BIOL 4100 Final Exam Material

Cell signaling deals with three things:

1. Signal reception

2. Signal transduction

3. Response/s

Endocrine signals

Produced in endocrine glands and secreted into the bloodstream and are distributed widely throughout the body

Paracrine signals

Released by the cells into the extracellular fluid in their neighborhood and act locally

Neuronal signals

Transmitted electrically along a nerve cell axon

Contact-dependent signaling

a cell-surface-bound signal molecule binds to a receptor protein on an adjacent cell

ABC of cell signaling

A. A limited set of extracellular signals can produce a huge variety of cell behaviors

B. A cell's response to a signal can be fast or slow

C. Some intracellular signaling proteins act as molecular switches

A of ABC of cell signaling

The same signal molecule can induce different responses in different target cells

Different cell types are configured to respond to the neurotransmitter acetylcholine in different ways

Every cell displays a set of receptors that enables it to respond to a specific signal

Each receptor is generally activated by only one type of signal molecule

B of ABC of cell signaling

Changes in gene expression and synthesis of new proteins; they therefore occur relatively slowly

Responses such as cell movement, secretion, or metabolism, occur more quickly

Which of the following signaling pathways would be likely to trigger the most rapid cell response?

B. Adrenaline binds to a GPCR to activate a cyclic AMP signaling pathway that triggers glycogen breakdown.

C. Adrenaline binds to a GPCR to activate a cyclic AMP signaling pathway that triggers the synthesis of hormones in endocrine cells.

D. Nerve growth factor binds to a receptor tyrosine kinase to activate a signaling pathway that enhances the transcription of Bcl2, a protein that suppresses cell death

C of ABCs of cell signaling

Transfers the terminal phosphate group from ATP or GTP to the signaling protein at a specific amino acid

- Tyrosine, threonine, and serine

- Catalyzed by a kinase enzyme

Phosphatases remove a phosphate group from their targets

Signal transduction

Many extracellular signals activate intracellular signaling pathways to change the behavior of the target cell

Intracellular signaling proteins

Can relay, amplify, integrate, distribute, and modulate via feedback an incoming signal

Positive and negative feedback

Cell-surface receptors

relay extracellular signals via intracellular signaling pathways

Ion-channel-coupled receptor

Opens in response to binding an extracellular signal molecule

Substance: Nicotine

Signal: Stimulate acetylcholine-activated ion channel-coupled results in elevation of blood pressure

Two categories: ligand-binding and membrane voltage

G-protein-coupled receptor (GPCR)

Activated receptor signals to a G protein on the cytosolic side

Binds to its extracellular signal molecule, activates a trimeric G protein on the cytosolic side of the plasma membrane

Substance: Morphine and heroin

Signal: Stimulate G-protein-coupled opiate receptors results in relief of pain and euphoria

Enzyme-coupled receptor

An enzyme activity is switched on at the other end of the receptor

Binds its extracellular signal molecule, an enzyme activity is switched on at the other end of the receptor

Substance: Many growth factor and hormones

Signal: Kinases and phosphatases- phosphorylation or dephosphorylation of molecules

Calcium signaling in B-cell

1. Glucose uptake results in increase in ATP

2. ATP increases result in closing of K ATP channels

3. Closed K ATP depolarize the PM and activate Ca2+ uptake

4. Ca2+ triggers the release of insulin

GPCRs

All possess a similar structure

The polypeptide chain traverses the membrane as seven a helices

Extracellular and intracellular loops

All GPCRs interact with nearby heterotrimeric G protein, with 3 subunits: alpha, beta, and gamma subunits

G protein a subunit

Switches itself OFF by hydrolyzing its bound GTP to GDP

Hydrolysis of GTP inactivates the a subunit, which dissociates from its target protein

- a subunit is phosphorylated first and

- Is associated to a Arestin Protein

- Can also associate with clathrin coated-pits to activate endocytosis

a subunit reassociates with a By complex to reform an inactive G protein

Which statement about signaling through GPCRs is correct?

