A novel function for the protein tyrosine phosphatase Shp2 during lung branching morphogenesis

Baliosperoid A attenuates lipopolysaccharide-induced acute lung injury by targeting SHP2 to inhibit inflammation and oxidative stress

Acute lung injury (ALI) remains a devastating clinical condition with limited therapeutic options. While Src homology-2 domain-containing protein tyrosine phosphatase 2 (SHP2) has emerged as a critical mediator in ALI pathogenesis, effective SHP2-targeting therapeutics remain largely elusive. Baliospermum solanifolium (Burm.) is a traditional medicine used to treat various diseases such as asthma, edema, bronchitis, jaundice, and constipation. Baliosperoid A (BA), a diterpenoid compound derived from Baliospermum solanifolium's roots, exhibits potent NO inhibitory activity in RAW264.7 cells. However, its anti-inflammatory activity and potential targets have never been reported. Here, we report BA acts as a selective SHP2 inhibitor with remarkable therapeutic potential against ALI. Through comprehensive molecular and functional analyses, we demonstrate that BA directly binds to and inhibits SHP2 phosphatase activity with high specificity (IC50 = 1.638 ± 0.324 μM). Mechanistically, BA orchestrates a dual-action therapeutic effect by simultaneously suppressing inflammatory cascades through SHP2-mediated MAPK and NF-κB pathway inhibition while activating the Nrf2-dependent antioxidant response. In preclinical models of ALI and sepsis, BA treatment significantly improved survival rates, preserved lung architecture, and prevented multi-organ dysfunction. Notably, BA demonstrated superior efficacy to the existing SHP2 inhibitor SHP099, particularly in sepsis survival outcomes (90 % vs 50 % survival at 24 h). Our findings not only identify BA as a promising therapeutic candidate for ALI but also establish a novel paradigm for targeting SHP2 in inflammatory diseases.

Setting sail: Maneuvering SHP2 activity and its effects in cancer

Since the discovery of tyrosine phosphorylation being a critical modulator of cancer signaling, proteins regulating phosphotyrosine levels in cells have fast become targets of therapeutic intervention. The nonreceptor protein tyrosine phosphatase (PTP) coded by the PTPN11 gene "SHP2" integrates phosphotyrosine signaling from growth factor receptors into the RAS/RAF/ERK pathway and is centrally positioned in processes regulating cell development and oncogenic transformation. Dysregulation of SHP2 expression or activity is linked to tumorigenesis and developmental defects. Even as a compelling anti-cancer target, SHP2 was considered "undruggable" for a long time owing to its conserved catalytic PTP domain that evaded drug development. Recently, SHP2 has risen from the "undruggable curse" with the discovery of small molecules that manipulate its intrinsic allostery for effective inhibition. SHP2's unique domain arrangement and conformation(s) allow for a truly novel paradigm of inhibitor development relying on skillful targeting of noncatalytic sites on proteins. In this review we summarize the biological functions, signaling properties, structural attributes, allostery and inhibitors of SHP2.

SHP2: The protein tyrosine phosphatase involved in chronic pulmonary inflammation and fibrosis

Chronic respiratory diseases (CRDs), including pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), lung cancer, and asthma, are significant global health problems due to their prevalence and rising incidence. The roles of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) in controlling tyrosine phosphorylation of targeting proteins modulate multiple physiological cellular responses and contribute to the pathogenesis of CRDs. Src homology-2 domain-containing PTP2 (SHP2) plays a pivotal role in modulating downstream growth factor receptor signaling and cytoplasmic PTKs, including MAPK/ERK, PI3K/AKT, and JAK/STAT pathways, to regulate cell survival and proliferation. In addition, SHP2 mutation and activation are commonly implicated in tumorigenesis. However, little is known about SHP2 in chronic pulmonary inflammation and fibrosis. This review discusses the potential involvement of SHP2 deregulation in chronic pulmonary inflammation and fibrosis, as well as the therapeutic effects of targeting SHP2 in CRDs.

