Restricted expression of R-cadherin by brain nuclei and neural circuits of the developing chicken brain

Differential expression of microRNAs in the developing avian auditory hindbrain

The avian auditory hindbrain is a longstanding model for studying neural circuit development. Information on gene regulatory network (GRN) components underlying this process, however, is scarce. Recently, the spatiotemporal expression of 12 microRNAs (miRNAs) was investigated in the mammalian auditory hindbrain. As a comparative study, we here investigated the spatiotemporal expression of the orthologous miRNAs during development of the chicken auditory hindbrain. All miRNAs were expressed both at E13, an immature stage, and P14, a mature stage of the auditory system. In most auditory nuclei, a homogeneous expression pattern was observed at both stages, like the mammalian system. An exception was the nucleus magnocellularis (NM). There, at E13, nine miRNAs showed a differential expression pattern along the cochleotopic axis with high expression at the rostromedial pole. One of them showed a gradient expression whereas eight showed a spatially selective expression at the rostral pole that reflected the different rhombomeric origins of this composite nucleus. The miRNA differential expression persisted in the NM to the mature stage, with the selective expression changed to linear gradients. Bioinformatics analysis predicted mRNA targets that are associated with neuronal developmental processes such as neurite and synapse organization, calcium and ephrin-Eph signaling, and neurotransmission. Overall, this first analysis of miRNAs in the chicken central auditory system reveals shared and strikingly distinct features between chicken and murine orthologues. The embryonic gradient expression of these GRN elements in the NM adds miRNA patterns to the list of cochleotopic and developmental gradients in the central auditory system.

Comparative analysis of protocadherin-11 X-linked expression among postnatal rodents, non-human primates, and songbirds suggests its possible involvement in brain evolution

Background

Protocadherin-11 is a cell adhesion molecule of the cadherin superfamily. Since, only in humans, its paralog is found on the Y chromosome, it is expected that protocadherin-11X/Y plays some role in human brain evolution or sex differences. Recently, a genetic mutation of protocadherin-11X/Y was reported to be associated with a language development disorder. Here, we compared the expression of protocadherin-11 X-linked in developing postnatal brains of mouse (rodent) and common marmoset (non-human primate) to explore its possible involvement in mammalian brain evolution. We also investigated its expression in the Bengalese finch (songbird) to explore a possible function in animal vocalization and human language faculties.

Methodology/Principal Findings

Protocadherin-11 X-linked was strongly expressed in the cerebral cortex, hippocampus, amygdala and brainstem. Comparative analysis between mice and marmosets revealed that in certain areas of marmoset brain, the expression was clearly enriched. In Bengalese finches, protocadherin-11 X-linked was expressed not only in nuclei of regions of the vocal production pathway and the tracheosyringeal hypoglossal nucleus, but also in areas homologous to the mammalian amygdala and hippocampus. In both marmosets and Bengalese finches, its expression in pallial vocal control areas was developmentally regulated, and no clear expression was seen in the dorsal striatum, indicating a similarity between songbirds and non-human primates.

Conclusions/Significance

Our results suggest that the enriched expression of protocadherin-11 X-linked is involved in primate brain evolution and that some similarity exists between songbirds and primates regarding the neural basis for vocalization.

Comparative analysis of cadherin expression and connectivity patterns in the cerebellar system of ferret and mouse

The cerebellum shows remarkable variations in the relative size of its divisions among vertebrate species. In the present study, we compare the cerebella of two mammals (ferret and mouse) by mapping the expression of three cadherins (cadherin-8, protocadherin-7, and protocadherin-10) at similar postnatal stages. The three cadherins are expressed differentially in parasagittal stripes in the cerebellar cortex, in the portions of the deep cerebellar nuclei, in the divisions of the inferior olivary nucleus, and in the lateral vestibular nucleus. The expression profiles suggest that the cadherin-positive structures are interconnected. The expression patterns resemble each other in ferret and mouse, although some differences can be observed. The general resemblance indicates that cerebellar organization is based on a common set of embryonic divisions in the two species. Consequently, the large differences in cerebellar morphology between the two species are more likely caused by differential growth of these embryonic divisions than by differences in early embryonic patterning. Based on the cadherin expression patterns, a model of corticonuclear projection territories in ferret and mouse is proposed. In summary, our results indicate that the cerebellar systems of rodents and carnivores display a relatively large degree of similarity in their molecular and functional organization.

Expression analysis of cadherins in the songbird brain: relationship to vocal system development

Songbirds learn their songs as juveniles. The brains of songbirds have a series of nuclei and neural circuits called the song system, which is indispensable for vocal learning and production. In the present study we analyzed the expression patterns of cell adhesion molecules, cadherins, in the Bengalese finch (Lonchura striata var. domestica) to investigate their potential involvement in song nuclei and neural circuit formation. We found that cadherin-6B was expressed in many song nuclei of the juvenile and adult brain, while R-cadherin was complementarily expressed in surrounding areas. On the other hand, cadherin-7 was expressed in the robust nucleus of the arcopallium (RA) in the sensory learning stage, and its expression was downregulated during the sensorimotor learning stage. This downregulation of cadherin-7 was sexually dimorphic, suggesting its involvement in song development. Other cadherins, including cadherin-9, -10, and -12, showed different song-nuclei-related expression profiles. These patterns of song nuclei related expression suggest the possibility that cadherins are involved in the formation and maintenance of the song nuclei or neural pathways of the song system.

Expression of classical cadherins in the cerebellar anlage: quantitative and functional aspects

During central nervous system (CNS) development, cell migration precedes and is key to the integration of diverse sets of cells. Mechanistically, CNS histogenesis is realized through a balanced interplay of cell-cell and cell-matrix adhesion molecules. Here, we summarize experiments that probe the developmental expression and potential significance of a set of cadherins, including M-, N- and R-cadherin, for patterning of the cerebellar cortex. We established a transgenic marker that allows cerebellar granule cells to be followed from the neuroblast stage to their final, postmitotic settlement. In conjunction with flow cytometry, this allowed us to derive a quantitative view of cadherin expression in differentiating granule cells and relate it to the expression of the same cadherins in cerebellar inhibitory interneuronal precursors. In vitro reaggregation analysis supports a role for cadherins in cell sorting and migration within the nascent cerebellar cortex that may be rationalized within the context of the differential adhesion hypothesis (Foty, R.A. and Steinberg, M.S., 2005. The differential adhesion hypothesis: a direct evaluation. Dev. Biol. 278, 255-263.).

