Retta - L19

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Questions and Answers

What role does cytochrome-c play in the intrinsic pathway of apoptosis?

It forms the apoptosome with Apaf1. (correct)

It triggers the opening of mitochondrial pores.

It is responsible for mitochondrial membrane potential maintenance.

It activates executioner caspases directly.

Which proteins are primarily responsible for the opening of pores in the outer membrane of mitochondria during apoptosis?

Bak and BAX (correct)

BAX and BCL-2

Cytochrome-c and serine proteases

Apaf1 and caspase 9

What occurs after cytochrome-c is released from mitochondria during the intrinsic apoptosis pathway?

It is degraded by proteasomes.

It directly activates executioner caspases.

It combines with Apaf1 to form the apoptosome. (correct)

It enters the nucleus to initiate DNA fragmentation.

What is the consequence of treating cells with stressful conditions like UV radiation in the context of apoptosis?

It induces the release of cytochrome-c into the cytoplasm. Signup and view all the answers

What characterizes the distribution of cytochrome-c observed under apoptotic conditions compared to normal conditions?

A diffuse distribution indicating release from mitochondria. Signup and view all the answers

What is the primary function of BH3 only proteins within the Bcl2 superfamily?

To promote apoptosis by inhibiting antiapoptotic proteins Signup and view all the answers

Which characteristic distinguishes BH3 only proteins from other Bcl2 superfamily members?

They contain only one BH domain. Signup and view all the answers

What is the role of Bax and Bak within the apoptosis pathway regulated by the Bcl2 superfamily?

To promote the formation of mitochondrial pores Signup and view all the answers

Which of the following correctly describes the roles of Bax and Bak in apoptosis?

Bax and Bak promote apoptosis by forming pores in the mitochondrial membrane. Signup and view all the answers

What is the function of the protein Bcl2 in the process of apoptosis?

Bcl2 inhibits apoptosis by preventing the release of cytochrome-c. Signup and view all the answers

What effect does the activation of BH3 only proteins have on Bcl2?

It inhibits Bcl2, promoting apoptosis. Signup and view all the answers

How many Bcl2 homology domains do proapoptotic proteins such as Bak and Bax contain?

Three BH domains. Signup and view all the answers

How does truncated Bid (tBID) influence mitochondrial function in the context of apoptosis?

tBID promotes the formation of pores in the mitochondrial outer membrane. Signup and view all the answers

In the context of apoptosis, what is an effect of a fully active Bcl2 protein?

Inhibition of proapoptotic factors. Signup and view all the answers

Which proteins act upstream of the intrinsic pathway of apoptosis by inhibiting Bcl2?

PUMA and NOXA Signup and view all the answers

What role does sMAC/diablo play in apoptosis signaling?

sMAC/diablo inhibits the inhibitor of apoptosis proteins, allowing caspases to become active. Signup and view all the answers

Which process is responsible for the initial response of cells to stress before apoptosis can occur?

Autophagy Signup and view all the answers

How does the intrinsic pathway of apoptosis become activated?

By mitochondrial dysfunction or autophagy inhibition Signup and view all the answers

Which of the following correctly describes a bystander effect in the context of viral infection-induced apoptosis?

Neighboring uninfected cells can also undergo apoptosis. Signup and view all the answers

What is the role of phagocytes in relation to apoptotic cells?

To recognize and remove apoptotic cells without causing inflammation Signup and view all the answers

Which of the following is NOT a characteristic of apoptotic cells recognized by phagocytes?

Release of proinflammatory cytokines Signup and view all the answers

What happens when autophagy is inhibited under basal stressful conditions?

Cell death can occur even with low levels of stress. Signup and view all the answers

What is the role of growth factors in relation to apoptosis?

They facilitate cell survival by promoting proliferation. Signup and view all the answers

How can survival factors inhibit apoptosis through Bcl2?

By increasing the production of Bcl2 proteins. Signup and view all the answers

What is the effect of neurotrophic factors on neuronal cells?

Insufficient neurotrophic factors can cause neuronal death. Signup and view all the answers

Which pathway can be activated by survival factors to suppress BH3 only proteins?

Inactivation of upstream regulators of Bcl2. Signup and view all the answers

What is the role of p53 in cellular responses to DNA damage?

p53 regulates both apoptosis and cell cycle arrest in response to DNA damage. Signup and view all the answers

Which of the following describes an oncogenic mutation related to Bcl2?

