Jinhu Xiong - US grants

With National Science Foundation support, Dr. Xiong will develop imaging and modeling strategies to study mechanisms underlying adaptive changes of the human brain. The focus of this proposal is to explore the mechanisms underlying motor learning. Learning-induced neural plasticity and functional reorganization are well-established and well-documented, but not well-understood. Current neuroimaging studies investigate neural mechanisms underlying learning by exploring the changes in regional neural activity and inter-regional activity of task-performance. Little effort has been given to studying the more fundamental changes of neural connections and synaptic weighting. On the technical front, human functional imaging research sorely needs more rigorous approaches, as can be provided by mathematical modeling. A modeling framework - Structural Equation Modeling - is now accepted as appropriate for human imaging data. Structural equation modeling however, is currently performed with anatomical constraints based on neuroanatomical studies in non-human species. The performance of structural equation modeling might be greatly enhanced if anatomical constraints are individually optimized using the same subject's task-independent anatomical connectivity data. To date, this strategy has not been reported by any laboratory. The present proposal seeks to develop system-level modeling strategies for neuroimaging and to apply these novel strategies to mechanisms of action of motor learning. The overall goal of this proposal will be accomplished through the following four goals. First, developing and optimizing imaging strategies for detecting anatomical connectivity for each individual subject. Second, developing a structural equation modeling strategy by incorporating individual anatomical constraints to enhance those models' performance. Third, investigating changes in regional neural activity and inter-regional activity of task-performance induced by motor learning using the enhanced modeling strategy. Fourth, investigating synaptic plasticity by applying the enhanced modeling and demonstrating that synaptic plasticity is an underlying mechanism of action of motor learning. When completed, this research project will increase the understanding of mechanisms of adaptive learning and has the potential of defining a new strategy by which functional imaging can be applied to study mechanisms of action and disease pathophysiology.

DESCRIPTION (provided by applicant): Direct MRI mapping of neuronal magnetic fields in the human brain Functional MRI technique has greatly enhanced our understanding of the functional organization of the human brain. Currently used fMRI techniques, however, depend on measuring regional cerebral hemodynamics to infer neural activation, rather than detecting neuronal activity directly. This indirect measurement has several limitations. First, regional cerebral hemodynamics does not necessarily always reflect neuronal activity and could change (for example, drug effects) without underlying neuronal activity change. Second, vascular geometry may not always overlap with the area of neural firing, so that the mediation of regional cerebral hemodynamics may degrade spatial localization. Third, the cerebral hemodynamics responses are much slower (seconds) than neuronal firing (milliseconds). Temporal resolution of the hemodynamic measurement is, therefore, limited and downgraded with respect to the underlying neural activation. To address shortcomings of current fMRI techniques, we reported a novel fMRI technique, magnetic source MRI (msMRI), for directly assessing neuronal function at 2003. The technique is based on directly detecting MRI signal changes in response to the changes in magnetic fields concomitant with neuronal firing and offers improved spatial localization and temporal resolution. While it offers promise, msMRI is still at its early developmental stage. Controversial results have been reported. The overall objectives of this developmental proposal are then to study mechanisms of signal contrast in msMRI and to clarify the controversies. Theoretical modeling will be performed to study mechanisms of msMRI and characteristic spatial and temporal signatures of msMRI signals (Aim1). The characteristic temporal signature of msMRI signals will be investigated by demonstrating that msMRI has high temporal resolution and can accurately detect the timing of both stimulation onset and offset (Aim2). Characteristic spatial signatures of msMRI signal will be investigated by demonstrating unique and different spatial distributions for phase and magnitude images (Aim3). Unique relationships between msMRI signals and experimental parameters will be explored (Aim4). Finally, the distinct sensitivity of msMRI signals to a symmetry SE sequence will be investigated (Aim5). Successful completion of the current project will enhance our understanding of mechanisms of signal contrast in msMRI;clarify the controversy surrounding msMRI detections;and provide a solid background for future developments, optimizations, and applications of the msMRI technique. PUBLIC HEALTH RELEVANCE: The overall objectives of this developmental proposal are to study mechanisms of signal contrast in magnetic source magnetic resonance imaging (msMRI) and to develop msMRI procedures for mapping human brain functions. Successful completion of the current project will enhance our understanding of mechanisms of signal contrast in msMRI;clarify the controversy surrounding msMRI detections;and provide a solid background for future developments, optimizations, and applications of the msMRI technique

