Spatial exploration in transgenic mice expressing human beta-S100

Environmental Enrichment as a Positive Behavioral Intervention Across the Lifespan

BACKGROUND

In recent decades, the interest in behavioral interventions has been growing due to the higher prevalence of age-related cognitive impairments. Hence, behavioral interventions, such as cognitive stimulation and physical activity, and along with these, our lifestyle (education level, work position, frequency of cognitive and social activities) have shown important benefits during the cognitive impairment, dementia and even recovery after brain injury. This is due to the fact that this type of intervention and activities promote the formation of a cognitive and brain reserve that allows tolerating brain damage during a long period of time without the appearance of cognitive symptoms. With regard to this, animal models have proved very useful in providing information about the brain mechanisms involved in the development of these cognitive and brain reserves and how they interact with each other.

METHODS

We summarize several studies showing the positive effects of Environmental Enrichment (EE), understood as a housing condition in which animals benefit from the sensory, physical, cognitive and social stimulation provided, on brain and cognitive functions usually impaired during aging.

RESULTS

Most of studies have shown that EE is a successful protocol to improve cognitive functions and reduce anxiety-related behaviors across the lifespan, as well as in animal models of neurodegenerative diseases.

CONCLUSION

Therefore, EE is a laboratory condition in which some aspects of an active lifestyle are reproduced.

Normal cognition in transgenic BRI2-Aβ mice

BACKGROUND

Recent research in Alzheimer's disease (AD) field has been focused on the potential role of the amyloid-β protein that is derived from the transmembrane amyloid precursor protein (APP) in directly mediating cognitive impairment in AD. Transgenic mouse models overexpressing APP develop robust AD-like amyloid pathology in the brain and show various levels of cognitive decline. In the present study, we examined the cognition of the BRI2-Aβ transgenic mouse model in which secreted extracellular Aβ1-40, Aβ1-42 or both Aβ1-40/Aβ1-42 peptides are generated from the BRI-Aβ fusion proteins encoded by the transgenes. BRI2-Aβ mice produce high levels of Aβ peptides and BRI2-Aβ1-42 mice develop amyloid pathology that is similar to the pathology observed in mutant human APP transgenic models.

RESULTS

Using established behavioral tests that reveal deficits in APP transgenic models, BRI2-Aβ1-42 mice showed completely intact cognitive performance at ages both pre and post amyloid plaque formation. BRI2-Aβ mice producing Aβ1-40 or both peptides were also cognitively intact.

CONCLUSIONS

These data indicate that high levels of Aβ1-40 or Aβ1-42, or both produced in the absence of APP overexpression do not reproduce memory deficits observed in APP transgenic mouse models. This outcome is supportive of recent data suggesting that APP processing derivatives or the overexpression of full length APP may contribute to cognitive decline in APP transgenic mouse models. Alternatively, Aβ aggregates may impact cognition by a mechanism that is not fully recapitulated in these BRI2-Aβ mouse models.

Novel object exploration in mice: not all objects are created equal

Object exploration is an increasingly popular experimental paradigm in behavioral sciences. We have begun a series of studies with mice specifically looking at the parameters that influence behaviors in this test. The aim of the present study was to examine the effect of object type on performance in the object exploration test. More specifically, adult male C57BL/6J mice were trained and tested using objects that could be climbed (CLIMB) or with those that could only be touched (TOUCH). The results show that activity is affected by the presentation of objects, with object type interacting with some of these changes. C57 mice explored objects that can be climbed over significantly longer than objects that can only be touched and a more rapid habituation was observed using objects that could only be touched. Robust recognition memory was observed in both groups of mice, however mice in the CLIMB group exhibited a significantly greater discrimination index compared to mice in the TOUCH group. Taken together, these findings demonstrate that the selection of objects is of critical importance and it is recommended that special attention be given to the functional (affordant) properties of the objects to-be-used in future studies.

