8 June 2010. Neurons control behavior. Antipsychotic drugs target neurotransmitter systems. These statements are among the most uncontroversial in neuroscience, but two surprising new studies suggest that even these seemingly settled issues may need to be reconsidered. What links them together is the unexpected roles of cells and drugs that we associate with the immune system.
In the May 28 issue of Cell, researchers at the University of Utah show that a pathological grooming behavior in mutant mice that has been proposed as a model of obsessive-compulsive disorder (OCD) is driven by a defect in bone-marrow derived glial cells, not neurons. In the May issue of the Journal of Clinical Psychiatry, Dutch researchers report that psychotic symptoms in schizophrenia patients were reduced when antipsychotic drug treatment was supplemented with aspirin, providing support for the idea that inflammatory processes may play a role in psychosis, a notion that has a large body of research behind it, though no major breakthroughs as yet.
A very well-groomed mouse
The Utah story begins in 2002, when Mario Capecchi and Joy Greer reported that mice with homozygous loss-of-function mutations in the Hoxb8 gene groomed themselves excessively, to the point that they developed large hairless patches and injured their skin; this phenotype had 100 percent penetrance in the mutants (Greer and Capecchi, 2002). (Interestingly, the Hoxb8 mutants also excessively groomed wild-type littermates.)
This was a remarkable finding, because the highly conserved Hox family of genes was primarily associated with establishment of the body plan during development (see, e.g., Lutz et al., 1996). The obvious parallels to trichotillomania, a form of OCD in which excessive pulling or twisting of the hair causes large bald patches on the scalp, led the Capecchi group to continue its analysis of Hoxb8 as a promising animal model of OCD.
In the new Hoxb8 study, a team in the Capecchi lab led by Shau-Kwaun Chen determined that the subset of cells showing Hoxb8 lineage in the adult brain of the mouse mutants were microglia, and that they are primarily found in the cerebral cortex, striatum, olfactory bulb, and brainstem. In newborn mutants, sparse cells of Hoxb8 lineage were seen primarily in the choroid plexus, meninges, and ventricular zone, suggesting that the microglia migrate to the forebrain postnatally. Adult mutant mice had at least 15 percent fewer of these microglia than did wild-type mice (the authors note that limitations in their labeling technique preclude a precise assessment, meaning that the reduction in microglia in Hoxb8 mutants could be higher).
“Although the origin of microglia is still debated,” the team writes, “there is general agreement that at least one subpopulation is of bone marrow origin (i.e., derived from circulating monocytes....)” To test this hypothesis, the group labeled white blood cells, and found that all hematopoietic lineages expressed Hoxb8, a finding that was replicated in bone marrow cells.
However, the most dramatic evidence for the linkage between hematopoietic cells and glial cells of Hoxb8 lineage was seen in bone marrow transplant experiments. When Hoxb8 mutants received transplants of wild-type bone marrow, they gradually stopped their excessive grooming. Their skin injuries healed, and their hair regrew. Of six Hoxb8-mutant animals that received wild-type transplants, “[f]our of them fully recovered and were indistinguishable from wild-type mice.” Conversely, two wild-type mice that received transplants of Hoxb8-mutant bone marrow exhibited the excessive grooming phenotype, though they spent less time grooming than is usually seen in Hoxb8 mutants.
In some ways, these results are as striking as the original description of the Hoxb8 excessive grooming phenotype itself. Though there is increasing appreciation of the diverse roles played by glial cells, particularly astrocytes, in the nervous system (see, e.g., Allen and Barres, 2009), microglia are phagocytes, immune system cells that engulf and degrade debris and pathogens. Other than their pathological roles in neurodegenerative diseases such as Alzheimer’s disease, these are hardly the type of cell that one associates with the regulation of a behavior as specific as grooming.
However, the authors write, “[f]rom an evolutionary perspective it may make perfect sense to couple a behavior such as grooming, whose purpose is to reduce pathogen count[,] with the cellular machinery, the innate and adaptive immune systems, used to eliminate pathogens.”
In terms of mechanism, the authors point to previous research showing that microglia physically interact with synapses, as well as their well-studied release of neuronally active cytokines, and speculate that microglia thereby might regulate the activity of synaptic circuits.
Take two aspirins, lower your PANSS score
In another study that links immune function with behavior, a team at University Medical Center Utrecht in The Netherlands conducted a randomized, double-blind, placebo-controlled trial in which 33 out of 70 patients with schizophrenia spectrum disorder (schizophrenia, schizoaffective disorder, or schizophreniform disorder) were provided with 1,000 mg of aspirin daily in addition to antipsychotic medication for three months.
The study builds upon three previous small trials with another non-steroidal anti-inflammatory drug, the COX-2 inhibitor celecoxib, two of which found symptom responses to the drug.
At the final follow-up, the patients who had received aspirin had mean Positive and Negative Syndrome Scale (PANSS) scores that were lower than those of the patients who had received placebo: in the aspirin-treated group, the total PANSS score was 4.86 lower, and the positive PANSS score was 1.57 points lower. No significant differences were seen in other measures, including negative and general PANSS scores, or performance on a battery of cognitive tests.
The lowest total and positive PANSS scores were seen in patients with the shortest disease duration. However, more relevant to hypotheses that inflammation contributes to psychosis was a correlation between the lowest PANNS scores in these two categories and altered immune function, defined as a shift in the balance of T helper cell levels toward anti-inflammatory cytokines versus pro-inflammatory cytokines (in this case, levels of interleukin-4 [IL-4] vs. those of interferon-γ [IFN-γ]).
“The inflammatory hypothesis is gaining some ground, and there are some interesting genetic findings related to it, including our own early GWAS study implicating a cytokine receptor gene in risk for schizophrenia,” Anil Malhotra of the Zucker Hillside Hospital told SRF (see SRF related news story). Jeffrey Lieberman of Columbia University agrees, telling SRF that “the inflammation model has much relevance to various, and in many cases seemingly unlikely, diseases. In addition, this builds on the prior studies with COX-2 inhibitors [in schizophrenia].”
The authors concede that their sample size was smaller than they had originally intended to enroll, and that a three-month follow-up cannot address questions about long-term use of aspirin in high doses (to avoid aspirin-associated gastric distress in the treatment group, the Dutch researchers provided all subjects with omeprazole, a proton-pump inhibitor). Nonetheless, they write that the study had “the largest sample size and longest time of follow-up to date” in research on the use of anti-inflammatory drugs in schizophrenia, adding that the sample size “agrees with the recommended 40 to 100 patients for drug augmentation studies in schizophrenia.”
New York University’s Daniel Javitt is cautious about the findings, noting that there is “a much smaller difference at three months than at two, and an effect size in the total symptoms of about 0.3 for the LOCF [last-observation-carried forward].” Javitt also questions why data were provided only for IFN-γ and IL-4 when the paper mentions that measures were also made of the monocyte cytokines IL-12 and IL-6. “The most compelling finding is the result that patients with most severe inflammation markers showed best response,” Javitt told SRF. “This would seem to be worth following up and selecting a potentially responsive subgroup.”—Pete Farley.
Chen SK, Tvrdik P, Peden E, Cho S, Wu S, Spangrude G, Capecchi MR. Hematopoietic origin of pathological grooming in Hoxb8 mutant mice. Cell. 2010 May 28;141(5):775-85. Abstract
Laan W, Grobbee DE, Selten JP, Heijnen CJ, Kahn RS, Burger H. Adjuvant aspirin therapy reduces symptoms of schizophrenia spectrum disorders: results from a randomized, double-blind, placebo-controlled trial. J Clin Psychiatry. 2010 May;71(5):520-7. Abstract