The salt-inducible kinase, SIK, is induced by depolarization in brain

Understanding the roles of salt-inducible kinases in cardiometabolic disease

Salt-inducible kinases (SIKs) are serine/threonine kinases of the adenosine monophosphate-activated protein kinase family. Acting as mediators of a broad array of neuronal and hormonal signaling pathways, SIKs play diverse roles in many physiological and pathological processes. Phosphorylation by the upstream kinase liver kinase B1 is required for SIK activation, while phosphorylation by protein kinase A induces the binding of 14-3-3 protein and leads to SIK inhibition. SIKs are subjected to auto-phosphorylation regulation and their activity can also be modulated by Ca2+/calmodulin-dependent protein kinase in response to cellular calcium influx. SIKs regulate the physiological processes through direct phosphorylation on various substrates, which include class IIa histone deacetylases, cAMP-regulated transcriptional coactivators, phosphatase methylesterase-1, among others. Accumulative body of studies have demonstrated that SIKs are important regulators of the cardiovascular system, including early works establishing their roles in sodium sensing and vascular homeostasis and recent progress in pulmonary arterial hypertension and pathological cardiac remodeling. SIKs also regulate inflammation, fibrosis, and metabolic homeostasis, which are essential pathological underpinnings of cardiovascular disease. The development of small molecule SIK inhibitors provides the translational opportunity to explore their potential as therapeutic targets for treating cardiometabolic disease in the future.

Noradrenaline release from the locus coeruleus shapes stress-induced hippocampal gene expression

Exposure to an acute stressor triggers a complex cascade of neurochemical events in the brain. However, deciphering their individual impact on stress-induced molecular changes remains a major challenge. Here, we combine RNA sequencing with selective pharmacological, chemogenetic, and optogenetic manipulations to isolate the contribution of the locus coeruleus-noradrenaline (LC-NA) system to the acute stress response in mice. We reveal that NA release during stress exposure regulates a large and reproducible set of genes in the dorsal and ventral hippocampus via β-adrenergic receptors. For a smaller subset of these genes, we show that NA release triggered by LC stimulation is sufficient to mimic the stress-induced transcriptional response. We observe these effects in both sexes, and independent of the pattern and frequency of LC activation. Using a retrograde optogenetic approach, we demonstrate that hippocampus-projecting LC neurons directly regulate hippocampal gene expression. Overall, a highly selective set of astrocyte-enriched genes emerges as key targets of LC-NA activation, most prominently several subunits of protein phosphatase 1 (Ppp1r3c, Ppp1r3d, Ppp1r3g) and type II iodothyronine deiodinase (Dio2). These results highlight the importance of astrocytic energy metabolism and thyroid hormone signaling in LC-mediated hippocampal function and offer new molecular targets for understanding how NA impacts brain function in health and disease.

Sleep need, the key regulator of sleep homeostasis, is indicated and controlled by phosphorylation of threonine 221 in salt-inducible kinase 3

Sleep need drives sleep and plays a key role in homeostatic regulation of sleep. So far sleep need can only be inferred by animal behaviors and indicated by electroencephalography (EEG). Here we report that phosphorylation of threonine (T) 221 of the salt-inducible kinase 3 (SIK3) increased the catalytic activity and stability of SIK3. T221 phosphorylation in the mouse brain indicates sleep need: more sleep resulting in less phosphorylation and less sleep more phosphorylation during daily sleep/wake cycle and after sleep deprivation (SD). Sleep need was reduced in SIK3 loss of function (LOF) mutants and by T221 mutation to alanine (T221A). Rebound after SD was also decreased in SIK3 LOF and T221A mutant mice. By contrast, SIK1 and SIK2 do not satisfy criteria to be both an indicator and a controller of sleep need. Our results reveal SIK3-T221 phosphorylation as a chemical modification which indicates and controls sleep need.

LKB1 Signaling and Patient Survival Outcomes in Hepatocellular Carcinoma

The liver is a major organ that is involved in essential biological functions such as digestion, nutrient storage, and detoxification. Furthermore, it is one of the most metabolically active organs with active roles in regulating carbohydrate, protein, and lipid metabolism. Hepatocellular carcinoma is a cancer of the liver that is associated in settings of chronic inflammation such as viral hepatitis, repeated toxin exposure, and fatty liver disease. Furthermore, liver cancer is the most common cause of death associated with cirrhosis and is the 3^rd leading cause of global cancer deaths. LKB1 signaling has been demonstrated to play a role in regulating cellular metabolism under normal and nutrient deficient conditions. Furthermore, LKB1 signaling has been found to be involved in many cancers with most reports identifying LKB1 to have a tumor suppressive role. In this review, we use the KMPlotter database to correlate RNA levels of LKB1 signaling genes and hepatocellular carcinoma patient survival outcomes with the hopes of identifying potential biomarkers clinical usage. Based on our results STRADß, CAB39L, AMPKα, MARK2, SIK1, SIK2, BRSK1, BRSK2, and SNRK expression has a statistically significant impact on patient survival.

An Epilepsy-Associated Mutation of Salt-Inducible Kinase 1 Increases the Susceptibility to Epileptic Seizures and Interferes with Adrenocorticotropic Hormone Therapy for Infantile Spasms in Mice

Six mutations in the salt-inducible kinase 1 (SIK1) have been identified in developmental and epileptic encephalopathy (DEE-30) patients, and two of the mutations are nonsense mutations that truncate the C-terminal region of SIK1. In a previous study, we generated SIK1 mutant (SIK1-MT) mice recapitulating the C-terminal truncated mutations using CRISPR/Cas9-mediated genome editing and found an increase in excitatory synaptic transmission and enhancement of neural excitability in neocortical neurons in SIK1-MT mice. NMDA was injected into SIK1-MT males to induce epileptic seizures in the mice. The severity of the NMDA-induced seizures was estimated by the latency and the number of tail flickering and hyperflexion. Activated brain regions were evaluated by immunohistochemistry against c-fos, Iba1, and GFAP. As another epilepsy model, pentylenetetrazol was injected into the adult SIK1 mutant mice. Seizure susceptibility induced by both NMDA and PTZ was enhanced in SIK1-MT mice. Brain regions including the thalamus and hypothalamus were strongly activated in NMDA-induced seizures. The epilepsy-associated mutation of SIK1 canceled the pharmacological effects of the ACTH treatment on NMDA-induced seizures. These results suggest that SIK1 may be involved in the neuropathological mechanisms of NMDA-induced spasms and the pharmacological mechanism of ACTH treatment.

Novel mutation of SIK1 gene causing a mild form of pediatric epilepsy in a Chinese patient

Developmental and Epileptic Encephalopathy (DEE) is a group of disorders affecting children at early stages of infancy, which is characterized by frequent seizures, epileptiform activity on EEG, and developmental delayor regression. Developmental and epileptic encephalopathy-30 (DEE30) is a severe neurologic disorder characterized by onset of refractory seizures soon after birth or in the first months of life. Which was recently found to be caused by heterozygous mutations in the salt-inducible kinase SIK1. In this study, we investigated a patient with early onset epilepsy. DNA sequencing of the whole coding region revealed a de novel heterozygous nucleotide substitution (c.880G > A) causing a missense mutation (p.A294T). This mutation was classified as variant of unknown significance (VUS) by American College of Medical Genetics and Genomics (ACMG). To further investigate the pathogenicity and pathogenesis of this mutation, we established a human neuroblastoma cell line (SH-SY5Y) stably-expressing wild type SIK1 and A294T mutant, and compared the transcriptome and metabolomics profiles. We presented a pediatric patient suffering from infantile onset epilepsy. Early EEG showed a boundary dysfunction of activity and MRI scan of the brain was normal. The patient responded well to single anti-epileptic drug treatment. Whole-exome sequencing found a missense mutation of SIK1 gene (c.880G > A chr21: 43,420,326 p. A294T). Dysregulated transcriptome and metabolome in cell models expressing WT and MUT SIK1 confirmed the pathogenicity of the mutation. Specifically, we found MEF2C target genes, certain epilepsy causing genes and metabolites are dysregulated by SIK1 mutation. We found MEF2C target genes, certain epilepsy causing genes and metabolites are dysregulated by SIK1 mutation. Our finding further expanded the disease spectrum and provided novel mechanistic insights of DEE30.

