Tiny but Mighty: The Role of Adipose-derived microRNAs in Chronic Primary Pain Conditions

Abstract

This dissertation investigates the contributions of adipose-derived microRNAs (miRNA) to the onset of chronic primary pain conditions (CPPCs). CPPCs, including temporomandibular disorder, fibromyalgia syndrome, and vestibulodynia, affect over 100 million Americans, predominately women, and pose a significant healthcare challenge. CPPCs arise from genetic and environmental factors that enhance catecholamine tone, potentially through miRNA dysregulation following catecholamine activation of beta-adrenergic receptors. Here, we identify the miR-374 family and miR-133a-3p as biomarkers of CPPC status in human and rodent cohorts and investigate its functions using in vivo and in vitro approaches. Downregulation of miR-374 in plasma is a consistent biomarker in individuals with temporomandibular disorder, fibromyalgia syndrome, or widespread pain following a motor vehicle collision. miR-374 was also downregulated in plasma, white adipose tissue, and spinal cord from mice with multi-site mechanical hypersensitivity. miR-374 downregulation in plasma and spinal cord was female-specific. Norepinephrine stimulation of primary adipocytes, but not DRG, led to decreased miR-374 expression in cultures derived from both sexes. This downregulation correlates with tissue- and sex-specific changes in the expression of predicted miR-374 mRNA targets, including known (HIF1A, NUMB, TGFBR2) and new (ATXN7, CRK-II) pain targets. Finally, miR-374 overexpression in DRG neurons reduced capsaicin-induced nociceptor activity. Downregulation of miR-374 represents an intermediate step between adrenergic receptor activation and mechanical hypersensitivity, and its adipocyte source implicates adipose signaling in nociception. Regarding miR-133a-3p, plasma levels this miRNA are consistently downregulated in humans with ≥1 CPPC (including fibromyalgia syndrome, vestibulodynia, and others) and in rat and mouse models of primary pain. Our data suggest that miR-133a-3p is packaged in extracellular vesicles that are secreted by adipocytes and trafficked to the spinal cord. Activation of adrenergic receptors on white adipocytes results in downregulation of miR-133a-3p which negatively regulates pain-related genes , such as MAP3K3, in the spinal cord that are critical to sensory neuron activation. Adipose-specific overexpression of miR-133a-3p in a mouse model of primary pain reverses mechanical hypersensitivity in both sexes. These findings implicate miR-133a-3p dysregulation in primary pain across conditions and species and establish its novel role in multi-site mechanical hypersensitivity. Together, this work links the fields of pain etiology, miRNA regulation, and adipose endocrinology to generate new hypotheses about the origins of CPPCs. Specifically, this dissertation proposes a new working model of CPPC onset whereby catecholamine signaling in adipose tissue causes the downregulation of miRNAs. These miRNAs, when packaged in extracellular vesicles that are secreted into circulation and taken up by the central nervous system, regulate gene networks related to pain and inflammation to promote mechanical hypersensitivity. Manipulation of miRNA expression directly alters sensory neuron activity in vitro and pain-like behaviors in vivo. These new insights will guide the design of future experiments that may help the development of gene therapies with the potential to improve conditions for the millions worldwide who live with CPPCs.