PROJECT SUMMARY/ABSTRACT Unbalanced bone remodeling often causes bone loss and leads to osteoporosis. Wnt/?-catenin signaling is essential for commitment and differentiation of bone forming osteoblasts from mesenchymal progenitors. Changes in mechanical loading can also influence bone formation. Osteocytes, former osteoblasts buried in bone matrix, are thought to be the cells that perceive mechanical signals and orchestrate bone remodeling. However, the molecular mechanisms by which osteocytes perceive and transduce mechanical signals are not fully understood. YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ-binding motif), two related transcriptional co-factors that shuttle between the cytoplasm and the nucleus, have emerged as potentially important transcriptional regulators in mechanotransduction. Their activity is dependent on their subcellular localization, which is tightly regulated by different extracellular cues. Cytoplasmic YAP and TAZ promote proteosomal degradation of ?-catenin and thereby inhibit Wnt signaling. Mechanical signals emanating from rigid extracellular matrix or from fluid flow promote YAP and TAZ translocation into nucleus where they stimulate transcription. Our preliminary studies show that deletion of YAP and TAZ from osteoblast progenitors increased osteoblast formation in mice, and this was associated with increased Wnt signaling. In contrast, deletion of YAP and TAZ from differentiated osteoblasts and osteocytes decreased osteoblast number and bone formation. These results suggest that YAP and TAZ perform different functions in mesenchymal progenitors versus mature osteoblasts and osteocytes. Based on this, we hypothesize that the less rigid environment of osteoblast progenitors retains YAP and TAZ in the cytoplasm, promotes ?-catenin degradation, and thereby inhibits osteoblast differentiation. In contrast, the more rigid environment of osteoblasts and osteocytes, together with other mechanical inputs (such as fluid shear), favors nuclear retention of YAP and TAZ and thereby increases osteoblast number. To test this hypothesis, we will determine whether YAP and TAZ in osteoblast progenitors inhibit their differentiation by promoting ?-catenin degradation using genetically modified mice in which YAP will be restricted to the cytoplasm or the nucleus (Aim 1). In addition, we will examine whether YAP and TAZ expression in osteocytes mediates the effects of changes in mechanical loading on osteoblast number using tail suspension and cyclic compression models (Aim 2). Further, we will perform in vitro studies to identify YAP and TAZ target genes in osteoblasts and osteocytes that are responsible for their effects on osteoblast number using genome-wide mRNA expression analysis (RNA-seq) in osteoblastic and osteocytic cells before and after suppression of the endogenous YAP and TAZ genes (Aim 3). Successful completion of these studies will shed light on novel mechanisms that control bone formation and will advance knowledge of how the skeleton responds to changes in mechanical load.

PROJECT SUMMARY/ABSTRACT Mechanical stimuli promote bone growth and are critical for skeletal homeostasis during adulthood. Loss of mechanical signals decreases bone mass and increases fracture risk. Osteocytes, which are cells buried in the bone matrix and derived from osteoblasts, are able to sense changes in mechanical load and orchestrate bone remodeling. Several lines of evidence suggest that calcium channels are involved in the sensing of mechanical load by osteocytes. For example, calcium influx is one of the earliest responses of osteocytes to mechanical stimuli in vitro and in vivo. Consistent with a functional role for calcium signaling in the response to mechanical forces, the response of osteocytes to mechanical stimuli can be inhibited by blocking calcium channels using chemical blockers. Moreover, load-induced bone formation in the rat ulna is significantly blunted by calcium channel inhibitors. However, the identity of the calcium channels activated by mechanical forces and their functional role as mechanosensors in bone remain unclear. We have found that Piezo1 calcium channel is highly expressed in osteocytes, and that its expression and activity are increased by mechanical stimulation in osteocytes. In addition, deletion of Piezo1 in osteoblasts and osteocytes decreases both bone mass and bone strength in mice, consistent with loss of skeletal responsiveness to mechanical stimulation. Moreover, the skeletal response to anabolic loading is blunted in mice lacking Piezo1 in osteoblasts and osteocytes. Wnt1, a ligand for Wnt signaling that is known to be upregulated by mechanical signals and stimulate bone formation, is downregulated in Piezo1 conditional knockout mice. Importantly, activation of Piezo1 by its chemical agonist, Yoda1, mimics the effects of fluid flow on osteocytes and increases bone mass in mice. Based on this evidence, we hypothesize that osteocytes sense changes in mechanical signals through Piezo1 and thereby promote bone formation in part by activating signaling pathways that increase the expression of Wnt1. To test this hypothesis, we will determine whether Piezo1 expression by osteocytes is required for mechanical sensing in the murine skeleton. We will generate mice in which Piezo1 is deleted from osteocytes, but not osteoblasts, and compare their skeletal phenotype to that observed in mice lacking Piezo1 in osteoblasts and osteocytes. We also will delete Piezo1 postnatally in adult mice and investigate their response to mechanical loads by tibia compression (Aim 1). In addition, to understand how Piezo1 promotes bone formation, we will determine the role of Wnt1 in Piezo1-mediated bone formation in vivo using a mouse genetic approach (Aim 2). In Aim 3, we will determine whether Piezo1 is responsible for the skeletal response to unloading using a tail-suspension model. Lastly, we will determine whether pharmacological activation of Piezo1 prevents bone loss associated with unloading or increases bone mass in old mice. Successful completion of this work should establish a new model for understanding the skeletal response to anabolic mechanical loading and may suggest new strategies to develop anabolic therapies for bone loss related to disuse or aging.