Specificity and efficiency of tamoxifen-mediated Cre induction is equivalent regardless of age

Temporally controlling Cre recombination through tamoxifen (Tam) induction has many advantages for biomedical research. Most studies report early post-natal/juvenile (<2 m.o.) Tam induction, but age-related neurodegeneration and aging studies can require Cre induction in older mice (>12 m.o.). While anecdotally reported as problematic, there are no published comparisons of Tam-mediated Cre induction at early and late ages. Here, microglial-specific Cx3cr1creERT2 mice were crossed to a floxed NuTRAP reporter to compare Cre induction at early (3-6 m.o.) and late (20 m.o.) ages. Specificity and efficiency of microglial labeling at 21-22 m.o. were identical in mice induced with Tam at early and late ages. Age-related microglial translatomic changes were also similar regardless of Tam induction age. Each Cre and flox mouse line should be independently validated, however, these findings demonstrate that Tam-mediated Cre induction can be performed even into older mouse ages and should be generalizable to other inducible Cre models.

Microglial MHC-I induction with aging and Alzheimer's is conserved in mouse models and humans

Major histocompatibility complex I (MHC-I) CNS cellular localization and function is still being determined after previously being thought to be absent from the brain. MHC-I expression has been reported to increase with brain aging in mouse, rat, and human whole tissue analyses, but the cellular localization was undetermined. Neuronal MHC-I is proposed to regulate developmental synapse elimination and tau pathology in Alzheimer's disease (AD). Here, we report that across newly generated and publicly available ribosomal profiling, cell sorting, and single-cell data, microglia are the primary source of classical and non-classical MHC-I in mice and humans. Translating ribosome affinity purification-qPCR analysis of 3-6- and 18-22-month-old (m.o.) mice revealed significant age-related microglial induction of MHC-I pathway genes B2m, H2-D1, H2-K1, H2-M3, H2-Q6, and Tap1 but not in astrocytes and neurons. Across a timecourse (12-23 m.o.), microglial MHC-I gradually increased until 21 m.o. and then accelerated. MHC-I protein was enriched in microglia and increased with aging. Microglial expression, and absence in astrocytes and neurons, of MHC-I-binding leukocyte immunoglobulin-like (Lilrs) and paired immunoglobin-like type 2 (Pilrs) receptor families could enable cell -autonomous MHC-I signaling and increased with aging in mice and humans. Increased microglial MHC-I, Lilrs, and Pilrs were observed in multiple AD mouse models and human AD data across methods and studies. MHC-I expression correlated with p16INK4A, suggesting an association with cellular senescence. Conserved induction of MHC-I, Lilrs, and Pilrs with aging and AD opens the possibility of cell-autonomous MHC-I signaling to regulate microglial reactivation with aging and neurodegeneration.

Consistent specificity and efficiency of tamoxifen-mediated cre induction across ages

Temporally controlling cre recombination through tamoxifen (Tam) induction has many advantages for biomedical research. Most studies report Tam induction at early post-natal/juvenile (<2 m.o.) mouse ages, but age-related neurodegeneration and aging studies can require cre induction in older mice (>12 m.o.). While anecdotally reported as problematic, there are no published comparisons of Tam mediated cre induction at early and late ages. Here, microglial-specific Cx3cr1 creERT 2 mice were crossed to a floxed NuTRAP reporter to compare cre induction at early (3-6 m.o.) and late (20 m.o.) ages. Specificity and efficiency of microglial labeling at 21-22 m.o. were identical in mice induced with Tam at 3-6 m.o. or 20 m.o. of age. Age-related microglial translatomic changes were also similar regardless of Tam induction age. Each cre and flox mouse line should be validated independently, however, these findings demonstrate that Tam-mediated cre induction can be performed even into older mouse ages.

Microglial senescence contributes to female-biased neuroinflammation in the aging mouse hippocampus: implications for Alzheimer's disease

BACKGROUND

Microglia, the brain's principal immune cells, have been implicated in the pathogenesis of Alzheimer's disease (AD), a condition shown to affect more females than males. Although sex differences in microglial function and transcriptomic programming have been described across development and in disease models of AD, no studies have comprehensively identified the sex divergences that emerge in the aging mouse hippocampus. Further, existing models of AD generally develop pathology (amyloid plaques and tau tangles) early in life and fail to recapitulate the aged brain environment associated with late-onset AD. Here, we examined and compared transcriptomic and translatomic sex effects in young and old murine hippocampal microglia.

METHODS

Hippocampal tissue from C57BL6/N and microglial NuTRAP mice of both sexes were collected at young (5-6 month-old [mo]) and old (22-25 mo) ages. Cell sorting and affinity purification techniques were used to isolate the microglial transcriptome and translatome for RNA-sequencing and differential expression analyses. Flow cytometry, qPCR, and imaging approaches were used to confirm the transcriptomic and translatomic findings.

