Sex and gender as critical and distinct contributors to the human brain-gut-microbiome axis.

The brain-gut-microbiome axis (BGMA) is a pivotal contributor to human health. A large body of research, especially from animal models, has revealed bidirectional, causal relationships between the BGMA and sex. In particular, sex steroids appear to be affected by the BGMA, to influence the BGMA, and to moderate environmental effects on the BGMA. However, animal research on the relationship between sex and the BGMA has not translated well to human models. We contend that this is due in part to an oversimplified approach to sex: although BGMA researchers have traditionally approached sex as a unidimensional, dichotomous variable, it is in fact multidimensional and is comprised of both multi-categorical and continuous dimensions. We also contend that research on the BGMA in humans should approach gender as a variable that is distinct from sex and that gender may influence the BGMA through pathways that are independent from the effects of sex alone. Research practices that consider the complexity and distinctiveness of sex and gender in relation to the human BGMA will not only yield improved understanding of this consequential system, but will also enhance the development of treatments for adverse health outcomes with BGMA-related etiologies. We conclude with recommendations for the implementation of such practices.

The effects of a probiotic formulation (Lactobacillus rhamnosus and L. helveticus) on developmental trajectories of emotional learning in stressed infant rats.

Recently, scientific interest in the brain-gut axis has grown dramatically, particularly with respect to the link between gastrointestinal and psychiatric dysfunction. However, the role of gut function in early emotional dysregulation is yet to be examined, despite the prevalence and treatment resistance of early-onset psychiatric disorders. The present studies utilized a developmental rodent model of early-life stress (ELS) to explore this gap. Rats were exposed to maternal separation (MS) on postnatal days 2-14. Throughout MS, dams received either vehicle or a probiotic formulation (previously shown to reduce gastrointestinal dysfunction) in their drinking water. Replicating past research, untreated MS infants exhibited an adult-like profile of long-lasting fear memories and fear relapse following extinction. In contrast, probiotic-exposed MS infants exhibited age-appropriate infantile amnesia and resistance to relapse. These effects were not mediated by changes in pups' or dams' anxiety at the time of training, nor by maternal responsiveness. Overall, probiotics acted as an effective and non-invasive treatment to restore normal developmental trajectories of emotion-related behaviors in infant rats exposed to ELS. These results provide promising initial evidence for this novel approach to reduce the risk of mental health problems in vulnerable individuals. Future studies are needed to test this treatment in humans exposed to ELS and to elucidate mechanisms for the observed behavioral changes.

The lasting impact of early-life adversity on individuals and their descendants: potential mechanisms and hope for intervention.

The adverse effects of early-life stress are pervasive, with well-established mental and physical health consequences for exposed individuals. The impact of early adverse experiences is also highly persistent, with documented increases in risk for mental illness across the life span that are accompanied by stable alterations in neural function and hormonal responses to stress. Here, we review some of these 'stress phenotypes', with a focus on intermediary factors that may signal risk for long-term mental health outcomes, such as altered development of the fear regulation system. Intriguingly, recent research suggests that such stress phenotypes may persist even beyond the life span of the individuals, with consequences for their offspring and grand-offspring. Phenotypic characteristics may be transmitted to future generations via either the matriline or the patriline, a phenomenon that has been demonstrated in both human and animal studies. In this review, we highlight behavioral and epigenetic factors that may contribute to this multigenerational transmission and discuss the potential of various treatment approaches that may halt the cycle of stress phenotypes.

The effect of adverse rearing environments on persistent memories in young rats: removing the brakes on infant fear memories.

Mental health problems are often assumed to have their roots in early-life experiences. However, memories acquired in infancy are rapidly forgotten in nearly all species (including humans). As yet, a testable mechanism on how early-life experiences have a lasting impact on mental health is lacking. In these experiments, we tested the idea that infant adversity leads to an early transition into adult-like fear retention, allowing infant memories to have a longer-lasting influence. Rats were exposed to maternal separation (3 h per day) across postnatal days (P) 2-14, or their mother was given corticosterone in her drinking water across the same period. Infant rats were then trained to fear a conditioned stimulus (CS) paired with an aversive unconditioned stimulus (US) on P17. Retention of the fear association was then tested 1-55 days later. When tested one day after the CS-US association was formed, both standard-reared (SR) and maternally-separated (MS) rats exhibited strong memory. However, when tested 10 days later, SR rats exhibited robust forgetting, whereas MS rats exhibited near-perfect retention. These effects were mimicked by exposing the mother to the stress hormone corticosterone in the drinking water. Finally, fear associations in P17 MS rats were retained for up to 30 days. Our findings point to differences in retention of fear as one factor that might underlie the propensity of stress-exposed individuals to exhibit early anxiety symptoms and suggest that manipulations of the corticosterone system may hold the key to ameliorating some of the effects of early stress on persistent retention of fear.

