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.

Nervous system function is critically dependent on the underlying neural architecture, including patterns of neuronal connectivity. Cell-cell recognition by cell surface receptors is central to establishing these functional neural circuits during development (Kiecker and Lumsden, 2005; Steinberg, 2007; Zipursky and Sanes, 2010). The cadherin superfamily is a large and diverse family of cell adhesion molecules that are strongly expressed in the developing nervous system (Hirano and Takeichi, 2012; Suzuki, 1996; Frank and Kemler, 2002; Shapiro et al., 2007; Gumbiner, 2005;Chen and Maniatis, 2013). The differential expression of classical cadherins and protocadherins, the largest groups within the cadherin superfamily, suggests that they play important roles in the development of neural circuitry (Weiner and Jontes, 2013; Hirano and Takeichi, 2012), an idea supported by their involvement in a range of neurodevelopmental disorders (Redies et al., 2012; Hirabayashi and Yagi, 2014). In particular, the non-clustered δ-protocadherins have been linked to autism spectrum disorders, intellectual disability, congenital microcephaly and epilepsy.

Protocadherin-19 (PCDH19) is a member of the non-clustered δ2-protocadherin subfamily (Wolverton and Lalande, 2001; Vanhalst et al., 2005; Gaitan and Bouchard, 2006; Emond et al., 2009; Liu et al., 2010) that is located on the X-chromosome. Mutations in PCDH19 cause an X-linked, female-limited form of infant-onset epilepsy (PCDH19 female epilepsy, PCDH19-FE; OMIM 300088) that is associated with intellectual disability, as well as compulsive or aggressive behavior and autistic features (Dibbens et al., 2008; Scheffer et al., 2008; Depienne and LeGuern, 2012; van Harssel et al., 2013; Leonardi et al., 2014; Thiffault et al., 2016; Terracciano et al., 2016; Walters et al., 2014). To date, well over 100 distinct mutations in PCDH19 have been identified in epilepsy patients, making it the second most clinically relevant gene in epilepsy. Approximately half of these mutations are missense mutations distributed throughout the extracellular domain of the PCDH19 protein. Despite the clear importance of PCDH19 and other non-clustered δ-protocadherins to neural development, their specific roles are only beginning to be revealed. For example, Pcdh7, Pcdh17 and Pcdh18b are involved in axon outgrowth or arborization (Piper et al., 2008; Hayashi et al., 2014; Biswas et al., 2014), while several δ-protocadherins, including Pcdh19, regulate cell motility during early development (Yamamoto et al., 1998; Aamar and Dawid, 2008; Biswas et al., 2010; Emond et al., 2009). In zebrafish, pcdh19, regulates the formation of neuronal columns in the optic tectum, and loss of pcdh19 degrades visually-guided behaviors (Cooper et al., 2015). However, it is not known how mutations in PCDH19 lead to PCDH19-FE.

Cadherins typically mediate adhesion using their extracellular domains, which are made of two or more consecutive extracellular cadherin (EC) repeats (Takeichi, 1990; Brasch et al., 2012). The adhesion mechanism used by classical cadherins is well known and involves a tip-to-tip interaction that is stabilized by the reciprocal exchange of tryptophan residues at the N-terminal EC1 repeat most distant from the membrane (Overduin et al., 1995; Shapiro et al., 1995; Nagar et al., 1996; Boggon et al., 2002; Patel et al., 2006; Zhang et al., 2009; Sivasankar et al., 2009; Harrison et al., 2010; Ciatto et al., 2010; Leckband and Sivasankar, 2012). However, PCDH19 along with the rest of the non-classical cadherins lack these critical tryptophan residues and must mediate adhesion by an alternative mechanism (Emond et al., 2011; Sotomayor et al., 2014; Biswas et al., 2010). In the case of the non-classical protocadherin-15 and cadherin-23 proteins, an adhesive interface is formed by overlapping, antiparallel interactions of their EC1 and EC2 tips (Elledge et al., 2010; Sotomayor et al., 2010; 2012; Geng et al., 2013). For clustered protocadherins, recent binding assays and structures suggest that adhesion is mediated by an antiparallel interaction of fully overlapping EC1 to EC4 domains (Rubinstein et al., 2015; Nicoludis et al., 2015; Goodman et al., 2016). Yet how non-clustered δ-protocadherins and PCDH19 form adhesive bonds and how these bonds are altered by disease-causing mutations is unknown.

