Does an infection also affect the immunization of subsequent generations?
Researchers at Radboud University (Netherlands) have studied this together with the Universities of Bonn, Saarland (Germany), Lausanne (Switzerland) and Athens (Greece).
Mouse fathers who had previously overcome an infection with fungi or were stimulated with fungal compounds also passed on their improved protection to their offspring across several generations. The team showed at the same time an improved immune response being passed on to the descendants.
The study has now been published in the renowned journal Nature Immunology.
Not just what is written in the DNA sequence is inherited. Scientific studies show that environmental influences are also passed on to the next generation.
Epigenetic research investigates such relationships on the molecular level. It examines changes in gene function. “Not all areas of DNA are equally accessible for reading the genetic information,” explains Prof. Dr. Andreas Schlitzer of the LIMES Institute at the University of Bonn.
A research team from the Radboud University Nijmegen (Netherlands), the University of Bonn, Saarland University, the University of Lausanne (Switzerland) as well as the National and Kapodistrian University of Athens (Greece) has now for the first time intensively investigated whether effects of the innate immune system are also passed on to the next generations in mammals.
Infection with fungi trains the immune system of mice
The researchers infected male mice with thrush fungi (Candida albicans). After recovering from the infection, the animals were mated with completely healthy females. The researchers compared the resulting offspring with offspring from pairs of mice that were not infected previously with Candida. In order to investigate the status of the immune system experimentally, the team infected the males of the subsequent generation of mice with coliform bacteria.
“The offspring of the male mice previously exposed to Candida were significantly better protected from a subsequent E. coli infection than the progeny of the uninfected male mice,” reports Prof. Mihai G. Netea of the Radboud Center for Infectious Diseases. This effect was still evident in the next generation.
How does this transmission of immunization to subsequent generations work?
The team examined typical immune cells such as monocytes or neutrophils. No differences were detectable between the offspring of Candida-infected male mice and the non-infected control group.
In the offspring of fathers previously infected with thrush fungi, it was found that genes associated with inflammation were easier to read in monocyte progenitors than in sons of uninfected fathers. “This shows that monocyte precursors are epigenetically rewired if the fathers have previously undergone infection with Candida albicans,” Schlitzer summarizes.
Shift in gene activity is detectable in sperm
How does the transmission of this information to the next generation take place? In cooperation with Saarland University, the researchers investigated the gene activity of the sperm of mouse fathers infected with Candida. They analyzed the extent to which methyl groups blocked access to genes.
“A shift in gene markers was evident here,” says Prof. Dr. Jörn Walter of Saarland University. Offspring of Candida-infected male mice showed fewer gene blockages in gene regions important for inflammatory processes and monocyte maturation. How the information about the sperm markings reaches the bone marrow, the birthplace of many immune cells, still needs to be explored in further studies.
“The results have been made possible by the very good and close cooperation of researchers from different disciplines and institutions,” emphasizes Prof. Netea. Together with Prof. Schlitzer, the researchers are also members of the Cluster of Excellence ImmunoSensation2 and part of the Life & Medical Sciences Institute (LIMES) of the University of Bonn.
“The study is the first to show in mammals that adaptations to infectious diseases are also passed on to the offspring,” Netea says. In contrast to the classical theory of evolution, which assumes slow adaptation through changes in the genetic code, this involves very rapid changes via the epigenetic regulation of gene activities, irrespective of the genetic code.
The researchers do not yet know whether the findings obtained in mice can also be transferred to humans. “But we are assuming that this is the case,” Schlitzer says. “The immune system mechanisms and cells involved are very similar in mice and humans.”
