Regional Brain Changes in Aging Healthy Adults: General Trends, Individual Differences and Modifiers

Brain aging research relies mostly on cross-sectional studies, which infer true changes from age differences. We present longitudinal measures of five-year change in the regional brain volumes in healthy adults. Average and individual differences in volume changes and the effects of age, sex and hypertension were assessed with latent difference score modeling. The caudate, the cerebellum, the hippocampus and the association cortices shrunk substantially. There was minimal change in the entorhinal and none in the primary visual cortex. Longitudinal measures of shrinkage exceeded cross-sectional estimates. All regions except the inferior parietal lobule showed individual differences in change. Shrinkage of the cerebellum decreased from young to middle adulthood, and increased from middle adulthood to old age. Shrinkage of the hippocampus, the entorhinal cortices, the inferior temporal cortex and the prefrontal white matter increased with age. Moreover, shrinkage in the hippocampus and the cerebellum accelerated with age. In the hippocampus, both linear and quadratic trends in incremental age-related shrinkage were limited to the hypertensive participants. Individual differences in shrinkage correlated across some regions, suggesting common causes. No sex differences in age trends except for the caudate were observed. We found no evidence of neuroprotective effects of larger brain size or educational attainment.

Invited review: Aging and sarcopenia

Aging is associated with progressive loss of neuromuscular function that often leads to progressive disability and loss of independence. The term sarcopenia is now commonly used to describe the loss of skeletal muscle mass and strength that occurs in concert with biological aging. By the seventh and eighth decade of life, maximal voluntary contractile strength is decreased, on average, by 20-40% for both men and women in proximal and distal muscles. Although age-associated decreases in strength per unit muscle mass, or muscle quality, may play a role, the majority of strength loss can be accounted for by decreased muscle mass. Multiple factors lead to the development of sarcopenia and the associated impact on function. Loss of skeletal muscle fibers secondary to decreased numbers of motoneurons appears to be a major contributing influence, but other factors, including decreased physical activity, altered hormonal status, decreased total caloric and protein intake, inflammatory mediators, and factors leading to altered protein synthesis, must also be considered. The prevalence of sarcopenia, which may be as high as 30% for those >/=60 yr, will increase as the percentage of the very old continues to grow in our populations. The link between sarcopenia and disability among elderly men and women highlights the need for continued research into the development of the most effective interventions to prevent or at least partially reverse sarcopenia, including the role of resistance exercise and other novel pharmacological and nutritional interventions.

Cortisol levels during human aging predict hippocampal atrophy and memory deficits

Elevated glucocorticoid levels produce hippocampal dysfunction and correlate with individual deficits in spatial learning in aged rats. Previously we related persistent cortisol increases to memory impairments in elderly humans studied over five years. Here we demonstrate that aged humans with significant prolonged cortisol elevations showed reduced hippocampal volume and deficits in hippocampus-dependent memory tasks compared to normal-cortisol controls. Moreover, the degree of hippocampal atrophy correlated strongly with both the degree of cortisol elevation over time and current basal cortisol levels. Therefore, basal cortisol elevation may cause hippocampal damage and impair hippocampus-dependent learning and memory in humans.

Stress induces atrophy of apical dendrites of hippocampal CA3 pyramidal neurons

The hippocampus is vulnerable to the damaging actions of insults such as transient ischemia and repetitive stimulation, as well as repeated exposure to exogenous glucocorticoids. This study investigated effects of a repeated psychological stressor, restraint, on the CA3 pyramidal neurons which are vulnerable to damage by repetitive stimulation. Repeated daily restraint stress for 21 days caused apical dendrites of CA3 pyramidal neurons to atrophy, while basal CA3 dendrites did not change. Rats undergoing this treatment were healthy and showed some adaptation of the glucocorticoid stress response over 21 days; however, stress reduced body weight gain by 14% and increased adrenal weight relative to body weight by 20%. Results are discussed in relation to the possible role of adrenal steroids and excitatory amino acids.

