Lec 9 Part 2 - Cel Adhesions

What are the four primary tissue types in the body?

Muscle, nerve, connective, and epithelial tissues

Which tissue is the model for studying cell‑cell adhesions in this course?

Epithelial tissue

What are homotypic adhesions?

Adhesions between the same cell type (e.g., epithelial-to-epithelial)

What are heterotypic adhesions?

Adhesions between different cell types (e.g., neuron to muscle, leukocyte to endothelium)

Name the four major adhesion molecule families

Cadherins, integrins, selectins, and Ig‑superfamily CAMs

What ion do cadherins require for adhesion?

Calcium (Ca2+)

IWhich protein subunits bound together do integrins function?

As α/β heterodimers

What ions commonly regulate integrin binding activity?

Magnesium (Mg2+) or manganese (Mn2+)

What unique domain defines selectins and what do they bind?

A lectin‑like domain that binds carbohydrates in a Ca2+‑dependent manner

What structural hallmark defines Ig‑superfamily CAMs?

Extracellular immunoglobulin‑like domains

List the four functional classes of junctions in epithelia

Anchoring junctions, occluding (tight) junctions, channel‑forming (gap) junctions, and signal‑relaying junctions

What cytoskeleton do adherens junctions connect to?

Actin filaments

What cytoskeleton do desmosomes connect to?

Intermediate filaments (keratins in epithelia)

What do tight junctions primarily do?

Seal the paracellular space to control permeability and maintain polarity

What do gap junctions do?

Form channels for electrical and metabolic coupling via small molecule and ion passage

What transmembrane proteins mediate focal adhesions?

α/β integrin dimers

What cytoskeleton is linked in focal adhesions?

Actin filaments

Name key intracellular linker proteins in focal adhesions

Talin, vinculin, and α‑actinin

What ECM ligands commonly bind integrins at focal adhesions?

Fibronectin and vitronectin

What is the role of talin in integrin activation?

Talin binding induces integrin conformational change and clustering to strengthen adhesion

What is the canonical short peptide motif in ECM proteins that mediates integrin binding?

RGD (Arg‑Gly‑Asp)

Do all collagens use RGD to bind integrins?

No; collagens IV and VII bind via the MIDAS (metal ion‑dependent adhesion site) motif

What integrin anchors hemidesmosomes to the basal lamina?

α6β4 integrin

What cytoskeleton do hemidesmosomes connect?

Intermediate filaments (keratins)

Name key hemidesmosome intracellular linkers

BP180 (type XVII collagen), BP230, and plectin

What basal lamina ligand do hemidesmosomes bind?

Laminin

What is fibronectin’s key role in ECM?

Cross‑links ECM by binding integrins, collagen, and glycosaminoglycans; organizes cell–ECM adhesion

What is the structural organization of laminins and their role?

Heterotrimers (α, β, γ) that self‑assemble with nidogen and collagen IV to form basal lamina networks

What initiates leukocyte tethering and rolling on endothelium?

Selectin–carbohydrate interactions (low‑affinity, Ca2+‑dependent)

What mediates firm adhesion after rolling?

Leukocyte integrins binding Ig‑superfamily ligands (e.g., ICAM‑1/VCAM‑1) on endothelium

What is transmigration (diapedesis) and which molecules assist it?

Leukocyte passage between endothelial cells aided by PECAM‑1 and VE‑cadherin

Can leukocytes cross directly through endothelial cells?

Rarely; transcellular migration occurs but paracellular is most common

What maintains close membrane apposition at chemical synapses?

Heterotypic adhesion molecules including neurexin, neuroligin, integrins, and Ig‑family CAMs

What is the functional purpose of synaptic adhesion?

To create a confined cleft for efficient neurotransmitter release and receptor activation

What transmembrane proteins form adherens junctions?

Classical cadherins (e.g., E‑cadherin)

What cytoskeleton is linked by adherens junctions?

Actin filaments organized into an adhesion belt

Name key linker proteins at adherens junctions

β‑catenin, α‑catenin, and p120‑catenin (delta‑catenin)

What is p120‑catenin’s role?

Regulates cadherin stability and clustering, modulating adhesion strength

Why do cadherins require extracellular Ca2+?

