Editor's Note: We put out a call for correspondents from the Society for Neuroscience meeting, and brave Susannah F. Locke of the University of Pennsylvania stepped up to the line. In this installment, she reports from one of the sessions she attended. We encourage you to contact us whenever you’re headed to a meeting. Your meeting reports can help speed information to your colleagues stuck at home.
31 October 2006. Paul Greengard, of The Rockefeller University in New York City, presented a special lecture at the Society for Neuroscience meeting in Atlanta, highlighting some essential findings from his fruitful career. His talk, entitled “Signal Transduction Pathways Used by Therapeutic Agents and Drugs of Abuse,” covered both old and recent data. He shared a summary of his continuing project on DARPP-32, a protein that inhibits PP-1 in response to a variety of neurotransmitters and drugs, and presented newer research on the regulation of the 5-HT1B receptor by p11.
DARPP-32 and schizophrenia
Greengard’s work on DARPP-32 was part of the signal transduction research for which he was awarded the Nobel Prize in Physiology or Medicine in 2000. He has been studying DARPP-32 for over 2 decades and is still adding to the DARPP-32 story today.
DARPP-32 (dopamine and cAMP regulated phosphoprotein of 32 kD) amplifies PKA signaling and acts as a potent inhibitor of protein phosphatase PP-1 in postsynaptic locations. By regulating phosphorylation cascades, DARPP-32 affects many key proteins, including ion channels, neurotransmitter receptors, and transcription factors. What is remarkable about DARPP-32 is its presence in different signaling cascades caused by a wide variety of neural events. Lying downstream of the dopamine, glutamate, and serotonin pathways, DARPP-32 acts as a signaling hub that elucidates the relationships between these various neurotransmitters. Greengard’s in vivo work has established DARPP-32’s roles in several psychiatric disorders, including schizophrenia.
DARPP-32 provides clues to why decreasing the ratio of D1 to D2 receptor activity has been a feature of many antipsychotic agents. D1 and D2 receptors have opposite effects on DARPP-32. D1 signaling activates PKA, which directly phosphorylates and activates DARPP-32. DARPP-32 can then inhibit PP-1, shifting the environment in favor of the phosphorylation of PKA targets. D2 signaling, however, activates PP2B, which dephosphorylates and inactivates DARPP-32. The phosphorylation of DARPP-32, and its consequences on other PKA substrates, can be viewed as a tug-of-war between D1 and D2 signaling cascades.
DARPP-32’s role in glutamatergic signaling lends support to the hypoglutamine hypothesis of schizophrenia. NMDA receptor activation results in the inactivation of DARPP-32 through PP2B. Similar to D1 signaling, a hypoglutamatergic environment would lead to active DARPP-32.
Greengard’s in vivo work has also linked DARPP-32 function with schizophrenia via psychotomimetic drugs. D-amphetamine, PCP, and LSD, all of which mimic aspects of psychosis, have common behavioral effects even though they target different receptors. Greengard’s data show that all three drugs activate a common substrate: DARPP-32. Mice lacking DARPP-32 function are less responsive to these drugs. In DARPP-32 knockout mice or mice expressing a nonfunctional DARPP-32 variant, D-amphetamine, PCP, and LSD cease to alter sensorimotor gating (such as prepulse inhibition of the startle reflex) and repetitive movements. Mice with nonfunctional DARPP-32 also have lower levels of c-fos mRNA after drug treatment than their wild-type counterparts.
Although not addressed in Greengard’s talk, a postmortem study has shown lower DARPP-32 in the prefrontal cortices of schizophrenic subjects than in non-schizophrenic subjects (Albert et al., 2002). It is not yet known whether this difference is a factor in the development of schizophrenia or a consequence of the disease or its treatment.
p11 regulation of the 5-HT1B receptor
Whereas the saga of DARPP-32 has been unfolding for over 2 decades, the story of p11’s regulation of the 5-HT1B receptor is a new tale, which was published this year in Science (Svenningsson et al., 2006; see also SRF related news story). The paper shows that p11 regulates the localization of the 5-HT1B receptor and that p11 is related to depression. In addition, it suggests that less p11 function could lead to depression and that increasing p11 activity could be a promising direction for antidepressant therapy.
p11 (annexin II light chain) is a member of the S100 protein family. It translocates annexin II’s heavy chain, as well as ion channels, to the cell membrane. p11 was identified as a subject of interest when it was picked up in a yeast two-hybrid screen of the intracellular portion of the 5-HT1B receptor. Greengard presented his in vitro data demonstrating that p11 and the 5-HT1B receptor interact specifically in co-immunoprecipitations and that they colocalize to the cell membrane. In addition, p11 cotransfection increases 5-HT1B receptor levels at the cell surface.
Greengard then shared in vivo work that correlates depression with lower p11 expression in both rodents and humans. The helpless H/Rouen mouse strain (an animal model of depression) has lower levels of p11 mRNA than non-helpless mice. A similar pattern of mRNA expression exists between the postmortem brains of depressed and non-depressed patients. In addition, various forms of antidepressant therapy, including imipramine treatment and electroconvulsive therapy, increase both p11 mRNA and protein in rodents. Overexpression of p11 in a transgenic mouse can mimic the effects of antidepressants, yielding mice that exhibit less depression-like and anxiety-like activity in behavioral tests. p11 knockout mice, on the other hand, have increased depression-like behavior.
The depression-like phenotype of the p11 knockout mouse could easily be due to the decrease in 5-HT1B receptor function observed in these animals. The mice have fewer functional 5-HT1B receptors on the cell surface. Primary cultures from the knockouts have reduced 5-HT1B receptor activity as shown by decreased inactivation of ERK1/2, more serotonin turnover (5-HT1B receptors act as autoreceptors), and less 5-HT1B-mediated inhibition of excitatory transmission.
Although Greengard did not address other neurotransmitter systems in the p11 portion of his talk, his Science paper suggests that p11 regulation of dopaminergic signaling is unlikely. p11 did not interact with D1 or D2 receptors through a yeast two-hybrid screen. In addition, long-term treatment with neither haloperidol nor risperidone increased p11 mRNA in mice.—Susannah F. Locke.
Albert KA, Hemmings HC Jr, Adamo AI, Potkin SG, Akbarian S, Sandman CA, Cotman CW, Bunney WE Jr, Greengard P. Evidence for decreased DARPP-32 in the prefrontal cortex of patients with schizophrenia. Arch Gen Psychiatry. 2002 Aug; 59(8):705-12. Abstract
Svenningsson P, Chergui K, Rachleff I, Flajolet M, Zhang X, El Yacoubi M, Vaugeois JM, Nomikos GG, Greengard P. Alterations in 5-HT1B receptor function by p11 in depression-like states. Science. 2006 Jan 6; 311(5757):77-80. Abstract