Figure 1: X chromosome with mutations in the FMR1
Fragile X Syndrome is the leading cause of inherited intellectual disability. It is also the largest known genetic cause of autism, though not the only genetic cause. The gene containing a mutation leading to Fragile X Syndrome in humans is called FMR1 for Fragile X mental retardation 1. The protein made from the gene is involved in learning and memory so a mutation in it leads to learning deficits. Previous research showed that FMR1 interacts with another protein implicated in intellectual disability called Rac1. Rac1 is increased in post-mortem brain tissue from patients with Fragile X Syndrome. Rac1 is one protein that Luis Martinez is studying as an early graduate student in Dr. Maria Tejada-Simon’s lab at the University of Houston College of Pharmacy. He is working to find out how Rac1 signaling may be involved in the memory problems in Fragile X Syndrome.
|Figure 2: Parts of a neuron. (Source)|
The way the FMR1 protein affects learning and memory is by changing synaptic plasticity, which is the way neurons adapt and change based on incoming information. Specifically, neurons have little bumps on the dendrites, called spines, that receive information from other neurons (Figure 2). These little bumps get larger and smaller and appear and disappear based on input from the environment, like learning something new. In transgenic mice without the FMR1 gene, there is a high density of spines but they are much thinner than in normal mice. Luis is using the FMR1 mouse model to study how the lack of the FMR1 protein yields increased Rac1, and this leads to learning and memory deficits. Also, he is trying to find out if blocking the Rac1 protein can improve learning.
Luis specifically studied associative memory, a form of memory where two factors become associated, such that the appearance of one induces the memory of the other. He studied this using contextual fear conditioning. In this paradigm, mice were placed in a chamber with a specific grid floor, walls, and lighting and were allowed to explore it for several minutes. Mice are curious little creatures and they will look, smell, and feel the chamber the whole time. However, near the end of their time in the chamber a tone was played and they received a small but unpleasant foot shock. Even after one training session, mice will learn to associate the tone with the shock but additional training sessions usually strengthen the association. Memory for the shock was tested one day after training by placing the mouse back in the chamber and measuring the amount of time they spent not moving, aka freezing. Freezing is a fear response specific to prey animals, which helps keep them safe from predators. If they remember the shock from the previous day, they freeze for more than half of the time.
In Luis’s experiment, the mice lacking the FMR1 gene had poor memory when compared with non-transgenic mice. Transgenic mice were treated with a Rac1 inhibitor before delay-fear conditioning wherein the foot shock is given at the same time as a tone. The transgenic mice treated with the Rac1-inhibitor exhibited greater freezing behavior during a contextual memory test, which means they remembered the shock. This type of memory depends on the hippocampus, a brain area involved in learning and memory, and amygdala, a region involved in processing emotions.
However, the inhibitor did not rescue the tone-memory, which is highly dependent on the amygdala. A subtle variation in the fear conditioning protocol known as trace-fear conditioning that does not involve the amygdala but does engage the prefrontal cortex, which is important for attention. During the conditioning phase, an interval of a few seconds is introduced between the foot shock and the tone, forcing mice to be more attentive. Transgenic mice treated with the Rac1 inhibitor expressed greater freezing behavior when the tone was replayed the following day. These observations suggest the inhibitor may be affecting the hippocampus only or its interaction with the prefrontal cortex to correct these memory deficits.
As Luis continues his graduate work, he plans to try to restore the cognitive deficits using other ways to block Rac1 activity. He will use experiments to see if synaptic plasticity, for example by measuring spine size, is restored as well.