“Since the discovery of atelectasis…we have learned quite a bit, but maybe not enough to have a significant impact on patient outcome.”
LUNG atelectasis was first described by the French physician René Théophile Hyacinthe Laënnec in his iconic book entitled, De L’Auscultation Médiate; ou Traité du Diagnostic des Maladies des Poumons et du Cœur published in 1819 in Paris,1 where he compiled his observations of lung and heart sounds derived from the use of his newly invented stethoscope, a simple wooden tube carved by himself. Europe was recovering from the French Revolutionary and the Napoleonic Wars and was battling with poverty, famine, and tuberculosis, a mysterious, untreatable pulmonary disease, reminiscent of today’s SARS-CoV-2.
In this issue of the Journal, Zeng et al.2 present their data exploring the molecular events related to atelectasis, a common clinical entity, by taking advantage of a translational large animal model with high fidelity to human disease. Atelectasis was induced with a bronchial blocker and thoracotomy on the left whole lung, while the right lung was ventilated in the presence or absence of systemic endotoxin infusion. Physiologic parameters were recorded, and imaging techniques were used to visualize regional aeration. After 8 h, lung tissue was collected from aerated and atelectatic lung regions and used for next generation RNA sequencing. The authors2 used gene set enrichment analysis, a pattern recognition tool assessing whether functionally related genes are coordinately and consistently regulated, and transcription factor prediction analysis, a tool pinpointing the transcription factors that drive the transcriptomic differences among groups, to reveal a wealth of novel findings. Downregulated genes associated with host defense, barrier integrity, and tissue regeneration or repair were found to be overrepresented in atelectatic tissue. Systemic endotoxin was capable of upregulating the immune response in atelectatic tissue, while genes linked to alveolar-capillary barrier integrity and function remained depressed. Results from transcription factor prediction analysis were even more intriguing. The data suggested the involvement of the interferon regulatory factor family of transcription factors in endotoxin-exposed atelectasis. These transcription factors, which play pivotal roles in many aspects of the immune response, including immune cell development and differentiation, are likely involved in the enhanced response of the atelectatic tissue to systemic inflammation. In addition, the authors found defective YES-associated protein signaling in atelectasis independent of endotoxin exposure.2 YES-associated protein, a transcriptional coactivator of the transcriptional enhanced associate domain transcription factor family, which controls cell proliferation and stem cell functions, appears to be involved in the loss of pulmonary barrier function of the atelectatic lung. These are important discoveries and fit very well to our current understanding where interferon-stimulated genes have been linked to severe outcomes in acute respiratory distress syndrome (ARDS) patients because of their role of predisposing to additional lung injury.3 Likewise, YES-associated protein signaling is known to play a major role in respiratory epithelial cell regeneration and regulation of the cytoskeleton,4 and it is conceivable that lack of cyclic stretch in atelectatic tissue downregulates this pathway.
Does this study have translational implications for the practicing clinician? Since the discovery of atelectasis, a frequent finding in patients with infectious pulmonary diseases, in patients undergoing general anesthesia as well as in patients with ARDS, we have learned quite a bit, but maybe not enough to have a significant impact on patient outcome. Much of our current understanding is based on physiologic studies and not on the fundamental underlying molecular events. Atelectasis in patients undergoing general anesthesia results from a reduction of functional residual capacity due to loss of respiratory muscle tone, positioning, denitrogenation, and surfactant impairment, and is believed to be a key culprit in the pathogenesis of postoperative pulmonary complications.5 In ARDS, atelectasis is accompanied by significant leukocyte infiltration and lung aeration is even more heterogeneously distributed (e.g., “baby lung” so named because of the occurrence of atelectasis in the dependent lung, causing baby-size aerated lungs). It is typically not amenable to rapid reversal. Previously applied “open lung ventilation strategies” used high sustained airway pressures followed by high positive end-expiratory pressure to achieve alveolar patency, but this aggressive approach has been abandoned in favor of a cautious approach to avoid mechanical lung injury in already jeopardized lungs. Some experts even promote “permissive hypercapnia,” the therapeutic use of hypercapnia, and more recently even “permissive atelectasis.”6 However, in this context, it is eye-opening to remember that atelectasis, when combined with higher tidal volumes, causes injury not only to the atelectatic airways but also to the nonatelectatic airways and alveoli, resulting in a generalized trauma to all distal airways in the entire ventilated lung.7 On the other hand, prolonged hypercapnia reduces host defense and repair mechanisms. For example, prolonged hypercapnia increases the bacterial load in pneumonia, increasing the severity of lung injury.8 This leaves anesthesiologists and intensivists alike caught in a veritable therapeutic dilemma. Although the study by Zeng et al.2 is unable to ultimately provide guidance to deal with this issue, it supports the concept that atelectasis, specifically in the context of systemic inflammation, may cause further harm, as evidenced at the molecular level.
What makes transcriptomics with next generation RNA sequencing to such a powerful means comparable to Laënnec’s wooden tube 200 yr ago? Next generation sequencing has fundamentally changed genome research in the biomedical sciences. In contrast to the classic sequencing technology (i.e., Sanger sequencing), which delivers “one target per reaction,” next generation sequencing is characterized by the ability to perform and capture data from millions of sequencing reactions simultaneously (massive parallel sequencing). This technology can be applied to transcriptomics studies by converting all RNA species into complementary DNA fragments (a complementary DNA library). Next generation RNA sequencing is considered superior to microarray hybridization, the more traditional technology used in transcriptomics,9 because owing to its single base resolution, it has the ability to distinguish between different isoforms and to explore allele-specific expression. Moreover, RNA sequencing is quantitative and can be used to determine RNA expression levels more accurately than microarrays, allowing for a robust comparison between diseased and healthy tissues. The technology has been further developed by combining next generation sequencing with cell sorting (single-cell RNA sequencing) or, most recently, to measure spatially resolved gene expression,10 opening the fascinating possibility of profiling the expression pattern of specific cell subtypes rather than RNA collected from inherently heterogeneous bulk tissue samples. It is unfortunate that Zeng et al.2 did not exploit this technology to its fullest by exploring the long noncoding RNAs, circular RNAs, and microRNAs, all of which are RNA transcripts not coding for proteins but important due to their chromatin and gene regulatory activities. In fact, the potential of RNAs as disease markers11 and as powerful innovative therapeutics12 has been widely recognized. Noncoding RNAs have been linked to human disease such as heart failure,13 and it would have been exciting to see whether the expression profile of noncoding RNAs as revealed by RNA sequencing could discriminate between the atelectatic and aerated lung. Such discoveries could indeed open new therapeutic approaches in pulmonary medicine as, for example, short-term induction of members of the microRNA family miR 130/301 activates YES-associated protein and could potentially ameliorate lung injury.14 Irrespectively, the authors should be congratulated for their contribution toward a fundamental understanding of the underlying molecular events triggered by atelectasis and inflammation. However, significant challenges remain, such as the mechanistic decoding of the new discoveries as well as the development of a therapeutic approach, potentially RNA-based, to mitigate lung injury in patients with atelectasis and inflammation. As we all know, the demonstration of a robust lung protective effect in a heterogeneous patient population requires a powerful treatment effect first and foremost.
This work was supported by grants from the Heart and Stroke Foundation of Canada (Ottawa, Canada) and the Swiss National Science Foundation (Berne, Switzerland, SINERGIA grant No. 177225).
The authors are not supported by, nor maintain any financial interest in, any commercial activity that may be associated with the topic of this article.