The resilient synapse: insights from genetic interference of synaptic cell adhesion molecules

Synaptic cell adhesion molecules (SCAMs) are mostly membrane-anchored molecules with extracellular domains that extend into the synaptic cleft. Prototypical SCAMs interact with homologous or heterologous molecules on the surface of adjacent cells, ensuring the precise apposition of pre- and postsynaptic elements. More recent definitions of SCAMs often include molecules involved in axon pathfinding, cell recognition and synaptic differentiation events, making SCAMs functionally and molecularly a highly diverse group. In this review, we summarize the proposed in vivo functions of a large variety of SCAMs. We mainly focus on results obtained from analyses of genetic model organisms, mostly mouse knockout mutants, lacking expression of the respective candidate genes. In contrast to the substantial effect yielded by some knockouts of molecules involved in synaptic vesicle release, no SCAM mutant has been reported thus far that shows a prominently altered structure of the majority of synapses or even lacks synapses altogether. This surprising resilience of synaptic structure might be explained by a high redundancy between different SCAMs, by the assumption that the crucial molecular players in synapse structure have yet to be discovered or by a grand variability in the mechanisms of synapse formation that underlies the diversity of synapses. Whatever the final answer turns out to be, the genetic dissection of the SCAM superfamilies has led to a much better understanding of the different steps required to form, differentiate and modify a synapse.

Temporally distinct demands for classic cadherins in synapse formation and maturation

Classic cadherins are synaptic adhesion proteins that have been implicated in synapse formation and targeting. Brief inactivation of classic cadherin function in young neurons appears to abrogate synapse formation when examined acutely. It remains unknown if such abrogation is unique to young neurons, whether it occurs by stalling neuronal maturation or by directly interfering with the process of synapse assembly, or whether synapse targeting is altered. Here we asked if sustained pan-cadherin blockade would prevent or alter the progression of axonal and dendritic outgrowth, synaptogenesis, or the stereotypic distribution of excitatory and inhibitory synapses on cultured hippocampal neurons. While pre- and postsynaptic cadherins are required for synapse assembly in young neurons, we find that in neurons older than 10 days, classic cadherins are entirely dispensable for joining and aligning presynaptic vesicle clusters with molecular markers of the postsynaptic density. Furthermore, we find that the proportion and relative distributions of excitatory and inhibitory terminals on single neurons are not altered. However, synapses that form on neurons in which cadherin function is blocked are smaller; they exhibit decreased synaptic vesicle recycling and a decreased frequency of spontaneous EPSCs. Moreover, they fail to acquire resistance to F-actin depolymerization, a hallmark of mature, stable contacts. These data provide new evidence that cadherins are required to promote synapse stabilization and structural and functional maturation, but dispensable for the correct subcellular distribution of excitatory and inhibitory synapses.

Differential expression of cadherins in the developing and adult zebrafish olfactory system

Cadherins are cell adhesion molecules that play important roles in development of a variety of tissues and maintenance of adult structures. Although cadherin expression has been studied in detail in the central nervous system of several vertebrate species, little is known of their distribution in the developing and adult olfactory structures, and there is no published report, to our knowledge, of cadherin expression in fish olfactory system. In this study, we examined expression patterns of three cadherins, cadherin-1 (E-cadherin), cadherin-2 (N-cadherin), and cadherin-4 (R-cadherin), in the olfactory system of developing and adult zebrafish by using both in situ hybridization and immunocytochemical methods. Cadherin-1 is detected in the newly formed olfactory placode, and its expression is maintained in the developing and adult olfactory epithelium and olfactory nerve. Cadherin-2 is expressed in the olfactory epithelium, olfactory nerve, and olfactory bulb of the embryonic and larval zebrafish, and its expression is reduced in the adult olfactory system. In contrast to the cadherin-1 and cadherin-2 expression, cadherin-4 is not found in the olfactory epithelium, but it is detected in the larval and adult olfactory bulb, in the olfactory tract, and its targets in the telencephalon. We hypothesize that the differential expression of these three cadherins in the developing zebrafish major olfactory structures reflects functionally different roles in the development of the vertebrate olfactory system.

Cadherins guide migrating Purkinje cells to specific parasagittal domains during cerebellar development

Several cadherins are expressed in parasagittal Purkinje cell domains, which can be defined by their afferent and efferent connectivity in the developing and mature cerebellum. By in vivo electroporation in chicken embryos, we demonstrate that Purkinje cell progenitors, which overexpress cadherin-6B or cadherin-7, distribute preferentially to those Purkinje cell domains, which express the respective cadherin endogenously. This differential distribution may be based, at least in part, on the guidance of migrating neurons along neurites that express the same cadherin. Selective induction of apoptosis and cadherin-based cell sorting within cortical domains do not seem to contribute to the differential distribution. These results show that cadherins can tell early neurons where to integrate in functional brain gray matter, possibly by a cadherin-based homotypic adhesive mechanism.

Cadherin-2 and cadherin-4 in developing, adult and regenerating zebrafish cerebellum

Cadherins are cell adhesion molecules that regulate development of a variety of tissues and maintenance of adult structures. In this study, we examined expression of two zebrafish classical cadherins, cadherin-2 and cadherin-4, in the cerebellum of developing, normal adult, and regenerating adult zebrafish using in situ hybridization and immunohistochemical methods. Cadherin-2 was widely expressed by the cerebellum of embryonic (24-50-h post fertilization) and larval zebrafish (3-14 days). Cadherin-2 expression became much reduced in the adult cerebellum, but it was greatly up-regulated in the regenerating adult cerebellum. Cadherin-4 was not detected in the embryonic cerebellum, but it was expressed in the Purkinje cells of the larval and adult cerebellum. To gain insight into cadherin-2 role in the formation of the cerebellum, we analyzed embryos injected with a specific cadherin-2 antisense morpholino oligonucleotide (cdh2MO1), and found that the cerebellar development of the cdh2MO1-injected embryos was severely disrupted. This phenotype was confirmed by examining a cadherin-2 mutant, glass onion. Our results suggest that cadherins are crucial for the normal development of the zebrafish cerebellum, and they may also be involved in the regeneration of injured fish cerebellum.

