Cell-Cell Junctions and Cadherins 2
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Questions and Answers

What is the primary function of cell–cell anchoring junctions?

  • To facilitate nutrient exchange
  • To promote cell division
  • To allow rapid communication between cells
  • To resist external forces that pull cells apart (correct)

    • Cadherins are present in fungi and plants.

    False

    What happens to cadherin-mediated adhesions if Ca2+ ions are removed from the extracellular medium?

    They come apart.

    E-cadherin is primarily found on many types of _______ cells.

    <p>epithelial</p> Signup and view all the answers

    Which of the following statements about nonclassical cadherins is correct?

    <p>They include protocadherins and are expressed in the brain.</p> Signup and view all the answers

    Cadherins are named for their dependency on _______ ions.

    <p>Ca2+</p> Signup and view all the answers

    Match the type of cadherin with its primary location:

    <p>E-cadherin = Epithelial cells N-cadherin = Nerve and muscle cells P-cadherin = Placenta and epidermis Protocadherins = Brain</p> Signup and view all the answers

    What type of junctions are best understood in cell–cell anchoring?

    <p>Cadherin-based junctions.</p> Signup and view all the answers

    How many members does the cadherin superfamily have in humans?

    <p>180</p> Signup and view all the answers

    Cadherins primarily bind heterophilically between cells.

    <p>False</p> Signup and view all the answers

    What type of interaction do cadherins typically mediate?

    <p>Homophilic adhesion</p> Signup and view all the answers

    The extracellular cadherin domain allows for _____________ binding.

    <p>homophilic</p> Signup and view all the answers

    Match the following cadherin features with their descriptions:

    <p>Classical cadherin = Contains 5 extracellular domains Desmogleins = Contains 4 to 5 extracellular domains Nonclassical cadherins = Can have more than 30 extracellular domains T-cadherin = Lacks a transmembrane domain</p> Signup and view all the answers

    What maintains the rigidity of cadherin domains?

    <p>Calcium ions</p> Signup and view all the answers

    The binding affinity of cadherins is generally high compared to receptors for soluble signaling molecules.

    <p>False</p> Signup and view all the answers

    What effect does the removal of Ca2+ ions have on cadherins?

    <p>Increases flexibility of hinge regions</p> Signup and view all the answers

    Cadherin molecules use a ____________ principle to hold cells together.

    <p>Velcro</p> Signup and view all the answers

    What role do adaptor proteins play in cadherin function?

    <p>Connect cadherins to the cytoskeleton</p> Signup and view all the answers

    All cadherins have an intracellular portion with the same structure.

    <p>False</p> Signup and view all the answers

    How many copies of the extracellular cadherin domain are typically present in classical cadherins?

    <p>5 copies</p> Signup and view all the answers

    The interaction between cadherin domains occurs at the ____________ of the molecules.

    <p>N-terminal tips</p> Signup and view all the answers

    Match the parts of cadherin interactions with their functions:

    <p>Hinge regions = Allow flexibility N-terminal tips = Site of binding Cadherin clusters = Increase adhesion strength Ca2+ ions = Maintain rigidity</p> Signup and view all the answers

    What was the significance of the experiments conducted in the 1950s on amphibian embryos?

    <p>They showed selective cell–cell recognition systems.</p> Signup and view all the answers

    Cadherins are only important during the embryonic development stage.

    <p>False</p> Signup and view all the answers

    What type of cadherin do neural tube cells lose during the formation of the neural tube?

    <p>E-cadherin</p> Signup and view all the answers

    Cells from different layers of an early amphibian embryo will sort out according to their ______.

    <p>origins</p> Signup and view all the answers

    Match the following cadherins with their associated function or location:

    <p>E-cadherin = Expressed by ectoderm cells N-cadherin = Associated with neural tube cells Cadherin 7 = Found during neural crest cell migration Cadherins = Enable homophilic cell binding</p> Signup and view all the answers

    What occurs when L cells are transfected with DNA encoding E-cadherin?

    <p>Cell–cell adhesion occurs.</p> Signup and view all the answers

    Cortical tension within a cell is low when it is unattached.

    <p>True</p> Signup and view all the answers

    What happens to cadherin expression as neural crest cells migrate away from the neural tube?

    <p>The expression of cadherins becomes scarcely detectable, and cadherin 7 appears.</p> Signup and view all the answers

    Strong cell–cell adhesions require changes in the ______ cytoskeleton.

