ATP is constantly released during normal and induced mechanical stimulation of the airways Button et al. CBF is temperature dependent Clary-Meinesz et al. The manipulation of temperature is a useful tool to study the relationship between CBF and mucociliary transport in controlled systems of airway cultures Sears et al. CBF can also be affected by pH changes. Although the human airway can tolerate variations in local pH 6. Cigarette smoke is known to be a major risk factor for the development of COPD.
Studies in humans and mice have shown that this common air pollutant significantly decreases CBF Elliott et al. The standard measurement of MCC rates in humans requires the inhalation of a nonpermeating radiolabeled marker that deposits on the airway surface. This method assumes that the marker moves out of the lung at the same rate as the airway secretions in which it is immersed. Although this method provides a direct measurement of MCC, it is not well suited for routine experimental studies.
Various methods have been used to study MCC in many different animal models, including sheep, dogs, pigs, and mice Sabater et al.
The use of genetically modified mouse models has provided a valuable tool to study the role of motile cilia; however, the small size of the mouse airways has made measurements of in vivo clearance rates using radioactivity challenging Foster et al. Other investigators studying mouse models have recorded the movement of fluorescent particles along the trachea or nasopharynx of the mouse e.
Alternatively, investigators have also used ex vivo preparations of trachea or bronchi to investigate MCC Cooper et al. Although these studies provide valuable information, the techniques are invasive and the system is not easy to manipulate. The development of well-differentiated human airway cultures Gray et al.
Air—liquid interface cultures of human airway cells with areas of coordinated ciliary activity spontaneously transport mucus in a circular pattern, forming what is commonly referred to as mucus hurricanes, which have been useful for the study of CF Matsui et al. However, in these cultures, the overall height of the mucus layer is variable and the areas of coordination are of various sizes. To avoid these disadvantages, a modified culture system was developed by gluing a mm plastic cylinder in the center of a mm millicell culture insert, generating a mucociliary transport device MCTD , in which human airway cells were seeded and allowed to differentiate Sears et al.
In this arrangement, cilia coordinate their activity to transport mucus in a continuous circular path around the culture track. This culture system enables the study of mucociliary transport, the visualization of ciliary coordination over time, and the evaluation of different factors that affect the rate of mucociliary transport. For example, using this system, it was shown that increasing CBF by increasing temperature produced a linear increase in mucociliary transport speed Sears et al.
A number of studies measuring the basal rates of MCC in healthy nonsmoking subjects showed that physiological factors as age, sex, posture, sleep, and exercise affect MCC Houtmeyers et al. Under low temperature and humidity, the depth of the PCL is reduced, ciliary cells decrease their activity, and MCC slows, potentially allowing for increased bacterial infection.
The ciliary factors that affect MCC rate include the number and length of the cilia, the coordination of ciliary beating, and the ciliary beat frequency and amplitude, which together determine the maximal velocity at the tips of the cilia and, therefore, the forward velocity of the mucous layer King However, a faster ciliary beat frequency does not necessarily imply improvement of MCC rates, as a lack of coordination can severely affect ciliary transport see below.
Serous factors include mucus rheology and hydration.
If the mucous layer is too thick because of mucin hypersecretion, as in asthma Kuyper et al. The transference of momentum between the cilia and the mucous layer during their forward stroke while minimally interacting with the mucus during the return is disrupted.
Cilia, Ciliated Epithelium, and Ciliary Activity reviews the state of knowledge of the activity of cilia and of the role these structures play in maintaining the normal . Cilia, Ciliated Epithelium and Ciliary Activity. Volume 15 of International Series of Monographs on Pure and Applied Biology–Modern Trends in Physiological.
Most of the current knowledge about how MCC is endogenously regulated at the organ and cellular level came from studies designed to explore the pathogenesis of chronic lung diseases. Studies of asthma and COPD have led to a better understanding of the effects of chronic inflammation and mucus hypersecretion Kuyper et al.
Studies of the effect of cigarette smoke on cilia performance provided insight into its effect on cilia length and its consequences for COPD Gensch et al. Finally, studies of PCD not only clearly showed the key role of ciliary motility in MCC, but also delineated the physiological role that motile cilia play elsewhere in the body Fliegauf et al.
Primary ciliary dyskinesia is a genetically heterogeneous autosomal-recessive disorder with an incidence of , live births. This disorder affects the proper biogenesis, assembly and activity of cilia, resulting in dysfunction of motile cilia and impairment of MCC. The disease affects the entire respiratory tract, with symptoms appearing soon after birth.
