Our understanding of amyloid precursor protein’s (APP’s) effects on the cell gained new insight this week, with a paper published December 16 in the Journal of Neuroscience showing that APP interacts with transcription factors in a triple complex bridged by Mint adaptor proteins. The work was led by joint first authors Andrzej Swistowski and Qiang Zhang in the laboratory of principal investigator Dale Bredesen at the Buck Institute for Age Research in Novato, California. The researchers showed that APP and Mint proteins interact with the transcription effectors TAZ and YAP. It is not yet clear if these interactions relate to the pathology of Alzheimer disease. “There is no proof that these molecules are critical in Alzheimer’s,” Bredesen said, but he believes AD is caused not only by the toxicity of the APP fragment Aβ, but also by signal transduction pathways gone awry. The current work “helps to define the signal transduction network downstream from APP,” he said. In other APP news this week, researchers report that APP can regulate calcium channel activity in GABAergic neuron in the mammalian brain (see ARF related news story).

The triple complex Bredesen and colleagues described seems to parallel another triumvirate, that of APP, the adaptor protein Fe65, and Tip60, which also influences transcription (see ARF related news story on Cao and Südhof, 2001 and ARF related news story on Stante et al., 2009). Both complexes appear to rely on the cleavage of APP by γ-secretase, which frees the APP intracellular domain (AICD) to join other cytoplasmic proteins in entering the nucleus (reviewed in Slomnicki and Lesniak, 2008). But the two complexes are not entirely similar, notes Suzanne Guenette of the MassGeneral Institute for Neurodegenerative Disease in Charlestown, Massachusetts, who was not involved with the current research. Tip60 is a histone acetyltransferase linked to DNA repair, whereas TAZ and YAP are transcriptional coactivators. “The two mechanisms might be completely different,” she said.

The Bredesen group was interested in the Mint family, Mint1, Mint2, and Mint3 (also known as X11 proteins), because they interact with APP (see ARF related news story on Ho et al., 2008). In mouse studies, Mint proteins seem to be involved in regulating neurotransmitter release (Ho et al., 2006). Mint1 and Mint2 are expressed mainly in neurons, and Mint3 is ubiquitous. They also appear to compete with Fe65 to bind APP (Lau et al., 2000).

To look for new proteins that might interact with the Mints, the researchers applied a phage-based screen developed in their lab (Kurakin et al., 2004). The goal of the screen was to identify, from a library of short peptides, amino acid sequences likely to interact with Mint1. They used beads bound with Mint1’s PDZ domain, a domain commonly involved in protein-protein interactions, as a target and exposed the beads to a collection of phage expressing a library of 16-amino-acid peptides on their surfaces. “You literally pour these [phage] over your target,” Bredesen said. The researchers then collected the beads to identify the peptides that interacted with the PDZ domain.

Any protein containing those amino acid sequences, then, would be a prime candidate for a Mint1 interactor. The researchers searched computer databases for proteins that fit the bill. Ultimately, they identified 46 novel proteins that may interact with Mint1. Among those were 10 transcriptional regulators. Membrane proteins, receptors, and enzymes were also well represented.

The researchers selected two transcriptional regulators for further study. They were interested in TAZ because it appears to repress the effects of PPARγ (Hong et al., 2005), a member of a protein family that appears to be protective in Alzheimer’s (reviewed in Kummer and Heneka, 2008). YAP was also intriguing because it appears to help control organ size (Dong et al., 2007) and, Alzheimer’s, of course, involves a shrinking brain.

The researchers showed that APP, Mint proteins, and TAZ and YAP interact by overexpressing the genes into 293T kidney cells and B103 neuroblastoma cells. They used co-immunoprecipitation to define a physical interaction among APP, Mint proteins, and TAZ in 293T cells. When they tagged TAZ with FLAG and used FLAG antibodies; APP came along for the ride—but only at very low levels unless Mint1 or Mint3 was overexpressed as well. (The researchers did not observe APP-Mint-YAP co-immunoprecipitation in a similar assay, although further experiments, described below, indicated that the three interact.)

Being transcriptional coactivators, TAZ and YAP must reach the DNA in the nucleus to carry out their function. Accordingly, the researchers used microscopy to look for an influence of Mint proteins over TAZ and YAP localization in 293T cells.

Mint1 and Mint2 tend to hang out in the cytoplasm, but Mint3 shuttles back and forth between the cytoplasm and the nucleus. When co-expressed with Mint1, both TAZ and YAP stayed in the cytoplasm. With Mint3, they were found in both compartments. The results suggest that the Mint proteins can dictate the location of the transcriptional co-activators.

Next, the researchers looked for a functional role for the APP-Mint-TAZ and APP-Mint-YAP complexes. They transfected B103 cells with an APP-GAL4 hybrid construct, as well as a GAL4-controlled luciferase reporter. If APP, Mint, and TAZ or YAP (and thus GAL4) reached the nucleus, the reporter should produce luciferase.

When the scientists expressed the reporter, APP-GAL4 and TAZ or YAP along with Mint3, they did see luciferase activity, in keeping with the localization results since Mint3 might be expected to carry the APP-GAL4 to the nucleus. But with Mint1 or Mint2, there was no reporter activity, presumably because the triple complex remained cytoplasmic.

The results suggest that Mint proteins act in concert with the APP cytoplasmic fragment to regulate transport of transcription factors to the nucleus. Bredesen was especially intrigued by the YAP connection, because in a mouse model of AD, brain size appears to decrease before any loss of neurons or synapses (Galvan et al., 2006). “It’s backwards,” Bredesen said. “Why would you have atrophy early, with no cell loss?” YAP’s role in organ size, influenced by APP cleavage, makes for an appealing but unproven explanation.

