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The Three Transmitters and DARPP-32—“All for One and One for All”

21 November 2003. Dopaminergic, serotonergic, and glutamatergic: three CNS neurotransmitter pathways that are distinct. Or are they? A paper in this week's Science reveals that all three pathways eventually lead to one protein, the dopamine- and cAMP-regulated protein of 32 kilodaltons, commonly known as DARPP-32.

DARPP-32 is an inhibitor of protein phosphastase-1 (PP1), one of the few phosphatases in the mammalian cell, and which has been implicated in the pathogenesis of neurodegenerative diseases, particularly Parkinson's disease (see Alzheimer Research Forum related news story). Now, the current finding provides an explanation for why three distinct types of drugs—dopaminergic agonists, serotonergic agonists, and glutamatergic antagonists—can all induce, in animals, a psychotomimetic state resembling schizophrenia.

Paul Greengard and colleagues at the Rockefeller University, New York, investigated the effect of knocking out the DARPP-32 gene in mice. First author Per Svenningsson and colleagues found that D-amphetamine, D-lysergic acid diethylamide (LSD), and phencyclidine (PCP), representatives of each drug class, respectively, all cause a loss of sensorimotor gating in wild-type mice but not in DARPP-32 knockouts. This suggests that the phosphatase inhibitor mediates the psychotomimetic actions of these compounds. Sensorimotor gating, or the processing and filtering of stimuli and information, is also compromised in non-medicated sufferers of schizophrenia.

To investigate how the three drugs may converge on DARPP-32, Svenningsson determined the phosphorylation status of the protein following administration of the drugs. The inhibitor is known to be phosphorylated at several sites. Modified at threonine 34, it becomes a potent inhibitor of PP1; at serine 97, it is rendered more susceptible to phosphorylation of T34 by protein kinase A (PKA), while with serine 130 phosphorylated, it becomes resistant to protein phosphatase 2B, which can dephosphorylate T34. Thus, phosphorylation at these three sites leads to more potent PP1 inhibition. In contrast, phosphorylation of threonine 75 attenuates PP1 inhibition because it converts DARPP-32 into an inhibitor of PKA.

When Svenningsson and colleagues treated tissue samples from frontal cortex and striatum with any of the three drugs, phosphorylation of T34 and S130 was significantly increased. Amphetamine had the strongest effect, almost tripling the T34-Pi content, while PCP increased it by about 50 percent.

To rule out some nonspecific cell-wide increase in phosphorylation in response to the drugs, the authors examined downstream targets of DARPP-32, including CREB and GSK-3β, and proteins thought to be regulated independently, including STAT-3 and CaMKII. While the three drugs caused phosphorylation of CREB and GSK-3β, STAT-3 and CaMKII were unaffected.

Svenningsson next used threonine-alanine mutants to confirm the relationship between DARPP-32, its downstream targets, and sensorimotor function. Compared to wild-type tissues, in those from T34A and S130A transgenic mice, phosphorylation of CREB and GSK-3β was significantly reduced in response to the three drugs. Furthermore, expression of c-fos, which is stimulated by activated CREB and GSK-3β, was suppressed in these mice. In addition, in transgenic animals the over twofold increase in repetitive movements caused by the psychotomimetics was significantly reduced. In contrast, T75A mutations had no effect on these movements or on DARPP-related phosphorylation activity.

Overall, the study suggests that it is through DARPP-32, and its target PP1 that these agonists and antagonists exert their psychotomimetic effects. Finding the substrates of PP1, the authors suggest, may lead to new insights into the behavioral effects of the drugs.—Tom Fagan.

Reference:
Svenningsson P, Tzavara ET, Carruthers R, Rachleff I, Wattler S, Nehls M, McKinzie DL, Fienberg AA, Nomikos GG, Greengard P. Diverse psychotomimetics act through a common signaling pathway. Science. 2003 Nov 21;302(5649):1412-5. Abstract

Comments on Related News


Related News: DARPP-32 Haplotype Affects Frontostriatal Cognition and Schizophrenia Risk

Comment by:  Jonathan Burns
Submitted 14 February 2007
Posted 14 February 2007

This study provides hard empirical evidence for the hypothesis that psychosis (and schizophrenia in particular) represents a costly "byproduct" of complex human (social) brain evolution. Interestingly, the activation paradigms in the fMRI study (N-back and emotional face-matching tasks) are both testing social cognition. And the demonstrated changes in frontostriatal connectivity support the hypothesis that schizophrenia is a disorder of evolved intrahemispheric circuits comprising the Social Brain in our species.

I would suggest that further candidates (conferring vulnerability to psychosis) should be sought from amongst those genes known to have played a significant role in human brain evolution.

References:

Burns J. (2007) The Descent of Madness: Evolutionary Origins of Psychosis and the Social Brain. Routledge Press: Hove, Sussex.

