Several lines of research have produced findings consistent with the hypothesis that alterations in GSK3 are connected with schizophrenia. However this association has not received the same intense scrutiny as it has in mood disorders, and contradictory findings have been reported, so the association of GSK3 with schizophrenia, while tantalizing, remains to be more thoroughly evaluated.
Schizophrenia is a prevalent and severe psychotic disorder with considerable variation among individuals in symptoms associated with thought content, perception, cognition, and affect [49
]. The causative factors of schizophrenia remain unknown, but dysregulated dopamine neurotransmission is likely the most widely investigated hypothesis of the pathophysiology of schizophrenia. This classical hyperdopaminergic hypothesis of schizophrenia pathology is supported by the therapeutic effects in schizophrenia of conventional antipsychotics that are dopamine D2 receptor antagonists and by the psychotogenic effects of dopamine enhancing drugs. However, although conventional antipsychotics can diminish symptoms in some patients, it is evident that the pathophysiology of schizophrenia is more complex, and diverse, than being caused only by increased dopaminergic activity in subcortical regions [49
]. Some symptoms of schizophrenia, such as cognitive impairments, are resistant to conventional antipsychotics, and the cognitive deficits in schizophrenia are thought to arise in part from hypodopaminergic neurotransmission at dopamine D1 receptors in the pre-frontal cortex [52
]. This is supported by clinical studies that show atypical antipsychotics, which increase dopamine neurotransmission at dopamine D1 receptors [53
], improve cognitive symptoms in schizophrenic patients [54
]. Especially intriguing is the evidence showing that dopamine D1 receptor hypoactivity in the pre-frontal cortex can result in dopamine D2 receptor hyperactivity in the striatum. Imaging studies of the function of dopamine D1 receptors in the prefrontal cortex [56
] and of dopamine D2 receptors in the striatum [51
] of schizophrenic patients lend further support to the view that an imbalance of cortical/subcortical dopaminergic function may be central to the pathology of schizophrenia [51
]. Thus, regarding dopaminergic neurotransmission, balanced activities of dopamine D1 and D2 receptors seems to be critical, and schizophrenia appears to be associated with low dopamine D1 and/or high dopamine D2 receptor function. Beyond the dopaminergic system, many other theories of the pathology of schizophrenia have been promulgated. One of the most widely considered is the evidence of neurodevelopmental abnormalities, supporting the concept that schizophrenia represents a spectrum of diseases with multi-factorial pathologies [49
]. Although studies connecting GSK3 to schizophrenia are few, they have identified links between GSK3 and these two major postulates, altered dopaminergic activity and disrupted neurodevelopment.
Abnormalities of GSK3 were first linked to schizophrenia in a series of studies reported by Agam and colleagues [59
]. They found approximately 40% lower GSK3β mRNA levels, GSK3β protein levels, and GSK3 kinase activity in postmortem samples of frontal cortex from subjects with schizophrenia, and a 30% lower GSK3β protein level in the cerebrospinal fluid, compared with controls. However, these differences were not detected, except for the lower GSK3 mRNA level, by the same investigators in samples from a different brain collection [63
]. This difference between brain collections was also encountered by another group who found differences of GSK3β protein levels in schizophrenic compared to control samples in one brain collection but not another [64
]. As noted earlier in this review, a lack of changes in GSK3 levels does not preclude changes in GSK3 actions because of the intracellular mechanisms that regulate its activity, such as the inhibitory effect of serine-phosphorylation, but this can not be studied in human brain samples because of the extensive loss of serine phosphorylation of GSK3 that occurs postmortem. Further studies with a greater number of samples will be necessary to draw concrete conclusions about whether or not alterations in GSK3 expression or protein levels reproducibly occur in subjects with schizophrenia.
Another approach to identifying potential links between schizophrenia and dysregulated GSK3 is to examine the modulatory influences of neurotransmitter systems that are thought to be involved in the illness, as discussed in the previous section concerning studies of the serotonergic system in mood disorders. Therefore, since there is evidence for dopaminergic dysregulation in schizophrenia, it is pertinent to consider whether dopaminergic activity has a role in regulating GSK3. This was first examined in brain in vivo
by Gil et al
] who found that administration of a dopamine D1 receptor agonist inhibited GSK3 activity in rabbit frontal cortex and hippocampus, and this was blunted in rabbits following prenatal cocaine exposure which itself caused inhibition of GSK3β activity. Thus, this study showed for the first time that dopaminergic activity has a regulatory influence on GSK3 in brain in vivo
, and that long term changes in the dopaminergic system can modulate GSK3. From this initial report, especially notable is the possibility that low dopamine D1 receptor activation that is reported to occur in schizophrenia would be associated with impaired inhibitory control of GSK3 ().
