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

Cognitive behavioral phenotyping of DSCAM heterozygosity as a model for autism spectrum disorder

It is estimated that 1 in 36 children are affected by autism spectrum disorder (ASD) in the United States, which is nearly a twofold increase from a decade ago. Recent genetic studies have identified de novo loss-of-function (dnLoF) mutations in the Down Syndrome Cell Adhesion Molecule (DSCAM) as a strong risk factor for ASD. Previous research has shown that DSCAM ablation confers social interaction deficits and perseverative behaviors in mouse models. However, it remains unknown to what extent DSCAM underexpression captures the full range of behaviors, specifically cognitive phenotypes, presented in ASD. Here, we conducted a comprehensive cognitive behavioral phenotyping which revealed that loss of one copy of DSCAM , as in the DSCAM 2J +/- mice, displayed hyperactivity, increased anxiety, and motor coordination impairments. Additionally, hippocampal-dependent learning and memory was affected, including working memory, long-term memory, and contextual fear learning. Interestingly, implicit learning processes remained intact. Therefore, DSCAM LoF produces autistic-like behaviors that are similar to human cases of ASD. These findings further support a role for DSCAM dnLoF mutations in ASD and suggest DSCAM 2J +/- as a suitable model for ASD research.

Summary Statement

Autism spectrum disorder represents a growing patient population. Loss of one copy of the DSCAM gene provides a promising mouse model that reproduces autistic-like behaviors for research and therapeutic testing.

Pten regulates endocytic trafficking of cell adhesion and Wnt signaling molecules to pattern the retina

The retina is exquisitely patterned, with neuronal somata positioned at regular intervals to completely sample the visual field. Here, we show that phosphatase and tensin homolog (Pten) controls starburst amacrine cell spacing by modulating vesicular trafficking of cell adhesion molecules and Wnt proteins. Single-cell transcriptomics and double-mutant analyses revealed that Pten and Down syndrome cell adhesion molecule Dscam) are co-expressed and function additively to pattern starburst amacrine cell mosaics. Mechanistically, Pten loss accelerates the endocytic trafficking of DSCAM, FAT3, and MEGF10 off the cell membrane and into endocytic vesicles in amacrine cells. Accordingly, the vesicular proteome, a molecular signature of the cell of origin, is enriched in exocytosis, vesicle-mediated transport, and receptor internalization proteins in Pten conditional knockout (PtencKO) retinas. Wnt signaling molecules are also enriched in PtencKO retinal vesicles, and the genetic or pharmacological disruption of Wnt signaling phenocopies amacrine cell patterning defects. Pten thus controls vesicular trafficking of cell adhesion and signaling molecules to establish retinal amacrine cell mosaics.

The role of Down syndrome cell adhesion molecule in Down syndrome

Down syndrome (DS) is caused by the presence of an extra copy of the entire or a portion of human chromosome 21 (HSA21). This genomic alteration leads to elevated expression of numerous HSA21 genes, resulting in a variety of health issues in individuals with DS. Among the genes located in the DS "critical region" of HSA21, Down syndrome cell adhesion molecule (DSCAM) plays an important role in neuronal development. There is a growing body of evidence underscoring DSCAM's involvement in various DS-related disorders. This review aims to provide a concise overview of the established functions of DSCAM, with a particular focus on its implications in DS. We delve into the roles that DSCAM plays in DS-associated diseases. In the concluding section of this review, we explore prospective avenues for future research to further unravel DSCAM's role in DS and opportunities for therapeutic treatments.

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.

Cell type diversity in a developing octopus brain

Octopuses are mollusks that have evolved intricate neural systems comparable with vertebrates in terms of cell number, complexity and size. The brain cell types that control their sophisticated behavioral repertoire are still unknown. Here, we profile the cell diversity of the paralarval Octopus vulgaris brain to build a cell type atlas that comprises mostly neural cells, but also multiple glial subtypes, endothelial cells and fibroblasts. We spatially map cell types to the vertical, subesophageal and optic lobes. Investigation of cell type conservation reveals a shared gene signature between glial cells of mouse, fly and octopus. Genes related to learning and memory are enriched in vertical lobe cells, which show molecular similarities with Kenyon cells in Drosophila. We construct a cell type taxonomy revealing transcriptionally related cell types, which tend to appear in the same brain region. Together, our data sheds light on cell type diversity and evolution in the octopus brain.

