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

Prevalence and diversity of retinal disease in adults with Down syndrome

While epidemiologic data exists for some ophthalmic diseases in people with Down Syndrome (DS), like strabismus and amblyopia, no studies explore the prevalence of retinal disease in people with DS on a large scale. This study utilized a literature review and exploratory epidemiology analysis to examine patterns of retinal disease in people with DS. To evaluate previous studies on physiology and/or anatomy in retinal models representing DS or in the retinas of people with DS, all relevant terms related to Down Syndrome, retina, and retinal diseases were searched in PubMed and Scopus. Data from the health platform TriNetX was then utilized to determine the prevalence and prevalence odds ratio (POR) of retinal disorders, including diabetic retinopathy and age-related macular degeneration (AMD), within the U.S. adult population with DS. The final literature review included 28 of 535 screened studies and found that a DS diagnosis was associated with atypical retinal vascularization, retinal thickening, and abnormal neuronal development. Of 55,198,979 individuals included in the population study, 97,795 (0.18%) had a recorded DS diagnosis. Compared to the population without DS, the population with DS had significantly increased PORs for any retinal diagnosis (3.78, 95% CI 3.63-3.93), for 16 of 18 recorded individual retinal diagnoses, and for 4 of 5 major diagnostic categories, including diabetic retinopathy (2.56, 95% CI 2.33-2.82) and macular degeneration (4.01, 95% CI 3.42-4.71). The conclusion is that retinal anomalies common to people with DS likely contribute to higher rates of recorded retinal disease. However, future studies should evaluate this relationship.

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 DSCAM2J+/-, that is, DSCAM heterozygous mice, displayed hyperactivity, increased anxiety-like behavior, and motor coordination deficits. Additionally, hippocampal-dependent learning and memory was affected, including impairments in 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 those observed in human cases of ASD. These findings further support a role for DSCAM dnLoF mutations in ASD and suggest DSCAM2J+/- as a suitable model for ASD research.

Synaptic cell adhesion molecules contribute to the pathogenesis and progression of fragile X syndrome

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and a monogenic cause of autism spectrum disorders. Deficiencies in the fragile X messenger ribonucleoprotein, encoded by the FMR1 gene, lead to various anatomical and pathophysiological abnormalities and behavioral deficits, such as spine dysmorphogenesis and learning and memory impairments. Synaptic cell adhesion molecules (CAMs) play crucial roles in synapse formation and neural signal transmission by promoting the formation of new synaptic contacts, accurately organizing presynaptic and postsynaptic protein complexes, and ensuring the accuracy of signal transmission. Recent studies have implicated synaptic CAMs such as the immunoglobulin superfamily, N-cadherin, leucine-rich repeat proteins, and neuroligin-1 in the pathogenesis of FXS and found that they contribute to defects in dendritic spines and synaptic plasticity in FXS animal models. This review systematically summarizes the biological associations between nine representative synaptic CAMs and FMRP, as well as the functional consequences of the interaction, to provide new insights into the mechanisms of abnormal synaptic development in FXS.

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.

A comparative overview of DSCAM and its multifunctional roles in Drosophila and vertebrates

DSCAM (Down syndrome cell adhesion molecule) is a unique neuronal adhesion protein with extensively documented multifaceted functionalities. DSCAM also has interesting properties in vertebrates and invertebrates, respectively. In Drosophila species, particularly, Dscam exhibits remarkable genetic diversity, with tens of thousands of splicing isoforms that modulate the specificity of neuronal wiring. Interestingly, this splice variant diversity of Dscam is absent in vertebrates. DSCAM plays a pivotal role in mitigating excessive adhesion between identical cell types, thereby maintaining the structural and functional coherence of neural networks. DSCAM contributes to the oversight of selective intercellular interactions such as synaptogenesis; however, the precise regulatory mechanisms underlying the promotion and inhibition of cell adhesion involved remain unclear. In this review, we aim to delineate the distinct molecules that interact with DSCAM and their specific roles within the biological landscapes of Drosophila and vertebrates. By integrating these comparative insights, we aim to elucidate the multifunctional nature of DSCAM, particularly its capacity to facilitate or deter intercellular adhesion.

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.

Activation of oxytocin receptors in mouse GABAergic amacrine cells modulates retinal dopaminergic signaling

Background

Oxytocin, secreted by oxytocin neurons in the hypothalamus, is an endogenous neuropeptide involved in modulating multiple sensory information processing pathways, and its roles in the brain have been associated with prosocial, maternal, and feeding-related behaviors. Visual information is necessary for initiating these behaviors, with the retina consisting of the first stage in the visual system mediating external stimulus perception. Oxytocin has been detected in the mammalian retina; however, the expression and possible function of oxytocin receptors (OxtR) in the retina remain unknown. Here, we explore the role of oxytocin in regulating visual information processing in the retina.

