Pharmacogenetics in model systems: defining a common mechanism of action for mood stabilisers

Abstract

Defining the underlying causes of psychiatric disorders has provided an ongoing and intractable problem. The analysis of the genetic basis of manic depression, in particular, has been impeded by the absence of a suitable model system and by the lack of candidate causative genes. One recent approach to overcome these problems has involved identifying those genes which control the sensitivity to anti-manic drugs in a model organism. Characterisation of the role of these genes and their encoded proteins in this model has allowed the analysis of their mammalian homologues to elucidate the therapeutic role of these drugs and the possible aetiology of manic depression. This approach has been used successfully with the cellular slime mould, Dictyostelium discoideum. This article introduces the use of model systems for pharmacogenetics research. It describes the identification of prolyl oligopeptidase in D. discoideum as a modulator of inositol phosphate signalling, and the subsequent identification of a common mechanism of action of three anti-manic drugs in mammalian neurons. The use of pharmacogenetics in model systems will provide a powerful tool for the ongoing analysis of both the treatment and cause of psychiatric disorders.

Fig. 1

Dictyostelium discoideum is a single-cell amoeba that develops into a multi-cellular fruiting body. A: In the single-cell stage, D. discoideum cells survive by consuming micro-organisms, or in culture conditions by fluid uptake. Here, a field of cells are shown containing the cytoplasmically located prolyl oligopeptidase protein linked to green fluorescent protein (black bar=10 μm). B: Upon starvation, cells aggregate and develop to form multi-cellular fruiting bodies composed of distinct cell types: stalk, spore and basal disk cells. Lithium functions, C, D: at 7 mM to reduce fruiting body size, and to alter developmental patterning by reducing spore cell and increasing stalk cell production, and E: at 10 mM to block aggregation. F: Elimination of the D. discoideum Gsk3 orthologue (GskA) partially phenocopies 7 mM lithium. G: These morphological effects on fruiting body development are dissimilar to that observed with valproate (VPA), whereby fruiting bodies developed on 1 mM VPA are small and show enlarged spore head and reduced stalk (white bar=0.5 mm).

Fig. 2

Restriction Enzyme Mediated Integration (REMI) bank can be used to isolate mutants resistant to therapeutic drugs. A, B: Dictyostelium discoideum cells (hatched) are electroporated in the presence of an antibiotic resistance gene and a restriction enzyme. C: Transformants containing the integrated resistance gene are selected in the presence of the antibiotic and individual mutants are grown and differentiate in the presence of the drug. D: Drug-resistant colonies are selected by their ability to overcome phenotypic changes caused by the drug. The integrating cassette is used to identify the ablated gene.

Fig. 3

The primary targets of lithium in the cell are the glycogen synthase kinase 3/A (Gsk3/A) and inositol trisphosphate (InsP₃) signalling pathways. Lithium inhibits mammalian Gsk3 or the Dictyostelium discoideum homologue, GskA, causing changes in the cytoskeletan and in gene transcription. Lithium also inhibits the recycling of inositol phosphates, by inhibiting inositol monophosphatase (IMPase) and inositol polyphosphatase (IPPase). The ‘inositol depletion’ theory of bipolar disorder treatment proposes that the therapeutic effect of lithium is to reduce inositol levels in the cell, resulting in the attenuation of an over-stimulated InsP₃ signalling pathway. Prolyl oligopeptidase presumably regulates the cleavage of an oligopeptide signal controlling the activity of multiple inositol polyphosphate phosphatase (MIPP), which functions to breakdown higher-order inositol phosphates to InsP₃. Abbreviations: DAG: diacylglycerol; GS: glycogen synthase; Gsk3/A, glycogen synthase kinase A/3; IMPase; inositol monophosphatase; InsP_{5 – 6}: inositol pentakisphosphate and hexakisphosphate; InsP₃: inositol (1,3,5) trisphosphate; IPPase: inositol polyphospatase; PIP₂: phosphatidylinositol bisphosphate; PLC: phospholipase C.

Fig. 4

Mammalian neuronal growth cones increase in size when treated with mood stabilizers. Rat dorsal root ganglia cells stained for actin (red) and acetylated tubulin (green) after treatment with mood stabilizers. A: In comparison to untreated cells treatment with B: 10 mM lithium and C: 3 mM valproate increases growth cone size. Only lithium treatment caused microtubule extension into the growth cone. Bar=10 μM.