The environment of an organism can convey a powerful influence over its biology. Environmental enrichment (EE), as a eustress model, has been used extensively in neuroscience to study neurogenesis ...and brain plasticity. EE has also been used as an intervention for the treatment and prevention of neurological and psychiatric disorders with limited clinical application. By contrast, the effects of EE on the immune system are relatively less investigated. Recently, accumulating evidence has demonstrated that EE can robustly impact immune function. In this review, we summarize the major components of EE, the impact of EE on natural killer (NK) cells, EE's immunoprotective roles in cancer, and the underlying mechanisms of EE-induced NK cell regulation. Moreover, we discuss opportunities for translational application based on insights from animal research of EE-induced NK cell regulation.
Macroenvironmental factors, including a patient's physical and social environment, play a role in cancer risk and progression. Our previous preclinical studies have shown that the enriched ...environment (EE) confers anti‐obesity and anti‐cancer phenotypes that are associated with enhanced adaptive immunity and are mediated by brain‐derived neurotrophic factor (BDNF). Natural killer (NK) cells have anti‐cancer and anti‐viral properties, and their absence or depletion is associated with inferior clinical outcomes. In this study, we investigated the effects of EE on NK cell maturation following their depletion. Mice living in EE displayed a higher proportion of NK cells in the spleen, bone marrow, and blood, compared to those living in the standard environment (SE). EE enhanced NK cell maturation in the spleen and was associated with upregulation of BDNF expression in the hypothalamus. Hypothalamic BDNF overexpression reproduced the EE effects on NK cell maturation in secondary lymphoid tissues. Conversely, hypothalamic BDNF knockdown blocked the EE modulation on NK cell maturation. Our results demonstrate that a bio‐behavior intervention enhanced NK cell maturation and was mediated at least in part by hypothalamic BDNF.
We hypothesized that mice living in an enriched environment would show enhanced maturation of natural killer cells mediated by hypothalamic brain‐derived neurotrophic factor. The results from this study provide new insight as to how an enriched environment mediates its effects on innate immunity via the CNS.
Despite the large body of work describing the tumor suppressor functions of Phosphatase and tensin homologue deleted on chromosome ten (PTEN), its roles in adipose homeostasis of adult animals are ...not yet fully understood. Here, we sought to determine the role of PTEN in whole-body adipose homeostasis.
We genetically manipulated PTEN in specific fat depots through recombinant adeno-associated viral vector (rAAV)-based gene transfer of Cre recombinase to adult PTENflox mice. Additionally, we used a denervation agent, 6OHDA, to assess the role of sympathetic signaling in PTEN-related adipose remodeling. Furthermore, we chemically manipulated AKT signaling via a pan-AKT inhibitor, MK-2206, to assess the role of AKT in PTEN-related adipose remodeling. Finally, to understand the role of leptin and central signaling on peripheral tissues, we knocked down hypothalamic leptin receptor with a microRNA delivered by a rAAV vector.
Knockdown PTEN in individual fat depot resulted in massive expansion of the affected fat depot through activation of AKT signaling associated with suppression of lipolysis and induction of leptin. This hypertrophic expansion of the affected fat depot led to upregulation of PTEN level, higher lipolysis, and induction of white fat browning in other fat depots, and the compensatory reduced fat mass to maintain a set point of whole-body adiposity. Administration of AKT inhibitor MK-2206 prevented the adipose PTEN knockdown-associated effects. 6OHDA-mediated denervation demonstrated that sympathetic innervation was required for the PTEN knockdown-induced adipose redistribution. Knockdown hypothalamic leptin receptor attenuated the adipose redistribution induced by PTEN deficiency in individual fat depot.
Our results demonstrate the essential role of PTEN in adipose homeostasis, including mass and distribution in adulthood, and reveal an “adipose PTEN-leptin-sympathetic nervous system” feedback loop to maintain a set point of adipose PTEN and whole-body adiposity.