Glia: the many ways to modulate synaptic plasticity

Synaptic plasticity consists in a change in synaptic strength that is believed to be the basis of learning and memory. Synaptic plasticity has been for a very long period of time a hallmark of neurons. Recent advances in physiology of glial cells indicate that astrocyte and microglia possess all the features to participate and modulate the various form of synaptic plasticity. Indeed beside their respective supportive and immune functions an increasing number of study demonstrate that astrocytes and microglia express receptors for most neurotransmitters and release neuroactive substances that have been shown to modulate neuronal activity and synaptic plasticity. Because glial cells are all around synapses and release a wide variety of neuroactive molecule during physiological and pathological conditions, glial cells have been reported to modulate synaptic plasticity in many different ways. From change in synaptic coverage, to release of chemokines and cytokines up to dedicated "glio" transmitters release, glia were reported to affect synaptic scaling, homeostatic plasticity, metaplasticity, long-term potentiation and long-term depression.

Mouse Models of Cognitive Disorders in Trisomy 21: A Review

Trisomy 21 (TRS21) is the most frequent genetic cause of mental retardation. Although the presence of an extra copy of HSA21 is known to be at the origin of the syndrome, we do not know which 225 HSA21 genes have an effect on cognitive processes. Mouse models of TRS21 have been developed using syntenies between HSA21 and MMU16, MMU10 and MMU17. Available mouse models carry extra fragments of MMU16 or of HSA21 that cover all of HSA21 (chimeric HSA21) or MMU16 (Ts16); some carry large parts of MMU16 (Ts65Dn, Ts1Cje, Ms1Cje), while others have reduced contiguous fragments covering the D21S17-ETS2 region or single transfected genes. This offers a nest design strategy for deciphering cognitive (learning, memory and exploration) and associated brain abnormalities involving each of these chromosomal regions. This review confirms the crucial but not exclusive contribution of the D21S17-ETS2 region encompassing 16 genes to cognitive disorders.

Temporal stability of novelty exploration in mice exposed to different open field tests

We investigated behavioural activity and temporal distribution (patterning) of mouse exploration in different open field (OF) arenas. Mice of 129S1 (S1) strain were subjected in parallel to three different OF arenas (Experiment 1), two different OF arenas in two trials (Experiment 2) or two trials of the same OF test (Experiment 3). Overall, mice demonstrated a high degree of similarity in the temporal profile of novelty-induced horizontal and vertical exploration (regardless of the size, colour and shape of the OF), which remained stable in subsequent OF exposures. In Experiments 4 and 5, we tested F1 hybrid mice (BALB/c-S1; NMRI-S1), and Vitamin D receptor knockout mice (generated on S1 genetic background), again showing strikingly similar temporal patterns of their OF exploration, despite marked behavioural strain differences in anxiety and activity. These results suggest that mice are characterised by stability of temporal organization of their exploration in different OF novelty situations.

Glia and their cytokines in progression of neurodegeneration

A glia-mediated, inflammatory immune response is an important component of the neuropathophysiology of Alzheimer's disease, of the midlife neurodegeneration of Down's syndrome, and of other age-related neurodegenerative conditions. All of these conditions are associated with early and often dramatic activation of, and cytokine overexpression in, microglia and astrocytes, sometimes decades before pathological changes consistent with a diagnosis of Alzheimer's disease are apparent, as in patients with Down's syndrome or head injury. Brains of normal elderly individuals also often show Alzheimer-type neuropathological changes, although to a lesser degree than those seen in Alzheimer's disease itself. These normal age-related glial changes, likely a response to the normal wear and tear of the aging process, raise the threshold of glial activation and thus may explain the fact that even genetically determined Alzheimer's disease, resulting from genetic mutations such as those in beta-amyloid precursor protein and presenilins or from genetic duplication such as of chromosome 21, only shows the full manifestation of the disease decades after birth. In the more common sporadic form of Alzheimer's disease, age-related increases in glial activation and expression of cytokines may act in synergy with other genetic and acquired environmental risks to culminate in the development of disease.

Altered hippocampal transcript profile accompanies an age-related spatial memory deficit in mice

We have carried out a global survey of age-related changes in mRNA levels in the C57BL/6NIA mouse hippocampus and found a difference in the hippocampal gene expression profile between 2-month-old young mice and 15-month-old middle-aged mice correlated with an age-related cognitive deficit in hippocampal-based explicit memory formation. Middle-aged mice displayed a mild but specific deficit in spatial memory in the Morris water maze. By using Affymetrix GeneChip microarrays, we found a distinct pattern of age-related change, consisting mostly of gene overexpression in the middle-aged mice, suggesting that the induction of negative regulators in the middle-aged hippocampus could be involved in impairment of learning. Interestingly, we report changes in transcript levels for genes that could affect synaptic plasticity. Those changes could be involved in the memory deficits we observed in the 15-month-old mice. In agreement with previous reports, we also found altered expression in genes related to inflammation, protein processing, and oxidative stress.

