Protocadherin-19 and N-cadherin interact to control cell movements during anterior neurulation

An analysis of contractile and protrusive cell behaviors at the superficial surface of the zebrafish neural plate

BACKGROUND

The forces underlying convergence and internalization of the teleost neural plate remain unknown. To help understand this morphogenesis, we analyzed collective and individual cell behaviors at the superficial surface of the neural plate as internalization begins to form the neural keel in the hindbrain region of the zebrafish embryo.

RESULTS

Convergence to the midline is not accompanied by anteroposterior elongation at this stage, and it is characterized by oscillatory contractile behaviors at the superficial surface of the neural plate, a punctate distribution of Cdh2 and medially polarized actin-rich protrusions at the surface of the neural plate. We also characterize the intimate relationship and dynamic protrusive cell behaviors between the surfaces of the motile neural plate and the stationary overlying non-neural enveloping layer.

CONCLUSIONS

Superficial neural plate cells are coupled by a punctate distribution of Cdh2-rich adhesions. At this surface, cells tug on neighbors using oscillatory contractions. Oscillatory contractions accompany convergence and shrinkage of the cells' superficial surface for internalization during keeling. Some shrinkage for internalization occurs without oscillations. The deep surface of the overlying non-neural enveloping layer is in contact with the superficial surface of the neural plate, suggesting that it may constrain the neural plate movements of convergence and internalization.

Altered cytoskeleton dynamics in patient-derived iPSC-based model of PCDH19 clustering epilepsy

Protocadherin 19 (PCDH19) is an adhesion molecule involved in cell-cell interaction whose mutations cause a drug-resistant form of epilepsy, named PCDH19-Clustering Epilepsy (PCDH19-CE, MIM 300088). The mechanism by which altered PCDH19 function drive pathogenesis is not yet fully understood. Our previous work showed that PCDH19 dysfunction is associated with altered orientation of the mitotic spindle and accelerated neurogenesis, suggesting a contribution of altered cytoskeleton organization in PCDH19-CE pathogenesis in the control of cell division and differentiation. Here, we evaluate the consequences of altered PCDH19 function on microfilaments and microtubules organization, using a disease model obtained from patient-derived induced pluripotent stem cells. We show that iPSC-derived cortical neurons are characterized by altered cytoskeletal dynamics, suggesting that this protocadherin has a role in modulating stability of MFs and MTs. Consistently, the levels of acetylated-tubulin, which is related with stable MTs, are significantly increased in cortical neurons derived from the patient's iPSCs compared to control cells, supporting the idea that the altered dynamics of the MTs depends on their increased stability. Finally, performing live-imaging experiments using fluorescence recovery after photobleaching and by monitoring GFP-tagged end binding protein 3 (EB3) "comets," we observe an impairment of the plus-end polymerization speed in PCDH19-mutated cortical neurons, therefore confirming the impaired MT dynamics. In addition to altering the mitotic spindle formation, the present data unveil that PCDH19 dysfunction leads to altered cytoskeletal rearrangement, providing therapeutic targets and pharmacological options to treat this disorder.

Protocadherin 19 regulates axon guidance in the developing Xenopus retinotectal pathway

Protocadherin 19 (Pcdh19) is a homophilic cell adhesion molecule and is involved in a variety of neuronal functions. Here, we tested whether Pcdh19 has a regulatory role in axon guidance using the developing Xenopus retinotectal system. We performed targeted microinjections of a translation blocking antisense morpholino oligonucleotide to knock down the expression of Pcdh19 selectively in the central nervous system. Knocking down Pcdh19 expression resulted in navigational errors of retinal ganglion cell (RGC) axons specifically at the optic chiasm. Instead of projecting to the contralateral optic tectum, RGC axons in the Pcdh19-depleted embryo misprojected ipsilaterally. Although incorrectly delivered into the ipsilateral brain hemisphere, these axons correctly reached the optic tectum. These data suggest that Pcdh19 has a critical role in preventing mixing of RGC axons originating from the opposite eyes at the optic chiasm, highlighting the importance of cell adhesion in bundling of RGC axons.

Neuronal network activity and connectivity are impaired in a conditional knockout mouse model with PCDH19 mosaic expression

Mutations in PCDH19 gene, which encodes protocadherin-19 (PCDH19), cause Developmental and Epileptic Encephalopathy 9 (DEE9). Heterogeneous loss of PCDH19 expression in neurons is considered a key determinant of the disorder; however, how PCDH19 mosaic expression affects neuronal network activity and circuits is largely unclear. Here, we show that the hippocampus of Pcdh19 mosaic mice is characterized by structural and functional synaptic defects and by the presence of PCDH19-negative hyperexcitable neurons. Furthermore, global reduction of network firing rate and increased neuronal synchronization have been observed in different limbic system areas. Finally, network activity analysis in freely behaving mice revealed a decrease in excitatory/inhibitory ratio and functional hyperconnectivity within the limbic system of Pcdh19 mosaic mice. Altogether, these results indicate that altered PCDH19 expression profoundly affects circuit wiring and functioning, and provide new key to interpret DEE9 pathogenesis.

PCDH19-clustering epilepsy, pathophysiology and clinical significance

PCDH19 clustering epilepsy (PCDH19-CE) is an X-linked epilepsy disorder associated with intellectual disability (ID) and behavioral disturbances, which is caused by PCDH19 gene variants. PCDH19 pathogenic variant leads to epilepsy in heterozygous females, not in hemizygous males and the inheritance pattern is unusual. The hypothesis of cellular interference was described as a key pathogenic mechanism. According to that, males do not develop the disease because of the uniform expression of PCDH19 (variant or wild type) unless they have a somatic variation. We conducted a literature review on PCDH19-CE pathophysiology and concluded that other significant mechanisms could contribute to pathogenesis including: asymmetric cell division and heterochrony, female-related allopregnanolone deficiency, altered steroid gene expression, decreased Gamma-aminobutyric acid receptor A (GABAA) function, and blood-brain barrier (BBB) dysfunction. Being aware of these mechanisms helps us when we should decide which therapeutic option is more suitable for which patient.

Abnormal cell sorting and altered early neurogenesis in a human cortical organoid model of Protocadherin-19 clustering epilepsy

Introduction

Protocadherin-19 (PCDH19)-Clustering Epilepsy (PCE) is a developmental and epileptic encephalopathy caused by loss-of-function variants of the PCDH19 gene on the X-chromosome. PCE affects females and mosaic males while male carriers are largely spared. Mosaic expression of the cell adhesion molecule PCDH19 due to random X-chromosome inactivation is thought to impair cell-cell interactions between mutant and wild type PCDH19-expressing cells to produce the disease. Progress has been made in understanding PCE using rodent models or patient induced pluripotent stem cells (iPSCs). However, rodents do not faithfully model key aspects of human brain development, and patient iPSC models are limited by issues with random X-chromosome inactivation.

Methods

To overcome these challenges and model mosaic PCDH19 expression in vitro, we generated isogenic female human embryonic stem cells with either HA-FLAG-tagged PCDH19 (WT) or homozygous PCDH19 knockout (KO) using genome editing. We then mixed GFP-labeled WT and RFP-labeled KO cells and generated human cortical organoids (hCOs).

Results

We found that PCDH19 is highly expressed in early (days 20-35) WT neural rosettes where it co-localizes with N-Cadherin in ventricular zone (VZ)-like regions. Mosaic PCE hCOs displayed abnormal cell sorting in the VZ with KO and WT cells completely segregated. This segregation remained robust when WT:KO cells were mixed at 2:1 or 1:2 ratios. PCE hCOs also exhibited altered expression of PCDH19 (in WT cells) and N-Cadherin, and abnormal deep layer neurogenesis. None of these abnormalities were observed in hCOs generated by mixing only WT or only KO (modeling male carrier) cells.