NUAK Kinases: Brain-Ovary Axis

Liver kinase B (LKB1) and adenosine monophosphate (AMP)-activated protein kinase (AMPK) are two major kinases that regulate cellular metabolism by acting as adenosine triphosphate (ATP) sensors. During starvation conditions, LKB1 and AMPK activate different downstream pathways to increase ATP production, while decreasing ATP consumption, which abrogates cellular proliferation and cell death. Initially, LKB1 was considered to be a tumor suppressor due to its loss of expression in various tumor types. Additional studies revealed amplifications in LKB1 and AMPK kinases in several cancers, suggesting a role in tumor progression. The AMPK-related proteins were described almost 20 years ago as a group of key kinases involved in the regulation of cellular metabolism. As LKB1-downstream targets, AMPK-related proteins were also initially considered to function as tumor suppressors. However, further research demonstrated that AMPK-related kinases play a major role not only in cellular physiology but also in tumor development. Furthermore, aside from their role as regulators of metabolism, additional functions have been described for these proteins, including roles in the cell cycle, cell migration, and cell death. In this review, we aim to highlight the major role of AMPK-related proteins beyond their functions in cellular metabolism, focusing on cancer progression based on their role in cell migration, invasion, and cell survival. Additionally, we describe two main AMPK-related kinases, Novel (nua) kinase family 1 (NUAK1) and 2 (NUAK2), which have been understudied, but play a major role in cellular physiology and tumor development.

The Small-Molecule Inhibitor MRIA9 Reveals Novel Insights into the Cell Cycle Roles of SIK2 in Ovarian Cancer Cells

Simple Summary

The current standard therapy of ovarian cancers comprises a reductive surgery followed by a combination of taxane-platinum-based primary chemotherapy. However, despite an initial positive response, patients in the advanced stage showed relapse within months or even weeks. Thus, there is a need to find combinatorial therapies that permit overcoming the paclitaxel-associated resistance in patients. Here, we found that MRIA9, a newly developed small-molecule inhibitor of the salt-inducible-kinase 2, interferes with the cell division of cancer cells. More importantly, MRIA9 increases paclitaxel efficiency in eliminating ovarian cancer cells and patient derived cancer cells by inducing apoptosis or programmed cell death. Thus, our study indicates that MRIA9 might represent a novel therapeutical tool for translational studies to overcome paclitaxel resistance in ovarian cancer.

Abstract

The activity of the Salt inducible kinase 2 (SIK2), a member of the AMP-activated protein kinase (AMPK)-related kinase family, has been linked to several biological processes that maintain cellular and energetic homeostasis. SIK2 is overexpressed in several cancers, including ovarian cancer, where it promotes the proliferation of metastases. Furthermore, as a centrosome kinase, SIK2 has been shown to regulate the G2/M transition, and its depletion sensitizes ovarian cancer to paclitaxel-based chemotherapy. Here, we report the consequences of SIK2 inhibition on mitosis and synergies with paclitaxel in ovarian cancer using a novel and selective inhibitor, MRIA9. We show that MRIA9-induced inhibition of SIK2 blocks the centrosome disjunction, impairs the centrosome alignment, and causes spindle mispositioning during mitosis. Furthermore, the inhibition of SIK2 using MRIA9 increases chromosomal instability, revealing the role of SIK2 in maintaining genomic stability. Finally, MRIA9 treatment enhances the sensitivity to paclitaxel in 3D-spheroids derived from ovarian cancer cell lines and ovarian cancer patients. Our study suggests selective targeting of SIK2 in ovarian cancer as a therapeutic strategy for overcoming paclitaxel resistance.

Risperidone Mitigates Enhanced Excitatory Neuronal Function and Repetitive Behavior Caused by an ASD-Associated Mutation of SIK1

Six mutations in the salt-inducible kinase 1 (SIK1)-coding gene have been identified in patients with early infantile epileptic encephalopathy (EIEE-30) accompanied by autistic symptoms. Two of the mutations are non-sense mutations that truncate the C-terminal region of SIK1. It has been shown that the C-terminal-truncated form of SIK1 protein affects the subcellular distribution of SIK1 protein, tempting to speculate the relevance to the pathophysiology of the disorders. We generated SIK1-mutant (SIK1-MT) mice recapitulating the C-terminal-truncated mutations using CRISPR/Cas9-mediated genome editing. SIK1-MT protein was distributed in the nucleus and cytoplasm, whereas the distribution of wild-type SIK1 was restricted to the nucleus. We found the disruption of excitatory and inhibitory (E/I) synaptic balance due to an increase in excitatory synaptic transmission and enhancement of neural excitability in the pyramidal neurons in layer 5 of the medial prefrontal cortex in SIK1-MT mice. We also found the increased repetitive behavior and social behavioral deficits in SIK1-MT mice. The risperidone administration attenuated the neural excitability and excitatory synaptic transmission, but the disrupted E/I synaptic balance was unchanged, because it also reduced the inhibitory synaptic transmission. Risperidone also eliminated the repetitive behavior but not social behavioral deficits. These results indicate that risperidone has a role in decreasing neuronal excitability and excitatory synapses, ameliorating repetitive behavior in the SIK1-truncated mice.

Nuts and bolts of the salt-inducible kinases (SIKs)

The salt-inducible kinases, SIK1, SIK2 and SIK3, most closely resemble the AMP-activated protein kinase (AMPK) and other AMPK-related kinases, and like these family members they require phosphorylation by LKB1 to be catalytically active. However, unlike other AMPK-related kinases they are phosphorylated by cyclic AMP-dependent protein kinase (PKA), which promotes their binding to 14-3-3 proteins and inactivation. The most well-established substrates of the SIKs are the CREB-regulated transcriptional co-activators (CRTCs), and the Class 2a histone deacetylases (HDAC4/5/7/9). Phosphorylation by SIKs promotes the translocation of CRTCs and Class 2a HDACs to the cytoplasm and their binding to 14-3-3s, preventing them from regulating their nuclear binding partners, the transcription factors CREB and MEF2. This process is reversed by PKA-dependent inactivation of the SIKs leading to dephosphorylation of CRTCs and Class 2a HDACs and their re-entry into the nucleus. Through the reversible regulation of these substrates and others that have not yet been identified, the SIKs regulate many physiological processes ranging from innate immunity, circadian rhythms and bone formation, to skin pigmentation and metabolism. This review summarises current knowledge of the SIKs and the evidence underpinning these findings, and discusses the therapeutic potential of SIK inhibitors for the treatment of disease.

Salt Sensation and Regulation

Taste sensation and regulation are highly conserved in insects and mammals. Research conducted over recent decades has yielded major advances in our understanding of the molecular mechanisms underlying the taste sensors for a variety of taste sensations and the processes underlying regulation of ingestion depending on our internal state. Salt (NaCl) is an essential ingested nutrient. The regulation of internal sodium concentrations for physiological processes, including neuronal activity, fluid volume, acid–base balance, and muscle contraction, are extremely important issues in animal health. Both mammals and flies detect low and high NaCl concentrations as attractive and aversive tastants, respectively. These attractive or aversive behaviors can be modulated by the internal nutrient state. However, the differential encoding of the tastes underlying low and high salt concentrations in the brain remain unclear. In this review, we discuss the current view of taste sensation and modulation in the brain with an emphasis on recent advances in this field. This work presents new questions that include but are not limited to, “How do the fly’s neuronal circuits process this complex salt code?” and “Why do high concentrations of salt induce a negative valence only when the need for salt is low?” A better understanding of regulation of salt homeostasis could improve our understanding of why our brains enjoy salty food so much.

