Dipalmitoyl-Phosphatidylcholine Biosynthesis is Induced by Non-Injurious Mechanical Stretch in a Model of Alveolar Type II Cells

The Role of Biophysical Factors in Organ Development: Insights from Current Organoid Models

Biophysical factors play a fundamental role in human embryonic development. Traditional in vitro models of organogenesis focused on the biochemical environment and did not consider the effects of mechanical forces on developing tissue. While most human tissue has a Young's modulus in the low kilopascal range, the standard cell culture substrate, plasma-treated polystyrene, has a Young's modulus of 3 gigapascals, making it 10,000-100,000 times stiffer than native tissues. Modern in vitro approaches attempt to recapitulate the biophysical niche of native organs and have yielded more clinically relevant models of human tissues. Since Clevers' conception of intestinal organoids in 2009, the field has expanded rapidly, generating stem-cell derived structures, which are transcriptionally similar to fetal tissues, for nearly every organ system in the human body. For this reason, we conjecture that organoids will make their first clinical impact in fetal regenerative medicine as the structures generated ex vivo will better match native fetal tissues. Moreover, autologously sourced transplanted tissues would be able to grow with the developing embryo in a dynamic, fetal environment. As organoid technologies evolve, the resultant tissues will approach the structure and function of adult human organs and may help bridge the gap between preclinical drug candidates and clinically approved therapeutics. In this review, we discuss roles of tissue stiffness, viscoelasticity, and shear forces in organ formation and disease development, suggesting that these physical parameters should be further integrated into organoid models to improve their physiological relevance and therapeutic applicability. It also points to the mechanotransductive Hippo-YAP/TAZ signaling pathway as a key player in the interplay between extracellular matrix stiffness, cellular mechanics, and biochemical pathways. We conclude by highlighting how frontiers in physics can be applied to biology, for example, how quantum entanglement may be applied to better predict spontaneous DNA mutations. In the future, contemporary physical theories may be leveraged to better understand seemingly stochastic events during organogenesis.

Mouse lung organoid responses to reduced, increased, and cyclic stretch

Most lung development occurs in the context of cyclic stretch. Alteration of the mechanical microenvironment is a common feature of many pulmonary diseases, with congenital diaphragmatic hernia (CDH) and fetal tracheal occlusion (FETO, a therapy for CDH) being extreme examples with changes in lung structure, cell differentiation, and function. To address limitations in cell culture and in vivo mechanotransductive models, we developed two mouse lung organoid (mLO) mechanotransductive models using postnatal day 5 (PND5) mouse lung CD326-positive cells and fibroblasts subjected to increased, decreased, and cyclic strain. In the first model, mLOs were exposed to forskolin (FSK) and/or disrupted (DIS) and evaluated at 20 h. mLO cross-sectional area changed by +59%, +24%, and -68% in FSK, control, and DIS mLOs, respectively. FSK-treated organoids had twice as many proliferating cells as other organoids. In the second model, 20 h of 10.25% biaxial cyclic strain increased the mRNAs of lung mesenchymal cell lineages compared with static stretch and no stretch. Cyclic stretch increased TGF-β and integrin-mediated signaling, with upstream analysis indicating roles for histone deacetylases, microRNAs, and long noncoding RNAs. Cyclic stretch mLOs increased αSMA-positive and αSMA-PDGFRα-double-positive cells compared with no stretch and static stretch mLOs. In this PND5 mLO mechanotransductive model, cell proliferation is increased by static stretch, and cyclic stretch induces mesenchymal gene expression changes important in postnatal lung development.

Phospholipases A2 as biomarkers in ARDS

Acute respiratory distress syndrome (ARDS) is a multifactorial life-threatening lung injury, characterized by diffuse lung inflammation and increased alveolocapillary barrier permeability. The different stages of ARDS have distinctive biochemical and clinical profiles. Despite the progress of our understanding on ARDS pathobiology, the mechanisms underlying its pathogenesis are still obscure. Herein, we review the existing literature about the implications of phospholipases 2 (PLA2s), a large family of enzymes that catalyze the hydrolysis of fatty acids at the sn-2 position of glycerophospholipids, in ARDS-related pathology. We emphasize on the versatile way of participation of different PLA2s isoforms in the distinct ARDS subgroup phenotypes by either potentiating lung inflammation and damage or by preserving the normal lung. Current research supports that PLA2s are associated with the progression and the outcome of ARDS. We herein discuss the transcellular communication of PLA2s through secreted extracellular vesicles and suggest it as a new mechanism of PLA2s involvement in ARDS. Thus, the elucidation of the spatiotemporal features of PLA2s expression may give new insights and provide valuable information about the risk of an individual to develop ARDS or advance to more severe stages, and potentially identify PLA2 isoforms as biomarkers and target for pharmacological intervention.