Results

We observed that OxtR mRNA and protein are expressed in the mouse retina. With Oxtr-Cre transgenic mice, immunostaining, and fluorescence in situ hybridization, we found that OxtRs are mainly expressed in GABAergic amacrine cells (ACs) in both the inner nuclear layer (INL) and ganglion cell layer (GCL). Further immunoreactivity studies showed that GABAergic OxtR⁺ neurons are mainly cholinergic and dopaminergic neurons in the INL and are cholinergic and corticotrophin-releasing hormone neurons in the GCL. Surprisingly, a high level of Oxtr mRNAs was detected in retinal dopaminergic neurons, and exogenous oxytocin application activated dopaminergic neurons to elevate the retinal dopamine level. Relying on in vivo electroretinographic recording, we found that activating retinal OxtRs reduced the activity of bipolar cells via OxtRs and dopamine receptors.

Conclusions

These data indicate the functional expression of OxtRs in retinal GABAergic ACs, especially dopaminergic ACs, and expand the interactions between oxytocinergic and dopaminergic systems. This study suggests that visual perception, from the first stage of information processing in the retina, is modulated by hypothalamic oxytocin signaling.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01405-0.

DSCAM is differentially patterned along the optic axon pathway in the developing Xenopus visual system and guides axon termination at the target

BACKGROUND

The Xenopus retinotectal circuit is organized topographically, where the dorsal-ventral axis of the retina maps respectively on to the ventral-dorsal axis of the tectum; axons from the nasal-temporal axis of the retina project respectively to the caudal-rostral axis of the tectum. Studies throughout the last two decades have shown that mechanisms involving molecular recognition of proper termination domains are at work guiding topographic organization. Such studies have shown that graded distribution of molecular cues is important for topographic mapping. However, the complement of molecular cues organizing topography along the developing optic nerve, and as retinal axons cross the chiasm and navigate towards and innervate their target in the tectum, remains unknown. Down syndrome cell adhesion molecule (DSCAM) has been characterized as a key molecule in axon guidance, making it a strong candidate involved in the topographic organization of retinal fibers along the optic path and at their target.

METHODS

Using a combination of whole-brain clearing and immunohistochemistry staining techniques we characterized DSCAM expression and the projection of ventral and dorsal retinal fibers starting from the eye, following to the optic nerve and chiasm, and into the terminal target in the optic tectum in Xenopus laevis tadpoles. We then assessed the effects of DSCAM on the establishment of retinotopic maps through spatially and temporally targeted DSCAM knockdown on retinal ganglion cells (RGCs) with axons innervating the optic tectum.

RESULTS

Highest expression of DSCAM was localized to the ventral posterior region of the optic nerve and chiasm; this expression pattern coincides with ventral fibers derived from ventral RGCs. Targeted downregulation of DSCAM expression on ventral RGCs affected the segregation of medial axon fibers from their dorsal counterparts within the tectal neuropil, indicating that DSCAM plays a role in retinotopic organization.

CONCLUSION

These findings together with previous studies demonstrating cell-autonomous roles for DSCAM during the development of pre- and postsynaptic arbors in the Xenopus retinotectal circuit indicates that DSCAM exerts multiple roles in coordinating axon targeting and structural connectivity in the developing vertebrate visual system.

Imaging of retina cellular and subcellular structures using ptychographic hard X-ray tomography

Ptychographic hard X-ray computed tomography (PXCT) is a recent method allowing imaging with quantitative electron-density contrast. Here, we imaged, at cryogenic temperature and without sectioning, cellular and subcellular structures of a chemically fixed and stained wild-type mouse retina, including axons and synapses, with complete isotropic 3D information over tens of microns. Comparison with tomograms of degenerative retina from a mouse model of retinitis pigmentosa illustrates the potential of this method for analyzing disease processes like neurodegeneration at sub-200 nm resolution. As a non-destructive imaging method, PXCT is very suitable for correlative imaging. Within the outer plexiform layer containing the photoreceptor synapses, we identified somatic synapses. We used a small region inside the X-ray-imaged sample for further high-resolution focused ion beam/scanning electron microscope tomography. The subcellular structures of synapses obtained with the X-ray technique matched the electron microscopy data, demonstrating that PXCT is a powerful scanning method for tissue volumes of more than 60 cells and sensitive enough for identification of regions as small as 200 nm, which remain available for further structural and biochemical investigations.