AMPKα-like proteins as LKB1 downstream targets in cell physiology and cancer

One of the key events in cancer development is the ability of tumor cells to overcome nutrient deprivation and hypoxia. Among proteins performing metabolic adaptation to the various cellular nutrient conditions, liver kinase B 1 (LKB1) and its main downstream target adenosine monophosphate (AMP)-activated protein kinase α (AMPKα) are important sensors of energy requirements within the cell. Although LKB1 was originally described as a tumor suppressor, given its role in metabolism, it potentially acts as a double-edged sword. AMPKα, a master regulator of cell energy demands, is activated when ATP level drops under a certain threshold, responding accordingly through its downstream targets. Twelve downstream kinase targets of LKB1 have been described as AMPKα-like proteins. This group is comprised of novel (nua) kinase family (NUAK) kinases (NUAK1 and 2) linked to cell cycle progression and ultraviolet (UV)-damage; microtubule affinity regulating kinases (MARKs) (MARK1, MARK2, MARK3, and MARK4) that are involved in cell polarity; salt inducible kinases (SIK) (SIK1, SIK2, also known as Qin-induced kinase or QIK and SIK3) that are implicated in cell metabolism and adipose tissue development and mitotic regulation; maternal embryonic leuzine zipper kinase (MELK) that regulate oocyte maturation; and finally brain selective kinases (BRSKs) (BRSK1 and 2), which have been mainly characterized in the brain due to their role in neuronal polarization. Thus, many efforts have been made in order to harness LKB1 kinase and its downstream targets as a possible therapeutic hub in tumor development and propagation. In this review, we describe LKB1 and its downstream target AMPK summarize major functions of various AMPK-like proteins, while focusing on biological functions of BRSK1 and 2 in different models.

Enhanced Meningeal Lymphatic Drainage Ameliorates Neuroinflammation and Hepatic Encephalopathy in Cirrhotic Rats

BACKGROUND & AIMS

Hepatic encephalopathy (HE) is a serious neurologic complication in patients with liver cirrhosis. Very little is known about the role of the meningeal lymphatic system in HE. We tested our hypothesis that enhancement of meningeal lymphatic drainage could decrease neuroinflammation and ameliorate HE.

METHODS

A 4-week bile duct ligation model was used to develop cirrhosis with HE in rats. Brain inflammation in patients with HE was evaluated by using archived GSE41919. The motor function of rats was assessed by the rotarod test. Adeno-associated virus 8-vascular endothelial growth factor C (AAV8-VEGF-C) was injected into the cisterna magna of HE rats 1 day after surgery to induce meningeal lymphangiogenesis.

RESULTS

Cirrhotic rats with HE showed significantly increased microglia activation in the middle region of the cortex (P < .001) as well as increased neuroinflammation, as indicated by significant increases in interleukin 1β, interferon γ, tumor necrosis factor α, and ionized calcium binding adaptor molecule 1 (Iba1) expression levels in at least 1 of the 3 regions of the cortex. Motor function was also impaired in rats with HE (P < .05). Human brains of patients with cirrhosis with HE also exhibited up-regulation of proinflammatory genes (NFKB1, IbA1, TNF-α, and IL1β) (n = 6). AAV8-VEGF-C injection significantly increased meningeal lymphangiogenesis (P = .035) and tracer dye uptake in the anterior and middle regions of the cortex (P = .006 and .003, respectively), their corresponding meninges (P = .086 and .006, respectively), and the draining lymph nodes (P = .02). Furthermore, AAV8-VEGF-C decreased microglia activation (P < .001) and neuroinflammation and ameliorated motor dysfunction (P = .024).

CONCLUSIONS

Promoting meningeal lymphatic drainage and enhancing waste clearance improves HE. Manipulation of meningeal lymphangiogenesis could be a new therapeutic strategy for the treatment of HE.

The Selective SIK2 Inhibitor ARN-3236 Produces Strong Antidepressant-Like Efficacy in Mice via the Hippocampal CRTC1-CREB-BDNF Pathway

Depression is a widespread chronic medical illness affecting thoughts, mood, and physical health. However, the limited and delayed therapeutic efficacy of monoaminergic drugs has led to intensive research efforts to develop novel antidepressants. ARN-3236 is the first potent and selective inhibitor of salt-inducible kinase 2 (SIK2). In this study, a multidisciplinary approach was used to explore the antidepressant-like actions of ARN-3236 in mice. Chronic social defeat stress (CSDS) and chronic unpredictable mild stress (CUMS) models of depression, various behavioral tests, high performance liquid chromatography-tandem mass spectrometry, stereotactic infusion, viral-mediated gene transfer, western blotting, co-immunoprecipitation and immunofluorescence were used together. It was found that ARN-3236 could penetrate the blood-brain barrier. Repeated ARN-3236 administration induced significant antidepressant-like effects in both the CSDS and CUMS models of depression, accompanied with fully preventing the stress-enhanced SIK2 expression and cytoplasmic translocation of cyclic adenosine monophosphate response element binding protein (CREB)-regulated transcription coactivator 1 (CRTC1) in the hippocampus. ARN-3236 treatment also completely reversed the down-regulating effects of CSDS and CUMS on the hippocampal brain-derived neurotrophic factor (BDNF) system and neurogenesis. Moreover, we demonstrated that the hippocampal CRTC1-CREB-BDNF pathway mediated the antidepressant-like efficacy of ARN-3236. Collectively, ARN-3236 possesses strong protecting effects against chronic stress, and could be a novel antidepressant beyond monoaminergic drugs.

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.

Salt-Inducible Kinase 2: An Oncogenic Signal Transmitter and Potential Target for Cancer Therapy

Salt-inducible kinase (SIK), which belongs to the sucrose non-fermenting 1/AMP-activated protein kinase family, was first discovered in the adrenal cortex of a rat on a high-salt diet. As an isoform of the SIK family, SIK2 modulates various biological functions and acts as a signal transmitter in various pathways. Compared with that in adjacent normal tissues, the expression of SIK2 is significantly higher in multiple types of tumors, which indicates its pivotal effect in oncogenesis. Studies on SIK2 have recently underlined its role in several signaling pathways, including the PI3K-Akt-mTOR pathway, the Hippo-YAP pathway, the LKB1-HDAC axis, and the cAMP-PKA axis. Moreover, a few small-molecule SIK2 inhibitors have been found to be able to rescue the oncogenicity of SIK2 during tumor development and reverse its abnormal activation of downstream pathways. In this mini-review, we discuss the results of in vivo and in vitro studies regarding the SIK2 mechanism in different signaling pathways, particularly their regulation of cancer cells. This work may provide new ideas for targeting SIK2 as a novel therapeutic strategy in tumor therapy.

The Salt-Inducible Kinases: Emerging Metabolic Regulators

The discovery of liver kinase B1 (LKB1) as an upstream kinase for AMP-activated protein kinase (AMPK) led to the identification of several related kinases that also rely on LKB1 for their catalytic activity. Among these, the salt-inducible kinases (SIKs) have emerged as key regulators of metabolism. Unlike AMPK, SIKs do not respond to nucleotides, but their function is regulated by extracellular signals, such as hormones, through complex LKB1-independent mechanisms. While AMPK acts on multiple targets, including metabolic enzymes, to maintain cellular ATP levels, SIKs primarily regulate gene expression, by acting on transcriptional regulators, such as the cAMP response element-binding protein-regulated transcription coactivators and class IIa histone deacetylases. This review describes the development of research on SIKs, from their discovery to the most recent findings on metabolic regulation.

Bosutinib Attenuates Inflammation via Inhibiting Salt-Inducible Kinases in Experimental Model of Intracerebral Hemorrhage on Mice

BACKGROUND AND PURPOSE

Intracerebral hemorrhage (ICH) is a subtype of stroke with highest mortality and morbidity. Pronounced inflammation plays a significant role in the development of the secondary brain injury after ICH. Recently, SIK-2 (salt-inducible kinase-2) was identified as an important component controlling inflammatory response. Here we sought to investigate the role of SIK-2 in post-ICH inflammation and potential protective effects of SIK-2 inhibition after ICH.

