Refining the identity of mesenchymal cell types associated with murine periosteal and endosteal bone.

Single-cell RNA-seq has led to novel designations for mesenchymal cells associated with bone as well as multiple designations for what appear to be the same cell type. The main goals of this study were to increase the amount of single-cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. We created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-Cxcl12 abundant reticular (CAR), osteo-CAR, preosteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by periostin expression. Osteo-X, osteo-CAR, and preosteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in preosteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single-cell RNA-seq datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

Piezo1 stimulates mitochondrial function via cAMP signaling.

Mechanical signals stimulate mitochondrial function but the molecular mechanisms are not clear. Here, we show that the mechanically sensitive ion channel Piezo1 plays a critical role in mitochondrial adaptation to mechanical stimulation. The activation of Piezo1 induced mitochondrial calcium uptake and oxidative phosphorylation (OXPHOS). In contrast, loss of Piezo1 reduced the mitochondrial oxygen consumption rate (OCR) and adenosine triphosphate (ATP) production in calvarial cells and these changes were associated with increased expression of the phosphodiesterases Pde4a and lower cyclic AMP (cAMP) levels. In addition, Piezo1 increased cAMP production and the activation of a cAMP-responsive transcriptional reporter. Consistent with this, cAMP was sufficient to increase mitochondrial OCR and the inhibition of phosphodiesterases augmented the increase in OCR induced by Piezo1. Moreover, the inhibition of cAMP production or activity of protein kinase A, a kinase activated by cAMP, prevented the increase in OCR induced by Piezo1. These results demonstrate that cAMP signaling contributes to the increase in mitochondrial OXPHOS induced by activation of Piezo1.

New Advances in Osteocyte Mechanotransduction.

PURPOSE OF REVIEW: Skeletal adaptation to mechanical loading plays a critical role in bone growth and the maintenance of bone homeostasis. Osteocytes are postulated to serve as a hub orchestrating bone remodeling. The recent findings on the molecular mechanisms by which osteocytes sense mechanical loads and the downstream bone-forming factors are reviewed. RECENT FINDINGS: Calcium channels have been implicated in mechanotransduction in bone cells for a long time. Efforts have been made to identify a specific calcium channel mediating the skeletal response to mechanical loads. Recent studies have revealed that Piezo1, a mechanosensitive ion channel, is critical for normal bone growth and is essential for the skeletal response to mechanical loading. Identification of mechanosensors and their downstream effectors in mechanosensing bone cells is essential for new strategies to modulate regenerative responses and develop therapies to treat the bone loss related to disuse or advanced age.

Characteristics of Invasive Breast Cancer Detected by Digital Breast Tomosynthesis on Screening and Diagnostic Mammograms.

OBJECTIVE: To determine whether there is added benefit for 3D mammography in the context of screening and diagnostic imaging, particularly relating to known prognostic characteristics, including histopathology, receptor status, and axillary lymph node involvement. METHODS: An institutional review board-approved retrospective review was performed of our mammography and pathology databases from October 2012 to May 2015 to identify biopsy-proven invasive breast carcinoma detected on screening and diagnostic mammograms by 2D plus 3D (2D + 3D) imaging. Percentages of cancer detection by 2D and 3D were compared. Correlation with histopathology and lymph node status was analyzed. RESULTS: Of 53 cancers diagnosed on 12 543 screening mammograms, 36 (67.9%) were better visualized on 3D (not visualized, equivocal, or only seen in retrospect on 2D). Of the 62 cancers diagnosed on 4090 diagnostic mammograms, 24 (38.7%) cancers were better detected on 3D. A statistically significant greater number of cancers were better detected on 3D in the screening compared to the diagnostic mammograms (67.9% vs 38.7%, P < .05). A significantly higher frequency of less aggressive tumors (grade I and grade II, positive estrogen/progesterone receptor, Her2 negative) was detected by 3D, with higher significance in the screening population. Additionally, there was a higher frequency of positive axillary lymph nodes in cancers detected by 3D in the screening group. CONCLUSION: Three-dimension increases invasive breast cancer detection, particularly pathologically less aggressive tumors, in both screening and diagnostic mammograms with more benefit for the screening population. Three-dimensional mammography detected more breast cancer associated with metastatic axillary lymph nodes in the screening population.