All GPCRs interact with nearby G protein

Which of the following steps are required in the activation of the G-protein signaling pathway?

A. Ligand binds to the G-protein-coupled receptor

C. G-a exchanges GDP for GTP

D. Activated Ga influences target proteins

GPCR regulated K+ channels

Some G proteins regulate K+ channels in heart muscle

Signal molecule: Acetylcholine

Receptor: GPCR

Signal transduction: binding of GTP to a subunit of G protein

Response: Opening of K+ channels and results in slowing of heartbeat

Inactivation: a subunit hydrolysis of its bound GTP to GDP

cAMP

Many G proteins activate membrane-bound enzymes that produce small messenger molecule

Cyclic AMP (cAMP) signaling pathway can activate enzymes and turn on genes

cAMP is synthesized from ATP by adeylyl cyclase

Deactivation involves breaking the cycle by phosphodiesterase, forming AMP

Epinephrine

Stimualtes glycogen breakdown in skeletal muscle cells via cAMP

Signal molecule: epinephrine

Receptor: GPCR

Signal transduction: GPCR > G-protein > adenylyl cyclase > cAMP > PKA > phosphorylase kinase

Response: Glycogen breakdown

Inactivation: Hydrolysis of GTP to GDP by a subunit of G protein

Rise in intracellular cAMP

Can also activate gene transcription

Signal transduction: GPCR > G-protein > adenylyl cyclase > cAMP > PKA > activate transcription regulator

Receptor: Transcription of target gene

Inositol phospholipid (IP)

Triggers a rise in intracellular Ca2+

Signal molecule: phospholipase C beta (PLCB)

Receptor: GPCR

Signal transduction: activated phospholipase C > hydrolyzes IP to IP3 and diacylglycerol

Response: Open ER Ca2+ channel and activated PKC by the help DAG and Ca2+

Inactivation: hydrolysis of GTP to GDP by Ga

Skin

Epithelial layers in certain organs, such as the intestine and skin, renew continuously through life

Tissue renewals involve cell divisions

GPCR signaling pathway

generates a dissolved Gas that carries a signal to adjacent cells

Open Ca2+ channel and activates nitric oxide synthase, stimulating the production of NO

NO diffuses out and causes muscle cells to relax

hematopoietic stem cells

Divides to generate more stem cells, that proliferate and differentiate into the mature blood cell types found in the circulation

Wnt proteins

Secreted by cells in and around the crypt base, especially by the Paneth cells- a gut stem cells

Wnt signaling pathway

maintains the proliferation of the stem cells and precursor cells in the intestinal crypt

Tumorigenesis

Initated by dysregulation of cell growth and developmental pathways

Tumor development

1. Most tumor cells produce constitutively active forms of one or more intracellular signal-transducing proteins

2. Inappropriate production of gene transcription factors can induce cell transformation

3. Loss of growth regulators or apoptotic proteins can contribute to cancer

Cancer

Develop by an accumulation of somatic mutations

Cancer-critical mutations

Cluster in a few fundamental pathways

Carcinomas

90% of cancers

In epithelial tissues

Leukemias

Blood cancer

Lymphoma

Lymph system cancer

Sarcomas

Connective tissues/bone cancer

Rising colorectal cancer in adults

Due to:

- Sedentary lifestyles

- Obesity

- Increased consumption of ultra-processed foods

- Environmental factors

Tumor evolution

Acquires an increasing competitive advantage

A single cell undergoes a mutation that enhances its ability to proliferate or survive (or both), so that its progeny becomes a dominant clone

Oncogenes

Gain of function mutation

Act in a dominant manner

Tumor suppressor genes

Loss of function mutation

Act in a recessive manner

Tumor-suppressor gene loss-of-function mutations

Cell-cycle control proteins, which function to restrain cell proliferation

DNA-repair proteins

Apoptic proteins

Tumor suppressors that promote apoptosis and oncoproteins that promote cell survival

