7 August 2009. 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.”
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.
Fonseca MI, Ager RR, Chu SH, Yazan O, Sanderson SD, Laferla FM, Taylor SM, Woodruff TM, Tenner AJ. 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. Abstract
Massaad CA, Washington TM, Pautler RG, Klann E. Overexpression of SOD-2 reduces hippocampal superoxide and prevents memory deficits in a mouse model of Alzheimer’s disease. PNAS Early Edition. 2009 August. Abstract