Skeletal muscle atrophy and the E3 ubiquitin ligases MuRF1 and MAFbx/atrogin-1

Muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/atrogin-1 were identified more than 10 years ago as two muscle-specific E3 ubiquitin ligases that are increased transcriptionally in skeletal muscle under atrophy-inducing conditions, making them excellent markers of muscle atrophy. In the past 10 years much has been published about MuRF1 and MAFbx with respect to their mRNA expression patterns under atrophy-inducing conditions, their transcriptional regulation, and their putative substrates. However, much remains to be learned about the physiological role of both genes in the regulation of mass and other cellular functions in striated muscle. Although both MuRF1 and MAFbx are enriched in skeletal, cardiac, and smooth muscle, this review will focus on the current understanding of MuRF1 and MAFbx in skeletal muscle, highlighting the critical questions that remain to be answered.

Appendicular skeletal muscle mass: effects of age, gender, and ethnicity

This study tested the hypothesis that skeletal muscle mass is reduced in elderly women and men after adjustment first for stature and body weight. The hypothesis was evaluated by estimating appendicular skeletal muscle mass with dual-energy X-ray absorptiometry in a healthy adult cohort. A second purpose was to test the hypothesis that whole body 40K counting-derived total body potassium (TBK) is a reliable indirect measure of skeletal muscle mass. The independent effects on both appendicular skeletal muscle and TBK of gender (n = 148 women and 136 men) and ethnicity (n = 152 African-Americans and 132 Caucasians) were also explored. Main findings were 1) for both appendicular skeletal muscle mass (total, leg, and arm) and TBK, age was an independent determinant after adjustment first by stepwise multiple regression for stature and weight (multiple regression model r2 = approximately 0.60); absolute decrease with greater age in men was almost double that in women; significantly larger absolute amounts were observed in men and African-Americans after adjustment first for stature, weight, and age; and >80% of within-gender or -ethnic group between-individual component variation was explained by stature, weight, age, gender, and ethnicity differences; and 2) most of between-individual TBK variation could be explained by total appendicular skeletal muscle (r2 = 0.865), whereas age, gender, and ethnicity were small but significant additional covariates (total r2 = 0.903). Our study supports the hypotheses that skeletal muscle is reduced in the elderly and that TBK provides a reasonable indirect assessment of skeletal muscle mass. These findings provide a foundation for investigating skeletal muscle mass in a wide range of health-related conditions.

Glomerular number and size in relation to age, kidney weight, and body surface in normal man

The number and size of glomeruli in normal, mature human kidneys were estimated by a direct and unbiased stereological method, the fractionator. The number was 617,000 on average, and the mean size 6.0 M microns3. Both glomerular number and size showed significant negative correlation to age and significant positive correlation to kidney weight. Apparently, humans loose glomeruli with age. Body surface area correlated positively to kidney weight and total glomerular volume but not to number of glomeruli. Body surface area correlates significantly with metabolic rate (Robertson and Reid, Lancet, 1: 940-943, 1952). Thus, intraspecies adaptation of kidney filtration capacity to the metabolic demand is performed by changing the size of glomeruli, i.e., the number of glomeruli in individuals of a given species is independent of the metabolic rate.

Increased dendritic spine densities on cortical projection neurons in autism spectrum disorders

Multiple types of indirect evidence have been used to support theories of altered cortical connectivity in autism spectrum disorders (ASD). In other developmental disorders reduced spine expression is commonly found, while conditions such as fragile X syndrome show increased spine densities. Despite its relevance to theories of altered cortical connectivity, synaptic spine expression has not been systematically explored in ASD. Here we examine dendritic spines on Golgi-impregnated cortical pyramidal cells in the cortex of ASD subjects and age-matched control cases. Pyramidal cells were studied within both the superficial and deep cortical layers of frontal, temporal, and parietal lobe regions. Relative to controls, spine densities were greater in ASD subjects. In analyses restricted to the apical dendrites of pyramidal cells, greater spine densities were found predominantly within layer II of each cortical location and within layer V of the temporal lobe. High spine densities were associated with decreased brain weights and were most commonly found in ASD subjects with lower levels of cognitive functioning. Greater spine densities in ASD subjects provide structural support for recent suggestions of connectional changes within the cerebral cortex that may result in altered cortical computations.

