The role of Nedd4 family E3 ubiquitin ligases in the regulation of neuronal cell surface receptors

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Signal transduction from trans-membrane cell surface receptors to nuclear transcription factors is regulated at multiple levels by protein ubiquitination, which can lead to downregulation of receptor surface expression, degradation of components of second messenger pathways, modification of the nuclear transport of transcription factors, or alterations in the epigenetic control of transcription by histone modification. In nonneuronal cells, monoubiquitination or K63-linked polyubiquitination are critical regulatory mechanisms that control the endocytosis and consequent downregulation of trans-membrane cell surface receptors, and thereby affect multiple cell biological processes - from cell differentiation to apoptosis. However, little is known about the role of monoubiquitination and K63-linked polyubiquitination of receptors on the nerve cell surface, although neuronal development and function are tightly controlled by extracellular signals. In the project proposed here, we will study the role of two HECT-type E3 ubiquitin ligases of the Nedd4 family, Nedd4-1 and Nedd4-2, in nerve cells - with a focus on their function in controlling cell surface receptors. Nedd4-1 and Nedd4-2 mediate monoubiquitination or K63-mediated polyubiquitination of substrates, are among the most abundant E3 ligases in neurons, and are particularly strongly expressed during early nerve cell development and differentiation, i.e. at developmental stages that are tightly controlled by extracellular signaling molecules such as growth factors. Moreover, Nedd4-1 and Nedd4-2 have been implicated in the control of multiple receptor proteins, although the corresponding evidence has often remained fragmentary. In preparation of our study, we generated conditional Nedd4-1 and Nedd4-2 KO mouse lines. We will employ these mouse models along with biochemical and cell biological approaches to identify transmembrane substrate proteins of Nedd4-1 and Nedd4-2 in neurons and to examine the consequences of such Nedd4- 1/Nedd4-2-mediated ubiquitination processes for neuron and brain development and function.