Ca2+ stabilizes cadherin ectodomains enabling homophilic binding

How does the actin adhesion belt contribute to morphogenesis?

Myosin II–driven contraction enables apical constriction and epithelial folding

How do different cadherins drive cell sorting?

Homophilic preference (E–E, N–N) causes segregation during development and EMT/MET transitions

Which transcription factors regulate cadherin switching during EMT? (epithelial-mesenchymal transition)

Snail, Twist, and Slug

Where is E‑cadherin primarily found and what happens if it is absent in mice?

In many epithelia; knockout causes death at the blastocyst stage due to failed compaction

Where is N‑cadherin expressed and what phenotype occurs if it is absent?

Neurons, heart, skeletal muscle, lens, fibroblasts; knockout causes embryonic death from heart defects

Where is P‑cadherin expressed and what phenotype occurs if it is absent?

Placenta, epidermis, breast epithelium; knockout shows abnormal mammary development

Where is VE‑cadherin expressed and what phenotype occurs if it is absent?

Endothelial cells; knockout causes abnormal vascular development with endothelial apoptosis

What cadherins are in desmosomes?

Desmogleins and desmocollins

What is the primary function of desmosomes in epithelia?

Provide strong mechanical coupling via keratin IFs to maintain tissue integrity

Name the intracellular plaque/linker proteins in desmosomes

Plakoglobin (γ‑catenin), plakophilin, and desmoplakin

What intermediate filaments connect at desmosomes in epithelial cells?

Keratin intermediate filaments

Why do IF linkages provide superior tensile strength compared with actin linkages?

IFs have rope‑like, flexible architecture that dissipates mechanical stress

Which epidermal layer forms hemidesmosomes, and which forms desmosomes?

Basal layer forms hemidesmosomes; desmosomes are present across layers for cohesion

What is the principal barrier function of tight junctions?

Prevent paracellular diffusion, maintaining distinct apical vs. basolateral environments

Name the core transmembrane tight junction proteins

Claudins, occludin, and JAMs

What are the main cytoplasmic scaffold proteins at tight junctions?

ZO‑1, ZO‑2, and ZO‑3 linking to actin/spectrin/microtubules

Where are tight junction sealing strands located relative to the apical surface?

At the apical‑lateral border as continuous sealing strands

How can some viruses exploit tight junctions?

By binding CAR, JAM, occludin, or claudins to disrupt barriers and increase leak

What proteins build gap junction channels?

Connexins assembling as hexameric connexons

What is the pore diameter of gap junction channels and what passes through?

~1.5 nm; small molecules such as ions, sugars, amino acids, fatty acids, and small drugs

How do connexons form functional channels between cells?

A connexon from each neighboring cell docks to create a continuous channel

Can connexons be homomeric (all same type) or heteromeric?

Yes; connexons may contain one or multiple connexin isoforms

What is the shared organizational logic of all adhesion junctions?

Transmembrane adhesion protein + cytoplasmic linker/plaque proteins + cytoskeletal attachment

Which junction uses cadherins to link to actin, and which uses desmosomal cadherins to link to IFs?

Adherens junctions link cadherins to actin; desmosomes link desmoglein/desmocollin to IFs via plak proteins

Which ECM adhesion links integrins to actin vs. to IFs?

Focal adhesions link to actin; hemidesmosomes link to intermediate filaments

What principal components form the basal lamina network?

Laminin, nidogen, collagen IV, and large proteoglycans forming cross‑linked networks

What do laminins bind to in basal lamina assembly?

Integrins, proteoglycans, and nidogen, and they self‑assemble into networks

Where are integrins before activation and how are they activated?

Distributed on the membrane and recycled; inside‑out signals induce talin binding and cation coordination for activation

What experimental peptide can competitively inhibit integrin–ECM binding?

RGD peptides, which detach cells by competing with ECM ligands

What is a clinical concept for RGD‑based strategies?

Disrupt tumor cell adhesion/migration to inhibit metastasis

Besides linkage, what are two additional roles of catenins at adherens junctions?

