To freeze, or not to freeze, that is the question!

Freezing behavior is a type of defensive responses, commonly observed in rodents when they encounter danger in a specific context. In animal studies investigating Pavlovian fear conditioning learning, animals learn to associate a particular context or a specific cue to a harmful stimulus (e.g. electric foot shock), and freezing behavior is used as a robust index to score fear memory strength.

Fragile X syndrome (FXS) is a leading cause of syndromic autism spectrum disorders. It is a neurodevelopmental disorder accompanied by intellectual disability. Mutations in the fragile X mental retardation 1 (FMR1) gene that encodes the fragile X mental retardation protein (FMRP) result in FXS. Fmr1 KO mice, the mouse model of FXS, exhibit various autistic-like behaviors, including deficits in fear learning and memory, making them excellent preclinical models to understand the mechanism of FXS.

In a newly published paper in the Journal of eLife, Stanford University School of Medicine scientists: Drs. Jie Li, Lu Chen and colleagues investigated how memory deficits in the FXS mouse was caused by altered activation patterns of neuronal population during memory recall. The research team showed that impaired fear memory recall in FXS is due to inappropriate memory engram reactivation, and improving network activation accuracy through an enriched environment may rescue the memory recall deficit.

In their studies the authors conducted behavioral tests and observed that the FXS mouse model with constitutive deletion of the Fmr1 gene (Fmr1 KO) mice displayed normal learning curve. However, they did exhibit significantly impaired memory recall in the behavioral tests 1–3 days after training. No deficit in pain perception was observed in the Fmr1 KO mice (normal pain perception is required for the appropriate formation of contextual fear memory, which is an association of a particular context with a painful experience involving a brief foot shock). Both the Fmr1 KO mice and their Wild type littermates had similar performance in various hippocampus-independent tests, indicating that the impaired memory recall was potentially caused by disorders within hippocampus region.

In CA1-Fmr1 KO mice (mice with specific FMRP deletion in the CA1 region of the hippocampus), the authors again found normal behavior during the learning phase of fear conditioning but impaired contextual memory recall the day after training, indicating that contextual memory formation requires normal expression of FMRP in hippocampal CA1.

The authors next focused on the CA1 region neural networks and found that during contextual memory test, CA1 engram reactivation efficacy has a positive correlation with the performance of the animal. In Fmr1 KO mice, CA1 memory engram reactivation efficacy is significantly lower than that of the WT mice, which correlates with their poor performance in memory recall. Experiencing an enriched environment prior to learning rescued the fear memory formation in Fmr1 KO mice. The authors further found that during enriched environment experience, a subset of hippocampal CA1 neurons are activated by the experience. Interestingly, these neurons are much more likely to become engram cells to encode memory in subsequent learning when compared to the neighboring neurons not activated by the enriched environment experience. Moreover, when the CA1 neurons activated by the enriched environment experience are inhibited specifically during the fear conditioning learning phase, enriched environment-induced memory improvement is prevented in Fmr1 KO mice.

Through these carefully designed studies, the authors used a well-established genetic labeling method to demonstrate that during memory recall, there is a preferential reactivation of learning-activated neurons in hippocampal CA1. In addition, the efficacy of this engram reactivation has a positive correlation with behavioral performances. “We would really like to find out what changes are induced in neurons by the enriched environment experience and how these changes increase a neuron’s chance of being recruited as an engram cell.” Dr. Chen commented when being asked about their next steps in this line of research. They suggest that future studies will be conducted using improved tools to explore in more detail the mechanisms underlying memory formation and recall.