Conserved Mechanisms in the Formation of the Airways and Alveoli of the Lung

Branching is an intrinsic property of respiratory epithelium that can be induced and modified by signals emerging from the mesenchyme. However, during stereotypic branching morphogenesis of the airway, the relatively thick upper respiratory epithelium extrudes through a mesenchymal orifice to form a new branch, whereas during alveologenesis the relatively thin lower respiratory epithelium extrudes to form sacs or bubbles. Thus, both branching morphogenesis of the upper airway and alveolarization in the lower airway seem to rely on the same fundamental physical process: epithelial extrusion through an orifice. Here I propose that it is the orientation and relative stiffness of the orifice boundary that determines the stereotypy of upper airway branching as well as the orientation of individual alveolar components of the gas exchange surface. The previously accepted dogma of the process of alveologenesis, largely based on 2D microscopy, is that alveoli arise by erection of finger-like interalveolar septae to form septal clefts that subdivide pre-existing saccules, a process for which the contractile properties of specialized alveolar myofibroblasts are necessary. Here I suggest that airway tip splitting and stereotypical side domain branching are actually conserved processes, but modified somewhat by evolution to achieve both airway tip splitting and side branching of the upper airway epithelium, as well as alveologenesis. Viewed in 3D it is clear that alveolar “septal tips” are in fact ring or purse string structures containing elastin and collagen that only appear as finger like projections in cross section. Therefore, I propose that airway branch orifices as well as alveolar mouth rings serve to delineate and stabilize the budding of both airway and alveolar epithelium, from the tips and sides of upper airways as well as from the sides and tips of alveolar ducts. Certainly, in the case of alveoli arising laterally and with radial symmetry from the sides of alveolar ducts, the mouth of each alveolus remains within the plane of the side of the ductal lumen. This suggests that the thin epithelium lining these lateral alveolar duct buds may extrude or “pop out” from the duct lumen through rings rather like soap or gum bubbles, whereas the thicker upper airway epithelium extrudes through a ring like toothpaste from a tube to form a new branch.

Targeting SHP2 as a therapeutic strategy for inflammatory diseases

With the change of lifestyle and the acceleration of aging process, inflammatory diseases have increasingly become one of the most vital threats to global human health. SHP2 protein is a non-receptor tyrosine phosphatase encoded by PTPN11 gene, and it is widely expressed in various tissues and cells. Numerous studies have shown that SHP2 plays important roles in the regulation of inflammatory diseases, including cancer-related inflammation, neurodegenerative diseases and metabolic diseases. In this paper, the roles of SHP2 in inflammatory diseases of various physiological systems were reviewed. At the same time, the latest SHP2 inhibitors were summarized, which will hold a promise for the therapeutic potential in future.

Regulation of vascular signalling by nuclear Sprouty2 in fetal lung epithelial cells: Implications for co-ordinated airway and vascular branching in lung development

Sprouty2 (Spry2) acts as a central regulator of tubular growth and branch patterning in the developing mammalian lung by controlling both magnitude and duration of growth factor signalling. To determine if this protein coordinates airway and vascular growth factor signalling, we tested the hypothesis that Spry2 links the primary cue for airway outgrowth, fibroblast growth factor-10 (FGF-10), to genomic events underpinning the expression and release of vascular endothelial growth factor-A (VEGF-A). Using primary fetal distal lung epithelial cells (FDLE) from rat, and immortalised human bronchial epithelial cells (16HBE14o-), we identified a nuclear sub-population of Spry2 which interacted with regions of the rat and human VEGF-A promoter spanning the hypoxia response element (HRE) and adjacent 3' sites. In FDLE cultured at the PO₂ of the fetal lung, FGF-10 relieved the Spry2 interaction at the HRE region by promoting clearance of a 39 kDa form and this was accompanied by histone-3 S10K14 phosphoacetylation, promoter de-methylation, hypoxia inducible factor-1α activation and VEGF-A expression. This repressive characteristic of nuclear Spry2 was relieved in 16HBE14o- by shRNA knockdown, and stable expression of mutants (C218A; C221A) that do not interact with the VEGF-A promoter HRE region. We conclude that nuclear Spry2 acts as a molecular link which co-ordinates airway and vascular growth of the cardiopulmonary system. This identifies Spry2 as a contributing determinant of design optimality in the mammalian lung.

Feedback regulation of RTK signaling in development

Precise regulation of the amplitude and duration of receptor tyrosine kinase (RTK) signaling is critical for the execution of cellular programs and behaviors. Understanding these control mechanisms has important implications for the field of developmental biology, and in recent years, the question of how augmentation or attenuation of RTK signaling via feedback loops modulates development has become of increasing interest. RTK feedback regulation is also important for human disease research; for example, germline mutations in genes that encode RTK signaling pathway components cause numerous human congenital syndromes, and somatic alterations contribute to the pathogenesis of diseases such as cancers. In this review, we survey regulators of RTK signaling that tune receptor activity and intracellular transduction cascades, with a focus on the roles of these genes in the developing embryo. We detail the diverse inhibitory mechanisms utilized by negative feedback regulators that, when lost or perturbed, lead to aberrant increases in RTK signaling. We also discuss recent biochemical and genetic insights into positive regulators of RTK signaling and how these proteins function in tandem with negative regulators to guide embryonic development.

