Nanocarrier-mediated inhibition of macrophage migration inhibitory factor attenuates secondary injury after spinal cord injury.

Spinal cord injury (SCI) can lead to permanent motor and sensory deficits. Following the initial traumatic insult, secondary injury mechanisms characterized by persistent heightened inflammation are initiated and lead to continued and pervasive cell death and tissue damage. Anti-inflammatory drugs such as methylprednisolone (MP) used clinically have ambiguous benefits with debilitating side effects. Typically, these drugs are administered systemically at high doses, resulting in toxicity and paradoxically increased inflammation. Furthermore, these drugs have a small time window postinjury (few hours) during which they need to be infused to be effective. As an alternative to MP, we investigated the effect of a small molecule inhibitor (Chicago sky blue, CSB) of macrophage migration inhibitory factor (MIF) for treating SCI. The pleiotropic cytokine MIF is known to contribute to upregulation of several pro-inflammatory cytokines in various disease and injury states. In vitro, CSB administration alleviated endotoxin-mediated inflammation in primary microglia and macrophages. Nanocarriers such as liposomes can potentially alleviate systemic side effects of high-dose therapy by enabling site-specific drug delivery to the spinal cord. However, the therapeutic window of 100 nm scale nanoparticle localization to the spinal cord after contusion injury is not fully known. Thus, we first investigated the ability of nanocarriers of different sizes to localize to the injured spinal cord up to 2 weeks postinjury. Results from the study showed that nanocarriers as large as 200 nm in diameter could extravasate into the injured spinal cord up to 96 h postinjury. We then formulated nanocarriers (liposomes) encapsulating CSB and administered them intravenously 48 h postinjury, within the previously determined 96 h therapeutic window. In vivo, in this clinically relevant contusion injury model in rats, CSB administration led to preservation of vascular and white matter integrity, improved wound healing, and an increase in levels of arginase and other transcripts indicative of a resolution phase of wound healing. This study demonstrates the potential of MIF inhibition in SCI and the utility of nanocarrier-mediated drug delivery selectively to the injured cord.

Extracellular matrix-based intracortical microelectrodes: Toward a microfabricated neural interface based on natural materials.

Extracellular matrix (ECM)-based implantable neural electrodes (NEs) were achieved using a microfabrication strategy on natural-substrate-based organic materials. The ECM-based design minimized the introduction of non-natural products into the brain. Further, it rendered the implants sufficiently rigid for penetration into the target brain region and allowed them subsequently to soften to match the elastic modulus of brain tissue upon exposure to physiological conditions, thereby reducing inflammatory strain fields in the tissue. Preliminary studies suggested that ECM-NEs produce a reduced inflammatory response compared with inorganic rigid and flexible approaches. In vivo intracortical recordings from the rat motor cortex illustrate one mode of use for these ECM-NEs.

Relationship between intracortical electrode design and chronic recording function.

Intracortical electrodes record neural signals directly from local populations of neurons in the brain, and conduct them to external electronics that control prosthetics. However, the relationship between electrode design, defined by shape, size and tethering; and long-term (chronic) stability of the neuron-electrode interface is poorly understood. Here, we studied the effects of various commercially available intracortical electrode designs that vary in shape (cylindrical, planar), size (15 µm, 50 µm and 75 µm), and tethering [electrode connections to connector with (tethered) and without tethering cable (untethered)] using histological, transcriptomic, and electrophysiological analyses over acute (3 day) and chronic (12 week) timepoints. Quantitative analysis of histological sections indicated that Michigan 50 µm (M50) and Michigan tethered (MT) electrodes induced significantly (p < 0.01) higher glial scarring, and lesser survival of neurons in regions of blood-brain barrier (BBB) breach when compared to microwire (MW) and Michigan 15 µm (M15) electrodes acutely and chronically. Gene expression analysis of the neurotoxic cytokines interleukin (Il)1 (Il1α, Il1ß), Il6, Il17 (Il17a, Il17b, Il17f), and tumor necrosis factor alpha (Tnf) indicated that MW electrodes induced significantly (p < 0.05) reduced expression of these transcripts when compared to M15, M50 and FMAA electrodes chronically. Finally, electrophysiological assessment of electrode function indicated that MW electrodes performed significantly (p < 0.05) better than all other electrodes over a period of 12 weeks. These studies reveal that intracortical electrodes with smaller size, cylindrical shape, and without tethering cables produce significantly diminished inflammatory responses when compared to large, planar and tethered electrodes. These studies provide a platform for the rational design and assessment of chronically functional intracortical electrode implants in the future.

