Faculty Bibliography

Purpose: Secondary cell death via gap junctions plays a critical role in the cell loss associated with neurodegenerative diseases (Belousov and Fontes 2013). In the retina we reported that secondary cell death accounts for most cell loss that occurs under excitotoxic and ischemic conditions (ARVO, 2011). Secondary cell death in CNS may be connexin specific and connexins may be up- or down-regulated under different pathological conditions (Rouach et al., 2002). Here we examined whether secondary cell death of retinal ganglion cells (RGCs) under excitotoxic and ischemic conditions is connexin-specific and whether the expression of Cx36 and Cx45 in inner retina is differentially effected. Methods: Excitotoxicity was induced in vitro by incubation of mouse retinas in NMDA. Transient retinal ischemia was induced in vivo by elevation of IOP. Levels of cell death were assayed histologically and antibodies against Cx36 and Cx45 were used to assess their levels in the IPL. Results: Consistent with our earlier work, we found that excitotoxic and ischemic conditions produced a significant loss of RGCs. Ablation of Cx36 in the Cx36 knockout (KO) mouse retina resulted in an ~70% decrease in RGC loss under excitotoxic conditions, whereas RGC loss in the Cx45 KO retina was not statistically different than that seen in WT mice. In contrast, RGC loss with ischemia was significantly reduced in Cx45 KO retinas, whereas the loss in Cx36 KO retinas was similar to that in the WT. In WT retinas the expression of Cx36 and Cx45 in the IPL followed a punctuate pattern typical for gap junctions. Under excitotoxic conditions the expression of Cx45 was down-regulated, whereas there were no detectable changes in Cx36 expression. In contrast, induction of ischemic conditions produced a dramatic change in Cx36 expression, which appeared as dense clusters around nuclei rather than as puncta. We found no change in the control punctate labeling pattern of Cx45 expression in ischemic retinas. Conclusions: Secondary cell death of RGCs is connexin specific where Cx36 gap junctions play a role under excitotoxic conditions and Cx45 gap junctions play a role during ischemia. These results are consistent with changes in connexin expression seen under these two conditions. These results suggest that targeting of specific connexins can be a novel therapeutic strategy for reducing RGC loss under different pathological conditions

Non-invasive fluorescence retinal imaging in small animals is an important requirement in an array of translational vision applications. Two-photon imaging has the potential for long-term investigation of healthy and diseased retinal function and structure in vivo. Here, we demonstrate that two-photon microscopy through a mouse's pupil can yield high-quality optically sectioned fundus images. By remotely scanning using an electronically tunable lens we acquire highly-resolved 3D fluorescein angiograms. These results provide an important step towards various applications that will benefit from the use of infrared light, including functional imaging of retinal responses to light stimulation.

Low-intensity ultrasonic waves can remotely and nondestructively excite central nervous system (CNS) neurons. While diverse applications for this effect are already emerging, the biophysical transduction mechanism underlying this excitation remains unclear. Recently, we suggested that ultrasound-induced intramembrane cavitation within the bilayer membrane could underlie the biomechanics of a range of observed acoustic bioeffects. In this paper, we show that, in CNS neurons, ultrasound-induced cavitation of these nanometric bilayer sonophores can induce a complex mechanoelectrical interplay leading to excitation, primarily through the effect of currents induced by membrane capacitance changes. Our model explains the basic features of CNS acoustostimulation and predicts how the experimentally observed efficacy of mouse motor cortical ultrasonic stimulation depends on stimulation parameters. These results support the hypothesis that neuronal intramembrane piezoelectricity underlies ultrasound-induced neurostimulation, and suggest that other interactions between the nervous system and pressure waves or perturbations could be explained by this new mode of biological piezoelectric transduction.

The precision, accuracy, and efficiency of a novel semi-automated renal segmentation technique for volumetric interpolated breath-hold sequence (VIBE) MRI sequences was analyzed using 7 clinical datasets (14 kidneys). Two observers performed whole-kidney segmentation using EdgeWave segmentation software based on constrained morphological growth. Ground truths were prepared by manual tracing of kidney on each of approximately 40 slices. Using the software, the average inter-observer disagreement was 2.7%+/- 2.1% for whole kidney volume, 2.1%+/- 1.8% for cortex, and 4.1%+/- 3.2% for medulla. In comparison to the ground truth kidney volume, the error was 2.8%+/- 2.5% for whole kidney volume, 3.1%+/- 1.7% for cortex, and 3.6%+/-.3.1% for medulla. It took an average of 4:14 +/- 1:42 minutes of operator time, plus 2:56 +/- 1:23 minutes of computer time to perform segmentation of one whole kidney, cortex, and medulla. Segmentation speed, inter-observer agreement and accuracy were several times better than those of our existing graph-cuts segmentation technique requiring approximately 20 minutes per case and with 7-10% error. With the expedited image processing, high inter-observer agreement, and volumes closely matching true values, kidney volumetry becomes a reality in many clinical applications.

