The junctions that don't fit the scheme: special symmetrical cell-cell junctions of their own kind

Abstract

Immunocytochemical, electron-, and immunoelectron-microscopical studies have revealed that, in addition to the four major "textbook categories" of cell-cell junctions (gap junctions, tight junctions, adherens junctions, and desmosomes), a broad range of other junctions exists, such as the tiny puncta adhaerentia minima, the taproot junctions (manubria adhaerentia), the plakophilin-2-containing adherens junctions of mesenchymal or mesenchymally derived cell types including malignantly transformed cells, the composite junctions (areae compositae) of the mature mammalian myocardium, the cortex adhaerens of the eye lens, the interdesmosomal "sandwich" or "stud" junctions in the subapical layers of stratified epithelia and the tumors derived therefrom, and the complexus adhaerentes of the endothelial and virgultar cells of the lymph node sinus. On the basis of their sizes and shapes, other morphological criteria, and their specific molecular ensembles, these junctions and the genes that encode them cannot be subsumed under one of the major categories mentioned above but represent special structures in their own right, appear to serve special functions, and can give rise to specific pathological disorders.

Fig. 1

Double-label immunofluorescence (a, e-g) and electron (b, c) and immunoelectron (d) microscopy showing cell processes of cultured human mesenchymal stem cells (MSCs), originally isolated from bone marrow (a-d) or ovine cardiac valve matrix (e-g). a Note that some of the cell processes are extremely long. The giant process extending in the lower part, for example, exceeds 450 µm in length and forms adherens junctions (AJs) of the puncta adhaerentia minima type (PAM) with at least five other cells. The microfilament-rich cell process is immunostained for the actin-binding protein, ezrin (Ezrin, red), and the numerous AJs have reacted with antibodies specific for β-catenin (β-cat, green). b Electron micrograph of the overlapping contact region of two cytoplasmic MSC processes that partly overlap in the contact region (bracket). c Higher magnification of the contact region demarcated in b showing a series of extremely small PAM (arrows; e.g., the diameter of the junction denoted by the arrow right is below 40 nm). d Immunoelectron microscopy of a similar region as that shown in c showing an overlap contact of processes of two cells (a, b); the processes are studded with PAM decorated with silver-enhanced immunogold-label for β-catenin (arrowheads). For details, see Wuchter et al. (2007). e–g Clusters of AJs at the tips of cell processes of cardiac valvular interstitial cells as visualized by immunostaining with antibodies to N-cadherin (N-cad; for details, see Barth et al. 2009). N-cadherin-positive (red) AJs connecting valvular interstitial cells (green, vimentin) are present as terminal punctate clusters at the tips of filopodium-like processes (e.g., the segment shown bottom in e exceeds 100 µm in length). Note the clusters of small AJs connecting the central bodies of three valvular interstitial cells (f) and the relatively large region densely studded with AJs connecting the terminal portions of two cell processes (g). For details, see Barth et al. . Bars 100 µm (a), 2 µm (b), 0.5 µm (c), 0.2 µm (d), 25 µm (e, f), 20 µm (g)

Fig. 2

Double-label immunofluorescence microscopy (a, b) and conventional ultrathin section transmission electron microscopy (c) showing connections of mesenchymal human-bone-marrow-derived stem cells (MSCs), including filopodia-like cytoplasmic processes of widely variable lengths that either form direct intercellular bridges. a Note that the cell shown here is connected to five other cells or deeply and tight-fittingly inserts into plasma membrane invaginations of an adjacent cell (manubrium adhaerens). b A series of such manubrial-type junctions of widely variable lengths, including examples up to 50 μm long (e.g., top). Most of these taproot junction formations are almost continuously positive for N-cadherin (N-cad, red in a, b) and α-catenin (α-cat, green in b), resulting in the yellow merge color. The same structures are also positive for β-catenin (β-cat, green in a), protein p120 (not shown here), and cadherin-11 (see also Wuchter et al. 2007). c Electron micrograph of a section through such a deep invagination tightly filled with a cell process from a neighboring cell forming a continuous plaque-like dense cytoplasmic coat over the entire length. d Representation showing a cell-cell junction of the manubrium adhaerens type and the resulting interlocking structure. Note that this form of structure essentially represents an extended AJ structure in a special form (inset cross-sectional image). Note also the continuous plaque system in the whole region. For further details, see Wuchter et al. (2007). Bars 50 µm (a), 20 µm (b), 0.2 µm (c)