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.

To Stick or Not to Stick: The Multiple Roles of Cell Adhesion Molecules in Neural Circuit Assembly

Precise wiring of neural circuits is essential for brain connectivity and function. During development, axons respond to diverse cues present in the extracellular matrix or at the surface of other cells to navigate to specific targets, where they establish precise connections with post-synaptic partners. Cell adhesion molecules (CAMs) represent a large group of structurally diverse proteins well known to mediate adhesion for neural circuit assembly. Through their adhesive properties, CAMs act as major regulators of axon navigation, fasciculation, and synapse formation. While the adhesive functions of CAMs have been known for decades, more recent studies have unraveled essential, non-adhesive functions as well. CAMs notably act as guidance cues and modulate guidance signaling pathways for axon pathfinding, initiate contact-mediated repulsion for spatial organization of axonal arbors, and refine neuronal projections during circuit maturation. In this review, we summarize the classical adhesive functions of CAMs in axonal development and further discuss the increasing number of other non-adhesive functions CAMs play in neural circuit assembly.

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.

Role of DSCAM in the Development of Neural Control of Movement and Locomotion

Locomotion results in an alternance of flexor and extensor muscles between left and right limbs generated by motoneurons that are controlled by the spinal interneuronal circuit. This spinal locomotor circuit is modulated by sensory afferents, which relay proprioceptive and cutaneous inputs that inform the spatial position of limbs in space and potential contacts with our environment respectively, but also by supraspinal descending commands of the brain that allow us to navigate in complex environments, avoid obstacles, chase prey, or flee predators. Although signaling pathways are important in the establishment and maintenance of motor circuits, the role of DSCAM, a cell adherence molecule associated with Down syndrome, has only recently been investigated in the context of motor control and locomotion in the rodent. DSCAM is known to be involved in lamination and delamination, synaptic targeting, axonal guidance, dendritic and cell tiling, axonal fasciculation and branching, programmed cell death, and synaptogenesis, all of which can impact the establishment of motor circuits during development, but also their maintenance through adulthood. We discuss herein how DSCAM is important for proper motor coordination, especially for breathing and locomotion.

Subtle Roles of Down Syndrome Cell Adhesion Molecules in Embryonic Forebrain Development and Neuronal Migration

Down Syndrome (DS) Cell Adhesion Molecules (DSCAMs) are transmembrane proteins of the immunoglobulin superfamily. Human DSCAM is located within the DS critical region of chromosome 21 (duplicated in Down Syndrome patients), and mutations or copy-number variations of this gene have also been associated to Fragile X syndrome, intellectual disability, autism, and bipolar disorder. The DSCAM paralogue DSCAM-like 1 (DSCAML1) maps to chromosome 11q23, implicated in the development of Jacobsen and Tourette syndromes. Additionally, a spontaneous mouse DSCAM deletion leads to motor coordination defects and seizures. Previous research has revealed roles for DSCAMs in several neurodevelopmental processes, including synaptogenesis, dendritic self-avoidance, cell sorting, axon growth and branching. However, their functions in embryonic mammalian forebrain development have yet to be completely elucidated. In this study, we revealed highly dynamic spatiotemporal patterns of Dscam and Dscaml1 expression in definite cortical layers of the embryonic mouse brain, as well as in structures and ganglionic eminence-derived neural populations within the embryonic subpallium. However, an in-depth histological analysis of cortical development, ventral forebrain morphogenesis, cortical interneuron migration, and cortical-subcortical connectivity formation processes in Dscam and Dscaml1 knockout mice (Dscam del17 and Dscaml1 GT ) at several embryonic stages indicated that constitutive loss of Dscam and Dscaml1 does not affect these developmental events in a significant manner. Given that several Dscam- and Dscaml1-linked neurodevelopmental disorders are associated to chromosomal region duplication events, we furthermore sought to examine the neurodevelopmental effects of Dscam and Dscaml1 gain of function (GOF). In vitro, ex vivo, and in vivo GOF negatively impacted neural migration processes important to cortical development, and affected the morphology of maturing neurons. Overall, these findings contribute to existing knowledge on the molecular etiology of human neurodevelopmental disorders by elucidating how dosage variations of genes encoding adhesive cues can disrupt cell-cell or cell-environment interactions crucial for neuronal migration.