Macrophage-Restricted Shp2 Tyrosine Phosphatase Acts as a Rheostat for MMP12 through TGF-β Activation in the Prevention of Age-Related Emphysema in Mice

Persistent activation of macrophages in lungs plays a critical role in the production of matrix metalloproteinases (MMPs) that contributes to the destruction of alveolar walls, a hallmark for pulmonary emphysema. Dysregulated TGF-β1 signaling has been an essential determinant in the elevation of MMPs during the development of emphysema. Nevertheless, the mechanism for this MMP-dependent pathogenesis has yet to be clearly investigated. Recently, we identified an important role for tyrosine phosphatase Src homology domain-containing protein tyrosine phosphatase 2 (Shp2) in regulating the activation of alveolar macrophages. Over a long-term observation period, mice with Shp2 deletion in macrophages (LysMCre:Shp2^fl/fl ) develop spontaneous, progressive emphysema-like injury in the lungs, characterized by massive destruction of alveolar morphology, interstitial extracellular matrix degradation, and elevated levels of MMPs, particularly, significant increases of macrophage elastase (MMP12) in aged mice. Further analysis demonstrated that MMP12 suppression by TGF-β1 activation was apparently abrogated in LysMCre:Shp2^fl/fl mice, whereas the TGF-β1 concentration in the lungs was relatively the same. Mechanistically, we found that loss of Shp2 resulted in attenuated SMAD2/3 phosphorylation and nuclear translocation in response to TGF-β activation, thereby upregulating MMP12 expression in macrophages. Together, our findings define a novel physiological function of Shp2 in TGF-β1/MMP12-dependent emphysema, adding insights into potential etiologies for this chronic lung disorder.

Shp2 Deficiency Impairs the Inflammatory Response Against Haemophilus influenzae by Regulating Macrophage Polarization

Macrophages can polarize and differentiate to regulate initiation, development, and cessation of inflammation during pulmonary infection with nontypeable Haemophilus influenzae (NTHi). However, the underlying molecular mechanisms driving macrophage phenotypic differentiation are largely unclear. Our study investigated the role of Shp2, a Src homology 2 domain-containing phosphatase, in the regulation of pulmonary inflammation and bacterial clearance. Shp2 levels were increased upon NTHi stimulation. Selective inhibition of Shp2 in mice led to an attenuated inflammatory response by skewing macrophages toward alternatively activated macrophage (M2) polarization. Upon pulmonary NTHi infection, Shp2(-/-) mice, in which the gene encoding Shp2 in monocytes/macrophages was deleted, showed an impaired inflammatory response and decreased antibacterial ability, compared with wild-type controls. In vitro data demonstrated that Shp2 regulated activated macrophage (M1) gene expression via activation of p65-nuclear factor-κB signaling, independent of p38 and extracellular regulated kinase-mitogen-activated proteins kinase signaling pathways. Taken together, our study indicates that Shp2 is required to orchestrate macrophage function and regulate host innate immunity against pulmonary bacterial infection.

SHP2 sails from physiology to pathology

Over the two past decades, mutations of the PTPN11 gene, encoding the ubiquitous protein tyrosine phosphatase SHP2 (SH2 domain-containing tyrosine phosphatase 2), have been identified as the causal factor of several developmental diseases (Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NS-ML), and metachondromatosis), and malignancies (juvenile myelomonocytic leukemia). SHP2 plays essential physiological functions in organism development and homeostasis maintenance by regulating fundamental intracellular signaling pathways in response to a wide range of growth factors and hormones, notably the pleiotropic Ras/Mitogen-Activated Protein Kinase (MAPK) and the Phosphoinositide-3 Kinase (PI3K)/AKT cascades. Analysis of the biochemical impacts of PTPN11 mutations first identified both loss-of-function and gain-of-function mutations, as well as more subtle defects, highlighting the major pathophysiological consequences of SHP2 dysregulation. Then, functional genetic studies provided insights into the molecular dysregulations that link SHP2 mutants to the development of specific traits of the diseases, paving the way for the design of specific therapies for affected patients. In this review, we first provide an overview of SHP2's structure and regulation, then describe its molecular roles, notably its functions in modulating the Ras/MAPK and PI3K/AKT signaling pathways, and its physiological roles in organism development and homeostasis. In the second part, we describe the different PTPN11 mutation-associated pathologies and their clinical manifestations, with particular focus on the biochemical and signaling outcomes of NS and NS-ML-associated mutations, and on the recent advances regarding the pathophysiology of these diseases.

