BRAIN Publication Roundup – April 2018

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Miniaturized carbon fiber electrode arrays for intra-nerve recording and stimulation… Reliable measurement and imaging of brain bioenergetics… Emerging medical imaging tool with superior contrast and sensitivity… Improved method for synaptic neural circuit tracing…

Advances in electrode array fabrication improve longevity of nervous system implants

Recent advances in neuromodulation have shown that chronic stimulation of the peripheral autonomic nervous system can be used to treat disease. However, the implants providing this stimulation typically have poor longevity. The electrode size (centimeter-scale) and stiffness, as well as the degree of tissue tolerance for the material, leads to insertion trauma and reactive tissue response. To address these issues, at Boston University, Dr. Timothy J. Gardner and colleagues have developed a method for assembling carbon fiber ultra-micro electrodes on a flexible substrate. They used 3D-printing and laser writing to precisely align the individual fibers and added indium (a malleable metal) to form robust contacts between the fine fibers and the electrical contacts, enabling further miniaturization of the device. These flexible carbon fiber ultra-micro electrodes have low tissue reactivity, and the small size allows many electrodes to be implanted within a small, limited-access area. Further, the carbon fiber stiffness permits easy penetration of biological tissue, including fine peripheral nerves, making it possible to make high-quality intra-nerve recordings at unprecedentedly small scales. A thin layer of iridium oxide was also electrodeposited on the carbon fiber tips, making the arrays usable for intra-nerve stimulation as well as recording. To test the arrays, the researchers conducted an acute in vivo preparation of a small peripheral nerve (125μm diameter, containing ~1000 axons) in anesthetized songbirds and recorded spontaneous multi-unit activity. When they stimulated the nerve using the carbon fiber array, they found that gradually increasing the stimulation intensity produced a graded evoked response. Although histology and long-term experiments have yet to be performed with these electrodes, the size, stiffness, and material suggest improved longevity over current implants. The promise of this technological advance – reducing tissue reactivity while still miniaturizing the electrode implants – holds great potential towards improving bio-electronic therapy.

Carbon fiber array (bottom) implanted in the tracheosyringeal nerve (horizontal white band). The fire-sharpened carbon fibers easily penetrated the intact epineurium sheath. Scale bar: 300μm.
A novel magnetic resonance spectroscopy design concept enables simultaneous assessment of phosphorus metabolism in two human brain regions

Many neurological disorders involve a behavioral and cognitive impairment that links to dysfunction in distinct brain regions, including metabolic dysfunction. While phosphorus-31 magnetic resonance (MR) spectroscopy represents a non-invasive, in vivo tool for researchers to assess metabolic impairments in different brain regions, the technology has been limited by certain deficiencies. Specifically, most MR scanners have had difficulty in obtaining simultaneous and high-quality phosphorus-31 signals from multiple brain regions, and low detection sensitivity has limited the signal-to-noise ratio and resolution. To address these technical shortcomings, Dr. Wei Chen and colleagues, at the University of Minnesota, have demonstrated a new dual-coil MR spectroscopy approach that provides simultaneous assessment of phosphorus-31-containing metabolites from two distinct human brain regions of interest. Their novel design enabled them to actively switch the nuclear radiofrequency (RF) transmitter–receiver between two phosphorus-31 RF coils (more than two coils is also feasible), enabling interleaved acquisition of two phosphorus metabolite signals, each from a different area of the brain. The team successfully collected data from human occipital and frontal lobes, simultaneously. This method, the concept of which can be used for other MR applications, may be capable of assessing bio-energetic abnormalities in target brain regions with high detection sensitivity and spectral resolution. The technology could provide insights into the normal function of the brain, as well as improve diagnosis and treatment of neurological disorders.