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.

Dysfunction of NMDA receptors in neuronal models of an autism spectrum disorder patient with a DSCAM mutation and in Dscam-knockout mice

Heterogeneity in the etiopathology of autism spectrum disorders (ASD) limits the development of generic remedies, requires individualistic and patient-specific research. Recent progress in human-induced pluripotent stem cell (iPSC) technology provides a novel platform for modeling ASDs for studying complex neuronal phenotypes. In this study, we generated telencephalic induced neuronal (iN) cells from iPSCs derived from an ASD patient with a heterozygous point mutation in the DSCAM gene. The mRNA of DSCAM and the density of DSCAM in dendrites were significantly decreased in ASD compared to control iN cells. RNA sequencing analysis revealed that several synaptic function-related genes including NMDA receptor subunits were downregulated in ASD iN cells. Moreover, NMDA receptor (R)-mediated currents were significantly reduced in ASD compared to control iN cells. Normal NMDA-R-mediated current levels were rescued by expressing wild-type DSCAM in ASD iN cells, and reduced currents were observed by truncated DSCAM expression in control iN cells. shRNA-mediated DSCAM knockdown in control iN cells resulted in the downregulation of an NMDA-R subunit, which was rescued by the overexpression of shRNA-resistant DSCAM. Furthermore, DSCAM was co-localized with NMDA-R components in the dendritic spines of iN cells whereas their co-localizations were significantly reduced in ASD iN cells. Levels of phospho-ERK1/2 were significantly lower in ASD iN cells, suggesting a potential mechanism. A neural stem cell-specific Dscam heterozygous knockout mouse model, showing deficits in social interaction and social memory with reduced NMDA-R currents. These data suggest that DSCAM mutation causes pathological symptoms of ASD by dysregulating NMDA-R function.

The γ-Protocadherins Regulate the Survival of GABAergic Interneurons during Developmental Cell Death

Inhibitory interneurons integrate into developing circuits in specific ratios and distributions. In the neocortex, inhibitory network formation occurs concurrently with the apoptotic elimination of a third of GABAergic interneurons. The cell surface molecules that select interneurons to survive or die are unknown. Here, we report that members of the clustered Protocadherins (cPCDHs) control GABAergic interneuron survival during developmentally-regulated cell death. Conditional deletion of the gene cluster encoding the γ-Protocadherins (Pcdhgs) from developing GABAergic neurons in mice of either sex causes a severe loss of inhibitory populations in multiple brain regions and results in neurologic deficits such as seizures. By focusing on the neocortex and the cerebellar cortex, we demonstrate that reductions of inhibitory interneurons result from elevated apoptosis during the critical postnatal period of programmed cell death (PCD). By contrast, cortical interneuron (cIN) populations are not affected by removal of Pcdhgs from pyramidal neurons or glial cells. Interneuron loss correlates with reduced AKT signaling in Pcdhg mutant interneurons, and is rescued by genetic blockade of the pro-apoptotic factor BAX. Together, these findings identify the PCDHGs as pro-survival transmembrane proteins that select inhibitory interneurons for survival and modulate the extent of PCD. We propose that the PCDHGs contribute to the formation of balanced inhibitory networks by controlling the size of GABAergic interneuron populations in the developing brain.SIGNIFICANCE STATEMENT A pivotal step for establishing appropriate excitatory-inhibitory ratios is adjustment of neuronal populations by cell death. In the mouse neocortex, a third of GABAergic interneurons are eliminated by BAX-dependent apoptosis during the first postnatal week. Interneuron cell death is modulated by neural activity and pro-survival pathways but the cell-surface molecules that select interneurons for survival or death are unknown. We demonstrate that members of the cadherin superfamily, the clustered γ-Protocadherins (PCDHGs), regulate the survival of inhibitory interneurons and the balance of cell death. Deletion of the Pcdhgs in mice causes inhibitory interneuron loss in the cortex and cerebellum, and leads to motor deficits and seizures. Our findings provide a molecular basis for controlling inhibitory interneuron population size during circuit formation.