Quantifying stretch and secretion in the embryonic lung: Implications for morphogenesis

Branching in the embryonic lung is controlled by a variety of morphogens. Mechanics is also believed to play a significant role in lung branching. The relative roles and interactions of these two broad factors are challenging to determine. We considered three hypotheses for explaining why tracheal occlusion triples branching with no overall increase in size. Both hypotheses are based on tracheal occlusion blocking the exit of secretions. (H1) Increased lumen pressure stretches tissues; stretch receptors at shoulders of growing tips increase local rate of branching. (H2) Blocking exit of secretions blocks advective transport of morphogens, leading to (H2a) increased overall concentration of morphogens or (H2b) increased flux of morphogens at specific locations. We constructed and analyzed computational models of tissue stretch and solute transport in a 3D lung geometry. Observed tissue stresses and stretches were predominantly in locations unrelated to subsequent branch locations, suggesting that tissue stretch (H1) is not the mechanism of enhancement of branching. Morphogen concentration in the mesenchyme (H2a) increased with tracheal occlusion, consistent with previously reported results. Morphogen flux at the epithelial surface (H2b) completely changed its distribution pattern when the trachea was occluded, tripling the number of locations at which it was elevated. Our results are consistent with the hypothesis that tracheal occlusion blocks outflow of secretions, leading to a higher number of high-flux locations at branching tips, in turn leading to a large increase in number of branching locations.

Hmga2 is required for canonical WNT signaling during lung development

Background

The high-mobility-group (HMG) proteins are the most abundant non-histone chromatin-associated proteins. HMG proteins are present at high levels in various undifferentiated tissues during embryonic development and their levels are strongly reduced in the corresponding adult tissues, where they have been implicated in maintaining and activating stem/progenitor cells. Here we deciphered the role of the high-mobility-group AT-hook protein 2 (HMGA2) during lung development by analyzing the lung of Hmga2-deficient mice (Hmga2^−/−).

Results

We found that Hmga2 is expressed in the mouse embryonic lung at the distal airways. Analysis of Hmga2^−/− mice showed that Hmga2 is required for proper cell proliferation and distal epithelium differentiation during embryonic lung development. Hmga2 knockout led to enhanced canonical WNT signaling due to an increased expression of secreted WNT glycoproteins Wnt2b, Wnt7b and Wnt11 as well as a reduction of the WNT signaling antagonizing proteins GATA-binding protein 6 and frizzled homolog 2. Analysis of siRNA-mediated loss-of-function experiments in embryonic lung explant culture confirmed the role of Hmga2 as a key regulator of distal lung epithelium differentiation and supported the causal involvement of enhanced canonical WNT signaling in mediating the effect of Hmga2-loss-of-fuction. Finally, we found that HMGA2 directly regulates Gata6 and thereby modulates Fzd2 expression.

Conclusions

Our results support that Hmga2 regulates canonical WNT signaling at different points of the pathway. Increased expression of the secreted WNT glycoproteins might explain a paracrine effect by which Hmga2-knockout enhanced cell proliferation in the mesenchyme of the developing lung. In addition, HMGA2-mediated direct regulation of Gata6 is crucial for fine-tuning the activity of WNT signaling in the airway epithelium. Our results are the starting point for future studies investigating the relevance of Hmga2-mediated regulation of WNT signaling in the adult lung within the context of proper balance between differentiation and self-renewal of lung stem/progenitor cells during lung regeneration in both homeostatic turnover and repair after injury.

Src homology 2-containing protein tyrosine phosphatase-2 acts as a negative regulator for MUC5AC transcription via the inhibition of the ERK1/2 MAPK signalling pathway in the airway

AIMS

Mucus hypersecretion has been frequently observed in inflammation respiratory diseases. However, the negative regulators for mucus overproduction have not been readily identified. Our work focused on identifying novel negative regulator that modulates mucus overproduction in the human respiratory system. Herein, we examined whether H2 O2 could induce MUC5AC transcription in a dose-dependent manner and activate tyrosine phosphatase (SHP)-2 in human airway epithelial cells.

METHODS

We performed qRT-PCR to detect the changes in MUC5AC transcription and dot-blotting analysis to investigate MUC5AC secretion as regulated by SHP-2.

RESULTS

H2 O2 induced MUC5AC transcription in a dose-dependent manner and dramatically activated SHP-2. In addition, whereas wild-type SHP-2 completely inhibited H2 O2 -induced MUC5AC transcription, siRNA-SHP-2 restored it interestingly, suggesting that SHP-2 may act as a negative regulator for mucus overproduction and hypersecretion in the human respiratory tract. Moreover, SHP-2 inhibited the ERK1/2 MAPK pathway, thus abolishing the signalling for MUC5AC transcription.