Learning and memory in S100-beta transgenic mice: an analysis of impaired and preserved function

S100-beta, a calcium-binding astrocytic protein from chromosome 21, has been implicated in CNS function generally and the hippocampus in particular. Elevated levels of S100-beta have been observed reliably in the brains of patients with Alzheimer's Disease and Down Syndrome. Groups of transgenic mice, carrying multiple S100-beta gene copies, and nontransgenic controls were administered a series of behavioral tests (delayed spatial and nonspatial non-matching-to-sample, radial arm maze, socially acquired food preference) that assessed a wide range of cognitive functions. Consistent with the widespread presence of S100-beta throughout the brain, transgenic mice exhibited learning or memory impairment on all tasks. The dementia-like cognitive profile of S100-beta mice represents a promising model for studying comparable cognitive deficits associated with neurodegenerative diseases.

Spatial learning in transgenic mice expressing human presenilin 1 (PS1) transgenes

Dominant mutations in the Presenilin 1 gene are linked to an aggressive, early-onset form of familial Alzheimer's Disease (FAD). Spatial memory of transgenic (Tg) mice expressing either mutant (lines Tg(M146L)1, Tg(M146L)76, Tg(L286V)198) or wild type (line Tg(PS1wt)195) human PS1 transgenes was investigated in the Morris water maze (WM) test at 6 and 9 months of age. The results showed that the mutated Tg mice had increased swim speed when compared to non-Tg littermates or Tg PS1 wild type mice. The swim speed difference did not, however, significantly affect the spatial learning in the WM test and all groups showed comparable search paths during training and similar spatial bias during probe trials. When re-tested at 9 months, all mice showed significantly improved learning acquisition of spatial information. The lack of progressive spatial learning impairment in mice expressing the mutated human PS1 transgene in the WM does not preclude impairments in other cognitive tasks but suggests that full phenotypic expression of mutant PS1 alleles may require co-expression of human versions of other AD-associated genes.

The trophic factors S-100beta and basic fibroblast growth factor are increased in the forebrain reactive astrocytes of adult callosotomized rat

S-100 is a calcium-binding protein that is predominantly found in astrocytes of the central nervous system. In the present study, we investigated the temporal and spatial changes of S-100beta immunoreactivity after a stereotaxic mechanical lesion of the adult rat corpus callosum performed with an adjustable wire knife. Rats were killed 7, 14 and 28 days after surgery. S-100beta immunoreactivity was found within the cytoplasm and processes of quiescent putative astrocytes that were observed throughout the gray and white matters of the forebrain of sham-operated rats. Following callosotomy, the S-100beta immunoreactive profiles showed increased size and thick processes, as well as increased amount of S-100beta immunoreactivity. Unbiased stereologic analysis revealed a sustained and widespread increase of the Areal Fraction of S-100beta immunoreactive profiles in the medial and lateral regions of the white matter of callosotomized rats at the studied time-intervals. In the cerebral cortex of callosotomized rats, the estimated total number of S-100beta immunoreactive profiles was also increased 7 and 14 days after the lesion. Since the cellular and temporal changes in S-100beta immunoreactivity were closely similar to those described for basic fibroblast growth factor (bFGF) following brain lesions, we co-localized the S-100beta and bFGF immunoreactivities after callosotomy. bFGF immunoreactivity was found in the nuclei of S-100beta immunoreactive glial profiles throughout the forebrain regions of the sham-operated rats. bFGF immunoreactivity was increased in the nuclei of reactive S-100beta immunoreactive putative astrocytes in the forebrain white matter and in the cerebral cortex of callosotomized rats. These results indicate that after transection of the corpus callosum of adult rats, the reactive astrocytes may exert paracrine trophic actions through S-100beta and bFGF. Interactions between S-100beta and bFGF may be relevant to the events related to neuronal maintenance and repair following brain injury.

