Identification and characterization of a constitutively T-loop phosphorylated and active recombinant S6K1: expression, purification, and enzymatic studies in a high capacity non-radioactive TR-FRET Lance assay

Co-expression of the RPS6KB1 and PDPK1 genes for production of activated p70S6K1 using bac-to-bac baculovirus expression system

BACKGROUND

Ribosomal protein S6 kinase 1 (p70S6K1) is a member of the AGC family of serine/threonine kinases which plays a role in various cellular processes, including protein synthesis, cell growth, and survival. Dysregulation of p70S6K1, characterized by its overexpression and/or hyperactivation, has been implicated in numerous human pathologies, particularly in several types of cancer. Therefore, generating active, recombinant p70S6K1 is critical for investigating its role in cancer biology and for developing novel diagnostic or therapeutic approaches.

METHODS

The baculovirus dual expression system was utilized, enabling the co-expression of two recombinant proteins in infected cells: (a) His-tagged S6K1 with a deletion of the C-terminal autoinhibitory motif and a phosphomimetic mutation at the mTORC1 phosphorylation site (T389D), and (b) untagged PDPK1 lacking the PH domain. The high activity of the purified kinase was confirmed by immunoblotting, as well as by Kinase-Glo and AlphaScreen kinase assays.

RESULTS

Efficient expression of both recombinant proteins was achieved, resulting in highly pure preparations of His-tagged p70S6K1. The high activity of the purified kinase was confirmed through multiple kinase assays, demonstrating significantly higher levels of substrate phosphorylation compared to the tested commercial product.

CONCLUSION

Here, we report a reliable and efficient methodology for the expression and purification of highly active p70S6K1 (His-actS6K1) in quantity and quality that is suitable for biochemical/biophysical studies and high-throughput enzymatic assays. Our developed methodology offers a rapid and cost-effective approach for producing constitutively active His-actS6K1, which can be utilized in academic research and biotechnology.

Systems Biology of Immunomodulation for Post-Stroke Neuroplasticity: Multimodal Implications of Pharmacotherapy and Neurorehabilitation

AIMS

Recent studies indicate that anti-inflammatory drugs, act as a double-edged sword, not only exacerbating secondary brain injury but also contributing to neurological recovery after stroke. Our aim is to explore whether there is a beneficial role for neuroprotection and functional recovery using anti-inflammatory drug along with neurorehabilitation therapy using transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), so as to improve functional recovery after ischemic stroke.

METHODS

We develop a computational systems biology approach from preclinical data, using ordinary differential equations, to study the behavior of both phenotypes of microglia, such as M1 type (pro-inflammatory) vis-à-vis M2 type (anti-inflammatory) under anti-inflammatory drug action (minocycline). We explore whether pharmacological treatment along with cerebral stimulation using tDCS and rTMS is beneficial or not. We utilize the systems pathway analysis of minocycline in nuclear factor kappa beta (NF-κB) signaling and neurorehabilitation therapy using tDCS and rTMS that act through brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB) signaling pathways.

RESULTS

We demarcate the role of neuroinflammation and immunomodulation in post-stroke recovery, under minocycline activated-microglia and neuroprotection together with improved neurogenesis, synaptogenesis, and functional recovery under the action of rTMS or tDCS. We elucidate the feasibility of utilizing rTMS/tDCS to increase neuroprotection across the reperfusion stage during minocycline administration. We delineate that the signaling pathways of minocycline by modulation of inflammatory genes in NF-κB and proteins activated by tDCS and rTMS through BDNF, TrkB, and calmodulin kinase (CaMK) signaling. Utilizing systems biology approach, we show that the activation pathways for pharmacotherapy (minocycline) and neurorehabilitation (rTMS applied to ipsilesional cortex and tDCS) results into increased neuronal and synaptic activity that commonly occur through activation of N-methyl-d-aspartate receptors. We construe that considerable additive neuroprotection effect would be obtained and delayed reperfusion injury can be remedied, if one uses multimodal intervention of minocycline together with tDCS and rTMS.

CONCLUSION

Additive beneficial effect is, thus, noticed for pharmacotherapy along with neurorehabilitation therapy, by maneuvering the dynamics of immunomodulation using anti-inflammatory drug and cerebral stimulation for augmenting the functional recovery after stroke, which may engender clinical applicability for enhancing plasticity, rehabilitation, and neurorestoration.

