Neural circuits revealed by axon tracing and mapping cadherin expression in the embryonic chicken cerebellum

Axonal Projection Patterns of the Dorsal Interneuron Populations in the Embryonic Hindbrain

Unraveling the inner workings of neural circuits entails understanding the cellular origin and axonal pathfinding of various neuronal groups during development. In the embryonic hindbrain, different subtypes of dorsal interneurons (dINs) evolve along the dorsal-ventral (DV) axis of rhombomeres and are imperative for the assembly of central brainstem circuits. dINs are divided into two classes, class A and class B, each containing four neuronal subgroups (dA1-4 and dB1-4) that are born in well-defined DV positions. While all interneurons belonging to class A express the transcription factor Olig3 and become excitatory, all class B interneurons express the transcription factor Lbx1 but are diverse in their excitatory or inhibitory fate. Moreover, within every class, each interneuron subtype displays its own specification genes and axonal projection patterns which are required to govern the stage-by-stage assembly of their connectivity toward their target sites. Remarkably, despite the similar genetic landmark of each dINs subgroup along the anterior-posterior (AP) axis of the hindbrain, genetic fate maps of some dA/dB neuronal subtypes uncovered their contribution to different nuclei centers in relation to their rhombomeric origin. Thus, DV and AP positional information has to be orchestrated in each dA/dB subpopulation to form distinct neuronal circuits in the hindbrain. Over the span of several decades, different axonal routes have been well-documented to dynamically emerge and grow throughout the hindbrain DV and AP positions. Yet, the genetic link between these distinct axonal bundles and their neuronal origin is not fully clear. In this study, we reviewed the available data regarding the association between the specification of early-born dorsal interneuron subpopulations in the hindbrain and their axonal circuitry development and fate, as well as the present existing knowledge on molecular effectors underlying the process of axonal growth.

Distribution of Cadherin in the Parahippocampal Area of Developing Domestic Chicken Embryos

Hippocampal formation is important in spatial learning and memory. Members of the cadherin superfamily are observed in the neural system with diverse spatial and temporal expression patterns and are involved in many biological processes. To date, the avian hippocampal formation is not well understood. In this study, we examined the expression of cadherin mRNA in chicken and mouse brains to investigate the morphological and cytoarchitectural bases of hippocampal formation. Profiles of the spatiotemporal expression of cadherin mRNAs in the developing chicken embryonic parahippocampal area (APH) are provided, and layer-specific expression and spatiotemporal expression were observed in different subdivisions of the APH. That fact that some cadherins (Cdh2, Cdh8, Pcdh8 and Pcdh10) showed conserved regional expression both in the hippocampus and entorhinal cortex of mice and the hippocampal formation of chickens partially confirmed the structural homology proposed by previous scientists. This study indicates that some cadherins can be used as special markers of the avian hippocampal formation.

Protocadherins and hypothalamic development: do they play an unappreciated role?

Normal brain development requires coordinated cell movements at precise times. It has long been established that cell-cell adhesion proteins of the cadherin superfamily are involved in the adhesion and sorting of cells during tissue morphogenesis. In the present review, we focus on protocadherins, which form the largest subfamily of the cadherin superfamily and mediate homophilic cell-cell adhesion in the developing brain. These molecules are highly expressed during neural development and the exact roles that they play are still emerging. Although, historically, protocadherins were considered to provide mechanical and chemical connections between adjacent cells, recent research suggests that they may also serve as molecular identity markers of neurones to help guide cell recognition and sorting, cell migration, outgrowth of neuronal processes, and synapse formation. This phenomenon of single cell diversity stems, in part, from the vast variation in protein structure, genomic organisation and molecular function of the protocadherins. Although expression profiles and genetic manipulations have provided evidence for the role of protocadherins in the developing brain, we have only begun to construct a complete understanding of protocadherin function. We examine our current understanding of how protocadherins influence brain development and discuss the possible roles for this large superfamily within the hypothalamus. We conclude that further research into these underappreciated but vitally important genes will shed insight into hypothalamic development and perhaps the underlying aetiology of neuroendocrine disorders.

