Deciphering the molecular mechanism of HD-PTP function in endosomal trafficking

The behaviour of cells within a tissue is controlled by their response to the environment. Receptor molecules at the cell surface receive a large number of chemical and physical stimuli that transmit signals to the interior of the cell and control important processes such cell migration, metabolism, cell proliferation and differentiation. Anomalies in the transmission of such signals result in pathological states that derive in diseases like cancer, diabetes, muscular dystrophy and neurological degeneration. Amongst the most important signals that cells receive are from circulating small proteins called growth factors. These bind to specialised receptors that alter their pattern of interactions with many molecules inside the cell to generate a mitogenic response and control cell growth. In order to prevent overstimulation of the cell response, there are mechanisms of regulation to ensure that these responses are not sustained endlessly, which would lead to uncontrolled cell division and proliferation. These mechanisms encompass the internalisation of the receptor upon stimulation leading it to specialised compartments or endosomes within the cell where they are ultimately destroyed.

We discovered a key protein, HD-PTP, which controls this mechanism of receptor down-regulation and identified several specific biological partners within the endocytic pathway. The endocytic pathway is also critical in protecting against viral and bacterial infections and to eliminate protein aggregates that otherwise accumulate inside the cells and result in neurodegeneration observed in Alzheimer, Parkinson and Huntington diseases. Our main aim is to characterise these interactions at the molecular level to understand in detail the important mechanism of action of HD-PTP. The knowledge gained with these investigations will bring new insights into the molecular basis of many diseases caused by mutations in effectors of the endocytic pathway and in the long-term will guide further efforts for pharmacological intervention.

Down-regulation of activated mitogenic receptors such as EGFR, is crucial to control the duration of signalling responses, and failure to turn them off is directly associated with cancer. Essential to this process is the internalization, ubiquitination and transport of the activated receptor for subsequent degradation in the lysosome.

We discovered that a novel regulator of this process is the tumour suppressor His Domain Phosphotyrosine Phosphatase (HD-PTP). HD-PTP is essential for sorting ubiquitinated cargo at the early endosome and its loss blocks transport of EGFR leading to accumulation of ubiquitinated proteins on endocytic compartments. We demonstrated that HD-PTP binds to key proteins of the endosomal sorting machinery (ESCRTs) and effectors of EGFR signalling. This provides a crucial link between receptor signalling and transport, to control the location and duration of signalling.

Defining in detail the specific interactions of HD-PTP with endosomal partners is central to understand the molecular mechanism that sustains its scaffolding role and its function as a tumour suppressor. The main aim of this project is to establish the structural basis for specific recognition and selective recruitment of endosomal proteins by HD-PTP.
We will use X-ray crystallography to directly reveal the details of the molecular interactions that underlie HD-PTP function. The structural work will be integrated with biochemical and functional analysis, to gain a full picture of the mechanism by which HD-PTP coordinates specific ESCRT binding with EGFR trafficking.

The structural analysis proposed here, would provide important new insight into how HD-PTP controls EGFR turnover that could be used in future cancer therapies. Likewise, deciphering the mechanisms that control ESCRT trafficking could in the future be translated to design new strategies to fight neurodegenerative diseases and bacterial infections.

The work proposed in this project addresses important questions in basic research that will interest the wider scientific community. However, as the target of our study is a tumour suppressor and interacts with a validated risk factor for neurodegenerative diseases (FTLD), the impact of the research will be of interest to the general public, the medical and clinical community and the pharmaceutical industry.

Both cancer and neurodegenerative diseases inflict a very high burden on the health system and society in general, as they require long-term specialised care and expensive treatments. This impacts negatively on the global economy as these diseases target both working age population as well as the elderly. Unfortunately, there is no cure for those diseases and clearly there is an unmet need to identify better and more specific approaches to prevention and pharmacological intervention. Any contribution towards the understanding of the molecular basis of such pathologies will have a long-term, yet significant, economic and strategic benefit for the UK and will result in a higher quality of life worldwide.

A detailed understanding of the role of our target in controlling endosomal trafficking at the molecular level will have a significant impact in future designing of new strategies to fight various diseases such as Alzheimer, bacterial infections and other neurodegenerative diseases. Furthermore, uncovering the mechanism of controlling EGFR turnover could provide a critical "switch-off" handle in cancer therapy.

The structural information that we will generate would provide important new insight into the molecular interactions between partners and a framework for future design of specific modulators of HD-PTP function. In the long-term, these strategies could be applied to control defects in pathways leading to cancer and to treat hereditary diseases associated with mutations in endosomal effectors. In particular, in UBAP1 we have identified a very exciting link between ESCRT function and frontotemporal lobar degeneration.