Time-Resolved Fluorescence Assays

Fluorescence-based detection techniques are popular in high throughput screening due to sensitivity and cost-effectiveness. Four commonly used techniques exist, each with distinct characteristics. Fluorescence intensity assays are the simplest to run, but suffer the most from signal interference. Fluorescence polarization assays show less interference from the compounds or the instrument, but require a design that results in change of fluorophore-containing moiety size and usually have narrow assay signal window. Fluorescence resonance energy transfer (FRET) is commonly used for detecting protein-protein interactions and is constrained not by the sizes of binding partners, but rather by the distance between fluorophores. Time-resolved fluorescence resonance energy transfer (TR-FRET), an advanced modification of FRET approach utilizes special fluorophores with long-lived fluorescence and earns its place near the top of fluorescent techniques list by its performance and robustness, characterized by larger assay window and minimized compound spectral interference. TR-FRET technology can be applied in biochemical or cell-based in vitro assays with ease. It is commonly used to detect modulation of protein-protein interactions and in detection of products of biochemical reactions and cellular activities.

mTORC2 targets AGC kinases through Sin1-dependent recruitment

The protein kinase TOR (target of rapamycin) is a key regulator of cell growth and metabolism with significant clinical relevance. In mammals, TOR signals through two distinct multi-protein complexes, mTORC1 and mTORC2 (mammalian TOR complex 1 and 2 respectively), the subunits of which appear to define the operational pathways. Rapamycin selectively targets mTORC1 function, and the emergence of specific ATP-competitive kinase inhibitors has enabled assessment of dual mTORC1 and mTORC2 blockade. Little is known, however, of the molecular action of mTORC2 components or the relative importance of targeting this pathway. In the present study, we have identified the mTORC2 subunit Sin1 as a direct binding partner of the PKC (protein kinase C) ε kinase domain and map the interaction to the central highly conserved region of Sin1. Exploiting the conformational dependence for PKC phosphorylation, we demonstrate that mTORC2 is essential for acute priming of PKC. Inducible expression of Sin1 mutants, lacking the PKC-interaction domain, displaces endogenous Sin1 from mTORC2 and disrupts PKC phosphorylation. PKB (protein kinase B)/Akt phosphorylation is also suppressed by these Sin1 mutants, but not the mTORC1 substrate p70(S6K) (S6 kinase), providing evidence that Sin1 serves as a selectivity adaptor for the recruitment of mTORC2 targets. This inducible selective mTORC2 intervention is used to demonstrate a key role for mTORC2 in cell proliferation in three-dimensional culture.

Serum- and glucocorticoid-induced kinase 3 in recycling endosomes mediates acute activation of Na+/H+ exchanger NHE3 by glucocorticoids

SGK1 plays an important role in regulation of Na⁺/H⁺ exchanger (NHE) 3 in vivo. We show that SGK3 colocalizes with NHE3 in recycling endosomes. These studies identify SGK3 as the effector of the PI3K pathway that activates NHE3 and show that endosomal localization of SGK3 is essential for acute activation of NHE3.

Na⁺/H⁺ exchanger 3 (NHE3) is the major Na⁺ transporter in the intestine. Serum- and glucocorticoid-induced kinase (SGK) 1 interacts with NHE regulatory factor 2 (NHERF2) and mediates activation of NHE3 by dexamethasone (Dex) in cultured epithelial cells. In this study, we compared short-term regulation of NHE3 by Dex in SGK1-null and NHERF2-null mice. In comparison to wild-type mice, loss of SGK1 or NHERF2 significantly attenuated regulation of NHE3 by Dex but did not completely obliterate the effect. We show that transfection of SGK2 or SGK3 in PS120 cells resulted in robust activation of NHE3 by Dex. However, unlike SGK1 or SGK2, SGK3 rapidly activated NHE3 within 15 min of Dex treatment in both PS120 and Caco-2bbe cells. Immunofluorescence analysis showed that SGK3 colocalized with NHE3 in recycling endosomes, whereas SGK1 and SGK2 were diffusely distributed. Mutation of Arg-90 of SGK3 disrupted the endosomal localization of SGK3 and delayed NHE3 activation. Activation of SGK3 and NHE3 by Dex was dependent on phosphoinositide 3-kinase (PI3K) and phosphoinositide-dependent kinase 1 (PDK1), and Dex induced translocation of PDK1 to endosomes. Our study identifies SGK3 as a novel endosomal kinase that acutely regulates NHE3 in a PI3K-dependent mechanism.

Human kinome drug discovery and the emerging importance of atypical allosteric inhibitors

IMPORTANCE OF THE FIELD

Protein kinases are important targets for drug discovery because they possess critical roles in many human diseases. Several protein kinase inhibitors have entered clinical development with others having already been approved for treating a host of diseases. However, many kinase inhibitors suffer from non-selectivity because they interact with the ATP binding region which has similar structures amongst the protein kinases and this non-selectivity sometimes can cause side effects. As a consequence, there is much interest in developing drugs that inhibit kinases through non-classical mechanisms with the hope of avoiding the side effects of previous kinase drugs.

AREAS COVERED IN THIS REVIEW

This review covers emerging information on kinase biology and discusses new approaches to design selective inhibitors that do not compete with ATP.

WHAT THE READER WILL GAIN

The reader will gain a better understanding of the importance of the field of allosteric inhibitor drug discovery and how this has required the adoption of a new generation of high-throughput screening techniques.