Selective synaptic cadherin expression by traced neurons of the chicken visual system

The stable and specific locking-in of pre- and postsynaptic membranes in synaptogenesis may be mediated by integral membrane proteins, such as members of the cadherin family. Cadherins are ideal candidate molecules for mediating synaptic specificity because they are differentially expressed in functionally connected brain structures. We studied the expression of four classic cadherins (R-cadherin, N-cadherin, cadherin-6B and cadherin-7) at the synaptic level on the somata and the proximal neurites of identified neuron populations that were traced selectively in the developing chicken visual system. Three major findings were observed. (1) Synapses on somata of shepherd's crook cells of the optic tectum are associated preferentially with one cadherin subtype. (2) In an isthmic nucleus that contains a mixed population of cells expressing different cadherins, somatic synapses tend to express the same cadherin subtype as the rest of the cell. (3) In the oculomotor complex, two cadherin subtypes are expressed only by synapses on the axon hillock. However, another neuron type that projects from the tectum to the isthmic nucleus does not show such selective synaptic cadherin staining. Our findings support the idea that a cadherin-based adhesive mechanism can mediate synaptic specificity.

Internal structure of the nucleus rotundus revealed by mapping cadherin expression in the embryonic chicken visual system

The nucleus rotundus is the largest nucleus of the avian thalamus. It is an important center of visual information processing and conveys information from the optic tectum to the ectostriatum in the telencephalon. The nucleus rotundus is generally believed to contain internal divisions processing information on color, form, motion, and looming of visual objects. The detailed arrangement of these internal divisions is unclear. Here, we map the expression of four classic cadherins (N-cadherin, R-cadherin, cadherin-6B, and cadherin-7), which are markers for specific functional gray matter divisions and their fiber connections in the vertebrate brain. Results show that each cadherin is expressed by one coherent part of the nucleus rotundus that is connected to other brain structures by fiber tracts expressing the same subtype of cadherin. Overall, the expression of the four cadherins encompasses almost the entire nucleus rotundus. The four cadherin-expressing parts show different degrees of overlap. For example, the cadherin-6B part and the cadherin-7 part overlap extensively, whereas the R-cadherin part and the cadherin-6B part show little overlap and are partially complementary. Regions with shallow gradients of cadherin expression alternate with regions that show relatively abrupt changes in cadherin expression. At some points, changes of cadherin expression are also arranged in a pinwheel-like fashion, alternating between clockwise and counterclockwise orientations. In general, these results are reminiscent of the organization of functional modules in the mammalian visual cortex. It is speculated that each domain of cadherin expression corresponds to a functional domain, which processes a specific stimulus feature.

Development of output connections from the inferior colliculus to the optic tectum in barn owls

We studied the development of the projection from the external nucleus of the inferior colliculus (ICX) to the optic tectum (OT) in the barn owl. The projection was labeled by tracer application in vitro to either the OT or the ICX, or by staining ICX cells intracellularly with biocytin. The axons of ICX neurons bifurcated into an ascending branch that projected toward the OT and a descending branch that coursed caudally to an unknown target in the brainstem. Axons of the ICX were observed to grow into the OT from embryonic day 16 (E16) on. From E22 on, side branches of the axonal projections could be found within the OT. At the day of hatching (E32), the projection displayed a dorsoventral topography comparable to the adult owl; however, atopically projecting cells remained. The complexity of the axonal arborization in the adult barn owl was found to be slightly increased compared with the hatchling. The terminal area of individual ICX cells in the OT of the adult barn owl was still broad, a finding that had not been expected from the sharply defined physiological response properties of the bimodal neurons in the space map of the OT. However, the width of the termination zone was in accordance with the large dendritic tree of the adult ICX cells, because both spanned comparable angles in their respective maps. Our data suggest that a coarse projection from the ICX to the OT can develop without coherent sensory input and may, therefore, be innately determined.

Cytoarchitecture of the avian optic tectum: neuronal substrate for cellular computation

To analyse cellular computation in the vertebrate brain, a thorough knowledge of the underlying anatomy, physiology and connectivity of the neuronal substrate is essential. This review compiles data on one of the best known structures of the vertebrate brain, the optic tectum of birds. The functions of this structure are multifold, but can be attributed largely to orientation and the basic analysis of sensory data in a spatial context. In the tectum, a wealth of data on physiology and anatomy has been gathered over more than a century and provides an excellent background for computational studies. The analysis of the optic tectum is facilitated by several principles of organisation, including the retinotopic input and the highly laminated layout with separated input and output layers. Moreover, the molecular mechanisms guiding the development and connectivity have been analysed in detail. As the avian tectum and the mammalian superior colliculus are partly homologous, the cellular mechanisms unraveled in the tectum can also be transferred to the colliculus and thus contribute to the understanding of the vertebrate visual system in general.

Spatial organization of the pigeon tectorotundal pathway: an interdigitating topographic arrangement

The retinotectofugal system is the main visual pathway projecting upon the telencephalon in birds and many other nonmammalian vertebrates. The ascending tectal projection arises exclusively from cells located in layer 13 of the optic tectum and is directed bilaterally toward the thalamic nucleus rotundus. Although previous studies provided evidence that different types of tectal layer 13 cells project to different subdivisions in Rt, apparently without maintaining a retinotopic organization, the detailed spatial organization of this projection remains obscure. We reexamined the pigeon tectorotundal projection using conventional tracing techniques plus a new method devised to perform small deep-brain microinjections of crystalline tracers. We found that discrete injections involving restricted zones within one subdivision retrogradely label a small fraction of layer 13 cells that are distributed throughout the layer, covering most of the tectal representation of the contralateral visual field. Double-tracer injections in one subdivision label distinct but intermingled sets of layer 13 neurons. These results, together with the tracing of tectal axonal terminal fields in the rotundus, lead us to propose a novel "interdigitating" topographic arrangement for the tectorotundal projection, in which intermingled sets of layer 13 cells, presumably of the same particular class and distributed in an organized fashion throughout the surface of the tectum, terminate in separate regions within one subdivision. This spatial organization has significant consequences for the understanding of the physiological and functional properties of the tectofugal pathway in birds.

