January 22nd - Common Mechanisms Underlying Growth Cone Guidance and Axon Branching

Katherine Kalil
Department of Anatomy
University of Wisconsin-Madison

To investigate how the cytoskeleton reorganizes at axon branch points, microtubules and actin filaments were fluorescently labeled in living cortical neurons and the behaviors of both cytoskeletal elements imaged during new growth from the axon shaft and the growth cone. In both regions microtubules were found to reorganize by splaying apart and fragmenting. Shorter microtubules then invade newly developing branches with anterograde and retrograde movements. Splaying of looped or bundled microtubules is accompanied by focal accumulation of f-actin. Dynamic microtubules were found to co-localize with f-actin in transition regions of growth cones and at axon branch points. In contrast, f-actin is excluded from the central region of the growth cone and the axon shaft. Interactions between dynamic microtubules and f-actin involve their coordinated polymerization. Application of drugs that attenuate either microtubule or f-actin dynamics also inhibits polymerization of the other cytoskeletal element. Importantly, inhibition of microtubule or f-actin dynamics prevents axon branching. Axons are still able to elongate but their outgrowth is undirected. These results show that interactions between dynamic microtubules and f-actin are required for axon branching and directed axon outgrowth. Taken together, these studies demonstrate that growth cone pausing is closely related to axon branching and suggest that common cytoskeletal mechanisms underlie directed axon growth from the terminal growth cone and the axon shaft.

February 26th - Multimodel Image Evaluation of NM404 in ApcMin+/ Mouse Mammary Carcinoma Model

Jamey Weichert
Contrast Agent Development Lab
Department of Radiology
University of Wisconsin-Madison

In vivo imaging has undergone explosive advancements in the past decade. In addition to providing exquisite anatomic details by computed tomography and magnetic resonance imaging, nuclear medicine and positron emission tomography can now provide biochemical information. These techniques can reliably detect tumors in humans less than 1 cm in diameter. Recent advances in these technologies, however, have improved the resolution capabilities 100 to 1000-fold so it in now possible to obtain spatial imaging resolution in live animals in the micrometer range. Our laboratory efforts focus on the discovery and development of targeted imaging agents. The design of these agents is based on biochemical pathways known to occur in the tissue or organ of interest. It is our goal to be able to not only provide high-resolution spatial images, but biochemical images in animals and humans as well. This presentation will focus on the evaluation of a new radiolabeled tumor imaging agent, NM404, in a mouse mammary carcinoma model by nuclear and microCT imaging techniques.

March 26th - Multi-Dimensional Calcium Signaling in Nerve Growth

Timothy Gomez
Department of Anatomy
University of Wisconsin-Madison

Filopodia are antenna-like extensions of neuronal growth cones that expand the environment sampled for extracellular guidance cues. Activation of receptors on their surface by interaction with ligands likely generates signals that are transmitted back to the growth cone where they are transduced into changes in motility, but the identity of these signals and whether they regulate filopodial movement directly is unknown. We find that filopodia generate localized transient elevations of intracellular calcium that propagate back to the growth cone and stimulate global calcium elevations. The frequency of filopodial calcium transients is substrate-dependent and may be due in part to influx of calcium through channels activated by integrin receptors. These transients slow neurite outgrowth by reducing filopodial motility, and promote turning when stimulated differentially within filopodia on one side of the growth cone. Calpain, a calcium-dependent protease, is one likely downstream target of these local calcium signals. These rapid transients appear to serve both as autonomous regulators of filopodial movement and as frequency-coded signals integrated within the growth cone and could be a common signaling process for many motile cells.

April 23rd - Optical Tomographic Imaging: Potentials of an Emerging Imaging Modality

Andreas Hielscher
Department of Biomedical Engineering
Columbia University

Optical Tomography (OT) is a fast developing new medical imaging modality that use near-infrared light to probe various sections of the human body. The light from laser diodes is delivered through optical fibers to several locations on the surface of the body, and measurements of back reflected and transmitted light intensities at other positions on the surface are recorded and analyzed. The technology for making these measurements is nowadays readily available and has mainly been applied to breast and brain imaging. However, a major challenge that still remains is the development of efficient numerical schemes that transform these data into useful cross-sectional images of the interior. Unlike x-rays, near-infrared photons do not cross the medium on a straight line from the source to the detector. Besides being absorbed, light is strongly scattered throughout the body. Therefore, back projection algorithms that are commonly used in computerized tomography (CT) cannot be applied for the tomographic image reconstruction in OT. Furthermore, the reconstruction problem is highly ill posed. Currently, the most promising type of algorithms for OT are so-called model-based iterative image reconstruction (MOBIIR) schemes. These schemes are computationally much more demanding than backprojection methods, but they more rigorously account for the underlying physics of light propagation in tissue. Furthermore, model-based algorithms allow incorporating prior knowledge about the region of interest into the reconstruction process, which makes the problem less ill posed. During this talk I will give a review of state-of-the-art reconstruction techniques for OT and present numerical and experimental results from various studies on tissue-phantoms, brains, limbs, and small animals. The potential of this promising new imaging modality for monitoring hemodynamics, functional brain imaging, as well as molecular imaging will be discussed.

May 28th - Molecular and Cellular Mechanisms of Synapse Formation and Maturation

Guosong Liu
Department of Brain and Cognitive Sciences
Massachusetts Institute of Technology

The human brain has roughly 10 billion neurons that communicate with one another through synaptic connections. Each neuron is capable of making 5000-10000 synapses to its targets, leading to 50-100 trillion synaptic connections in the brain. The connections between neurons give rise to functional neural networks that provide the cellular substrate for higher cognitive functions such as learning, memory, and ultimately consciousness. How these complex connections are established during the early phases of nervous system development is an important question that remains largely unknown. Comparisons among immature and mature nervous systems have shown that individual neurons make a large number of supernumerary connections during early development. From this work, it is believed that correlated neural activity initiated by external stimulation could then sculpt the pattern of connections through a selective strengthening of appropriate sets of latent connections, while eliminating the inappropriate. We have carried out a series of experiments to study functional maturation of individual synaptic connections during early development. I will discuss the dynamic localization of AMPA receptors during synaptogenesis, and switch in presynaptic transmitter release mechanisms at individual visualized synapses. These studies portray a dynamic picture of functional maturation of synapses during the development of synaptic transmission.