Cadherin-6 mediates axon-target matching in a non-image-forming visual circuit

The M6 cell: A small-field bistratified photosensitive retinal ganglion cell

We have identified a novel, sixth type of intrinsically photosensitive retinal ganglion cell (ipRGC) in the mouse-the M6 cell. Its spiny, highly branched dendritic arbor is bistratified, with dendrites restricted to the inner and outer margins of the inner plexiform layer, co-stratifying with the processes of other ipRGC types. We show that M6 cells are by far the most abundant ganglion cell type labeled in adult pigmented Cdh3-GFP BAC transgenic mice. A few M5 ipRGCs are also labeled, but no other RGC types were encountered. Several distinct subnuclei in the geniculate complex and the pretectum contain labeled retinofugal axons in the Cdh3-GFP mouse. These are presumably the principle central targets of M6 cells (as well as M5 cells). Projections from M6 cells to the dorsal lateral geniculate nucleus were confirmed by retrograde tracing, suggesting they contribute to pattern vision. M6 cells have low levels of melanopsin expression and relatively weak melanopsin-dependent light responses. They also exhibit strong synaptically driven light responses. Their dendritic fields are the smallest and most abundantly branched of all ipRGCs. They have small receptive fields and strong antagonistic surrounds. Despite deploying dendrites partly in the OFF sublamina, M6 cells appear to be driven exclusively by the ON pathway, suggesting that their OFF arbor, like those of certain other ipRGCs, may receive ectopic input from passing ON bipolar cells axons in the OFF sublayer.

Sub-topographic maps for regionally enhanced analysis of visual space in the mouse retina

In many species, neurons are unevenly distributed across the retina, leading to nonuniform analysis of specific visual features at certain locations in visual space. In recent years, the mouse has emerged as a premiere model for probing visual system function, development, and disease. Thus, achieving a detailed understanding of mouse visual circuit architecture is of paramount importance. The general belief is that mice possess a relatively even topographic distribution of retinal ganglion cells (RGCs)-the output neurons of the eye. However, mouse RGCs include ∼30 subtypes; each responds best to a specific feature in the visual scene and conveys that information to central targets. Given the crucial role of RGCs and the prominence of the mouse as a model, we asked how different RGC subtypes are distributed across the retina. We targeted and filled individual fluorescently tagged RGC subtypes from across the retinal surface and evaluated the dendritic arbor extent and soma size of each cell according to its specific retinotopic position. Three prominent RGC subtypes: On-Off direction selective RGCs, object-motion-sensitive RGCs, and a specialized subclass of nonimage-forming RGCs each had marked topographic variations in their dendritic arbor sizes. Moreover, the pattern of variation was distinct for each RGC subtype. Thus, there is increasing evidence that the mouse retina encodes visual space in a region-specific manner. As a consequence, some visual features are sampled far more densely at certain retinal locations than others. These findings have implications for central visual processing, perception, and behavior in this prominent model species.

Distribution and development of molecularly distinct perineuronal nets in visual thalamus

Visual information is detected by the retina and transmitted into the brain by retinal ganglion cells. In rodents, the visual thalamus is a major recipient of retinal ganglion cells axons and is divided into three functionally distinct nuclei: the dorsal lateral geniculate nucleus (dLGN), ventral LGN (vLGN), and intergeniculate leaflet. Despite being densely innervated by retinal input, each nucleus in rodent visual thalamus possesses diverse molecular profiles which underpin their unique circuitry and cytoarchitecture. Here, we combined large-scale unbiased proteomic and transcriptomic analyses to elucidate the molecular expression profiles of the developing mouse dLGN and vLGN. We identified several extracellular matrix proteins as differentially expressed in these regions, particularly constituent molecules of perineuronal nets (PNNs). Remarkably, we discovered at least two types of molecularly distinct Aggrecan-rich PNN populations in vLGN, exhibiting non-overlapping spatial, temporal, and cell-type specific expression patterns. The mechanisms responsible for the formation of these two populations of PNNs also differ as the formation of Cat315⁺ PNNs (but not WFA⁺ PNNs) required input from the retina. This study is first to suggest that cell type- and molecularly specific supramolecular assemblies of extracellular matrix may play important roles in the circuitry associated with the subcortical visual system and in the processing of visual information. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Cover Image for this issue: doi: 10.1111/jnc.14203.

Distribution and development of molecularly distinct perineuronal nets in visual thalamus

Visual information is detected by the retina and transmitted into the brain by retinal ganglion cells. In rodents, the visual thalamus is a major recipient of retinal ganglion cells axons and is divided into three functionally distinct nuclei: the dorsal lateral geniculate nucleus (dLGN), ventral LGN (vLGN), and intergeniculate leaflet. Despite being densely innervated by retinal input, each nucleus in rodent visual thalamus possesses diverse molecular profiles which underpin their unique circuitry and cytoarchitecture. Here, we combined large-scale unbiased proteomic and transcriptomic analyses to elucidate the molecular expression profiles of the developing mouse dLGN and vLGN. We identified several extracellular matrix proteins as differentially expressed in these regions, particularly constituent molecules of perineuronal nets (PNNs). Remarkably, we discovered at least two types of molecularly distinct Aggrecan-rich PNN populations in vLGN, exhibiting non-overlapping spatial, temporal, and cell-type specific expression patterns. The mechanisms responsible for the formation of these two populations of PNNs also differ as the formation of Cat315⁺ PNNs (but not WFA⁺ PNNs) required input from the retina. This study is first to suggest that cell type- and molecularly specific supramolecular assemblies of extracellular matrix may play important roles in the circuitry associated with the subcortical visual system and in the processing of visual information. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Cover Image for this issue: doi: 10.1111/jnc.14203.

Assembly and repair of eye-to-brain connections

Vision is the sense humans rely on most to navigate the world and survive. A tremendous amount of research has focused on understanding the neural circuits for vision and the developmental mechanisms that establish them. The eye-to-brain, or 'retinofugal' pathway remains a particularly important model in these contexts because it is essential for sight, its overt anatomical features relate to distinct functional attributes and those features develop in a tractable sequence. Much progress has been made in understanding the growth of retinal axons out of the eye, their selection of targets in the brain, the development of laminar and cell type-specific connectivity within those targets, and also dendritic connectivity within the retina itself. Moreover, because the retinofugal pathway is prone to degeneration in many common blinding diseases, understanding the cellular and molecular mechanisms that establish connectivity early in life stands to provide valuable insights into approaches that re-wire this pathway after damage or loss. Here we review recent progress in understanding the development of retinofugal pathways and how this information is important for improving visual circuit regeneration.