Identification and localization of multiple classic cadherins in developing rat limbic system

Classic cadherins are multifunctional adhesion proteins that play roles in tissue histogenesis, neural differentiation, neurite outgrowth and synapse formation. Several lines of evidence suggest that classic cadherins may establish regional or laminar recognition cues by virtue of their differential expression and tight, and principally homophilic, cell adhesion. As a first step toward investigating the role this family plays in generating limbic system connectivity, we used RT-PCR to amplify type I and type II classic cadherins present in rat hippocampus during the principal period of synaptogenesis. We identified nine different cadherins, one of which, cadherin-9, is novel in hippocampus. Using in situ hybridization, we compared the cellular and regional distribution of five of the cadherins (N, 6, 8, 9 and 10) during the first two postnatal weeks in hippocampus, subiculum, entorhinal cortex, cingulate cortex, anterior thalamus, hypothalamus and amygdala. We find that each cadherin is differentially distributed in distinct, but highly overlapping fields that largely correspond to known anatomical boundaries and are often coordinately expressed in interconnected regions. For example, cadherin-6 expression defines CA1 and its principal target, the subiculum; cadherin-10 is differentially expressed in CA1 and CA3 in a manner correlating with the organization of interconnecting Schaffer collateral axons; and cadherin-9 shows a striking concentration in CA3. Some cadherin mRNAs are highly restricted to particular anatomical fields over the entire time course, while others are more broadly expressed and become concentrated within particular domains coincident with the timing of afferent ingrowth. Our data indicate that classic cadherins are sufficiently diverse and differentially distributed to support a role in cell surface recognition and adhesion during the formation of limbic system connectivity.

Small-scale pattern formation in a cortical area of the embryonic chicken telencephalon

The parahippocampal area is a cortical region of the avian dorsomedial telencephalon. In the chicken embryo, it contains discrete clusters of cadherin-7-positive cells, which are embedded in a cadherin-7-negative matrix. In the present work, the development and spatial distribution of these clusters is studied in whole-mount specimens. The clusters form a complex, coherent pattern of patches of variable size, spacing, and staining intensity. The pattern is especially prominent and regularly spaced in the rostral part of the caudolateral parahippocampal area. Here, it consists of stripes and connecting bridges with an average periodicity of approximately 0.3 mm. This pattern vaguely resembles some animal fur patterns and the ocular dominance domain of the mammalian visual cortex. The cadherin-7-positive patches also differ from their surrounding area by their cytoarchitecture and their increased acetylcholinesterase activity, suggesting that they represent functionally specialized subregions within the parahippocampal area. During development, the patchiness is first observed between 9 and 10 days of incubation and gradually becomes more prominent until 15 days of incubation. Our results indicate that the patchy organization of cortical gray matter on a small scale of periodicity (below 1 mm), which is well studied in the mammalian neocortex, is also found in the avian telencephalon.

Developmental patterns of cadherin expression and localization in relation to compartmentalized thalamocortical terminations in rat barrel cortex

The wiring of synaptic circuitry during development is remarkably precise, but the molecular interactions that enable such precision remain largely to be defined. Cadherins are cell adhesion molecules hypothesized to play roles in axon growth and synaptic targeting during development. We previously showed that N-cadherin localizes to ventrobasal (VB) thalamocortical synapses in rat somatosensory (barrel) cortex during formation of the whisker-map in layer IV (Huntley and Benson [1999] J. Comp. Neurol. 407:453-471). Such specific association of N-cadherin with one identified afferent pathway raises the prediction that other cadherins are expressed in barrel cortex and that these are, in some combination, also differentially associated with distinct inputs. Here, we first show that N-cadherin and three other classic cadherins (cadherin-6, -8, and -10) are expressed contemporaneously in barrel cortex with relative levels of postnatal expression that are highest during the first 2 weeks, when afferent and intrinsic circuitries are forming and synaptogenesis is maximal. Each displayed distinct, but partly overlapping laminar patterns of expression that changed over time. Cadherin-8 probe hybridization formed a particularly striking pattern of intermittent, columnar patches extending from layer V through layer III, which was first detectable at approximately postnatal day 3. The patches were centered precisely over regions of dysgranular layer IV and, in the whisker barrel field, over barrel septa. This pattern is similar to that formed by the terminal distribution of thalamocortical afferents arising from the posterior nucleus (POm), suggesting cadherin-8 association with the POm thalamocortical synaptic circuit. Consistent with this, cadherin-8 mRNAs were enriched in the POm nucleus, and cadherin-8 immunolabeling in layer IV was enriched in barrel septa and codistributed with labeled POm thalamocortical synaptic-like puncta. The striking molecular parcellation of at least two different cadherins to the two, converging thalamic pathways that terminated in non-overlapping barrel center and septal compartments in layer IV strongly suggested that cadherins provide requisite molecular recognition and targeting that enable precise construction of thalamocortical and other synaptic circuitry.

Granule cell raphes in the cerebellar cortex of chicken and mouse

The cerebellar cortex of the chicken embryo contains parasagittal segments of Purkinje cells. At intermediate stages of development, cell-dense ribbons of migrating granule cells ("raphes") are found between the segments. The complementary pattern of granule cell raphes and Purkinje cell segments represents a basic scheme of cerebellar organization that coincides with the expression domains of various genes, such as cadherins, gene regulatory proteins, and ephrins and their receptors. We have recently found the raphe/segment pattern also in a mammalian species, the postnatal mouse. Like in the chicken, the parasagittal raphes of granule cells were observed at the boundaries of Purkinje cell segments that differentially express cadherins. The number and arrangement of the raphes in the different cerebellar lobules is roughly similar in both species. The raphe/segment pattern is thus more widely distributed in vertebrates than previously assumed.

