Questions: Cell Polarity and Establishment of Asymmetry

Question 1

A researcher uses a genetic approach to eliminate aPKC activity in fully polarized epithelial cells. What is the most likely consequence for the apical and basolateral membrane domains?

• A. The apical domain expands and overruns the basolateral domain, since aPKC normally restrains apical identity
• B. Polarity collapses — without aPKC activity, the mutual antagonism that excludes basolateral proteins from the apical zone is lost, allowing the domains to mix
• C. The basolateral domain is lost but apical identity is maintained, since tight junctions physically prevent protein mixing after initial polarization
• D. Nothing changes immediately, because once tight junctions are established they are sufficient to maintain polarity without ongoing PAR signaling

Answer: (B)

aPKC (part of the Par3/Par6/aPKC complex) maintains polarity through active, ongoing phosphorylation: it phosphorylates Par1 and Lgl to exclude them from the apical domain. Without aPKC activity, this exclusion stops. Par1 and other basolateral determinants can invade the apical zone, while Par3 is no longer maintained away from the basolateral side. The polarity boundary collapses because it is dynamic — maintained by continuous mutual antagonism, not by physical structure alone. Tight junctions do create a fence, but they cannot prevent mixing when the upstream biochemical machinery that establishes distinct domains is disrupted.

Question 2

Which statement accurately describes the full role of tight junctions in maintaining epithelial cell polarity?

• A. Tight junctions establish polarity by recruiting PAR proteins to the apical domain during initial polarization
• B. Tight junctions function only as a paracellular barrier, preventing molecules from passing between adjacent cells
• C. Tight junctions serve both as a paracellular barrier between cells AND as a membrane fence that prevents apical proteins from drifting laterally into the basolateral domain
• D. Tight junctions act as scaffolds that anchor the PAR complex and prevent it from being degraded

Answer: (C)

Tight junctions have two distinct and equally important functions in epithelial polarity. First, as a paracellular barrier: composed of claudins, occludin, and JAMs, they stitch adjacent cells so tightly that even small molecules cannot pass between them — forcing transcellular transport and giving the epithelium control over what crosses. Second, as a membrane fence: by sitting at the boundary between apical and basolateral domains, tight junctions prevent membrane proteins from diffusing laterally from one domain to the other. Without this fence, even a well-established apical domain would gradually lose its identity as proteins mix. Option B is the common misconception — recognizing only the paracellular barrier function and missing the membrane fence role.

Question 3

Loss of epithelial cell polarity is associated with cancer metastasis because cells that lose their organized architecture can detach from their tissue, invade surrounding structures, and spread to distant sites.

• T. True
• F. False

Answer: True

This connection between polarity loss and cancer is mechanistically grounded. In epithelial-to-mesenchymal transition (EMT), cancer cells disrupt PAR signaling, downregulate tight junction components, and lose the apicobasal organization that anchors them to the epithelium. Without polarity, cells no longer maintain their position within the tissue layer, can degrade the basement membrane, and acquire migratory capacity. The same PAR proteins that establish developmental polarity are tumor suppressors — loss of Lgl, for example, correlates with aggressive cancer phenotypes. Polarity is therefore not merely an organizational feature but a determinant of tissue integrity.

Question 4

The PAR complex establishes apical cell polarity by attracting and converting basolateral proteins to an apical identity.

• T. True
• F. False

Answer: False

The PAR complex does not convert basolateral proteins — it excludes them through phosphorylation. aPKC phosphorylates Par1 and Lgl, causing them to be removed from the apical zone and confined to the basolateral domain. Reciprocally, Par1 phosphorylates Par3 to exclude it from the basolateral side. This is mutual antagonism, not conversion: each side actively kicks out the other's determinants. The mechanism creates a sharp, self-reinforcing boundary because each domain's machinery is continuously working to prevent the other from encroaching. Describing this as attraction or conversion fundamentally misrepresents how the polarity boundary is established and maintained.

Question 5

How does the mutual antagonism between apical and basolateral polarity determinants create a self-reinforcing boundary, and why is this mechanism more robust than simple physical separation would be?

Model answer: Mutual antagonism creates positive feedback: apical determinants (aPKC) phosphorylate and exclude basolateral determinants (Par1, Lgl), while basolateral determinants (Par1) phosphorylate and exclude apical determinants (Par3). Each domain's machinery actively maintains its own identity while attacking the other. This creates a bistable system — the boundary is sharp because any encroachment by one side triggers increased exclusion by the other. Physical separation alone (like a static fence) is passive and fragile; a membrane protein that diffuses across a physical barrier is not removed. Mutual antagonism is dynamic: even if a basolateral protein transiently reaches the apical zone, the aPKC present there phosphorylates and expels it, restoring the boundary without outside intervention.

The mutual antagonism model explains why polarity is so stable once established and why disrupting even one component can collapse it globally. It also explains why cell polarity requires continuous active maintenance — it is not a structure that persists passively, but a dynamic equilibrium maintained by competing molecular processes. This principle appears in many other biological contexts: bistable gene regulatory networks, cell fate decisions, and developmental patterning all use similar mutual exclusion logic to create sharp, stable boundaries.