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From CellBiology

Bcl-2

Anti-apoptoptotic protein of Bcl-2 protein family


Introduction

The Bcl-2 protein family is involved in the major regulating mechanism of the apoptosis, and it consists of 25 different protein members known to date. The Bcl-2 protein family has both pro-and anti-apoptotic protein groups. Bcl-2 is one of the anti-apoptotic proteins of this family and its main role is suppression of apoptosis progression. Action of anti-apoptotic protein is important since it gives cells time to fix damage and return to their normal state. Without the anti-apoptoic protein family, apoptosis may be uncontrollable and normal cells with the apoptotic mechanism initiated are unable to stop the process, and it may pass the point where it can no longer return to their normal state but instead die.The proto-oncogene Bcl-2 and its role in regulating apoptosis.[1]

Figure1:antiapoptotic vs proapoptotic proteins of Bcl-2 family


Location

Bcl-2 and other anti-apoptotic members are found in the mitochondria, endoplasmic reticulum (ER), or nuclear membrane. It's one of the integral membrane proteins. In contrast, a substantial fraction of the pro-apoptotic members are localized in cytosol or cytoskeleton prior to a death signal. [2]:


Structure

Bcl-2 family members possess up to four conserved Bcl-2 homology (BH) domains designated BH1, BH2, BH3, and BH4. Bcl-2 and many of the anti-apoptotic members display sequence conservation in all four domains. In contrast, pro-apoptotic molecules frequently display less conservation of the first alpha-helical segment. [3]:

Of these four homology domains, BH-3 domain serves as a critical death domain, which has an important role in protein-protein interactions and cell death. Bcl-2 protein has a hydrophobic face of BH3 domain burried with amino terminus, disabling its BH3 action. In contrast pro-apoptotic protein subunit has BH3 domain constituitively exposed or exposed following the death signals. This conformational difference enables proapoptotic protein to interact with other proteins to perform proapoptotic role. Interestingly cleavage of the amino terminus of Bcl-2 can lead to the exposure their BH3 domain surface, and it may convert them into pro-apoptotic molecules. [4]:

Many Bcl-2 family members also contain a carboxy-terminal hydro-phobic domain, which in the case of Bcl-2 is essential for its targeting to membranes such as the mitochondrial outer membrane.


Function

A wide variety of stimuli can trigger apoptosis. The stimuli can be physical such as intracellular Ca2+ concentration increase, oxidative stress, raditation, or it can be pathological such as viruses. The stimuli commonly induces activation of apoptosis pathway. There are various mechanisms in the cell regulating the progress of apoptosis. Bcl-2 is the one of the protein involved in this regulating mechanisms.


  • Guarding of the mitochondria membrane
Figure2:Apoptosis mitochondrial pathway

A major site of activity of the Bcl-2 proteins is the mitochondrial membrane. They are constituitively activated in the normal healthy cell, and it guards the mitochondrial membrane from the proapoptotic or pore forming molecules as it may disturb the mitochondria membrane and lead to the formation of pore on the membrane which may lead to releases cytochome c and other proteolytic molecules.[4]

Following a death signal, the pro-apoptotic members undergo a conformational change that enables them to target and integrate into membranes, especially the mitochondrial outer membrane. Integrated pro-apoptotic members can form the The permeability transition pore (PTP). Opening of the PTP results the release of the cytochrome c which activates the cascades of proteolytic protein activation and progression of the apoptosis. Following the opening of the PTP also results in mitochondrial depolarization, uncoupling of oxidative phosphorylation, swelling of the mitochondria and release of Pro-caspase-2 and pro caspase-9.

Bcl-2 can regulate the activation of the mitochondrial pool and release of cytochrome c as it is phosphorylated following the anti-apoptotic stimuli and this modification activate its anti-apoptotic activity. [4].

BAD antagonizes both cell cycle and antiapoptotic functions of Bcl-2 through BH3 binding. The General process is shown in figure 2.


  • Cell cycle regulation

Bcl-2 directly controls the cell cycle. Main activity of the Bcl-2 in the cell cycle is antiproliferative by arresting the cell cycle in G0 phase and inhibition of G0 to S-phase progression, in contrast BAX(proapoptotic) accelerates S-phase progression. Bcl-2 doesn't directly repair the cell damage, but it gives cell a time of delayed G0 phase and G1 phase to repair the damage. And upon the stimuli, the cell can go back to their normal cell cycle. With this mechanisms cells can keep its viability upon death signal, and avoid apoptosis. [5]


  • Prevention of capase activation

A central checkpoint of apoptosis is the activation of Caspase-9 by mitochondria. Bcl-2 can bind to the C terminal part of Apaf-1 thus inhibiting the association of Caspase-9 with Apaf-1, consequently inhibiting apoptosis. [6]


abnormalities

Pro- and anti-apoptotic mechanisms enables cell to regulate the rate of the apoptosis. Any imbalance of this activity could lead to the pathological conditions of the cell. Bcl-2 protein is known as pro-oncogenic, as it suppresses apoptosis which may lead to tumorgenesis.