    <p>actin</p> Signup and view all the answers

    Match the following cell types with their associated cadherins:

    <p>Mesoderm cells = Green Neural plate cells = Blue Epidermal cells = Red Neural crest cells = Cadherin 7</p> Signup and view all the answers

    What role do cadherins play during tissue segregation?

    <p>They facilitate strong cell adhesion.</p> Signup and view all the answers

    Cells expressing different amounts of the same cadherin will not segregate.

    <p>False</p> Signup and view all the answers

    What happens during the assembly of adherens junctions?

    <p>Decreases in cortical tension and changes in the actin cytoskeleton are required.</p> Signup and view all the answers

    Cells are known to reassociate into structures resembling their original ______.

    <p>embryo</p> Signup and view all the answers

    What is the primary role of adherens junctions in relation to mechanical forces?

    <p>Tension sensing and force balance</p> Signup and view all the answers

    Adherens junctions are merely passive attachments between cells.

    <p>False</p> Signup and view all the answers

    What occurs to α-catenin when contractile activity increases at the junction?

    <p>α-catenin is stretched from a folded to an extended conformation.</p> Signup and view all the answers

    The process by which adherens junctions adapt to tension involves proteins in the ___________ complex.

    <p>cadherin</p> Signup and view all the answers

    Match the following components of adherens junctions with their functions:

    <p>α-catenin = Exposes binding site for vinculin when stretched Vinculin = Promotes actin recruitment Actin filaments = Provide structural support Myosin II = Generates contractile forces</p> Signup and view all the answers

    How does pulling on a junction affect connected cells in a tissue?

    <p>It increases contractile force in the attached cell.</p> Signup and view all the answers

    What process allows cells to insert themselves between other cells, resulting in a longer and narrower epithelium?

    <p>Intercalation</p> Signup and view all the answers

    Adherens junctions primarily serve to anchor cells in place without any mechanotransductive capabilities.

    <p>False</p> Signup and view all the answers

    Desmosomes are important for providing mechanical strength in epithelial tissues.

    <p>True</p> Signup and view all the answers

    In which type of tissue do adherens junctions often form a continuous adhesion belt?

    <p>Epithelial tissue</p> Signup and view all the answers

    In Drosophila, contraction of cells in the __________ results in mechanical stresses that influence wing development.

    <p>hinge</p> Signup and view all the answers

    What cellular mechanism is suggested to weaken adhesion during cell contraction?

    <p>Clathrin-mediated endocytosis</p> Signup and view all the answers

    Desmosomes are linked to __________ filaments in most epithelial cells.

    <p>keratin</p> Signup and view all the answers

    Which protein is revealed as a binding site for vinculin when α-catenin is unfolded?

    <p>A cryptic binding site on α-catenin</p> Signup and view all the answers

    Match the following components with their functions:

    <p>Cadherins = Cell adhesion Intermediate filaments = Structural strength E-cadherin = Epithelial tissue adhesion Clathrin = Endocytosis mechanism</p> Signup and view all the answers

    Actin filaments in epithelial cells only provide structural support and do not engage in any contractility.

    <p>False</p> Signup and view all the answers

    What is the significance of vinculin in the context of adherens junctions?

    <p>Vinculin promotes the recruitment of actin to strengthen the junction.</p> Signup and view all the answers

    Which of the following statements is true regarding tight junctions?

    <p>They form a seal between cells.</p> Signup and view all the answers

    The coordinated contraction of actin–myosin bundles is essential for the __________ of epithelial-cell sheets.

    <p>folding</p> Signup and view all the answers

    Drosophila has desmosomes in its epithelial tissues.

    <p>False</p> Signup and view all the answers

    What is a potential consequence of antibodies binding to desmosomal cadherins in affected individuals?

    <p>Severe blistering of the skin</p> Signup and view all the answers

    What effect does mechanotransduction have on epithelial tissue?

    <p>It leads to rapid signaling for modifying cell behavior.</p> Signup and view all the answers

    The __________ side of the epithelial tissue is attached to other tissues.

    <p>basal</p> Signup and view all the answers

    What type of filaments are desmosomes attached to in heart muscle cells?

    <p>Desmin filaments</p> Signup and view all the answers

    Cadherins are responsible for linking intermediate filaments to adjacent cells.

    <p>False</p> Signup and view all the answers

    What anatomical feature lies at the interface between epithelial tissue and underlying tissues?