Discussion

Our results using the mosaic PCE hCO model suggest that PCDH19 plays a critical role in human VZ radial glial organization and early cortical development. This model should offer a key platform for exploring mechanisms underlying PCE-related cortical hyperexcitability and testing of potential precision therapies.

CELSR1, a core planar cell polarity protein, features a weakly adhesive and flexible cadherin ectodomain

Planar cell polarity (PCP), essential to multicellular developmental processes, arises when cells polarize and align across tissues. Central to PCP is CELSR1, an atypical cadherin featuring a long ectodomain with nine extracellular cadherin (EC) repeats, a membrane adjacent domain (MAD10), and several characteristic adhesion GPCR domains. Cell-based aggregation assays have demonstrated CELSR1's homophilic adhesive nature, but mechanistic details are missing. Here, we investigate the possible adhesive properties and structures of CELSR1 EC repeats. Our bead aggregation assays do not support strong adhesion by EC repeats alone. Consistently, EC1-4 only dimerizes at high concentration in solution. Crystal structures of human CELSR1 EC1-4 and EC4-7 reveal typical folds and a non-canonical linker between EC5 and EC6. Simulations and experiments using EC4-7 indicate flexibility at EC5-6, and solution experiments show EC7-MAD10-mediated dimerization. Our results suggest weak homophilic adhesion by CELSR1 cadherin repeats and provide mechanistic insights into the structural determinants of CELSR1 function.

Mapping combinatorial expression of non-clustered protocadherins in the developing brain identifies novel PCDH19-mediated cell adhesion properties

Non-clustered protocadherins (ncPcdhs) are adhesive molecules with spatio-temporally regulated overlapping expression in the developing nervous system. Although their unique role in neurogenesis has been widely studied, their combinatorial role in brain physiology and pathology is poorly understood. Using probabilistic cell typing by in situ sequencing, we demonstrate combinatorial inter- and intra-familial expression of ncPcdhs in the developing mouse cortex and hippocampus, at single-cell resolution. We discovered the combinatorial expression of Protocadherin-19 (Pcdh19), a protein involved in PCDH19-clustering epilepsy, with Pcdh1, Pcdh9 or Cadherin 13 (Cdh13) in excitatory neurons. Using aggregation assays, we demonstrate a code-specific adhesion function of PCDH19; mosaic PCDH19 absence in PCDH19+9 and PCDH19 + CDH13, but not in PCDH19+1 codes, alters cell-cell interaction. Interestingly, we found that PCDH19 as a dominant protein in two heterophilic adhesion codes could promote trans-interaction between them. In addition, we discovered increased CDH13-mediated cell adhesion in the presence of PCDH19, suggesting a potential role of PCDH19 as an adhesion mediator of CDH13. Finally, we demonstrated novel cis-interactions between PCDH19 and PCDH1, PCDH9 and CDH13. These observations suggest that there is a unique combinatorial code with a cell- and region-specific characteristic where a single molecule defines the heterophilic cell-cell adhesion properties of each code.

Prediction of acute fish toxicity (AFT) and fish embryo toxicity (FET) tests by cytotoxicity assays using liver and embryo zebrafish cell lines (ZFL and ZEM2S)

Fish cell-based assays represent potential alternative methods to vertebrates' use in ecotoxicology. In this study, we evaluated the cytotoxicity of thirteen chemicals, chosen from OECD guidelines 236 and 249, in two zebrafish cell lines (ZEM2S and ZFL). We aimed to investigate whether the IC50 values obtained by viability assays (alamar blue, MTT, CFDA-AM, and neutral red) can predict the LC50 values of Acute Fish Toxicity (AFT) test and Fish Embryo Toxicity (FET) test. There was no significant difference between the values obtained by the different viability assays. ZFL strongly correlated with AFT and FET tests (R2AFT = 0.73-0.90; R2FET48h = 0.79-0.90; R2FET96h = 0.76-0.87), while ZEM2S correlated better with the FET test (48h) (R2 = 0.70-0.86) and weakly with AFT and FET tests (96h) (R2AFT = 0.68-0.74 and R2FET96h = 0.62-0.64). The predicted LC50 values allowed the correct categorization of the chemicals in 76.9% (AFT test) - 90.9% (FET test) using ZFL and in 30.7% (AFT test) - 63.6% (FET test) using ZEM2S considering the US EPA criterion for classifying acute aquatic toxicity. ZFL is a promising cell line to be used in alternative methods to adult fish and fish embryos in ecotoxicity assessments, and the method performed in 96-well plates is advantageous in promoting high-throughput cytotoxicity assessment.

Stepwise modulation of apical orientational cell adhesions for vertebrate neurulation

Neurulation transforms the neuroectoderm into the neural tube. This transformation relies on reorganising the configurational relationships between the orientations of intrinsic polarities of neighbouring cells. These orientational intercellular relationships are established, maintained, and modulated by orientational cell adhesions (OCAs). Here, using zebrafish (Danio rerio) neurulation as a major model, we propose a new perspective on how OCAs contribute to the parallel, antiparallel, and opposing intercellular relationships that underlie the neural plate-keel-rod-tube transformation, a stepwise process of cell aggregation followed by cord hollowing. We also discuss how OCAs in neurulation may be regulated by various adhesion molecules, including cadherins, Eph/Ephrins, Claudins, Occludins, Crumbs, Na+ /K+ -ATPase, and integrins. By comparing neurulation among species, we reveal that antiparallel OCAs represent a conserved mechanism for the fusion of the neural tube. Throughout, we highlight some outstanding questions regarding OCAs in neurulation. Answers to these questions will help us understand better the mechanisms of tubulogenesis of many tissues.

PCDH19 in Males: Are Hemizygous Variants Linked to Autism?

Background: Autism spectrum disorder (ASD) is a complex developmental disability that impairs the social communication and interaction of affected individuals and leads to restricted or repetitive behaviors or interests. ASD is genetically heterogeneous, with inheritable and de novo genetic variants in more than hundreds of genes contributing to the disease. However, these account for only around 20% of cases, while the molecular basis of the majority of cases remains unelucidated as of yet. Material and methods: Two unrelated Lebanese patients, a 7-year-old boy (patient A) and a 4-year-old boy (patient B), presenting with ASD were included in this study. Whole-exome sequencing (WES) was carried out for these patients to identify the molecular cause of their diseases. Results: WES analysis revealed hemizygous variants in PCDH19 (NM_001184880.1) as being the candidate causative variants: p.Arg787Leu was detected in patient A and p.Asp1024Asn in patient B. PCDH19, located on chromosome X, encodes a membrane glycoprotein belonging to the protocadherin family. Heterozygous PCDH19 variants have been linked to epilepsy in females with mental retardation (EFMR), while mosaic PCDH19 mutations in males are responsible for treatment-resistant epilepsy presenting similarly to EFMR, with some reported cases of comorbid intellectual disability and autism. Interestingly, a hemizygous PCDH19 variant affecting the same amino acid that is altered in patient A was previously reported in a male patient with ASD. Conclusion: Here, we report hemizygous PCDH19 variants in two males with autism without epilepsy. Reporting further PCDH19 variants in male patients with ASD is important to assess the possible involvement of this gene in autism.