Because of impaired MCC, mucus and pathogens accumulate in the upper and lower airways causing year-round daily cough and nasal congestion, chronic sinusitis, and recurrent lower respiratory infections, leading to bronchiectasis, and, in severe cases, lung transplantation. Currently, mutations in 33 genes have been associated with PCD Zariwala et al. The most prevalent ultrastructural defect in the ciliary axoneme of patients with PCD as evaluated by transmission electron microscopy , is the lack of ODAs Fig.
The structural defect in the ODAs can be caused by mutations in genes encoding proteins that are structural components of the ODA e. Mutations found in PCD patients and their association to ultrastructural axonemal defects. Schematic representation of the axoneme in A cross-sectional, and B the longitudinal views of the 96 nm repeat. The affected gene and the corresponding axoneme defect are listed.
Other PCD-causing mutations have been identified in genes that code for proteins involved in the cytoplasmic assembly of dynein arms e. When examined by transmission electron microscopy, the majority of cilia from these patients appear normal; a subset of ciliary axonemes display abnormalities of the central pair Knowles et al. In these patients, the CBF is in the normal range but parameters related to the amplitude and velocity of cilia bend are affected; as a result, ciliary beat is abnormal with a circular beat pattern Castleman et al.
Analysis of isolated cilia from a patient with RSPH1 mutations by cryoelectron tomography revealed that, although the RSPH1 mutation resulted in the loss of radial spoke heads 1 and 2, the third radial spoke head remained intact Lin et al. These data suggest that, although mutations in RSPH1 cause disease, the atypical circular beat pattern may be able to provide a low level of MCC. In addition, PCD may occur without discernible axonemal ultrastructural defects.
This new observation highlights the difficulties of diagnosing PCD by TEM, and shows the importance of using other techniques, including whole exome sequencing, to obtain a definitive diagnosis. Cilia from patients with DNAH11 mutations have a stiff but typically hyperkinetic beat frequency Schwabe et al. In these cases, although the axonemal structure appears normal and ciliary beat frequency ranges from immotile to hyperkinetic, the abnormal ciliary motion results in inefficient MCC.
Future studies will no doubt uncover mutations in additional genes that cause PCD. As expected, PCD patients show markedly reduced tracheobronchial clearance compared with healthy individuals Walker et al. To compensate for the deficient mucociliary transport, patients with PCD rely almost entirely on cough clearance to remove secretions from their bronchial airways Noone et al. Although the ability to clear mucus by coughing partially compensates for the lack of effective MCC, PCD patients suffer from chronic, life-long, respiratory disease, and additional research into new therapies is warranted.
In the last decade, important advances have been made in understanding the structure of the cilia as well as the regulation of ciliary activity.
The standardization of techniques to measure MCC in humans in health and disease, the development of mouse models of PCD and other airway diseases, and the development of well-differentiated culture systems have yielded considerable knowledge about the role of cilia in MCC. The studies performed in patients with PCD highlight the crucial role that ciliary motility plays in MCC, and have identified many mutations that can affect ciliary function.
However, there are still many important questions remaining, including 1 How are the cilia of the airway coordinated at both the cellular and organelle level? The collaborative efforts of many scientists from different fields will be required to uncover the answers to these questions. Additional Perspectives on Cilia available at www. Cilia and Mucociliary Clearance Ximena M. Bustamante-Marin and Lawrence E. Previous Section Next Section. Figure 1. Secretory Cells The goblet cells are the principal secretory cells in the superficial epithelium of the tracheobronchial airway Sleigh et al.
Ciliated Cells Ciliated airway epithelial cells are elongated columnar cells that make limited contact with the basement membrane Mercer et al. Figure 2. The Mucous Layer The major macromolecular constituents of the mucous layer are the mucin glycoproteins. Increased production and secretion of mucins is a feature of many chronic airway diseases, including asthma, COPD, and cystic fibrosis Rose and Voynow The rheological properties of mucus i. Figure 3. Cilia and MCC in PCD Primary ciliary dyskinesia is a genetically heterogeneous autosomal-recessive disorder with an incidence of , live births.
Viscosity increased. Regulation of flagellar dynein by phosphorylation of a kD inner arm dynein intermediate chain. Wanner A Clinical aspects of mucociliary transport. Hydrodynamics of soft active matter. Motile cilia are readily visible on the lateral sides of the head region. Figure 2 with 2 supplements see all. Correlation values were calculated relative to the prototroch.