Cell Phys

Basel Lectures - cytoskeleton, adhesions, and matrix

Cells are organized and held together by a set of structural proteins that are divided into three categories

Microtubules, Microfilaments, and Intermediate Filaments

Cell Shape • Cell Motility • Cell Strength • Cell Processes • Cell Polarization • Cell Signaling • Organelle Organization • Movement of Macromolecules • Membrane Organization are all important functions of the _____

Cytoskeleton

Hollow tubes that are often centered around nucleus radiating outward

Microtubule

- end = _____ exposed, and regress faster than Beta tubulin

Alpha

+ end = ____ exposed, and grow faster than Alpha tubulin

Beta

Micro tubules can be found in the

General cytoplasmic organization and cilia

During cell division (prophase, metaphase, etc.) where are microtubules present?

Mitotic spindle

T/F: Axons and dendrites have microtubules present within them.

True

Cell scaffolding, polarization, and Polarized movement of organelles, proteins, and DNA are primary functions of

microtubules

What other very important function do microtubes have?

Resists compression

Kinesin (toward ____ end) away from nucleus

positive

Dynein (toward ____ end) toward the nucleus

negative

Microtubule Transport Bind to cargo and ‘walk’ down the tubules, this is known as

polarized movement

T/F: Microtubules are the largest component of the microtubule complex

True

Two major nucleation sites of the Microtubule are

Centrosomes and Basal Bodies

Which component allow anchor of microtubules on the negative side?

Centrosomes

Nucleation _____ microtubule assembly

polarizes

Microtubules are hollow tubes ~___ nm in diameter made of tubulin dimers.

24

The microtubule network can be used for polarized transport of proteins and vesicles using motor proteins called

kinesins/dyneins

Microtubules are _____ structures regulated by nucleation, dynamic instability, and microtubule associated proteins.

dynamic

GDP - Tubulin Dimer =

Inactive/Shrinking State

GTP-Tubulin Dimer =

Active/Growing State

Nucleotide required to be bound to tubulin dimers for addition to the positive (+) end of a microtubule

GTP - dimers

Tubulin subunit responsible for the slow hydrolysis of bound GTP to GDP

$$\beta\text{-tubulin}$$

Effect of high GTP-bound tubulin dimer concentrations on microtubule dynamics

Promotes microtubule growth and stabilization through the formation of a protective GTP cap (assembly occurs faster than GTP hydrolysis)

Effect of low GTP-bound tubulin dimer concentrations on microtubule dynamics

Promotes loss of the GTP cap, leading to dynamic instability, catastrophe, and rapid shrinking (GTP hydrolysis occurs faster than assembly)

Primary cellular advantage of dynamic instability despite its high energy cost

Allows rapid structural remodeling and fine-tuned spatial regulation of the microtubule network

Microtubule Associated Proteins (MAPs) have Many different proteins bind to MT that

regulate MT assembly and function

General functions of Microtubule Associated Proteins (MAPs) in a cell

Stabilizing, destabilizing, bundling, branching, mediating interactions with other cytoskeletal fibers or cell junctions, and transporting cargo

Double helical fibers and Found throughout the cell, but especially toward periphery

Microfilament

Monomeric protein subunit that polymerizes to form microfilaments

G-actin (globular actin)

Filamentous polymer formed by the assembly of actin subunits

F-actin (filamentous actin)

Key cellular structures and regions where microfilaments are found

Cell cortex, microvilli and stereocilia, lamellipodia and filopodia, and muscle fibers

Primary cellular functions of microfilaments

Cell membrane organization, ameboid movement, muscle contractions, cytokinesis, and resisting stretching

Type of mechanical stress that microfilaments primarily resist

Stretching

Microfilaments can be used for transport of proteins/vesicles or bulk cell movement using motor proteins called

myosins

Microfilaments are dynamic structures regulated by

nucleation, dynamic instability/treadmilling, and actin binding proteins

Functions of Microfilament Contraction and Transport

move vesicles, protrude membranes, and contract the cell

Microfilament Muscle fibers are specialized and densely organized fibers of

Actin and Myosin

Formation of G-Actin Monomers into tetramers is called ___ and occurs near the membrane

nucleation

After G-Actin binds to the (+) end of F-Actin, G-Actin binds ____ and becomes hydrolyzes to remove phosphate and becomes ____ as it moves towards the (-) end.

ATP ; ADP

Microfilament regulate length by

Rate of nucleation, rate of (+) end assembly, (-) end disassembly, and overall actin stability

Intermediate Filament functions

Mechanical strength (rigidity and flexibility), Non-motile scaffolding network, and resists shearing

T/F: All IF have the same functions for all tissue types and experience the same forces.

False

IF are made up of _____ proteins

fibrous (non globular)

The long fibrous proteins bundle and twist together into ____ like structures

rope

IF proteins are found in

Epithelium - cytokeratin - Connective tissue (fibroblasts) - Muscle - Glial cells/astrocytes - Nerves - Neuronal stem cells - Nucleus (all cells)

T/F: IF can deform and are able to spring back into position without breaking

True

Though relatively permanent, IF still need to change in response to changing conditions, a good example are ____ _____ because they support the nuclear membrane, are phosphorylated by lamin kinases during entry into prophase which destabilize the IF and are dephosphorylated on entry to telophase which allows reassembly.

