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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CorCir (Cortical circuit assembly in the developing mouse neocortex)

Teaser

Summary

Imbalance of the excitatory-inhibitory neurons is a hallmark of several neurodevelopmental disorders such as epilepsy, autism and schizophrenia. Despite being a common theme in these disorders, the establishment of this balance in the developing brain and how it may contribute to cortical circuit assembly remains unknown.

Understanding the molecular mechanisms involved in the establishment of this excitatory-inhibitory balance is vital in (1) understanding the critical developmental time period in which this balance is established, (2) providing insights into the aetiology of neurodevelopmental disorders and (3) identifying potential therapeutic avenues that can prevent and/or correct this imbalance during development.

Consequently, one of the main aims of the research project is to determine the molecular mechanisms that regulate inhibitory cell survival in the developing cortex. More specifically, the aim of this project is listed as follows

(1) Characterisation of programmed cell death within the neuronal population in the developing mouse brain.
(2) Establishment of the molecular mechanisms driving interneuron cell death during mice postnatal development.

Work performed

In order to achieve the main objectives of elucidating the molecular mechanisms regulating inhibitory cell survival in the developing cortex, the project is divided into several work projects as described below, together with its main objectives and key findings.

Work Package 1
The main objective for this work package is to characterise neuronal cell death in the developing mouse cortex. To achieve this, we have performed stereology, an unbiased and systematic method of estimating the number of cells in a tissue. By using this method, we were able to determine changes in the number of pyramidal cells and the main subtype of interneurons, namely the medial ganglion eminence (MGE) interneurons at different time points during the first three weeks of postnatal development.

The key findings are as follows:
(1) Pyramidal cells undergo programmed cell death during the first week of postnatal development where around 13% of excitatory cells die by postnatal (P) day 5.
(2) MGE interneurons undergo programmed cell death between P5 and P10, where around 30% of MGE interneurons die.

Together, this data suggest that there is a consecutive wave of programmed cell death in early postnatal cortex and that this two events may be linked during development.

Work Package 2
The main objective for this work project is to determine the molecular mechanisms underlying interneuron cell death. To achieve this, we have divided this work package into the following subproject as listed below

(a) Impact of pyramidal cell activity and number on interneuron survival
(b) Identification of the molecular mechanisms driving MGE interneuron cell death
(c) Identification of the molecular mechanisms driving caudal ganglionic eminence (CGE), the other interneuron subtype cell death

The key findings are as follows
(1) Alteration of pyramidal cell activity or numbers can impact both MGE and CGE interneurons survival during a critical time window in development. For instance, increasing pyramidal cell activity or number enhanced the survival of interneurons during this critical time period in development. Alteration of pyramidal cell activity beyond this critical time window has no impact on interneuron survival.
(2) PTEN, a known inhibitor of PI3 kinase (a molecule that has been associated with cell survival), is expressed at different levels during the first two weeks of postnatal development. A peak expression of PTEN was observed at P7-P8 within the MGE interneuron population, corresponding to the peak of their cell death.
(3) Pyramidal cell can regulate the survival of inhibitory cell via an activity-dependent modulation of interneuron PTEN levels.

Together, our findings have demonstrated that early postnatal activity-dependent mechanisms dynamically adjusted inhibitory cell numbers and ultimately led to the establishment of the appropriate excitatory to inhibitory balance during early postnatal development. Current work is still ongoing on the molecular mechanisms regulating CGE interneuron cell death.

This work has resulted in a high-impact publication (Wong et al. Nature 2018) and has been disseminated in both national and international conferences. Furthermore, to increase the exposure of the work to the general public, a layman summary of this work has been written in international newspapers such as the Financial Times (https://preview.tinyurl.com/yy654p6m), science blogs (https://www.nature.com/articles/s41586-018-0139-6/metrics) and podcast (https://tinyurl.com/y2tfn9bc).

Final results

As mentioned previously, this work has successfully achieved its main objective and have resulted in the publication in a high-impact journal. Current ongoing investigation focusses on the CGE interneuron subtypes and we hope to complete this work for publication by the summer of 2020. The work from this project has provided novel insights into the mechanisms underlying the establishing the excitatory-inhibitory balance in the developing cortex that is independent of brain shape and size.

The work from this project has several impacts as listed below
(1) This work has identified the potential mechanisms to the aetiology of neurodevelopmental disorders such as autism spectrum disorder, by identifying a novel role for PTEN. Identification of this novel role of PTEN will provide new therapeutic avenues in treatment for neurodevelopmental disorders in the future.
(2) This work has also identified a critical time window during development in which changes in pyramidal cell activity and number can have a significant consequence on the establishment of the excitatory-inhibitory balance. This will provide the basis of identification of a similar time window in the developing human cortex and will be key in the aetiology of neurodevelopmental disorders. Identification of this critical time window will also have a significant impact on the diagnosis of neurodevelopmental disorders.
(3) Basis for new projects within the Marin Lab. The work from this project has form the basis of the ERC grant awarded to Oscar MarÃn (DEVINCI) to study the developmental principles underlying the functional specialisation of inhibitory circuits. This will provide more insights into the role of programmed cell death in sculpting cortical circuits and how its dysregulation can lead to neurodevelopmental disorders.
(4) This work has been cited by 38 peer-review journals since it publication in May 2018 (as of October 2019), thus contributing to the overall scientific impact within the field.
(5) During the course of this work, I have trained 2 lab technicians and 1 Master students. The transfer of scientific knowledge and skill has led to the enrichment of the skilled worked pool within the UK and Europe.