DSCAM Mutation Impairs Motor Cortex Network Dynamic and Voluntary Motor Functions

While it is well known that netrin-1 and its receptors UNC5 and UNC40 family members are involved in the normal establishment of the motor cortex and its corticospinal tract, less is known about its other receptor Down syndrome cell adherence molecule (DSCAM). DSCAM is expressed in the developing motor cortex, regulates axonal outgrowth of cortical neurons, and its mutation impairs the dendritic arborization of cortical neurons, thus suggesting that it might be involved in the normal development and functioning of the motor cortex. In comparison to WT littermates, DSCAM2J mutant mice slipped and misplaced their paw while walking on the rungs of a horizontal ladder, and exhibited more difficulties in stepping over an obstacle while walking at slow speed. Anterograde tracing showed a normal pyramidal decussation and corticospinal projection, but a more dorsal distribution of their axonal terminals in the spinal gray matter. Intracortical microstimulations showed a reduced corticospinal and intracortical efficacy, whereas stimulations of the pyramidal tract revealed a normal spinal efficacy and excitability of corticospinal tract axons, thus arguing for a dysfunctional cortical development. Our study reveals impairment of the network dynamics within the motor cortex, reducing corticospinal drive and impairing voluntary locomotor functions upon DSCAM2J mutation.

Nuclear import of the DSCAM-cytoplasmic domain drives signaling capable of inhibiting synapse formation

DSCAM and DSCAML1 are immunoglobulin and cell adhesion-type receptors serving important neurodevelopmental functions including control of axon growth, branching, neurite self-avoidance, and neuronal cell death. The signal transduction mechanisms or effectors of DSCAM receptors, however, remain poorly characterized. We used a human ORFeome library to perform a high-throughput screen in mammalian cells and identified novel cytoplasmic signaling effector candidates including the Down syndrome kinase Dyrk1a, STAT3, USP21, and SH2D2A. Unexpectedly, we also found that the intracellular domains (ICDs) of DSCAM and DSCAML1 specifically and directly interact with IPO5, a nuclear import protein of the importin beta family, via a conserved nuclear localization signal. The DSCAM ICD is released by γ-secretase-dependent cleavage, and both the DSCAM and DSCAML1 ICDs efficiently translocate to the nucleus. Furthermore, RNA sequencing confirms that expression of the DSCAM as well as the DSCAML1 ICDs alone can profoundly alter the expression of genes associated with neuronal differentiation and apoptosis, as well as synapse formation and function. Gain-of-function experiments using primary cortical neurons show that increasing the levels of either the DSCAM or the DSCAML1 ICD leads to an impairment of neurite growth. Strikingly, increased expression of either full-length DSCAM or the DSCAM ICD, but not the DSCAML1 ICD, significantly decreases synapse numbers in primary hippocampal neurons. Taken together, we identified a novel membrane-to-nucleus signaling mechanism by which DSCAM receptors can alter the expression of regulators of neuronal differentiation and synapse formation and function. Considering that chromosomal duplications lead to increased DSCAM expression in trisomy 21, our findings may help uncover novel mechanisms contributing to intellectual disability in Down syndrome.

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.

