Annotated Bibliography
1.
Kotton, D.N. & Morrisey, E.E. Lung regeneration: mechanisms, applications and
emerging stem cell populations. Nature Medicine 20, 822-832 (2014).
This was the primary review article I used for my research. It outlines the
pathways and progenitor cells involved in lung development and regeneration
in response to injury in the mouse. It describes the in vivo research used to
construct the models of the adult and developmental mouse lung, including
differentiation repertoires of stem cell candidates and the mechanisms of celllineage labeling. It also goes into detail with regards to the mechanisms of
epithelial regeneration in specific regions of the lung, including the proximal
and distal airways, the bronchoalveolar junction and the alveoli. Lastly, it
reviews the progress made in de novo regenerative therapies for the mouse,
including those that use induced pluripotent stem cells in vitro. It concludes
with the current therapeutic approaches being employed for human lung
regeneration, such as the implantation of stromal cells isolated from human
bone marrow.
2.
Hogan, B.L. et al. Repair and regeneration of the respiratory system: complexity,
plasticity, and mechanisms of lung stem cell function. Cell Stem Cell 15, 123-138
(2014).
This was another review article used for developing my background knowledge
on the topic of adult lung regenerative mechanisms in vivo. It starts by providing
a very detailed outline of the stages of lung development in the embryo of the
mouse, including branching morphogenesis and alveologenesis. It then
proceeds to outline the epithelial progenitor cell populations that mediate adult
lung homeostasis and regeneration in the various regions of the lung (just like
the review article above). It additionally provides comprehensive diagrams of
the adult mouse bronchioles and alveoli and their epithelial populations, which
complimented the extensive written descriptions very nicely. The article also
describes the responses of the various regions of the adult mouse lung to
different injury-inducing treatments, including naphthalene and bleomycin.
Lastly, like the above article, it goes into the current methods for bioengineering
lung tissue, including whole-lung decellularization strategies.
3.
Rock, J.R. et al. Basal cells as stem cells of the mouse trachea and human airway
epithelium. Proc. Natl. Acad. Sci. USA 106, 12771-12775 (2009).
This primary research article used lineage tracing with KRT5-CreERT2;Rosa26ReYFP transgenic mice to track basal cell descendents in the trachea during
postnatal growth, adult homeostasis and in response to SO2 injury, which leads
to extensive damage of the tracheal epithelium. In all three cases, they found
that basal cells self-proliferate and also give rise to ciliated and secretory club
cells. However, they found that they gave rise to a much higher percentage of
secretory cells than ciliated cells, suggesting that secretory cells themselves
likely give rise to ciliated cells, as opposed to basal cells directly differentiating
into them. The researchers then analyzed the transcriptome profile of basal
cells in the adult mouse lung in response to injury, which were isolated from
other cells of the lung using flow cytometry. They found a large increase in
signaling ligands and receptors, as well as increase in antagonists of
intercellular signaling pathways. Lastly, using a clonal assay, they demonstrated
that basal cells of both mouse and human proximal airways can self-renew and
differentiate in the absence of stroma or columnar epithelial cells.
4.
Rawlins, E.L. et al. The role of Scb1a1+ Clara cells in the long-term maintenance
and repair of lung airway, but not alveolar, epithelium. Cell Stem Cell 4, 525-534
(2009).
In this primary research article, the authors performed cell-lineage tracing on
Scgb1a1-CreERTM ; Rosa26R-eYFP transgenic mice to track the descendents of
Scgb1a1+ (i.e. secretory) cells in the tracheal and distal airways, and alveoli over
time. They analyzed the lungs of these mice during postnatal growth, adult
homeostasis, and in response to naphthalene- and hyperoxia-induced injury.
They found that while these cells give rise to secretory and ciliated cells in the
distal airways during postnatal growth and adult homeostasis, they do not give
rise to alveolar epithelial cells. This was also found to hold in response to the
given lung injuries, despite previous conjectures that bronchiolar epithelial cells
contribute to alveoli during repair. The authors also found that Scgb1a1+ cells
contribute only very minimally to epithelial repair in response to tracheal injury
and during postnatal growth, supporting the hypothesis that basal cells are the
primary contributors to proximal airway epithelial regeneration.
5.
Barkauskas, C.E. et al. Type 2 alveolar cells are stem cells in adult lung. J. Clin.
Invest. 123, 3025-3036 (2013).
This article describes lineage-tracing experiments using Sftpc-CreERT2;Rosa26RtdTm doubly transgenic mice to determine the progenitor cells of the alveoli in
the adult mouse lung during maintenance and repair. They found that surfactant
protein C-positive (Sftpc+) alveolar epithelial type 2 cells (AEC2s) self renew
and differentiate over about a year in the adult homeostatic lung, additionally
giving rise to AEC1s. Interestingly, authors found that the percentage of lineagelabeled Sftpc+ cells does not increase in response to bleomycin injury,
suggesting that a Sftpc- cell population helps to restore the AEC2 population in
this instance, potentially including the Scgb1a1+ secretory club cells mentioned
in the previous article. Additionally, they found that single lineage-labeled
AEC2s grown in culture with Pdgfra+ lung stromal cells give rise to sphere-like
colonies, which they termed alveolospheres, containing both AEC2s and AEC1s.
