Selection for age of sexual maturation in mice and the sequences for population regulation

Early Life Interventions: Impact on Aging and Longevity

Across mammals, lifespans vary remarkably, spanning over a hundredfold difference. Comparative studies consistently reveal a strong inverse relationship between developmental pace and lifespan, hinting at the potential for early-life interventions (ELIs) to influence aging and lifespan trajectories. Focusing on postnatal interventions in mice, this review explores how ELIs influence development, lifespan, and the underlying mechanisms. Previous ELI studies have employed a diverse array of approaches, including dietary modifications, manipulations of the somatotropic axis, and various chemical treatments. Notably, these interventions have demonstrated significant impacts on aging and lifespan in mice. The underlying mechanisms likely involve pathways related to mitochondrial function, mTOR and AMPK signaling, cellular senescence, and epigenetic alterations. Interestingly, ELI studies may serve as valuable models for investigating the complex regulatory mechanisms of development and aging, particularly regarding the interplay among somatic growth, sexual maturation, and lifespan. In addition, prior research has highlighted the intricacies of experimental design and data interpretation. Factors such as timing, sex-specific effects, administration methods, and animal husbandry practices must be carefully considered to ensure the reliability and reproducibility of results, as well as rigorous interpretation. Addressing these factors is essential for advancing our understanding of how development, aging, and lifespan are regulated, potentially opening avenues for interventions that promote healthy aging.

The ameliorative effects of a phenolic derivative of Moringa oleifera leave against vanadium-induced neurotoxicity in mice

Vanadium, a transition series metal released during some industrial activities, induces oxidative stress and lipid peroxidation. Ameliorative effect of a pure compound from the methanolic extract of Moringa oleifera leaves, code-named MIMO2, in 14-day old mice administered with vanadium (as sodium metavanadate 3 mg/kg) for 2 weeks was assessed. Results from body weight monitoring, muscular strength, and open field showed slight reduction in body weight and locomotion deficit in vanadium-exposed mice, ameliorated with MIMO2 co-administration. Degeneration of the Purkinje cell layer and neuronal death in the hippocampal CA1 region were observed in vanadium-exposed mice and both appeared significantly reduced with MIMO2 co-administration. Demyelination involving the midline of the corpus callosum, somatosensory and retrosplenial cortices was also reduced with MIMO2. Microglia activation and astrogliosis observed through immunohistochemistry were also alleviated. Immunohistochemistry for myelin, axons and oligodendrocyte lineage cells were also carried out and showed that in vanadium-treated mice brains, oligodendrocyte progenitor cells increased NG2 immunolabelling with hypertrophy and bushy, ramified appearance of their processes. MIMO2 displayed ameliorative and antioxidative effects in vanadium-induced neurotoxicity in experimental murine species. This is likely the first time MIMO2 is being used in vivo in an animal model.

Long-Term Stress and Concomitant Marijuana Smoke Exposure Affect Physiology, Behavior and Adult Hippocampal Neurogenesis

Marijuana is a widely used recreational drug with increasing legalization worldwide for medical purposes. Most experimental studies use either synthetic or plant-derived cannabinoids to investigate the effect of cannabinoids on anxiety and cognitive functions. The aim of this study was to mimic real life situations where young people smoke cannabis regularly to relax from everyday stress. Therefore, we exposed young adult male NMRI mice to daily stress and concomitant marijuana smoke for 2 months and investigated the consequences on physiology, behavior and adult hippocampal neurogenesis. Animals were restrained for 6-h/day for 5-days a week. During the stress, mice were exposed to cannabis smoke for 2 × 30 min/day. We burned 2 “joints” (2 × 0.8 g marijuana) per occasion in a whole body smoking chamber. Cannabinoid content of the smoke and urine samples was measured by HPLC and SFC-MS/MS. Body weight gain was recorded daily and we did unrestrained, whole body plethysmography to investigate pulmonary functions. The cognitive performance of the animals was evaluated by the novel object recognition and Y maze tests. Anxietyrelated spontaneous locomotor activity and self-grooming were assessed in the open field test (OFT). Adult neurogenesis was quantified post mortem in the hippocampal dentate gyrus. The proliferative activity of the precursor cells was detected by the use of the exogenous marker 5-bromo-20-deoxyuridine. Treatment effects on maturing neurons were studied by the examination of doublecortin-positive neurons. Both stress and cannabis exposure significantly reduced body weight gain. Cannabis smoke had no effect on pulmonary functions, but stress delayed the maturation of several lung functions. Neither stress, nor cannabis smoke affected the cognitive functioning of the animals. Results of the OFT revealed that cannabis had a mild anxiolytic effect and markedly increased self-grooming behavior. Stress blocked cell proliferation in the dentate gyrus, but cannabis had no effect on this parameter. Marijuana smoke however had a pronounced impact on doublecortin-positive neurons influencing their number, morphology and migration. In summary, we report here that long-term stress in combination with cannabis smoke exposure can alter several health-related measures, but the present experimental design could not reveal any interaction between these two treatment factors except for body weight gain.

