29 March 2011. Bita Moghaddam of the University of Pittsburgh kicked off the final session of the NYAS meeting on Advancing Drug Discovery in Schizophrenia on the afternoon of 11 March 2011. Dissatisfied with how simple circuit versions of the glutamate hypothesis of schizophrenia fail to deliver for the whole brain, she proposed an alternative way of thinking about the disorder. Instead of seeing the main clues we have for schizophrenia—NMDA receptor hypofunction and decreased GAD67, an enzyme that synthesizes GABA—as factors that contribute to the disorder, she suggested that decreased GAD67 may instead reflect a compensation for abnormal levels of glutamate. If markers of disease like these are compensatory, rather than primary, then trying to correct them would give opposite effects than expected. With this in mind, she cited a tightly regulated GABA shunt in mitochondria that recycles GABA molecules from glutamate. Moghaddam suggested that imbalances in GABA and glutamate levels could stem from improper mitochondrial function—something with repercussions for excitatory and inhibitory signaling throughout the brain.
Seeking to better simulate the molecular processes underway in neurons in disease, Akira Sawa of Johns Hopkins University reported on the garden of human neural cells growing in his lab. He has had some success in maintaining olfactory neurons from humans obtained through a relatively non-invasive nasal biopsy, deriving human neurons from iPSCs, and for the first time, deriving neurons directly from human skin cells. After two weeks of treatment in media, these latter neurons, called induced neurons (or iN), showed neuron-specific markers and fired action potentials.
As an example of the dividends these approaches may provide, he showed a comparison of mRNA profiles in olfactory neurons between people with schizophrenia and controls. The groups differed in genes related to actin binding, the NF-κB protein complex involved in DNA transcription, intracellular protein transport, and immune and stress responses. Olfactory neurons from people with schizophrenia showed abnormalities in DISC1 phosphorylation, a state which dictates whether neuron proliferation or migration will proceed during brain development.
Dopamine strikes back
The last three speakers discussed whether there might be ways to refine dopamine signaling in the brain to treat schizophrenia more effectively, with fewer side effects. Starting with D2Rs, the target of all antipsychotics, Marc Caron of Duke University noted that D2Rs engage both a G protein-coupled pathway and one involving the scaffolding protein β-arrestin-2 (see SRF related news story), which in turn activates the AKT/GSK3β signaling independently associated with schizophrenia. To probe how much this β-arrestin-2 pathway mediates psychosis-like behaviors in mice, he disabled it by removing GSK3β in D2R-containing neurons. This interfered with apomorphine-induced rearing, and amphetamine-induced hyperlocomotion and prepulse inhibition, but not cognitive tasks. Removing GSK3β's target, β-catenin, from D1R-containing neurons induced more amphetamine-induced psychosis, whereas removing it from D2R-containing neurons induced less. Together, the results implicate this pathway in psychosis-like behaviors and antipsychotic response, and the extent of its contribution may be further delineated with D2R mutants engineered to selectively signal through either the G protein or the β-arrestin-2 pathway.
In a later talk (see below), John Allen of the University of North Carolina reported on efforts to find D2R ligands that selectively activate the β-arrestin-2 pathway. After screening hundreds of compounds, he came up with two, which. when administered to mice, decreased PCP- or amphetamine-induced hyperlocomotion, without increasing catalepsy. These results suggest that selective β-arrestin-2 signal recruitment may contribute to antipsychotic effects without motor side effects.
In another twist for dopamine signaling, Susan George of the University of Toronto presented her evidence for a novel dopamine receptor made up of a D1R and a D2R. This combination-receptor offers a new mode of signaling for dopamine because it activates a G protein pathway that is not induced when either receptor is activated alone (see SRF related news story). The D1-D2 heteromers occur endogenously in the brain, and are enriched in the nucleus accumbens and globus pallidus of rats. Using a competitive binding assay to detect a high-affinity state for a D2 agonist in the D1-D2 heteromer, George found this state is enhanced in rats treated with amphetamine and in the globus pallidus of postmortem brain in schizophrenia (Perreault et al., 2010). The work suggests that abnormal coupling between D1 and D2 could be a molecular marker for pathological states in the brain, and that this coupling may be a new treatment target—something that has already been explored in mouse models of depression (see SRF related news story).
A sprinkling of serotonin
Serotonin signaling is also thought to contribute to schizophrenia symptoms, and atypical antipsychotics have been designed to target 5HT-2A receptors, following suggestions that this activity set clozapine apart from the typical antipsychotics. Prompted by a rare CNV found in the caveolin-1 (CAV1) gene in one case of schizophrenia (Walsh et al., 2008), John Allen also explored the state of serotonin signaling in knockout mice missing CAV1. CAV1 encodes a scaffolding protein involved in clustering diverse signaling molecules together, including 5HT-2A receptors. Loss of CAV1 attenuated 5HT-2A signaling in these mice, as revealed by an increase in PCP-induced hyperlocomotion and disrupted prepulse inhibition, and these could not be reversed as usual by clozapine, an atypical antipsychotic. Similarly, these mice made fewer head twitches in response to a hallucinogenic 5HT-2A agonist. The number of 5HT-2A receptors was normal in these mice, which suggests that loss of the CAV1 scaffold mislocalized 5HT-2A receptors and their downstream effector molecules, compromising their function.
Although the crowd was dwindling, energy remained high at the end of the talks, with several people saying that they felt optimistic about the prospects for drug discovery in schizophrenia. One participant raised the issue of specificity, asking how disturbances to intracellular signaling pathways available to all cells can produce the malfunctions in specific brain circuits observed in schizophrenia. Moghaddam suggested that patterns of metabolic activity, combined with aberrant signaling, somehow targeted certain circuits for dysfunction. John Krystal of Yale University suggested a genetic explanation, noting that susceptible brain regions in schizophrenia are the most recently evolved and thus could be the most genetically labile. Either way, a challenge will be to deliver treatment to ailing brain circuits without disrupting those that are functioning normally.
In his closing remarks, Krystal called the meeting "a next-generation conference" with research beginning to meet the urgent need for mechanistically novel compounds. "What's exciting is not how far we've come, but the possibility that we might be getting to the point where we can use science to guide psychiatry," he said.—Michele Solis.