Salt-inducible kinase is involved in the regulation of corticotropin-releasing hormone transcription in hypothalamic neurons in rats

Salt-inducible kinase 1-CREB-regulated transcription coactivator 1 signalling in the paraventricular nucleus of the hypothalamus plays a role in depression by regulating the hypothalamic-pituitary-adrenal axis

Elucidating the molecular mechanism underlying the hyperactivity of the hypothalamic-pituitary-adrenal axis during chronic stress is critical for understanding depression and treating depression. The secretion of corticotropin-releasing hormone (CRH) from neurons in the paraventricular nucleus (PVN) of the hypothalamus is controlled by salt-inducible kinases (SIKs) and CREB-regulated transcription co-activators (CRTCs). We hypothesised that the SIK-CRTC system in the PVN might contribute to the pathogenesis of depression. Thus, the present study employed chronic social defeat stress (CSDS) and chronic unpredictable mild stress (CUMS) models of depression, various behavioural tests, virus-mediated gene transfer, enzyme-linked immunosorbent assay, western blotting, co-immunoprecipitation, quantitative real-time reverse transcription polymerase chain reaction, and immunofluorescence to investigate this connection. Our results revealed that both CSDS and CUMS induced significant changes in SIK1-CRTC1 signalling in PVN neurons. Both genetic knockdown of SIK1 and genetic overexpression of CRTC1 in the PVN simulated chronic stress, producing a depression-like phenotype in naive mice, and the CRTC1-CREB-CRH pathway mediates the pro-depressant actions induced by SIK1 knockdown in the PVN. In contrast, both genetic overexpression of SIK1 and genetic knockdown of CRTC1 in the PVN protected against CSDS and CUMS, leading to antidepressant-like effects in mice. Moreover, stereotactic infusion of TAT-SIK1 into the PVN also produced beneficial effects against chronic stress. Furthermore, the SIK1-CRTC1 system in the PVN played a role in the antidepressant actions of fluoxetine, paroxetine, venlafaxine, and duloxetine. Collectively, SIK1 and CRTC1 in PVN neurons are closely involved in depression neurobiology, and they could be viable targets for novel antidepressants.

Blunted diurnal firing in lateral habenula projections to dorsal raphe nucleus and delayed photoentrainment in stress-susceptible mice

Daily rhythms are disrupted in patients with mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the 15-day chronic social defeat stress (CSDS) paradigm and in vitro slice electrophysiology, we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cells^LHb→DRN) and unlabeled DRN cells. We also performed optogenetic experiments to investigate if increased firing in cells^LHb→DRN during exposure to a weak 7-day social defeat stress (SDS) paradigm induces stress-susceptibility. Last, we investigated whether exposure to CSDS affected the ability of mice to photoentrain to a new light–dark (LD) cycle. The cells^LHb→DRN and unlabeled DRN cells of stress-susceptible mice express greater blunted diurnal firing compared to stress-näive (control) and stress-resilient mice. Daytime optogenetic activation of cells^LHb→DRN during SDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that exposure to strong stressors induces blunted daily rhythms in firing in cells^LHb→DRN, DRN cells and decreases the initial rate of photoentrainment in susceptible-mice. In contrast, resilient-mice may undergo homeostatic adaptations that maintain daily rhythms in firing in cells^LHb→DRN and also show rapid photoentrainment to a new LD cycle.

Daily rhythms are disrupted in patients suffering from mood disorders, and it is known that the lateral habenula and dorsal raphe nucleus contribute to circadian timekeeping and regulate mood. This study shows that stress-susceptible mice have blunted and inverted diurnal firing rhythms in lateral habenula cells that project to the dorsal raphe nucleus, and have a slow rate of photoentrainment to a new light cycle.

The potent roles of salt-inducible kinases (SIKs) in metabolic homeostasis and tumorigenesis

Salt-inducible kinases (SIKs) belong to AMP-activated protein kinase (AMPK) family, and functions mainly involve in regulating energy response-related physiological processes, such as gluconeogenesis and lipid metabolism. However, compared with another well-established energy-response kinase AMPK, SIK roles in human diseases, especially in diabetes and tumorigenesis, are rarely investigated. Recently, the pilot roles of SIKs in tumorigenesis have begun to attract more attention due to the finding that the tumor suppressor role of LKB1 in non-small-cell lung cancers (NSCLCs) is unexpectedly mediated by the SIK but not AMPK kinases. Thus, here we tend to comprehensively summarize the emerging upstream regulators, downstream substrates, mouse models, clinical relevance, and candidate inhibitors for SIKs, and shed light on SIKs as the potential therapeutic targets for cancer therapies.

