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Molecular Mechanisms Associated With Thyroid Hormone Induced Differentiation And Maturation Of Astrocytes Using Primary Cultures From Rat Brain

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Title Molecular Mechanisms Associated With Thyroid
Hormone Induced Differentiation And Maturation Of
Astrocytes Using Primary Cultures From Rat Brain
 
Creator Ghosh, Mausam
 
Subject Cell Biology & Physiology
 
Description Thyroid hormones (TH) have profound effects on brain development both in vivo and in
vitro. Deficiency of the hormones severely impair the various stages of neuronal development
like proliferation, differentiation, axonal migration, myelinogenesis, synaptogenesis etc. Other
neural cells like oligodendroglia, astroglia also respond to the hormones as they bear TH
receptors. Astrocytes undergo dynamic changes in morphology during normal brain
development. Such changes in astrocyte morphology also occur during various
pathophysiological changes. Previous workers from this laboratory had established an in vitro
long-term primary culture of astrocytes derived from neonatal rat brains covering the entire span
of morphological maturation of astrocytes and demonstrated the successive changes in
morphology of the cells from radial glia to flat polygonal cells with epithelioid morphology and
finally to mature process bearing cells with stellate morphology. It was observed that deficiency
of TH delayed the first step and completely prevented the final step of differentiation. In another
study it was demonstrated that β-adrenergic receptor (β-AR) antagonists could completely block
TH-induced morphogenesis of astrocytes.
With this background, the present work was conceived. Two major issues were
addressed. We investigated the possible signaling mechanisms associated with TH-induced
differentiation of astrocytes. We also studied the effect of TH-induced differentiation on the β-
adrenergic system in astrocytes.
In this dissertation work, it was observed that, TH initially activates protein kinase A
(PKA), with a peak activity at 2 hr, which then falls back to basal level. Although there is no
visible change in morphology of the cells during this time, this activation of PKA was sufficient
to drive the process of transformation to completion. It was further observed that TH treatment
resulted in a biphasic response on cellular p-MAP kinase (p-ERK) level, an initial decline in p-
ERK level followed by an induction at 18-24 hr, which in turn activate p-CREB. Moreover, the
induction in p-ERK level coincides with initiation of morphological differentiation of the
astrocytes. Contrary to a short-term decline in p-ERK activity during TH treatment, a long
lasting activation of p-ERK activity at a later stage appeared to be critical for the transformation
of astrocytes. Ontogenic studies showed a gradual increase in the specific binding of 125I-PIN to
β2-AR in response to TH treatment due to the increased affinity of the receptors. Although, β2-
AR didn’t shown any significant change in the transcriptional level, overexpression of the same
gene in the hypothyroid astrocyte caused morphological transformation even in absence of TH.
While β-arrestin showed a steady increase in total protein level as well as in mRNA level after
TH-treatment, an initial decrease in 125I-PIN binding to β2-AR was also encountered during early
phase (2 – 12 hr) of hormone exposure which coincides with a decline in phospho-β-arrestin
protein level at the same time. The result indicates a regulatory influence of the hormone in
increasing the affinity of the said receptor and a putative role of its regulator in TH-induced
morphological differentiation of astrocytes.
 
Date 2007
 
Type Thesis
NonPeerReviewed
 
Format application/pdf
 
Identifier http://www.eprints.iicb.res.in/575/1/Thesis%2DMausam_Ghosh.pdf
Ghosh, Mausam (2007) Molecular Mechanisms Associated With Thyroid Hormone Induced Differentiation And Maturation Of Astrocytes Using Primary Cultures From Rat Brain. PhD thesis, Jadavpur University.
 
Relation http://www.eprints.iicb.res.in/575/