1. ¹Department of Biological Sciences, Columbia University, New York, New York, USA
2. ²Department of Pathology and Cell Biology, Columbia University, New York, New York, USA
3. ³Department of Pediatrics, University of Wisconsin Madison-School of Medicine and Public Health, Madison, Wisconsin, USA
4. ⁴Department of Pediatrics, Columbia University, New York, New York, USA
5. ⁵Key Laboratory of Molecular and Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, China

Correspondence: Lloyd A Greene, Fax: 212 305-5498
E-mail: lag3@columbia.edu

Received 3 November 2008; Revised 15 February 2009; Accepted 2 March 2009.

Abstract

Here, we explore the role of Cbl proteins in regulation of neuronal apoptosis. In two paradigms of neuron apoptosis — nerve growth factor (NGF) deprivation and DNA damage — cellular levels of c-Cbl and Cbl-b fell well before the onset of cell death. NGF deprivation also induced rapid loss of tyrosine phosphorylation (and most likely, activation) of c-Cbl. Targeting c-Cbl and Cbl-b with siRNAs to mimic their loss/inactivation sensitized neuronal cells to death promoted by NGF deprivation or DNA damage. One potential mechanism by which Cbl proteins might affect neuronal death is by regulation of apoptotic c-Jun N-terminal kinase (JNK) signaling. We demonstrate that Cbl proteins interact with the JNK pathway components mixed lineage kinase (MLK) 3 and POSH and that knockdown of Cbl proteins is sufficient to increase JNK pathway activity. Furthermore, expression of c-Cbl blocks the ability of MLKs to signal to downstream components of the kinase cascade leading to JNK activation and protects neuronal cells from death induced by MLKs, but not from downstream JNK activators. On the basis of these findings, we propose that Cbls suppress cell death in healthy neurons at least in part by inhibiting the ability of MLKs to activate JNK signaling. Apoptotic stimuli lead to loss of Cbl protein/activity, thereby removing a critical brake on JNK activation and on cell death.