DSCAM promotes axon fasciculation and growth in the developing optic pathway

Although many aspects of optic pathway development are beginning to be understood, the mechanisms promoting the growth of retinal ganglion cell (RGC) axons toward visual targets remain largely unknown. Down syndrome cell adhesion molecule (Dscam) is expressed by mouse RGCs shortly after they differentiate at embryonic day 12 and is essential for multiple aspects of postnatal visual system development. Here we show that Dscam is also required during embryonic development for the fasciculation and growth of RGC axons. Dscam is expressed along the developing optic pathway in a pattern consistent with a role in regulating RGC axon outgrowth. In mice carrying spontaneous mutations in Dscam (Dscam^del17 ; Dscam^2J), RGC axons pathfind normally, but growth from the chiasm toward their targets is impaired, resulting in a delay in RGC axons reaching the dorsal thalamus compared with that seen in wild-type littermates. Conversely, Dscam gain of function results in exuberant growth into the dorsal thalamus. The growth of ipsilaterally projecting axons is particularly affected. Axon organization in the optic chiasm and tract and RGC growth cone morphologies are also altered in Dscam mutants. In vitro DSCAM promotes RGC axon growth and fasciculation, and can act independently of cell contact. In vitro and in situ DSCAM is required both in the RGC axons and in their environment for the promotion of axon outgrowth, consistent with a homotypic mode of action. These findings identify DSCAM as a permissive signal that promotes the growth and fasciculation of RGC axons, controlling the timing of when RGC axons reach their targets.

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.

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.

Motor hypertonia and lack of locomotor coordination in mutant mice lacking DSCAM

Down syndrome cell adherence molecule (DSCAM) contributes to the normal establishment and maintenance of neural circuits. Whereas there is abundant literature regarding the role of DSCAM in the neural patterning of the mammalian retina, less is known about motor circuits. Recently, DSCAM mutation has been shown to impair bilateral motor coordination during respiration, thus causing death at birth. DSCAM mutants that survive through adulthood display a lack of locomotor endurance and coordination in the rotarod test, thus suggesting that the DSCAM mutation impairs motor control. We investigated the motor and locomotor functions of DSCAM(2J) mutant mice through a combination of anatomical, kinematic, force, and electromyographic recordings. With respect to wild-type mice, DSCAM(2J) mice displayed a longer swing phase with a limb hyperflexion at the expense of a shorter stance phase during locomotion. Furthermore, electromyographic activity in the flexor and extensor muscles was increased and coactivated over 20% of the step cycle over a wide range of walking speeds. In contrast to wild-type mice, which used lateral walk and trot at walking speed, DSCAM(2J) mice used preferentially less coordinated gaits, such as out-of-phase walk and pace. The neuromuscular junction and the contractile properties of muscles, as well as their muscle spindles, were normal, and no signs of motor rigidity or spasticity were observed during passive limb movements. Our study demonstrates that the DSCAM mutation induces dystonic hypertonia and a disruption of locomotor gaits.

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.

Role of DSCAM in the development of the spinal locomotor and sensorimotor circuits

Locomotion is controlled by spinal circuits that generate rhythm and coordinate left-right and flexor-extensor motoneuronal activities. The outputs of motoneurons and spinal interneuronal circuits are shaped by sensory feedback, relaying peripheral signals that are critical to the locomotor and postural control. Several studies in invertebrates and vertebrates have argued that the Down syndrome cell adhesion molecule (DSCAM) would play an important role in the normal development of neural circuits through cell spacing and targeting, axonal and dendritic branching, and synapse establishment and maintenance. Although there is evidence that DSCAM is important for the normal development of neural circuits, little is known about its functional contribution to spinal motor circuits. We show here that adult DSCAM(2J) mutant mice, lacking DSCAM, exhibit a higher variability in their locomotor pattern and rhythm during treadmill locomotion. Retrograde tracing studies in neonatal isolated spinal cords show an increased number of spinal commissural interneurons, which likely contributes to reducing the left-right alternation and to increasing the flexor/swing duration during neonatal and adult locomotion. Moreover, our results argue that, by reducing the peripheral excitatory drive onto spinal motoneurons, the DSCAM mutation reduces or abolishes spinal reflexes in both neonatal isolated spinal cords and adult mice, thus likely impairing sensorimotor control. Collectively, our functional, electrophysiological, and anatomical studies suggest that the mammalian DSCAM protein is involved in the normal development of spinal locomotor and sensorimotor circuits.

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.

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.