Cellular composition characterizing postnatal development and maturation of the mouse brain and spinal cord

The process of development, maturation, and regression in the central nervous system (CNS) are genetically programmed and influenced by environment. Hitherto, most research efforts have focused on either the early development of the CNS or the late changes associated with aging, whereas an important period corresponding to adolescence has been overlooked. In this study, we searched for age-dependent changes in the number of cells that compose the CNS (divided into isocortex, hippocampus, olfactory bulb, cerebellum, 'rest of the brain', and spinal cord) and the pituitary gland in 4-40-week-old C57BL6 mice, using the isotropic fractionator method in combination with neuronal nuclear protein as a marker for neuronal cells. We found that all CNS structures, except for the isocortex, increased in mass in the period of 4-15 weeks. Over the same period, the absolute number of neurons significantly increased in the olfactory bulb and cerebellum while non-neuronal cell numbers increased in the 'rest of the brain' and isocortex. Along with the gain in body length and weight, the pituitary gland also increased in mass and cell number, the latter correlating well with changes of the brain and spinal cord mass. The majority of the age-dependent alterations (e.g., somatic parameters, relative brain mass, number of pituitary cells, and cellular composition of the cerebellum, isocortex, rest of the brain, and spinal cord) occur rapidly between the 4th and 11th postnatal weeks. This period includes murine adolescence, underscoring the significance of this stage in the postnatal development of the mouse CNS.

Effect of boar exposure on expression of genetic potential for age of puberty in gilts

Genotype × environment interaction for age at puberty was evaluated using gilts from the Nebraska Gene Pool population derived from lines selected randomly (RS; no. = 48) or for seven generations for decreased age at puberty (AP; no. = 73). Age at puberty was evaluated in two environments: boar exposure for 15 min daily (BE) or no boar exposure (NBE). Pigs were randomly assigned to treatment and mixed into groups of 20 in pasture lots. Oestrous detection was initiated when the oldest gilt in a pen was 125 days of age. Blood samples were taken weekly and assayed for progesterone. Gilts were considered to have expressed oestrus if they exhibited lordosis or had consecutive weekly blood samples with progesterone values above 6·4 × 109 mol/1 (2 μ, g/l). Gilts were removed from the pen upon confirmation of puberty or at 250 days of age. Two gilts failed to reach puberty by 250 days so this value was assigned as their age at puberty. Least-squares analyses of variance were used to analyse the data. A model including line, sire within line, farrowing group, treatment and line × treatment interaction was fitted to the dependent variables age at puberty and percentage cycling by 185 days. Line and treatment each affected age at puberty (P < 0·05) but did not interact. Least-square mean ages at puberty were 154 (s.e. 4·5), 164 (s.e. 4·7), 164 (s.e. 6·1) and 179 (s.e. 5·9) days for AP-BE, AP-NBE, RS-BE and RS-NBE, respectively. Treatment affected percentage of gilts cycling by 185 days (P < 0·05). Least-square percentages were 91 (s.e. 6·9), 76 (s.e. 71), 84 (s.e. 9·2) and 65 (s.e. 8·9) % for AP-BE, AP-NBE, RS-BE and RS-NBE, respectively. These results indicate that expression of response to selection for decreased age at puberty in the gilt is not dependent on stimuli from the boar.

