Cell Signaling

What is cell signaling?

Process by which a cell interacts with itself, other cells, and the environment.

What are the 4 main functions of cell signaling?

1. Development (embryogenesis and organogenesis)

2. Homeostasis (maintains tissue balance)

3. Response to Injury

4. Immune Responses (innate and adaptive)

What are the 4 types of cell signaling?

1. Autocrine

2. Paracrine

3. Endocrine

4. Direct (Juxtacrine)

What is autocrine signaling?

A cell that signals itself.

What is paracrine signaling?

A cell that targets nearby cells.

Synaptic signaling in nerve cells and blood-clotting are examples of this.

What is endocrine signaling?

Signaling of distant cells

• long-range communication

• Slower but LONGER-lasting effects

What type of signaling is Insulin and PTH an example of?

Endocrine

What is direct (Juxtacrine) signaling?

Fast, highly specific, and direct contact signaling by membrane-bound receptors or gap junctions.

What do gap junctions use for direct signaling?

Connexons

What are the speeds of the cell signaling pathways?

Autocrine»fast

Paracrine»fast

Endocrine»SLOW

Direct»fast

Which 2 key pathways does tooth formation undergo in order to develop enamel and dentin?

BMP and Wnt

Which 2 key pathways does periodontitis go through which further leads to chronic inflammation and tooth destruction?

TLR and NF-kB

Which 2 key pathways does the tooth go through for wound healing, specifically soft tissue regeneration?

PDGF and TGF-B

What are the 2 key pathways that lead to oral cancer (uncontrolled growth)?

EGFR and PI3K-AKT

What are intracellular receptors?

Proteins located inside a cell's cytoplasm or nucleus that bind to small, hydrophobic signaling molecules

What is another name for intracellular receptors?

Nuclear Receptors

What are the 2 types of nuclear receptors?

Type 1 and Type 2

What is Type 1 Nuclear receptor and what is it’s function?

A type of receptor that is located in the cytoplasm that are bound to a chaperone protein.

Function»After binding to a ligand, it releases its chaperone protein»» gets into the nucleus and acts as a TRANSCRIPTION factor.

What is Type 2 Nuclear receptor and what is it’s function?

A type of receptor inside the nucleus (whether bound to ligand or not), which when inactive are bound to co-repressor proteins.

Function» Ligand binding releases co-repressor proteins allowing TYPE 2 to bind to DNA and MODULATE gene expression.

What is an example of a Type 2 nuclear receptor protein and what is it important for?

VDR (Vitamin-D receptor)

Crucial for formation and mineralization of dentin.

What are 3 examples of a Type 1 Nuclear receptor?

glucocorticoid

androgen

progesterone

What are the major classes of Nuclear Receptors?

Class 1- Steroid Receptors

Class 2- Retinoid X Receptors (RXR) Heterodimers

Class 3- Dimeric Orphan Receptors (DOR)

Class 4- Monomeric Orphan Receptors (MOR)

Which 2 classes of Nuclear Receptors require a ligand for activation?

Class 1 and Class 2

Which 2 classes of Nuclear Receptors DO NOT require a ligand for activation?

Class 3 and Class 4

Where are Class 1 nuclear receptors located and what are examples of them?

Located in cytoplasm

Examples are steroid hormones

(glucocorticoid, progesterone, estrogen, mineralocorticoid, androgen)

How are class 1 estrogen receptors (ERa and ERB) important in dentistry?

They have an osteogenic function (formation of bone in teeth)»»»specifically ERB

They encourage the differentiation of periodontal ligament stem cells.

Where are class 2 nuclear receptors located and what are their main function? Which specific receptors are involved in this function?

Located in Nucleus (already bound to DNA)

Function» Tooth development/formation

Retinoic Acid Receptors (RARs and RXRs)

What do class 2 Peroxisome Proliferator-Activated Receptors (PPARs) play a role in?

Lipid and glucose metabolism

What are transmembrane proteins (extracellular receptors)?

Transmembrane receptors are embedded in the cell membrane that allow communication between the cell and its external environment.

What do transmembrane receptors do?

Trigger cascade of signals which may include production of secondary messengers.

What is the JAK-STAT signaling pathway?

Enzyme-linked transmembrane receptors that lack intrinsic enzymatic activity but activate intracellular enzymes.

After activation, they directly activate a non-covalently associated cytoplasmic kinase.

