The Neuropeptide Tac2 Controls a Distributed Brain State Induced by Chronic Social Isolation Stress.

Chronic social isolation causes severe psychological effects in humans, but their neural bases remain poorly understood. 2 weeks (but not 24 hr) of social isolation stress (SIS) caused multiple behavioral changes in mice and induced brain-wide upregulation of the neuropeptide tachykinin 2 (Tac2)/neurokinin B (NkB). Systemic administration of an Nk3R antagonist prevented virtually all of the behavioral effects of chronic SIS. Conversely, enhancing NkB expression and release phenocopied SIS in group-housed mice, promoting aggression and converting stimulus-locked defensive behaviors to persistent responses. Multiplexed analysis of Tac2/NkB function in multiple brain areas revealed dissociable, region-specific requirements for both the peptide and its receptor in different SIS-induced behavioral changes. Thus, Tac2 coordinates a pleiotropic brain state caused by SIS via a distributed mode of action. These data reveal the profound effects of prolonged social isolation on brain chemistry and function and suggest potential new therapeutic applications for Nk3R antagonists.

Adolescent Parvalbumin Expression in the Left Orbitofrontal Cortex Shapes Sociability in Female Mice.

The adolescent social experience is essential for the maturation of the prefrontal cortex in mammalian species. However, it still needs to be determined which cortical circuits mature with such experience and how it shapes adult social behaviors in a sex-specific manner. Here, we examined social-approaching behaviors in male and female mice after postweaning social isolation (PWSI), which deprives social experience during adolescence. We found that the PWSI, particularly isolation during late adolescence, caused an abnormal increase in social approaches (hypersociability) only in female mice. We further found that the PWSI female mice showed reduced parvalbumin (PV) expression in the left orbitofrontal cortex (OFCL). When we measured neural activity in the female OFCL, a substantial number of neurons showed higher activity when mice sniffed other mice (social sniffing) than when they sniffed an object (object sniffing). Interestingly, the PWSI significantly reduced both the number of activated neurons and the activity level during social sniffing in female mice. Similarly, the CRISPR/Cas9-mediated knockdown of PV in the OFCL during late adolescence enhanced sociability and reduced the social sniffing-induced activity in adult female mice via decreased excitability of PV+ neurons and reduced synaptic inhibition in the OFCL Moreover, optogenetic activation of excitatory neurons or optogenetic inhibition of PV+ neurons in the OFCL enhanced sociability in female mice. Our data demonstrate that the adolescent social experience is critical for the maturation of PV+ inhibitory circuits in the OFCL; this maturation shapes female social behavior via enhancing social representation in the OFCL SIGNIFICANCE STATEMENT Adolescent social isolation often changes adult social behaviors in mammals. Yet, we do not fully understand the sex-specific effects of social isolation and the brain areas and circuits that mediate such changes. Here, we found that adolescent social isolation causes three abnormal phenotypes in female but not male mice: hypersociability, decreased PV+ neurons in the left orbitofrontal cortex (OFCL), and decreased socially evoked activity in the OFCL Moreover, parvalbumin (PV) deletion in the OFCL in vivo caused the same phenotypes in female mice by increasing excitation compared with inhibition within the OFCL Our data suggest that adolescent social experience is required for PV maturation in the OFCL, which is critical for evoking OFCL activity that shapes social behaviors in female mice.

Systemic histone deacetylase inhibition ameliorates the aberrant responses to acute stress in socially isolated male mice.

Adverse experiences in early life can induce maladaptive responses to acute stress in later life. Chronic social isolation during adolescence is an early life adversity that can precipitate stress-related psychiatric disorders. We found that male mice after 8 weeks of adolescent social isolation (SI) have markedly increased aggression after being exposed to 2 h of restraint stress (RS), which was accompanied by a significant increase of AMPA receptor- and NMDA receptor-mediated synaptic transmission in prefrontal cortex (PFC) pyramidal neurons of SIRS males. Compared to group-housed counterparts, SIRS males exhibited a significantly decreased level of histone H3 acetylation in PFC. Systemic administration of class I histone deacetylase inhibitors, romidepsin or MS-275, ameliorated the aggressive behaviour, as well as general social interaction deficits, of SIRS males. Electrophysiological recordings also found normalization of PFC glutamatergic currents by romidepsin treatment of SIRS male mice. These results revealed an epigenetic mechanism and intervention avenue for aggression induced by chronic social isolation. KEY POINTS: Adolescent chronic social isolation can precipitate stress-related psychiatric disorders. A significant increase of glutamatergic transmission is found in the prefrontal cortex (PFC) of socially isolated male mice exposed to an acute stress (SIRS). Treatment with class I histone deacetylase (HDAC) inhibitors ameliorates the aggressive behaviour and social interaction deficits of SIRS males, and normalizes glutamatergic currents in PFC neurons. It provides an epigenetic mechanism and intervention avenue for aberrant stress responses induced by chronic social isolation.

