A molecular link between distinct neuronal asymmetries

Striking functional lateralization of the human brain across the left–right axis has long been known, the most notable of which is the localization of language to the left side of the brain. Mechanisms used to establish brain asymmetry have remained elusive due to the difficulty of identifying molecular correlates of functional asymmetries. The nematode Caenorhabditis elegans has been a valuable system to address molecular mechanisms used to confer asymmetry on a largely symmetric nervous system; it is one of the few organisms in which clear molecular-to-functional asymmetric correlates have been identified.

Two pairs of C. elegans sensory neurons display molecular and functional asymmetries: AWC olfactory neurons and ASE taste neurons.^1,2 Each pair of these neurons is morphologically and anatomically symmetric, but expresses distinct chemoreceptors in an asymmetric fashion and has distinct functions. They display unique forms of asymmetry: AWC neurons represent an example of stochastic asymmetry or antisymmetry, in which asymmetric traits are randomly distributed in either the left or the right side of the animal, while ASE neurons exhibit directional asymmetry, in which asymmetric traits are fixed to one side of the body. Stochastic AWC asymmetry and directional ASE asymmetry are established using completely distinct mechanisms at different developmental stages.

Stochastic AWC asymmetry is defined by antisymmetric expression of G protein-coupled receptor (GPCR) genes. The AWC^ON neuron expresses a particular set of GPCR chemoreceptor genes such as str-2, while the AWC^OFF neuron expresses a different set of chemoreceptor genes such as srsx-3 (Fig. 1).² AWC^OFF is a default fate and is specified by a calcium-activated CaMKII–MAPK cascade. Intercellular calcium signaling between the 2 AWC and other neurons in a network connected via NSY-5 gap junctions establish stochastic AWC asymmetry during late embryogenesis.^4-6 Differential calcium levels between the 2 AWC neurons are essential for asymmetric AWC differentiation. The cell with a higher calcium level remains as the default AWC^OFF fate, while the cell with a lower calcium level becomes the induced AWC^ON fate.⁶ Several mechanisms have been identified to inhibit the calcium–kinase pathway to induce AWC^ON. For example, the NSY-5 gap junction protein and NSY-4 claudin-like protein act in parallel to inhibit the calcium-activated kinase pathway in the induced AWC^ON fate (Fig. 1).^5,7

Figure 1. Schematic of AWC and ASE asymmetry. Intercellular communication mediated by gap junctions (GJ) and claudins coordinates antisymmetric expression of chemosensory receptor genes in AWC. Directional ASE asymmetry is established through transcriptional and posttranscriptional regulatory cascades. Not all molecules involved are listed. Molecules in black or red are active and gray are less active or inactive.

In contrast to AWC neurons, ASE asymmetry is established at the 4-cell stage and is lineage-dependent. ASE left (ASEL) and ASE right (ASER) neurons express different putative chemoreceptors of the receptor-type quanylyl cyclase family in a directionally asymmetric manner. Notch signaling and priming of the chromatin of a microRNA (miRNA) locus in the early lineage of ASEL and ASER are crucial for ASE asymmetry and result in a complex transcriptional regulatory network and a bi-stable negative feedback loop. In ASEL, the DIE-1 transcription factor and lsy-6 miRNA suppress expression of the COG-1 transcription factor to promote ASEL-specific genes. In ASER, COG-1 represses DIE-1 at a transcriptional and post-transcriptional level¹ (Fig. 1).

Although AWC and ASE neurons appear to use distinct mechanisms to establish asymmetry, a recent study discovered an interesting molecular link between these 2 types of asymmetries.³ The zinc finger transcription factor DIE-1 has long been known to be important for ASE asymmetry, but its role in AWC neurons was not known. In a study aimed at examining the regulation of directionally asymmetric die-1 expression in ASEL, it was discovered that die-1 is also asymmetrically expressed in AWC neurons. Intriguingly, asymmetric expression of die-1 in AWC neurons is stochastic, reflective of the stochastic nature of AWC asymmetry.

Expression of die-1 is regulated in distinct ways in AWC and ASE neurons. In ASE neurons, 3 phases of die-1 regulation have been identified:³ (1) the early bilateral phase, during which die-1 is initially expressed in both ASE neurons; (2) restriction of die-1 to only ASEL through transient activity of the afore-mentioned cog-1, which suppresses die-1 in ASER; (3) maintenance of asymmetric die-1 expression in ASEL using a transcription factor called CEH-36 as well as DIE-1 to maintain its own expression in ASEL. Asymmetric expression of die-1 is also maintained using the FOZI-1 transcription factor, which represses die-1 in ASER throughout life.

In AWC neurons, die-1 is expressed in a stochastically asymmetric fashion in the AWC^OFF neuron; antisymmetric die-1 expression is dependent on NSY-4 tight junctions and NSY-5 gap junctions. Consistent with asymmetric die-1 expression in AWC^OFF, die-1 is required for the calcium-activated CaMKII-MAPK pathway to specify the AWC^OFF fate in a cell autonomous fashion (Fig. 1).³ Although hypomorphic alleles for die-1 affect ASE asymmetry, only die-1-null mutants affect AWC asymmetry, suggesting a differential requirement for die-1 activity in AWC and ASE neurons.

Overall, die-1 is essential for the establishment of 2 distinct forms of neuronal asymmetries. Although the mechanisms used to regulate die-1 are distinct in the 2 pairs of neurons described, this molecular link may represent an evolutionary tie between stochastic and directional asymmetries, in which directional asymmetry is perhaps a more evolved system used to confer functional diversity on a largely symmetric nervous system.

Cochella L, et al. Genes Dev. 2014;28:34–43. doi: 10.1101/gad.233643.113.