  • Over expression of the Bcl-2 increase resistance of the cell to the apoptosis, consequently leads to uncontrolled proliferations of the cell and lead to cancer. It is also involved in the growth of resistance to anticancer chemotherapy.
  • Defects in expression of Bcl-2, or unable to transcribe Bcl-2 may cause the cell unable to stop the progress of the apoptosis, and lead to the neurodegenerative disease such as Schizophrenia, and Alzheimers disease.[6]




References

  1. roemer G.The proto-oncogene Bcl-2 and its role in regulating apoptosis. 2007 Mar;50(1):2-9. Review. [1]
  2. Zhu, A Cowie, G W Wasfy, L Z Penn, B Leber, and D W Andrews. EBcl-2 mutants with restricted subcellular location reveal spatially distinct pathways for apoptosis in different cell types. EMBO J. 1996 August 15; 15(16): 4130–4141. [2]
  3. Chou, J.J., H. Li, G.S. Salvesen, J. Yuan, and G. Wagner. 1999. Solution structure of BID, an intracellular amplifier of apoptotic signaling. Cell 96:615-624. [3]
  4. 4.0 4.1 4.2 Gross. A, McDonnell JM, Korsmeyer SJ.BCL-2 family members and the mitochondria in apoptosis. Genes Dev. 1999 Aug 1;13(15):1899-911. [4]
  5. Zinkel S, Gross A, Yang E. BCL2 family in DNA damage and cell cycle control. Cell Death Differ. 2006 Aug;13(8):1351-9. Epub 2006 Jun 9. [5]
  6. 6.0 6.1 Reed JC. Dysregulation of apoptosis in cancer.J Clin Oncol. 1999 Sep;17(9):2941-53. Review. [6]



Glossary

  • G0 phase : itis a resting phase where the cell has left the cycle and has stopped dividing
  • G1 phase : Cells increase in size in Gap 1. The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.
  • S phase : DNA replication occurs during this phase.
  • Proto-oncogene :A normal cellular gene that, when mutated or inappropriately expressed, can cause a cell to become cancerous.
  • PTP : The PTP (permeability transition pore) is a large conductance pore that evolves in mitochondria following necrotic or apoptotic signals. The PTP is permeable to solutes with a molecular mass of ~1500 daltons when studies in vitro. Opening of the PTP results in mitochondrial depolarization, uncoupling of oxidative phosphorylation, annd swelling of the mitochondria. Pro-caspase-2 and pro caspase-9 are released from the mitochondria following PTP opening. Bcl-2 can regulate the activation of the mitochondrial pool, thus suppresses release of cytochrome c, which activates a downstream caspase program.


--Mark Hill 09:26, 20 April 2009 (EST) Please read my comments on your discussion page.

HOMEWORK

week 4

Q: Type of cellular process requires large energy

A: Muscle, sperm tail, flagella

Lecture 5

- Exocytosis -

Q:What concept about exocytosis did you find difficult to understand?

A:Generally, lecture material was clear to understand. But I was curious of the mechanisms how the vesicles are transported within the cell. I understand that vesicles uses microtubules as a railway, but in cytoplasm there are lots of cytoskeleton such as microtubule, intermediate filament, and micro filament which are interwoven, wouldn't this cytoskeleton structure interfere the vescicle transportation? And how does vescicle know where they are destined?

Lecture 7

- Mitochondria -

Q:What types of cellular processes require lots of energy from the mitochondria?

A:Detoxification in the hepatocyte, contraction of the skeletal muscle cell and movement of flagella of the sperm cell as well as the activity of neuronal cells are the process with high metabolic activity. They need large amount of energy in the form of ATP, and these ATP is largely produced from mitochondria via aerobic respiration.

Lecture 8

- Adhesion -

Q:What do the different "CAM" acronyms stand for?

A: CAM is abbreviation of cell adhesion molecule, and there are different kinds of CAMs,

1)N-CAM ; neural cell adhesion molecule

2)Ng-CAM ; neuron-glia cell adhesion molecule

3)L-CAM ; liver cell adhesion molecule

4)I-CAM ; intercellular adhesion molecule

and others..

5)VCAM ; vascular cell adhesion molecule

6)PECAM ; Platelet-endothelial Cell Adhesion Molecule

Lecture 10

Q: What is the name of the epidermal layer between the basal and granulosa layer and how does it relate to intermediate filaments?

A: Stratum spinosum layer is found between the basal and granulosa layer, this is the layer where keratinization begins.Keratin is the type of intermediate filaments, keratin protein polymer form filamentous polymers and undergoes series of assembly steps to dimer, tetramer, octamer and eventually unit-length-filament which are capable of anneal end to end into a long filament.

Lab 6

Q: "If you've seen differences in the distribution of phenotypes in Tm4 over-expressing B35 cells versus control B35 cells, describe these differences. Formulate a hypothesis with regards to what changes on the molecular level may have occurred due to the over-expression of Tm4 that lead to morphological changes that you have observed"

A:

Total for [A] Phenotypes: A - 0 B - 7 C - 41 D - 81 E - 64 F - 17

Total for [B] Phenotypes: A - 8 B - 93 C - 58 D - 75 E - 27 F - 9

This was the result, and Phenotype A shows more prolonged type of cells, while Phenotype be shows large number of broken fan type. This shows Tm4 over expression of the cell affects the cytoskelton structure of the cell, leading to the morphological change of the neuron cell.

Lecture 14

- Confocal Microscopy -

Q:What are the 2 main forms of generating confocal microscopy?

A:Confocal microscopy has adventages over conventional widefield optical microscopy as it can obtain high resolution optical images.This is achieved with lasers and aperture of the light detector of the microscope.

Confocal microscopy uses monochromatic laser and specific fluorescent techniques which stimulate light and amplifies into intense and focused beams, this significantly enhances contrast of the image.

And the detector aperture obstructs the light that is not coming from the focal point. The out-of-focus light is eliminated by the detector aperture and only the focused light is passed through the aperture and collected throughout the scanning. This enhanced the images resolution, it also permits the microscope to obtain images of planes at various depths within the sample and enables to make 2D and 3D images.

Lecture 15

-cell cycle-

Q: Origin of the "S" in the s phase of cell cycle

A: "s" in the s phase of cell cycle originated from word "synthesis", during this phase DNS replication and synthesis occurs