Infant Gut Microbiome Composition Is Associated with Non-Social Fear Behavior in a Pilot Study
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ARTICLE https://doi.org/10.1038/s41467-021-23281-y OPEN Infant gut microbiome composition is associated with non-social fear behavior in a pilot study Alexander L. Carlson 1, Kai Xia2, M. Andrea Azcarate-Peril3,4, Samuel P. Rosin 5, Jason P. Fine5, Wancen Mu5, Jared B. Zopp2, Mary C. Kimmel 2, Martin A. Styner 2,6, Amanda L. Thompson7,8, ✉ Cathi B. Propper1 & Rebecca C. Knickmeyer 2,9,10,11 Experimental manipulation of gut microbes in animal models alters fear behavior and relevant 1234567890():,; neurocircuitry. In humans, the first year of life is a key period for brain development, the emergence of fearfulness, and the establishment of the gut microbiome. Variation in the infant gut microbiome has previously been linked to cognitive development, but its rela- tionship with fear behavior and neurocircuitry is unknown. In this pilot study of 34 infants, we find that 1-year gut microbiome composition (Weighted Unifrac; lower abundance of Bac- teroides, increased abundance of Veillonella, Dialister, and Clostridiales) is significantly asso- ciated with increased fear behavior during a non-social fear paradigm. Infants with increased richness and reduced evenness of the 1-month microbiome also display increased non-social fear. This study indicates associations of the human infant gut microbiome with fear behavior and possible relationships with fear-related brain structures on the basis of a small cohort. As such, it represents an important step in understanding the role of the gut microbiome in the development of human fear behaviors, but requires further validation with a larger number of participants. 1 Frank Porter Graham Child Development Institute, University of North Carolina, Chapel Hill, NC, USA. 2 Department of Psychiatry, University of North Carolina, Chapel Hill, NC, USA. 3 Department of Medicine, University of North Carolina, Chapel Hill, NC, USA. 4 Microbiome Core Facility, University of North Carolina, Chapel Hill, NC, USA. 5 Department of Biostatistics, University of North Carolina, Chapel Hill, NC, USA. 6 Department of Computer Science, University of North Carolina, Chapel Hill, NC, USA. 7 Department of Anthropology, University of North Carolina, Chapel Hill, NC, USA. 8 Department of Nutrition, University of North Carolina, Chapel Hill, NC, USA. 9 Department of Pediatrics and Human Development, Michigan State University, East Lansing, MI, USA. 10 Institute for Quantitative Health Science and Engineering, Michigan State University, East Lansing, MI, USA. 11 C-RAIND Fellow and Co-Director, ✉ Michigan State University, East Lansing, MI, USA. email: knickmey@msu.edu NATURE COMMUNICATIONS | (2021) 12:3294 | https://doi.org/10.1038/s41467-021-23281-y | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-23281-y ear—a behavioral response to threat—is an evolutionarily volumes increase ~105%, 84%, and 104% from birth to 1 year of Fconserved mechanism that promotes survival. The emer- age, respectively37. Cellular processes underlying gross volumetric gence of fear is an important component of normal human changes include myelination of axons, synaptogenesis, prolifera- development. Fear behavior in response to different environ- tion and migration of glia, and differentiation of mental stimuli emerge on a schedule that appears to parallel oligodendrocytes38–40. The connectivity of these structures— developmentally-relevant fitness threats, supporting an evolu- especially bottom-up signaling from amygdalae to prefrontal tionary non-associative model of fear acquisition1,2. Around cortex—are crucially important for early affective learning and 6 months of age, infants demonstrate fear processing through behavior during infancy and childhood41. In addition, early-life discrimination of fearful faces from other facial expressions3–5. stressors have been shown to have long-term consequences on Fear of heights, strangers, and strange objects can be reliably amygdala function and anxious behavior42. Given the structural detected in the subsequent months6 and behavioral responses growth and long-term consequences of perturbations occurring increase in intensity until at least 11 months of age, with many during this time, the first year of life may represent a critical fear-evoking paradigms eliciting peak intensities around period for development of fear circuitry. 