CONCLUSION

We found that H2 O2 induced SHP-2 activation, which acted as a suppressor in H2 O2 signalling to regulate MUC5AC transcription in the airway.

Shp2 plays an important role in acute cigarette smoke-mediated lung inflammation

Cigarette smoke (CS), the major cause of chronic obstructive pulmonary disease, contains a variety of oxidative components that were implicated in the regulation of Src homology domain 2-containing protein tyrosine phosphatase 2 (Shp2) activity. However, the contribution of Shp2 enzyme to chronic obstructive pulmonary disease pathogenesis remains unclear. We investigated the role of Shp2 enzyme in blockading CS-induced pulmonary inflammation. Shp2 levels were assessed in vivo and in vitro. Mice (C57BL/6) or pulmonary epithelial cells (NCI-H292) were exposed to CS or cigarette smoke extract (CSE) to induce acute injury and inflammation. Lungs of smoking mice showed increased levels of Shp2, compared with those of controls. Treatment of lung epithelial cells with CSE showed elevated levels of Shp2 associated with the increased release of IL-8. Selective inhibition or knockdown of Shp2 resulted in decreased IL-8 release in response to CSE treatment in pulmonary epithelial cells. In comparison with CS-exposed wild-type mice, selective inhibition or conditional knockout of Shp2 in lung epithelia reduced IL-8 release and pulmonary inflammation in CS-exposed mice. In vitro biochemical data correlate CSE-mediated IL-8 release with Shp2-regulated epidermal growth factor receptor/Grb-2-associated binders/MAPK signaling. Our data suggest an important role for Shp2 in the pathological alteration associated with CS-mediated inflammation. Shp2 may be a potential target for therapeutic intervention for inflammation in CS-induced pulmonary diseases.

Eya1 protein phosphatase regulates tight junction formation in lung distal epithelium

Little is known about the regulatory mechanisms underlying lung epithelial tight junction (TJ) assembly, which is inextricably linked to the preservation of epithelial polarity, and is highly coordinated by proteins that regulate epithelial cell polarity, such as aPKCζ. We recently reported that Eya1 phosphatase functions through aPKCζ-Notch1 signaling to control cell polarity in the lung epithelium. Here, we have extended these observations to TJ formation to demonstrate that Eya1 is crucial for the maintenance of TJ protein assembly in the lung epithelium, probably by controlling aPKCζ phosphorylation levels, aPKCζ-mediated TJ protein phosphorylation and Notch1-Cdc42 activity. Thus, TJs are disassembled after interfering with Eya1 function in vivo or during calcium-induced TJ assembly in vitro. These effects are reversed by reintroduction of wild-type Eya1 or partially inhibiting aPKCζ in Eya1siRNA cells. Moreover, genetic activation of Notch1 rescues Eya1(-/-) lung epithelial TJ defects. These findings uncover novel functions for the Eya1-aPKCζ-Notch1-Cdc42 pathway as a crucial regulatory mechanism of TJ assembly and polarity of the lung epithelium, providing a conceptual framework for future mechanistic and translational studies in this area.

Loss of Shp2 in alveoli epithelia induces deregulated surfactant homeostasis, resulting in spontaneous pulmonary fibrosis

Type II alveolar epithelial (AT-II) cells produce pulmonary surfactant proteins that are essential for alveolar function. AT-II is a major target in lung injury, and ineffective repair of damaged alveolar epithelia has been postulated to cause pulmonary fibrosis. Previous studies have shown that tyrosine phosphatase Shp2 is expressed highly in the embryonic lung epithelial buds, and Shp2 activity is required for FGF-induced lung branching morphogenesis. To investigate in vivo function of pulmonary Shp2, we generated alveoli epithelia-specific Shp2-knockout (Shp2(Δ/Δ)) mice. Shp2(Δ/Δ) mice exhibit marked reduction in surfactant proteins, disorganized lamellar bodies, increased alveolar epithelial apoptosis, and interstitial pulmonary fibrosis without preceding inflammation. Mechanistically, Shp2 acts to mediate expression of thyroid transcription factor 1 (TTF1) and ATP-binding cassette subfamily A member 3 (ABCA3). Shp2 also plays a central role in mediating FGF/GAB/ERK activity, required for epithelial repair program. Together, our results identify a novel role of tyrosine phosphatase Shp2 in surfactant homeostasis, and deregulation of Shp2 triggers spontaneous pulmonary fibrosis with minimal inflammation.