Imaging Features of Nonmalignant and Malignant Architectural Distortion Detected by Tomosynthesis.

OBJECTIVE: The objective of our study was to determine the ability of tomosynthesis (3D) to detect nonmalignant and malignant architectural distortion (AD) on 3D screening mammograms compared with digital mammography (2D) only and to correlate the 3D imaging features of nonmalignant and malignant AD with pathology findings. MATERIALS AND METHODS: For this single-institution retrospective study, screening mammography reports from October 1, 2012, to December 1, 2016, that included AD as a finding were reviewed. Associated additional imaging studies and pathology results were also reviewed. RESULTS: Three-dimensional mammography showed statistically significant increased detection of both nonmalignant and malignant AD compared with 2D only (0.10% [24/24,902 examinations] vs 0.01% [1/9470 examinations], p < 0.05; and 0.21% [52/24,902 examinations] vs 0.07% [7/9470 examinations], p < 0.05, respectively). Higher percentages of nonmalignant AD (16%) were occult on ultrasound compared with malignant AD (3%). The pathologic diagnoses of nonmalignant AD included radial scar (42%), sclerosing adenosis (16%), stromal or dense fibrosis (16%), and other miscellaneous benign causes (25%). Morphologically, nonmalignant AD was more likely to show symmetric or spoke-wheel spiculation appearance (58% vs 2%, p < 0.05) and central lucency (25% vs 0%, p < 0.05) than malignant AD, whereas malignant AD was more likely to show asymmetric spiculation (98% vs 42%, p < 0.05) and central mass 60% vs 0%, p < 0.05) than nonmalignant AD. CONCLUSION: Malignant AD and nonmalignant AD are more readily detected by 3D mammography than 2D mammography. Three-dimensional imaging features of AD can help to distinguish nonmalignant types in which symmetric or spoke-wheel spiculation with central lucency are more often seen and are more often occult on ultrasound.

RANKL (Receptor Activator of NFκB Ligand) Produced by Osteocytes Is Required for the Increase in B Cells and Bone Loss Caused by Estrogen Deficiency in Mice.

The cytokine receptor activator of NFκB ligand (RANKL) produced by osteocytes is essential for osteoclast formation in cancellous bone under physiological conditions, and RANKL production by B lymphocytes is required for the bone loss caused by estrogen deficiency. Here, we examined whether RANKL produced by osteocytes is also required for the bone loss caused by estrogen deficiency. Mice lacking RANKL in osteocytes were protected from the increase in osteoclast number and the bone loss caused by ovariectomy. Moreover, these mice did not exhibit the increase in bone marrow B lymphocytes caused by ovariectomy that occurred in control littermates. Deletion of estrogen receptor α from B cells did not alter B cell number or bone mass and did not alter the response to ovariectomy. In addition, lineage-tracing studies demonstrated that B cells do not act as osteoclast progenitors in estrogen-replete or estrogen-deficient mice. Taken together, these results demonstrate that RANKL expressed by osteocytes is required for the bone loss as well as the increase in B cell number caused by estrogen deficiency. Moreover, they suggest that estrogen control of B cell number is indirect via osteocytes and that the increase in bone marrow B cells may be a necessary component of the cascade of events that lead to cancellous bone loss during estrogen deficiency. However, the role of B cells is not to act as osteoclast progenitors but may be to act as osteoclast support cells.

Cortical bone loss caused by glucocorticoid excess requires RANKL production by osteocytes and is associated with reduced OPG expression in mice.

