Akbar M, Calderon F, Wen Z, Kim HY.
Docosahexaenoic acid: a positive modulator of Akt signaling in neuronal survival.
Proc Natl Acad Sci U S A. 2005 Aug 2;102(31):10858-63.
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This report from Akbar and Kim adds to their earlier work in this area by providing compelling evidence for an impact of DHA on enhancing the rate of PI3K>Akt signaling in neurons and brain by increasing phosphatidylserine (PS) and translocation-dependent activation of Akt through its pleckstrin homology domain. Because of the well-established importance of Akt in "survival signaling" in multiple neurotrophic factor pathways, the paper establishes the significance of maintaining adequate CNS DHA levels in neurodegenerative diseases, including Alzheimer disease (AD). Average DHA intake in the USA is clearly deficient and on the order of 60-80 mg per day in contrast to expert panel recommendations in the range of 200-300 mg per day, and multiple studies have associated increased Alzheimer risk with reduced fish or omega-3 fatty acid intake and reduced DHA blood levels (MacLean, 2005). Part of the beauty of DHA's Akt enhancement mechanism shown by Akbar and Kim is that it should enhance the neuroprotective activity of appropriately presented neurotrophic factors rather than flood the brain with potentially inappropriate exogenous stimulation.
Because Akt phosphorylation of "BAD" inhibits caspase activation, the new findings provide additional insight into one likely mechanism underlying our findings that DHA effectively increases phosphoBAD and reduces postsynaptic caspase activation and neurodegenerative changes at postsynaptic sites in aging Tg2576 on a DHA-depleting, high safflower oil diet (Calon et al., 2004). At first sight, the author's observations that DHA influences Akt activity without altering PI3-K's p85α subunit might seem to contradict the report from our group, which also found that Tg2576 transgene positive animals on an omega-3 depleted diet had reduced levels of p85α message and protein, both partially restored or prevented by adding back DHA to the diet. There are several likely explanations for the apparent differences in the two studies.
1. The effects we saw in aged Tg2576 were clearly dependent on both transgene and diet interactions, while Akbar and Kim's study showed that lack of impact on PI3 kinase in vitro in N2A cells was due to acute effects. Akbar and Kim’s in vivo studies (depletion during embryogenesis) showed vulnerability of hippocampal neurons to apoptosis without measuring PI3 kinase activity in that paradigm. In general, in our study, transgene-negative mice were much less affected by a DHA-depleting diet during aging than were APPsw transgenics, and, in fact, p85α PI3-K was unchanged by DHA in transgene negative mice ( 7.44 +/- 0.906 without DHA vs. 7.988 +/- 0.649 with DHA, p = 0.638). The new PNAS manuscript by Akbar would suggest that DHA depletion is not directly involved in the PI3-K defect in transgene-positive mice. It, however, does not eliminate the possibility that DHA depletion is indirectly involved (e.g., via an Aβ or transgene interaction).
2. The restoration of p85α levels in transgene-positive animals on DHA-depleting safflower oil diet by DHA was partial and less than 50 percent despite full restoration of DHA, suggesting that factors other than DHA (e.g., oxidative damage) were involved in the APPsw transgene X safflower oil diet effects. There is a clear defect of the PI3 kinase pathway in Alzheimer disease (Jolles et al., 1992; Bothmer et al., 1994a, 1994b; Zubenko et al., 1999; Zhao et al., 2004; Calon et al 2004) that is likely significant for pathogenesis, so a PI3-K defect in the APPsw transgene X diet phenotype remains AD-relevant whether or not it is directly dependent on DHA levels.
3. While in the Akbar and Kim paradigm DHA-depletion per se does not regulate levels of either PI3-K p85α or Akt protein, it should be pointed out that in the context of caspase-activating stimuli, signaling molecules are differentially altered depending on the paradigm. For example, in an in vitro study, PI3-K p85α protein was selectively lost with UV-induced caspase activation, but not with etoposide or Fas ligation. In contrast, Akt-1 protein was lost with all three caspase-activating stimuli, along with RasGAP, Raf-1, and Cbl, while 21 other signal-related proteins were unaffected (Widmann et al., 1998). Thus, both papers together would suggest that transgene-dependent p85α loss with a DHA-depleting diet and recovery with DHA may be pathology-dependent and indirect.
While the report from Akbar and Kim makes it unlikely that DHA directly regulates PI3-K itself, it provides convincing data to show an impact on PI3-K>Akt survival signal transduction at the level of Akt translocation without enhancing PI3-K activity (measured by IGF responsiveness in N2A cells). Beneficial as this is likely to be, one should not forget that DHA has other important activities. For example, Bazan's group has in vitro data to show that enzymatic DHA metabolites like "neuroprotectin D1" can reduce proapoptotic proteins like BAD and Bcl-x, and increase protective antiapoptotic proteins (Mukherjee et al., 2004). Consistent with this additional mechanism, we have recently reported that DHA reduced both BAD and caspase activation in aging Tg2576 (Calon et al., 2005). Finally, DHA also appears to modulate the amyloid accumulation in aging Tg2576 (Lim et al., 2005), and although the mechanism is not entirely clear, we have been able to repeat the observation of amyloid reduction in aged animals with late-life DHA treatment (Lim et al., unpublished). It is important to keep in mind that DHA repletion is both very cheap and very safe. Thus, while it is unlikely that there will be only one important mechanism for DHA's relevance to AD in particular, the results from this new study from Akbar and Kim, showing a mechanism promoting survival signaling, makes DHA very likely relevant to multiple neurodegenerative diseases.
