Profiles of DARPP-32 in the insular cortex with schizophrenia: a postmortem brain study

Darpp-32 is regulated by dopamine and is required for the formation of GABAergic neurons in the developing telencephalon

DARPP-32 (dopamine and cAMP-regulated phosphoprotein Mr. 32 kDa) is a phosphoprotein that is modulated by multiple receptors integrating intracellular pathways and playing roles in various physiological functions. It is regulated by dopaminergic receptors through the cAMP/protein kinase A (PKA) pathway, which modulates the phosphorylation of threonine 34 (Thr34). When phosphorylated at Thr34, DARPP-32 becomes a potent protein phosphatase-1 (PP1) inhibitor. Since dopamine is involved in the development of GABAergic neurons and DARPP-32 is expressed in the developing brain, it is possible that DARPP-32 has a role in GABAergic neuronal development. We cloned the zebrafish darpp-32 gene (ppp1r1b) gene and observed that it is evolutionarily conserved in its inhibitory domain (Thr34 and surrounding residues) and the docking motif (residues 7-11 (KKIQF)). We also characterized darpp-32 protein expression throughout the 5 days post-fertilization (dpf) zebrafish larval brain by immunofluorescence and demonstrated that darpp-32 is mainly expressed in regions that receive dopaminergic projections (pallium, subpallium, preoptic region, and hypothalamus). We demonstrated that dopamine acutely suppressed darpp-32 activity by reducing the levels of p-darpp-32 in the 5dpf zebrafish larval brain. In addition, the knockdown of darpp-32 resulted in a decrease in the number of GABAergic neurons in the subpallium of the 5dpf larval brain, with a concomitant increase in the number of DAergic neurons. Finally, we demonstrated that darpp-32 downregulation during development reduced the motor behavior of 5dpf zebrafish larvae. Thus, our observations suggest that darpp-32 is an evolutionarily conserved regulator of dopamine receptor signaling and is required for the formation of GABAergic neurons in the developing telencephalon.

Mass Spectrometry based identification of site-specific proteomic alterations and potential pathways underlying the pathophysiology of schizophrenia

BACKGROUND

Schizophrenia (SZ) is a complex multifactorial disorder that affects 1% of the population worldwide with no available effective treatment. Although proteomic alterations are reported in SZ however proteomic expression aberrations among different brain regions are not fully determined. Therefore, the present study aimed spatial differential protein expression profiling of three distinct regions of SZ brain and identification of associated affected biological pathways in SZ progression.

METHODS AND RESULTS

Comparative protein expression profiling of three distinct autopsied human brain regions (i.e., substantia nigra, hippocampus and prefrontal cortex) of SZ was performed with respective healthy controls. Using two-dimensional electrophoresis (2DE)-based nano liquid chromatography tandem mass spectrometry (Nano-LC MS /MS) analysis, 1443 proteins were identified out of which 58 connote to be significantly dysregulated, representing 26 of substantia nigra,14 of hippocampus and 18 of prefrontal cortex. The 58 differentially expressed proteins were further analyzed using Ingenuity pathway analysis (IPA). The IPA analysis provided protein-protein interaction networks of several proteins including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kb), extracellular signal regulated kinases 1/2 (ERK1/2), alpha serine / Threonine-protein kinase (AKT1), cellular tumor antigen p53 (TP53) and amyloid precursor protein (APP), holding prime positions in networks and interacts with most of the identified proteins and their closely interacting partners.

CONCLUSION

These findings provide conceptual insights of novel SZ related pathways and the cross talk of co and contra regulated proteins. This spatial proteomic analysis will further broaden the conceptual framework for schizophrenia research in future.

