Toward a crystal-clear view of lithium's site of action.

Even though lithium is remarkably effective in treating many patients with manicdepressive illness, this treatment has several serious drawbacks. Particularly troublesome is the slim margin between therapeutic and toxic levels. Furthermore, because the clearance of lithium is intimately tied to sodium and water excretion, slight changes in electrolyte balance produced by spending a hot summer day at the beach or succumbing to an intestinal flu can precipitate life-threatening rises in lithium levels. In addition, even tight regulation of this drug within its therapeutic window is associated with a wide range of side effects, such as tremor, renal dysfunction, thyroid abnormalities, and birth defects (1). Accordingly, these side effects have fueled interest in identifying lithium substitutes, less toxic drugs that mimic the therapeutic efficacy of lithium. As part of this drug-development effort, researchers have focused their attention on a putative target of lithium's therapeutic action, inositol monophosphatase. This key enzyme in the inositol phospholipid cycle, which is responsible for generating inositol needed to synthesize membrane inositol phospholipids (2), is inhibited by lithium at therapeutic concentrations (3). Thus, the detailed structural model of how this enzyme works and is inhibited by lithium, proposed by Pollack et al. (4) in this issue, represents an extremely important step forward in the rational design of lithium-mimetic drugs. Use of a scientific strategy to identify effective drugs for treatment of manicdepressive illness contrasts sharply with the largely fortuitous discovery of lithium's anti-manic properties nearly 50 years ago (5). From the observation that lithium salts blocked urea toxicity in guinea pigs, Dr. John Cade, an Australian psychiatrist, followed a highly tenuous line of reasoning, which led him to conduct clinical trials of lithium in manic patients. As the effectiveness of lithium was confirmed by others and it was introduced into general psychiatric practice, the search began for clues to decipher how this metal ion exerted its remarkable therapeutic effects. In pursuing reports that lithium affected carbohydrate transport, researchers investigated the effects of lithium on inositol disposition and found a modest reduction in brain inositol levels (6) that was associated with a large increase in inositol monophosphate (7). Subsequent in vitro studies ofinositol monophosphate breakdown in brain homogenates revealed that lithium strongly inhibited the phosphatase involved (3). However, when this seminal discovery was made in the 1970s, it was unclear whether or not this effect was relevant to the therapeutic action of lithium. At that time, it had been established that a wide variety of receptor agonists could enhance incorporation of 32p into phosphatidylinositol in peripheral tissues and brain (8-10). However, how this signaling system worked was far from clear. In fact, introduction of lithium as a tool to simplify measurements of inositol phospholipid turnover was instrumental in clarifying this question (11). This approach exploited inhibition by lithium of inositol monophosphate breakdown to allow accumulation of this intermediate in response to agonist stimulation of the system. This innovation greatly simplified assays of inositol phospholipid turnover and unexpectedly led to the identification of inositol 1,4,5-trisphosphate, a key second messenger in this pathway (12). In light of these advances in understanding the workings of the inositol phospholipid system, Berridge and coworkers (13) proposed that the therapeutic action of lithium stemmed from its ability to block recycling of inositol into membrane inositol phospholipid precursors. These authors argued persuasively that, in the presence of lithium, continued stimulation of the system would lead to depletion of inositol and a run-down of this signaling pathway. This hypothesis was particularly attractive because it could account for the ability of lithium to exert profound effects in psychiatric patients, even though the drug appears to have little effect in normal individuals. Characterization of lithium's blockade of inositol monophosphatase revealed that this blockade becomes more effective as the substrate concentration increases (3, 14). This paradoxical behavior, referred to as uncompetitive inhibition, occurs if the inhibitor binds to the enzyme-substrate or enzyme-product complex. Accordingly, lithium would be preferentially effective in the presence of hyperactive inositol phospholipid turnover, possibly associated with bipolar disorder. The inhibitory action of lithium would kick in as inositol monophosphate levels rose. This elevation in substrate concentration would potentiate the inhibitory action of lithium, generating a feedback spiral that would choke off the supply of inositol needed to sustain the inositol phospholipid signaling pathway. The structural model of inositol monophosphatase proposed by Pollack et al. (4), based on crystallographic studies of recombinantly expressed wild-type and mutant forms of the enzyme, provides a mechanistic explanation for the uncompetitive behavior oflithium. According to this model, two magnesium ions are critical for enzyme activity. One ion remains bound to the enzyme throughout the catalytic cycle, while the second ion hops aboard after the substrate is bound and hops off after the substrate is cleaved. These workers suggest that lithium binds at the second magnesium site when the phosphate generated by hydrolysis of inositol monophosphate is still bound to the enzyme. The preferential affinity of lithium for this enzyme-product complex results in the uncompetitive type of inhibition observed. Synthesis of inositol analogues that inhibit inositol monophosphatase is already in progress (15). Hopefully, the detailed structural information obtained in these studies will facilitate the design of drugs that mimic the action of lithium. Although the proposal that inositol depletion underlies the therapeutic action of lithium has been extremely influential in guiding research in this area, this hypothesis has met with criticism because it has been difficult to obtain experimental confirmation of a central tenet of this theory-namely, depletion of inositol. Careful reevaluation of the effect of lithium treatment on brain inositol levels in vivo indicates that a significant inositol reduction becomes apparent only at lithium concentrations that exceed therapeutic levels of this drug (16). Investigators agree that at therapeutic concentrations lithium causes substantial inositol monophosphatase inhibition and inositol monophosphate accumulation in vivo. How-