    <p>Basal lamina</p> Signup and view all the answers

    The __________ are junctions that act like rivets, providing mechanical strength between cells.

    <p>desmosomes</p> Signup and view all the answers

    Match the following junction types with their primary functions:

    <p>Adherens junctions = Cell-cell adhesion Desmosomes = Mechanical strength Tight junctions = Sealing and compartmentalization Hemidesmosomes = Attachment to the basal lamina</p> Signup and view all the answers

    What role does actin play in the process of intercalation?

    <p>Coupling contraction to adhesion disassembly</p> Signup and view all the answers

    What is a primary function of claudin proteins in tight junctions?

    <p>Forming paracellular pores</p> Signup and view all the answers

    Occludin is essential for the assembly and structure of tight junctions.

    <p>False</p> Signup and view all the answers

    What happens to baby mice that lack the claudin-1 gene?

    <p>They lose water rapidly and die within a day after birth.</p> Signup and view all the answers

    The _____ junctions create direct channels between adjacent cells' cytoplasm.

    <p>gap</p> Signup and view all the answers

    Match the following tight junction proteins with their primary functions:

    <p>Claudin = Formation of sealing strands Occludin = Limiting junction permeability Tricellulin = Sealing cell membranes where three cells meet ZO proteins = Providing structural support</p> Signup and view all the answers

    Which of the following is NOT true about tight junctions?

    <p>They are solely responsible for cell adhesion.</p> Signup and view all the answers

    Tricellulin is essential for sealing cell membranes at points where two cells meet.

    <p>False</p> Signup and view all the answers

    Explain the primary role of zonula occludens (ZO) proteins in tight junctions.

    <p>ZO proteins provide structural support for tight junctions and organize protein complexes.</p> Signup and view all the answers

    A mutation in the gene encoding a specific claudin can lead to excessive loss of _____ in the urine.

    <p>Mg2+</p> Signup and view all the answers

    Match the following junction types with their corresponding characteristics:

    <p>Tight junctions = Block passageways between cells Gap junctions = Allow direct cytoplasmic communication Adherens junctions = Bond cells together mechanically Desmosomes = Provide mechanical strength to tissues</p> Signup and view all the answers

    What is the role of Rac in the initial interaction between epithelial precursor cells?

    <p>It inhibits the formation of stress fibers.</p> Signup and view all the answers

    What is a characteristic of the claudin protein family?

    <p>They are expressed in various combinations in different epithelia.</p> Signup and view all the answers

    Rho generally promotes the formation of branched actin networks.

    <p>False</p> Signup and view all the answers

    The sealing strands of tight junctions consist of cadherin proteins.

    <p>False</p> Signup and view all the answers

    What is the term used for the whole assembly of tight junctions, adherens junctions, and desmosomes?

    <p>Junctional complex</p> Signup and view all the answers

    What is the function of cadherins at adherens junctions?

    <p>To mediate homophilic binding between adjacent cells.</p> Signup and view all the answers

    The process by which adherens junctions sense mechanical stresses and generate biochemical signals is called ______.

    <p>mechanotransduction</p> Signup and view all the answers

    The major transmembrane proteins that form sealing strands in tight junctions are called _____ proteins.

    <p>claudin</p> Signup and view all the answers

    Match the following proteins with their corresponding roles at adherens junctions:

    <p>β-catenin = Links cadherins to actin filaments p120-catenin = Stabilizes cadherin-linkages α-catenin = Recruiter of actin-associated proteins vinculin = Associates with α-catenin to link to actin</p> Signup and view all the answers

    Which of the following contributes to the expansion of the junction during cell adhesion?

    <p>Disassembly of actin–myosin fibers</p> Signup and view all the answers

    Actin protrusions do not contribute to the junction expansion.

    <p>False</p> Signup and view all the answers

    How do adherens junctions maintain balance in pulling forces?

    <p>By sensing the forces acting on them and modifying local actin and myosin behavior.</p> Signup and view all the answers

    The assembly of a large adhesion surface depends on local reduction of cortical tension achieved through inhibition of ______.

    <p>cortical actin–myosin fiber formation</p> Signup and view all the answers

    Which proteins provide a dynamic linkage to actin filaments at adherens junctions?

    <p>Catenins</p> Signup and view all the answers

    Catenins are involved in linking cadherins to intermediate filaments at desmosomes.

    <p>True</p> Signup and view all the answers

    What effect does inhibition of myosin activity have on adherens junctions?

