternating pattern was observed between dscam and sdk2 paralogs and between sdk1 and sdk2 paralogs. The expression of dscaml1 was observed in the INL and the GCL, with some cells in the basal portion of the INL showing co-expression of dscaml1 and dscama.

Conclusions

These findings suggest that zebrafish dscam and sdk2 paralogs were likely the result of the teleost whole genome duplication and that all CAM duplicates show some differential expression patterns. We also demonstrate that the comparative expression patterns of CAM genes in the zebrafish are distinct from the exclusive expression patterns observed in chick retina, in which retinal ganglion cells express one of the four chick Dscam or Sdk genes only. The patterns in zebrafish are more similar to those of mice, in which co-expression of Dscam and Sdk genes is observed. These findings provide the groundwork for future functional analysis of the roles of the CAM paralogs in zebrafish.

Analysis of Retinal Vascular Plexuses and Interplexus Connections

The retina is a highly organized neural tissue consisting of three neural layers and two synaptic layers. Blood vessels that nourish the mouse and human neural retina mirror this organization consisting of three plexus layers, or plexuses, that run parallel within the retina, connected by interplexus vessels to create a closed vascular network. Here, we describe a methodology to describe this organization that can be used to interrogate factors mediating retinal vessel patterning including: coverage of the vascular plexuses, branching and orientation of the interplexus connections, and digital reconstruction of the retinal vasculature to measure vessel length and density. The methodology focuses on the mouse retina, but can easily be adapted to study retinal vessels of other species.

DSCAM-mediated control of dendritic and axonal arbor outgrowth enforces tiling and inhibits synaptic plasticity

Mature mammalian neurons have a limited ability to extend neurites and make new synaptic connections, but the mechanisms that inhibit such plasticity remain poorly understood. Here, we report that OFF-type retinal bipolar cells in mice are an exception to this rule, as they form new anatomical connections within their tiled dendritic fields well after retinal maturity. The Down syndrome cell-adhesion molecule (Dscam) confines these anatomical rearrangements within the normal tiled fields, as conditional deletion of the gene permits extension of dendrite and axon arbors beyond these borders. Dscam deletion in the mature retina results in expanded dendritic fields and increased cone photoreceptor contacts, demonstrating that DSCAM actively inhibits circuit-level plasticity. Electrophysiological recordings from Dscam-/- OFF bipolar cells showed enlarged visual receptive fields, demonstrating that expanded dendritic territories comprise functional synapses. Our results identify cell-adhesion molecule-mediated inhibition as a regulator of circuit-level neuronal plasticity in the adult retina.

Characterization and Evolution of the Spotted Gar Retina

In this study, we characterize the retina of the spotted gar, Lepisosteus oculatus, a ray-finned fish. Gar did not undergo the whole genome duplication event that occurred at the base of the teleost fish lineage, which includes the model species zebrafish and medaka. The divergence of gars from the teleost lineage and the availability of a high-quality genome sequence make it a uniquely useful species to understand how genome duplication sculpted features of the teleost visual system, including photoreceptor diversity. We developed reagents to characterize the cellular organization of the spotted gar retina, including representative markers for all major classes of retinal neurons and Müller glia. We report that the gar has a preponderance of predicted short-wavelength shifted (SWS) opsin genes, including a duplicated set of SWS1 (ultraviolet) sensitive opsin encoding genes, a SWS2 (blue) opsin encoding gene, and two rod opsin encoding genes, all of which were expressed in retinal photoreceptors. We also report that gar SWS1 cones lack the geometric organization of photoreceptors observed in teleost fish species, consistent with the crystalline photoreceptor mosaic being a teleost innovation. Of note the spotted gar expresses both exo-rhodopsin (RH1-1) and rhodopsin (RH1-2) in rods. Exo-rhodopsin is an opsin that is not expressed in the retina of zebrafish and other teleosts, but rather is expressed in regions of the brain. This study suggests that exo-rhodopsin is an ancestral actinopterygian (ray finned fish) retinal opsin, and in teleosts its expression has possibly been subfunctionalized to the pineal gland.

