Most nervous systems are largely bilaterally symmetric on a structural level, but are usually functionally lateralized. How left/right asymmetry is developmentally controlled is poorly understood, in part because there are few molecular correlates to functional asymmetries in any nervous system. The C.elegans nervous system displays a number of striking asymmetries in its nervous system . We have focused much of our efforts on the two bilaterally symmetric gustatory neurons ASE(Left) and ASE(Right) as an entry point into studying how asymmetry develops.
ASE(L) and ASE(R) express in a stereotyped, left/right asymmetric manner many members of a receptor-type guanylyl cyclase (rGCY) family that we found to be required for left/right asymmetric taste perception (Figure 1)[2, 3]. The ability to easily visualize this functional asymmetry through gfp reporters that tag therGCY-encoding gcy loci has therefore provided us with a unique opportunity to ask how asymmetry is developmentally programmed. Using three different approaches, we have provided the following insights:
(i) We have dissected cis-regulatory regions of the gcy genes to probe the regulatory logic of their left/right asymmetric restriction, revealing that they are under direct control of the bilaterally expressed terminal selector CHE-1, but that additional cis-regulatory motifs promote or inhibit the activity of CHE-1 in either ASE(L) or ASE(R)(Figure 2).
(ii) Through genetic screens we have elucidated a complex network of regulatory RNAs, transcription factors and chromatin modifiers, many under direct control of the terminal selector CHE-1, that regulate asymmetric gcy expression (Figure 2)[4-16].
(iii) Early embryonic blastomere manipulations revealed that the initial trigger for ASE(L/R) asymmetry lies in a Notch signal sent in the early embryo, long before the ASE neurons are born (Figure 3). Via intermediary T-box transcription factors, this signal induces a decompaction of chromatin at the lsy-6 miRNA locus throughout embryonic development in the ABa lineage. lsy-6 expression is then amplified in the ASE(L) neuron through the terminal selector che-1 and the bistable feedback loop shown in Figure 3 to trigger postmitotic, asymmetric differentiation of the ASE(L/R) neurons.
We have also found that asymmetry in the C.elegans nervous system extends beyond chemosensory asymmetry. We have identified left/right asymmetric gene expression programs in several interneurons and motoneurons and have shown that they are induced by a novel, mesodermally derived Notch signal(Figure 4). These findings reveal a previously underappreciated complexity in the neuronal circuitry of the worm.
A key factor in ASE left/right asymmetry control is the ASEL-specific miRNA, lsy-6, which we had discovered previously . Aside from placing a miRNA in a complex gene regulatory network (Figure 3), we have used the lsy-6 interaction with its genetically defined target cog-1, to probe concepts of miRNA/target interactions, thereby challenging some commonly accepted rules[20, <21]. As we detailed in an essay in Science, this and other work reveals striking conceptual similarities between miRNA and transcription factor action.
Figure 1: Left/right asymmetric expression of receptor-type guanylyl-cyclases in the ASEL/R neuron pair.
Figure 2: Gene regulatory network controlling ASE left/right asymmetry. Most factors shown here were retrieved through our screens for asymmetry mutants. lsy-6 is the first miRNA with a demonstrated function in the nervous system . A combination of genetic analysis and mutational dissection of the cis-regulatory control regions of left/right asymmetrically expressed genes revealed how the bilateral differentiation program of both ASE neurons intersects with the execution of left/right asymmetric regulatory programs: Bilaterally expressed terminal differentiation genes contain cis-regulatory elements (“ASE motif”) that bind the bilaterally expressed terminal selector transcription factor che-1. Asymmetrically expressed genes also contain ASE motifs, but their activity is modulated in a left/right asymmetric manner by required co-activator elements or repressor elements .
Figure 3: Control of left/right asymmetric terminal differentiation of the ASE neurons – early embryonic predetermination and postmitotic execution. The early Notch signal is memorized via tbx-37/38-dependent choromatin decompaction of the lsy-6 locus in Abalpp. Chromatin decompaction of the lsy-6 locus in ABalpp descendants is a prerequisite for amplification by che-1 and by components of the bistable feedback loop shown in the figure, thereby resulting in lsy-6 expression only in ASE(L) and not ASE(R). From .
Figure 4: Other examples of asymmetries in left/right pairs of neurons. This diagram shows a selected number of connected neurons from a navigation circuit [23-25]. Asymmetries in hlh-16 are shown in same color code as in Figure 1C. ASE neurons are asymmetric based on the expression of many genes, including chemoreceptors. AWC is another sensory neuron that inputs into this circuit and is left/right asymmetric (this asymmetry is, however, stochastic). From .
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