Regulation of metabotropic glutamate receptors 4 and 7 by microRNAs in the dorsolateral prefrontal cortex of subjects with schizophrenia

Metabotropic glutamate receptors (mGluRs) have been proposed as targets for the development of new compounds with antipsychotic (AP) potential. Group I (mGluR1/5) and group II (mGluR2/3) mGluRs have received more attention as targets for novel AP drugs than group III (mGluR4/6/7 and 8). However, several genetic and pharmacological evidence points to the involvement of group III mGluRs in schizophrenia (SZ). Unpublished results from our laboratory show that the immunodensities of mGluR4 and mGluR7, measured through Western Blot, were significantly reduced in the dorsolateral prefrontal cortex (dlPFC) of subjects with SZ, compared to control subjects, without alterations in the corresponding gene expression levels. Here, we evaluated the potential role of selected mGluR-targeting microRNAs in the regulation of group III mGluR densities in the dlPFC of subjects with SZ. The effect of AP use on the levels of these microRNAs was also evaluated.

dlPFC samples of 24 subjects with SZ and age-, sex-, and postmortem delay-matched controls were obtained at autopsies in the Basque Institute of Legal Medicine. Toxicological analyses identified SZ subjects who were (AP+; n=12) or were not (AP-, n=12) taking AP drugs at the time of death, according to AP drug plasma levels. microRNAs reported to inhibit the translation of GRM4 (miR-1202, miR-29b-3p, miR-335-5p, miR-1908-5p and miR-328-3p) or GRM7 (miR-137-3p, miR-15a-5p, miR-15b-5p and miR-34a-5p) were selected according to the existence of experimental evidence of mGluR4 or mGluR7 regulation and predictions of targeting GRM4 or GRM7 mRNA by TargetScanHuman. Paired t-test was used to compare microRNA expression levels determined by PCR assays between both experimental groups. Pearson’s correlations evaluated the association between the levels of each microRNA and the protein density of the receptor (mGluR4 or mGluR7) they target.

Gene expression of miR-328-3p, miR-29b-3p, miR-1908-5p, miR-137-3p, miR-15a-5p, miR-15b-5p and miR-34a-5p were not significantly different (p>0.05) between subjects with SZ and controls, irrespective of AP treatment. miR-1202 expression levels were significantly higher in SZ cases than in control subjects (+37%; p<0.05), but this effect was not observable in the SZ subgroups when divided by AP presence in blood. miR-335-5p expression levels were significantly higher in subjects with SZ in comparison to controls (+61%; p<0.01). Subjects with SZ who were AP- (but not AP+ cases) displayed higher miR-335-5p expression levels than their matched control subjects (+86%; p<0.05). Correlation analyses showed no significant associations between mGluR4 protein density and the expression levels of miR-1202 (r=-0.15; p>0.05) or miR-335-5p (r=-0.06; p>0.05). No significant associations were observed between other individual microRNAs and the protein density of the targeted receptor (mGluR4 or mGluR7) (p>0.05).

In conclusion, miR-1202 and miR-335-5p expression levels were enhanced in the dlPFC of SZ subjects. None of these effects were attributed to AP medication, indicating that these upregulations may be a core component of the illness. While elevated levels of miR-1202 and miR-335-5p could explain reduced mGluR4/7 densities in SZ dlPFC, no direct evidence of microRNA-protein correlations was observed. Confounding factors such as illness duration, or perimortem conditions could affect RNA and protein molecules differently and may contribute to microRNA-protein dissociations.

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