. APP binds DR6 to trigger axon pruning and neuron death via distinct caspases. Nature. 2009 Feb 19;457(7232):981-9. PubMed.

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  1. This paper provides extensive and compelling evidence for β-APPs (shed from the cell surface by β-secretase) being a major agonist for the cell death receptor DR6, a key signaling pathway in the neuronal pruning that takes place during development. Finally, a clear study on the normal biological function of APP: a seminal paper, in my view. However, too much is made of possible implications to AD. For instance, this mechanism does not explain any of the FAD mutations in either APP, PS1, or PS2 (except perhaps adding an additional mechanism for the pathogenesis of the APP Swedish mutation near the β-secretase cleavage site).

  2. The paper from Marc Tessier-Lavigne and colleagues provides compelling evidence that DR6 mediates axonal pruning and degeneration induced by trophic-factor withdrawal in developing neurons, and that the extracellular sequences of APP bind to DR6 with high affinity and specificity. The identification of DR6 as the receptor for secreted APP and the link of APP to axonal degeneration through the DR6/caspase 6 pathway are extremely exciting. However, its physiological significance needs to be further established both in developing neurons and in adult brain. The findings that APP/DR6 activation requires BACE, but not α-secretase cleavage, that BACE processing is followed by other cleavage events, and that this pathway is Aβ independent are intriguing and leave open many interesting questions. Although the study addresses a developmental function, it could offer novel insights to AD pathogenesis and therapeutic intervention as APP/DR6/caspase 6 are expressed in both developing neurons and adult brains.

    View all comments by Hui Zheng
  3. It's interesting that an androgen receptor coactivator interacts with DR6 and that androgen induces the expression of APP.

    References:

    . Androgen receptor coregulator ARA267-alpha interacts with death receptor-6 revealed by the yeast two-hybrid. Sci China C Life Sci. 2004 Oct;47(5):442-8. PubMed.

    . Amyloid precursor protein is a primary androgen target gene that promotes prostate cancer growth. Cancer Res. 2009 Jan 1;69(1):137-42. PubMed.

    View all comments by Mary Reid
  4. Reading this remarkable paper from the Tessier-Lavigne group feels like drinking water from a fire hose (six figures in the main text + 18 multipanel, supplementary figures). This study unambiguously shows that DR6 is involved in axonal pruning and neuronal degeneration with caspase 3 and 6 as the downstream players. The readers of this forum might be wondering how this study fits in with what we know about APP function and whether the proposed ligand (~35 kDa N-terminal fragment) is relevant to AD pathogenesis.

    1. It seems likely that APP performs different functions in the peripheral nervous system (PNS) as opposed to the central nervous system (CNS). This study is focused on PNS neurons and the uncanny similarity in neuromuscular abnormality seen in DR6-/- mice (Figure 6) and APP/APLP2 double-KO mice (1) is consistent with APP regulating axonal elongation/pruning in the PNS. However, in the CNS the story seems to be different. The lack of APP and APLP2 (and APLP1) results in abnormal migration of neurons (2), a finding supported by other studies (3) and interaction with Fe65 as a possible underlying mechanism (4) for the observed migration phenotype. Nonetheless, this can’t be the complete story since APP is also expressed in non-neuronal tissues where axonal pruning or neuronal migration is irrelevant. Obviously, more surprises await us.

    2. The relevance of the ~35 kDa N-terminal fragment of APP to AD pathology, an exciting idea as it is to all Aβ skeptics including me (5), is at best tenuous. One, these observations are made in the PNS neurons and one doesn’t know whether this will also hold true for the CNS neurons. Second, there is no evidence that the ~35 kDa fragment observed in trophic factor deprived medium and which binds DR6-AP (Figure 4) is actually toxic to neurons; all the subsequent toxicity experiments performed in the paper used recombinant APP1-286-His protein. Indeed, the crucial piece of data—IP using DR6-Fc followed by Western blot using APP-Nt antibody—is not shown in the paper.

    3. One piece of information that cannot be easily reconciled with the current findings is that more than 90 percent of APP is cleaved by the α-secretase (at least in the CNS neurons) and far too many studies have shown the sAPP-α to be neurotrophic. This group finds little or no sAPP-α in their culture medium. Perhaps PNS neurons behave differently from the CNS neurons.

    4. Finally, it is interesting that DAPT treatment also protected neurons from anti-NGF induced degeneration (Suppl. Figure 17). Inhibition of γ-cleavage should block generation and release of AICD, which is known to induce apoptosis. Could AICD be an important player, too?

