11 June 2009. While anti-amyloid therapies and other frontline drug candidates hover in that “close, but no cigar” segment of the Alzheimer disease clinical pipeline, interest in hitting nicotinic acetylcholine receptors for AD treatment is smoldering just below the surface. For example, a study in last month’s Archives of Neurology offers a whiff of hope to efforts aimed at the α7 subtype. Led by Steven DeKosky at the University of Pittsburgh School of Medicine in Pennsylvania, who since moved to become dean of the University of Virginia School of Medicine, researchers report that α7 nicotinic acetylcholine receptor (nAChR) levels in the superior frontal cortex remain stable during the transition from normal cognition to mild AD, even as Aβ peptide concentrations rise. Albeit with modest sample size, the study calls into question earlier observations of waning brain α7 nAChR expression in AD, and bears out the idea that this receptor holds promise as an AD drug target.
The new analysis represents the latest in a series DeKosky’s team has done to examine cholinergic dysfunction in early AD. The researchers have studied brain samples of elderly nuns and priests who received clinical evaluations, including neuropsychological tests, as part of the ongoing Religious Orders Study at Rush University Medical Center in Chicago. Most of the clergy had also agreed to donate their brains for research upon death. “This population is unique in that it’s very well characterized clinically,” said first author Milos Ikonomovic. “When they die, we have the opportunity to examine structural and neurochemical changes in the brain and to correlate these changes with the detailed clinical profiles in these patients, most of whom had their last clinical evaluation within one year before death.”
Previous studies on α7 and other nAChR isoforms have suggested that the cholinergic system steadily fades in AD patients (e.g., Burghaus et al., 2000; Perry et al., 2001), fueling doubts about the long-term utility of therapeutics that target these neurons. Indeed, the cholinergic boosters currently used to treat AD offer only modest, temporary benefit.
Several investigations by DeKosky and colleagues offer a more nuanced view of the nAChR system as it relates to AD. In their 2002 study (DeKosky et al., 2002 and ARF related news story), hippocampus and frontal cortex tissue from seniors with early AD did not show the predicted drop in acetylcholine enzyme function. Instead, ACh function—measured as amounts of choline acetyl transferase (ChAT), an acetylcholine-synthesizing enzyme—took a surprising trajectory. Relative to their levels in the cognitively normal group, ChAT amounts actually rose in seniors with mild cognitive impairment (MCI), returned to normal levels in mild AD patients, and declined in people with more advanced disease.
The curious rise in ChAT levels among MCI patients did not appear to stem from a parallel increase in regional cholinergic fibers. Studying members of the Religious Orders cohort, the Pittsburgh team found that cholinergic fiber densities remained normal in the MCI group but dropped markedly in mild AD patients (Ikonomovic et al., 2007). This suggested that structural changes were not responsible for the temporary elevation in cholinergic enzyme activity at the MCI stage. Rather, as patients progress to mild AD, “there may be a biochemical upregulation (of ChAT protein or enzyme activity levels) that possibly compensates for structural loss of axonal terminals in the hippocampus and frontal cortex,” Ikonomovic said.
Conflicting results from other studies further complicate the picture of what happens to cholinergic neurons during progression from normal cognition to AD—particularly those expressing α7 nAChR. Alongside the earlier studies showing decreased expression of this receptor in AD brains, some analyses have found that α7 nAChR levels remain stable (e.g., Martin-Ruiz et al., 1999; Court et al., 2001) or even increase (e.g., Nordberg, 2001) during AD. “The issue has been controversial,” Ikonomovic said.
Sprinkle into this intriguing picture evidence that Aβ binds α7 nAChRs with high affinity (Wang et al., 2000) and that this interaction may induce tau phosphorylation (Wang et al., 2003 and ARF related news story), and AD researchers have even more reason to home in on α7 nAChRs. “Our approach was to use brain tissue homogenates from the frontal cortex—this area of potential plasticity that we identified in MCI cases—to quantify α7 nAChR binding and Aβ concentration,” Ikonomovic told ARF. His team measured these features using [3H]methyllycaconitine binding and enzyme-linked immunosorbent assays, respectively, on superior frontal cortical samples from 29 retired clergy who were diagnosed with no clinical impairment (n = 12), MCI (n = 9), or mild AD (n = 8).
