Dr. Ian Shih and his team were recently awarded a BRAIN Initiative R01 grant by the National Institute of Mental Health. Their project, in response to the RFA http://grants.nih.gov/grants/guide/rfa-files/RFA-MH-16-750.html, will dissect the neuronal and astrocytic compartment of the BOLD signal using cutting-edge fMRI techniques.
Blood-oxygenation-level-dependent functional magnetic resonance imaging (BOLD fMRI) is widely used in to study human brain function; however the cellular and molecular mechanisms underlying the BOLD signal remain poorly understood. The BOLD signal is highly complex as it represents disproportionate interactions of cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen (CMRO2) during neuronal activation. On the cellular level, while lactate generated from the astrocytes is used to sustain neuronal activity, astrocytic signaling also releases vasoactive compounds, indicating that BOLD could reflect a combined response of both neurons and astrocytes. Dissecting the fractional contribution of neurons, astrocytes, their crosstalk, and specific molecular signaling cascades to BOLD, CBF, CBV, and CMRO2 is crucial to more accurately model and interpret BOLD data.
Unlike neurons, astrocytes lack the appropriate ion channels to propagate action potentials but rather mediate their activity predominantly through G-protein-coupled receptors (GPCRs). Substantial pharmacological evidence has suggested that astrocytic GPCRs are key molecular players in their control of CBF through their binding of various paracrine compounds released by neurons. Interestingly, some studies have questioned this conclusion, demonstrating that activation of astrocytic Gq-GPCRs are not critical for CBF modulation. Further, it remains unclear how other GPCR subfamilies (i.e., Gs and Gi) affect BOLD. These controversies and missing data prompted us to systematically investigate the following questions for the first time: 1) whether selective activation of astrocytic Gq-, Gs-, or Gi-GPCR signaling pathways modulate hemodynamic or BOLD responses in vivo, 2) can neurons or astrocytes independently elicit hemodynamic and BOLD responses without the involvement of the other, and 3) what molecular mechanisms contribute to the BOLD signal disruption in disease states where astrogliosis and neuronal remodeling occur.
Dr. Shih and his team will employ cutting-edge chemogenetic tools, a.k.a. Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), to selectively modulate Gq-, Gs- and Gi-signaling cascades in neurons and astrocytes. They will also utilize multimodal fMRI tools that allow measurement of BOLD, CBV, CBF, and CMRO2 changes in a single setting. Additionally, they will perform immunohistochemistry in all subjects, allowing within-subject comparison of the number or ratio of activated/suppressed cells and the observed hemodynamic responses. This study will shed light on the mechanisms by which the BOLD signal is formed and how it is disrupted in disease states, and ultimately build a more solid foundation for human brain mapping.
BRIC would like to recognize Dr. Pew-Thian Yap, for receiving National Institutes of Health (NIH) R01 funding in February 2016 for his project entitled, “Robust White Matter Morphometry with Small Databases.
Over the next three years, Yap and his investigation team will tackle a common challenge in neuroimaging research – producing reliable outcomes when study subject databases of limited size are statistically compared. Through developing a novel set of statistical computational tools, Yap’s team seeks to advance the neuroscience research community’s ability to detect meaningful differences when analyzing diffusion magnetic resonance imaging (MRI) data associated with smaller and moderately-sized databases. One of the tools to be developed aims to better estimate diffusion MRI statistics and their variability through significantly increasing samples via identifying and assembling repetitive local information throughout an image. An additional comparative method to be developed will improve upon those used for limited-size databases through correcting for registration errors that cause structural variability in diffusion MRI data in smaller sample sizes.
In meeting its aims, Yap’s study will remove critical technological hurdles linked to poor statistical power amongst limited sample studies that have hindered investigation of brain development, aging, and disease pathologies linked to abnormalities driving neurodegenerative disorders.
As applied to his scope of research, Yap notes: "This funding will allow me to develop cutting-edge statistical tools for making full use of diffusion MRI data when sample size is small and when image quality is low. This project is part of my effort to improve the utility of diffusion MRI in both research and clinical settings, with the hope of ultimately arriving at greater insights into how the human brain works.”
