November is COPD Awareness Month in the United States. COPD, chronic obstructive pulmonary disease, affects an estimated 65 million people, according to the World Health Organization. In recognition of this awareness month, we spoke with Biophysical Society member Thomas J. Brett about his lab's research into the molecular mechanisms that contribute to airway health and disease.
What is the connection between your research and COPD?
My lab uses structural, biophysical, biochemical, and cellular techniques to investigate molecular mechanisms that contribute to airway health and disease. One of our main projects has focused on structure/function studies of a family of important, yet very understudied, proteins known as calcium-activated chloride channel regulators (CLCAs). Regulation of anion channel activity is central to airway health and disease, as it directly influences the physical and innate immunological properties of mucus. Anion channels pass chloride ions (which are followed by water molecules) required for proper mucus hydration and mucocilliary clearance of allergens and pathogens. They also pass bicarbonate anions required to adjust mucus pH, which is crucial for the innate immunological (antimicrobial) functions of mucus. In addition, the CLCA proteins contain an active metalloprotease domain, which has been shown to proteolytically cleave mucin proteins (the main protein component of mucus), which triggers conformational changes required for proper mucus function. In other projects, we are investigating the molecular mechanisms of inflammatory signaling in COPD.
Why is your research important to those concerned about COPD?
Mucus is a central component of airway health, and is also directly linked to the morbidity and mortality associated with chronic lung disease, including COPD. For example, overproduction of mucus can cause plugging of small airways. Conversely, mucus is required for mucocilliary clearance of allergens and pathogens, and partially protects airway epithelial cells from viral and microbial pathogens. One of the CLCA family members, CLCA1 has long been linked to COPD and suggested as a drug target. However, before our work little was known about the biochemical function of CLCA1. We have since shown that CLCA1 is a secreted protein that is activated by proteolytically cleaving itself, and can bind and potentiate TMEM16A, an important chloride channel expressed in the airway epithelium. In addition, our findings suggest a potential translational application, directed potentiation of TMEM16A to stimulate secretion to assist clearance of mucus during COPD exacerbations. Finally, understanding the molecular mechanisms responsible for chronic inflammatory signaling are important for developing new anti-inflammatory therapeutics, which are sorely needed in COPD.
How did you get into this area of research?
I had a background in structural biology, biophysics, cell biology, and immunology when I was recruited to start my lab in the Pulmonary Division in the Department of Medicine. I had never studied lung diseases prior to that, so that was when I initiated my studies related to asthma and COPD. COPD is the third leading cause of death in the US, yet it represents one of the diseases for which research is most underfunded in terms of NIH research dollars per patient.
How long have you been working on it?
I have been working in the area of chronic inflammatory lung diseases for the last 9 years.
Do you receive public funding for this work? If so, from what agency?
This work has been funded by the NIH/NHLBI, American Lung Association, Institute for Clinical and Translational Science, American Cancer Society, American Heart Association, Alzheimer’s Association, and the Cystic Fibrosis Foundation.
Have you had any surprise findings thus far?
I would say everything we have learned regarding the mechanism and function of CLCA1 has been unique and surprising, because there was so little known about the CLCA proteins when we started our work. They had originally been reported to be chloride channels themselves, but when we started working on them they had just been demonstrated to be secreted and soluble proteins that must activate an unknown endogenous channel. The first surprise was when we found that CLCA1 needed to be proteolytically cleaved by a novel metalloprotease domain within itself in order to produce the active form of the protein that can potentiate calcium activated chloride channels. The next surprise was that CLCA1 bound to and potentiated currents through the calcium activated chloride channel TMEM16A, and that this interaction increased TMEM16A expression on the surface of cells. The third big surprise was that this binding is mediated by a von Willebrand factor type A (VWA) domain in CLCA1, and this CLCA1 VWA domain alone is sufficient to potentiate anion currents through TMEM16A.
What is particularly interesting about the work from the perspective of other researchers?
Our work has highlighted a novel mechanism by which channel activity is modified. CLCA1 can bind to TMEM16A on the cell surface and inhibit its internalization, thereby increasing surface channel density and currents. It does not appear to impact channel gating. In addition, both CLCA1 and TMEM16A belong to larger protein families which, coincidently, have been co-implicated in a number of diseases including COPD, asthma, pulmonary hypertension, inflammatory bowel disease and certain cancers. Our findings suggest there may be distinct family members that functionally pair-up and contribute to disease pathogenesis.
What is particularly interesting about the work from the perspective of the public?
As mentioned above, CLCA1 had long been associated with chronic inflammatory lung diseases like COPD and asthma, but its role was unknown and the protein was not characterized functionally. Some of our collaborative work suggested that it may signal for mucus overproduction in the setting of COPD and asthma. However, CLCA1 is a multi-function, multidomain molecule, and we recently demonstrated that CLCA1 potentiation of TMEM16A does not induce signals for mucus overproduction. This suggests that specific potentiation of anion currents through TMEM16A using CLCA1 VWA domain or some derivative based on this mechanism could potentially be useful in the setting of COPD or asthma (to stimulate fluid secretion and clearance of mucus) or cystic fibrosis (to compensate for loss of anion channel activity by dysfunctional CFTR). These are avenues we are currently pursuing.