Analysis of Distinct Roles of CaMKK Isoforms Using STO-609-Resistant Mutants in Living Cells

CaMKK2 is not involved in contraction-stimulated AMPK activation and glucose uptake in skeletal muscle

OBJECTIVE

The AMP-activated protein kinase (AMPK) gets activated in response to energetic stress such as contractions and plays a vital role in regulating various metabolic processes such as insulin-independent glucose uptake in skeletal muscle. The main upstream kinase that activates AMPK through phosphorylation of α-AMPK Thr172 in skeletal muscle is LKB1, however some studies have suggested that Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) acts as an alternative kinase to activate AMPK. We aimed to establish whether CaMKK2 is involved in activation of AMPK and promotion of glucose uptake following contractions in skeletal muscle.

METHODS

A recently developed CaMKK2 inhibitor (SGC-CAMKK2-1) alongside a structurally related but inactive compound (SGC-CAMKK2-1N), as well as CaMKK2 knock-out (KO) mice were used. In vitro kinase inhibition selectivity and efficacy assays, as well as cellular inhibition efficacy analyses of CaMKK inhibitors (STO-609 and SGC-CAMKK2-1) were performed. Phosphorylation and activity of AMPK following contractions (ex vivo) in mouse skeletal muscles treated with/without CaMKK inhibitors or isolated from wild-type (WT)/CaMKK2 KO mice were assessed. Camkk2 mRNA in mouse tissues was measured by qPCR. CaMKK2 protein expression was assessed by immunoblotting with or without prior enrichment of calmodulin-binding proteins from skeletal muscle extracts, as well as by mass spectrometry-based proteomics of mouse skeletal muscle and C2C12 myotubes.

RESULTS

STO-609 and SGC-CAMKK2-1 were equally potent and effective in inhibiting CaMKK2 in cell-free and cell-based assays, but SGC-CAMKK2-1 was much more selective. Contraction-stimulated phosphorylation and activation of AMPK were not affected with CaMKK inhibitors or in CaMKK2 null muscles. Contraction-stimulated glucose uptake was comparable between WT and CaMKK2 KO muscle. Both CaMKK inhibitors (STO-609 and SGC-CAMKK2-1) and the inactive compound (SGC-CAMKK2-1N) significantly inhibited contraction-stimulated glucose uptake. SGC-CAMKK2-1 also inhibited glucose uptake induced by a pharmacological AMPK activator or insulin. Relatively low levels of Camkk2 mRNA were detected in mouse skeletal muscle, but neither CaMKK2 protein nor its derived peptides were detectable in mouse skeletal muscle tissue.

CONCLUSIONS

We demonstrate that pharmacological inhibition or genetic loss of CaMKK2 does not affect contraction-stimulated AMPK phosphorylation and activation, as well as glucose uptake in skeletal muscle. Previously observed inhibitory effect of STO-609 on AMPK activity and glucose uptake is likely due to off-target effects. CaMKK2 protein is either absent from adult murine skeletal muscle or below the detection limit of currently available methods.

Exogenous AMPA downregulates gamma-frequency network oscillation in CA3 of rat hippocampal slices

Pharmacologically-induced persistent hippocampal γ oscillation in area CA3 requires activation of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs). However, we demonstrated that exogenous AMPA dose-dependently inhibited carbachol (CCH)-induced γ oscillation in the CA3 area of rat hippocampal slices, but the underlying mechanism is not clear. Application of AMPARs antagonist NBQX (1 μM) did not affect γ oscillation power (γ power), nor AMPA-mediated γ power reduction. At 3 μM, NBQX had no effect on γ power but largely blocked AMPA-mediated γ power reduction. Ca2+-permeable AMPA receptor (CP-AMPAR) antagonist IEM1460 or CaMKK inhibitor STO-609 but not CaMKIIα inhibitor KN93 enhanced γ power, indicating that activation of CP-AMPAR or CaMKK negatively modulated CCH-induced γ oscillation. Either CP-AMPAR antagonist or CaMKK inhibitor alone did not affected AMPA-mediated γ power reduction, but co-administration of IEM1460 and NBQX (1 μM) largely prevented AMPA-mediated downregulation of γ suggesting that CP-AMPARs and CI-AMPARs are involved in AMPA downregulation of γ oscillation. The recurrent excitation recorded at CA3 stratum pyramidale was significantly reduced by AMPA application. Our results indicate that AMPA downregulation of γ oscillation may be related to the reduced recurrent excitation within CA3 local neuronal network due to rapid CI-AMPAR and CP-AMPAR activation.

