Bryan L. Benson

Transcallosal Sensorimotor Fiber Tract Structure-Function Relationships

Recent studies have demonstrated neuroanatomically selective relationships among white matter tract microstructure, physiological function, and task performance. Such findings suggest that the microstructure of transcallosal motor fibers may reflect the capacity for interhemispheric inhibition between the primary motor cortices, although full characterization of the transcallosal inhibitory sensorimotor network is lacking. Thus, the goal of this study was to provide a comprehensive description of transcallosal fibers connecting homologous sensorimotor cortical regions and to identify the relationship(s) between fiber tract microstructure and interhemispheric inhibition during voluntary cortical activity. To this end, we assessed microstructure of fiber tracts connecting homologous sensorimotor regions of the cortex with diffusion tensor imaging. We also assessed interhemispheric inhibition by eliciting the ipsilateral silent period (iSP) within the same participants. We mapped mutually exclusive transcallosal connections between homologous sensorimotor regions and computed quantitative metrics of each fiber tract. Paralleling work in non‐human primates, we found the densest interhemispheric sensorimotor connections to be between the medial motor areas. Additionally, we provide a midsagittal callosal atlas in normalized Montreal Neurological Institute (MNI) space for future studies to use when investigating callosal fiber tracts connecting primary and secondary sensorimotor cortices. Finally, we report a strong, positive relationship (r = 0.76) between strength of interhemispheric inhibition (iSP) and microstructure of interhemispheric fibers that is specific to tracts connecting the primary motor cortices. Thus, increased fiber microstructure in young adults predicts interhemispheric inhibitory capacity. Hum Brain Mapp, 2013.

A spatial explicit strategy reduces error but interferes with sensorimotor adaptation

Although sensorimotor adaptation is typically thought of as an implicit form of learning, it has been shown that participants who gain explicit awareness of the nature of the perturbation during adaptation exhibit more learning than those who do not. With rare exceptions, however, explicit awareness is typically polled at the end of the study. Here, we provided participants with either an explicit spatial strategy or no instructions before learning. Early in learning, explicit instructions greatly reduced movement errors but also resulted in increased trial-to-trial variability and longer reaction times. Late in adaptation, performance was indistinguishable between the explicit and implicit groups, but the mechanisms underlying performance improvements remained fundamentally different, as revealed by catch trials. The progression of implicit recalibration in the explicit group was modulated by the use of an explicit strategy: these participants showed a lower level of recalibration as well as decreased aftereffects. This phenomenon may be due to the reduced magnitude of errors made to the target during adaptation or inhibition of implicit learning mechanisms by explicit processing.

The effects of working memory resource depletion and training on sensorimotor adaptation

We have recently demonstrated that visuospatial working memory performance predicts the rate of motor skill learning, particularly during the early phase of visuomotor adaptation. Here, we follow up these correlational findings with direct manipulations of working memory resources to determine the impact on visuomotor adaptation, a form of motor learning. We conducted two separate experiments. In the first one, we used a resource depletion strategy to investigate whether the rate of early visuomotor adaptation would be negatively affected by fatigue of spatial working memory resources. In the second study, we employed a dual n-back task training paradigm that has been shown to result in transfer effects [1] over five weeks to determine whether training-related improvements would boost the rate of early visuomotor adaptation. The depletion of spatial working memory resources negatively affected the rate of early visuomotor adaptation. However, enhancing working memory capacity via training did not lead to improved rates of visuomotor adaptation, suggesting that working memory capacity may not be the factor limiting maximal rate of visuomotor adaptation in young adults. These findings are discussed from a resource limitation/capacity framework with respect to current views of motor learning.

