We previously identified Neuregulin1 (NRG1) as a gene contributing to the risk of developing schizophrenia. Furthermore, we showed that NRG1 ϩ/Ϫ mutant mice display behavioral abnormalities that are reversed by clozapine, an atypical antipsychotic drug used for the treatment of schizophrenia. We now present evidence that ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4), the tyrosine kinase receptor for NRG1 in hippocampal neurons, interacts with two nonreceptor tyrosine kinases, Fyn and Pyk2 (proline-rich tyrosine kinase 2). NRG1 stimulation of cells expressing ErbB4 and Fyn leads to the association of Fyn with ErbB4 and consequent activation. Furthermore, we show that NRG1 signaling, through activation of Fyn and Pyk2 kinases, stimulates phosphorylation of Y1472 on the NR2B subunit of the NMDA receptor (NMDAR), a key regulatory site that modulates channel properties. NR2B Y1472 is hypophosphorylated in NRG1 ϩ/Ϫ mutant mice, and this defect can be reversed by clozapine at a dose that reverses their behavioral abnormalities. We also demonstrate that short-term synaptic plasticity is altered and theta-burst long-term potentiation is impaired in NRG1 ϩ/Ϫ mutant mice, and incubation of hippocampal slices from these mice with NRG1 reversed those effects. Attenuated NRG1 signaling through ErbB4 may contribute to the pathophysiology of schizophrenia through dysfunction of NMDAR modulation. Thus, our data support the glutamate hypothesis of schizophrenia.

High ErbB2 levels are associated with cancer, and impaired endocytosis of ErbB2 could contribute to its overexpression. Therefore, knowledge about the mechanisms underlying endocytic down-regulation of ErbB2 is warranted. The Cterminus of ErbB2 can be cleaved after various stimuli, and after inhibition of HSP90 with geldanamycin this cleavage is accompanied by proteasome-dependent endocytosis of ErbB2. However, it is unknown whether C-terminal cleavage is linked to endocytosis. To study ErbB2 cleavage and endocytic trafficking, we fused yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP) to the N-and C-terminus of ErbB2, respectively (YFP-ErbB2-CFP). After geldanamycin stimulation YFP-ErbB2-CFP became cleaved in nonapoptotic cells in a proteasome-dependent manner, and a markedly larger relative amount of cleaved YFP-ErbB2-CFP was observed in early endosomes than in the plasma membrane. Furthermore, cleavage took place at the plasma membrane, and cleaved ErbB2 was internalized and degraded far more efficiently than full-length ErbB2. Concordantly, a C-terminally truncated ErbB2 was also readily endocytosed and degraded in lysosomes compared with full-length ErbB2. Altogether, we suggest that geldanamycin leads to C-terminal cleavage of ErbB2, which releases the receptor from a retention mechanism and causes endocytosis and lysosomal degradation of ErbB2. INTRODUCTIONThe receptor tyrosine kinase ErbB2 is a potent oncoprotein, and a high ErbB2 level can by itself activate ErbB receptors (Worthylake et al., 1999). Concordantly, increased ErbB2 levels can be found in several cancers (Klapper et al., 2000;Yarden, 2001), and high ErbB2 levels are correlated to a worse prognosis for breast cancer patients (Slamon et al., 1987;Hynes and Stern, 1994;De Placido et al., 1998;Pegram et al., 1998). Gene amplification is the most studied mechanisms causing high ErbB2 levels, but also posttranscriptional regulation of ErbB2 plays a role in cancers (Vernimmen et al., 2003;Magnifico et al., 2007). Little attention has been paid to the role of ErbB2 degradation in cancers, although when compromised it also would lead to increased ErbB2 levels and activity. Several studies have shown that endocytic downregulation of ErbB2 is impaired in cancer cells (Sorkin et al., 1993; Baulida et al., 1996;Wang et al., 1999;Hommelgaard et al., 2004;Austin et al., 2004;Longva et al., 2005), and ErbB2 can even transmit this property to the related epidermal growth factor receptor (EGFR; Muthuswamy et al., 1999;Wang et al., 1999;Worthylake et al., 1999;Haslekas et al., 2005). Inhibition of HSP90 (e.g., with geldanamycin [GA]) leads to increased internalization and lysosomal degradation of ErbB2 in a manner depending on proteasomal activity (Tikhomirov and Carpenter, 2000;Austin et al., 2004;Lerdrup et al., 2006). HSP90 inhibition also induces cleavage of the cytoplasmic part of ErbB2, resulting in a transmembrane 135-kDa ErbB2 and short-lived cytoplasmic fragments (Tikhomirov and Carpenter, 2000;Lerdrup et al., 2006). The cleavage oc...

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Human papillomavirus type 16 (HPV-16) is the major risk factor for development of cervical cancer. The major oncoprotein E7 enhances cell growth control. However, E7 has in some reports been shown to induce apoptosis suggesting that there is a delicate balance between cell proliferation and induction of cell death. We have used the osteosarcoma cell line U2OS cells provided with E7 and the cdk2 inhibitor p21 (cip1/waf1) under inducible control, as a model system for the analysis of E7-mediated apoptosis. Our data shows that simultaneous expression of E7 and p21 proteins induces cell death, possibly because of conflicting growth control. Interestingly, E7/p21-induced cell death is associated with the activation of a newly identified mediator of apoptosis, namely cathepsin B. Activation of the cellular caspases is undetectable in cells undergoing E7/p21-induced apoptosis. To our knowledge, this is the first time a role for cathepsin B is reported in HPV-induced apoptotic signalling.