Biology of mast cells and the inflammatory response

Dr John Hunt, Dr Nicole Jackson

Our current research focuses on the role of novel mast cell-derived tryptases. This family of proteases is of interest clinically as they are implicated in the development of inflammation-based diseases such as asthma and arthritis. Tryptase inhibitors have been successful in treating asthma and inflammatory bowel disease in patients. I have discovered a new human tryptase gene (delta tryptase), and a whole family of novel isoforms (splice variants) with unique activities. The biggest impact of these findings is the immediate need for the pharmaceutical industry to take the existence of these new tryptases into consideration when designing the new generation of tryptase inhibitors.
The discovery of these isoforms was the beginning of a long term study. In 2008 we published an initial characterization of these novel tryptases (Jackson et al J. Biol. Chem., 2008; 283: 34178-87). Excitement surrounding the discovery of the splice variant tryptases stems from the fact that it solves a long standing conundrum concerning the appropriate size for tryptase substrates. Tryptases are unique in that, under physiological conditions, they spontaneously assemble into ring-shaped tetramers with their active sites facing a small central pore. This structure dictates that only small peptide substrates can be cleaved, as large proteins are physically excluded from the active sites. A major conundrum existed because purified native tryptases, in addition to cleaving small peptide substrates, were able to efficiently cleave large substrates such as fibronectin. To cleave large substrates, tryptases must, by some mechanism, exist as active monomers.
Although various in vitro mechanisms had been proposed to explain this, none were likely to work in vivo. Our discovery of tryptase splice variants solves this conundrum. The identical pattern of alternate splicing that occurs in the three human tryptase genes (alpha, beta, and delta) results in complete excision of the key motif or "loop" that mediates tetramer binding. We have preliminary results which demonstrate the "proof of concept" that splice variant tryptase was more efficient than full length trypase at cleaving a large substrate (fibronectin).