Cell Biology Terms Starting With J
Cell Biology Glossary: J
Jump to Cell Biology Term
Junction Complex
/ JUNK-shun KOM-pleks / · Latin junctio, joining; Latin complexus, embrace
Junction complex is an organized assembly of distinct cell-cell junctions arranged in a defined sequence near the apical surface of epithelial cells to seal the paracellular space and maintain tissue integrity.
A junction complex typically consists of tight junctions, adherens junctions, and desmosomes arranged in a continuous band just below the apical membrane of epithelial cells. Tight junctions, built from claudin and occludin proteins, seal the paracellular space and restrict passage of molecules between cells. Adherens junctions use E-cadherin to mechanically link adjacent cells, while desmosomes anchor intermediate filaments and provide tensile strength through desmoplakin.
Each junction type contributes a distinct mechanical or barrier property, and disruption of any single component compromises the function of the entire assembly.
In the early 1960s, Marilyn Farquhar and George Palade used electron microscopy to identify and name the three components of the junction complex in intestinal epithelium, establishing the structural framework that cell biologists still use today. Their work revealed that what had appeared as a single dense band under light microscopy was actually three biochemically and functionally distinct structures.
One junction type handles every function at a cell border. The tight junction seals the paracellular space, the adherens junction provides mechanical coupling, and the desmosome resists tensile forces, and no single junction type can substitute for the others.
In the intestinal epithelium of mice, the apical junction complex restricts paracellular permeability so effectively that molecules larger than roughly 4 angstroms in diameter cannot pass between enterocytes. Disruption of claudin-1, a key tight junction protein, increases paracellular flux by more than tenfold and allows luminal bacteria to reach the lamina propria.
Junctional Adhesion Molecule
/ JUNK-shuh-nul ad-HEE-zhun MOL-eh-kyool / · From Latin junctionem meaning joining, adhaerere meaning to stick, and molecula meaning small mass.
Junctional adhesion molecule is a transmembrane immunoglobulin-family protein concentrated at tight junctions in epithelial and endothelial cells that mediates cell-cell adhesion and organizes the molecular scaffold of the tight junction.
The junctional adhesion molecule family includes JAM-A, JAM-B, JAM-C, JAM-4, and JAM-L, each carrying two extracellular immunoglobulin-like domains that form homophilic or heterophilic contacts between adjacent cells. JAM-A, the most extensively characterized member, accumulates at concentrations exceeding 100,000 molecules per epithelial cell and recruits scaffold proteins ZO-1 and PAR-3 to organize the tight junction complex and establish apical-basal polarity. Its cytoplasmic domain binds signaling adaptors that regulate cell proliferation and directed migration, extending its influence well beyond simple adhesion.
JAM proteins also bind certain viruses; reovirus docks onto JAM-A with nanomolar affinity, exploiting the junction as an entry point into epithelial tissue. During leukocyte transmigration, endothelial JAM-A and JAM-C form transient adhesive contacts that guide immune cells across blood vessel walls without permanently disrupting barrier integrity.
JAM-C shows a striking polarized distribution in epithelial cells, appearing exclusively on the apical side of tight junctions, while JAM-A localizes throughout the junction. This spatial segregation within a structure only nanometers wide suggests the two closely related proteins perform distinct regulatory functions at the same cell border.
Junctional adhesion molecules only mediate cell adhesion. Their cytoplasmic domains bind signaling adaptors that regulate cell polarity, proliferation, and directed migration, making them active participants in intracellular signaling rather than passive structural anchors.
In JAM-A knockout mice, intestinal epithelial barrier permeability increases measurably, and the animals develop significantly more severe colitis than wild-type controls when exposed to dextran sodium sulfate. Transepithelial electrical resistance, a standard measure of barrier tightness, drops by roughly 30 percent in JAM-A-deficient monolayers cultured in vitro.
Juxtacrine Signaling
/ JUK-stuh-krin SIG-nuh-ling / · From Latin juxta meaning near or beside, and Greek krinein meaning to separate or secrete.
Juxtacrine signaling is a form of cell-to-cell communication in which a membrane-bound ligand on one cell binds a receptor on a directly adjacent cell, requiring physical contact between the two cells.
Juxtacrine signaling operates exclusively between cells in immediate contact because the ligand remains anchored to the plasma membrane of the signaling cell rather than diffusing through extracellular space. The Notch-Delta pathway is the best-characterized example: Delta-like ligands on one cell bind Notch receptors on a neighboring cell, triggering two successive proteolytic cleavages, the second by gamma-secretase, that release the Notch intracellular domain to enter the nucleus and alter gene expression. During vertebrate embryogenesis, this mechanism drives lateral inhibition, a process in which one cell that begins differentiating toward a particular fate suppresses the same fate in its neighbors, generating precise cellular patterns from initially equivalent progenitors.
T cell activation also depends on juxtacrine contact, requiring direct interaction between CD28 on the T cell and B7 ligands on an antigen-presenting cell to deliver the co-stimulatory signal needed for full immune activation.
Juxtacrine signaling can be bidirectional: in Notch-Delta signaling, the Delta ligand undergoes its own intracellular processing after binding Notch, so both the sending and receiving cell experience changes in gene expression simultaneously. This mutual signaling was demonstrated experimentally by Sprinzak and colleagues in 2010 using single-cell reporters in mammalian cell culture.
Juxtacrine signaling is the same as gap junction communication. Gap junctions form cytoplasmic channels that pass small molecules directly between cells, whereas juxtacrine signaling involves membrane-bound ligand-receptor binding with no cytoplasmic continuity between the two cells.
During development of the fruit fly (Drosophila melanogaster) peripheral nervous system, juxtacrine Notch-Delta signaling selects individual cells from clusters of roughly 20 equivalent progenitors to become sensory organ precursors. The selected cell upregulates Delta expression within 2 to 3 hours, suppressing Notch activity in all immediate neighbors and preventing them from adopting the same fate.