Early life experience shapes neural genome

Early life adversity as a risk factor for cognitive impairment and Alzheimer's disease

Neurological conditions, including cognitive impairment and Alzheimer's disease (AD), impose a huge burden on society, affecting millions of people globally. In addition to genetic factors, recent studies indicate that environmental and experiential factors may contribute to the pathogenesis of these diseases. Early life adversity (ELA) has a profound impact on brain function and health later in life. In rodent models, exposure to ELA results in specific cognitive deficits and aggravated AD pathology. Extensive concerns have been raised regarding the higher risk of developing cognitive impairments in people with a history of ELA. In this review, we scrutinize findings from human and animal studies focusing on the connection of ELA with cognitive impairment and AD. These discoveries suggest that ELA, especially at early postnatal stages, increases susceptibility to cognitive impairment and AD later in life. In terms of mechanisms, ELA could lead to dysregulation of the hypothalamus-pituitary-adrenal axis, altered gut microbiome, persistent inflammation, oligodendrocyte dysfunction, hypomyelination, and aberrant adult hippocampal neurogenesis. Crosstalks among these events may synergistically contribute to cognitive impairment later in life. Additionally, we discuss several interventions that may alleviate adverse consequences of ELA. Further investigation into this crucial area will help improve ELA management and reduce the burden of related neurological conditions.

Early life adversity affecting the attachment bond alters ventral tegmental area transcriptomic patterning and behavior almost exclusively in female mice

Early life experiences that affect the attachment bond formation can alter developmental trajectories and result in pathological outcomes in a sex-related manner. However, the molecular basis of sex differences is quite unknown. The dopaminergic system originating from the ventral tegmental area has been proposed to be a key mediator of this process. Here we exploited a murine model of early adversity (Repeated Cross Fostering, RCF) to test how interfering with the attachment bond formation affects the VTA-related functions in a sex-specific manner. Through a comprehensive behavioral screening, within the NiH RDoC framework, and by next-generation RNA-Seq experiments, we analyzed the long-lasting effect of RCF on behavioral and transcriptional profiles related to the VTA, across two different inbred strains of mouse in both sexes. We found that RCF impacted to an extremely greater extent VTA-related behaviors in females than in males and this result mirrored the transcriptional alterations in the VTA that were almost exclusively observed in females. The sexual dimorphism was conserved across two different inbred strains in spite of their divergent long lasting consequences of RCF exposure. Our data suggest that to be female primes a sub-set of genes to respond to early environmental perturbations. This is, to the best of our knowledge, the first evidence of an almost exclusive effect of early life experiences on females, thus mirroring the extremely stronger impact of precocious aversive events reported in clinical studies in women.

Resilience to anhedonia-passive coping induced by early life experience is linked to a long-lasting reduction of Ih current in VTA dopaminergic neurons

Exposure to aversive events during sensitive developmental periods can affect the preferential coping strategy adopted by individuals later in life, leading to either stress-related psychiatric disorders, including depression, or to well-adaptation to future adversity and sources of stress, a behavior phenotype termed “resilience”.

We have previously shown that interfering with the development of mother-pups bond with the Repeated Cross Fostering (RCF) stress protocol can induce resilience to depression-like phenotype in adult C57BL/6J female mice. Here, we used patch-clamp recording in midbrain slice combined with both in vivo and ex vivo pharmacology to test our hypothesis of a link between electrophysiological modifications of dopaminergic neurons in the intermediate Ventral Tegmental Area (VTA) of RCF animals and behavioral resilience. We found reduced hyperpolarization-activated (I_h) cation current amplitude and evoked firing in VTA dopaminergic neurons from both young and adult RCF female mice. In vivo, VTA-specific pharmacological manipulation of the I_h current reverted the pro-resilient phenotype in adult early-stressed mice or mimicked behavioral resilience in adult control animals.

This is the first evidence showing how pro-resilience behavior induced by early events is linked to a long-lasting reduction of I_h current and excitability in VTA dopaminergic neurons.