Cadherin 8 regulates proliferation of cortical interneuron progenitors

Cortical interneurons are born in the ventral forebrain and migrate tangentially in two streams at the levels of the intermediate zone (IZ) and the pre-plate/marginal zone to the developing cortex where they switch to radial migration before settling in their final positions in the cortical plate. In a previous attempt to identify the molecules that regulate stream specification, we performed transcriptomic analysis of GFP-labelled interneurons taken from the two migratory streams during corticogenesis. A number of cadherins were found to be expressed differentially, with Cadherin-8 (Cdh8) selectively present in the IZ stream. We verified this expression pattern at the mRNA and protein levels on tissue sections and found approximately half of the interneurons of the IZ expressed Cdh8. Furthermore, this cadherin was also detected in the germinal zones of the subpallium, suggesting that it might be involved not only in the migration of interneurons but also in their generation. Quantitative analysis of cortical interneurons in animals lacking the cadherin at E18.5 revealed a significant increase in their numbers. Subsequent functional in vitro experiments showed that blocking Cdh8 function led to increased cell proliferation, with the opposite results observed with over-expression, supporting its role in interneuron generation.

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.

Retinal ganglion cell axon sorting at the optic chiasm requires dystroglycan

In the developing visual system, retinal ganglion cell (RGC) axons project from the retina to several distal retinorecipient regions in the brain. Several molecules have been implicated in guiding RGC axons in vivo, but the role of extracellular matrix molecules in this process remains poorly understood. Dystroglycan is a laminin-binding transmembrane protein important for formation and maintenance of the extracellular matrix and basement membranes and has previously been implicated in axon guidance in the developing spinal cord. Using two genetic models of functional dystroglycan loss, we show that dystroglycan is necessary for correct sorting of contralateral and ipsilateral RGC axons at the optic chiasm. Mis-sorted axons still target retinorecipient brain regions and persist in adult mice, even after axon pruning is complete. Our results highlight the importance of the extracellular matrix for axon sorting at an intermediate choice point in the developing visual circuit.

The Role of Wnt/β-Catenin Signaling and K-Cadherin in the Regulation of Intraocular Pressure

Purpose

Wnt/β-catenin signaling in the trabecular meshwork (TM) is required for maintaining normal intraocular pressure (IOP), although the mechanism(s) behind this are unknown. We hypothesize that Wnt/β-catenin signaling regulates IOP via β-catenin's effects on cadherin junctions.

Methods

Nonglaucomatous primary human TM (NTM) cells were treated with or without 100 ng/ml Wnt3a, 1 μg/ml sFRP1, or both for 4 to 48 hours. Cells were immunostained for β-catenin, total cadherins, or cadherin isoforms. Membrane proteins or whole-cell lysates were isolated for Western immunoblotting and probed for cadherin isoforms. RNA was extracted for cDNA synthesis and qPCR analysis of cadherin expression. Some NTM cells were cultured on electric plates for cell impedance assays. Ad5.CMV recombinant adenoviruses encoding K-cadherin, and/or sFRP1 were injected into eyes of 4- to 6-month-old female BALB/cJ mice (n = 8–10). Conscious IOPs were assessed for 35 days.

Results

Upon Wnt3a treatment, total cadherin expression increased and β-catenin accumulated at the TM cell membrane and on processes formed between TM cells. qPCR showed that Wnt3a significantly increased K-cadherin expression in NTM cells (P < 0.01, n = 3), and Western immunoblotting showed that Wnt3a increased K-cadherin in NTM cells, which was inhibited by the addition of sFRP1. Cell impedance assays showed that Wnt3a treatment increased transcellular resistance and anti-K-cadherin siRNA decreased transcellular resistance (P < 0.001, n = 4–6). Our in vivo study showed that K-cadherin significantly decreased sFRP1-induced ocular hypertension (P < 0.05, n = 6). Western immunoblotting also showed that K-cadherin alleviated sFRP1-induced β-catenin decrease in mouse anterior segments.

Conclusions

Our results suggest that cadherins play important roles in the regulation of TM homeostasis and IOP via the Wnt/β-catenin pathway.

The Cadherin Superfamily in Neural Circuit Assembly

The cadherin superfamily comprises a large, diverse collection of cell surface receptors that are expressed in the nervous system throughout development and have been shown to be essential for the proper assembly of the vertebrate nervous system. As our knowledge of each family member has grown, it has become increasingly clear that the functions of various cadherin subfamilies are intertwined: they can be present in the same protein complexes, impinge on the same developmental processes, and influence the same signaling pathways. This interconnectedness may illustrate a central way in which core developmental events are controlled to bring about the robust and precise assembly of neural circuitry.

Subtype-specific response of retinal ganglion cells to optic nerve crush

Glaucoma is a neurodegenerative disease with retinal ganglion cell (RGC) loss, optic nerve degeneration and subsequent vision loss. There are about 30 different subtypes of RGCs whose response to glaucomatous injury is not well characterized. The purpose of this study was to evaluate the response of 4 RGC subtypes in a mouse model of optic nerve crush (ONC). In this study, we also evaluated the pattern of axonal degeneration in RGC subtypes after nerve injury. We found that out of the 4 subtypes, transient-Off α RGCs are the most susceptible to injury followed by On-Off direction selective RGCs (DSGC). Non-image forming RGCs are more resilient with ipRGCs exhibiting the most resistance of them all. In contrast, axons degenerate irrespective of their retinal soma after ONC injury. In conclusion, we show that RGCs have subtype specific cell death response to ONC injury and that RGC axons disintegrate in an autonomous fashion undergoing Wallerian degeneration. These discoveries can further direct us towards effective diagnostic and therapeutic approaches to treat optic neuropathies, such as glaucoma.