Blunted Diurnal Firing in Lateral Habenula Projections to Dorsal Raphe Nucleus and Delayed Photoentrainment in Stress-Susceptible Mice.. [DOI: 10.1101/2020.03.19.998732] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Daily rhythms are disrupted in patients suffering from mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the chronic social defeat stress (CSDS) paradigm and in-vitro slice electrophysiology we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cellsLHb→DRN) and DRN cells alone. We also performed optogenetic experiments to investigate if increased firing in cellsLHb→DRN during exposure to subthreshold social defeat stress (SSDS), induces stress-susceptibility. Last we investigated whether exposure to CSDS affected the ability of mice to phototentrain to a new LD cycle. The cellsLHb→DRN and DRN cells alone of stress-susceptible mice express greater blunted diurnal firing compared to stress-naive (control) and stress-resilient mice. Day-time optogenetic activation of cellsLHb→DRN during SSDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that CSDS induces blunted daily rhythms in firing in cellsLHb→DRN and slow rate of photoentrainment in susceptible-mice. In contrast, resilientmice may undergo homeostatic adaptations that maintain daily rhythms in firing in cellsLHb→DRN and also show rapid photoentrainment to a new LD-cycle.

SIK1 Regulates CRTC2-Mediated Gluconeogenesis Signaling Pathway in Human and Mouse Liver Cells

The regulation of hepatic gluconeogenesis is of great significance to improve insulin resistance and benefit diabetes therapy. cAMP-Regulated Transcriptional Co-activator 2 (CRTC2) plays a key role in regulating hepatic gluconeogenesis through controlling the expression of gluconeogenic rate-limiting enzymes such as glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK). Recently, salt-inducible kinase 1 (SIK1) has been identified to play an important role in glucose metabolism disorders, but whether and how SIK1 regulates the CTRC2 signaling in liver cells under high glucose conditions has rarely been intensively elucidated. Here, we show that high glucose stimulation resulted in time-dependent down-regulated expression of SIK1, phosphorylated SIK1 at T182 site, and phosphorylated CRTC2 at S171 site, as well as upregulated expression of total CRTC2 and its downstream targets G6Pase and PEPCK in the human liver cell line HepG2. The nuclear expression levels of SIK1 and CRTC2 were time-dependently upregulated upon high glucose challenge, which was accompanied by enhanced cytoplasm-to-nucleus translocation of SIK1. Manipulation of SIK1 activity using plasmid-mediated SIK1 over-expression and the use of the SIKs inhibitor HG-9-91-01 confirmed that SIK1 regulated the CRTC2 signaling pathway in HepG2 cells. Furthermore, in mouse primary hepatocytes, high glucose exposure down-regulated SIK1 expression, and promoted SIK1 nuclear accumulation. While HG-9-91-01 treatment suppressed SIK1 expression and released the inhibitory effects of SIK1 on the expressions of key molecules involved in the CRTC2 signaling pathway, additional ectopic expression of SIK1 using adenovirus infection reversed the impacts of HG-9-91-01 on the expressions of these molecules in mouse hepatocytes. Therefore, SIK1 regulates CRTC2-mediated gluconeogenesis signaling pathway under both physiological and high glucose-induced pathological conditions. The modulation of the SIK1-CRTC2 signaling axis could provide an attractive means for treating diabetes.

Hippocampal Salt-Inducible Kinase 2 Plays a Role in Depression via the CREB-Regulated Transcription Coactivator 1-cAMP Response Element Binding-Brain-Derived Neurotrophic Factor Pathway

BACKGROUND

Developing novel pharmacological targets beyond monoaminergic systems is now a popular strategy for finding new ways to treat depression. Salt-inducible kinase (SIK) is a kinase that regulates the nuclear translocation of cyclic adenosine monophosphate response element binding protein (CREB)-regulated transcription coactivator (CRTC) by phosphorylation. Here, we hypothesize that dysfunction of the central SIK-CRTC system may contribute to the pathogenesis of depression.

METHODS

Chronic social defeat stress (CSDS) and chronic unpredictable mild stress (CUMS) models of depression, various behavioral tests, viral-mediated gene transfer, Western blotting, coimmunoprecipitation, quantitative real-time reverse transcription polymerase chain reaction, and immunohistochemistry were used in this study (for in vivo studies, n = 10; for in vitro studies, n = 5).

RESULTS

Both CSDS and CUMS markedly increased the expression of hippocampal SIK2, which reduced CRTC1 nuclear translocation and binding of CRTC1 and CREB in the hippocampus. Genetic overexpression of hippocampal SIK2 in naïve mice simulated chronic stress, inducing depressive-like behaviors in the forced swim test, tail suspension test, sucrose preference test, and social interaction test, as well as decreasing the brain-derived neurotrophic factor signaling cascade and neurogenesis in the hippocampus. In contrast, genetic knockdown and knockout of hippocampal SIK2 protected against CSDS and CUMS, exerting significant antidepressant-like effects that were mediated via the downstream CRTC1-CREB-brain-derived neurotrophic factor pathway. Moreover, fluoxetine, venlafaxine, and mirtazapine all significantly restored the effects of CSDS and CUMS on the hippocampal SIK2-CRTC1 pathway, which was necessary for their antidepressant actions.

CONCLUSIONS

The hippocampal SIK2-CRTC1 pathway is involved in the pathogenesis of depression, and hippocampal SIK2 could be a novel target for the development of antidepressants.