Here we present crystals structures of the highly homologous zebrafish Protocadherin-19 (Pcdh19) encompassing repeats EC1-4 and EC3-4. The structures allow us to map >70% of the disease-causing missense mutations and provide a structural framework to interpret their functional impact. In addition, the structures suggest two possible homophilic adhesive interfaces, and complementary binding assays validate one of them, which is affected by multiple PCDH19-FE mutations. This interface involves fully overlapping EC1 to EC4 domains and likely represents a general interaction mechanism for the non-clustered δ-protocadherins.

To understand the mechanism of Pcdh19 function and to determine the structural role of PCDH19-FE mutations, the Danio rerio Pcdh19 EC1-4 and the EC3-4 fragments (70% identity, 83% similarity to Homo sapiens EC1-4) were produced in E. coli, refolded from inclusion bodies, and used for crystallization and structural determination (see Materials and methods). The solved structure for Pcdh19 EC3-4 (2.51 Å, Table 1, Figure 1—figure supplement 1A) includes four molecules in the asymmetric unit, each starting from Pro 213 and continuing to Asp 422 (numbering corresponds to the processed Danio rerio protein, see Materials and methods). Root-mean-square-deviation (RMSD) among these four molecules is <2.4 Å. One of the Pcdh19 EC3-4 molecules was used to solve the Pcdh19 EC1-4 structure (3.59 Å, Table 1, Figure 1—figure supplement 1B), which contains two molecules in the asymmetric unit, each starting from Val one to Asp 422 (RMSD of 1.4 Å). The EC3-4 repeats from both structures superpose well (RMSD 2.1 Å), and good quality electron density maps allowed us to unambiguously position side chains for most residues (Materials and methods and Figure 1—figure supplement 1). Given the similarities among our structures and chains, we will describe features as seen in the more complete chain B of Pcdh19 EC1-4, unless otherwise explicitly stated.

The architecture of all Pcdh19 EC repeats matches that observed for other cadherins (Shapiro et al., 1995; Overduin et al., 1995), with the typical Greek-key motif comprised of seven β strands (A-G) forming a β sandwich fold (Figure 1A). The EC1 repeat has a disulfide bond at the E-F loop, typical of clustered protocadherins, as well as one of two α-helices (at the B-C loop) also found in structures of clustered protocadherins (Morishita et al., 2006; Nicoludis et al., 2015; Rubinstein et al., 2015; Goodman et al., 2016) (Figure 1A,B). The three linker regions of Pcdh19 (EC1-2, EC2-3, EC3-4) have canonical cadherin calcium-binding sites (Nagar et al., 1996) (Figure 1E–G). Overall, our structures show canonical features and provide a unique framework to analyze >70% of the PCDH19-FE mutations.

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PCDH19-FE mutations.

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Antiparallel interfaces in crystal contacts of the Pcdh19 EC1-4 structure

Crystal structures have previously revealed the adhesive interfaces for classical cadherins, clustered protocadherins, and the protocadherin-15 and cadherin-23 complex (Nagar et al., 1996; Boggon et al., 2002; Patel et al., 2006; Ciatto et al., 2010; Sotomayor et al., 2012; Nicoludis et al., 2015; Goodman et al., 2016). Although the Pcdh19 EC3-4 structure does not reveal any relevant interface, the Pcdh19 EC1-4 structure does. The purified Pcdh19 EC1-4 fragment elutes in two well-defined peaks in size exclusion chromatography experiments (SEC), with these peaks most likely representing monomeric and dimeric states in solution (Figure 2—figure supplement 1). Pcdh19 EC1-4 crystals were grown from the putative dimer SEC peak elution, and two plausible adhesive interfaces are observed in our Pcdh19 EC1-4 structure. The first one, which we will refer to as Pcdh19-I1, arises from contacts between the two Pcdh19 EC1-4 molecules in the asymmetric unit, and involves a fully-overlapped antiparallel dimer in which EC1 from one molecule interacts with EC4 from the other (EC1:EC4), EC2 with EC3 (EC2:EC3), EC3 with EC2 (EC3:EC2), and EC4 with EC1 (EC4:EC1; Figure 2A,B). Within the same protein crystal structure, the second antiparallel interface (Pcdh19-I2) involves the opposite side of Pcdh19 with observed EC2:EC4, EC3:EC3, and EC4:EC2 interactions, as well as potential (not observed) EC1:EC5 and EC5:EC1 contacts (Figure 2—figure supplement 2A). Several lines of evidence favor the first interface Pcdh19-I1 as the most likely to mediate biological function.

