The theme “don’t go whole hog” might come to mind for those who check out two recent papers suggesting potential therapeutic strategies for Alzheimer disease. In one study, published in the July 15 Journal of Immunology, researchers report that blocking the receptor of a downstream complement activation product (C5a) relieves pathology and cognitive decline in AD mice. In the second study, featured in this week’s PNAS Early Edition, scientists reduced amyloid plaques and memory loss in an AD mouse model by overexpressing an antioxidant specific to mitochondria (SOD-2). By targeting select components instead of hitting entire systems—one mediating inflammation, the other manhandling oxidative stress—these studies hint that fine-tuning may be key to designing effective AD therapies.

Tweaking the Complement Pathway
In the first study, researchers led by Andrea Tenner of the University of California, Irvine, treated AD mice with a compound that inhibits a downstream component of the complement system—a cascade of reactions that rallies inflammatory cells to help fight pathogens. By now, connections between AD and complement run deep. Fibrillar Aβ can trigger the complement pathway (Rogers et al., 1992; Bradt et al., 1998), and complement factors cozy up with Aβ in fibrillar amyloid plaques (Akiyama et al., 2000; Loeffler et al., 2008). Several years ago, Tenner’s group showed that Tg2576 AD transgenic mice lacking C1q (a protein that helps initiate the complement cascade) developed milder neuropathology than AD mice with an intact complement system (Fonseca et al., 2004).

Based on those findings, complement activation seemed harmful in AD; however, other studies have suggested the opposite—by showing that deficiency in complement component C3 exacerbates pathology and neuron loss in AD mice (Wyss-Coray et al., 2002; Maier et al., 2008 and ARF related news story). That work, along with recent data from Tenner’s own lab indicating that even C1q could be neuroprotective (Pisalyaput and Tenner, 2008), led her team to target C5a, a complement component downstream of C1q and C3, in the current study.

First author Maria Fonseca and colleagues administered an oral C5a receptor antagonist to two AD mouse strains, and to wild-type littermates, at an age when Aβ deposition had begun in the AD mice. Twelve-week treatment began at 12 to 15 months in Tg2576 mice, which develop Aβ pathology and memory loss, and at 17 to 20 months in 3xTg mice, which additionally have tau pathology. The drug candidate (PMX205), a cyclic hexapeptide, is under development at Cephalon, Inc., Frazer, Pennsylvania. A related compound (PMX53) has shown therapeutic benefit in animal models of peripheral inflammation and appeared safe in Phase 1 human testing, Tenner said. PMX205 was designed with enhanced lipophilic qualities and presumably penetrates the brain more easily, but neither this capability nor the chemical’s pharmacokinetic properties have been explored, and the compound remains to be tested in humans.

Meanwhile, the current study shows that PMX205 treatment reduced fibrillar Aβ plaques in the cortex and hippocampus of Tg2576 mice. Treated animals also had fewer activated glia surrounding the plaques, and increased hippocampal staining of synaptophysin (a presynaptic protein used to assess neuronal integrity). In hippocampal neurons of 3xTg mice, the C5a antagonist also brought a sharp reduction in hyperphosphorylated tau.

Alongside the decreased AD-like pathology, PMX205 improved some measures of cognition in the mice. AD mice that got the compound did better than non-treated controls at learning to avoid a dark chamber—a passive avoidance task with hippocampal (memory) and amygdala (anxiety) components.

While the compound’s pathological and behavioral effects look promising in mice, scientists understand very little about its mechanism of action. “We don’t know whether it’s working on peripheral inflammation, keeping that down, or if it’s getting into the brain and doing things there,” Tenner told ARF. Bruce Lamb of Cleveland Clinic, Ohio, noted in an e-mail to ARF that C5a receptor antagonists have also been protective in rodent models of amyotrophic lateral sclerosis (see, e.g., Woodruff et al., 2008), and thus wonders if there is a more general mechanism involved.

Tenner believes PMX205 has succeeded in mice thus far because it preserves the benefits of complement activation while modulating its detrimental effects. By targeting the receptor for C5a, her team left intact C1q, C3, and other upstream complement components that help lyse and kill pathogens. “You’re not bludgeoning the immune system; you’re curtailing it,” she said, noting that even the proinflammatory cytokines can be neuroprotective in small doses. However, problems can arise when complement activation lures glial cells to the vicinity of Aβ. These inflammatory cells spew out more cytokines, which can interact with neurons and enhance their pathologic cleavage of amyloid precursor protein (APP). “That’s what’s going to drive and accelerate this whole process,” Tenner said. “If you can knock that out, you can have things a lot better under control.”

