I received my Ph.D. from the Department of Molecular Biophysics and Biochemistry at Yale University in 1975 and subsequently conducted research as a postdoctoral fellow with Paul Greengard (Yale University School of Medicine) and Mordecai Blaustein (Washington University School of Medicine). I came to the University of Maryland School of Medicine as Assistant Professor of Physiology in 1979 and was promoted to Professor in 1990. At the present time my laboratory is funded by the NINDS (NIH) and the Department of Defense (USAMRMC). I have have been the recipient of Alfred P. Sloan Foundation, Guggenheim, and Fogarty International fellowships. During the 1991-92 academic year, I was on sabbatical leave in the laboratory of Martin Raff (University College London) studying programmed cell death (apoptosis) during brain development. Until recently, my research focused on the molecular structure and function of ion channels in excitable membranes and I continue to be co-coursemaster of our course on "Ion Channels" offered annually to second-year graduate students. Since 1999, I have been coursemaster of "Introduction to Neuroscience".

Neurotrophin Signaling in the Developing, Adult and Aging Brain

The principal research interests of this laboratory are the cellular and molecular mechanisms that underlie brain development and cognitive behavior. Most of our recent work has focused on the mechanism of action of the neurotrophin, brain-derived neurotrophic factor (BDNF), acting via its receptor, trkB. BDNF exerts a wide variety of effects on neuronal function. During development, it promotes neuron survival and regulates the growth of axons and the specificity of synaptic connections. In the mature brain, BDNF not only maintains neuron survival but also influences cognitive function by modulating synaptic plasticity. Finally, BDNF/trkB signaling has been implicated in neurodegenerative and psychiatric disorders such as Alzheimer’s disease and depression.

Defective BDNF/trkB Signaling and Impaired Neuron Survival in the Trisomy 16 (Ts16) Mouse. The Ts16 mouse has a naturally-occurring triplication of chromosome 16, which contains most of the human chromosome 21 genes that are triplicated in Down syndrome (Ts21). The development of the cerebral cortex is defective in Ts16 mice and cortical neurons undergo accelerated programmed cell death (apoptosis). We have found that Ts16 hippocampal neurons in vitro undergo accelerated apoptosis and that this is due to their failure to respond to the neuroprotective actions of BDNF. A similar defect could result in neuron vulnerability in neurodegenerative disorders such as Alzheimer's disease. This defect is due to overexpression of the inactive, truncated isoform of trkB (Fig. 1), which acts as a dominant-negative inhibitor of BDNF signaling. Overexpression of the active, full-length trkB in Ts16 neurons, introduced by an adenoviral vector, restored the neuroprotective effect of BDNF.

Ca2+-dependent Regulation of TrkB Expression in Neurons. Neuronal depolarization results in increased expression of the full-length, catalytically active isoform of trkB without affecting expression of the truncated isoform. The expression of trkB is regulated by neuronal activity via Ca2+-dependent regulatory elements in the trkB gene. TrkB transcription can be initiated at either of two start sites, designated P1 and P2. P1 is inhibited by Ca2+ while P2 is stimulated by Ca2+. The Ca2+-dependence of P2 is conferred by the transcription factor CREB, acting at a pair of CRE sites upstream from the transcriptional start site. Our results demonstrate that excitation can enhance neuronal BDNF responsiveness by selectively stimulating the expression of full-length trkB and suggest that this modulation is mediated by Ca2+-dependent promoter selection.

Abnormal Cortical Development in the Ts16 Mouse. We have determined that the cerebral cortex of the Ts16 mouse develops abnormally. There is a delay in the radial expansion of the cortex during the embryonic period when postmitotic neurons leave the proliferative ventricular zone and migrate to their normal adult positions. Since the timing of this delay coincides with initial formation of cortical connections, we hypothesize that the delay could lead to abnormal connectivity in the Ts16 cortex. We have determined that the development of the Ts16 cortex begins with a shortage of "founder cells" and that the signal to send cells out of the cell cycle and on to become postmitotic neurons migrating to the cortical plate occurs with a one-day delay. Thus, daughter cells of proliferating neuroblasts in the Ts16 cortex are more likely to return to the cell cycle than to become postmitotic neurons. The delay in generating cortical neurons is especially severe for the subplate, a small population of "pioneering" neurons that normally precedes the majority cortical plate neurons and is thought to direct the subsequent lamination and synaptic connectivity of the cortex. In the Ts16 cortex, the subplate and cortical plate neurons are generated concurrently. The consequence of this timing error is that thalamocortical neurons fail to innervate the subplate and cortical plate in the Ts16 cortex (Fig. 2). These developmental disorders in the embryonic Ts16 mouse cortex predict parallel defects during cortical development in Down syndrome. We are also investigating whether defective BDNF/trkB signaling in Ts16 cells and altered Ca2+ homeostasis (see below) may contribute to these developmental abnormalities.

Defective Ca2+ Homeostasis in Ts16 Astrocytes and Neurons. We have also found that astrocytes and neurons from Ts16 mice maintain elevated levels of intracellular Ca2+ and that intracellular, endoplasmic reticulum Ca2+ stores are overloaded in the Ts16 cells. Such defects would be expected to severely compromise cell function and intercellular signaling. We are investigating the cellular mechanisms of this defect as well as its possible role in the impaired survival of Ts16 neurons and abnormal development of the Ts16 cortex.

• Recombinant DNA technology
• Measurement of protein and RNA expression by in situ hybridization, northern and western blotting
• Neurohistology, neural pathway mapping
• Cell culture (neurons, astrocytes, organotypic brain slices)
• Cell survival measurements; assays of apoptosis (pyknosis, TUNEL)
• Computer assisted calcium imaging
• Confocal microscopy
• Immunohistochemistry
• Behavioral measurements of learning and memory in live animals
• Production of animal models of neurodegenerative disorders
• Electrophysiology (patch clamp, planar bilayer)