Reciprocal Connections Between Cortex and Thalamus Contribute to Retinal Axon Targeting to Dorsal Lateral Geniculate Nucleus

The dorsal Lateral Geniculate Nucleus (dLGN) is the primary image-forming target of the retina and shares a reciprocal connection with primary visual cortex (V1). Previous studies showed that corticothalamic input is essential for the development of thalamocortical projections, but less is known about the potential role of this reciprocal connection in the development of retinal projections. Here, we show a deficit of retinal innervation in the dLGN around E18.5 in Tra2β conditional knockout (cKO) "cortexless" mice, an age when apoptosis occurs along the thalamocortical tract and in some dLGN neurons. In vivo electrophysiology experiments in the dLGN further confirmed the loss of functional retinal input. Experiments with N-methyl-d-aspartic acid-induced V1 lesion as well as Fezf2 cKO mice confirmed that the disruption of connections between the dLGN and V1 lead to abnormal retinal projections to the dLGN. Interestingly, retinal projections to the ventral Lateral Geniculate Nucleus (vLGN) and Superior Colliculus (SC) were normal in all 3 mice models. Finally, we show that the cortexless mice had worse performance than control mice in a go-no go task with visual cues. Our results provide evidence that the wiring of visual circuit from the retina to the dLGN and V1 thereafter is coordinated at a surprisingly early stage of circuit development.

Architecture, Function, and Assembly of the Mouse Visual System

Vision is the sense humans rely on most to navigate the world, make decisions, and perform complex tasks. Understanding how humans see thus represents one of the most fundamental and important goals of neuroscience. The use of the mouse as a model for parsing how vision works at a fundamental level started approximately a decade ago, ushered in by the mouse's convenient size, relatively low cost, and, above all, amenability to genetic perturbations. In the course of that effort, a large cadre of new and powerful tools for in vivo labeling, monitoring, and manipulation of neurons were applied to this species. As a consequence, a significant body of work now exists on the architecture, function, and development of mouse central visual pathways. Excitingly, much of that work includes causal testing of the role of specific cell types and circuits in visual perception and behavior-something rare to find in studies of the visual system of other species. Indeed, one could argue that more information is now available about the mouse visual system than any other sensory system, in any species, including humans. As such, the mouse visual system has become a platform for multilevel analysis of the mammalian central nervous system generally. Here we review the mouse visual system structure, function, and development literature and comment on the similarities and differences between the visual system of this and other model species. We also make it a point to highlight the aspects of mouse visual circuitry that remain opaque and that are in need of additional experimentation to enrich our understanding of how vision works on a broad scale.

Strict Independence of Parallel and Poly-synaptic Axon-Target Matching during Visual Reflex Circuit Assembly

The use of sensory information to drive specific behaviors relies on circuits spanning long distances that wire up through a range of axon-target recognition events. Mechanisms assembling poly-synaptic circuits and the extent to which parallel pathways can "cross-wire" to compensate for loss of one another remain unclear and are crucial to our understanding of brain development and models of regeneration. In the visual system, specific retinal ganglion cells (RGCs) project to designated midbrain targets connected to downstream circuits driving visuomotor reflexes. Here, we deleted RGCs connecting to pupillary light reflex (PLR) midbrain targets and discovered that axon-target matching is tightly regulated. RGC axons of the eye-reflex pathway avoided vacated PLR targets. Moreover, downstream PLR circuitry is maintained; hindbrain and peripheral components retained their proper connectivity and function. These findings point to a model in which poly-synaptic circuit development reflects independent, highly stringent wiring of each parallel pathway and downstream station.

Guidance of retinal axons in mammals

In order to navigate through the surrounding environment many mammals, including humans, primarily rely on vision. The eye, composed of the choroid, sclera, retinal pigmented epithelium, cornea, lens, iris and retina, is the structure that receives the light and converts it into electrical impulses. The retina contains six major types of neurons involving in receiving and modifying visual information and passing it onto higher visual processing centres in the brain. Visual information is relayed to the brain via the axons of retinal ganglion cells (RGCs), a projection known as the optic pathway. The proper formation of this pathway during development is essential for normal vision in the adult individual. Along this pathway there are several points where visual axons face 'choices' in their direction of growth. Understanding how these choices are made has advanced significantly our knowledge of axon guidance mechanisms. Thus, the development of the visual pathway has served as an extremely useful model to reveal general principles of axon pathfinding throughout the nervous system. However, due to its particularities, some cellular and molecular mechanisms are specific for the visual circuit. Here we review both general and specific mechanisms involved in the guidance of mammalian RGC axons when they are traveling from the retina to the brain to establish precise and stereotyped connections that will sustain vision.

Heterophilic Type II Cadherins Are Required for High-Magnitude Synaptic Potentiation in the Hippocampus

Hippocampal CA3 neurons form synapses with CA1 neurons in two layers, stratum oriens (SO) and stratum radiatum (SR). Each layer develops unique synaptic properties but molecular mechanisms that mediate these differences are unknown. Here, we show that SO synapses normally have significantly more mushroom spines and higher-magnitude long-term potentiation (LTP) than SR synapses. Further, we discovered that these differences require the Type II classic cadherins, cadherins-6, -9, and -10. Though cadherins typically function via trans-cellular homophilic interactions, our results suggest presynaptic cadherin-9 binds postsynaptic cadherins-6 and -10 to regulate mushroom spine density and high-magnitude LTP in the SO layer. Loss of these cadherins has no effect on the lower-magnitude LTP typically observed in the SR layer, demonstrating that cadherins-6, -9, and -10 are gatekeepers for high-magnitude LTP. Thus, Type II cadherins may uniquely contribute to the specificity and strength of synaptic changes associated with learning and memory.

