A new paper in Cell by Nakano et al describes the first mutant histone allele recovered from a genetic screen of a multicellular organism. This gain of function mutation in a histone H3 gene of C. elegans causes a very specific defect: a transformation in the fate of a single asymmetric motor neuron. To account for these findings the authors put forward a radical model in which differential epigenetic regulation between sister chromatids leads to asymmetric fate determination upon cell division.
The nematode worm C. elegans has an invariant cell lineage, meaning that any particular cell is generated from a specific series of mother and grandmother cells. Differences between daughter cells are determined either by non-cell autonomous mechanisms such as signalling by neighbouring cells, or by cell autonomous mechanisms such as the asymmetric inheritance of cell fate determinants, or by both.
The MI motor neuron is a left-right unpaired neuron located in the pharynx. The great-great-grandmother cell of MI gives rise to left and right paired lineages of cells, symmetrical, except for one left-right asymmetry: the MI motor neuron and the e3D pharyngeal epithelial cell. The researchers had previously shown that the MI-e3D asymmetry was dependent on a cascade of transcription factors asymmetrically expressed in the grandmother and mother cells of MI: CEH-36 (an Otx homeodomain protein) promoted the expression of the bHLH containing proneural proteins NGN-1 and HLH-2. When any of these proteins are inactivated, the MI neuron is transformed into an e3D-like cell.
In a genetic screen to find other factors involved in the MI-e3D asymmetry, Nakano et al identified a gain of function allele in the gene his-9 as causing MI-e3D transformation. his-9 encodes one of 14 identical replication-dependent histone H3 proteins in C. elegans.
In eukaryotes, chromosomal DNA is condensed by being wound around octamers of various histone proteins to form nucleosomes. Alterations to nucleosome structure or density can determine the accessibility of the DNA to the transcriptional apparatus, and hence the transcription state of that piece of chromatin. These variable chromatin states are said to be ‘epigenetically’ determined, as they can be maintained through mitoses by the inheritance of the modification status of histones (and other non-DNA sequence chromosomal features).
The nucleosome core contains a tetramer composed of two histone H3/ H4 dimers. This dimerisation occurs due to interactions between the two H3 molecules. It was these H3-H3 interactions that were compromised in the original mutant allele. The addition of similarly mutated versions of other replication dependent histone H3 genes into wild type worms also had the ability to transform the fate of the MI and yet showed no other gross abnormalisties. This showed that MI cell fate specification is very sensitive to gain of function mutations in histone H3 genes.
By generating worms that carried mutant his-9 transgenes on an extrachromasomal array that is mitotically unstable (hence creating mosaic worms), Nakano et al showed that the histone H3 gain of function activity acts cell autonomously within the MI mother cell.
Histone H3-H4 dimers are deposited into the nucleosome by a histone chaperone complex called CAF-1. Compromising the activity of any of the CAF-1 subunits in C. elegans also caused MI transformation. Therefore, replication dependent nucleosome formation mediated by CAF-1 is necessary to generate MI-e3D asymmetry.
To integrate their earlier findings with their new data, Nakano et al suggest that the NGN-1/HLH-2 complex recruits histone modifying enzymes that act on CAF-1 assembled nucleosomal arrays to generate an epigenetically marked MI-neuronal state. They combine this with the idea that CAF-1 can generate differences in the densities of nucleosomes between sister chromatids that upon mitotic segregation would generate a difference between sister cells. MI neuronal fate determination would require NGN-1/HLH-2 mediated histone modifications to be found at a specific (CAF-1 mediated) density.
To my knowledge, the idea that epigenetic marks, asymmetrically inherited, can act as cell fate determinants is novel and potentially a very important mechanism of development. In this case it is only a model that will require a lot more experimentation, however the authors go on to suggest that it could be a conserved mechanism generating bilateral asymmetries in the nervous systems of mammals as well. Mutations in a microtubule-based motor protein called left-right dynein (LRD) randomize visceral left-right asymmetry in the mouse due to defective cilia causing a left-right determining flow in the node to fail. LRD has also been implicated in biased chromatid segregation and interestingly rather than randomized asymmetry in the brain, LRD mutant mouse hippocampuses exhibit a loss of bilateral asymmetry that the authors suggest could be caused by parallel mechanisms as MI-e3D asymmetry. This is probably a leap too far, but fun anyway.
Nakano, S., Stillman, B., & Horvitz, H. (2011). Replication-Coupled Chromatin Assembly Generates a Neuronal Bilateral Asymmetry in C. elegans Cell, 147 (7), 1525-1536 DOI: 10.1016/j.cell.2011.11.053