It prevents apoptosis through the overexpression of Bcl2. Signup and view all the answers

Which effect does radiation therapy aim for in damaged cells?

To induce apoptosis to prevent mutation propagation. Signup and view all the answers

What results from the excessive upregulation of survival mechanisms in cancer therapy?

Resistance to chemotherapy and radiotherapy. Signup and view all the answers

How do inhibitors of apoptosis (IAP) function in cell survival?

They block signals that induce apoptosis. Signup and view all the answers

What role do pro-apoptotic proteins Bak and Bax play in apoptosis?

They induce mitochondrial outer membrane permeabilization. Signup and view all the answers

How does the overexpression of Bcl2 relate to cancer progression?

It provides an intrinsic survival advantage to cancer cells. Signup and view all the answers

What is the function of IAP proteins in relation to apoptosis?

They prevent the activity of caspases to inhibit apoptosis. Signup and view all the answers

What is the consequence of excessive mitochondrial hyperpermeabilization?

It triggers necrosis instead of apoptosis. Signup and view all the answers

Which of the following statements about BH3-only proteins is correct?

They inhibit Bcl2, enabling Bak and Bax to promote apoptosis. Signup and view all the answers

What initiates the activation of BH3-only proteins such as PUMA?

Intracellular damage including DNA damage. Signup and view all the answers

What is the role of SMAC/DIABLO released by the mitochondria?

To inhibit the IAP proteins. Signup and view all the answers

Which protein contributes to both inhibition of Bcl2 and activation of Bak and Bax?

Bid Signup and view all the answers

What is the significance of the diffuse distribution of cytochrome-c observed during apoptosis?

It signifies that cytochrome-c is being released from mitochondria. Signup and view all the answers

How do Bak and Bax proteins facilitate the process of apoptosis?

They promote pore formation in the outer mitochondrial membrane. Signup and view all the answers

What role does the adaptor protein Apaf1 play in the intrinsic pathway of apoptosis?

It recruits inactive caspase 9 to form the apoptosome. Signup and view all the answers

What cellular event is triggered by the opening of pores in the outer mitochondrial membrane?

The release of cytochrome-c into the cytoplasm. Signup and view all the answers

In the context of mitochondrial membrane permeabilization, what does MMP stand for?

Mitochondrial membrane permeability. Signup and view all the answers

What is the primary mechanism by which Bcl2 exerts its anti-apoptotic effects?

Inhibits the formation of pores in the outer mitochondrial membrane. Signup and view all the answers

In the context of the Bcl2 superfamily, how do PUMA and NOXA influence apoptosis?

They promote the oligomerization of Bax and Bak by inhibiting Bcl2. Signup and view all the answers

What role does truncated Bid (tBID) play in the crosstalk between intrinsic and extrinsic apoptosis pathways?

tBID activates the formation of mitochondrial pores through Bax and Bak. Signup and view all the answers

Which of the following statements best describes the role of sMAC/diablo in apoptosis signaling?

sMAC/diablo activates caspases by inhibiting their inhibitors. Signup and view all the answers

How does the activation of p53 relate to apoptosis following DNA damage?

p53 activates proteins that stimulate the intrinsic pathway by inhibiting Bcl2. Signup and view all the answers

What distinguishes the proapoptotic proteins Bak and Bax from the antiapoptotic proteins in the Bcl2 superfamily?

They contain three BH domains. Signup and view all the answers

What is the effect of BH3 only proteins on the Bcl2 and Bax signaling pathways?

They activate proapoptotic signals and inhibit antiapoptotic proteins. Signup and view all the answers

In the context of the Bcl2 superfamily, which statement correctly describes the BH3 only proteins?

They are upstream regulators with a single BH3 domain. Signup and view all the answers

How do antiapoptotic Bcl2 proteins prevent apoptosis from occurring?

By inhibiting the release of proapoptotic factors from mitochondria. Signup and view all the answers

Which upstream signal is known to activate BH3 only proteins such as PUMA?

DNA damage. Signup and view all the answers

What is the principal characteristic of the Bcl2 superfamily of proteins?

They regulate apoptosis through a set of shared BH domains. Signup and view all the answers

What is the primary consequence of Bcl2 inhibition during apoptosis?