Reprint of "review of Dscam-mediated immunity in shrimp and other arthropods"

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

Roles for DSCAM and DSCAML1 in central nervous system development and disease

DSCAMs (Down syndrome cell adhesion molecules) are a group of immunoglobulin-like transmembrane proteins that contain fibronectin III domains. The founding member of the family was isolated in a positional cloning study that sought to identify genes located on chromosome 21 at the locus 21q22.2-q22.3 that is implicated in the neurological and cardiac phenotypes associated with Down's syndrome. In Drosophila, Dscam proteins are involved in neuronal wiring, while in vertebrates, the role of these cell adhesion molecules in neurogenesis, dendritogenesis, axonal outgrowth, synaptogenesis, and synaptic plasticity is only just beginning to be understood. In this chapter, we will review the functions ascribed to the two paralogous proteins found in humans, DSCAM and DSCAML1 (DSCAM-like 1), based on findings in knockout mice. The signaling pathways downstream of DSCAM activation and the role of DSCAM miss-expression in disease will be also discussed, particularly with regard to the intellectual disability in Down's syndrome.

Developmentally dynamic colocalization patterns of DSCAM with adhesion and synaptic proteins in the mouse retina

PURPOSE

The Down syndrome cell adhesion molecule (Dscam) gene is required for normal dendrite arborization and lamination in the mouse retina. In this study, we characterized the developmental localization of the DSCAM protein to better understand the postnatal stages of retinal development during which laminar disorganization occur in the absence of the protein.

METHODS

Immunohistochemistry and colocalization analysis software were used to assay the localization of the DSCAM protein during development of the retina.

RESULTS

We found that DSCAM was initially localized diffusely throughout mouse retinal neurites but then adopted a punctate distribution. DSCAM colocalized with catenins in the adult retina but was not detected at the active zone of chemical synapses, electrical synapses, and tight junctions. Further analysis identified a wave of colocalization between DSCAM and numerous synaptic and junction proteins coinciding with synaptogenesis between bipolar and retinal ganglion cells.

CONCLUSIONS

Research presented in this study expands our understanding of DSCAM function by characterizing its location during the development of the retina and identifies temporally regulated localization patterns as an important consideration in understanding the function of adhesion molecules in neural development.

Review of Dscam-mediated immunity in shrimp and other arthropods

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

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

The Down syndrome cell adhesion molecule (DSCAM) is an Ig containing cell adhesion molecule with remarkable structural conservation throughout metazoans. In insects, DSCAM has 38,000 potential isoforms that convey axon guidance, fasciculation, and dendrite morphogenesis during neurodevelopment. In vertebrates, DSCAM is expressed throughout the nervous system and seems to also mediate proper axonal guidance and synaptogenesis without the isoform diversity found in insects. Differences in DSCAM function among several vertebrate species complicate the understanding of an evolutionarily conserved role during embryogenesis. We take advantage of the frog developmental model Xenopus tropicalis to study DSCAM function in early development by expression analysis and morpholino-mediated knockdown. Our results indicate that DSCAM is expressed early in development and restricted to the head and nervous system. Knockdown of protein expression results in early morphogenetic phenotypes characterized by failed gastrulation and improper posterior neural tube closure. Our results reveal a specific, fundamental role of DSCAM in early morphogenetic movements, presumably through its well-known role in homophilic cell adhesion.

Sema6B, Sema6C, and Sema6D expression and function during mammalian retinal development

In the vertebrate retina, the formation of neural circuits within discrete laminae is critical for the establishment of retinal visual function. Precise formation of retinal circuits requires the coordinated actions of adhesive and repulsive molecules, including repulsive transmembrane semaphorins (Sema6A, Sema5A, and Sema5B). These semaphorins signal through different Plexin A (PlexA) receptors, thereby regulating distinct aspects of retinal circuit assembly. Here, we investigate the physiological roles of three Class 6 transmembrane semaphorins (Sema6B, Sema6C, and Sema6D), previously identified as PlexA receptor ligands in non-retinal tissues, in mammalian retinal development. We performed expression analysis and also phenotypic analyses of mice that carry null mutations in each of genes encoding these proteins using a broad range of inner and outer retinal markers. We find that these Class 6 semaphorins are uniquely expressed throughout postnatal retinal development in specific domains and cell types of the developing retina. However, we do not observe defects in stereotypical lamina-specific neurite stratification of retinal neuron subtypes in Sema6B-/- or Sema6C-/-; Sema6D-/- retinas. These findings indicate these Class 6 transmembrane semaphorins are unlikely to serve as major PlexA receptor ligands for the assembly of murine retinal circuit laminar organization.

