Desmosomes are adhesive intercellular junctions that provide strength and integrity to the epidermis. However, little is known about how desmosomes are integrated with subcellular organelles essential for epidermal structure and differentiation. Using electron microscopy and live-cell fluorescence imaging, we found that peripheral ER tubules associate with desmosomal cell-cell contacts in a variety of cultured epithelia and tissues, including skin. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and 3D reconstructions at nanometer resolution revealed intricate associations of ER tubules with the desmosome plaque. Keratin filaments, which anchor to desmosomes, aligned and intertwined with ER tubules on either side of the desmosome. Manual and semi-automated segmentation of volume EM data revealed mirror image-like arrangements of ER and keratin at virtually all mature desmosomes. Live-cell imaging demonstrated that ER tubules, which are normally highly dynamic, were immobilized at junctional desmosomes, both during steady-state conditions and dynamic processes like desmosome fusion. Further, ER at cell-cell borders remained anchored to desmosomes following microtubule depolymerization, whereas ER at cell-free edges retracted toward the cell interior. Genetic disruption of desmosomes altered ER morphology and dynamics. Additionally, expression of a keratin 14 mutant that causes the epidermal blistering disease, epidermolysis bullosa simplex, shifted ER morphology from tubular to sheet-like. These data demonstrate that desmosomes and keratins are key regulators of ER organization and mobility. Our results reveal a previously unknown subcellular complex comprising the ER, desmosomes, and keratin filaments, and expose an unappreciated therapeutic target for desmosomal and keratin skin diseases.

The endoplasmic reticulum (ER) is an expansive, membrane-enclosed organelle that plays crucial roles in numerous cellular functions. We used emerging superresolution imaging technologies to clarify the morphology and dynamics of the peripheral ER, which contacts and modulates most other intracellular organelles. Peripheral components of the ER have classically been described as comprising both tubules and flat sheets. We show that this system consists almost exclusively of tubules at varying densities, including structures that we term ER matrices. Conventional optical imaging technologies had led to misidentification of these structures as sheets because of the dense clustering of tubular junctions and a previously uncharacterized rapid form of ER motion. The existence of ER matrices explains previous confounding evidence that had indicated the occurrence of ER "sheet" proliferation after overexpression of tubular junction-forming proteins.