Mitochondrial-targeted Antioxidants
In the second paper, Eric Klann, New York University, and colleagues were able to relieve amyloid pathology and memory loss in Tg2576 mice not by eradicating but instead by boosting something. That something was a form of an antioxidant specifically localized to mitochondria—superoxide dismutase 2 (SOD-2).

A growing literature links mitochondrial dysfunction in general to AD (for reviews, see Reddy and Beal, 2008 and Wang et al., 2007) and, in particular, SOD-2 deficits to Aβ. Aβ deposition takes the wind out of SOD-2, leading to an excess of free radicals (Anantharaman et al., 2006). Further support for the SOD-2/Aβ connection comes from work showing that reduced levels of SOD-2 intensify pathology and behavioral impairments in AD mice (Li et al., 2004 and ARF related news story; Esposito et al., 2006). “We build upon that, showing that if we overexpress that enzyme, we can prevent those deficits by just having some extra SOD-2,” Klann told ARF.

First author Cynthia Massaad and colleagues crossed Tg2576 animals with mice that overexpress SOD-2. These double transgenic mice had reduced amyloid plaques in the cortex and hippocampus, lower levels of hippocampal superoxide, and better associative and spatial memory, compared to Tg2576 mice lacking the SOD-2 transgene. Curiously, SOD-2 overexpression did not affect absolute Aβ levels but did encourage a less pathogenic Aβ composition by lowering the Aβ1-42 to Aβ1-40 ratio.

In regard to interpreting these data, the authors write that the studies “explored the therapeutic effectiveness of SOD-2 from a genetic standpoint and hence, at this stage, do not offer any insight for temporal effectiveness.” In a phone interview with ARF, Klann described two types of mouse experiments that could more closely approximate a human therapy begun in mid- to late life. The first is pharmacological. Toward this end, Klann’s group has treated hippocampal slices with mitoquinone (MitoQ)—a compound under development at Antipodean Pharmaceuticals, Auckland, New Zealand—showing it can alleviate impaired synaptic plasticity induced by soluble Aβ. In the future, he wants to test whether the quinone can relieve memory loss in an AD mouse model. MitoQ has not been tested in people with AD but has been used in a Phase 2 study of newly diagnosed Parkinson disease patients, in whom it showed no therapeutic benefit (see MedPageToday article on company data presented at the 2008 American Academy of Neurology meeting).

In AD patients, general antioxidants (e.g., vitamins C and E) have also shown no success in clinical trials—most likely because they are not very specific, Klann said. “We know that reactive oxygen species have roles in normal physiological processes. That’s probably why [antioxidants] haven’t been all that effective in treating many disorders. We’ll have to be specific in targeting the sources responsible for enhanced oxygen levels on a disease-by-disease basis,” he said.

Hemachandra Reddy of Oregon Health and Science University, Beaverton, seems to agree. “Given the limited success of recent clinical trials using natural antioxidants in AD patients, findings from this new study may have some important implications for the development of mitochondria-targeted therapeutics for AD patients,” he wrote in an e-mail to ARF. Using a genetic approach similar to the current study, Reddy’s group has crossed Tg2576 AD mice with transgenic mice that overexpress mitochondria-targeted catalase—to see if they have delayed pathology. These mice make more catalase in the mitochondrial matrix and thus neutralize free radicals more quickly.

In the meantime, Klann hopes his existing data can offer proof-of-principle for funding to make a tet-on/tet-off SOD-2 mouse. “That would be really nice because you could let the mouse develop, and then turn on SOD-2, determine whether or not the animal has memory deficits, and then turn it off and see if the memory deficits come back.”—Esther Landhuis

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Comments on News and Primary Papers

  1. The findings of Massaad and colleagues will advance our basic understanding of the neuroprotective role of mitochondrially targeted antioxidants in Alzheimer disease (AD) pathogenesis. Their findings suggest that mitochondrial superoxide dismutase 2 (SOD2) decreases hippocampal superoxide radicals, ameliorates learning/memory deficits, and decreases amyloid-β (Aβ) plaques in double transgenic mice that overexpress SOD2 and mutant human amyloid precursor protein. Interestingly, they also found a decreased ratio of Aβ1-42 to 1-40 in double transgenic mice. These findings further support the mitochondrial oxidative damage hypothesis of AD, and may have important implications for mitochondrially targeted antioxidant therapeutics in AD.

