Ursula Greferath | PubMed Health

PubMed2006

Eph tyrosine kinase receptor EphA4 is required for the topographic mapping of the corticospinal tract.

Fine movement in the body is controlled by the motor cortex, which signals in a topographically specific manner to neurons in the spinal cord by means of the corticospinal tract (CST). How the correct topography of the CST is established is unknown. To investigate the possibility that the Eph tyrosine kinase receptor EphA4 is involved in this process, we have traced CST axons in mice in which the EphA4 gene has been deleted. The forelimb subpopulation of CST axons is unaffected in the EphA4-/- mice, but the hindlimb subpopulation branches too early within the cord, both temporally and spatially. EphA4 shows a dynamic expression pattern in the environment of the developing CST in the spinal cord: high at the time of forelimb branching and down-regulated before hindlimb branching. To examine whether the fore- and hindlimb subpopulations of CST axons respond differently to EphA4 in their environment, neurons from fore- and hindlimb motor cortex were cultured on a substrate containing EphA4. Neurons from the hindlimb cortex showed reduced branching on the EphA4 substrate compared with their forelimb counterparts. Neurons from the hindlimb cortex express ephrinA5, a high-affinity ligand for EphA4, at higher levels compared with forelimb cortex neurons, and this expression is down-regulated before hindlimb branching. Together, these findings suggest that EphA4 regulates topographic mapping of the CST by controlling the branching of CST axons in the spinal cord.

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PubMed2004

Tracing functional circuits using c-Fos regulated expression of marker genes targeted to neuronal projections.

We have developed novel techniques to trace functionally activated circuits and synaptic plasticity within the brain. We have generated transgenic mice, FTL, which contain a tau-lacZ fusion gene regulated by the promoter for c-fos. Following a particular nervous system stimulation in these mice, only neurons, which are functionally activated, will express LacZ, which is targeted to neuronal processes by the tau protein. In the FTL mice, we found highly inducible expression of lacZ by a range of different stimuli, and successful targeting of expression to neuronal cell bodies, axons and dendrites. To test if a functionally activated circuit could be visualized, the mice were deprived of water, which activates nuclei involved in body fluid homeostasis. LacZ was induced in these nuclei and their projections, allowing the mapping of a neuroendocrine circuit. Further studies have employed these mice in the analysis of neurons and circuits activated in vision, and learning and memory. We have also developed methods to measure markers of synaptic plasticity in the brain, and found significant experience dependent changes in the levels of these markers in different parts of the brain. We believe these techniques will aid in the identification of circuits for many different brain functions, and within those circuits, the locations of synaptic plasticity.

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PubMed2004

Fos-tau-LacZ mice expose light-activated pathways in the visual system.

We have employed fos-tau-LacZ (FTL) transgenic mice to examine functional activation in the visual areas of the nervous system. The FTL mice express the marker gene lacZ in neurons and their processes following many different stimuli, and allow the imaging of activation from the level of the entire brain surface through individual neurons and their projections. Analysis of FTL expression in the retinas of mice following diurnal exposure to light shows that bipolar cells, specific classes of amacrine cells, ganglion cells, and a dense network of processes in the inner plexiform layer are functionally activated. In animals deprived of light, there is almost no activity in the retina. In the lateral geniculate nucleus (LGN), light exposure appears responsible for FTL expression in dorsal nuclei, but not for expression in the ventral nuclei or the intergeniculate leaflet. In the superficial layers of the superior colliculus, FTL expression is highly dependent on light exposure. Similarly, light exposure is required for FTL expression in primary visual cortex (area 17), but some expression remains in area 18 of dark-adapted animals. Finally, using mice with one or both eyes missing, we have determined which parts of the visual system are dependent on the presence of a functional connectivity from the eye. These data demonstrate the usefulness of the FTL mice to map functional activation within the entire visual system. Furthermore, we can capture visual activation in a conscious animal. Our findings give an insight into the architecture of activity within the retina and throughout the visual system.

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PubMed2002

Estrogen treatment suppresses forebrain p75 neurotrophin receptor expression in aged, noncycling female rats.

There is increasing evidence that estrogen has beneficial effects on cognition, both in humans and in rodents, and may delay Alzheimer's disease onset in postmenopausal women. Several rodent studies have utilised the ovariectomy model to show estrogen regulation of the p75 neurotrophin receptor, TrkA, and markers of acetylcholine synthesis in the cholinergic basal forebrain. We studied estrogenic effects in aged (16-17-month-old), noncycling rats. Estrogen treatment for 10 days drastically reduced p75(NTR) immunoreactivity in the rostral parts of the basal forebrain. The number of p75(NTR)-immunoreactive neurons was decreased, and those neurons remaining positive for p75(NTR) showed reduced p75(NTR) staining intensity. In vehicle-treated rats, almost all choline acetyltransferase-immunoreactive neurons were p75(NTR) positive (and vice versa), but, in estrogen treated rats, large numbers of choline acetyltransferase-immunoreactive cells were negative for p75(NTR). Similar levels of p75(NTR) down-regulation in the rostral basal forebrain were found when estrogen treatment was extended to 6 weeks. There was no reduction in the number of p75(NTR)-immunoreactive neurons in the caudal basal forebrain after 10 days of treatment. After 6 weeks of treatment, however, there was evidence of p75(NTR) down-regulation in the caudal basal forebrain. There was no evidence of hypertrophy or atrophy of cholinergic neurons even after 6 weeks of estrogen treatment. Considering the evidence for the role of p75(NTR) in regulating survival, growth and nerve growth factor responsiveness of cholinergic basal forebrain neurons, the results indicate an important aspect of estrogen's effects on the nervous system.