Modifying PCDH19 levels affects cortical interneuron migration

PCDH19 is a transmembrane protein and member of the protocadherin family. It is encoded by the X-chromosome and more than 200 mutations have been linked to the neurodevelopmental PCDH-clustering epilepsy (PCDH19-CE) syndrome. A disturbed cell-cell contact that arises when random X-inactivation creates mosaic absence of PCDH19 has been proposed to cause the syndrome. Several studies have shown roles for PCDH19 in neuronal proliferation, migration, and synapse function, yet most of them have focused on cortical and hippocampal neurons. As epilepsy can also be caused by impaired interneuron migration, we studied the role of PCDH19 in cortical interneurons during embryogenesis. We show that cortical interneuron migration is affected by altering PCDH19 dosage by means of overexpression in brain slices and medial ganglionic eminence (MGE) explants. We also detect subtle defects when PCDH19 expression was reduced in MGE explants, suggesting that the dosage of PCDH19 is important for proper interneuron migration. We confirm this finding in vivo by showing a mild reduction in interneuron migration in heterozygote, but not in homozygote PCDH19 knockout animals. In addition, we provide evidence that subdomains of PCDH19 have a different impact on cell survival and interneuron migration. Intriguingly, we also observed domain-dependent differences in migration of the non-targeted cell population in explants, demonstrating a non-cell-autonomous effect of PCDH19 dosage changes. Overall, our findings suggest new roles for the extracellular and cytoplasmic domains of PCDH19 and support that cortical interneuron migration is dependent on balanced PCDH19 dosage.

Understanding Protein Protocadherin-19 (PCDH19) Syndrome: A Literature Review of the Pathophysiology

PCDH19 syndrome is a monogenic epilepsy related to the protein protocadherin-19 (PCDH19) gene, which encodes for a protein important for brain development. The protein also seems to regulate gamma-aminobutyric acid type A receptors (GABA(A)(R)). The disease presents with refractory epilepsy that is characterized by seizures occurring in clusters. Till now, the pathophysiology of the disease is mainly unknown, so we conducted a literature review to elucidate the pathophysiology of PCDH19-related epilepsy. We used two databases to investigate this literature review (Google Scholar and PubMed). We selected full-text papers that are published in the English language and published after the year 2000. We selected initially 64 papers and ended up with 29 to conduct this literature review. We found four main theories for the pathophysiology of PCDH19-related epilepsy: GABA(A)(R) dysregulation, blood-brain barrier (BBB) dysfunction, cellular interference, and the AKR1C1-3 gene product deficiency.

GABA(A)(R) dysfunction and expression cause decreased effective inhibitory currents predisposing patients to epilepsy. BBB dysfunction allows the passage of methyl-D-aspartate (NMDA)-type glutamate receptor antibodies (abs-NR) through the BBB susceptible membrane. The cellular interference hypothesis establishes that the mutant and non-mutant cells interfere with each other’s communication within the same tissue. Women are more susceptible to being affected by this hypothesis as men only have one copy of the x gene and interference is mediated by this gene, meaning that it cannot occur in them. Finally, downregulation and deficiency of the AKR1C3/AKR1C2 products lead to decreasing levels of allopregnanolone, which diminish the regulation of GABA(A)(R).

A rat model of a focal mosaic expression of PCDH19 replicates human brain developmental abnormalities and behaviors

Protocadherin 19 gene-related epilepsy or protocadherin 19 clustering epilepsy is an infantile-onset epilepsy syndrome characterized by psychiatric (including autism-related), sensory, and cognitive impairment of varying degrees. Protocadherin 19 clustering epilepsy is caused by X-linked protocadherin 19 protein loss of function. Due to random X-chromosome inactivation, protocadherin 19 clustering epilepsy-affected females present a mosaic population of healthy and protocadherin 19-mutant cells. Unfortunately, to date, no current mouse model can fully recapitulate both the brain histological and behavioural deficits present in people with protocadherin 19 clustering epilepsy. Thus, the search for a proper understanding of the disease and possible future treatment is hampered. By inducing a focal mosaicism of protocadherin 19 expression using in utero electroporation in rats, we found here that protocadherin 19 signalling in specific brain areas is implicated in neuronal migration, heat-induced epileptic seizures, core/comorbid behaviours related to autism and cognitive function.

Modeling PCDH19-CE: From 2D Stem Cell Model to 3D Brain Organoids

PCDH19 clustering epilepsy (PCDH19-CE) is a genetic disease characterized by a heterogeneous phenotypic spectrum ranging from focal epilepsy with rare seizures and normal cognitive development to severe drug-resistant epilepsy associated with intellectual disability and autism. Unfortunately, little is known about the pathogenic mechanism underlying this disease and an effective treatment is lacking. Studies with zebrafish and murine models have provided insights on the function of PCDH19 during brain development and how its altered function causes the disease, but these models fail to reproduce the human phenotype. Induced pluripotent stem cell (iPSC) technology has provided a complementary experimental approach for investigating the pathogenic mechanisms implicated in PCDH19-CE during neurogenesis and studying the pathology in a more physiological three-dimensional (3D) environment through the development of brain organoids. We report on recent progress in the development of human brain organoids with a particular focus on how this 3D model may shed light on the pathomechanisms implicated in PCDH19-CE.

Elastic versus brittle mechanical responses predicted for dimeric cadherin complexes

Cadherins are a superfamily of adhesion proteins involved in a variety of biological processes that include the formation of intercellular contacts, the maintenance of tissue integrity, and the development of neuronal circuits. These transmembrane proteins are characterized by ectodomains composed of a variable number of extracellular cadherin (EC) repeats that are similar but not identical in sequence and fold. E-cadherin, along with desmoglein and desmocollin proteins, are three classical-type cadherins that have slightly curved ectodomains and engage in homophilic and heterophilic interactions through an exchange of conserved tryptophan residues in their N-terminal EC1 repeat. In contrast, clustered protocadherins are straighter than classical cadherins and interact through an antiparallel homophilic binding interface that involves overlapped EC1 to EC4 repeats. Here we present molecular dynamics simulations that model the adhesive domains of these cadherins using available crystal structures, with systems encompassing up to 2.8 million atoms. Simulations of complete classical cadherin ectodomain dimers predict a two-phased elastic response to force in which these complexes first softly unbend and then stiffen to unbind without unfolding. Simulated α, β, and γ clustered protocadherin homodimers lack a two-phased elastic response, are brittle and stiffer than classical cadherins and exhibit complex unbinding pathways that in some cases involve transient intermediates. We propose that these distinct mechanical responses are important for function, with classical cadherin ectodomains acting as molecular shock absorbers and with stiffer clustered protocadherin ectodomains facilitating overlap that favors binding specificity over mechanical resilience. Overall, our simulations provide insights into the molecular mechanics of single cadherin dimers relevant in the formation of cellular junctions essential for tissue function.

The Broad Clinical Spectrum of Epilepsies Associated With Protocadherin 19 Gene Mutation

Protocadherin 19 (PCDH19) gene is one of the most common genes involved in epilepsy syndromes. According to literature data PCDH19 is among the 6 genes most involved in genetic epilepsies. PCDH19 is located on chromosome Xq22.1 and is involved in neuronal connections and signal transduction. The most frequent clinical expression of PCDH19 mutation is epilepsy and mental retardation limited to female (EFMR) characterized by epileptic and non-epileptic symptoms affecting mainly females. However, the phenotypic spectrum of these mutations is considerably variable from genetic epilepsy with febrile seizure plus to epileptic encephalopathies. The peculiar exclusive involvement of females seems to be caused by a cellular interference in heterozygosity, however, affected mosaic-males have been reported. Seizure types range from focal seizure to generalized tonic-clonic, tonic, atonic, absences, and myoclonic jerks. Treatment of PCDH19-related epilepsy is limited by drug resistance and by the absence of specific treatment indications. However, seizures become less severe with adolescence and some patients may even become seizure-free. Non-epileptic symptoms represent the main disabilities of adult patients with PCDH19 mutation. This review aims to analyze the highly variable phenotypic expression of PCDH19 gene mutation associated with epilepsy.

