Sarah Libbrecht

High-density optrode-electrode neural probe using SixNy photonics for in vivo optogenetics

Moore s law in neural sciences: we present an optical neural probe (optoprobe) with the highest integration density of optrodes-electrodes using a CMOS process platform in 193 nm lithography. We designed, developed, and packaged an ultrathin (30 μm) optical neural probe, co-integrating silicon nitride (SixNy) photonics and biocompatible titanium nitride (TiN) electrodes (1). Functionality was verified in vivo by optically evoking and electrically recording neuronal activity in a mouse brain. Our design takes advantage of CMOS technology and incorporates 12 miniaturized optical outputs (optrodes) placed symmetrically next to 24 recording electrodes on a narrow 100-μm wide shank. We achieved an unprecedented optrode density by integrating grating couplers (GCs) instead of traditional end-butt coupling. The size of each optrode and electrode is 6 × 20 μm2 and 10 × 10 μm2 respectively, which is the typical size of a neuron. The system was capable of local excitation and recording of transduced neurons, a breakthrough achieved by the high-density integration.

Long-Term Fate Mapping Using Conditional Lentiviral Vectors Reveals a Continuous Contribution of Radial Glia-Like Cells to Adult Hippocampal Neurogenesis in Mice

Newborn neurons are generated throughout life in two neurogenic regions, the subventricular zone and the hippocampal dentate gyrus. Stimulation of adult neurogenesis is considered as an attractive endogenous repair mechanism to treat different neurological disorders. Although tremendous progress has been made in our understanding of adult hippocampal neurogenesis, important questions remain unanswered, regarding the identity and the behavior of neural stem cells in the dentate gyrus. We previously showed that conditional Cre-Flex lentiviral vectors can be used to label neural stem cells in the subventricular zone and to track the migration of their progeny with non-invasive bioluminescence imaging. Here, we applied these Cre-Flex lentiviral vectors to study neurogenesis in the dentate gyrus with bioluminescence imaging and histological techniques. Stereotactic injection of the Cre-Flex vectors into the dentate gyrus of transgenic Nestin-Cre mice resulted in specific labeling of the nestin-positive neural stem cells. The labeled cell population could be detected with bioluminescence imaging until 9 months post injection, but no significant increase in the number of labeled cells over time was observed with this imaging technique. Nevertheless, the specific labeling of the nestin-positive neural stem cells, combined with histological analysis at different time points, allowed detailed analysis of their neurogenic potential. This long-term fate mapping revealed that a stable pool of labeled nestin-positive neural stem cells continuously contributes to the generation of newborn neurons in the mouse brain until 9 months post injection. In conclusion, the Cre-Flex technology is a valuable tool to address remaining questions regarding neural stem cell identity and behavior in the dentate gyrus.

Proximal and distal modulation of neural activity by spatially confined optogenetic activation with an integrated high-density optoelectrode

Optogenetic manipulations are widely used for investigating the contribution of genetically identified cell types to behavior. Simultaneous electrophysiological recordings are less common, although they are critical for characterizing the specific impact of optogenetic manipulations on neural circuits in vivo. This is at least in part because combining photostimulation with large-scale electrophysiological recordings remains technically challenging, which also poses a limitation for performing extracellular identification experiments. Currently available interfaces that guide light of the appropriate wavelength into the brain combined with an electrophysiological modality suffer from various drawbacks such as a bulky size, low spatial resolution, heat dissipation, or photovoltaic artifacts. To address these challenges, we have designed and fabricated an integrated ultrathin neural interface with 12 optical outputs and 24 electrodes. We used the device to measure the effect of localized stimulation in the anterior olfactory cortex, a paleocortical structure involved in olfactory processing. Our experiments in adult mice demonstrate that because of its small dimensions, our novel tool causes far less tissue damage than commercially available devices. Moreover, optical stimulation and recording can be performed simultaneously, with no measurable electrical artifact during optical stimulation. Importantly, optical stimulation can be confined to small volumes with approximately single-cortical layer thickness. Finally, we find that even highly localized optical stimulation causes inhibition at more distant sites. NEW & NOTEWORTHY In this study, we establish a novel tool for simultaneous extracellular recording and optogenetic photostimulation. Because the device is built using established microchip technology, it can be fabricated with high reproducibility and reliability. We further show that even very localized stimulation affects neural firing far beyond the stimulation site. This demonstrates the difficulty in predicting circuit-level effects of optogenetic manipulations and highlights the importance of closely monitoring neural activity in optogenetic experiments.

The potential of stem cells in tackling neurodegenerative diseases

In the second part of this chapter, we will discuss adult neurogenesis in the context of neurodegeneration and the possible use of this stem cells source as a therapeutic target. Both in mammals as in humans, neurogenesis persists during adulthood in two germinal regions: the subventricular zone of the lateral ventricles and the dentate gyrus in the hippocampus. In neurodegenerative diseases, this neurogenesis cascade is impaired and might account for some of the presented preclinical symptoms. Hence, harnessing this endogenous stem cell pool as a therapeutic target might tackle the progressive neurodegeneration and alleviate some of these symptoms.