Postnatal morphological lung development of wild type and CD26/DPP4 deficient rat pups in dependency of LPS exposure

The expression of the surfactant proteins SP-A and SP-B during postnatal alveolarization of the rat lung

OBJECTIVE

Surfactant-specific proteins (SP) are responsible for the functional and structural integrity as well as for the stabilization of the intra-alveolar surfactant. Morphological lung maturation starts in rat lungs after birth. The aim of this study was to investigate whether the expression of the hydrophilic SP-A and the hydrophobic SP-B is associated with characteristic postnatal changes characterizing morphological lung maturation.

METHODS

Stereological methods were performed on the light microscope. Using immunohistochemical and molecular biological methods (Western Blot, RT-qPCR), the SP-A and SP-B of adult rat lungs and of those with different postnatal developmental stages (3, 7, 14 and 21 days after birth) were characterized.

RESULTS

As signs of alveolarization the total septal surface and volume increased and the septal thickness decreased. The significantly highest relative surface fraction of SP-A labeled alveolar epithelial cells type II (AEII) was found together with the highest relative SP-A gene expression before the alveolarization (3th postnatal day). With the downregulation of SP-A gene expression during and after alveolarization (between postnatal days 7 and 14), the surface fraction of the SP-A labeled AEII also decreased, so they are lowest in adult animals. The surface fraction of SP-B labeled AEII and the SP-B gene expression showed the significantly highest levels in adults, the protein expression increased also significantly at the end of morphological lung maturation. There were no alterations in the SP-B expression before and during alveolarization until postnatal day 14. The protein expression as well as the gene expression of SP-A and SP-B correlated very well with the total surface of alveolar septa independent of the postnatal age.

CONCLUSION

The expression of SP-A and SP-B is differentially associated with morphological lung maturation and correlates with increased septation of alveoli as indirect clue for alveolarization.

Divergent Pattern of Development in Rats and Humans

Rattus norvegicus is the second most used laboratory species and the most widely used model in neuroscience. Nonetheless, there is still no agreement regarding the temporal relationship of development between humans and rats. We addressed this question by examining the time required to reach a set of homologous developmental milestones in both species. With this purpose, a database was generated with data collected through a bibliographic survey. This database was in turn compared with other databases about the same topic present in the literature. Finally, the databases were combined, covering for the first time the entire development of the rat including the prenatal, perinatal, and postnatal periods. This combined database includes 362 dates of 181 developmental events for each species. The developmental relationship between humans and rats was better fit by a logarithmic function than by a linear function. As development progresses, an increase in the dispersion of the data is observed. Developmental relationships should not be interpreted as a univocal equivalence. In this work is proposed an alternative interpretation where the age of one species is translated into a range of ages in the other.

Lung development and immune status under chronic LPS exposure in rat pups with and without CD26/DPP4 deficiency

Dipeptidyl-peptidase IV (CD26), a multifactorial integral type II protein, is expressed in the lungs during development and is involved in inflammation processes. We tested whether daily LPS administration influences the CD26-dependent retardation in morphological lung development and induces alterations in the immune status. Newborn Fischer rats with and without CD26 deficiency were nebulized with 1 µg LPS/2 ml NaCl for 10 min from days postpartum (dpp) 3 to 9. We used stereological methods and fluorescence activated cell sorting (FACS) to determine morphological lung maturation and alterations in the pulmonary leukocyte content on dpp 7, 10, and 14. Daily LPS application did not change the lung volume but resulted in a significant retardation of alveolarization in both substrains proved by significantly lower values of septal surface and volume as well as higher mean free distances in airspaces. Looking at the immune status after LPS exposure compared to controls, a significantly higher percentage of B lymphocytes and decrease of CD4⁺CD25⁺ T cells were found in both subtypes, on dpp7 a significantly higher percentage of CD4 T⁺ cells in CD26⁺ pups, and a significantly higher percentage of monocytes in CD26⁻ pups. The percentage of T cells was significantly higher in the CD26-deficient group on each dpp. Thus, daily postnatal exposition to low doses of LPS for 1 week resulted in a delay in formation of secondary septa, which remained up to dpp 14 in CD26⁻ pups. The retardation was accompanied by moderate parenchymal inflammation and CD26-dependent changes in the pulmonary immune cell composition.

A comparison of airway pressures for inflation fixation of developing mouse lungs for stereological analyses

The morphometric analysis of lung structure using the principles of stereology has emerged as a powerful tool to describe the structural changes in lung architecture that accompany the development of lung disease that is experimentally modelled in adult mice. These stereological principles are now being applied to the study of the evolution of the lung architecture over the course of prenatal and postnatal lung development in mouse neonates and adolescents. The immature lung is structurally and functionally distinct from the adult lung, and has a smaller volume than does the adult lung. These differences have raised concerns about whether the inflation fixation of neonatal mouse lungs with the airway pressure (P_aw) used for the inflation fixation of adult mouse lungs may cause distortion of the neonatal mouse lung structure, leading to the generation of artefacts in subsequent analyses. The objective of this study was to examine the impact of a P_aw of 10, 20 and 30 cmH₂O on the estimation of lung volumes and stereologically assessed parameters that describe the lung structure in developing mouse lungs. The data presented demonstrate that low P_aw (10 cmH₂O) leads to heterogeneity in the unfolding of alveolar structures within the lungs, and that high P_aw (30 cmH₂O) leads to an overestimation of the lung volume, and thus, affects the estimation of volume-dependent parameters, such as total alveoli number and gas-exchange surface area. Thus, these data support the use of a P_aw of 20 cmH₂O for inflation fixation in morphometric studies on neonatal mouse lungs.

Impact of litter size on survival, growth and lung alveolarization of newborn mouse pups

BACKGROUND

Lung alveolarization, the development of the alveoli, is disturbed in preterm infants with bronchopulmonary dysplasia (BPD), the most common complication of preterm birth. Animal models based on oxygen toxicity to the developing mouse lung are used to understand the mechanisms of stunted alveolarization in BPD, and to develop new medical management strategies for affected infants. The toxicity of genetic and pharmacological interventions, together with maternal cannibalism, reduce mouse litter sizes in experimental studies. The impact of litter size on normal and stunted lung alveolarization is unknown, but may influence data interpretation. The aim of the study was to assess the impact of litter size on normal and oxygen-stunted lung alveolarization in mice.

METHODS

BPD was experimentally modelled in newborn C57BL/6J mice by exposure to 85% O₂ in the inspired air for the first 14 days of post-natal life. Perturbations to mouse lung architecture were assessed by design-based stereology, in which the alveolar density, total number of alveoli, gas-exchange surface area, and the septal thickness were estimated.

RESULTS

Litter sizes of a single mouse were not viable to post-natal day 14. Normal lung alveolarization was comparable in mouse pups in litters of 2, 4, 6, and 8 pups per litter. Hyperoxia was equally effective at stunting lung alveolarization in mouse pups in litters of 2, 4, 6, and 8 pups per litter.

CONCLUSIONS

Studies on normal lung alveolarization as well as alveolarization stunted by oxygen toxicity can be undertaken in mouse litters as small as two pups, and as large as eight pups. There is no evidence to suggest that data cannot be compared within and between litters of two to eight mouse pups.