A novel mouse Dscam mutation inhibits localization and shedding of DSCAM

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 DSCAM^R1018P 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.

DSCAMs: restoring balance to developmental forces

Many of the models of neurodevelopmental processes such as cell migration, axon outgrowth, and dendrite arborization involve cell adhesion and chemoattraction as critical physical or mechanical aspects of the mechanism. However, the prevention of adhesion or attraction is under-appreciated as a necessary, active process that balances these forces, insuring that the correct cells are present and adhering in the correct place at the correct time. The phenomenon of not adhering is often viewed as the passive alternative to adhesion, and in some cases this may be true. However, it is becoming increasingly clear that active signaling pathways are involved in preventing adhesion. These provide a balancing force during development that prevents overly exuberant adhesion, which would otherwise disrupt normal cellular and tissue morphogenesis. The strength of chemoattractive signals may be similarly modulated. Recent studies, described here, suggest that Down Syndrome Cell Adhesion Molecule (DSCAM), and closely related proteins such as DSCAML1, may play an important developmental role as such balancers in multiple systems.

Candidate molecular mechanisms for establishing cell identity in the developing retina

In the developing nervous system, individual neurons must occupy appropriate positions within circuits. This requires that these neurons recognize and form connections with specific pre- and postsynaptic partners. Cellular recognition is also required for the spacing of cell bodies and the arborization of dendrites, factors that determine the inputs onto a given neuron. These issues are particularly evident in the retina, where different types of neurons are evenly spaced relative to other cells of the same type. This establishes a reiterated columnar circuitry resembling the insect retina. Establishing these mosaic patterns requires that cells of a given type (homotypic cells) be able to sense their neighbors. Therefore, both synaptic specificity and mosaic spacing require cellular identifiers. In synaptic specificity, recognition often occurs between different types of cells in a pre- and postsynaptic pairing. In mosaic spacing, recognition is often occurring between different cells of the same type, orhomotypic self-recognition. Dendritic arborization can require recognition of different neurites of the same cell, or isoneuronal self-recognition. The retina is an extremely amenable system for studying the molecular identifiers that drive these various forms of recognition. The different neuronal types in the retina are well defined, and the genetic tools for marking cell types are increasingly available. In this review we will summarize retinal anatomy and describe cell types in the retina and how they are defined. We will then describe the requirements of a recognition code and discuss newly emerging candidate molecular mechanisms for recognition that may meet these requirements.

Integrins regulate repulsion-mediated dendritic patterning of drosophila sensory neurons by restricting dendrites in a 2D space

Dendrites of the same neuron usually avoid each other. Some neurons also repel similar neurons through dendrite-dendrite interaction to tile the receptive field. Nonoverlapping coverage based on such contact-dependent repulsion requires dendrites to compete for limited space. Here we show that Drosophila class IV dendritic arborization (da) neurons, which tile the larval body wall, grow their dendrites mainly in a 2D space on the extracellular matrix (ECM) secreted by the epidermis. Removing neuronal integrins or blocking epidermal laminin production causes dendrites to grow into the epidermis, suggesting that integrin-laminin interaction attaches dendrites to the ECM. We further show that some of the previously identified tiling mutants fail to confine dendrites in a 2D plane. Expansion of these mutant dendrites in three dimensions results in overlap of dendritic fields. Moreover, overexpression of integrins in these mutant neurons effectively reduces dendritic crossing and restores tiling, revealing an additional mechanism for tiling.

Cell-type specific roles for PTEN in establishing a functional retinal architecture

Background

The retina has a unique three-dimensional architecture, the precise organization of which allows for complete sampling of the visual field. Along the radial or apicobasal axis, retinal neurons and their dendritic and axonal arbors are segregated into layers, while perpendicular to this axis, in the tangential plane, four of the six neuronal types form patterned cellular arrays, or mosaics. Currently, the molecular cues that control retinal cell positioning are not well-understood, especially those that operate in the tangential plane. Here we investigated the role of the PTEN phosphatase in establishing a functional retinal architecture.

