Airway smooth muscle in asthma: phenotype plasticity and function.

Clinical asthma is characterized by reversible airway obstruction which is commonly due to an exaggerated airway narrowing referred to as airway hyperresponsiveness (AHR). Although debate exists on the complex etiology of AHR, it is clear that airway smooth muscle (ASM) mediated airway narrowing is a major contributor to airway dysfunction. More importantly, it is now appreciated that smooth muscle is far from being a simple cell with only contractile ability properties. Rather, it is more versatile with the capacity to exhibit numerous cellular functions as it adapts to the microenvironment to which it is exposed. The emerging ability of individual smooth muscle cells to undergo changes in their phenotype (phenotype plasticity) and function (functional plasticity) in response to physiological and pathological cues is an important and active area of research. This article provides a brief review of the current knowledge and emerging concepts in the field of ASM phenotype and function both under healthy and asthmatic conditions.