    <p>It results in the disassembly of many adherens junctions.</p> Signup and view all the answers

    The link between cadherins and actin filaments includes adaptor proteins such as ______ and ______.

    <p>β-catenin, p120-catenin</p> Signup and view all the answers

    What is the initial consequence of cadherin clustering at cell interaction sites?

    <p>Local activation of Rac</p> Signup and view all the answers

    Rac activation in the long term leads to increased cortical tension.

    <p>False</p> Signup and view all the answers

    What is one primary function of epithelial cells?

    <p>Selective permeability barriers</p> Signup and view all the answers

    Tight junctions allow for free movement of molecules across the epithelial barrier.

    <p>False</p> Signup and view all the answers

    What type of structure does the epithelium of the small intestine have?

    <p>simple columnar</p> Signup and view all the answers

    Tight junctions help prevent the diffusion of proteins between the ______ and ______ membranes.

    <p>apical, basolateral</p> Signup and view all the answers

    Match the terms with their descriptions.

    <p>Apical surface = Faces the lumen of the gut Basolateral surface = Faces the extracellular fluid Tight junctions = Prevent leakage between cells Transcellular transport = Movement of substances through a cell</p> Signup and view all the answers

    What type of transport is used to bring glucose into the absorptive cell?

    <p>Active transport</p> Signup and view all the answers

    The permeability of tight junctions is the same across all types of epithelium.

    <p>False</p> Signup and view all the answers

    What is the size of the gap in gap junctions?

    <p>2–4 nm</p> Signup and view all the answers

    What role do transport proteins play in epithelial cells?

    <p>Transport selected molecules across the epithelium.</p> Signup and view all the answers

    Innexins are the predominant gap-junction proteins in vertebrates.

    <p>False</p> Signup and view all the answers

    The sealing function of tight junctions is easy to demonstrate by adding a ______ to one side of the epithelium.

    <p>low-molecular-mass tracer</p> Signup and view all the answers

    Match the following types of transport with their characteristics.

    <p>Active transport = Requires energy Passive transport = No energy required Paracellular transport = Between adjacent cells Transcellular transport = Through cells</p> Signup and view all the answers

    What type of molecules can pass through gap junctions?

    <p>Inorganic ions and small water-soluble molecules</p> Signup and view all the answers

    What occurs at the basolateral surface of absorptive cells?

    <p>Passive transport into extracellular fluid</p> Signup and view all the answers

    A connexon is made up of six __________ subunits.

    <p>connexin</p> Signup and view all the answers

    Match the following types of junction proteins to their respective characteristics:

    <p>Connexins = Found in vertebrates Innexins = Found in invertebrates Cadherins = Mediate cell adhesion Claudins = Form tight junctions</p> Signup and view all the answers

    Tight junctions only serve to seal gaps between cells and do not affect protein distribution.

    <p>False</p> Signup and view all the answers

    Which of the following functions do gap junctions serve in electrically excitable tissues?

    <p>Synchronized muscle contractions</p> Signup and view all the answers

    What do tight junctions prevent in the context of molecular movement?

    <p>Backflow of transported molecules</p> Signup and view all the answers

    Tight junctions are considered barriers to ______ diffusion.

    <p>solute</p> Signup and view all the answers

    What kind of signals can regulate some subtypes of gap junctions?

    <p>Extracellular signals such as neurotransmitters</p> Signup and view all the answers

    The pore size of a gap junction channel is approximately 1.4 nm.

    <p>True</p> Signup and view all the answers

    Which of the following statements about tight junctions is true?

    <p>They vary in permeability depending on tissue type.</p> Signup and view all the answers

    The mechanism of _________ through gap junctions allows for rapid action potential propagation.

    <p>electrical coupling</p> Signup and view all the answers

    What are the two families of channel-forming proteins found in gap junctions?

    <p>Connexins and innexins</p> Signup and view all the answers

    What process allows cells to share small metabolites and ions?

    <p>Communication through gap junctions</p> Signup and view all the answers

    Gap junctions allow the exchange of macromolecules such as proteins and nucleic acids.

    <p>False</p> Signup and view all the answers

    The assembly and disassembly of gap junctions are dependent on various __________ stimuli.

    <p>physiological</p> Signup and view all the answers

    What is the typical shape of proteins that form gap junctions?

    <p>Four-pass</p> Signup and view all the answers

    Match the terms with their definitions:

    <p>Homomeric = Connexons composed of identical connexins Heteromeric = Connexons composed of different connexins Homotypic = Channels formed by identical connexons from adjacent cells Heterotypic = Channels formed by different connexons from adjacent cells</p> Signup and view all the answers

    What is the half-life of connexin molecules?