METHODS

Two hundred and ninety-three male CD-1 mice were used. ICH was induced via injection of 30 μL of autologous blood. Recombinant SIK-2 was administrated 1 hour after ICH intracerebroventricularly. SIK-2 small interfering RNA was injected intracerebroventricularly 24 hours before ICH. Bosutinib, a clinically approved tyrosine kinase inhibitor with affinity to SIK-2, was given intranasally 1 hour or 6 hours after ICH. Effects of treatments were evaluated by neurological tests and brain water content calculation. Molecular pathways were investigated by Western blots and immunofluorescence studies.

RESULTS

Endogenous SIK-2 was expressed in microglia and neurons. SIK-2 expression was reduced after ICH. Exogenous SIK-2 aggravated post-ICH inflammation, leading to brain edema and the neurobehavioral deficits. SIK-2 inhibition attenuated post-ICH inflammation, reducing brain edema and ameliorating neurological dysfunctions. Bosutinib inhibited SIK-2-attenuating ICH-induced brain damage. Protective effects of Bosutinib were mediated, at least partly, by CRTC3 (cyclic amp-response element binding protein-regulated transcription coactivator 3)/cyclic amp-response element binding protein/NF-κB (nuclear factor-κB) pathway.

CONCLUSIONS

SIK-2 participates in inflammation induction after ICH. SIK-2 inhibition via Bosutinib or small interfering RNA decreased inflammation, attenuating brain injury. SIK-2 effects are, at least partly, mediated by CRTC3-cyclic amp-response element binding protein-NF-κB signaling pathway.

A Novel Role of Salt-Inducible Kinase 1 (SIK1) in the Post-Translational Regulation of Scavenger Receptor Class B Type 1 Activity

Salt-inducible kinase 1 (SIK1) is a serine/threonine kinase that belongs to the stress- and energy-sensing AMPK family of kinases. SIK1 expression is rapidly induced in Y1 adrenal cells in response to ACTH via the cAMP-PKA signaling cascade, and it has been suggested that an increased level of SIK1 expression inhibits adrenal steroidogenesis by repressing the cAMP-dependent transcription of steroidogenic proteins, CYP11A1 and StAR, by attenuating CREB transcriptional activity. Here we show that SIK1 stimulates adrenal steroidogenesis by modulating the selective HDL-CE transport activity of SR-B1. Overexpression of SIK1 increases cAMP-stimulated and SR-B1-mediated selective HDL-BODIPY-CE uptake in cell lines without impacting SR-B1 protein levels, whereas knockdown of SIK1 attenuated cAMP-stimulated selective HDL-BODIPY-CE uptake. SIK1 forms a complex with SR-B1 by interacting with its cytoplasmic C-terminal domain, and in vitro kinase activity measurements indicate that SIK1 can phosphorylate the C-terminal domain of SR-B1. Among potential phosphorylation sites, SIK1-catalyzed phosphorylation of Ser496 is critical for SIK1 stimulation of the selective CE transport activity of SR-B1. Mutational studies further demonstrated that both the intact catalytic activity of SIK1 and its PKA-catalyzed phosphorylation are essential for SIK1 stimulation of SR-B1 activity. Finally, overexpression of SIK1 caused time-dependent increases in SR-B1-mediated and HDL-supported steroid production in Y1 cells; however, these effects were lost with knockdown of SR-B1. Taken together, these studies establish a role for SIK1 in the positive regulation of selective HDL-CE transport function of SR-B1 and steroidogenesis and suggest a potential mechanism for SIK1 signaling in modulating SR-B1-mediated selective CE uptake and associated steroidogenesis.

Salt-inducible kinase 1 is involved in high glucose-induced mesangial cell proliferation mediated by the ALK5 signaling pathway

High glucose levels can induce mesangial cell proliferation and extracellular matrix (ECM) accumulation through the type I activin receptor-like kinase 5 (ALK5) signaling pathway. Salt-inducible kinase 1 (SIK1) prevents fibrosis by downregulating ALK5, while the expression level of the SIK1 protein itself is downregulated by glucose in neuronal cells following ischemia. In this study, we investigated the correlation between SIK1 and the ALK5 signaling pathway in a rat glomerular mesangial cell line (HBZY-1 cells). We found that high glucose levels downregulated the expression level of SIK1 and suppressed the phosphorylation of SIK1 at Thr-182. The downregulation of SIK1 by high glucose was accompanied by the activation of the ALK5 signaling pathway, while the overexpression of SIK1 in the HBZY-1 cells resulted in a decrease in the ALK5 protein level, as well in the levels of its downstream targets, including fibronectin and plasminogen activator inhibitor type I. In conclusion, high glucose may activate the ALK5 signaling pathway by downregulating SIK1, and SIK1 may be a protective factor against cellular proliferation and ECM accumulation in glomerular mesangial cells under high glucose conditions.

The tumor suppressor kinase LKB1 activates the downstream kinases SIK2 and SIK3 to stimulate nuclear export of class IIa histone deacetylases

Histone deacetylases 4 (HDAC4), -5, -7, and -9 form class IIa within the HDAC superfamily and regulate diverse physiological and pathological cellular programs. With conserved motifs for phosphorylation-dependent 14-3-3 binding, these deacetylases serve as novel signal transducers that are able to modulate histone acetylation and gene expression in response to extracellular cues. Here, we report that in a PKA-sensitive manner the tumor suppressor kinase LKB1 acts through salt-inducible kinase 2 (SIK2) and SIK3 to promote nucleocytoplasmic trafficking of class IIa HDACs. Both SIK2 and SIK3 phosphorylate the deacetylases at the conserved motifs and stimulate 14-3-3 binding. SIK2 activates MEF2-dependent transcription and relieves repression of myogenesis by the deacetylases. Distinct from SIK2, SIK3 induces nuclear export of the deacetylases independent of kinase activity and 14-3-3 binding. These findings highlight the difference among members of the SIK family and indicate that LKB1-dependent SIK activation constitutes an important signaling module upstream from class IIa deacetylases for regulating cellular programs controlled by MEF2 and other transcription factors.

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.

Possible role of brain salt-inducible kinase 1 in responses to central sodium in Dahl rats

In Dahl salt-sensitive (S) rats, Na+ entry into the cerebrospinal fluid (CSF) and sympathoexcitatory and pressor responses to CSF Na+ are enhanced. Salt-inducible kinase 1 (SIK1) increases Na+/K+-ATPase activity in kidney cells. We tested the possible role of SIK1 in regulation of CSF [Na+] and responses to Na+ in the brain. SIK1 protein and activity were lower in hypothalamic tissue of Dahl S (SS/Mcw) compared with salt-resistant SS.BN13 rats. Intracerebroventricular infusion of the protein kinase inhibitor staurosporine at 25 ng/day, to inhibit SIK1 further increased mean arterial pressure (MAP) and HR but did not affect the increase in CSF [Na+] or hypothalamic aldosterone in Dahl S on a high-salt diet. Intracerebroventricular infusion of Na+-rich artificial CSF caused significantly larger increases in renal sympathetic nerve activity, MAP, and HR in Dahl S vs. SS.BN13 or Wistar rats on a normal-salt diet. Intracerebroventricular injection of 5 ng staurosporine enhanced these responses, but the enhancement in Dahl S rats was only one-third that in SS.BN13 and Wistar rats. Staurosporine had no effect on MAP and HR responses to intracerebroventricular ANG II or carbachol, whereas the specific protein kinase C inhibitor GF109203X inhibited pressor responses to intracerebroventricular Na+-rich artificial CSF or ANG II. These results suggest that the SIK1-Na+/K+-ATPase network in neurons acts to attenuate sympathoexcitatory and pressor responses to increases in brain [Na+]. The lower hypothalamic SIK1 activity and smaller effect of staurosporine in Dahl S rats suggest that impaired activation of neuronal SIK1 by Na+ may contribute to their enhanced central responses to sodium.