Control of axon guidance and neurotransmitter phenotype of dB1 hindbrain interneurons by Lim-HD code

Hindbrain dorsal interneurons (HDIs) are implicated in receiving, processing, integrating, and transmitting sensory inputs from the periphery and spinal cord, including the vestibular, auditory, and proprioceptive systems. During development, multiple molecularly defined HDI types are set in columns along the dorsoventral axis, before migrating along well-defined trajectories to generate various brainstem nuclei. Major brainstem functions rely on the precise assembly of different interneuron groups and higher brain domains into common circuitries. Yet, knowledge regarding interneuron axonal patterns, synaptic targets, and the transcriptional control that govern their connectivity is sparse. The dB1 class of HDIs is formed in a district dorsomedial position along the hindbrain and gives rise to the inferior olive nuclei, dorsal cochlear nuclei, and vestibular nuclei. dB1 interneurons express various transcription factors (TFs): the pancreatic transcription factor 1a (Ptf1a), the homeobox TF-Lbx1 and the Lim-homeodomain (Lim-HD), and TF Lhx1 and Lhx5. To decipher the axonal and synaptic connectivity of dB1 cells, we have used advanced enhancer tools combined with conditional expression systems and the PiggyBac-mediated DNA transposition system in avian embryos. Multiple ipsilateral and contralateral axonal projections were identified ascending toward higher brain centers, where they formed synapses in the Purkinje cerebellar layer as well as at discrete midbrain auditory and vestibular centers. Decoding the mechanisms that instruct dB1 circuit formation revealed a fundamental role for Lim-HD proteins in regulating their axonal patterns, synaptic targets, and neurotransmitter choice. Together, this study provides new insights into the assembly and heterogeneity of HDIs connectivity and its establishment through the central action of Lim-HD governed programs.

Central topography of cranial motor nuclei controlled by differential cadherin expression

Neuronal nuclei are prominent, evolutionarily conserved features of vertebrate central nervous system (CNS) organization [1]. Nuclei are clusters of soma of functionally related neurons and are located in highly stereotyped positions. Establishment of this CNS topography is critical to neural circuit assembly. However, little is known of either the cellular or molecular mechanisms that drive nucleus formation during development, a process termed nucleogenesis [2, 3, 4, 5]. Brainstem motor neurons, which contribute axons to distinct cranial nerves and whose functions are essential to vertebrate survival, are organized exclusively as nuclei. Cranial motor nuclei are composed of two main classes, termed branchiomotor/visceromotor and somatomotor [6]. Each of these classes innervates evolutionarily distinct structures, for example, the branchial arches and eyes, respectively. Additionally, each class is generated by distinct progenitor cell populations and is defined by differential transcription factor expression [7, 8]; for example, Hb9 distinguishes somatomotor from branchiomotor neurons. We characterized the time course of cranial motornucleogenesis, finding that despite differences in cellular origin, segregation of branchiomotor and somatomotor nuclei occurs actively, passing through a phase of each being intermingled. We also found that differential expression of cadherin cell adhesion family members uniquely defines each motor nucleus. We show that cadherin expression is critical to nucleogenesis as its perturbation degrades nucleus topography predictably.

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Cranial motor nucleogenesis occurs through an active process of segregation

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Differential cadherin expression defines cranial motor nuclei

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Cadherin expression drives specificity of cranial motor nucleus segregation

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Cadherin expression does not affect cranial motor neuron migration

Cadherin expression delineates the divisions of the postnatal and adult mouse amygdala