Representative in vivo phosphorus‐31 three‐dimensional chemical shift imaging (CSI) data detected in the human occipital lobe, as well as maps of phosphorus metabolite signals (phosphocreatine (PCr) and γ‐adenosine triphosphate (γ‐ATP)) overlaid on the anatomical images. (A) Location of the coil placed near the occipital lobe. (B–D) Phosphorus-31 MR spectroscopy (MRS) profiles from three selected CSI slices. (E) Single-voxel phosphorus-31 spectrum denoted by the yellow circle in (C) shows excellent detection sensitivity. (F–H) Corresponding PCr maps. (I–K) Corresponding γ-ATP maps. Similar images were obtained simultaneously from the frontal lobe.

A new imaging technique capable of monitoring the healing process in traumatic brain injury

In the United States alone, there are approximately 2.5 million emergency visits for traumatic brain injury (TBI) every year, but quickly identifying the severity of injury remains a challenge. Currently-used subjective methods and imaging-based modalities are appropriate for diagnosing severe TBI injuries, but many mild-to-medium grade impacts go un-imaged, undiagnosed, and untreated. At the University of California, Berkeley, Dr. Steven Conolly and colleagues have presented data and methods illustrating the ground-breaking non-invasive technology, magnetic particle imaging (MPI). MPI, which uses a different device than magnetic resonance imaging, images a distribution of superparamagnetic iron oxide (SPIO) nanoparticles (a safe tracer) with excellent contrast and sensitivity, anywhere in the body. MPI has no radiation, offers high resolution, and the nanoparticles can be tracked for long periods of time in both the blood and inside cells. Furthermore, because the nanoparticles stay inside the vasculature, only blood appears in the resulting image. With no signal from background tissue, MPI can yield an impressive contrast-to-noise ratio. Conolly’s team demonstrated MPI’s ability to detect internal hemorrhaging in a closed-head TBI rat model. They acquired MPI images that showed SPIO nanoparticles had accumulated rapidly at the impact site and persisted for two weeks. The signal half-life in the impact region was approximately four days, while no signal was detected in the same area of the control animal. The scientific potential of this finding suggests that MPI could eventually improve the diagnosis of internal bleeding for patients suffering from trauma.

(Left) Maximum intensity projection (MIP) of control and TBI animals. Note the large hemorrhage in the TBI animal. (Right) MIP of the animals after 3 days; blue circle represents the impact site, green circles indicate lymph nodes. The TBI rat continues to have significant signal from the hemorrhage and inside the lymph nodes, unlike the control.
A second-generation rabies viral vector is non-toxic to host neurons and enables long-term study of neural circuits

The use of genetically-modified rabies viral (RV) vectors, in labeling neurons and their synaptic connections, has enhanced the study of neural circuit organization. However, their use has been limited to short-term experiments because the virus replicates copiously inside infected neurons, consequently killing them approximately two weeks after infection. At the Massachusetts Institute of Technology and the Allen Institute for Brain Science, Dr. Ian Wickersham and colleagues have introduced a methodology for eliminating the toxicity of RV vectors by using the virus as a retrograde vector and monosynaptic tracer. In the original method, the only viral gene deleted was that which encodes a vital protein within the virus’ outer covering, thus stopping the virus from spreading to other cells but not from replicating within its host cell. Wickersham’s team engineered a new class of RV vector, with a second gene deleted: the gene encoding the viral polymerase, an essential enzyme for viral gene transcription as well as for replication of the viral genome. Therefore, the new version of the virus does not replicate after infecting the cell, leaving infected cells alive and healthy for much longer. In addition, the new virus has been modified to express a DNA recombinase, which is capable (even at low levels) of deleting specific DNA sequences of interest within the infected neuron’s genome. As a result, instead of relying on the virus to, for example, express a fluorescent labeling protein and then proliferate to reach a suitable level of detectability, the new virus triggers its host cell to detectibly express the label. The team injected the new virus into chosen regions within the brains of mice. Among infected neurons, electrophysiological properties in the amygdala were normal after two months, and structural and functional imaging in the cortex appeared normal after four months. This new approach may transform how researchers are able to study the organization and connectivity of the brain.

Example two-photon structural images of the same volume of mouse cortex labeled by the new class of RV vector, at two different imaging time points. Every neuron visible at 4 weeks is still alive and structurally normal at 8 weeks. Scale bar, 100μm.