A fundamental task in neuroscience is to understand how neural ensembles represent information. Population decoding is a useful tool to extract information from neuronal populations based on the ensemble spiking activity. We propose a novel Bayesian decoding paradigm to decode unsorted spikes in the rat hippocampus. Our approach uses a direct mapping between spike waveform features and covariates of interest and avoids accumulation of spike sorting errors. Our decoding paradigm is nonparametric, encoding model-free for representing stimuli, and extracts information from all available spikes and their waveform features. We apply the proposed Bayesian decoding algorithm to a position reconstruction task for freely behaving rats based on tetrode recordings of rat hippocampal neuronal activity. Our detailed decoding analyses demonstrate that our approach is efficient and better utilizes the available information in the nonsortable hash than the standard sorting-based decoding algorithm. Our approach can be adapted to an online encoding/decoding framework for applications that require real-time decoding, such as brain-machine interfaces.

Pyramidal cells in the rodent hippocampus often exhibit clear spatial tuning in navigation. Although it has been long suggested that pyramidal cell activity may underlie a topological code rather than a topographic code, it remains unclear whether an abstract spatial topology can be encoded in the ensemble spiking activity of hippocampal place cells. Using a statistical approach developed previously, we investigate this question and related issues in greater detail. We recorded ensembles of hippocampal neurons as rodents freely foraged in one- and two-dimensional spatial environments and used a "decode-to-uncover" strategy to examine the temporally structured patterns embedded in the ensemble spiking activity in the absence of observed spatial correlates during periods of rodent navigation or awake immobility. Specifically, the spatial environment was represented by a finite discrete state space. Trajectories across spatial locations ("states") were associated with consistent hippocampal ensemble spiking patterns, which were characterized by a state transition matrix. From this state transition matrix, we inferred a topology graph that defined the connectivity in the state space. In both one- and two-dimensional environments, the extracted behavior patterns from the rodent hippocampal population codes were compared against randomly shuffled spike data. In contrast to a topographic code, our results support the efficiency of topological coding in the presence of sparse sample size and fuzzy space mapping. This computational approach allows us to quantify the variability of ensemble spiking activity, examine hippocampal population codes during off-line states, and quantify the topological complexity of the environment.

Sunlight is the ultimate driver of biosynthesis but photochemical steps late in biosynthetic pathways are very rare. They appear to play a role in the formation of certain furanocembranoids isolated from Caribbean corals. One of these compounds, intricarene, has been suspected to arise from an intramolecular 1,3-dipolar cycloaddition involving an oxidopyrylium. Here we show, by a combination of experiments and theory, that the oxidopyrylium forms under photochemical conditions and that its cycloaddition occurs via a triplet state. The formation of a complex by-product can be rationalized by another photochemical step that involves a conical intersection. Our work raises the question whether intricarene is biosynthesized in the natural habitat of the corals or is an artefact formed during workup. It also demonstrates that the determination of exact irradiation spectra, in combination with quantum chemical calculations, enables the rationalization of complex reaction pathways that involve multiple excited states.

Opioid receptors (ORs) are widely distributed in the brain, the spinal cord, and the digestive tract and play an important role in nociception. All known ORs are G-protein-coupled receptors (GPCRs) of family A. Another well-known member of this family, rhodopsin, is activated by light through the cis/trans isomerization of a covalently bound chromophore, retinal. We now show how an OR can be combined with a synthetic azobenzene photoswitch to gain light sensitivity. Our work extends the reach of photopharmacology and outlines a general strategy for converting Family A GPCRs, which account for the majority of drug targets, into photoreceptors.

Periodic mesoporous organosilica (PMO) materials offer a strategy to position molecular semiconductors within a highly defined, porous network. We developed thin films of a new semiconducting zinc phthalocyanine-bridged PMO exhibiting a face-centered orthorhombic pore structure with an average pore diameter of 11 nm. The exceptional degree of order achieved with this PMO enabled us to create thin films consisting of a single porous domain throughout their entire thickness, thus providing maximal accessibility for subsequent incorporation of a complementary phase. The phthalocyanine building blocks inside the pore walls were found to be well-aggregated, enabling electronic conductivity and extending the light-harvesting capabilities to the near IR region. Ordered 3D heterojunctions capable of promoting photo-induced charge transfer were constructed by impregnation of the PMO with a fullerene derivative. When integrated into a photovoltaic device, the infiltrated PMO is capable of producing a high open-circuit voltage and a considerable photocurrent, which represents a significant step towards potential applications of PMOs in optoelectronics.