Composition and size of type I, IIA, IID/X, and IIB fibers and citrate synthase activity of rat muscle

A population of muscle fibers containing a myosin heavy-chain isoform IId (or 2x) has recently been identified in rat muscle. The purpose of this study was to histochemically determine the relative population and size of muscle fibers composed of type IID/X fibers as well as type I, IIA, and IIB fibers to estimate the absolute mass of the different types of fibers in rat muscle. In addition, muscle citrate synthase activity was measured to determine the relationship between fiber composition and muscle oxidative capacity. Seventy-six muscles or muscle parts from the face, neck, shoulder, arm, trunk, hip, thigh, and leg of three adult (4.5-5 mo of age) male Sprague-Dawley rats were removed, weighed, and frozen for histochemical and biochemical analyses. The data demonstrated that type IIB fibers make up 71% of the total muscle mass, type IID/X fibers 18%, type IIA fibers 5%, and type I fibers 6%. The mean cross-sectional area across all muscles was 5,078 +/- 175 microns 2 for type IIB fibers, 3,078 +/- 105 microns2 for type IID/X fibers, 2,045 +/- 80 microns2 for type IIA fibers, and 1,898 +/- 90 microns2 for type I fibers. Citrate synthase activity, an indicator of muscle mitochondrial content, was most closely related to the population of type IIA fibers and was in the rank order of type IIA > I > IID/X > IIB. NADH-tetrazolium reductase staining intensity also confirmed this order. These data demonstrate that type IID/X fibers make up a significant portion of the adult rat muscle mass and are intermediate to type IIA and IIB fibers in regard to fiber size and oxidative potential.

Big brains, enhanced cognition, and response of birds to novel environments

The widely held hypothesis that enlarged brains have evolved as an adaptation to cope with novel or altered environmental conditions lacks firm empirical support. Here, we test this hypothesis for a major animal group (birds) by examining whether large-brained species show higher survival than small-brained species when introduced to nonnative locations. Using a global database documenting the outcome of >600 introduction events, we confirm that avian species with larger brains, relative to their body mass, tend to be more successful at establishing themselves in novel environments. Moreover, we provide evidence that larger brains help birds respond to novel conditions by enhancing their innovation propensity rather than indirectly through noncognitive mechanisms. These findings provide strong evidence for the hypothesis that enlarged brains function, and hence may have evolved, to deal with changes in the environment.

Phosphorylation of p70(S6k) correlates with increased skeletal muscle mass following resistance exercise

High-resistance exercise training results in an increase in muscle wet mass and protein content. To begin to address the acute changes following a single bout of high-resistance exercise, a new model has been developed. Training rats twice a week for 6 wk resulted in 13.9 and 14.4% hypertrophy in the extensor digitorum longus (EDL) and tibialis anterior (TA) muscles, respectively. Polysome profiles after high-resistance lengthening contractions suggest that the rate of initiation is increased. The activity of the 70-kDa S6 protein kinase (p70(S6k)), a regulator of translation initiation, is also increased following high-resistance lengthening contractions (TA, 363 +/- 29%; EDL, 353 +/- 39%). Furthermore, the increase in p70(S6k) activity 6 h after exercise correlates with the percent change in muscle mass after 6 wk of training (r = 0.998). The tight correlation between the activation of p70(S6k) and the long-term increase in muscle mass suggests that p70(S6k) phosphorylation may be a good marker for the phenotypic changes that characterize muscle hypertrophy and may play a role in load-induced skeletal muscle growth.

