The role of cell adhesion molecule-1 in human mast cell biology

Mast cells are white blood cells which are important in maintaining normal organ function and protection from bacterial infections, but also important for contributing to the development of many diseases such as asthma, allergy, lung fibrosis (scarring) and atherosclerosis (hardening of the arteries). They exert their effects through the release of many diverse chemicals: when release of these is limited the effects are beneficial, when excessive then they become harmful. For mast cells to function normally, they interact with many cell types present in the tissues by sticking to them, a process known as adhesion. By adhering or sticking to other cells, this allows mast cells to direct their chemicals to specific cell types accurately. We have found that mast cells express an interesting protein on their surface called cell adhesion molecule 1 (CADM1). Mast cells are unusual in that this protein is not expressed by other white blood cells. CADM1 facilitates the interaction of mast cells with airway muscle cells and cells called fibroblasts, allowing the cells to stick together and thus communicate. Very little is known about how CADM1 functions, or what it is it binds to on airway muscle and fibroblasts. In this project we therefore wish to work out how CADM1 regulates mast cell function, and how it interacts with other cell types that mast cells stick to. We will use methods that are well established in our laboratory to tackle this. These include a specialist technique called RNA interference which inhibits the gene which produces CADM1 in mast cells. The results of this research project will highlight ways in which the effects of CADM1 can be controlled and may provide entirely new ways of controlling mast cell function in both health and disease.

Mast cells play a key role in the pathophysiology of asthma and many other diverse diseases including pulmonary fibrosis. Current treatments for these diseases are either ineffective or carry unacceptable side effects for a number of patients; in consequence, new approaches to therapy are needed urgently. Mast cells express cell adhesion molecule 1 (CADM1) which plays an important role in their interaction with structural cells including airway smooth muscle (ASM) and fibroblasts, facilitating both cell-cell adhesion and mast cell proliferation. The ligand for CADM1 on ASM and the mechanisms through which CADM1 achieves its biological effects in human mast cells are unknown. Our central hypotheses are that CADM1 plays a central role in mast cell interactions with airway smooth muscle cells and fibroblasts, and operates in a critical and co-operative manner with the stem cell factor receptor (c-kit, CD117). The aim of this proposal is to identify:

1. The effects of CADM1 overexpression, knockdown and mutation on human mast cell adhesion, proliferation, survival, migration and secretion. This will be investigated in the human mast cell line HMC-1 and primary human lung mast cells (HLMC) using adenoviral delivery of CADM1 dominant-negative mutants and short hairpin (sh)RNA.
2. The effects of CADM1 overexpression, knockdown and mutation on CD117 expression, function and tyrosine phosphorylation in HLMC and HMC-1.
3. The ligand(s) for CADM1 expressed by primary human ASM (likely candidates are Nectin-3 and Necl-1: to be assessed by RT-PCR, Western blotting, flow cytometry, adhesion assays).
4. The effects of CADM1 mediated signals on primary human ASM proliferation and differentiation (shRNA knockdown, dominant-negative mutants, blocking antibodies).
5. The role of CADM1 in human mast cell adhesion to primary human airway fibroblasts and the functional consequences (adhesion and proliferation assays in the presence of CADM1 blocking antibodies and following CADM1 knockdown/mutation).

The results of this research project will delineate the role of CADM1 in human mast cell biology, and may offer a novel approach to the modulation of mast cell function in health and disease.