Gene Level Regulation of Na,K-ATPase in the Renal Proximal Tubule Is Controlled by Two Independent but Interacting Regulatory Mechanisms Involving Salt Inducible Kinase 1 and CREB-Regulated Transcriptional Coactivators

For many years, studies concerning the regulation of Na,K-ATPase were restricted to acute regulatory mechanisms, which affected the phosphorylation of Na,K-ATPase, and thus its retention on the plasma membrane. However, in recent years, this focus has changed. Na,K-ATPase has been established as a signal transducer, which becomes part of a signaling complex as a consequence of ouabain binding. Na,K-ATPase within this signaling complex is localized in caveolae, where Na,K-ATPase has also been observed to regulate Inositol 1,4,5-Trisphosphate Receptor (IP3R)-mediated calcium release. This latter association has been implicated as playing a role in signaling by G Protein Coupled Receptors (GPCRs). Here, the consequences of signaling by renal effectors that act via such GPCRs are reviewed, including their regulatory effects on Na,K-ATPase gene expression in the renal proximal tubule (RPT). Two major types of gene regulation entail signaling by Salt Inducible Kinase 1 (SIK1). On one hand, SIK1 acts so as to block signaling via cAMP Response Element (CRE) Binding Protein (CREB) Regulated Transcriptional Coactivators (CRTCs) and on the other hand, SIK1 acts so as to stimulate signaling via the Myocyte Enhancer Factor 2 (MEF2)/nuclear factor of activated T cell (NFAT) regulated genes. Ultimate consequences of these pathways include regulatory effects which alter the rate of transcription of the Na,K-ATPase β1 subunit gene atp1b1 by CREB, as well as by MEF2/NFAT.

Expression of stress hormones AVP and CRH in the hypothalamus of Mus musculus following water and food deprivation

Neurohypophyseal hormone, arginine vasopressin (AVP), in addition to acting as antidiuretic hormone is also considered to be stress hormone like hypothalamic corticotropin-releasing hormone (CRH). Present study was designed to investigate the relative response of these stress hormones during water and food deprivation. In this study, male laboratory mice of Swiss strain were divided in 5 groups, control - provided water and food ad libitum, two experimental groups water deprived for 2 and 4days respectively (WD2 and WD4) and another two groups food deprived for 2 and 4days respectively (FD2 and FD4). Results indicate an increased expression of AVP mRNA as well as peptide in the hypothalamus of WD2 mice and the expression was further upregulated after 4days of water deprivation but the expression of CRH remained unchanged compare to their respective controls. On the other hand no change was observed in the expression of hypothalamic AVP mRNA while AVP peptide increased significantly in FD2 and FD4 mice compare to control. Further, the expression of CRH mRNA although increased in hypothalamus of both FD2 and FD4 mice, the immunofluorescent staining shows decreased expression of CRH in PVN of food deprived mice. Based on these findings it is concluded that since during osmotic stress only AVP expression is upregulated but during metabolic stress i.e. food deprivation transcription and translation of both the stress hormones are differentially regulated. Further, it is suggested that role of AVP and CRH may be stress specific.

Activation of salt-inducible kinase 2 promotes the viability of peritoneal mesothelial cells exposed to stress of peritoneal dialysis

Maintaining mesothelial cell viability is critical to long-term successful peritoneal dialysis (PD) treatment. To clarify the viability mechanism of peritoneal mesothelial cells under PD solutions exposure, we examined the mechanisms of cellular response to this stress conditions. Here we report that the proteasome activity is inhibited when treated with PD solutions. Proteasome inhibition-mediated activation of salt-inducible kinase 2 (SIK2), an endoplasmic reticulum-resident protein, is important for mesothelial cell viability. SIK2 is mobilized to promote autophagy and protect the cells from apoptosis under PD solution or MG132 treatment. Immunofluorescence staining showed that SIK2 is colocalized with LC3B in the autophagosomes of mesothelial cells treated with PD solution or derived from patients undergoing PD treatment. SIK2 activation is likely via a two-step mechanism, upstream kinases relieving the autoinhibitory conformation of SIK2 molecule followed by autophosphorylation of Thr175 and activation of kinase activity. These results suggest that activation of SIK2 is required for the cell viability when proteasome activity is inhibited by PD solutions. Maintaining or boosting the activity of SIK2 may promote peritoneal mesothelial cell viability and evolve as a potential therapeutic target for maintaining or restoring peritoneal membrane integrity in PD therapy.

Phosphodiesterase-4 inhibition as a therapeutic strategy for metabolic disorders

Phosphodiesterase-4 (PDE4) hydrolyses cyclic adenosine monophosphate (cAMP), a crucial secondary messenger for cellular adaptation to diverse external stimuli. The activity of PDE4 is tightly controlled by post-translational regulation, structure-based auto-regulation and locus specific 'compartmentalization' of PDE4 with its interactive proteins (signalsomes). Through these mechanisms, PDE4 regulates cAMP levels and shapes the cAMP signalling, directing signals from the diverse external stimuli to distinct microenvironments exquisitely. Derangement of the PDE4-cAMP signalling represents a pathophysiologically relevant pathway in metabolic disorders as demonstrated through a critical role in the processes including inflammation, disordered glucose and lipid metabolism, hepatic steatosis, abnormal lipolysis, suppressed thermogenic function and deranged neuroendocrine functions. A limited number of PDE4 inhibitors are currently undergoing clinical evaluation for treating disorders such as type 2 diabetes and non-alcoholic steatohepatitis. The discovery of novel PDE4 allosteric inhibitors and signalsome-based strategies targeting individual PDE4 variants may allow PDE4 isoform selective inhibition, which may offer safer strategies for chronic treatment of metabolic disorders.