Live-cell visualization of gasdermin D-driven pyroptotic cell death

Pyroptosis is a form of cell death important in defenses against pathogens that can also result in a potent and sometimes pathological inflammatory response. During pyroptosis, GSDMD (gasdermin D), the pore-forming effector protein, is cleaved, forms oligomers, and inserts into the membranes of the cell, resulting in rapid cell death. However, the potent cell death induction caused by GSDMD has complicated our ability to understand the biology of this protein. Studies aimed at visualizing GSDMD have relied on expression of GSDMD fragments in epithelial cell lines that naturally lack GSDMD expression and also lack the proteases necessary to cleave GSDMD. In this work, we performed mutagenesis and molecular modeling to strategically place tags and fluorescent proteins within GSDMD that support native pyroptosis and facilitate live-cell imaging of pyroptotic cell death. Here, we demonstrate that these fusion proteins are cleaved by caspases-1 and -11 at Asp-276. Mutations that disrupted the predicted p30-p20 autoinhibitory interface resulted in GSDMD aggregation, supporting the oligomerizing activity of these mutations. Furthermore, we show that these novel GSDMD fusions execute inflammasome-dependent pyroptotic cell death in response to multiple stimuli and allow for visualization of the morphological changes associated with pyroptotic cell death in real time. This work therefore provides new tools that not only expand the molecular understanding of pyroptosis but also enable its direct visualization.

Spatiotemporal ablation of CXCR2 on oligodendrocyte lineage cells: Role in myelin repair

Background: Residual CXCR2 expression on CNS cells in Cxcr2+/−→Cxcr2−/− chimeric animals slowed remyelination after both experimental autoimmune encephalomyelitis and cuprizone-induced demyelination. Methods: We generated Cxcr2fl/−:PLPCre-ER(T) mice enabling an inducible, conditional deletion of Cxcr2 on oligodendrocyte lineage cells of the CNS. Cxcr2fl/−:PLPCre-ER(T) mice were evaluated in 2 demyelination/remyelination models: cuprizone-feeding and in vitro lysophosphatidylcholine (LPC) treatment of cerebellar slice cultures. Results: Cxcr2fl/−:PLPCre-ER(T)+ (termed Cxcr2-cKO) mice showed better myelin repair 4 days after LPC-induced demyelination of cerebellar slice cultures. Cxcr2-cKOs also displayed enhanced hippocampal remyelination after a 2-week recovery from 6-week cuprizone feeding. Conclusion: Using 2 independent demyelination/remyelination models, our data document enhanced myelin repair in Cxcr2-cKO mice, consistent with the data obtained from radiation chimerism studies of germline CXCR2. Further experiments are appropriate to explore how CXCR2 function in the oligodendrocyte lineage accelerates myelin repair.

Biomimetic post-capillary venule expansions for leukocyte adhesion studies

Leukocyte adhesion and extravasation are maximal near the transition from capillary to post-capillary venule, and are strongly influenced by a confluence of scale-dependent physical effects. Mimicking the scale of physiological vessels using in vitro microfluidic systems allows the capture of these effects on leukocyte adhesion assays, but imposes practical limits on reproducibility and reliable quantification. Here we present a microfluidic platform that provides multiple (54–512) technical replicates within a 15-minute sample collection time, coupled with an automated computer vision analysis pipeline that captures leukocyte adhesion probabilities as a function of shear and extensional stresses. We report that in post-capillary channels of physiological scale, efficient leukocyte adhesion requires erythrocytes forcing leukocytes against the wall, a phenomenon that is promoted by the transitional flow in post-capillary venule expansions and dependent on the adhesion molecule ICAM-1.

Chemical disruption of the pyroptotic pore-forming protein gasdermin D inhibits inflammatory cell death and sepsis

Necrosulfonamide inhibits pyroptosis through direct inhibition of gasdermin D. Targeting gasdermin D Gasdermin D (GSDMD) is a key downstream effector in inflammasome-driven, caspase-1–dependent and lipopolysaccharide-driven, caspase-11–dependent pyroptosis. Upon activation and caspase-dependent proteolytic cleavage, GSDMD oligomerizes to form pores that facilitate pyroptotic cell death. Here, Rathkey et al. report necrosulfonamide (NSA) to be an inhibitor of GSDMD and GSDMD-mediated pyroptosis. They propose that NSA binds to cysteine 191 and inhibits the oligomerization of GSDMD dimers. NSA could be used as a template to develop more potent inhibitors of GSDMD, an important goal for the treatment of septic shock. In the same issue, Sollberger et al. independently report a pyrazolo-oxazepine scaffold–based molecule to be an inhibitor of GSDMD. Dysregulation of inflammatory cell death is a key driver of many inflammatory diseases. Pyroptosis, a highly inflammatory form of cell death, uses intracellularly generated pores to disrupt electrolyte homeostasis and execute cell death. Gasdermin D, the pore-forming effector protein of pyroptosis, coordinates membrane lysis and the release of highly inflammatory molecules, such as interleukin-1β, which potentiate the overactivation of the innate immune response. However, to date, there is no pharmacologic mechanism to disrupt pyroptosis. Here, we identify necrosulfonamide as a direct chemical inhibitor of gasdermin D, the pyroptotic pore-forming protein, which binds directly to gasdermin D to inhibit pyroptosis. Pharmacologic inhibition of pyroptotic cell death by necrosulfonamide is efficacious in sepsis models and suggests that gasdermin D inhibitors may be efficacious clinically in inflammatory diseases.