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Analysis of the Pcdh19-I1 antiparallel interface with the Protein Interfaces, Surfaces and Assemblies (PISA) server (Krissinel and Henrick, 2007) and with the NOXclass classifier (Zhu et al., 2006) revealed a large interface (~1650 Ų), that is unlikely to be a crystal packing artifact (89.21% biological, 81% obligate). In contrast, the possible antiparallel Pcdh19-I2 interface is predicted by NOXclass to be non physiological, as its smaller interface area (~930 Ų) and the nature of its contacts matches those of crystal packing interactions (42.97% biological, 20.21% obligate). Yet, both interface areas are larger than 856 Ų, an empirical cut-off that can distinguish biological interfaces from crystal contacts with 85% accuracy (Ponstingl et al., 2000), and our analysis of the Pcdh19-I2 interface lacks contributions from possible EC1-EC5 contacts, which might be significant. Moreover, shape correlation (Lawrence and Colman, 1993) for Pcdh19-I1 is lower than for the Pcdh19-I2 interface (S_c-I1 = 0.44 vs. S_c-I2 = 0.61), as there is a large gap between the main EC2-EC3:EC3-EC2 contacts and the EC1-EC4:EC4-EC1 interactions zones (Figure 2A).

To further differentiate between the possible Pcdh19-I1 and Pcdh19-I2 interfaces, we evaluated whether any of the six PCDH19-FE mutations altering surface residues at crystal contacts, but not necessarily protein structure, could interfere with binding. Five of these mutations (S139L, T146R, P149S, E313K, and T404I; all at conserved sites) change residues involved in the Pcdh19-I1 interface (S116, T123, P126, E290, T381 respectively in Figure 2B–E), where we define residues at a given interface as those with a buried surface area that is at least 20% of their accessible surface area according to PISA. In all cases we predict altered EC1-4 homophilic binding, as the size and nature of the residue is changed by each mutation (hydrophobic vs. hydrophilic; charged vs. non-charged). The remaining mutation (H203P) involves a non-conserved residue at the Pcdh19-I2 interface, which could impair its formation (R180 in Figure 2—figure supplement 2A). However, the patient with the H203P mutation also carries another PCDH19-FE mutation (F206C) at a location mutated in other epilepsy patients (Marini et al., 2012; Depienne et al., 2011); thus it is unclear if H203P is contributing to epilepsy. In contrast, all five PCDH19-FE mutations at the Pcdh19-I1 interface are likely causal, which suggests that Pcdh19-I1 is relevant in vivo.

We also analyzed predicted glycosylation sites that might interfere with binding and thereby reveal non-physiological interfaces, as observed for VE-cadherin (Brasch et al., 2011). There are 14 glycosylation sites within EC1-4, and none of them involve residues at the Pcdh19-I1 interface (Figure 2—figure supplement 3A). An O-linked glycosylation site is predicted to be at the Pcdh19-I2 interface (T232), and an additional O-linked glycosylation site is predicted for the human PCDH19 protein at S204 (the equivalent N202 in Pcdh19 is predicted to be non-glycosylated), also at the Pcdh19-I2 interface (Figure 2—figure supplement 3B). Glycation sites, for which sugar molecules might be added randomly and to long-lived proteins, are predicted at both interfaces (K156 and K308 in Pcdh19-I1 and K204 in Pcdh19-I2), but may not interfere directly with either, since glycation depends on environmental conditions and it has never been reported for cadherins (Salahuddin et al., 2014; Simm et al., 2015). Thus the lack of glycosylation sites at the Pcdh19-I1 interface renders it as the most likely to be functional.

While not conclusive, all the analyses presented above favor the Pcdh19-I1 antiparallel dimer over the Pcdh19-I2 interface in terms of physiological relevance. The larger surface area of the Pcdh19-I1 dimer, the nature of the residues involved, the number of PCDH19-FE mutations at this interface, and the lack of predicted glycosylation sites, all suggest that the Pcdh19-I1 interface may occur and be functional in vivo.

