The integrity and permeability of epithelial and endothelial barriers depend on the formation of tight junctions, adherens junctions, and a junction-associated cytoskeleton. The establishment of this junction–cytoskeletal module relies on the correct folding and oligomerization of its protein components. Molecular chaperones are known regulators of protein folding and complex formation in different cellular compartments. Mammalian cells possess an elaborate chaperone network consisting of several hundred chaperones and co-chaperones. Only a small part of this network has been linked, however, to the regulation of intercellular adhesions, and the systematic analysis of chaperone functions at epithelial and endothelial barriers is lacking. This review describes the functions and mechanisms of the chaperone-assisted regulation of intercellular junctions. The major focus of this review is on heat shock protein chaperones, their co-chaperones, and chaperonins since these molecules are the focus of the majority of the articles published on the chaperone-mediated control of tissue barriers. This review discusses the roles of chaperones in the regulation of the steady-state integrity of epithelial and vascular barriers as well as the disruption of these barriers by pathogenic factors and extracellular stressors. Since cytoskeletal coupling is essential for junctional integrity and remodeling, chaperone-assisted assembly of the actomyosin cytoskeleton is also discussed.

Intestinal epithelial barrier integrity is essential for human health, and its disruption induces and exacerbates intestinal inflammatory disorders. While the epithelial cytoskeleton is critical for maintaining gut barrier-integrity, the role of septins — a family of GTP-binding, cytoskeletal proteins — is largely unknown. This highlights an important knowledge gap, as dysfunction of septins, and specifically septin 9 (SEPT9), is associated with intestinal pathologies. We determined that SEPT9 localizes to the apical junctions of intestinal epithelial cells (IECs), overlapping with both tight and adherens junctions. IEC-specific ablation of SEPT9 in mice resulted in leaky gut, due to mislocalization of junctional proteins, and increased susceptibility to experimental colitis. Consistently, SEPT9 expression was significantly reduced in intestinal mucosa of patients with inflammatory bowel disease (IBD). Using affinity-purification mass spectrometry, super-resolution imaging, and genetic KO, we determined that SEPT9 interacts with and is necessary to recruit nonmuscle myosin IIC (NMIIC) to the IEC perijunctional actomyosin belt. Loss of NMIIC also caused IEC barrier disruption. In summary, SEPT9 regulates intestinal barrier integrity by supporting the assembly of tight and adherens junctions through NMIIC recruitment to the actomyosin belt. The septin cytoskeleton safeguards the intestinal mucosa during acute inflammation, and its disruption in IBD suggests a loss of this protective function.