Replacing the PDZ-interacting C-termini of DSCAM and DSCAML1 with epitope tags causes different phenotypic severity in different cell populations

Different types of neurons in the retina are organized vertically into layers and horizontally in a mosaic pattern that helps ensure proper neural network formation and information processing throughout the visual field. The vertebrate Dscams (DSCAM and DSCAML1) are cell adhesion molecules that support the development of this organization by promoting self-avoidance at the level of cell types, promoting normal developmental cell death, and directing vertical neurite stratification. To understand the molecular interactions required for these activities, we tested the functional significance of the interaction between the C-terminus of the Dscams and multi-PDZ domain-containing scaffolding proteins in mouse. We hypothesized that this PDZ-interacting domain would mediate a subset of the Dscams' functions. Instead, we found that in the absence of these interactions, some cell types developed almost normally, while others resembled complete loss of function. Thus, we show differential dependence on this domain for Dscams' functions in different cell types.

Pou4f2 knock-in Cre mouse: A multifaceted genetic tool for vision researchers

PURPOSE

A transgenic mouse that expresses Cre recombinase under control of the Pou4f2-promoter (also referred to as Brn-3b and Brn-3.2) was characterized. Pou4f2 expression has been reported in a subset of retinal ganglion cells (RGCs) in the retina, in the midbrain, and in the germline. In this study, we characterize the expression pattern of this Cre-recombinase line and report its utility in targeted deletion, temporal deletion, RGC depletion, and germline targeting, which can be regulated by the sex of the Cre-carrying mouse.

METHODS

Pou4f2(Cre) was mapped by using a combination of PCR and sequencing of PCR products to better understand the construct and to locate where it was inserted within the Pou4f2 locus. Cre expression patterns were examined by crossing Pou4f2(Cre/+) mice to Cre reporter mice. Immunohistochemistry was used to further define the pattern of Cre expression and Cre-mediated recombination within the retina, brain, and other tissues.

RESULTS

An internal ribosome entry site (IRES)-Cre cassette was inserted into the Pou4f2 gene disrupting normal gene function, as verified by the depletion of RGCs in mice homozygous for the insert. Pou4f2(Cre) expression was observed in the retina, brain, peripheral neurons, and male germ cells. Germline recombination was observed when the sire carried the Cre and the target for recombination. In all other breeding schemes, recombination was observed within subsets of cells within the retina, brain, intestines, heart, and gonads. In the retina, Cre efficiently targets recombination in neurons within the RGC layer (RGL), the inner nuclear layer (INL), and a small percentage of photoreceptors, activity that has not been previously reported. Unlike most other Cre lines active in the inner retina, recombination in Müller and other glia was not observed in mice carrying Pou4f2(Cre) . Within the visual centers of the brain, Cre targets recombination in about 15% of cells within the superchiasmatic nucleus, lateral geniculate nucleus, and superior colliculus.

CONCLUSIONS

Pou4f2(Cre) provides multiple uses for the vision researcher's genetic toolkit. First, Pou4f2(Cre) is a knock-in allele that can be used to eliminate Pou4f2, resulting in depletion of RGCs. Second, expression of Cre in male germ cells makes this strain an efficient germline activator of recombination, for example, to target LoxP-flanked sequences in the whole mouse. Third, Pou4f2(Cre) efficiently targets RGCs, amacrine cells, bipolar cells, horizontal cells, and a small number of photoreceptors within the retina, as well as the visual centers in the brain. Unlike other Cre recombinase lines that target retinal neurons, no recombination was observed in Müller or other retinal glia. These properties make this Cre recombinase line a useful tool for vision researchers.

Defective Angiogenesis and Intraretinal Bleeding in Mouse Models With Disrupted Inner Retinal Lamination

PURPOSE

Abnormal retinal angiogenesis leads to visual impairment and blindness. Understanding how retinal vessels develop normally has dramatically improved treatments for people with retinal vasculopathies, but additional information about development is required. Abnormal neuron patterning in the outer retina has been shown to result in abnormal vessel development and blindness, for example, in people and mouse models with Crumbs homologue 1 (CRB1) mutations. In this study, we report and characterize a mouse model of inner retinal lamination disruption and bleeding, the Down syndrome cell adhesion molecule (Dscam) mutant, and test how neuron-neurite placement within the inner retina guides development of intraretinal vessels.

METHODS

Bax mutant mice (increased neuron cell number), Dscam mutant mice (increased neuron cell number, disorganized lamination), Fat3 mutant mice (disorganized neuron lamination), and Dscam gain-of-function mice (Dscam(GOF)) (decreased neuron cell number) were used to manipulate neuron placement and number. Immunohistochemistry was used to assay organization of blood vessels, glia, and neurons. In situ hybridization was used to map the expression of angiogenic factors.