The adrenergic-regulated CRTC1 and CRTC2 phosphorylation and cellular distribution is independent of endogenous SIK1 in the male rat pinealocyte

Salt inducible kinase 1 (SIK1) has been reported to repress cAMP-response element binding protein (CREB)-mediated gene transcription by causing the nuclear export of CREB-regulated transcription coactivators (CRTCs) through phosphorylation. Although the repressor role of SIK1 in suppressing the expression of arylalkylamine N-acetyltransferase, the enzyme that controls the daily rhythm in melatonin production in the rat pineal gland, has been established, whether SIK1 regulates the phosphorylation and localization of CRTC1 and CRTC2 in this tissue remains unclear. The present study found that overexpressing SIK1 in NE-stimulated rat pinealocytes could increase the phosphorylation of CRTC1 and CRTC2, reduced selectively the nuclear level of CRTC2 (but not that of CRTC1), and elevated the cytosolic levels of both CRTC1 and CRTC2. In contrast, transient knockdown of endogenous SIK1 had no effect on the phosphorylation or distribution of CRTC1 and CRTC2 in norepinephrine (NE)-stimulated pinealocytes. Our results also showed that adrenergic blockade during NE stimulation led to a rapid rephosphorylation and decline in the nucleus levels of CRTC1 and CRTC2; however SIK1 knockdown had no effect on this rapid rephosphorylation. Moreover, studies with kinase inhibitors revealed that kinase(s) sensitive to KT5823 appeared to be involved in this rapid rephosphorylation. Together, these results indicate that although overexpressing SIK1 can phosphorylate CRTC1 and CRTC2 in the NE-stimulated pinealocyte, the endogenous SIK1, in spite of its induction by NE, does not appear to be the main regulator of the phosphorylation and intracellular localization of these two coactivators.

Oxytocin Regulates Stress-Induced Crf Gene Transcription through CREB-Regulated Transcription Coactivator 3

The major regulator of the neuroendocrine stress response in the brain is corticotropin releasing factor (CRF), whose transcription is controlled by CREB and its cofactors CRTC2/3 (TORC2/3). Phosphorylated CRTCs are sequestered in the cytoplasm, but rapidly dephosphorylated and translocated into the nucleus following a stressful stimulus. As the stress response is attenuated by oxytocin (OT), we tested whether OT interferes with CRTC translocation and, thereby, Crf expression. OT (1 nmol, i.c.v.) delayed the stress-induced increase of nuclear CRTC3 and Crf hnRNA levels in the paraventricular nucleus of male rats and mice, but did not affect either parameter in the absence of the stressor. The increase in Crf hnRNA levels at later time points was parallel to elevated nuclear CRTC2/3 levels. A direct effect of Thr(4) Gly(7)-OT (TGOT) on CRTC3 translocation and Crf expression was found in rat primary hypothalamic neurons, amygdaloid (Ar-5), hypothalamic (H32), and human neuroblastoma (Be(2)M17) cell lines. CRTC3, but not CRCT2, knockdown using siRNA in Be(2)M17 cells prevented the effect of TGOT on Crf hnRNA levels. Chromatin-immunoprecipitation demonstrated that TGOT reduced CRTC3, but not CRTC2, binding to the Crf promoter after 10 min of forskolin stimulation. Together, the results indicate that OT modulates CRTC3 translocation, the binding of CRTC3 to the Crf promoter and, ultimately, transcription of the Crf gene.

SIGNIFICANCE STATEMENT

The neuropeptide oxytocin has been proposed to reduce hypothalamic-pituitary-adrenal (HPA) axis activation during stress. The underlying mechanisms are, however, elusive. In this study we show that activation of the oxytocin receptor in the paraventricular nucleus delays transcription of the gene encoding corticotropin releasing factor (Crf), the main regulator of the stress response. It does so by sequestering the coactivator of the transcription factor CREB, CRTC3, in the cytosol, resulting in reduced binding of CRTC3 to the Crf gene promoter and subsequent Crf gene expression. This novel oxytocin receptor-mediated intracellular mechanism might provide a basis for the treatment of exaggerated stress responses in the future.