•PTEN deficiency leads to mass expansion, lipolysis suppression, and leptin induction in affected fat depot.•Knockdown PTEN in one fat pad results in whole-body fat redistribution.•AKT signaling mediates PTEN-related adipose remodeling.•An “adipose PTEN-leptin-sympathetic nervous system” loop implicates in whole-body adipose homeostasis.
Environmental enrichment (EE) is a physiological model to investigate brain-fat interactions. We previously discovered that EE activates the hypothalamic-sympathoneural adipocyte (HSA) axis via ...induction of brain-derived neurotrophic factor (BDNF), thus leading to sympathetic stimulation of white adipose tissue (WAT) and an anti-obesity phenotype. Here, we investigate whether PTEN acts as a downstream mediator of the HSA axis in the EE.
Mice were housed in EE for 4- and 16-week periods to determine how EE regulates adipose PTEN. Hypothalamic injections of adeno-associated viral (AAV) vectors expressing BDNF and a dominant negative form of its receptor were performed to assess the role of the HSA axis in adipose PTEN upregulation. A β-blocker, propranolol, and a denervation agent, 6-hydroydopamine, were administered to assess sympathetic signaling in the observed EE-PTEN phenotype. To determine whether inducing PTEN is sufficient to reproduce certain EE adipose remodeling, we overexpressed PTEN in WAT using an AAV vector. To determine whether adipose PTEN is necessary for the EE-mediated reduction in adipocyte size, we injected a rAAV vector expressing Cre recombinase to the WAT of adult PTENflox mice and placed the mice in EE.
EE upregulated adipose PTEN expression, which was associated with suppression of AKT and ERK phosphorylation, increased hormone-sensitive lipase (HSL) phosphorylation, and reduced adiposity. PTEN regulation was found to be controlled by the HSA axis—with the hypothalamic BDNF acting as the upstream mediator—and dependent on sympathetic innervation. AAV-mediated adipose PTEN overexpression recapitulated EE-mediated adipose changes including suppression of AKT and ERK phosphorylation, increased HSL phosphorylation, and reduced adipose mass, whereas PTEN knockdown blocked the EE-induced reduction of adipocyte size.
These data suggest that adipose PTEN responds to environmental stimuli and serves as downstream mediator of WAT remodeling in the EE paradigm, resulting in decreased adipose mass and decreased adipocyte size.
•Environmental enrichment (EE) induces adipose PTEN expression and is associated with (1) suppression of AKT phosphorylation, (2) increased hormone-sensitive lipase phosphorylation, and (3) decreased adiposity•The hypothalamic-sympathoneural-adipocyte (HSA) axis mediates EE-induced adipose PTEN•rAAV-mediated gene delivery of PTEN to adipose tissues mimics EE-related adipose remodeling•Knockdown of adipose PTEN blocks EE-induced reductions in adipocyte size
The aging process and age-related diseases all involve metabolic decline and impaired ability to cope with adversity. Environmental enrichment (EE)-a housing environment which recapitulates aspects ...of active lifestyle-exerts a wide range of health benefits in laboratory rodents. Brain-derived neurotrophic factor (BDNF) in the hypothalamus orchestrates autonomic and neuroendocrine processes, serving as one key brain mediator of EE-induced resistance to obesity, cancer, and autoimmunity. Recombinant adeno-associated virus (AAV)-mediated hypothalamic BDNF gene transfer alleviates obesity, diabetes, and metabolic syndromes in both diet-induced and genetic models. One recent study by our lab demonstrates the efficacy and safety of a built-in autoregulatory system to control transgene BDNF expression, mimicking the body's natural feedback systems in middle-age mice. Twelve-month old mice were treated with autoregulatory BDNF vector and monitored for 7months. BDNF gene transfer prevented age-associated metabolic decline by: reducing adiposity, preventing the decline of brown fat activity, increasing adiponectin while reducing leptin and insulin in circulation, improving glucose tolerance, increasing energy expenditure, alleviating hepatic steatosis, and suppressing inflammatory genes in the hypothalamus and adipose tissues. Furthermore, BDNF treatment reduced anxiety-like and depression-like behaviors. This chapter summarizes this work and discusses potential roles that hypothalamic BDNF might play in promoting healthy aging.
