L14 - Cells Junctions & Cytoskeleton

What is the Cytoskeleton?

A 3-dimensional network of protein filaments throughout the cell that provides structure and supports movement, intracellular transport, changes in cell shape, adhesion, and epithelial organization

What are Major Functions of the Cytoskeleton?

1. Cell movement

2. Changes in cell shape

3. Intracellular transport

4. Cell division/cytokinesis

5. Phagocytosis

6. Cellular support and strength

7. Cell-cell and cell-ECM adhesion

8. Maintenance of specialized epithelial structures

What are the 3 Major Families of Cytoskeletal Filaments?

1. Actin filaments (microfilaments) → dynamic

2. Intermediate filaments → stable/non-dynamic

3. Microtubules → dynamic

What are the Major Characteristics of Actin Filaments?

• Smallest cytoskeletal filaments: ~5–9 nm

• Made of actin monomers

• Dynamic + flexible

• Form bundles or networks

• Concentrated near the cell periphery/apical surface

What is the major relationship between actin and cellular movement/contractility?

Actin forms an extensive network in contractile cells and is a key component of all types of muscle

How does actin contribute to epithelial structure?

Actin forms the structural basis of microvilli, which make up the brush border on the apical surface of certain epithelial cells

Where are brush borders idenitfy?

Apical surface of:

• Intestinal columnar epithelium

• Proximal convoluted tubule cuboidal epithelial cells

What are the Major Characteristics of Microtubules?

• Largest cytoskeletal filaments: ~25 nm

• Long, hollow cylinders

• Made of α- and β-tubulin heterodimers

• Dynamic

• More rigid than actin

• One end is associated with the centrosome

What are the Major Functions of Microtubules?

1. Maintain cell structure/shape

2. Necessary for cell division

3. Provide tracks for intracellular cargo/vesicle transport

4. Form the structural basis of cilia

What motor proteins transport cargo along microtubules?

Kinesin + dynein

Where do Microtubules Originate?

From a microtubule-organizing center (MTOC), the centrosome

What is the Microtubule arrangement of motile cilia?

• 9 + 2 arrangement

• Dynein arms allow movement

• Dynein functions as an ATPase molecular motor

• Cilia are attached to a basal body

How do Primary/Non-motile Cilia differ from Motile Cilia?

• Primary cilia lack the central microtubules

• Dynein arms found in motile cilia

How can abnormal microtubules affect health?

Defects can interfere with structures/functions that depend on microtubules, including cilia and neurons

• EX: ciliary dyskinesia and neurological/developmental disorders

What happens in congenital ciliary dyskinesia?

Abnormal microtubules and/or loss of dynein arms cause defective motile cilia

• Can lead to abnormal respiratory mucosa, recurrent respiratory infections, digestive issues, and infertility

What are the Major Characteristics of Intermediate Filaments?

• ~10 nm diameter

• Rope-like fibers

• Very stable/non-dynamic

• Provide mechanical strength

• Can span the cytoplasm from one junction to another, transmitting mechanical stresses through tissue layers

What is the Major Function of Intermediate Filaments in epithelial tissues?

1. Provide mechanical strength and help tissues withstand and distribute mechanical stress.

What are Examples of Intermediate Filaments?

• Neurofilaments → neurons

• Vimentin → mesenchymal cells

• Keratin → nails, hooves

What are the 5 Major Cell Junctions to know?

1. Tight junctions

2. Adherens junctions

3. Desmosomes

4. Gap junctions

5. Hemidesmosomes

Where are the Major Junctions located on a polarized epithelial cell?

1. Apical: Tight junction

2. Lateral: Adherens junctions, desmosomes, gap junctions

3. Basal: Hemidesmosomes

Which cell junctions connect to which cytoskeletal components?

1. Tight junction → Actin

2. Adherens junction → Actin

3. Desmosome → Intermediate filaments

4. Hemidesmosome → Intermediate filaments

5. Gap junction → NONE

What is the Major Function of Tight Junctions?

Seal the space between epithelial cells and regulate the movement of water, ions, and solutes through the paracellular pathway

Where are Tight Junctions located?

At the apical/luminal end of the lateral cell membrane, forming circumferential belts around epithelial cells

What are the Major Protein components of Tight Junctions?

1. Claudins

2. Occludins

3. Tricellulin

Which cytoskeletal filament is associated with tight junctions?