Fig. 3

Demonstration of the acquisition of plakophilin-2 (Pkp2) by some of the AJ-related cell-cell junctions between human mesenchymal cells in culture. a Double-label immunofluorescence microscopy of cultured human bone-marrow-derived mesenchymal cells (same culture as shown in Fig. 1a-d) immunostained for plakophilin-2 (red), in combination with the AJ protein, β-catenin (β-cat, green). Co-localization of the two plaque proteins appears in yellow in limited regions of some of the cell-cell contacts. b Plakophilin-2 also shows co-localization with the AJ-typical proteins, here with N-cadherin (N-cad, green), in cells of cultures of cardiac valvular interstitial cells of human origin. Bars 20 µm (a), 100 µm (b)

Fig. 4

Double-label immunofluorescence microscopy of cryostat sections through myocardium of an adult human heart, as seen after reactions with antibodies to desmoplakin (DP, green), in combination with antibodies to (red in each case) desmoglein 2 (Dsg2, a), N-cadherin (N-cad, b), or the plaque protein ARVCF (c). Only the merged color (yellow) is seen presenting near-complete colocalization in the composite junctions (areae compositae) of the intercalated disks and thus representing the amalgamated form containing both desmosomal and AJ proteins. Bars 20 μm

Fig. 5

Immunoelectron micrographs of sections through intercalated disks (IDs) of adult human heart. a Survey image showing the localization of a desmosomal protein, desmoplakin, by an immunogold-silver enhancement reaction in the entire ID plaque of the area composita. b Details of the intense plaque reaction in both small and large ID subdivisions. c An extended, continuous, completely plaque-covered, desmoplakin-rich junction (for details, see Franke et al. 2006). Bars 2 μm (a), 0.5 µm (b, c)

Fig. 6

Double-label immunofluorescence microscopy of cryostat sections through bovine lens tissue presenting details of the cortex adhaerens. A comparison of the reaction for N-cadherin (N-cad, red) with that for the prominently gap-junction-associated protein ZO-1 (a, green), the actin-filament-associated protein ezrin (b, green), and the AJ plaque protein α-catenin (c, α-cat, green). Only the merged images are shown. Note that here the ZO-1 reaction appears to be restricted to a limited region in the longer lateral wall, whereas N-cadherin and α-catenin are highly enriched at junction-like structures in the short wall elements. Ezrin is seen in the entire cell cortex. For further details, see Straub et al. (2003). Bars 10 μm

Fig. 7

Immunoelectron microscopy of ultrathin sections through the stratified squamous epithelium of bovine tongue mucosa (a-d) or a Hassall corpuscle of bovine thymus (e), as seen after reaction with antibodies to occludin. Immunogold label is not only seen in the uppermost living cell layer, the stratum granulosum-equivalent (for details see, e.g., Brandner et al. ; Langbein et al. , ; Schlüter et al. 2004), but also in inconspicuous interdesmosomal regions (arrows in a, b, d, e) and in special junctions (iuncturae structae) with an electron-dense middle layer (arrowheads in c, d). Tight junction (TJ) proteins are not restricted to typical TJs but at least some of them also occur in additional, yet insufficiently characterized junctions (D desmosomes). Bars 0.2 μm (a, b), 0.1 μm (c-e)

Fig. 8

Double-label immunofluorescence laser-scanning microscopy images of cryostat cross sections through human lymph nodes, showing the specific, mutually exclusive localization of VE-cadherin in the endothelium of small blood vessels (V) and the desmoplakin and α-catenin immunoreactions in the complexus adhaerentes of the endothelial and virgultar cells (SEVCs) of the sinus (S). a Colocalization of desmoplakin and VE-cadherin in the complexus adhaerentes of SEVCs cells in the sinus (S) can be seen with special clarity in the yellow merged image (VE-cadherin, red versus Desmoplakin, green). b Corresponding merged image showing co-localization (yellow) of desmoplakin (red) and α-catenin (green) at distinct small junctional structures (for details, see the review of Moll et al. 2009). Bars 50 μm