Development of 3D cultures of zebrafish liver and embryo cell lines: a comparison of different spheroid formation methods

Fish cell spheroids are promising 3D culture models for vertebrate replacement in ecotoxicology. However, new alternative ecotoxicological methods must be adapted for applications in industry and for regulatory purposes; such methods must be cost-effective, simple to manipulate and provide rapid results. Therefore, we compared the effectiveness of the traditional hanging drop (HD), orbital shaking (OS), and HD combined with OS (HD+OS) methods on the formation of zebrafish cell line spheroids (ZFL and ZEM2S). Time in HD (3-5 days) and different 96-well plates [flat-bottom or ultra-low attachment of round-bottom (ULA-plates)] in OS were evaluated. Easy handling, rapid spheroid formation, uniform-sized spheroids, and circularity were assessed to identify the best spheroid protocol. Traditional HD alone did not result in ZFL spheroid formation, whereas HD (5 days)+OS did. When using the OS, spheroids only formed on the ULA-plate. Both HD+OS and OS were reproducible in size (177.50 ± 2.81 µm and 225.62 ± 19.20 µm, respectively) and circularity (0.83 ± 0.02 and 0.80 ± 0.01, respectively) of ZFL spheroids. Nevertheless, HD+OS required a considerable time to completely form spheroids (10 days) and intensive handling, whereas the OS was fast (5 days of incubation) and simple. OS also yielded reproducible ZEM2S spheroids in 1 day (226.23 ± 0.57 µm diameter and 0.80 ± 0.01 circularity). In conclusion, OS in ULA-plate is an effective and simple spheroid protocol for high-throughput ecotoxicity testing. This study contributes to identify a fast, reproducible, and simple protocol of single piscine spheroid formation in 96-well plates and supports the application of fish 3D model in industry and academia.

A complete Protocadherin-19 ectodomain model for evaluating epilepsy-causing mutations and potential protein interaction sites

Cadherin superfamily members play a critical role in differential adhesion during neurodevelopment, and their disruption has been linked to several neurodevelopmental disorders. Mutations in protocadherin-19 (PCDH19), a member of the δ-protocadherin subfamily of cadherins, cause a unique form of epilepsy called PCDH19 clustering epilepsy. While PCDH19 and other non-clustered δ-protocadherins form multimers with other members of the cadherin superfamily to alter adhesiveness, the specific protein surfaces responsible for these interactions are unknown. Only portions of the PCDH19 extracellular domain structure had been solved previously. Here, we present a structure of the missing segment from zebrafish Protocadherin-19 (Pcdh19) and create a complete ectodomain model. This model shows the structural environment for 97% of disease-causing missense mutations and reveals two potential surfaces for intermolecular interactions that could modify Pcdh19's adhesive strength and specificity.

δ-Protocadherins regulate neural progenitor cell division by antagonizing Ryk and Wnt/β-catenin signaling

The division of neural progenitor cells provides the cellular substrate from which the nervous system is sculpted during development. The δ-protocadherin family of homophilic cell adhesion molecules is essential for the development of the vertebrate nervous system and is implicated in an array of neurodevelopmental disorders. We show that lesions in any of six, individual δ-protocadherins increases cell divisions of neural progenitors in the hindbrain. This increase is due to mis-regulation of Wnt/β-catenin signaling, as this pathway is upregulated in δ-protocadherin mutants and inhibition of this pathway blocks the increase in cell division. Furthermore, the δ-protocadherins can be present in complex with the Wnt receptor Ryk, and Ryk is required for the increased proliferation in protocadherin mutants. Thus, δ-protocadherins are novel regulators of Wnt/β-catenin signaling that may control the development of neural circuits by defining a molecular code for the identity of neural progenitor cells and differentially regulating their proliferation.

Dissecting the Role of PCDH19 in Clustering Epilepsy by Exploiting Patient-Specific Models of Neurogenesis

PCDH19-related epilepsy is a rare genetic disease caused by defective function of PCDH19, a calcium-dependent cell–cell adhesion protein of the cadherin superfamily. This disorder is characterized by a heterogeneous phenotypic spectrum, with partial and generalized febrile convulsions that are gradually increasing in frequency. Developmental regression may occur during disease progression. Patients may present with intellectual disability (ID), behavioral problems, motor and language delay, and a low motor tone. In most cases, seizures are resistant to treatment, but their frequency decreases with age, and some patients may even become seizure-free. ID generally persists after seizure remission, making neurological abnormalities the main clinical issue in affected individuals. An effective treatment is lacking. In vitro studies using patient-derived induced pluripotent stem cells (iPSCs) reported accelerated neural differentiation as a major endophenotype associated with PCDH19 mutations. By using this in vitro model system, we show that accelerated in vitro neurogenesis is associated with a defect in the cell division plane at the neural progenitors stage. We also provide evidence that altered PCDH19 function affects proper mitotic spindle orientation. Our findings identify an altered equilibrium between symmetric versus asymmetric cell division as a previously unrecognized mechanism contributing to the pathogenesis of this rare epileptic encephalopathy.

Female-specific synaptic dysfunction and cognitive impairment in a mouse model of PCDH19 disorder

Protocadherin-19 (PCDH19) mutations cause early-onset seizures and cognitive impairment. The PCDH19 gene is on the X-chromosome. Unlike most X-linked disorders, PCDH19 mutations affect heterozygous females (PCDH19^HET♀ ) but not hemizygous males (PCDH19^HEMI♂ ); however, the reason why remains to be elucidated. We demonstrate that PCDH19, a cell-adhesion molecule, is enriched at hippocampal mossy fiber synapses. Pcdh19^HET♀ but not Pcdh19^HEMI♂ mice show impaired mossy fiber synaptic structure and physiology. Consistently, Pcdh19^HET♀ but not Pcdh19^HEMI♂ mice exhibit reduced pattern completion and separation abilities, which require mossy fiber synaptic function. Furthermore, PCDH19 appears to interact with N-cadherin at mossy fiber synapses. In Pcdh19^HET♀ conditions, mismatch between PCDH19 and N-cadherin diminishes N-cadherin-dependent signaling and impairs mossy fiber synapse development; N-cadherin overexpression rescues Pcdh19^HET♀ phenotypes. These results reveal previously unknown molecular and cellular mechanisms underlying the female-specific PCDH19 disorder phenotype.

Right Place at the Right Time: How Changes in Protocadherins Affect Synaptic Connections Contributing to the Etiology of Neurodevelopmental Disorders

During brain development, neurons need to form the correct connections with one another in order to give rise to a functional neuronal circuitry. Mistakes during this process, leading to the formation of improper neuronal connectivity, can result in a number of brain abnormalities and impairments collectively referred to as neurodevelopmental disorders. Cell adhesion molecules (CAMs), present on the cell surface, take part in the neurodevelopmental process regulating migration and recognition of specific cells to form functional neuronal assemblies. Among CAMs, the members of the protocadherin (PCDH) group stand out because they are involved in cell adhesion, neurite initiation and outgrowth, axon pathfinding and fasciculation, and synapse formation and stabilization. Given the critical role of these macromolecules in the major neurodevelopmental processes, it is not surprising that clinical and basic research in the past two decades has identified several PCDH genes as responsible for a large fraction of neurodevelopmental disorders. In the present article, we review these findings with a focus on the non-clustered PCDH sub-group, discussing the proteins implicated in the main neurodevelopmental disorders.

