Behavioral effects of SGK1 knockout in VTA and dopamine neurons

Serum/glucocorticoid regulated kinase 1 (SGK1) in neurological disorders: pain or gain

Serum/Glucocorticoid Regulated Kinase 1 (SGK1), a serine/threonine kinase, is ubiquitous across a wide range of tissues, orchestrating numerous signaling pathways and associated with various human diseases. SGK1 has been extensively explored in diverse types of immune and inflammatory diseases, cardiovascular disorders, as well as cancer metastasis. These studies link SGK1 to cellular proliferation, survival, metabolism, membrane transport, and drug resistance. Recently, increasing research has focused on SGK1's role in neurological disorders, including a variety of neurodegenerative diseases (e.g., Alzheimer's disease, Huntington's disease and Parkinson's disease), brain injuries (e.g., cerebral ischemia and traumatic brain injury), psychiatric conditions (e.g., depression and drug addiction). SGK1 is emerging as an increasingly compelling therapeutic target across the spectrum of neurological disorders, supported by the availability of several effective agents. However, the conclusions of many studies observing the prevalence and function of SGK1 in neurological disorders are contradictory, necessitating a review of the SGK1 research within neurological disorders. Herein, we review recent literature on SGK1's primary functions within the nervous system and its impacts within different neurological disorders. We summarize significant findings, identify research gaps, and outline possible future research directions based on the current understanding of SGK1 to help further progress the understanding and treatment of neurological disorders.

BNST GluN2D-containing NMDARs contribute to ethanol intake but not negative affective behaviors in female mice

BACKGROUND

Alcohol use disorder (AUD) is a chronic, relapsing disease, highly comorbid with anxiety and depression. The bed nucleus of the stria terminalis (BNST) and Crh+ neurons in this region play a key role in chronic ethanol-induced increases in volitional intake, hypothesized to be driven by emergent negative affective behaviors. Excitatory N-methyl-d-aspartate receptors (NMDARs) are a major target of ethanol, and chronic ethanol exposure has been shown to regulate NMDAR function and expression. Specifically, GluN2D subunit-containing NMDARs have emerged as a target of interest due to their limited distribution and potential roles in affective behavior.

METHODS

Male and female mice underwent a home cage chronic drinking forced abstinence model (CDFA) to assess the impact of 1 day or 2 weeks of ethanol abstinence on BNST synaptic transmission and BNST Grin gene expression. Constitutive and conditional BNST GluN2D knockout mice were used to assess the impact of GluN2D deletion on continuous access ethanol intake as well as negative affect behaviors, using the open field, elevated zero maze, and forced swim tasks.

RESULTS

We report here that excitatory transmission undergoes time-dependent upregulation in BNST Crh+ cells. Further, knockdown of dorsal BNST (dBNST) GluN2D expression significantly decreases ethanol intake in female, but not male, mice. While BNST Grin2b expression was significantly increased in protracted abstinence following CDFA, no differences in Grin2d expression were observed in the dBNST or dBNST Crh+ neurons. Finally, we find that deletion of GluN2D fails to alter negative affect in ethanol-naïve female mice.

CONCLUSIONS

These data suggest a role for BNST GluN2D-containing NMDARs in ethanol drinking but not ethanol abstinence, highlighting sex differences and behavioral specificity. Overall, these data further suggest roles for BNST synaptic signaling in volitional ethanol intake that are partially independent of actions on affective behavior.

The role of serum/glucocorticoid-regulated kinase 1 in brain function following cerebral ischemia

Cardiopulmonary arrest (CA) is a major cause of death/disability in the U.S. with poor prognosis and survival rates. Current therapeutic challenges are physiologically complex because they involve hypoperfusion (decreased cerebral blood flow), neuroinflammation, and mitochondrial dysfunction. We previously discovered novel serum/glucocorticoid-regulated kinase 1 (SGK1) is highly expressed in brain of neurons that are susceptible to ischemia (hippocampus and cortex). We inhibited SGK1 and utilized pharmacological (specific inhibitor, GSK650394) and neuron-specific genetic approaches (shRNA) in rodent models of CA to determine if SGK1 is responsible for hypoperfusion, neuroinflammation, mitochondrial dysfunctional, and neurological deficits after CA. Inhibition of SGK1 alleviated cortical hypoperfusion and neuroinflammation (via Iba1, GFAP, and cytokine array). Treatment with GSK650394 enhanced mitochondrial function (via Seahorse respirometry) in the hippocampus 3 and 7 days after CA. Neuronal injury (via MAP2, dMBP, and Golgi staining) in the hippocampus and cortex was observed 7 days after CA but ameliorated with SGK1-shRNA. Moreover, SGK1 mediated neuronal injury by regulating the Ndrg1-SOX10 axis. Finally, animals subjected to CA exhibited learning/memory, motor, and anxiety deficits after CA, whereas SGK1 inhibition via SGK1-shRNA improved neurocognitive function. The present study suggests the fundamental roles of SGK1 in brain circulation and neuronal survival/death in cerebral ischemia-related diseases.