Epigenetic changes

DNA methylation, histone modification, RNAi

HDAC inhibitors act as anticancer therapy

Drugs are being developed to target RNAi

Oncogene activation methods

Gene variants/mutations

Epigenetic changes

Chromosome rearrangements

Gene duplication

HER2 receptor mutation

Changes transmembrane region causes dimerization and constitutive activation of the receptor

Transforms receptor into the NEU oncoprotein, a constituvely active kinase

E.g., some types of breast cancer

EGF receptor mutation

Deletion causing loss of the extracellular ligand-binding domain causes receptor dimerization

Transforms receptor into constitutively activated ErbB oncoprotein kinase

E.g., wide variety of solid tumor

RTK/RAS/MAP kinase pathway

Oncogenic mutations in human cancers

Inactivating mutations found in cancer cells

Ras activity mutations

G12 mutation to any other amino acid inhibits Ras GTPase activity, maintaining Ras in the ON state

Recessive loss-of-function mutation in a GTPase-activating protein (GAP) - Ras in GTP ON state

Breast cancer cell

Multiple translocations

p53

Regulates cell divison by keeping cells from growing and dividing (proliferating) too fast or in an uncontrolled way

Most frequently mutated gene (>50%) in human cancer

Colorectal cancer

Illustrates how loss of a tumor suppressor gene can lead to cancer

Thousands of small polyps, and a few much larger ones, are seen in the lining of the colon of a patient with an inherited APC mutation

The extracellular Wnt signal stimulates several intracellular signal transduction cascades

The APC protein keeps the Wnt signaling pathway inactive when the cell is not exposed to a secreted Wnt signal protein

Wnt proteins in the intestinal crypt are primarily secreted by which cells?

Paneth cells

Where are intestinal stem cells primarily located?

At the base of the crypts

High Wnt signaling in the intestinal crypt mainly promotes what process?

Cell proliferation

As intestinal cells migrate toward the villi, Wnt signaling generally what?

Decreases

Reduced Wnt signaling as cells leave the crypt most directly promotes what?

Differentiation

Which intestinal cell type helps maintain the stem-cell niche by releasing signals such as Wnt?

Paneth cells

If Wnt signaling is experimentally blocked, what is the likely outcome?

Reduced stem cell proliferation

The intestinal epithelium requires continual renewal because it is what?

Rapidly turned over tissue

A mutation causes permanent activation of the Wnt pathway in intestinal crypt cells. What is the most likely result?

Excessive proliferation leading to tumor formation

A patient taking a Wnt inhibitor develops chronic diarrhea. Why?

Impaired epithelial renewal

The inositol phospholipid (IP3) pathway primarily leads to:

Increases in intracellular Ca2+

What is the role of IP3 in signaling?

Opens ER Ca2+ channels

Upon activation, what happens to the G protein subunits?

a separates from By complex

Diacylglycerol (DAG) helps activate:

Protein kinase C

Nitric oxide (NO) produced via GPCR signaling causes:

Smooth muscle relaxation

cAMP is produced from ATP by:

Adenylyl cyclase

What role does arrestin play in GPCR signaling?

Promotes receptor internalization

Clathrin-coated pits are involved in:

Endocytosis of receptors

How is the G protein α subunit inactivated?

Hydrolysis of GTP to GDP

Which sequence correctly represents GPCR signaling?

Ligand binding → G protein activation → effector activation

Which of the following best describes the structure of GPCRs?

Seven transmembrane α-helices with extracellular and intracellular loops

In the unstimulated state, the G protein is:

Bound to GDP and inactive

What directly activates the G protein after ligand binding to a GPCR?

Exchange of GDP for GTP on the α subunit

A mutation in the HER2 receptor transmembrane domain causes spontaneous receptor dimerization in the absence of ligand. What is the most direct consequence?

ligand-independent (constitutive) activation of the HER2 kinase domain

In the experiment below, MEK inhibitor stops proliferation of cells.

Why does the MEK inhibitor also stop proliferation?

Cell cycle arrest, induction of apoptosis

Researchers are studying why a tumor cell line shows continuous cell proliferation even in the absence of growth hormone. They suspect mutations in the RTK/Ras/MAP kinase pathway.

