Nedd4 proteins in physiology and disease
Natasha Boase, Hazel Dalton and Natalie Foot
Collaborators: David Cook (University of Sydney), Philip Poronnik (University of Queensland), Seong Seng Tan (Howard Florey Institute), Grigori Rychkov (University of Adelaide)
Aberrations in the ubiquitin system underlie the pathogenesis of many
diseases including malignancies, neurodegenerative disorders and channelopathies.
Ubiquitin-protein ligases (E3s) determine the substrate specificity of the
ubiquitination process. The Nedd4 family of E3s is evolutionarily conserved
and required for the ubiquitination of numerous cellular targets involved in
processes such as transcription, stability and trafficking of plasma membrane
proteins, and the degradation of misfolded proteins. Nedd4 is a gene initially
identified in our laboratory. Members of the Nedd4-family can ubiquitinate a
range of membrane proteins, resulting in their internalisation and degradation.
We have shown that Nedd4, and the closely related protein Nedd4-2, interacts
with and ubiquitinates the epithelial sodium channel (ENaC). ENaC is required
for sodium absorption across a range of epithelial tissues such as the lungs,
colon and kidney and is an important regulator of blood sodium concentration.
Ubiquitination of ENaC by Nedd4 and Nedd4-2 leads to its internalisation and
degradation. Defects in this process disrupt sodium homeostasis and can cause
hypertension. Our current focus is to characterise the mechanisms of regulation
of ENaC and other ion channels (such as voltage-gated sodium channels) by
Nedd4 and Nedd4-2.
Regulation of ENaC by Nedd4-2. When intracellular Na+ levels are low,
various hormones activate Sgk1, Akt or PKA, which phosphorylate Nedd4-2.
14-3-3 binds to phosphorylated Nedd4-2 to prevent its interaction with
ENaC, resulting in an increase in the levels of ENaC at the plasma membrane.
When intracellular Na+ is high, Nedd4-2 ubiquitinates ENaC, which leads
to its internalisation and degradation.
We have also identified a number of other Nedd4-interacting proteins. Two such proteins are Ndfip1 and Ndfip2, which display Golgi and endosomal localisation, suggestive of a role in protein trafficking. Ndfip1 also appears to have a neuroprotective role in response to traumatic brain injury. An increasing number of Nedd4 targets do not contain discernible Nedd4-binding motifs suggesting the involvement of accessory proteins. Based on our published and preliminary data, Ndfip1 and Ndfip2 are predicted to function as adaptor proteins that recruit Nedd4 family E3s to their substrates to provide specificity and regulatory complexity to the ubiquitination system.
Our recent work suggests that Ndfips regulate DMT1, the primary non-heme iron transporter in mammals. Interestingly, Bsd2p, the yeast Ndfip homologue, is involved in the negative regulation of the divalent metal ion transporter Smf1p (Figure). Golgi-localised Bsd2p is required for delivery of Smf1p to the vacuole (yeast equivalent of the lysosome) for degradation in the presence of intracellular heavy metal. Loss of Bsd2p function is also associated with enhanced Smf1p expression on the cell surface and enhanced sensitivity to metal ion. Bsd2p appears to complex with either one of two functionally redundant Tf-like receptors, Tre1p and Tre2p. Both Bsd2p and Tre1/2p bind the Nedd4 homologue Rsp5p, and together are critical for Smf1p ubiquitination by Rsp5p and trafficking to the vacuole for degradation.
Bsd2p mediates the regulation of Smf1p by Rsp5p