Binding assays probing Pcdh19 interfaces

To conclusively test which binding interface mediates Pcdh19 adhesion, and whether PCDH19-FE mutations at the protein surface can interfere with one of the two possible Pcdh19 interfaces described above, we used modified bead aggregations assays, mutagenesis, and size exclusion chromatography experiments. Previous cell-based assays showed weak homophilic adhesion for the chicken Pcdh19 (Tai et al., 2010). In addition, previous assays in which the full-length Pcdh19 extracellular cadherin domain fused to Fc (Pcdh19ECFc) was incubated with protein A beads showed calcium-dependent aggregation only when it was co-purified with N-cadherin (Biswas et al., 2010; Emond et al., 2011). To study Pcdh19 homophilic interactions, we modified the previous protocol (Emond and Jontes, 2014) and added a final step in which beads were rocked (Sano et al., 1993) in a controlled fashion for up to two minutes (see Materials and methods). The modified protocol allowed us to identify clear bead aggregates mediated by Pcdh19ECFc alone (Figure 3—figure supplement 1).

To identify the minimal adhesive unit of Pcdh19 we used our modified protocol with truncated versions of Pcdh19 containing different numbers of EC repeats: Pcdh19ECFc (EC1-6), Pcdh19EC1-5Fc, Pcdh19EC1-4Fc, Pcdh19EC1-3Fc, Pcdh19EC1-2Fc, and Pcdh19EC2-6Fc (Figure 3). Bead aggregation was observed only when using Pcdh19ECFc, Pcdh19EC1-5Fc, and Pcdh19EC1-4Fc, thus suggesting that Pcdh19EC1-4 is the minimal adhesive unit and highlighting the biological relevance of the antiparallel Pcdh19-I1 interface, which involves EC1-4 only.

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Quantification of aggregation assays.

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Next, we introduced two PCDH19-FE mutations (T146R and E313K located at the Pcdh19-I1 interface; T123R and E290K in Figure 2B) in the full-length Pcdh19 extracellular domain and tested bead aggregation with these protein constructs (Figure 4). Bead aggregates were not detected when the Pcdh19ECFc carried these mutations under the conditions tested (Figure 4B–C,E–H). In contrast, the mutation R364E, predicted to impair the Pcdh19-I2 interface, did not eliminate bead aggregation (Figure 2—figure supplement 2B–D). Moreover, the presence of a N-cadherin (Ncad) fragment known to enhance Pcdh19-mediated adhesion (Emond et al., 2011) did not qualitatively change the effect of the T146R and E313K mutations. Bead aggregates were greatly diminished for T146R and abolished for E313K in the presence of NcadEC W2A/R14E His, a non-adhesive Ncad mutant previously used to study Pcdh19-mediated homophilic adhesion (Harrison et al., 2010; Emond et al., 2011) (Figure 4—figure supplement 1B–C,E–F). In addition, these mutations did not abolish the interaction between Pcdh19ECFc and NcadEC W2A/R14E His (Figure 4—figure supplement 2). It is possible that the T146R and 313K mutations affect interactions with N-cadherin in a subtle way (directly or allosterically), yet our experimental results suggest that these mutations directly impair Pcdh19 homophilic adhesion.

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We also introduced the T146R and E313K mutations at the Pcdh19-I1 interface into the bacterially expressed Pcdh19 EC1-4 protein fragment, and used analytical size exclusion chromatography to determine whether the mutant fragments were eluting as putative dimers or monomers in solution. Both mutations resulted in a shift of the elution peak that indicated a smaller, monomeric state (Figure 4I). Taken together, our crystallographic structural analyses and binding assays including PCDH19-FE mutations strongly support a model in which fully overlapped EC1-4 domains (Pcdh19-I1 interface) form the functional adhesive unit of Pcdh19 (Figure 4J).

Model for PCDH19 adhesive interaction and implications for other protocadherins

The antiparallel Pcdh19-I1 dimer interface validated above reveals a homophilic 'forearm handshake' binding mechanism for PCDH19, involving overlap of 4 ECs from each protomer wrapping around each other. This is different from the mechanism used by classical cadherins, only involving EC1 (Brasch et al., 2012) or the heterophilic 'extended handshake' used by protocadherin-15 and cadherin-23, involving overlap of only EC1-2 of each protein (Sotomayor et al., 2012). The forearm handshake is similar to the binding mechanism recently reported for clustered protocadherins (Goodman et al., 2016; Rubinstein et al., 2015; Nicoludis et al., 2015) and might be used by other non-clustered protocadherins.