    Increasing evidence suggests that mitochondrial abnormalities are involved in the development and progression of AD (reviewed in Reddy, 2009). Further, it has been proposed that mitochondrially generated free radicals and oxidative damage are involved in abnormal processing of APP and in generating Aβ peptide by activating β- and γ-secretases (Reddy, 2006; Reddy and Beal, 2008). There is some evidence to support this hypothesis (Tamagno et al., 2008; Jin et al., 2008). Further, recently several groups (Crouch et al., 2005; Caspersen et al., 2005; Manczak et al., 2006; Devi et al., 2006; Hanson Petersen et al., 2008) found that Aβ peptide is localized to mitochondrial membranes and the mitochondrial matrix, and that mitochondrially localized Aβ peptide interacts with mitochondrial proteins, induces free radical production, decreases cytochrome oxidase activity, inhibits ATP production, and damages AD neurons. In addition, recent structural studies revealed that Aβ fragments mitochondria, suggesting that mitochondrial structural abnormalities (caused by Aβ) may be critical for mitochondrial dysfunction in AD (Wang et al., 2009; Mao et al., 2009). Overall, these studies suggest that Aβ and mitochondrial dysfunction play a big role in AD pathogenesis.

    To determine the role of overexpressed SOD2 in AD pathogenesis, Massaad et al. crossed SOD2 transgenic mice with Tg2576 mice and studied cognitive deficits, Aβ1-42 and 1-40 levels, and Aβ deposits in Tg2576 mice and double mutant mice (SOD2xTg2576 mice). They found decreased Aβ deposits and reduced cognitive deficits in double transgenic mice relative to Tg2576 mice, suggesting that mitochondrial superoxide dismutase improves cognitive functions in AD. However, it is still unclear 1) how overexpressed mitochondrial superoxide dismutase alters the ratio of Aβ1-42 to 1-40; and 2) how overexpressed mitochondrial superoxide dismutase improves learning and memory functions in double mutant mice. Further research is needed to find answers to these questions, and the answers may have some important implications to AD patients.

    Overall, findings of the study by Massaad and colleagues, together with previous studies (Hirai et al., 2001; Swerdlow et al., 1997; Reddy et al., 2004; Manczak et al., 2004; Caspersen et al., 2005; Manczak et al., 2006; Devi et al., 2006; Hansson Petersen et al., 2008; Wang et al., 2009), improve our understanding of mitochondrial oxidative damage in AD pathogenesis. Given the limited success of recent clinical trials using natural antioxidants in AD patients, findings from this new study may have some important implications for the development of mitochondrially targeted therapeutics for AD patients.

    See also:

    Mao P, Manczak M, Shree D and Reddy PH. Abnormal mitochondrial structural and functional changes caused by amyloid beta in Alzheimer’s disease. Paper presented at the International Conference on Alzheimer’s disease held Vienna, Austria July 11-16, 2009.

    References:

    . Mitochondrial Abeta: a potential focal point for neuronal metabolic dysfunction in Alzheimer's disease. FASEB J. 2005 Dec;19(14):2040-1. PubMed.

    . Accumulation of amyloid precursor protein in the mitochondrial import channels of human Alzheimer's disease brain is associated with mitochondrial dysfunction. J Neurosci. 2006 Aug 30;26(35):9057-68. PubMed.

    . The amyloid beta-peptide is imported into mitochondria via the TOM import machinery and localized to mitochondrial cristae. Proc Natl Acad Sci U S A. 2008 Sep 2;105(35):13145-50. PubMed.

    . Mitochondrial abnormalities in Alzheimer's disease. J Neurosci. 2001 May 1;21(9):3017-23. PubMed.

    . DNA damage-inducing agents elicit gamma-secretase activation mediated by oxidative stress. Cell Death Differ. 2008 Sep;15(9):1375-84. PubMed.

    . Mitochondria are a direct site of A beta accumulation in Alzheimer's disease neurons: implications for free radical generation and oxidative damage in disease progression. Hum Mol Genet. 2006 May 1;15(9):1437-49. PubMed.

    . Differential expression of oxidative phosphorylation genes in patients with Alzheimer's disease: implications for early mitochondrial dysfunction and oxidative damage. Neuromolecular Med. 2004;5(2):147-62. PubMed.

    . Amyloid precursor protein-mediated free radicals and oxidative damage: implications for the development and progression of Alzheimer's disease. J Neurochem. 2006 Jan;96(1):1-13. PubMed.

    . Amyloid beta, mitochondrial structural and functional dynamics in Alzheimer's disease. Exp Neurol. 2009 Aug;218(2):286-92. PubMed.

    . Amyloid beta, mitochondrial dysfunction and synaptic damage: implications for cognitive decline in aging and Alzheimer's disease. Trends Mol Med. 2008 Feb;14(2):45-53. PubMed.