Expression of R-cadherin and N-cadherin by cell groups and fiber tracts in the developing mouse forebrain: relation to the formation of functional circuits

The expression of R-cadherin and N-cadherin was mapped in the postnatal forebrain of the mouse by immunohistochemistry and in situ hybridization. Results show that the two molecules are expressed in specific and restricted patterns in numerous brain nuclei, gray matter areas and cortical layers that are widely distributed throughout the mouse forebrain at postnatal day 1. The expression pattern of R-cadherin is clearly distinct from that of N-cadherin, but overlap is observed in many areas. In many cortical areas, the two cadherins have a laminar-specific distribution that varies from region to region. In addition, immunohistochemical data revealed expression of R-cadherin protein and N-cadherin protein in the neuropil of many brain regions as well as in the axons that travel in fiber tracts such as the olfactory tract, the anterior commissure, the corpus callosum, the stria terminalis and the fornix. Often, subsets of axons within the same fiber tract differentially express R-cadherin and N-cadherin, with partial overlap of expression. The targets of the cadherin-immunoreactive fiber bundles often contain neuropil as well as cell bodies of neurons that also express the same type(s) of cadherin, suggesting that R-cadherin and N-cadherin may be involved in target recognition and the establishment of connections. Specifically, the expression of R-cadherin and N-cadherin is related to the maturation of thalamocortical sensory pathways, corticofugal pathways, and pathways associated with the hippocampal complex, the piriform cortex, and the amygdala. It is also related to the development of the cell groups associated with these pathways.Together, the results from the present study indicate the possibility that the selective adhesion of neural structures that express the same type(s) of cadherin contributes to the formation of gray matter areas, neural circuits and functional connections in the postnatal forebrain of the mouse.

Modularity in vertebrate brain development and evolution

Embryonic modularity and functional modularity are two principles of brain organization. Embryonic modules are histogenetic fields that are specified by position-dependent expression of patterning genes. Within each embryonic module, secondary and higher-level pattern formation takes places during development, finally giving rise to brain nuclei and cortical layers. Defined subsets of these structures become connected by fiber tracts to form the information-processing neural circuits, which represent the functional modules of the brain. We review evidence that a group of cell adhesion molecules, the cadherins, provides an adhesive code for both types of modularity, based on a preferentially homotypic binding mechanism. Embryonic modularity is transformed into functional modularity, in part by translating early-generated positional information into an array of adhesive cues, which regulate the binding of functional neural structures distributed across the embryonic modules. Brain modularity may provide a basis for adaptability in evolution.

N- and R-cadherins expression in the rat sciatic nerve demonstrated by postembedding immunogold method on semi-thin sections

The calcium-dependent adhesion molecules N- and R-cadherins take part in the morphogenesis of the nervous system. The data about their localization in the peripheral nerves postnatum are limited and somewhat controversial. In the present study, a new methodological approach to the immunohistochemical cadherin detection has been applied. The sciatic nerves of 10-day-old rats were immunotested on semi-thin Epon sections by anti-N- and R-cadherins polyclonal antibody and visualized by IgG conjugated to colloidal gold and silver amplification. In the myelinated nerve fibers, an immuno-like activity was associated with the axoplasm and axolemma, the surface of the myelinating Schwann cells and their extracellular matrix. The units of nonmyelinated fibres and their associated glial cells were also intensely immunostained. The myelin sheaths and the glial cytoplasm were immunonegative. The significance of the localization is discussed in regard to the possible role of N- and R-cadherins in the anatomical and functional compartmentation of peripheral nerves.

Calcium-dependent adhesion is necessary for the maintenance of prosomeres

Cell adhesion has been suggested to function in the establishment and maintenance of the segmental organization of the central nervous system. Here we tested the role of different classes of adhesion molecules in prosencephalic segmentation. Specifically, we examined the ability of progenitors from different prosomeres to reintegrate and differentiate within various brain regions after selective maintenance or removal of different classes of calcium-dependent versus -independent surface molecules. This analysis implicates calcium-dependent adhesion molecules as central to the maintenance of prosomeres. Only conditions that spared calcium-dependent adhesion systems but ablated more general (calcium-independent) adhesion systems resulted in prosomere-specific integration after transplantation. Among the members of this class of adhesion molecules, R-cadherin shows a striking pattern of prosomeric expression during development. To test whether expression of this molecule was sufficient to direct progenitor integration to prosomeres expressing R-cadherin, we used a retroviral-mediated gain-of-function approach. We found that progenitors originally isolated from prosomere P2 (a region which does not express R-cadherin), when forced to express this molecule, can now integrate more readily into R-cadherin-expressing regions, such as the cortex, the ventral thalamus, and the hypothalamus. Nonetheless, our analysis suggests that while calcium-dependent molecules are able to direct prosomere-specific integration, they are not sufficient to induce progenitors to change their regional identity. While diencephalic progenitors from R-cadherin-expressing regions of prosomere 5 could integrate into R-cadherin-expressing regions of the cortex, they did not express the cortex-specific gene Emx1 or the telencephalic-specific gene Bf-1. Furthermore, diencephalic progenitors that integrate heterotopically into the cortex do not persist postnatally, whereas the same progenitors survive and differentiate when they integrate homotopically into the diencephalon. Together our results implicate calcium-dependent adhesion molecules as key mediators of prosomeric organization but suggest that they are not sufficient to bestow regional identities.

Cadherins in the central nervous system

The central nervous system (CNS) is divided into diverse embryological and functional compartments. The early embryonic CNS consists of a series of transverse subdivisions (neuromeres) and longitudinal domains. These embryonic subdivisions represent histogenetic fields in which neurons are born and aggregate in distinct cell groups (brain nuclei and layers). Different subsets of these aggregates become selectively connected by nerve fiber tracts and, finally, by synapses, thus forming the neural circuits of the functional systems in the CNS. Recent work has shown that 30 or more members of the cadherin family of morphoregulatory molecules are differentially expressed in the developing and mature brain at almost all stages of development. In a regionally specific fashion, most cadherins studied to date are expressed by the embryonic subdivisions of the early embryonic brain, by developing brain nuclei, cortical layers and regions, and by fiber tracts, neural circuits and synapses. Each cadherin shows a unique expression pattern that is distinct from that of other cadherins. Experimental evidence suggests that cadherins contribute to CNS regionalization, morphogenesis and fiber tract formation, possibly by conferring preferentially homotypic adhesiveness (or other types of interactions) between the diverse structural elements of the CNS. Cadherin-mediated adhesive specificity may thus provide a molecular code for early embryonic CNS regionalization as well as for the development and maintenance of functional structures in the CNS, from embryonic subdivisions to brain nuclei, cortical layers and neural circuits, down to the level of individual synapses.

