Respiratory mechanics following chronic cigarette smoke exposure in the Apoe$^{-/-}$ mouse model

Abstract

Even though cigarette smoking (CS) has been on the decline over the past 50 years, it is still the leading cause of preventable premature death in the United States. Preclinical models have investigated the cardiopulmonary effects of CS exposure (CSE), but the structure-function relationship in the respiratory system has not yet been fully explored. To evaluate these relationships, we exposed female apolipoprotein E-deficient (Apoe$^{-/-}$) mice to mainstream CS ($n=8$) for 5 days/week over 24 weeks with room air as a control (AE, $n=8$). To contextualize the impact of CSE, we also assessed the natural aging effects over 24 weeks of air exposure (baseline, $n=8$). Functional assessments were performed on a small animal mechanical ventilator (flexiVent, SCIREQ), where pressure–volume curves and impedance data at four levels of positive end-expiratory pressure ($P_{\text {peep}}$) and with increasing doses of methacholine were collected. Constant phase model parameters ($R_{\text {N}}$: Newtonian resistance, H: coefficient of tissue elastance, and G: coefficient of tissue resistance) were calculated from the impedance data. Perfusion fixed-left lung tissue was utilized for quantification of parenchyma airspace size and tissue thickness, airway wall thickness, and measurements of elastin, cytoplasm + nucleus, fibrin, and collagen content for the parenchyma and airways. Aging caused the lung to become more compliant, with an upward–leftward shift of the pressure–volume curve and a reduction in all constant phase model parameters. This was supported by larger parenchyma airspace sizes, with a reduction in cell cytoplasm + nucleus area. Airway walls became thinner, even though low-density collagen content increased. In contrast, CSE caused a downward–rightward shift of the pressure–volume curve along with an increase in H, G, and hysteresivity ($\eta = G/H$). Organ stiffening was accompanied by enhanced airway hyper-responsiveness following methacholine challenge. Structurally, parenchyma airspaces enlarged, as indicated by an increase in equivalent airspace diameter ($D_0$), and the septum thickened with significant deposition of low-density collagen along with an influx of cells. Airway walls thickened due to deposition of both high and low-density collagen, infiltration of cells, and epithelial cell elongation. In all, our data suggest that CSE in female Apoe$^{-/-}$ mice reduces respiratory functionality and causes morphological alterations in both central and peripheral airways that results in lung stiffening, compared to AE controls.

Appendix

$\alpha$ :

$\begin{aligned} \frac{2}{\pi } {\text {tan}}^{-1} \Big ( \frac{1}{\eta } \Big ) \end{aligned}$

$\eta$ :

hysteresivity, $\Big ( \frac{G}{H} \Big )$

$\Lambda$ :

Non-dimensional P-V curve shape parameters

$\omega$ :

Angular frequency, $\omega=2{\pi}f$

$\omega_{n}$ :

Angular frequency normalized by 1 rad/s

$\overline{R _{rs}}$ :

Real impedances, group average, Eqn. 5

$\overline{X _{rs}}$ :

Imaginary impedances, group average, Eqn. 5

$\theta$ :

Vector of parameters minimizing, Eqn. 5

$C_{st}$ :

Static compliance

d :

Pressure range where largest change in volume occurs, Fig. 2A

$D_{0}$ :

Parenchyma airspace diameter

f :

Frequency

G :

Coeffcient of tissue resistance

H :

Coeffcient of tissue elastance

HSL :

Hue, Saturation, and Light

$L{m}$ :

Mean linear intercept

N :

Number of frequencies samples, Eqn. 5

$P_{0}$ :

In ection pressure, Fig. 2A

$P_{cu}$ :

Lower corner pressure, Fig. 2A

$P_{cu}$ :

Upper corner pressure, Fig. 2A

$P _{PV}(t)$ :

Time dependent pressure

$R_{N}$ :

Newtonian resistance

$R_{rs}$ :

Real part of impedance, $\mathfrak{R}(Z)$

$V_{L}$ :

Lower asymptotic volume, Fig. 2A

$V_{U}$ :

Upper asymptotic volume, Fig. 2A

$V _{PV}(t)$ :

Time dependent volume

$X_{rs}$ :

Imaginary part of impedance, $\mathfrak{R}(Z)$

Z :

Impedance

AE:

Air Exposure

CS:

Cigarette Smoke

CSE:

Cigarette Smoke Exposure

FRC:

Functional Residual Capacity

Mch:

Methacholine

TLC:

Total Lung Capacity