Comparison of Neurological Function in Males and Females from Two Substrains of C57BL/6 Mice

Early Life Outcomes of Prenatal Exposure to Alcohol and Synthetic Cannabinoids in Mice

This study explores the effects of prenatal co-exposure to alcohol and synthetic cannabinoids on offspring viability, physical development, and neurobehavioral outcomes in young adulthood. The aim is to identify distinct outcomes of co-exposure compared to single-drug exposures and to examine potential sex-specific vulnerabilities in motor coordination and exploratory behaviors. Pregnant C57Bl/6J mice were assigned to one of four treatment groups: Control, Alcohol-exposed, Cannabinoid-exposed, or Alcohol+Cannabinoid-exposed, with drug administration occurring between Gestational Days 12-15. Offspring were first evaluated at birth for survival, physical malformations, and developmental delays. Subsequently, young adult offspring were assessed for motor coordination using rotarod tests and exploratory behavior using open field tests. Our results indicate that alcohol and cannabinoid co-exposure significantly reduced offspring survival and litter sizes compared to controls. Non-viable offspring displayed craniofacial abnormalities, limb malformations, and developmental delays. Behavioral assessments in young adulthood demonstrated that all forms of prenatal drug exposure impaired motor coordination in males, while alcohol and cannabinoid exposures independently produced impairments in females. In the open field test, co-exposed male offspring exhibited reduced center exploration, indicative of anxiety-like behavior. Co-exposed offspring, regardless of sex, demonstrated hyperactivity, characterized by increased speed and distance traveled. Together, these findings underscore the heightened risks associated with prenatal polysubstance exposure, which exacerbates offspring mortality and induces sex-specific neurobehavioral deficits. This study highlights the distinct outcomes associated with prenatal co-exposure, and the need for future research to investigate underlying mechanisms driving these developmental disruptions and sex-specific susceptibilities.

Effects of aging on otolith morphology and functions in mice

Background

Increased fall risk caused by vestibular system impairment is a significant problem associated with aging. A vestibule is composed of linear acceleration-sensing otoliths and rotation-sensing semicircular canals. Otoliths, composed of utricle and saccule, detect linear accelerations. Otolithic organs partially play a role in falls due to aging. Aging possibly changes the morphology and functions of otoliths. However, the specific associations between aging and otolith changes remain unknown. Therefore, this study aimed to clarify these associations in mice.

Methods

Young C56BL/6 N (8 week old) and old (108-117 weeks old) mice were used in a micro-computed tomography (μCT) experiment for morphological analysis and a linear acceleration experiment for functional analysis. Young C56BL/6 N (8 week old) and middle-aged (50 week old) mice were used in electron microscopy experiments for morphological analysis.

Results

μCT revealed no significant differences in the otolith volume (p = 0.11) but significant differences in the otolith density (p = 0.001) between young and old mice. μCT and electron microscopy revealed significant differences in the structure of striola at the center of the otolith (μCT; p = 0.029, electron microscopy; p = 0.017). Significant differences were also observed in the amplitude of the eye movement during the vestibulo-ocular reflex induced by linear acceleration (maximum amplitude of stimulation = 1.3G [p = 0.014]; maximum amplitude of stimulation = 0.7G [p = 0.015]), indicating that the otolith function was worse in old mice than in young mice.

Discussion

This study demonstrated the decline in otolith function with age caused by age-related morphological changes. Specifically, when otolith density decreased, inertial force acting on the hair cells decreased, and when the structure of striola collapsed, the function of cross-striolar inhibition decreased, thereby causing a decline in the overall otolith function.