A novel role of epigenetic modifications in infection-mediated neurodevelopmental disorders
A short overview of epigenetic mechanisms
‘Epigenetics’ refers to the combination of mechanisms that confer long-term and heritable changes in gene expression without altering the DNA sequence itself.21 Epigenetic programming is dynamic and responsive to different environmental exposures during development and includes several interrelated processes (Figure 2), including chromatin remodeling, histone modifications, DNA methylation and expression of microRNAs (miRNAs).21, 22, 45
Histone modifications are covalent post-translational modifications of histone proteins, which include, among others, methylation, phosphorylation, acetylation, ubiquitylation and sumoylation.21, 22, 45 Histone modifications can define the extent to which DNA is wrapped around the nucleosome core, thereby influencing the accessibility of the gene transcription machinery and subsequent gene expression. DNA methylation, on the other hand, consists of covalent methylation of cytosine rings that are found at cytosine–phosphodiester–guanine (CpG) dinucleotides.21, 22, 45
When located in distinct genomic regions such as gene promoter or enhancer sites, DNA methylation typically acts to repress gene transcription.21, 22, 45 Silencing of gene transcription by DNA methylation can be mediated by direct interference with the binding of transcription factors or enhancers to recognition elements that contain CpG dinucleotides, or through recruitment of methylated DNA-binding factors that in turn attract chromatin-inactivating complexes including histone deacetylases and histone methyltransferases.
Finally, miRNAs are a class of small non-encoding RNAs (~22 nucleotides long) that can control target gene expression post-transcriptionally.21, 22, 45 Interestingly, recent studies have demonstrated that epigenetic mechanisms, such as DNA methylation, not only regulate the transcription of protein-encoding genes, but also the expression of miRNAs.46
Conversely, miRNAs can control the expression of important epigenetic regulators, including DNA methyltransferases and histone deacetylases. Hence, there is a dynamic regulatory network between different epigenetic pathways, which altogether organize gene expression profiles through transcriptional or post-transcriptional mechanisms.21, 22, 45, 46
Schematic representation of major epigenetic mechanisms. The DNA–protein complex is referred to as chromatin. The functional unit of the chromatin is the nucleosome, which is composed of DNA wrapped around a core octamer of histone proteins. The DNA–histone interaction occurs at the N-terminal tails of these histones, which face outward and are sites for epigenetic marking known as histone modifications. These modifications represent a first major epigenetic mechanism modulating gene expression and involve methylation, phosphorylation, acetylation, ubiquitylation and sumoylation. DNA methylations at cytosine rings, typically found at CpG dinucleotides, are another important epigenetic mark that can influence gene expression. Finally, micro-RNAs (miRNAs), a class of small, non-coding RNAs, can control target gene expression post-transcriptionally. CpG, cytosine–phosphodiester–guanine.
How can epigenetic modifications influence brain development?
A growing number of studies highlight the importance of epigenetic mechanisms in normal brain development.45 Interestingly, transcriptional changes in the brain occur more frequently during prenatal life as compared to any other age in life.47 A temporally precise and specific regulation of gene expression thus seems indispensable for normal brain development. This regulatory process involves methylation-related epigenetic modifications, which allow a fine-tuning of fetal gene expression according to specific stages of brain development.48, 49, 50
A critical role for DNA methylation in early brain development is also supported by the dynamic expression of DNA methyltransferases during prenatal life,51 and by studies using transgenic mouse models showing neurodevelopmental deficits in mice with mutations in the methyl-CpG-binding protein 2 (Mecp2) gene.52, 53 Furthermore, mice lacking distinct DNA methyltransferases (DNMTs) in the forebrain have been found to display severe learning and memory deficits, as well as impaired synaptic plasticity.54, 55
In addition to epigenetic processes involving DNA methylation, several other epigenetic mechanisms appear to be critical for neuronal development and functions. For example, it has been shown that histone modifications can regulate the conversion of oligodendrocytes into neural stem cells.56 Furthermore, Fischer et al.57 demonstrated in a mouse model of neurodegenerative disorders that increasing histone acetylation could promote synaptogenesis and augment cognitive functions.
Accumulating evidence suggests that miRNAs may similarly play a role in central nervous system development. For example, it has been shown that miRNAs in specific neuronal or glial cell populations show a dynamic expression pattern during brain development.58, 59
Moreover, a recent study reported distinct temporal patterns of miRNA expression in the brain throughout gestation, and from early neonatal to adult life.60 The developmentally regulated pattern of miRNA expression is indicative of a functional role of these molecules in normal brain development. However, our understanding of how miRNA can influence neurodevelopmental processes is still in its infancy and warrants further investigations.