Kang and co-workers examined the effect of PS1 and PS2 deficiency on PI3/Akt and ERK pathways, highlighting the importance of upstream cell-surface receptors in PS1- and PS2-mediated Akt and ERK signaling. The authors demonstrate that the loss of PS1 and PS2 inhibits the PI3/Akt pathway, increasing tau phosphorylation via GSK-3 activation and suppressing the ERK pathway. Although GSK-3 activation, regulated by PI3/Akt signaling (or by Wnt/β-catenin pathways), is viewed as the main player in the phosphorylation of tau, ERK dysregulation is also likely to play an important role in the increased phosphorylation of tau protein (Perry et al., 1999; Roder et al., 1993). In addition to the phosphorylation of tau, the PI3/Akt and ERK pathways play key roles in the survival of neurons and synaptic plasticity, which collectively are involved in the pathophysiology of a variety of neurodegenerative disorders, in particular Alzheimer disease and tauopathy. This paper complements the two recent papers by Saura et al. (2004) and Feng et al. (2004) in showing that the loss of both PS1 and PS2 causes tau phosphorylation. The papers by Saura and Feng further present compelling evidence of the marked neurodegeneration that occurs in the brain of double knockout mice. Kang and colleagues, while they did not examine the effect of PI3/Akt and ERK suppression on cell survival, did demonstrate the important finding that PS2 but not PS1 selectively restores PDGFR expression and PDGF-induced PDGFR tyrosine phosphorylation. The authors further demonstrate the requirement of the N-terminal fragment of PS2 in the PDGF-induced activation of Akt/ERK. Characterizing signaling receptors that regulate the Akt/ERK signaling pathways increases our understanding of the intracellular mechanisms that govern tau phosphorylation and neurodegeneration. This study and those by Saura and Feng indicate the extent to which presenilin 1 and 2, together, regulate tau phosphorylation and cell survival. These presenilin-mediated events add to the previous data showing the involvement of PS1 mutations in tauopathy and in the Aβ deposition found in familial Alzheimer disease. Overall, with the recent paper in Science by Santacruz and colleagues (Santacruz et al., 2005) showing that suppression of transgenic tau in mice restores memory function and suppresses neuronal death, these studies may put tau back on the frontline of the Alzheimer disease/etiology controversy.
Kang and colleagues described a new role of presenilins on the highly conserved ERK signaling cascade. Specifically, these authors demonstrate that loss of presenilins alters the expression, degradation, and function of the tyrosine kinase receptor PDGF, which results in dysregulation of the ERK1/2 signaling pathway. Interestingly, the regulation of PDGF receptor is mediated by the NTF of PS2 through its interaction with the transcription factor FHL2. Unlike PDGF signaling, altered serum-induced Akt/ERK activation in PS-/- cells is reconstituted by expression of PS1 or PS2. This indicates that under normal conditions presenilins regulate directly or indirectly the Akt and ERK signaling pathways by affecting still unknown cell surface receptors or signaling molecules. While these results open an interesting avenue for future investigations, it will be important to dissect the physiological relevance of these findings in cellular events regulated by normal and mutant presenilins such as cell proliferation and survival.
Familial forms of Alzheimer´s and frontotemporal dementia, characterized by abnormal tau phosphorylation, are linked to mutations in the presenilin genes. In addition, loss of presenilins in the mouse brain causes abnormal tau phosphorylation and neurodegeneration (Saura et al., 2004). In neurons, PS1 facilitates PI3K/Akt signaling and phosphorylation of GSK3β (Pigino et al., 2003), a kinase that phosphorylates tau protein. Using presenilin-null fibroblasts, Kang et al. also nicely demonstrate that total loss of presenilin function compromises GSK3β phosphorylation, which results in increased tau phosphorylation. Although it has not been fully demonstrated, it is tempting to speculate that disturbed Akt and ERK signaling caused by presenilin dysfunction may contribute to tau phosphorylation in such neurodegenerative diseases and mouse models.
In their highly interesting paper, Kang and colleagues demonstrate an important additional role for the presenilins independent of their role in γ-secretase activity. This role is one of modulating several signal transduction pathways by influencing upstream receptors in the pathways. They further characterize presenilin’s role in the PI3 kinase/Akt pathway and its link to tau phosphorylation. A specific novel function for PS2 in the modulation of the MEK/ERK pathway is also demonstrated. This is shown to happen by a direct effect of PS2 on the PDGF receptor. This is an exciting finding that potentially directly links the presenilins to pathways shown to be important in learning and memory.
The paper by Kang et al. is very interesting, both because it confirms the important role presenilins play in the activation of the PI3K/Akt pathway, and because it describes a novel role of PS2 in PDGF signaling. More important, for the mechanism involved in the induction of AD by FAD mutations, this paper supports the hypothesis by Baki et al., 2004) that presenilins may prevent AD pathology, including tau hyperphosphorylation, by activating the PI3K/Akt pathway, while presenilin FAD mutations may promote AD pathology by inhibiting this pathway. Interestingly, the paper confirms evidence that these novel presenilin functions are independent of γ-secretase activity (Baki et al., 2004). It is known that absence of PS1 or presence of PS1 mutations promotes apoptotic changes including activation of caspase-3, and that these changes depend on the cadherin/PS1/PI3K/Akt/GSK-3 pathway. It is not unreasonable to hypothesize that these apoptotic changes, triggered by presenilin FAD mutations, may promote neurodegeneration and dementia. Furthermore, there is plenty of evidence that neurodegeneration promotesAβ production. Clearly, the new data together with published reports on these novel presenilin functions may put the AD field on a new avenue.