Schematic depiction of the regulation of GSK3 by dopamine D1 and dopamine D2 receptors and changes associated with schizophrenia
Subsequently, more extensive in vivo
interactions between the dopaminergic system and GSK3 in mammalian brain were reported. In a recent definitive study, Beaulieu et al
] found that increased dopaminergic stimulation of dopamine D2 receptors in the striatum induced by administration of the indirect dopamine stimulant amphetamine or present in dopamine transporter knockout mice (DAT-KO), caused activation of GSK3α and GSK3β in mouse striatum. This appeared to occur because of decreased Akt activity which resulted in decreased serine-phosphorylation of GSK3. Hyperactive GSK3 was shown to contribute to the behavioral phenotype because administration of GSK3 inhibitors, including lithium, antagonized dopamine-dependent hyperactivity and stereotypy in the DAT-KO mice, and amphetamine-induced hyperactivity was lower in GSK3β +/− mice [67
]. These findings clearly demonstrated that GSK3 is a downstream target of dopamine D2 receptor-mediated signaling in vivo
and that GSK3 mediates some of the behavioral effects of dopamine, supporting the possibility that alterations in GSK3 activity may be relevant to schizophrenia and other dopamine-related disorders [67
]. This study notably raises the possibility that GSK3 is abnormally activated in schizophrenia since dopamine D2 receptor activation is elevated, which in conjunction with impaired dopamine D1 receptor-mediated inhibition of GSK3 may synergistically contribute to hyperactivated GSK3 ().
More direct evidence of impaired Akt/GSK3β signaling in subjects with schizophrenia was recently reported. Emamian et al.
] found approximately 50% decreases in the protein levels of one isoform of Akt, called Akt1, in the frontal cortex and lymphocytes of subjects with schizophrenia compared with controls. Administration to mice of haloperidol, a typical antipsychotic which is an antagonist of dopamine D2 receptors, increased the activating phosphorylation of Akt and the inhibitory serine-phosphorylation of GSK3β in brain. The decreased Akt signaling to GSK3β in schizophrenia and corrective modulation by the dopaminergic antagonist supports the potential role of dopamine receptor-coupled signaling to Akt and GSK3β in the pathogenesis of schizophrenia. This study also reported lower phospho-Ser9-GSK3β levels in samples from subjects with schizophrenia compared with controls as determined by immunoblot analysis [68
]. However, the recently reported rapid postmortem serine-dephosphorylation of GSK3β [16
] suggests that more thorough examination of immunoreactive bands is necessary to unequivocally identify levels of phosphoserine-GSK3 in postmortem samples. Most interestingly, Emamian et al.
] found that a haplotype of Akt1 that was preferably transmitted to schizophrenic probands is related to a lower protein level of Akt1 and that amphetamine administration to Akt1-depleted mice showed disruption of prepulse inhibition, a representative model of impaired sensorimotor gating of schizophrenia. A recent report confirmed that Akt1 is a susceptibility gene for schizophrenia in a large population study [69
] but it was not confirmed in another study [70
]. This finding suggests that reduced Akt1 may contribute to schizophrenia, supporting the possibility of an association between impaired control of GSK3 and schizophrenia.
The role of GSK3β in association with the Wnt signaling pathway () is a well known factor regulating CNS development [6
], so altered GSK3β signaling in the brain of subjects with schizophrenia also could contribute to the neurodevelopmental abnormalities that have been linked to schizophrenia. There have been several reports of alterations in the Wnt signaling pathway, which regulates the action of GSK3, associated with schizophrenia [71
]. Since Wnt signaling is a key component of neurodevelopment, and much evidence indicates neurodevelopmental abnormalities in schizophrenia [75
], these are tantalizing reports, but the significance of these findings for the pathophysiology of schizophrenia remain to be investigated in greater detail.
In addition to dopaminergic activity, alterations of cholinergic and glutamatergic neurotransmission also have been linked to the pathology of schizophrenia, so it is of interest that each of these neurotransmitter systems recently was found to influence the regulation of brain GSK3 in vivo
. Schizophrenia has been linked to dysregulated cholinergic neurotransmission in several studies, and especially strong is the evidence indicating association with the cognitive impairment of schizophrenia [78
]. Cognitive impairment is often evident in schizophrenia, and schizophrenia has been reported to be associated with reduced choline acetyltransferase, the enzyme that synthesizes acetylcholine which is a critical neuro-transmitter for cognition, and choline acetyltransferase activity was reported to be inversely correlated with cognitive impairments in schizophrenia [80
]. There have been several reports of decreased levels of muscarinic receptors in specific brain regions of schizophrenic patients, including frontal cortex [83
], anterior cingulate gyrus [86
], hippocampus [87
], Broadmann’s area 9 [88
], and caudate-putamen [85
]. Taken together, these and other findings suggest that muscarinic receptor stimulation can be impaired in schizophrenia either at the level of acetylcholine synthesis or receptor activation. Therefore, it is of interest that a regulatory influence of cholinergic activity modulating the phosphorylation of brain GSK3 in vivo
was recently identified. De Sarno et al.
