The Effects of Environmental Adversities on Human Neocortical Neurogenesis Modeled in Brain Organoids

Abstract

Over the past decades, a growing body of evidence has demonstrated the impact of prenatal environmental adversity on the development of the human embryonic and fetal brain. Prenatal environmental adversity includes infectious agents, medication, and substances of use as well as inherently maternal factors, such as diabetes and stress. These adversities may cause long-lasting effects if occurring in sensitive time windows and, therefore, have high clinical relevance. However, our knowledge of their influence on specific cellular and molecular processes of in utero brain development remains scarce. This gap of knowledge can be partially explained by the restricted experimental access to the human embryonic and fetal brain and limited recapitulation of human-specific neurodevelopmental events in model organisms. In the past years, novel 3D human stem cell-based in vitro modeling systems, so-called brain organoids, have proven their applicability for modeling early events of human brain development in health and disease. Since their emergence, brain organoids have been successfully employed to study molecular mechanisms of Zika and Herpes simplex virus-associated microcephaly, as well as more subtle events happening upon maternal alcohol and nicotine consumption. These studies converge on pathological mechanisms targeting neural stem cells. In this review, we discuss how brain organoids have recently revealed commonalities and differences in the effects of environmental adversities on human neurogenesis. We highlight both the breakthroughs in understanding the molecular consequences of environmental exposures achieved using organoids as well as the on-going challenges in the field related to variability in protocols and a lack of benchmarking, which make cross-study comparisons difficult.

Keywords: brain organoid; corticogenesis; environmental programming; neural stem/progenitor cells; neurogenesis.

FIGURE 1

Scope of the review. Environmental adversities affect the pregnant woman and may pass through a series of biological barriers to the embryonic/fetal brain causing neurogenic defects. Purple color represents the maternal compartment and red color corresponds to the embryo/fetus.

FIGURE 2

(A) Cellular view of the normal human neocortical development. Human neocortical development starts from neuroepithelial cells (NEC), which further transform to ventricular radial glia (vRG). Both NEC and vRG have nuclei next to the ventricle (neuroeplithelium (NE) and ventricular zone (VZ), respectively) and a basal process spanning through the cortical wall and marginal zone (MZ) to reach the pial surface. vRG can either divide symmetrically to generate two vRGs or differentiate to produce excitatory neurons or basal neural progenitor cells. Basal progenitor cells reside in the inner (iSVZ) and outer (oSVZ) subventricular zones and include intermediate progenitor cells (IPC) and outer radial glia (oRG). Both types of basal neural progenitor cells can self-amplify or differentiate into excitatory neurons. Later in development, vRG lose connection to the pial surface and transform into truncated radial glia (tRG) while still generating basal progenitors and excitatory neurons. Newborn neurons of different origins migrate along the radial fibers through the intermediate zone (IZ) and subplate (SP) and take their place in the cortical plate (CP). A low level of cell death in neural cells is normal over neurogenesis. (B) Brain organoids infected with Zika virus reveal cellular mechanisms of microcephaly. B’. Multiple studies suggest preferential infection of neural progenitor cells over neurons. Infection in vRG leads to the apoptotic cell death, attenuation of proliferation as well as shift to direct neurogenesis. (B’’) upper panel. Zika virus (ZIKV) activates Toll-like receptor 3 in vRGs that leads to transcriptional deregulation with activation of apoptotic pathways and inhibition of regulators of neurogenesis. Adapted from Dang et al. (2016). (B’’) lower panel. NS2A, a protein of ZIKV envelop, binds the components of adherens junctions (AJ) in the cytoplasm and prevents them from forming functional AJ and apical polarity complex. Reprinted from Yoon et al. 2017 with permission from Elsevier. Copyright (2017). (C) Hypoxic exposure in brain organoids at different time points results in distinct defects on the cellular level. (C’) Hypoxic exposure (1% O₂, 72 h) at day 10 of organoid differentiation results in the increased cell death, presumably, of NECs. (C’’) When hypoxia (3% O₂, 24 h) is applied to brain organoids at day 28, it results in the immediate cell death across the cortical wall followed by proliferation in vRG. Subsequently, vRG tend to differentiate into neurons at the expense of generation of both IPCs and oRG, which results in the decreased number of these cells 14 days after hypoxic exposure. (C’’’) Cortical spheroids that are exposed to hypoxia (1% O₂, 48 h) at day 75 of differentiation show decreased numbers of IPCs resulting from premature differentiation but not from cell death in these cells.