THE ROLE OF NEURONAL PRECURSOR VARIETY IN NEOCORTICAL NEUROGENESIS AND CONNECTIVITY DURING THE DEVELOPMENT OF BRAINS WITH AND WITHOUT ANEUPLOIDY

Abstract

The cerebral cortex is a six-layered brain tissue that develops during early gestation from a simple sheet of cells into one of the most complex structures in the known universe. Neurons generated from distinct progenitor populations contribute to circuits that regulate cognition, social behavior, speech, and motor function. All these behaviors are also known to be detrimentally impacted by trisomy 21/Down’s syndrome (DS). Prior research using mouse models of DS has revealed that fewer neurons are born in the developing cortex due to the trisomy, and that they display altered identity and function. These neuronal changes likely underpin some of the behavioral issues commonly seen in DS children. Despite these incremental advances in understanding DS-associated brain changes, we currently lack a comprehensive description of the cellular, molecular, and physiological mechanisms behind DS. Addressing this gap requires linking changes in neural progenitor biology to alterations in cortical organization, neuronal activity, and ultimately behavior. This dissertation includes five chapters examining how neural progenitor diversity shapes excitatory lineage development and cortical circuit assembly under typical conditions, and how these processes are altered by trisomy 21-associated aneuploidy. In Chapter 1, I present an overview of mammalian neocortical development, emphasizing the mechanisms and consequences of neural progenitor and stem cell lineage diversification during development of the brain. I synthesize emerging findings on cortical precursor diversity, clarify mechanisms of lineage specification, and evaluate how changes to lineage specification contributes both to neurotypical neocortical expansion and to altered brain development. This chapter is an in-depth examination of the biological consequences of neural precursor heterogeneity, its impacts on cortical circuitry, and the evidence that alterations in precursor variety are a key feature of multiple intellectual and developmental disabilities, including DS. In addition, it reviews the existing mouse models of DS that have been developed to investigate developmental processes, outlining their genetic composition, phenotypic features, and respective strengths and limitations. Particular attention is given to the genetic shortcomings of earlier models such as incomplete gene dosage representation and species-specific genomic differences and how these limitations motivated the development of more accurate models. These challenges are addressed by the TcMAC21 mouse model, which incorporates a nearly complete copy of human chromosome 21. Cortical development of this mouse model is characterized in detail in Chapter 3. In Chapter 2, I characterize the structural connectivity patterns of the two principal excitatory neuron lineages in the neurotypically developing neocortex, establishing a framework for understanding lineage-specific contributions to cortical circuit assembly. In Chapter 3, I investigate how neuronal progenitor pool composition and lineage-specific developmental trajectories are altered in a pre-clinical DS model, TcMAC21. Because TcMAC21 is a newly developed model, without extensive characterization of its neurodevelopmental features, this chapter focuses on defining baseline alterations in cortical neurogenesis. The findings in this study revealed altered patterns of population activity consistent with changes in cortical organization, potentially reflecting changes in the positional distribution of excitatory neurons. In this study, I identified subtle but significant lineage-specific delays in neurogenesis which may result in altered contributions of specific progenitor lineages to the neocortical circuitry. In Chapter 4, I examine how these developmental alterations (seen in Chapter 3) relate to circuit-level dysfunction in the trisomic cortex. To determine how these circuit alterations manifest during behavior, I performed in vivo multiphoton calcium imaging of individual cortical neurons in awake TcMAC21 mice during locomotion. Together, the findings in this chapter demonstrate that trisomic animals exhibit abnormal neuronal population activity dynamics and behavioral-state encoding in the cortex, as well as alterations in synaptogenesis and calcium signaling. More broadly, Chapters 3 and 4 connect early disruptions in neuronal lineage composition and laminar positioning to later-emerging circuit dysfunction in a genetically faithful mouse model of DS. By integrating developmental lineage analyses with ex vivo and in vivo measurements of cortical activity, this dissertation establishes a multiscale framework for understanding how subtle neurodevelopmental perturbations can give rise to profound circuit-level and behavioral deficits in neurodevelopmental disorders. Taken together, this dissertation helped generate a new understanding of how neocortical excitatory lineages develop and integrate into functional circuits during typical brain development and how these processes change under conditions of trisomy 21. Furthermore, it sets the stage for deeper characterization of the relationship between neuronal lineage of origin and cortical development, as well as neuronal activity patterns and behavior in a novel pre-clinical DS model.