12 months of age7. Fear may act as a regulatory or protective The studies reviewed above suggest that the infant gut mechanism to balance the increase in mobility and exploratory microbiome may be a key component in the normal development behavior during this time8. This general developmental pattern is of fear behavior and associated brain structures. In addition, it observed in different individuals, but the intensity of fear in may have a role in subsequent psychopathology, since fear response to a specific threat can vary with high levels of devel- behavior represents a fundamental behavioral dimension that is opmental fear predicting the future emergence of anxiety disrupted in multiple psychiatric disorders. Altered microbial disorders9,10 while a lack of early fear behavior may be associated composition has been reported in individuals with generalized with future callous-unemotional traits11. anxiety disorder, autism, depression, schizophrenia, attention The microbiome has been associated with animal behavior deficit hyperactivity disorder (ADHD), and eating disorders43–47. across the Chordata phylum12 and may be one mechanism A meta-analysis of probiotic interventions48 suggests positive impacting fear behavior. Several studies have found that admin- effects on anxiety symptoms, and transplant of microbial com- istering probiotic species of Lactobacillus, Bifidobacterium,or munities from people with psychiatric illnesses into rodent Bacteroides yields anxiolytic effects in rodents13–16. In germ-free models can induce relevant behaviors not seen when microbial mouse models and a study in germ-free zebrafish, manipulating communities from healthy controls are transplanted22,49–53. the gut microbiome has marked effects on fear behavior17–23. For However, to date, there have been no studies testing the asso- example, germ-free mice demonstrate reduced expression of fear ciation of the gut microbiome with observed fear behavior and behavior compared to conventionally colonized mice in the open fear-related brain structures during early human development. field, elevated plus maze, and light-dark box, whereas germ-free For this pilot study, we recruited a prospective longitudinal rats display increased fear behavior24. In non-mammalian ver- cohort of 34 infants and followed them from 1 month to 1 year of tebrates, germ-free zebrafish display similar reductions in fear age. These timepoints were selected because they index key per- behavior compared to conventionalized controls in an open-field iods in the development of the microbiome, brain, and fear analog. The timing of exposure to the microbiome appears to be behavior. The developmental phase of the microbiome spans particularly important in the development of fear behavior. Germ these ages54, brain volumes show the greatest developmental free mice colonized with specific pathogen free microbiota at change55, and peak reactivity to fear-inducing stimuli occurs 3 weeks of age exhibit normal fear behavior. However, when around 1 year of age7. To reduce confounding effects of birth colonized later in development at 10 weeks of age, fear behavior mode, antibiotic exposure, and feeding practices, all infants were remains abnormal19,25. Germ-free animal models are a unique vaginally delivered, antibiotic naïve, and exclusively breastfed experimental system that do not completely reflect biological until their first study visit at 1 month. In this study, we tested reality. Never-the-less, these studies do suggest there are critical whether features of 1-month and 1-year gut microbiomes were neurodevelopmental windows in which individual differences in associated with the volume of brain regions involved in fear composition of the microbiome may impact fear behavior circuitry at each age as well as fear behaviors at 1 year of age. acquisition and expression. Specifically, we examined behavioral inhibition, the tendency to Microbiome manipulation in rodent models also results in display fear and withdrawal when presented with novel stimuli or structural and biochemical changes to brain structures canonically situations. Behavioral inhibition measured in infancy has been involved in fear circuitry including the amygdala, hippocampus, shown to predict internalizing psychopathology in adulthood10. andmedialprefrontalcortex26–30. For example, germ-free condi- Traditionally, behavioral inhibition has been treated as a unitary tions resulted in changes to the morphology of cells in the amygdala construct, but recent work suggests it may be useful to distinguish where dendritic length and spine density was increased along with between reactions to non-social and social stimuli which have increased overall amygdala volume31. There are also broad changes differing implications for predicting later pathology56.We in amygdala microRNA expression along with mRNA expression of hypothesized that infants with relatively greater amounts of NGFI-A, BDNF, and NR2B in germ free conditions18,20,32. Similar microbiota linked to reduced fear in mice, specifically effects are observed in the hippocampus where germ free mice have Lactobacillus13, Bifidobacterium15 and Bacteroides16,57, would larger