Desmosome dualism – most of the junction is stable, but a plakophilin moiety is persistently dynamic

Desmosomes, strong cell–cell junctions of epithelia and cardiac muscle, link intermediate filaments to cell membranes and mechanically integrate cells across tissues, dissipating mechanical stress. They comprise five major protein classes – desmocollins and desmogleins (the desmosomal cadherins), plakoglobin, plakophilins and desmoplakin – whose individual contribution to the structure and turnover of desmosomes is poorly understood.

In this article, live-cell imaging together with fluorescence recovery after photobleaching (FRAP) and fluorescence loss and localisation after photobleaching (FLAP) is used to show that desmosomes consist of two contrasting protein moieties or modules: a very stable moiety of desmosomal cadherins, desmoplakin and plakoglobin, and a highly mobile plakophilin (Pkp2a).

As desmosomes mature from Ca²⁺ dependence to Ca²⁺-independent hyper-adhesion, their stability increases, but Pkp2a remains highly mobile. It is shown that desmosome downregulation during growth-factor-induced cell scattering proceeds by internalisation of whole desmosomes, which still retain a stable moiety and highly mobile Pkp2a. This molecular mobility of Pkp2a suggests a transient and probably regulatory role for Pkp2a in desmosomes.

Ca²⁺-dependent desmosomes exhibit differential protein dynamics.

(A) Representative time-lapse images showing FRAP in MDCK cells transfected with neonGreen-labelled desmoglein 2 (Dsg2), desmocollin 2a (Dsc2a), plakoglobin (PG) and plakophilin 2a (Pkp2a), and eGFP tagged desmoplakin (DP). The cells were cultured at a confluent cell density for 1 d before recording, with the majority of desmosomes being Ca2+ dependent. Images before photobleaching (Pre), directly after bleaching (0 s) and at 300 s post bleaching are presented using a colour-coded fluorescence intensity scale (A.U., arbitrary units). Graphs show the line profile fluorescence intensity plots of the bleached ROIs (dashed box at 0 s) over time, which is indicated using a colour-coded time scale. Scale bars: 5 μm. Note the recovery of neonGreen–Pkp2a fluorescence in similar spots to those present pre-bleaching. (B) Graphs show the mean fluorescence recovery curves for all recorded desmosomes (mean±s.e.m.; n=24–47, N=3). Dashed red lines show the recovery of paraformaldehyde-fixed samples (reversible photobleaching). (C) Mean mobile fraction values for the indicated desmosomal proteins in MDCK cells (n=24–47, N=3). The box represents the 25–75th percentiles, and the median is indicated in red. The whiskers show the range of values. **P≤0.01; *P≤0.05 (Kruskal–Wallis with Dunn’s multiple comparisons test).

The full article can be accessed here.

¹Wellcome Trust Centre for Cell-Matrix Research, University of Manchester, Manchester M13 9PT, UK. ²Skin Research Institute of Singapore, Agency of Science Technology and Research (A*STAR), 8A Biomedical Grove, #06-06 Immunos, 138648 Singapore, Singapore. ³Institute of Medical Biology, Agency of Science Technology and Research (A*STAR), 61 Biopolis Dr, 138673 Singapore, Singapore. ⁴A*STAR Microscopy Platform, Research Support Centre, Agency ofScience Technology and Research (A*STAR), Biopolis 138673 Singapore, Singapore.