The importance of epigenetic mechanisms in brain development is also supported by a plethora of findings demonstrating epigenetic alterations in neurodevelopmental disease models that are based on exposure to environmental adversities in early life. As reviewed in detail elsewhere21, 45, 61 stable epigenetic modifications may represent an important mechanism by which exposures to early-life environmental adversities can induce pathological consequences, even across multiple generations.
Transgenerational transmission of disease susceptibility or epigenetic modification has been observed following early-life exposure to various environmental adversities (see Table 1 for prenatal environmental adversities), including prenatal or neonatal stress,62, 63, 64, 65, 66, 67, 68, 69 prenatal malnutrition,70, 71, 72, 73, 74, 75 endocrine disruptors76, 77, 78, 79, 80, 81 and chronic psychostimulant or alcohol intake.82, 83 The phenomenon of non-genetic transgenerational transmission of behavioral traits has gained increasing recognition in view of its potential importance in the etiology and treatment of multifactorial disorders.61, 84, 85 As discussed in the subsequent sections, recent research now suggests that similar effects can be induced by prenatal exposure to infection.
Epigenetic and transgenerational effects of prenatal infection
Epigenetic modifications induced by prenatal infection
Several recent studies sought to examine the putative effects of prenatal infection on epigenetic processes. Using a rat model of poly(I:C)-induced maternal immune activation, Hollins et al.86 revealed that prenatally infected offspring exhibited significant differences in the expression of miRNA in the entorhinal cortex, a brain area implicated in neurodevelopmental disorders. Interestingly, a large subset of these miRNAs were clustered within the Dlk1-Dio3-imprinted domain on 6q32, which is associated with schizophrenia, and were predicted to regulate pathways involved in synaptic remodeling, learning and memory formation.86
Maternal immune activation by poly(I:C) treatment in mice has also been found to alter histone modifications in the offspring. More specifically, offspring of poly(I:C)-treated mice displayed changes in promoter-specific histone acetylation and corresponding transcriptional changes, the latter of which affected genes associated with neuronal development, synaptic transmission and immune signaling.87
Accumulating evidence suggest that prenatal infection can also cause stable changes in DNA methylation. For example, Basil et al.88 found that prenatal exposure to the viral mimic poly(I:C) caused global changes in the level of DNA methylation in the adolescent mouse brain, including Mecp2 promoter hypomethylation.
A recent study by Labouesse et al.89 extended those findings by assessing correlations between DNA-related epigenetic modifications, expression levels of corresponding genes and behavioral deficits. The authors showed that prenatal viral-like immune activation by poly(I:C) in mice induced methylation-related promoter remodeling of GAD1 and GAD2 in the prefrontal cortex. GAD1 and GAD2 encode for two isoforms of the rate-limiting enzyme for γ-aminobutyric acid biosynthesis.
It was shown that offspring born to immune-challenged mothers displayed GAD1 and GAD2 promoter hypermethylation and associated reductions in the expression of the corresponding mRNA transcripts (GAD67 and GAD65), which in turn correlated with deficits in social interaction and impairments in working memory.89 These findings suggest that methylation-related epigenetic modifications at presynaptic GABAergic systems may represent a mechanism whereby maternal infection during pregnancy can induce long-term behavioral impairments in the offspring.
Using the same mouse model of poly(I:C)-induced maternal immune activation, a recent study by Richetto et al.90 examined genome-wide DNA methylation differences at single-nucleotide resolution by capture array bisulfite sequencing in the adult prefrontal cortex.
It was shown that offspring of immune-challenged mothers displayed hyper- and hypomethylated CpGs at numerous loci and at distinct genomic regions.90 The differences in methylation were again associated with transcriptional changes of the corresponding genes, suggesting that the infection-induced epigenetic modifications had a functional impact on gene expression.90
Taken together, these findings indicate that prenatal infection can cause lasting changes in the offspring’s epigenome. The available data thus far show that viral-like maternal immune activation in early/middle (between gestation day (GD) 9 and 12)87, 88 or in late (GD15 and beyond)86, 89 gestation causes such changes. Recent evidence suggests, however, that the timing of viral-like immune challenge critically determines the specificity of infection-mediated epigenetic modifications,90 since early and late gestational window clearly differ in terms of methylation-related epigenetic modifications they induce.