Actin microfilaments

What is the Major Function of Adherens Junctions?

Provide cell-cell adhesion and help maintain the sheet-like organization of epithelial cells through the actin cytoskeleton

How are adherens junctions connected to the cytoskeleton?

→ Cadherins → catenins → actin

What analogy does the lecture use for adherens junctions?

Think “zipper/Velcro”

• Adjacent cells expressing the same type of transmembrane cadherin adhere to one another

What is the Major Function of Desmosomes?

Form very strong cell-cell adhesions that are crucial for the structural integrity of tissues

Which cytoskeletal component attaches to desmosomes?

Intermediate filaments, especially keratin filaments

What Proteins are important in Desmosomes?

Desmogleins (cadherins) interact with plakoglobin to control cell-cell adhesion and connect adjacent cells to intermediate filaments

What is the Major Function of Gap Junctions?

Cell-cell communication by allowing ions and small molecules to pass between cell

What Proteins form Gap Junction channels?

Connexins

Are gap junctions connected to the cytoskeleton?

No → This makes them the major exception among the junctions

How can connexin abnormalities affect health?

Connexin deficits can cause progressive peripheral neuropathy and/or deafness

Where are Hemidesmosomes located?

At the basal surface of epithelial cells

What is the Major Function of Hemidesmosomes?

1. Attach epithelial cells to the basal lamina/ECM, helping anchor the epithelial sheet to the underlying matrix

How do hemidesmosomes connect the ECM to the cytoskeleton?

Specialized integrins bind ECM proteins of the basal lamina extracellularly and connect to intermediate filaments intracellularly

Why can defects in cell junctions cause disease?

• Junctions connect cells to each other, their cytoskeleton, or the ECM

  • If junction proteins are abnormal, epithelial sheets may lose their normal adhesion, organization, or structural integrity

How does pemphigus foliaceus demonstrate the importance of desmosomes?

1. Animals produce antibodies against desmoglein 1, a desmosomal protein

2. Desmosomal adhesion is disrupted

3. Epithelial tissues fail to maintain normal structure

What is the overall lesson from diseases involving cytoskeletal/junction proteins?

Structure determines tissue integrity. Altering a cytoskeletal protein or its connection to a junction can disrupt cell adhesion, movement, communication, mechanical strength, or epithelial maintenance

Cytoskeleton Comparison SUMMARY

Feature	Actin Filaments	Intermediate Filaments	Microtubules
Size	Smallest: 5–9 nm	10 nm	Largest: 25 nm
Made of	Actin monomers	Various IF proteins	α + β tubulin
Dynamic	Yes	No — very stable	Yes
Structure	Thin, flexible	Rope-like	Long, hollow cylinders
Location/orientation	Cell periphery + apical side	Span cytoplasm/junctions	Extend from centrosome/MTOC
Main function	Movement, contraction, cell shape	Mechanical strength	Structure + intracellular transport
Special structure	Microvilli/brush border	Structural backbone	Cilia
Junction association	Tight + adherens	Desmosomes + hemidesmosomes

Cell Junctions SUMMARY

Junction	Location	Main Protein(s)	Cytoskeleton	Main Function
Tight (zonula occludens)	Apical	Claudins, occludins, tricellulin	Actin	Controls paracellular diffusion
Adherens (zonula adherens)	Lateral	Cadherins + catenins	Actin	Strong cell-cell adhesion
Desmosome (macula adherens)	Lateral	Cadherins	Intermediate filaments	Strong cell-cell adhesion
Gap	Lateral	Connexins	None	Ions/small molecules → communication
Hemidesmosome	Basal	Integrins + laminins	Intermediate filaments	Anchors cell to basement membrane

Cytoskeleton → Structure → Function SUMMARY. Contractility/movement and maintenance of epitheliu.

Cytoskeleton	Important Structure/Process	Why It Matters
Actin	Muscle	⭐ Contractility
Actin	Microvilli/brush border	Maintains specialized apical structure
Actin	Tight + adherens junctions	Maintains epithelial organization
Microtubules	Intracellular tracks	Vesicle/cargo transport
Microtubules	Cilia	⭐ Ciliary movement
Microtubules	Cell division	Mitotic function
Intermediate filaments	Desmosomes	⭐ Mechanical strength between cells
Intermediate filaments	Hemidesmosomes	⭐ Anchors epithelium to basal lamina