] found that cholinergic stimulation with the muscarinic receptor-selective agonist pilocarpine or the acetylcholinesterase inhibitor physostigmine rapidly increased the serine-phosphorylation of GSK3α and of GSK3β by several-fold in three mouse brain regions. This finding raised the possibility that dysfunctional cholinergic activity may cause inadequate inhibitory control of GSK3 which can be restored by stimulation of muscarinic receptors.
Much research has linked altered glutamatergic neurotransmission to schizophrenia [91
]. One of the most widely used models of schizophrenia involves application of glutamatergic N-methyl D-aspartate (NMDA) receptor antagonists to animals because in healthy human subjects these agents can induce symptoms similar to those seen in schizophrenia [92
]. Thus, administration of the noncompetitive NMDA receptor antagonists phencyclidine or ketamine can induce several symptoms of schizophrenia in normal control individuals, and can worsen symptoms in schizophrenic subjects [reviewed in 51
]. Conversely, administration of NMDA receptor agonists examined as adjunctive treatments have been reported to improve psychotic symptoms in schizophrenia [93
]. These findings support the hypothesis that activation of NMDA receptors may be impaired in schizophrenia [93
This connection between NMDA receptor activity and schizophrenia raises the question of whether this may contribute to the regulation of GSK3, and several recent studies have provided support for this regulatory interaction. NMDA treatment of cultured hippocampal neurons caused a rapid and nearly complete dephosphorylation of phospho-Ser9-GSK3β , indicating that GSK3β is activated by NMDA receptor signaling [94
]. In accordance with that conclusion, in vivo
blockade of NMDA receptors by administration of the antagonist phencyclidine increased mouse brain serine-phosphorylation of GSK3 [95
], a response was also observed in mouse brain following administration of memantine, an NMDA antagonist approved for use in humans [90
]. A conflicting report showed that in immature rats blockade of NMDA receptors by in vivo
administration of the antagonist MK-801 transiently decreased the serine-phosphorylation of GSK3 [96
], a difference from the other reports that could be due to age-dependent differences in responses to NMDA receptor modulation or differences between NMDA antagonists. Thus, although still only few, the majority of studies indicate that NMDA receptor stimulation dephosphorylates GSK3 and that blockade of NMDA receptors in vivo
is sufficient to cause increased levels of serine-phosphorylated GSK3.
Overall, quite a few connections have been identified between GSK3 and schizophrenia, but there are also serious contradictions in this data. Thus, some data suggests the action of GSK3 is reduced, whereas other data suggests it is increased, in association with schizophrenia. Schizophrenia-associated reductions of GSK3 are indicated by the measurements in postmortem brain samples and by the inhibitory effects of the NMDA antagonists phencyclidine and memantine. Postmortem measurements are the most direct approach to identifying disease-related links, but this strategy is also fraught with difficulties inherent in using postmortem tissue and in studying such a heterogeneous sample population. These difficulties are exemplified by the mixed results obtained in different sample sets. Countering the indications of reduced GSK3, there is substantial evidence of increased GSK3 actions in schizophrenia. This evidence comes from studies showing reduced Akt in schizophrenia and studies of neurotransmitter effects on regulating GSK3. The two studies implicating Akt deficits in schizophrenia provide strong evidence that this inhibitory regulator of GSK3 is dysfunctional, thus allowing hyperactivation of GSK3. This is corroborated by some indications in schizophrenia of low dopamine D1 receptors which inhibit GSK3, and elevated dopamine D2 receptors that activate GSK3, and that typical antipsychotics block D2 receptors, which would cause inhibition of GSK3. The altered balance of D1 and D2 receptors, along with a possible deficit in cholinergic neurotransmission, lead to the prediction that GSK3 is inadequately controlled in schizophrenia. However, lithium, a GSK3 inhibitor, has very limited therapeutic effects in schizophrenia, suggesting that if GSK3 activity is abnormal in schizophrenia it may only contribute to a subset of symptoms. Thus, although intriguing connections between schizophrenia and alterations of GSK3 have been identified, much more research is necessary to integrate the findings from studies of GSK3 in postmortem tissue, the developmental influences of GSK3, and the regulatory effects on GSK3 of neurotransmitter systems that have been shown to be involved in schizophrenia.