Effects of leptin deficiency on postnatal lung development in mice

Abstract

Leptin modulates energy metabolism and lung development. We hypothesize that the effects of leptin on postnatal lung development are volume dependent from 2 to 10 wk of age and are independent of hypometabolism associated with leptin deficiency. To test the hypotheses, effects of leptin deficiency on lung maturation were characterized in age groups of C57BL/6J mice with varying Lep(ob) genotypes. Quasi-static pressure-volume curves and respiratory impedance measurements were performed to profile differences in respiratory system mechanics. Morphometric analysis was conducted to estimate alveolar size and number. Oxygen consumption was measured to assess metabolic rate. Lung volume at 40-cmH(2)O airway pressure (V(40)) increased with age in each genotypic group, and V(40) was significantly (P < 0.05) lower in leptin-deficient (ob/ob) mice beginning at 2 wk. Differences were amplified through 7 wk of age relative to wild-type (+/+) mice. Morphometric analysis showed that alveolar surface area was lower in ob/ob compared with +/+ and heterozygote (ob/+) mice beginning at 2 wk. Unlike the other genotypic groups, alveolar size did not increase with age in ob/ob mice. In another experiment, ob/ob at 4 wk received leptin replacement (5 microg.g(-1) x day(-1)) for 8 days, and expression levels of the Col1a1, Col3a1, Col6a3, Mmp2, Tieg1, and Stat1 genes were significantly increased concomitantly with elevated V(40). Leptin-induced increases in V(40) corresponded with enlarged alveolar size and surface area. Gene expression suggested a remodeling event of lung parenchyma after exogenous leptin replacement. These data support the hypothesis that leptin is critical to postnatal lung remodeling, particularly related to increased V(40) and enlarged alveolar surface area.

Fig. 1.

A: age-dependent changes in body weight in the three genotypic mice. B: age-dependent changes in lung volume at 40 cmH₂O (V₄₀) in the three genotypic mice. +/+, Wild type; ob/+, heterozygous; ob/ob, leptin deficient. Values are means ± SE. †P < 0.05 vs. +/+; *P < 0.01, ob/ob vs. +/+; §P < 0.01, ob/ob vs. ob/+.

Fig. 2.

Effects of chest wall mechanics on respiratory system impedance parameters expressed as percent change with the chest wall opened relative to the chest wall intact. Impedance parameters are as follows: Raw, airway resistance; G, tissue damping; H, tissue elastance; η, hysteresivity. Measurements were obtained in 10-wk-old +/+ (n = 7) and ob/ob (n = 8) mice. *P < 0.01, chest wall opened vs. intact.

Fig. 3.

Age-dependent changes in O₂ consumption (V̇o₂) in three genotypic mice. A: absolute V̇o₂, expressed as ml/min. B: V̇o₂ corrected for body weight, expressed as ml·min⁻¹·kg⁻¹. Values are means ± SE. *P < 0.01, ob/ob vs. +/+; §P < 0.01, ob/ob vs. ob/+.

Fig. 4.

A: effects of leptin on lung volume at airway pressures of 30 cmH₂O in 4-wk-old mice are dose dependent. An effective dose of 5 μg/g for 8 days significantly increased lung volume in both ob/+ and ob/ob mice, whereas 10 μg/g for 4 days did not significantly alter lung volume. B: time-dependent serum concentration levels of leptin after a single dose at 5 μg/g. †P < 0.01, leptin vs. saline treatment.

Fig. 5.

Effect of leptin treatment in ob/ob mice after an 8-day treatment with saline or leptin. A: changes in body weight. B: changes in food intake. Values are means ± SE. *P < 0.01, leptin vs. saline treatment.

Fig. 6.

Effect of leptin treatment on the quasi-static pressure-volume deflation curve in ob/ob mice. A: before normalizing to V₄₀. B: after normalizing to V₄₀. Values are means ± SE. *P < 0.01, leptin vs. saline treatment.

Fig. 7.

Effect of leptin treatment in ob/ob mice on lung architecture. A: alveolar mean chord length. B: alveolar number. C: alveolar surface area. D: absolute V̇o₂. Values are means ± SE. †P < 0.05, leptin vs. saline treatment.

Fig. 8.

Leptin treatment in ob/ob mice significantly upregulated the gene expression of procollagen I (Col1a1), III (Col3a1), and VI (Col6a3), insulin-like growth factor binding protein-2 (Igfbp2), matrix metalloproteinase 2 (Mmp2), signal transducer and activator of transcription 1 (Stat1) in lung tissue, and transforming growth factor-β-inducible early growth response 1 (Tieg1). The gene expression levels of elastin (Eln), leptin receptor (Lepr), insulin-like growth factor I (Igf1), insulin-like growth factor binding protein-2 (Igfbp2) and -4 (Igfbp4), serine peptidase inhibitor h1 (Serpinh1), and the surfactant proteins a1 (Sftpa1), b (Sftpb), c (Sftpc), and d (Sftpd) were not significantly altered by leptin treatment in ob/ob mice. Average values (±SE) are derived from quantitative PCR data referenced to the expression levels of GADPH. The vertical line at a gene expression level of 1 represents no detectable change in expression. †P < 0.05, leptin vs. saline treatment.