Stretch-induced alternative splicing of serum response factor promotes bronchial myogenesis and is defective in lung hypoplasia

Abstract

IntroductionVisceral smooth muscle (SM) originates from local mesenchymal cells that in early-midgestation begin to synthesize SM proteins, including SM α-actin, desmin, SM myosin, SM22, and calponin in a specific periairway distribution (1-5). In the mouse developing respiratory system, cells expressing SM proteins are first detected in the trachea on day 11 of gestation (3, 4), and then SM differentiation proceeds in a cranial-tocaudal fashion to form the bronchial musculature (1-5). The other type of visceral SM cells found in the lung are interstitial SM cells, also known as interstitial contractile cells, or myofibroblasts. Interstitial SM cells are originally located at the sites of future alveolar septae, and, in the mature organ, they form part of the septae tips (6). Except for the aorta, the development of the vascular musculature lags behind that of visceral SM by several days (4, 7-9).Unlike striated muscle differentiation, on which considerable information was gathered over the years, the mechanisms and genetic program that control SM myogenesis remain, for the most part, unknown. We and others have observed that lung mesenchymal cell precursors change their shape from round to elongated before undergoing bronchial SM differentiation (ref.3; Y. Yang and L. Schuger, unpublished observations). Based on this observation we recently examined whether changes in cell shape might play a role in airway myogenesis. Unexpectedly, our studies demonstrated that essentially all undifferentiated embryonic mesenchymal cells are potential SM precursors (10-12). These studies also confirmed the critical role of cell shape in myogenesis. Specifically, we found that cell rounding prezvents myogenesis, regardless of the normal fate of the cell in vivo, whereas cell spreading/elongation induces SM differentiation, even in mesenchymal cells from nonmuscular organs (10-12).Developing tubular tissues, such as those of the respiratory, gastrointestinal, and urinary systems, are filled with liquid. As a consequence, the periluminal mesenchymal cells are subjected to mechanical tension/stretch exerted by the liquid's hydrostatic pressure (13). These forces likely represent a significant factor in determining the periluminal mesenchymal cell shape. In the developing lung, cells are additionally subjected to repeated stretch caused by intrauterine breathing (13). The fact that mechanical stretch causes cell elongation and that cell elongation is likely to be sensed by the cell as a mechanical stimulus suggested to us that cell tension/stretch may play an important role in the process of visceral myogenesis.Here we used a combination of lung cell and organ cultures from fetal mouse and human origin to determine the effect of mechanical stretch upon SM myogenesis. Smooth muscle (SM) develops only in organs and sites that sustain mechanical tensions. Therefore, we determined the role of stretch in mouse and human bronchial myogenesis. Sustained stretch induced expression of SM proteins in undifferentiated mesenchymal cells and acc...

Peristalsis of airway smooth muscle is developmentally regulated and uncoupled from hypoplastic lung growth

“…Lung hypoplasia is associated with near-term and postnatal ASM-related dysfunction (3,38). However, in the present study, hypoplastic lungs showed abnormal ASM activity far earlier during embryonic development.…”

“…Previous studies have demonstrated coupling of AP frequency and lung growth in vitro: pharmacological alteration of one consistently yields parallel changes in the other (and vice versa) (17). Finally, lung hypoplasia has been associated with perinatal ASM-related abnormalities (3,38).…”

“…Finally, when lung growth is impaired, ASM abnormalities coincide: human congenital diaphragmatic hernia (CDH; ϳ1:2,500 births) remains a premier example where hypoplastic lung growth is responsible for significant death (ϳ50%) and disability (34). This human lung hypoplasia accompanies abnormal expression of serum response factor (SRF) isoforms with impairment of SRF-mediated, ASM-specific gene expression (38). Nitrofen administration to pregnant rats leads to pulmonary hypoplasia in all pups and CDH in a proportion (18).…”

Role of serum response factor in the pathogenesis of disease

“…In hypoplastic lungs of humans, where a stretch signal known to promote alternate SRF splice variants favoring normal pulmonary myogenesis is thought to be missing, the levels of full-length SRF are reduced. 96 Finally, in a mouse model of pulmonary interstitial fibrosis, where activated myofibroblasts contribute to pathology, the levels of SRF increase and presumably activate genes favoring the fibrotic phenotype in this model. 97 Future work should be directed toward the development of SRF-knockout models using established pulmonary epithelial cell-restricted Cre drivers such as Ccsp, 98 and toward assessing SRF expression and activity in asthma, pulmonary hypertension, and chronic obstructive pulmonary disease.…”

The expression and role of serum response factor in papillary carcinoma of the thyroid

“…These investigators have also suggested that SRFΔ5 contributes to epithelial tumorigenesis through the inhibition of apoptosis. Expression of SRFΔ5 is enhanced in human hypoplastic lungs whereas the spliced form is suppressed during normal bronchial myogenesis (20). In addition, Yang et al have shown that in bleomycin-exposed mouse lungs, the level of SRFΔ5 is increased mainly due to de novo synthesis (21).…”