Are responses to artificial selection for reproductive fitness characters consistently asymmetrical?

Non-linear offspring-parent regressions and heritabilities are expected for characters showing genetic asymmetry due to directional dominance and/or asymmetrical gene frequencies. Since reproductive fitness characters exhibit these characteristics, they should show consistently nonlinear heritabilities, with greater heritabilities in the direction of lower fitness. As a consequence, responses to bi-directional selection on fitness traits should be asymmetrical in the same direction. This prediction has been tested by an analysis of published bi-directional selection experiments for reproductive fitness traits. Significant asymmetry (24 of 30 studies) in the predicted direction was found. For studies reporting realized heritabilities, the means were 0·173 and 0·259 for lines selected for higher and lower reproductive fitness, respectively, the high lines being 33% less than the low lines. Asymmetry was evident for studies reporting realized heritabilities and for those with random mating controls of the same size as the selection lines. Consequently, it is argued that the asymmetry results from genetic asymmetries. This asymmetry has important implications in the improvement of reproductive fitness traits in plant and animal breeding.

Life history and bioeconomy of the house mouse

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.

Behavioral selection of odor cues by young female mice affects age of puberty

Female house mice were reared from weaning at 21 days of age until first vaginal estrus in 40 1 aquaria in which they were given a choice of exposing themselves to bedding placed on opposite halves of the aquarium floor and sprayed with water or urine containing puberty-influencing chemosignals. In Experimental 1, mice housed with only male urine cues sprayed on the bedding matured significantly earlier and mice housed with only grouped female urine sprayed on the bedding matured significantly later than control mice where water was sprayed on the bedding for both halves of the aquarium. In Experiment 2, there were no significant differences in mean ages at vaginal introitus or first estrus for females reared with choices between (a) bedding sprayed with male urine versus bedding sprayed with water, (b) bedding sprayed with urine from grouped females versus bedding sprayed with water, (c) bedding sprayed with male urine versus bedding sprayed with urine from grouped females, or (d) the control condition where both sides of the aquarium contained bedding sprayed with water. Analysis of continuous video tapes of the locations of the females for Experiment 2 revealed that females chose initially to spend more time on the half of the floor with bedding that delayed puberty relative to the other side, but shifted their preference toward a more puberty-enhancing signal at about the time of first estrus. Female house mice appear to be able to exert some behavioral control over their own sexual maturation.

Environmental factors and age of puberty in female house mice

Genetics, urinary chemosignals and related social influences, and ambient conditions affect reproduction in female mice. Five experiments tested the effects of environmental stressors on age at first vaginal estrus in female house mice. Environmental disruption in the form of changing the cage bedding and/or nesting material at various prescribed intervals resulted in different degrees of puberty delay relative to non-disrupted control mice. Disruption in the form of a male chasing the female for one to three 15-min intervals each day or the female being trapped and held in a live-trap for one to three 15-min intervals each day resulted in delays in puberty for treatments involving multiple daily disruptions. Food deprivation, but not water deprivation, influenced the onset of puberty. Variations in temperature and humidity resulted in differences in the age of puberty; low but not high temperatures and extremely low humidity levels delayed sexual maturation.

Social mediation of puberty: an adaptive female strategy?

Females of two hamster species with contrasting degrees of gregariousness were tested for social influences on the timing of sexual maturation. When female dwarf hamsters (Phodopus sungorus campbelli), a gregarious species, were housed with an adult male at weaning, they began estrous cycles significantly earlier than when they were housed alone or with their family. Females housed with one or two female siblings at weaning matured significantly later than did all other females. Once mature, females housed with familiar males conceived later than did females housed with unfamiliar males. Rearing conditions also affected body weight; those housed with female siblings grew more slowly and attained a lower weight at 8 weeks of age than did females in all other groups. In contrast, when weanling female golden hamsters (Mesocricetus auratus), a solitary species, were housed with either an adult male, two female siblings, or alone, they did not differ in their rate of sexual maturation or growth. It is suggested that the degree to which social factors can accelerate or delay puberty is an adaptive characteristic and may be correlated with the degree of gregariousness of a species.