Psychosocial Health and the Association Between Cerebral Small Vessel Disease Markers With Dementia: The ARIC Study.

BACKGROUND: Associations between magnetic resonance imaging markers of cerebral small vessel disease (CSVD) and dementia risk in older adults have been established, but it remains unclear how lifestyle factors, including psychosocial health, may modify this association. METHODS: Social support and social isolation were assessed among participants of the community-based ARIC (Atherosclerosis Risk in Communities) Study, via self-reported questionnaires (1990-1992). Following categorization of both factors, participants were classified as having strong or poor mid-life social relationships. At visit 5 (2011-2013), participants underwent 3T brain magnetic resonance imaging quantifying CSVD measures: white matter hyperintensity volume, microbleeds (subcortical), infarcts (lacunar), and white matter integrity (diffusion tensor imaging). Incident dementia cases were identified from the time of imaging through December 31, 2020 with ongoing surveillance. Associations between CSVD magnetic resonance imaging markers and incident dementia were evaluated using Cox proportional-hazard regressions adjusted for demographic and additional risk factors (from visit 2). Effect modification by mid-life social relationships was evaluated. RESULTS: Of the 1977 participants with magnetic resonance imaging, 1617 participants (60.7% women; 26.5% Black participants; mean age at visit 2, 55.4 years) were examined. In this sample, mid-life social relationships significantly modified the association between white matter hyperintensity volume and dementia risk (P interaction=0.001). Greater white matter hyperintensity volume was significantly associated with risk of dementia in all participants, yet, more substantially in those with poor (hazard ratio, 1.84 [95% CI, 1.49-2.27]) versus strong (hazard ratio, 1.26 [95% CI, 1.08-1.47]) mid-life social relationships. Although not statistically significant, subcortical microbleeds in participants with poor mid-life social relationships were associated with a greater risk of dementia, relative to those with strong social relationships, in whom subcortical microbleeds were no longer associated with elevated dementia risk. CONCLUSIONS: The elevated risk of dementia associated with CSVD may be reduced in participants with strong mid-life social relationships. Future studies evaluating psychosocial health through the life course and the mechanisms by which they modify the relationship between CSVD and dementia are needed.

The effect of acute social isolation on neural molecular responses in components of the social decision-making network.

Prolonged or chronic social isolation has pronounced effects on animals, ranging from altered stress responses, increased anxiety and aggressive behaviour, and even increased mortality. The effects of shorter periods of isolation are much less well researched; however, short periods of isolation are used routinely for testing animal behaviour and physiology. Here, we studied how a 3 h period of isolation from a cagemate affected neural gene expression in three brain regions that contain important components of the social decision-making network, the hypothalamus, the nucleus taeniae of the amygdala, and the bed nucleus of the stria terminalis, using a gregarious bird as a model (zebra finches). We found evidence suggestive of altered neural activity, synaptic transmission, metabolism, and even potentially pain perception, all of which could create cofounding effects on experimental tests that involve isolating animals. We recommend that the effects of short-term social isolation need to be better understood and propose alternatives to isolating animals for testing.

Multiple integrated social stress induces depressive-like behavioral and neural adaptations in female C57BL/6J mice.

