Questions: Cell Junctions: Adhesion and Communication

Question 1

A drug blocks a specific membrane channel that allows small ions and second messengers to pass directly between adjacent cardiac muscle cell cytoplasms. The cardiac muscle cells begin to contract asynchronously. Which junction type is being disrupted?

• A. Tight junctions, because they control the movement of molecules between cells
• B. Desmosomes, because they link the cytoskeletons of adjacent cardiac cells
• C. Gap junctions, because connexon channels electrically couple cardiac cells by allowing current to spread directly between cytoplasms
• D. Hemidesmosomes, because they anchor cardiac cells to the extracellular matrix

Answer: (C)

Gap junctions are composed of connexin proteins that form connexon channels docking between adjacent cells. These channels allow ions (including K⁺ and Ca²⁺) and small second messengers to pass directly from one cytoplasm to the next, spreading electrical depolarization without requiring neurotransmitter release. Cardiac synchrony depends critically on this electrical coupling — blocking gap junctions severs the cell-to-cell current flow and disrupts coordinated contraction. Tight junctions control paracellular flow (between cells, not through them), and desmosomes provide mechanical anchoring, not electrical communication.

Question 2

Patients with pemphigus vulgaris develop severe skin blistering because autoimmune antibodies attack desmosomal cadherins (desmogleins). This outcome best illustrates which property of desmosomes?

• A. Desmosomes form the primary barrier preventing paracellular passage of toxins through skin
• B. Desmosomes anchor intermediate filament networks of adjacent cells into a continuous mechanical network; their disruption destroys the tissue's ability to resist shear and tensile forces
• C. Desmosomes couple skin cells electrically, and their loss desynchronizes the shedding cycle
• D. Desmosomes attach the basal layer of skin to the basement membrane, and their loss causes cells to detach from below

Answer: (B)

Desmosomes act as spot-welds between cells, with cadherin family proteins (desmogleins, desmocollins) linking the intermediate filament cytoskeletons of neighboring cells. This creates a continuous mechanical network that distributes tensile and shear forces across the entire epithelium. When autoantibodies in pemphigus target desmogleins, the rivets are dissolved — adjacent cells lose their attachment to each other and the tissue tears apart under normal mechanical stress, causing blisters. This is a direct demonstration that desmosomes are load-bearing structures essential for tissue integrity under stress, not just 'sticky proteins.'

Question 3

Tight junctions form an absolute impermeable seal between epithelial cells, preventing most transport between cells.

• T. True
• F. False

Answer: False

Tight junctions are selectively permeable, not absolutely impermeable. Different epithelial tissues express different claudin isoforms that create junctions of varying tightness. The kidney tubule, for example, strategically places 'leaky' tight junctions (expressing claudins that allow paracellular ion flow) in segments where passive reabsorption is needed, and 'tight' junctions in segments requiring precise concentration control. The intestinal epithelium uses tight junctions to prevent most luminal contents from entering the bloodstream while still permitting regulated paracellular transport. 'Tight' is a relative term — the defining feature is selectivity and controlled permeability, not absolute impermeability.

Question 4

Gap junctions primarily serve mechanical adhesion between neighboring cells, which is why their disruption in cardiac tissue impairs the structural integrity of the heart wall.

• T. True
• F. False

Answer: False

Gap junctions serve communication, not mechanical adhesion. They allow ions, small second messengers, and metabolites to pass directly between cytoplasms — this is what electrically couples cardiac cells for synchronized contraction. Mechanical integrity in the heart is provided by desmosomes (spot-welding intermediate filaments of adjacent cells) and adherens junctions (actin-linked belts). If gap junctions in cardiac tissue were disrupted, the structural integrity would be largely unaffected, but the cells would lose electrical coupling and contract asynchronously — which is exactly what happens in some arrhythmia-associated gap junction diseases.

Question 5

How does loss of E-cadherin function contribute to cancer metastasis, and why does this make mechanistic sense given E-cadherin's role in adherens junctions?

Model answer: E-cadherin in adherens junctions physically links adjacent epithelial cells into a continuous sheet via actin cytoskeleton connections, and also suppresses pro-migratory signaling intracellularly. When E-cadherin is lost (through mutation, epigenetic silencing, or proteolytic cleavage), cells lose both physical adhesion to neighbors and the restraining signal. This triggers epithelial-to-mesenchymal transition: cells adopt a migratory phenotype, detach from the epithelial layer, and invade surrounding tissue. Loss of E-cadherin is thus not merely a loss of 'glue' — it simultaneously removes a structural anchor and a tumor suppressor signal.

E-cadherin loss is one of the most common molecular events in epithelial cancer progression. It is a canonical hallmark of the transition from carcinoma in situ (confined) to invasive cancer. Understanding it mechanistically — as disruption of a junction that integrates both structural and signaling functions — explains why simple adhesion mutants (which only affect stickiness) do not fully recapitulate metastasis, while E-cadherin loss does: the full phenotypic consequence requires loss of both the physical linkage and the downstream signaling suppression.