Cell states along oligodendrocyte development and disease

Abstract

The brain, one of the most complex organs in the body, where an immense diversity of cell states emerges from simple structure, where function arises from sets of regulatory principles and pattern persist where individual cells do not. Revealing the regulatory underpinnings of the brain, from unspecified cell states to diversity, is paramount for achieving a thorough understanding of the development process and generating insight into the disease states of the brain. This thesis is an exploration into how canonical regulatory factors and elements, such as transcription factors and genes, lock a regulatory system in a multi-outcome network with limited possible states.

The work in this thesis focuses on the oligodendrocyte lineage, a glial cell known for it’s supportive role in the central nervous system, where it facilitates electrical transmission through the enscheathment of axons. Oligodendrocytes (OLs) lie at the heart of multiple sclerosis (MS), a disease where an immune response is mounted against myelin. As a response, oligodendrocyte precursor cells (OPCs) move towards lesions and remyelinate axons, however, this mechanism fails in later stages of the disease. Thus, an understanding to how OPCs develop is vital to amelioration of the altered oligodendrocyte population. In Paper I we reveal a previously underestimated heterogeneity within the oligodendrocyte lineage in mouse. We show that OL maturation is an ongoing process, albeit, decreasing in frequency with age. Furthermore, complex wheel training in mice revealed that the OLs respond to this challenge through an increase in differentiation. Paper II investigates the cellular response in the experimental autoimmune encephalomyelitis (EAE) disease mouse model of MS, where we find a tailored response by the resident OL population, changed from its normal transcriptional program, expressing a spectrum of genes related to survival, immunological stimulation, phagocytosis, and active differentiation. Furthermore, we provide evidence that OLs can elicit responses from T cells. In Paper III we explore the different waves of OPC generation in the developing mouse brain at embryonic day 13.5 and postnatal day 7. We show that recently Pdgfra expressing cells at the E13.5 time point exhibit a multitude of patterning genes, and we show the emergence of a possible OPC progenitor through the inclusion of a bridging E17.5 time point population. This pre-OPC population is biased towards expressing glial and OL lineage specifying genes such as Olig1, Olig2, Ptprz1, and Bcan. Furthermore, lineage tracing of OPC developmental waves, shows no transcriptional differences, leading us to conclude that OPCs are generally naïve to the time or region of specification. In Paper IV we show that we are able to detect OPC formation in the developing human forebrain. We detect OPCs at the earliest sampled time point post conception week 8. We attempt to recover the path of OPC formation, and investigate the regulatory dynamics in the specification of OPCs.