Despite women representing most of those affected by major depression, preclinical studies have focused almost exclusively on male subjects, partially due to a lack of ideal animal paradigms. As the persistent need regarding the sex balance of neuroscience research and female-specific pathology of mental disorders surges, the establishment of natural etiology-based and systematically validated animal paradigms for depression with female subjects becomes an urgent scientific problem. This study aims to establish, characterize, and validate a "Multiple Integrated Social Stress (MISS)" model of depression in female C57BL/6J mice by manipulating and integrating daily social stressors that females are experiencing. Female C57BL/6J mice randomly experienced social competition failure in tube test, modified vicarious social defeat stress, unescapable overcrowding stress followed by social isolation on each day, for ten consecutive days. Compared with their controls, female MISS mice exhibited a relatively decreased preference for social interaction and sucrose, along with increased immobility in the tail suspension test, which could last for at least one month. These MISS mice also exhibited increased levels of blood serum corticosterone, interleukin-6 L and 1ß. In the pharmacological experiment, MISS-induced dysfunctions in social interaction, sucrose preference, and tail suspension tests were amended by systematically administrating a single dose of sub-anesthetic ketamine, a rapid-onset antidepressant. Compared with controls, MISS females exhibited decreased c-Fos activation in their anterior cingulate cortex, prefrontal cortex, nucleus accumbens and some other depression-related brain regions. Furthermore, 24 h after the last exposure to the paradigm, MISS mice demonstrated a decreased center zone time in the open field test and decreased open arm time in the elevated plus-maze test, indicating anxiety-like behavioral phenotypes. Interestingly, MISS mice developed an excessive nesting ability, suggesting a likely behavioral phenotype of obsessive-compulsive disorder. These data showed that the MISS paradigm was sufficient to generate pathological profiles in female mice to mimic core symptoms, serum biochemistry and neural adaptations of depression in clinical patients. The present study offers a multiple integrated natural etiology-based animal model tool for studying female stress susceptibility.

Does Tobacco Smoking Increase Social Isolation? A Mendelian Randomization Study.

In this study, we aimed to investigate the causal effect of smoking on social isolation among older adults in England. Data from older adults of European ancestry who participated in 1 or more waves of the English Longitudinal Study of Ageing, from wave 1 (2002/2003) to wave 9 (2018/2019), were analyzed (n = 43,687 observations from 7,008 individuals; mean age = 68.50 years). The effect of current smoking on social isolation (ranging from 0 to 5) was estimated by 2-stage least squares regression using a polygenic score (PGS) for smoking cessation as the instrument. A low PGS for smoking cessation predicted current smoking (per 1-standard-deviation lower PGS, coefficient = 0.023, 95% confidence interval (CI): 0.015, 0.030; F = 36.420). The second-stage regression showed that current smoking increased social isolation by 1.205 points (95% CI: 0.308, 2.101). The association was larger for persons with higher socioeconomic backgrounds: 2.501 (95% CI: -0.024, 5.026) and 0.696 (95% CI: -0.294, 1.686) for those with higher and lower educational levels, respectively. This study showed that current smoking instrumented by a PGS for smoking cessation was associated with social isolation. Assuming that the PGS served as a valid instrument in this study, the findings support an effect of smoking on social isolation.

The impact of loneliness and social isolation on the development of cognitive decline and Alzheimer's Disease.

Alzheimer's Disease (AD) is the leading cause of dementia, observed at a higher incidence in women compared with men. Treatments aimed at improving pathology in AD remain ineffective to stop disease progression. This makes the detection of the early intervention strategies to reduce future disease risk extremely important. Isolation and loneliness have been identified among the major risk factors for AD. The increasing prevalence of both loneliness and AD emphasizes the urgent need to understand this association to inform treatment. Here we present a comprehensive review of both clinical and preclinical studies that investigated loneliness and social isolation as risk factors for AD. We discuss that understanding the mechanisms of how loneliness exacerbates cognitive impairment and AD with a focus on sex differences will shed the light for the underlying mechanisms regarding loneliness as a risk factor for AD and to develop effective prevention or treatment strategies.

Social isolation, depression, and anxiety among young adult cancer survivors: The mediating role of social connectedness.