Regional Cellular Environment Shapes Phenotypic Variations of Hippocampal and Neocortical Chandelier Cells

Different cortical regions processing distinct information, such as the hippocampus and the neocortex, share common cellular components and circuit motifs but form unique networks by modifying these cardinal units. Cortical circuits include diverse types of GABAergic interneurons (INs) that shape activity of excitatory principal neurons (PNs). Canonical IN types conserved across distinct cortical regions have been defined by their morphological, electrophysiological, and neurochemical properties. However, it remains largely unknown whether canonical IN types undergo specific modifications in distinct cortical regions and display "regional variants." It is also poorly understood whether such phenotypic variations are shaped by early specification or regional cellular environment. The chandelier cell (ChC) is a highly stereotyped IN type that innervates axon initial segments of PNs and thus serves as a good model with which to address this issue. Here, we show that Cadherin-6 (Cdh6), a homophilic cell adhesion molecule, is a reliable marker of ChCs and Cdh6-CreER mice (both sexes) provide genetic access to hippocampal ChCs (h-ChCs). We demonstrate that, compared with neocortical ChCs (nc-ChCs), h-ChCs cover twice as much area and innervate twice as many PNs. Interestingly, a subclass of h-ChCs exhibits calretinin (CR) expression, which is not found in nc-ChCs. Furthermore, we find that h-ChCs appear to be born earlier than nc-ChCs. Surprisingly, despite the difference in temporal origins, ChCs display host-region-dependent axonal/synaptic organization and CR expression when transplanted heterotopically. These results suggest that local cellular environment plays a critical role in shaping terminal phenotypes of regional IN variants in the hippocampus and the neocortex.SIGNIFICANCE STATEMENT Canonical interneuron (IN) types conserved across distinct cortical regions such as the hippocampus and the neocortex are defined by morphology, physiology, and gene expression. However, it remains unknown whether they display phenotypic variations in different cortical regions. In addition, it is unclear whether terminal phenotypes of regional IN variants belonging to a canonical IN type are determined intrinsically or extrinsically. Our results provide evidence of striking differences in axonal/synaptic organization and calretinin expression between hippocampal chandelier cells (ChCs) and neocortical ChCs. They also reveal that local cellular environment in distinct cortical regions regulates these terminal phenotypes. Therefore, our study suggests that local cortical environment shapes the phenotypes of regional IN variants, which may be required for unique circuit operations in distinct cortical regions.

Satb1 Regulates Contactin 5 to Pattern Dendrites of a Mammalian Retinal Ganglion Cell

The size and shape of dendritic arbors are prime determinants of neuronal connectivity and function. We asked how ON-OFF direction-selective ganglion cells (ooDSGCs) in mouse retina acquire their bistratified dendrites, in which responses to light onset and light offset are segregated to distinct strata. We found that the transcriptional regulator Satb1 is selectively expressed by ooDSGCs. In Satb1 mutant mice, ooDSGC dendrites lack ON arbors, and the cells selectively lose ON responses. Satb1 regulates expression of a homophilic adhesion molecule, Contactin 5 (Cntn5). Both Cntn5 and its co-receptor Caspr4 are expressed not only by ooDSGCs, but also by interneurons that form a scaffold on which ooDSGC ON dendrites fasciculate. Removing Cntn5 from either ooDSGCs or interneurons partially phenocopies Satb1 mutants, demonstrating that Satb1-dependent Cntn5 expression in ooDSGCs leads to branch-specific homophilic interactions with interneurons. Thus, Satb1 directs formation of a morphologically and functionally specialized compartment within a complex dendritic arbor.

Reconnecting Eye to Brain

Although much is known about the regenerative capacity of retinal ganglion cells, very significant barriers remain in our ability to restore visual function following traumatic injury or disease-induced degeneration. Here we summarize our current understanding of the factors regulating axon guidance and target engagement in regenerating axons, and review the state of the field of neural regeneration, focusing on the visual system and highlighting studies using other model systems that can inform analysis of visual system regeneration. This overview is motivated by a Society for Neuroscience Satellite meeting, "Reconnecting Neurons in the Visual System," held in October 2015 sponsored by the National Eye Institute as part of their "Audacious Goals Initiative" and co-organized by Carol Mason (Columbia University) and Michael Crair (Yale University). The collective wisdom of the conference participants pointed to important gaps in our knowledge and barriers to progress in promoting the restoration of visual system function. This article is thus a summary of our existing understanding of visual system regeneration and provides a blueprint for future progress in the field.

Corticothalamic Axons Are Essential for Retinal Ganglion Cell Axon Targeting to the Mouse Dorsal Lateral Geniculate Nucleus

Retinal ganglion cells (RGCs) relay information about the outside world to multiple subcortical targets within the brain. This information is either used to dictate reflexive behaviors or relayed to the visual cortex for further processing. Many subcortical visual nuclei also receive descending inputs from projection neurons in the visual cortex. Most areas receive inputs from layer 5 cortical neurons in the visual cortex but one exception is the dorsal lateral geniculate nucleus (dLGN), which receives layer 6 inputs and is also the only RGC target that sends direct projections to the cortex. Here we ask how visual system development and function changes in mice that develop without a cortex. We find that the development of a cortex is essential for RGC axons to terminate in the dLGN, but is not required for targeting RGC axons to other subcortical nuclei. RGC axons also fail to target to the dLGN in mice that specifically lack cortical layer 6 projections to the dLGN. Finally, we show that when mice develop without a cortex they can still perform a number of vision-dependent tasks.

SIGNIFICANCE STATEMENT

The dorsal lateral geniculate nucleus (dLGN) is a sensory thalamic relay area that receives feedforward inputs from retinal ganglion cells (RGCs) in the retina, and feed back inputs from layer 6 neurons in the visual cortex. In this study we examined genetically manipulated mice that develop without a cortex or without cortical layer 6 axonal projections, and find that RGC axons fail to project to the dLGN. Other RGC recipient areas, such as the superior colliculus and suprachiasmatic nucleus, are targeted normally. These results provide support for a new mechanism of target selection that may be specific to the thalamus, whereby descending cortical axons provide an activity that promotes feedforward targeting of RGC axons to the dLGN.

The role of cell adhesion molecules in brain wiring and neuropsychiatric disorders

Cell adhesion molecules (CAMs) in the nervous system have long been a research focus, but many mice lacking CAMs show very subtle phenotypes, giving an impression that CAMs may not be major players in constructing the nervous system. However, recent human genetic studies suggest CAM involvement in many neuropsychiatric disorders, implicating that they must have significant functions in nervous system development, namely in circuitry formation. As CAMs can provide specificity through their molecular interactions, this review summarizes possible mechanisms on how alterations of CAMs can result in neuropsychiatric disorders through circuitry modification.

