Faculty Bibliography

Hedgehog signaling is required for multiple aspects of brain development, including growth, the establishment of both dorsal and ventral midline patterning and the generation of specific cell types such as oligodendrocytes and interneurons. To identify more precisely when during development hedgehog signaling mediates these events, we directed the removal of hedgehog signaling within the brain by embryonic day 9 of development, using a FoxG1(Cre) driver line to mediate the removal of a conditional smoothened null allele. We observed a loss of ventral telencephalic patterning that appears to result from an initial lack of specification of these structures rather than by changes in proliferation or cell death. A further consequence of the removal of smoothened in these mice is the near absence of both oligodendrocytes and interneurons. Surprisingly, the dorsal midline appears to be patterned normally in these mutants. Together with previous analyses, the present results demonstrate that hedgehog signaling in the period between E9.0 and E12 is essential for the patterning of ventral regions and the generation of cell types that are thought to largely arise from them

We distinguish the cardiac pacemaking and conduction system (CPCS) from neighboring working cardiomyocytes by its function to generate and deliver electrical impulses within the heart. Yet the CPCS is a series of integrated but distinct components. The components must act in a coordinated fashion, but they are also functionally, molecularly, and electrophysiologically unique. Understanding the differentiation and function of this elegant and complex system is an exciting challenge. Knowledge of genes and signaling pathways that direct CPCS development is at present minimal, but the use of transgenic mice represents an enormous opportunity for elucidating the unknown. Transgenic marker lines have enabled us to image and manipulate the CPCS in new ways. These tools are now being used to examine the CPCS in mutants where its formation and function is altered, generating new information and directions for study of the genetics of CPCS development

Although recent research has focused on the fundamental role(s) of steroids synthesized de novo in the brain on development, the mechanism by which production of these neurosteroids is regulated remains unclear. Steroid production in peripheral tissues is acutely regulated by the steroidogenic acute regulatory (StAR) protein, which mediates the rate-limiting step in steroid biosynthesis: the intramitochondrial delivery of cholesterol to cytochrome P450scc for conversion to steroid. We recently demonstrated that StAR is present in discrete cell types in the adult brain, suggesting that neurosteroid production is mediated by StAR. Nevertheless, little is known regarding the presence of StAR in the developing brain. In the present study, the presence of StAR and for the first time, its homolog, the putative cholesterol transport protein metastatic lymph node 64 (MLN64), were defined in the neonatal mouse brain using immunocytochemical techniques. Both StAR and MLN64 were found to be present in the brain with staining patterns characteristic to each protein, indicating the authenticity of StAR and MLN64 immunoreactivity. Furthermore, we found MLN64 to be expressed in the adult brain as well, apparently at higher levels than StAR. Importantly, StAR protein is present in cells that also express P450scc. These data suggest that, as with the adult, neurosteroid production during development occurs through a StAR-mediated pathway

Degeneration of cholinergic nucleus basalis (NB) cortical projection neurons is associated with cognitive decline in late-stage Alzheimer's disease (AD). NB neuron survival is dependent on coexpression of the nerve growth factor (NGF) receptors p75(NTR) and TrkA, which bind NGF in cortical projection sites. We have shown previously a significant reduction of NB perikarya expressing p75(NTR) and TrkA protein during the early stages of AD. Whether there is a concomitant reduction in cortical levels of these receptors during the progression of AD is unknown. p75(NTR) and TrkA protein was evaluated by quantitative immunoblotting in five cortical regions (anterior cingulate, superior frontal, superior temporal, inferior parietal, and visual cortex) of individuals clinically diagnosed with no cognitive impairment (NCI), mild cognitive impairment (MCI), mild/moderate AD, or severe AD. Cortical p75(NTR) levels were stable across the diagnostic groups. In contrast, TrkA levels were reduced approximately 50% in mild/moderate and severe AD compared with NCI and MCI in all regions except visual cortex. Mini-Mental Status Examination scores correlated with TrkA levels in anterior cingulate, superior frontal, and superior temporal cortex. The selective reduction of cortical TrkA levels relative to p75(NTR) may have important consequences for cholinergic NB function during the transition from MCI to AD

Expression profiling data is available for many diverse tissues throughout the body, allowing for exciting hypothesis testing of critical concepts such as cellular development, differentiation, normative function, and disease pathogenesis. The central nervous system is an ideal structure to evaluate relationships between functional genomics and expression data. Recent developments in gene array technologies, specifically cDNA microarray platforms, have made it easier to try to understand the multiplicity of gene alterations that occur within the brains of animal models and postmortem human tissues. However, unlike structures have one principal cell type, the brain contains diverse populations of phenotypically distinct cell types. A goal of modern molecular and cellular neuroscience is to assay gene expression from homogeneous populations of cells within a defined region without potential contamination by expression profiles of adjacent neuronal subtypes and non-neuronal cells. This is a difficult task that demands a multidisciplinary approach that is highlighted in this review within the context of neurodegenerative pathology

