Polycomb Repressive Complexes, PRC1 and PRC2, establish and maintain chromatin architectures that are critical for cell type–specific transcriptional programs. In the developing spinal cord, PRC1 is required to implement Hox-dependent transcriptional programs that diversify motor neuron identities along the rostrocaudal axis. The relationship between canonical Polycomb components, their higher-order assemblies, and maintenance of neuronal subtype identity was addressed using chick and mouse models.
The canonical PRC1 components BMI1, PHC2, CBX2, and RING1B were observed to coalesce into discrete subnuclear assemblies as neural progenitors differentiate into motor neurons. These assemblies contain BMI1 and are spatially organized within the nucleus, indicating that PRC1 can form higher-order structures during the transition from progenitor to postmitotic neuronal states. The finding that multiple core PRC1 subunits are present in these assemblies suggests a coordinated assembly process linked to motor neuron differentiation.
Experimental disruption of PHC2 polymerization, applied either in neural progenitors or in postmitotic neurons, led to dismantling of BMI1-containing PRC1 assemblies. Loss of these assemblies correlated with derepression of caudal Hox genes and with a cell-autonomous loss of segment-specific motor neuron fates. These results indicate that PHC2 polymerization is necessary to maintain the integrity of BMI1-containing PRC1 assemblies and that those assemblies contribute to repression of inappropriate caudal Hox programs during motor neuron specification.
Mouse mutants lacking the core PRC1 subunit Ring1 failed to form BMI1-containing assemblies despite the persistence of PHC2 and CBX2 polymers. This observation indicates that intact PHC2/CBX2 polymers alone are insufficient for formation or maintenance of BMI1-positive PRC1 assemblies when core subunits such as RING1 are absent. The data suggest that core enzymatic or structural PRC1 components are required for full incorporation of BMI1 into higher-order assemblies.
Selective deletion of core PRC2 subunits (Ezh1/2) in motor neurons depleted H3K27me3 histone marks, as expected from loss of PRC2 methyltransferase activity. Despite this reduction in H3K27me3, PRC1 assemblies containing BMI1 were retained and Hox expression boundaries remained intact. These results indicate that PRC1 assembly formation and maintenance of Hox transcriptional boundaries can persist in the absence of PRC2-mediated H3K27 methylation in the neuronal context examined.
Taken together, the observations support a model in which PHC2-dependent PRC1 subnuclear assemblies stabilize Hox transcriptional boundaries and thereby maintain motor neuron segmental identities. Disruption of PHC2 polymerization dismantles these assemblies and permits derepression of caudal Hox genes, leading to loss of segment-specific fates. Conversely, loss of PRC2 activity and associated H3K27me3 does not abolish PRC1 assemblies or Hox boundaries in the conditions reported.
The work was performed using chick and mouse models. Specific experimental details, including methods, quantitative results, and full datasets, were reported in the source preprint but are not reproduced here. The source notes mouse genetic mutants for Ring1 and motor neuron–restricted deletion of Ezh1/2; further methodological specifics and statistical outcomes were not reported in the summary provided.
These findings imply that higher-order organization of PRC1, dependent on PHC2 polymerization and incorporation of core subunits like RING1 and BMI1, contributes to stable repression of inappropriate Hox programs during motor neuron differentiation. The preservation of PRC1 assemblies despite PRC2 loss suggests separable functions for PRC1 structural assemblies and PRC2-catalyzed H3K27 methylation in maintaining neuronal subtype identities.
The study acknowledges support from the National Institute of Neurological Disorders and Stroke and the National Cancer Institute. The authors declared no competing interests. The original work is available as a bioRxiv preprint with the DOI provided in the source.