Aging↗

PubMed2002

Visualization of functionally activated circuitry in the brain.

We have used a transgenic approach to visualize functionally activated neurons and their projections. The transgenic mice contain a tau-lacZ fusion gene regulated by the promoter for c-fos, an immediate early gene that is rapidly induced in neurons after functional stimulation. Constitutive expression of beta-galactosidase (beta-gal), the lacZ product, was low and in accord with previous reports of c-fos expression. However, expression of beta-gal in positive neurons was clearly in cell bodies, axons, and dendrites. Treatment of the mice with kainic acid, a strong inducer of c-fos expression, resulted in high induction of beta-gal. beta-gal was induced in the same defined populations of neurons in the brain as those that express c-fos after kainic acid induction. Furthermore, the pattern of beta-gal expression within the neurons changed over time after kainic acid treatment. Early after kainate treatment, beta-gal was found mainly in cell bodies; at later times, expression extended further along the neuronal processes. This expression pattern is consistent with induction and anterograde transport of the Fos-Tau-beta-gal protein in the neurons. To test whether a functionally activated pathway could be visualized, transgenic mice were deprived of water, which activates nuclei involved in body fluid homeostasis. beta-gal induction was traced in neurons and their processes in the lamina terminalis, in magnocellular neurons of the supraoptic and paraventricular nuclei, and in their projections to the posterior pituitary gland. This strategy allowed the mapping of an activated osmoregulatory pathway. This transgenic approach may have general application in the mapping of functionally activated circuitry in the brain.

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PubMed2002

Developmental expression of EphA4-tyrosine kinase receptor in the mouse brain and spinal cord.

Eph receptor tyrosine kinases and their ephrin ligands are involved in some of the most important steps during the development of the central nervous system, including cell migration, axon guidance, topographic mapping and synapse formation. Moreover, in the adult, they have been implicated in plasticity and regulation of neural stem cell function. One member of the Eph family, EphA4, can bind to both classes of ephrins and may have multiple functions in nervous system development. In order to look for potential sites of EphA4 action during central nervous system development, we conducted a spatio-temporal analysis of EphA4 protein expression. We used immunohistochemistry in preference to in situ hybridization because of the high likelihood that EphA4 protein is expressed on axon tracts, long distances from EphA4 mRNA. In the telencephalon, EphA4 was expressed in the developing cortex from embryonic day 11 (E11) and, later, on major cortical tracts including the corpus callosum and cortico-spinal tract. Robust EphA4 expression was also found in the hippocampus and fornix, and cells and tracts in the striatum. In the diencephalon, the thalamus, the hypothalamus and thalamo-cortical projection were strongly positive. In the mesencephalon, a number of different nuclei were weakly positive, most prominently the red nucleus. In the rhombencephalon, many nuclei were strongly positive including the cerebellum and one of its afferent paths, the inferior cerebellar peduncle, as well as the olivary region. In the spinal cord, there was a dynamic pattern of expression through development, with persistent expression in the dorsal funiculus and ventral grey matter.

Animals↗

PubMed2002

Developmental expression of EphA4-tyrosine kinase receptor in the mouse brain and spinal cord.

Eph receptor tyrosine kinases and their ephrin ligands are involved in some of the most important steps during the development of the central nervous system, including cell migration, axon guidance, topographic mapping and synapse formation. Moreover, in the adult, they have been implicated in plasticity and regulation of neural stem cell function. One member of the Eph family, EphA4, can bind to both classes of ephrins and may have multiple functions in nervous system development. In order to look for potential sites of EphA4 action during central nervous system development, we conducted a spatio-temporal analysis of EphA4 protein expression. We used immunohistochemistry in preference to in situ hybridization because of the high likelihood that EphA4 protein is expressed on axon tracts, long distances from EphA4 mRNA. In the telencephalon, EphA4 was expressed in the developing cortex from embryonic day 11 (E11) and, later, on major cortical tracts including the corpus callosum and cortico-spinal tract. Robust EphA4 expression was also found in the hippocampus and fornix, and cells and tracts in the striatum. In the diencephalon, the thalamus, the hypothalamus and thalamo-cortical projection were strongly positive. In the mesencephalon, a number of different nuclei were weakly positive, most prominently the red nucleus. In the rhombencephalon, many nuclei were strongly positive including the cerebellum and one of its afferent paths, the inferior cerebellar peduncle, as well as the olivary region. In the spinal cord, there was a dynamic pattern of expression through development, with persistent expression in the dorsal funiculus and ventral grey matter.

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