Methodology/Principal Findings

In the developing retina, PTEN was localized preferentially to ganglion, amacrine and horizontal cells, whose somata are distributed in mosaic patterns in the tangential plane. Generation of a retina-specific Pten knock-out resulted in retinal ganglion, amacrine and horizontal cell hypertrophy, and expansion of the inner plexiform layer. The spacing of Pten mutant mosaic populations was also aberrant, as were the arborization and fasciculation patterns of their processes, displaying cell type-specific defects in the radial and tangential dimensions. Irregular oscillatory potentials were also observed in Pten mutant electroretinograms, indicative of asynchronous amacrine cell firing. Furthermore, while Pten mutant RGC axons targeted appropriate brain regions, optokinetic spatial acuity was reduced in Pten mutant animals. Finally, while some features of the Pten mutant retina appeared similar to those reported in Dscam-mutant mice, PTEN expression and activity were normal in the absence of Dscam.

Conclusions/Significance

We conclude that Pten regulates somal positioning and neurite arborization patterns of a subset of retinal cells that form mosaics, likely functioning independently of Dscam, at least during the embryonic period. Our findings thus reveal an unexpected level of cellular specificity for the multi-purpose phosphatase, and identify Pten as an integral component of a novel cell positioning pathway in the retina.

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.

Class 5 transmembrane semaphorins control selective Mammalian retinal lamination and function

In the vertebrate retina, neurites from distinct neuronal cell types are constrained within the plexiform layers, allowing for establishment of retinal lamination. However, the mechanisms by which retinal neurites are segregated within the inner or outer plexiform layers are not known. We find that the transmembrane semaphorins Sema5A and Sema5B constrain neurites from multiple retinal neuron subtypes within the inner plexiform layer (IPL). In Sema5A⁻/⁻; Sema5B⁻/⁻ mice, retinal ganglion cells (RGCs) and amacrine and bipolar cells exhibit severe defects leading to neurite mistargeting into the outer portions of the retina. These targeting abnormalities are more prominent in the outer (OFF) layers of the IPL and result in functional defects in select RGC response properties. Sema5A and Sema5B inhibit retinal neurite outgrowth through PlexinA1 and PlexinA3 receptors both in vitro and in vivo. These findings define a set of ligands and receptors required for the establishment of inner retinal lamination and function.

Transmembrane semaphorin signalling controls laminar stratification in the mammalian retina

In the vertebrate retina, establishment of precise synaptic connections among distinct retinal neuron cell types is critical for processing visual information and for accurate visual perception. Retinal ganglion cells (RGCs), amacrine cells and bipolar cells establish stereotypic neurite arborization patterns to form functional neural circuits in the inner plexiform layer (IPL), a laminar region that is conventionally divided into five major parallel sublaminae. However, the molecular mechanisms governing distinct retinal subtype targeting to specific sublaminae within the IPL remain to be elucidated. Here we show that the transmembrane semaphorin Sema6A signals through its receptor PlexinA4 (PlexA4) to control lamina-specific neuronal stratification in the mouse retina. Expression analyses demonstrate that Sema6A and PlexA4 proteins are expressed in a complementary fashion in the developing retina: Sema6A in most ON sublaminae and PlexA4 in OFF sublaminae of the IPL. Mice with null mutations in PlexA4 or Sema6A exhibit severe defects in stereotypic lamina-specific neurite arborization of tyrosine hydroxylase (TH)-expressing dopaminergic amacrine cells, intrinsically photosensitive RGCs (ipRGCs) and calbindin-positive cells in the IPL. Sema6A and PlexA4 genetically interact in vivo for the regulation of dopaminergic amacrine cell laminar targeting. Therefore, neuronal targeting to subdivisions of the IPL in the mammalian retina is directed by repulsive transmembrane guidance cues present on neuronal processes.

Circuit assembly: the repulsive side of lamination

Identifying the mechanisms that shape neuronal circuit architecture remains a major challenge. A recent study shows that repulsive signaling between parallel visual pathways helps organize their connections into laminar circuits in the inner retina.