Paul Yarowsky

Diffusion tensor imaging of the developing mouse brain

It is shown that diffusion tensor MR imaging (DTI) can discretely delineate the microstructure of white matter and gray matter in embryonic and early postnatal mouse brains based on the existence and orientation of ordered structures. This order was found not only in white matter but also in the cortical plate and the periventricular zone, which are precursors of the cerebral cortex. This DTI‐based information could be used to accomplish the automated spatial definition of the cortical plate and various axonal tracts. The DTI studies also revealed a characteristic evolution of diffusion anisotropy in the cortex of the developing brain. This ability to detect changes in the organization of the brain during development will greatly enhance morphological studies of transgenic and knockout models of cortical dysfunction. Magn Reson Med 46:18–23, 2001.

Anatomical characterization of human fetal brain development with diffusion tensor magnetic resonance imaging

The human brain is extraordinarily complex, and yet its origin is a simple tubular structure. Characterizing its anatomy at different stages of human fetal brain development not only aids in understanding this highly ordered process but also provides clues to detecting abnormalities caused by genetic or environmental factors. During the second trimester of human fetal development, neural structures in the brain undergo significant morphological changes. Diffusion tensor imaging (DTI), a novel method of magnetic resonance imaging, is capable of delineating anatomical components with high contrast and revealing structures at the microscopic level. In this study, high-resolution and high-signal-to-noise-ratio DTI data of fixed tissues of second-trimester human fetal brains were acquired and analyzed. DTI color maps and tractography revealed that important white matter tracts, such as the corpus callosum and uncinate and inferior longitudinal fasciculi, become apparent during this period. Three-dimensional reconstruction shows that major brain fissures appear while most of the cerebral surface remains smooth until the end of the second trimester. A dominant radial organization was identified at 15 gestational weeks, followed by both laminar and radial architectures in the cerebral wall throughout the remainder of the second trimester. Volumetric measurements of different structures indicate that the volumes of basal ganglia and ganglionic eminence increase along with that of the whole brain, while the ventricle size decreases in the later second trimester. The developing fetal brain DTI database presented can be used for education, as an anatomical research reference, and for data registration.

Three-dimensional anatomical characterization of the developing mouse brain by diffusion tensor microimaging.

Investigation of three-dimensional (3D) morphometry of developing brains has been hindered by a lack of imaging modalities that can monitor the 3D evolution of various anatomical structures without sectioning and staining processes. In this study, we combined magnetic resonance microimaging and diffusion tensor imaging techniques to accomplish such visualization. The application of this approach to developing mouse embryos revealed that it could clearly delineate early critical structures such as neuroepithelium, cortical plate, and various axonal structures, and follow their developmental evolution. The technique was applied to the study of the Netrin-1 mutant, allowing verification of its anatomical phenotype.

Spatial memory deficits in segmental trisomic Ts65Dn mice

Spatial memory was assessed in the segmental trisomic 16 mouse (Ts65Dn), a potential model for Down syndrome (DS), using the 12-arm radial maze (RAM). Ts65Dn mice have a portion of mouse chromosome 16 syntenic to the distal end of human chromosome 21 triplicated. On each of 8 daily trials of the RAM, Ts65Dn mice made fewer correct choices than control mice and performed at or near chance levels, indicating a deficit in spatial working memory. On trials 9 and 10, Ts65Dn mice performed as well as control mice on the initial 12 choices, but required a greater number of choices to complete the RAM. The improved performance of Ts65Dn mice on trials 9 and 10 was lost when the animals were retested after a 50-day retention period, suggesting that long-term memory is also defective. These results are not likely explained by differences in either response bias or perceptual discrimination. Ts65Dn and control mice displayed comparable levels of performance in spontaneous alternation in a T-maze, demonstrating that simple spatial memory was not impaired. In the elevated plus maze, Ts65Dn mice did not display higher anxiety levels which could affect their performance in the RAM. In fact, Ts65Dn mice visited open arms on the elevated plus maze more frequently and spent more time on open arms than did control mice. Taken together, these results provide evidence for short- and long-term spatial memory deficits in Ts65Dn mice.

Fluoxetine rescues deficient neurogenesis in hippocampus of the Ts65Dn mouse model for Down syndrome.

The Ts65Dn mouse, an adult model of Down syndrome displays behavioral deficits consistent with a dysfunctional hippocampus, similar to that seen with DS. In looking for mechanisms underlying these performance deficits, we have assessed adult neurogenesis in the dentate gyrus of Ts65Dn. Under untreated conditions, Ts65Dn mice (2-5 months old) showed markedly fewer BrdU-labeled cells than euploid animals. Chronic antidepressant treatment for over 3 weeks with the serotonin selective reuptake inhibitor, fluoxetine, increased neurogenesis in the Ts65Dn to comparable levels seen in the euploid by augmenting both proliferation and survival of BrdU-labeled cells in the subgranular layer and granule cell layer of the hippocampus, respectively.