The Pcdh19-I1 interface involves extended and mostly symmetric, in-register contacts between repeats EC2:EC3 that account for ~58% of the interfacial area, as well as smaller, separate EC1:EC4 contacts (~350 Ų × 2) that are slightly off-register. The EC1:EC4 contacts arise as both repeats bend to meet after the C-terminal end of EC3 and the N-terminal end of EC2 separate from each other. In this arrangement, the EC2-3 linkers from each protomer are right next to each other, while the EC3-EC4 linker in one protomer is separated from the EC1-2 linker of the binding partner by a large opening. The interface is generally amphiphilic, with ~49% of the interfacial area involving hydrophobic residues, ~28% hydrophilic, and ~23% charged residues (Figure 5—figure supplement 1). Interestingly, the contact formed by EC1:EC4 is more hydrophobic (58%; 22%; 20%) than the one formed by EC2:EC3 (41%; 33%; 26%), yet salt-bridge pairs across protomers are present in both: R40-E328 and E81-R39 enhance the EC1 to EC4 contacts (Figure 2C) and R158-E290 links EC2 to EC3 (Figure 2D). While the R40-E328 pair seems to be zebrafish specific, the other two salt-bridges are highly conserved across sequenced species, along with most of the residues involved in the Pcdh19 EC1-4 interface (Figure 5A and Figure 5—figure supplement 2). The same set of residues is highly variable across different members of the δ1, δ2, and the clustered protocadherins (Figure 5B and Figure 5—figure supplement 3), suggesting that binding mechanisms might differ across subfamilies or that residue variability might confer specificity within a common binding mechanism.

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Protocadherin-19 sequences.

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Sequences for selected clustered and δ-protocadherins.

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A comparison of our Pcdh19-I1 interface to recently reported models and structures of clustered protocadherin interfaces (Nicoludis et al., 2015; Goodman et al., 2016) reveals multiple similarities among them. The most complete models of α and β-protocadherins show similar, fully overlapped antiparallel EC1-4 dimers (Figure 5—figure supplement 4A–D), with the same extended EC2:EC3 antiparallel connection accompanied with smaller EC1:EC4 contacts and salt-bridges across protomers. Structural alignments show that the relative arrangements of protomers within the antiparallel dimers for Pcdh19, Mm Pcdhα4 (5DZW), and Mm Pcdhα7 (5DZV) are the most similar to each other with slight shifting in some EC repeats (Figure 5—figure supplement 4A,B). The Mm Pcdhβ6 (5DZX) and Mm Pcdhβ8 (5DZY) structures show similar dimeric interfaces, but the relative arrangement of protomers within the dimer is slightly shifted for all EC repeats (Figure 5—figure supplement 4C,D). Similarly, the Mm PcdhγA1 EC1-3 interface (4ZI9) matches and aligns well with the Pcdh19 EC1-4 dimer (Figure 5—figure supplement 4E). Mapping of all interaction sites to the Pcdh19 EC1-4 topology diagram (Figure 5C) reveals a pattern for common interacting domains in odd and even EC repeats across these structures, which include the F-G β hairpin and β strand A for repeats EC1 and EC3, as well as the A-B and D-E β hairpins for EC2 and EC4. While there are differences in some of the interacting domains, dimeric arrangements, and contact details, including diversity of interfacial residues, clustered protocadherins seem to use the same binding mechanism that Pcdh19 uses to mediate adhesion.

A conserved RGD sequence motif within Pcdh19 EC2 (residues 158 to 160) at its D-E loop is similar to an integrin-binding RGD site within EC1 (C-D loop) in the α-protocadherins (Ruoslahti, 1996; Mutoh et al., 2004). The EC1 RGD motif is exposed in the Mm Pcdhα4 and Mm Pcdhα7 homo-dimers while the EC2 RGD motif of Pcdh19 (also present in Pcdh17 [Kim et al., 2011]) is buried at the EC2:EC3 contacts in the Pcdh19-I1 interface. This suggests that homophilic binding could regulate the availability of this potential, untested, integrin-binding site.