Impact of high tidal volume ventilation on surfactant metabolism and lung injury in infant rats

The poorly understood tolerance toward high tidal volume (V_T) ventilation observed in critically ill children and age-equivalent animal models may be explained by surfactant homeostasis. The aim of our prospective animal study was to test whether high V_T with adequate positive end-expiratory pressure (PEEP) is associated with surfactant de novo synthesis and secretion, leading to improved lung function, and whether extreme mechanical ventilation affects intracellular lamellar body formation and exocytosis. Rats (14 days old) were allocated to five groups: nonventilated controls, PEEP 5 cmH₂O with V_T of 8, 16, and 24 mL/kg, and PEEP 1 cmH₂O with V_T 24 mL/kg. Following 6 h of ventilation, lung function, surfactant proteins and phospholipids, and lamellar bodies were assessed by forced oscillation technique, quantitative real-time polymerase chain reaction, mass spectrometry, immunohistochemistry, and transmission electron microscopy. High V_T (24 mL/kg) with PEEP of 5 cmH₂O improved respiratory system mechanics and was not associated with lung injury, elevated surfactant protein expression, or surfactant phospholipid content. Extreme ventilation with V_T 24 mL/kg and PEEP 1 cmH₂O produced a mild inflammatory response and correlated with higher surfactant phospholipid concentrations in bronchoalveolar lavage fluid without affecting lamellar body count and morphology. Elevated phospholipid concentrations in the potentially most injurious strategy (V_T 24 mL/kg, PEEP 1 cmH₂O) need further evaluation and might reflect accumulation of biophysically inactive small aggregates. In conclusion, our data confirm the resilience of infant rats toward high V_T-induced lung injury and challenge the relevance of surfactant synthesis, storage, and secretion as protective factors.