Stimulation of StAR expression by cAMP is controlled by inhibition of highly inducible SIK1 via CRTC2, a co-activator of CREB

In mouse steroidogenic cells the activation of cholesterol metabolism is mediated by steroidogenic acute regulatory protein (StAR). Here, we visualized a coordinated regulation of StAR transcription, splicing and post-transcriptional processing, which are synchronized by salt inducible kinase (SIK1) and CREB-regulated transcription coactivator (CRTC2). To detect primary RNA (pRNA), spliced primary RNA (Sp-RNA) and mRNA in single cells, we generated probe sets by using fluorescence in situ hybridization (FISH). These methods allowed us to address the nature of StAR gene expression and to visualize protein-nucleic acid interactions through direct detection. We show that SIK1 represses StAR expression in Y1 adrenal and MA10 testis cells through inhibition of processing mediated by CRTC2. Digital image analysis matches qPCR analyses of the total cell culture. Evidence is presented for spatially separate accumulation of StAR pRNA and Sp-RNA at the gene loci in the nucleus. These findings establish that cAMP, SIK and CRTC mediate StAR expression through activation of individual StAR gene loci.

De novo mutations in SIK1 cause a spectrum of developmental epilepsies

Developmental epilepsies are age-dependent seizure disorders for which genetic causes have been increasingly identified. Here we report six unrelated individuals with mutations in salt-inducible kinase 1 (SIK1) in a series of 101 persons with early myoclonic encephalopathy, Ohtahara syndrome, and infantile spasms. Individuals with SIK1 mutations had short survival in cases with neonatal epilepsy onset, and an autism plus developmental syndrome after infantile spasms in others. All six mutations occurred outside the kinase domain of SIK1 and each of the mutants displayed autophosphorylation and kinase activity toward HDAC5. Three mutations generated truncated forms of SIK1 that were resistant to degradation and also showed changes in sub-cellular localization compared to wild-type SIK1. We also report the human neuropathologic examination of SIK1-related developmental epilepsy, with normal neuronal morphology and lamination but abnormal SIK1 protein cellular localization. Therefore, these results expand the genetic etiologies of developmental epilepsies by demonstrating SIK1 mutations as a cause of severe developmental epilepsy.

Novel aspects of glucocorticoid actions

Normal hypothalamic-pituitary-adrenal (HPA) axis activity leading to the rhythmic and episodic release of adrenal glucocorticoids (GCs) is essential for body homeostasis and survival during stress. Acting through specific intracellular receptors in the brain and periphery, GCs regulate behaviour, as well as metabolic, cardiovascular, immune and neuroendocrine activities. By contrast to chronic elevated levels, circadian and acute stress-induced increases in GCs are necessary for hippocampal neuronal survival and memory acquisition and consolidation, as a result of the inhibition of apoptosis, the facilitation of glutamatergic neurotransmission and the formation of excitatory synapses, and the induction of immediate early genes and dendritic spine formation. In addition to metabolic actions leading to increased energy availability, GCs have profound effects on feeding behaviour, mainly via the modulation of orexigenic and anorixegenic neuropeptides. Evidence is also emerging that, in addition to the recognised immune suppressive actions of GCs by counteracting adrenergic pro-inflammatory actions, circadian elevations have priming effects in the immune system, potentiating acute defensive responses. In addition, negative-feedback by GCs involves multiple mechanisms leading to limited HPA axis activation and prevention of the deleterious effects of excessive GC production. Adequate GC secretion to meet body demands is tightly regulated by a complex neural circuitry controlling hypothalamic corticotrophin-releasing hormone (CRH) and vasopressin secretion, which are the main regulators of pituitary adrenocorticotrophic hormone (ACTH). Rapid feedback mechanisms, likely involving nongenomic actions of GCs, mediate the immediate inhibition of hypothalamic CRH and ACTH secretion, whereas intermediate and delayed mechanisms mediated by genomic actions involve the modulation of limbic circuitry and peripheral metabolic messengers. Consistent with their key adaptive roles, HPA axis components are evolutionarily conserved, being present in the earliest vertebrates. An understanding of these basic mechanisms may lead to novel approaches for the development of diagnostic and therapeutic tools for disorders related to stress and alterations of GC secretion.

Role of the SIK2-p35-PJA2 complex in pancreatic β-cell functional compensation

Energy sensing by the AMP-activated protein kinase (AMPK) is of fundamental importance in cell biology. In the pancreatic β-cell, AMPK is a central regulator of insulin secretion. The capacity of the β-cell to increase insulin output is a critical compensatory mechanism in prediabetes, yet its molecular underpinnings are unclear. Here we delineate a complex consisting of the AMPK-related kinase SIK2, the CDK5 activator CDK5R1 (also known as p35) and the E3 ligase PJA2 essential for β-cell functional compensation. Following glucose stimulation, SIK2 phosphorylates p35 at Ser 91, to trigger its ubiquitylation by PJA2 and promote insulin secretion. Furthermore, SIK2 accumulates in β-cells in models of metabolic syndrome to permit compensatory secretion; in contrast, β-cell knockout of SIK2 leads to accumulation of p35 and impaired secretion. This work demonstrates that the SIK2-p35-PJA2 complex is essential for glucose homeostasis and provides a link between p35-CDK5 and the AMPK family in excitable cells.