RESULTS

There were marginal sex differences identified in the young hippocampal microglia, with most differentially expressed genes (DEGs) restricted to the sex chromosomes. Both sex chromosomally and autosomally encoded sex differences emerged with aging. These sex DEGs identified at old age were primarily female-biased and enriched in senescent and disease-associated microglial signatures. Normalized gene expression values can be accessed through a searchable web interface ( https://neuroepigenomics.omrf.org/ ). Pathway analyses identified upstream regulators induced to a greater extent in females than in males, including inflammatory mediators IFNG, TNF, and IL1B, as well as AD-risk genes TREM2 and APP.

CONCLUSIONS

These data suggest that female microglia adopt disease-associated and senescent phenotypes in the aging mouse hippocampus, even in the absence of disease pathology, to a greater extent than males. This sexually divergent microglial phenotype may explain the difference in susceptibility and disease progression in the case of AD pathology. Future studies will need to explore sex differences in microglial heterogeneity in response to AD pathology and determine how sex-specific regulators (i.e., sex chromosomal or hormonal) elicit these sex effects.

Microglial MHC-I induction with aging and Alzheimer's is conserved in mouse models and humans

Major Histocompatibility Complex I (MHC-I) CNS cellular localization and function is still being determined after previously being thought to be absent from the brain. MHC-I expression has been reported to increase with brain aging in mouse, rat, and human whole tissue analyses but the cellular localization was undetermined. Neuronal MHC-I is proposed to regulate developmental synapse elimination and tau pathology in Alzheimer's disease (AD). Here we report that across newly generated and publicly available ribosomal profiling, cell sorting, and single-cell data, microglia are the primary source of classical and non-classical MHC-I in mice and humans. Translating Ribosome Affinity Purification-qPCR analysis of 3-6 and 18-22 month old (m.o.) mice revealed significant age-related microglial induction of MHC-I pathway genes B2m , H2-D1 , H2-K1 , H2-M3 , H2-Q6 , and Tap1 but not in astrocytes and neurons. Across a timecourse (12-23 m.o.), microglial MHC-I gradually increased until 21 m.o. and then accelerated. MHC-I protein was enriched in microglia and increased with aging. Microglial expression, and absence in astrocytes and neurons, of MHC-I binding Leukocyte Immunoglobulin-like (Lilrs) and Paired immunoglobin-like type 2 (Pilrs) receptor families could enable cell-autonomous MHC-I signaling and increased with aging in mice and humans. Increased microglial MHC-I, Lilrs, and Pilrs were observed in multiple AD mouse models and human AD data across methods and studies. MHC-I expression correlated with p16INK4A , suggesting an association with cellular senescence. Conserved induction of MHC-I, Lilrs, and Pilrs with aging and AD opens the possibility of cell-autonomous MHC-I signaling to regulate microglial reactivation with aging and neurodegeneration.

Microglial senescence contributes to female-biased neuroinflammation in the aging mouse hippocampus: implications for Alzheimer's disease

Background

Microglia, the brain's principal immune cells, have been implicated in the pathogenesis of Alzheimer's disease (AD), a condition shown to affect more females than males. Although sex differences in microglial function and transcriptomic programming have been described across development and in disease models of AD, no studies have comprehensively identified the sex divergences that emerge in the aging mouse hippocampus. Further, existing models of AD generally develop pathology (amyloid plaques and tau tangles) early in life and fail to recapitulate the aged brain environment associated with late-onset AD. Here, we examined and compared transcriptomic and translatomic sex effects in young and old murine hippocampal microglia.

Methods

Hippocampal tissue from C57BL6/N and microglial NuTRAP mice of both sexes were collected at young (5-6 month-old [mo]) and old (22-25 mo) ages. Cell sorting and affinity purification techniques were used to isolate the microglial transcriptome and translatome for RNA-sequencing and differential expression analyses. Flow cytometry, qPCR, and imaging approaches were used to confirm the transcriptomic and translatomic findings.

Results

There were marginal sex differences identified in the young hippocampal microglia, with most differentially expressed genes (DEGs) restricted to the sex chromosomes. Both sex chromosomally-and autosomally-encoded sex differences emerged with aging. These sex DEGs identified at old age were primarily female-biased and enriched in senescent and disease-associated microglial signatures. Normalized gene expression values can be accessed through a searchable web interface ( https://neuroepigenomics.omrf.org/ ). Pathway analyses identified upstream regulators induced to a greater extent in females than in males, including inflammatory mediators IFNG, TNF, and IL1B, as well as AD-risk genes TREM2 and APP.

Conclusions

These data suggest that female microglia adopt disease-associated and senescent phenotypes in the aging mouse hippocampus, even in the absence of disease pathology, to a greater extent than males. This sexually divergent microglial phenotype may explain the difference in susceptibility and disease progression in the case of AD pathology. Future studies will need to explore sex differences in microglial heterogeneity in response to AD pathology and determine how sex-specific regulators (i.e., sex chromosomal or hormonal) elicit these sex effects.

