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From CellBiology

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Apoptosis Inducing Factor (AIF)

What is AIF?

The AIF gene has been conserved throughout the eukaryotes. It is a phylogenetically old flavoprotien. It is called a flavoprotein because it is able to be stably bound to FAD (flavin adenine dinucleotide). AIF is a positive intrinsic regulator of apoptosis and a redox enzyme needed for normal respiratory function within the mitochondria (Feraud et al, 2006). It is a mitochondrial oxidoreductase. AIF resides within the inter-membranous layer of the mitochondria. There are said to be three isoforms , AIF, AIF-exB (different exon), and AIFsh (of short).

Topology and Gene location

Crystal structure

ZAIF crystal structure.JPG

  • AIF consists of a three domains, a mitochondrial localization sequence (MLS), a spacer sequence, and a oxidoreductase amino acid domain (Cande et al, 2002).

Gene location

Xq25-q26. The gene is made up of 17 exons. Image:

How it works

There are two proposed functions in which AIF has been associated with, free radical scavengers and DNA fragmentation in apoptosis. The first has been well documented in AIF knockout mice, which has seen resulting in neuronal degeneration (Klein et al, 2002). It should be mentioned that mouse AIF has strong 92% amino acid similarity homology to human AIF (Cande et al, 2002). This study by Klein et al (2002) demonstrated that without AIF expression there is oxidative stress from too many free radicals. The second is a caspase independent pathway via which apoptosis is induced. AIF in this pathway contributed to the fragmentation of DNA and chromatin condensation (Millan et al, 2007). The later pathway is the focus of this report.

Arriving in the mitochondria

AIF is transcribed from X chromosome gene within in the nucleus. It is translated to the cytosol of the cell where if forms a precursor protein (Modjtahedi et al, 2006). It was mentioned that AIF resides within the mitochondrion, and it does. AIF is translocated to the inner membranous region of the organelle where it truncated to become matures. Here the AIF is responsible for metabolite redox and vital bioenergetics within the mitochondria (Modjtahedi et al, 2006). It remains within the mitochondria until sufficient stimulus has altered the mitochondrial outer membrane allowing translation of the AIF back into the cytosol.

Getting out of the Mitochondria

The exact mechanism by which the outer membrane of the mitochondria becomes more permeable to AIF is not fully understood. It has been suggested that death stimuli such as oxidative stress and DNA damage begin the cascade. There are two mechanisms which have been hypothesised to induce AIF translocating out of the mitochondria from death stimuli (Vosler et al, 2009). Recent studies by Volser et al (2009) show that PARP-1, and calcium dependent cystien protease calprin are the triggers of AIF release from the mitochondria as well as nuclear translocation (Wang et al., 2004; Culmsee et al., 2005; Moubarak et al., 2007). This study showed that calprin activity is dependent on that of PARP-1. This downstream regulation was shown to be modulated by calcium levels within the mitochondria; however this exact mechanism is not known (Vosler et al, 2009).

In the cytosol

Once in the cytosol AIF cleaves into a smaller protein, catalysted by the enzymatic reaction with the activated from of calprins and cathepsin. The AIF in the cytosol can be inhibited by Heat shock protein 70 family (Hsp70). If it is not inhibited the AIF translocates to the nucleus. One hypothesis made by Daugas et al (2000) suggested that this translocation may be due to ATP depletion or NAD+ depletion. It must be emphasised that many of these AIF pathways are speculations.

To the nucleus

Once the AIF is within the nucleus, it has been shown to bind to cyclophilians A (Cyp A). The AIF and the Cyp A bind to DNA where there is the formation of a trimolecular complex. This interaction allows for the degeneration of the DNA (Modjtahedi et al, 2006). Images from Ye et al (2002) immunostaining showed colocalization of AIF with DNA during the first stage of chromatin condensation. This process converts chromatin to high molecular weight segments (Bajti et al, 2006). This idea that AIF binding to DNA is significant for cell death has been shown in AIF mutants. Studies have indicated that AIF mutants are defective in DNA binding and in consequence are defective at inducing cell death.

The overall mechanism of DNA degeneration via AIF has not been fully uncovered. Like many of the other processes ideas of their workings have been hypothesised. One idea is that the AIF binding to DNA recruits the enzymes proteases and nucleases inducing the chromatin condensation. Another idea is that the binding of AIF to DNA may displace specific chromatin-associated proteins, disrupting the normal structure and leading to a collapse of the DNA.

Signaling

Diagram

ZAIF Diagram.jpg

Current studies

AIF and Diabetes in Animal models

A recent study by Schulthess et al (2009) demonstrated the importance of AIF in maintaining beta cell mass in mice. It is this decline of beta cell mass which is a major contributor to diabetes within humans. Harlequin (Hq) mutant mice have been used in previous studies to demonstrate the role of AIF in scavenging for free radicals to prevent apoptosis (Modjtahedi et al, 2006). Hq mice mutant are inserted with a provirus into the gene which results in 80% less AIF production. As a consequence these mice show evidence of a significant reduction in oxidative phosphorylation (OxP) and subsequently a reduction of ATP. This effect of reduced OxP and mitochondrial function has been shown to influence insulin resistance (Schulthess et al, 2009). In this current study by Schulthess et al (2009) the depletion of AIF in mice showed a significant decrease of beta cell mass and a continual decline with age. It was shown that the proliferation of the beta cells did not reduce, but it was the cell cycle that was disrupted. It was hypothesised that the G2 phase of the cell’s development was interrupted by the presence of oxidative stress. it could then be hypothesised that it is the AIF which scavenge for oxidants. A G2 check point is triggered by the oxidative stress and this server disruption induces apoptosis. The effect of a depletion of AIF could be a significant factor in beta cell mass loss.