Proximity-dependent Proteomics Reveals Extensive Interactions of Protocadherin-19 with Regulators of Rho GTPases and the Microtubule Cytoskeleton

Protocadherin-19 belongs to the cadherin family of cell surface receptors and has been shown to play essential roles in the development of the vertebrate nervous system. Mutations in human Protocadherin-19 (PCDH19) lead to PCDH19 Female-limited epilepsy (PCDH19 FLE) in humans, characterized by the early onset of epileptic seizures in children and a range of cognitive and behavioral problems in adults. Despite being considered the second most prevalent gene in epilepsy, very little is known about the intercellular pathways in which it participates. In order to characterize the protein complexes within which Pcdh19 functions, we generated Pcdh19-BioID fusion proteins and utilized proximity-dependent biotinylation to identify neighboring proteins. Proteomic identification and analysis revealed that the Pcdh19 interactome is enriched in proteins that regulate Rho family GTPases, microtubule binding proteins and proteins that regulate cell divisions. We cloned the centrosomal protein Nedd1 and the RacGEF Dock7 and verified their interactions with Pcdh19 in vitro. Our findings provide the first comprehensive insights into the interactome of Pcdh19, and provide a platform for future investigations into the cellular and molecular biology of this protein critical to the proper development of the nervous system.

Protocadherins at the Crossroad of Signaling Pathways

Protocadherins (Pcdhs) are cell adhesion molecules that belong to the cadherin superfamily, and are subdivided into clustered (cPcdhs) and non-clustered Pcdhs (ncPcdhs) in vertebrates. In this review, we summarize their discovery, expression mechanisms, and roles in neuronal development and cancer, thereby highlighting the context-dependent nature of their actions. We furthermore provide an extensive overview of current structural knowledge, and its implications concerning extracellular interactions between cPcdhs, ncPcdhs, and classical cadherins. Next, we survey the known molecular action mechanisms of Pcdhs, emphasizing the regulatory functions of proteolytic processing and domain shedding. In addition, we outline the importance of Pcdh intracellular domains in the regulation of downstream signaling cascades, and we describe putative Pcdh interactions with intracellular molecules including components of the WAVE complex, the Wnt pathway, and apoptotic cascades. Our overview combines molecular interaction data from different contexts, such as neural development and cancer. This comprehensive approach reveals potential common Pcdh signaling hubs, and points out future directions for research. Functional studies of such key factors within the context of neural development might yield innovative insights into the molecular etiology of Pcdh-related neurodevelopmental disorders.

Quantitative MRI-Based Analysis Identifies Developmental Limbic Abnormalities in PCDH19 Encephalopathy

Protocadherin-19 (PCDH19) is a calcium dependent cell-adhesion molecule involved in neuronal circuit formation with prevalent expression in the limbic structures. PCDH19-gene mutations cause a developmental encephalopathy with prominent infantile onset focal seizures, variably associated with intellectual disability and autistic features. Diagnostic neuroimaging is usually unrevealing. We used quantitative MRI to investigate the cortex and white matter in a group of 20 PCDH19-mutated patients. By a statistical comparison between quantitative features in PCDH19 brains and in a group of age and sex matched controls, we found that patients exhibited bilateral reductions of local gyrification index (lGI) in limbic cortical areas, including the parahippocampal and entorhinal cortex and the fusiform and lingual gyri, and altered diffusivity features in the underlying white matter. In patients with an earlier onset of seizures, worse psychiatric manifestations and cognitive impairment, reductions of lGI and diffusivity abnormalities in the limbic areas were more pronounced. Developmental abnormalities involving the limbic structures likely represent a measurable anatomic counterpart of the reduced contribution of the PCDH19 protein to local cortical folding and white matter organization and are functionally reflected in the phenotypic features involving cognitive and communicative skills as well as local epileptogenesis.

Identification of an adhesive interface for the non-clustered δ1 protocadherin-1 involved in respiratory diseases

Cadherins form a large family of calcium-dependent adhesive proteins involved in morphogenesis, cell differentiation, and neuronal connectivity. Non-clustered δ1 protocadherins form a cadherin subgroup of proteins with seven extracellular cadherin (EC) repeats and cytoplasmic domains distinct from those of classical cadherins. Non-clustered δ1 protocadherins mediate homophilic adhesion and have been implicated in various diseases including asthma, autism, and cancer. Here we present X-ray crystal structures of human Protocadherin-1 (PCDH1), a δ1-protocadherin member essential for New World Hantavirus infection that is typically expressed in the brain, airway epithelium, skin keratinocytes, and lungs. The structures suggest a binding mode that involves antiparallel overlap of repeats EC1 to EC4. Mutagenesis combined with binding assays and biochemical experiments validated this mode of adhesion. Overall, these results reveal the molecular mechanism underlying adhesiveness of PCDH1 and δ1-protocadherins, also shedding light on PCDH1's role in maintaining airway epithelial integrity, the loss of which causes respiratory diseases.

Multiplane Calcium Imaging Reveals Disrupted Development of Network Topology in Zebrafish pcdh19 Mutants

Functional brain networks self-assemble during development, although the molecular basis of network assembly is poorly understood. Protocadherin-19 (pcdh19) is a homophilic cell adhesion molecule that is linked to neurodevelopmental disorders, and influences multiple cellular and developmental events in zebrafish. Although loss of PCDH19 in humans and model organisms leads to functional deficits, the underlying network defects remain unknown. Here, we employ multiplane, resonant-scanning in vivo two-photon calcium imaging of developing zebrafish, and use graph theory to characterize the development of resting state functional networks in both wild-type and pcdh19 mutant larvae. We find that the brain networks of pcdh19 mutants display enhanced clustering and an altered developmental trajectory of network assembly. Our results show that functional imaging and network analysis in zebrafish larvae is an effective approach for characterizing the developmental impact of lesions in genes of clinical interest.

The female epilepsy protein PCDH19 is a new GABAAR-binding partner that regulates GABAergic transmission as well as migration and morphological maturation of hippocampal neurons

The PCDH19 gene (Xp22.1) encodes the cell-adhesion protein protocadherin-19 (PCDH19) and is responsible for a neurodevelopmental pathology characterized by female-limited epilepsy, cognitive impairment and autistic features, the pathogenic mechanisms of which remain to be elucidated. Here, we identified a new interaction between PCDH19 and GABAA receptor (GABAAR) alpha subunits in the rat brain. PCDH19 shRNA-mediated downregulation reduces GABAAR surface expression and affects the frequency and kinetics of miniature inhibitory postsynaptic currents (mIPSCs) in cultured hippocampal neurons. In vivo, PCDH19 downregulation impairs migration, orientation and dendritic arborization of CA1 hippocampal neurons and increases rat seizure susceptibility. In sum, these data indicate a role for PCDH19 in GABAergic transmission as well as migration and morphological maturation of neurons.

The Role of Protocadherin 19 (PCDH19) in Neurodevelopment and in the Pathophysiology of Early Infantile Epileptic Encephalopathy-9 (EIEE9)

PCDH19 is considered one of the most clinically relevant genes in epilepsy, second only to SCN1A. To date about 150 mutations have been identified as causative for PCDH19-female epilepsy (also known as early infantile epileptic encephalopathy-9, EIEE9), which is characterized by early onset epilepsy, intellectual disabilities, and behavioral disturbances. Although little is known about the physiological role of PCDH19 and the pathogenic mechanisms that lead to EIEE9, in this review, we will present latest researches focused on these aspects, underlining protein expression, its known functions and the mechanisms by which the protein acts, with particular interest in PCDH19 extracellular and intracellular roles in neurons.