    <p>A few hours</p> Signup and view all the answers

    The central part of the gap-junction plaque consists of new connexin molecules.

    <p>False</p> Signup and view all the answers

    What type of experimental tag is used to label connexin molecules in the experiment?

    <p>Tetracysteine tag</p> Signup and view all the answers

    In plants, _____ perform many of the same functions as gap junctions.

    <p>plasmodesmata</p> Signup and view all the answers

    What happens to connexins synthesized during the 4 or 8-hour incubation?

    <p>They are labeled red</p> Signup and view all the answers

    Plasmodesmata can be removed when no longer required.

    <p>True</p> Signup and view all the answers

    What structure connects the cytoplasms of two cells in plasmodesmata?

    <p>Desmotubule</p> Signup and view all the answers

    Old connexin molecules take up _____ dye.

    <p>green</p> Signup and view all the answers

    Match the following components with their respective descriptions:

    <p>Desmotubule = Structurally linked to smooth ER of adjacent cells Connexin = Molecule involved in gap junctions Plasmodesmata = Channels connecting plant cell cytoplasms Hemichannel = Unpaired connexin structure in the plasma membrane</p> Signup and view all the answers

    What regulates the transport through plasmodesmata?

    <p>Size of the molecules</p> Signup and view all the answers

    Plasmodesmata can only be formed de novo and cannot penetrate existing cell walls.

    <p>False</p> Signup and view all the answers

    What is the diameter range of the channels formed by plasmodesmata?

    <p>20-40 nm</p> Signup and view all the answers

    Connexons are connected through _____ in gap junctions.

    <p>cell membranes</p> Signup and view all the answers

    Which of these statements best describes the outer part of a gap junction plaque after 8 hours of incubation?

    <p>It shows a mixture of old and new connexin molecules</p> Signup and view all the answers

    Connexins contained in hemichannels are typically open in resting cells.

    <p>False</p> Signup and view all the answers

    Which of the following cell adhesion proteins requires Ca2+ for its function?

    <p>Both A and C</p> Signup and view all the answers

    Selectins mediate long-lasting cell-cell adhesion interactions.

    <p>False</p> Signup and view all the answers

    Name the selectin found on blood platelets and endothelial cells activated by inflammatory responses.

    <p>P-selectin</p> Signup and view all the answers

    Selectins bind to specific __________ on glycoproteins and glycolipids.

    <p>oligosaccharides</p> Signup and view all the answers

    Match the type of selectin with its respective characteristic.

    <p>L-selectin = Found on lymphocytes P-selectin = Found on platelets E-selectin = Expressed later during inflammation</p> Signup and view all the answers

    What do selectins primarily govern in vertebrates?

    <p>Traffic of white blood cells</p> Signup and view all the answers

    Ig superfamily members mediate cell-cell adhesion in a Ca2+-dependent manner.

    <p>False</p> Signup and view all the answers

    What happens when white blood cells activate their integrins?

    <p>They latch onto specific macromolecules on endothelial cells, allowing them to exit the bloodstream.</p> Signup and view all the answers

    White blood cells use __________ to roll along the blood vessel walls before entering tissues.

    <p>selectins</p> Signup and view all the answers

    Match the following adhesion molecules with their interactions.

    <p>ICAMs = Bind to integrins on white blood cells NCAM = Homophilic binding VCAMs = Bind to integrins on leukocytes</p> Signup and view all the answers

    Which of the following correctly describes the interaction mediated by selectins?

    <p>Heterophilic binding to different types of cells</p> Signup and view all the answers

    Endothelial cells express selectins to signal for white blood cells to enter tissues during inflammation.

    <p>True</p> Signup and view all the answers

    What is the consequence of the weak adhesion mediated by selectins?

    <p>It allows white blood cells to roll along the endothelium before activating integrins to adhere more strongly.</p> Signup and view all the answers

    Integrins bind to specific proteins called __________ on endothelial cells.

    <p>ICAMs</p> Signup and view all the answers

    What type of cadherins are closely related in sequence throughout their extracellular and intracellular domains?

    <p>Classical cadherins</p> Signup and view all the answers

    Selectins mediate transient, Ca2+-independent cell–cell adhesion in the bloodstream.

    <p>False</p> Signup and view all the answers

    What binds together to form a gap junction?