Pcdh19 belongs to the δ2-protocadherin subfamily, and given the sequence similarity among subfamily members, it is likely that all use the same dimer interface to mediate adhesion. This is less obvious for the δ1 subfamily, with members that have seven EC repeats and that display some critical differences at interaction sites, such as the presence of a positively charged residue (R or K) at position 290, where most δ2 members have a negatively charged glutamate that interacts with an arginine at position 158 (Figure 2D, Figure 5—figure supplements 3 and 5). The PCDH19-FE E313K mutation at this site (E290) prevents binding (Figure 4C,H–I and Figure 4—figure supplement 1), suggesting that δ1-protocadherins, which effectively carry the same mutation, should use a different interface to mediate adhesion. Yet, residues at position 157 and 158 in δ1-protocadherins are also charge swapped, with aspartates and glutamates that would restore this critical salt-bridge interaction at the EC2:EC3 interface, and at the same time prevent heterophilic interactions with δ2-protocadherins (Figure 5—figure supplements 3 and 5). Thus, it is likely that all non-clustered δ-protocadherins use fully overlapped EC1-4 antiparallel interfaces, like the one observed for Pcdh19, to mediate adhesion.

The non-clustered δ-protocadherins are increasingly linked to human neurodevelopmental disorders, emphasizing both their importance to brain development and their relevance to human health (Redies et al., 2005; Redies et al., 2012; Hirabayashi and Yagi, 2014). In particular, mutations in PCDH19 cause a female-limited form of infant-onset epilepsy (Dibbens et al., 2008; Scheffer et al., 2008; Depienne and LeGuern, 2012; van Harssel et al., 2013; Leonardi et al., 2014; Thiffault et al., 2016; Terracciano et al., 2016). Therefore, it is imperative to understand the developmental roles of PCDH19 and other non-clustered δ-protocadherins, the structural basis of homophilic adhesion by these molecules, and the functional impact of pathogenic missense mutations. The structural and biochemical data presented here provide a first view on the molecular mechanism of Pcdh19 adhesion, which is likely used by all non-clustered δ and clustered protocadherins. Moreover, our Pcdh19 EC1-4 structural model shows > 70% of the missense mutations identified in PCDH19-FE patients, and reveals the biochemical basis for the deleterious effects for many of these mutations.

The Pcdh19 EC1-4 structure reveals an antiparallel dimer that is consistent with a trans adhesive interface, a conclusion supported by multiple lines of evidence. Notably, several missense mutations identified in PCDH19-FE patients localize to this interface. Two of these missense mutations (T146R and E313K) impair dimerization, as assessed by analytical gel filtration, and adhesion as assessed in bead aggregation assays, with and without N-cadherin. Sequence analysis suggests that the antiparallel adhesive mechanism presented here is broadly relevant to other, related δ-protocadherins. Recent work with clustered protocadherins, implicated in self-avoidance and self/non-self recognition (Lefebvre et al., 2012; Kostadinov and Sanes, 2015; Yagi, 2012), have revealed a similar antiparallel adhesive interface for these clustered protocadherins (Rubinstein et al., 2015; Nicoludis et al., 2015; Goodman et al., 2016). Thus, the Pcdh19-I1 adhesive interface observed in our Pcdh19 EC1-4 structure likely represents the mechanism used by both non-clustered δ-protocadherins and clustered protocadherins, which, together, represent the largest group within the cadherin superfamily.

Our structural data for Pcdh19, as well as recent work with the clustered protocadherins raises an interesting conundrum. The adhesive interface for protocadherins is extensive and involves interactions extending throughout EC1-4. This contrasts sharply with the adhesive interface of classical cadherins, which is restricted to EC1 and involves the reciprocal swap of Aβ-strands that is stabilized by burying Trp2 in a hydrophobic pocket (Brasch et al., 2012). However, the K_D for dimerization of α- and β-protocadherins is in the micromolar range (similar to classical cadherins), bead aggregation and cell-based assays have consistently shown weak adhesion by both non-clustered and clustered protocadherins, and protocadherins are widely recognized as being only weakly adhesive (Schreiner and Weiner, 2010; Thu et al., 2014; Sano et al., 1993; Rubinstein et al., 2015). This disparity suggests that other mechanisms could modulate protocadherin adhesion in vivo. For instance, cis-oligomerization could compete with trans adhesive interactions, or interactions with other proteins, including N-cadherin, could sequester protocadherins or mask their adhesive interface. Further studies will be required to better understand protocadherin adhesion, how it may be altered in the presence of N-cadherin, and how it is regulated in vivo.

