Switching proteins on and off
A number of cellular processes are initiated by the binding of ligands, such as hormones, neurotransmitters or cytokines to specific receptors in the cell membrane. "Signal transduction has to be strictly regulated in a spacial and temporal manner, in order to trigger an appropriate cellular response on the one hand and to avoid excessive responses on the other," explains Konstanze Winklhofer. Regulation can be achieved by temporarily modifying proteins, for example by attaching phosphate groups or chains of small ubiquitin proteins.
To facilitate rapid regulation, signalling molecules and enzymes regulating their activity can accumulate in so-called biomolecular condensates. "Think of this as a temporary accumulation of these molecules at specific sites in the cell," outlines Konstanze Winklhofer. This accumulation of molecules resembles droplets without an outer barrier in form of a membrane.
NEMO needs ubiquitin chains
Konstanze Winklhofer's research group has shown that such biomolecular condensates are formed during the activation of the transcription factor NF-κB. This transcription factor is activated by various signalling pathways, for example in innate immune responses induced by the cytokines interleukin-1 (IL-1) and tumour necrosis factor (TNF) or by a number of neurotrophic factors in the nervous system.
The protein NEMO (NF-κB essential modulator) plays a central role in the activation of NF-κB. Various NF-κB-activating stimuli trigger the formation of chains of ubiquitin molecules. NEMO binds to these chains, alters its conformation and, as a result, activates certain enzymes that are required for signal transduction.
Konstanze Winklhofer and her team have discovered that NEMO can only form biomolecular condensates when it interacts with ubiquitin chains. If the binding of NEMO to ubiquitin chains is impaired by a mutation in the NEMO gene, no condensates are formed and NF-κB is not activated.
A pathogenic mutation inhibits the function of NEMO
"We have identified a mutation in the NEMO gene located on the X chromosome that inhibits the ability of NEMO to bind to ubiquitin chains," says Konstanze Winklhofer. As a result, NEMO is unable to promote signalling. The NEMO mutant is associated with a disease called Incontinentia pigmenti. Clinical manifestations of this disease include skin abnormalities and neurological symptoms. In male patients, this disease is usually fatal, as they have only one X chromosome. "The characterisation of this NEMO mutant in cellular models has contributed significantly to our understanding of the signalling processes involved in NF-κB activation," stresses Konstanze Winklhofer. "We are currently investigating specific functions of NEMO in neuronal and immune cells and its possible role in mediating crosstalks between the nervous and immune system."