    <p>Connexons</p> Signup and view all the answers

    The _______ junction prevents the passage of most dissolved molecules between adjacent epithelial cells.

    <p>tight</p> Signup and view all the answers

    Match the following types of junctions with their functions:

    <p>Adherens junctions = Cell–cell adhesion Tight junctions = Seals adjacent cells Gap junctions = Communicating channels Plasmodesmata = In plant cells</p> Signup and view all the answers

    Which type of junction forms a continuous belt around interacting epithelial cells?

    <p>Adherens junctions</p> Signup and view all the answers

    All cadherins belong to the same family of proteins without any structural variations.

    <p>False</p> Signup and view all the answers

    What is the role of PDZ domains in protein interactions?

    <p>Docking sites for intracellular tails of transmembrane proteins</p> Signup and view all the answers

    What is the primary role of nectins in tissues?

    <p>To provide cell strength during tissue formation</p> Signup and view all the answers

    Cadherins mediate stronger cell adhesion than Ig superfamily members.

    <p>True</p> Signup and view all the answers

    What role does ICAM play in cell adhesion?

    <p>It binds heterophilically to integrins on white blood cells.</p> Signup and view all the answers

    The extracellular portion of cadherins typically binds to _______ on adjacent cells.

    <p>cadherins</p> Signup and view all the answers

    Match the following adhesion molecules with their primary characteristics:

    <p>Cadherins = Strong adhesion and homophilic binding Nectins = Collaborate with cadherins at adherens junctions Selectins = Mediates transient adhesion and binds to carbohydrates Ig superfamily = Involves both homophilic and heterophilic binding</p> Signup and view all the answers

    What effect does the removal of nectin expression have in epithelial tissues?

    <p>Causes defects in tissue formation</p> Signup and view all the answers

    Claudins are responsible for cell signaling in epithelial tissues.

    <p>False</p> Signup and view all the answers

    What is the main function of tight junctions in epithelial cells?

    <p>To seal gaps between cells and prevent molecule diffusion.</p> Signup and view all the answers

    Cells connected by gap junctions can share _______ and small molecules.

    <p>ions</p> Signup and view all the answers

    Match each adhesion element with their relevant tissue type:

    <p>N-cadherins = Neurons ICAM = Endothelial cells Claudins = Epithelia Selectins = White blood cells</p> Signup and view all the answers

    Which of the following best describes the intracellular function of the adaptor protein catenins?

    <p>Link cadherins to the actin cytoskeleton</p> Signup and view all the answers

    Gap junctions consist of clustered cadherins.

    <p>False</p> Signup and view all the answers

    What is the typical molecular size that gap junctions allow to pass through?

    <p>Molecules smaller than about 1000 daltons.</p> Signup and view all the answers

    The _______ superfamily includes proteins that can bind both homophilically and heterophilically.

    <p>Ig</p> Signup and view all the answers

    Which type of proteins regulate the sorting of cell populations during development?

    <p>Cadherins</p> Signup and view all the answers

    Study Notes

    Cell-Cell Junctions

    • Cell-cell junctions are essential for multicellular organisms, enabling cells to adhere to one another and form tissues.
    • Cadherins are the primary adhesion molecules in cell-cell anchoring junctions, and are crucial for resisting forces that could pull cells apart.
    • These junctions are dynamic and adaptable, allowing tissues to remodel and repair.
    • Cadherins are a diverse family of proteins found in all multicellular animals and some choanoflagellates.
    • Cadherins are calcium-dependent adhesion molecules, meaning they require calcium ions for their adhesive function.
    • Cadherins mediate homophilic adhesion, meaning they bind to cadherins of the same or similar subtype on adjacent cells.
    • Cadherin binding relies on a knob-and-pocket mechanism, where the N-terminal domains of cadherin molecules interact.
    • The strength of cadherin-mediated adhesion comes from the cumulative effect of many weak interactions between cadherin molecules.
    • Changes in cadherin expression are critical for tissue development and organization, as cells sort out and segregate based on their cadherin profiles.
    • Assembly of strong cell-cell adhesions requires local changes in the actin cytoskeleton, specifically a reduction in cortical tension.
    • The GTPases Rac and Rho play a key role in regulating cortical tension and promoting adherens junction formation.
    • Catenins are adaptor proteins that link classical cadherins to the actin cytoskeleton, facilitating the connection of cadherins to the cell's internal framework.
    • Adherens junctions are capable of mechanotransduction, sensing mechanical stresses and responding to changes in tension.
    • The contractile forces generated by actin and myosin II play a crucial role in maintaining the integrity of adherens junctions and ensuring balanced tension between cells.
    • Adherens junctions can regulate local actin and myosin activity in response to mechanical changes, ensuring that forces across the junction are balanced.