A new continuous alternation task in T-maze detects hippocampal dysfunction in mice. A strain comparison and lesion study

The mammalian hippocampus has been the focus of several neurobiology studies because of its important behavioral function and because long-term potentiation (LTP) is a prominent feature of this brain region. Converging evidence suggests that hippocampal function is associated with learning multiple relationships of environmental cues. In this paper a novel behavioral test procedure is introduced, a modified T-maze continuous alternation task (T-CAT), that may serve as a simple, automatable, and quick test of hippocampal function in addition to the frequently applied water maze and fear conditioning paradigms. A comparison is made between mice (strain C57BL/6) with ibotenic acid lesioned or vehicle injected hippocampus, two transgenic strains (on CD1 background) overexpressing a calcium binding protein, S100beta, and inbred (C57BL/6, DBA/2, 129/SV and 129/SVEV) and outbred (CD1) strains of mice. This study shows that hippocampal lesioning led to a significant impairment in T-CAT. Furthermore, overexpression of S100beta, which impairs hippocampal LTP, also led to an impairment demonstrating that T-CAT is sensitive to detect hippocampal dysfunction. Analysis of the mouse strains revealed that C57BL/6 and CD1 mice performed well in T-CAT, whereas 129/SV, 129/SVEV and DBA/2 were significantly impaired, a finding that underscores the importance of strain differences in pharmacological or single gene manipulation studies of hippocampal function in mice.

Senescence-accelerated overexpression of S100beta in brain of SAMP6 mice

S100beta is an astrocyte-derived protein with paracrine and autocrine effects on neurons and glia. Brain S100beta expression increases progressively with age, and this increased expression has been implicated as a factor underlying the increasing risk of Alzheimer's disease that accompanies aging. Senescence acceleration-prone (SAMP) mice are a group of inbred strains that provide animal models of aging and of various age-related disease processes in the brain and peripheral tissues. One of these strains, the osteopenic SAMP6, has not been previously associated with central nervous system alterations. We used Northern and Western immunoblot analysis and immunohistochemical labeling to examine S100beta expression in brains of SAMP6 mice. Cerebral tissue levels of S100beta and of S100beta mRNA were 2.2-fold and 1.6-fold those of senescence-resistant (control) mice at 4 months of age (p < 0.05 in each case), and were 3.7-fold and 1.9-fold those of control mice at 6 months of age (p < 0.01 in each case). In contrast, levels of glial fibrillary acidic protein (GFAP) in cerebral hemispheres were not different from those of controls. Image analysis of immunohistochemical preparations showed increased numbers and immunoreactive intensity of S100beta-immunoreactive astrocytes in both the hippocampus and cerebral cortex of SAMP6 mice at 4 months of age (p < 0.05 or better in each case). These increases were greater in the hippocampus than in the cerebral cortex. In contrast, increases in numbers of GFAP immunoreactive astrocytes were noted only in the hippocampus. Our finding of increased S100beta gene expression in brains of SAMP6 mice mirror age-associated increases in S100beta expression in human brain and suggest that SAMP6 may provide insights into age-associated brain alterations and diseases.

Mice lacking metabotropic glutamate receptor 5 show impaired learning and reduced CA1 long-term potentiation (LTP) but normal CA3 LTP

Class I metabotropic glutamate receptors (mGluRs) have been postulated to play a role in synaptic plasticity. To test the involvement of one member of this class, we have recently generated mutant mice that express no mGluR5 but normal levels of other glutamate receptors. The CNS revealed normal development of gross anatomical features. To examine synaptic functions we measured evoked field EPSPs in the hippocampal slice. Measures of presynaptic function, such as paired pulse facilitation in mutant CA1 neurons, were normal. The response of mutant CA1 neurons to low concentrations of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was missing, which suggests that mGluR5 may be the primary high affinity ACPD receptor in these neurons. Long-term potentiation (LTP) in mGluR5 mutants was significantly reduced in the NMDA receptor (NMDAR)-dependent pathways such as the CA1 region and dentate gyrus of the hippocampus, whereas LTP remained intact in the mossy fiber synapses on the CA3 region, an NMDAR-independent pathway. Some of the difference in CA1 LTP could lie at the level of expression, because the reduction of LTP in the mutants was no longer observed 20 min after tetanus in the presence of 2-amino-5-phosphonopentanoate. We propose that mGluR5 plays a key regulatory role in NMDAR-dependent LTP. These mutant mice were also impaired in the acquisition and use of spatial information in both the Morris water maze and contextual information in the fear-conditioning test. This is consistent with the hypothesis that LTP in the CA1 region may underlie spatial learning and memory.