Physiological Roles of the Sodium‐Calcium Exchanger in Nerve and Musclea

Sodium-calcium exchange has been well studied in cardiac muscle and in photoreceptors; the physiological roles of the exchanger in these tissues are widely recognized and appreciated (see other articles in this volume). A prominent Na-Ca exchanger has also been identified in both vertebrate and invertebrate neurons, but the physiological role(s) of the exchanger in neurons is (are) poorly understood. In other types of cells, such as vascular smooth muscle (VSM) cells, the physiological significance of an Na-Ca exchanger, and even its presence, have been questioned. This uncertainty has arisen because even large changes in the trans-plasmalemmal Na+ electrochemical gradient, ApNa, often do not markedly alter the resting cytosolic free-Ca2+ concentration, [Ca2+]c(rest), or, in VSM, “resting” tension. Here we review the evidence that there is a prominent, physiologically important Na-Ca exchanger in the plasmalemma of mammalian neurons as well as in astrocytes and VSM cells, and in barnacle “skeletal” muscle fibers. The exchangers in these cells clearly modulate [Ca2f]c(mt), even though this parameter is mainly under the control of the ATP-driven Ca2+ pumps in the plasmalemma and in the endoplasmic reticulum (ER) or, in muscle, the sarcoplasmic reticulum (SR). A very different situation prevails during cell activation, however, because even small effects of the exchanger on [Ca2+Idr,,) are reflected by relatively large changes in the amount of Caz+ stored in the ER or SR. Thus, a key role of the exchanger in many types of cells appears to be

Impaired spatial working and reference memory in segmental trisomy (Ts65Dn) mice

To evaluate the cognitive phenotype of the segmental trisomy 16 (Ts65Dn) mouse, a model of Down Syndrome (DS, trisomy 21), we assessed spatial working and reference memory using a 12-arm radial maze (RAM). Ts65Dn mice made a greater number of reference memory errors across trials compared to control mice. Both genotypes showed improvement across trials, although improvement was slower in Ts65Dn mice. Ts65Dn mice also made a greater number of working memory errors on the RAM, and in contrast to control mice, did not improve across trials, always performing at near-chance levels. These results provide evidence for both spatial working and reference memory deficits in Ts65Dn mice, characteristics of cognitive dysfunction.

Detection of amyloid plaques in mouse models of Alzheimer's disease by magnetic resonance imaging

We performed three‐dimensional, high‐resolution magnetic resonance imaging (MRI) of fixed mouse brains to determine whether MRI can detect amyloid plaques in transgenic mouse models of Alzheimers disease. Plaque‐like structures in the cortex and hippocampus could be clearly identified in T2‐weighted images with an image resolution of 46 μm × 72 μm × 72 μm. The locations of plaques were confirmed in coregistration studies comparing MR images with Congo red‐stained histological results. This technique is quantitative, less labor‐intensive compared to histology, and is free from artifacts related to sectioning process (deformation and missing tissues). It enabled us to study the distribution of plaques in the entire brain in 3D. The results of this study suggest that this method may be useful for assessing treatment efficacy in mouse models of Alzheimers disease (AD). Magn Reson Med 51:452–457, 2004.

Modulation of two functionally distinct Ca2+ stores in astrocytes: Role of the plasmalemmal Na/Ca exchanger

Mechanisms that regulate the amount of releasable Ca2+ in intracellular stores of cultured mouse astrocytes were investigated using digital imaging of fura‐2 loaded cells. At rest, the cytoplasmic Ca2+ concentration, [Ca2+]cyt, was about 110 nM. In the absence of extracellular Ca2+, cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca2+‐ATPase, induced a transient, four‐fold increase in [Ca2+]cyt due to the release of Ca2+ from inositol triphosphate (IP3) sensitive stores. Caffeine (CAF), which releases Ca2+ from Ca2+‐sensitive stores, induced a two‐fold increase in [Ca2+]cyt. The CPA‐ and CAF‐sensitive stores could be released independently. Changes in the amplitudes of the Ca2+ transients were taken as a measure of changes in store content. Removal of extracellular Na+ or addition of ouabain, which inhibit Ca2+ extrusion and promote Ca2+ entry across the plasmalemma via the Na/Ca exchanger, caused minimal increases in resting [Ca2+]cyt but greatly potentiated both CPA‐ and CAF‐induced Ca2+ transients. The amount of Ca2+ releasable from the IP3 (CPA) sensitive store was directly proportional to cytosolic Na+ concentration (i.e., inversely proportional to the transmembrane Na+ electrochemical gradient). Under these reduced Na+ gradient conditions, little, if any, Ca2+ destined for the ER stores enters the cells through voltage‐dependent Ca2+ channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2+ stores, the larger, IP3‐ (CPA‐) sensitive, and the smaller, Ca2+‐ (CAF‐) sensitive. The Ca2+ content of both ER stores can be regulated by the Na/Ca exchanger. Thus, the magnitude of cellular responses to signals that are mediated by Ca2+ release induced by the two second messengers, IP3 and Ca2+, can be modulated by factors that affect the net transport of Ca2+ across the plasmalemma.

Mapping postnatal mouse brain development with diffusion tensor microimaging

While mouse brain development has been extensively studied using histology, quantitative characterization of morphological changes is still a challenging task. This paper presents how developing brain structures can be quantitatively characterized with magnetic resonance diffusion tensor microimaging coupled with techniques of computational anatomy. High resolution diffusion tensor images of ex vivo postnatal mouse brains provide excellent contrasts to reveal the evolutions of mouse forebrain structures. Using anatomical landmarks defined on diffusion tensor images, tissue level growth patterns of mouse brains were quantified. The results demonstrate the use of these techniques to three-dimensionally and quantitatively characterize brain growth.