Each week we showcase a hot new cell signaling article from a Nature Publishing Group journal. Free full text access to the paper will be maintained for three months, after which the paper will be accessible via the Research Library.
Vaccinium related kinase 3 (VRK3) suppresses ERK activity by activating the Mitogen-activated kinase phosphatase VHR.
Mitogen-activated protein kinase phosphatases (MKPs) inactivate extracellular signal regulated kinases (ERKs), which are pivotal to many cellular responses such as proliferation, differentiation and death. Both the transcription and post-transcriptional stabilization of MKPs are well-established negative-feedback mechanisms for ERK signaling. A study in Nature Cell Biology by Kang et al. reveals that Vaccinium related kinase 3 (VRK3), binds to the MKP vaccinia H1-related (VHR), which has previously been shown to inactivate ERK in the nucleus, and increases the phosphatase activity of VHR1 through a mechanism that is independent of its kinase activity.
VRK1 is a member of the VRK family of kinases and is known to phosphorylate several transcription factors such as p53 and cJun. However, much less is known about the roles of VRK2 and VRK3. The authors examined the phosphorylation events induced by VRK3 overexpression. While VRK3 reduced the levels of nuclear phospho-ERK, VRK3 siRNA knockdown led to an increase of phospho-ERK. VRK3 expression also reduced the levels of phospho-E26-like kinase 1 (Elk1), a transcription factor downstream of ERK.
The authors show that VHR, VRK3 and ERK co-localize in the nucleus, and immunoprecipitation studies reveal that VHR and VRK3 are binding partners. More importantly, siRNA-mediated depletion of VHR seems to be required for VRK3-induced suppression of ERK phosphorylation.
MAPKs regulate MKP activity in one of two ways; either by direct interaction with- or phosphorylation of MKPs. Therefore, the authors investigated how VRK3 might regulate the phosphatase activity of VHR. They found no evidence to suggest that VRK3 phosphorylates VHR. However, VHR activity was higher in cells expressing VRK3, and the kinase domain of VRK3 was found to be critical for VHR activation. Somewhat surprisingly, VRK3-mediated activation of VHR in vitro did not require VRK3 kinase activity.
Phorbol 12-myristate 13-acetate (PMA) has been previously shown to induce an accumulation of active ERK in the nucleus. In order to assess the role of ERK activation by PMA, the authors monitored the levels of phospho-ERK, VRK3 and VHR after PMA treatment. They show that the increase in phospho-ERK is accompanied by an increase in the levels of VRK3 and in VHR activity. The induction of VRK3 appears to be due to de novo synthesis in response to PMA and occurs downstream of ERK activation, resulting in a negative feedback loop that promotes ERK dephosphorylation and inactivation.
It is known that ERK regulates neurite outgrowth in PC12 cells and the authors show that the ERK-VRK3 feedback also functions in this system: VRK3 expression suppresses ERK activity and outgrowth, whereas VRK3 knockdown promotes neurite outgrowth. VHR phosphatase activity in these cells was shown to be modulated by VRK3 expression. Thus it seems that VRK3's interaction with VHR represents a novel mechanism for negative feedback regulation of ERK signaling with important implications for neurite outgrowth.
Clare Garvey, Assistant Editor Signaling Gateway
letter Tae-Hong Kang & Kyong-Tai Kim Negative regulation of ERK activity by VRK3-mediated activation of VHR phosphatase Nature Cell Biology, (16 July 2006) | doi:10.1038/ncb1447 Full text | PDF | Subscribe to Nature Cell Biology
Krüppel-like factor 2 (Klf2) positively regulates the expression of proteins required for thymocyte and T cell migration.
The Krüppel-like family (Klf) of zinc-finger transcription factors regulates many growth-related signal transduction pathways and has been implicated in cell proliferation, apoptosis and angiogenesis. A study in Nature now reveals a novel role for Klf2 in thymocyte (T cell precursor) and T cell emigration and peripheral trafficking.