Dr. Tamara Branca was recently awarded a R01 grant by the National Institute of Health to investigate the sensitive and specific detection of Brown Adipose Tissue and activity by magnetic resonance with hyperpolarized xenon gas.
Approximately 65% of Americans are overweight or obese and, despite the many efforts to stop and reverse this trend, the obesity epidemic keeps worsening, suggesting that new and more effective treatments are urgently needed. At the fundamental level, obesity results from an imbalance between energy intake and energy expenditure. The latter is very difficult to quantify, but recent studies show that in humans, as in other mammals, it is strongly modulated by the activity of Brown Adipose Tissue (BAT). BAT is a fatty tissue present in all mammals and is uniquely adapted to dissipate large amounts of energy through heat in a process called Non-Shivering Thermogenesis (NST). Interestingly, BAT seems to regulate not only energy expenditure and susceptibility to weight gain, but also glucose homeostasis and insulin sensitivity, suggesting the possibility to up-regulate this tissue to correct for hyperglycemia in subjects with diabetes.
As the biochemical mechanisms that trigger BAT activity are being discovered, and as new therapeutic treatments that specifically target this tissue are being considered, non-invasive detection of BAT tissue and thermogenic activity remains challenging. BAT is especially difficult to detect in overweight and obese subjects, the target population for BAT interventions, in which this tissue is present but metabolically inactive. In addition, BAT thermogenic activity can be detected non-invasively only indirectly, either through measurement of tissue uptake of potential NST substrates or through measurements of physiological changes that may or may not correlate with NST.
The Branca Lab has recently developed a magnetic resonance imaging method that will enable researchers to clearly detect this tissue in humans with unprecedented sensitivity and specificity. This new imaging method, recently described in the Proceedings of the National Academy of Sciences, leverages on the exquisite temperature sensitivity of the lipophilic xenon gas that has had its nuclear polarization enhanced up to 5 orders of magnitude, to produce background free maps of brown fat and its temperature. Preliminary magnetic resonance studies at the BRIC performed with HyperPolarized 129Xe gas (HP 129Xe) in rodents and humans show that stimulation of NST leads to a selective downstream accumulation of inhaled HP 129Xe gas into BAT, whether or not stimulation of NST is followed by BAT activation. At the same time, when xenon dissolves in BAT, the temperature dependence of its chemical shift can be used to directly measure BAT temperature and thermogenic activity in real time and with unprecedented accuracy.
Now, thanks to a funded NIH grant, the Branca Lab, which includes clinical collaborators from the UNC School of Medicine as well as other BRIC faculty members with complementary imaging expertise, will be able to fully validate this novel methodology for the non-invasive detection of brown adipose tissue and for the characterization of its function. Specifically we will establish whether 129Xe MRI can quantify BAT tissue volume and mass with better accuracy and sensitivity than conventional imaging methodologies like 18FDG-PET, 1H MRI, and contrast ultrasound in animals with different BAT functional states, while using histology as ground truth.
Finally, sensitivity and specificity of HP 129Xe MR with respect to 18FDG-PET will be assessed directly in humans using the hybrid PET/MRI scanner, while the temperature sensitivity of HP 129Xe will be used to directly measure, for the first time, human BAT thermogenic activity. Reliable identification of BAT by HP129Xe MR, coupled with direct measurement of its thermogenic activity, will not only enhance the success of preclinical research studies aimed at understanding the primary factors that regulate the development, the differentiation, and the activation of this tissue, but in the clinical research setting, such a tool will allow us to correctly assess, in a larger number of human subjects, normal and abnormal BAT function, and to determine the efficacy of new anti-obesity and anti-diabetes therapies that specifically target this tissue.
Dr. Ian Shih and his team were awarded an R01 grant by the National Institute of Neurological Disorders and Stroke to map therapeutic circuitry of deep brain stimulation in Parkinson’s disease.