Decompression sickness, fatness and active hydrophobic spots

Since decompression sickness (DCS) in humans was first described, mankind has embarked on an odyssey to prevent it. The demonstration that decompression releases bubbles, which mainly contain inert gas (nitrogen, helium), into the circulation and that the slower the decompression rate the lesser the incidence of DCS, resulted in 1908 in the publication of the first, reasonably safe diving tables. Besides the development of proper diving tables, the selection of divers is also of importance. A relationship between body composition and DCS was observed in dogs as long ago as the nineteenth century, an observation supported early in the twentieth century: "Really fat men should never be allowed to work in compressed air, and plump men should be excluded from high pressure caissons…or in diving to more than about 10 fathoms, and at this depth the time of their exposure should be curtailed. If deep diving is to be undertaken…. skinny men should be selected." Alas, nothing is that simple! From my own experience it was not always the fat diver who ended up in the treatment chamber with DCS. Therefore, other factors must be at play; gender, age, physical fitness, and the existence of a persistent foramen ovale (PFO) have all been studied as possible factors for the development of vascular gas bubbles and, therefore, for DCS. However, none of these factors, alone or in combination, explain why there are intra-individual or intra-cohort differences in bubble grades (BG). In other words, why does a dive I did today led to a high BG but the same dive next week lead to a low one? Or, why is there such a difference in BG amongst divers of more or less the same age, gender, body composition and physical fitness? In a letter in this issue, a novel hypothesis is postulated that may fill in these gaps; active hydrophobic spots (AHS). These AHS can be found at the luminal side of capillary, venous and arterial walls and have an oligolamellar lining. In an in vitro experiment, nanobubbles developed on AHS after a 'dive' to 1,000 kPa (90 msw). It appears that AHS consist of dipalmitoylphosphatidylcholine (DPPC), which is the main component of surfactant. It is proposed that DPPC may leak from the alveoli into the alveolar capillary and be transported to veins and arteries where it precipitates and forms AHS. Based on these ideas, it is hypothesized that AHS generate nanobubbles that can grow into microbubbles. When these microbubbles detach from the AHS they might also take along pieces of the AHS membrane making the AHS smaller or even disappear. This phenomenon could explain some of the earlier findings regarding the formation of microbubbles in divers. The fact that the presence of microbubbles differs between younger and older divers, after repetitive dives, and between experienced divers and novice divers can be explained by this model, and AHS may be the missing link we are looking for in our quest to understand and treat DCS. However, some reservations must be made. Firstly, these observations are derived from in vitro and animal experiments and whether or not they reflect a similar process in man remains unclear. Secondly, it appears that female divers have lower bubble grades after similar dives compared to male divers, suggesting lower decompression stress. If AHS is the main generator for microbubbles, there should be a difference in the presence of AHS between men and women. We do not know from these animal experiments whether there is a gender difference, neither does a literature search in PubMed provide us with an answer. Thirdly, as said before, DPPC is the main component of surfactant. All alveolar surfactant phospholipids, such as DPPC, are secreted to the alveolar space via exocystosis of the lamellar bodies (LB) from alveolar type II (ATII) cells. To form a functional air-blood barrier, alveolar type I and ATII cells are connected to each other by tight junctions. These tight junctions constitute the seal of the intercellular cleft and in that way form a true barrier between the alveolus and the capillary. Only small molecules like oxygen, carbon dioxide, etc. can penetrate through this barrier by themselves due to passive diffusion. All other (macro)molecules, including DPPC, need intermediate processes such as ion transport proteins, channels, metabolic pumps, etc. to gain access to the pulmonary capillary lumen. To my knowledge, no such mechanisms for DPPC or LB are known. A theoretical explanation might be the fact that the production of DPPC and the exocystosis of DPPC-containing LBs into the alveolar space can be stimulated by stretch. Stretch of the alveoli can switch on Ca2+ entry by either mechanosensitive channels, store-operated channels or second messenger-operated channels, which induces LB exocystosis. Furthermore, an ATP-release mechanism might also be responsible for the pulmonary alveolar mechanotransduction of LB. During diving, transpulmonary pressure changes occur which might induce additional alveolar stretch and thus, theoretically, an extra release of LB. However, whether or not such exocystosis of LB is vascularly orientated remains unclear. Besides which, the leakage of DPPC from the alveolus to the pulmonary capillary might also be as simple as a malfunction of the tight junction due to epithelial membrane damage as a result of diving. Finally, it is also possible that DPPC is produced in other non-ATII cells in our body of which we are currently unaware. To conclude, this is an interesting hypothesis regarding the origin of microbubbles. Whether or not DPPC and LB are the main reason for individual sensitivity to DCS remains unclear. Further research will hopefully identify if DPPC and LB are indeed the missing link or just another branch on the big tree of the genesis of decompression sickness.

The phospholipase A2 activity of peroxiredoxin 6

Peroxiredoxin 6 (Prdx6) is a Ca²⁺-independent intracellular phospholipase A₂ (called aiPLA₂) that is localized to cytosol, lysosomes, and lysosomal-related organelles. Activity is minimal at cytosolic pH but is increased significantly with enzyme phosphorylation, at acidic pH, and in the presence of oxidized phospholipid substrate; maximal activity with phosphorylated aiPLA₂ is ∼2 µmol/min/mg protein. Prdx6 is a "moonlighting" protein that also expresses glutathione peroxidase and lysophosphatidylcholine acyl transferase activities. The catalytic site for aiPLA₂ activity is an S32-H26-D140 triad; S32-H26 is also the phospholipid binding site. Activity is inhibited by a serine "protease" inhibitor (diethyl p-nitrophenyl phosphate), an analog of the PLA₂ transition state [1-hexadecyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol (MJ33)], and by two naturally occurring proteins (surfactant protein A and p67^phox), but not by bromoenol lactone. aiPLA₂ activity has important physiological roles in the turnover (synthesis and degradation) of lung surfactant phospholipids, in the repair of peroxidized cell membranes, and in the activation of NADPH oxidase type 2 (NOX2). The enzyme has been implicated in acute lung injury, carcinogenesis, neurodegenerative diseases, diabetes, male infertility, and sundry other conditions, although its specific roles have not been well defined. Protein mutations and animal models are now available to further investigate the roles of Prdx6-aiPLA₂ activity in normal and pathological physiology.