A dual-strategy expression screen for candidate connectivity labels in the developing thalamus

The thalamus or “inner chamber” of the brain is divided into ~30 discrete nuclei, with highly specific patterns of afferent and efferent connectivity. To identify genes that may direct these patterns of connectivity, we used two strategies. First, we used a bioinformatics pipeline to survey the predicted proteomes of nematode, fruitfly, mouse and human for extracellular proteins containing any of a list of motifs found in known guidance or connectivity molecules. Second, we performed clustering analyses on the Allen Developing Mouse Brain Atlas data to identify genes encoding surface proteins expressed with temporal profiles similar to known guidance or connectivity molecules. In both cases, we then screened the resultant genes for selective expression patterns in the developing thalamus. These approaches identified 82 candidate connectivity labels in the developing thalamus. These molecules include many members of the Ephrin, Eph-receptor, cadherin, protocadherin, semaphorin, plexin, Odz/teneurin, Neto, cerebellin, calsyntenin and Netrin-G families, as well as diverse members of the immunoglobulin (Ig) and leucine-rich receptor (LRR) superfamilies, receptor tyrosine kinases and phosphatases, a variety of growth factors and receptors, and a large number of miscellaneous membrane-associated or secreted proteins not previously implicated in axonal guidance or neuronal connectivity. The diversity of their expression patterns indicates that thalamic nuclei are highly differentiated from each other, with each one displaying a unique repertoire of these molecules, consistent with a combinatorial logic to the specification of thalamic connectivity.

Establishing Wiring Specificity in Visual System Circuits: From the Retina to the Brain

The retina is a tremendously complex image processor, containing numerous cell types that form microcircuits encoding different aspects of the visual scene. Each microcircuit exhibits a distinct pattern of synaptic connectivity. The developmental mechanisms responsible for this patterning are just beginning to be revealed. Furthermore, signals processed by different retinal circuits are relayed to specific, often distinct, brain regions. Thus, much work has focused on understanding the mechanisms that wire retinal axonal projections to their appropriate central targets. Here, we highlight recently discovered cellular and molecular mechanisms that together shape stereotypic wiring patterns along the visual pathway, from within the retina to the brain. Although some mechanisms are common across circuits, others play unconventional and circuit-specific roles. Indeed, the highly organized connectivity of the visual system has greatly facilitated the discovery of novel mechanisms that establish precise synaptic connections within the nervous system.

Expression of transcription factors divides retinal ganglion cells into distinct classes

Retinal ganglion cells (RGCs) are tasked with transmitting all light information from the eye to the retinal recipient areas of the brain. RGCs can be classified into many different types by morphology, gene expression, axonal projections, and functional responses to different light stimuli. Ultimately, these classification systems should be unified into an all-encompassing taxonomy. Toward that end, we show here that nearly all RGCs express either Islet-2 (Isl2), Tbr2, or a combination of Satb1 and Satb2. We present gene expression data supporting the hypothesis that Satb1 and Satb2 are expressed in ON-OFF direction-selective (DS) RGCs, complementing our previous work demonstrating that RGCs that express Isl2 and Tbr2 are non-DS and non-image-forming, respectively. Expression of these transcription factors emerges at distinct embryonic ages and only in postmitotic cells. Finally, we demonstrate that these transcription factor-defined RGC classes are born throughout RGC genesis.

Diverse Central Projection Patterns of Retinal Ganglion Cells

Understanding how >30 types of retinal ganglion cells (RGCs) in the mouse retina each contribute to visual processing in the brain will require more tools that label and manipulate specific RGCs. We screened and analyzed retinal expression of Cre recombinase using 88 transgenic driver lines. In many lines, Cre was expressed in multiple RGC types and retinal cell classes, but several exhibited more selective expression. We comprehensively mapped central projections from RGCs labeled in 26 Cre lines using viral tracers, high-throughput imaging, and a data processing pipeline. We identified over 50 retinorecipient regions and present a quantitative retina-to-brain connectivity map, enabling comparisons of target-specificity across lines. Projections to two major central targets were notably correlated: RGCs projecting to the outer shell or core regions of the lateral geniculate projected to superficial or deep layers within the superior colliculus, respectively. Retinal images and projection data are available online at http://connectivity.brain-map.org.

Retinal axon guidance at the midline: Chiasmatic misrouting and consequences

The visual representation of the outside world relies on the appropriate connectivity between the eyes and the brain. Retinal ganglion cells are the sole neurons that send an axon from the retina to the brain, and thus the guidance decisions of retinal axons en route to their targets in the brain shape the neural circuitry that forms the basis of vision. Here, we focus on the choice made by retinal axons to cross or avoid the midline at the optic chiasm. This decision allows each brain hemisphere to receive inputs from both eyes corresponding to the same visual hemifield, and is thus crucial for binocular vision. In achiasmatic conditions, all retinal axons from one eye project to the ipsilateral brain hemisphere. In albinism, abnormal guidance of retinal axons at the optic chiasm leads to a change in the ratio of contralateral and ipsilateral projections with the consequence that each brain hemisphere receives inputs primarily from the contralateral eye instead of an almost equal distribution from both eyes in humans. In both cases, this misrouting of retinal axons leads to reduced visual acuity and poor depth perception. While this defect has been known for decades, mouse genetics have led to a better understanding of the molecular mechanisms at play in retinal axon guidance and at the origin of the guidance defect in albinism. In addition, fMRI studies on humans have now confirmed the anatomical and functional consequences of axonal misrouting at the chiasm that were previously only assumed from animal models. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 844-860, 2017.

Congenital Cataract in Gpr161vl/vl Mice Is Modified by Proximal Chromosome 15

The morphology and severity of human congenital cataract varies even among individuals with the same mutation, suggesting that genetic background modifies phenotypic penetrance. The spontaneous mouse mutant, vacuolated lens (vl), arose on the C3H/HeSnJ background. The mutation disrupts secondary lens fiber development by E16.5, leading to full penetrance of congenital cataract. The vl locus was mapped to a frameshift deletion in the orphan G protein-coupled receptor, Gpr161, which is expressed in differentiating lens fiber cells. When Gpr161^vl/vl C3H mice are crossed to MOLF/EiJ mice an unexpected rescue of cataract is observed, suggesting that MOLF modifiers affect cataract penetrance. Subsequent QTL analysis mapped three modifiers (Modvl3-5: Modifier of vl) and in this study we characterized Modvl4 (Chr15; LOD = 4.4). A Modvl4^MOLF congenic was generated and is sufficient to rescue congenital cataract and the lens fiber defect at E16.5. Additional phenotypic analysis on three subcongenic lines narrowed down the interval from 55 to 15Mb. In total only 18 protein-coding genes and 2 micro-RNAs are in this region. Fifteen of the 20 genes show detectable expression in the E16.5 eye. Subsequent expression studies in Gpr161^vl/vl and subcongenic E16.5 eyes, bioinformatics analysis of C3H/MOLF polymorphisms, and the biological relevancy of the genes in the interval identified three genes (Cdh6, Ank and Trio) that likely contribute to the rescue of the lens phenotype. These studies demonstrate that modification of the Gpr161^vl/vl cataract phenotype is likely due to genetic variants in at least one of three closely linked candidate genes on proximal Chr15.