RESULTS

Significant changes in the organization of vessels within mutant retinas were found. Displaced neurons and microglia were associated with the attraction of vessels. Using Fat3 mutant and Dscam(GOF) retinas, we provide experimental evidence that vessel branching is induced at the neuron-neurite interface, but that other factors are required for full plexus layer formation. We further demonstrate that the displacement of neurons results in the mislocalization of angiogenic factors.

CONCLUSIONS

Inner retina neuron lamination is required for development of intraretinal vessels.

Morphological Diversity of the Rod Spherule: A Study of Serially Reconstructed Electron Micrographs

Purpose

Rod spherules are the site of the first synaptic contact in the retina’s rod pathway, linking rods to horizontal and bipolar cells. Rod spherules have been described and characterized through electron micrograph (EM) and other studies, but their morphological diversity related to retinal circuitry and their intracellular structures have not been quantified. Most rod spherules are connected to their soma by an axon, but spherules of rods on the surface of the Mus musculus outer plexiform layer often lack an axon and have a spherule structure that is morphologically distinct from rod spherules connected to their soma by an axon. Retraction of the rod axon and spherule is often observed in disease processes and aging, and the retracted rod spherule superficially resembles rod spherules lacking an axon. We hypothesized that retracted spherules take on an axonless spherule morphology, which may be easier to maintain in a diseased state. To test our hypothesis, we quantified the spatial organization and subcellular structures of rod spherules with and without axons. We then compared them to the retracted spherules in a disease model, mice that overexpress Dscam (Down syndrome cell adhesion molecule), to gain a better understanding of the rod synapse in health and disease.

Methods

We reconstructed serial EM images of wild type and Dscam^GoF (gain of function) rod spherules at a resolution of 7 nm in the X-Y axis and 60 nm in the Z axis. Rod spherules with and without axons, and retracted spherules in the Dscam^GoF retina, were reconstructed. The rod spherule intracellular organelles, the invaginating dendrites of rod bipolar cells and horizontal cell axon tips were also reconstructed for statistical analysis.

Results

Stereotypical rod (R1) spherules occupy the outer two-thirds of the outer plexiform layer (OPL), where they present as spherical terminals with large mitochondria. This spherule group is highly uniform and composed more than 90% of the rod spherule population. Rod spherules lacking an axon (R2) were also described and characterized. This rod spherule group consists of a specific spatial organization that is strictly located at the apical OPL-facing layer of the Outer Nuclear Layer (ONL). The R2 spherule displays a large bowl-shaped synaptic terminal that hugs the rod soma. Retracted spherules in the Dscam^GoF retina were also reconstructed to test if they are structurally similar to R2 spherules. The misplaced rod spherules in Dscam^GoF have a gross morphology that is similar to R2 spherules but have significant disruption in internal synapse organization.

Conclusion

We described a morphological diversity within Mus musculus rod spherules. This diversity is correlated with rod location in the ONL and contributes to the intracellular differences within spherules. Analysis of the Dscam^GoF retina indicated that their R2 spherules are not significantly different than wild type R2 spherules, but that their retracted rod spherules have abnormal synaptic organization.

IPLaminator: an ImageJ plugin for automated binning and quantification of retinal lamination

Background

Information in the brain is often segregated into spatially organized layers that reflect the function of the embedded circuits. This is perhaps best exemplified in the layering, or lamination, of the retinal inner plexiform layer (IPL). The neurites of the retinal ganglion, amacrine and bipolar cell subtypes that form synapses in the IPL are precisely organized in highly refined strata within the IPL. Studies focused on developmental organization and cell morphology often use this layered stratification to characterize cells and identify the function of genes in development of the retina. A current limitation to such analysis is the lack of standardized tools to quantitatively analyze this complex structure. Most previous work on neuron stratification in the IPL is qualitative and descriptive.

Results

In this study we report the development of an intuitive platform to rapidly and reproducibly assay IPL lamination. The novel ImageJ based software plugin we developed: IPLaminator, rapidly analyzes neurite stratification patterns in the retina and other neural tissues. A range of user options allows researchers to bin IPL stratification based on fixed points, such as the neurites of cholinergic amacrine cells, or to define a number of bins into which the IPL will be divided. Options to analyze tissues such as cortex were also added. Statistical analysis of the output then allows a quantitative value to be assigned to differences in laminar patterning observed in different models, genotypes or across developmental time.

Conclusion

IPLaminator is an easy to use software application that will greatly speed and standardize quantification of neuron organization.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-0876-1) contains supplementary material, which is available to authorized users.