Signaling to the cytoskeleton (e.g., via Rho GTPases) and regulating cadherin clustering/affinity

What broader signaling pathway is β‑catenin involved in?

Wnt signaling, linking adhesion to gene regulation

Clinical: What happens to epithelial sheets without functional IF–desmosome linkage under stretch?

Cells rupture and the sheet tears, causing fragility disorders

Clinical: What diseases result from keratin mutations affecting desmosomal anchorage?

Epidermolysis bullosa simplex with skin blistering

Clinical: What desmosomal gene mutations are associated with palmoplantar keratoderma? (thickening of skin onpalms of hands and soles of feet)

Mutations in keratins and plakoglobin can cause palmoplantar keratoderma

Clinical: Which desmosomal proteins are implicated in ARVC/D and what organ is affected?

Plakophilin‑2, desmocolin‑2, desmoglein‑2, and desmoplakin; the heart (arrhythmogenic right ventricular cardiomyopathy)

Clinical: Why might skin blistering co‑present with woolly hair and cardiomyopathy?

Shared use of keratins and desmosomal proteins in skin, hair, and cardiac intercalated discs

Clinical: What autoantibody mechanism can cause skin blistering?

Autoantibodies against desmosomal components disrupt adhesion

Clinical: How can adenovirus and coxsackievirus alter epithelial barriers?

By binding tight junction components (e.g., CAR/JAM/occludin/claudin) to induce opening and leak

Clinical: Which connexin mutations are linked to Charcot‑Marie‑Tooth disease? (hereditary neurological disorder that affects the peripheral nerves, leading to muscle weakness, atrophy, and sensory loss, especially in the hands and feet.)

Cx32 mutations

Clinical: Which connexins are associated with nonsyndromic deafness?

Cx26, Cx30, Cx31, Cx32, and Cx43

Clinical: Which connexins are linked to cataracts?

Cx46 and Cx50

Clinical: How can connexin defects affect the heart?

They can cause cardiomyopathy via impaired electrical coupling of myocytes

What tracer experiment demonstrates tight junction sealing?

Apical tracer fails to enter the paracellular space; basal tracer fills intercellular space but does not cross apically

How do tight junctions support transepithelial transport and polarity?

By blocking paracellular diffusion, enabling vectorial transport via apical/basolateral transporters

Where is the actin adhesion belt typically located?

In the upper half of the lateral membrane beneath the apical surface

What motor drives adhesion belt constriction?

Myosin II sliding antiparallel actin filaments

What makes selectin–carbohydrate adhesion low affinity but effective for rolling?

Electrostatic interactions enable transient, reversible tethering under flow

What are endothelial Ig‑superfamily ligands for leukocyte integrins?

ICAM‑1 and VCAM‑1

Which junction is prominent in epithelia vs. mesenchymal cells: hemidesmosome or focal adhesion?

Hemidesmosomes in epithelia; focal adhesions in mesenchymal cells

Which linker bridges integrins to IFs in hemidesmosomes and to actin in focal adhesions?

Plectin bridges to IFs; talin/vinculin/α‑actinin bridge to actin

How do cadherins achieve strong adhesion despite low individual affinity?

Lateral packing and clustering create high‑avidity interfaces coupled to the cytoskeleton

What distance separates opposing membranes in cadherin adhesion?

Approximately 38.5 nm between opposing cadherin N‑termini

How does switching from E‑ to N‑cadherin affect tissue morphogenesis?

Drives cell sorting and segregation (e.g., neural tube formation) via homophilic specificity

Which cells express VE‑cadherin and why is this important?

Endothelial cells; it marks vascular adherens junctions distinct from epithelial ones

Which junction uses ZO‑1/2/3 as scaffolds?

Tight junctions

Which junction uses plakoglobin and plakophilin?

Desmosomes

Which junction lacks a cytoskeletal linkage yet permits intercellular diffusion?

Gap junctions (no cytoskeletal linkage)

What EM feature helps identify desmosomes?

Dense cytoplasmic plaques with keratin IFs inserting

What freeze‑fracture feature identifies tight junctions?

Continuous sealing strands at the apical‑lateral border

What is the phenotype of E‑cadherin knockout mice?

Lethal at blastocyst stage due to failed compaction