The Helico Maze Detects Early Impairment of Reference Memory at Three Months of Age in the 5XFAD Mouse Model of Alzheimer's Disease.

BACKGROUND: The 5XFAD model of Alzheimer's disease (AD) bearing five familial mutations of Alzheimer's disease on human APP and PSEN1 transgenes shows deposits of amyloid-ß peptide (Aß) as early as 2 months, while deficits in long-term memory can be detected at 4 months using the highly sensitive olfactory-dependent tests that we previously reported. OBJECTIVE: Given that detecting early dysfunctions in AD prior to overt pathology is of major interest in the field, we sought to detect memory deficits at earlier stages of the disease in 3-month-old male 5XFAD mice. METHODS: To this end, we used the Helico Maze, a behavioral task that was recently developed and patented. This device allows deeper analysis of learning and subcategories of hippocampal-dependent long-term memory using olfactory cues. RESULTS: Eight male 5XFAD and 6 male wild-type (WT: C57Bl6 background) mice of 3 months of age were tested in the Helico Maze. The results demonstrated, for the first time, a starting deficit of pure reference long-term memory. Interestingly, memory impairment was clearly correlated with Aß deposits in the hippocampus. While we also found significant differences in astrogliosis between 5XFAD and WT mice, this was not correlated with memory abilities. CONCLUSION: Our results underline the efficiency of this new olfactory-dependent behavioral task, which is easy to use, with a small cohort of mice. Using the Helico Maze may open new avenues to validate the efficacy of treatments that target early events related to the amyloid-dependent pathway of the disease and AD progression.

The Helico Maze allows testing of early learning and subcategories of long-term memory in mice.

Different memory systems operate in parallel to support behaviour. To evaluate procedural and reference subcategories of long-term memory as early as possible in the mouse, the Helico Maze (HM) was developed. BALB/c AnNCrl (BALB), C57BL/6JRj (C57) and DBA/2 JRj (DBA) mice were trained on this new maze. The three strains learned how to use the HM (procedural memory), and they then learned and remembered four odour-reward associations (reference memory). The three strains differed in the number of correct responses. BALB mice showed better performance than C57 and DBA mice. The results of the first block of each session revealed that only the BALB and C57 mice remembered the odour-reward associations. DBA mice needed to relearn the associations each day. With this new apparatus, the number of olfactory cue-reward associations was increased from 2 to 4 in comparison to a previous olfactory tubing maze. Consequently, a supplementary effort of memory was required, and the chance level was decreased from 50 % to 25 %. Thus, in several important respects, the HM can be considered to measure the hippocampus-dependent behaviour of the mouse, allowing to study, as early as possible in young mice, the different subcategories of long-term memory, such as those observed in humans.

Chronic treatments with a 5-HT4 receptor agonist decrease amyloid pathology in the entorhinal cortex and learning and memory deficits in the 5xFAD mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is the main cause of dementia and a major health issue worldwide. The complexity of the pathology continues to challenge its comprehension and the implementation of effective treatments. In the last decade, a number of possible targets of intervention have been pointed out, among which the stimulation of 5-HT4 receptors (5-HT4Rs) seems very promising. 5-HT4R agonists exert pro-cognitive effects, inhibit amyloid-ß peptide (Aß) production and therefore directly and positively impact AD progression. In the present work, we investigated the effects of RS 67333, a partial 5-HT4R agonist, after chronic administration in the 5xFAD mouse model of AD. 5xFAD male mice and their wild type (WT) male littermates received either RS 67333 or vehicle solution i.p., twice a week, for 2 or 4 months. Cognitive performance was evaluated in a hippocampal-dependent behavioral task, the olfactory tubing maze (OTM). Mice were then sacrificed to evaluate the metabolism of the amyloid precursor protein (APP), amyloidosis and neuroinflammatory processes. No beneficial effects of RS 67333 were observed in 5xFAD mice after 2 months of treatment, while 5xFAD mice treated for 4 months showed better cognitive abilities compared to vehicle-treated 5xFAD mice. The beneficial effects of RS 67333 on learning and memory correlated with the decrease in both amyloid plaque load and neuroinflammation, more specifically in the entorhinal cortex. The most significant improvements in learning and memory and reduction of pathology stigmata were observed after the 4-month administration of RS 67333, demonstrating that treatment duration is important to alleviate amyloidosis and glial reactivity, particularly in the entorhinal cortex. These results confirm the 5-HT4R as a promising target for AD pathogenesis and highlight the need for further investigations to characterize fully the underlying mechanisms of action.

Dynamic expression of the polysialyltransferase in adult rat hippocampus performing an olfactory associative task.

Neural cell adhesion molecule (NCAM) is associated with polysialic acid (PSA), and its function is highly dependent on the extent of polysialylation through the activity of two polysialyltransferases, sialyltransferase-X (STX) and polysialyltransferase (PST). PSA-NCAM plays an important role in synaptic plasticity in the hippocampus. The involvement of STX and PST during mnesic processes was assessed in the adult rat hippocampus. We investigated whether different levels in learning and memory using an olfactory associative task influenced STX and PST gene expression in the hippocampus using semiquantitative transcription-polymerase chain reaction. Then, NCAM polysialylation and cell proliferation were quantified in the dentate gyrus of a "Learning and Memory" group using immunohistochemistry. We found that only the expression level of PST mRNA increased with learning performance and returned to an initial level when learned associations were consolidated in long-term memory, while STX mRNA levels remained unchanged. This phenomenon was accompanied by an increase in PSA on NCAM but not by cell proliferation in the dentate gyrus. Our results suggest a different involvement for STX and PST in neural plasticity: while STX is probably involved in the proliferation of neural progenitor cells, PST could play a key role in synaptic plasticity of mature neural networks. The expression of the STX and PST genes could, therefore, be useful markers of neurobiological plasticity in the brain, allowing to follow chronological events in limbic and cortical structures related first to learning and memory processes (for PST) and, second, to adult neurogenesis processes (for STX).

Onset of hippocampus-dependent memory impairments in 5XFAD transgenic mouse model of Alzheimer's disease.

The 5XFAD mice are an early-onset transgenic model of Alzheimer's disease (AD) in which amyloid plaques are first observed between two and four months of age in the cortical layer five and in the subiculum of the hippocampal formation. Although cognitive alterations have been described in these mice, there are no studies that focused on the onset of hippocampus-dependent memory deficits, which are a hallmark of the prodromal stage of AD. To identify when the first learning and memory impairments appear, 5XFAD mice of two, four, and six months of age were compared with their respective wild-type littermates using the olfactory tubing maze, which is a very sensitive hippocampal-dependent task. Deficits in learning and memory started at four months with a substantial increase at six months of age while no olfactory impairments were observed. The volumetric study using magnetic resonance imaging of the whole brain and specific areas (olfactory bulb, striatum, and hippocampus) did not reveal neuro-anatomical difference. Slight memory deficits appeared at 4 months of age in correlation with an increased astrogliosis and amyloid plaque formation. This early impairment in learning and memory related to the hippocampal dysfunction is particularly suited to assess preclinical therapeutic strategies aiming to delay or suppress the onset of AD.