Alveolar Epithelial cell differentiation and response to injury.
immaturity is the major cause of morbidity and mortality in premature infants,
especially those born <28 weeks gestation. Proper lung development from 23-28 weeks
requires coordinated cell proliferation and differentiation. Infants born at this age
are at high risk for respiratory distress syndrome (RDS), a lung disease characterized
by insufficient surfactant production due to ... read moreimmaturity of the saccular/alveolar type II
epithelial cells in the lung. Neuregulin (NRG) stimulation of ErbB4 signaling is
important for development of type II cell surfactant synthesis. ErbB4 may mediate gene
expression via a non-canonical pathway involving enzymatic cleavage releasing its
intracellular domain (4ICD) for nuclear trafficking and gene regulation. The accepted
model for release of 4ICD is consecutive cleavage by Tumor necrosis factor alpha
Converting Enzyme (TACE) and γ-secretase enzymes. Here we hypothesize that
cleavage of ErbB4 by the two enzymes TACE and gamma secretase is necessary for
surfactant production in type 2 epithelial cells. Our data show that Presenilin-1
(PSEN-1) is the active enzyme in the γ-secretase complex which cleaves ErbB4 to
release 4ICD. We further show that release of 4ICD by PSEN-1 cleavage is not dependent
on previous TACE cleavage. We used siRNA to silence PSEN-1 expression in a mouse lung
type II epithelial cell line (MLE12 cells), and both siRNA knockdown and chemical
inhibition of TACE. Knockdown of PSEN-1 significantly decreased baseline and
NRG-stimulated surfactant phospholipid synthesis, expression of the surfactant proteins
SP-B and SP-C, as well as 4ICD levels, with no change in ErbB4 ectodomain shedding.
Neither siRNA knockdown nor chemical inhibition of TACE inhibited 4ICD release or
surfactant synthesis. PSEN-1 cleavage of ErbB4 for non-canonical signaling through 4ICD
release does not require prior cleavage by TACE. Preterm babies with RDS are treated
with oxygen and supportive breathing using ventilators. Higher inspired oxygen levels
improve preterm infants' survival but increase the risk for a condition of chronic lung
injury, called Bronchopulmonary Dysplasia (BPD). BPD is characterized by arrested
alveolarization and airway hyperreactivity. The mechanisms regulating normal alveolar
development and BPD are not well understood. Of novel interest is the potential role of
the matricellular protein CCN5 (Cysteine-rich protein 61/Connective tissue growth
factor/Nephroblastoma-overexpressed protein), which the Castellot laboratory previously
reported is present in developing alveolar epithelium. The mechanism of action and
biological function of CCN5 in smooth muscle cells (SMCs) is well-studied. CCN5 is a
cell cycle regulator that inhibits SMC proliferation and promotes SMC differentiation.
The cell-specific expression and function of CCN5 in alveolar development and injury is
unknown. Here we hypothesize that CCN5 is a negative regulator of type2 cell
proliferation and motility during alveolar development and response to injury. Our data
show that CCN5 knockdown in MLE12 cells reduces proliferation and motility in these
cells. A mouse model of BPD, in which neonatal pups are exposed to 90% oxygen from post
natal (P) 5 through P13 was used to study CCN5 and alveolar epithelial cell fate.
Western Blot analysis of whole lung lysates showed that CCN5 expression is significantly
reduced in hyperoxic versus room air P13 lungs. Using immunofluorescence labeling CCN5
was not expressed in alveolar epithelial type II cells in RA lungs. In oxygen-exposed
lungs an intermediate cell type that expressed markers for both type II and I epithelial
cells was observed. These intermediate cells showed proliferation activity and labeled
positive for CCN5. Type I cells also showed proliferation activity. Overall, only
proliferating alveolar epithelial cells demonstrated CCN5 expression. In conclusion,
CCN5 expression correlates positively with proliferation in alveolar epithelial cells in
response to neonatal oxygen injury. This contrasts strongly with studies in SMCs in
which CCN5 is highly expressed in non-proliferating cells and is down regulated in
proliferating SMCs. Neonatal hyperoxic lung injury induces a transdifferentiation of
type II epithelial cells to type I epithelial cells, which is accompanied by both cell
proliferation and by CCN5 expression. We propose that CCN5 is a functional regulator of
alveolar epithelial proliferation during development and during the response of the
neonatal lung to oxygen injury.
Thesis (Ph.D.)--Tufts University, 2014.
Submitted to the Dept. of Pharmacology & Experimental Therapeutics.
Advisors: John Castellot, and Heber Nielsen.
Committee: Theoharis Theoharides, Martin Bienborn, MaryAnn Volpe, and Maria Ramirez.
Keyword: Biology.read less