A high-concentrate diet induces inflammatory injury via regulating Ca2+/CaMKKβ-mediated autophagy in mammary gland tissue of dairy cows

Introduction

Calmodulin-dependent protein kinase β (CaMKKβ) is closely related to Ca²⁺ concentration. An increase in Ca²⁺ concentration in the cytoplasm activates CaMKKβ, and activated CaMKKβ affects the activities of AMPK and mTOR and induces autophagy. A high-concentrate diet leads to Ca²⁺ disorder in mammary gland tissue.

Objectives

Therefore, this study mainly investigated the induction of mammary gland tissue autophagy by a high-concentrate diet and the specific mechanism of lipopolysaccharide (LPS)-induced autophagy in bovine mammary epithelial cells (BMECs).

Material and Methods

Twelve mid-lactation Holstein dairy cows were fed with a 40% concentrate diet (LC) and a 60% concentrate diet (HC) for 3 weeks. At the end of the trial, rumen fluid, lacteal vein blood, and mammary gland tissue were collected. The results showed that the HC diet significantly decreased rumen fluid pH, with a pH lower than 5.6 for more than 3 h, indicating successfully induction of subacute rumen acidosis (SARA). The mechanism of LPS-induced autophagy in BMECs was studied in vitro. First, the cells were divided into a Ctrl group and LPS group to study the effects of LPS on the concentration of Ca²⁺ and autophagy in BMECs. Then, cells were pretreated with an AMPK inhibitor (compound C) or CaMKKβ inhibitor (STO-609) to investigate whether the CaMKKβ-AMPK signaling pathway is involved in LPS-induced BMEC autophagy.

Results

The HC diet increased the concentration of Ca²⁺ in mammary gland tissue and pro-inflammatory factors in plasma. The HC diet also significantly increased the expression of CaMKKβ, AMPK, and autophagy-related proteins, resulting in mammary gland tissue injury. In vitro cell experiments showed that LPS increased intracellular Ca²⁺ concentration and upregulated protein expression of CaMKKβ, AMPK, and autophagy-related proteins. Compound C pretreatment decreased the expression of proteins related to autophagy and inflammation. In addition, STO-609 pretreatment not only reversed LPS-induced BMECs autophagy but also inhibited the protein expression of AMPK, thereby alleviating the inflammatory response in BMECs. These results suggest that inhibition of the Ca²⁺/CaMKKβ-AMPK signaling pathway reduces LPS-induced autophagy, thereby alleviating inflammatory injury of BMECs.

Conclusion

Therefore, SARA may increase the expression of CaMKKβ by increasing Ca²⁺ levels and activate autophagy through the AMPK signaling pathway, thereby inducing inflammatory injury in mammary gland tissue of dairy cows.

Molecular Mechanisms Underlying Ca2+/Calmodulin-Dependent Protein Kinase Kinase Signal Transduction

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) is the activating kinase for multiple downstream kinases, including CaM-kinase I (CaMKI), CaM-kinase IV (CaMKIV), protein kinase B (PKB/Akt), and 5′AMP-kinase (AMPK), through the phosphorylation of their activation-loop Thr residues in response to increasing the intracellular Ca²⁺ concentration, as CaMKK itself is a Ca²⁺/CaM-dependent enzyme. The CaMKK-mediated kinase cascade plays important roles in a number of Ca²⁺-dependent pathways, such as neuronal morphogenesis and plasticity, transcriptional activation, autophagy, and metabolic regulation, as well as in pathophysiological pathways, including cancer progression, metabolic syndrome, and mental disorders. This review focuses on the molecular mechanism underlying CaMKK-mediated signal transduction in normal and pathophysiological conditions. We summarize the current knowledge of the structural, functional, and physiological properties of the regulatory kinase, CaMKK, and the development and application of its pharmacological inhibitors.