BNST GluN2D-containing NMDARs contribute to ethanol intake but not negative affective behaviors in female mice

Alcohol use disorder (AUD) is a chronic, relapsing disease, highly comorbid with anxiety and depression. The bed nucleus of the stria terminalis (BNST), and Crh + neurons in this region are thought to play a key role in chronic ethanol-induced increases in volitional ethanol intake. This role has been hypothesized to be driven by emergent BNST-dependent negative affective behaviors. Indeed, we report here that in female mice undergoing a home cage chronic drinking forced abstinence model (CDFA), excitatory transmission undergoes time-dependent upregulation in BNST Crh + cells. Excitatory NMDA receptors (NMDARs) are a major target of ethanol, and chronic ethanol exposure has been shown to regulate NMDAR function and expression. GluN2D subunit-containing NMDARs have emerged as a target of interest due to their limited distribution and potential roles in affective behavior. We find that knockdown of dorsal BNST (dBNST) GluN2D expression significantly decreases ethanol intake in female, but not male, mice. While BNST Grin2b expression was significantly increased in protracted abstinence following CDFA, no differences in Grin2d expression were observed in dBNST or specifically in dBNST Crh + neurons. Finally, to determine the impact of GluN2D expression on negative affective behaviors, open field, elevated zero maze, and forced swim tasks were used to measure anxiety- and depressive-like behaviors in constitutive and conditional BNST GluN2D knockout mice. Surprisingly, we find that deletion of GluN2D fails to alter negative affect in ethanol-naïve female mice. Together, these data suggest a role for BNST GluN2D-containing NMDARs in ethanol drinking behaviors but not abstinence from ethanol, highlighting potential sex differences and behavioral specificity in the context of AUD behaviors. Overall, these data further suggest roles for BNST synaptic signaling in volitional ethanol intake that are partially independent of actions on affective behavior.

Dexmedetomidine improves the acute stress reactivity of male rat through interventions of serum- and glucocorticoid-inducible kinase 1 and nNOS in the bed nucleus of the stria terminalis

Moderate acute stress responses are beneficial for adaptation and maintenance of homeostasis. Exposure of male rat to stress induces effects in the bed nucleus of the stria terminalis (BNST), for it can be activated by the same stimuli that induce activation of the hypothalamic-pituitary-adrenal axis. However, the underlying mechanism of the BNST on male stress reactivity remains unclear. In this study, we explored whether systematic administration of dexmedetomidine (DEXM) altered the acute stress reactivity through its effect on the BNST. Male Sprague-Dawley rats in the stress (STRE) group, DEXM group, and the DEXM + GSK-650394 (GSK, an antagonist of serum- and glucocorticoid-inducible kinase 1 (SGK1)) group, except those in the vehicle (VEH) group, underwent 1-h restraint plus water-immersion (RPWI) exposure. All the rats proceeded the open field test (OFT) 24 h before RPWI and 1 h after RPWI. After the second OFT, the rats received VEH, DEXM (75 μg/kg i.p.), or were pretreated with GSK (2 μM i.p.) 0.5 h ahead of DEXM respectively. The third OFT was conducted 6 h after drug administration and then the rats were sacrificed. The rats that experienced RPWI showed dramatically elevated serum corticosterone (CORT), multiplied neuronal nitric oxide synthase (nNOS) and SGK1 in the BNST, and terrible OFT behavior. We discovered when the nNOS and SGK1 were decreased in the rat BNST through DEXM treatment, the serum CORT was reduced and the OFT manifestation was ameliorated, whereas these were restrained by GSK application. Our results reveal that modest interventions to SGK1 and nNOS in the BNST improve the male rat reactivity to acute stress, and DEXM was one modulator of these effects.

Serum- and glucocorticoid-inducible kinase 1 activity in ventral tegmental area dopamine neurons regulates cocaine conditioned place preference but not cocaine self-administration

Drugs of abuse regulate the activity of the mesolimbic dopamine (DA) system, and drug-induced changes in ventral tegmental area (VTA) cellular activity and gene regulation are linked to behavioral outputs associated with addiction. Previous work from our lab determined that VTA serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription and catalytic activity were increased by repeated cocaine administration; however, it was unknown if these biochemical changes contributed to cocaine-elicited behaviors. Using transgenic and viral-mediated manipulations, we investigated the role of VTA SGK1 catalytic activity in regulating cocaine conditioned place preference and self-administration. We showed intra-VTA infusion of a catalytically inactive SGK1 mutant (K127Q) significantly decreased cocaine conditioned place preference (CPP). Further, we found that K127Q expression in VTA DA neurons significantly decreased cocaine CPP, while this same manipulation in VTA GABA neurons had no effect. However, blunted VTA DA SGK1 catalytic activity did not alter cocaine self-administration. Altogether, these studies identify the specific VTA cells critical for SGK1-mediated effects on cocaine CPP but not self-administration.

Sex-dependent alterations in behavior, drug responses and dopamine transporter expression in heterozygous DAT-Cre mice

Heterozygous mice that express Cre-recombinase under the dopamine transporter promoter (DAT-Cre knock in mice, or KI) are widely used for targeting midbrain dopamine neurons, under the assumption that their constitutive physiology is not affected. We report here that these mice display striking sex-dependent behavioral and molecular differences in relation to wildtypes (WT). Male and female KI mice were constitutively hyperactive, and male KI mice showed attenuated hyperlocomotor responses to amphetamine. In contrast, female KIs displayed a marked reduction in locomotion ("calming" effect) in response to the same dose of amphetamine. Furthermore, male and female DAT-Cre KI mice showed opposing differences in reinforcement learning, with females showing faster conditioning and males showing slower extinction. Other behavioral variables, including working memory and novelty preference, were not changed compared to WT. These effects were paralleled by differences in striatal DAT expression that disproportionately affected female KI mice. Our findings reveal clear limitations of the DAT-Cre line that must be considered when using this model.