Chromosomal and gonadal factors regulate microglial sex effects in the aging brain

Biological sex contributes to phenotypic sex effects through genetic (sex chromosomal) and hormonal (gonadal) mechanisms. There are profound sex differences in the prevalence and progression of age-related brain diseases, including neurodegenerative diseases. Inflammation of neural tissue is one of the most consistent age-related phenotypes seen with healthy aging and disease. The pro-inflammatory environment of the aging brain has primarily been attributed to microglial reactivity and adoption of heterogeneous reactive states dependent upon intrinsic (i.e., sex) and extrinsic (i.e., age, disease state) factors. Here, we review sex effects in microglia across the lifespan, explore potential genetic and hormonal molecular mechanisms of microglial sex effects, and discuss currently available models and methods to study sex effects in the aging brain. Despite recent attention to this area, significant further research is needed to mechanistically understand the regulation of microglial sex effects across the lifespan, which may open new avenues for sex informed prevention and treatment strategies.

Promotion of colorectal cancer by transcription factor BHLHE40 involves upregulation of ADAM19 and KLF7

BHLHE40 is a transcription factor, whose role in colorectal cancer has remained elusive. We demonstrate that the BHLHE40 gene is upregulated in colorectal tumors. Transcription of BHLHE40 was jointly stimulated by the DNA-binding ETV1 protein and two associated histone demethylases, JMJD1A/KDM3A and JMJD2A/KDM4A, which were shown to also form complexes on their own and whose enzymatic activity was required for BHLHE40 upregulation. Chromatin immunoprecipitation assays revealed that ETV1, JMJD1A and JMJD2A interacted with several regions within the BHLHE40 gene promoter, suggesting that these three factors directly control BHLHE40 transcription. BHLHE40 downregulation suppressed both growth and clonogenic activity of human HCT116 colorectal cancer cells, strongly hinting at a pro-tumorigenic role of BHLHE40. Through RNA sequencing, the transcription factor KLF7 and the metalloproteinase ADAM19 were identified as putative BHLHE40 downstream effectors. Bioinformatic analyses showed that both KLF7 and ADAM19 are upregulated in colorectal tumors as well as associated with worse survival and their downregulation impaired HCT116 clonogenic activity. In addition, ADAM19, but not KLF7, downregulation reduced HCT116 cell growth. Overall, these data have revealed a ETV1/JMJD1A/JMJD2A→BHLHE40 axis that may stimulate colorectal tumorigenesis through upregulation of genes such as KLF7 and ADAM19, suggesting that targeting this axis represents a potential novel therapeutic avenue.

Accrual to breast cancer clinical trials at a university-affiliated hospital in metropolitan Detroit

Despite the large number of available studies, most women with breast cancer do not participate in clinical trials, and this is especially true among lower income and minority women. In this study the authors surveyed the practice patterns of four medical oncologists who comprised the clinical breast service at a large urban university hospital to develop a better understanding of the clinical trials enrollment process for women with breast cancer. Of 136 new female breast cancer patients seen by the four physicians over a 7-month period, there were 47 women (34%) offered participation in a clinical trial, and 16 women (12%) were eventually enrolled. Women who were offered participation were more likely to be younger (p = 0.068) and to have earlier stage disease then were women not offered participation (p = 0.008). Women enrolled into a trial were also more likely to be younger, although this difference was not statistically significant (p = 0.114). Patient race was not associated with the accrual or enrollment process. Over half of the women were not offered participation in clinical trials because of the lack of available studies. Further work evaluating the process of patient enrollment and physician and patient barriers is necessary to develop effective strategies for recruitment into breast cancer clinical trials.

Immunoenzymometric assay and radioimmunoassay measure different populations of antibody against human acetylcholine receptor

We used a "sandwich"-type immunoenzymometric assay (IEMA) and a radioimmunoassay (RIA) to measure antibody against the human nicotinic acetylcholine receptor in serum from individuals with myasthenia gravis, with markedly different results for certain specimens, as measured by the two techniques. In some cases, antibody concentrations were high by RIA but low by IEMA; in others, the reverse was found. Such differences persisted through 30 months after thymectomy. An investigation of potential causes of this disparity suggests that high IEMA measurements reflect specific anti-receptor antibody and are not artifactual. The IEMA is recommended as an adjunct to the RIA because some patients with myasthenia gravis who have low concentrations of anti-receptor antibodies as measured by RIA have significantly above-normal concentrations of anti-receptor antibodies as measured by IEMA.

Immunoenzymometric assay and radioimmunoassay measure different populations of antibody against human acetylcholine receptor

We used a "sandwich"-type immunoenzymometric assay (IEMA) and a radioimmunoassay (RIA) to measure antibody against the human nicotinic acetylcholine receptor in serum from individuals with myasthenia gravis, with markedly different results for certain specimens, as measured by the two techniques. In some cases, antibody concentrations were high by RIA but low by IEMA; in others, the reverse was found. Such differences persisted through 30 months after thymectomy. An investigation of potential causes of this disparity suggests that high IEMA measurements reflect specific anti-receptor antibody and are not artifactual. The IEMA is recommended as an adjunct to the RIA because some patients with myasthenia gravis who have low concentrations of anti-receptor antibodies as measured by RIA have significantly above-normal concentrations of anti-receptor antibodies as measured by IEMA.