Regulation of the time-sensitive period in neuronal circuit development by NTRK2/TrkB signalling.

Maturation of the mammalian brain occurs postnatally and is based on specific developmental programmes of gene transcription. Early in postnatal development, activity shapes neuronal circuits during specific time frames. Neuronal dysfunction at these critical times leads to neurodevelopmental disorders, such as intellectual disabilities, autism and schizophrenia, affecting adult brain functions.
Several studies have reported that the neurotransmitter GABA drives early developmental events such as forming and refining neuronal circuits through its early excitatory effects on neural precursors and immature neurons at critical times. This effect occurs in diverse brain regions, including the hippocampus, a structure involved in higher brain functions, such as learning and memory, and spatial coding. Altering this early excitatory effect of GABA gives rise to an array of neurodevelopmental disorders.
We have recently established the genetic importance of the neurotrophin receptor TrkB shaping hippocampal neuronal circuitry at a critical period by controlling the GABAergic system development. Namely, altering TrkB signalling in specific hippocampal cells at an early stage of development induces neuronal defects similar to those found in many neurodevelopmental disorders and leads to permanent cognitive dysfunction in adulthood. However, we still lack a deeper understanding of the molecular mechanisms shaping development during critical periods of plasticity.

Here, we propose integrated approaches to understand how TrkB signalling-dependent transcription at critical times promotes the refinement and plasticity of neural circuits for cognitive function and what could go wrong in disease. To achieve this, we will combine precise genetics with cutting-edge sequencing techniques, including a computational integrated analysis of single-cell transcriptome and epigenome that will help to select top candidate genes for functional validation analysis. Together, this will provide new insights and new potential genetic factors that regulate time-sensitive periods during the development of hippocampal circuits. These studies are vital for understanding the molecular mechanisms responsible for the onset of neurodevelopmental disorders, both in terms of basic mechanisms and clinical application. Thus, this project is timely and innovative in the field of neurodevelopmental disorders.

This project will examine the molecular mechanisms underlying time-sensitive periods in dentate granule cells (DGC) differentiation continuum leading to sequential maturation of intrinsic hippocampal circuits in normal and perturbed conditions.

The guiding hypothesis is that early in postnatal development, BDNF-TrkB activation in immature DGCs provides an instructive signal that drives the sequential maturation of intrinsic hippocampal circuits via the depolarizing action of GABA. Disrupting this signal at a critical period leads to neurodevelopmental defects, followed by synaptic plasticity and cognitive impairments in adulthood.

A key step to elucidate the mechanisms underlying the crucial transition of immature DGCs into the GCL is the coordinated analysis of these neurons' transcriptional and epigenetic properties at single-cell level revealing their cellular state and cellular capacity simultaneously.

We have established platforms for genetic labelling and lineage tracing of developing DG neurons, single-cell RNA and ATAC sequencing, and in vivo viral transduction of the hippocampus.

This project will carry out transcriptional and epigenetic profile analysis using scRNAseq and scATACseq, respectively, from wild type and Trkb mutant DGC populations at two critical time points of postnatal development. We will then perform a computational integrative analysis of epigenome vs transcriptome data obtained to select top candidate gene regulators of the DGC differentiation continuum. Finally, by functionally validating top candidates by rescuing the DGC differentiation block occurring in the absence of TrkB signalling, we will identify key molecular pathways that regulate the DGC differentiation continuum during time-sensitive periods of postnatal development.