Developmental neurotoxicity of inhaled ambient ultrafine particle air pollution: Parallels with neuropathological and behavioral features of autism and other neurodevelopmental disorders

Abstract

Accumulating evidence from both human and animal studies show that brain is a target of air pollution. Multiple epidemiological studies have now linked components of air pollution to diagnosis of autism spectrum disorder (ASD), a linkage with plausibility based on the shared mechanisms of inflammation. Additional plausibility appears to be provided by findings from our studies in mice of exposures from postnatal day (PND) 4-7 and 10-13 (human 3rd trimester equivalent), to concentrated ambient ultrafine (UFP) particles, considered the most reactive component of air pollution, at levels consistent with high traffic areas of major U.S. cities and thus highly relevant to human exposures. These exposures, occurring during a period of marked neuro- and gliogenesis, unexpectedly produced a pattern of developmental neurotoxicity notably similar to multiple hypothesized mechanistic underpinnings of ASD, including its greater impact in males. UFP exposures induced inflammation/microglial activation, reductions in size of the corpus callosum (CC) and associated hypomyelination, aberrant white matter development and/or structural integrity with ventriculomegaly (VM), elevated glutamate and excitatory/inhibitory imbalance, increased amygdala astrocytic activation, and repetitive and impulsive behaviors. Collectively, these findings suggest the human 3rd trimester equivalent as a period of potential vulnerability to neurodevelopmental toxicity to UFP, particularly in males, and point to the possibility that UFP air pollution exposure during periods of rapid neuro- and gliogenesis may be a risk factor not only for ASD, but also for other neurodevelopmental disorders that share features with ASD, such as schizophrenia, attention deficit disorder, and periventricular leukomalacia.

Keywords: Air pollution; Autism; Corpus callosum; Excitatory–inhibitory imbalance; Lateral ventricle; Myelination; Periventricular leukomalacia; Schizophrenia; Ultrafine particles; White matter.

Figure 1

CAPS exposure leads to male specific and time dependent increases in IBA-1. IBA-1 levels (group mean ± S.E.) as percent of time point control plotted separately for males and females for Air and CAPS-treated groups at PND14 and PND55 in anterior commissure and hippocampus (Panel A) and at PND 270 in corpus callosum (Panel B). CAPS = concentrated ambient ultrafine particles; PND=postnatal day; Air-14= brains from Air exposed offspring at postnatal day 14; Air-55=brains from Air exposed offspring at postnatal day 55; CAPS-14= brains collected at postnatal day 55 from offspring exposed to concentrated ambient ultrafine particles; CAPS-55= brains collected at postnatal day 14 from offspring exposed to concentrated ambient ultrafine particles; brains collected at postnatal day 55 from offspring exposed to concentrated ambient ultrafine particles; * = significantly different from corresponding time point control. N=4-5/group.

Figure 2

CAPS exposure leads to increase in lateral ventricle size; effects were graded with the example demonstrating the most severe effects shown here. Panels A-C: Rostral to caudal section of Air exposed males at PND 14 with lateral ventricle (LV) becoming visible indicted by arrowhead in Panel C. Panels D-E Matched Rostral to caudal sections of CAPS-treated males at PND14. Note: LV and CC damage apparent rostrally; Arrowheads in Panel D shows bridge across ventricle (fusion) and coarctation in LV in Panel E. Magnification 2×; CAPS = concentrated ambient ultrafine particles; PND=postnatal day; FCtx=frontal cortex; Cpu=caudate putamen. n=4-5/group.

Figure 3

CAPS exposure leads to male specific increased ventricular size. Lateral ventricle area (group mean ± S.E) of males (left panel) and females (right panel) treated with Air or postnatal CAPS were measured at three time points: PND 14 (defined 24 hr post exposure), PND 55 and PND 270; n=4-6/group, average of at least 3 sections/brain. CAPS = concentrated ambient ultrafine particles; PND=postnatal day. Male data modified from [80] and female data unpublished.

Figure 4

CAPS treatment reduces size of CC in both sexes at PND14 with a trend towards greater magnitude effects in males. Panels A–D: CC size (μm²) of individual male and female mice following Air of postnatal CAPS treatment as measured at PND14 and PND270. Each data point represents an individual value. Sample sizes of 3-5 for PND14 and 7-11 for PND270. Panel E. Group mean ± S.E. size of CC across time points for male and females in treatment groups as indicated. CAPS = concentrated ambient ultrafine particles; PND=postnatal day; CC=corpus callosum.

Figure 5

CAPS treatment induces male specific CC hypomyelination at PND14. Panels A-D. Fluorescent intensity of myelin basic protein labeling expressed as percent of total CC area at PND14 (Panels A,B) and PND270 (Panels C,D) for male and female Air vs. postnatal CAPS-treated mice as measured at 3 different sites along the CC: truncus, medial external capsule and lateral external capsule for treatment groups as indicated. Shown is group mean ± S.E. Panels E-G. Relative area of MBP labeling across time points for male and females in treatment groups as indicated for truncus, medial external capsule (MEC) and lateral external capsule (LEC), respectively. Treatment × sex × time point indicates significant 3-way interaction in overall ANOVAs. Sample sizes of 3-5 for PND14 and 4-11 for PND270. Shown is group mean ± S.E. CAPS = concentrated ambient ultrafine particles; PND=postnatal day; CC=corpus callosum.

Figure 6

CAPS exposure does not alter striatal myelination in either sex. Panels A-B: Fluorescent intensity of myelin basic protein labeling expressed as arbitrary units in striatum at PND14 and PND270, respectively, for male and female Air vs. postnatal CAPS-treated mice. Panel C: Trajectory of density of myelin basic protein staining between PND14 and PND270 for males and females for indicated treatment groups. Sample sizes of 3-5 for PND14 and 4-11 for PND270. Shown is group mean ± S.E. CAPS = concentrated ambient ultrafine particles; PND=postnatal day.

Figure 7

CAPS treatment does not alter frontal cortex myelination in either sex. Panels A-B: Density of myelin basic protein staining (μm²) in frontal cortex at PND14 and PND270, respectively, for male and female Air vs. postnatal CAPS-treated mice. Panel C: Trajectory of density of myelin basic protein staining between PND14 and PND270 for males and females for indicated treatment groups. Sample sizes of 3-5 for PND14 and 4-11 for PND270. Data are group mean ± S.E. CAPS = concentrated ambient ultrafine particles; PND=postnatal day.

Figure 8

CAPS produced a male specific frontal cortex excitatory/inhibitory imbalance. Glutamate/GABA ratios based on levels of frontal cortex at PND14 and PND60 as based on extracellular glutamate, glutamine and GABA in Air vs. postnatal CAPS treated males (left panel) and female (right panel) mice. CAPS = concentrated ambient ultrafine particles; PND=postnatal day; Treatment= main effect of CAPS in statistical analysis; timepoint=main effect of time point in statistical analysis, n=9-12/group. Data modified from [80] and [81]. Shown are group means ± S.E

Figure 9

CAPS exposure increased GFAP labeling in amygdala. Fluorescent intensity of GFAP labeling in amygdala in male mice exposed to postnatal CAPS at PND14 and normalized to Air exposed controls. n=5/gp. Shown is group mean ± S.E. CAPS = concentrated ambient ultrafine particles; PND=postnatal day; GFAP=glial fibrillary acidic protein.

Figure 10

Cumulative records of individual male from indicated treatment groups on the fixed ratio (FR) wait schedule. Y axis response counter resets to baseline after each 30 responses; pips (backslashes) = reward delivery, horizontal lines = periods of waiting, vertical lines = FR responding. Modified from [79].