The impact of chronic blood-brain barrier breach on intracortical electrode function.

Brain-computer interfaces (BCIs) have allowed control of prosthetic limbs in paralyzed patients. Unfortunately, the electrodes of the BCI that interface with the brain only function for a short period of time before the signal quality on these electrodes becomes substantially diminished. To truly realize the potential of BCIs, it is imperative to have electrodes that function chronically. In order to elucidate the physiological determinants of a chronically functional neural interface, we studied the role of the blood-brain barrier (BBB) in electrode function, because it is a key mediator of neuronal hemostasis. We monitored the status of the BBB and the consequences of BBB breach on electrode function using non-invasive imaging, electrophysiology, genomic, and histological analyses. Rats implanted with commercially available intracortical electrodes demonstrated an inverse correlation between electrode performance and BBB breach over a period of 16 weeks. Genomic analysis showed that chronically functional electrodes elicit an enhanced wound healing response. Conversely, in poorly functioning electrodes, chronic BBB breach led to local accumulation of neurotoxic factors and an influx of pro-inflammatory myeloid cells, which negatively affect neuronal health. These findings were further verified in a subset of electrodes with graded electrophysiological performance. In this study, we determine the mechanistic link between intracortical electrode function and failure. Our results indicate that BBB status is a critical physiological determinant of intracortical electrode function and can inform future electrode design and biochemical intervention strategies to enhance the functional longevity of BCIs.

The upregulation of specific interleukin (IL) receptor antagonists and paradoxical enhancement of neuronal apoptosis due to electrode induced strain and brain micromotion.

The high mechanical mismatch between stiffness of silicon and metal microelectrodes and soft cortical tissue, induces strain at the neural interface which likely contributes to failure of the neural interface. However, little is known about the molecular outcomes of electrode induced low-magnitude strain (1-5%) on primary astrocytes, microglia and neurons. In this study we simulated brain micromotion at the electrode-brain interface by subjecting astrocytes, microglia and primary cortical neurons to low-magnitude cyclical strain using a biaxial stretch device, and investigated the molecular outcomes of induced strain in vitro. In addition, we explored the functional consequence of astrocytic and microglial strain on neural health, when they are themselves subjected to strain. Quantitative real-time PCR array (qRT-PCR Array) analysis of stretched astrocytes and microglia showed strain specific upregulation of an Interleukin receptor antagonist - IL-36Ra (previously IL-1F5), to ≈ 1018 and ≈ 236 fold respectively. Further, IL-36Ra gene expression remained unchanged in astrocytes and microglia treated with bacterial lipopolysaccharide (LPS) indicating that the observed upregulation in stretched astrocytes and microglia is potentially strain specific. Zymogram and western blot analysis revealed that mechanically strained astrocytes and microglia upregulated matrix metalloproteinases (MMPs) 2 and 9, and other markers of reactive gliosis such as glial fibrillary acidic protein (GFAP) and neurocan when compared to controls. Primary cortical neurons when stretched with and without IL-36Ra, showed a ≈ 400 fold downregulation of tumor necrosis factor receptor superfamily, member 11b (TNFRSF11b). Significant upregulation of members of the caspase cysteine proteinase family and other pro-apoptotic genes was also observed in the presence of IL-36Ra than in the absence of IL-36Ra. Adult rats when implanted with microwire electrodes showed upregulation of IL-36Ra (≈ 20 fold) and IL-1Ra (≈ 1500 fold) 3 days post-implantation (3 DPI), corroborating in vitro results, although these transcripts were drastically down regulated by ≈ 20 fold and ≈ 1488 fold relative to expression levels 3 DPI, at the end of 12 weeks post-implantation (12 WPI). These results demonstrate that IL receptor antagonists may be negatively contributing to neuronal health at acute time-points post-electrode implantation.