Research
Cell Lineage Specification: Cortical interneurons are generated in the embryonic brain from a progenitor domain collectively known as the ganglionic eminences (GE). In addition to cINs (which are themselves highly diverse), the GE produces many different classes of neurons that are involved in diverse brain functions such as motor control, memory formation, sensory processing and fear. We are interested in the molecular factors that regulate GE neuron production. To date, we have completed 2 projects in this area:
The first is the role of extrinsic patterning factors such as Wnt. We recently published a study demonstrating the role of Wnt/Ryk signaling in the generation of the 2 main subclasses of cINs (McKenzie et al., 2019). We have also
The second examines the role of the transcription factor St18. Here, we find that St18 is necessary and sufficient to direct MGE progenitors towards a specific lineage: prototypic projection neurons of the globus pallidus pars externa (Nunnelly et al, BioRxiv).
Stem Cell Directed Differentiation: Both of these studies made use of a model system we employ regularly in the lab – an embryonic stem cell-based model of GE development (Au et al., 2013). With this system, we direct pluripotent stem cells to adopt cIN identity. Over the course of their differentiation process, we can intercede by programming with transcription factors and signaling cascades, which allows us to infer which molecular mechanisms dictate neuronal cell type identity. We use this approach to generate hypotheses that we can test using mouse genetics and then further refine with more ES model experiments. To bridge the divide between ES-derived cINs and mouse brain development, we can introduce ES-derived cINs into host mouse brain using an approach known as ultrasound backscatter microscopy-guided cell transplantation. With this method, ES-derived cINs mature and integrate with host mouse cortex, allowing us to directly link our molecular perturbations with bona fide cell identity.
Cortical Interneuron Migration: cINs originate in the GE but the ultimately end up in the cortex. Thus, interneurons must migrate long distances to somehow arrive at the correct area of the brain prior to forming functional connections. The molecular regulation of this process is largely unknown. We recently examined an aspect of this process by asking the question: what initiates cortical interneuron migration into the cortex? We found that interneuron migration is regulated by external cues provided by blood vessels. During embryonic development, the brain is vascularized in a stepwise fashion and as new blood vessels penetrate the GE, they provide paracrine cues such as SPARC and SerpinE1. We found that these 2 factors regulated the initiation of interneuron migration and that it does so with both mouse and human interneurons. Comparative studies have shown that the rate of maturation in human neurons is protracted versus mice. By treating human interneurons with SPARC and SerpinE1, we were able to accelerate maturation rate of human interneurons, suggesting that the rate of brain vascularization is one of the mechanisms that regulates the rate of maturation across disparate species.
Cortical Interneuron Synaptogenesis: Once cINs have completed migration into the cortex, they form inhibitory synaptic connections throughout the cortex. The means by which this is accomplished is largely unknown. In fact, we don’t even know how many different kinds of connections are formed by cINs. In the mature brain, cINs establish a distributed network that refines cortical signals and regulates cortical oscillations through different forms of inhibition.
Currently, we are working on a system that allows us to analyze interneuron synaptic connections at a population level. By employing this approach, we hope to address some of the aforementioned questions. Among other applications, this will allow us to assess interneuron synapses developmentally and to compare between healthy and diseased brains.