Conformation-Dependent Reversible Interaction of Ca2+/Calmodulin-Dependent Protein Kinase Kinase with an Inhibitor, TIM-063

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK), a Ca²⁺/CaM-dependent enzyme that phosphorylates and activates multifunctional kinases, including CaMKI, CaMKIV, protein kinase B/Akt, and 5'AMP-activated protein kinase, is involved in various Ca²⁺-signaling pathways in cells. Recently, we developed an ATP-competitive CaMKK inhibitor, TIM-063 (2-hydroxy-3-nitro-7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one, Ohtsuka et al. Biochemistry 2020, 59, 1701-1710). To gain mechanistic insights into the interaction of CaMKK with TIM-063, we prepared TIM-063-coupled sepharose (TIM-127-sepharose) for association/dissociation analysis of the enzyme/inhibitor complex. CaMKKα/β in transfected COS-7 cells and in mouse brain extracts specifically bound to TIM-127-sepharose and dissociated following the addition of TIM-063 in a manner similar to that of recombinant GST-CaMKKα/β, which could bind to TIM-127-sepharose in a Ca²⁺/CaM-dependent fashion and dissociate from the sepharose following the addition of TIM-063 in a dose-dependent manner. In contrast to GST-CaMKKα, GST-CaMKKβ was able to weakly bind to TIM-127-sepharose in the presence of EGTA, probably due to the partially active conformation of recombinant GST-CaMKKβ without Ca²⁺/CaM-binding. These results suggested that the regulatory domain of CaMKKα prevented the inhibitor from interacting with the catalytic domain as the GST-CaMKKα mutant (residues 126-434) lacking the regulatory domain (residues 438-463) interacted with TIM-127-sepharose regardless of the presence or absence of Ca²⁺/CaM. Furthermore, CaMKKα bound to TIM-127-sepharose in the presence of Ca²⁺/CaM completely dissociated from TIM-127-sepharose following the addition of excess EGTA. These results indicated that TIM-063 interacted with and inhibited CaMKK in its active state but not in its autoinhibited state and that this interaction is likely reversible, depending on the concentration of intracellular Ca²⁺.

Discovery of AdipoRon analogues as novel AMPK activators without inhibiting mitochondrial complex I

Activation of AMPK emerges as a potential therapeutic approach to metabolic diseases. AdipoRon is claimed to be an adiponectin receptor agonist that activates AMPK through adiponectin receptor 1 (AdipoR1). However, AdipoRon also exhibits moderate inhibition of mitochondrial complex I, leading to increased risk of lactic acidosis. In order to find novel AdipoRon analogues that activate AMPK without inhibition of complex I, 27 analogues of AdipoRon were designed, synthesized and biologically evaluated. As results, benzyloxy arylamide B10 was identified as a potent AMPK activator without inhibition of complex I. B10 dose-dependently improved glucose tolerance in normal mice, and significantly lowered fasting blood glucose level and ameliorated insulin resistance in db/db diabetic mice. More importantly, unlike the pan-AMPK activator MK-8722, B10 did not cause cardiac hypertrophy, probably owing to its selective activation of AMPK in the muscle tissue but not in the heart tissue. Together, B10 represents a novel class of AMPK activators with promising therapeutic potential against metabolic disease.

Development and Characterization of Novel Molecular Probes for Ca2+/Calmodulin-Dependent Protein Kinase Kinase, Derived from STO-609