They found, unexpectedly, that α7 nAChR binding did not differ across clinical categories; if anything, it was slightly higher, but not significantly, in people with neuropathologically diagnosed AD by CERAD criteria. (Consortium to Establish a Registry for Alzheimer's Disease scores are largely based on Aβ neuritic plaque frequency, accounting as well for clinical history and age.) Concentrations of Aβ42 and total Aβ, but not Aβ40, were higher in CERAD-diagnosed AD patients and correlated with lower scores on the Mini-Mental State Examination (MMSE).
“The reason we think we did not find differences in α7 binding across clinical groups is because cases in each of these groups had Aβ plaques. This is consistent with many studies,” Ikonomovic told ARF. He noted, in particular, a recent analysis showing that 20 to 40 percent of non-demented seniors had some level of neuropathological AD (Price et al., 2009). “Most likely α7 receptor changes are associated with neuropathology, possibly with Aβ plaques,” Ikonomovic said. “Different studies use different classifications of their cases. I think it's very important to examine the receptors in groups that are defined both clinically and neuropathologically.”
To extend the current analysis, his team plans to use immunohistochemistry to determine which cell types show the α7 nAChR changes, and to see whether these changes correlate with local Aβ pathology. One could envision a situation where α7 nAChR expression is going up in astrocytes and dropping in neurons due to AD-related cell loss. “Overall, you would not detect any change,” Ikonomovic said, noting a study that described these parallel phenomena in the brains of people with sporadic AD and of patients carrying the Swedish amyloid precursor protein (APP) mutation (Yu et al., 2005). “In reality,” he said, “there can be very interesting changes that we are missing using biochemical assays.”
Because α7 nAChRs are found in brains areas important for cognition, some believe that selective activation of these receptors will relieve the cholinergic dysfunction in AD without undesired side effects that come from overactivation of other AChRs involved in addiction or other non-CNS functions. Several companies—including En Vivo Pharmaceuticals, Memory Pharmaceuticals/Roche, AstraZeneca, and Siena Biotech/Wyeth—are developing α7 nAChR agonists for possible treatment of AD. Memory Pharmaceuticals has completed a Phase 2 safety and efficacy trial of its lead AD α7 compound, MEM 3454, in mild to moderate AD patients, and is recruiting for a study of this drug as an add-on to donepezil treatment in the same population. En Vivo has moved its α7 nAChR agonist, EVP-6124, past a Phase 1 study. Many companies are also working on α7 nAChR partial agonists and positive allosteric modulators for other indications, including schizophrenia, Parkinson disease, chronic pain, depression, epilepsy and attention deficit hyperactivity disorder (ADHD).
Recent publications offer insight into the molecular mechanisms of an α7 nAChR allosteric modulator (Barron et al., 2009) and describe cognitive and neuroprotective effects of selective α7 nAChR agonists in animal models (Roncarati et al., 2009; Feuerbach et al., 2009). There’s even a new α7 receptor—the α7β2 heteromeric nAChR—that seems more sensitive to oligomeric Aβ than the known homomeric α7 receptors (see ARF related news story). And if that’s not enough to whet your appetite, stay tuned for a for meeting on α7 nAChR-based therapeutic approaches to be held 14-16 October in Chicago, immediately preceding this year’s Society for Neuroscience conference.—Esther Landhuis.
Ikonomovic MD, Wecker L, Abrahamson EE, Wuu J, Counts SE, Ginsberg SD, Mufson EJ, DeKosky ST. Cortical alpha7 nicotinic acetylcholine receptor and beta-amyloid levels in early Alzheimer disease. Arch Neurol. 2009 May;66(5):646-51. Abstract