The expression of specific transcription factors determines the differentiated features of postmitotic neurons. However, the mechanism by which specific molecules determine neuronal cell fate and the extent to which the functions of transcription factors are conserved in evolution are not fully understood. In C. elegans, the cholinergic and peptidergic SMB sensory/inter/motor neurons innervate muscle quadrants in the head and control the amplitude of sinusoidal movement. Here I showed that the LIM homeobox protein LIM-4 determines neuronal characteristics of the SMB neurons. In lim-4mutant animals, expression of terminal differentiation genes, such as the cholinergic gene battery and the flp-12 neuropeptide gene, is completely abolished and thus the function of the SMB neurons is compromised. LIM-4 activity promotes SMB identity by directly regulating the expression of the SMB marker genes via a distinct cis-regulatory motif. Two human LIM-4 orthologs, LHX6 and LHX8, functionally substitute for LIM-4 in C. elegans. Furthermore, C. elegans LIM-4 or human LHX6 can induce cholinergic and peptidergic characteristics in the human neuronal cell lines. My results indicate that the evolutionarily conserved LIM-4/LHX6 homeodomain proteins function in generation of precise neuronal subtypes.
Table Of Contents
I Introduction 1-- 1.1 Historical background 1-- 1.2 Terminal selector transcription factors coregulate termi- nal differentiation genes 11-- 1.3 Terminal selectors initiate and maintain the terminally differentiated state 15-- 1.4 Regulations of pan-neuronal features 17-- II Materials and Methods 20-- III Results 28-- 3.1 Expression of a neuropeptide gene in the SMB neurons is abolished in lim-4 mutants 28-- 3.2 Expression of terminally differentiated SMB markers including cholinergic genes is abolished in lim-4 mu- tants 39-- 3.3 The function of the SMB neurons is compromised in lim-4 mutants 47-- 3.4 lim-4 is expressed and functions in the SMB neurons to regulate their terminal specification 50-- 3.5 Postdevelopmental expression of LIM-4 is sufficient to restore the SMB-specific defects of lim-4 mutants 58-- 3.6 Expression of lim-4 is sufficient to induce the SMB identity in other cell-types 61-- 3.7 LIM-4 regulates gene expression via a cis-regulatory motif in the SMB markers 66-- 3.8 The function of lim-4 is conserved in human 76-- IV. Discussions 81-- V. References 89-- VI. Summary in Koreans 97-- VII. Appendix 99