A type of junction that connect two cells together very tightly (cells act in unison to compartmentalize) differentiate between apical and basolateral compartments

Tight Junctions

What are the two primary protein families of tight junctions?

Claudins and occludins

cells interact with each other is through ____ _____ ______ proteins expressed on the cell surface that bind the cell to other cells

cell adhesion molecules

Which tight junction protein has a much larger intracellular domains that regulatory domains

on them which can interact with different signaling pathways, changing how tightly the cells are in unison

occludin

There are 2 fluids that the tight junctions separate from the apical and basolateral

Extracellular fluid and Fat

Tight junctions forms 2 different types of regulations

Barrier (gate) and fence

Sodium Glucose Transport (SGLT) is a _____ active transport

secondary

Glucose Transporter (GLUT) is a ______ transport

passive

Tight Junctions form a permeability barrier, through action on claudins/occludins and interactions with the actin cytoskeleton can change permeability resulting in

leakiness

TJ leakiness and cytokine regulation may be associated with

Inflammatory Bowel Disease IBD

_____ are immune signals that activate pathways to make TJ more leaky.

Cytokines

A junction that causes cells to adhere to one another (cells act as one mechanical unit – ______network is joined across cells)

microfilament

Adhesion Belts are made of bundles of

Actin

What proteins are present on Adhesion Belts between intracellular space of both cells forming a zipper network

Cadherin Dimers

What is an example of small gTPAs in Adhesion regulation

Rho

When a stimulus is sent through Rho/ROCK what happens to the actin cytoskeleton that leasd to tension through the adherens junction?

contraction

Adherens junctions interact significantly with the ____ _______ and can be regulated through that interaction

actin cytoskeleton

Rho can also be important with

tissue modeling

What are the mechanical units in the intermediate filament network that join across the cell found in desmosomes

Keratin

What cadherin proteins are found in desmosomes that link the adjacent cell together

desmoglein and desmocolin

What adaptor protein is vital for preventing the separation of desmoglein and desmocolin from the membrane

desmoplankin

Desmosomes are abundant in tissues exposed to mechanical stress, including

stratified epithelia and cardiac muscle

Desmosomes are relatively _____ structures, but they still need to be remodeled at times during tissue growth and wound healing.

permanent

Protein kinase (PKCa) is a signal that acts on desmosomes, causing the desmoglein and desmocolin to form a weaker bond and become Ca dependent when

phosphorylated

_______ causes the desmoglein and desmocolin to form a stronger bond between desmosome complexes and become Ca independent, forming “hyperadhesive” state

dephosphorylation

______ allow direct communication between the cytoplasm of two cells, allowing passage of ions and small molecules between neighboring cells

Gap junctions

Gap Junctions have six connexin subunits that form one connexon (hemichannel); connexons from adjacent cells dock to form a gap-junction channel. What is their function?

allow passage of different sizes and types of molecules

T/F: Most proteins can go through gap junctions.

False

T/F: If one cell becomes depolarized, the depolarization travels between the Gap junctions and the neighboring cells become depolarized as well.

true

Phosphorylation can change how permeable gap junctions are, but also ___, _____, and _____ can too.

pH, Ca, and Voltage

T/F: Cadherins usually bind to the same cadherin (homophilic)

True

Cadherins (“calcium-sensitive adhesion protein”) are important in

Anchoring junctions, initiation of anchoring junctions and tissue development and sorting

Usually Ca²⁺-independent; homophilic or heterophilic

Immunoglobulin-superfamily CAMs (IgCAMs)

I-CAM binds to LFA (Leukocyte Function Antigen) are important in

immune response

A family of cell adhesion molecules that bind carbohydrates

Selectins

T/F: Selectins are important when recognizing self vs non-self

True

Selectins bind to specific carbohydrates (or glycoprotein) on the cell surface (heterophilic) which are important in

Inflammation/Immune Response and Uterine Implantation

primarily extracellular matrix binding proteins are called

Integrins

Integrins is a type of LFA, when inactive they are bent over, when stimulated, the Integrins errect and find to an I-CAM. This is important as adhesion molecule for

immune response

A family of fibrous proteins that give strength and rigidity to the extracellular matrix

Collagen

A family of fibrous proteins that give flexibility and elasticity to the extracellular matrix

Elastin

complex network of extracellular macromolecules that provides structural, mechanical, and biochemical support to cells

Extracellular Matrix

ECM is made up of generally 3 components:

Protein fibers, Complex carbohydrates, Adhesive glycoproteins

Triple helix Gives tensile strength and rigidity to extracellular matrix, is a Fibrous, helical protein

Collagen

Collagen Fibers can be oriented in _____ or ______ to create strength in different dimensions

parallel or obliquely

T/F: Triple helix are almost impervious to proteolytic degradation, making it difficult for proteases to it break down.

True

ECM strength and rigidity influenced by

Type, density, crosslinking and orientation of collagen.

Protein structures cannot make a tight bend on their own so they require the imino-acid ______.

Proline

To form a tighter kink in a triple helix structure proline is modified by a hydroxyl group to form

hydroxyproline