Functioning of Bak and Bax to promote apoptosis Signup and view all the answers

Which proteins can directly activate Bak and Bax in the intrinsic pathway of apoptosis?

Bid and Bad Signup and view all the answers

How do IAPs (inhibitors of apoptosis) primarily achieve their role in regulating apoptosis?

By inhibiting the activity of both initiator and effector caspases Signup and view all the answers

What role does the protein SMAC/DIABLO play in apoptosis regulation?

It inhibits IAP proteins, allowing apoptosis to proceed. Signup and view all the answers

What can happen if the mitochondrial permeability transition (mPT) pore is excessively opened?

Only necrosis may happen under certain conditions. Signup and view all the answers

What is the significance of mutations in pro-apoptotic proteins Bax and Bak in cancer progression?

They provide cancer cells with resistance to apoptosis. Signup and view all the answers

What initiates the activation of BH3-only proteins within the intrinsic pathway of apoptosis?

Upstream signals like DNA damage Signup and view all the answers

Which condition results from the action of anti-apoptotic factors like Bcl-2 in relation to apoptosis?

Inhibition of apoptotic signals and cell survival Signup and view all the answers

Which of the following best describes a mechanism by which survival factors inhibit apoptosis?

Survival factors can increase the expression of Bcl2 proteins. Signup and view all the answers

What is the consequence of insufficient neurotrophic factors on neurons?

They promote apoptosis in some neurons. Signup and view all the answers

What can lead to the activation of the intrinsic pathway of apoptosis?

Compounds causing mitochondrial dysfunction Signup and view all the answers

What role does p53 play in cellular responses to DNA damage?

It can induce apoptosis or block the cell cycle. Signup and view all the answers

Through which mechanism does upregulation of IAP likely block apoptosis?

By inhibiting the activation of pro-apoptotic proteins. Signup and view all the answers

Which factor can inhibit autophagy and consequently lead to cell death under otherwise basal conditions?

High levels of ROS production Signup and view all the answers

How can growth factor receptors contribute to apoptosis inhibition?

By activating signal transduction pathways such as MAP kinase. Signup and view all the answers

What is the relationship between viral infection and apoptosis in host cells?

Infected cells can undergo apoptosis through both intrinsic and extrinsic pathways. Signup and view all the answers

What effect does a gain of function mutation in Bcl2 have on apoptosis?

It can lead to cancer by preventing apoptosis. Signup and view all the answers

Which mechanism allows phagocytes to recognize and remove apoptotic cells?

Exposure of PtdSer on the membrane of apoptotic cells Signup and view all the answers

How does the upregulation of death receptors by viral infection affect bystander cells?

It can initiate apoptosis in bystander cells through extrinsic signaling. Signup and view all the answers

Which statement accurately reflects how BH3 only proteins affect cell survival?

They activate pro-apoptotic signals when necessary. Signup and view all the answers

Which of the following factors is primarily associated with the removal of apoptotic cells by phagocytes?

Ligand-receptor interactions promoting cell internalization Signup and view all the answers

What is a potential consequence of radiotherapy in relation to apoptosis?

It can stimulate survival pathways in cancer cells. Signup and view all the answers

What happens when BH3 only proteins are inactivated?

Survival of cells is promoted by normal Bcl2 levels. Signup and view all the answers

Study Notes

Intrinsic Pathway of Apoptosis

The intrinsic pathway is crucially dependent on mitochondria for apoptosis regulation.
Cytochrome c release from mitochondria is a hallmark of the intrinsic apoptotic pathway, monitored using a GFP-cytochrome c fusion protein.
Under stressful conditions (e.g., UV treatment), cytochrome c exhibits a diffuse distribution indicating its release into the cytoplasm.
The release results from the opening of pores in the outer mitochondrial membrane, regulated by BCL-2 family proteins.

Role of BCL-2 Family Proteins

The BCL-2 family comprises both pro-apoptotic (e.g., Bak, Bax) and anti-apoptotic proteins (e.g., Bcl2).
Bak is present on the outer membrane and oligomerizes in response to apoptotic stimuli, while Bax translocates from the cytosol to the membrane to form pores.
Pro-apoptotic proteins promote apoptosis by enabling cytochrome c release, while anti-apoptotic proteins like Bcl2 inhibit this process, preventing apoptosis.