The amygdaloid complex represents a group of telencephalic nuclei and cortical areas that control emotional and social behavior. Amygdalar development is poorly understood. It is generally accepted that the structures of the amygdala originate from the neuroepithelium at both sides of the pallial-subpallial boundary. In the present study, we mapped the expression of 13 members of the cadherin superfamily of cell adhesion molecules, which provide an adhesive code for the development and maintenance of functional structures in the central nervous system (CNS). Five classic cadherins (Cdh4, Cdh6, Cdh7, Cdh8, Cdh11) and eight delta-protocadherins (Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, Pcdh11, PCdh17, PCdh19) were studied by in situ hybridization in the postnatal (P5) and adult mouse amygdala. In the different parts of the amygdala, each of these (proto-) cadherins shows a distinct and spatially restricted expression pattern that is highly similar at postnatal and adult stages. The combinatorial expression of (proto-) cadherins allows the distinction of multiple molecular subdivisions within the amygdala that partially coincide with previously described morphological divisions. Beyond these expected results, a number of novel molecular subdivisions and subpopulations of cells were identified; for example, additional molecular subdomains, patches, or cell aggregates with distinct (proto-) cadherin expression in several nuclei/areas of the amygdala. We also show that several cadherins are molecular markers for particular functional subsystems within the amygdala, such as in the olfactory projections. In summary, (proto-) cadherins provide a code of potentially adhesive cues that can aid the understanding of functional organization in the amygdala.

Expression of delta-protocadherins in the spinal cord of the chicken embryo

Protocadherins constitute the largest subfamily of cadherin genes and are widely expressed in the nervous system. In the present study, we cloned eight members of the delta-protocadherin subfamily of cadherins (Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, Pcdh17, Pcdh18, and Pcdh19) from the chicken, and investigated their expression in the developing chicken spinal cord by in situ hybridization. Our results showed that each of the investigated delta-protocadherins exhibits a spatially restricted and temporally regulated pattern of expression. Pcdh1, Pcdh8, Pcdh18, and Pcdh19 are expressed in restricted dorsoventral domains of the neuroepithelial layer at early developmental stages (E2.5–E4). In the differentiating mantle layer, specific expression profiles are observed for all eight delta-protocadherins along the dorsoventral, mediolateral, and rostrocaudal dimensions at intermediate stages of development (E6–E10). Expression profiles are especially diverse in the motor column, where different pools of motor neurons exhibit signal for subsets of delta-protocadherins. In the dorsal root ganglion, subpopulations of cells express combinations of Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, and Pcdh17. The ventral boundary cap cells are positive for Pcdh7, Pcdh9, and Pcdh10. Signals for Pcdh8, Pcdh18, and Pcdh19 are found in the meninges. Surrounding tissues, such as the notochord, dermomyotome, and sclerotome also exhibit differential expression patterns. The highly regulated spatiotemporal expression patterns of delta-protocadherins suggest that they have multiple and diverse functions during development of the spinal cord and its surrounding tissues.

Pattern formation during development of the embryonic cerebellum

The patterning of the embryonic cerebellum is vital to establish the elaborate zone and stripe architecture of the adult. This review considers early stages in cerebellar Purkinje cell patterning, from the organization of the ventricular zone to the development of Purkinje cell clusters—the precursors of the adult stripes.

Cerebellar zones: history, development, and function

The longitudinal and transverse zonal arrangement of axonal projections to and from the cerebellum, even more than the well-known laminar cytoarchitecture, is the hallmark of cerebellar anatomy. No model of cerebellar function, whether in motor control, cognition, or emotion, will be complete without understanding the development and function of zones. To this end, a special issue of this journal is dedicated to zones, and the purpose of this article is to summarize the research and review articles that are contained within. The special issue begins by considering some of the very first studies in the 1960s and 1970s that led to our modern understanding of this unique and defining anatomical substructure. Then, it considers the molecular analogs of longitudinal zones in the form of stripes in the cerebellar cortex and related sub-areas in the deep cerebellar nuclei, and it includes studies on the genetic underpinnings of stripes and zones. Several articles address the evolution of both embryonic clusters and adult zones across vertebrate species, and others discuss the functional and clinical relevance of zones. While we do not yet fully understand the role of zones with respect to motor behavior in all of its complexities, cerebellar function is clearly modular, and combinatorial models of complex motor movements based on multi-purpose modules are beginning to emerge. This special issue, by refocusing attention on this fundamental organization of the cerebellum, sets the stage for future studies that will more fully reveal the cellular, developmental, behavioral, and clinical relevance of zones.