BACKGROUND: Social isolation and social connectedness are health determinants and aspects of social well-being with strong associations with psychological distress. This study evaluated relationships among social isolation, social connectedness, and psychological distress (i.e., depression, anxiety) over 1 year in young adult (YA) cancer survivors 18-39 years old. METHODS: Participants were YAs in a large cohort study that completed questionnaires every 2 months for 1 year. Social isolation, aspects of social connectedness (i.e., companionship, emotional support, instrumental support, and informational support), depression, and anxiety were assessed with Patient-Reported Outcomes Measurement Information System short form measures. Mixed-effect models were used to evaluate changes over time. Confirmatory factor analysis and multilevel structural equation modeling were used to define social connectedness as a latent construct and determine whether relationships between social isolation and psychological distress were mediated by social connectedness. RESULTS: Participants (N = 304) were mean (M) = 33.5 years old (SD = 4.7) and M = 4.5 years (SD = 3.5) post-initial cancer diagnosis. Most participants were female (67.4%) and non-Hispanic White (68.4%). Average scores for social well-being and psychological distress were within normative ranges and did not change (p values >.05). However, large proportions of participants reported at least mild social isolation (27%-30%), depressive symptoms (36%-37%), and symptoms of anxiety (49%-51%) at each time point. Across participants, more social isolation was related to less social connectedness (p values <.001), more depressive symptoms (p < .001), and more symptoms of anxiety (p < .001). Social connectedness mediated the relationship between social isolation and depression (p = .004), but not anxiety (p > .05). CONCLUSIONS: Social isolation and connectedness could be intervention targets for reducing depression among YA cancer survivors.

Living alone and cancer mortality by race/ethnicity and socioeconomic status among US working-age adults.

BACKGROUND: Previous studies have shown an association between living alone and cancer mortality; however, findings by sex and race/ethnicity have generally been inconsistent, and data by socioeconomic status are sparse. The association between living alone and cancer mortality by sex, race/ethnicity, and socioeconomic status in a nationally representative US cohort was examined. METHODS: Pooled 1998-2019 data for adults aged 18-64 years at enrollment from the National Health Interview Survey linked to the National Death Index (N = 473,648) with up to 22 years of follow-up were used to calculate hazard ratios (HRs) for the association between living alone and cancer mortality. RESULTS: Compared to adults living with others, adults living alone were at a higher risk of cancer death in the age-adjusted model (HR, 1.32; 95% CI, 1.25-1.39) and after additional adjustments for multiple sociodemographic characteristics and cancer risk factors (HR, 1.10; 95% CI, 1.04-1.16). Age-adjusted models stratified by sex, poverty level, and educational attainment showed similar associations between living alone and cancer mortality, but the association was stronger among non-Hispanic White adults (HR, 1.33; 95% CI, 1.25-1.42) than non-Hispanic Black adults (HR, 1.18; 95% CI, 1.05-1.32; p value for difference < .05) and did not exist in other racial/ethnic groups. These associations were attenuated but persisted in fully adjusted models among men (HR, 1.13; 95% CI, 1.05-1.23), women (HR, 1.09; 95% CI, 1.01-1.18), non-Hispanic White adults (HR, 1.13; 95% CI, 1.05-1.20), and adults with a college degree (HR, 1.22; 95% CI, 1.07-1.39). CONCLUSIONS: In this nationally representative study in the United States, adults living alone were at a higher risk of cancer death in several sociodemographic groups.

Social Isolation, Loneliness, and Risk of Microvascular Complications Among Individuals With Type 2 Diabetes Mellitus.

RATIONALE & OBJECTIVE: Social disconnection has been associated with poor cardiometabolic health. This study sought to investigate the associations of social isolation and loneliness with diabetic microvascular complications (DMCs) among individuals with type 2 diabetes mellitus (T2DM) and compare these associations versus those related to traditional risk factors. STUDY DESIGN: Prospective cohort study. SETTING & PARTICIPANTS: A total of 24,297 UK Biobank participants with T2DM and no DMCs at baseline. EXPOSURE: Social isolation and loneliness were measured using self-reported questionnaires. OUTCOME: The incidence of DMCs defined as a composite of diabetic kidney disease, diabetic retinopathy, or diabetic neuropathy. ANALYTICAL APPROACH: Multivariable cause-specific hazards regression. To compare the relative importance of social disconnection with other established factors, the R2 values of the Cox models were calculated. RESULTS: During a median follow-up of 12.6 years, 5,530 patients were documented to experience DMCs (3,458 with diabetic kidney disease, 2,255 with diabetic retinopathy, and 1,146 with diabetic neuropathy). The highest level of social isolation was associated with an increased risk of any DMC component (most vs least: HR, 1.13; 95% CI, 1.05-1.22), especially diabetic kidney disease (HR, 1.14; 95% CI, 1.04-1.25) and neuropathy (HR, 1.31; 95% CI, 1.11-1.53). Any level of loneliness was associated with an increased risk of any DMC component (HR, 1.12; 95% CI, 1.02-1.23) and diabetic kidney disease (HR, 1.16; 95% CI, 1.03-1.30). Social isolation and loneliness exhibited associations with DMCs comparable to those of other conventional risk factors, including smoking, blood pressure, and physical activity. LIMITATIONS: Limited generalizability related to the composition of participants in the UK Biobank Study. CONCLUSIONS: Social isolation and loneliness were independently associated with a higher risk of incident DMCs among individuals with T2DM, with comparable importance to other traditional risk factors. These findings underscore social isolation and loneliness as novel and potentially modifiable risk factors for DMCs. PLAIN-LANGUAGE SUMMARY: Social isolation and loneliness are important social determinants that are associated with adverse cardiometabolic health. Individuals with diabetes are particularly vulnerable to social isolation and loneliness. However, the relationship of social isolation or loneliness with diabetic microvascular complications (DMCs) remains unclear. Our study used the UK Biobank study data to investigate the associations of social isolation and loneliness with the development of DMCs. We found that social isolation and loneliness were independently associated with a higher risk of incident DMCs. Remarkably, their association with DMCs was comparable to those of other lifestyle factors such as smoking, blood pressure, and physical activity. These findings collectively imply that social isolation and loneliness are 2 important potentially modifiable risk factors for DMCs among individuals with type 2 diabetes mellitus.