Aging population demographics show that the age structure of the United States is shifting, with more individuals over the age of 65 than those under 18 projected as soon as 2034. These trends are ...expected to continue, mirroring population changes happening globally. The obesity epidemic is also worsening, and high body mass index and associated complications continue to contribute significantly to morbidity and mortality. Obesity in the United States impacts the middle-aged most severely, and research shows obesity is one of the strongest predictors of mortality at midlife. The conjunction of these population trends is a progressing public health concern. Environmental enrichment (EE), a housing model for animals implemented for animal welfare, has been shown to improve health in aging mice. Recent evidence suggests that age-related metabolic decline, obesity, and age- and obesity-associated illness stems from shared pathophysiology. This dissertation addresses hypothalamic inflammation as an overlapping etiology of aging and obesity and defines how it is impacted by environmental enrichment.Enrichment housing of female C57BL/6 mice over 12 months starting in middle age improves metabolism and reduces adiposity. We examined hypothalamic neuroinflammatory markers and observed reduced inflammation following EE. Microglia are the resident immune cells of the brain which are chronically and progressively more pro-inflammatory across the lifespan. Based on these studies, we prepared and visualized immunohistochemical staining of microglia. After 7.5 months of EE started in middle age, microglia throughout the brain exhibited increased ramification. This suggests microglial surveillance of the central nervous system (CNS) parenchyma is enhanced or activated by EE. Morphometric analyses performed using FIJI, an image processing package of ImageJ provided by the National Institutes of Health, showed increased area, branch number, and branch length of microglia. These features were region- and nucleus- specific, but were pronounced in the hypothalamus, hippocampus, and amygdala.Microglia are also CNS-supportive glia. As resident macrophages in the brain, they aid in synaptic formation, elimination, and plasticity by providing trophic support and phagocytosing apoptotic cells and circuit debris. We investigated the role of microglia in the CNS changes important for EE metabolic outcomes. Specifically, EE activates the hypothalamic-sympathoneural-adipocyte axis, in which brain-derived neurotrophic factor (BDNF) is produced by the hypothalamus and activates the sympathetic nervous system, which signals onto adipocytes for adipolysis and lowering circulating leptin. Using the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622, we depleted microglia from the CNS of middle-aged female mice before housing them in either standard or EE housing. PLX5622 did not impair or enhance the hypothalamic expression of BDNF in response to enrichment. Removal of microglia in middle age in combination with EE resulted in a robustly improved metabolic phenotype when compared to either PLX5622 or EE alone. The ability of PLX5622 to remove pro-inflammatory microglia in the CNS, and to reduce specific classes of adipose tissue macrophages in the periphery, may both play a role in the enhanced combined effect of the housing and drug interventions.BDNF is the key mediator of the improved whole-body metabolism seen in EE. Hypothalamic expression of BDNF with an adeno-associated virus (AAV-BDNF) in young male mice reduced adiposity and neuroinflammatory markers. Microglia expressed reduced fractalkine receptor (CX3CR1) in response to EE, and this increased CX3CR1 expression was recapitulated in dissected block hypothalamus by AAV-BDNF. Blockade of neuronal BDNF with an AAV expressing a dominant negative version of the BDNF receptor (AAV-TrkB.T1) in neurons resulted in the reversal of these trends: increased weight gain and increased hypothalamic inflammation. AAV-TrkB.T1 was sufficient to block adiposity reduction by EE in middle-aged mice, and AAV-TrkB.T1 treatment resulted in significant microgliosis in the hypothalamus.Together this research demonstrates the utility of EE in improving health in animals. Modulation of microglia by EE has implications for other neuroinflammatory conditions, and reducing hypothalamic inflammation may be one mechanism through which EE and lifestyle interventions can improve metabolism. Complex neuron-microglia communication in the hypothalamus is impacted by neuronal BDNF signaling. We are continuing to explore this and other lines of research.