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) activates particular multifunctional kinases, including CaMKI, CaMKIV, and 5'AMP-activated protein kinase (AMPK), resulting in the regulation of various Ca²⁺-dependent cellular processes, including neuronal, metabolic, and pathophysiological pathways. We developed and characterized a novel pan-CaMKK inhibitor, TIM-063 (2-hydroxy-3-nitro-7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one) derived from STO-609 (7H-benzimidazo[2,1-a]benz[de]isoquinoline-7-one-3-carboxylic acid), and an inactive analogue (TIM-062) as molecular probes for the analysis of CaMKK-mediated cellular responses. Unlike STO-609, TIM-063 had an inhibitory activity against CaMKK isoforms (CaMKKα and CaMKKβ) with a similar potency (K_i = 0.35 μM for CaMKKα, and K_i = 0.2 μM for CaMKKβ) in vitro. Two TIM-063 analogues lacking a nitro group (TIM-062) or a hydroxy group (TIM-064) completely impaired CaMKK inhibitory activities, indicating that both substituents are necessary for the CaMKK inhibitory activity of TIM-063. Enzymatic analysis revealed that TIM-063 is an ATP-competitive inhibitor that directly targets the catalytic domain of CaMKK, similar to STO-609. TIM-063 suppressed the ionomycin-induced phosphorylation of exogenously expressed CaMKI, CaMKIV, and endogenous AMPKα in HeLa cells with an IC₅₀ of ∼0.3 μM, and it suppressed CaMKK isoform-mediated CaMKIV phosphorylation in transfected COS-7 cells. Thus, TIM-063, but not the inactive analogue (TIM-062), displayed cell permeability and the ability to inhibit CaMKK activity in cells. Taken together, these results indicate that TIM-063 could be a useful tool for the precise analysis of CaMKK-mediated signaling pathways and may be a promising lead compound for the development of therapeutic agents for the treatment of CaMKK-related diseases.

Ca2+/calmodulin-dependent protein kinase II and Dimethyl Sulfoxide affect the sealing frequencies of transected hippocampal neurons

Traumatic injury often results in axonal severance, initiating obligatory Wallerian degeneration of distal segments, whereas proximal segments often survive. Calcium ion (Ca²⁺ ) influx at severed proximal axonal ends activates pathways that can induce apoptosis. However, this same Ca²⁺ -influx also activates multiple parallel pathways that seal the plasmalemma by inducing accumulation and fusion of vesicles at the lesion site that reduce Ca²⁺ -influx and enhance survival. We examined whether various inhibitors of Ca²⁺ /calmodulin-dependent protein kinases (CaMKs), and/or dimethyl sulfoxide (DMSO), a common solvent for biologically active substances, affected the ability of a hippocampal-derived neuronal cell line (B104 cells) to seal membrane damage following axotomy. Axolemmal sealing frequencies were assessed at different transection distances from the axon hillock and at various times after Ca²⁺ -influx (PC times) by observing whether transected cells took-up fluorescent dyes. Inhibition of CaMKII by tatCN21 and KN-93, but not inhibition of CaMKI and CaMKIV by STO-609, affected axonal sealing frequencies. That is, CaMKII is a component of previously reported parallel pathways that induce membrane sealing, whereas CaMKI and CaMKIV are not involved. The effects of these CaMKII inhibitors on plasmalemmal sealing depended on their mechanism of inhibition, transection distance, and PC time. DMSO at low concentrations (90 µM-28 mM or 0.00064%-0.2% v/v) significantly increased membrane-sealing frequencies at most PC times and transection distances, possibly by permeabilizing the plasmalemma to Ca²⁺ . Inhibition of CaMKII, DMSO, PC time, and the transection distance significantly affect plasmalemmal sealing that is critical to somal survival in traumatic lesions.

AMP-activated protein kinase-mediated feedback phosphorylation controls the Ca2+/calmodulin (CaM) dependence of Ca2+/CaM-dependent protein kinase kinase β

The Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ)/5'-AMP-activated protein kinase (AMPK) phosphorylation cascade affects various Ca²⁺-dependent metabolic pathways and cancer growth. Unlike recombinant CaMKKβ that exhibits higher basal activity (autonomous activity), activation of the CaMKKβ/AMPK signaling pathway requires increased intracellular Ca²⁺ concentrations. Moreover, the Ca²⁺/CaM dependence of CaMKKβ appears to arise from multiple phosphorylation events, including autophosphorylation and activities furnished by other protein kinases. However, the effects of proximal downstream kinases on CaMKKβ activity have not yet been evaluated. Here, we demonstrate feedback phosphorylation of CaMKKβ at multiple residues by CaMKKβ-activated AMPK in addition to autophosphorylation in vitro, leading to reduced autonomous, but not Ca²⁺/CaM-activated, CaMKKβ activity. MS analysis and site-directed mutagenesis of AMPK phosphorylation sites in CaMKKβ indicated that Thr¹⁴⁴ phosphorylation by activated AMPK converts CaMKKβ into a Ca²⁺/CaM-dependent enzyme as shown by completely Ca²⁺/CaM-dependent CaMKK activity of a phosphomimetic T144E CaMKKβ mutant. CaMKKβ mutant analysis indicated that the C-terminal domain (residues 471-587), including the autoinhibitory region, plays an important role in stabilizing an inactive conformation in a Thr¹⁴⁴ phosphorylation-dependent manner. Furthermore, immunoblot analysis with anti-phospho-Thr¹⁴⁴ antibody revealed phosphorylation of Thr¹⁴⁴ in CaMKKβ in transfected COS-7 cells that was further enhanced by exogenous expression of AMPKα. These results indicate that AMPK-mediated feedback phosphorylation of CaMKKβ regulates the CaMKKβ/AMPK signaling cascade and may be physiologically important for intracellular maintenance of Ca²⁺-dependent AMPK activation by CaMKKβ.

Phosphorylation of AMPK by upstream kinases is required for activity in mammalian cells

AMP-activated protein kinase (AMPK) plays a major role in regulating metabolism and has attracted significant attention as a therapeutic target for treating metabolic disorders. AMPK activity is stimulated more than 100-fold by phosphorylation of threonine 172 (Thr¹⁷²). Binding of AMP to the γ subunit allosterically activates the kinase. Additionally, many small molecules, e.g. 991, have been identified that bind between the kinase domain and the carbohydrate-binding module of the β subunit, stabilising their interaction and leading to activation. It was reported recently that non-phosphorylated Thr¹⁷² AMPK is activated by AMP and A769662. We present here the crystal structure of non-phosphorylated Thr¹⁷² AMPK in complex with AMP and 991. This structure reveals that the activation loop, as well as the complex overall, is similar to the Thr¹⁷² phosphorylated complex. We find that in the presence of AMP and 991 non-phosphorylated Thr¹⁷², AMPK is much less active than the Thr¹⁷² phosphorylated enzyme. In human cells, the basal level of Thr¹⁷² phosphorylation is very low (∼1%), but is increased 10-fold by treatment with 2-deoxyglucose. In cells lacking the major Thr¹⁷² kinases, LKB1 and CaMKKβ, Thr¹⁷² phosphorylation is almost completely abolished, and AMPK activity is virtually undetectable. Our data show that AMP and 991 binding to non-phosphorylated Thr¹⁷² AMPK can induce an ordered, active-like, conformation of the activation loop explaining how AMPK activity can be measured in vitro without Thr¹⁷² phosphorylation. However, in a cellular context, phosphorylation of Thr¹⁷² is critical for significant activation of AMPK.

Elimination of cancer stem cells and reactivation of latent HIV-1 via AMPK activation: Common mechanism of action linking inhibition of tumorigenesis and the potential eradication of HIV-1