Environmental and microbial factors influence affective and cognitive behavior in C57BL/6 sub-strains

C57BL/6 mice are one of the most widely used inbred strains in biomedical research. Early separation of the breeding colony has led to the development of several sub-strains. Colony separation led to genetic variation development driving numerous phenotypic discrepancies. The reported phenotypic behavior differences between the sub-strains were, however; not consistent in the literature, suggesting the involvement of factors other than host genes. Here, we characterized the cognitive and affective behavior of C57BL/6J and C57BL/6N mice in correlation with the immune cell profile in the brain. Furthermore, faecal microbiota transfer and mice co-housing techniques were used to dissect microbial and environmental factors' contribution, respectively, to cognitive and affective behavior patterns. We first noted a unique profile of locomotor activity, immobility pattern, and spatial and non-spatial learning and memory abilities between the two sub-strains. The phenotypic behavior profile was associated with a distinct difference in the dynamics of type 2 cytokines in the meninges and brain parenchyma. Analysing the contribution of microbiome and environmental factors to the noted behavioral profile, our data indicated that while immobility pattern was genetically driven, locomotor activity and cognitive abilities were highly sensitive to alterations in the gut microbiome and environmental factors. Changes in the phenotypic behavior in response to these factors were associated with changes in immune cell profile. While microglia were highly sensitive to alteration in gut microbiome, immune cells in meninges were more resilient. Collectively, our findings demonstrated a direct impact of environmental conditions on gut microbiota which subsequently impacts the brain immune cell profile that could modulate cognitive and affective behavior. Our data further highlight the importance of characterizing the laboratory available strain/sub-strain to select the most appropriate one that fits best the study purpose.

Metabolic differences and differentially expressed genes between C57BL/6J and C57BL/6N mice substrains

C57BL/6J (B6J) and C57BL/6N (B6N) mice are the most frequently used substrains in C57BL/6 (B6) inbred mice, serving as physiological models for in vivo studies and as background strains to build transgenic mice. However, the differences in metabolic phenotypes between B6J and B6N mice are not coherent, and genotypic differences in metabolically important tissues have not been well studied. The phenotypic differences between B6J and B6N substrains have often been attributed to the role of the nicotinamide nucleotide transhydrogenase (Nnt) gene, whereby B6J has a spontaneous missense mutation of Nnt. Nevertheless, phenotypic differences between the two cannot be explained by Nnt mutations alone, especially in metabolic traits. Therefore, we aimed to investigate the genetic cause of the phenotypic differences between B6J and B6N mice. Determining consistent genetic differences across multiple tissues involved in metabolic traits such as subcutaneous and visceral white adipose tissues, brown adipose tissue, skeletal muscle, liver, hypothalamus, and hippocampus, may help explain phenotypic differences in metabolism between the two substrains. We report candidate genes along with comparative data on body weight, tissue weight, blood components involved in metabolism, and energy balance of B6J and B6N mice. Insulin degrading enzyme, adenylosuccinate synthase 2, and ectonucleotide triphosphate diphosphohydrolase 4 were highly expressed in B6J mice compared with those in B6N mice, and Nnt, WD repeat and FYVE domain containing 1, and dynein light chain Tctex-type 1 were less expressed in B6J mice compared with those in B6N mice in all seven tissues. Considering the extremely wide use of both substrains and their critical importance in generating transgenic and knock-out models, these findings guide future research across several interrelated fields.

Distinct Age-Specific Effects on Olfactory Associative Learning in C57BL/6 Substrains