    Cadherin Structure and Function

    • Cadherins are transmembrane proteins with an extracellular domain containing multiple copies of the extracellular cadherin (EC) domain.
    • Calcium ions bind to hinge regions between EC domains, providing structural rigidity and preventing flexibility.
    • Homophilic binding occurs at the N-terminal tips of cadherin molecules, where the knob of one domain inserts into the pocket of another.

    Implications of Cadherin Function

    • Cadherins contribute to cell-sorting processes during embryogenesis, enabling cells of the same lineage to preferentially adhere to one another.
    • Cadherin-dependent cell sorting occurs in both in vivo and in vitro settings, demonstrating the importance of cadherins in tissue organization.
    • Assembly of adherens junctions depends on a balance of forces between cortical tension and cadherin-mediated adhesion, regulated by the actin cytoskeleton.
    • The process of adherens junction formation involves local changes in cortical tension, regulated by the GTPases Rac and Rho.
    • Catenins link cadherins to the actin cytoskeleton, enabling the transmission of mechanical forces across junctions.
    • Adherens junctions are capable of sensing and responding to mechanical stresses, demonstrating their role in mechanotransduction.

    Adherens Junctions

    • Adherens junctions are not passive but actively regulate themselves in response to mechanical changes
    • Tension sensors in the cell's cadherin complex alter shape when stretched
    • α-catenin protein unfolds, exposes a cryptic binding site for vinculin, leading to more actin recruitment at the junction
    • This change also triggers local regulation of small GTPases, that control actin and myosin behavior
    • Pulling on a junction in one cell increases contractile force in the attached cell

    Tissue Remodeling

    • Adherens junctions are essential for tissue modeling
    • Connect actin filaments in neighboring cells, allowing coordinated use of cytoskeletons
    • In epithelia, they form a continuous adhesion belt encircling the apical surface of the cell
    • Contractile bundles of actin filaments and myosin II link, enabling coordinated contraction for epithelial tissue folding
    • During germ-band extension, actin-dependent contraction leads to loss of specific adherens junctions
    • Cells rearrange via intercalation, extending epithelium
    • Cadherin removal through clathrin-mediated endocytosis may be a possible mechanism for adherens junction disassembly

    Desmosomes

    • Provide mechanical strength to epithelia
    • Structurally similar to adherens junctions, but link to intermediate filaments instead of actin filaments
    • Found in tissues subject to high mechanical stress such as heart muscle and the epidermis
    • Appear as button-like spots of adhesion, connecting cells together
    • Intermediate filaments anchored to desmosomes form a strong structural framework
    • The type of intermediate filaments depends on the cell type: keratin filaments in most epithelial cells, desmin filaments in heart muscle cells

    Tight Junctions

    • Seal epithelial cells together, preventing molecular leakage
    • Act as a fence, prohibiting apical or basolateral proteins from diffusing into the wrong region
    • Permeability to small molecules varies between tight junctions in different tissues
    • Composed of sealing strands that encircle the apical end of each cell
    • Claudins are the main transmembrane proteins forming these strands
    • Occludin is another major transmembrane protein that helps regulate permeability
    • Tricellulin is a third protein required to seal cells together near cell junctions

    Tight Junctions

    • Form paracellular pores
    • Allow specific ions to pass between cells, e.g. Mg2+ in kidney tubules
    • Claudins and occludins interact extracellularly to promote junction assembly
    • Zonula occludens (ZO) proteins are scaffold proteins that organize the junction:
      • They consist of protein-binding domains, including PDZ domains, which bind to claudins, occludin, and the cytoskeleton.
      • Form a meshwork that links sealing strands to the actin cytoskeleton.
    • Tight junctions are typically located just apical to adherens and desmosome junctions, forming a junctional complex
      • Formation of these junctions relies on each other
        • Blocking adherens junction formation prevents tight junction formation.