Morphologic fate of diencephalic prosomeres and their subdivisions revealed by mapping cadherin expression

The expression of four cadherins (cadherin-6B, cadherin-7, R-cadherin, and N-cadherin) was mapped in the diencephalon of chicken embryos at 11 days and 15 days of incubation and was compared with Nissl stains and radial glial topology. Results showed that each cadherin is expressed in a restricted manner by a different set of embryonic divisions, brain nuclei, and their subregions. An analysis of the segmental organization based on the prosomeric model indicated that, in the mature diencephalon, each prosomere persists and forms a coherent domain of gray matter extending across the entire transverse dimension of the neural tube, from the ventricular surface to the pial surface. Moreover, the results suggest the presence of a novel set of secondary subdivisions for the dorsal thalamus (dorsal, intermediate, and ventral tiers and anteroventral subregion). They also confirm the presence of secondary subdivisions in the pretectum (commissural, juxtacommissural, and precommissural). At most of the borders between the prosomeres and their secondary subdivisions, changes in radial glial fiber density were observed. The diencephalic brain nuclei that derive from each of the subdivisions were determined. In addition, a number of previously less well-characterized gray matter regions of the diencephalon were defined in more detail based on the mapping of cadherin expression. The results demonstrate in detail how the divisions of the early embryonic diencephalon persist and transform into mature gray matter architecture during brain morphogenesis, and they support the hypothesis that cadherins play a role in this process by providing a framework of potentially adhesive specificities.

Molecular modification of N-cadherin in response to synaptic activity

The relationship between adhesive interactions across the synaptic cleft and synaptic function has remained elusive. At certain CNS synapses, pre- to postsynaptic adhesion is mediated at least in part by neural (N-) cadherin. Here, we demonstrate that upon depolarization of hippocampal neurons in culture by K+ treatment, or application of NMDA or alpha-latrotoxin, synaptic N-cadherin dimerizes and becomes markedly protease resistant. These properties are indices of strong, stable, enhanced cadherin-mediated intercellular adhesion. N-cadherin retained protease resistance for at least 2 hr after recovery, while other surface molecules, including other cadherins, were completely degraded. The acquisition of protease resistance and dimerization of N-cadherin is not dependent on new protein synthesis, nor is it accompanied by internalization of N-cadherin. By immunocytochemistry, we found that high K+ selectively induces surface dispersion of N-cadherin, which, after recovery, returns to synaptic puncta. N-cadherin dispersion under K+ treatment parallels the rapid expansion of the presynaptic membrane consequent to the massive vesicle fusion that occurs with this type of depolarization. In contrast, with NMDA application, N-cadherin does not disperse but does acquire enhanced protease resistance and dimerizes. Our data strongly suggest that synaptic adhesion is dynamically and locally controlled, and modulated by synaptic activity.

Spatial correspondence between R-cadherin expression domains and retinal ganglion cell axons in developing zebrafish

Mechanisms underlying axonal pathfinding have been investigated for decades, and numerous molecules have been shown to play roles in this process, including members of the cadherin family of cell adhesion molecules. We showed in the companion paper that a member of the cadherin family (zebrafish R-cadherin) is expressed in retinal ganglion cells, and in presumptive visual structures in zebrafish brain, during periods when the axons were actively extending toward their targets. The present study extends the earlier work by using 1,1'-dioctadecyl-3,3,3',3', tetramethylindocarbocyanine perchlorate (DiI) anterograde tracing techniques to label retinal ganglion cell axons combined with R-cadherin in situ hybridization to explicitly examine the association ofretinal axons and brain regions expressing R-cadherin message. We found that in zebrafish embryos at 46-54 hours postfertilization, DiI-labeled retinal axons were closely associated with cells expressing R-cadherin message in the hypothalamus, the pretectum, and the anterolateral optic tectum. These results demonstrate that R-cadherin is appropriately distributed to play a role in regulating development of the zebrafish visual system, and in particular, pathfinding and synaptogenesis of retinal ganglion cell axons.

R-cadherin expression in the developing and adult zebrafish visual system

Cell adhesion molecules in the cadherin family have been implicated in histogenesis and maintenance of cellular structure and function in several organs. Zebrafish have emerged as an important new developmental model, but only three zebrafish cadherin molecules have been identified to date (N-cadherin, paraxial protocadherin, and VN-cadherin). We began a systematic study to identify other zebrafish cadherins by screening zebrafish cDNA libraries using an antibody raised to the cytoplasmic domain of mouse E-cadherin. Here, we report a partial cDNA with extensive sequence homology to R-cadherin. Spatial and temporal expression of this putative zebrafish R-cadherin was examined in embryos and adults by Northern analysis, RNase protection, and in situ hybridization. R-cadherin message increased during embryogenesis up to 80 hours postfertilization (hpf) and persisted in adults. In the embryonic brain, R-cadherin was first expressed in groups of cells in the diencephalon and pretectum. In adult zebrafish brain, R-cadherin continued to be expressed in several specific regions including primary visual targets. In the retina, R-cadherin was first detected at about 33 hours postfertilization in the retinal ganglion cell layer and the inner part of the inner nuclear layer. Expression levels were highest during periods of axon outgrowth and synaptogenesis. Retrograde labeling of the optic nerve with 1,1'-dioctadecyl-3,3,3',3', tetramethylindocarbocyanine perchlorate (DiI) followed by in situ hybridization confirmed that a subset of retinal ganglion cells in the embryo expressed R-cadherin message. In the adult, R-cadherin expression continued in a subpopulation of retinal ganglion cells. These results suggest that R-cadherin-mediated adhesion plays a role in development and maintenance of neuronal connections in zebrafish visual system.