Not a one-trick pony: Diverse connectivity and functions of the rodent lateral geniculate complex

Often mislabeled as a simple relay of sensory information, the thalamus is a complicated structure with diverse functions. This diversity is exemplified by roles visual thalamus plays in processing and transmitting light-derived stimuli. Such light-derived signals are transmitted to the thalamus by retinal ganglion cells (RGCs), the sole projection neurons of the retina. Axons from RGCs innervate more than ten distinct nuclei within thalamus, including those of the lateral geniculate complex. Nuclei within the lateral geniculate complex of nocturnal rodents, which include the dorsal lateral geniculate nucleus (dLGN), ventral lateral geniculate nucleus (vLGN), and intergeniculate leaflet (IGL), are each densely innervated by retinal projections, yet, exhibit distinct cytoarchitecture and connectivity. These features suggest that each nucleus within this complex plays a unique role in processing and transmitting light-derived signals. Here, we review the diverse cytoarchitecture and connectivity of these nuclei in nocturnal rodents, in an effort to highlight roles for dLGN in vision and for vLGN and IGL in visuomotor, vestibular, ocular, and circadian function.

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.

Two Pairs of ON and OFF Retinal Ganglion Cells Are Defined by Intersectional Patterns of Transcription Factor Expression

Visual information is conveyed to the brain by axons of >30 retinal ganglion cell (RGC) types. Characterization of these types is a prerequisite to understanding visual perception. Here, we identify a family of RGCs that we call F-RGCs on the basis of expression of the transcription factor Foxp2. Intersectional expression of Foxp1 and Brn3 transcription factors divides F-RGCs into four types, comprising two pairs, each composed of closely related cells. One pair, F-mini(ON) and F-mini(OFF), shows robust direction selectivity. They are among the smallest RGCs in the mouse retina. The other pair, F-midi(ON) and F-midi(OFF), is larger and not direction selective. Together, F-RGCs comprise >20% of RGCs in the mouse retina, halving the number that remain to be classified and doubling the number of known direction-selective cells. Co-expression of Foxp and Brn3 genes also marks subsets of RGCs in macaques that could be primate homologs of F-RGCs.

Corticothalamic Axons Are Essential for Retinal Ganglion Cell Axon Targeting to the Mouse Dorsal Lateral Geniculate Nucleus

Retinal ganglion cells (RGCs) relay information about the outside world to multiple subcortical targets within the brain. This information is either used to dictate reflexive behaviors or relayed to the visual cortex for further processing. Many subcortical visual nuclei also receive descending inputs from projection neurons in the visual cortex. Most areas receive inputs from layer 5 cortical neurons in the visual cortex but one exception is the dorsal lateral geniculate nucleus (dLGN), which receives layer 6 inputs and is also the only RGC target that sends direct projections to the cortex. Here we ask how visual system development and function changes in mice that develop without a cortex. We find that the development of a cortex is essential for RGC axons to terminate in the dLGN, but is not required for targeting RGC axons to other subcortical nuclei. RGC axons also fail to target to the dLGN in mice that specifically lack cortical layer 6 projections to the dLGN. Finally, we show that when mice develop without a cortex they can still perform a number of vision-dependent tasks.

SIGNIFICANCE STATEMENT

The dorsal lateral geniculate nucleus (dLGN) is a sensory thalamic relay area that receives feedforward inputs from retinal ganglion cells (RGCs) in the retina, and feed back inputs from layer 6 neurons in the visual cortex. In this study we examined genetically manipulated mice that develop without a cortex or without cortical layer 6 axonal projections, and find that RGC axons fail to project to the dLGN. Other RGC recipient areas, such as the superior colliculus and suprachiasmatic nucleus, are targeted normally. These results provide support for a new mechanism of target selection that may be specific to the thalamus, whereby descending cortical axons provide an activity that promotes feedforward targeting of RGC axons to the dLGN.

Contributions of Retinal Ganglion Cells to Subcortical Visual Processing and Behaviors

Every aspect of visual perception and behavior is built from the neural activity of retinal ganglion cells (RGCs), the output neurons of the eye. Here, we review progress toward understanding the many types of RGCs that communicate visual signals to the brain, along with the subcortical brain regions that use those signals to build and respond to representations of the outside world. We emphasize recent progress in the use of mouse genetics, viral circuit tracing, and behavioral psychophysics to define and map the various RGCs and their associated networks. We also address questions about the homology of RGC types in mice and other species including nonhuman primates and humans. Finally, we propose a framework for understanding RGC typology and for highlighting the relationship between RGC type-specific circuitry and the processing stations in the brain that support and give rise to the perception of sight.

Cell type-specific manipulation with GFP-dependent Cre recombinase

There are many transgenic GFP reporter lines that allow visualization of specific populations of cells. Using such lines for functional studies requires a method that transforms GFP into a molecule that enables genetic manipulation. Here we report the creation of a method that exploits GFP for gene manipulation, Cre Recombinase Dependent on GFP (CRE-DOG), a split component system that uses GFP and its derivatives to directly induce Cre/loxP recombination. Using plasmid electroporation and AAV viral vectors, we delivered CRE-DOG to multiple GFP mouse lines, leading to effective recombination selectively in GFP-labeled cells. Further, CRE-DOG enabled optogenetic control of these neurons. Beyond providing a new set of tools for manipulation of gene expression selectively in GFP+ cells, we demonstrate that GFP can be used to reconstitute the activity of a protein not known to have a modular structure, suggesting that this strategy might be applicable to a wide range of proteins.