Differential expression, alternative splicing, and adhesive properties of the zebrafish δ1-protocadherins

Protocadherins comprise the largest family within the cadherin superfamily of cell surface receptors. Here, we characterize the δ1-protocadherin subfamily during the development of the zebrafish nervous system. In zebrafish, there are five δ1-protocadherins: pcdh1a, pcdh1b, pcdh7a, pcdh7b, andpcdh9. Each protocadherin gene is highly homologous to its human ortholog. While the expression pattern in the developing CNS is similar for each δ1-protocadherin, with labeling observed in all major subdivisions, the detailed patterns are distinct. In addition, we provide evidence for alternative splicing of the pcdh7b and pcdh9 genes, resulting in variation in their respective cytoplasmic domains. As protocadherins are widely regarded to act as cell adhesion molecules, we used in vitro assays of δ1-pcdh ectodomains to directly test their adhesive properties. We found no evidence for calcium-dependent, homophilic adhesion, contrasting sharply with the behavior of classical cadherins.

Antigenic compartmentation of the cerebellar cortex in the chicken (Gallus domesticus)

The chick is a well-understood developmental model of cerebellar pattern formation,but we know much less about the patterning of the adult chicken cerebellum. Therefore an expression study of two Purkinje cell stripe antigens-zebrin II/aldolase C and phospholipase Cbeta4 (PLCbeta4)-has been carried out in the adult chicken (Gallus domesticus). The mammalian cerebellar cortex is built around transverse expression domains ("transverse zones"), each of which is further subdivided into parasagittally oriented stripes. The results from the adult chicken reveal a similar pattern. Five distinct transverse domains were identified. In the anterior lobe a uniformly zebrin II-immunopositive/PLCbeta4-immunonegative lingular zone (LZ; lobule I) and a striped anterior zone (AZ; lobules II-VIa) were distinguished. A central zone (CZ; approximately lobules VIa-VIIIa,b) and a posterior zone (PZ; approximately lobules VIIIa,b-IXc,d) were distinguished in the posterior lobe. Finally, the nodular zone (NZ; lobule X) is uniformly zebrin II-immunoreactive and is innervated by vestibular mossy fibers. Lobule IXc,d is considered as a transitional region between the PZ and the NZ, because the vestibular mossy fiber projection extends into these lobules and because they receive optokinetic mossy and climbing fiber input. It is proposed that the zebrin II-immunonegative P3- stripe corresponds to the lateral vermal B zone of the mammalian cerebellum and that the border between the avian homologs of the mammalian vermis and hemispheres is located immediately lateral to P3-. Thus, there seem to be transverse zones in chicken that are plausible homologs of those identified in mammals, together with an LZ that is characteristic of birds.

Organization of the marmoset cerebellum in three-dimensional space: lobulation, aldolase C compartmentalization and axonal projection

The cerebellar cortex is organized by transverse foliation and longitudinal compartmentalization. Although the latter, which is recognized through the molecular expression in subsets of Purkinje cells (PCs), is closely related to topographic axonal projection and represents functional divisions, the details have not been fully clarified in mammals other than rodents. Therefore, we examined folial and compartmental organization of the marmoset cerebellum, which resembles the macaque cerebellum, and compared it with that of the rodent cerebellum by aldolase C immunostaining, three-dimensional reconstruction of the PC layer, and labeling of olivocerebellar and corticonuclear projections. Longitudinal stripes of different aldolase C expression intensities separated the entire cerebellar cortex into multiple compartments. Lobule VIIAb-d was equivalent to rodent lobule VIc in that it contained a transverse gap in the cortical layers and served as the rostrocaudal boundary for compartments and axonal branching. Olivocortical and corticonuclear projection patterns in major compartments indicated that the compartmental organization in the marmoset cerebellum was generally equivalent to that in the rodent cerebellum, although two compartments were missing in the pars intermedia and several compartments that have not been seen in rodents were recognized in the flocculus, nodulus, and the most lateral hemisphere. Reconstruction showed that the paraflocculus and flocculus were formed by a single longitudinal sheet, the axis of which was parallel to the aldolase C compartments, PC dendrites, and olivocerebellar climbing fiber distribution. The results indicate that molecular compartmentalization in the marmoset cerebellum reflected both the common fundamental organization of the mammalian cerebellum and species-dependent differentiation.

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.