Blood-oxygenation-level-dependent functional magnetic resonance imaging (BOLD fMRI) is the most widely used functional brain mapping technique in humans. While BOLD serves as an indirect proxy for neuronal activity, the cellular and molecular mechanisms underlying the BOLD signal remain poorly understood. Among various brain cells, astrocytes are central in neurovascular coupling due to their unique anatomical placement between neurons and blood vessels and their ability to release vasoactive factors downstream of G-protein-coupled receptors (GPCRs) signaling pathways. Recent studies selectively modulating astrocytic Gq-GPCR signaling have challenged the involvement of astrocytes in neurovascular coupling, hinting at the possibility of unexplored roles in other GPCR signaling pathways. This controversy, insufficient data, paired with the difficulty to selectively control cellular signaling during fMRI, have impeded our understanding of the biology behind BOLD. By utilizing genetic methods to manipulate neurons and astrocytes at the molecular signaling level, we will empirically define the fractional contribution of neurons and astrocytic GPCRs to BOLD, thus developing a more complete model of the BOLD signal. We project that the information gathered in these studies can clarify a crucial link to improve our understanding of how specific cellular processes generate the macroscopic changes in BOLD signal and build a stronger foundation for human brain mapping. Specifically, we propose to address the following questions:
1. How selective recruitment of specific astrocytic GPCR signaling pathways contribute to BOLD?
2. Can BOLD be generated in brain regions with evoked neuronal activation and silenced astrocytes?
3. Can regional activation of astrocytes elicits BOLD without the involvement of neurons?
4. Why is BOLD altered in diseases characterized by neuroinflammation?
Dissection of BOLD has been challenging due to the inability to selectively manipulate cellular activity during fMRI, as well as the complex, disproportionate susceptibility of BOLD attributed to other hemodynamic contributors such as cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen (CMRO2). Our team is uniquely positioned to address these challenges. We are capable to employ novel chemogenetic tools, namely Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), to selectively manipulate neuronal activity and different astrocytic GPCR signaling cascades (i.e., Gq, Gs, and Gi) during fMRI. We are also experienced with multimodal fMRI tools, thus allowing quantitative modeling of how CBF, CBV, and CMRO2 contribute to BOLD under specific cellular manipulations. This project is in collaboration with Drs. Garret Stuber (http://www.stuberlab.org/), Tom Kash (http://www.kash-lab.org/), and Hongtu Zhu (http://www.bios.unc.edu/research/bias/).