Genomic Variation, Evolvability, and the Paradox of Mental Illness

Twentieth-century genetics was hard put to explain the irregular behavior of neuropsychiatric disorders. Autism and schizophrenia defy a principle of natural selection; they are highly heritable but associated with low reproductive success. Nevertheless, they persist. The genetic origins of such conditions are confounded by the problem of variable expression, that is, when a given genetic aberration can lead to any one of several distinct disorders. Also, autism and schizophrenia occur on a spectrum of severity, from mild and subclinical cases to the overt and disabling. Such irregularities reflect the problem of missing heritability; although hundreds of genes may be associated with autism or schizophrenia, together they account for only a small proportion of cases. Techniques for higher resolution, genomewide analysis have begun to illuminate the irregular and unpredictable behavior of the human genome. Thus, the origins of neuropsychiatric disorders in particular and complex disease in general have been illuminated. The human genome is characterized by a high degree of structural and behavioral variability: DNA content variation, epistasis, stochasticity in gene expression, and epigenetic changes. These elements have grown more complex as evolution scaled the phylogenetic tree. They are especially pertinent to brain development and function. Genomic variability is a window on the origins of complex disease, neuropsychiatric disorders, and neurodevelopmental disorders in particular. Genomic variability, as it happens, is also the fuel of evolvability. The genomic events that presided over the evolution of the primate and hominid lineages are over-represented in patients with autism and schizophrenia, as well as intellectual disability and epilepsy. That the special qualities of the human genome that drove evolution might, in some way, contribute to neuropsychiatric disorders is a matter of no little interest.

Somatic mutations in neurodegeneration: An update

Mosaicism, the presence of genomic differences between cells due to post-zygotic somatic mutations, is widespread in the human body, including within the brain. A role for this in neurodegenerative diseases has long been hypothesised, and technical developments are now allowing the question to be addressed in detail. The rapidly accumulating evidence is discussed in this review, with a focus on recent developments. Somatic mutations of numerous types may occur, including single nucleotide variants (SNVs), copy number variants (CNVs), and retrotransposon insertions. They could act as initiators or risk factors, especially if they arise in development, although they could also result from the disease process, potentially contributing to progression. In common sporadic neurodegenerative disorders, relevant mutations have been reported in synucleinopathies, comprising somatic gains of SNCA in Parkinson's disease and multiple system atrophy, and in Alzheimer's disease, where a novel recombination mechanism leading to somatic variants of APP, as well as an excess of somatic SNVs affecting tau phosphorylation, have been reported. In Mendelian repeat expansion disorders, mosaicism due to somatic instability, first detected 25 years ago, has come to the forefront. Brain somatic SNVs occur in DNA repair disorders, and there is evidence for a role of several ALS genes in DNA repair. While numerous challenges, and need for further validation, remain, this new, or perhaps rediscovered, area of research has the potential to transform our understanding of neurodegeneration.

Nutritional stress timing differentially programs cognitive abilities in young adult male mice

Objectives: The impact of chronic exposure to environmental adversities on brain regions involved in cognition and mental health depends on whether it occurs during the perinatal period, childhood, adolescence or adulthood. The effects of these adversities on the brain and behavior arise as a function of the timing of the exposure and their co-occurrence with the development of specific regions. Here we aimed to explore the behavioral phenotypes derived from two nutritional stress paradigms which differed in the timing of exposure: a low-protein perinatal diet during gestation and lactation and a low-protein diet during adolescence.Methods: Locomotor and exploratory activity, recognition memory and aversive memory were measured in CF-1 8-week-old male mice subjected to perinatal malnutrition (LP-P) or adolescent malnutrition (LP-A), and their respective controls with normal protein diet (NP-P and NP-A).Results: By using the open field test, we found that LP-P and LP-A mice showed reduced exploratory activity compared to controls, but no alterations in their locomotor activity. Recognition memory was impaired only in LP-P mice. Interestingly, aversive memory was not altered in LP-P mice but was enhanced in LP-A mice. Considering the stress-inoculation theory, we hypothesized that protein malnutrition during adolescence represents a challenging but still moderate stressful environment, which promotes active coping in face of later adversity.Conclusion: Our results indicate that while perinatal malnutrition impairs recognition memory, adolescent malnutrition enhances aversive memory, showing dissimilar adaptive responses.

Xlr4 as a new candidate gene underlying vulnerability to cocaine effects

Although several studies have been performed in rodents, non-human primates and humans, the biological basis of vulnerability to develop cocaine addiction remains largely unknown. Exposure to critical early events (as Repeated Cross Fostering (RCF)) has been reported to increase sensitivity to cocaine effects in adult C57BL/6J female mice. Using a microarray approach, here we report data showing a strong engagement of X-linked lymphocyte-regulated 4a and 4b (Xlr4) genes in cocaine effects. The expression of Xlr4, a gene involved in chromatin remodeling and dendritic spine morphology, was reduced into the Nucleus Accumbens (NAc) of adult RCF C57BL/6J female. We used virally mediated accumbal Xlr4 down-modulation (AAVXlr4-KD) to investigate the role of this gene in vulnerability to cocaine effects. AAVXlr4-KD animals show a potentiated behavioral and neurochemical response to cocaine, reinstatement following cocaine withdrawal and cocaine-induced spine density alterations in the Medium-Sized Spiny Neurons of NAc. We propose Xlr4 as a new candidate gene mediating the cocaine effects.