BDNF-dependent signaling in the olfactory bulb modulates social recognition memory in mice.

An operative olfactory bulb (OB) is critical to social recognition memory (SRM) in rodents, which involves identifying conspecifics. Furthermore, OB also allocates synaptic plasticity events related to olfactory memories in their intricate neural circuit. Here, we asked whether the OB is a target for brain-derived neurotrophic factor (BDNF), a well-known mediator of plasticity and memory. Adult ICR-CD1 male mice had their SRM evaluated under the inhibition of BDNF-dependent signaling directly in the OB. We also quantified the expression of BDNF in the OB, after SRM acquisition. Our results presented an amnesic effect of anti-BDNF administered 12 h post-training. Although the western blot showed no statistical difference in pro-BDNF and BDNF expression, the analysis of fluorescence intensity in slices suggests SRM acquisition decreases BDNF in the granular cell layer of the OB. Next, to test the ability of BDNF to rescue SRM deficit, we administered the human recombinant BDNF (rBDNF) directly in the OB of socially isolated (SI) mice. Unexpectedly, rBDNF did not rescue SRM in SI mice. Furthermore, BDNF and pro-BDNF expression in the OB was unchanged by SI. Our study reinforces the OB as a plasticity locus in memory-related events. It also adds SRM as another type of memory sensitive to BDNF-dependent signaling.

Social cognition and social motivation in schizophrenia and bipolar disorder: are impairments linked to the disorder or to being socially isolated?

BACKGROUND: People with schizophrenia on average are more socially isolated, lonelier, have more social cognitive impairment, and are less socially motivated than healthy individuals. People with bipolar disorder also have social isolation, though typically less than that seen in schizophrenia. We aimed to disentangle whether the social cognitive and social motivation impairments observed in schizophrenia are a specific feature of the clinical condition v. social isolation generally. METHODS: We compared four groups (clinically stable patients with schizophrenia or bipolar disorder, individuals drawn from the community with self-described social isolation, and a socially connected community control group) on loneliness, social cognition, and approach and avoidance social motivation. RESULTS: Individuals with schizophrenia (n = 72) showed intermediate levels of social isolation, loneliness, and social approach motivation between the isolated (n = 96) and connected control (n = 55) groups. However, they showed significant deficits in social cognition compared to both community groups. Individuals with bipolar disorder (n = 48) were intermediate between isolated and control groups for loneliness and social approach. They did not show deficits on social cognition tasks. Both clinical groups had higher social avoidance than both community groups. CONCLUSIONS: The results suggest that social cognitive deficits in schizophrenia, and high social avoidance motivation in both schizophrenia and bipolar disorder, are distinct features of the clinical conditions and not byproducts of social isolation. In contrast, differences between clinical and control groups on levels of loneliness and social approach motivation were congruent with the groups' degree of social isolation.

The temporal association between social isolation, distress, and psychotic experiences in individuals at clinical high-risk for psychosis.