Cocaine induces the expression of MEF2C transcription factor in rat striatum through activation of SIK1 and phosphorylation of the histone deacetylase HDAC5

Distinct forms of MEF2 transcription factor act as positive or negative regulators of dendritic spine formation, with MEF2C playing a key regulator role in synapse plasticity. We report here that acute cocaine treatment of rats induced the expression of MEF2C in the striatum through a recently discovered transduction pathway. Repeated injections were found to induce MEF2C to a lesser extent. The mechanism by which MEF2C was induced involves the subsequent activation of the salt-inducible kinase SIK1 and the phosphorylation of HDAC5, a member of the class IIa of HDACs. Cocaine activated SIK1 by phosphorylation on Thr-182 residue, which was accompanied by the nuclear import of the kinase. In the nuclear compartment, SIK1 then phosphorylated HDAC5 causing the shuttling of its phospho-form from the nucleus to the cytoplasm of striatal cells. Activation of SIK1 by cocaine was further validated by the phosphorylation of TORC1/3, which was followed by the shuttling of TORC proteins from the nucleus to the cytoplasm. Activation of MEF2C was assessed by measuring the expression of the MEF2C gene itself, since the gene is known to be under the control of its own product. Since MEF2C plays a key role in memory/learning processes, activation of this pathway by cocaine is probably involved in plasticity mechanisms whereby the drug establishes its long-term effects such as drug dependence.

Salt-inducible kinase 1 is present in lung alveolar epithelial cells and regulates active sodium transport

Salt-inducible kinase 1 (SIK1) in epithelial cells mediates the increases in active sodium transport (Na(+), K(+)-ATPase-mediated) in response to elevations in the intracellular concentration of sodium. In lung alveolar epithelial cells increases in active sodium transport in response to β-adrenergic stimulation increases pulmonary edema clearance. Therefore, we sought to determine whether SIK1 is present in lung epithelial cells and to examine whether isoproterenol-dependent stimulation of Na(+), K(+)-ATPase is mediated via SIK1 activity. All three SIK isoforms were present in airway epithelial cells, and in alveolar epithelial cells type 1 and type 2 from rat and mouse lungs, as well as from human and mouse cell lines representative of lung alveolar epithelium. In mouse lung epithelial cells, SIK1 associated with the Na(+), K(+)-ATPase α-subunit, and isoproterenol increased SIK1 activity. Isoproterenol increased Na(+), K(+)-ATPase activity and the incorporation of Na(+), K(+)-ATPase molecules at the plasma membrane. Furthermore, those effects were abolished in cells depleted of SIK1 using shRNA, or in cells overexpressing a SIK1 kinase-deficient mutant. These results provide evidence that SIK1 is present in lung epithelial cells and that its function is relevant for the action of isoproterenol during regulation of active sodium transport. As such, SIK1 may constitute an important target for drug discovery aimed at improving the clearance of pulmonary edema.

SIK2 is a key regulator for neuronal survival after ischemia via TORC1-CREB

The cAMP responsive element-binding protein (CREB) functions in a broad array of biological and pathophysiological processes. We found that salt-inducible kinase 2 (SIK2) was abundantly expressed in neurons and suppressed CREB-mediated gene expression after oxygen-glucose deprivation (OGD). OGD induced the degradation of SIK2 protein concomitantly with the dephosphorylation of the CREB-specific coactivator transducer of regulated CREB activity 1 (TORC1), resulting in the activation of CREB and its downstream gene targets. Ca(2+)/calmodulin-dependent protein kinase I/IV are capable of phosphorylating SIK2 at Thr484, resulting in SIK2 degradation in cortical neurons. Neuronal survival after OGD was significantly increased in neurons isolated from sik2(-/-) mice, and ischemic neuronal injury was significantly reduced in the brains of sik2(-)(/-) mice subjected to transient focal ischemia. These findings suggest that SIK2 plays critical roles in neuronal survival, is modulated by CaMK I/IV, and regulates CREB via TORC1.

Drosophila salt-inducible kinase (SIK) regulates starvation resistance through cAMP-response element-binding protein (CREB)-regulated transcription coactivator (CRTC)

Salt-inducible kinase (SIK), one of the AMP-activated kinase (AMPK)-related kinases, has been suggested to play important functions in glucose homeostasis by inhibiting the cAMP-response element-binding protein (CREB)-regulated transcription coactivator (CRTC). To examine the role of SIK in vivo, we generated Drosophila SIK mutant and found that the mutant flies have higher amounts of lipid and glycogen stores and are resistant to starvation. Interestingly, SIK transcripts are highly enriched in the brain, and we found that neuron-specific expression of exogenous SIK fully rescued lipid and glycogen storage phenotypes as well as starvation resistance of the mutant. Using genetic and biochemical analyses, we demonstrated that CRTC Ser-157 phosphorylation by SIK is critical for inhibiting CRTC activity in vivo. Furthermore, double mutants of SIK and CRTC became sensitive to starvation, and the Ser-157 phosphomimetic mutation of CRTC reduced lipid and glycogen levels in the SIK mutant, suggesting that CRTC mediates the effects of SIK signaling. Collectively, our results strongly support the importance of the SIK-CRTC signaling axis that functions in the brain to maintain energy homeostasis in Drosophila.

Role of salt-induced kinase 1 in androgen neuroprotection against cerebral ischemia

Androgens within physiological ranges protect castrated male mice from cerebral ischemic injury. Yet, underlying mechanisms are unclear. Here, we report that, after middle cerebral artery occlusion (MCAO), salt-induced kinase 1 (SIK1) was induced by a potent androgen-dihydrotestosterone (DHT) at protective doses. To investigate whether SIK1 contributes to DHT neuroprotection after cerebral ischemia, we constructed lentivirus-expressing small interference RNA (siRNA) against SIK1. The SIK1 knockdown by siRNA exacerbated oxygen-glucose deprivation (OGD)-induced cell death in primary cortical neurons, suggesting that SIK1 is an endogenous neuroprotective gene against cerebral ischemia. Furthermore, lentivirus-mediated SIK1 knockdown increased both cortical and striatal infarct sizes in castrated mice treated with a protective dose of DHT. Earlier studies show that SIK1 inhibits histone deacetylase (HDAC) activities by acting as a class IIa HDAC kinase. We observed that SIK1 knockdown decreased histone H3 acetylation in primary neurons. The SIK1 siRNA also exacerbated OGD-induced neuronal death in the presence of trichostatin A (TSA), an HDAC inhibitor, and decreased histone H3 acetylation at 4 hours reoxygenation in TSA-treated neurons. Finally, we showed that DHT at protective doses prevented ischemia-induced histone deacetylation after MCAO. Our finding suggests that SIK1 contributes to neuroprotection by androgens within physiological ranges by inhibiting histone deacetylation.

Salt-inducible kinase 1 in the rat pinealocyte: adrenergic regulation and role in arylalkylamine N-acetyltransferase gene transcription

The recognition of the basic leucine zipper domain in the regulation of transcriptional activity of cAMP response element-binding protein by salt-inducible kinase (SIK) prompted our investigation of the regulatory role of this kinase in the induction of Aa-nat and other cAMP-regulated genes in the rat pineal gland. Here we report Sik1 expression was induced by norepinephrine (NE) in rat pinealocytes primarily through activation of beta-adrenergic receptors, with a minor contribution from activation of alpha-adrenergic receptors. Treatments with dibutyryl cAMP, and to a lesser extent, agents that elevate intracellular Ca(2+) mimicked the effect of NE on Sik1 expression. In parallel to the results of the pineal cell culture studies, a marked nocturnal induction of Sik1 transcription was found in whole-animal studies. Knockdown of Sik1 by short hairpin RNA amplified the NE-stimulated Aa-nat transcription and other adrenergic-regulated genes, including Mapk phosphatase 1, inducible cAMP repressor, and type 2 iodothyronine deiodinase in a time-dependent manner. In contrast, overexpressing Sik1 had an inhibitory effect on the NE induction of Aa-nat and other adrenergic-regulated genes. Together, our results indicate that the adrenergic induction of Sik1 in the rat pineal gland is primarily through the beta-adrenergic receptor --> protein kinase A pathway. SIK1 appears to function as part of an endogenous repressive mechanism that regulates the peak and indirectly the duration of expression of Aa-nat and other cAMP-regulated genes. These findings support a role for SIK1 in framing the temporal expression profile of Aa-nat and other adrenergic-regulated genes in the rat pineal gland.

