Using the head direction system of rats to model neuronal circuit alterations during development in Fragile X syndrome

Abstract

Fragile X Syndrome (FXS) is one of the most common monogenic causes of autism and intellectual disability and it is caused by epigenetic silencing of the FMR1 gene (Fragile X messenger ribonucleoprotein 1) and subsequent reduction or lack of production of the FMRP protein. An Fmr1 knock-out (Fmr1-/y) rat model has been developed to study the pathophysiology of FXS. In this thesis, I am focussing on the head direction (HD) system to study cellular and circuit changes in the absence of FMRP at different stages of development and how these may cause cognitive alterations.

The HD system acts as an internal neural compass, using self-motion and external landmark cues to signal an animal’s current head direction. It is composed by HD cells, each of which has a preferred direction in which it exhibits an increased firing rate. HD cells are found in many areas throughout the brain; the HD signal is generated subcortically and is relayed through the thalamus onto the cortex, with the postsubiculum being the first cortical area to receive the HD signal. The HD system can be a great model to study neurodevelopmental disorders because the relationship between its wiring and function is well studied, it can be examined from a very young developmental stage, and it can be probed both in the presence and absence of external sensory stimuli – i.e., during wakefulness and sleep.

In this thesis, I use in vivo and ex vivo electrophysiological recordings in juvenile (22-28 days postnatally) and adult wild-type and Fmr1-/y rats to test the hypothesis that FMRP is necessary for the normal development of the HD system. Using ex vivo slice electrophysiology, I found that excitatory neurons in the postsubiculum did not show detectable alterations in their intrinsic membrane properties, other than a subtle agespecific change in excitability. However, spontaneous excitatory postsynaptic currents onto these neurons showed age-specific alterations: they were more frequent in Fmr1-/y juvenile rats up to 25 days old, but of higher amplitude in adult Fmr1-/y rats, both compared to wild-type rats of the same age.

To examine the function of the HD signal in vivo, I implanted silicon probes into the postsubiculum of juvenile and adult wild-type and Fmr1-/y rats and recorded the activity of neuronal assembles while these animals foraged in a simple cylindrical environment with a cue card. I found that surprisingly, juvenile Fmr1-/y rats had a sharper HD signal, where each regular-spiking neuron conveyed more HD information per spike, and the preferred direction of each neuron was more stable within an exploration, compared to wild-type. In contrast, adult Fmr1-/y rats showed decreased stability of the HD signal, compared to wild-type.

I then asked whether the differences observed in the HD signal were due to changes in the wiring of the HD system itself, or due to differences in the self-motion or external sensory inputs used to generate and maintain it. The wiring of the HD system creates predictable co-activity patterns and ensemble dynamics that are maintained during REM sleep. In juveniles, there were differences between genotypes consistent with the genotype effects found in the HD signal, and they were consistent during wake and REM sleep, indicating that differences in the interconnectivity of HD cells may at least in part play a role in the altered HD signal of Fmr1-/y juvenile rats. On the other hand, the underlying HD network seemed to be unaffected in Fmr1-/y adults, indicating that the instability seen in these rats was caused by a difference in the visual or self-motion cues used by the HD system instead.

Finally, I recorded postsubicular ensembles in a series of protocols including cue rotations and a novel, cue-rich environment. I found that an environment rich in prominent visual cues rescued the instability the HD signal in adult Fmr1-/y rats, which is evidence that Fmr1-/y adults are capable of using visual cues to stabilise the HD signal when those are abundant, thus the instability is likely caused by deficits in the self-motion inputs that reach the HD system.

Overall, the results reported in this thesis show that the HD signal in the postsubiculum of Fmr1-/y rats shows age-specific alterations, with higher sharpness and stability during early development and lower stability in adulthood, compared to control, with both phenotypes arising due to different underlying mechanisms. The exact mechanism causing these changes is not clear, however alterations in the underlying ring attractor network and the encoding or integration of vestibular inputs are likely to contribute at least partially, in an age-specific manner.