Aryl Hydrocarbon Receptor Nuclear Translocator in Vascular Smooth Muscle Cells Is Required for Optimal Peripheral Perfusion Recovery

Background Limb ischemia resulting from peripheral vascular disease is a common cause of morbidity. Vessel occlusion limits blood flow, creating a hypoxic environment that damages distal tissue, requiring therapeutic revascularization. Hypoxia‐inducible factors (HIFs) are key transcriptional regulators of hypoxic vascular responses, including angiogenesis and arteriogenesis. Despite vascular smooth muscle cells’ (VSMCs’) importance in vessel integrity, little is known about their functional responses to hypoxia in peripheral vascular disease. This study investigated the role of VSMC HIF in mediating peripheral ischemic responses. Methods and Results We used Arnt SMKO mice with smooth muscle–specific deletion of aryl hydrocarbon receptor nuclear translocator (ARNT, HIF‐1β), required for HIF transcriptional activity, in a femoral artery ligation model of peripheral vascular disease. Arnt SMKO mice exhibit impaired perfusion recovery despite normal collateral vessel dilation and angiogenic capillary responses. Decreased blood flow manifests in extensive tissue damage and hypoxia in ligated limbs of Arnt SMKO mice. Furthermore, loss of aryl hydrocarbon receptor nuclear translocator changes the proliferation, migration, and transcriptional profile of cultured VSMCs. Arnt SMKO mice display disrupted VSMC morphologic features and wrapping around arterioles and increased vascular permeability linked to decreased local blood flow. Conclusions Our data demonstrate that traditional vascular remodeling responses are insufficient to provide robust peripheral tissue reperfusion in Arnt SMKO mice. In all, this study highlights HIF responses to hypoxia in arteriole VSMCs critical for the phenotypic and functional stability of vessels that aid in the recovery of blood flow in ischemic peripheral tissues.

Novel insights into cell-cell interactions at the blood-brain barrier revealed by a fully-human flow-based in vitro model

properties of the reactive astrocytes at the BBB.We have previously shown the importance of the vitamin Ametabolite retinoic acid (RA) in embryonic BBB development. Our recent analyses reveal that RA counteracts the deleterious effects of inflammatory cytokines on BBB function, decreases monocyte migration, and induces an immune quiescent phenotype in resting and immune activated brain endothelial cells in vitro. Interestingly, low vitamin A serum levels have been correlated to a worsened disease course in relapsing-remitting MS patients. To investigate the role of RA in neuroinflammation, we analysed the RA synthetic pathway in post-mortemMS lesions.We show that reactive astrocytes in MS lesions re-express the enzyme responsible for RA production. Furthermore, increased RA receptor expression indicates active RA signalling in lesions. Using primary human astrocyte cultures, we have reproduced inflammation-induced RA synthesis release in vitro. Assays using human brain endothelium show that RA secretion by reactive astrocytes is an endogenous protective mechanism that reduces functional BBB damage and immune activation during neuroinflammation. Our ongoing research points towards anti-oxidant transcription factor signalling as one of themechanisms bywhich astrocyte-derived RA protects the inflamed BBB. Detailed knowledge on the regulation of RA levels in the CNS in astrocytes and its protective effect on the BBB may lead to the development of novel therapies aimed at restoring the BBB and reducing the inflammatory cascade in MS lesions.