26 Jan 2017
by Lesley McCollum
A new study published online January 23 in Nature Medicine reports that an alteration in the human nicotinic cholinergic α5 receptor subunit leads to reduced activity in the prefrontal cortex (PFC) in mice, an abnormality that parallels a defining feature of schizophrenia and addiction called hypofrontality. Led by Uwe Maskos of the Pasteur Institute in France, the study showed that the effects of the α5 variant can be attributed to a pathway involving specific subtypes of GABAergic interneurons found in layer II/III of the PFC. Treating the mice with nicotine restored the signaling deficits in the region.
Smoking, which heavily burdens the schizophrenia population, can actually improve cognition and mitigate some cognitive deficits associated with hypofrontality. Restoration of the impaired cholinergic signaling in the PFC suggests a mechanism behind nicotine’s beneficial effects.
The new findings bolster support for the role of dysfunctional nicotinic acetylcholine receptors (nAChRs) in the cognitive deficits associated with schizophrenia (see SRF related story) by outlining a mechanism that starts with a genetic variant identified in genomewide association studies (GWAS) of smoking: A single nucleotide polymorphism (SNP) in the α5 subunit gene (CHRNA5) increases risk for smoking, and has been associated with alterations in PFC circuitry. Recently, the landmark 2014 schizophrenia GWAS (see SRF related story) highlighted a link between the disorder and the genetic locus that contains CHRNA5 (along with the CHRNA3 and CHRNB4 nicotinic receptor genes).
“Mechanistically, this is beautiful,” said Bita Moghaddam of Oregon Health & Science University in Portland, who was not involved in the study. She added that the study is a great use of advanced tools to demonstrate a mechanism that could be disrupted from genetic findings in schizophrenia.
α5 controls circuit behavior
Rather than pharmacological manipulation or removing the gene altogether, first author Fani Koukouli, also of the Pasteur Institute, and colleagues developed a mouse line expressing the human α5 SNP identified in the smoking GWAS, which results in partial loss of function of α5-containing nAChRs, to understand its effects in the brain.
The human α5 SNP-expressing mice showed impaired social behavior and reduced prepulse inhibition (a measure of cognitive filtering), both considered common features of schizophrenia. Using two-photon calcium imaging to measure Ca2+ transients as an indicator of neuronal firing, Koukouli and colleagues found that human α5 SNP-expressing mice had significantly lower neuronal activity in layer II/III of the PFC.
To identify the specific pathway, the researchers used the genome editing technique CRISPR-Cas9 to target the Chrna5 gene in layer II/III of the PFC, and reduced expression of the α5 subunit specifically in vasoactive intestinal polypeptide (VIP) interneurons. Lower activity of the VIP interneurons disinhibited somatostatin interneurons, which then exerted increased inhibitory drive over pyramidal neurons—the primary excitatory neurons in the region—to reduce their activity.
The authors suggest this decrease in layer II/III circuit activity parallels hypofrontality in patients, though Moghaddam notes that large-scale recordings of the PFC by clinically relevant measurements, such as BOLD signals or EEG, could have shown an association with hypofrontality more directly.
In transgenic mice lacking the α5 subunit (α5 knockout), re-expression of the normal α5 subunit in PFC layer II/III completely restored the reduced activity of pyramidal neurons, whereas re-expression of the human α5 SNP only partially restored activity. Restoration of the activity indicates a dominant effect of α5 on circuit behavior.
“In a way, it was completely unexpected,” said Maskos. Although the role of nicotinic receptors in cortical signaling itself had been indicated by previous studies, the findings show these specific interneurons exert critical control over pyramidal neurons.
“That was interesting because it also relates to cognitive function in the prefrontal cortex and gamma oscillations,” said Moghaddam, which were not measured in this study. Interneurons regulate gamma band oscillations in the PFC, and disruptions in the oscillations have been linked to cognitive deficits (see SRF related news story). “So the mechanism was nice; it aligned with how prefrontal cortex could be regulating cognitive function.”
Importantly, the researchers showed that systemic administration of nicotine—similar to the exposure when a person smokes—restored the deficits in pyramidal neuron signaling. The authors had thought that nicotine might have a beneficial effect, based on behavioral studies in animals. But previous studies have also shown that nicotine enhances activity too much in normal mice and makes healthy people worse. The new findings show the mechanism behind this phenomenon at the circuit level—there must first be a cholinergic deficit.
“Nicotine restores cholinergic activity to just the right level,” said Maskos, adding that the findings provide an explanation of the long-standing hypothesis that patients with schizophrenia use nicotine to self-medicate.
A new treatment strategy?
Nicotinic receptors have been considered as a target for treatment, said Maskos. “But frankly, without understanding the mechanism.” Previous drugs targeting the α7 nicotinic receptor generated initial support (see SRF related news story), only to fail in advanced clinical trials. Now, the authors have highlighted specific cell types and the α5-containing receptors that Maskos sees as potential targets for developing medications.
“There’s clearly potential in terms of having a target that’s so well defined,” said Moghaddam. “On the other hand, we have nicotine for that treatment,” which was demonstrated to be effective in the study. While she agrees as to the therapeutic value, experiments demonstrating that an α5 ligand could be superior to nicotine, without its side effects, would be needed to ignite interest in new drug development.
“What is interesting is that this mutation is also relevant to heavy smokers,” said Moghaddam, noting that the findings could have implications for treating addiction. If partial α5 agonists could be developed, they may be used to potentially reduce nicotine cravings and aid smoking cessation.