Transgenerational effects of prenatal infection
Recent studies extended the abovementioned findings by assessing whether behavioral and cognitive abnormalities emerging in the direct descendants (F1) of gestationally immune-challenged mothers could be transmitted across subsequent generations (F2 and F3) without any further immune exposures. Using the prenatal poly(I:C) administration model in mice, it was repeatedly shown that some of the behavioral abnormalities are not only present in the direct descendants of immune-challenged mothers, but are transmitted to the subsequent generations,91, 92 at least when the immune challenge was induced in early/middle pregnancy (that is, between GD9 and GD12).
Interestingly, one study reported a transgenerational transmission of behavioral abnormalities mostly via the paternal lineage, which extended to the third (F3) generation of offspring (Figure 3).92 The paternal mode of transmission is consistent with other models of early-life adversities, such as pre- and neonatal stress and prenatal malnutrition.62, 63, 64, 65, 66, 70, 71
The fact that prenatal immune activation can cause transgenerational transmission of pathological phenotypes via the paternal lineage strongly suggests the involvement of epigenetic modifications in male gametes, which in turn could mediate epigenetic inheritance across generations.21, 45, 63, 66 Prenatal immune activation thus likely alters epigenetic marks in the germ line of the direct offspring, which resists erasure and epigenetic reestablishment during germ cell development.21
Summary of the transgenerational transmission and modification of behavioral deficits induced by prenatal immune activation. The use of a mouse model of viral-like immune activation, which was induced by maternal treatment with the viral mimetic poly(I:C), led to the recent discovery of transgenerational effects following prenatal immune activation (for details, see Weber-Stadlbauer et al.92). In this model, reduced sociability and increased fear-related behavior were similarly present in first-generation (F1) and second-generation (F2) offspring of immune-challenged ancestors. Sensorimotor gating impairments were confined to the direct descendants of infected mothers, whereas increased behavioral despair emerged as a novel phenotype in the second generation. The transgenerational effects were transmitted via the paternal lineage (not shown) and were stable until the third generation (F3), demonstrating transgenerational non-genetic inheritance of pathological traits following prenatal immune activation.
It remains to be determined further, however, why early-life adversities such as maternal infection largely spare the female germ cells. One possible explanation may relate to the differential developmental dynamics of male and female gametes. Germ cells start developing shortly after fertilization and rapidly proliferate until they migrate to the genital ridge, where sex determination occurs.21 Once the primordial germ cells are sexually differentiated, oocytes and male germ cells have different developmental dynamics; following sexual differentiation, the oocyte enters into meiosis and arrests until puberty, while male germ cells go into arrest until birth, when they undergo a phase of proliferation and then complete meiosis during puberty.93
In addition to its transgenerational effects on behavior, prenatal exposure to viral-like immune activation in mice was also found to modify transcriptional activity across generations.92 Using next-generation mRNA sequencing, it was shown that prenatal poly(I:C)-induced immune activation caused widespread gene expression changes in the brains of both the F1 and F2 generations. Intriguingly, while some transcriptional changes were uniquely present in either F1 or F2 offspring, others were common to both generations. Hence, the prenatal infection-induced transgenerational transmission of behavioral abnormalities (Figure 3) is associated with, and perhaps even mediated by, transgenerational modifications of gene expression.
Depending on the timing of the environmental insults or genetic loci, some of the epigenetic marks may affect only the germ line while sparing somatic tissues in the first generation. This may explain why certain behavioral deficits emerge as a novel phenotype only in the second and third generation of infected ancestors (Figure 3). Future studies are warranted in order to compare infection-induced epigenetic modifications in somatic cells and gametes. Such studies will help to further elucidate the mechanisms underlying the transgenerational modification and inheritance of brain pathology following prenatal immune activation.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5534947/
More information: Jorge Domínguez-Andrés, Transmission of trained immunity and heterologous resistance to infections across generations, Nature Immunology (2021). DOI: 10.1038/s41590-021-01052-7. www.nature.com/articles/s41590-021-01052-7