Cloning and expression of mouse Cadherin-7, a type-II cadherin isolated from the developing eye

We report the molecular cloning of Cadherin-7 from the embryonic mouse eye. The deduced amino acid sequence shows it to be a type-II cadherin similar to Xenopus F-cadherin and chick Cadherin-7. The mouse Cadherin-7 gene maps to chromosome 1, outside the conserved linkage group of cadherin genes on chromosome 8. Cadherin-7 is expressed throughout the entire period of neural development and mRNA levels are developmentally regulated in both the embryonic and the postnatal central nervous system (CNS). In adult mice, Cadherin-7 expression is restricted to the CNS, with highest levels in the retina. In the developing eye, Cadherin-7 mRNA is found only in the neural retina. It is expressed by all retinal neuroblasts from E11 onward, but becomes progressively restricted to neurons in the inner neuroblast and developing ganglion cell layers (GCL). In the adult retina it is confined to subpopulations of cells in the GCL and to amacrine cells in the inner part of the inner nuclear layer. This expression pattern suggests a role for Cadherin-7 in mouse retinal development, particularly in the formation and maintenance of the GCL.

Multiple cadherin mRNA expression and developmental regulation of a novel cadherin in the developing mouse eye

Cadherins form a large family of transmembrane glycoproteins whose members include the classical cadherins, the desmosomal cadherins, and the protocadherins. The classical cadherins mediate homophilic cell-cell adhesion and are key regulators of many morphogenetic processes. More than a dozen classical cadherins are expressed in both the developing and the mature central nervous system. Although individual cadherins have been identified in the retina of various species, we wished to determine the range of cadherins expressed at distinct developmental stages in the mouse retina. Using a PCR-based cloning strategy, we detected 10 different classical cadherin mRNAs of both type I and type II subtypes. The most abundant cDNA was that encoding the type II cadherin, Cadherin-11. The other type II cadherins detected were VE- and T2-cadherin and Cadherin-6 and -12. Four type I cadherins, N-, R-, P-, and E-cadherin, were also present. One cadherin cDNA encoded a novel cadherin, called EY-cadherin for cloned from eye. EY-cadherin is most closely related to human Cadherin-14 (93% identical). EY-cadherin mRNA was detected in the adult mouse eye, brain, and testis with a 20-fold increase in expression levels in the embryonic head from E11 to E19 and a 50-fold increase in expression levels in the postnatal eye from PN1 to PN16. Multiple cadherin gene expression is consistent with the hypothesis that different cadherins regulate morphogenetic processes, such as neuronal migration and lamination, and determine the specific interneuronal connections found in the mature retina.

Combinatorial expression of cadherins in the tectum and the sorting of neurites in the tectofugal pathways of the chicken embryo

The expression of four cadherins (N-cadherin, R-cadherin, cadherin-6B and cadherin-7) was mapped in the developing tectal system of the chicken embryo from four to 19 days of incubation. Each of the cadherins is expressed in a restricted fashion in specific tectal layers, with partial overlap between the cadherins. In some layers, subpopulations of neurons differentially express the cadherins, e.g., in the stratum griseum centrale. Double labeling demonstrates that many of the projection neurons in this layer co-express at least two cadherins. Fibers of the efferent (tectofugal) pathways originating in these neurons also differentially express the cadherins, most prominently at around 1 1 days of incubation. While the different subpopulations of cadherin-expressing projection neurons are dispersed and mixed within the tectum, their neurites sort out and fasciculate according to which cadherin they express, as they collect in the major output of the tectum, the brachium colliculi superioris. From here, cadherin-expressing fascicles follow separate paths to their respective target areas, some of which also express the respective cadherins, in a matching fashion. We propose that the preferentially homophilic binding of cadherins provides a potential adhesive basis for the sorting and selective fasciculation of specific subpopulations of neurites, similar to the well-established sorting and aggregation of cells expressing cadherins. The combinatorial expression of cadherins by the tectal projection neurons may contribute to the complexity and specificity of functional connections in this system.

Expression of a novel protocadherin, OL-protocadherin, in a subset of functional systems of the developing mouse brain

We cloned a novel protocadherin cDNA, which we named OL-protocadherin (OL-pc), from mouse brain cDNA libraries. Its cytoplasmic region showed no similarities to other protocadherins, indicating that it belongs to a novel subfamily of protocadherins. Experiments using transfectants showed that OL-pc is a homophilic cell-cell adhesion molecule. The molecular mass of OL-pc was 140 kDa in the brain. Expression of OL-pc mRNA was specific to the nervous system, changing over time from the embryonic stage to the adult stage. The OL-pc expression seemed to be restricted to a subset of functionally related brain nuclei and regions such as the nuclei in the main olfactory system, the limbic system, and the olivocortical projection. There were at least two distinct patterns of distribution for the OL-pc protein. First, it was localized in particular brain nuclei or compartments, such as the stripes of the developing cerebellum. Second, it was found at the synapse in regions such as the glomeruli of the olfactory bulb. In addition, the OL-pc protein seemed not to be detected or was detected only weakly in some regions, such as hippocampus in which the mRNA was expressed at high levels. These results indicate that the expression of OL-pc is developmentally regulated in a subset of the functional systems and that it may be involved in the formation of the neural network by segregation of the brain nuclei and mediation of the axonal connections.

Heterogeneity of cadherin-8 expression in the neonatal rat striatum: comparison with striatal compartments

Mechanisms of organization of the striatal compartments are poorly understood, although involvement of cell adhesion molecules in the compartmentalization has been suggested. Cadherin-8 distribution in the neonatal rat striatum was immunohistochemically studied using a rabbit anti-cadherin-8 antiserum. Intensity of cadherin-8 immunolabeling in the striatum was heterogeneous from postnatal day 0 to postnatal day 7. At postnatal day 9, cadherin-8 immunoreactivity was so weak that heterogeneity was no longer clearly seen. Cadherin-8 immunoreactivity was not detectable at postnatal day 14. Cadherin-8-rich and cadherin-8-poor areas were identical to calbindin-rich areas and tyrosine hydroxylase-rich patches, respectively, in allocation, indicating that cadherin-8 was predominantly expressed in the striatal matrix. These results suggest that cadherin-8 is involved in formation of the striatal compartmentalized structures during brain development.