Impact of ventilation-induced lung injury on the structure and function of lamellar bodies

Alterations to the pulmonary surfactant system have been observed consistently in ventilation-induced lung injury (VILI) including composition changes and impairments in the surface tension reducing ability of the isolated extracellular surfactant. However, there is limited information about the effects of VILI on the intracellular form of surfactant, the lamellar body. It is hypothesized that VILI leads to alterations of lamellar body numbers and function. To test this hypothesis, rats were randomized to one of three groups, nonventilated controls, control ventilation, and high tidal volume ventilation (VILI). Following physiological assessment to confirm lung injury, isolated lamellar bodies were tested for surfactant function on a constrained sessile drop surfactometer. A separate cohort of animals was used to fix the lungs followed by examination of lamellar body numbers and morphology using transmission electron microscopy. The results showed an impaired ability of reducing surface tension for the lamellar bodies isolated from the VILI group as compared with the two other groups. The morphological assessment revealed that the number, and the relative area covered by, lamellar bodies were significantly decreased in animals with VILI animals as compared with the other groups. It is concluded that VILI causes significant alterations to lamellar bodies. It is speculated that increased secretion causes a depletion of lamellar bodies that cannot be compensated by de novo synthesis of surfactant in these injured lungs.

Mechanisms of ventilator-induced lung injury in healthy lungs

Mechanical ventilation is an essential method of patient support, but it may induce lung damage, leading to ventilator-induced lung injury (VILI). VILI is the result of a complex interplay among various mechanical forces that act on lung structures, such as type I and II epithelial cells, endothelial cells, macrophages, peripheral airways, and the extracellular matrix (ECM), during mechanical ventilation. This article discusses ongoing research focusing on mechanisms of VILI in previously healthy lungs, such as in the perioperative period, and the development of new ventilator strategies for surgical patients. Several experimental and clinical studies have been conducted to evaluate the mechanisms of mechanotransduction in each cell type and in the ECM, as well as the role of different ventilator parameters in inducing or preventing VILI. VILI may be attenuated by reducing the tidal volume; however, the use of higher or lower levels of positive end-expiratory pressure (PEEP) and recruitment maneuvers during the perioperative period is a matter of debate. Many questions concerning the mechanisms of VILI in surgical patients remain unanswered. The optimal threshold value of each ventilator parameter to reduce VILI is also unclear. Further experimental and clinical studies are necessary to better evaluate ventilator settings during the perioperative period in different types of surgery.

Alterations in rat pulmonary phosphatidylcholines after chronic exposure to ambient fine particulate matter

This study elucidated the underlying pathophysiological changes that occur after chronic ambient fine particulate matter (PM2.5) exposure via a lipidomic approach. Five male Sprague-Dawley rats were continually whole-body exposed to ambient air containing PM2.5 at 16.7 ± 10.1 μg m(-3) from the outside of the building for 8 months, whereas a control group (n = 5) inhaled filtered air. Phosphorylcholine-containing lipids were extracted from lung tissue and profiled using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The phosphatidylcholine (PC) signal features of the two groups were compared using partial least squares discriminant analysis (PLS-DA) and Wilcoxon rank sum tests. The PC profile of the exposure group differed from that of the control group; the R(2)Y and Q(2) were 0.953 and 0.677, respectively, in the PLS-DA model. In the exposure group, a significant 0.66- to 0.80-fold reduction in lyso-PC levels, which may have resulted from repeated inflammation, was observed. Decreased surfactant PCs by 16% at most may indicate injuries to alveolar type II cells. Cell function and cell signalling are likely to be altered because the decrease in unsaturated PCs may reduce membrane fluidity. Accompanied by the decline in plasmenylcholines, decreased unsaturated PCs may indicate the attack of reactive oxygen species generated by PM2.5 exposure. The physiological findings conformed to the histopathological changes in the exposed animals. PC profiling using UPLC-MS/MS-based lipidomics is sensitive for reflecting pathophysiological perturbations in the lung after long-term and low concentration PM2.5 exposure.