Glucocorticoid excess is a major cause of low bone mass and fractures. Glucocorticoid administration decreases cortical thickness and increases cortical porosity in mice, and these changes are associated with increased osteoclast number at the endocortical surface. Receptor activator of NF-κB ligand (RANKL) produced by osteocytes is required for osteoclast formation in cancellous bone as well as the increase in cortical bone resorption caused by mechanical unloading or dietary calcium deficiency. However, whether osteocyte-derived RANKL also participates in the increase in bone resorption caused by glucocorticoid excess is unknown. To address this question, we examined the effects of prednisolone on cortical bone of mice lacking RANKL production in osteocytes. Prednisolone administration increased osteoclast number at the endocortical surface, increased cortical porosity, and reduced cortical thickness in control mice, but none of these effects occurred in mice lacking RANKL in osteocytes. Prednisolone administration did not alter RANKL mRNA abundance but did reduce osteoprotegerin (OPG) mRNA abundance in osteocyte-enriched cortical bone. Similarly, dexamethasone suppressed OPG but did not increase RANKL production in cortical bone organ cultures and primary osteoblasts. These results demonstrate that RANKL produced by osteocytes is required for the cortical bone loss caused by glucocorticoid excess but suggest that the changes in endocortical resorption are driven by reduced OPG rather than elevated RANKL expression.

Detection and Clinical Significance of Sternal Lesions on Breast MRI.

The purpose of this study is to characterize sternal lesions detected on breast magnetic resonance imaging (MRI), compare MRI detection of sternal lesions with other imaging modalities (bone scan, positron emission tomography/computed tomography (PET/CT) and chest CT), and ascertain how often patient management is altered by discovery of sternal lesions. Retrospective review of 1143 breast MRIs between 2007 and 2012 identified 17 patients with sternal lesions including 15 patients with newly diagnosed breast cancer and two patients with remote history of breast cancer. Tumor size, histopathology, receptor status, nodal and distant metastasis, and images of breast MRI, and other modalities were reviewed. Sternal lesions in 9 of the 17 patients were determined to be malignant (metastasis) either by biopsy or presence of widespread metastases. Sternal lesions in 8 of the 17 were benign, confirmed by biopsy or presumed benign as not detected by other modalities. The malignant group had statistically significant larger breast cancer size (malignant: 6.4 cm; benign: 2.3 cm), a higher percentage of diffuse sternal lesions (malignant: 56%; benign: 0%), and more frequently showed rapid initial enhancing (malignant: 100%; benign: 63%) and delayed washout curves (malignant: 67%; benign: 13%). Although not statistically significant, the malignant group had a higher frequency of invasive lobular carcinoma (malignant: 44%; benign: 13%) and more lymph node involvement (malignant: 78%; benign: 50%). Breast MRI detected more sternal lesions than did bone scan, PET/CT and chest CT. Four of the 17 (24%) patients were upgraded to stage 4 due to unsuspected metastatic sternal lesions on breast MRI. In conclusion, breast MRI is more sensitive than other modalities in detecting sternal lesions. Sternal metastases occur more frequently in aggressive breast cancer and exhibit malignant-type dynamics on breast MRI. Detection of unsuspected sternal metastasis alters staging and improves patient management with more appropriate treatment.

Suppression of autophagy in osteocytes mimics skeletal aging.

Bone mass declines with age but the mechanisms responsible remain unclear. Here we demonstrate that deletion of a conditional allele for Atg7, a gene essential for autophagy, from osteocytes caused low bone mass in 6-month-old male and female mice. Cancellous bone volume and cortical thickness were decreased, and cortical porosity increased, in conditional knock-out mice compared with control littermates. These changes were associated with low osteoclast number, osteoblast number, bone formation rate, and wall width in the cancellous bone of conditional knock-out mice. In addition, oxidative stress was higher in the bones of conditional knock-out mice as measured by reactive oxygen species levels in the bone marrow and by p66(shc) phosphorylation in L6 vertebra. Each of these changes has been previously demonstrated in the bones of old versus young adult mice. Thus, these results demonstrate that suppression of autophagy in osteocytes mimics, in many aspects, the impact of aging on the skeleton and suggest that a decline in autophagy with age may contribute to the low bone mass associated with aging.

A high performance 3D cluster-based test of unsmoothed fMRI data.