In addition to mutations that disrupt adhesion, our data reveal the potential effects of two other classes of mutations. In the first class, many mutations are predicted to directly impair folding and stability, which could lead to reduced levels of protein on the surface, due to impaired trafficking or enhanced protein degradation. In the second, PCDH19-FE mutations affecting calcium-binding sites are likely to cause shifts in calcium affinity as well as protein instability. Similar mutations in cadherin-23 and protocadherin-15 have been shown to decrease protein affinity for calcium, with K_D shifts that are relevant in the context of the low calcium concentration to which these proteins are exposed (Sotomayor et al., 2010). Yet PCDH19 is expected to be in interstitial space with high calcium concentration, so it is more likely that the relevant effect of PCDH19-FE mutations at calcium-binding sites is compromised stability (even at saturating calcium concentrations), as shown here for the N340S mutation. Finally, analysis of one PCDH19-FE mutation within EC1-4, and three within EC5-6, reveal no obvious predicted consequences at the structural level, as they are exposed residues that should not affect calcium-binding, protein stability or adhesion. These mutations may impact a variety of protein-protein interactions. Although the physiological relevance is unclear, both non-clustered and clustered protocadherins can form cis-homo- or cis-hetero-oligomers (Chen et al., 2007; Schreiner and Weiner, 2010), and mutations affecting the formation of cis-oligomers could adversely impact protocadherin function. Similarly, protocadherins participate in a variety of protein complexes beyond homophilic trans adhesion: Pcdh19 has been shown to associate in cis with N-cadherin (Emond et al., 2011); protocadherins associate with the Wnt co-receptor, RYK (Berndt et al., 2011); PAPC interacts with Frizzled-7 and FLRT3 (Chen et al., 2009; Kraft et al., 2012); and Pcdh17 and Pcdh19 have highly conserved RGD sequences, suggesting that they may interact with integrins (Ruoslahti, 1996; Mutoh et al., 2004; Kim et al., 2011). Thus, further experimental characterization of key mutants in vitro and in vivo will continue to reveal correlations between structural defects, cellular-level defects, and different aspects of PCDH19-FE.

The non-clustered protocadherins are increasingly recognized as a family of molecules that play important roles during neural development. In addition to the role of PCDH19 in epilepsy, mutation of PCDH12 was found to underlie a syndrome of microcephaly that is associated with epilepsy and developmental disability (Aran et al., 2016). Moreover, both PCDH9 and PCDH10 have been associated with autism spectrum disorders (Marshall et al., 2008; Morrow et al., 2008). Ongoing work will likely uncover further links between members of this family and neurodevelopmental disorders. Our Pcdh19 EC1-4 model is the first to show the structural basis of adhesion by the non-clustered δ-protocadherins, and reveals that some of the missense mutations identified in PCDH19-FE occur at the adhesive interface and act by abolishing adhesion. This represents an initial stage in understanding the mechanisms of non-clustered δ-protocadherin homophilic adhesion and provides insight into the biochemical basis of protocadherin-based neurodevelopmental disease.

1. 1. Cooper SR
   2. Jontes JD
   3. Sotomayor M

   (2015) Crystal Structure of Zebrafish Protocadherin-19 EC3-4

   Publicly available at the RCSB Protein Data Bank (accession no: 5CO1).

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Author details

1. Sharon R Cooper

   1. Department of Chemistry and Biochemistry, The Ohio State University, Columbus, United States
   2. Department of Neuroscience, The Ohio State University, Columbus, United States

   Contribution

   SRC, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

2. James D Jontes

   Department of Neuroscience, The Ohio State University, Columbus, United States

   Contribution

   JDJ, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   jontes.1@osu.edu

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0002-8954-6127

3. Marcos Sotomayor

   Department of Chemistry and Biochemistry, The Ohio State University, Columbus, United States

   Contribution

   MS, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   sotomayor.8@osu.edu

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0002-3333-1805

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