Engineered Axonal Tracts as “Living Electrodes” for Synaptic‐Based Modulation of Neural Circuitry

Abstract

Brain-computer interface and neuromodulation strategies relying on penetrating non-organic electrodes/optrodes are limited by an inflammatory foreign body response that ultimately diminishes performance. A novel "biohybrid" strategy is advanced, whereby living neurons, biomaterials, and microelectrode/optical technology are used together to provide a biologicallybased vehicle to probe and modulate nervous-system activity. Microtissue engineering techniques are employed to create axon-based "living electrodes", which are columnar microstructures comprised of neuronal population(s) projecting long axonal tracts within the lumen of a hydrogel designed to chaperone delivery into the brain. Upon microinjection, the axonal segment penetrates to prescribed depth for synaptic integration with local host neurons, with the perikaryal segment remaining externalized below conforming electrical-optical arrays. In this paradigm, only the biological component ultimately remains in the brain, potentially attenuating a chronic foreign-body This article is protected by copyright. All rights reserved. 4 response. Axon-based living electrodes are constructed using multiple neuronal subtypes, each with differential capacity to stimulate, inhibit, and/or modulate neural circuitry based on specificity uniquely afforded by synaptic integration, yet ultimately computer controlled by optical/electrical components on the brain surface. Current efforts are assessing the efficacy of this biohybrid interface for targeted, synaptic-based neuromodulation, and the specificity, spatial density and longterm fidelity versus conventional microelectronic or optical substrates alone.

Assessing functional connectivity across 3D tissue engineered axonal tracts using calcium fluorescence imaging

“…Micro-TENNs as a Platform Technology for Brain Pathway Reconstruction or as "Living Electrodes"[1],[4]-[6]. (A) Micro-TENNs initially consist of a 3D hydrogel cylinder (gray) encasing an extracellular matrix core of collagen and laminin (red), to date measuring up to several centimeters in length.…”

Innervation: the missing link for biofabricated tissues and organs

“…In a different fabrication process, we have created engineered constructs called micro-TENNs that consist of an aggregate of neurons seeded within the collagen-filled lumen of a hydrogel micro-column (345-710 μm in diameter) and projecting millimeter-to centimeter-scale aligned axonal tracts (Fig. 4a, b) 19,21,[207][208][209][210] . These micro-TENNs have been traditionally designed for minimally invasive implantation in the CNS featuring neurons isolated from the cerebral cortex ( Fig.…”

“…4c-e) or the ventral midbrain ( Fig. 4f-i) for applications in cortical-thalamic pathway reconstruction 208 and as synaptic-based interfaces with cortical circuitry 210,211 , or as replacements for the degenerated nigrostriatal pathway in Parkinson's disease 21 , respectively. These constructs replicate the structure of axonal pathways projecting from a discrete neuronal population, and thus may also mimic the ganglia-axon tract architecture in the PNS/ANS.…”

“…d, e High magnification images of the aggregate and axon tract regions of the micro-TENN construct, respectively. (Reprinted with permission from Serruya et al 210 ; authors of content, John Wiley and Sons). f Bidirectional micro-TENN at 14 days fabricated with an aggregate of rat embryonic ventral midbrain neurons (left) and striatal neurons (right) and stained for dopaminergic neurons (tyrosine hydroxylase (TH); red), medium spiny (striatal) neurons (DARPP-32; green), synapses (synapsin I; purple), and nuclei (Hoechst; blue).…”

Emerging regenerative medicine and tissue engineering strategies for Parkinson’s disease

“…15,19 To date, micro-TENNs have been fabricated using dorsal root ganglia neurons, cerebral cortical neurons (e.g., mixed glutamatergic and GABAergic), embryonic rodent ventral mesencephalic dopaminergic neurons, and human ESC-derived dopaminergic neurons. 5,125,126,129,[141][142][143][144] As our understanding of PD pathophysiology expands, it is possible that multiple micro-TENNs could be transplanted to reconnect different damaged regions, comprised of alternative neuronal phenotypes, such as noradrenergic, serotoninergic, and cholinergic cell types. However, it is difficult to predict how transplant therapies, including either transplanted cells or pathways, such as micro-TENNs, could be used to recapitulate widely dispersed innervation of cerebral cortex from cholinergic, serotonergic, or noradrenergic brainstem nuclei.…”