Six1 transcription factor is critical for coordination of epithelial, mesenchymal and vascular morphogenesis in the mammalian lung

Six1 is a member of the six-homeodomain family of transcription factors. Six1 is expressed in multiple embryonic cell types and plays important roles in proliferation, differentiation and survival of precursor cells of different organs, yet its function during lung development was hitherto unknown. Herein we show that Six1(-/-) lungs are severely hypoplastic with greatly reduced epithelial branching and increased mesenchymal cellularity. Six1 is expressed at the distal epithelial tips of branching tubules as well as in the surrounding distal mesenchyme. Six1(-/-) lung epithelial cells show increased expression of differentiation markers, but loss of progenitor cell markers. Six1 overexpression in MLE15 lung epithelial cells in vitro inhibited cell differentiation, but increases the expression of progenitor cell markers. In addition, Six1(-/-) embryos and newborn mice exhibit mesenchymal overproliferation, decreased Fgf10 expression and severe defects in the smooth muscle component of the bronchi and major pulmonary vessels. These defects lead to rupture of major vessels in mutant lungs after birth. Treatment of Six1(-/-) epithelial explants in culture with recombinant Fgf10 protein restores epithelial branching. As Shh expression is abnormally increased in Six1(-/-) lungs, we also treated mutant mesenchymal explants with recombinant Shh protein and found that these explants were competent to respond to Shh and continued to grow in culture. Furthermore, inhibition of Shh signaling with cyclopamine stimulated Six1(-/-) lungs to grow and branch in culture. This study provides the first evidence for the requirement of Six1 in coordinating Shh-Fgf10 signaling in embryonic lung to ensure proper levels of proliferation and differentiation along the proximodistal axis of epithelial, mesenchymal and endothelial cells. These findings uncover novel and essential functions for Six1 as a critical coordinator of Shh-Fgf10 signaling during embryonic lung development. We propose that Six1 is hence critical for coordination of proper lung epithelial, mesenchymal and vascular development.

Eya1 controls cell polarity, spindle orientation, cell fate and Notch signaling in distal embryonic lung epithelium

Cell polarity, mitotic spindle orientation and asymmetric division play a crucial role in the self-renewal/differentiation of epithelial cells, yet little is known about these processes and the molecular programs that control them in embryonic lung distal epithelium. Herein, we provide the first evidence that embryonic lung distal epithelium is polarized with characteristic perpendicular cell divisions. Consistent with these findings, spindle orientation-regulatory proteins Insc, LGN (Gpsm2) and NuMA, and the cell fate determinant Numb are asymmetrically localized in embryonic lung distal epithelium. Interfering with the function of these proteins in vitro randomizes spindle orientation and changes cell fate. We further show that Eya1 protein regulates cell polarity, spindle orientation and the localization of Numb, which inhibits Notch signaling. Hence, Eya1 promotes both perpendicular division as well as Numb asymmetric segregation to one daughter in mitotic distal lung epithelium, probably by controlling aPKCζ phosphorylation. Thus, epithelial cell polarity and mitotic spindle orientation are defective after interfering with Eya1 function in vivo or in vitro. In addition, in Eya1(-/-) lungs, perpendicular division is not maintained and Numb is segregated to both daughter cells in mitotic epithelial cells, leading to inactivation of Notch signaling. As Notch signaling promotes progenitor cell identity at the expense of differentiated cell phenotypes, we test whether genetic activation of Notch could rescue the Eya1(-/-) lung phenotype, which is characterized by loss of epithelial progenitors, increased epithelial differentiation but reduced branching. Indeed, genetic activation of Notch partially rescues Eya1(-/-) lung epithelial defects. These findings uncover novel functions for Eya1 as a crucial regulator of the complex behavior of distal embryonic lung epithelium.

Eyes absent 1 (Eya1) is a critical coordinator of epithelial, mesenchymal and vascular morphogenesis in the mammalian lung