X-linked epileptic syndrome by protocadherin 19 mutation associated with leukoencephalopathy and posterior reversible tractopathy

Epilepsy and mental retardation produced by mutations in gene PCDH19 (protocadherin 19) is an X-linked syndrome restricted to females. It starts with global and speech developmental delay and epilepsy; intellectual disability may continue in adults. At least in 20% of cases, there are no seizures or intellectual retardation. We report the case of a girl with epilepsy, developmental delay, and autistic conversion associated with posterior reversible leukoencephalopathy and tractopathy produced by PCDH19 mutation (c.142G>T/ p.Glu48X).

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.

PCDH19 regulation of neural progenitor cell differentiation suggests asynchrony of neurogenesis as a mechanism contributing to PCDH19 Girls Clustering Epilepsy

PCDH19-Girls Clustering Epilepsy (PCDH19-GCE) is a childhood epileptic encephalopathy characterised by a spectrum of neurodevelopmental problems. PCDH19-GCE is caused by heterozygous loss-of-function mutations in the X-chromosome gene, Protocadherin 19 (PCDH19) encoding a cell-cell adhesion molecule. Intriguingly, hemizygous males are generally unaffected. As PCDH19 is subjected to random X-inactivation, heterozygous females are comprised of a mosaic of cells expressing either the normal or mutant allele, which is thought to drive pathology. Despite being the second most prevalent monogeneic cause of epilepsy, little is known about the role of PCDH19 in brain development. In this study we show that PCDH19 is highly expressed in human neural stem and progenitor cells (NSPCs) and investigate its function in vitro in these cells of both mouse and human origin. Transcriptomic analysis of mouse NSPCs lacking Pcdh19 revealed changes to genes involved in regulation of neuronal differentiation, and we subsequently show that loss of Pcdh19 causes increased NSPC neurogenesis. We reprogramed human fibroblast cells harbouring a pathogenic PCDH19 mutation into human induced pluripotent stem cells (hiPSC) and employed neural differentiation of these to extend our studies into human NSPCs. As in mouse, loss of PCDH19 function caused increased neurogenesis, and furthermore, we show this is associated with a loss of human NSPC polarity. Overall our data suggests a conserved role for PCDH19 in regulating mammalian cortical neurogenesis and has implications for the pathogenesis of PCDH19-GCE. We propose that the difference in timing or "heterochrony" of neuronal cell production originating from PCDH19 wildtype and mutant NSPCs within the same individual may lead to downstream asynchronies and abnormalities in neuronal network formation, which in-part predispose the individual to network dysfunction and epileptic activity.

PCDHGA9 acts as a tumor suppressor to induce tumor cell apoptosis and autophagy and inhibit the EMT process in human gastric cancer

The results of a cDNA  array revealed that protocadherin gamma subfamily A, 9 (PCDHGA9) was significantly decreased in SGC-7901 gastric cancer (GC) cells compared with GES-1 normal gastric cells and was strongly associated with the Wnt/β-catenin and transforming growth factor-β (TGF-β)/Smad2/3 signaling pathway. As a member of the cadherin family, PCDHGA9 functions in both cell-cell adhesion and nuclear signaling. However, its role in tumorigenicity or metastasis has not been reported. In the present study, we found that PCDHGA9 was decreased in GC tissues compared with corresponding normal mucosae and its expression was correlated with the GC TNM stage, the UICC stage, differentiation, relapse, and metastasis (p < 0.01). Multivariate Cox analysis revealed that PCDHGA9 was an independent prognostic indicator for overall survival (OS) and disease-free survival (DFS) (p < 0.01). The effects of PCDHGA9 on GC tumor growth and metastasis were examined both in vivo and in vitro. PCDHGA9 knockdown promoted GC cell proliferation, migration, and invasion, whereas PCDHGA9 overexpression inhibited GC tumor growth and metastasis but induced apoptosis, autophagy, and G1 cell cycle arrest. Furthermore, PCDHGA9 suppressed epithelial-mesenchymal transition (EMT) induced by TGF-β, decreased the phosphorylation of Smad2/3, and inhibited the nuclear translocation of pSmad2/3. Our results suggest that PCDHGA9 might interact with β-catenin to prevent β-catenin from dissociating in the cytoplasm and translocating to the nucleus. Moreover, PCDHGA9 overexpression restrained cell proliferation and reduced the nuclear β-catenin, an indicator of Wnt/β-catenin pathway activation, suggesting that PCDHGA9 negatively regulates Wnt signaling. Together, these data indicate that PCDHGA9 acts as a tumor suppressor with anti-proliferative activity and anti-invasive ability, and the reduction of PCDHGA9 could serve as an independent prognostic biomarker in GC.

Protocadherin 19 (PCDH19) interacts with paraspeckle protein NONO to co-regulate gene expression with estrogen receptor alpha (ERα)

De novo and inherited mutations of X-chromosome cell adhesion molecule protocadherin 19 (PCDH19) cause frequent, highly variable epilepsy, autism, cognitive decline and behavioural problems syndrome. Intriguingly, hemizygous null males are not affected while heterozygous females are, contradicting established X-chromosome inheritance. The disease mechanism is not known. Cellular mosaicism is the likely driver. We have identified p54nrb/NONO, a multifunctional nuclear paraspeckle protein with known roles in nuclear hormone receptor gene regulation, as a PCDH19 protein interacting partner. Using breast cancer cells we show that PCDH19-NONO complex is a positive co-regulator of ERα-mediated gene expression. Expression of mutant PCDH19 affects at least a subset of known ERα-regulated genes. These data are consistent with our findings that genes regulated by nuclear hormone receptors and those involved in the metabolism of neurosteroids in particular are dysregulated in PCDH19-epilepsy girls and affected mosaic males. Overall we define and characterize a novel mechanism of gene regulation driven by PCDH19, which is mediated by paraspeckle constituent NONO and is ERα-dependent. This PCDH19-NONO-ERα axis is of relevance not only to PCDH19-epilepsy and its comorbidities but likely also to ERα and generally nuclear hormone receptor-associated cancers.

δ-Protocadherins: Organizers of neural circuit assembly

The δ-protocadherins comprise a small family of homophilic cell adhesion molecules within the larger cadherin superfamily. They are essential for neural development as mutations in these molecules give rise to human neurodevelopmental disorders, such as schizophrenia and epilepsy, and result in behavioral defects in animal models. Despite their importance to neural development, a detailed understanding of their mechanisms and the ways in which their loss leads to changes in neural function is lacking. However, recent results have begun to reveal roles for the δ-protocadherins in both regulation of neurogenesis and lineage-dependent circuit assembly, as well as in contact-dependent motility and selective axon fasciculation. These evolutionarily conserved mechanisms could have a profound impact on the robust assembly of the vertebrate nervous system. Future work should be focused on unraveling the molecular mechanisms of the δ-protocadherins and understanding how this family functions broadly to regulate neural development.