Cluster-size tests (CST) based on random field theory have been widely adopted in fMRI data analysis to detect brain activation. However, most existing approaches can be used appropriately only when the image is highly smoothed in the spatial domain. Unfortunately, spatial smoothing degrades spatial specificity. Recently, a threshold-free cluster enhancement technique was proposed which does not require spatial smoothing, but this method can be used only for group level analysis. Advances in imaging technology now yield high quality high spatial resolution imaging data in single subjects and an inference approach that retains the benefits of greater spatial resolution is called for. In this work, we present a new CST with a correction for voxelation to address this problem. The theoretical formulation of the new approach based on Gaussian random fields is developed to estimate statistical significance using 3D statistical parametric maps without assuming spatial smoothness. Simulated phantom and resting-state fMRI experimental data are then used to compare the voxelation-corrected procedure to the widely used standard random field theory. Unlike standard random field theory approaches, which require heavy spatial smoothing, the new approach has a higher sensitivity for localizing activation regions without the requirement of spatial smoothness.

Application of mean-shift clustering to blood oxygen level dependent functional MRI activation detection.

BACKGROUND: Functional magnetic resonance imaging (fMRI) analysis is commonly done with cross-correlation analysis (CCA) and the General Linear Model (GLM). Both CCA and GLM techniques, however, typically perform calculations on a per-voxel basis and do not consider relationships neighboring voxels may have. Clustered voxel analyses have then been developed to improve fMRI signal detections by taking advantages of relationships of neighboring voxels. Mean-shift clustering (MSC) is another technique which takes into account properties of neighboring voxels and can be considered for enhancing fMRI activation detection. METHODS: This study examines the adoption of MSC to fMRI analysis. MSC was applied to a Statistical Parameter Image generated with the CCA technique on both simulated and real fMRI data. The MSC technique was then compared with CCA and CCA plus cluster analysis. A range of kernel sizes were used to examine how the technique behaves. RESULTS: Receiver Operating Characteristic curves shows an improvement over CCA and Cluster analysis. False positive rates are lower with the proposed technique. MSC allows the use of a low intensity threshold and also does not require the use of a cluster size threshold, which improves detection of weak activations and highly focused activations. CONCLUSION: The proposed technique shows improved activation detection for both simulated and real Blood Oxygen Level Dependent fMRI data. More detailed studies are required to further develop the proposed technique.

Receptor activator of nuclear factor κB ligand (RANKL) protein expression by B lymphocytes contributes to ovariectomy-induced bone loss.

Production of the cytokine receptor activator of NFκB ligand (RANKL) by lymphocytes has been proposed as a mechanism by which sex steroid deficiency causes bone loss. However, there have been no studies that functionally link RANKL expression in lymphocytes with bone loss in this condition. Herein, we examined whether RANKL expression in either B or T lymphocytes contributes to ovariectomy-induced bone loss in mice. Mice harboring a conditional RANKL allele were crossed with CD19-Cre or Lck-Cre mice to delete RANKL in B or T lymphocytes, respectively. Deletion of RANKL from either cell type had no impact on bone mass in estrogen-replete mice up to 7 months of age. However, mice lacking RANKL in B lymphocytes were partially protected from the bone loss caused by ovariectomy. This protection occurred in cancellous, but not cortical, bone and was associated with a failure to increase osteoclast numbers in the conditional knock-out mice. Deletion of RANKL from T lymphocytes had no impact on ovariectomy-induced bone loss. These results demonstrate that lymphocyte RANKL is not involved in basal bone remodeling, but B cell RANKL does contribute to the increase in osteoclasts and cancellous bone loss that occurs after loss of estrogen.

Piezo1 opposes age-associated cortical bone loss.

As we age, our bones undergo a process of loss, often accompanied by muscle weakness and reduced physical activity. This is exacerbated by decreased responsiveness to mechanical stimulation in aged skeleton, leading to the hypothesis that decreased mechanical stimulation plays an important role in age-related bone loss. Piezo1, a mechanosensitive ion channel, is critical for bone homeostasis and mechanotransduction. Here, we observed a decrease in Piezo1 expression with age in both murine and human cortical bone. Furthermore, loss of Piezo1 in osteoblasts and osteocytes resulted in an increase in age-associated cortical bone loss compared to control mice. The loss of cortical bone was due to an expansion of the endosteal perimeter resulting from increased endocortical resorption. In addition, expression of Tnfrsf11b, encoding anti-osteoclastogenic protein OPG, decreases with Piezo1 in vitro and in vivo in bone cells, suggesting that Piezo1 suppresses osteoclast formation by promoting Tnfrsf11b expression. Our results highlight the importance of Piezo1-mediated mechanical signaling in protecting against age-associated cortical bone loss by inhibiting bone resorption in mice.