Genetic selection for reproductive photoresponsiveness in deer mice

Seasonal breeding is common in mammals, particularly in habitats outside the tropics. Climate and availability of food are the ultimate factors that usually dictate the optimal time of year for a mammal to breed; however, day length (photoperiod) often serves as the proximal cue to signal the onset or cessation of seasonal reproduction. Some individuals in some populations of deer mice are reproductively responsive to photoperiod, while other individuals in the same population are not. As shown here, selection can dramatically alter the frequency of photoresponsiveness in a laboratory population in only two generations. To our knowledge this is the first demonstration of selection for reproductive photoresponsiveness in any mammal. By implication, some wild populations of deer mice must use multiple, genetic-based reproductive strategies, and the degree to which each such strategy is exhibited must be subject to rapid change in response to both seasonally and momentarily changing climatic and dietary conditions.

Postnatal effects on reproduction and maternal care in early- and late-maturing gerbils

In previous papers, we have identified a number of differences in both the reproductive profiles and patterns of maternal care shown by female gerbils exhibiting vaginal opening either before or after weaning. Early-maturing females delivered their first litters when younger, produced a greater percentage of female offspring/litter, and a greater percentage of early-maturing daughters than late-maturing females. Further, early-maturing females exhibited reduced levels of maternal care, as assessed by time spent nursing, time pups were gathered in the nest, and probability of retrieval of displaced young. In the present experiments, we found that equating age at first parity of early- and late-maturing females (Experiment 1) eliminated differences between them in pup gathering, but not in pup retrieval or nursing. Further, equating age at first parity affected neither sex-ratios of litters at birth nor rate of maturation of daughters. Equating litter size at birth, sex-ratio of litters at birth, and maternal age at first parturition (Experiment 2) eliminated differences in the nursing behavior of early- and late-maturing females and reduced differences in retrieval behavior, but left rate of maturation of daughters unaffected. Last (Experiment 3), both daughters born to early-maturing females, reared by late-maturing females, and daughters born to late-maturing females, reared by early-maturing females, exhibited rates of maturation typical of daughters reared by their natural mothers.

Measures of growth, development, and sexual maturation in mongolian gerbils (Meriones unguiculatus): effects of photic period during ontogeny

We examined, in Mongolian gerbils, the adequacy of two commonly used indices of rate of development, age at eye-opening and age at vaginal introitus, as predictors of biologically important events: initiation of feeding on solid food and age at first parturition. We then enquired as to the interrelationships among body weight, eye-opening, and vaginal introitus in Mongolian gerbils whose rates of development were manipulated by altering the extent of their exposure to illumination during ontogeny. We found that (1) age at vaginal introitus was significantly, but weakly, correlated with age at first parturition (Experiment I); (2) age at eye-opening strongly predicted age of initiation of gnawing on solid food (Experiment II); (3) eye-opening occurred at markedly different ages, but at the same body weight in gerbils reared under 0/24-, 6/18-, 12/12-, and 24/0-hr day-night cycles (Experiment III); and (4) vaginal introitus occurred at both different ages and different body weights in females reared under different day-night cycles (Experiment III).

Weight and age at puberty in female and male mice of strains selected for large and small body size

Puberty was studied in mice of the ninth selected generation of the Q-strain. There were 6 replicate lines selected for large body size (6-week weight), 6 replicates selected for small size and 6 replicate unselected controls. Female puberty was assessed by the opening of the vagina and male puberty by the first copulation plug. The sexes differed in the mean age at puberty, males being older by 13 days in the large, 4 days in the control and 8 days in the small lines. The sexes differed also in the way size affected puberty. In males the large and small lines reached puberty at the same age and both were older than the controls. In females the large lines on average were heavier and younger at puberty than the controls, and the small lines were lighter and older than the controls, though not significantly older. The replicates within each size-group, however, reached puberty at about the same weight, irrespective of their differences in growth rate. Thus, the differences of growth between the large, control and small groups affected both the weight and the age of females at puberty, but the differences of growth between the replicate lines within each size affected only the age at puberty. No explanation was found for this inconsistency between size-groups and replicates. Several lines of evidence led to the conclusion that in females puberty is partly or mainly weight-dependent, whereas in males it is almost wholly age-dependent.