The work opens up many new questions. For one, Bredesen would like to explore more of the 46 novel Mint1 interactors identified in the phage assay. In addition, Guenette suggested that it would be important to find evidence for the APP-Mint-YAP and TAZ interactions in cells or animals expressing normal levels of the proteins. At this point, the results say that the three can interact—but not that they actually do come together in any physiologically relevant system. Future work could also address the potential gene targets of YAP and TAZ, which may or may not be involved in AD pathology.—Amber Dance

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Comments on News and Primary Papers

  1. This study from Dale Bredesen’s group identifies TAZ and YAP as two potential downstream agents of APP-modulated gene transcription in a luciferase-reporter system. Since the γ-secretase inhibitor DAPT blocks the reporter activity, the authors suggest that it is mediated via the APP Intracellular domain (AICD), which, upon release from the membrane, enters the nucleus and activates the reporter gene in a tripartite complex with Mint3 and TAZ/YAP. This paper adds Mint3 to the list of proteins (Fe65 and JIP-1B) that are known to bind APP/AICD and to activate gene transcription.

    As Bredesen admits, the relevance of APP/AICD interaction with TAZ/YAP to AD pathology remains to be established. Nonetheless, it is clear that these studies add further support to the view that APP cytoplasmic domain (ACD) and the free, cleaved AICD are important players in APP biology. ACD/AICD is known to interact with about a dozen or so proteins, which provide the molecular basis for the functional importance of ACD/AICD and the diverse array of biological effects it exerts. The challenge faced by the field, and succinctly stated by Suzanne Guennette above, is how to ensure that these interactions occur at the physiological protein level and are not induced by protein overexpression. Such experiments are technically not easy, and for a field so focused on “Aβ-ology,” perhaps not compelling. Yet, it is difficult to see how we can get a complete picture of APP pathophysiology, and hence of AD pathogenesis, without such lines of investigation.

    View all comments by Sanjay Pimplikar

References

News Citations

  1. APP Makes a Calcium Connection in Neurons
  2. Long-elusive Function for APP Cleavage Product Comes into View: It's Gene Transcription
  3. APP Tail May Wag DNA Repair Pathway
  4. Aβ Boosts Memory; Mint/X11 Proteins Boost Aβ?

Paper Citations

  1. . A transcriptionally [correction of transcriptively] active complex of APP with Fe65 and histone acetyltransferase Tip60. Science. 2001 Jul 6;293(5527):115-20. PubMed.
  2. . Fe65 is required for Tip60-directed histone H4 acetylation at DNA strand breaks. Proc Natl Acad Sci U S A. 2009 Mar 31;106(13):5093-8. PubMed.
  3. . A putative role of the Amyloid Precursor Protein Intracellular Domain (AICD) in transcription. Acta Neurobiol Exp (Wars). 2008;68(2):219-28. PubMed.
  4. . Deletion of Mint proteins decreases amyloid production in transgenic mouse models of Alzheimer's disease. J Neurosci. 2008 Dec 31;28(53):14392-400. PubMed.
  5. . Genetic analysis of Mint/X11 proteins: essential presynaptic functions of a neuronal adaptor protein family. J Neurosci. 2006 Dec 13;26(50):13089-101. PubMed.
  6. . Fe65 and X11beta co-localize with and compete for binding to the amyloid precursor protein. Neuroreport. 2000 Nov 9;11(16):3607-10. PubMed.
  7. . Target-assisted iterative screening of phage surface display cDNA libraries. Methods Mol Biol. 2004;264:47-60. PubMed.
  8. . TAZ, a transcriptional modulator of mesenchymal stem cell differentiation. Science. 2005 Aug 12;309(5737):1074-8. PubMed.

External Citations

  1. Kummer and Heneka, 2008
  2. Dong et al., 2007
  3. Galvan et al., 2006

Further Reading

Papers

  1. . X11alpha haploinsufficiency enhances Abeta amyloid deposition in Alzheimer's disease transgenic mice. Neurobiol Dis. 2009 Oct;36(1):162-8. PubMed.
  2. . The cytoplasmic adaptor protein X11alpha and extracellular matrix protein Reelin regulate ApoE receptor 2 trafficking and cell movement. FASEB J. 2010 Jan;24(1):58-69. PubMed.
  3. . Phosphorylation of the amino-terminal region of X11L regulates its interaction with APP. J Neurochem. 2009 Apr;109(2):465-75. PubMed.
  4. . X11 proteins regulate the translocation of amyloid beta-protein precursor (APP) into detergent-resistant membrane and suppress the amyloidogenic cleavage of APP by beta-site-cleaving enzyme in brain. J Biol Chem. 2008 Dec 19;283(51):35763-71. PubMed.
  5. . Role of X11 and ubiquilin as in vivo regulators of the amyloid precursor protein in Drosophila. PLoS One. 2008;3(6):e2495. PubMed.
  6. . Mint3/X11gamma is an ADP-ribosylation factor-dependent adaptor that regulates the traffic of the Alzheimer's Precursor protein from the trans-Golgi network. Mol Biol Cell. 2008 Jan;19(1):51-64. PubMed.

Primary Papers

  1. . Novel mediators of amyloid precursor protein signaling. J Neurosci. 2009 Dec 16;29(50):15703-12. PubMed.