Complex and dynamic expression of cadherins in the embryonic marmoset cerebral cortex

Cadherin is a cell adhesion molecule widely expressed in the nervous system. Previously, we analyzed the expression of nine classic cadherins (Cdh4, Cdh6, Cdh7, Cdh8, Cdh9, Cdh10, Cdh11, Cdh12, and Cdh20) and T-cadherin (Cdh13) in the developing postnatal common marmoset (Callithrix jacchus) brain, and found differential expressions between mice and marmosets. In this study, to explore primate-specific cadherin expression at the embryonic stage, we extensively analyzed the expression of these cadherins in the developing embryonic marmoset brain. Each cadherin showed differential spatial and temporal expression and exhibited temporally complicated expression. Furthermore, the expression of some cadherins differed from that in rodent brains, even at the embryonic stage. These results suggest the possibility that the differential expressions of diverse cadherins are involved in primate specific cortical development, from the prenatal to postnatal period.

Functional assembly of accessory optic system circuitry critical for compensatory eye movements

Accurate motion detection requires neural circuitry that compensates for global visual field motion. Select subtypes of retinal ganglion cells perceive image motion and connect to the accessory optic system (AOS) in the brain, which generates compensatory eye movements that stabilize images during slow visual field motion. Here, we show that the murine transmembrane semaphorin 6A (Sema6A) is expressed in a subset of On direction-selective ganglion cells (On DSGCs) and is required for retinorecipient axonal targeting to the medial terminal nucleus (MTN) of the AOS. Plexin A2 and A4, two Sema6A binding partners, are expressed in MTN cells, attract Sema6A(+) On DSGC axons, and mediate MTN targeting of Sema6A(+) RGC projections. Furthermore, Sema6A/Plexin-A2/A4 signaling is required for the functional output of the AOS. These data reveal molecular mechanisms underlying the assembly of AOS circuits critical for moving image perception.

Contactin-4 mediates axon-target specificity and functional development of the accessory optic system

The mammalian eye-to-brain pathway includes more than 20 parallel circuits, each consisting of precise long-range connections between specific sets of retinal ganglion cells (RGCs) and target structures in the brain. The mechanisms that drive assembly of these parallel connections and the functional implications of their specificity remain unresolved. Here we show that in the absence of contactin 4 (CNTN4) or one of its binding partners, amyloid precursor protein (APP), a subset of direction-selective RGCs fail to target the nucleus of the optic tract (NOT)--the accessory optic system (AOS) target controlling horizontal image stabilization. Conversely, ectopic expression of CNTN4 biases RGCs to arborize in the NOT, and that process also requires APP. Our data reveal critical and novel roles for CNTN4/APP in promoting target-specific axon arborization, and they highlight the importance of this process for functional development of a behaviorally relevant parallel visual pathway.

Characteristic patterns of dendritic remodeling in early-stage glaucoma: evidence from genetically identified retinal ganglion cell types

Retinal ganglion cell (RGC) loss is a hallmark of glaucoma and the second leading cause of blindness worldwide. The type and timing of cellular changes leading to RGC loss in glaucoma remain incompletely understood, including whether specific RGC subtypes are preferentially impacted at early stages of this disease. Here we applied the microbead occlusion model of glaucoma to different transgenic mouse lines, each expressing green fluorescent protein in 1-2 specific RGC subtypes. Targeted filling, reconstruction, and subsequent comparison of the genetically identified RGCs in control and bead-injected eyes revealed that some subtypes undergo significant dendritic rearrangements as early as 7 d following induction of elevated intraocular pressure (IOP). By comparing specific On-type, On-Off-type and Off-type RGCs, we found that RGCs that target the majority of their dendritic arbors to the scleral half or "Off" sublamina of the inner plexiform layer (IPL) undergo the greatest changes, whereas RGCs with the majority of their dendrites in the On sublamina did not alter their structure at this time point. Moreover, M1 intrinsically photosensitive RGCs, which functionally are On RGCs but structurally stratify their dendrites in the Off sublamina of the IPL, also underwent significant changes in dendritic structure 1 week after elevated IOP. Thus, our findings reveal that certain RGC subtypes manifest significant changes in dendritic structure after very brief exposure to elevated IOP. The observation that RGCs stratifying most of their dendrites in the Off sublamina are first to alter their structure may inform the development of new strategies to detect, monitor, and treat glaucoma in humans.

The classic cadherins in synaptic specificity

During brain development, billions of neurons organize into highly specific circuits. To form specific circuits, neurons must build the appropriate types of synapses with appropriate types of synaptic partners while avoiding incorrect partners in a dense cellular environment. Defining the cellular and molecular rules that govern specific circuit formation has significant scientific and clinical relevance because fine scale connectivity defects are thought to underlie many cognitive and psychiatric disorders. Organizing specific neural circuits is an enormously complicated developmental process that requires the concerted action of many molecules, neural activity, and temporal events. This review focuses on one class of molecules postulated to play an important role in target selection and specific synapse formation: the classic cadherins. Cadherins have a well-established role in epithelial cell adhesion, and although it has long been appreciated that most cadherins are expressed in the brain, their role in synaptic specificity is just beginning to be unraveled. Here, we review past and present studies implicating cadherins as active participants in the formation, function, and dysfunction of specific neural circuits and pose some of the major remaining questions.

Transient ipsilateral retinal ganglion cell projections to the brain: Extent, targeting, and disappearance

During development of the mammalian eye, the first retinal ganglion cells (RGCs) that extend to the brain are located in the dorsocentral (DC) retina. These RGCs extend to either ipsilateral or contralateral targets, but the ipsilateral projections do not survive into postnatal periods. The function and means of disappearance of the transient ipsilateral projection are not known. We have followed the course of this transient early ipsilateral cohort of RGCs, paying attention to how far they extend, whether they enter targets and if so, which ones, and the time course of their disappearance. The DC ipsilateral RGC axons were traced using DiI labeling at E13.5 and E15.5 to compare the proportion of ipsi- versus contralateral projections during the first period of growth. In utero electroporation of E12.5 retina with GFP constructs was used to label axons that could be visualized at succeeding time points into postnatal ages. Our results show that the earliest ipsilateral axons grow along the cellular border of the brain, and are segregated from the laterally positioned contralateral axons from the same retinal origin. In agreement with previous reports, although many early RGCs extend ipsilaterally, after E16 their number rapidly declines. Nonetheless, some ipsilateral axons from the DC retina enter the superior colliculus and arborize minimally, but very few enter the dorsal lateral geniculate nucleus and those that do extend only short branches. While the mechanism of selective axonal disappearance remains elusive, these data give further insight into establishment of the visual pathways.