Corticotropin releasing factor in neuroplasticity

Stress is among the strongest signals promoting neuroplasticity: Stress signals, indicating real or perceived danger, lead to alterations of neuronal function and often structure, designed to adapt to the changed conditions and promote survival. Corticotropin releasing factor (CRF) is expressed and released in several types of neuronal populations that are involved in cognition, emotion and the regulation of autonomic and endocrine function. CRF expressing neurons undergo functional and structural plasticity during stress and, in addition, the peptide acts via specific receptors to promote plasticity of target neurons.

Differential contribution of CBP:CREB binding to corticotropin-releasing hormone expression in the infant and adult hypothalamus

Corticotropin-releasing hormone (CRH) contributes crucially to the regulation of central and peripheral responses to stress. Because of the importance of a finely tuned stress system, CRH expression is tightly regulated in an organ- and brain region-specific manner. Thus, in the hypothalamus, CRH is constitutively expressed and this expression is further enhanced by stress; however, the underlying regulatory mechanisms are not fully understood. The regulatory region of the crh gene contains several elements, including the cyclic-AMP response element (CRE), and the role of the CRE interaction with the cyclic-AMP response element binding protein (CREB) in CRH expression has been a focus of intensive research. Notably, whereas thousands of genes contain a CRE, the functional regulation of gene expression by the CRE:CREB system is limited to ∼100 genes, and likely requires additional proteins. Here, we investigated the role of a member of the CREB complex, CREB binding protein (CBP), in basal and stress-induced CRH expression during development and in the adult. Using mice with a deficient CREB-binding site on CBP, we found that CBP:CREB interaction is necessary for normal basal CRH expression at the mRNA and protein level in the nine-day-old mouse, prior to onset of functional regulation of hypothalamic CRH expression by glucocorticoids. This interaction, which functions directly on crh or indirectly via regulation of other genes, was no longer required for maintenance of basal CRH expression levels in the adult. However, CBP:CREB binding contributed to stress-induced CRH expression in the adult, enabling rapid CRH synthesis in hypothalamus. CBP:CREB binding deficiency did not disrupt basal corticosterone plasma levels or acute stress-evoked corticosterone release. Because dysregulation of CRH expression occurs in stress-related disorders including depression, a full understanding of the complex regulation of this gene is important in both health and disease.

Interaction between salt-inducible kinase 2 (SIK2) and p97/valosin-containing protein (VCP) regulates endoplasmic reticulum (ER)-associated protein degradation in mammalian cells

Salt-inducible kinase 2 (SIK2) is an important regulator of cAMP response element-binding protein-mediated gene expression in various cell types and is the only AMP-activated protein kinase family member known to interact with the p97/valosin-containing protein (VCP) ATPase. Previously, we have demonstrated that SIK2 can regulate autophagy when proteasomal function is compromised. Here we report that physical and functional interactions between SIK2 and p97/VCP underlie the regulation of endoplasmic reticulum (ER)-associated protein degradation (ERAD). SIK2 co-localizes with p97/VCP in the ER membrane and stimulates its ATPase activity through direct phosphorylation. Although the expression of wild-type recombinant SIK2 accelerated the degradation and removal of ERAD substrates, the kinase-deficient variant conversely had no effect. Furthermore, down-regulation of endogenous SIK2 or mutation of the SIK2 target site on p97/VCP led to impaired degradation of ERAD substrates and disruption of ER homeostasis. Collectively, these findings highlight a mechanism by which the interplay between SIK2 and p97/VCP contributes to the regulation of ERAD in mammalian cells.

Regulation of hypothalamic corticotropin-releasing hormone transcription by elevated glucocorticoids

Negative glucocorticoid feedback is essential for preventing the deleterious effects of excessive hypothalamic pituitary adrenal axis axis activation, with an important target being CRH transcription in the hypothalamic paraventricular nucleus. The aim of these studies was to determine whether glucocorticoids repress CRH transcription directly in CRH neurons, by examining glucocorticoid effects on glucocorticoid receptor (GR)-CRH promoter interaction and the activation of proteins required for CRH transcription. Immunoprecipitation of hypothalamic chromatin from intact or adrenalectomized rats subjected to either stress or corticosterone injections showed minor association of the proximal CRH promoter with the GR compared with that with phospho-CREB (pCREB). In contrast, the Period-1 (Per1, a glucocorticoid-responsive gene) promoter markedly recruited GR. Stress increased pCREB recruitment by the CRH but not the Per1 promoter, irrespective of circulating glucocorticoids. In vitro, corticosterone pretreatment (30 minutes or 18 hours) only slightly inhibited basal and forskolin-stimulated CRH heteronuclear RNA in primary hypothalamic neuronal cultures and CRH promoter activity in hypothalamic 4B cells. In 4B cells, 30 minutes or 18 hours of corticosterone exposure had no effect on forskolin-induced nuclear accumulation of the recognized CRH transcriptional regulators, pCREB and transducer of regulated CREB activity 2. The data show that inhibition of CRH transcription by physiological glucocorticoids in vitro is minor and that direct interaction of GR with DNA in the proximal CRH promoter may not be a major mechanism of CRH gene repression. Although GR interaction with distal promoter elements may have a role, the data suggest that transcriptional repression of CRH by glucocorticoids involves protein-protein interactions and/or modulation of afferent inputs to the hypothalamic paraventricular nucleus.