Although promising treatments are currently in development to slow disease progression and increase patient survival, cancer remains the second leading cause of death in the United States. Cancer treatment modalities commonly include chemoradiation and therapies that target components of aberrantly activated signaling pathways. However, treatment resistance is a common occurrence and recent evidence indicates that the existence of cancer stem cells (CSCs) may underlie the limited efficacy and inability of current treatments to effectuate a cure. CSCs, which are largely resistant to chemoradiation therapy, are a subpopulation of cancer cells that exhibit characteristics similar to embryonic stem cells (ESCs), including self-renewal, multi-lineage differentiation, and the ability to initiate tumorigenesis. Interestingly, intracellular mechanisms that sustain quiescence and promote self-renewal in adult stem cells (ASCs) and CSCs likely also function to maintain latency of HIV-1 in CD4⁺ memory T cells. Although antiretroviral therapy is highly effective in controlling HIV-1 replication, the persistence of latent but replication-competent proviruses necessitates the development of compounds that are capable of selectively reactivating the latent virus, a method known as the "shock and kill" approach. Homeostatic proliferation in central CD4⁺ memory T (T_CM) cells, a memory T cell subset that exhibits limited self-renewal and differentiation and is a primary reservoir for latent HIV-1, has been shown to reinforce and stabilize the latent reservoir in the absence of T cell activation and differentiation. HIV-1 has also been found to establish durable and long-lasting latency in a recently discovered subset of CD4⁺ T cells known as T memory stem (T_SCM) cells. T_SCM cells, compared to T_CM cells, exhibit stem cell properties that more closely match those of ESCs and ASCs, including self-renewal and differentiation into all memory T cell subsets. It is our hypothesis that activation of AMPK, a master regulator of cellular metabolism that plays a critical role in T cell activation and differentiation of ESCs and ASCs, will lead to both T cell activation-induced latent HIV-1 reactivation, facilitating virus destruction, as well as "activation", differentiation, and/or apoptosis of CSCs, thus inhibiting tumorigenesis. We also propose the novel observation that compounds that have been shown to both facilitate latent HIV-1 reactivation and promote CSC differentiation/apoptosis (e.g. bryostatin-1, JQ1, metformin, butyrate, etc.) likely do so through a common mechanism of AMPK activation.

Impact of Genetic Variation on Human CaMKK2 Regulation by Ca2+-Calmodulin and Multisite Phosphorylation

The Ca²⁺-calmodulin dependent protein kinase kinase-2 (CaMKK2) is a key regulator of neuronal function and whole-body energy metabolism. Elevated CaMKK2 activity is strongly associated with prostate and hepatic cancers, whereas reduced CaMKK2 activity has been linked to schizophrenia and bipolar disease in humans. Here we report the functional effects of nine rare-variant point mutations that were detected in large-scale human genetic studies and cancer tissues, all of which occur close to two regulatory phosphorylation sites and the catalytic site on human CaMKK2. Four mutations (G87R, R139W, R142W and E268K) cause a marked decrease in Ca²⁺-independent autonomous activity, however S137L and P138S mutants displayed increased autonomous and Ca²⁺-CaM stimulated activities. Furthermore, the G87R mutant is defective in Thr85-autophosphorylation dependent autonomous activity, whereas the A329T mutation rendered CaMKK2 virtually insensitive to Ca²⁺-CaM stimulation. The G87R and R139W mutants behave as dominant-negative inhibitors of CaMKK2 signaling in cells as they block phosphorylation of the downstream substrate AMP-activated protein kinase (AMPK) in response to ionomycin. Our study provides insight into functionally disruptive, rare-variant mutations in human CaMKK2, which have the potential to influence risk and burden of disease associated with aberrant CaMKK2 activity in human populations carrying these variants.

Oocyte activation and latent HIV-1 reactivation: AMPK as a common mechanism of action linking the beginnings of life and the potential eradication of HIV-1