Cadherins mediate sequential roles through a hierarchy of mechanisms in the developing mammillary body

Expression of intricate combinations of cadherins (a family of adhesive membrane proteins) is common in the developing central nervous system. On this basis, a combinatorial cadherin code has long been proposed to underlie neuronal sorting and to be ultimately responsible for the layers, columns and nuclei of the brain. However, experimental proof of this particular function of cadherins has proven difficult to obtain and the question is still not clear. Alternatively, non-specific, non-combinatorial, purely quantitative adhesive differentials have been proposed to explain neuronal sorting in the brain. Do cadherin combinations underlie brain cytoarchitecture? We approached this question using as model a well-defined forebrain nucleus, the mammillary body (MBO), which shows strong, homogeneous expression of one single cadherin (Cdh11) and patterned, combinatorial expression of Cdh6, −8 and −10. We found that, besides the known combinatorial Cdh pattern, MBO cells are organized into a second, non-overlapping pattern grouping neurons with the same date of neurogenesis. We report that, in the Foxb1 mouse mutant, Cdh11 expression fails to be maintained during MBO development. This disrupted the combination-based as well as the birthdate-based sorting in the mutant MBO. In utero RNA interference (RNAi) experiments knocking down Cdh11 in MBO-fated migrating neurons at one specific age showed that Cdh11 expression is required for chronological entrance in the MBO. Our results suggest that neuronal sorting in the developing MBO is caused by adhesion-based, non-combinatorial mechanisms that keep neurons sorted according to birthdate information (possibly matching them to target neurons chronologically sorted in the same manner). Non-specific adhesion mechanisms would also prevent cadherin combinations from altering the birthdate-based sorting. Cadherin combinations would presumably act later to support specific synaptogenesis through specific axonal fasciculation and final target recognition.

The role of cell adhesion molecules in visual circuit formation: from neurite outgrowth to maps and synaptic specificity

The formation of visual circuitry is a multistep process that involves cell–cell interactions based on a range of molecular mechanisms. The correct implementation of individual events, including axon outgrowth and guidance, the formation of the topographic map, or the synaptic targeting of specific cellular subtypes, are prerequisites for a fully functional visual system that is able to appropriately process the information captured by the eyes. Cell adhesion molecules (CAMs) with their adhesive properties and their high functional diversity have been identified as key actors in several of these fundamental processes. Because of their growth‐promoting properties, CAMs play an important role in neuritogenesis. Furthermore, they are necessary to control additional neurite development, regulating dendritic spacing and axon pathfinding. Finally, trans‐synaptic interactions of CAMs ensure cell type‐specific connectivity as a basis for the establishment of circuits processing distinct visual features. Recent discoveries implicating CAMs in novel mechanisms have led to a better general understanding of neural circuit formation, but also revealed an increasing complexity of their function. This review aims at describing the different levels of action for CAMs to shape neural connectivity, with a special focus on the visual system. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 75: 569–583, 2015

Cadherin-based transsynaptic networks in establishing and modifying neural connectivity

It is tacitly understood that cell adhesion molecules (CAMs) are critically important for the development of cells, circuits, and synapses in the brain. What is less clear is what CAMs continue to contribute to brain structure and function after the early period of development. Here, we focus on the cadherin family of CAMs to first briefly recap their multidimensional roles in neural development and then to highlight emerging data showing that with maturity, cadherins become largely dispensible for maintaining neuronal and synaptic structure, instead displaying new and narrower roles at mature synapses where they critically regulate dynamic aspects of synaptic signaling, structural plasticity, and cognitive function. At mature synapses, cadherins are an integral component of multiprotein networks, modifying synaptic signaling, morphology, and plasticity through collaborative interactions with other CAM family members as well as a variety of neurotransmitter receptors, scaffolding proteins, and other effector molecules. Such recognition of the ever-evolving functions of synaptic cadherins may yield insight into the pathophysiology of brain disorders in which cadherins have been implicated and that manifest at different times of life.

Neuronal cell types and connectivity: lessons from the retina

We describe recent progress toward defining neuronal cell types in the mouse retina and attempt to extract lessons that may be generally useful in the mammalian brain. Achieving a comprehensive catalog of retinal cell types now appears within reach, because researchers have achieved consensus concerning two fundamental challenges. The first is accuracy-defining pure cell types rather than settling for neuronal classes that are mixtures of types. The second is completeness-developing methods guaranteed to eventually identify all cell types, as well as criteria for determining when all types have been found. Case studies illustrate how these two challenges are handled by combining state-of-the-art molecular, anatomical, and physiological techniques. Progress is also being made in observing and modeling connectivity between cell types. Scaling up to larger brain regions, such as the cortex, will require not only technical advances but also careful consideration of the challenges of accuracy and completeness.

Connecting the retina to the brain

The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed.

Type II cadherins guide assembly of a direction-selective retinal circuit

Complex retinal circuits process visual information and deliver it to the brain. Few molecular determinants of synaptic specificity in this system are known. Using genetic and optogenetic methods, we identified two types of bipolar interneurons that convey visual input from photoreceptors to a circuit that computes the direction in which objects are moving. We then sought recognition molecules that promote selective connections of these cells with previously characterized components of the circuit. We found that the type II cadherins, cdh8 and cdh9, are each expressed selectively by one of the two bipolar cell types. Using loss- and gain-of-function methods, we showed that they are critical determinants of connectivity in this circuit and that perturbation of their expression leads to distinct defects in visually evoked responses. Our results reveal cellular components of a retinal circuit and demonstrate roles of type II cadherins in synaptic choice and circuit function.

So many pieces, one puzzle: cell type specification and visual circuitry in flies and mice

The visual system is a powerful model for probing the development, connectivity, and function of neural circuits. Two genetically tractable species, mice and flies, are together providing a great deal of understanding of these processes. Current efforts focus on integrating knowledge gained from three cross-fostering fields of research: (1) understanding how the fates of different cell types are specified during development, (2) revealing the synaptic connections between identified cell types ("connectomics") by high-resolution three-dimensional circuit anatomy, and (3) causal testing of how identified circuit elements contribute to visual perception and behavior. Here we discuss representative examples from fly and mouse models to illustrate the ongoing success of this tripartite strategy, focusing on the ways it is enhancing our understanding of visual processing and other sensory systems.