Structural determinants of adhesion by Protocadherin-19 and implications for its role in epilepsy

Non-clustered δ-protocadherins are homophilic cell adhesion molecules essential for the development of the vertebrate nervous system, as several are closely linked to neurodevelopmental disorders. Mutations in protocadherin-19 (PCDH19) result in a female-limited, infant-onset form of epilepsy (PCDH19-FE). Over 100 mutations in PCDH19 have been identified in patients with PCDH19-FE, about half of which are missense mutations in the adhesive extracellular domain. Neither the mechanism of homophilic adhesion by PCDH19, nor the biochemical effects of missense mutations are understood. Here we present a crystallographic structure of the minimal adhesive fragment of the zebrafish Pcdh19 extracellular domain. This structure reveals the adhesive interface for Pcdh19, which is broadly relevant to both non-clustered δ and clustered protocadherin subfamilies. In addition, we show that several PCDH19-FE missense mutations localize to the adhesive interface and abolish Pcdh19 adhesion in in vitro assays, thus revealing the biochemical basis of their pathogenic effects during brain development.

DOI:http://dx.doi.org/10.7554/eLife.18529.001

As the brain develops, its basic building blocks – cells called neurons – need to form the correct connections with one another in order to give rise to neural circuits. A mistake that leads to the formation of incorrect connections can result in a number of disorders or brain abnormalities.

Proteins called cadherins that are present on the surface of neurons enable them to stick to their correct partners like Velcro. One of these proteins is called Protocadherin-19. However, it was not fully understood how this protein forms an adhesive bond with other Protocadherin-19 molecules, or how some of the proteins within the cadherin family are able to distinguish between one another.

Cooper et al. used X-ray crystallography to visualize the molecular structure of Protocadherin-19 taken from zebrafish in order to better understand the adhesive bond that these proteins form with each other. In addition, the new structure showed the sites of the mutations that cause a form of epilepsy in infant females. From this, Cooper et al. could predict how the mutations would disrupt Protocadherin-19’s shape and function.

The structures revealed that Protocadherin-19 molecules from adjacent cells engage in a “forearm handshake” to form the bond that connects neurons. Some of the mutations that cause epilepsy occur in the region responsible for this Protocadherin-19 forearm handshake. Laboratory experiments confirmed that these mutations impair the formation of the adhesive bond, revealing the molecular basis for some of the mutations that underlie Protocadherin-19-female-limited epilepsy.

Other cadherin molecules may interact via a similar forearm handshake; this could be investigated in future experiments. It also remains to be discovered how brain wiring depends on Protocadherin-19 adhesion in animal development, and how altering these proteins can rewire developing brain circuits.

DOI:http://dx.doi.org/10.7554/eLife.18529.002

Characterizing PCDH19 in human induced pluripotent stem cells (iPSCs) and iPSC-derived developing neurons: emerging role of a protein involved in controlling polarity during neurogenesis

PCDH19 (Protocadherin 19), a member of the cadherin superfamily, is involved in the pathogenic mechanism of an X-linked model of neurological disease. The biological function of PCHD19 in human neurons and during neurogenesis is currently unknown. Therefore, we decided to use the model of the induced pluripotent stem cells (iPSCs) to characterize the location and timing of expression of PCDH19 during cortical neuronal differentiation. Our data show that PCDH19 is expressed in pluripotent cells before differentiation in a homogeneous pattern, despite its localization is often limited to one pole of the cell. During neuronal differentiation, positional information on the progenitor cells assumes an important role in acquiring polarization. The proper control of the cell orientation ensures a fine balancing between symmetric (giving rise to two progenitor sister cells) versus asymmetric (giving rise to one progenitor cell and one newborn neuron) division. This process results in the polar organization of the neural tube with a lumen indicating the basal part of the polarized neuronal progenitor cell; in the iPSC model the cells are organized in the ‘neural rosette’ and interestingly, PCDH19 is located at the center of the rosette, with other well-known markers of the lumen (N-cadherin and ZO-1). These data suggest that PCDH19 has a role in instructing the apico-basal polarity of the progenitor cells, thus regulating the development of a properly organized human brain.

Aberrant expression and functions of protocadherins in human malignant tumors

Protocadherins (PCDHs) are a group of transmembrane proteins belonging to the cadherin superfamily and are subdivided into "clustered" and "non-clustered" groups. PCDHs vary in both structure and interaction partners and thus regulate multiple biological responses in complex and versatile patterns. Previous researches showed that PCDHs regulated the development of brain and were involved in some neuronal diseases. Recently, studies have revealed aberrant expression of PCDHs in various human malignant tumors. The down-regulation or absence of PCDHs in malignant cells has been associated with cancer progression. Further researches suggest that PCDHs may play major functions as tumor suppressor by inhibiting the proliferation and metastasis of cancer cells. In this review, we focus on the altered expression of PCDHs and their roles in the development of cancer progression. We also discuss the potential mechanisms, by which PCDHs are aberrantly expressed, and its implications in regulating cancers.

Pcdh19 Loss-of-Function Increases Neuronal Migration In Vitro but is Dispensable for Brain Development in Mice

Protocadherin 19 (Pcdh19) is an X-linked gene belonging to the protocadherin superfamily, whose members are predominantly expressed in the central nervous system and have been implicated in cell-cell adhesion, axon guidance and dendrite self-avoidance. Heterozygous loss-of-function mutations in humans result in the childhood epilepsy disorder PCDH19 Girls Clustering Epilepsy (PCDH19 GCE) indicating that PCDH19 is required for brain development. However, understanding PCDH19 function in vivo has proven challenging and has not been studied in mammalian models. Here, we validate a murine Pcdh19 null allele in which a β-Geo reporter cassette is expressed under the control of the endogenous promoter. Analysis of β-Geo reporter activity revealed widespread but restricted expression of PCDH19 in embryonic, postnatal and adult brains. No gross morphological defects were identified in Pcdh19(+/β-Geo) and Pcdh19(Y/β-Geo) brains and the location of Pcdh19 null cells was normal. However, in vitro migration assays revealed that the motility of Pcdh19 null neurons was significantly elevated, potentially contributing to pathogenesis in patients with PCDH19 mutations. Overall our initial characterization of Pcdh19(+/β-Geo), Pcdh19(β-Geo/β-Geo) and Pcdh19(Y/β-Geo)mice reveals that despite widespread expression of Pcdh19 in the CNS, and its role in human epilepsy, its function in mice is not essential for brain development.

Loss of function of PCDH12 underlies recessive microcephaly mimicking intrauterine infection

OBJECTIVE

To identify the genetic basis of a recessive syndrome characterized by prenatal hyperechogenic brain foci, congenital microcephaly, hypothalamic midbrain dysplasia, epilepsy, and profound global developmental disability.

METHODS

Identification of the responsible gene by whole exome sequencing and homozygosity mapping.

RESULTS

Ten patients from 4 consanguineous Palestinian families manifested in utero with hyperechogenic brain foci, microcephaly, and intrauterine growth retardation. Postnatally, patients had progressive severe microcephaly, neonatal seizures, and virtually no developmental milestones. Brain imaging revealed dysplastic elongated masses in the midbrain-hypothalamus-optic tract area. Whole exome sequencing of one affected child revealed only PCDH12 c.2515C>T, p.R839X, to be homozygous in the proband and to cosegregate with the condition in her family. The allele frequency of PCDH12 p.R839X is <0.00001 worldwide. Genotyping PCDH12 p.R839X in 3 other families with affected children yielded perfect cosegregation with the phenotype (probability by chance is 2.0 × 10(-12)). Homozygosity mapping revealed that PCDH12 p.R839X lies in the largest homozygous region (11.7 MB) shared by all affected patients. The mutation reduces transcript expression by 84% (p < 2.4 × 10(-13)). PCDH12 is a vascular endothelial protocadherin that promotes cellular adhesion. Endothelial adhesion disruptions due to mutations in OCLN or JAM3 also cause congenital microcephaly, intracranial calcifications, and profound psychomotor disability.

