| Summary: | 博士 === 國立陽明大學 === 神經科學研究所 === 95 === Retinoic acid (RA) is a vitamin A derivative that plays an important role in morphogenesis and homeostasis in vertebrates. During development, retinoids as well as their receptors (RA receptors-α, β, γ and retinoid X receptors-α, β, γ) are expressed in the striatal primordium, suggesting that retinoid signaling may regulate striatal development. However, little is yet known about the downstream genes regulated by RA and the underlying molecular mechanisms during striatal development. We have previously shown that RA signaling coordinately up-regulates a set of dopamine signal transduction molecules. In this dissertation, I demonstrated that activation of RA signaling temporally down-regulates the expression of the LIM-HD transcription factor Isl-1 in the developing striatum both at mRNA and protein levels. In contrast, blocking of RA signaling by over-expression of dominant-negative RARα derivative (RAR403) results in up-regulation of Isl-1 expression in striatal culture. Moreover, RARα and RARβ were involved in regulation of striatal Isl-1 expression. Interestingly, in vivo analysis of Isl-1 expression in RA-treated embryos showed that a different competence to RA signals was present in the rostral and caudal striatum, which was likely to reflect endogenous RA distribution along the rostrocaudal axis of the developing striatum. However, high levels of RA, presumably released from RALDH3+ cells, were distributed nearby the medial band where Isl-1 was highly expressed. Double immunostaining of Isl-1 and RARb showed that many Isl1-positive cells did not co-express RARβ, which might render them incompetent to transduce RA signaling.
Over-expression of wild type Isl-1 and N230S Isl-1 mutants into N2a and striatal cells showed that Isl-1 inhibited neurite outgrowth in these two types of cells. To comprehensively explore Isl-1 function in the developing striatum, we generated a pCCALL2-Isl-1N230S transgenic mouse line and cross-mated these mice with Nestin-Cre and Dlx5/6-Cre driver lines. When the Line 6 of Isl-1N230S transgenic mice were mated to Nestin-Cre drivers, aberrant apoptosis occurred in the developing telencephalon of double transgenic NestinCre;Isl-1N230S embryos. Moreover, aberrant expression of class III β-tubulin, an early differentiating neuronal marker, was found in the SVZ of the telencephalon of double transgenic mice. Because over-expression of Isl-1 mutants might interfere with the endogenous functions of Isl-1 and/or other LIM-HD transcription factors, these findings suggested that Isl-1 and its associated molecules are important for neuronal survival and differentiation in forebrain development. In summary, my thesis work indicates that the spatial and temporal expression of striatal Isl-1 is regulated by RA signaling, and further suggests that Isl-1 may regulate survival and differentiation processes of striatal neurons during development.
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