Spiny mice (Acomys) cells fail to engraft in NOD scid gamma.

Immune cells and stromal cells regulate wound healing and regeneration through complex activation patterns with spatiotemporal variation. The scarless regeneration of Spiny mice (Acomys species) is no exception; differential activation of immune and stromal cell populations seems to play a role in its remarkable regenerative capacity. To elucidate the role and interplay of Acomys immune cells in mammalian regeneration, we sought to create Acomys-Mus chimeras by transplanting bone marrow (BM) from Acomys into NOD Scid Gamma (NSG), a severely immunodeficient mouse line often used in creating humanized mice. Here, we report that Acomys BM cells fail to reconstitute and differentiate when transferred to irradiated NSG adults and neonates. In addition, we did not detect the presence of donor cells nor observe the onset of Graft versus Host Disease (GvHD)-like pathology, even after transplanting Acomys splenocytes in Acomys-Mus chimeras suggesting early graft failure. Overall, these results demonstrate the adoptive transfer of Acomys BM alone is not sufficient to establish Acomys hematopoietic system in NSG mouse.

Breathing patterns and CO2 production in adult spiny mice (Acomys cahirinus).

The spiny mouse (Acomys) is a precocial mammal with unique regenerative abilities. We used whole-body plethysmography to describe the breathing patterns and CO2 production (VCO2) of adult spiny mice (n = 10 male, 10 female) and C57BL/6 mice (n = 9 male, 11 female). During quiet breathing, female but not male spiny mice had lower tidal volumes and CO2 production vs. C57BL/6 mice. During extended hypoxia (30 min), male and female spiny mice decreased VCO2 and tidal volume to a greater degree than C57BL/6 mice. During an acute hypoxic-hypercapnic respiratory challenge (10% O2, 7% CO2), male and female spiny mice had blunted ventilatory responses as compared to C57BL/6 mice, primarily from a diminished increase in respiratory rate. These data establish a baseline for studies of respiratory physiology and neurobiology in spiny mice in the context of their remarkable regenerative capacity and their unique background of a desert dwelling species.

Spiny mouse (Acomys): an emerging research organism for regenerative medicine with applications beyond the skin.

The spiny mouse (Acomys species) has emerged as an exciting research organism due to its remarkable ability to undergo scarless regeneration of skin wounds and ear punches. Excitingly, Acomys species demonstrate scar-free healing in a wide-range of tissues beyond the skin. In this perspective article, we discuss published findings from a variety of tissues to highlight how this emerging research organism could shed light on numerous clinically relevant human diseases. We also discuss the challenges of working with this emerging research organism and suggest strategies for future Acomys-inspired research.

Animal Models to Study Emerging Technologies Against SARS-CoV-2.

New technologies are being developed toward the novel coronavirus SARS-CoV-2 to understand its pathogenesis and transmission, to develop therapeutics and vaccines, and to formulate preventive strategies. Animal models are indispensable to understand these processes and develop and test emerging technologies; however, the mechanism of infection for SARS-CoV-2 requires certain similarities to humans that do not exist in common laboratory rodents. Here, we review important elements of viral infection, transmission, and clinical presentation reflected by various animal models readily available or being developed and studied for SARS-CoV-2 to help bioengineers evaluate appropriate preclinical models for their emerging technologies. Importantly, applications of traditional mice and rat models are limited for studying SARS-CoV-2 and development of COVID-19. Non-human primates, Syrian hamsters, ferrets, cats, and engineered chimeras mimic the human infection more closely and hold strong potential as animal models of SARS-CoV-2 infection and progression of resulting human disease.

Second Harmonic Generation Imaging of Collagen in Chronically Implantable Electrodes in Brain Tissue.

Advances in neural engineering have brought about a number of implantable devices for improved brain stimulation and recording. Unfortunately, many of these micro-implants have not been adopted due to issues of signal loss, deterioration, and host response to the device. While glial scar characterization is critical to better understand the mechanisms that affect device functionality or tissue viability, analysis is frequently hindered by immunohistochemical tissue processing methods that result in device shattering and tissue tearing artifacts. Devices are commonly removed prior to sectioning, which can itself disturb the quality of the study. In this methods implementation study, we use the label free, optical sectioning method of second harmonic generation (SHG) to examine brain slices of various implanted intracortical electrodes and demonstrate collagen fiber distribution not found in normal brain tissue. SHG can easily be used in conjunction with multiphoton microscopy to allow direct intrinsic visualization of collagen-containing glial scars on the surface of cortically implanted electrode probes without imposing the physical strain of tissue sectioning methods required for other high resolution light microscopy modalities. Identification and future measurements of these collagen fibers may be useful in predicting host immune response and device signal fidelity.

The role of inflammation on the functionality of intracortical microelectrodes.