Although Klf2-/- thymocytes have been shown to develop normally, few Klf2-/- T cells actually migrate to peripheral lymphoid tissues. Those that do reach peripheral tissues are activated and appear to undergo apoptosis, leading to the suggestion that Klf2 regulates T cell quiescence and survival. To further test this hypothesis in vivo, Jameson and colleagues generated fetal liver chimaeras with fetal livers from Klf2-/- or Klf2+/- embryonic mice injected into irradiated Rag2-/- hosts that are unable to generate mature T or B lymphocytes.
In agreement with previous findings, a remarkable deficit in peripheral T cells was observed. Interestingly, when Klf2-/- thymocytes were transferred into congenic mice and tracked, the number of Klf2-deficient cells did not decline, challenging the notion that Klf2 is required for their survival. Furthermore, BrdU (5-bromodeoxyuridine) incorporation experiments showed that following adoptive transfer, Klf2-/- and Klf2+/- T cells proliferate at similar rates, suggesting that Klf2 does not affect T cell proliferation or survival, at least in the short term.
Instead, Klf2 seems to affect the tissue distribution of T cells. Unlike Klf2+/- T cells, which are found in the blood, lymph nodes and spleen, Klf2-/- cells only localize in the spleen. Further examination of Klf2-/- T cells reveals a decrease in the expression of CD62L, CCR7 and β7 integrin, proteins that are known to be essential for trafficking into peripheral lymphoid tissues. The failure of Klf2-/- thymocytes to egress could be due to a downregulation in the expression of sphingosine-1-phophate receptor (SIP1), a critical factor for thymocyte emigration. Chromatin immunoprecipitation and reporter assays show that Klf2 binds and transactivates the SIP1 promoter, shedding some light on the mechanism through which Klf2 promotes thymocyte emigration
In summary, this paper disputes previous suggestions that Klf2 regulates the quiescence and survival of T cells. The failure of thymocytes and T cells to reach peripheral tissues can be attributed to a direct effect of Klf2 on the expression of proteins required for thymocyte emigration and T cell trafficking. It will be interesting to determine the factors regulating Klf2 in this context, as well as the effects of Klf2 on T cell activation and survival in the long term.
Monica Hoyos Flight Cell Migration Gateway
letter Corey M. Carlson, Bart T. Endrizzi, Jinghai Wu, Xiaojie Ding, Michael A. Weinreich, Elizabeth R. Walsh, Maqsood A. Wani, Jerry B. Lingrel, Kristin A. Hogquist and Stephen C. Jameson Kruppel-like factor 2 regulates thymocyte and T-cell migration Nature, 442, 299-302 (20 July 2006) | doi:10.1038/nature04882 Full text | PDF | Subscribe to Nature
Enhanced NRG1 signaling may contribute to NMDA hypofunction in schizophrenia
Neuregulin1 (NRG1) and its receptor erbB4 are implicated in the pathophysiology of schizophrenia. However, the signaling pathways underlying this link are poorly understood. A study in Nature Medicine by Arnold and colleagues reveals that levels of erbB4 and NRG1 remain unchanged in schizophrenic individuals. However, erbB4 is hyperactivated, consistent with its increased association with postsynaptic density protein of 95kDa (PSD-95). The authors also show that in the frontal lobe of schizophrenic individuals erbB4 hyperactivation inhibits the NMDA receptor, which has previously been linked to the onset of schizophrenia.
NRG1 is a susceptibility gene for schizophrenia. NRG1 family proteins contain an epidermal growth factor (EGF)-like domain that mediates activation of the erb-family of receptor tyrosine kinases. NRG1 signaling has previously been shown to have roles in neural and glial development, as well as in the regulation of neurotransmitter receptors.
Treatment of brain tissue samples with NRG1 increases the incidence of erbB4 tyrosine phosphorylation and activation. This is accompanied by a parallel increase in Erk and Akt activation, and erbB4 and B2 heterodimerization. Prefrontal lobe cortices of schizophrenic individuals exhibit higher levels of NRG1-induced erbB4 phosphorylation and downstream Erk and Akt activation compared to wild types. Could it be that the anti-psychotic drug treatment of schizophrenic individuals is responsible for the elevated erb signaling? To this end the authors treated mice with the anti-psychotic drug haloperidol and found that NRG1-induced erbB4 activation was reduced.
PSD-95 is associated with enhanced erbB4 signaling and indeed this association is induced in the brains of schizophrenic individuals independently of ligand stimulation or changes in PSD-95 protein levels.
