Exploring expression of neurodevelopmental susceptibility genes in the foetal human thalamus and other related structures

Abstract

The thalamus is a brain region consisting of neuronal clusters and with a large number of connections which are responsible for several important functions including cognitive functions. It serves as a major relay centre, transmitting and modulating information between the cerebral cortex and subcortical structures. Given its involvement in higher-order cognitive functions, abnormalities in thalamic development have been implicated in various neurodevelopmental disorders, including schizophrenia and autism spectrum disorders. However, the developmental process of these nuclei in the human brain is still unknown. Understanding the developmental trajectory of the thalamus in humans is essential for several reasons. Firstly, while rodent models have provided insights into thalamic development, there are significant species-specific differences in the timing, organization, and molecular regulation of thalamic nuclei formation. These differences necessitate direct investigation of human developmental processes to bridge the gap between animal models and human neurodevelopment. Secondly, delineating how thalamic nuclei emerge during early foetal development may provide critical information on the origins of functional specialization within the thalamus, shedding light on how distinct neuronal populations are specified and how their connectivity is established. Finally, by identifying molecular markers and gene expression patterns specific to early thalamic development, we can gain a deeper understanding of the genetic and cellular mechanisms that may contribute to neurodevelopmental disorders, potentially informing future diagnostic and therapeutic approaches.

This study aims to investigate gene expression patterns in the human thalamus, extending from 8 to 21 PCW, in order to track the development of each thalamic nucleus. Additionally, we focus on 14 PCW to identify distinct thalamic nuclei based on the expression of a unique combination of transcription factors and other genes/proteins. We also aim to investigate the expression of susceptibility genes linked to neurological diseases such as FEZ1, NRXN1 regarding their expression in specific thalamic nuclei.

The methods we used are immunostaining and RNAscope in situ hybridisation, including double staining methods for multiple markers. Sections were taken from human early foetal brains (ethically sourced and supplied by the Human Developmental Biology Resource), in all planes and covering the extent of the diencephalon. Sections taken at 14 PCW were aligned with 3-D maps of the forebrain collected by structural MRI scanning. The expression of combinations of markers were localised to particular regions of the thalamus. We also analysed open source scRNAseq data with the aim of identifying clusters of cells grouped by shared gene expression patterns. We also investigated the expression of neurodevelopmental disease susceptibility genes in specific nuclei and cell types of the thalamus and telencephalon.

This study provides significant insights into the early development of the human thalamus and telencephalon. The study highlights distinct gene expression patterns and the emergence of thalamic nuclei from a protomap. Our findings demonstrate that different transcription factors and molecular markers define specific thalamic regions, reinforcing the concept that a structured thalamic map begins to emerge by 14 PCW. Furthermore, we identified 15 distinct groups of cells with functional characteristics, supporting the notion that thalamic differentiation is an intricate and highly regulated process. It also implicates that neurogenesis and extensive cellular migration are critical processes during this crucial period.

The differential expression of neurodevelopmental disease susceptibility genes in the thalamus further underscores the importance of studying early thalamic development in the context of neurological disorders. The high expression of FEZ1 in progenitor cells, transitioning to glutamatergic neurons, and the elevated presence of NRXN1 in the thalamus suggest potential roles in neuronal connectivity and function. These findings may provide critical clues for understanding the etiology of conditions such as schizophrenia, where thalamic dysfunction has been implicated.

Overall, this study bridges a crucial gap in our knowledge of human thalamic development, and lay the foundation for future research into the molecular mechanisms underlying thalamic organization. Further investigations, including functional studies and longitudinal analyses, will be essential for uncovering how early developmental events shape thalamic function and its implications for neurodevelopmental and neuropsychiatric disorders.