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To advance our ability to effect earlier diagnosis, prevent, and possibly restore healthy lung function in patients with chronic obstructive pulmonary disease (COPD) may require novel thinking. One avenue to explore is the concept of COPD as a trans-generational disease. Early development and COPD may be related first by failure of normal growth development leading to an increased risk of disease, and second by recapitulation of some developmental pathways that may be key to lung repair after injury. While we should be mindful that "aging" may not be only thought of as "late" development in a COPD context, the aging process in the lung is probably fundamentally different from emphysema. However, injury of the aging lung may result in emphysema. Finally, taking a more holistic view of COPD, aging and development in extrapulmonary contexts (e.g., musculoskeletal or immune systems) may also impact on COPD initiation and progression. Addressing the impact of development and the aging process on the natural history of the disease, both in men and in women, may open up research avenues that will drive new advances in disease classification, diagnosis, prognosis, and therapy for this chronic debilitating lung disease.

At least two populations of epithelial stem/progenitor cells give rise to the lung anlage, comprising the laryngo-tracheal complex versus the distal lung below the first bronchial bifurcation. Amplification of the distal population requires FGF9-FGF10-FGFR2b-Sprouty signaling. Residual pools of adult stem cells are hypothesized to be the source of lung regeneration and repair. These pools have been located within the basal layer of the upper airways, within or near pulmonary neuroendocrine cell rests, at the bronchoalveolar junction as well as within the alveolar epithelial surface. Rapid repair of the denuded alveolar surface after injury is clearly key to survival. Strategies to enhance endogenous alveolar epithelial repair could include protection of epithelial progenitors from injury and/or stimulation of endogenous progenitor cell function. Protection with inosine or FGF signaling are possible small molecule therapeutic options. Alternatively, exogenous stem/progenitor cells can be delivered into the lung either intravenously, intratracheally, or by direct injection. Sources of exogenous stem/progenitor cells that are currently under evaluation in the context of acute lung injury repair include embryonic stem cells, bone marrow- or fat-derived mesenchymal stem cells, circulating endothelial progenitors, and, recently, amniotic fluid stem/progenitor cells. Further work will be needed to translate stem/progenitor cell therapy for the lung.

Chronic obstructive pulmonary disease with emphysema has been considered to be an accelerated involutional disease of aging smokers. However, because only a proportion ( approximately 15%) of smokers develop chronic obstructive pulmonary disease with emphysema, clearly genetic susceptibility must play a significant part in determining both the age of onset and the rapidity of decline in lung function. In mice, interference with key genes, either by null mutation, hypomorphism, or gain or loss of function, results in phenotypes comprising either neonatal lethal respiratory distress if the structural effect is severe, or reduced alveolarization and/or early-onset emphysema if the effect is milder. Likewise, null mutants that interfere with matrix assembly and/or integrity, such as elastin, lysyl oxidase, or fibrillin, also result in alveolar dysplasia. Importantly, null mutation of Smad3, which encodes a receptor-activated Smad in the transforming growth factor-beta signaling pathway, results in a more subtle failure to correctly organize the alveolar matrix, which is in turn antecedent to early-onset emphysema mediated by matrix metalloproteinase-9. Furthermore, exposure to side-stream smoke profoundly exacerbates and accelerates alveolar destruction, leading to more severe early-onset emphysema in young Smad3-null mice (unpublished data). Interestingly, polymorphisms in the fibrillin, transforming growth factor-beta type II receptor, and matrix metalloproteinase-9 genes have been described in humans with emphysema. Thus, dysplastic or degraded matrix cannot provide the structural niche for alveolar stem/progenitor cells to assume the correct phenotype and/or repair the alveolar cell lineage niche. The hope is that providing the correct exogenous signals can coax them into doing so.

The incidence of finding evidence of both emphysema and pulmonary fibrosis in the same patient has received increased attention. Several investigators have found on biopsy the presence of emphysema of the upper zones and diffuse parenchymal disease with fibrosis of the lower zones of the lung, especially associated with current or previous heavy smokers. Believed previously to be two different disease mechanisms, there are now data to implicate some common pathways of cell and molecular activation leading to the different morphologic and physiologic outcomes. According to a current view, emphysema may originate from a protease/antiprotease imbalance, whereas a role for antiproteases has been proposed in the modulation of fibrosis. Overexpression of transforming growth factor beta (TGF-beta) in experimental rodent models leads to progressive pulmonary fibrosis, accompanied with marked up-regulation of protease inhibitors, such as tissue inhibitor of metalloproteinases (TIMP) and plasminogen activator inhibitor-1 (PAI-1) genes, along with excessive matrix accumulation. It may be that a "matrix degrading" pulmonary microenvironment, one in which metalloproteinase activities prevail, favors the development of emphysema, whereas a "matrix nondegrading" microenvironment, with enhanced presence of TIMPs, would lead to matrix accumulation and fibrosis. Surprisingly, although Smad3 null mice, deficient in TGF-beta signal transmission, are resistant to bleomycin- and TGF-beta-mediated fibrosis, they develop spontaneous age-related airspace enlargement, consistent with emphysema, with a lack of ability to repair tissue damage appropriately. A common element is tissue damage and repair, with TGF-beta and the Smad signaling pathway playing prominent molecular roles. Both changes can be followed in experimental models with noninvasive imaging and physiologic measurements.