BACKGROUND: Psychotic experiences (PEs) and social isolation (SI) seem related during early stages of psychosis, but the temporal dynamics between the two are not clear. Literature so far suggests a self-perpetuating cycle wherein momentary increases in PEs lead to social withdrawal, which, subsequently, triggers PEs at a next point in time, especially when SI is associated with increased distress. The current study investigated the daily-life temporal associations between SI and PEs, as well as the role of SI-related and general affective distress in individuals at clinical high risk (CHR) for psychosis. METHODS: We used experience sampling methodology in a sample of 137 CHR participants. We analyzed the association between SI, PEs, and distress using time-lagged linear mixed-effects models. RESULTS: SI did not predict next-moment fluctuations in PEs, or vice versa. Furthermore, although SI-related distress was not predictive of subsequent PEs, general affective distress during SI was a robust predictor of next-moment PEs. CONCLUSIONS: Our results suggest that SI and PEs are not directly related on a moment-to-moment level, but a negative emotional state when alone does contribute to the risk of PEs. These findings highlight the role of affective wellbeing during early-stage psychosis development.

Uncovering associations between gender nonconformity, psychosocial factors, and mental health in adolescents: a birth cohort study.

BACKGROUND: Little information is available on the association between gender nonconformity during adolescence and subsequent mental health. While the distress related to gender nonconformity may be socially produced rather than attributed to individual-level factors, further research is needed to better understand the role of psychosocial factors in this context. METHOD: We analyzed data from the Tokyo Teen Cohort, obtained through random sampling of adolescents born between 2002 and 2004. We used inverse probability weighting to examine the association of gender nonconformity at ages 12 and 14 as a time-varying variable with subsequent mental health at age 16, while accounting for time-fixed and time-varying confounders. Furthermore, we used a weighting approach to investigate the mediating role of modifiable psychosocial factors in this association, addressing exposure-mediator and mediator-mediator interactions. RESULTS: A total of 3171 participants were analyzed. Persistent gender nonconforming behavior at ages 12 and 14 was associated with subsequent depression (ß = 2.02, 95% confidence interval [CI] 0.85 to 3.19) and psychotic experiences (ß = 0.33, 95% CI 0.14 to 0.52) at age 16. The results remained robust in sensitivity analyses. Approximately 30% of the association between gender nonconformity and depression was consistently mediated by a set of psychosocial factors, namely loneliness, bullying victimization, and relationships with mother, father, and friends. CONCLUSIONS: Persistent gender nonconformity during adolescence is associated with subsequent mental health. Psychosocial factors play a vital mediating role in this association, highlighting the essential need for social intervention and change to reduce stigmatization and ameliorate mental health challenges.

Stress-related gene regulation: Do isolated and connected individuals differ?

BACKGROUND: Social isolation and loneliness (known as social disconnection, collectively) lead to serious downstream health effects, including shortening of lifespan and higher risk for cardiac disease. We must better understand how isolation and loneliness lead to these negative health outcomes. Previous literature has demonstrated that social motivation and social ability are contributors to the likelihood of social isolation and loneliness. We examined the effect of the above social factors on immune gene expression in socially-connected and -isolated individuals. METHODS: Recruitment occurred via two online advertisements, one for socially isolated individuals and another for general research participants. Participants (n = 102) were separated into groups (isolated versus connected) based on which ad they responded to, and provided data on isolation, loneliness, social motivation, and social ability. The Conserved Transcriptional Response to Adversity (CTRA) stress gene regulation program was assessed with genome-wide transcriptional profiling. RESULTS: CTRA gene expression patterns were reversed between connected and isolated groups across several variables. Social isolation was associated with higher CTRA levels in the connected group, but lower levels in the isolated group. Social approach was associated with lower CTRA levels in the connected group, but higher in the isolated group, and the converse was true for social avoidance. CTRA levels were minimally affected by social ability measures. CONCLUSION: Prior work on social isolation and loneliness has focused on loneliness and has identified many negative downstream health effects. In this study we demonstrate that objective social isolation may not be associated with the same negative downstream health effects, and in fact, social interaction may be more stressful than social isolation for some socially-isolated individuals.

A critical analysis of stress-related gene regulation in isolated and connected individuals.