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 localization and function at the mouse cone synapse

The Down syndrome cell adhesion molecule (DSCAM) is required for regulation of cell number, soma spacing, and cell type-specific dendrite avoidance in many types of retinal ganglion and amacrine cells. In this study we assay the organization of cells making up the outer plexiform layer of the retina in the absence of Dscam. Some types of OFF bipolar cells, type 3b and type 4 bipolar cells, had defects in dendrite arborization in the Dscam mutant retina, whereas other cell types appeared similar to wild type. The cone synapses that these cells project their dendrites to were intact, as visualized by electron microscopy, and had a distribution and density that was not significantly different from that of wild type. The spacing of type 3b bipolar cell dendrites was further analyzed by Voronoi domain analysis, density recovery profiling (DRP) analysis, and nearest neighbor analysis. Spacing was found to be significantly different when wild-type and mutant type 3b bipolar cell dendrites were compared. Defects in arborization of these bipolar cells could not be attributed to the disorganization of inner plexiform layer cells that occurs in the Dscam mutant retina or an increase in cell number, as they arborized when Dscam was targeted in retinal ganglion cells only or in the bax null retina. Localization of DSCAM was assayed and the protein was localized near to cone synapses in mouse, macaque, and ground squirrel retinas. DSCAM protein was detected in several types of bipolar cells, including type 3b and type 4 bipolar cells.

Neuronal clustering and fasciculation phenotype in Dscam- and Bax-deficient mouse retinas

Individual types of retinal neurons are distributed to minimize proximity to neighboring cells. Many of these same cell types extend dendrites to provide coverage of the retinal surface. These two cardinal features of retinal mosaics are disrupted, for certain cell types, in mice deficient for the Down syndrome cell adhesion molecule, Dscam, exhibiting an aberrant clustering of somata and fasciculation of dendrites. The Dscam mutant mouse retina also exhibits excess numbers of these same cell types. The present study compared these two features in Dscam mutant retinas with the Bax knockout retina, in which excess numbers of two of these cell types, the melanopsin-positive retinal ganglion cells (MRGCs) and the dopaminergic amacrine cells (DACs), are also present. Whole retinas were immunolabeled for both populations, and every labeled soma was plotted. For the MRGCs, we found a gene dosage effect for Dscam, with the Dscam+/- retinas showing smaller increases in cell number, clustering, and fasciculation. Curiously, Bax-/- retinas, showing numbers of MRGCs intermediate to those found in the Dscam-/- and Dscam+/- retinas, also had clustering and fasciculation phenotypes that were intermediate to retinas with those genotypes. DACs, by comparison, showed changes in both the Dscam-/- and the Bax-/- retinas that did not correlate with their increases in DAC number. The fasciculation phenotype in the Dscam-/- retina was particularly prominent despite only modest clustering. These results demonstrate that the somal clustering and fasciculation observed in the Dscam mutant retina are not unique to Dscam deficiency and are manifested distinctively by different retinal cell types.

Cell autonomy of DSCAM function in retinal development

Cell adhesion molecules (CAMs) provide identifying cues by which neural architecture is sculpted. The Down Syndrome Cell Adhesion Molecule (DSCAM) is required for many neurodevelopmental processes in different species and also has several potential mechanisms of activity, including homophilic adhesion, homophilic repulsion and heterophilic interactions. In the mouse retina, Dscam is expressed in many, but not all neuronal subtypes. Mutations in Dscam cause the fasciculation of dendrites of neighboring homotypic neurons, indicating a role in self-avoidance among cells of a given type, a disruption of the non-random patterning of their cell bodies, and a decrease in developmental cell death in affected cell populations. In order to address how DSCAM facilitates retinal pattering, we developed a conditional allele of Dscam to use alongside existing Dscam mutant mouse strains. Conditional deletion of Dscam reproduces cell spacing, cell number and dendrite arborization defects. Inducible deletion of Dscam and retinal ganglion cell depletion in Brn3b mutant retinas both indicate that these DSCAM-mediated phenotypes can occur independently. In chimeric retinas, in which wild type and Dscam mutant cells are comingled, Dscam mutant cells entangle adjacent wild type cells of the same type, as if both cells were lacking Dscam, consistent with DSCAM-dependent cell spacing and neurite arborization being mediated through homophilic binding cell-to-cell. Deletion of Dscam in specific cell types causes cell-type-autonomous cell body spacing defects, indicating that DSCAM mediates arborization and spacing by acting within given cell types. We also examine the cell autonomy of DSCAM in laminar stratification and find that laminar disorganization can be caused in a non-cell autonomous fashion. Finally, we find Dscam dosage-dependent defects in developmental cell death and amacrine cell spacing, relevant to the increased cell death and other disorders observed in Down syndrome mouse models and human patients, in which Dscam is present in three copies.

