Modelling cortical network development at the cellular scale and disruption by Mecp2 deficiency.

Disruption to the refinement of brain connectivity during early postnatal development can have profound effects on cognitive functions. In Rett syndrome, young girls develop normally for the first 6-18 months-of-age followed by regression of cognitive, language, social, sensory and motor abilities. In 95% of cases, Rett syndrome is caused by loss-of-function mutations in the chromatin-remodeller MECP2 (Chahrour & Zoghbi, 2007). However, there are no treatments available for Rett syndrome, and further understanding of the neurophysiological consequences of MeCP2 deficiency is needed.
Mecp2-deficient mice also show regression of behavioural, sensory and motor function following an initially normal development. Thus, Mecp2-deficient mice provide a useful way of modelling pathological processes in order to identify innovative targets for treatment of Rett syndrome.
Electrophysiological recordings have previously shown cell-type specific effects of Mecp2 deficiency - inhibitory Parvalbumin-expressing (PV) cells develop prematurely whereas excitatory pyramidal neurons show delayed development (Mierau et al., 2016). This may contribute to disruption of the excitatory/inhibitory (EI) balance in the cortex during a critical developmental period (Goffin & Zhou, 2012). However, the downstream effects on network function are unknown.
The goal of my interdisciplinary PhD research will be to assess developmental network-level defects due to Mecp2 deficiency and how specific excitatory and inhibitory cells contribute to this pathology. To do this, I will record spontaneous activity in cultured cortical cells using MEAs before applying various computational methods for detecting spiking and bursting activity. Following this I will characterise the effects of Mecp2 deficiency in terms of more detailed network features including clustering, path length and small-world topology (derived from graph theory). I will then apply machine learning classification in order to determine the primary features of Mecp2-efficient networks. I have set the following aims:
1. Assess changes network measures (such as at sequential time points and compare this development between wild-type and Mecp2-deficient cultures. This will elucidate to how Mecp2 deficiency affects network-level parameters during development. I will include a comparison of different burst detection methods.
2. Compare the contribution of PV cell-mediated inhibition to the development of network connectivity in wild-type and Mecp2-deficicient cultures. I will employ an optogenetic approach using Mecp2-deficient mice that express a light-silencing channel (archaerhodopsin) selectively in PV cells. This will enable me to silence PV cells during MEA recordings and assess changes in network activity.
3. As Mecp2 deficiency accelerates PV cell maturation, the final aim will be to test whether preventing this process may recover network function by restoring the excitatory-inhibitory balance in the network. I will use chronic, intermittent silencing of PV cells through optogenetic manipulation.
I hypothesise that Mecp2-deficient cultures will show hyper-activity and hyper-connectivity. This may be shown by an reduced characteristic path length and an increased clustering coefficient. These findings would suggest that Mecp2 deficiency leads to a alterations in both local and global information-processing efficiency. Additionally, I hypothesise that optogenetic manipulations PV cells will show that decreasing PV cell activity alters neuronal activity such that overall network synchrony is reduced. Furthermore, chronic intermittent silencing of PV cells in Mecp2-deficient cultures may ameliorate these differences between Mecp2-deficient and wild-type cultures. This would have implications for the treatment of Rett Syndrome by suggesting premature PV cell activity as a target for therapeutic interventions.