The proper level of proliferation and differentiation along the proximodistal axis is crucial for lung organogenesis. Elucidation of the factors that control these processes will therefore provide important insights into embryonic lung development and regeneration. Eya1 is a transcription factor/protein phosphatase that regulates cell lineage specification and proliferation. Yet its functions during lung development are unknown. In this paper we show that Eya1(-/-) lungs are severely hypoplastic with reduced epithelial branching and increased mesenchymal cellularity. Eya1 is expressed at the distal epithelial tips of branching tubules as well as in the surrounding distal mesenchyme. Eya1(-/-) lung epithelial cells show loss of progenitor cell markers with increased expression of differentiation markers and cell cycle exit. In addition, Eya1(-/-) embryos and newborn mice exhibit severe defects in the smooth muscle component of the bronchi and major pulmonary vessels with decreased Fgf10 expression. These defects lead to rupture of the major vessels and hemorrhage into the lungs after birth. Treatment of Eya1(-/-) epithelial explants in culture with recombinant Fgf10 stimulates epithelial branching. Since Shh expression and activity are abnormally increased in Eya1(-/-) lungs, we tested whether genetically lowering Shh activity could rescue the Eya1(-/-) lung phenotype. Indeed, genetic reduction of Shh partially rescues Eya1(-/-) lung defects while restoring Fgf10 expression. This study provides the first evidence that Eya1 regulates Shh signaling in embryonic lung, thus ensuring the proper level of proliferation and differentiation along the proximodistal axis of epithelial, mesenchymal and endothelial cells. These findings uncover novel functions for Eya1 as a critical upstream coordinator of Shh-Fgf10 signaling during embryonic lung development. We conclude, therefore, that Eya1 function is critical for proper coordination of lung epithelial, mesenchymal and vascular development.

Control of HIF-1{alpha} and vascular signaling in fetal lung involves cross talk between mTORC1 and the FGF-10/FGFR2b/Spry2 airway branching periodicity clock

Lung development requires coordinated signaling between airway and vascular growth, but the link between these processes remains unclear. Mammalian target of rapamycin complex-1 (mTORC1) can amplify hypoxia-inducible factor-1α (HIF-1α) vasculogenic activity through an NH(2)-terminal mTOR binding (TOS) motif. We hypothesized that this mechanism coordinates vasculogenesis with the fibroblast growth factor (FGF)-10/FGF-receptor2b/Spry2 regulator of airway branching. First, we tested if the HIF-1α TOS motif participated in epithelial-mesenchymal vascular signaling. mTORC1 activation by insulin significantly amplified HIF-1α activity at fetal Po(2) (23 mmHg) in human bronchial epithelium (16HBE14o-) and induced vascular traits (Flk1, sprouting) in cocultured human embryonic lung mesenchyme (HEL-12469). This enhanced activation of HIF-1α by mTORC1 was abolished on expression of a HIF-1α (F99A) TOS-mutant and also suppressed vascular differentiation of HEL-12469 cocultures. Next, we determined if vasculogenesis in fetal lung involved regulation of mTORC1 by the FGF-10/FGFR2b/Spry2 pathway. Fetal airway epithelium displayed distinct mTORC1 activity in situ, and its hyperactivation by TSC1(-/-) knockout induced widespread VEGF expression and disaggregation of Tie2-positive vascular bundles. FGF-10-coated beads grafted into fetal lung explants from Tie2-LacZ transgenic mice induced localized vascular differentiation in the peripheral mesenchyme. In rat fetal distal lung epithelial (FDLE) cells cultured at fetal Po(2), FGF-10 induced mTORC1 and amplified HIF-1α activity and VEGF secretion without induction of ERK1/2. This was accompanied by the formation of a complex between Spry2, the cCBL ubiquitin ligase, and the mTOR repressor, TSC2, which abolished GTPase activity directed against Rheb, the G protein inducer of mTORC1. Thus, mTORC1 links HIF-1α-driven vasculogenesis with the FGF-10/FGFR2b/Spry2 airway branching periodicity regulator.

STAT1 is required for redifferentiation during Madin-Darby canine kidney tubulogenesis

Signal transducers and activators of transcription (STAT)1 is the key to the sequential control of Madin-Darby canine kidney tubulogenesis. Loss of STAT1 prevents redifferentiation. Constitutively active STAT1 is sufficient to restore cord formation but not mature lumens. These data suggest that STAT1 is necessary for the redifferentiation phase of tubulogenesis and that mature lumenogenesis requires a distinct signal.

Tubule formation in vitro using Madin-Darby canine kidney (MDCK) epithelial cells consists mainly of two processes. First, the cells undergo a partial epithelial–mesenchymal transition (pEMT), losing polarity and migrating. Second, the cells redifferentiate, forming cords and then tubules with continuous lumens. We have shown previously that extracellular signal-regulated kinase activation is required for pEMT. However, the mechanism of how the pEMT phase is turned off and the redifferentiation phase is initiated is largely unknown. To address the central question of the sequential control of these two phases, we used MDCK cells grown as cysts and treated with hepatocyte growth factor to model tubulogenesis. We show that signal transducer and activator of transcription (STAT)1 controls the sequential progression from the pEMT phase to the redifferentiation phase. Loss of STAT1 prevents redifferentiation. Constitutively active STAT1 allows redifferentiation to occur even when cells are otherwise prevented from progressing beyond the pEMT phase by exogenous activation of Raf. Moreover, tyrosine phosphorylation defective STAT1 partially restored cord formation in such cells, suggesting that STAT1 functions in part as nonnuclear protein mediating signal transduction in this process. Constitutively active or inactive forms of STAT1 did not promote lumen maturation, suggesting this requires a distinct signal.

