Inhibition of Protein Kinase C Signaling Protects Prefrontal Cortex Dendritic
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Inhibition of protein kinase C signaling protects SEE COMMENTARY prefrontal cortex dendritic spines and cognition from the effects of chronic stress Avis Brennan Hainsa, Mai Anh T. Vua, Paul K. Maciejewskib,1, Christopher H. van Dycka,b, Melissa Gottrona, and Amy F. T. Arnstena,2 aDepartment of Neurobiology, Yale University School of Medicine, 333 Cedar Street SHM C-303, New Haven, CT 06510; and bDepartment of Psychiatry, Yale University School of Medicine, 300 George Street, New Haven, CT 06511 Communicated by Bruce S. McEwen, The Rockefeller University, New York, NY, August 13, 2009 (received for review April 7, 2009) The prefrontal cortex r regulates behavior, cognition, and emotion form the networks subserving representational knowledge (1), and by using working memory. Prefrontal functions are impaired by particularly in distal apical dendrites (11). However, the molecular stress exposure. Acute, stress-induced deficits arise from excessive changes underlying this spine loss are not yet understood. protein kinase C (PKC) signaling, which diminishes prefrontal It is likely that many molecular ‘‘culprits’’ contribute to architectural neuronal firing. Chronic stress additionally produces architectural changes in prefrontal neurons during chronic stress. One likely candi- changes, reducing dendritic complexity and spine density of cor- date contributing to dendritic spine loss is PKC intracellular signaling, tico-cortical pyramidal neurons, thereby disrupting excitatory as in vitro studies of hippocampal cell cultures have shown that elevated working memory networks. In vitro studies have found that PKC signaling induces spine collapse through disruption of the actin sustained PKC activity leads to spine loss from hippocampal- cytoskeleton (20). Excessive PKC signaling also has immediate clinical cultured neurons, suggesting that PKC may contribute to spine loss relevance, as either genetic or environmental insults can overactivate during chronic stress exposure. The present study tested whether this pathway. As illustrated in Fig. 1, several molecules that normally inhibition of PKC with chelerythrine before daily stress would inhibit PKC signaling are genetically altered and/or dramatically re- protect prefrontal spines and working memory. We found that duced in patients with mental illness who have impaired prefrontal NEUROSCIENCE inhibition of PKC rescued working memory impairments and re- function. The bipolar disorder genome has recently identified alter- versed distal apical dendritic spine loss in layer II/III pyramidal ations in diacylglycerol (DAG) kinase eta (DGKH) (21, 22), which neurons of rat prelimbic cortex. Greater spine density predicted encodes for the kinase that normally inhibits DAG activation of PKC better cognitive performance, the first direct correlation between signaling. Loss of function of DGKH in bipolar disorder would lead to pyramidal cell structure and working memory abilities. These excessive PKC signaling, as has been reported previously (23). Another findings suggest that PKC inhibitors may be neuroprotective in key factor is regulator of G protein signaling 4 (RGS4), which normally disorders with dysregulated PKC signaling such as bipolar disorder, inhibits Gq signaling (Fig. 1). RGS4 levels are greatly reduced in the schizophrenia, post-traumatic stress disorder, and lead poison- prefrontal cortex of patients with schizophrenia (24). It is noteworthy ing—conditions characterized by impoverished prefrontal struc- that pharmacological treatments for both bipolar disorder and schizo- tural and functional integrity. phrenia indirectly inhibit PKC signaling: Lithium indirectly reduces PKC activity by inhibiting inositol monophosphatase, whereas atypical bipolar disorder ͉ post-traumatic stress disorder ͉ working memory ͉ antipsychotics block ␣1 adrenoceptors (␣1-ARs) and 5HT2 receptors chelerythrine ͉ lead poisoning that are coupled to Gq signaling (25–29). Environmental factors can also activate PKC signaling and result in prefrontal impairment. For example, lead can mimic calcium he prefrontal cortex is the most highly evolved brain region and ϩ Tis essential for the intelligent regulation of attention, action, and (Ca2 ) and potently activate PKC (Fig. 1). Lead poisoning is emotion by representational knowledge. The ability to represent associated with prefrontal deficits including altered attention reg- information is often referred to as working memory or the ‘‘mental ulation and behavioral disinhibition in animals (30, 31) and humans sketch-pad’’. Networks of prefrontal cortical neurons interconnect (32–34). Exposure to a traumatic stressor can lead to post-traumatic on dendritic spines to maintain information in