OBJECTIVE: Neuroinflammation has long been associated with the performance decline of intracortical microelectrodes (IMEs). Consequently, several strategies, including the use of anti-inflammatories, have been employed to mitigate the inflammation surrounding IMEs. However, these strategies have had limited success towards achieving a chronically viable cortical neural interface, questioning the efficacy of anti-inflammatory approach. APPROACH: Herein, we conducted a systematic study in rats implanted with functional devices by modulating inflammation via systemic injection of lipopolysaccharide (LPS), dexamethasone (DEX), a combination of both, or none to assess the degree of inflammation on device functionality. We hypothesized that implanted rats treated with LPS will have a negative impact, and rats treated with DEX will have a positive impact on functionality IMEs and histological outcome. MAIN RESULTS: Contrary to our hypothesis, we did not observe adverse effects in recording metrics among different groups with LPS and/or DEX treatment despite alterations in initial pro-inflammatory markers. We also did not observe any functional benefit of anti-inflammatory treatment. Regardless of the treatment conditions, the recording quality degraded at chronic time points. In end-point histology, implanted rats that received LPS had significantly lower NeuN density and higher levels of CD68 surrounding the implant site, indicative of the pro-inflammatory effect of LPS, which, however, contradicted with the recorded results. SIGNIFICANCE: Collectively, our results suggest that acute inflammatory events may not be the key driver for functional degradation of IMEs. Future intervention strategies geared towards improving the functional longevity of intracortical devices may benefit using multi-modal approaches rather than a single approach, such as controlling the initial inflammatory response.

PrismPlus: a mouse line expressing distinct fluorophores in four different brain cell types.

To screen the complex central nervous system (CNS) injury responses, we created a quadruple-labelled 'PrismPlus' mouse line with a genetically encoded distinct fluorescent tag in oligodendrocytes, microglia, neurons, and astrocytes. Cx3cr1-gfp and Prism mice originally developed by Jung et al., 2000 and Dougherty et al., 2012, respectively, were cross-bred. First, we confirmed the presence of fluorophores in appropriate cell types in PrismPlus mice. PrismPlus mice were then used to examine the cellular responses to brain implanted micro-devices. We observed an increase in microglial response at earlier time points as compared to 4 weeks, a progressive astrocytic response, and fewer neurons at the vicinity of an implanted device. These results are similar to what has been described in literature using other rodent strains, previously attainable only through time-consuming and variable immunohistochemistry methods. Finally, we demonstrate the compatibility of PrismPlus brain tissue with CLARITY, an advanced tissue clearing technique, opening the door to future thick tissue imaging studies. This report confirms PrismPlus transgenic fluorescence and highlights the utility of these mice to study CNS injuries. The work herein seeks to establish a novel transgenic mouse line to improve experimental scope, consistency, and efficiency for CNS researchers.

Placing Sites on the Edge of Planar Silicon Microelectrodes Enhances Chronic Recording Functionality.

OBJECTIVE: This study aims to identify the impact of using edge sites over center sites on a planar silicon microelectrode array. METHODS: We used custom-designed, silicon-substrate multisite microelectrode arrays with sites on the center, edge, and tip. We compared their single unit recording capability, noise level, impedance, and histology to identify the differences between each site location. Wide and narrow devices were used to evaluate if the differences are consistent and meet theoretical expectations. RESULTS: On the wide device, significantly more number of edge sites were functional than center sites over the course of 8 weeks with generally higher signal-to-noise amplitude ratio. On the narrow device, edge sites also performed generally better than center sites, but the differences were not significant and smaller than wide devices. The data from the tip sites were inconclusive. CONCLUSION: Edge sites outperformed center sites in terms of single unit recording capability. This benefit decreased as the device gets narrower and the distance to center sites decreases. SIGNIFICANCE: We showed that a simple alteration to the site placement can greatly enhance the functionality of silicon microelectrodes. This study promotes the idea that not only the substrate but also the site architecture needs attention to lengthen the lifetime of neural implants.

Foreign Body Response to Intracortical Microelectrodes Is Not Altered with Dip-Coating of Polyethylene Glycol (PEG).

Poly(ethylene glycol) (PEG) is a frequently used polymer for neural implants due to its biocompatible property. As a follow-up to our recent study that used PEG for stiffening flexible neural probes, we have evaluated the biological implications of using devices dip-coated with PEG for chronic neural implants. Mice (wild-type and CX3CR1-GFP) received bilateral implants within the sensorimotor cortex, one hemisphere with a PEG-coated probe and the other with a non-coated probe for 4 weeks. Quantitative analyses were performed using biomarkers for activated microglia/macrophages, astrocytes, blood-brain barrier leakage, and neuronal nuclei to determine the degree of foreign body response (FBR) resulting from the implanted microelectrodes. Despite its well-known acute anti-biofouling property, we observed that PEG-coated devices caused no significantly different FBR compared to non-coated controls at 4 weeks. A repetition using CX3CR1-GFP mice cohort showed similar results. Our histological findings suggest that there is no significant impact of acute delivery of PEG on the FBR in the long-term, and that temporary increase in the device footprint due to the coating of PEG also does not have a significant impact. Large variability seen within the same treatment group also implies that avoiding large superficial vasculature during implantation is not sufficient to minimize inter-animal variability.