Structure and Functions of Sidekicks

Many of the immunoglobulin superfamily (IgSF) molecules play pivotal roles in cell communication. The Sidekick (Sdk) gene, first described in Drosophila, encodes the single-pass transmembrane protein, Sdk, which is one of the largest among IgSF membrane proteins. Sdk first appeared in multicellular animals during the Precambrian age and later evolved to Sdk1 and Sdk2 in vertebrates by gene duplication. In flies, a single Sdk is involved in positioning photoreceptor neurons and their axons in the visual system and is responsible for dynamically rearranging cell shapes by strictly populating tricellular adherens junctions in epithelia. In vertebrates, Sdk1 and Sdk2 are expressed by unique sets of cell types and distinctively participate in the formation and/or maintenance of neural circuits in the retina, indicating that they are determinants of synaptic specificity. These functions are mediated by specific homophilic binding of their ectodomains and by intracellular association with PDZ scaffold proteins. Recent human genetic studies as well as animal experiments implicate that Sdk genes may influence various neurodevelopmental and psychiatric disorders, such as autism spectrum disorders, attention-deficit hyperactivity disorder, addiction, and depression. The gigantic Sdk1 gene is susceptible to erratic gene rearrangements or mutations in both somatic and germ-line cells, potentially contributing to neurological disorders and some types of cancers. This review summarizes what is known about the structure and roles of Sdks.

Signalling Pathways Implicated in Alzheimer's Disease Neurodegeneration in Individuals with and without Down Syndrome

Down syndrome (DS), the most common cause of intellectual disability of genetic origin, is characterized by alterations in central nervous system morphology and function that appear from early prenatal stages. However, by the fourth decade of life, all individuals with DS develop neuropathology identical to that found in sporadic Alzheimer's disease (AD), including the development of amyloid plaques and neurofibrillary tangles due to hyperphosphorylation of tau protein, loss of neurons and synapses, reduced neurogenesis, enhanced oxidative stress, and mitochondrial dysfunction and neuroinflammation. It has been proposed that DS could be a useful model for studying the etiopathology of AD and to search for therapeutic targets. There is increasing evidence that the neuropathological events associated with AD are interrelated and that many of them not only are implicated in the onset of this pathology but are also a consequence of other alterations. Thus, a feedback mechanism exists between them. In this review, we summarize the signalling pathways implicated in each of the main neuropathological aspects of AD in individuals with and without DS as well as the interrelation of these pathways.

LKB1 coordinates neurite remodeling to drive synapse layer emergence in the outer retina

Structural changes in pre and postsynaptic neurons that accompany synapse formation often temporally and spatially overlap. Thus, it has been difficult to resolve which processes drive patterned connectivity. To overcome this, we use the laminated outer murine retina. We identify the serine/threonine kinase LKB1 as a key driver of synapse layer emergence. The absence of LKB1 in the retina caused a marked mislocalization and delay in synapse layer formation. In parallel, LKB1 modulated postsynaptic horizontal cell refinement and presynaptic photoreceptor axon growth. Mislocalized horizontal cell processes contacted aberrant cone axons in LKB1 mutants. These defects coincided with altered synapse protein organization, and horizontal cell neurites were misdirected to ectopic synapse protein regions. Together, these data suggest that LKB1 instructs the timing and location of connectivity in the outer retina via coordinate regulation of pre and postsynaptic neuron structure and the localization of synapse-associated proteins.

Developmental Cell Death in the Cerebral Cortex

In spite of the high metabolic cost of cellular production, the brain contains only a fraction of the neurons generated during embryonic development. In the rodent cerebral cortex, a first wave of programmed cell death surges at embryonic stages and affects primarily progenitor cells. A second, larger wave unfolds during early postnatal development and ultimately determines the final number of cortical neurons. Programmed cell death in the developing cortex is particularly dependent on neuronal activity and unfolds in a cell-specific manner with precise temporal control. Pyramidal cells and interneurons adjust their numbers in sync, which is likely crucial for the establishment of balanced networks of excitatory and inhibitory neurons. In contrast, several other neuronal populations are almost completely eliminated through apoptosis during the first two weeks of postnatal development, highlighting the importance of programmed cell death in sculpting the mature cerebral cortex.

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.

