A tale of the epidermal growth factor receptor: The quest for structural resolution on cells

Highlights

• We describe how key concepts in EGFR research have evolved over the preceding decades.

• Atomic resolution structures provided by X-ray crystallography provide the basis for understanding EGFR.

• Lower resolution microscopy data from receptors in cell membranes often disagrees.

• Super-resolution techniques are beginning to bridge the gap between the two.

Abstract

The challenge of determining the architecture and geometry of oligomers of the epidermal growth factor receptor (EGFR) on the cell surface has been approached using a variety of biochemical and biophysical methods. This review is intended to provide a narrative of how key concepts in the field of EGFR research have evolved over the years, from the origins of the prevalent EGFR signalling dimer hypothesis through to the development and implementation of methods that are now challenging the conventional view. The synergy between X-ray crystallography and cellular fluorescence microscopy has become particularly important, precisely because the results from these two methods diverged and highlighted the complexity of the challenge. We illustrate how developments in super-resolution microscopy are now bridging this gap. Exciting times lie ahead where knowledge of the nature of the complexes can assist with the development of a new generation of anti-cancer drugs.

Introduction

The human epidermal growth factor receptor (hEGFR; aka HER1) is the founding member of the growth factor receptor tyrosine kinase (RTK) super-family [1]. This family also comprises 18 sub-groups of cell surface receptors for many growth factors, cytokines and hormones [2]. The evolution of the EGFR family can be traced from one receptor/ligand pair in C. elegans, through one receptor with multiple ligands in D. melanogaster, to a family comprising four receptors (HER1 and ErbB2-4, known as HER2-4 in humans) and at least 13 ligands in mammals [3], [4].

The EGFR family are key regulators of cell-to-cell inductive processes and cell fate [5]. Their function is to transmit growth factor signals from the outside to the inside of the cell where changes in gene expression allow the cell to respond to the new circumstances. Deregulated signalling by cell surface HER1 receptors (e.g. via activating mutations in the HER1 gene) is implicated in a substantial percentage of lung cancers [6]. As activation of these receptors has been shown to result in the growth and progression of the malignancy, there have been considerable research efforts directed toward the development of effective inhibitors of HER1. Several of these cancer drugs are in different stages of pre-clinical and clinical trials [7].

Here we review the efforts towards understanding the structure–function relationships underlying the transduction of the EGF signal and the activation of EGFR across the plasma membrane. This understanding requires the determination of structure at high resolution. Because biological function usually requires the interaction of many molecules within a complex system, a full understanding of the structure–function relationship is only possible if we are able to obtain structural detail of molecules within the context of their natural environment; the cell. Although this inevitably leads us to discussions of how fluorescence based microscopy techniques have been used for this purpose, we have restricted the scope of this review to efforts to elucidate EGFR macromolecular structure. For an excellent review that discusses the imaging of other aspects of EGFR biology such as interaction dynamics and the events subsequent to receptor activation please see [8]. Ultimately the goal would be to obtain atomic resolution structures from molecules in living cells, in real time, but this is likely to remain out of reach for the foreseeable future.