Burns J. The social brain hypothesis of schizophrenia. World Psychiatry. 2006 Jun;5(2):77-81. Abstract

Burns JK. Psychosis: a costly by-product of social brain evolution in Homo sapiens. Prog Neuropsychopharmacol Biol Psychiatry. 2006 Jul;30(5):797-814. Epub 2006 Mar 3. Review. Abstract

Burns JK. An evolutionary theory of schizophrenia: cortical connectivity, metarepresentation, and the social brain. Behav Brain Sci. 2004 Dec;27(6):831-55; discussion 855-85. Review. Abstract

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Related News: Genetics and Schizophrenia—Calcineurin Connection Grows

Comment by:  Mary Reid
Submitted 26 February 2007
Posted 27 February 2007

Tom Fagan mentions that calcineurin regulates phosphorylations elicited by both glutamatergic and dopaminergic signaling. The activity of D-amino acid oxidase is increased in schizophrenia, and this also affects signaling through both these pathways. Is there any clinical benefit with the use of sodium benzoate which inhibits DAO activity?

He also mentions that EGR3 can be regulated by the activity of neuregulin. Interestingly, Roberts et al. suggest that BDNF, which is decreased in first-episode psychosis (Buckley et al., 2007), induces synthesis of EGR3 to regulate activity of GABRA4. Ma and colleagues (Ma et al., 2005) conclude that GABRA4 is involved in the etiology of autism and it has also has been implicated in nicotine dependence (Saccone et al., 2007).

Glorioso and colleagues (Glorioso et al., 2006) report changes in genes encoding early-immediate genes such as EGR1 and EGR2 and RGS4 which is involved in cellular signaling and has been implicated in schizophrenia following BDNF gene ablation. Several studies report that zinc increases BDNF expression. Does this give support to the zinc deficiency theory of schizophrenia?

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Related News: DARPP-32 Haplotype Affects Frontostriatal Cognition and Schizophrenia Risk

Comment by:  Daniel Durstewitz
Submitted 8 June 2007
Posted 8 June 2007
  I recommend the Primary Papers

The phosphoprotein DARPP-32 occupies a central position in the dopamine-regulated intracellular cascades of cortical and striatal neurons (Greengard et al., 1999). It is a point of convergence for multiple signaling pathways, is differentially affected by D1- vs. D2-class receptor activation, and mainly through inhibition of protein-phosphotase-1 mediates or contributes to a number of the dopaminergic effects on voltage- and ligand-gated ion channels. These, in turn, by regulating intracellular Ca2+ levels, themselves influence phosphorylation of DARPP-32 and thereby interact with dopamine-induced processes.

Given its central, vital role in dopamine-regulated signaling pathways, it is quite surprising that (to my knowledge) only a few studies exist on the implications of DARPP-32 variations for cognitive functions and brain activity. Therefore, this comprehensive series of studies by Meyer-Lindenberg et al. combining human genetics, structural and functional MRI, and behavioral testing represents an important milestone. Meyer-Lindenberg et al. identified different functionally relevant DARPP haplotypes, associated with differential DARPP mRNA activity in postmortem studies, and found that these were linked to significant differences on a number of cognitive tests probing “executive functions,” as well as to differences in putamen volume and activity, and structural and functional covariation between striatal and prefrontal cortical areas. Thereby, they paved the way for detailed investigations of the role of DARPP-32 in human cognition.

Since DARPP-32 is so intricately interwoven into so many intracellular and physiological feedback loops, as with dopamine itself (Durstewitz and Seamans, 2002), mechanistic accounts for the functional involvement of DARPP-32 variations in neural network dynamics may be hard to obtain. “Linear” causal thinking usually breaks down in such complex functional networks constituted of so many interacting positive and negative feedback loops on different time scales. Thus it may still be a while until we gain a deeper, biophysically based understanding of the neural processes that mediate the influence of DARPP variations on cognition, and integrative computational approaches may be required to help resolving these issues. Given the complexity of DARPP-regulated networks, I also would expect that fine-grained behavioral testing and analysis of error types of human subjects on different cognitive tasks may ultimately reveal quite subtle and differential effects of DARPP polymorphisms. Moreover, the effects on neural network dynamics may be such (e.g., changing the temporal organization of spiking patterns) that they may not always be detectable by current neuroimaging methods, meaning that while the most dramatic effects were found on activation and volume of striatum, where DARPP-32 is most abundantly expressed, a significant contribution of other brain areas in DARPP-associated cognitive differences may not be ruled out. Regardless of these difficulties in unraveling the underlying neural mechanisms, the work by Meyer-Lindenberg et al. allows us to tackle the question of how the balance in dopamine-regulated intracellular networks relates to cognition in humans, and points toward the neural structures and interactions most interesting to look at.

View all comments by Daniel Durstewitz