    . Gene expression profiles of transcripts in amyloid precursor protein transgenic mice: up-regulation of mitochondrial metabolism and apoptotic genes is an early cellular change in Alzheimer's disease. Hum Mol Genet. 2004 Jun 15;13(12):1225-40. PubMed.

    . Cybrids in Alzheimer's disease: a cellular model of the disease?. Neurology. 1997 Oct;49(4):918-25. PubMed.

    . Oxidative stress activates a positive feedback between the gamma- and beta-secretase cleavages of the beta-amyloid precursor protein. J Neurochem. 2008 Feb;104(3):683-95. PubMed.

    . Impaired balance of mitochondrial fission and fusion in Alzheimer's disease. J Neurosci. 2009 Jul 15;29(28):9090-103. PubMed.

  2. In this study the researchers examined the role of the complement system in murine models of Alzheimer disease (AD). In human AD brains immunohistochemical studies have demonstrated that both early and late complement factors are found in amyloid plaques. Immunohistochemical studies in transgenic mice models for AD have shown the presence of the early complement factors in amyloid plaques also, but there is a lack of information about the presence of late complement factors (C5-C9) in such plaques (see Schwab et al., 2004). The present paper describes the effect of C5aT antagonists and the finding suggests indirectly the involvement of C5 in pathology. However, no direct information about an increase of C5 is given and also no immunohistochemical data demonstrating the presence of C5 in the amyloid plaques in the mouse.

    But despite these points, the paper is potentially very interesting. It seems that the role of the early complement factors (c1q, C3) is especially important in the aggregation, deposition, and removal of Aβ. Studies from Wyss-Coray, Tenner and Lemere in transgenic murine AD models point also in this direction. So, studying these early complement factors could be beneficial and helpful. C5a is a powerfull inflammatory mediator and could play a most important role in the organisation of the inflammation that could be toxic for neurons. It is, in this respect, highly interesting that the treatment with C5aR antagonist enhances behavorial performance. As written in this Alzforum news, the story is, at the moment, confusing. One of the possibities that emerges from the present paper from Fonseca et al., 2009 and that is in line with the earlier papers in transgenic mice, is that the early complement factors could be helpful in the removal of Aβ. If indeed C5a is the most powerful pro-inflammatory peptide of the complement cascade system for inducing an inflammatory response, then we can have an unraveling of the beneficial (early complement factors in Aβ removal) and the detrimental aspects of complement activation in AD (C5a as the toxic neuroinflammatory component).

    In summary, the present paper implies that we should pay more attention to the possible role of the late complement factors (from C5-C9) and to studies that demonstrate the presence and involvement of the late complement factors in animal AD models.

    View all comments by Piet Eikelenboom
  3. Strengthening the Links Between Amyloid-β, Mitochondria and Oxidative Stress
    Striking features of Alzheimer disease pathophysiology are amyloid-β deposits, reduced brain metabolism, oxidative stress, and cognitive decline. The links between these features are strengthened by the Massaad study. They show that reduction of mitochondrial oxidative stress through overexpression of SOD-2 in a mouse model of Aβ overproduction results not in a reduction in total Aβ, but an alteration in the 40/42 ratio, so that less Aβ is deposited. Further, they show SOD-2 expression improves memory in these mice. These findings support the view that Aβ’s response to oxidative stress has important adaptive features.

    Massaad et al. demonstrate it is not just that Aβ is a source for reactive oxygen, or that the enzymes that control Aβ production touch oxidative balance, but rather that almost every critical feature of the cell controls the Aβ system and response to it. Stress in oxidative balance is no error on nature’s part; this critical feature of cellular protection has been adapted, modulated, and finely honed to one of the most intricate networks in the body. Aβ may be one of the critical find modulators of oxidative balance.

    View all comments by Xiongwei Zhu
  4. This paper shows the beneficial effects of SOD-2 overexpression in transgenic Alzheimer mice. Massaad and colleagues’ data confirmed our previous findings showing that in transgenic Alzheimer mice, overexpression of SOD-2 reduces oxidative stress and amyloid deposition, and improves memory impairments (Dumont et al., 2009).

    The mechanism of these effects still remains unclear. In both our and Massaad’s work, the SDS-soluble or formic acid (FA)-soluble pools of Aβ1-42 and Aβ1-40 remained unchanged. Moreover, we also demonstrated that levels of β and α C-terminal fragments of APP were unaffected by SOD-2 overexpression. An interesting outcome of the paper of Massaad is the decrease of Aβ42/40 ratio in the FA-soluble fraction, suggesting a reduction of the most pathogenic isoform of Aβ.