The TEAD/TEF family of transcription factor Scalloped mediates Hippo signaling in organ size control

The Hippo (Hpo) signaling pathway governs cell growth, proliferation, and apoptosis by controlling key regulatory genes that execute these processes; however, the transcription factor of the pathway has remained elusive. Here we provide evidence that the TEAD/TEF family transcription factor Scalloped (Sd) acts together with the coactivator Yorkie (Yki) to regulate Hpo pathway-responsive genes. Sd and Yki form a transcriptional complex whose activity is inhibited by Hpo signaling. Sd overexpression enhances, whereas its inactivation suppresses, tissue overgrowth caused by Yki overexpression or tumor suppressor mutations in the Hpo pathway. Inactivation of Sd diminishes Hpo target gene expression and reduces organ size, whereas a constitutively active Sd promotes tissue overgrowth. Sd promotes Yki nuclear localization, whereas Hpo signaling retains Yki in the cytoplasm by phosphorylating Yki at S168. Finally, Sd recruits Yki to the enhancer of the pathway-responsive gene diap1, suggesting that diap1 is a direct transcriptional target of the Hpo pathway.

Sex differences in cortical thickness mapped in 176 healthy individuals between 7 and 87 years of age

Findings from previous magnetic resonance imaging studies of sex differences in gray matter have been inconsistent, with some showing proportionally increased gray matter in women and some showing no differences between the sexes. Regional sex differences in gray matter thickness have not yet been mapped over the entire cortical surface in a large sample of subjects spanning the age range from early childhood to old age. We applied algorithms for cortical pattern matching and techniques for measuring cortical thickness to the structural magnetic resonance images of 176 healthy individuals between the ages of 7 and 87 years. We also mapped localized differences in brain size. Maps of sex differences in cortical thickness revealed thicker cortices in women in right inferior parietal and posterior temporal regions even without correcting for total brain volume. In these regions, the cortical mantle is up to 0.45 mm thicker, on average, in women than in men. Analysis of a subset of 18 female and 18 male subjects matched for age and brain volume confirmed the significance of thicker gray matter in temporal and parietal cortices in females, independent of brain size differences. Further analyses were conducted in the adult subjects where gender differences were evaluated using height as a covariate, and similar sex differences were observed even when body size differences between the sexes were controlled. Together, these results suggest that greater cortical thickness in posterior temporal inferior parietal regions in females relative to males are independent of differences in brain or body size. Age-by-sex interactions were not significant in the temporoparietal region, suggesting that sex differences in these regions are present from at least late childhood and then are maintained throughout life. Male brains were larger than female brains in all locations, though male enlargement was most prominent in the frontal and occipital poles, bilaterally. Given the large sample and the large range of ages studied, these results help to address controversies in the study of central nervous system sexual dimorphisms.

The ontogeny of human gyrification

During development the human cortex changes from a smooth lissencephalic structure to one that is highly convoluted. Increases in the degree of cortical folding are associated with brain size only for the first part of brain growth; during the second half, differences in cortical folding match those of brain size, resulting in no change in the degree of folding. When the degree of cortical folding is studied as a function of age, a brief postnatal overshoot, an effect of brain size, is observed. The analysis suggests that the mechanical hypothesis of cortical buckling can best explain the degree of cortical folding, but that other hypotheses, like gyrogenesis, are required to explain the placement and orientation of sulci. The adult asymptote in degree of cortical folding is associated with the onset and disappearance of single subplate lamina, suggesting that subplate:cortical plate associations should be examined as causal for gyrification. Areas whose sulci differ in length between the two hemispheres have similar degrees of convolutedness, supporting interpretations that the sizes of gyri are asymmetric in the two hemispheres. The ontogenetic data support the thesis that human cortical proportions evolved when the brain enlarged in size and that the process was not one of neoteny.

Role for Akt3/protein kinase Bgamma in attainment of normal brain size

Studies of Drosophila and mammals have revealed the importance of insulin signaling through phosphatidylinositol 3-kinase and the serine/threonine kinase Akt/protein kinase B for the regulation of cell, organ, and organismal growth. In mammals, three highly conserved proteins, Akt1, Akt2, and Akt3, comprise the Akt family, of which the first two are required for normal growth and metabolism, respectively. Here we address the function of Akt3. Like Akt1, Akt3 is not required for the maintenance of normal carbohydrate metabolism but is essential for the attainment of normal organ size. However, in contrast to Akt1-/- mice, which display a proportional decrease in the sizes of all organs, Akt3-/- mice present a selective 20% decrease in brain size. Moreover, although Akt1- and Akt3-deficient brains are reduced in size to approximately the same degree, the absence of Akt1 leads to a reduction in cell number, whereas the lack of Akt3 results in smaller and fewer cells. Finally, mammalian target of rapamycin signaling is attenuated in the brains of Akt3-/- but not Akt1-/- mice, suggesting that differential regulation of this pathway contributes to an isoform-specific regulation of cell growth.

