Journal of Molecular Biology
Assembly of Tight Junction Strands: Claudin-10b and Claudin-3 Form Homo-Tetrameric Building Blocks that Polymerise in a Channel-Independent Manner
Graphical abstract
Introduction
Tight junctions (TJs) regulate paracellular passage of solutes and water across epithelia and endothelia. TJs contain a panel of transmembrane proteins as well as cytoplasmic membrane-associated scaffolding and signalling proteins [1]. Strikingly, claudins constitute the backbone of TJ strands [2]. In recent years, several claudin crystal structures have become available [[3], [4], [5], [6]], and this has greatly enhanced our understanding of the structure of TJ proteins. However, the oligomeric architecture of claudin-based strands is still nebulous. Moreover, formation of paracellular barriers and/or pores depends on the claudin subtype, but the underlying structural principles are still unclear. It is assumed that polymeric claudin strands are formed by cis-interactions between claudin protomers in one membrane and trans-interactions between protomers in opposing membranes [[7], [8], [9]]. It was suggested that cis-oligomerisation occurs at least partly before trans-interaction triggers polymerisation into TJ strands [10,11]. On the basis of the Cldn15 crystal structure and experimental data, Suzuki et al. proposed a model for TJ strands containing paracellular channels [12]. The model consists of anti-parallel double rows of claudin protomers in each membrane, which are joined at cell–cell contacts (Joined anti-parallel Double–Row model, “JDR model”). However, concerns (e.g. C1–C3 below) about the validity of the model have been raised [13,14] due to inconsistencies. C1: The model leads to steric clashes at trans-interfaces, including the turn region of extracellular segment 2 (ECS2) and the loop between the β1- and β2-strand of ECS1 (β1β2loop). C2: The model does not contain cis-interaction between transmembrane helices (TMHs); this is rather surprising for oligomers of transmembrane proteins, especially since it has been suggested that TMHs are involved in claudin dimerisation [15]. For instance, TMH3 residues of Cldn3 and Cldn5 [16] and residues at the TMH1-ECS1 transition of Cldn10a/10b [17,18] have been found to affect TJ-strand formation. C3: Pore-lining residues in the JDR model of Cldn15 do not correspond with the accessibility of residues sensitive to thiol probes within the Cldn2 cation pore [13,19]. One possible explanation for discrepancies C2 and C3 might be that there are differences between Cldn2, Cldn10 and Cldn15 with respect to their protomer arrangement. Although this is conceivable, we hypothesised that the overall protomer arrangement is similar in the classic claudins (including Cldn2, Cldn10 and Cldn15), due to their high sequence homology, which implies mechanistic similarities [20]. Beside the JDR model [12], other potential protomer arrangements have been proposed that could lead to the formation of TJ strands and channels [[21], [22], [23], [24], [25], [26]].
The objective of the present study is to improve our understanding of the molecular architecture of claudin-based TJ strands and paracellular channels, using cellular reconstitution of TJ strands, structure-guided mutagenesis, in silico protein docking and oligomer modelling. We focussed on (i) Cldn10b (with high homology to Cldn15) as a prototypic channel-forming claudin and (ii) Cldn3 as a prototypic barrier-forming claudin [27]. The results indicate that classic claudins share a common overall oligomeric protomer arrangement, which represents a refinement of the JDR model suggested for Cldn15 [12]. Furthermore, we propose novel trans-interfaces, keys for structural compatibilities between claudin subtypes and a mechanism that can lead to formation of either channels or barriers.
Section snippets
Starting models of claudin homo-oligomers
For Cldn3 and Cldn10b protomers, homology models were generated using Cldn15 as template (PDB ID: 4P79 [3]) and sequence alignments (Figure S11). In silico sampling of potential protomer interfaces within Cldn3- or Cldn10b homo-oligomers was performed (e.g. with HADDOCK platform [28]). This suggested different potential cis- and trans-interfaces. However, none of these interfaces could be convincingly combined to cis-/trans-oligomers, which were fully consistent with published experimental
Discussion
The aim of this study was to improve the understanding of the molecular architecture of claudin-based TJ strands and paracellular channels (Figure 1). For Cldn15, a polymer model has been proposed, the joined double-row (JDR) model [12]. Reservations have been expressed about the validity of the JDR model, for instance, with respect to steric interference within the modelled oligomer (see C1 in Introduction). However, recent molecular dynamics (MD) simulation and protein–protein docking studies
Conclusion
We provide experimental and modelling data that enhance our understanding of the structural foundation of claudin-based formation of paracellular channels and barriers. In particular, we provide the following:
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Evidence for multistep claudin polymerisation, starting with cis-oligomerisation that is specific to the claudin subtype and extends by trans-interaction-triggered cis-polymerisation (Figure 9)
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Support for an anti-parallel double-row arrangement of protomers in Cldn10b- and Cldn3 strands,
Plasmids
Murine Cldn3-wt EYFP fusion constructs in pEYFP-N1 (Clontech, Mountain View, USA) have been described previously [41]. Cysteine residues were introduced resulting in Cldn3-A33C, -F34C, -I39C, -D67C, -L69C, -L70C, -N140C and R144C by site-directed mutagenesis as reported before [8]. N- and C-terminal CFP/YFP tagged human Cldn3 (in pcDNA3-YFP, pcDNA3-YFP, pEYFP-N1, pECFP-N1) [42], plasmids of pcDNA3-YFP/CFP-huCldn10b and -10a isoforms and corresponding TMH1/ECS1 chimeras -10b_a and -10a_b [18]
CRediT authorship contribution statement
C. Hempel: Investigation, Methodology, Writing original draft, Writing – review & editing. J. Protze:Investigation, Methodology, Visualization, Writing original draft, Writing – review & editing. E. Altun: Investigation. B. Riebe: Investigation. A. Piontek: Investigation, Methodology. A. Fromm: Investigation, Formal analysis. I.M. Lee: Investigation. T. Saleh: Investigation. D. Günzel: Conceptualization, Writing – review & editing. G. Krause: Supervision, Project administration, Funding
Acknowledgments
This research was funded by the Deutsche Forschungsgemeinschaft (DFG PI 837/4-1, DFG KR1273/8-1) and Promotionsabschlussstipendium of the Charité – Universitätsmedizin Berlin for Caroline Hempel. We thank Dr. Michael Fromm for critical reading of the manuscript.
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