Paradoxical effects of exercise on hippocampal plasticity and cognition in mice with a heterozygous null mutation in the serotonin transporter gene

BACKGROUND AND PURPOSE

Exercise is known to improve cognitive function, but the exact synaptic and cellular mechanisms remain unclear. We investigated the potential role of the serotonin (5-HT) transporter (SERT) in mediating these effects.

EXPERIMENTAL APPROACH

Hippocampal long-term potentiation (LTP) and neurogenesis were measured in standard-housed and exercising (wheel running) wild-type (WT) and SERT heterozygous (HET) mice. We also assessed hippocampal-dependent cognition using the Morris water maze (MWM) and a spatial pattern separation touchscreen task.

KEY RESULTS

SERT HET mice had impaired hippocampal LTP regardless of the housing conditions. Exercise increased hippocampal neurogenesis in WT mice. However, this was not observed in SERT HET animals, even though both genotypes used the running wheels to a similar extent. We also found that standard-housed SERT HET mice displayed altered cognitive flexibility than WT littermate controls in the MWM reversal learning task. However, SERT HET mice no longer exhibited this phenotype after exercise. Cognitive changes, specific to SERT HET mice in the exercise condition, were also revealed on the touchscreen spatial pattern separation task, especially when the cognitive pattern separation load was at its highest.

CONCLUSIONS AND IMPLICATIONS

Our study is the first evidence of reduced hippocampal LTP in SERT HET mice. We also show that functional SERT is required for exercise-induced increase in adult neurogenesis. Paradoxically, exercise had a negative impact on hippocampal-dependent cognitive tasks, especially in SERT HET mice. Taken together, our results suggest unique complex interactions between exercise and altered 5-HT homeostasis.

Somatic mutations in neurons during aging and neurodegeneration

The nervous system is composed of a large variety of neurons with a diverse array of morphological and functional properties. This heterogeneity is essential for the construction and maintenance of a distinct set of neural networks with unique characteristics. Accumulating evidence now indicates that neurons do not only differ at a functional level, but also at the genomic level. These genomic discrepancies seem to be the result of somatic mutations that emerge in nervous tissue during development and aging. Ultimately, these mutations bring about a genetically heterogeneous population of neurons, a phenomenon that is commonly referred to as "somatic brain mosaicism". Improved understanding of the development and consequences of somatic brain mosaicism is crucial to understand the impact of somatic mutations on neuronal function in human aging and disease. Here, we highlight a number of topics related to somatic brain mosaicism, including some early experimental evidence for somatic mutations in post-mitotic neurons of the hypothalamo-neurohypophyseal system. We propose that age-related somatic mutations are particularly interesting, because aging is a major risk factor for a variety of neuronal diseases, including Alzheimer's disease. We highlight potential links between somatic mutations and the development of these diseases and argue that recent advances in single-cell genomics and in vivo physiology have now finally made it possible to dissect the origins and consequences of neuronal mutations in unprecedented detail.

Family Childcare Types and Conduct Problem Behaviors in Young Children: The Mediation Role of Caregiver-Child Interaction

Background: Previous studies have demonstrated the impacts of genetic, family, and community factors on child conduct problems (CPs). However, little is understood regarding the association between family childcare types and child conduct problem behaviors, as well as whether and to what extent caregiver-child interaction mediates the above association. Methods: 9,289 children first entering kindergartens in the Longhua New District of Shenzhen, China were enrolled in this cross-sectional study. Primary caregivers were invited to fulfill a self-administered structured questionnaire containing data regarding socio-demographics, family childcare types, caregiver-child interaction, and child conduct problem behaviors (measured by the Conners' Conduct Problem Subscale). A series of multiple logistic and linear regression models were employed to assess the associations among family childcare, caregiver-child interaction, and child conduct problem behaviors. Results: Family childcare types other than by parents together (i.e., mother alone, mother with others, grandparents, or changing caregivers) were all significantly associated with higher risks of conduct problem behaviors in young children (adjusted ORs ranged from 2.18 to 2.55, and adjusted βs ranged from 0.043 to 0.073; all p < 0.05), after adjusting for confounders including child age, gender, parental education level, parental age at pregnancy, marital status, and family income. The following family childcare types (mother alone, or grandparents, or changing caregivers) vs. the childcare by parents together showed significant relative indirect effects on conduct problem behaviors through caregiver-child interaction, indicating the significant mediation effect of caregiver-child interaction on the above associations. Mediation of caregiver-child interaction on the effect of being cared by mother with others relative to care by parents together on child conduct problem behaviors was yet non-significant. Conclusions: Family childcare types other than by parents together are associated with increased risks for conduct problem behaviors in young children. Caregiver-child interaction may function as a potential mediator for the above association.