A novel null allele of mouse DSCAM survives to adulthood on an inbred C3H background with reduced phenotypic variability

DSCAMs are cell adhesion molecules that play several important roles in neurodevelopment. Mouse alleles of Dscam identified to date do not survive on an inbred C57BL/6 background, complicating analysis of DSCAM-dependent developmental processes because of phenotypic variability related to the segregating backgrounds needed for postnatal survival. A novel spontaneous allele of Dscam, hereafter referred to as Dscam²(J), has been identified. This allele contains a four base pair duplication in exon 19, leading to a frameshift and truncation of the open reading frame. Mice homozygous for the Dscam²(J) mutant allele survive into adulthood on the C3H/HeJ background on which the mutation was identified. Using the Dscam²(J) allele, retinal phenotypes that have variable severity on a segregating background were examined. A neurite lamination defect similar to that described in chick was discovered in mice. These results indicate that, in the retina, additional DSCAM-dependent processes can be found by analysis of mutations on different genetic backgrounds.

Distinct expression patterns of mitochondrially localized YFP in neuronal subsets in the retina of three transgenic mouse lines

Background

Transgenic labels that allow the visualization of specific populations of neurons have proven to be powerful tools for research. Further developing such resources to label additional cell types and specific organelles within these cell will provide additional experimental opportunities.

Findings

The retinal expression profile of a mitochondria-localized yellow fluorescent protein (YFP) in each of three transgenic mouse lines was determined. Each line, Mito-R, Mito-Y and Mito-Z, expresses YFP in distinct and reproducible populations of retinal neurons. In the Mito-R line, YFP is expressed in most or all retinal ganglion cells (RGCs) and photoreceptors making this line useful for studying axonal transport in diseases such as glaucoma and photoreceptor degeneration related to transport of mitochondria into the inner segments. In the Mito-Y line, YFP is expressed in many cell types in the dorsal retina and in a rough mosaic population of RGCs in the rest of the retina, making this line useful for study of how retinal mosaics are organized. In the Mito-Z line, YFP is expressed in a subset of RGCs, amacrine cells, bipolar cells and photoreceptors. The Mito-Z line is inserted on the X-Chromosome, resulting in X-inactivation mosaicism in female mice carrying a single copy of the transgene. In the female hemizygous retina, expression is present in distinct clonal columns, making this transgenic line useful for analysis of clonal proliferation and lateral migration of retinal neurons.

Conclusion

The retinal expression profiles of three transgenic mouse lines that express a mitochondrially localized YFP were characterized in this study. These lines will allow researchers to isolate and identify cell types within the retina and to study retinal mitochondrial trafficking and disease.

DSCAM and DSCAML1 function in self-avoidance in multiple cell types in the developing mouse retina

DSCAM and DSCAM-LIKE1 (DSCAML1) serve diverse neurodevelopmental functions, including axon guidance, synaptic adhesion, and self-avoidance, depending on the species, cell type, and gene family member studied. We examined the function of DSCAM and DSCAML1 in the developing mouse retina. In addition to a subset of amacrine cells, Dscam was expressed in most retinal ganglion cells (RGCs). RGCs had fasciculated dendrites and clumped cell bodies in Dscam(-/-) mice, suggesting a role in self-avoidance. Dscaml1 was expressed in the rod circuit, and mice lacking Dscaml1 had fasciculated rod bipolar cell dendrites and clumped AII amacrine cell bodies, also indicating a role in self-avoidance. Neurons in Dscam or Dscaml1 mutant retinas stratified their processes appropriately in synaptic laminae in the inner plexiform layer, and functional synapses formed in the rod circuit in mice lacking Dscaml1. Therefore, DSCAM and DSCAML1 function similarly in self-avoidance, and are not essential for synaptic specificity in the mouse retina.