C57BL/6 is the most widely used mouse strain in the laboratories. Two substrains of C57BL/6, C57BL/6J (B6J), and C57BL/6N (B6N) are well-known backgrounds for genetic modification and have been shown difference in quite a few tests, including open field test, rotarod test, and Morris water maze. However, difference between these two substrains in olfaction-dependent behaviors remains unknown. Here, we used olfactory two-alternative choice task, which is modified to have two training stages, to evaluate animals’ ability in instrumental learning and olfactory association. In the first (rule learning) stage, the mice were trained to use the operant chamber to collect water rewards. An odor cue was provided in the procedure, with no indication about reward locations. In the following (discrimination learning) stage, two odor cues were provided, with each indicating a specific water port. The animals were rewarded upon correct port choices following cue deliveries. We found that during young adulthood (7–10 weeks old), proportionally more B6J than B6N mice were able to pass rule learning (58.3% vs. 29.2%) and ultimately acquire this task (54.2% vs. 25%), with the two substrains showing similar pass rates in discrimination learning (92.9% vs. 85.7%). Surprisingly, at a more mature age (17 weeks old), this substrain difference disappeared. Mature B6N mice had a significant improvement in pass percentages of rule learning and overall task, whereas similar improvement was not observed in the B6J counterparts. Instead, mature B6J mice had an improved speed in rule learning and overall task. We further examined behavioral patterns of 8-week-old B6J and B6N mice in the olfactory habituation or dishabituation test. We observed normal olfactory habituation from subjects of both substrains, with the B6J mice exhibiting stronger investigative responses to newly presented odorants. These results reveal for the first time that B6J and B6N mice are different in acquisition processes of a behavioral task that requires instrumental learning and olfactory association, and that maturation appears to employ different effects on these two substrains during these processes. Furthermore, young adult B6J and B6N mice might be similar in olfactory habituation but different in the olfactory aspects of novelty seeking.

Supplier-origin mouse microbiomes significantly influence locomotor and anxiety-related behavior, body morphology, and metabolism

The mouse is the most commonly used model species in biomedical research. Just as human physical and mental health are influenced by the commensal gut bacteria, mouse models of disease are influenced by the fecal microbiome (FM). The source of mice represents one of the strongest influences on the FM and can influence the phenotype of disease models. The FM influences behavior in mice leading to the hypothesis that mice of the same genetic background from different vendors, will have different behavioral phenotypes. To test this hypothesis, colonies of CD-1 mice, rederived via embryo transfer into surrogate dams from four different suppliers, were subjected to phenotyping assays assessing behavior and physiological parameters. Significant differences in behavior, growth rate, metabolism, and hematological parameters were observed. Collectively, these findings show the profound influence of supplier-origin FMs on host behavior and physiology in healthy, genetically similar, wild-type mice maintained in identical environments.

Consideration of Gut Microbiome in Murine Models of Diseases

The gut microbiome (GM), a complex community of bacteria, viruses, protozoa, and fungi located in the gut of humans and animals, plays significant roles in host health and disease. Animal models are widely used to investigate human diseases in biomedical research and the GM within animal models can change due to the impact of many factors, such as the vendor, husbandry, and environment. Notably, variations in GM can contribute to differences in disease model phenotypes, which can result in poor reproducibility in biomedical research. Variation in the gut microbiome can also impact the translatability of animal models. For example, standard lab mice have different pathogen exposure experiences when compared to wild or pet store mice. As humans have antigen experiences that are more similar to the latter, the use of lab mice with more simplified microbiomes may not yield optimally translatable data. Additionally, the literature describes many methods to manipulate the GM and differences between these methods can also result in differing interpretations of outcomes measures. In this review, we focus on the GM as a potential contributor to the poor reproducibility and translatability of mouse models of disease. First, we summarize the important role of GM in host disease and health through different gut–organ axes and the close association between GM and disease susceptibility through colonization resistance, immune response, and metabolic pathways. Then, we focus on the variation in the microbiome in mouse models of disease and address how this variation can potentially impact disease phenotypes and subsequently influence research reproducibility and translatability. We also discuss the variations between genetic substrains as potential factors that cause poor reproducibility via their effects on the microbiome. In addition, we discuss the utility of complex microbiomes in prospective studies and how manipulation of the GM through differing transfer methods can impact model phenotypes. Lastly, we emphasize the need to explore appropriate methods of GM characterization and manipulation.