working memory (1, stress disorder (PTSD), which is associated with weakened medial 2). However, the prefrontal cortex is also the brain region that is prefrontal function (35). Importantly, PTSD, lead poisoning, most sensitive to the detrimental effects of stress. Even mild acute, schizophrenia, and bipolar disorder are all associated with prefron- uncontrollable stress rapidly impairs prefrontal functions in animals tal gray matter loss (10), and where studied, dendritic spine loss (3–5) and humans (6), whereas regions such as the hippocampus from prefrontal pyramidal neurons (36). Recent data indicate that require more severe or prolonged stress to show functional changes lithium can rescue prefrontal gray matter in bipolar disorder (7, 8). Acute stress impairs prefrontal cortical function through a (37–40) and restore stress-induced dendritic retraction in rat hip- cascade of events: There is a large increase in catecholamine release in the prefrontal cortex, which in turn leads to high levels of cyclic adenosine monophosphate (cAMP) and protein kinase C (PKC) Author contributions: A.B.H. and A.F.T.A. designed research; A.B.H., M.A.T.V., and M.G. performed research; A.B.H., P.K.M., C.H.v.D., and A.F.T.A. analyzed data; and A.B.H. and intracellular signaling, which reduce prefrontal neuronal firing, and A.F.T.A. wrote the paper. rapidly impair working memory (9, 10). Conflict of interest statement: Drs. Arnsten and Hains, together with Yale University, have When stressors are repeated over many days and weeks, there are a patent and patent applications, and are pursuing a license agreement to develop CHEL further architectural changes in the prefrontal cortex: a retraction for the treatment of PTSD, bipolar disorder, and related illnesses. of dendrites and a loss of spine density (11–15). These dendritic Freely available online through the PNAS open access option. changes are associated with impaired prefrontal cortical attentional See Commentary on page 17613. set-shifting in rats and, it is likely, in human subjects as well (16, 17). 1Present address: Department of Psychiatry, Brigham and Women’s Hospital, Boston, MA Dendritic changes in the prefrontal cortex can occur with repeated 02115. exposure to even mild stressors (18, 19), indicating that this region 2To whom correspondence should be addressed: E-mail: [email protected]. is remarkably sensitive to chronic stress as well as to acute stress. This article contains supporting information online at www.pnas.org/cgi/content/full/ Dendritic changes are evident in layer II/III pyramidal cells, which 0908563106/DCSupplemental. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0908563106 PNAS ͉ October 20, 2009 ͉ vol. 106 ͉ no. 42 ͉ 17957–17962 Downloaded by guest on October 2, 2021 function in rats (11, 13, 17), and examined whether chronic treat- ment with the PKC inhibitor, chelerythrine (CHEL), before daily stress would rescue prefrontal cognition and spine density. Systemic administration of CHEL has previously been shown to be effective in reversing the biochemical and cognitive effects of acute stress on PKC signaling (9). The current study reports that treatment with daily CHEL rescued both working memory and dendritic spine density from the detrimental effects of chronic stress exposure. Results PKC Inhibition Rescues Stress-Induced Working Memory Impairment. Rats were exposed to a restraint stress paradigm (21 days, 6 h/day) that induces distal apical dendritic retraction and spine loss in layer II/III pyramidal neurons in the medial prefrontal cortex (11, 13, 17). Half of the stressed animals received vehicle and half received CHEL (1.0 mg/kg, s.c.), daily, before stress exposure. Nonstressed Fig. 1. Stress results in elevated catecholamine release in the prefrontal cortex. Increased noradrenergic signaling activates the PI-PKC cascade via controls also received vehicle or CHEL daily. All rats had been Gq-coupled ␣1-AR. RGS4 and diacylglycerol kinase, including DGKH, inhibit pretrained on a spatial delayed alternation task in a T maze to an this pathway. Genetic studies indicate that RGS4 and DGKH are compromised equivalent level of performance, and were then tested prior to in schizophrenia and bipolar disorder, which would lead to overactivation of drug/stress treatment each day. This task requires working memory PKC signaling. Lead (Pbϩϩ) mimics Ca2ϩ, a necessary cofactor for the activation as well as behavioral inhibition and the ability to overcome dis- of PKC. Thus, PKC is likely dysregulated in stress-related disorders such as PTSD traction, and necessitates an intact medial prefrontal cortical func- as well as in lead poisoning, bipolar disorder, and schizophrenia. PKC dys- tion in rats (41). Rats were assessed for cognitive ability 10 times regulation may be related to pathology and symptomology as overactive PKC