Differential protein expression of DARPP-32 versus Calcineurin in the prefrontal cortex and nucleus accumbens in schizophrenia and bipolar disorder

Dopamine- and cAMP-regulated phosphoprotein of molecular weight 32 kDa (DARPP-32) integrates dopaminergic signaling into that of several other neurotransmitters. Calcineurin (CaN), located downstream of dopaminergic pathways, inactivates DARPP-32 by dephosphorylation. Despite several studies have examined their expression levels of gene and protein in postmortem patients’ brains, they rendered inconsistent results. In this study, protein expression levels of DARPP-32 and CaN were measured by enzyme-linked immunosorbent assay (ELISA) in the prefrontal cortex (PFC), and nucleus accumbens (NAc) of 49 postmortem samples from subjects with schizophrenia, bipolar disorder, and normal controls. We also examined the association between this expression and genetic variants of 8 dopaminergic system-associated molecules for 55 SNPs in the same postmortem samples. In the PFC of patients with schizophrenia, levels of DARPP-32 were significantly decreased, while those of CaN tended to increase. In the NAc, both of DARPP-32 and CaN showed no significant alternations in patients with schizophrenia or bipolar disorder. Further analysis of the correlation of DARPP-32 and CaN expressions, we found that positive correlations in controls and schizophrenia in PFC, and schizophrenia in NAc. In PFC, the expression ratio of DARPP-32/CaN were significantly lower in schizophrenia than controls. We also found that several of the aforementioned SNPs may predict protein expression, one of which was confirmed in a second independent sample set. This differential expression of DARPP-32 and CaN may reflect potential molecular mechanisms underlying the pathogenesis of schizophrenia and bipolar disorder, or differences between these two major psychiatric diseases.

The involvement of DARPP-32 in the pathophysiology of schizophrenia

Schizophrenia is one of the most devastating heterogeneous psychiatric disorders. The dopamine hypothesis is the longest standing pathoetiologic theory of schizophrenia based on neurochemical evidences of elevated brain striatal dopamine synthesis capacity and increased dopamine release in response to stress. Dopamine and cyclic AMP-regulated phosphoprotein of relative molecular mass 32,000 (DARPP-32) is a cytosolic protein highly enriched in the medium spiny neurons of the neostriatum, considered as the most important integrator between the cortical input and the basal ganglia, and associated with motor control. Accumulating evidences has indicated the involvement of DARPP-32 in the development of schizophrenia; i. DARPP-32 phosphorylation is regulated by several neurotransmitters, including dopamine and glutamate, neurotransmitters implicated in schizophrenia pathogenesis; ii. decrease of both total and phosphorylated DARPP-32 in the prefrontal cortex are observed in schizophrenic animal models; iii. postmortem brain studies indicated decreased expression of DARPP-32 protein in the superior temporal gyrus and dorsolateral prefrontal cortex in patients with schizophrenia; iv. DARPP-32 phosphorylation is increased upon therapy with antipsychotic drugs, such as haloperidol and risperidone which improve behavioral performance in experimental animal models and patients; v. Genetic analysis of the gene coding for DARPP-32 propose an association with schizophrenia. Cumulatively, these findings implicate DARPP-32 protein in schizophrenia and propose it as a potential therapeutic target. Here, we summarize the possible roles of DARPP-32 during the development of schizophrenia and make some recommendations for future research. We propose that DARPP-32 and its interacting proteins may serve as potential therapeutic targets in the treatment of schizophrenia.

Elevated postmortem striatal t-DARPP expression in schizophrenia and associations with DRD2/ANKK1 polymorphism

BACKGROUND

Dopamine- and cAMP-regulated phosphoprotein of molecular weight 32 kDa (DARPP-32) and calcineurin (CaN) have been implicated in the pathogenesis of schizophrenia because they function as molecular integrators of dopamine and glutamate signaling. DARPP-32 and CaN are mainly expressed in the caudate nucleus and putamen; however, a few postmortem brain studies have focused on DARPP-32 expression in striatum from patients with schizophrenia.

METHODS

We used immunoblotting techniques and postmortem tissue samples from patients with schizophrenia and from normal control individuals to examine the expression of two major DARPP-32 isoforms, full-length (FL-DARPP) and truncated (t-DARPP), and of CaN in the striatum. We also assessed whether there was any significant correlation between the expression levels of either protein and the A1 allele of Taq1A genotype in the dopamine D2 receptor (DRD2) gene/ankyrin-repeat containing kinase 1 (ANKK1) gene.