CONCLUSIONS

Loss of function of PCDH12 leads to recessive congenital microcephaly with profound developmental disability. The phenotype resembles Aicardi-Goutières syndrome and in utero infections. In cases with similar manifestations but no evidence of infection, our results suggest consideration of an additional, albeit rare, cause of congenital microcephaly.

Streptococcus pneumoniae Cell-Wall-Localized Phosphoenolpyruvate Protein Phosphotransferase Can Function as an Adhesin: Identification of Its Host Target Molecules and Evaluation of Its Potential as a Vaccine

In Streptococcus pneumonia, phosphoenolpyruvate protein phosphotransferase (PtsA) is an intracellular protein of the monosaccharide phosphotransferase systems. Biochemical and immunostaining methods were applied to show that PtsA also localizes to the bacterial cell-wall. Thus, it was suspected that PtsA has functions other than its main cytoplasmic enzymatic role. Indeed, recombinant PtsA and anti-rPtsA antiserum were shown to inhibit adhesion of S. pneumoniae to cultured human lung adenocarcinoma A549 cells. Screening of a combinatorial peptide library expressed in a filamentous phage with rPtsA identified epitopes that were capable of inhibiting S. pneumoniae adhesion to A549 cells. The insert peptides in the phages were sequenced, and homologous sequences were found in human BMPER, multimerin1, protocadherin19, integrinβ4, epsin1 and collagen type VIIα1 proteins, all of which can be found in A549 cells except the latter. Six peptides, synthesized according to the homologous sequences in the human proteins, specifically bound rPtsA in the micromolar range and significantly inhibited pneumococcal adhesion in vitro to lung- and tracheal-derived cell lines. In addition, the tested peptides inhibited lung colonization after intranasal inoculation of mice with S. pneumoniae. Immunization with rPtsA protected the mice against a sublethal intranasal and a lethal intravenous pneumococcal challenge. In addition, mouse anti rPtsA antiserum reduced bacterial virulence in the intravenous inoculation mouse model. These findings showed that the surface-localized PtsA functions as an adhesin, PtsA binding peptides derived from its putative target molecules can be considered for future development of therapeutics, and rPtsA should be regarded as a candidate for vaccine development.

Coordinating cell and tissue behavior during zebrafish neural tube morphogenesis

The development of a vertebrate neural epithelium with well-organized apico-basal polarity and a central lumen is essential for its proper function. However, how this polarity is established during embryonic development and the potential influence of surrounding signals and tissues on such organization has remained less understood. In recent years the combined superior transparency and genetics of the zebrafish embryo has allowed for in vivo visualization and quantification of the cellular and molecular dynamics that govern neural tube structure. Here, we discuss recent studies revealing how co-ordinated cell-cell interactions coupled with adjacent tissue dynamics are critical to regulate final neural tissue architecture. Furthermore, new findings show how the spatial regulation and timing of orientated cell division is key in defining precise lumen formation at the tissue midline. In addition, we compare zebrafish neurulation with that of amniotes and amphibians in an attempt to understand the conserved cellular mechanisms driving neurulation and resolve the apparent differences among animals. Zebrafish neurulation not only offers fundamental insights into early vertebrate brain development but also the opportunity to explore in vivo cell and tissue dynamics during complex three-dimensional animal morphogenesis.

INGA: protein function prediction combining interaction networks, domain assignments and sequence similarity

Identifying protein functions can be useful for numerous applications in biology. The prediction of gene ontology (GO) functional terms from sequence remains however a challenging task, as shown by the recent CAFA experiments. Here we present INGA, a web server developed to predict protein function from a combination of three orthogonal approaches. Sequence similarity and domain architecture searches are combined with protein-protein interaction network data to derive consensus predictions for GO terms using functional enrichment. The INGA server can be queried both programmatically through RESTful services and through a web interface designed for usability. The latter provides output supporting the GO term predictions with the annotating sequences. INGA is validated on the CAFA-1 data set and was recently shown to perform consistently well in the CAFA-2 blind test. The INGA web server is available from URL: http://protein.bio.unipd.it/inga.

Protocadherins branch out: Multiple roles in dendrite development

The proper formation of dendritic arbors is a critical step in neural circuit formation, and as such defects in arborization are associated with a variety of neurodevelopmental disorders. Among the best gene candidates are those encoding cell adhesion molecules, including members of the diverse cadherin superfamily characterized by distinctive, repeated adhesive domains in their extracellular regions. Protocadherins (Pcdhs) make up the largest group within this superfamily, encompassing over 80 genes, including the ∼60 genes of the α-, β-, and γ-Pcdh gene clusters and the non-clustered δ-Pcdh genes. An additional group includes the atypical cadherin genes encoding the giant Fat and Dachsous proteins and the 7-transmembrane cadherins. In this review we highlight the many roles that Pcdhs and atypical cadherins have been demonstrated to play in dendritogenesis, dendrite arborization, and dendritic spine regulation. Together, the published studies we discuss implicate these members of the cadherin superfamily as key regulators of dendrite development and function, and as potential therapeutic targets for future interventions in neurodevelopmental disorders.

Differential expression of protocadherin-19, protocadherin-17, and cadherin-6 in adult zebrafish brain

Cell adhesion molecule cadherins play important roles in both development and maintenance of adult structures. Most studies on cadherin expression have been carried out in developing organisms, but information on cadherin distribution in adult vertebrate brains is limited. In this study we used in situ hybridization to examine mRNA expression of three cadherins, protocadherin-19, protocadherin-17, and cadherin-6 in adult zebrafish brain. Each cadherin exhibits a distinct expression pattern in the fish brain, with protocadherin-19 and protocadherin-17 showing much wider and stronger expression than that of cadherin-6. Both protocadherin-19 and protocadherin-17-expressing cells occur throughout the brain, with strong expression in the ventromedial telencephalon, periventricular regions of the thalamus and anterior hypothalamus, stratum periventriculare of the optic tectum, dorsal tegmental nucleus, granular regions of the cerebellar body and valvula, and superficial layers of the facial and vagal lobes. Numerous sensory structures (e.g., auditory, gustatory, lateral line, olfactory, and visual nuclei) and motor nuclei (e.g., oculomotor, trochlear, trigeminal motor, abducens, and vagal motor nuclei) contain protocadherin-19 and/or protocadherin-17-expressing cell. Expression of these two protocadherins is similar in the ventromedial telencephalon, thalamus, hypothalamus, facial, and vagal lobes, but substantially different in the dorsolateral telencephalon, intermediate layers of the optic tectum, and cerebellar valvula. In contrast to the two protocadherins, cadherin-6 expression is much weaker and limited in the adult fish brain.

Emerging roles of protocadherins: from self-avoidance to enhancement of motility

Protocadherins are a group of transmembrane proteins belonging to the cadherin superfamily that are subgrouped into 'clustered' and 'non-clustered' protocadherins. Although cadherin superfamily members are known to regulate various forms of cell-cell interactions, including cell-cell adhesion, the functions of protocadherins have long been elusive. Recent studies are, however, uncovering their unique roles. The clustered protocadherins regulate neuronal survival, as well as dendrite self-avoidance. Combinatorial expression of clustered protocadherin isoforms creates a great diversity of adhesive specificity for cells, and this process is likely to underlie the dendritic self-avoidance. Non-clustered protocadherins promote cell motility rather than the stabilization of cell adhesion, unlike the classic cadherins, and mediate dynamic cellular processes, such as growth cone migration. Protocadherin dysfunction in humans is implicated in neurological disorders, such as epilepsy and mental retardation. This Commentary provides an overview of recent findings regarding protocadherin functions, as well as a discussion of the molecular basis underlying these functions.