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

Dscamb regulates cone mosaic formation in zebrafish via filopodium-mediated homotypic recognition

Cone photoreceptors assemble to form a regular mosaic pattern in vertebrate retinas. In zebrafish, four distinct spectral cone types (red, green, blue, and ultraviolet), form a lattice-like pattern. However, the mechanism of cone mosaic formation has been unknown. Here we show that Down Syndrome Cell Adhesion Molecule b (Dscamb) regulates the cone mosaic pattern in zebrafish, especially via red-cone spacing. During photoreceptor differentiation, newly formed cones extend filopodium-like processes laterally to apical surfaces of neighboring cones. Interestingly, red cones extend filopodia, but promptly retract them when they meet their own cone type, suggesting filopodium-mediated, homotypic recognition and self-avoidance. This self-avoidance is compromised in zebrafish dscamb mutants, leading to abnormal clustering of red cones and subsequent disruption of regular cone spacing. Thus, apical filopodium-mediated spacing of the same cone type depends on Dscamb and is essential for cone mosaic formation in zebrafish.

Two new mouse models of Gjb1-associated Charcot-Marie-Tooth disease type 1X

BACKGROUND

Charcot-Marie-Tooth disease type 1X is caused by mutations in GJB1, which is the second most common gene associated with inherited peripheral neuropathy. The GJB1 gene encodes connexin 32 (CX32), a gap junction protein expressed in myelinating glial cells. The gene is X-linked, and the mutations cause a loss of function.

AIMS

A large number of disease-associated variants have been identified, and many result in mistrafficking and mislocalization of the protein. An existing knockout mouse lacking Gjb1 expression provides a valid animal model of CMT1X, but the complete lack of protein may not fully recapitulate the disease mechanisms caused by aberrant CX32 proteins. To better represent the spectrum of human CMT1X-associated mutations, we have generated a new Gjb1 knockin mouse model.

METHODS

CRISPR/Cas9 genome editing was used to produce mice carrying the R15Q mutation in Gjb1. In addition, we identified a second allele with an early frame shift mutation in codon 7 (del2). Mice were analyzed using clinically relevant molecular, histological, neurophysiological, and behavioral assays.

RESULTS

Both alleles produce protein detectable by immunofluorescence in Schwann cells, with some protein properly localizing to nodes of Ranvier. However, both alleles also result in peripheral neuropathy with thinly myelinated and demyelinated axons, as well as degenerating and regenerating axons, predominantly in distal motor nerves. Nerve conduction velocities were only mildly reduced at later ages and compound muscle action potential amplitudes were not reduced. Levels of neurofilament light chain in plasma were elevated in both alleles. The del2 mice have an onset at ~3 months of age, whereas the R15Q mice had a later onset at 5-6 months of age, suggesting a milder loss of function. Both alleles performed comparably to wild type littermates in accelerating rotarod and grip strength tests of neuromuscular performance.

INTERPRETATION

We have generated and characterized two new mouse models of CMT1X that will be useful for future mechanistic and preclinical studies.

A Unique Role for Protocadherin γC3 in Promoting Dendrite Arborization through an Axin1-Dependent Mechanism

The establishment of a functional cerebral cortex depends on the proper execution of multiple developmental steps, culminating in dendritic and axonal outgrowth and the formation and maturation of synaptic connections. Dysregulation of these processes can result in improper neuronal connectivity, including that associated with various neurodevelopmental disorders. The γ-Protocadherins (γ-Pcdhs), a family of 22 distinct cell adhesion molecules that share a C-terminal cytoplasmic domain, are involved in multiple aspects of neurodevelopment including neuronal survival, dendrite arborization, and synapse development. The extent to which individual γ-Pcdh family members play unique versus common roles remains unclear. We demonstrated previously that the γ-Pcdh-C3 isoform (γC3), via its unique "variable" cytoplasmic domain (VCD), interacts in cultured cells with Axin1, a Wnt-pathway scaffold protein that regulates the differentiation and morphology of neurons. Here, we confirm that γC3 and Axin1 interact in the cortex in vivo and show that both male and female mice specifically lacking γC3 exhibit disrupted Axin1 localization to synaptic fractions, without obvious changes in dendritic spine density or morphology. However, both male and female γC3 knock-out mice exhibit severely decreased dendritic complexity of cortical pyramidal neurons that is not observed in mouse lines lacking several other γ-Pcdh isoforms. Combining knock-out with rescue constructs in cultured cortical neurons pooled from both male and female mice, we show that γC3 promotes dendritic arborization through an Axin1-dependent mechanism mediated through its VCD. Together, these data identify a novel mechanism through which γC3 uniquely regulates the formation of cortical circuitry.SIGNIFICANCE STATEMENT The complexity of a neuron's dendritic arbor is critical for its function. We showed previously that the γ-Protocadherin (γ-Pcdh) family of 22 cell adhesion molecules promotes arborization during development; it remained unclear whether individual family members played unique roles. Here, we show that one γ-Pcdh isoform, γC3, interacts in the brain with Axin1, a scaffolding protein known to influence dendrite development. A CRISPR/Cas9-generated mutant mouse line lacking γC3 (but not lines lacking other γ-Pcdhs) exhibits severely reduced dendritic complexity of cerebral cortex neurons. Using cultured γC3 knock-out neurons and a variety of rescue constructs, we confirm that the γC3 cytoplasmic domain promotes arborization through an Axin1-dependent mechanism. Thus, γ-Pcdh isoforms are not interchangeable, but rather can play unique neurodevelopmental roles.