In all mammalian species studied to date, the initiation of oocyte activation is orchestrated through alterations in intracellular calcium (Ca(2+)) signaling. Upon sperm binding to the oocyte plasma membrane, a sperm-associated phospholipase C (PLC) isoform, PLC zeta (PLCζ), is released into the oocyte cytoplasm. PLCζ hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce diacylglycerol (DAG), which activates protein kinase C (PKC), and inositol 1,4,5-trisphosphate (IP3), which induces the release of Ca(2+) from endoplasmic reticulum (ER) Ca(2+) stores. Subsequent Ca(2+) oscillations are generated that drive oocyte activation to completion. Ca(2+) ionophores such as ionomycin have been successfully used to induce artificial human oocyte activation, facilitating fertilization during intra-cytoplasmic sperm injection (ICSI) procedures. Early studies have also demonstrated that the PKC activator phorbol 12-myristate 13-acetate (PMA) acts synergistically with Ca(2+) ionophores to induce parthenogenetic activation of mouse oocytes. Interestingly, the Ca(2+)-induced signaling cascade characterizing sperm or chemically-induced oocyte activation, i.e. the "shock and live" approach, bears a striking resemblance to the reactivation of latently infected HIV-1 viral reservoirs via the so called "shock and kill" approach, a method currently being pursued to eradicate HIV-1 from infected individuals. PMA and ionomycin combined, used as positive controls in HIV-1 latency reversal studies, have been shown to be extremely efficient in reactivating latent HIV-1 in CD4(+) memory T cells by inducing T cell activation. Similar to oocyte activation, T cell activation by PMA and ionomycin induces an increase in intracellular Ca(2+) concentrations and activation of DAG, PKC, and downstream Ca(2+)-dependent signaling pathways necessary for proviral transcription. Interestingly, AMPK, a master regulator of cell metabolism that is activated thorough the induction of cellular stress (e.g. increase in Ca(2+) concentration, reactive oxygen species generation, increase in AMP/ATP ratio) is essential for oocyte maturation, T cell activation, and mitochondrial function. In addition to the AMPK kinase LKB1, CaMKK2, a Ca(2+)/calmodulin-dependent kinase that also activates AMPK, is present in and activated on T cell activation and is also present in mouse oocytes and persists until the zygote and two-cell stages. It is our hypothesis that AMPK activation represents a central node linking T cell activation-induced latent HIV-1 reactivation and both physiological and artificial oocyte activation. We further propose the novel observation that various compounds that have been shown to reactivate latent HIV-1 (e.g. PMA, ionomycin, metformin, bryostatin, resveratrol, etc.) or activate oocytes (PMA, ionomycin, ethanol, puromycin, etc.) either alone or in combination likely do so via stress-induced activation of AMPK.

Differential AMP-activated Protein Kinase (AMPK) Recognition Mechanism of Ca2+/Calmodulin-dependent Protein Kinase Kinase Isoforms

Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) is a known activating kinase for AMP-activated protein kinase (AMPK). In vitro, CaMKKβ phosphorylates Thr(172) in the AMPKα subunit more efficiently than CaMKKα, with a lower Km (∼2 μm) for AMPK, whereas the CaMKIα phosphorylation efficiencies by both CaMKKs are indistinguishable. Here we found that subdomain VIII of CaMKK is involved in the discrimination of AMPK as a native substrate by measuring the activities of various CaMKKα/CaMKKβ chimera mutants. Site-directed mutagenesis analysis revealed that Leu(358) in CaMKKβ/Ile(322) in CaMKKα confer, at least in part, a distinct recognition of AMPK but not of CaMKIα.

Facilitation of axon outgrowth via a Wnt5a-CaMKK-CaMKIα pathway during neuronal polarization

Background

Wnt5a, originally identified as a guidance cue for commissural axons, activates a non-canonical pathway critical for cortical axonal morphogenesis. The molecular signaling cascade underlying this event remains obscure.

Results

Through Ca²⁺ imaging in acute embryonic cortical slices, we tested if radially migrating cortical excitatory neurons that already bore primitive axons were sensitive to Wnt5a. While Wnt5a only evoked brief Ca²⁺ transients in immature neurons present in the intermediate zone (IZ), Wnt5a-induced Ca²⁺ oscillations were sustained in neurons that migrated out to the cortical plate (CP). We wondered whether this early Wnt5a-Ca²⁺ signaling during neuronal polarization has a morphogenetic consequence. During transition from round to polarized shape, Wnt5a administration to immature cultured cortical neurons specifically promoted axonal, but not dendritic, outgrowth. Pharmacological and genetic inhibition of the CaMKK-CaMKIα pathway abolished Wnt5a-induced axonal elongation, and rescue of CaMKIα in CaMKIα-knockdown neurons restored Wnt5a-mediated axon outgrowth.

Conclusions

This study suggests that Wnt5a activates Ca²⁺ signaling during a neuronal morphogenetic time window when axon outgrowth is critically facilitated. Furthermore, the CaMKK-CaMKIα cascade is required for the axonal growth effect of Wnt5a during neuronal polarization.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0189-3) contains supplementary material, which is available to authorized users.