Glucose regulation of the acetyl-CoA carboxylase promoter PI in rat hepatocytes.

The rat acetyl-CoA carboxylase (ACC) alpha gene is transcribed from two promoters, denoted PI and PII, that direct regulated expression in a tissue-specific manner. Induction of ACC, the rate-controlling enzyme of fatty acid biosynthesis, occurs in the liver in response to feeding of a high carbohydrate, low fat diet, conditions that favor enhanced lipogenesis. This induction is mainly due to increases in PI promoter activity. We have used primary cultured hepatocytes from the rat to investigate glucose regulation of ACC expression. Glucose and insulin synergistically activated expression of ACC mRNAs transcribed from the PI promoter with little or no effect on PII mRNAs. Glucose treatment stimulated PI promoter activity in transfection assays and a glucose-regulated element was identified (-126/-102), homologous to those previously described in other responsive genes, including l-type pyruvate kinase, S(14) and fatty acid synthase. Mutation of this element eliminated the response to glucose. This region of the ACC PI promoter was able to bind a liver nuclear factor designated ChoRF that interacts with other conserved glucose-regulated elements. This ACC PI element is also capable of conferring a strong response to glucose when linked to a heterologous promoter. We conclude that induction of ACC gene expression under lipogenic conditions in hepatocytes is mediated in part by the activation of a glucose-regulated transcription factor, ChoRF, which stimulates transcription from the PI promoter. Similar mechanisms operate on related genes permitting the coordinate induction of the lipogenic pathway.

Mapping the polarity of changes that occur in interrupted CAG repeat tracts in yeast.

To explore the mechanisms by which CAG trinucleotide repeat tracts undergo length changes in yeast cells, we examined the polarity of alterations with respect to an interrupting CAT trinucleotide near the center of the tract. In wild-type cells, in which most tract changes are large contractions, the changes that retain the interruption are biased toward the 3' end of the repeat tract (in reference to the direction of lagging-strand synthesis). In rth1/rad27 mutant cells that are defective in Okazaki fragment maturation, the tract expansions are biased to the 5' end of the repeat tract, while the tract contractions that do not remove the interruption occur randomly on either side of the interruption. In msh2 mutant cells that are defective in the mismatch repair machinery, neither the small changes of one or two repeat units nor the larger contractions attributable to this mutation are biased to either side of the interruption. The results of this study are discussed in terms of the molecular paths leading to expansions and contractions of repeat tracts.

Orientation dependence of trinucleotide CAG repeat instability in Saccharomyces cerevisiae.

To examine the chromosomal stability of repetitions of the trinucleotide CAG, we have cloned CAG repeat tracts onto the 3' end of the Saccharomyces cerevisiae ADE2 gene and placed the appended gene into the ARO2 locus of chromosome VII. Examination of chromosomal DNA from sibling colonies arising from clonal expansion of strains harboring repeat tracts showed that repeat tracts often change in length. Most changes in tract length are decreases, but rare increases also occur. Longer tracts are more unstable than smaller tracts. The most unstable tracts, of 80 to 90 repeats, undergo changes at rates as high as 3 x 10(-2) changes per cell per generation. To examine whether repeat orientation or adjacent sequences alter repeat stability, we constructed strains with repeat tracts in both orientations, either with or without sequences 5' to ADE2 harboring an autonomously replicating sequence (ARS; replication origin). When CAG is in the ADE2 coding strand of strains harboring the ARS, the repeat tract is relatively stable regardless of the orientation of ADE2. When CTG is in the ADE2 coding strand of strains harboring the ARS, the repeat tract is relatively unstable regardless of the orientation of ADE2. Removal of the ARS as well as other sequences adjacent to the 5' end of ADE2 alters the orientation dependence such that stability now depends on the orientation of ADE2 in the chromosome. These results suggest that the proximity of an ARS or another sequence has a profound effect on repeat stability.