RESULTS

We found that the mean t-DARPP expression level in the caudate was higher in patients with schizophrenia than in control individuals (P<0.05) and the A1 allele of Taq1A genotype in DRD2/ANKK1 was significantly associated with elevated expression of t-DARPP in the caudate. Also, the A1 allele was significantly correlated with the total score of antemortem psychiatric symptoms.

CONCLUSION

These results may reflect potential molecular mechanisms important to the pathogenesis of schizophrenia.

Revisiting DARPP-32 in postmortem human brain: changes in schizophrenia and bipolar disorder and genetic associations with t-DARPP-32 expression

Dopamine- and cAMP-regulated phosphoprotein of molecular weight 32 kDa (DARPP-32 or PPP1R1B) has been of interest in schizophrenia owing to its critical function in integrating dopaminergic and glutaminergic signaling. In a previous study, we identified single-nucleotide polymorphisms (SNPs) and a frequent haplotype associated with cognitive and imaging phenotypes that have been linked with schizophrenia, as well as with expression of prefrontal cortical DARPP-32 messenger RNA (mRNA) in a relatively small sample of postmortem brains. In this study, we examined the association of expression of two major DARPP-32 transcripts, full-length (FL-DARPP-32) and truncated (t-DARPP-32), with genetic variants of DARPP-32 in three brain regions receiving dopaminergic input and implicated in schizophrenia (the dorsolateral prefrontal cortex (DLPFC), hippocampus and caudate) in a much larger set of postmortem samples from patients with schizophrenia, bipolar disorder, major depression and normal controls (>700 subjects). We found that the expression of t-DARPP-32 was increased in the DLPFC of patients with schizophrenia and bipolar disorder, and was strongly associated with genotypes at SNPs (rs879606, rs90974 and rs3764352), as well as the previously identified 7-SNP haplotype related to cognitive functioning. The genetic variants that predicted worse cognitive performance were associated with higher t-DARPP-32 expression. Our results suggest that variation in PPP1R1B affects the abundance of the splice variant t-DARPP-32 mRNA and may reflect potential molecular mechanisms implicated in schizophrenia and affective disorders.

Lack of insula reactivity to aversive stimuli in schizophrenia

Patients with schizophrenia may have altered pain perception, as suggested by clinical reports of pain insensitivity, and recent neuroimaging findings. Here, we examined neural responses to an aversive electrical stimulus and the immediate anticipation of such a stimulus using fMRI and a classical conditioning paradigm, which involved pairing an electrical shock with a neutral photograph. Fifteen men with schizophrenia and 13 healthy men, matched for demographic characteristics, electrical stimulation level and scan movement, were studied. The shock induced robust responses in midbrain, thalamus, cingulate gyrus, insula and somatosensory cortex in both groups. However, compared to controls, the schizophrenic patients displayed significantly lower activation of the middle insula (p(FWE)=0.002, T=5.72, cluster size=24 voxels). Moreover, the lack of insula reactivity in the schizophrenia group was predicted by the magnitude of positive symptoms (r=-0.46, p=0.04). In contrast, there were no significant differences between the two groups in the magnitude of neural responses during anticipation of the shock. These findings provide support for the existence of a basic deficit in interoceptive perception in schizophrenia, which could play a role in the generation and/or maintenance of psychotic states.

Glutamatergic transmission in schizophrenia: from basic research to clinical practice

PURPOSE OF REVIEW

The past 20 years have seen the glutamatergic hypothesis go from theory to phase III trials of novel mechanism antipsychotics.

RECENT FINDINGS

We review the recent literature on glutamatergic theory, covering assessment and genetic studies, as well as drug development in animals and humans.

SUMMARY

Although evidence continues to accumulate in support of glutamate hypotheses, further research continues to be required and interactions with other key systems need to be explored.