Dynamic Changes and Interactions Between Cerebrospinal Fluid and Neural Progenitors in Early Brain Development

Abstract

Throughout cerebral cortical development, cell-intrinsic genetic programs and cell-extrinsic signals work together to regulate neural progenitor cell survival, proliferation and differentiation. From the earliest stages of forebrain development, one of the major sources of extrinsic signals is the cerebrospinal fluid (CSF), which directly contacts the progenitors along the brain’s ventricular surface. CSF first develops during neurulation, when amniotic fluid (AF) captured inside the neural tube becomes the nascent CSF. However, very little is known about the composition of these fluids and how they interact with the neural progenitors around the time of neurulation. Similarly, not much is known about the intrinsic changes in the neural progenitors at this early developmental stage. In this study, we developed techniques to isolate these early fluids for functional testing. Using mass spectrometry, we revealed how AF and CSF proteomes diverge into distinct fluid compartments from a common origin, and demonstrated that many ribosome constituents are dynamically available in early AF/CSF. We showed that age-matched fluid is needed to support the identity of Sox2-positive neural progenitors, and identified LIF as a signaling factor in early CSF that could regulate progenitors cell fate. Transcriptome analysis of the developing neuroepithelium uncovered a global down-regulation of the ribosomal and translational machinery after neurulation, resulting in decrease in ribosome biogenesis and protein synthesis. We identified MYC as one potential upstream regulator of these processes. MYC expression is high in progenitors before neurulation but down-regulated thereafter. Loss of function experiments confirm previous studies that MYC is important for multiple aspects of development, including neurulation. On the other hand, forced MYC expression in neural progenitors after neurulation leads to increased expression of ribosome constituents, macrocephaly and tumorigenesis. This study deciphers the proteomes and demonstrates the functional, instructive roles of the early fluids in brain development. Paired with the transcriptome and changes in protein synthesis in neighboring neural progenitors, this work opens up avenues for future studies on this important but understudied stage of development, with direct implications on pathologies such as neural tube defects and brain tumor.