Sprouty2-modulated Kras signaling rescues Shp2 deficiency during lens and lacrimal gland development

Shp2/Ptpn11 tyrosine phosphatase is a general regulator of the RTK pathways. By genetic ablation, we demonstrate that Shp2 is required for lacrimal gland budding, lens cell proliferation, survival and differentiation. Shp2 deletion disrupted ERK signaling and cell cycle regulation, which could be partially compensated by activated Kras signaling, confirming that Ras signaling was the main downstream target of Shp2 in lens and lacrimal gland development. We also showed that Sprouty2, a general suppressor of Ras signaling, was regulated by Shp2 positively at the transcriptional level and negatively at the post-translational level. Only in the absence of Sprouty2 could activated Kras signaling robustly rescue the lens proliferation and lacrimal-gland-budding defects in the Shp2 mutants. We propose that the dynamic regulation of Sprouty by Shp2 might be important not only for modulating Ras signaling in lens and lacrimal gland development, but also for RTK signaling in general.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

Development, repair and fibrosis: what is common and why it matters

The complex structure of the lung is developed sequentially, initially by epithelial tube branching and later by septation of terminal air sacs with accompanying coordinated growth of a variety of lung epithelial and mesenchymal cells. Groups of transcriptional factors, peptide growth factors and their intracellular signaling regulators, as well as extracellular matrix proteins are programmed to be expressed at appropriate levels in the right place at the right time to control normal lung formation. Studies of lung development and lung repair/fibrosis to date have discovered that many of the same factors that control normal development are also key players in lung injury repair and fibrosis. Transforming growth factor-beta (TGF-beta) family peptide signaling is a prime example. Lack of TGF-beta signaling results in abnormal lung branching morphogenesis and alveolarization during development, whereas excessive amounts of TGF-beta signaling cause severe hypoplasia in the immature lung and fibrosis in mature lung. This leads us to propose the 'Goldilocks' hypothesis of regulatory signaling in lung development and injury repair that everything must be done just right!

Stem/progenitor cells in lung development, injury repair, and regeneration

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.

Bud specific N-sulfation of heparan sulfate regulates Shp2-dependent FGF signaling during lacrimal gland induction

Preferential outgrowth of the bud cells forms the basis of branching morphogenesis. Here, we show that lacrimal gland development requires specific modification of heparan sulfates by Ndst genes at the tip of the lacrimal gland bud. Systemic and conditional knockout experiments demonstrate the tissue specific requirement of Ndst1 and Ndst2 in the lacrimal gland epithelial, but not mesenchymal, cells, and the functional importance of Ndst1 in Fgf10-Fgfr2b, but not of Fgf1-Fgfr2b, complex formation. Consistent with this, Fgf10-induced ectopic lacrimal gland budding in explant cultures is dependent upon Ndst gene dose, and epithelial deletion of Fgfr2 abolishes lacrimal gland budding, its specific modification of heparan sulfate and its phosphorylation of Shp2 (Ptpn11 - Mouse Genome Informatics). Finally, we show that genetic ablation of Ndst1, Fgfr2 or Shp2 disrupts ERK signaling in lacrimal gland budding. Given the evolutionarily conserved roles of these genes, the localized activation of the Ndst-Fgfr-Shp2 genetic cascade is probably a general regulatory mechanism of FGF signaling in branching morphogenesis.

The molecular functions of Shp2 in the Ras/Mitogen-activated protein kinase (ERK1/2) pathway

Shp2 is a ubiquitous tyrosine phosphatase containing Src Homology 2 domains which plays major biological functions in response to various growth factors, hormones or cytokines. This is essentially due to its particularity of promoting the activation of the Ras/Mitogen-activated protein kinase pathway. Recent progresses have been made in the understanding of the molecular mechanisms involved in this regulation. We review here, and discuss the physiological relevance, of the following molecular functions of Shp2 that have been proposed to couple the phosphatase to Ras activation: promoter of Grb2/Sos recruitment through direct binding to Grb2, binding partner and regulator of SHPS-1, negative regulator of Sprouty, negative regulator of RasGAP recruitment, and activator of Src through dephosphorylation of Src-regulatory proteins.