What are ionotropic transmembrane receptors?

Ligand-gated ion channels that open upon ligand binding, allowing the passage of specific ions (eg, Na+, K+, Ca²+, or Cl-) across the plasma membrane.

What are metabotropic transmembrane receptors?

G-protein-coupled Receptors (GPCRs) that initiate a cascade of intracellular events rather than direct opening of channel (ionotropic).

Why are GPCRs important in dentistry?

They play roles in tooth development, mineralization, and potentially even regeneration.

What are primary (Orthosteric) ligands?

Molecules that bind to the primary binding site of a protein or receptor, often referred to as the active site.

This binding typically a change in protein conformation or activation of downstream signaling pathways.

What are examples of Orthosteric ligands? (2)

Neurotransmitters» glutamate and GABA

ATP» bind to active site of kinases

What are allosteric ligands?

molecules that bind to an allosteric site, causing a conformational change that affects the protein's function.

Do allosteric ligands activate the receptor ?

No

Do Orthosteric ligands activate the receptor?

Yes, they either activate or inhibit.

What allosteric ligands do? (3)

-increase/decrease binding affinity

-increase/decrease ACTIVATION strength

-change receptor’s location

What is apoptosis?

programmed cell death mechanism that can be triggered through two distinct pathways

• the extrinsic and intrinsic pathways

What are intrinsic pathways?

Activated by factors originating inside the system

What are 2 main examples of intrinsic pathways?

1. Blood clotting (collagen in blood vessels for clotting)

2. Apoptosis

What are extrinsic pathways?

Activated by factors coming from OUTSIDE of the system.

What is the process of the extrinsic pathway in APOPTOSIS?

External signals binding to death receptors on the cell surface (i.e. FAS receptor), activating a caspase cascade that leads to cell death.

What is the process of the intrinsic pathway in APOPTOSIS?

Initiated within the cell in response to stress signals, causing mitochondria to release pro-apoptotic proteins, such as SMAC and cytochrome C, which then activate caspases and lead to apoptosis.

What do enzyme-linked transmembrane receptors do?

Ligand that binds to extracellular receptor initiates enzymatic activity intracellularly.

What is second messenger signaling?

Intracellular molecules or ions relay signals received at the cell surface to target molecules within the cell, ultimately triggering a cellular response.

What are examples of secondary messengers?

IP3

DAG

cAMP

Ca2+ ions

What is a signal transduction?

Cells convert an external signal, like a hormone, into a specific internal action.

What is the significance of the gene BCL2 in apoptosis and teeth?

Inhibits apoptosis

Involved in odontoblast function and dentin repair in teeth.

Describe cAMP’s pathway?

Generated by adenylyl cyclase»»»ACTIVATES protein kinase A (PKA)»»»Phosphorylates target proteins

What happens when there is an INCREASE in intracellular Ca2+ ions?

Activates downstream targets

What do IP3 and DAG do?

Activate protein kinase C (PKC)

What are the types of cellular responses ? (5)

1. Changes in protein synthesis and activity

2. Cell growth and division

3. Cell movement and shape change

4. Metabolic changes

5. Gene expression

What are the KEY aspects of cellular responses? (4)

1. Specificity» HIGHLY specific

2. Integration» integrate multiple signals

3. Complexity» complex (involves multiple steps)

4. Variability» different cell types can have different responses to the SAME signal

How many transmembrane domains do GPCRs have? Describe them.

7

Where N-terminal is located extracellularly and C-terminal is located intracellularly.

3 loops are outside and 3 loops are inside

Made of alpha-helices

What are the 3 parts of GPCRs?

1. Transmembrane receptor

2. G-protein

3. Effector

How do GPCRs crucial for humans?

They are therapeutic (drug therapy)

What does the G protein in GPCRs consist of?

Alpha, beta, and gamma subunits (heterotrimers)

GTP-binding proteins

What is the ligand binding process of GPCRs?

1. Ligand binds to extracellular part of GPCR

2. Binding causes a CONFORMATIONAL change to the GPCR

3. GPCR is activated and interacts with the intracellular G protein

4. G-protein dissociates where subunits of G-protein interact with other proteins/enzymes (especially alpha)

5. Specific cellular response is produced

What does the Ga i subunit do?

INHIBITS AC

What does the Gq a do to PLC?