Comprehensive characterization of motor and coordination functions in three adolescent wild-type mouse strains

Neuropsychiatric disorders are often associated with motor and coordination abnormalities that have important implications on the etiology, pathophysiology, and management of these disorders. Although the onset of many neuropsychiatric disorders including autism spectrum disorder, schizophrenia, and attention-deficit hyperactivity disorder emerges mainly during infancy and adolescence, most of the behavioral studies in mice modeling neuropsychiatric phenotypes are performed in adult animals, possibly missing valuable phenotypic information related to the effect of synaptic maturation during development. Here, we examined which behavioral tests assessing both motor and coordination functions can be performed in mice at two different adolescent stages. As strain and sex affect mouse behavior, our experiments covered both male and female mice of three inbred wild-type strains, C57BL/6N, DBA/2, and FVB/N. Adolescent mice of both postnatal days (P)22-30 and P32-40 developmental stages were capable of mastering common motor and coordination tests. However, results differed significantly between strains and sexes. Moreover, the 10-day interval between the two tested cohorts uncovered a strong difference in the behavioral results, confirming the significant impact of maturation on behavioral patterns. Interestingly, the results of distinct behavioral experiments were directly correlated with the weight of mice, which may explain the lack of reproducibility of some behavioral results in genetically-modified mice. Our study paves the way for better reproducibility of behavioral tests by addressing the effect of the developmental stage, strain, sex, and weight of mice on achieving the face validity of neuropsychiatric disorder-associated motor dysfunctions.

Mitochondrial respiration in C57BL/6 substrains varies in response to myocardial infarction

The C57BL/6 mouse strain have been commonly used for the genetic background animal models and experimental research. There are several major sources of C57BL/6 substrains for the biomedical research community which display genetic and phenotypic differences. Previous studies have suggested that the varies in baseline of cardiovascular phenotypes as well as in response to pressure overload by transverse aortic constriction (TAC). To investigate whether there exist substrain specific differences in response to heart failure post myocardial infarction (MI), consequently the impaired mitochondrial respiration, we performed MI surgery on two commonly used C57BL/6 substrains: C57BL/6J (BL/6J) and C57BL/6NCrl (BL/6N) mice. Subsequently, measurements about cardiac function, histology and mitochondrial respiration capacities were conducted to evaluate the differences. The data showed that C57BL/6J(BL/6J) mice is more resistant to the attack of MI, evidenced by lower mortality, less infarct size and better preserved cardiac function after MI, especially exhibited better mitochondrial respiration capacities, compared with the C57BL/6NCrl(BL/6N) mice.

Substrains matter in phenotyping of C57BL/6 mice

The inbred mouse strain C57BL/6 has been widely used as a background strain for spontaneous and induced mutations. Developed in the 1930s, the C57BL/6 strain diverged into two major groups in the 1950s, namely, C57BL/6J and C57BL/6N, and more than 20 substrains have been established from them worldwide. We previously reported genetic differences among C57BL/6 substrains in 2009 and 2015. Since then, dozens of reports have been published on phenotypic differences in behavioral, neurological, cardiovascular, and metabolic traits. Substrains need to be chosen according to the purpose of the study because phenotypic differences might affect the experimental results. In this paper, we review recent reports of phenotypic and genetic differences among C57BL/6 substrains, focus our attention on the proper use of C57BL/6 and other inbred strains in the era of genome editing, and provide the life science research community wider knowledge about this subject.

Behavioral tests assessing neuropsychiatric phenotypes in adolescent mice reveal strain- and sex-specific effects

In humans, infancy and adolescence are associated with major changes in synaptic functions and ongoing maturation of neural networks, which underlie the major behavioral changes during these periods. Among adult cases with neuropsychiatric disorders including autism spectrum disorder, schizophrenia, attention deficit hyperactivity, and bipolar disorders, 50% have developed behavioral symptoms and received a diagnosis before 15 years of age. However, most of the behavioral studies in mice modeling neuropsychiatric phenotypes are performed in adult animals, missing valuable phenotypic information related to the effect of synaptic maturation during development. Here, we explored which behavioral experiments assessing neuropsychiatric phenotypes can be performed during a specific window of development in adolescent male and female C57BL/6N, DBA/2, and FVB/N mice that are typically used as background strains for generating genetically-modified mouse models. The three wild-type strains were evaluated across anxiety, social behaviors, and cognitive functions in order to cover the main behavioral impairments that occur in neuropsychiatric disorders. During adolescence, the three strains displayed significant differences under certain behavioral paradigms. In addition, C57BL/6N and FVB/N, but not DBA/2 mice revealed some sex-related differences. Our results provide new insights into discrete behaviors during development and emphasize the crucial importance of the genetic background, sex, and experimental settings in the age-dependent regulation of different behaviors.