Cadherin-8 protein expression in gray matter structures and nerve fibers of the neonatal and adult mouse brain

The topological distribution of mouse cadherin-8 protein in the neonatal and adult mouse brain was studied immunohistochemically using a rabbit antiserum. Cadherin-8 expression was restricted to several areas in neonatal brains constituting particular neural circuits, i.e. the limbic system, the basal ganglia-thalamocortical circuit, and the cerebellum and related nuclei. In addition, the nerve fibers linking some of the cadherin-8-positive areas, i.e. the habenulo-interpeduncular tract, decussation of the dorsal tegmentum, the medial longitudinal fasciculus, transverse pontine fibers, the brachium conjunctivum and the inferior cerebellar peduncle were cadherin-8 positive, as were the spinal tract of the trigeminal nerve, oculomotor nerve, facial nerve and trigeminal nerve. Cadherin-8 expression also showed a patch-like distribution in the intermediate gray layer of the superior colliculus, resembling acetylcholinesterase-rich patches in allocation. Segmentally organized cadherin-8-positive areas were found in the neonatal cerebellar Purkinje cell layer. Some nuclei and fibers in the brainstem and cerebellum, expressing cadherin-8 at neonatal stages, were also stained in the adult mouse brain. These findings suggest that cadherin-8 is involved in the formation of particular neural circuits by connecting areas expressing this molecule with positive nerve fibers, and indicate its possible implication in subdivisional organization in the superior colliculus and cerebellum.

Expression of multiple cadherins and catenins in the chick optic tectum

Cadherins form a large family of homophilic cell adhesion molecules that are involved in numerous aspects of neural development. The best-studied neural cadherin, N-cadherin, is concentrated at synapses made by retinal axons in the chick optic tectum and is required for the arborization of retinal axons in their target (retinorecipient) laminae. By analogy, other cadherins might mediate arborization or synaptogenesis in other tectal laminae. Here we consider which cadherins are expressed in tectum, which cells express them, and how their expression is regulated. First, using N-cadherin as a model, we show that synaptic input regulates both cadherin gene expression and the subcellular distribution of cadherin protein. Second, we demonstrate that N-, R-, and T-cadherin are each expressed in distinct laminar patterns during retinotectal synaptogenesis and that N- and R- are enriched in nonoverlapping synaptic subsets. Third, we show that over 20 cadherin superfamily genes are expressed in the tectum during the time that synapses are forming and that many of them are expressed in restricted groups of cells. Finally, we report that both beta-catenin and gamma-catenin (plakoglobin), cytoplasmic proteins required for cadherin signaling, are enriched at synapses and associated with N-cadherin. However, beta- and gamma-catenins are differentially distributed and regulated, and form mutually exclusive complexes. This result suggests that cadherin-based specificity involves multiple cadherin-dependent signaling pathways as well as multiple cadherins.

N-cadherin redistribution during synaptogenesis in hippocampal neurons

Cadherins are homophilic adhesion molecules that, together with their intracellular binding partners the catenins, mediate adhesion and signaling at a variety of intercellular junctions. This study shows that neural (N)-cadherin and beta-catenin, an intracellular binding partner for the classic cadherins, are present in axons and dendrites before synapse formation and then cluster at developing synapses between hippocampal neurons. N-cadherin is expressed initially at all synaptic sites but rapidly becomes restricted to a subpopulation of excitatory synaptic sites. Sites of GABAergic, inhibitory synapses in mature cultures therefore lack N-cadherin but are associated with clusters of beta-catenin, implying that they contain a different classic cadherin. These findings indicate that N-cadherin adhesion may stabilize early synapses that can then be remodeled to express a different cadherin and that cadherins systematically differentiate between functionally (excitatory and inhibitory) and spatially distinct synaptic sites on single neurons. These results suggest that differential cadherin expression may orchestrate the point-to-point specificity displayed by developing synapses.

Blocking N-cadherin function disrupts the epithelial structure of differentiating neural tissue in the embryonic chicken brain

The cell adhesion molecule N-cadherin is ubiquitously expressed in the early neuroepithelium, with strongest expression in the ependymal lining. We blocked the function of N-cadherin during early chicken brain development by injecting antibodies against N-cadherin into the tectal ventricle of embryos at 4-5 d of incubation [embryonic day 4 (E4)-E5]. N-cadherin blockage results in massive morphological changes in restricted brain regions. At approximately E6, these changes consist of invaginations of pieces of the ependymal lining and the formation of neuroepithelial rosettes. The rosettes are composed of central fragments of ependymal lining, surrounded by an inner ventricular layer and an outer mantle layer. Radial glia processes are radially arranged around the ependymal centers of the rosettes. The normal layering of the neural tissue is thus preserved, but its coherent epithelial structure is disrupted. The observed morphological changes are restricted to specific brain regions such as the tectum and the dorsal thalamus, whereas the ventral thalamus and the pretectum are almost undisturbed. At E10-E11, analysis of late effects of N-cadherin blockage reveals that in the dorsal thalamus, gray matter is fragmented and disorganized; in the tectum, additional layers have formed at the ventricular surface. Together, these results indicate that N-cadherin function is required for the maintenance of a coherent sheet of neuroepithelium in specific brain regions. Disruption of this sheet results in an abnormal morphogenesis of brain gray matter.

Cadherin-Defined Segments and Parasagittal Cell Ribbons in the Developing Chicken Cerebellum

In the developing chicken cerebellar cortex, three cadherins (Cad6B, Cad7, and R-cadherin) are expressed in distinct parasagittal segments that are separated from each other by ribbons of migrating interneurons and granule cells which express R-cadherin and Cad7, respectively. The segment/ribbon pattern is respected by the expression of other types of molecules, such as engrailed-2 and SC1/BEN/DM-GRASP. The cadherin-defined segments contain young Purkinje cells which are connected to underlying nuclear zones expressing the same cadherin, thereby forming parasagittal cortico-nuclear zones of topographically organized connections. In addition, R-cadherin-positive mossy fiber terminals display a periodic pattern in the internal granular layer. In this layer, Cad7 and R-cadherin are associated with synaptic complexes. These results suggest that cadherins play a pivotal role in the formation of functional cerebellar architecture by providing a three-dimensional scaffold of adhesive information. Copyright 1998 Academic Press.