While cardiac function in the mature heart is dependent on a properly functioning His-Purkinje system, the early embryonic tubular heart efficiently pumps blood without a distinct specialized conduction system. Although His-Purkinje system precursors have been identified using immunohistological techniques in the looped heart, little is known whether these precursors function electrically. To address this question, we used high-resolution optical mapping and fluorescent dyes with two CCD cameras to describe the motion-corrected activation patterns of 76 embryonic chick hearts from tubular stages (stage 10) to mature septated hearts (stage 35). Ventricular activation in the tubular looped heart (stages 10-17) using both calcium-sensitive fluo-4 and voltage-sensitive di-4-ANEPPS shows sequentially uniform propagation. In late looped hearts (stages 18-22), domains of the dorsal and lateral ventricle are preferentially activated before spreading to the remaining myocardium and show alternating regions of fast and slow propagation. During stages 22-26, action potentials arise from the dorsal ventricle. By stages 27-29, action potential breakthrough is also observed at the right ventricle apex. By stage 31, activation of the heart proceeds from foci at the apex and dorsal surface of the heart. The breakthrough foci correspond to regions where putative conduction system precursors have been identified immunohistologically. To date, our study represents the most detailed electrophysiological characterization of the embryonic heart between the looped and preseptated stages and suggests that ventricular activation undergoes a gradual transformation from sequential to a mature pattern with right and left epicardial breakthroughs. Our investigation suggests that cardiac conduction system precursors may be electrophysiologically distinct and mature gradually throughout cardiac morphogenesis in the chick

The improvement of MRI speed with parallel acquisition is ultimately an SNR-limited process. To offset acquisition- and reconstruction-related SNR losses, practical parallel imaging at high accelerations should include the use of a many-element array with a high intrinsic signal-to-noise ratio (SNR) and spatial-encoding capability, and an advantageous imaging paradigm. We present a 32-element receive-coil array and a volumetric paradigm that address the SNR challenge at high accelerations by maximally exploiting multidimensional acceleration in conjunction with noise averaging. Geometric details beyond an initial design concept for the array were determined with the guidance of simulations. Imaging with the support of 32-channel data acquisition systems produced in vivo results with up to 16-fold acceleration, including images from rapid abdominal and MRA studies

Neural activity in primary motor cortex (MI) is known to correlate with hand position and velocity. Previous descriptions of this tuning have (1) been linear in position or velocity, (2) depended only instantaneously on these signals, and/or (3) not incorporated the effects of interneuronal dependencies on firing rate. We show here that many MI cells encode a superlinear function of the full time-varying hand trajectory. Approximately 20% of MI cells carry information in the hand trajectory beyond just the position, velocity, and acceleration at a single time lag. Moreover, approximately one-third of MI cells encode the trajectory in a significantly superlinear manner; as one consequence, even small position changes can dramatically modulate the gain of the velocity tuning of MI cells, in agreement with recent psychophysical evidence. We introduce a compact nonlinear "preferred trajectory" model that predicts the complex structure of the spatiotemporal tuning functions described in previous work. Finally, observing the activity of neighboring cells in the MI network significantly increases the predictability of the firing rate of a single MI cell; however, we find interneuronal dependencies in MI to be much more locked to external kinematic parameters than those described recently in the hippocampus. Nevertheless, this neighbor activity is approximately as informative as the hand velocity, supporting the view that neural encoding in MI is best understood at a population level.

To address computational 'scale-up' issues in modeling large regions of the cortex, many coarse-graining procedures have been invoked to obtain effective descriptions of neuronal network dynamics. However, because of local averaging in space and time, these methods do not contain detailed spike information and, thus, cannot be used to investigate, e.g., cortical mechanisms that are encoded through detailed spike-timing statistics. To retain high-order statistical information of spikes, we develop a hybrid theoretical framework that embeds a subnetwork of point neurons within, and fully interacting with, a coarse-grained network of dynamical background. We use a newly developed kinetic theory for the description of the coarse-grained background, in combination with a Poisson spike reconstruction procedure to ensure that our method applies to the fluctuation-driven regime as well as to the mean-driven regime. This embedded-network approach is verified to be dynamically accurate and numerically efficient. As an example, we use this embedded representation to construct 'reverse-time correlations' as spiked-triggered averages in a ring model of orientation-tuning dynamics