Histological evaluation of flexible neural implants; flexibility limit for reducing the tissue response?

OBJECTIVE: Flexible neural probes are hypothesized to reduce the chronic foreign body response (FBR) mainly by reducing the strain-stress caused by an interplay between the tethered probe and the brain's micromotion. However, a large discrepancy of Young's modulus still exists (3-6 orders of magnitude) between the flexible probes and the brain tissue. This raises the question of whether we need to bridge this gap; would increasing the probe flexibility proportionally reduce the FBR? APPROACH: Using novel off-stoichiometry thiol-enes-epoxy (OSTE+) polymer probes developed in our previous work, we quantitatively evaluated the FBR to four types of probes with different softness: silicon (~150 GPa), polyimide (1.5 GPa), OSTE+Hard (300 MPa), and OSTE+Soft (6 MPa). MAIN RESULTS: We observed a significant reduction in the fluorescence intensity of biomarkers for activated microglia/macrophages and blood-brain barrier (BBB) leakiness around the three soft polymer probes compared to the silicon probe, both at 4 weeks and 8 weeks post-implantation. However, we did not observe any consistent differences in the biomarkers among the polymer probes. SIGNIFICANCE: The results suggest that the mechanical compliance of neural probes can mediate the degree of FBR, but its impact diminishes after a hypothetical threshold level. This infers that resolving the mechanical mismatch alone has a limited effect on improving the lifetime of neural implants.

Materials approaches for modulating neural tissue responses to implanted microelectrodes through mechanical and biochemical means.

Implantable intracortical microelectrodes face an uphill struggle for widespread clinical use. Their potential for treating a wide range of traumatic and degenerative neural disease is hampered by their unreliability in chronic settings. A major factor in this decline in chronic performance is a reactive response of brain tissue, which aims to isolate the implanted device from the rest of the healthy tissue. In this review we present a discussion of materials approaches aimed at modulating the reactive tissue response through mechanical and biochemical means. Benefits and challenges associated with these approaches are analyzed, and the importance of multimodal solutions tested in emerging animal models are presented.

Resistive and reactive changes to the impedance of intracortical microelectrodes can be mitigated with polyethylene glycol under acute in vitro and in vivo settings.

The reactive response of brain tissue to implantable intracortical microelectrodes is thought to negatively affect their recordable signal quality and impedance, resulting in unreliable longitudinal performance. The relationship between the progression of the reactive tissue into a glial scar and the decline in device performance is unclear. We show that exposure to a model protein solution in vitro and acute implantation result in both resistive and capacitive changes to electrode impedance, rather than purely resistive changes. We also show that applying 4000 MW polyethylene glycol (PEG) prevents impedance increases in vitro, and reduces the percent change in impedance in vivo following implantation. Our results highlight the importance of considering the contributions of non-cellular components to the decline in neural microelectrode performance, and present a proof of concept for using a simple dip-coated PEG film to modulate changes in microelectrode impedance.

The effect of site placement within silicon microelectrodes on the long-term electrophysiological recordings.

Intracortical microelectrodes can be used to treat various neurological disorders given their capabilities to interface with single or multiple populations of neurons. However, most of these penetrating devices have been reported to fail over time, within weeks to months, putatively due to the foreign body response (FBR) which persistently aggravates the surrounding brain tissues. A number of studies have confirmed that various electrode properties, such as size, shape, and surface area, may play a role in the biological responses to the microelectrode. Further experimental data is needed to determine the effect of these properties on the FBR and the recording performance. In this paper, we evaluate the effect of site placement using Michigan arrays with sites on the center, edge, and tip of the shank. The results show that there is significant performance variance between the center, edge, and tip sites.

Intact histological characterization of brain-implanted microdevices and surrounding tissue.

Research into the design and utilization of brain-implanted microdevices, such as microelectrode arrays, aims to produce clinically relevant devices that interface chronically with surrounding brain tissue. Tissue surrounding these implants is thought to react to the presence of the devices over time, which includes the formation of an insulating "glial scar" around the devices. However, histological analysis of these tissue changes is typically performed after explanting the device, in a process that can disrupt the morphology of the tissue of interest. Here we demonstrate a protocol in which cortical-implanted devices are collected intact in surrounding rodent brain tissue. We describe how, once perfused with fixative, brains are removed and sliced in such a way as to avoid explanting devices. We outline fluorescent antibody labeling and optical clearing methods useful for producing an informative, yet thick tissue section. Finally, we demonstrate the mounting and imaging of these tissue sections in order to investigate the biological interface around brain-implanted devices.