Dr. Yen-Yu Ian Shih
Deep brain stimulation (DBS) is a well-established neurosurgical therapy for multiple neurological and psychiatric disorders. In DBS, an electrode is stereotactically guided to a target cerebral nucleus and high frequency (~130 Hz) electrical stimulation is delivered through a pacemaker-like subcutaneous stimulating device. It is most commonly employed in the treatment of Parkinson’s disease (PD), generally in cases where other medical therapies have become inadequate or dyskinesias have become intolerable. When applied for the symptomatic treatment of PD, the subthalamic nucleus (STN) is frequently targeted, often resulting in a marked reduction in several hallmark PD symptoms, including resting tremor and rigidity. However, despite these benefits, many parkinsonian symptoms are frequently refractory to, or may worsen during STN-DBS. The STN is both anatomically heterogeneous and fiber-dense, and thus there is a high likelihood of recruitment of off-target circuits during STN-DBS, even with accurate electrode placements. A better understanding of how DBS exerts its therapeutic effects will allow optimization of this procedure to enhance therapeutic outcomes and reduce unwanted side-effects.
Dr. Shih and his team aim to address three critical, yet elusive questions of: 1) which neural circuits represent on- and off-target STN-DBS effects, 2) whether selective optogenetic stimulation of STN neurons ameliorate parkinsonian motor deficits, and 3) which neural circuits are necessary for therapeutic STN-DBS. To these ends, they will use state-of-the-art functional magnetic resonance imaging (fMRI), functional connectivity MRI (fcMRI), electrophysiology, optogenetics, and behavioral assessment to dissect therapeutic DBS circuitry in an animal model of PD, in which the amelioration of motor deficits are strongly DBS-dependent. Their central hypotheses are that: 1) on- and off-target DBS exhibit behavior-correlated, distinct brain activity and connectivity patterns, 2) high frequency optogenetic stimulation of the STN cell bodies mimics STN-DBS therapy and suppresses pathological oscillatory activity, and 3) suppressing pivotal circuit elements using optogenetics during therapeutic DBS attenuates motor deficit rescue, and thus allowing effective therapeutic DBS circuits to be separated from DBS side effects. This project will provide novel insights into DBS mechanisms, and lay a foundation to establish new DBS treatment targets and stimulus paradigms for a wide variety of neurological and psychiatric disorders.
Dynamic MRI of tPA-induced Peri-Infract Spreading Depolarizations: Outcome Correlates and Potential Therapy
Dr. Yen-Yu Ian Shih, Shih Lab
Peri-infarct spreading depolarization (PID) describes a series of propagating electrical potentials that silence brain activity, alter cerebral blood flow (CBF), induce cell swelling, and appear during the hyperacute phase of stroke. PID has been shown to accelerate stroke progress, and suppressing PID is known to reduce the final infarct size. Although PID is a potential therapeutic target for stroke, the spatiotemporal characteristics of PIDs remain to be characterized and how reperfusion affects PID signatures is largely unexplored.
The Shih lab at BRIC recently developed a novel means to map PIDs in nearly real-time using quantitative MRI. Interestingly, their recent data also demonstrated for the first time that infusion of tissue plasminogen activator (tPA), a clinically used agent for recanalization, could increase, but not decrease, the number of PIDs under certain pathophysiological conditions, indicating a potential mechanism for tPA tissue toxicity that reduces the efficacy of its thrombolytic action. This prompts the Shih lab to address two critical questions, namely: 1) can PID serve as a marker for tPA tissue toxicity and 2) whether co-administration of a glutamate antagonist, topiramate, reduces tPA toxicity and improves outcomes. Dr. Shih and his lab members will use a photothrombotic model to induce controllable ischemic lesions in rats during MRI scans so as to capture the entire spatiotemporal evolution of PIDs. It is their hope that their results will shed light on whether strategies to reduce tPA-induced tissue toxicity can improve the overall efficacy of IV tPA in stroke patients.
This work recently received a 4-year Scientist Development Grant Award (ranked at 1%) from the American Heart Association.