SIK1/SOS2 networks: decoding sodium signals via calcium-responsive protein kinase pathways

Changes in cellular ion levels can modulate distinct signaling networks aimed at correcting major disruptions in ion balances that might otherwise threaten cell growth and development. Salt-inducible kinase 1 (SIK1) and salt overly sensitive 2 (SOS2) are key protein kinases within such networks in mammalian and plant cells, respectively. In animals, SIK1 expression and activity are regulated in response to the salt content of the diet, and in plants SOS2 activity is controlled by the salinity of the soil. The specific ionic stress (elevated intracellular sodium) is followed by changes in intracellular calcium; the calcium signals are sensed by calcium-binding proteins and lead to activation of SIK1 or SOS2. These kinases target major plasma membrane transporters such as the Na⁺,K⁺-ATPase in mammalian cells, and Na⁺/H⁺ exchangers in the plasma membrane and membranes of intracellular vacuoles of plant cells. Activation of these networks prevents abnormal increases in intracellular sodium concentration.

Importance of autophosphorylation at Ser186 in the A-loop of salt inducible kinase 1 for its sustained kinase activity

Autophosphorylation is an important mechanism by which protein kinases regulate their own biological activities. Salt inducible kinase 1 (SIK1) is a regulator in the feedback cascades of cAMP-mediated gene expression, while its kinase domain also features autophosphorylation activity. We provide evidence that Ser186 in the activation loop is the site of autophosphorylation and essential for the kinase activity. Ser186 is located at the +4 position of the critical Thr residue Thr182, which is phosphorylated by upstream kinases such as LKB1. The relationship between phosphorylation at Ser186 and at Thr182 in COS-7 cells indicates that the former is a prerequisite for the latter. Glycogen synthase kinase-3beta (GSK-3beta) phosphorylates Ser/Thr residues located at the fourth position ahead of the pre-phosphorylated Ser/Thr residues, and inhibitors of GSK-3beta reduce the phosphorylation at Thr182. The results of an in vitro reconstitution assay also indicate that GSK-3beta could be the SIK1 kinase. However, overexpression and knockdown of GSK-3beta in LKB1-defective HeLa cells suggests that GSK-3beta alone may not be able to phosphorylate or activate SIK1, indicating that LKB1 may play a crucial role by phosphorylating SIK1 at Thr182, possibly as an initiator of the autophosphorylation cascade, and GSK-3beta may phosphorylate SIK1 at Thr182 by recognizing the priming-autophosphorylation at Ser186 in cultured cells. This may also be the case for the other isoform SIK2, but not for SIK3.

The MAPK pathway is required for depolarization-induced "promiscuous" immediate-early gene expression but not for depolarization-restricted immediate-early gene expression in neurons

Depolarization, growth factors, neurotrophins, and other stimuli induce expression of immediate early genes (IEGs) in neurons. We identified a subset of IEGs, IPD-IEGs, which are induced preferentially by depolarization, but not by neurotrophins or growth factors, in PC12 cells. The "promiscuous" IEGs Egr1 and c-fos, induced by growth factors and neurotrophins, in addition to depolarization, require activation of the MAP kinase signaling pathway for induction in response to KCl depolarization in PC12 cells; MEK1/2 inhibitors block KCl-induced Egr1 and c-fos expression. In contrast, MEK1/2 inhibition has no effect on KCl-induced expression of the known IPD-IEGs in PC12 cells. Additional "candidate" IDP-IEGs were identified by a microarray comparison of genes induced by KCl in the presence vs. the absence of an MEK1/2 inhibitor in PC12 cells. Northern blot analyses demonstrated that representative newly identified candidate IPD-IEGs, as with the known IPD-IEGs, are also induced by a MAP kinase- independent pathway in response to depolarization, both in PC12 cells and in rat primary cortical neurons. Nerve growth factor and epidermal growth factor are unable to induce the expression of the Crem/Icer, Nur77, Nor1, Rgs2, Dusp1 (Mkp1), and Dscr1 genes in PC12 cells, validating their identification as IPD-IEGs. Inhibiting calcium/calmodulin-dependent kinase II (CaMKII), calcineurin, or protein kinase A (PKA) activity prevents KCl-induced IPD-IEG mRNA accumulation, suggesting that the IPD-IEG genes are induced by depolarization in neurons via a combination of calcineurin/PKA- and CaMKII-dependent pathways.

SIK1 is part of a cell sodium-sensing network that regulates active sodium transport through a calcium-dependent process

In mammalian cells, active sodium transport and its derived functions (e.g., plasma membrane potential) are dictated by the activity of the Na(+),K(+)-ATPase (NK), whose regulation is essential for maintaining cell volume and composition, as well as other vital cell functions. Here we report the existence of a salt-inducible kinase-1 (SIK1) that associates constitutively with the NK regulatory complex and is responsible for increases in its catalytic activity following small elevations in intracellular sodium concentrations. Increases in intracellular sodium are paralleled by elevations in intracellular calcium through the reversible Na(+)/Ca(2+) exchanger, leading to the activation of SIK1 (Thr-322 phosphorylation) by a calcium calmodulin-dependent kinase. Activation of SIK1 results in the dephosphorylation of the NK alpha-subunit and an increase in its catalytic activity. A protein phosphatase 2A/phosphatase methylesterase-1 (PME-1) complex, which constitutively associates with the NK alpha-subunit, is activated by SIK1 through phosphorylation of PME-1 and its dissociation from the complex. These observations illustrate the existence of a distinct intracellular signaling network, with SIK1 at its core, which is triggered by a monovalent cation (Na(+)) and links sodium permeability to its active transport.

TORC-SIK cascade regulates CREB activity through the basic leucine zipper domain

The transcription factor cAMP response element-binding protein (CREB) plays important roles in gene expression induced by cAMP signaling and is believed to be activated when its Ser133 is phosphorylated. However, the discovery of Ser133-independent activation by the activation of transducer of regulated CREB activity coactivators (TORC) and repression by salt inducible kinase cascades suggests that Ser133-independent regulation of CREB is also important. The activation and repression are mediated by the basic leucine zipper domain of CREB. In this review, we focus on the basic leucine zipper domain in the regulation of transcriptional activity of CREB and describe the functions of TORC and salt inducible kinase.

LKB1-dependent signaling pathways

This review focuses on remarkable recent findings concerning the mechanism by which the LKB1 protein kinase that is mutated in Peutz-Jeghers cancer syndrome operates as a tumor suppressor. We discuss evidence that the cellular localization and activity of LKB1 is controlled through its interaction with a catalytically inactive protein resembling a protein kinase, termed STRAD, and an armadillo repeat-containing protein, named mouse protein 25 (MO25). The data suggest that LKB1 functions as a tumor suppressor by not only inhibiting proliferation, but also by exerting profound effects on cell polarity and, most unexpectedly, on the ability of a cell to detect and respond to low cellular energy levels. Genetic and biochemical findings indicate that LKB1 exerts its effects by phosphorylating and activating 14 protein kinases, all related to the AMP-activated protein kinase. The work described in this review shows how a study of an obscure cancer syndrome can uncover new and important regulatory pathways, relevant to the understanding of multiple human diseases.