Mechanism of Apoptosome Formation

Cytochrome c released from mitochondria interacts with Apaf1 to form the apoptosome, activating caspase 9 and downstream caspases (caspase 3 and 7) to facilitate cell death.

Cross Talk Between Intrinsic and Extrinsic Pathways

The apoptotic signal can be amplified through common regulators like Bid, which links extrinsic signals (triggering caspase 8) to the intrinsic pathway's mitochondrial responses.
Apoptosis requires more than just cytochrome c release; factors like SMAC/DIABLO are necessary to inhibit anti-apoptotic mechanisms and facilitate caspase activation.

Upstream Regulators

p53 can induce apoptosis by activating proteins like PUMA and NOXA that inhibit Bcl2 and promote Bak and Bax activity, triggering the intrinsic pathway.
BH3-only proteins play a significant role in regulating apoptosis, acting upstream to modulate the balance between pro- and anti-apoptotic factors.

Mitochondrial Permeability Transition (MPT)

MPT pores, formed by various proteins including VDAC and ANT, can lead to necrosis if overly permeabilized, transitioning a cell from apoptosis to necrosis.
Reactive oxygen species (ROS) can induce mitochondrial damage and influence apoptosis and necrosis decisions.

Inhibitor of Apoptosis Proteins (IAPs)

IAPs inhibit both initiator and effector caspases, preventing apoptosis; they were first identified in insect viruses that exploit these proteins to enhance viral replication by inhibiting host cell death.
Cancer progression can involve mutations in pro-apoptotic proteins or overexpression of IAPs, granting survival advantages to tumor cells and contributing to chemotherapy resistance.

Summary of BCL-2 Superfamily Structure

The BCL-2 family consists of three groups:
- Anti-apoptotic proteins (e.g., Bcl2, Bcl-XL) with multiple BH domains.
- Pro-apoptotic proteins (e.g., Bax, Bak) with fewer BH domains.
- BH3-only proteins (e.g., Bad, Bid, Puma, NOXA) with a single BH3 domain, serving as key regulatory elements that inhibit anti-apoptotic factors and promote pro-apoptotic activities.

Final Notes

Understanding the interplay between these proteins and the intrinsic apoptosis pathway is crucial in the context of diseases, especially cancer, where apoptosis regulation is often disrupted.
Targeting these pathways offers potential therapeutic strategies to induce apoptosis in cancer cells or to manage cellular responses in various pathological conditions.### IAP and Caspases
IAP (Inhibitor of Apoptosis Proteins) inhibits caspase activation, preventing apoptosis even after apoptosome formation.
XIAP acts downstream to block caspases specifically to inhibit apoptosis.

Extracellular Survival Factors

Signals promoting cell survival include:
- Cell attachment to the extracellular matrix.
- The presence of growth factors which support cell proliferation.
Limited survival factors can induce apoptosis in certain cells while allowing others to thrive.

Neurotrophic Factors

Insufficient neurotrophic factors lead to neuronal death.
Neurotrophic factors stimulate prosurvival signals and inhibit apoptosis.

Mechanisms of Action of Survival Factors

Survival factors increase production of antiapoptotic Bcl2 proteins, blocking apoptosis.
They may inactivate BH3-only proteins, which are upstream apoptotic regulators.
Survival factors can inhibit IAP inhibitors, contributing to apoptosis suppression.

p53 and Apoptosis Regulation

p53 can activate apoptotic regulators like Bax and BH3-only proteins in response to DNA damage.
Mutations in p53 can lead to tumorigenesis, affecting cell cycle and apoptosis balance.
DNA damage triggers p53 to upregulate apoptosis or induce cell cycle arrest for repair.

Chemotherapy and Radiotherapy

Both therapies aim to induce apoptosis in damaged cells, but tumor cells may evade this by activating survival mechanisms.

Autophagy and Apoptosis

Autophagy first occurs in response to stress to remove damaged components.
Blocked autophagy under stress can lead to apoptosis, even under low stress conditions.

Mitochondrial Dysfunction and Apoptosis

Compounds that induce ROS can lead to mitochondrial membrane potential loss, facilitating apoptosis.
Cyanide, for example, inhibits cytochrome oxidase to enhance ROS production and apoptosis.