    Gap Junctions

    • Connect the cytoplasm of adjacent cells
    • Present in most animal tissues, including epithelia and heart muscle
    • Appear as patches with 2-4 nm gap between membranes
    • Connexons are channel-forming proteins:
      • 2 families: connexins (vertebrates) and innexins (invertebrates)
      • 21 isoforms of  connexins in humans
      • 6 connexins assemble to form a hemichannel.
    • Pore size is about 1.4 nm, allowing the passage of small molecules, but not proteins or nucleic acids.
    • Functions:
      • Enable electrical and metabolic communication between cells.
      • Synchronize contractions in heart and smooth muscle tissues.
    • Gap junction channels are dynamic and can be regulated:
      • Can open or close in response to stimuli like voltage, membrane potential, or cytoplasmic changes, such as pH and Ca2+ levels.
      • Turnover: new connexons are added at the periphery, while old ones are removed from the middle of the plaque.

    Plasmodesmata

    • Cytoplasmic channels found in plant cell walls
    • Plasma membrane of adjacent cells is continuous at plasmodesmata
    • Desmotubule: a narrower cylindrical structure that runs through the center of the channel and is connected to the smooth ER of both cells.
    • Allow the passage of small molecules, with a size cutoff of about 800 daltons, similar to gap junctions
    • Regulation: transport can be restricted, with barriers to the movement of even small molecules between certain cells.

    Selectins

    • Carbohydrate-binding proteins that mediate transient cell-cell adhesion
    • Functions:
      • Control traffic of white blood cells in the bloodstream.
      • Facilitate white blood cell migration from the bloodstream to tissues.
    • Types: L-selectin (on white blood cells), P-selectin (on platelets and activated endothelial cells), and E-selectin (on endothelial cells)
    • Heterophilic binding: selectins bind to specific oligosaccharides on other cells
    • Function in inflammation:
      • Endothelial cells express selectins that bind to white blood cells and platelets at sites of inflammation.
    • Interaction with integrins: selectins work together with integrins to strengthen cell adhesion.
    • Rolling: selectins mediate weak adhesion, allowing cells to roll on the endothelial surfaces.
      • Integrins are activated, leading to strong cell adhesion and migration through the vessel walls.

    Adhesion Molecules In The Immune System

    • White blood cells adhere to the vessel wall via integrins.
    • Integrins bind to ICAMs (intercellular cell adhesion molecules) and VCAMs (vascular cell adhesion molecules) on the endothelial cells.
    • ICAMs and VCAMs belong to the immunoglobulin (Ig) superfamily which contain Ig-like domains similar to antibodies.

    Cell-Cell Adhesion: The Ig Superfamily

    • The Ig superfamily mediates both homophilic and heterophilic binding.
    • Neural cell adhesion molecule (NCAM) is a member of the Ig superfamily that can inhibit adhesion due to its sialic acid content.
    • Nectins, another Ig superfamily member, collaborate with cadherins to strengthen adherens junctions and establish cell-cell interaction during tissue formation.

    Cadherins and Cell-Cell Adhesion

    • Cadherins are transmembrane proteins responsible for strong cell-cell adhesion in tissues.
    • Cadherins mediate homophilic binding and play a role in sorting cells based on their cadherin expression.
    • "Classical" cadherins are linked to the actin cytoskeleton through catenins, enabling physical anchorage and response to tension.

    Tight Junctions: Sealing The Gaps

    • Tight junctions seal the gaps between epithelial cells, preventing molecule diffusion across the cell sheet and separating protein populations in the plasma membrane.
    • Claudins form the major transmembrane proteins of tight junctions.
    • Intracellular scaffold proteins organize claudins into a complex protein network directly linked to the actin cytoskeleton

    Gap Junctions: Direct Cell-Cell Communication

    • Gap junctions are channels that allow molecules smaller than 1000 daltons to pass directly between connected cells.
    • Cells connected by gap junctions share inorganic ions, small molecules and are chemically and electrically coupled.
    • Gap junctions are formed by connexons, which are made up of six connexin protein subunits.

    Other Adhesion Mechanisms

    • Selectins mediate transient, Ca2+-dependent cell-cell adhesion in the bloodstream.
    • Ig superfamily members are involved in various transient interactions, some homophilic, some heterophilic.
    • Integrins mainly attach cells to the extracellular matrix, but can mediate cell-cell adhesion by binding to specific Ig superfamily proteins.

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    Explore the essential role of cell-cell junctions in multicellular organisms through this quiz. Learn about cadherins, their function as adhesion molecules, and the mechanisms through which they enable tissue formation and remodeling. Delve into the dynamic nature of these junctions and the importance of calcium ions in their functionality.

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