TAKE HOME MESSAGE

Discovery and development of allosteric modulators will result in a family of novel kinase therapies with greater selectivity and more varied ways to control activity of disease causing kinase targets.

Structural basis of human p70 ribosomal S6 kinase-1 regulation by activation loop phosphorylation

p70 ribosomal S6 kinase (p70S6K) is a downstream effector of the mTOR signaling pathway involved in cell proliferation, cell growth, cell-cycle progression, and glucose homeostasis. Multiple phosphorylation events within the catalytic, autoinhibitory, and hydrophobic motif domains contribute to the regulation of p70S6K. We report the crystal structures of the kinase domain of p70S6K1 bound to staurosporine in both the unphosphorylated state and in the 3'-phosphoinositide-dependent kinase-1-phosphorylated state in which Thr-252 of the activation loop is phosphorylated. Unphosphorylated p70S6K1 exists in two crystal forms, one in which the p70S6K1 kinase domain exists as a monomer and the other as a domain-swapped dimer. The crystal structure of the partially activated kinase domain that is phosphorylated within the activation loop reveals conformational ordering of the activation loop that is consistent with a role in activation. The structures offer insights into the structural basis of the 3'-phosphoinositide-dependent kinase-1-induced activation of p70S6K and provide a platform for the rational structure-guided design of specific p70S6K inhibitors.

Specification of neuronal polarity regulated by local translation of CRMP2 and Tau via the mTOR-p70S6K pathway

Mammalian target of rapamycin (mTOR) is an important regulator of neuronal development and functions. Although it was reported recently that mTOR signaling is critical for neuronal polarity, the underlying mechanism remains unclear. Here, we describe the molecular pathway of mTOR-dependent axon specification, in which the collapsing response mediator protein 2 (CRMP2) and Tau are major downstream targets. The activity of mTOR effector 70-kDa ribosomal protein S6 kinase (p70S6K) specifically increases in the axon during neuronal polarity formation. The mTOR inhibitor rapamycin suppresses the translation of some neuronal polarity proteins, including CRMP2 and Tau, thereby inhibiting axon formation. In contrast, constitutively active p70S6K up-regulates the translation of these molecules, thus inducing multiple axons. Exogenous CRMP2 and Tau facilitate axon formation, even in the presence of rapamycin. In the 5'-untranslated region of Tau and CRMP2 mRNAs, we identified a 5'-terminal oligopyrimidine tract, which mediates mTOR-governed protein synthesis. The 5'-terminal oligopyrimidine tract sequences of CRMP2 and Tau mRNAs strongly contribute to the up-regulation of their translation in the axon in response to the axonal activation of the mTOR-p70S6K pathway. Taken together, we conclude that the local translation of CRMP2 and Tau, regulated by mTOR-p70S6K, is critical for the specification of neuronal polarity.

Baculovirus-mediated expression, purification, and characterization of a fully activated catalytic kinase domain construct of the 70 kDa 40S ribosomal protein S6 kinase-1 alphaII isoform (S6K1alphaII)

S6K1alphaII is a member of the AGC subfamily of serine-threonine protein kinases, whereby catalytic activation requires dual phosphorylation of critical residues in the conserved T-loop (T229) and hydrophobic motif (HM; T389) regions of its catalytic kinase domain [S6K1alphaII(DeltaAID); deletion of C-terminal autoinhibitory domain residues 399-502]. With regard to mimicking the synergistic effect of full dual site phosphorylation, baculovirus-mediated expression and affinity purification of the His(6)-S6K1alphaII(DeltaAID)-T229E,T389E double mutant from Sf9 insect cells yielded enzyme with compromised activity. Higher activity preparations were generated using the Sf9 purified His(6)-S6K1alphaII(DeltaAID)-T389E single mutant isoform, which was in vitro phosphorylated by the upstream T229 kinase, PDK1 ( approximately 75 nmol/min/mg). Most significantly, we report that the His(6)-S6K1alphaII(DeltaAID)-T389E construct was generated in its most highly active form (250 nmol/min/mg) by baculovirus-mediated expression and purification from Sf9 insect cells that were coinfected with recombinant baculovirus expressing the catalytic kinase domain of PDK1 [His(6)-PDK1(DeltaPH)]. Approximately equal amounts of fully activated His(6)-S6K1alphaII(DeltaAID)-T389E (5+/-1 mg) and His(6)-PDK1(DeltaPH) (8+/-2 mg) were His(6) affinity co-purified 60 h after initial coinfection of 200 mL of Sf9 insect cells (2x10(6) cells/mL), which were resolved by MonoQ anion exchange chromatography. ESI-TOF mass spectrometry, MonoQ anion exchange chromatography, and kinetic assays showed His(6)-PDK1(DeltaPH) to phosphorylate T229 to approximately 100% after co-expression in Sf9 insect cells as compared to approximately 50% under in vitro conditions, raising interest to mechanistic components not fully achieved in the in vitro reaction. Generation of fully activated S6K1 will facilitate more rigorous analysis of its structure and mechanism.