The Drosophila forkhead transcription factor FOXO mediates the reduction in cell number associated with reduced insulin signaling

BACKGROUND

Forkhead transcription factors belonging to the FOXO subfamily are negatively regulated by protein kinase B (PKB) in response to signaling by insulin and insulin-like growth factor in Caenorhabditis elegans and mammals. In Drosophila, the insulin-signaling pathway regulates the size of cells, organs, and the entire body in response to nutrient availability, by controlling both cell size and cell number. In this study, we present a genetic characterization of dFOXO, the only Drosophila FOXO ortholog.

RESULTS

Ectopic expression of dFOXO and human FOXO3a induced organ-size reduction and cell death in a manner dependent on phosphoinositide (PI) 3-kinase and nutrient levels. Surprisingly, flies homozygous for dFOXO null alleles are viable and of normal size. They are, however, more sensitive to oxidative stress. Furthermore, dFOXO function is required for growth inhibition associated with reduced insulin signaling. Loss of dFOXO suppresses the reduction in cell number but not the cell-size reduction elicited by mutations in the insulin-signaling pathway. By microarray analysis and subsequent genetic validation, we have identified d4E-BP, which encodes a translation inhibitor, as a relevant dFOXO target gene.

CONCLUSION

Our results show that dFOXO is a crucial mediator of insulin signaling in Drosophila, mediating the reduction in cell number in insulin-signaling mutants. We propose that in response to cellular stresses, such as nutrient deprivation or increased levels of reactive oxygen species, dFOXO is activated and inhibits growth through the action of target genes such as d4E-BP.

Relationship of obesity with osteoporosis

CONTEXT

The relationship between obesity and osteoporosis has been widely studied, and epidemiological evidence shows that obesity is correlated with increased bone mass. Previous analyses, however, did not control for the mechanical loading effects of total body weight on bone mass and may have generated a confounded or even biased relationship between obesity and osteoporosis.

OBJECTIVE

The objective of this study was to reevaluate the relationship between obesity and osteoporosis by accounting for the mechanical loading effects of total body weight on bone mass.

METHODS

We measured whole body fat mass, lean mass, percentage fat mass, body mass index, and bone mass in two large samples of different ethnicity: 1988 unrelated Chinese subjects and 4489 Caucasian subjects from 512 pedigrees. We first evaluated the Pearson correlations among different phenotypes. We then dissected the phenotypic correlations into genetic and environmental components with bone mass unadjusted or adjusted for body weight. This allowed us to compare the results with and without controlling for mechanical loading effects of body weight on bone mass.

RESULTS

In both Chinese and Caucasian subjects, when the mechanical loading effect of body weight on bone mass was adjusted for, the phenotypic correlation (including its genetic and environmental components) between fat mass (or percentage fat mass) and bone mass was negative. Further multivariate analyses in subjects stratified by body weight confirmed the inverse relationship between bone mass and fat mass, after mechanical loading effects due to total body weight were controlled.

CONCLUSIONS

Increasing fat mass may not have a beneficial effect on bone mass.

Anhedonia and motivational deficits in rats: impact of chronic social stress

Stress, especially chronic stress, is one of the most important factors responsible for precipitation of affective disorders in humans. The animal models commonly used in the investigation of stress effects are based mainly on powerful physical stressors. In the majority of cases, these models are not relevant to situations that human beings encounter in everyday life. In our study, an animal model for chronic social stress has been developed for rats using a resident-intruder paradigm. This paradigm is considered a model of social defeat or subordination, and therefore may mimic situations occurring in humans. Rats were subjected daily to subordination stress for a period of five weeks and, in parallel, tested with a battery of behavioural tests. Chronically stressed rats showed behavioural changes, including decreased motility and exploratory activity, increased immobility in a forced swim test, and reduced preference for sweet sucrose solution (anhedonia). Reduced locomotor and exploratory activity represents a loss of interest in new stimulating situations, implying a deficit in motivation. Increased immobility in the forced swim test indicates behavioural despair, a characteristic of depressive disorders. Decreased sucrose preference may indicate desensitisation of the brain reward mechanism. Since anhedonia is one of the core symptoms of depression in humans, our findings suggest that the rat chronic social stress model may be an appropriate model for depressive disorders.