6.
Kumar, P.A. et al. Distal airway stem cells yield alveoli in vitro and during lung
regeneration following H1N1 influenza infection. Cell 147, 525-538 (2011).
In this primary research article, the authors examine the role of basal cells of
the proximal airways in response to H1N1 influenza infection. They find that
p63-expressing basal cells are intermingled in the bronchiolar epithelium 11
days post-inoculation, and then fall in concentration by 21 days, though they are
not present at all in the bronchioles of normal mice. Additionally, basal cells
were also found in the damaged lung parenchyma, and formed discrete clusters
of pods in the interstitial lung (the area between the pulmonary alveoli and
bloodstream). Then, they examined human tracheal airway stem cells in vitro
using pedigree tracking and found that they display significantly robust
differentiation into ciliated and mucin-producing goblet cells, far greater than
that found in distal airway stem cells. Furthermore, they found that mouse basal
cells in vitro stain positive for Aqp5, a marker of AEC1, in response to influenza
infection. Lastly, using a poorly designed lineage tracing experiment, they found
that Krt5+ cells and their descendents migrated from the bronchioles to local
sites of interbronchiolar damage in response to influenza-induced injury.
7.
Zheng, D. et al. A cellular pathway involved in Clara cell to alveolar type II cell
differentiation after severe lung injury. PLoS ONE 8, e71028 (2013).
This article examines the role of Clara cells, also known as secretory club cells,
in the regeneration of alveolar epithelia in response to severe alveolar damage
induced by bleomycin and influenza infection. They utilize lineage tracing of
Scgb1a1+ cells to determine the distribution of their descendents up to three
weeks post-treatment. The authors determine the presence of lineage-labeled
Sftpc+ bronchiolar epithelial cells (SBECs) that eventually lose Scgb1a1
expression, giving rise to Sftpc+ cells in ring structures of the damaged lung
parenchyma. These structures appear to differentiate into AEC2s via a process
mimicking mouse alveolar epithelial development. Through the lack of SBEC
presence in response to naphthalene treatment, which induces bronchiolar, but
not alveolar damage, the authors conclude that SBECs most probably arise from
Clara cells as opposed to AEC2s.
8.
Kretzschmar, K. & Watt, F.M. Lineage tracing. Cell 148, 33-45 (2012).
This review article outlines the development of lineage tracing as a mechanism
for tracking descendents of specific progenitor cells. In particular, it delves into
the use of genetic lineage tracing in mice using the Cre-loxP system. It describes
the general mechanism employed, involving the placement of the Cre
recombinase gene under the control of a lineage-specific promoter in one
transgenic line and the production of a reporter gene under the control of a
ubiquitous promoter, flanked by a loxP-STOP-loxP sequence in a second
transgenic line. In animals expressing both constructs, Cre specifically activates
the reporter in cells that express the lineage-specific promoter by excising the
STOP sequence. The article then elaborates on recent improvements in this
system, including temporal and spatial control of Cre activity through the use of
the human estrogen receptor and tamoxifen, as well as the advent of multicolor
reporter constructs for lineage tracing with two or more markers.
9.
Li, F. et al. Diversity of epithelial stem cell types in adult lung. Stem Cells
International 2015, 728307 (2015).
This review article serves as a compliment to the first two references of this
annotated bibliography, providing an even more detailed look into the epithelial
progenitor cells of the adult mouse lung. In addition to discussing the usual
stem cell candidates, such as basal cells in the proximal airway, alveolar type II
epithelial cells and naphthalene-resistant variant club cells within
neuroepithelial bodies of the distal airways, it also describes a subpopulation of
as yet unidentified cells in the ducts of the submucosal glands (in the proximal
airway) and secretory cells in the bronchoalveolar duct junction. It provides
excellent illustrations of the various cell populations of mouse lung epithelia,
and a comprehensive description of potential niches for such progenitor cells,
which I could not find in other review articles. Lastly, it discusses lung cancer
stem cells and the role of their niches in supporting their capacity for selfrenewal proliferation.
10. Beers, M.F. & Morrisey E.E. The three R’s of lung health and disease: repair,
remodeling and regeneration. J. Clin. Invest. 121, 2065-2073 (2011).
This article aims not to provide an overly thorough overview of lung
development, adult lung injury or the pathways involved, but instead to
elucidate similarities between what occurs during the development process and
injury response required to properly regenerate damaged cell lineages. It
outlines the possible avenues for future regenerative therapies, including the
activation of local progenitor populations, the insertion of exogenous lung
progenitors and the promotion of local proliferation of undamaged epithelium.
Furthermore, it outlines the development of parenchymal lung disease, in which
the response stage can either result in appropriate repair, as is seen in injury
models of the mouse that were examined in the previous references, or in
aberrant remodeling, including excessive apoptosis and dysfunctional states of
differentiation.