This letter responds to the article by Yvonne S. Yang et al. (2024) on the relationship between social isolation, loneliness, and stress-related gene expression, published in Brain, Behavior, and Immunity. The authors' recruitment methods and use of genome-wide transcriptional profiling to assess the Conserved Transcriptional Response to Adversity (CTRA) are highly commendable, offering a comprehensive understanding of how social isolation impacts immune gene expression. However, we raise concerns about the small sample size, demographic limitations, cross-sectional design, and lack of correction for multiple comparisons, which may affect the study's generalizability and validity. Despite these limitations, the study provides valuable insights, paving the way for future research to explore the complex interactions between social isolation, loneliness, and health. We recommend that future studies address these limitations to enhance the robustness of findings and the development of effective interventions.

Social isolation promotes tumor immune evasion via ß2-adrenergic receptor.

Social isolation is a recognized risk factor for tumor initiation and mortality, but the role and mechanisms responsible for social isolation on tumor progression are poorly understood. In this study, we found that social isolation contributed to accelerated tumor growth and induced a remodeling of the tumor immune microenvironment, resulting in immunosuppression. Mechanistically, social isolation triggered the activation of the sympathetic nervous system, leading to impaired CD8+ T cell antitumor immune responses by activating ß-adrenergic receptor 2 (ß2-AR), which highly expressed on tumor-infiltrating CD8+ T cells. Pharmacological inhibition of ß2-AR signaling effectively enhanced CD8+ T cell anti-tumor immune responses and improved the efficacy of anti-PD-1 immunotherapy in the context of social isolation. Thus, our study uncovers a mechanism through which social isolation induces tumor immune evasion and offers potential directions for cancer immunotherapy in socially isolated patients.

Social isolation, loneliness, and inflammation: A multi-cohort investigation in early and mid-adulthood.

Social isolation and loneliness have been associated with poor health and increased risk for mortality, and inflammation might explain this link. We used data from the Danish TRIAGE Study of acutely admitted medical patients (N = 6,144, mean age 60 years), and from two population-representative birth cohorts: the New Zealand Dunedin Longitudinal Study (N = 881, age 45) and the UK Environmental Risk (E-Risk) Longitudinal Twin Study (N = 1448, age 18), to investigate associations of social isolation with three markers of systemic inflammation: C-reactive protein (CRP), interleukin-6 (IL-6), and a newer inflammation marker, soluble urokinase plasminogen activator receptor (suPAR), which is thought to index systemic chronic inflammation. In the TRIAGE Study, socially isolated patients (those living alone) had significantly higher median levels of suPAR (but not CRP or IL-6) compared with patients not living by themselves. Social isolation prospectively measured in childhood was longitudinally associated with higher CRP, IL-6, and suPAR levels in adulthood (at age 45 in the Dunedin Study and age 18 in the E-Risk Study), but only suPAR remained associated after controlling for covariates. Dunedin Study participants who reported loneliness at age 38 or age 45 had elevated suPAR at age 45. In contrast, E-Risk Study participants reporting loneliness at age 18 did not show any elevated markers of inflammation. In conclusion, social isolation was robustly associated with increased inflammation in adulthood, both in medical patients and in the general population. It was associated in particular with systemic chronic inflammation, evident from the consistently stronger associations with suPAR than other inflammation biomarkers.

Fatty Acid Amide Hydrolase and Cannabinoid Receptor Type 1 Genes Regulation is Modulated by Social Isolation in Rats.

Social isolation is a state of lack of social connections, involving the modulation of different molecular signalling cascades and associated with high risk of mental health issues. To investigate if and how gene expression is modulated by social experience at the central level, we analyzed the effects of 5 weeks of social isolation in rats focusing on endocannabinoid system genes transcription in key brain regions involved in emotional control. We observed selective reduction in mRNA levels for fatty acid amide hydrolase (Faah) and cannabinoid receptor type 1 (Cnr1) genes in the amygdala complex and of Cnr1 in the prefrontal cortex of socially isolated rats when compared to controls, and these changes appear to be partially driven by trimethylation of Lysine 27 and acetylation of Lysine 9 at Histone 3. The alterations of Cnr1 transcriptional regulation result also directly correlated with those of oxytocin receptor gene. We here suggest that to counteract the effects of SI, it is of relevance to restore the endocannabinoid system homeostasis via the use of environmental triggers able to revert those epigenetic mechanisms accounting for the alterations observed.