Stuart Tobet, PhDProfessor Phone: 970-491-1672 Member Education |
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Research Interests -- Determination of Cell Positions in the Developing
Neuroendocrine Brain
The long-term goal of my research is to determine cellular and molecular events underlying the differentiation of regions of the brain that underlie neuroendocrine function. Neuroendocrine structures are important because they regulate behavior and hoemostasis and are susceptible to a variety of diseases or syndromes such as Kallmann's, Prader-Willi, or Rubenstein-Taybi. I began my career by examining the long-term consequences of early hormone action on sexual behaviors, reproductive physiology, and hypothalamic structure. I proceeded to focus on molecular actions of gonadal steroids during critical periods of hormone action. We are now determining how multiple signals affect migration and cell position in the developing nervous system.
Cellular organization and differentiation may follow several courses leading to primarily layered (e.g., cerebral or cerebellar cortices) or nuclear (cell groupings within the thalamus and hypothalamus) structures. We concentrate on the formation of nuclear structures using the ventromedial and paraventricular nuclei of the hypothalamus (VMH and PVN) as model systems. The VMH and PVN are part of complex neural circuitries that regulate homeostatic, neuroendocrine, and behavioral functions. They migrate to specific positions within the respective nuclei characterized by neurochemical environment, phenotype of neighboring cells, and the pattern of anatomical connections. Our model system allows us to follow the formation of nuclei in vitro (video microscopy) and is easily accessible to manipulation. Our discovery of a unique relationship of the neurotransmitter GABA to the developing VMH, and more recently the PVN, has led to hypotheses that are being tested directly through pharmacological manipulations and selecting transgenic mouse models. In particular, we are taking advantage of two lines of gene-disrupted mice. In one line, a single gene-deletion (the nuclear orphan receptor, steroidogenic factor-1 or SF-1) leads to failure of VMH formation. In the other, the R1 subunit of the GABAB receptor is disrupted and GABAB receptors are functionally impaired. The unique targeting of the SF-1 gene to the VMH also leads to our efforts to examine transgenic mice in which green fluorescent protein (GFP) expression is driven by the SF-1 gene promoter and another line of mice in which GFP expression is driven by the NPY promoter (see images).
Click here for movie of cell movement in the developing neuroendocrine brain
We are also studying GABAergic mechanisms in the development
of neurons that synthesize gonadotropin-releasing hormone (GnRH).
During embryonic development, neurons containing GnRH migrate from
the nasal compartment, across the cribriform plate, into the brain
to reside in the basal forebrain. Over their early migration, we have
found GABA either in neighboring cells or within GnRH neurons in mice,
rats, humans and lamprey. Using pharmacology and a unique in vitro
slice paradigm that keeps the relationship of the head and brain intact,
we are testing mechanisms of migration directly. Here again, we are
taking advantage of lines of transgenic and gene-disrupted mice. In
particular, we are examining GnRH neuron migration in mice deficient
in the netrin-1 receptor, deleted in colon cancer (DCC). In addition,
we are using live video microscopy to examine the migration of GnRH
neurons that are revealed by GFP expression driven by the GnRH gene
promoter. We are characterizing intracellular, extracellular, and
cell surface molecules as they relate to this instance of neuronal
migration from the periphery into the CNS, a unique event.
Finally, we were led to examine the role of gonadal
steroids in cell migration when a monoclonal antibody we generated
revealed sex differences in antigen expression in radial glia in the
hypothalamus of perinatal rats. We further discovered a transient
radial glial scaffold that reaches from the lateral ventricles across
the anterior commissure through the sexually dimorphic preoptic/anterior
hypothalamus (POAA/AH) to the pial surface at its base. We are exploring
which cells might utilize this unique radial glial pathway using immunocytochemical
and live video microscopy techniques. We are particularly focused
on following the migratory behavior of cells that might contribute
to sex differences in structure or function. Our live video microscopy
allowed us to discover the first evidence of sex differences in neuronal
migration. Now, it is providing for our examination of hypotheses
of the mechanism(s) of sex differences. By focusing our efforts primarily
in murine models, we combine the utility of in vitro work and pharmacology
with the power of mouse genetics. Thus we are taking advantage of
different transgenic mice or mice in which selected genes have been
disrupted (e.g., SF-1 mentioned above, and others) to explore the
importance of specific steroid hormone systems in the POA/AH.
Representative Publications
Tobet SA. 2002. Genes controlling hypothalamic development and sexual differentiation. Eur J Neurosci 16:373-376.
Tobet SA, Walker HJ, Seney ML, Yu KW. 2003. Viewing cell movements in the developing neuroendocrine brain. Integr Comp Biol 43:794-801.
Bless EP, Walker HJ, Yu KW, Knoll JG, Moenter SM, Schwarting GA, Tobet SA. 2005. Live view of gonadotropin-releasing hormone containing neuron migration. Endocrinology 146:463-468.
Wolfe CA, Van Doren M, Walker HJ, Seney ML, McClellan KM, Tobet SA. 2005. Sex differences in the location of immunochemically defined cell populations in the mouse preoptic area/anterior hypothalamus. Dev Brain Res 157:34-41.
Tobet S, Schwarting G. 2006. Minireview: Recent Progress in GnRH neuronal migration. Endocrinology 147:1159-1165.
McClellan K, Parker K, Tobet S. 2006. Development of the ventromedial nucleus of the hypothalamus. Front Neuroendocrinol 27:193-209.
Edelmann M, Wolfe C, Scordalakes EM, Rissman EF, Tobet S. 2007. Neuronal nitric oxide synthase and calbindin delineate sex differences in the developing hypothalamus and preoptic area. Dev Neurobiol 67:1371-1381.
Knoll JG, Wolfe CA, Tobet SA. 2007. Estrogen modulates neuronal movements within the developing preoptic area/anterior hypothalamus. Eur J Neurosci 26:1091-1099.
McClellan KM, Calver AR, Tobet SA. 2008. GABAB Receptors role in cell migration and positioning within the ventromedial nucleus of the hypothalamus. Neuroscience. In Press.