Aging is associated with increased central nervous system inflammation, in large part due to dysfunctional microglia. Environmental enrichment (EE) provides a model for studying the dynamics of ...lifestyle factors in the development of age-related neuroinflammation and microglial dysfunction. EE results in improvements in learning and memory, metabolism, and mental health in a variety of animal models. We recently reported that implementing EE in middle age promotes healthy aging. In the present study, we investigated whether EE influences microglial morphology, and whether EE is associated with changes in expression of microglial and neuroinflammatory markers. Inflammatory cytokines and MHC-II were reduced following 12-month EE in 10-month-old mice. Long-term EE for 7.5 months resulted in broad increases in Iba1 expression in hippocampus, hypothalamus, and amygdala detected by immunohistochemistry. Quantification of microglial morphology reveal both hypertrophy and ramification in these three brain regions, without increases in microglial cell density. These data indicate that long-term EE implemented in middle age results in a microglial state distinct from that of normal aging in standard laboratory housing, in specific brain regions, associated with reduced neuroinflammatory markers and improvement of systemic metabolism.
•Environmental enrichment (EE) modulates thymocyte maturation and selection in mice.•Hypothalamic BDNF mediates the EE-induced thymic phenotypes.•Adrenalectomy and thymocyte glucocorticoid receptor ...knockout block EE’s thymic effects.•EE protects mice against autoimmune EAE via regulation of type 1 T helper cells.•Adoptive transfer and thymocyte GR knockout link EE thymic modulation to EAE outcome.
Environmental and social factors have profound impacts on immune homeostasis. Our work on environmental enrichment (EE) has revealed a novel anti-obesity and anticancer phenotype associated with enhanced activity of CD8+ cytotoxic T lymphocytes in secondary lymphoid tissues. Here we investigated how an EE modulated thymus and thymocyte development. EE decreased thymus mass and cellularity, decreased the double positive thymocyte population, increased the proportion of CD8+ T cells, reduced the CD4:CD8 ratio, and downregulated CD69 expression in T cells. In a model of multiple sclerosis: experimental autoimmune encephalomyelitis (EAE), EE alleviated symptoms, inhibited spinal cord inflammation through regulation of type 1 T-helper cells mediated by glucocorticoid receptor signaling, and prevented EAE-induced thymic disturbance. Our mechanistic studies demonstrated that hypothalamic BDNF activated a hypothalamic-pituitary-adrenal axis mediating the EE’s thymic effects. Our results indicate that a lifestyle intervention links the nervous, endocrine, and adaptive immune system, allowing the body to adapt to internal and external environments.
With increases in life expectancy, it is vital to understand the dynamics of aging, their interaction with lifestyle factors, and the connections to age-related disease processes. Our work on ...environmental enrichment (EE), a housing environment boosting mental health, has revealed a novel anticancer and anti-obesity phenotype mediated by a brain-fat axis: the hypothalamic-sympathoneural-adipocyte (HSA) axis in young animals. Here we investigated EE effects on healthspan and lifespan when initiated after middle age. Short-term EE for six weeks activated the HSA axis in 10-month-old mice. Long-term EE for twelve months reduced adiposity, improved glucose tolerance, decreased leptin levels, enhanced motor abilities, and inhibited anxiety. In addition to adipose remodeling, EE decreased age-related liver steatosis, reduced hepatic glucose production, and increased glucose uptake by liver and adipose tissue contributing to the improved glycemic control. The EE-induced liver modulation was associated with a suppression of protein kinase Cε. Moreover, EE down-regulated the expression of inflammatory genes in the brain, adipose, and liver. EE initiated at 18-month of age significantly improved glycemic control and showed a trend of positive impact on mean lifespan. These data suggest that EE induces metabolic and behavioral adaptations that are shared by factors known to increase healthspan and lifespan.
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