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

The mitogen-activated protein kinase (MAPK) pathway has been shown to be involved in both neurodevelopment and neurodegeneration. c-Jun N-terminal kinase (JNK), a MAPK important in retinal development and after optic nerve crush injury, is regulated by two upstream kinases: MKK4 and MKK7. The specific requirements of MKK4 and MKK7 in retinal development and retinal ganglion cell (RGC) death after axonal injury, however, are currently undefined. Optic nerve injury is an important insult in many neurologic conditions including traumatic, ischemic, inflammatory, and glaucomatous optic neuropathies. Mice deficient in Mkk4, Mkk7, and both Mkk4 and Mkk7 were generated. Immunohistochemistry was used to study the distribution and structure of retinal cell types and to assess RGC survival after optic nerve injury (mechanical controlled optic nerve crush (CONC)). Adult Mkk4- and Mkk7-deficient retinas had all retinal cell types, and with the exception of small areas of disrupted photoreceptor lamination in Mkk4-deficient mice, the retinas of both mutants were grossly normal. Deficiency of Mkk4 or Mkk7 reduced JNK signaling in RGCs after axonal injury and resulted in a significantly greater percentage of surviving RGCs 35 days after CONC as compared to wild-type controls (Mkk4: 51.5%, Mkk7: 29.1%, WT: 15.2%; p < 0.001). Combined deficiency of Mkk4 and Mkk7 caused failure of optic nerve formation, irregular retinal axonal trajectories, disruption of retinal lamination, clumping of RGC bodies, and dendritic fasciculation of dopaminergic amacrine cells. These results suggest that MKK4 and MKK7 may serve redundant and unique roles in molecular signaling important for retinal development and injury response following axonal insult.

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.

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

BACKGROUND

Proper patterning of dendritic and axonal arbors is a critical step in the formation of functional neuronal circuits. Developing circuits rely on an array of molecular cues to shape arbor morphology, but the underlying mechanisms guiding the structural formation and interconnectivity of pre- and postsynaptic arbors in real time remain unclear. Here we explore how Down syndrome cell adhesion molecule (DSCAM) differentially shapes the dendritic morphology of central neurons and their presynaptic retinal ganglion cell (RGC) axons in the developing vertebrate visual system.

METHODS

The cell-autonomous role of DSCAM, in tectal neurons and in RGCs, was examined using targeted single-cell knockdown and overexpression approaches in developing Xenopus laevis tadpoles. Axonal arbors of RGCs and dendritic arbors of tectal neurons were visualized using real-time in vivo confocal microscopy imaging over the course of 3 days.

RESULTS

In the Xenopus visual system, DSCAM immunoreactivity is present in RGCs, cells in the optic tectum and the tectal neuropil at the time retinotectal synaptic connections are made. Downregulating DSCAM in tectal neurons significantly increased dendritic growth and branching rates while inducing dendrites to take on tortuous paths. Overexpression of DSCAM, in contrast, reduced dendritic branching and growth rate. Functional deficits mediated by tectal DSCAM knockdown were examined using visually guided behavioral assays in swimming tadpoles, revealing irregular behavioral responses to visual stimulus. Functional deficits in visual behavior also corresponded with changes in VGLUT/VGAT expression, markers of excitatory and inhibitory transmission, in the tectum. Conversely, single-cell DSCAM knockdown in the retina revealed that RGC axon arborization at the target is influenced by DSCAM, where axons grew at a slower rate and remained relatively simple. In the retina, dendritic arbors of RGCs were not affected by the reduction of DSCAM expression.

CONCLUSIONS

Together, our observations implicate DSCAM in the control of both pre- and postsynaptic structural and functional connectivity in the developing retinotectal circuit, where it primarily acts as a neuronal brake to limit and guide postsynaptic dendrite growth of tectal neurons while it also facilitates arborization of presynaptic RGC axons cell autonomously.

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.

Estrogen Action in the Epithelial Cells of the Mouse Vagina Regulates Neutrophil Infiltration and Vaginal Tissue Integrity

In the female reproductive tract, the innate immune system is modulated by two sex steroid hormones, estrogen and progesterone. A cyclical wave of neutrophils in the vaginal lumen is triggered by chemokines and correlates with circulating estrogen levels. Classical estrogen signaling in the female reproductive tract is activated through estrogen receptor α (encoded by the Esr1 gene). To study the role of estrogen action in the vagina, we used a mouse model in which Esr1 was conditionally ablated from the epithelial cells (Wnt7a^cre/+; Esr1^f/f). Histological evidence showed that in response to a physical stress, the lack of ESR1 caused the vaginal epithelium to deteriorate due to the absence of a protective cornified layer and a reduction in keratin production. In the absence of ESR1 in the vaginal epithelial tissue, we also observed an excess of neutrophil infiltration, regardless of the estrous cycle stage. The histological presence of neutrophils was found to correlate with persistent enzymatic activity in the cervical-vaginal fluid. Together, these findings suggest that ESR1 activity in the vaginal epithelial cells is required to maintain proper structural integrity of the vagina and immune response, both of which are necessary for protecting the vagina against physical damage and resetting the vaginal environment.