Effects of aging on muscle fibre type and size

Aging has been associated with a loss of muscle mass that is referred to as 'sarcopenia'. This decrease in muscle tissue begins around the age of 50 years, but becomes more dramatic beyond the 60th year of life. Loss of muscle mass among the aged directly results in diminished muscle function. Decreased strength and power contribute to the high incidence of accidental falls observed among the elderly and can compromise quality of life. Moreover, sarcopenia has been linked to several chronic afflictions that are common among the aged, including osteoporosis, insulin resistance and arthritis. Loss of muscle fibre number is the principal cause of sarcopenia, although fibre atrophy--particularly among type II fibres--is also involved. Several physiological mechanisms have been implicated in the development of sarcopenia. Denervation results in the loss of motor units and thus, muscle fibres. A decrease in the production of anabolic hormones such as testosterone, growth hormone and insulin-like growth factor-1 impairs the capacity of skeletal muscle to incorporate amino acids and synthesise proteins. An increase in the release of catabolic agents, specifically interleukin-6, amplifies the rate of muscle wasting among the elderly. Given the demographic trends evident in most western societies, i.e. increased number of those considered aged, management interventions for sarcopenia must become a major goal of the healthcare profession.

Longitudinal pattern of regional brain volume change differentiates normal aging from MCI

BACKGROUND

Neuroimaging measures have potential as surrogate markers of disease through identification of consistent features that occur prior to clinical symptoms. Despite numerous investigations, especially in relation to the transition to clinical impairment, the regional pattern of brain changes in clinically normal older adults has not been established. We predict that the regions that show early pathologic changes in association with Alzheimer disease will show accelerated volume loss in mild cognitive impairment (MCI) compared to normal aging.

METHODS

Through the Baltimore Longitudinal Study of Aging, we prospectively evaluated 138 nondemented individuals (age 64-86 years) annually for up to 10 consecutive years. Eighteen participants were diagnosed with MCI over the course of the study. Mixed-effects regression was used to compare regional brain volume trajectories of clinically normal individuals to those with MCI based on a total of 1,017 observations.

RESULTS

All investigated volumes declined with normal aging (p < 0.05). Accelerated change with age was observed for ventricular CSF (vCSF), frontal gray matter, superior, middle, and medial frontal, and superior parietal regions (p < or = 0.04). The MCI group showed accelerated changes compared to normal controls in whole brain volume, vCSF, temporal gray matter, and orbitofrontal and temporal association cortices, including the hippocampus (p < or = 0.04).

CONCLUSION

Although age-related regional volume loss is apparent and widespread in nondemented individuals, mild cognitive impairment is associated with a unique pattern of structural vulnerability reflected in differential volume loss in specific regions. Early identification of patterns of abnormality is of fundamental importance for detecting disease onset and tracking progression.

Deletion, but not antagonism, of the mouse growth hormone receptor results in severely decreased body weights, insulin, and insulin-like growth factor I levels and increased life span

GH participates in growth, metabolism, and cellular differentiation. To study these roles, we previously generated two different dwarf mouse lines, one expressing a GH antagonist (GHA) and the other having a disrupted GH receptor and binding protein gene (GHR -/-). In this study we compared the two dwarf lines in the same genetic background (C57BL/6J). One of the most striking differences between the mouse lines was their weight gain profile after weaning. The weights of the GHA dwarfs gradually approached controls over time, but the weights of the GHR -/- dwarfs remained low throughout the analysis period. Additionally, fasting insulin and glucose levels were reduced in the GHR -/- mice but normal in the GHA mice. IGF-I and IGF binding protein 3 (IGFBP-3) levels were significantly reduced, but by different degrees, in both mouse lines, but IGFBP-1 and -4 levels were reduced and IGFBP-2 levels increased in GHR -/- mice but unaltered in GHA mice. Finally, life span was significantly extended for the GHR -/- mice but remained unchanged for GHA dwarfs. These results suggest that the degree of blockade of GH signaling can lead to dramatically different phenotypes.