KIN-29 SIK regulates chemoreceptor gene expression via an MEF2 transcription factor and a class II HDAC

The expression of individual chemoreceptor (CR) genes in Caenorhabditis elegans is regulated by multiple environmental and developmental cues, possibly enabling C. elegans to modulate its sensory responses. We had previously shown that KIN-29, a member of the salt-inducible kinase family, acts in a subset of chemosensory neurons to regulate the expression of CR genes, body size and entry into the alternate dauer developmental stage. Here, we show that KIN-29 regulates these processes by phosphorylating the HDA-4 class II histone deacetylase (HDAC) and inhibiting the gene repression functions of HDA-4 and an MEF-2 MADS domain transcription factor. MEF-2 binds directly to the CR gene regulatory sequences, and is required only to repress but not activate CR gene expression. A calcineurin phosphatase antagonizes the KIN-29/MEF-2-regulated pathway to modulate levels of CR gene expression. Our results identify KIN-29 as a new regulator of MEF2/HDAC functions in the nervous system, reveal cell-specific mechanisms of action of this pathway in vivo and demonstrate remarkable complexity in the regulation of CR gene expression in C. elegans.

14-3-3 cooperates with LKB1 to regulate the activity and localization of QSK and SIK

The LKB1 tumour suppressor kinase phosphorylates and activates a number of protein kinases belonging to the AMP-activated protein kinase (AMPK) subfamily. We have used a modified tandem affinity purification strategy to identify proteins that interact with AMPKalpha, as well as the twelve AMPK-related kinases that are activated by LKB1. The AMPKbeta and AMPKgamma regulatory subunits were associated with AMPKalpha, but not with any of the AMPK-related kinases, explaining why AMP does not influence the activity of these enzymes. In addition, we identified novel binding partners that interacted with one or more of the AMPK subfamily enzymes, including fat facets/ubiquitin specific protease-9 (USP9), AAA-ATPase-p97, adenine nucleotide translocase, protein phosphatase 2A holoenzyme and isoforms of the phospho-protein binding adaptor 14-3-3. Interestingly, the 14-3-3 isoforms bound directly to the T-loop Thr residue of QSK and SIK, after these were phosphorylated by LKB1. Consistent with this, the 14-3-3 isoforms failed to interact with non-phosphorylated QSK and SIK, in LKB1 knockout muscle or in HeLa cells in which LKB1 is not expressed. Moreover, mutation of the T-loop Thr phosphorylated by LKB1, prevented QSK and SIK from interacting with 14-3-3 in vitro. Binding of 14-3-3 to QSK and SIK, enhanced catalytic activity towards the TORC2 protein and the AMARA peptide, and was required for the cytoplasmic localization of SIK and for localization of QSK to punctate structures within the cytoplasm. To our knowledge, this study provides the first example of 14-3-3 binding directly to the T-loop of a protein kinase and influencing its catalytic activity and cellular localization.

MAPKAP kinase-2 is a primary response gene induced by depolarization in PC12 cells and in brain

Using a combination of targeted differential display for induced protein kinases and differential library screening, we identified mitogen-activated protein kinase activated protein kinase 2 (MAPKAPK2), as a primary response gene whose transcription is stimulated by membrane depolarization and by forskolin in rat PC12 pheochromocytoma cells. MAPKAPK3 was neither induced nor repressed by similar treatments. The increase in MAPKAPK2 mRNA is preceded by an increase in a MAPKAPK2 intron-containing RNA precursor, indicating that the increase in message is due at least in part to increased transcription. The open reading frame of full-length rat MAPKAPK2 cDNA is 99% identical to mouse MAPKAPK2 and 92% identical to human MAPKAPK2. The human MAPKAPK2 predicted protein contains 14 additional amino acids in the proline-rich N-terminal domain, when compared to murine and rat MAPKAPK2 predicted proteins. The MAPKAPK2 form found in PC12 cells corresponds to variant 2 in the human; this ortholog carries a nuclear translocation signal near its C-terminus. MAPKAPK2 message is also induced in the dentate gyrus, CA1, and CA3 of the rat hippocampus between 2-4 hr after the onset of kainic acid-induced seizures.

Salt-inducible kinase-1 represses cAMP response element-binding protein activity both in the nucleus and in the cytoplasm

Salt-inducible kinase-1 (SIK1) is phosphorylated at Ser577 by protein kinase A in adrenocorticotropic hormone-stimulated Y1 cells, and the phospho-SIK1 translocates from the nucleus to the cytoplasm. The phospho-SIK1 is dephosphorylated in the cytoplasm and re-enters the nucleus several hours later. By using green-fluorescent protein-tagged SIK1 fragments, we found that a peptide region (586-612) was responsible for the nuclear localization of SIK1. The region was named the 'RK-rich region' because of its Arg- and Lys-rich nature. SIK1s mutated in the RK-rich region were localized mainly in the cytoplasm. Because SIK1 represses cAMP-response element (CRE)-mediated transcription of steroidogenic genes, the mutants were examined for their effect on transcription. To our surprise, the cytoplasmic mutants strongly repressed the CRE-binding protein (CREB) activity, the extent of repression being similar to that of SIK1(S577A), a mutant localized exclusively in the nucleus. Several chimeras were constructed from SIK1 and from its isoform SIK2, which was localized mainly in the cytoplasm, and they were examined for intracellular localization as well as CREB-repression activity. A SIK1-derived chimera, where the RK-rich region had been replaced with the corresponding region of SIK2, was found in the cytoplasm, its CREB-modulating activity being similar to that of wild-type SIK1. On the other hand, a SIK2-derived chimera with the RK-rich region of SIK1 was localized in both the nucleus and the cytoplasm, and had a CREB-repressing activity similar to that of the wild-type SIK2. Green fluorescent protein-fused transducer of regulated CREB activity 2 (TORC2), a CREB-specific co-activator, was localized in the cytoplasm and nucleus of Y1 cells, and, after treatment with adrenocorticotropic hormone, cytoplasmic TORC2 entered the nucleus, activating CREB. The SIK1 mutants, having a strong CRE-repressing activity, completely inhibited the adrenocorticotropic hormone-induced nuclear entry of green fluorescent protein-fused TORC2. This suggests that SIK1 may regulate the intracellular movement of TORC2, and as a result modulates the CREB-dependent transcription activity. Together, these results indicate that the RK-rich region of SIK1 is important for determining the nuclear localization and attenuating CREB-repressing activity, but the degree of the nuclear localization of SIK1 itself does not necessarily reflect the degree of SIK1-mediated CREB repression.

Synaptotagmin IV overexpression inhibits depolarization-induced exocytosis in PC12 cells

Depolarization-induced vesicle exocytosis is a complex mechanism involving a number of proteins. In this process, synaptotagmins work as members of the Ca(2+)-sensing system that triggers the fusion of the synaptic vesicle with the plasma membrane. Synaptotagmin IV (SytIV), an immediate-early gene induced by depolarization in PC12 pheochromocytoma cells and in the hippocampus, has been suggested to work as a negative regulator of neurotransmitter release. Unlike other synaptotagmins, SytIV has an evolutionarily conserved substitution of an aspartate to a serine in the Ca(2+) coordination site of its C2A domain, preventing SytIV from binding anionic lipids in a Ca(2+)-dependent fashion. We used the secretion of human growth hormone (hGH) as a reporter system with which to examine the effects of overexpressing SytIV and other depolarization-induced immediate-early genes (the protein kinases KID-1, SIK, and PIM-1 and the transcription factors rTLE3 and Nurr1) on depolarization-induced vesicle exocytosis in PC12 cells. SytIV overexpression resulted in decreased depolarization-induced hGH release. However, conversion of the unique serine in SytIV to an aspartate eliminated this inhibitory activity. In addition, rTLE3 overexpression produced only a modest increase in spontaneous vesicle exocytosis, whereas KID-1, SIK, PIM-1, and Nurr1 overexpression had no effect on depolarization-induced exocytosis.

snf1lk encodes a protein kinase that may function in cell cycle regulation

msk, myocardial SNF1-like kinase, was originally isolated in a screen for kinases expressed during early cardiogenesis in the mouse. msk maps to the proximal end of mouse chromosome 17 in a region that is syntenic with human chromosome 21q22.3, where the gene for SNF1LK, a predicted protein that shares 80% identity at the amino acid level with Msk, is located. Accordingly, msk has been redesignated snf1lk. Interestingly, the region encompassing the SNF1LK locus has been implicated in congenital heart defects often observed in patients with Down syndrome. snf1lk is also expressed in skeletal muscle progenitor cells of the somite beginning at 9.5 dpc. These data suggest a more general role for snf1lk in the earliest stages of muscle growth and/or differentiation. Consistent with a role in cell cycling, we observe that Chinese hamster ovary cells that express a tetracycline-inducible SNF1LK kinase domain do not divide, but undergo additional rounds of replication to yield 8N and 16N cells. These data suggest a possible function for SNF1LK in G2/M regulation. We show data that indicate that SNF1LK does not share functional homology with other SNF1-related kinases, but represents a new subclass with novel molecular activities.