ACTIVATE PLC (phospholipids C)»PLC cleaves PIP2 into DAG and IP3 (HYDROLYSIS)» IP3 liberates Ca2+ intracellularly» DAG activates protein kinase C (PKC)

What does G12/13 a do?

Couple through guanine factors for small GTP binding proteins (Rho and Rac)

What does Ga s do to cAMP?

Stimulates AC» converts ATP to cAMP» cAMP activates protein kinase A (PKA)

What does Ga do to PLA2?

Activates Phospholipase A2 (PLA2)»release of arachidonic acid» metabolized to thrombonaxes, etc.

Which pathway do Odonotblasts use?

G-protein/PLC pathway

What is PLC activity tied to in teeth?

Odontoblasts and Mineralization

Why is the Wnt signaling pathway critical for teeth?

Influences formation of tooth structures, including enamel, dentin, and cementum.

What happens to the teeth if the Wnt signaling pathway is dsyregulated?

Dental abnormalities

Arrested tooth development

Describe an activated Wnt signaling pathway.

In the presence of a WNT ligand, phosphorylation and degradation of β-catenin are inhibited, allowing it to accumulate in the cytoplasm and translocate into the nucleus FOR GENE TRANSCRIPTION!

Describe an INactivated Wnt signaling pathway.

In the absence of a WNT ligand, cytoplasmic β-catenin is associated proteins that CAUSE DEGRADATION!!

Proteolysis occurs and NO GENE TRANSCRIPTION

What is the significance of the BMP signaling pathway?

Crucial for development/embryonic and homeostasis

Bone formation

Cell differentiation

Describe the signaling cascade of the BMP signaling pathway.

1. A BMP ligand binds to the BMP type I and type II receptors on the cell surface.

2. binding triggers a conformational change, and the Type II receptor phosphorylates the Type I receptor»ACTIVATING it

3. activated Type I receptor then phosphorylates Smad proteins» forming Smad4 complex

4. Smad complexes move into the nucleus

5. Smad complexes regulate the transcription of target genes

What are BMPs?

Bone morphogenetic proteins (BMPs) that are GROWTH factors.

What is Dentinogenesis imperfecta (DGI)?

Disrupted BMP signaling leading to DEFECTIVE dentin formation and REDUCED expression of key dentin matrix proteins.

Dysregulation of the BMP pathway impairs what?

Odontoblast differentiation» crucial for proper dentin mineralization» results in tooth abnormalities

What does an activated EGFR pathway do?

EGF binds to receptor»trigger downstream signaling pathways, including the MAPK, PI3K/Akt, and JAK/STAT pathways!

What are EGF receptors ?

a type of receptor tyrosine kinase (RTK), which is a transmembrane protein that, upon binding to its ligand, initiates intracellular signaling pathways.

What is the NFkB pathway?

crucial signaling cascade that regulates gene expression related to immunity, inflammation, cell survival, and stress responses.

How many pathways does the NFkB pathway have and what are they called?

2

Canonical and NON-canonical

Describe the Canonical NFKB pathway.

1. Triggered by LPS, inflammatory cytokines, and cellular stress.

2. IKK complex phosphorylates IkB proteins» leads to release of NFkB complex

3. NFkB complex translocates into the nucleus and activates gene expression

What is the non-canonical pathway important for?

non-canonical pathway is important for the development of lymphoid organs, B cell maturation, and immune responses.

What are the MAJOR differences between the canonical and non-canonical pathway?

1. Canonical» more broad

   1. Non-canonical» specific to TNF

2. Canonical»primary relies on IKKB

   1. Non-canonical relies on IKKA

3. Canonical» IKB degradation

   1. Non-canonical »p100 processing

4. Canonical» p65/p50 dimers

   1. Non-canonical»p52/RelB dimers

5. Canonical is FASTER

   1. Non-canonical is SLOWER

What is notch signaling?

a ligand on a sending cell binds to a Notch receptor on a receiving cell, triggering two proteolysis events (cleavages) that release the Notch Intracellular Domain (NICD)

NICD then moves to the nucleus» activates the transcription of specific genes, thereby affecting cell fate and function.

What type of cell signaling is Notch?

Juxtacrine (direct cell to cell)

What does Notch signaling govern in teeth?

Craniofacial patterning and tooth shape

What do disruptions to notch signaling govern?

Abnormalities in the jaw and dentition

Which 1 pathway is important for periodontics?

NFKB pathway