AIF in Colon Cancer

It is known that some cellular changes that inhibit apoptosis can induce the development of abnormal growths, such as cancer. It has been seen that blockage of the AIF signalling pathway could be implicated in chemo-resistance in some cancer types such as non small cell lung carcinoma. Unlike these cancers colon cancer has shown to have a different interaction with AIF. AIF has been seen to reduce chemical induced apoptosis and it sustains the abnormal formation of the malignant cell. Although this concept has some unsolved issues it has been shown by Urbano et al (2009) that AIF deficient cells were excessively susceptible to apoptosis. AIF could in the future be a potential target for cancer drug therapy. This enables a potential drug alternative to those patients whom have developed drug resistance to regular chemoradiotherapeutic intervention.

Conclusion

Terms

  • oxidoreductase
  • free radical
  • redox
  • bioenergetics
  • PARP-1
  • calprins
  • cathepsin
  • ATP
  • NAD+
  • immunostaining
  • chromatin

References

Candé C, Vahsen N, Kouranti I, Schmitt E, Daugas E, Spahr C, Luban J, Kroemer RT, Giordanetto F, Garrido C, Penninger JM, Kroemer G (2004) AIF and cyclophilin A cooperate in apoptosis-associated chromatinolysis. Oncogene. 26;23(8):1514-21.

Candé C, Cohen I, Daugas E, Ravagnan L, Larochette N, Zamzami N, Kroemer G (2002). Apoptosis-inducing factor (AIF): a novel caspase-independent death effector released from mitochondria. Biochimie. 84(2-3):215-22.

Millan A, Huerta S (2009) Apoptosis-inducing factor and colon cancer. J Surg Res. 151(1):163-70. Epub 2007 Dec 3

Modjtahedi N, Giordanetto F, Madeo F, Kroemer G (2006) Apoptosis-inducing factor: vital and lethal. Trends Cell Biol. 16(5):264-72. Epub 2006 Apr 18.

Joza N, Susin SA, Daugas E, Stanford WL, Cho SK, Li CY, Sasaki T, Elia AJ, Cheng HY, Ravagnan L, Ferri KF, Zamzami N, Wakeham A, Hakem R, Yoshida H, Kong YY, Mak TW, Zúñiga-Pflücker JC, Kroemer G, Penninger JM.(2000)Essential role of the mitochondrial apoptosis-inducing factor in programmed cell death.Nature.410(6828):549-54

Lorenzo HK, Susin SA, Penninger J, Kroemer G.(1999). Apoptosis inducing factor (AIF): a phylogenetically old, caspase-independent effector of cell death.Cell Death Differ.6(6):516-24.

New Articles:

Porter AG, Urbano AGL. (2006) Does apoptosis-inducing factor (AIF) have both life and death functions in cells?. BioEssays 28:834-843.

Knockout AIF mice: The embroyoid cavity fails to form killing mice before birth. Demonstrating the need for apoptosis in tissue morphology.

Feedback

--Beatrix Palmer 15:52, 11 May 2009 (EST) mark, could you please remove an uploaded pic, z crystal structure AIF. thanks.

--Mark Hill 09:22, 16 April 2009 (EST) All on track, kee up the good work!

--Mark Hill 18:13, 19 March 2009 (EST) Thank you for your feedback. Yes nucleosomes are important, not only for DNA packing, but also in our initial understanding of how apoptosis works.

  • Formed by DNA wrapped around histones.

--Mark Hill 15:25, 25 March 2009 (EST) The complete ribosome does not exist unless attached to a messenger RNA, it occurs as the 2 major subunits in the cytoplasm. Initial binding of a subunit occurs at the 5' amino- encoding end of the mRNA and the other subunit now assembles to form the complete ribosome. t-RNA now docks and brings the first amino acid and protein synthesis at AA1 has begun.The next AA is added and synthesis occurs as the ribosome translocates (moves) along the mRNA to the 3' carboxy- terminal at the end of the forming protein.

--Mark Hill 22:44, 31 March 2009 (EST) Yes both these processes, but I had told you these in the lecture, so it was not as if you have tried to find additional cellular processes that require lots of energy. --Beatrix Palmer 15:56, 5 April 2009 (EST) I did look on the internet after class as well, i just thought you wanted a couple of examples. sorry.

  • Muscle cells (cardiac, skeletal and smooth muscle cels), for contraction.
  • Sperm, within the tail for motility.

--Mark Hill 09:39, 3 April 2009 (EST) Correct for CAM terms, lots of science today uses acronyms (because the terms or gene names are very long), but what this means is that several acronyms can mean different things to different people.