A Breath of Fresh Air in Lung Regeneration

Abstract

Enhancing the ability of the lungs to regenerate following injury could revolutionize the treatment of a wide range of different diseases. In this issue, Kumar et al. (2011) and Ding et al. (2011) dissect the cellular and molecular mechanisms of murine lung regeneration following injury and provide insights into the basic biology of the organ with implications for development of future therapeutic approaches.

Nature’s extravagant examples of regenerative potential—such as those displayed by planaria and plants, which can produce a replicate from a mere fragment of an adult, or by Axolotl, which can fully regrow a lost appendage—seem utterly superior in comparison to ours as humans. As inferior as we may seem, humans and other mammals regularly employ less dramatic versions of similar processes to repair tissues following injury. Studies by Kumar et al. (2011) and Ding et al. (2011) in this issue now start to unravel the recovery process that occurs in the mammalian lung in response to infection and injury.

Irrespective of its place in the grand scheme of evolution, an animal’s regenerative potential resides within long-lived stem cells that can both divide to self-renew and differentiate to replace lost tissue during normal homeostasis or wound repair. Tapping into the mastery of stem cells has enormous potential to aid recovery from a number of debilitating injuries and degenerative diseases. This painstaking process begins with identifying the progenitors of a tissue, illuminating their intrinsic molecular and cellular properties, and defining the often complex interactions they have with the specialized environments that they occupy.

The mammalian respiratory system facilitates gas exchange between the external environment and fixed physiology of the circulatory system. At the proximal end of the conducting airway is the trachea, a cartilaginous tube lined with pseudostratified columnar epithelium. The trachea branches into similarly organized bronchi, which narrow and sprout into noncartilaginous bronchioles covered by a simple columnar epithelium. In turn, these terminate in a distal network of alveolar sacs lined by a squamous epithelium, the site of gas exchange.

Considering such varied epithelial organization and the multiple cell types represented, it is perhaps not surprising to find that, despite intensive study, dissecting progenitor-progeny relationships in the mammalian airway has been a considerable challenge, and the complete picture remains elusive (for a thoughtful review, see Rock and Hogan, 2011). Moreover, in the adult lung, under steady-state conditions, progenitors cycle much more slowly than they do in the skin, gut, or hematopoietic systems. Thus, it is only by exposing the lung to trauma that appreciable levels of progenitors become activated in a manner conducive for study within the physiological context of the tissue.

Kumar et al. infect the mouse airways with a sublethal dose of influenza A (H1N1) virus, which results in rapid, profound, and nonselective damage to the epithelium. However, ~2 weeks later, the virus clears, and by 2 months, the distal bronchio-alveolar epithelium has recovered fully, enabling the researchers to analyze temporally the cellular mechanisms involved. The authors trace this remarkable regenerative process to a transient population of p63⁺ Krt5⁺ basal-like cells that appear in the bronchioles and peribronchiolar region following infection. These cells expand locally, organize into growing spheres with a lumen, and subsequently assume expression of alveolar specific proteins, all indicative of proper differentiation and regeneration in vivo (Figure 1A).

Figure 1. Lung Regeneration in Response to Reduction in Pulmonary Function.

(A) Alveolar epithelium regeneration after infection with influenza A virus starts with the appearance of p63⁺ Krt5⁺ basal cells in the bronchial epithelium, clonal expansion of basal cells into the peribronchiolar space, and their organization and differentiation into alveolar epithelial cells. dpi, days postinfection.

(B) Compensatory lung growth following unilateral pneumonectomy occurs predominantly at the level of the alveolar sac, where pulmonary endothelial and alveolar epithelial cells are in close proximity. Vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) signaling cascades that originate in the pulmonary endothelial cell induce expression of matrix metalloproteinase 14 (MMP14). MMP14 generates EGF-like ligands, including heparin binding EGF-like growth factor (HB-EGF) and EGF-like fragment of laminin5 γ2, which activate EGF receptor (EGFR) on the alveolar epithelial cell, driving it to proliferate.

Previous work has implicated p63-expressing basal cells as epithelial progenitors in the proximal trachea and bronchi, yet they are normally absent from the distal regions of the mouse lung (Hong et al., 2004; Rock et al., 2009). This raises the question of their origin. Have they been overlooked until now and, as in humans, normally exist as a rare population in the distal airway? Or are they reseeded from the proximal airway following injury? It is also possible that the cells arise from a p63-negative bronchiolar cell type. A good candidate may be a rare but broadly distributed population of c-kit⁺ cells, recently identified as early progenitors in the human airway (Kajstura et al., 2011). Alternatively, in the injured trachea, T1α⁺ basal cells can develop from a small population of secretory Clara cells (Rawlins et al., 2009). Given these findings, the possibility of a similar scenario in the distal lung should now be investigated.

When the McKeon and Xian groups analyzed gene expression in regenerating lung tissue, they discovered an enrichment in signature transcripts of endothelial cell differentiation, consistent with a link between capillary growth and alveolar regeneration from the basal-like stem cells. Interestingly, the study by the Rafii group focuses on just such an interplay and provides a potential mechanism for earlier observations that alveolar remodeling occurs in tandem with angiogenesis of the associated vascular network (White et al., 2007; Yamamoto et al., 2007). Ding et al. analyze compensatory growth in the right lung following unilateral removal of the left lung. Shortly after pneumonectomy, they observe a burst of proliferation and expansion of the progenitors of the bronchiolar and alveolar epithelia. They make a similar observation in endothelial cells of associated pulmonary capillaries, consistent with previous reports that compensatory lung growth occurs at the level of the distal airway (Nolen-Walston et al., 2008).

Seeking the essential inducer of this compensatory growth, the authors identified VEGF and FGF signaling-dependent expression of matrix metalloproteinase MMP14 in the endothelial cells. Thus, when MMP14 function is inhibited with an MMP14-neutralizing antibody administered shortly after pneumonectomy, alveolar expansion and lung re-growth are reduced, but endothelial proliferation is not. Moreover, MMP14 activation in this context appears to generate EGF-like fragments through cleavage of proteins in the extracellular space, and these in turn stimulate EGF signaling and proliferation in the alveolar epithelium (Figure 1B). Importantly, the authors demonstrate that systemic administration of EGF can drive compensatory growth even in the absence of endothelial VEGF signaling and MMP14 expression.

Though Ding et al., place endothelial cells at the center of the compensatory lung growth, it remains unclear whether reciprocal signaling from the expanding epithelial cells exists, which could finely coordinate the development of the functional air exchange unit. The systemic and/or local events that activate the pulmonary endothelial cells in the first place are also awaiting elucidation. The studies by these two groups open the door for these future explorations, which may offer an entry point for therapeutic regimes to increase pulmonary function.