    Our study and that of Massaad provide strong evidence that the mitochondrial antioxidant system plays an important role in Alzheimer disease pathogenesis. This is consistent with other evidence that mitochondrial dysfunction and oxidative damage play an important role in AD.

    View all comments by Magali Dumont

References

News Citations

  1. Complement: AD Friend or Foe? New Work Tips Balance to Former
  2. Aβ Production Linked to Oxidative Stress

Paper Citations

  1. . Complement activation by beta-amyloid in Alzheimer disease. Proc Natl Acad Sci U S A. 1992 Nov 1;89(21):10016-20. PubMed.
  2. . Complement-dependent proinflammatory properties of the Alzheimer's disease beta-peptide. J Exp Med. 1998 Aug 3;188(3):431-8. PubMed.
  3. . Inflammation and Alzheimer's disease. Neurobiol Aging. 2000 May-Jun;21(3):383-421. PubMed.
  4. . Plaque complement activation and cognitive loss in Alzheimer's disease. J Neuroinflammation. 2008;5:9. PubMed.
  5. . Absence of C1q leads to less neuropathology in transgenic mouse models of Alzheimer's disease. J Neurosci. 2004 Jul 21;24(29):6457-65. PubMed.
  6. . Prominent neurodegeneration and increased plaque formation in complement-inhibited Alzheimer's mice. Proc Natl Acad Sci U S A. 2002 Aug 6;99(16):10837-42. PubMed.
  7. . Complement C3 deficiency leads to accelerated amyloid beta plaque deposition and neurodegeneration and modulation of the microglia/macrophage phenotype in amyloid precursor protein transgenic mice. J Neurosci. 2008 Jun 18;28(25):6333-41. PubMed.
  8. . Complement component C1q inhibits beta-amyloid- and serum amyloid P-induced neurotoxicity via caspase- and calpain-independent mechanisms. J Neurochem. 2008 Feb;104(3):696-707. PubMed.
  9. . The complement factor C5a contributes to pathology in a rat model of amyotrophic lateral sclerosis. J Immunol. 2008 Dec 15;181(12):8727-34. PubMed.
  10. . Amyloid beta, mitochondrial dysfunction and synaptic damage: implications for cognitive decline in aging and Alzheimer's disease. Trends Mol Med. 2008 Feb;14(2):45-53. PubMed.
  11. . Insights into amyloid-beta-induced mitochondrial dysfunction in Alzheimer disease. Free Radic Biol Med. 2007 Dec 15;43(12):1569-73. PubMed.
  12. . Beta-amyloid mediated nitration of manganese superoxide dismutase: implication for oxidative stress in a APPNLH/NLH X PS-1P264L/P264L double knock-in mouse model of Alzheimer's disease. Am J Pathol. 2006 May;168(5):1608-18. PubMed.
  13. . Increased plaque burden in brains of APP mutant MnSOD heterozygous knockout mice. J Neurochem. 2004 Jun;89(5):1308-12. PubMed.
  14. . Reduction in mitochondrial superoxide dismutase modulates Alzheimer's disease-like pathology and accelerates the onset of behavioral changes in human amyloid precursor protein transgenic mice. J Neurosci. 2006 May 10;26(19):5167-79. PubMed.

Other Citations

  1. Tg2576

External Citations

  1. Cephalon, Inc.
  2. Antipodean Pharmaceuticals
  3. Phase 2 study
  4. MedPageToday article

Further Reading

Papers

  1. . Amyloid beta, mitochondrial dysfunction and synaptic damage: implications for cognitive decline in aging and Alzheimer's disease. Trends Mol Med. 2008 Feb;14(2):45-53. PubMed.
  2. . Atypical dementia: when it is not Alzheimer's disease. J Med Assoc Thai. 2007 Oct;90(10):2222-32. PubMed.
  3. . Prominent neurodegeneration and increased plaque formation in complement-inhibited Alzheimer's mice. Proc Natl Acad Sci U S A. 2002 Aug 6;99(16):10837-42. PubMed.
  4. . Amyloid precursor protein-mediated free radicals and oxidative damage: implications for the development and progression of Alzheimer's disease. J Neurochem. 2006 Jan;96(1):1-13. PubMed.

Primary Papers

  1. . Treatment with a C5aR antagonist decreases pathology and enhances behavioral performance in murine models of Alzheimer's disease. J Immunol. 2009 Jul 15;183(2):1375-83. PubMed.
  2. . Overexpression of SOD-2 reduces hippocampal superoxide and prevents memory deficits in a mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 2009 Aug 11;106(32):13576-81. PubMed.