Deficiency in the cell-adhesion molecule dscaml1 impairs hypothalamic CRH neuron development and perturbs normal neuroendocrine stress axis function

The corticotropin-releasing hormone (CRH)-expressing neurons in the hypothalamus are critical regulators of the neuroendocrine stress response pathway, known as the hypothalamic-pituitary-adrenal (HPA) axis. As developmental vulnerabilities of CRH neurons contribute to stress-associated neurological and behavioral dysfunctions, it is critical to identify the mechanisms underlying normal and abnormal CRH neuron development. Using zebrafish, we identified Down syndrome cell adhesion molecule like-1 (dscaml1) as an integral mediator of CRH neuron development and necessary for establishing normal stress axis function. In dscaml1 mutant animals, hypothalamic CRH neurons had higher crhb (the CRH homolog in zebrafish) expression, increased cell number, and reduced cell death compared to wild-type controls. Physiologically, dscaml1 mutant animals had higher baseline stress hormone (cortisol) levels and attenuated responses to acute stressors. Together, these findings identify dscaml1 as an essential factor for stress axis development and suggest that HPA axis dysregulation may contribute to the etiology of human DSCAML1-linked neuropsychiatric disorders.

Structure and evolution of neuronal wiring receptors and ligands

One of the fundamental properties of a neuronal circuit is the map of its connections. The cellular and developmental processes that allow for the growth of axons and dendrites, selection of synaptic targets, and formation of functional synapses use neuronal surface receptors and their interactions with other surface receptors, secreted ligands, and matrix molecules. Spatiotemporal regulation of the expression of these receptors and cues allows for specificity in the developmental pathways that wire stereotyped circuits. The families of molecules controlling axon guidance and synapse formation are generally conserved across animals, with some important exceptions, which have consequences for neuronal connectivity. Here, we summarize the distribution of such molecules across multiple taxa, with a focus on model organisms, evolutionary processes that led to the multitude of such molecules, and functional consequences for the diversification or loss of these receptors.

Structure of cell-cell adhesion mediated by the Down syndrome cell adhesion molecule

The Down syndrome cell adhesion molecule (DSCAM) belongs to the immunoglobulin superfamily (IgSF) and plays important roles in neural development. It has a large ectodomain, including 10 Ig-like domains and 6 fibronectin III (FnIII) domains. Previous data have shown that DSCAM can mediate cell adhesion by forming homophilic dimers between cells and contributes to self-avoidance of neurites or neuronal tiling, which is important for neural network formation. However, the organization and assembly of DSCAM at cell adhesion interfaces has not been fully understood. Here we combine electron microscopy and other biophysical methods to characterize the structure of the DSCAM-mediated cell adhesion and generate three-dimensional views of the adhesion interfaces of DSCAM by electron tomography. The results show that mouse DSCAM forms a regular pattern at the adhesion interfaces. The Ig-like domains contribute to both trans homophilic interactions and cis assembly of the pattern, and the FnIII domains are crucial for the cis pattern formation as well as the interaction with the cell membrane. By contrast, no obvious assembly pattern is observed at the adhesion interfaces mediated by mouse DSCAML1 or Drosophila DSCAMs, suggesting the different structural roles and mechanisms of DSCAMs in mediating cell adhesion and neural network formation.

Single-cell RNA sequencing: A new opportunity for retinal research

Single-cell RNA sequencing (scRNA-seq) is a technology for single-cell transcriptome analysis that can be used to characterize complex dynamics of various retinal cell types. It provides deep scrutiny into the gene expression character of diverse cell types, lending insight into all the biological processes being carried out. The scRNA-seq is an alternative to regular RNA-seq, which does not achieve cellular heterogeneity. The retina, is a part of the central nervous system (CNS) and consists of six types of neurons and several types of glial cells. Studying retinal cell heterogeneity is important for understanding retinal diseases. Currently, scRNA-seq is employed to assess retina development and retinal disease pathogenesis and has improved our understanding of the relationship between the retina, its visual pathways, and the brain. Moreover, this technology provides new ideas on the sensitivity and molecular mechanisms of cell subtypes involved in retinal-related diseases. The application of scRNA-seq technology has given us a deeper understanding of the latest advancements and challenges in retinal development and diseases. We advocate scRNA-seq as one of the important tools for developing novel therapies for retinal diseases. This article is categorized under: RNA Methods > RNA Analyses in Cells RNA in Disease and Development > RNA in Development RNA in Disease and Development > RNA in Disease.