Viral Infection and Apoptosis

Viral infections can activate both intrinsic and extrinsic apoptotic pathways.
Infected cells express death receptors recognized by immune cells, promoting apoptosis.

Phagocytosis of Apoptotic Cells

Apoptotic cells send "find me" signals recognized by phagocytes for removal.
Anti-inflammatory cytokines are produced to prevent damage to surrounding tissues during phagocytosis.

Cell Junctions and Extracellular Matrix

Cells interact through cell-cell adhesion or extracellular matrix to form tissues and organs.
Major tissue types include nerve, muscle, blood, lymphoid, epithelial, and connective tissues.

Types of Cell Junctions

Anchoring junctions connect cells to each other or the extracellular matrix.
Occluding junctions form barriers, preventing molecule passage between cells.
Communicating junctions (gap junctions) allow rapid intercellular communication.
Signal relaying junctions mediate neuromuscular interactions.

Junction Classification

Cell-cell junctions: adherent junctions (actin-based) and desmosomes (intermediate filament-based).
Cell-matrix junctions: focal adhesions (actin-based) and hemidesmosomes (intermediate filament-based).
Tight junctions prevent leakage between epithelial cells, crucial for barrier function.

Cadherin Family in Junctions

Cadherins mediate adherent junctions and desmosome formation.
They engage in homophilic interactions, facilitating intercellular adhesion.### Adhesive Protein Dynamics
Adhesive proteins interact with the cytoskeleton via adaptor proteins, creating a connection between the proteins and the cytoskeleton.
Desmosomes utilize specific desmosomal cadherins, whereas adherent junctions use various cadherins specialized for their functions.
Cytoskeletal components differ: desmosomes utilize intermediate filaments, while adherent junctions rely on actin filaments.

Key Junction Proteins

Tight junctions are formed by proteins like claudin, occludin, and JAM, which contribute to cell-cell adhesion integrity.
Anchoring junctions provide strength and resilience to tissues against mechanical stress through a robust membrane structure anchored to cytoskeletal filaments.

Mechanical Stress Resistance

The interaction of adhesive proteins and the cytoskeleton allows cells to withstand substantial mechanical stress, ensuring tissue cohesion.
Traction forces can disrupt tissues, highlighting the importance of anchoring junctions.

Intracellular Adaptation Mechanism

Adaptor proteins mediate interactions between cadherins and cytoskeletal elements, similar to how integrins attach to the extracellular matrix.
Adaptor proteins form distinct plaques on the cytoplasmic side of the membrane, linking junctional complexes to the cytoskeleton.

Cadherins and Their Functions

Cell-cell adherent junctions utilize classical cadherins such as E-cadherin, establishing homophilic interactions with neighboring cells.
Non-classical cadherins in desmosomes interact homophilically with similar cadherins on adjacent cells, supported by specific adaptor proteins like γ-catenin and desmoplakin.

Cell-Matrix Adhesion

Integrins mediate cell-matrix junctions, facilitating heterophilic interactions between integrins and extracellular matrix proteins.
Actin-linked cell-matrix junctions incorporate multiple adaptor proteins, including talin, kindlin, and vinculin, which provide structural and regulatory functions.

Signaling Mechanisms

Both integrins and cadherins contribute to signaling transduction, playing roles beyond mechanical support.
Hemidesmosomes, formed by integrin α/β4 heterodimers, link extracellular matrix proteins to intermediate filaments using adaptor proteins like plectin.

Junctional Structures Overview

Tight junctions involve actin filaments and tight junction proteins, whereas adherent junctions include classical cadherins connected to the actin cytoskeleton.
Desmosomes, mediated by non-classical cadherins, connect to intermediate filaments, and cell-matrix adhesion relies on integrins linking to actin and extracellular matrix proteins.

Cadherin Interactions

Cadherins interact through their extracellular domains, often forming homodimers, reinforced by adaptor proteins like α/β-catenin, linking to actin cytoskeleton.
Focal adhesions consist of integrin heterodimers that connect to extracellular matrix proteins and the actin cytoskeleton, orchestrated by adaptor proteins and kinases.

Dual Role of Adhesive Molecules

Cadherins and integrins serve dual purposes: they provide mechanical stability through adhesive interactions and modulate signaling pathways, including receptor tyrosine kinase activity affecting signal transduction dynamics.