Glial-Glial Interactions

During development, billions of neurons and trillions of glia are specified, migrate to precise locations, interact in a highly orchestrated manner, and ultimately differentiate into morphologically and functionally unique cells that create our nervous system. This allows for the assembly of a robust nervous system hard-wired enough to allow for life sustaining activities like locomotion and reproduction but equally as plastic to allow for learning and repair. Despite our broad knowledge of the major processes that drive neural development, our understanding of how these trillions of cells interact and coordinate their developmental processes is still very superficial. Historically, studies have mainly focused on neuron-glia interactions. However, given the number of glia in the nervous system and emerging studies demonstrating interactions between both homogenous and heterogenous types of glia, we are intensely interested in characterizing glial-glial interactions during nervous system assembly. Understanding the full repertoire of the cellular and molecular mechanisms that engineer vertebrate nervous systems will not only provide an extraordinary opportunity to uncover novel mechanisms that build and regenerate nervous systems, it will also lead to a more fundamental understanding of how glial-glial interactions impact neurodevelopment and provide new insights into mechanisms that may be perturbed in disease or harnessed for future therapeutic targets.

Glial Tiling

Important to nervous system development is the process of glial tiling (or spacing). We define glial tiling as the process by which glial cells coordinate their development so that they occupy discrete, non-overlapping territories with neighboring glia. Tiling itself is an emergent behavior which includes the processes of migration, a balance of proliferation and apoptosis, and cell “contact” that can be mediated directly via contact-mediated repulsion (CMR), or indirectly via cell signaling. Previous studies have mostly investigated tiling in neuronal populations. However, glia also tile and this occurs between glia found in the CNS, PNS, or between glial cells where one cell is in the CNS and the other is in the PNS. My lab is very interested in glial tiling because of our work investigating the selective migration of glia across nervous system transition zones (TZs). However, we still only appreciate the tip of the iceberg when it comes to the cellular and molecular mechanisms that drive this behavior, what role local spacing plays in the global distribution of glia during development and disease/injury, and if perturbations to glial tiling underlie neurodevelopmental disorders. The studies we are doing will provide insight into how glial cells interact and tile under physiological conditions in both the CNS and at MEP TZs and provide avenues for future discovery to determine if these events are perturbed in disease/after injury.

Perineurial Glial Development

Nervous system TZs are the unique locations in the nervous system where the CNS and PNS meet. In vertebrates, these structures are found at both the cranial and trunk level, and in the spinal cord, TZs come in two flavors: 1) motor exit point (MEP) TZs, where motor axons exit the CNS and make connections with peripheral targets, including somatic muscle, and 2) dorsal root entry zones (DREZ), where sensory axons from peripheral neurons enter the CNS. TZ boundaries are delineated by a layer of astrocytic endfeet that are continuous with the glia limitans with the exception of perforations that historically, were thought to be only permissible to motor and sensory axons. Recent work shows that MEP TZs are far more dynamic than previously believed, and we are still fascinated by a class of glia integral to these structures known as perineurial glia, which exit the spinal cord and differentiate into the mature perineurium, a component of the blood-nerve-barrier. 

---

Glial Diversity

Assembly of an efficient vertebrate nervous system requires that axons be myelinated in both the central and peripheral nervous systems (CNS and PNS, respectively) by myelinating glia during development. The process of myelination is complex, and we know that it involves a series of orchestrated mechanisms that govern both axon-glial and glial-glial interactions. However, despite knowing several distinct mechanisms that direct myelination in both the CNS and PNS, we likely still don’t have a complete picture of all the cellular and molecular mediators of this essential process. Identifying additional pathways (and cells) involved in regulating myelination will provide a deeper understanding of the fundamental rules that build vertebrate nervous systems as well as reveal possible novel therapeutic avenues for treating human disorders where myelination is perturbed. To identify novel mediators of myelination in the developing nervous system, we are taking a very broad and unbiased approach. In the last decade, a significant number of publicly available single-cell RNA-sequencing (scRNAseq) and proteome datasets from zebrafish, mouse, and human that include oligodendrocyte lineage cells (OLs) and Schwann cells (SCs) have been published. And yet, despite this enormous amount of data, it has sat idle with relatively few studies using it to discover new glial biology. We have decided to use these vast resources and mine for conserved genes that could be putative regulators of developmental myelination and lead to the discovery of new subpopulations of glia.

Glial-Mediated Debris Clearance

During neural development, billions of neurons and glia are specified, migrate to specific locations, interact, and differentiate into morphologically unique cells. These developmental events form a robust system of neural circuits that is hard-wired enough to allow for life sustaining activities like breathing but is equally as plastic to allow for learning and repair. Intriguingly, during these early developmental processes, the nervous system is not simply assembled. Instead, it is precisely sculpted from an excess of cells and cellular processes that must be efficiently removed. Despite our understanding that extraneous cells and cellular processes must be cleared during neurodevelopment, the importance of this phenomenon and the cellular and molecular mechanisms that govern it are still not fully understood. We are very interested in the role of glia as non-professional phagocytes and the role of debris clearance during early nervous system assembly.

Myelin Targeting, Growth, and Refinement

Efficient vertebrate nervous systems require the precise insulation of central nervous system (CNS) axons by myelinating oligodendrocytes. However, we know very little about how axons are selected for myelination and why some CNS structures are avoided. We set out to investigate these questions and identified several genes that regulate CNS myelin targeting and growth. Using novel zebrafish CRISPR mutants, in vivo, time-lapse imaging, and proximity proteomics, we've set our sights on elucidating how OLs myelinate axons which will lead to new avenues that may be harnessed for future therapeutics for diseases like multiple sclerosis where appropriate new myelin targeting is a goal.

---