Retrotransposon target site selection by imitation of a cellular protein

Mobile elements rely on cellular processes to replicate, and therefore, mobile element proteins frequently interact with a variety of cellular factors. The integrase (IN) encoded by the retrotransposon Ty5 interacts with the heterochromatin protein Sir4, and this interaction determines Ty5's preference to integrate into heterochromatin. We explored the hypothesis that Ty5's targeting mechanism arose by mimicking an interaction between Sir4 and another cellular protein(s). Mutational analyses defined the requirements for the IN-Sir4 interaction, providing criteria to screen for cellular analogues. Esc1, a protein associated with the inner nuclear membrane, interacted with the same domain of Sir4 as IN, and 75% of mutations that disrupted IN-Sir4 interactions also abrogated Esc1-Sir4 interactions. A small motif critical for recognizing Sir4 was identified in Esc1. The functional equivalency of this motif and the Sir4-interacting domain of IN was demonstrated by swapping these motifs and showing that the chimeric IN and Esc1 proteins effectively target integration and partition DNA, respectively. We conclude that Ty5 targets integration by imitating the Esc1-Sir4 interaction and suggest molecular mimicry as a general mechanism that enables mobile elements to interface with cellular processes.

Defects in eye development in transgenic mice overexpressing the heparan sulfate proteoglycan agrin

The importance of heparan sulfate proteoglycans (HSPGs) in neurodevelopment is becoming increasingly clear. However, studies on HSPGs are hampered by pleiotropic effects when synthesis or modification of heparan sulfate itself is targeted, and by redundancy when the core proteins are altered. Gain-of-function experiments can sometimes circumvent these issues. Here we establish that transgenic mice overexpressing the HSPG agrin have severe ocular dysgenesis. The defects occur through a gain-of-function mechanism and penetrance is dependent on agrin dosage. The agrin-induced developmental defects are highly variable, and include anophthalmia, persistence of vitreous vessels, and fusion of anterior chamber structures. A frequently observed defect is an optic stalk coloboma leading to the misdifferentiation of the optic stalk as retina, which becomes continuous with the forebrain. The defects in optic-stalk differentiation correlate with reduced sonic hedgehog immunoreactivity and overexpansion of the PAX6 domain from the retina into the optic stalk. The ocular phenotypes associated with agrin overexpression are dependent on genetic background, occurring with high penetrance in inbred C57BL/6J mice. Distinct loci sensitizing C57BL/6J mice to agrin-induced dysgenesis were identified. These results indicate that agrin overexpression will provide a tool to explore the molecular interactions of the extracellular matrix and cell surface in eye development, and provide a means for identifying modifier loci that sensitize mice to developmental eye defects.

CEN plasmid segregation is destabilized by tethered determinants of Ty 5 integration specificity: a role for double-strand breaks in CEN antagonism

The yeast retrotransposon Ty 5 integrates preferentially into heterochromatin at the telomeres and HM loci. Target specificity is mediated by a six amino acid sequence motif (the targeting domain, TD) of integrase that interacts with Sir4p, a structural component of heterochromatin. When tethered to CEN plasmids as part of a Gal4p DNA binding domain (GBD) fusion protein, TD destabilizes plasmid segregation in a manner similar to that observed for CEN + HM or CEN +TEL antagonism. This instability is caused by the ability of TD to nucleate components of heterochromatin on the CEN plasmid, because CEN +TD antagonism is abrogated by sir2, sir3 and sir4 mutations and by TD mutations that prevent interaction with Sir4p. In strains that acquire resistance to CEN +TD antagonism, the CEN plasmid has either recombined with a 2 mu plasmid or sustained deletions in sequences required to bind GBD-TD. CEN +TD and CEN + HM antagonism is exacerbated by mutations in components of the Ku-mediated non-homologous end-joining pathway. These observations suggest that CEN antagonism is caused by DNA breaks that result from competition between CEN - and Sir-specific segregation pathways.

Controlling integration specificity of a yeast retrotransposon

Retrotransposons and retroviruses integrate nonrandomly into eukaryotic genomes. For the yeast retrotransposon Ty5, integration preferentially occurs within domains of heterochromatin. Targeting to these locations is determined by interactions between an amino acid sequence motif at the C terminus of Ty5 integrase (IN) called the targeting domain, and the heterochromatin protein Sir4p. Here we show that new Ty5 integration hot spots are created when Sir4p is tethered to ectopic DNA sites. Targeting to sites of tethered Sir4p is abrogated by single amino acid substitutions in either IN or Sir4p that prevent their interaction. Ty5 target specificity can be altered by replacing the IN-targeting domain with other peptide motifs that interact with known protein partners. Integration occurs at high efficiency and in close proximity to DNA sites where the protein partners are tethered. These findings define a mechanism by which retrotransposons shape their host genomes and suggest ways in which retroviral integration can be controlled.