A framework for defining mesenchymal cell types associated with murine periosteal and endosteal bone.

Single-cell RNA sequencing has led to numerous novel designations for mesenchymal cell types associated with bone. Consequently, there are now multiple designations for what appear to be the same cell type. In addition, existing datasets contain relatively small numbers of mature osteoblasts and osteocytes and there has been no comparison of periosteal bone cells to those at the endosteum and trabecular bone. The main goals of this study were to increase the amount of single cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. To do this, we created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-CAR, osteo-CAR, pre-osteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by Postn expression. Osteo-X, osteo-CAR, and pre-osteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in any cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of skeletal stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in pre-osteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single cell RNA sequencing datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

Changes occur in resting state network of motor system during 4 weeks of motor skill learning.

We tested whether the resting state functional connectivity of the motor system changed during 4 weeks of motor skill learning using functional magnetic resonance imaging (fMRI). Ten healthy volunteers learned to produce a sequential finger movement by daily practice of the task over a 4 week period. Changes in the resting state motor network were examined before training (Week 0), two weeks after the onset of training (Week 2), and immediately at the end of the training (Week 4). The resting state motor system was analyzed using group independent component analysis (ICA). Statistical Parametric Mapping (SPM) second-level analysis was conducted on independent z-maps generated by the group ICA. Three regions, namely right postcentral gyrus, and bilateral supramarginal gyri were found to be sensitive to the training duration. Specifically, the strength of resting state functional connectivity in the right postcentral gyrus and right supramarginal gyrus increased from Week 0 to Week 2, during which the behavioral performance improved significantly, and decreased from Week 2 to Week 4, during which there was no more significant improvement in behavioral performance. The strength of resting state functional connectivity in left supramarginal gyrus increased throughout the training. These results confirm changes in the resting state network during slow-learning stage of motor skill learning, and support the premise that the resting state networks play a role in improving performance.

Deletion of a putative promoter-proximal Tnfsf11 regulatory region in mice does not alter bone mass or Tnfsf11 expression in vivo.

The cytokine RANKL is essential for osteoclast formation during physiological and pathological bone resorption. RANKL also contributes to lymphocyte production, development of lymph nodes and mammary glands, as well as other biological activities. Transcriptional control of the Tnfsf11 gene, which encodes RANKL, is complex and involves distant regulatory regions. Nevertheless, cell culture studies suggest that an enhancer region near the transcription start site is involved in the control of Tnfsf11 expression by hormones such as 1,25-(OH)2 vitamin D3 and parathyroid hormone, as well as the sympathetic nervous system. To address the significance of this region in vivo, we deleted the sequence between -510 to -1413 bp, relative to Tnfsf11 exon 1, from mice using CRISPR-based gene editing. MicroCT analysis of the femur and fourth lumbar vertebra of enhancer knockout mice showed no differences in bone mass compared to wild type littermates at 5 weeks and 6 months of age, suggesting no changes in osteoclast formation. RNA extracted from the tibia, fifth lumbar vertebra, thymus, and spleen at 6 months of age also showed no reduction in Tnfsf11 mRNA abundance between these groups. However, maximal stimulation of Tnfsf11 mRNA abundance in cultured stromal cells by PTH was reduced approximately 40% by enhancer deletion, while stimulation by 1,25-(OH)2 vitamin D3 was unaffected. The abundance of B and T lymphocytes in the bone marrow did not differ between genotypes. These results demonstrate that the region between -510 and -1413 does not contribute to Tnfsf11 expression, osteoclast support, or lymphocyte production in mice under normal physiological conditions but may be involved in situations of elevated parathyroid hormone.

Local Production of Osteoprotegerin by Osteoblasts Suppresses Bone Resorption.