To identify the defective component, they measure MAP kinase activation under different experimental conditions.

Which pathway component is most likely constitutively active in the tumor cell?

Ras

Deletion of the extracellular ligand-binding domain in the EGF receptor can still lead to activation because:

It allows the receptor to form active dimer pairs and activate downstream signaling pathways without ligand binding

Researchers are studying a signaling pathway that requires Protein A, Protein B, and Protein C for cell division. They use mutant cell lines and a constitutively active form of Protein B. Results are shown below:

Based on the experimental results, which of the following represents the most likely order of proteins in the signaling pathway?

A -> B -> C -> Cell division

Researchers treat intestinal tissue with a drug that blocks the Wnt signaling pathway.

What is the most likely effect on the intestinal crypts after several days?

Loss of intestinal stem cells and transient-amplifying cells

What role does the Wnt signaling pathway play in the intestinal crypts?

Maintains intestinal homeostasis, drives ISC maintenance, proliferation, and differentiation

Which tissue renews fastest?

Epithelial

In the presence of ethylene gas, which of the following components of the signaling pathway is correctly described?

Transcription regulator is active

In a hypothetical signaling pathway involving two proteins A and B being disabled by mutation of one protein at a time. What would likely occur if a continuously active form of Ras were introduced into the cell with mutant protein?

Signaling is restored if the mutant Protein is located before Ras in the signaling cascade

Which of the following best represents the signaling pathway under investigation presented below?

RTK → Protein B → Ras → Protein A → → cell response

Endothelial cells regulate vascular tone through signaling pathways that influence adjacent smooth muscle cells. In one pathway, a ligand binds to a G protein-coupled receptor (GPCR) on endothelial cells, leading to activation of phospholipase C (PLC). This results in increased intracellular Ca²⁺, which activates nitric oxide synthase (eNOS). Nitric oxide (NO) then diffuses into nearby smooth muscle cells, where it stimulates soluble guanylyl cyclase, increasing cyclic GMP (cGMP) levels and promoting relaxation.

Which of the following mechanisms most directly explains how increased cGMP levels lead to smooth muscle relaxation?

Activates cGMP-dependent protein kinase, which in turn activates myosin light-chain phosphatase and decreases intracellular Ca2+

Epinephrine stimulates glycogen breakdown through which pathway?

GPCR-cAMP signaling pathway

Acetylcholine acting through GPCRs in heart muscle leads to:

Opening of K⁺ channels → slower heartbeat

Which statement is TRUE about GPCRs?

All GPCRs interact with nearby G proteins

When KATP channels close, the cell membrane becomes __________, which triggers the opening of voltage-sensitive calcium channels.

Hint: Pick more than one answer.

Depolarized

Less negative

Which of the following best describes the sequence of events in calcium signaling in a β-cell that leads to insulin release?

Glucose uptake/metabolism -> ATP increase -> KATP channel closure -> Membrane depolarization -> Voltage gated Ca2+ channel opening -> Ca2+ influx -> insulin vesicle exocytosis

What is the function of phosphatases in signaling pathways?

Remove phosphate groups from proteins to act as "off switches" to deactivate kinases and terminate signals

What happens when a kinase acts on a signaling protein?

Either activate or deactivates it, thereby transmitting signals for processes like growth, metabolism, and gene expression

Which cellular signaling pathway is most likely to produce a rapid response?

Ligand-gated ion channels

Acetylcholine causes contraction in skeletal muscle cells but slows heart rate in cardiac cells.

What explains this difference?

Distinct receptor types

Tyrosine kinase

The largest class of enzyme-coupled receptors relay signal to a cytoplasmic domain that function as this

Receptor Tyrosine Kinases (RTKs)

Activated RTKs recruit a complex of intracellular signaling proteins

Two receptor molecules come together in the plasma membrane to form a dimer

Each receptor tail phosphorylates the other

Triggers intracellular signaling complex

Most activate the monomeric GTPase Ras

Attract an adaptor