Transcripts within rod photoreceptors of the Zebrafish retina

BACKGROUND

The purpose of this study was to identify transcripts of retinal rod photoreceptors of the zebrafish. The zebrafish is an important animal model for vision science due to rapid and tractable development, persistent neurogenesis of rods throughout the lifespan, and capacity for functional retinal regeneration.

RESULTS

Zebrafish rods, and non-rod retinal cells of the xops:eGFP transgenic line, were separated by cell dissociation and fluorescence-activated cell sorting (FACS), followed by RNA-seq. At a false discovery rate of < 0.01, 597 transcripts were upregulated ("enriched") in rods vs. other retinal cells, and 1032 were downregulated ("depleted"). Thirteen thousand three hundred twenty four total transcripts were detected in rods, including many not previously known to be expressed by rods. Forty five transcripts were validated by qPCR in FACS-sorted rods vs. other retinal cells. Transcripts enriched in rods from adult retinas were also enriched in rods from larval and juvenile retinas, and were also enriched in rods sorted from retinas subjected to a neurotoxic lesion and allowed to regenerate. Many transcripts enriched in rods were upregulated in retinas of wildtype retinas vs. those of a zebrafish model for rod degeneration.

CONCLUSIONS

We report the generation and validation of an RNA-seq dataset describing the rod transcriptome of the zebrafish, which is now available as a resource for further studies of rod photoreceptor biology and comparative transcriptomics.

A third copy of the Down syndrome cell adhesion molecule (Dscam) causes synaptic and locomotor dysfunction in Drosophila

Down syndrome (DS) is caused by triplication of chromosome 21 (HSA21). It is characterised by intellectual disability and impaired motor coordination that arise from changes in brain volume, structure and function. However, the contribution of each HSA21 gene to these various phenotypes and to the causal alterations in neuronal and synaptic structure and function are largely unknown. Here we have investigated the effect of overexpression of the HSA21 gene DSCAM (Down syndrome cell adhesion molecule), on glutamatergic synaptic transmission and motor coordination, using Drosophila expressing three copies of Dscam1. Electrophysiological recordings of miniature and evoked excitatory junction potentials at the glutamatergic neuromuscular junction of Drosophila larvae showed that the extra copy of Dscam1 changed the properties of spontaneous and electrically-evoked transmitter release and strengthened short-term synaptic depression during high-frequency firing of the motor nerve. Behavioural analyses uncovered impaired locomotor coordination despite preserved gross motor function. This work identifies DSCAM as a candidate causative gene in DS that is sufficient to modify synaptic transmission and synaptic plasticity and cause a DS behavioural phenotype.

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Drosophila expressing a third copy of Dscam have altered neuromuscular transmission.

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Drosophila expressing a third copy of Dscam have deficits in locomotor coordination.

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Drosophila are a powerful system for studying single-gene effects in Down syndrome.

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.

Age- and speed-dependent modulation of gaits in DSCAM2J mutant mice

Gaits depend on the interplay between distributed spinal neural networks, termed central pattern generators, generating rhythmic and coordinated movements, primary afferents, and descending supraspinal inputs. Recent studies demonstrated that the mouse displays a rich repertoire of gaits. Changes in gaits occur in mutant mice lacking particular neurons or molecular signaling pathways implicated in the normal establishment of these neural networks. Given the role of the Down syndrome cell adherence molecule (DSCAM) to the formation and maintenance of spinal interneuronal circuits and sensorimotor integration, we have investigated its functional contribution to gaits over a wide range of locomotor speeds using freely walking mice. We show in this study that the DSCAM^2J mutation, while not precluding any gait, impairs the age- and speed-dependent modulation of gaits. It impairs the ability of mice to maintain their locomotion at high treadmill speeds. DSCAM^2J mutation induces the dominance of lateral walk over trot and the emergence of aberrant gaits for mice, such as pace and diagonal walk. Gaits were also more labile in DSCAM^2J mutant mice, i.e., less stable, less attractive, and less predictable than in their wild-type littermates. Our results suggest that the DSCAM mutation affects the behavioral repertoire of gaits in an age- and speed-dependent manner. NEW & NOTEWORTHY Gaits evolve throughout development, up to adulthood, and according to the genetic background. Using mutant mice lacking DSCAM (a cell adherence molecule associated with Down syndrome), we show that the DSCAM^2J mutation alters the repertoire of gaits according to the mouse's age and speed, and prevents fast gaits. Such an incapacity suggests a reorganization of spinal, propriospinal, and supraspinal neuronal circuits underlying locomotor control in DSCAM^2J mutant mice.