Osteoprotegerin (OPG) inhibits the ability of receptor activator of nuclear factor κB (NF-κB) ligand (RANKL) to stimulate the differentiation, activity, and survival of bone-resorbing osteoclasts. Genetic studies in mice show that osteocytes are an important source of RANKL, but the cellular sources of OPG are unclear. We use conditional deletion of Tnfrsf11b, which encodes OPG, from different cell populations to identify functionally relevant sources of OPG in mice. Deletion from B lymphocytes and osteocytes, two cell types commonly thought to supply OPG, has little or no impact on bone mass. By contrast, deletion of Tnfrsf11b from osteoblasts increases bone resorption and reduces bone mass to an extent similar to germline deletion, demonstrating that osteoblasts are an essential source of OPG. These results suggest that, in addition to producing new bone matrix, osteoblasts also play an active role in terminating the resorption phase of the bone remodeling cycle by suppressing RANKL activity.

Osteocyte RANKL is required for cortical bone loss with age and is induced by senescence.

In aging mice, osteoclast number increases in cortical bone but declines in trabecular bone, suggesting that different mechanisms underlie age-associated bone loss in these 2 compartments. Osteocytes produce the osteoclastogenic cytokine RANKL, encoded by Tnfsf11. Tnfsf11 mRNA increases in cortical bone of aged mice, suggesting a mechanism underlying the bone loss. To address this possibility, we aged mice lacking RANKL in osteocytes. Whereas control mice lost cortical bone between 8 and 24 months of age, mice lacking RANKL in osteocytes gained cortical bone during this period. Mice of both genotypes lost trabecular bone with age. Osteoclasts increased with age in cortical bone of control mice but not in RANKL conditional knockout mice. Induction of cellular senescence increased RANKL production in murine and human cell culture models, suggesting an explanation for elevated RANKL levels with age. Overexpression of the senescence-associated transcription factor Gata4 stimulated Tnfsf11 expression in cultured murine osteoblastic cells. Finally, elimination of senescent cells from aged mice using senolytic compounds reduced Tnfsf11 mRNA in cortical bone. Our results demonstrate the requirement of osteocyte-derived RANKL for age-associated cortical bone loss and suggest that increased Tnfsf11 expression with age results from accumulation of senescent cells in cortical bone.

Long-term motor training induced changes in regional cerebral blood flow in both task and resting states.

Neuroimaging studies of functional activation often only reflect differentiated involvement of brain regions compared between task performance and control states. Signals common for both states are typically not revealed. Previous motor learning studies have shown that extensive motor skill training can induce profound changes in regional activity in both task and control states. To address the issue of brain activity changes in the resting-state, we explored long-term motor training induced neuronal and physiological changes in normal human subjects using functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). Ten healthy subjects performed a finger movement task daily for four weeks, during which three sessions of fMRI images and two sessions of PET images were acquired. Using a classical data analysis strategy, we found that the brain activation increased first and then returned to the pre-training, replicating previous findings. Interestingly, we also observed that motor skill training induced significant increases in regional cerebral blood flow (rCBF) in both task and resting states as the practice progressed. The apparent decrease in activation may actually result from a greater increase in activity in the resting state, rather than a decrease in the task state. By showing that training can affect the resting state, our findings have profound implications for the interpretation of functional activations in neuroimaging studies. Combining changes in resting state with activation data should greatly enhance our understanding of the mechanisms of motor-skill learning.

Direct MRI mapping of neuronal activity evoked by electrical stimulation of the median nerve at the right wrist.

Magnetic source MRI (msMRI) has being developed recently for direct detections of neuronal magnetic fields to map brain activity. However, controversial results have been reported by different research groups. In this study, more evidence was provided to demonstrate that the neuronal current signal could be detected by MRI using a rapid median nerve stimulation paradigm. The experiments were performed on six normal human participants to investigate the temporal specificity of the effect, as well as inter- and intrasubject reproducibility. Significant activation of contralateral primary sensory cortex (S1) was detected 80 ms after stimulation onset (corresponding to the P80 evoked potential peak). The 80-ms latency S1 activation was observed over three independent sessions for one subject and for all six participants. The magnitude of the signal change was 0.2-0.3%. Coinciding with our expectations, no S1 activation was found when MRI data acquisitions were targeted at the N20 and P30 peaks because of mutual cancellation of magnetic fields generated by those peaks. The results demonstrated good reproducibility of S1 activations and indicated that the S1 activations most likely originated from neuronal magnetic field rather than hemodynamic response.