LKB1 is a master kinase that activates 13 kinases of the AMPK subfamily, including MARK/PAR-1

We recently demonstrated that the LKB1 tumour suppressor kinase, in complex with the pseudokinase STRAD and the scaffolding protein MO25, phosphorylates and activates AMP-activated protein kinase (AMPK). A total of 12 human kinases (NUAK1, NUAK2, BRSK1, BRSK2, QIK, QSK, SIK, MARK1, MARK2, MARK3, MARK4 and MELK) are related to AMPK. Here we demonstrate that LKB1 can phosphorylate the T-loop of all the members of this subfamily, apart from MELK, increasing their activity >50-fold. LKB1 catalytic activity and the presence of MO25 and STRAD are required for activation. Mutation of the T-loop Thr phosphorylated by LKB1 to Ala prevented activation, while mutation to glutamate produced active forms of many of the AMPK-related kinases. Activities of endogenous NUAK2, QIK, QSK, SIK, MARK1, MARK2/3 and MARK4 were markedly reduced in LKB1-deficient cells. Neither LKB1 activity nor that of AMPK-related kinases was stimulated by phenformin or AICAR, which activate AMPK. Our results show that LKB1 functions as a master upstream protein kinase, regulating AMPK-related kinases as well as AMPK. Between them, these kinases may mediate the physiological effects of LKB1, including its tumour suppressor function.

The dual response of protein kinase Fyn to neural trauma: early induction in neurons and delayed induction in reactive astrocytes

In the developing central nervous system, a src-related protein-tyrosine kinase fyn participates in the myelination process, neuronal growth, and cytoskeletal organization. In adults, fyn has been implicated in learning and memory formation. To test if fyn expression is modulated by neuronal activity, we performed quantitative in situ hybridization (ISH) using brain sections of the adult rats that had undergone either kainic acid (KA)-induced seizures or neuronal deafferentation (entorhinal cortex lesion, ECL). In the KA model, a few hours after seizure activities, fyn mRNA was elevated in the dentate gyrus (DG) (+45%), cerebral cortex layer III (+35%), and piriform cortex (+25%). Conversely, fyn mRNA consistently decreased in the hippocampal neurons after transection of the major axonal inputs from the entorhinal cortex. Although fyn expression in the brain has been allegedly limited to neurons and oligodendrocytes, we provide in this study the first evidence that fyn mRNA is highly expressed in the astrocytes involved in reactive gliosis. In the KA model, the occurrence of fyn-overexpressing astrocytes increased with the progress of neuronal damage in the CA1 and CA3 regions of the hippocampus. In contrast, fyn-overexpressing astrocytes were not observed in the granular cell layer of dentate gyrus (DG), where neurons were not damaged. Likewise, in the ECL model, the most drastic change in fyn mRNA expression took place at the reactive astrocytes near the stab wound sites, where fyn mRNA levels were doubled 4-10 d after the lesion. Collectively, our data suggest that (i) an early induction of fyn mRNA in neurons is linked to neuronal activity, and (ii) the delayed induction of fyn mRNA in reactive astrocytes near the damaged cells may play novel signaling roles during glial response.

Salt-inducible kinase in steroidogenesis and adipogenesis

The salt-inducible kinases (SIKs) are a family of related serine-threonine kinases. In cultured adrenocortical cells, SIK1 is rapidly but transiently induced by adrenocorticotropin (ACTH) treatment, suggesting that it contributes to ACTH-mediated induction of steroidogenic enzymes. However, ACTH treatment of Y1 mouse adrenocortical cells stimulates a rapid translocation of SIK1 from the nucleus to the cytoplasm, and SIK1 represses the transcription of a steroidogenic enzyme by inhibiting the action of cAMP-responsive elements in the promoter. These studies suggest that SIK1 has a role in the fine tuning of steroidogenic enzyme production during the initial phase of steroidogenesis. SIK2 is found in adipocytes and phosphorylates a specific serine residue in insulin receptor substrate-1. This finding, along with the fact that its expression is raised in the white adipose tissue of mice with type 2 diabetes mellitus, suggests that SIK2 might be involved in metabolic regulation in adipose tissue. Thus, members of the SIK family are emerging as important modulators of key processes such as steroid hormone biosynthesis by the adrenal cortex and insulin signaling in adipocytes.

Expression of depolarization-induced immediate early gene proteins in PC12 cells

Immediate early genes induced by depolarization are thought to be important in mediating neuronal functional plasticity. We previously identified a group of immediate early genes that are preferentially induced by depolarization and forskolin but not by nerve growth factor or epidermal growth factor in PC12 pheochromocytoma cells. These depolarization-induced genes include synaptotagmin 4; the protein kinases KID-1, PIM-1, and SIK; an orphan transcription factor, Nurr-1; and a transcription corepressor, rTLE-3. All these genes are also induced in the hippocampus in response to kainic-acid induced depolarization. To characterize further the unique functions of these genes in plasticity, we used recombinant proteins to generate and purify antibodies against KID-1 and SIK proteins. Immunoblotting experiments were performed to examine the induced expression of the KID-1 and SIK proteins in PC12 cells. PIM-1 and Nurr-1 protein expression was also examined following stimulation, using commercially available antibodies. There is an increase in synthesis, in PC12 cells, of these four IEG proteins after KCl plus forskolin treatment. Nurr-1 protein peaks between 2 and 4 hr and decreases by 6 hr after the treatment. PIM-1 and KID-1 proteins rise by 1 hr, peak between 2 and 4 hr, and return to their basal levels at 6 hr. SIK protein increases significantly at 2 hr after treatment, peaks between 4 and 6 hr, and returns to the basal level at 8 hr. Immunofluorescence studies demonstrate distinct distribution patterns of each of these depolarization-induced IEG proteins in PC12 cells.

Genes preferentially induced by depolarization after concussive brain injury: effects of age and injury severity

Fluid percussion (FP) brain injury leads to immediate indiscriminate depolarization and massive potassium efflux from neurons. Using Northern blotting, we examined the post-FP expression of primary response/immediate early genes previously described as induced by depolarization in brain. RNA from ipsilateral and contralateral hippocampus was harvested from immature and adult rats 1 h following mild, moderate, or severe lateral fluid percussion injury and compared against age-matched sham animals. C-fos gene expression was used as a positive control and showed marked induction in both pups (6-25-fold with increasing severity) and adults (9.7-17.1-fold). Kinase-induced-by-depolarization-1 (KID-1) and salt-inducible kinase (SIK) gene expression was increased in adult (KID-1 1.5-1.6-fold; SIK 1.3-3.9-fold) but not developing rats. NGFI-b RNA was elevated after injury in both ages (pups 1.8-6.1-fold; adults 3.5-5-fold), in a pattern similar to that seen for c-fos. Secretogranin I (sec I) demonstrated no significant changes. Synaptotagmin IV (syt IV) was induced only following severe injury in the immature rats (1.4-fold). Our results reveal specific severity- and age-dependent patterns of hippocampal immediate early gene expression for these depolarization-induced genes following traumatic brain injury. Differential expression of these genes may be an important determinant of the distinct molecular responses of the brain to varying severities of trauma experienced at different ages.