In this study, we found that mRNA expression levels for mGluR1α, but not mGluR5, are significantly higher in the PFC in schizophrenia subjects. Consistent with prior reports (15
), we also found lower mRNA levels for RGS4, which reduces the duration of G protein-mediated intracellular signaling from mGluR1α activation (13
); lower RGS4 mRNA levels were commonly found in the same schizophrenia subjects who had higher mGluR1α mRNA levels. In contrast, no differences were found in mRNA levels for the synthesizing and metabolizing enzymes for 2-AG, DAGL (both α and β isoforms) and MGL, respectively, or for FAAH, the metabolizing enzyme of the other cortical endocannabinoid, anandamide, in schizophrenia subjects. Taken together, these data suggest that altered mGluR1α and RGS4 mRNA levels may be common in schizophrenia subjects and may represent a disturbed “molecular hub” that has an important impact on multiple components of neuronal communication in the PFC.
Increased mGluR1α mRNA levels appear to be specific to the disease process of schizophrenia, or at least not attributable to factors frequently associated with the illness. For example, treatment with antipsychotic medications does not appear to affect mGluR1α mRNA levels since mGluR1α mRNA levels were similar in schizophrenia subjects on and off medications at time of death. In addition, in contrast to our findings in schizophrenia subjects, mGluR1α mRNA levels were slightly lower in antipsychotic-exposed monkeys, although these differences did not achieve statistical significance. Furthermore, differences in sex, suicide as a cause of death, diagnosis of substance abuse or dependence current at the time of death, use of antidepressants, benzodiazepines or sodium valproate at the time of death, or a history of cannabis use did not appear to affect mGluR1α levels in schizophrenia subjects. Interestingly, mGluR1α mRNA levels and mGluR5 mRNA levels appeared to be higher in schizophrenia subjects than in schizoaffective disorder subjects; however, no other markers showed a difference between schizophrenia and schizoaffective disorder subjects, including CB1R (25
Several lines of evidence suggest that the capacity for mGluR1α activation may be increased in the PFC in schizophrenia. First, our findings of elevated mGluR1α mRNA levels parallel a previous report of higher mGluR1α protein levels in the PFC in schizophrenia (12
), suggesting that higher levels of mGluR1α mRNA expression may lead to translation of more protein, although this hypothesis needs to be directly tested in the same subjects. Second, RGS4 reduces the duration of intracellular signaling from mGluR1α (13
). Consistent with evidence that RGS4 also regulates G protein coupling for multiple other receptors (36
), RGS4 and mGluR1α mRNA levels were not correlated in individual subjects and differences in RGS4 and mGluR1α mRNA levels between schizophrenia and matched normal comparison subjects were not correlated in subject pairs. However, out of the 42 subject pairs, 27 pairs showed both higher mGluR1α and lower RGS4 mRNA levels in the schizophrenia subject compared to the matched normal comparison subject. Taken together, these data suggest that higher mGluR1α and lower RGS4 mRNA levels are generally found in the same schizophrenia subjects and, if accompanied by changes in levels of the corresponding proteins, may lead to a greater capacity for, and a longer duration of intracellular signaling from, mGluR1α activation in the PFC in schizophrenia ().
Potential downstream effects of alterations in mGluR1α and RGS4 mRNA levels on neurotransmitter systems in schizophrenia
As mentioned above, alterations in RGS4 mRNA levels in schizophrenia may have additional effects beyond modifying the intracellular signaling efficacy of mGluR1α. First, RGS4 is a GTPase-activating protein that reduces the duration of activity of several different G protein-coupled receptors in addition to metabotropic glutamate receptors, including dopamine, serotonin, and opioid receptors (36
). Thus, lower RGS4 mRNA levels, if also present at the protein level (16
), may enhance the functioning of an array of receptors in schizophrenia (15
) (). Second, some, though not all (40
), case control and family based association studies have identified single nucleotide polymorphisms (SNPs) of the RGS4 gene as susceptibility factors for schizophrenia, although different alleles of these SNPs have been associated with schizophrenia in different subject populations (14
). Some of the risk haplotypes for RGS4 are also associated with smaller volume of the PFC (42
) and altered activation of the PFC during working memory tasks (43
) in schizophrenia subjects. Thus, alterations in RGS4 at the genomic and transcript level may have a broad impact on diverse neurotransmitter systems and on functional properties of the PFC in schizophrenia.
Higher mRNA levels for mGluR1α and lower mRNA levels for RGS4, if accompanied by altered levels of the corresponding proteins (12
), are consistent with a higher capacity for mGluR1α-mediated neurotransmission in schizophrenia, which may have a diverse and complicated impact on multiple components of neural transmission in the disorder. For example, immunoreactivity for mGluR1α and RGS4 is found in both pyramidal neurons and GABA neurons in primate PFC (44
). An important limitation of our study is that a tissue level analysis of mRNA levels does not allow a determination of a potential cell-type specificity of higher mGluR1α and lower RGS4 mRNA levels in schizophrenia; thus, follow up studies are needed to determine whether altered levels of mGluR1α and RGS4 are predominantly found in pyramidal neurons or GABA neurons or both. With this caveat in mind and for the purpose of discussion, we will focus on the potential downstream impact of greater mGluR1α-mediated neurotransmission on the NMDA receptor, endocannabinoid, and GABA systems in schizophrenia. For example, activation of group I mGluR results in long-term potentiation of NMDA receptor function (11
), and NMDA receptor function appears to be impaired in schizophrenia (5
). Thus, these lines of evidence invite speculation that in schizophrenia, greater mGluR1α activity may have a partially compensatory effect for NMDA receptor hypofunction. In addition, activation of group I mGluR has been reported to increase synthesis of 2-AG (24
) and suppress GABA release (6
). mGluR1α-mediated activation of 2-AG synthesis has been reported to occur through a G protein coupled mechanism that leads to increased levels of substrate (i.e. diacylglycerol) for DAGL (46
). Thus, while mRNA levels for the synthesizing and metabolizing enzymes for 2-AG are not altered in schizophrenia, higher mGluR1α activity may increase the substrate-dependent activity of DAGL, even in the absence of changes in DAGL enzyme level. Thus, higher mGluR1α-mediated activation of 2-AG synthesis may further suppress GABA release, which may worsen GABAergic deficits in schizophrenia. Higher mGluR1α mRNA levels and lower RGS4 mRNA levels may also affect additional components of neural transmission, including AMPA, dopamine, serotonin, and opioid receptors (11
), (). Taken together, these data suggest that altered mGluR1α and RGS4 mRNA levels may represent a disturbed “molecular hub” that has an important impact on multiple neurotransmitter systems in schizophrenia. Additional studies characterizing the cell-type specificity of transcript and protein level alterations in mGluR1α and RGS4 may provide greater insight into their downstream effects on PFC circuitry, and potentially their relationship to cognitive impairments, in schizophrenia.