    In any case, these are truly exciting and important findings that will, undoubtedly, be repeated in other labs and will further stimulate studies focused on APP function. Even if some of the details might change as more groups repeat these observations, this study provides a potential mechanism for the action of APP in the PNS during embryonic development. That is a solid step forward.

    References:

    . Defective neuromuscular synapses in mice lacking amyloid precursor protein (APP) and APP-Like protein 2. J Neurosci. 2005 Feb 2;25(5):1219-25. PubMed.

    . Cortical dysplasia resembling human type 2 lissencephaly in mice lacking all three APP family members. EMBO J. 2004 Oct 13;23(20):4106-15. PubMed.

    . A critical function for beta-amyloid precursor protein in neuronal migration revealed by in utero RNA interference. J Neurosci. 2007 Dec 26;27(52):14459-69. PubMed.

    . Essential roles for the FE65 amyloid precursor protein-interacting proteins in brain development. EMBO J. 2006 Jan 25;25(2):420-31. PubMed.

    . Reassessing the amyloid cascade hypothesis of Alzheimer's disease. Int J Biochem Cell Biol. 2009 Jun;41(6):1261-8. PubMed.

    View all comments by Sanjay Pimplikar
  5. This is a compelling study that provides a biological “raison d’être” for an N-terminal secreted APP fragment in axonal pruning. Its interaction with the DR6 receptor is triggered by trophic factor deprivation in cultured motor, commissural and sensory neurons. A related role for the DR6 receptor is also evident in vivo in DR6 deficient mice since axons of the phrenic nerve overshoot the motor endplate at the developing neuromuscular junction of the diaphragm muscle. Interestingly, axonal sprouting was previously reported in the diaphragm muscle of P0 APP/APLP2 double knockout mice (Wang et al., 2005). Since the β-secretase derived ectodomain of APLP2 is also a ligand for DR6, failure of DR6 signaling during development of the neuromuscular junction in the APP/APLP2 DKO may be operative. The demonstrated involvement of BACE activity in this process suggests that BACE1 and 2 knockout mice might also display this phenotype.

    sAPPβ and the N-terminal 35 kDa APP fragment are generated by NGF withdrawal and bind the DR6 receptor to activate axonal pruning. Furthermore, APP(1-286) (~35 kDa) is shown to be sufficient for this effect. Since α-secretase acts at the cell surface, this fragment could just as readily be produced by cleavage of sAPPα. Is BACE activity predominant in neurons under these conditions and how does DR6-mediated axonal pruning occur without cell body death during development? Is there local recruitment of β-secretase activity to targeted axons? The observation that an Aβ antibody (Aβ33-42) capable of blocking Aβ1-42 toxicity is unable to reverse axonal pruning resulting from NGF deprivation suggests that Aβ does not mediate axonal pruning. However, incubation of γ-secretase inhibitors partly blocked axonal degeneration induced by NGF deprivation suggesting that other γ-secretase derived products contribute to axonal degeneration in this paradigm. Alternatively, γ-secretase inhibition may lead to a redistribution of surface APP away from the axon surface. Could this be sufficient to produce partial rescue of axonal degeneration mediated by trophic deprivation? Abundant DR6 expression is observed in the adult mouse hippocampus and cortex raising the possibility that this death receptor contributes to the selective vulnerability of these regions in AD. Is there a sufficient amount of APP on the surface of adult nerve axons, and if not, is there a physiological event that can trigger surface APP expression and this pathway in Alzheimer disease?

    References:

    . Defective neuromuscular synapses in mice lacking amyloid precursor protein (APP) and APP-Like protein 2. J Neurosci. 2005 Feb 2;25(5):1219-25. PubMed.

    View all comments by Suzanne Guenette
  6. I recommend the primary paper.

    View all comments by J. Lucy Boyd
  7. The data reported in this exciting paper can potentially open new ways to interpret AD pathogenesis at the molecular level. Of course, much more data are needed to assess the relevance for AD of the APP-DR6 interaction; however, when confirmed, such a new pathway may not necessarily be alternative to the amyloid hypothesis. In my opinion, one out of the many points that will need consideration is determining DR6 location on the cell membrane. This could link the APP-DR6 pathway with the effect of cell membrane cholesterol on AD pathogenesis, providing further clues on the cholesterol-AD relation. For example, it could be of interest to study the effects, if any, on the APP-DR6 pathway of increasing or reducing membrane cholesterol, and the resulting modifications of lipid raft stability. The finding that APP/DR6 activation requires BACE, but not α-secretase cleavage, adds further points to be addressed in the light of a possible link with membrane cholesterol content, raft stability, and behavior.

    View all comments by Massimo Stefani