Sustained adrenergic stimulation is required for the nuclear retention of TORC1 in male rat pinealocytes

The process involved in relocation of the coactivator, transducer of regulated cAMP-regulated element-binding protein (TORC) to the cytoplasm, unlike its activation, is not well understood. Using cultured pineal cells prepared from male rats, we found that although both α- and β-adrenergic stimulation could cause TORC1 dephosphorylation, only α-adrenergic stimulation was effective in the norepinephrine (NE)-mediated translocation of TORC1 into the nucleus. In contrast, blockade of either the α- or the β-adrenergic receptor after NE stimulation was effective in causing the rephosphorylation and rapid relocation of TORC1 into the cytoplasm. Studies with phosphoprotein phosphatase (PP) inhibitors indicated that although both PP2A and PP2B could dephosphorylate TORC1, only PP2B could cause translocation into the nucleus. However, after NE stimulation, treatment with either PP2A or PP2B inhibitors could cause the rephosphorylation and cytoplasmic relocation of TORC1. These results indicate a requirement of continuous activation of both α- and β-adrenergic receptors as well as PP2A and PP2B activities for the nuclear retention of TORC1 during NE stimulation. Knockdown of salt-inducible kinase 1 (SIK1) had no effect on the phosphorylation or localization of TORC1. Although overexpressing SIK1 could induce TORC1 phosphorylation in the nucleus, it did not reduce TORC1 level in the nucleus, indicating that SIK1-mediated TORC1 phosphorylation may not be sufficient for its relocation into the cytoplasm. Together, these results demonstrate that, in the rat pineal gland, different mechanisms are involved in regulating the nuclear entry and exit of TORC1 and that the SIK1-mediated phosphorylation of TORC1 may not lead to its nuclear exit.

Transcriptional regulation of episodic glucocorticoid secretion

Circadian and ultradian variations of basal glucocorticoid secretion and transient elevations during stress are essential for homeostasis. Using intronic qRT-PCR to measure changes in primary transcript (hnRNA) we have shown that secretory events induced by stress or ACTH injection are followed by episodic increases in transcription of rate limiting steroidogenic proteins, such as steroidogenic acute regulatory protein (StAR), cytochrome P450 side chain cleavage and melanocortin receptor associated protein. These transcriptional episodes imply rapid turnover of steroidogenic proteins and the need of de novo synthesis following each secretory event. In addition to episodic ACTH secretion, it is likely that intracellular feedback mechanisms at the adrenal fasciculata level contribute to the generation of episodes of transcription. The time relationship between activation and translocation of the CREB co-activator, transducer of regulated CREB activity (TORC) to the nucleus preceding transcriptional episodes suggest the involvement of TORC in the transcriptional activation of StAR and other steroidogenic proteins.

LKB1 tumor suppressor and salt-inducible kinases negatively regulate human T-cell leukemia virus type 1 transcription

BACKGROUND

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL). Treatment options are limited and prophylactic agents are not available. We have previously demonstrated an essential role for CREB-regulating transcriptional coactivators (CRTCs) in HTLV-1 transcription.

RESULTS

In this study we report on the negative regulatory role of LKB1 tumor suppressor and salt-inducible kinases (SIKs) in the activation of HTLV-1 long terminal repeats (LTR) by the oncoprotein Tax. Activation of LKB1 and SIKs effectively blunted Tax activity in a phosphorylation-dependent manner, whereas compromising these kinases, but not AMP-dependent protein kinases, augmented Tax function. Activated LKB1 and SIKs associated with Tax and suppressed Tax-induced LTR activation by counteracting CRTCs and CREB. Enforced expression of LKB1 or SIK1 in cells transfected with HTLV-1 molecular clone pX1MT repressed proviral transcription. On the contrary, depletion of LKB1 in pX1MT-transfected cells and in HTLV-1-transformed T cells boosted the expression of Tax. Treatment of HTLV-1 transformed cells with metformin led to LKB1/SIK1 activation, reduction in Tax expression, and inhibition of cell proliferation.

CONCLUSIONS

Our findings revealed a new function of LKB1 and SIKs as negative regulators of HTLV-1 transcription. Pharmaceutical activation of LKB1 and SIKs might be considered as a new strategy in anti-HTLV-1 and anti-ATL therapy.