The Hippo-YAP pathway: new connections between regulation of organ size and cancer

The control of organ size is a basic biological question. In the past several years, the Hippo signaling pathway has been delineated and shown to be crucial in control of organ size in both Drosophila and mammals. Acting downstream of the Hippo pathway is the Yki/YAP/TAZ transcription co-activators. In mammalian cells, the Hippo pathway kinase cascade inhibits YAP and its paralog TAZ by phosphorylation and promotion of their cytoplasmic localization. The TEAD family transcription factors have recently been identified as evolutionarily conserved key mediators of YAP biological functions. yap is a candidate oncogene, and several other components of the Hippo pathway are tumor suppressors. Dysregulation of the Hippo pathway contributes to the loss of contact inhibition observed in cancer cells. Therefore, the Hippo-YAP pathway connects the regulation of organ size and tumorigenesis.

Infant Gut Microbiome Associated With Cognitive Development

BACKGROUND

Studies in rodents provide compelling evidence that microorganisms inhabiting the gut influence neurodevelopment. In particular, experimental manipulations that alter intestinal microbiota impact exploratory and communicative behaviors and cognitive performance. In humans, the first years of life are a dynamic time in gut colonization and brain development, but little is known about the relationship between these two processes.

METHODS

We tested whether microbial composition at 1 year of age is associated with cognitive outcomes using the Mullen Scales of Early Learning and with global and regional brain volumes using structural magnetic resonance imaging at 1 and 2 years of age. Fecal samples were collected from 89 typically developing 1-year-olds. 16S ribosomal RNA amplicon sequencing was used for identification and relative quantification of bacterial taxa.

RESULTS

Cluster analysis identified 3 groups of infants defined by their bacterial composition. Mullen scores at 2 years of age differed significantly between clusters. In addition, higher alpha diversity was associated with lower scores on the overall composite score, visual reception scale, and expressive language scale at 2 years of age. Exploratory analyses of neuroimaging data suggest the gut microbiome has minimal effects on regional brain volumes at 1 and 2 years of age.

CONCLUSIONS

This is the first study to demonstrate associations between the gut microbiota and cognition in human infants. As such, it represents an essential first step in translating animal data into the clinic.

Visible burrow system as a model of chronic social stress: behavioral and neuroendocrine correlates

In mixed-sex rat groups maintained in visible burrow systems (VBS), consistent asymmetries in offensive and defensive behaviors of male dyads are associated with the development of dominance hierarchies. Subordinate males are characterized by particular wound patterns, severe weight loss, and a variety of behavioral changes, many of them isomorphic to target symptoms of clinical depression. In two VBS studies, subordinate males showed increased basal levels of plasma corticosterone (CORT), and increased adjusted adrenal and spleen weights compared to controls, and often, to dominants as well. Thymus weights and testosterone levels of subordinates were not reliably different in one study using highly aggressive males, but were reduced, along with testes weights, in a second study using unselected males. Glucocorticoid receptor binding levels in hippocampus, hypothalamus, and pituitary were not different, nor were aldosterone levels. When tested in a restraint stress procedure, subordinates had higher basal CORT levels, but about 40% of these animals showed a reduced, or absent, CORT response to restraint. These findings indicate that subordination may be reflected in high magnitude changes consistent with physiological indices of prolonged stress. Dominant rats of such groups may also show physiological changes suggesting stress, particularly when the groups are comprised of highly aggressive males only. The VBS colony model thus appears to enable rat groups to produce natural, stress-engendering, social interactions that constitute a particularly relevant model for investigating the behavioral, neural, and endocrine correlates of chronic stress.