Zebrafish dscaml1 Deficiency Impairs Retinal Patterning and Oculomotor Function

Down syndrome cell adhesion molecules (dscam and dscaml1) are essential regulators of neural circuit assembly, but their roles in vertebrate neural circuit function are still mostly unexplored. We investigated the functional consequences of dscaml1 deficiency in the larval zebrafish (sexually undifferentiated) oculomotor system, where behavior, circuit function, and neuronal activity can be precisely quantified. Genetic perturbation of dscaml1 resulted in deficits in retinal patterning and light adaptation, consistent with its known roles in mammals. Oculomotor analyses revealed specific deficits related to the dscaml1 mutation, including severe fatigue during gaze stabilization, reduced saccade amplitude and velocity in the light, greater disconjugacy, and impaired fixation. Two-photon calcium imaging of abducens neurons in control and dscaml1 mutant animals confirmed deficits in saccade-command signals (indicative of an impairment in the saccadic premotor pathway), whereas abducens activation by the pretectum-vestibular pathway was not affected. Together, we show that loss of dscaml1 resulted in impairments in specific oculomotor circuits, providing a new animal model to investigate the development of oculomotor premotor pathways and their associated human ocular disorders.SIGNIFICANCE STATEMENT Dscaml1 is a neural developmental gene with unknown behavioral significance. Using the zebrafish model, this study shows that dscaml1 mutants have a host of oculomotor (eye movement) deficits. Notably, the oculomotor phenotypes in dscaml1 mutants are reminiscent of human ocular motor apraxia, a neurodevelopmental disorder characterized by reduced saccade amplitude and gaze stabilization deficits. Population-level recording of neuronal activity further revealed potential subcircuit-specific requirements for dscaml1 during oculomotor behavior. These findings underscore the importance of dscaml1 in the development of visuomotor function and characterize a new model to investigate potential circuit deficits underlying human oculomotor disorders.

Expression and Roles of the Immunoglobulin Superfamily Recognition Molecule Sidekick1 in Mouse Retina

Processes of >100 types of interneurons (bipolar and amacrine cells) and projection neurons (retinal ganglion cells, RGCs) form specific and stereotyped patterns of connections in the inner plexiform layer (IPL) of the mouse retina. Four closely related homophilic immunoglobulin superfamily recognition molecules (Sidekick [Sdk] 1, Sdk 2, Dscam, and DscamL1) have been shown to play roles in patterning neuronal arbors and connections in chick retina, and all but Sdk1 have been shown to play related roles in mice. Here, we compare patterns of Sdk1 and Sdk2 expression in mouse retina and use genetic methods to assess roles of Sdk1. In adult retina, 3 neuronal types express sdk1 but not sdk2 at detectable levels, 5 express sdk2 but not sdk1 and 3 express both. Patterns of gene expression and protein localization at or near synapses are established during the first postnatal week. Dendrites of amacrine cells and RGCs that express sdk1 but not sdk2 arborize in the same narrow stratum in the center of the IPL. In the absence of Sdk1, this laminar restriction is degraded. Overexpression of sdk1 in developing cells that normally express sdk2 reorients their dendrites to resemble those of endogenously Sdk1-positive cells, indicating that Sdk1 plays an instructive role in patterning the IPL. Sdk1 fails to affect arbors when introduced after they are mature, suggesting that it is required to form but not maintain laminar restrictions. The effect of ectopically expressed sdk1 requires the presence of endogenous Sdk1, suggesting that the effect requires homophilic interactions among Sdk1-positive neurites. Together with previous results on Sdk2, Dscam, DscamL1, as well as the related Contactins, our results support the idea that an elaborate immunoglobulin superfamily code plays a prominent role in establishing neural circuits in the retina by means of tightly regulated cell type-specific expression and homophilically restricted intercellular interactions.

DSCAM promotes self-avoidance in the developing mouse retina by masking the functions of cadherin superfamily members

During neural development, self-avoidance ensures that a neuron's processes arborize to evenly fill a particular spatial domain. At the individual cell level, self-avoidance is promoted by genes encoding cell-surface molecules capable of generating thousands of diverse isoforms, such as Dscam1 (Down syndrome cell adhesion molecule 1) in Drosophila Isoform choice differs between neighboring cells, allowing neurons to distinguish "self" from "nonself". In the mouse retina, Dscam promotes self-avoidance at the level of cell types, but without extreme isoform diversity. Therefore, we hypothesize that DSCAM is a general self-avoidance cue that "masks" other cell type-specific adhesion systems to prevent overly exuberant adhesion. Here, we provide in vivo and in vitro evidence that DSCAM masks the functions of members of the cadherin superfamily, supporting this hypothesis. Thus, unlike the isoform-rich molecules tasked with self-avoidance at the individual cell level, here the diversity resides on the adhesive side, positioning DSCAM as a generalized modulator of cell adhesion during neural development.

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.

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.