Understanding stress-effects in the brain via transcriptional signal transduction pathways

Glucocorticoid hormones exert crucial effects on the brain in relation to physiology, endocrine regulation, mood and cognition. Their two receptor types, glucocorticoid and mineralocorticoid receptors (GR and MR), are members of the nuclear receptor superfamily and act in large measure as transcription factors. The outcome of MR/GR action on the genome depends on interaction with members from different protein families, which are of crucial importance for cross-talk with other neuronal and hormonal signals that impinge on the glucocorticoid sensitive circuitry. Relevant interacting proteins include other transcription factors that may either tether the receptor to the DNA, or that bind in the vicinity of GR and MR to tune the transcriptional response. In addition, transcriptional coregulator proteins constitute the actual signal transduction pathway to the transcription machinery. We review the current evidence for involvement of individual coregulators in GR-dependent effects on stress responses, and learning and memory. We discuss the use of in vitro and in silico tools to predict those coregulators that are of importance for particular brain processes. Finally, we discuss the potential of selective receptor modulators that may only allow a subset of all interactions, thus allowing more selective targeting of glucocorticoid-dependent processes in the brain.

Role of salt-inducible kinase 1 in the activation of MEF2-dependent transcription by BDNF

Substantial evidence supports a role for myocyte enhancer factor 2 (MEF2)-mediated transcription in neuronal survival, differentiation and synaptic function. In developing neurons, it has been shown that MEF2-dependent transcription is regulated by neurotrophins. Despite these observations, little is known about the cellular mechanisms by which neurotrophins activate MEF2 transcriptional activity. In this study, we examined the role of salt-inducible kinase 1 (SIK1), a member of the AMP-activated protein kinase (AMPK) family, in the regulation of MEF2-mediated transcription by the neurotrophin brain-derived neurotrophic factor (BDNF). We show that BDNF increases the expression of SIK1 in primary cultures of rat cortical neurons through the extracellular signal-regulated kinase 1/2 (ERK1/2)-signaling pathway. In addition to inducing SIK1 expression, BDNF triggers the phosphorylation of SIK1 at Thr182 and its translocation from the cytoplasm to the nucleus of cortical neurons. The effects of BDNF on the expression, phosphorylation and, translocation of SIK1 are followed by the phosphorylation and nuclear export of histone deacetylase 5 (HDAC5). Blockade of SIK activity with a low concentration of staurosporine abolished BDNF-induced phosphorylation and nuclear export of HDAC5 in cortical neurons. Importantly, stimulation of HDAC5 phosphorylation and nuclear export by BDNF is accompanied by the activation of MEF2-mediated transcription, an effect that is suppressed by staurosporine. Consistent with these data, BDNF induces the expression of the MEF2 target genes Arc and Nur77, in a staurosporine-sensitive manner. In further support of the role of SIK1 in the regulation of MEF2-dependent transcription by BDNF, we found that expression of wild-type SIK1 or S577A SIK1, a mutated form of SIK1 which is retained in the nucleus of transfected cells, is sufficient to enhance MEF2 transcriptional activity in cortical neurons. Together, these data identify a previously unrecognized mechanism by which SIK1 mediates the activation of MEF2-dependent transcription by BDNF.

Reversible acetylation regulates salt-inducible kinase (SIK2) and its function in autophagy

Salt-inducible kinase 2 (SIK2) is a serine/threonine protein kinase belonging to the AMP-activated protein kinase (AMPK) family. SIK2 has been shown to function in the insulin-signaling pathway during adipocyte differentiation and to modulate CREB-mediated gene expression in response to hormones and nutrients. However, molecular mechanisms underlying the regulation of SIK2 kinase activity remains largely elusive. Here we report a dynamic, post-translational regulation of its kinase activity that is coordinated by an acetylation-deacetylation switch, p300/CBP-mediated Lys-53 acetylation inhibits SIK2 kinase activity, whereas HDAC6-mediated deacetylation restores the activity. Interestingly, overexpression of acetylation-mimetic mutant of SIK2 (SIK2-K53Q), but not the nonacetylatable K53R variant, resulted in accumulation of autophagosomes. Further consistent with a role in autophagy, knockdown of SIK2 abrogated autophagosome and lysosome fusion. Consequently, SIK2 and its kinase activity are indispensable for the removal of TDP-43Δ inclusion bodies. Our findings uncover SIK2 as a critical determinant in autophagy progression and further suggest a mechanism in which the interplay among kinase and deacetylase activities contributes to cellular protein pool homeostasis.

Identifying links in the chain: the dynamic coupling of catecholamines, peptide synthesis, and peptide release in hypothalamic neuroendocrine neurons

Compared to neurons that communicate using synapses, some neuroendocrine neurons release relatively large quantities of peptide into the vasculature to control neuroendocrine function. Maintaining adequate amounts of peptide for release through controlled biosynthesis is therefore critical for their function. But how neuroendocrine-or in fact, any neuropeptide-neurons link appropriate levels of peptide biosynthesis with the action potentials that drive peptide release is unknown. Here, we review possible mechanisms in paraventricular hypothalamic CRH neuroendocrine neurons to coordinate these processes in response to catecholaminergic inputs. We show that CRH synthesis and release mechanisms are not invariably linked as CRH neurons are activated. Instead, coupling mechanisms exist in the premotor network that provides their synaptic inputs and in their intracellular signal transduction mechanisms, where transmitter-regulated phosphorylation of p44/42 mitogen-activated protein kinases (ERK1/2) may play a prominent role. These versatile and dynamic coupling mechanisms provide a way to link peptide biosynthesis and release.