Autism spectrum disorder: neuropathology and animal models

Autism spectrum disorder (ASD) has a major impact on the development and social integration of affected individuals and is the most heritable of psychiatric disorders. An increase in the incidence of ASD cases has prompted a surge in research efforts on the underlying neuropathologic processes. We present an overview of current findings in neuropathology studies of ASD using two investigational approaches, postmortem human brains and ASD animal models, and discuss the overlap, limitations, and significance of each. Postmortem examination of ASD brains has revealed global changes including disorganized gray and white matter, increased number of neurons, decreased volume of neuronal soma, and increased neuropil, the last reflecting changes in densities of dendritic spines, cerebral vasculature and glia. Both cortical and non-cortical areas show region-specific abnormalities in neuronal morphology and cytoarchitectural organization, with consistent findings reported from the prefrontal cortex, fusiform gyrus, frontoinsular cortex, cingulate cortex, hippocampus, amygdala, cerebellum and brainstem. The paucity of postmortem human studies linking neuropathology to the underlying etiology has been partly addressed using animal models to explore the impact of genetic and non-genetic factors clinically relevant for the ASD phenotype. Genetically modified models include those based on well-studied monogenic ASD genes (NLGN3, NLGN4, NRXN1, CNTNAP2, SHANK3, MECP2, FMR1, TSC1/2), emerging risk genes (CHD8, SCN2A, SYNGAP1, ARID1B, GRIN2B, DSCAM, TBR1), and copy number variants (15q11-q13 deletion, 15q13.3 microdeletion, 15q11-13 duplication, 16p11.2 deletion and duplication, 22q11.2 deletion). Models of idiopathic ASD include inbred rodent strains that mimic ASD behaviors as well as models developed by environmental interventions such as prenatal exposure to sodium valproate, maternal autoantibodies, and maternal immune activation. In addition to replicating some of the neuropathologic features seen in postmortem studies, a common finding in several animal models of ASD is altered density of dendritic spines, with the direction of the change depending on the specific genetic modification, age and brain region. Overall, postmortem neuropathologic studies with larger sample sizes representative of the various ASD risk genes and diverse clinical phenotypes are warranted to clarify putative etiopathogenic pathways further and to promote the emergence of clinically relevant diagnostic and therapeutic tools. In addition, as genetic alterations may render certain individuals more vulnerable to developing the pathological changes at the synapse underlying the behavioral manifestations of ASD, neuropathologic investigation using genetically modified animal models will help to improve our understanding of the disease mechanisms and enhance the development of targeted treatments.

Dystroglycan Maintains Inner Limiting Membrane Integrity to Coordinate Retinal Development

Proper neural circuit formation requires the precise regulation of neuronal migration, axon guidance, and dendritic arborization. Mutations affecting the function of the transmembrane glycoprotein dystroglycan cause a form of congenital muscular dystrophy that is frequently associated with neurodevelopmental abnormalities. Despite its importance in brain development, the role of dystroglycan in regulating retinal development remains poorly understood. Using a mouse model of dystroglycanopathy (ISPD^L79* ) and conditional dystroglycan mutants of both sexes, we show that dystroglycan is critical for the proper migration, axon guidance, and dendritic stratification of neurons in the inner retina. Using genetic approaches, we show that dystroglycan functions in neuroepithelial cells as an extracellular scaffold to maintain the integrity of the retinal inner limiting membrane. Surprisingly, despite the profound disruptions in inner retinal circuit formation, spontaneous retinal activity is preserved. These results highlight the importance of dystroglycan in coordinating multiple aspects of retinal development.SIGNIFICANCE STATEMENT The extracellular environment plays a critical role in coordinating neuronal migration and neurite outgrowth during neural circuit development. The transmembrane glycoprotein dystroglycan functions as a receptor for multiple extracellular matrix proteins and its dysfunction leads to a form of muscular dystrophy frequently associated with neurodevelopmental defects. Our results demonstrate that dystroglycan is required for maintaining the structural integrity of the inner limiting membrane (ILM) in the developing retina. In the absence of functional dystroglycan, ILM degeneration leads to defective migration, axon guidance, and mosaic spacing of neurons and a loss of multiple neuron types during retinal development. These results demonstrate that disorganization of retinal circuit development is a likely contributor to visual dysfunction in patients with dystroglycanopathy.

