Raised monophosphatase activity in schizophrenic patients

From membrane phospholipid defects to altered neurotransmission: is arachidonic acid a nexus in the pathophysiology of schizophrenia?

Schizophrenia (SZ) is a devastating neuropsychiatric disorder affecting 1% of the general population, and is characterized by symptoms such as delusions, hallucinations, and blunted affect. While many ideas regarding SZ pathogenesis have been put forth, the majority of research has focused on neurotransmitter function, particularly in relation to altered dopamine activity. However, treatments based on this paradigm have met with only modest success, and current medications fail to alleviate symptoms in 30-60% of patients. An alternative idea postulated a quarter of a century ago by Feldberg (Psychol. Med. 6 (1976) 359) and Horrobin (Lancet 1 (1977) 936) involves the theory that SZ is associated in part with phospholipid/fatty acid abnormalities. Since then, it has been repeatedly shown that in both central and peripheral tissue, SZ patients demonstrate increased phospholipid breakdown and decreased levels of various polyunsaturated fatty acids (PUFAs), particularly arachidonic acid (AA). Given the diverse physiological function of membrane phospholipids and PUFAs, an elucidation of their role in SZ pathophysiology may provide novel strategies in the treatment of this disorder. The purpose of this review is to summarize the relevant data on membrane phospholipid/PUFA defects in SZ, the physiological consequence of altered AA signaling, and how they relate to the neurobiological manifestations of SZ and therapeutic outcome.

Membrane polyunsaturated fatty acids and CSF cytokines in patients with schizophrenia

Findings to date provide evidence that altered membrane structure and function are present in patients with either first-episode or chronic schizophrenia, suggesting defects in phospholipid metabolism and cell signaling in schizophrenia. The purpose of this investigation is to test whether decreased membrane polyunsaturated fatty acids (PUFAs) were associated with an increased secretion of proinflammatory cytokines. Thus, we measured interleukin 6 (IL-6) and interleukin 10 (IL-10) in cerebrospinal fluid (CSF) of patients with chronic schizophrenia as well as PUFAs of red blood cell (RBC) membranes from the same individuals. A significant and inverse correlation was found between CSF IL-6 (not IL-10) and RBC membrane PUFAs levels in both haloperidol-treated and medication-free patients with schizophrenia. Specifically, such an association was found in the n-6 (18:2, 20:4, and 22:4) and, to a lesser extent, the n-3 fatty acids. Taken together, the present findings suggest that decreased membrane PUFAs may be related to an immune disturbance in schizophrenia, possibly resulting from an increased phospholipase A2 activity mediated through the proinflammatory cytokines.

Inositol levels are decreased in postmortem brain of schizophrenic patients

BACKGROUND

A previous study reported decreased levels of inositol in frontal cortex of postmortem brain from bipolar patients and suicide victims. The aim of the present study was to test the specificity of this finding.

METHODS

Inositol and the enzyme that synthesizes it, inositol monophosphatase, were measured in postmortem brain tissue from frontal and occipital cortex and cerebellum from 10 schizophrenic patients and the previously reported controls. Inositol levels were assayed gas-chromatographically as trimethylsilyl derivatives with mannitol as an internal standard. Inositol monophosphatase activity in brain homogenates was measured as the difference between phosphate release from inositol-l-phosphate in the absence and in the presence of Li+.

RESULTS

Inositol was significantly reduced in all three areas in the schizophrenic patient' brains: inositol monophosphatase was unchanged. Postmortem interval did not correlate with inositol levels and did not differ between control group and schizophrenic patients.

CONCLUSIONS

These results suggest an abnormality of second messenger precursor availability in common with schizophrenia and affective psychopathology.

Cerebrospinal fluid inositol monophosphatase: elevated activity in depression and neuroleptic-treated schizophrenia

BACKGROUND

Inositol monophosphatase (IMPase) is a key enzyme in the regulation of the activity of the phosphatidyl inositol (PI) signaling pathway. This enzyme is also found in the cerebrospinal fluid (CSF), where it may prove useful as a marker of dysfunctional PI signal transduction.

METHODS

IMPase activity was measured in lumbar CSF of depressed and neuroleptic-treated schizophrenic patients. In addition, and to gain an insight into the factors that influence the levels of CSF IMPase, enzyme activity was measured in subgroups of schizophrenic patients treated for 3-7 days with lithium or 7 days with inositol.

RESULTS

CSF IMPase activity was significantly increased by 88% in depressed and by 172% in schizophrenic patients relative to control subjects. Lithium produced a marked increase in CSF IMPase activity in the group as a whole, and this group effect could be more specifically attributed to 3 of the 8 individuals in whom enzyme activity increased by over 300%. On the other hand, inositol had no effect on CSF IMPase activity.

CONCLUSIONS

In the absence of a clear relationship between CSF IMPase activity and neuronal PI signaling pathways it is not possible to correlate these changes with altered neuronal function. Nevertheless, increased CSF IMPase activity in depression and schizophrenia may be a marker of the pathophysiological processes underlying these disorders. Moreover, the large lithium-induced increase in IMPase activity seen in a subgroup of schizophrenic subjects suggests a differential regulation of CSF enzyme activity in these patients.