C57BL/6J and C57BL/6N substrains differentially influence phenotype severity in the Scn1a +/- mouse model of Dravet syndrome

Many disease-relevant phenotypes modeled in inbred mice have been shown to be strain-dependent, indicating the important influence of genetic background on disease phenotypes. Although C57BL/6 mice are one of the most commonly used inbred strains in laboratory research, there are multiple substrains (eg, B6J vs B6N) that have been separated for more than 50 years. Thus, understanding the substrain differences is important for scientific rigor and reproducibility. In this study, seizure susceptibility, spontaneous seizures, and survival were compared between Scn1a +/- mice on (C57BL/6J × 129S6/SvEvTac)F1 (F1J) vs (C57BL/6N × 129S6/SvEvTac)F1 (F1N) strain backgrounds. F1N.Scn1a +/- mice were more susceptible to hyperthermia-induced seizures, yet had milder spontaneous seizures and improved survival relative to F1J.Scn1a +/- mice. Our results indicate that choice of C57BL/6 substrain may significantly alter disease phenotypes and should be considered carefully in experimental design using the Scn1a +/- Dravet mouse model, as well as other mouse models of epilepsy.

Analysis of Motor Function in Amyloid Precursor-Like Protein 2 Knockout Mice: The Effects of Ageing and Sex

The amyloid precursor protein (APP) is a member of a conserved gene family that includes the amyloid precursor-like proteins 1 (APLP1) and 2 (APLP2). APP and APLP2 share a high degree of similarity, and have overlapping patterns of spatial and temporal expression in the central and peripheral tissues, in particular at the neuromuscular junction. APP-family knockout (KO) studies have helped elucidate aspects of function and functional redundancy amongst the APP-family members. In the present study, we investigated motor performance of APLP2-KO mice and the effect sex differences and age-related changes have on motor performance. APLP2-KO and WT (on C57Bl6 background) littermates control mice from 8 (young adulthood) to 48 weeks (middle age) were investigated. Analysis of motor neuron and muscle morphology showed APLP2-KO females but not males, had less age-related motor function impairments. We observed age and sex differences in both motor neuron number and muscle fiber size distribution for APLP2-KO mice compared to WT (C57Bl6). These alterations in the motor neuron number and muscle fiber distribution pattern may explain why female APLP2-KO mice have far better motor function behaviour during ageing.

Protein-restricted diet during pregnancy after insemination alters behavioral phenotypes of the progeny

Background

Epidemiological studies suggest that hyponutrition during the fetal period increases the risk of mental disorders such as attention deficit hyperactivity disorder and autism-spectrum disorder, which has been experimentally supported using animal models. However, previous experimental hyponutrition or protein-restricted (PR) diets affected stages other than the fetal stage, such as formation of the egg before insemination, milk composition during lactation, and maternal nursing behavior.

Results

We conducted in vitro fertilization and embryo transfer in mice and allowed PR diet and folic acid-supplemented PR diet to affect only fetal environments. Comprehensive phenotyping of PR and control-diet progenies showed moderate differences in fear/anxiety-like, novelty-seeking, and prosocial behaviors, irrespective of folic-acid supplementation. Changes were also detected in gene expression and genomic methylation in the brain.

Conclusions

These results suggest that epigenetic factors in the embryo/fetus influence behavioral and epigenetic phenotypes of progenies. Significant epigenetic alterations in the brains of the progenies induced by the maternal-protein restriction were observed in the present study. To our knowledge, this is first study to evaluate the effect of maternal hyponutrition on behavioral phenotypes using reproductive technology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12263-016-0550-2) contains supplementary material, which is available to authorized users.