There’s a new player in town or, to be precise, in the rat brain. Based on a report in yesterday’s Journal of Neuroscience, scientists can now add the α7β2 heteromeric nicotinic acetylcholine receptor (nAChR) to the α7 homomeric receptors already known to exist in the basal forebrain—a site of cholinergic neuron loss in early Alzheimer disease (AD). “We think that, at least in the basal forebrain, the majority of nicotinic receptors are heteromeric α7β2, though there are also some homomeric α7s,” said principal investigator Jie Wu, from the Barrow Neurological Institute in Phoenix, Arizona. More importantly for Alzheimer researchers, these heteromeric receptors appear to be much more sensitive than their known cousins to blockage by oligomers of amyloid-β (Aβ), raising questions about what role these novel receptors may play in the human disease.
“Nicotinic acetylcholine receptors are perhaps the most complex neurotransmitter receptors in the brain,” Wu told ARF. The pentameric structures can form from various combinations of 16 different subunits, which can sometimes make data difficult to interpret. While homomeric α7 receptors have been seen as the predominant nicotinic receptor in the basal forebrain, recent evidence showed that β2 subunits are highly expressed in that region, as well. This is the first data showing that α7 and β2 subunits form a complex and are functional in the basal forebrain, said Wu.
First author Qiang Liu and colleagues used a mixture of electrophysiology, pharmacology, cell biology, and genetics to confirm the presence of the heteromeric receptor in rat basal forebrain. The researchers discovered that nicotinic receptors in acutely isolated cholinergic neurons from the medial septum/diagonal band (MS/DB) of the forebrain have different properties than the α7 homomeric receptors in dopaminergic neurons from the ventral tegmental area (VTA). In the latter neurons, the transmitter choline elicited the rapid induction of current indicative of α7 homomeric stimulation, whereas in MS/DB neurons the kinetics were significantly slower, suggesting the receptors were not identical. Moreover, receptors on MS/DB neurons were much more susceptible to blockage by the β2-selective antagonist dihydro-β-erythroidine (DHβE), suggesting the β2 subunit may be modulating choline responses. Finally, the researchers found that α7 and β2 subunits co-immunoprecipitate from basal forebrain neurons but not VTA neurons. “These results are consistent with the hypothesis that functional α7-nAChRs on MS/DB cholinergic neurons also contain DHβE-sensitive β2 subunits,” write the authors. They found similar kinetics and pharmacological responses in Xenopus oocytes co-expressing α7 and β2 subunits, but not in neurons from β2 subunit knockout mice. This further supports the hypothesis that α7β2 receptors assemble and function in forebrain neurons. (Due to questionable antibody specificity, immunoprecipitation experiments with nAChRs can be difficult to interpret, hence multiple lines of evidence are needed).
Nicotinic ACh receptors are intimately tied into the pathophysiology of AD. These receptors disappear from the cortex and hippocampus during the course of the disease (see Burghaus et al., 2000). Four of the five AD drugs approved by the U.S. Food and Drug Administration are cholinesterase inhibitors, which boost acetylcholine levels. There are also indications that cholinergic stimulation attenuates Aβ production (see Mousavi et al., 2008) and that Aβ can bind to ACh receptors and spur phosphorylation of tau, the major component of neurofibrillary tangles (see ARF related news story). Nicotinic agonists are among the more commonly pursued drug candidates in the pharma industry’s pipeline for AD and even schizophrenia.
To test how α7β2 receptors might fit into the larger cholinergic picture, Liu and colleagues tested their response to Aβ42. Aβ oligomers inhibited the response to choline of MS/DB neurons (but not of VTA neurons) even at concentrations as low as 1 nM. “That’s a more physiological level of Aβ, which is usually about 5 nM in the AD brain, or at least lower than 10 nM,” said Wu—100nM Aβ is typically used to block α7 homomeric receptors. The authors confirmed Aβ oligomer toxicity in oocytes expressing the two receptor subunits, and also found that forebrain neurons from β2 subunit knockout mice were unaffected by 1 nM Aβ42. “Based on the current findings, we suggest that selective, high-affinity effects of oligomeric Aβ1-42 on basal forebrain, cholinergic neuronal α7β2-nAChRs could acutely contribute to disruption of cholinergic signaling and diminished learning and memory abilities,” wrote the authors.
Wu said it is unclear why these receptors should be more sensitive to Aβ. He noted that adding the β2 subunit seems to increase the receptor’s affinity for nicotine dramatically, so it somehow changes the properties of the receptor. Whether the heteromeric receptors are expressed and functional in the human basal forebrain is a question Wu plans to investigate next.—Tom Fagan
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