Regional proton magnetic resonance spectroscopy in schizophrenia and exploration of drug effect

Schizophrenia is a disorder with an unclear pathophysiology, despite numerous attempts to elucidate its etiology. We have employed proton magnetic resonance spectroscopy in vivo to explore the neurochemistry of several brain regions (left frontal and temporal cortices, left basal ganglia, and left and right thalamus) in patients with schizophrenia and in normal control subjects. We have also examined patients in different medication states. A trend toward a decreased level of inositol/creatine was found in the left temporal lobe of patients with schizophrenia, as was a trend toward a reduced level of N-acetylaspartate/creatine in the left thalamus of patients. In schizophrenic patients treated with atypical antipsychotics, decreased levels of choline were found in the left basal ganglia, while increased levels of N-acetylaspartate were found in the left frontal cortex. These results suggest altered metabolism in patients with schizophrenia, and imply that further study is needed to clarify the effects of the more recently available antipsychotics.

Incorporation of 3H-arachidonic acid into platelet phospholipids of patients with schizophrenia

Incorporation of 3H-arachidonic acid (AA) into resting platelets was carried out in normal control subjects as well as in schizophrenic patients before and after haloperidol (HD) withdrawal. Metabolic turnover of membrane phospholipids was subsequently evaluated in prelabelled platelets at various time intervals after thrombin activation. 3H-AA was mainly incorporated into phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylinositol (PI) and phosphatidylserine (PS) of resting platelets. Very minute amounts of 3H-labelling were found in phosphatidic acid (PA). Following thrombin activation, however, substantial amounts of 3H-labelling were found in PA. Such an increase in thrombin-induced PA formation was not reduced in schizophrenic patients both receiving and not receiving HD treatment. Increased labelling has been found in platelet diacylglycerol (DAG) after thrombin activation. It is therefore not likely that a decreased DAG kinase activity contributes to the accumulation of DAG. However, the thrombin-induced PA production was temporally associated with a decreased 3H-labelling in PI, but not in PC, PS and PE. The present data taken together with our previous findings suggest that the increased production of second messengers (DAG, PA and inositol phosphates) in schizophrenia may result from an increased phospholipase C (PLC) activity in schizophrenia, because thrombin-induced platelet activation is mediated by polyphosphoinositide hydrolysis through the G-protein activation.

CSF inositol in schizophrenia and high-dose inositol treatment of schizophrenia

Inositol is a key metabolite in the phosphatidylinositol cycle, which is a second messenger system for serotonin-2 receptors that have been implicated in the pathophysiology of schizophrenia. Cerebrospinal fluid inositol levels were measured in 20 schizophrenic patients and 19 age- and sex-matched controls and no difference was found. However, the patients were all neuroleptic-treated. A controlled double-blind crossover trial of 12 g daily of inositol for a month in 12 anergic schizophrenic patients, twice the dose given before in schizophrenia, did not show any beneficial effects. However, the number of patients studied was small and the length of time of inositol administration may not have been sufficient.

Ziskind-Somerfeld Research Award 1993. Biochemical, behavioral, and clinical studies of the role of inositol in lithium treatment and depression

Lithium (Li) reduces brain inositol levels by inhibiting the enzyme inositol monophosphatase. The enzyme inositol-1-phosphatase was measured in human red blood cells of controls, Li-free bipolar patients, and Li-treated bipolar patients and was found to be reduced by 80% in Li-treated bipolars, thus supporting the concept that chronic Li at therapeutic concentrations inhibits this enzyme. Two behaviors in rats caused by Li, reduction of rearing, and Li-pilocarpine seizures, are reversed by intracerebroventricular replenishment of inositol. The reversal is stereospecific to the naturally occurring myo-inositol; whereas the stereoisomer L-chiro-inositol is ineffective. The reversal is dose-dependent, requiring a dose consistent with known quantities of brain inositol depletion; and is time-dependent, as inositol must be given 1-8 h before stimulation. High-dose peripheral inositol also reverses the limbic seizures induced by Li-pilocarpine, and using gas chromatography was shown to increase brain inositol levels that had been reduced by Li treatment. Low-dose inositol could be shown to reverse a peripheral Li-induced side effect, polyuria/polydipsia, in rats and in patients treated with Li. A higher dose of inositol markedly reduced Hamilton Depression Ratings in 9 of 11 unipolar major depressive disorder patients previously unresponsive to tricyclics, in an open design, but had no effect on chronic schizophrenics in a controlled double-blind randomized crossover trial. A new inositol monophosphatase inhibitor, a fungal product originally discovered as a complement inhibitor, was found to act like Li and lower the seizure threshold for subconvulsant doses of pilocarpine. These data suggest that inositol monophosphatase inhibition is a key mechanism of Li's therapeutic action and that design of new inositol monophosphatase inhibitors may be a practical strategy to create new compounds with Li-like therapeutic effects.

Novel inositol containing phospholipids and phosphates: their synthesis and possible new roles in cellular signalling

Details of the widely employed PtdIns(4,5)P2 hydrolysis receptor-stimulated signalling pathway continue to be elucidated rapidly. However, it has recently become apparent that numerous other inositol lipids and phosphates are widespread and are likely to have important cellular functions. In this review, we focus particularly on three rapidly progressing areas: the synthesis and possible functions of 3-phosphorylated inositol lipids, particularly phosphatidylinositol 3,4,5-trisphosphate; the roles of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in coordinating intracellular Ca2+ mobilization and Ca2+ influx in stimulated cells; and the metabolism and possible functions of other inositol polyphosphates and of inositol polyphosphate pyrophosphates.