👤 Rebecca Aucott

HP1 proteins are thought to be modulators of chromatin organization in all mammals, yet their exact physiological function remains unknown. In a first attempt to elucidate the function of these proteins in vivo, we disrupted the murine Cbx1 gene, which encodes the HP1-beta isotype, and show that the Cbx1(-/-) -null mutation leads to perinatal lethality. The newborn mice succumbed to acute respiratory failure, whose likely cause is the defective development of neuromuscular junctions within the endplate of the diaphragm. We also observe aberrant cerebral cortex development in Cbx1(-/-) mutant brains, which have reduced proliferation of neuronal precursors, widespread cell death, and edema. In vitro cultures of neurospheres from Cbx1(-/-) mutant brains reveal a dramatic genomic instability. Our results demonstrate that HP1 proteins are not functionally redundant and that they are likely to regulate lineage-specific changes in heterochromatin organization. Show less

We have compared the distribution of endogenous heterochromatin protein 1 (HP1) proteins (alpha, beta and gamma) in different epithelial lines, pluripotent stem cells and embryonic fibroblasts. In parallel, we have interrogated assembly and dynamics of newly expressed HP1-GFP proteins in cells lacking both HP1alpha and HP1beta alleles, blocked at the G1-S boundary, or cultured in the presence of HDAC and HAT inhibitors. The results reveal a range of cell type and differentiation state-specific patterns that do not correlate with 'fast' or 'slow' subunit exchange in heterochromatin. Furthermore, our observations show that targeting of HP1gamma to heterochromatic sites depends on HP1alpha and H1beta and that, on an architectural level, HP1alpha is the most polymorphic variant of the HP1 family. These data provide evidence for HP1 plasticity under shifting microenvironmental conditions and offer a new conceptual framework for understanding chromatin dynamics at the molecular level. Show less