Cigarette smoke induces DNA damage and alters base-excision repair and tau levels in the brain of neonatal mice

Abstract

The prenatal and perinatal periods of brain development are especially vulnerable to insults by environmental agents. Early life exposure to cigarette smoke (CS), which contains both genotoxicants and oxidants, is considered an important risk factor for both neurodevelopmental and neurodegenerative disorders. Yet, little is known regarding the underlying pathogenetic mechanisms. In the present study, neonatal Swiss ICR (CD-1) albino mice were exposed to various concentrations of CS for 4 weeks and the brain examined for lipid peroxides, DNA damage, base-excision repair (BER) enzymes, apoptosis, and levels of the microtubule protein tau. CS induced a dose-dependent increase in both malondialdehyde and various types of DNA damage, including single-strand breaks, double-strand breaks, and DNA-protein cross-links. However, the CS-induced DNA damage in the brain returned to basal levels 1 week after smoking cessation. CS also modulated the activity and distribution of the BER enzymes 8-oxoguanine-DNA-glycosylase (OGG1) and apyrimidinic/apurinic endonuclease (APE1) in several brain regions. Normal tau (i.e., three-repeat tau, 3R tau) and various pathological forms of tau were also measured in the brain of CS-exposed neonatal mice, but only 3R tau and tau phosphorylated at serine 199 were significantly elevated. The oxidative stress, genomic dysregulation, and alterations in tau metabolism caused by CS during a critical period of brain development could explain why CS is an important risk factor for both neurodevelopmental and neurodegenerative disorders appearing in later life.

FIG. 1.

Distribution of SCGE data generated either in neutral (A) or in alkaline (B) environment, as related to the dose of whole-body CS exposure of mice. The data were generated by testing 100 nuclei per brain sample obtained from five mice in Group A, eight mice in Group B, eight mice in Group C, and five mice in Group D. The horizontal bold lines indicate the medians within each experimental group. *p < 0.01, as compared with the corresponding sham.

FIG. 2.

Distribution of SCGE data generated in alkaline environment, either in the absence (PK−) or presence (PK+) of proteinase K, as related to the dose of whole-body CS exposure of mice. The horizontal bold lines indicate the medians within each experimental group. Columns identified with (w) refer to mice that were sacrificed 1 week after discontinuation of CS exposure rather than the day after. *p < 0.05 and **p < 0.001, as compared with the corresponding sham. #p < 0.001, as compared with PK−.

FIG. 3.

Nissl staining and immunohistochemical detection of OGG1 and APE1 in the hippocampus (A) and cerebellum (B) of a sham-exposed mouse and a CS-exposed mouse. CA1, CA2 and CA3 indicate the three hippocampal pyramidal cell layers and DG indicates the dentate gyrus. Note the heavy staining of OGG1 in the CA1 (arrowheads) and DG (star) regions of the hippocampus. GC, granule cell; ML, molecular layer; PC, Purkinje cell.

FIG. 4.

OGG1 and APE1 activities in various brain regions of sham exposed (empty columns, n = 5) and mice exposed to CS, at a TPM of 438 mg/m³ (full columns, n = 3–5). *p < 0.01 and **p < 0.001, as compared with sham.

FIG. 5.

3R tau and pS199 levels in various brain regions of sham exposed (empty columns, n = 5) and mice exposed to CS, at a TPM of 438 mg/m³ (full columns, n = 3–5). *p < 0.05, as compared with sham.