The microbiota influences cell death and microglial colonization in the perinatal mouse brain

Abstract

The mammalian fetus develops in a largely sterile environment, and direct exposure to a complex microbiota does not occur until birth. We took advantage of this to examine the effect of the microbiota on brain development during the first few days of life. The expression of anti- and pro-inflammatory cytokines, developmental cell death, and microglial colonization in the brain were compared between newborn conventionally colonized mice and mice born in sterile, germ-free (GF) conditions. Expression of the pro-inflammatory cytokines interleukin 1β and tumor necrosis factor α was markedly suppressed in GF newborns. GF mice also had altered cell death, with some regions exhibiting higher rates (paraventricular nucleus of the hypothalamus and the CA1 oriens layer of the hippocampus) and other regions exhibiting no change or lower rates (arcuate nucleus of the hypothalamus) of cell death. Microglial labeling was elevated in GF mice, due to an increase in both microglial cell size and number. The changes in cytokine expression, cell death and microglial labeling were evident on the day of birth, but were absent on embryonic day 18.5, approximately one-half day prior to expected delivery. Taken together, our results suggest that direct exposure to the microbiota at birth influences key neurodevelopmental events and does so within hours. These findings may help to explain some of the behavioral and neurochemical alterations previously seen in adult GF mice.

Microglial Depletion Causes Region‐Specific Changes to Developmental Neuronal Cell Death in the Mouse Brain

“…Data from males and females were combined in all analyses. Previous studies from our lab found no sex differences in cell death during the time points analyzed here (Ahern et al, 2013;Mosley et al, 2017a;Castillo-Ruiz et al, 2018a;2018b). We, and others, also have not found sex differences in microglial number or morphology in newborn mice (Strahan et al, 2017;Hanamsagar et al, 2017;Castillo-Ruiz et al, 2018a), although such sex differences have been reported in rats (Schwarz et al, 2012;VanRyzin et al, 2019) and adult mice (Hanamsagar et al, 2017;Guneykaya et al, 2018;Villa et al, 2018).…”

“…Previous studies from our lab found no sex differences in cell death during the time points analyzed here (Ahern et al , ; Mosley et al , ; Castillo‐Ruiz et al , ; ). We, and others, also have not found sex differences in microglial number or morphology in newborn mice (Strahan et al , ; Hanamsagar et al , ; Castillo‐Ruiz et al , ), although such sex differences have been reported in rats (Schwarz et al , ; VanRyzin et al ., ) and adult mice (Hanamsagar et al , ; Guneykaya et al , ; Villa et al , ). Two‐tailed independent t‐tests were used to compare saline and clodronate liposome groups for density of Iba1+ and AC3+ cells.…”

Does Birth Trigger Cell Death in the Developing Brain?

“…Together with birth timing, other extrinsic factors also modulate cell death. For example, delivery mode (Castillo-Ruiz et al, 2018b) and the exposure to a microbiota at birth (Castillo-Ruiz et al, 2018a) affect the magnitude of cell death in the neonatal mouse brain. Moreover, effects are regionally specific in both cases, as in the current study.…”

“…We estimated overall brain size by outlining the left side of the entire FB from the point where the medial border of the anterior commissure is located ventral to the tip of the lateral ventricle to the rostral most appearance of the dorsomedial nucleus of the hypothalamus (Paxinos et al, 2007, their Figs. 57-70 of the developing mouse brain atlas) as described previously (Castillo-Ruiz et al, 2018a).…”

Microbiome–microglia connections via the gut–brain axis

“…For example, microglia from E18.5 embryos from GF mothers show an increased density in various brain regions, with increased numbers of microglia exhibiting a ramified morphology, indicative of a resting state. Altered microglial morphology and density, as well as attenuated inflammatory responses, are also observed in offspring of GF dams immediately after birth, a time when microglia typically exhibit an activated phenotype (Castillo-Ruiz et al, 2018).…”