Project 1: Neural regulation of marrow adipose tissue

Figure 1. Specialized imaging techniques for neural regulation of marrow adiposity. (A) Osmium tetroxide staining of marrow fat (dark grey) overlaid on bone (light grey) showing marrow adipose tissue (MAT) distribution in a mouse tibia. MAT in the proximal tibia is depleted in response to intracerebroventricular injection of leptin (right vs left). (B) Imaging of sensory nerves (red) and sympathetic nerves (green) in the periosteum. (C) Multilocular MAT adipocytes form in response to the β3-adrenergic agonist CL316,243. Confocal imaging, nuclei in blue, perilipin+ cells in red. (D) Three-dimensional electron microscopic imaging of the MAT microenvironment. MAT lipid droplet in blue, mitochondrial network in green.

Figure 2. Neuropathy as a component of bone loss in diabetes. (A) Severe periodontal disease in a 12 year old patient with minimal plaque  and type 1 diabetes mellitus. Images courtsey of Dr. Juan Schilling, University of Talca, Chile. (B) Schematic of CGRP+ nerve fibers in the periosteum, cortical bone, and bone marrow from Mach et al. Neuroscience, v114(1), 2002.  

The skeleton and bone marrow are extensively innervated by both somatosensory afferent and sympathetic efferent nerve fibers. Despite their prevalence, the role of nerves in bone remains almost entirely unknown. In addition to blood and bone-forming units, the skeleton contains a third population of metabolically active cells – the marrow adipose tissue (MAT). Until recently, MAT was thought to be a passive, space-filler within the marrow. However, we now realize that, like white adipose tissue (WAT), MAT is dynamically regulated in metabolic diseases including diabetes and lipodystrophy and is capable of contributing to circulating adipokines in times of caloric need. Though there is significant precedent for neural regulation of WAT, the regulation of MAT by the skeleton’s neural supply remains unstudied. We recently found that both sympathetic and sensory nerve endings within the bone marrow cluster around a population of marrow adipocytes. We hypothesize that both sympathetic and sensory neurotransmission are necessary for coordination of marrow adipose tissue turnover and that marrow adipose tissue undergoes differential innervation and connection of peripheral nerves to central sites, depending on skeletal site, with significant downstream consequences for regulation of MAT in the context of local and systemic metabolic homeostasis

The extracellular matrix (ECM) establishes local signaling gradients in both time and space by sequestering or presenting signaling molecules, like growth factors, to their cell surface receptors. The mechanical rigidity of the matrix mediates cell spreading and motility. In bone, the ECM directs the process of mineralization by providing a scaffold for hydroxyapatite deposition. Despite its importance, the composition and organization of the ECM within the bone marrow remains poorly understood. The goal of this project is to characterize the supramolecular structures that create niches specialized for either hematopoiesis, marrow adipose tissue homeostasis and bone remodeling. 

Diabetes, a major risk factor for the development of periodontal disease, has increased in prevalence by 400% since 1980. Of the 25 million diabetic patients in the United States, it is estimated that 7.5 million have severe periodontitis. It is generally accepted that the severity of periodontal disease is correlated with the presence of inflammation. However, inflammation fails to completely explain the rapid progression of periodontal disease, generalized skeletal bone loss, and marrow fat accumulation that can be observed in young insulin-dependent diabetics with low plaque levels and good oral hygiene. Therefore, it is likely that other mechanisms are mediating the coupling of bone loss and diabetes. Since 1967 it has been repeatedly noted that clinical neuropathy is correlated with both the prevalence and severity of periodontal disease in diabetic patients. The skeleton and periodontal complex are highly innervated and local changes in nerve function have the capacity to regulate both the bone microenvironment and the overlying inflammatory response. However, despite significant correlative evidence linking neuropathy and periodontal disease in diabetes, the ability of local neuropathic change to contribute to bone loss, marrow fat accumulation and periodontal disease development in diabetes remains unexplored. Our central hypothesis is that in diabetes, sensory denervation and depletion of sensory neuropeptides in the skeleton enhances vasoconstriction and accumulation of marrow fat while limiting bone regeneration. These changes would predispose diabetic patients to development of osteopenia and progression of periodontal disease. 

Figure 3. Microfibril-associated glycoprotein-1 (MAGP1) is a small 20 kDa matricellular (non-structural) component of the ECM. Incredibly, deletion of MAGP1 alone is sufficient to cause severe physiological complications. MAGP1-deficient mice become obese, diabetic, and osteopenic. (A) Immunofluroescence of MAGP1 (green) assembled into fibers in the ECM of primary stromal cells isolated from mouse adipose tissue. (B) Image showing increased adiposity in MAGP1-deficient mouse, relative to control mouse. (C) Microcomputed tomography showing increased marrow fat volume in the bones of MAGP1-deficient mice. (D) Skeleton of MAGP1-deficient mouse with arrows highlighting spontaneous fractures.     

Project 3: Extracellular Matrix

Project 2: Diabetic neuropathy and skeletal disease