Lhx9 Is Required for the Development of Retinal Nitric Oxide-Synthesizing Amacrine Cell Subtype

Amacrine cells are the most diverse group of retinal neurons. Various subtypes of amacrine interneurons mediate a vast majority of image forming and non-image forming visual functions. The transcriptional regulation governing the development of individual amacrine cell subtypes is not well understood. One such amacrine cell subtype comprises neuronal nitric oxide synthase (nNOS/bNOS/NOS1)-expressing amacrine cells (NOACs) that regulate the release of nitric oxide (NO), a neurotransmitter with physiological and clinical implications in the retina. We have identified the LIM-homeodomain transcription factor LHX9 to be necessary for the genesis of NOACs. During retinal development, NOACs express Lhx9, and Lhx9-null retinas lack NOACs. Lhx9-null retinas also display aberrations in dendritic stratification at the inner plexiform layer. Our cell lineage-tracing studies show that Lhx9-expressing cells give rise to both the GAD65 and GAD67 expressing sub-populations of GABAergic amacrine cells. As development proceeds, Lhx9 is downregulated in the GAD65 sub-population of GABAergic cells and is largely restricted to the GAD67 sub-population of amacrine cells that NOACs are a part of. Taken together, we have uncovered Lhx9 as a new molecular marker that defines a subset of amacrine cells and show that it is necessary for the development of the NOAC subtype of amacrine cells.

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.

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.

Controlled microfluidics to examine growth-factor induced migration of neural progenitors in the Drosophila visual system

BACKGROUND

The developing visual system in Drosophila melanogaster provides an excellent model with which to examine the effects of changing microenvironments on neural cell migration via microfluidics, because the combined experimental system enables direct genetic manipulation, in vivo observation, and in vitro imaging of cells, post-embryo. Exogenous signaling from ligands such as fibroblast growth factor (FGF) is well-known to control glia differentiation, cell migration, and axonal wrapping central to vision.

NEW METHOD

The current study employs a microfluidic device to examine how controlled concentration gradient fields of FGF are able to regulate the migration of vision-critical glia cells with and without cellular contact with neuronal progenitors.

RESULTS

Our findings quantitatively illustrate a concentration-gradient dependent chemotaxis toward FGF, and further demonstrate that glia require collective and coordinated neuronal locomotion to achieve directionality, sustain motility, and propagate long cell distances in the visual system.

COMPARISON WITH EXISTING METHOD(S)

Conventional assays are unable to examine concentration- and gradient-dependent migration. Our data illustrate quantitative correlations between ligand concentration/gradient and glial cell distance traveled, independent or in contact with neurons.

CONCLUSIONS

Microfluidic systems in combination with a genetically-amenable experimental system empowers researchers to dissect the signaling pathways that underlie cellular migration during nervous system development. Our findings illustrate the need for coordinated neuron-glia migration in the Drosophila visual system, as only glia within heterogeneous populations exhibited increasing motility along distances that increased with increasing FGF concentration. Such coordinated migration and chemotactic dependence can be manipulated for potential therapeutic avenues for NS repair and/or disease treatment.

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.

Netrin-1 induces local translation of down syndrome cell adhesion molecule in axonal growth cones

Down syndrome cell adhesion molecule (DSCAM) plays an important role in many neurodevelopmental processes such as axon guidance, dendrite arborization, and synapse formation. DSCAM is located in the Down syndrome trisomic region of human chromosome 21 and may contribute to the Down syndrome brain phenotype, which includes a reduction in the formation of long-distance connectivity. The local translation of a select group of mRNA transcripts within growth cones is necessary for the formation of appropriate neuronal connectivity. Interestingly, we have found that Dscam mRNA is localized to growth cones of mouse hippocampal neurons, and is dynamically regulated in response to the axon guidance molecule, netrin-1. Furthermore, netrin-1 stimulation results in an increase in locally translated DSCAM protein in growth cones. Deleted in colorectal cancer (DCC), a netrin-1 receptor, is required for the netrin-1-induced increase in Dscam mRNA local translation. We also find that two RNA-binding proteins-fragile X mental retardation protein (FMRP) and cytoplasmic polyadenylation element binding protein (CPEB)-colocalize with Dscam mRNA in growth cones, suggesting their regulation of Dscam mRNA localization and translation. Finally, overexpression of DSCAM in mouse cortical neurons results in a severe stunting of axon outgrowth and branching, suggesting that an increase in DSCAM protein results in a structural change having functional consequences. Taken together, these results suggest that netrin-1-induced local translation of Dscam mRNA during embryonic development may be an important mechanism to regulate axon growth and guidance in the developing nervous system. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 799-816, 2016.