Both molecular and genetic studies have previously shown a link between NRG1-erbB4 signaling, NMDAR hypoactivation and schizophrenia. Now, the authors show that like PSD-95, NMDAR also associates with activated erbB4 in schizophrenic individuals. A decrease in NMDA-mediated ionic currents after NRG1 stimulation has been documented in rodent brain cells and both NMDAR phosphorylation and NMDA mediated ionic currents are reduced after NMDA and NRG1 co-stimulation in schizophrenia patients. This study provides the first evidence of an association between decreased NMDA receptor function and schizophrenia, and implicates NRG1 signaling in this process.
Clare Garvey, Assistant Editor Signaling Gateway
letter Chang-Gyu Hahn, Hoau-Yan Wang, Dan-Sung Cho, Konrad Talbot, Raquel E Gur, Wade H Berrettini, Kalindi Bakshi, Joshua Kamins, Karin E Borgmann-Winter, Steven J Siegel, Robert J Gallop & Steven E Arnold Altered neuregulin 1-erbB4 signaling contributes to NMDA> receptor hypofunction in schizophrenia Nature Medicine, Published online: 11 June 2006 doi:10.1038/nm1418 Full text | PDF | Subscribe to Nature Medicine
news and views Gerald D Fischbach Schizophrenia: signals from the other side Nature Medicine, Published online: 11 June 2006 doi:10.1038/nm0706-734 Full text | PDF | Subscribe to Nature Medicine
Canonical Wnt signaling acts at early stages of development to induce hepatoblast cell fate.
The vertebrate liver develops from ventral foregut endoderm. Hhex and prox1 are the earliest known markers of hepatocyte progenitors, but little is known about the signals that regulate this cell population. Wnt signaling has a pivotal role in many developmental contexts, including the regulation of liver growth and regeneration. A study in Nature by Elke Ober et al. now shows that canonical Wnt signaling acts earlier in development to induce liver cell fate.
A forward genetic screen, using a transgenic zebrafish line that expresses GFP throughout the developing endoderm, identified prometheus (prt) as a key gene for liver development. Hhex and prox1 were either absent or expressed at very low levels in prt mutants, indicating a defect in hepatic cell fate specification. Transplantation of rhodamine-labeled wild type cells into the lateral plate mesoderm of prt mutants, but not into mutant endoderm, rescued the prt phenotype and led to the development of wild type livers. These findings suggest that prt functions in a cell non-autonomous manner and that crosstalk between the mesoderm and endoderm is required for liver specification.
Interestingly, positional cloning reveals that prt encodes a Wnt2b homologue, and Wnt2b morpholino antisense knock-down phenocopies the prt mutations. Moreover, Wnt2b is expressed in discrete domains in the lateral plate mesoderm adjacent to the liver-forming endoderm, consistent with its role in promoting liver development.
Wnt proteins signal through the non-canonical Wnt/Ca2+ and Wnt/Jun N-terminal kinase (JNK) pathways, as well as the canonical Wnt/β-catenin pathway. To shed light on the signaling pathway regulating hepatic specification, the authors induced expression of the β-catenin signaling inhibitor dnTcf3-GFP under the heat-shock promoter. Inhibition of β-catenin signaling between 16 and 24 hours post fertilization strongly reduced the development of hepatic tissue, suggesting a role for canonical Wnt signaling in liver cell fate specification.
Intriguingly, prt/Wnt2b mutants do not exhibit any other morphological phenotypes and liver tissue does form, albeit at later stages in development, indicating that Wnt2b is specifically required for liver formation but that in its absence other factors may compensate. This study also highlights the role of mesodermal signals in the formation of endodermal organs. Whether this crosstalk is important for the specification of other organs along the primitive gut tube remains to be determined.
letter Elke A. Ober, Heather Verkade, Holly A. Field and Didier Y. R. Stainier Mesodermal Wnt2b signalling positively regulates liver specification Nature, advance online publication, 21 June 2006 doi:10.1038/nature04888 Full text | PDF | Subscribe to Nature
Canonical Wnt signaling acts at early stages of development to induce hepatoblast cell fate.