Figure 5.

Dissociation of desmoplakin from the cardiac adhering junctions (areae compositae) in the absence of plakophilin 2. Electron (a and b) and immunoelectron (c–l) microscopy of ultrathin sections through intercalated disk junctions that connect cardiomyocytes of forming hearts in wt and plakophilin 2–deficient (pkp2−/−) E10.75 embryos. Desmosome-like structures (an example is seen in the top part of a) are not seen in the pkp2−/− embryos (b). Although the electron microscopic appearance of plaque-bearing junctions of the fascia adhaerens type is similar in both the wt and the mutant embryos (bottom part of a and b), the localization of desmoplakin (DP), as seen in immunoelectron micrographs (c–g, i, and j), is drastically altered: in the wt embryos, desmoplakin immunogold label is enriched in plaques of both morphotypes of adhering junctions (c and e), usually with higher label intensity at desmosome-like structures (e, compare the right hand junction with the fascia adhaerens–like junction in the left), including “fused” junctions with a practically continuous DP positivity (g). The same localization is seen for plakophilin 2 immunolabel in the wt hearts (h). By contrast, the pkp2−/− embryos show no desmoplakin enrichment at junctional plaques. Instead, DP label is dispersed over the cytoplasm, showing no significant enrichment at plaques (d and f), and is markedly enriched in dense granular aggregates of various sizes occurring throughout the cytoplasm, where they are interspersed between myofibrillar bundles (i) or in close association with loose arrays of IFs (j, for higher magnification see the inset). In contrast, other constituent proteins of these junctions are detected at comparable intensity in both wt and pkp2−/− embryos (k and l present the comparison for N-cadherin). Bars: 1 μm (b), 0.5 mm (a, c–g, i, and j), 0.2 μm (h, k, and l), and 0.1 mm (j, inset). Asterisk marks a cell junction devoid of desmoplakin.