Tight junctions and gap junctions · Chapter 10

Anchoring junctions hold cells together, but they don’t stop things from slipping through the cracks between cells, and they don’t let cells share their contents. Animals use two other junctions for those jobs: one seals, the other connects.

Tight junctions seal a cell layer

The lining of your intestine is a sheet just one cell thick. One side of each cell faces the lumen (the hollow inside of the intestine, where food passes), and the other side faces the ECM and a blood vessel. If material could seep between the cells, the contents of your gut would leak straight into your blood, and blood components would leak out into your gut.

Tight junctions prevent that. They form a tight seal between neighboring cells so material can’t leak across the cell layer between them.

The seal is made of two membrane-spanning proteins, occludin and claudin. In each cell, they form strands woven through the plasma membrane like a net. The strands of one cell bind to the strands of the next, zipping the two membranes together.

Tight junctions are not mechanically strong, because they lack firm connections to the cytoskeleton. So cells joined by tight junctions also have anchoring junctions to hold them in place.

Three jobs of tight junctions

Prevent leaks. In the intestine, they stop materials from passing between the lumen and the blood in either direction.
Keep the cell’s two sides different. Membrane proteins can drift sideways, and tight junctions act as a fence that stops them. The side facing the lumen is the apical side; the side facing the blood is the basolateral side. Proteins for taking material in (receptor-mediated endocytosis) stay apical, and proteins for releasing it (exocytosis) stay basolateral. This difference between the sides is cell polarity. It lets intestinal cells absorb nutrients from the gut and release them into the blood, moving material across the cell layer, which is called transepithelial transport.
Block microbes. Skin and the lining of the digestive tract are sealed by tight junctions. Some disease-causing microbes, including some that cause diarrhea, infect by breaking these seals.

Testing the seal

Researchers showed how well tight junctions work by injecting a rat’s bloodstream with lanthanum, a metal that shows up dark under the electron microscope. The lanthanum seeped into the spaces between intestinal cells on the blood side, but stopped dead at the tight junction and never reached the lumen side.

Gap junctions connect cells directly

A gap junction is named for the small gap left between the plasma membranes of the two cells it connects. Gap junctions are clusters of tiny tunnels that join the cytosols of neighboring cells, so small things can pass straight from one cell to the next.

The tunnels are built from a membrane protein. In vertebrates it is connexin; invertebrates use a similar protein called innexin.

Six connexin proteins in one cell’s plasma membrane form a ring with a channel in the middle. This ring is a connexon.
A connexon in one cell lines up with a connexon in the neighboring cell, forming one continuous channel between the two cells.
Many of these paired connexons cluster together in the membrane. That cluster is the gap junction.

Two neighboring cell membranes separated by a narrow gap, joined by pairs of connexons that line up to form channels. A close-up shows one connexon as a ring of connexin proteins; small ions and molecules pass through the channel while a large protein is blocked — Figure 10.5 Each channel is two connexons, one from each cell, joined across the gap. Small molecules pass through; large ones are turned away.

What fits through

Gap junction channels pass ions such as Ca²⁺ and small molecules such as amino acids, sugars, and signaling molecules like cAMP. They are too small for RNA, proteins, or polysaccharides. So connected cells can share small metabolites and signal each other directly, yet each still keeps its own set of large molecules.

The size limit is usually stated in daltons (Da), a unit of mass for molecules; one dalton is about the mass of a hydrogen atom.

Researchers found the cutoff with glowing dyes. They grew a single layer of rat liver cells, which formed gap junctions with each other, and injected one cell with a fluorescent dye of known mass. Under a fluorescence microscope, they then checked whether the glow spread into the neighboring cells. Repeating this with dyes of different masses gave a clear pattern:

Dyes up to about 900 Da spread to neighboring cells.
Heavier dyes stayed in the injected cell.

Later work in other cell types and species found some variation, but molecules heavier than about 1000 Da generally don’t pass.

Why the heart needs them

Gap junctions are abundant wherever cells must act together. Heart muscle cells are joined by many of them. Because ions flow through, an electrical change in one heart cell spreads quickly to its neighbors, so the cells contract together as a coordinated beat.

Check yourselfWhat would go wrong if the cells lining your small intestine had no tight junctions?

Substances in the gut, including any toxins you swallowed, could leak between the cells directly into your blood, and materials in your blood could leak out into the intestine. The cells would also lose their cell polarity, so absorbing nutrients and passing them into the blood would break down.