Mouse Heterochromatin Adopts Digital Compaction States without Showing Hallmarks of HP1-Driven Liquid-Liquid Phase Separation
Authors: Ahmad Al Alwash
Anne Rademacher
Karsten Rippe
Jana Tünnermann
Rifka Vlijm
Lukas Frank
Johan Hummert
Sabrina Schumacher
Caroline Bauer
Klaus Yserentant
Christophe Normand
Dirk-Peter Herten
Robin Weinmann
Elisabeth Schweigert
Johann Engelhardt
Fabian Erdel
Journal: Molecular Cell (2020)
Discipline: MOLECULAR BIOLOGY & GENETICS
Keywords: Heterochromatin protein 1; chromatin accessibility; chromatin compartmentalization; epigenetic editing; intracellular viscosity
Affiliations: LBME, Centre de Biologie Intégrative (CBI), CNRS, UPS, Toulouse, France; Division of Chromatin Networks, German Cancer Research Center (DKFZ) and Bioquant, Heidelberg, Germany. Electronic address: ., Division of Chromatin Networks, German Cancer Research Center (DKFZ) and Bioquant, Heidelberg, Germany., Department of Optical Nanoscopy, Max Planck Institute for Medical Research, Heidelberg, Germany., Department for Physical Chemistry, Heidelberg University, Heidelberg, Germany., LBME, Centre de Biologie Intégrative (CBI), CNRS, UPS, Toulouse, France., Department for Physical Chemistry, Heidelberg University, Heidelberg, Germany; Institute of Cardiovascular Sciences, College of Medical and Dental Sciences and School of Chemistry, University of Birmingham, Birmingham, UK; Centre of Membrane Proteins and Receptors (COMPARE), Universities of Birmingham and Nottingham, United Kingdom., Division of Chromatin Networks, German Cancer Research Center (DKFZ) and Bioquant, Heidelberg, Germany. Electronic address: .

Abstract:  The formation of silenced and condensed heterochromatin foci involves enrichment of heterochromatin protein 1 (HP1). HP1 can bridge chromatin segments and form liquid droplets, but the biophysical principles underlying heterochromatin compartmentalization in the cell nucleus are elusive. Here, we assess mechanistically relevant features of pericentric heterochromatin compaction in mouse fibroblasts. We find that (1) HP1 has only a weak capacity to form liquid droplets in living cells; (2) the size, global accessibility, and compaction of heterochromatin foci are independent of HP1; (3) heterochromatin foci lack a separated liquid HP1 pool; and (4) heterochromatin compaction can toggle between two "digital" states depending on the presence of a strong transcriptional activator. These findings indicate that heterochromatin foci resemble collapsed polymer globules that are percolated with the same nucleoplasmic liquid as the surrounding euchromatin, which has implications for our understanding of chromatin compartmentalization and its functional consequences. 

Retracted: No

PubPeer comments:
 There is an interesting thread that discusses related work at: https: pubpeer.com publications E0F04DC414EAF3D4C8B22967A10470 I do not see any reference to the HP1 knock-out mice in this paper. , 
 #1 In this paper (Erdel et al) they discuss earlier work in knock-out mice. They reference the Suv39 knock-out mice published by Peters et al, in which chromocenters lose most H3K9me3 and HP1. , 
 #2  *In this paper (Erdel et al) they discuss earlier work in knock-out mice. They reference the Suv39 knock-out mice published by Peters et al, in which chromocenters lose most H3K9me3 and HP1* **This reply is both incorrect and evasive**. Suv39 knock-out mice are **not** the same as HP1 knock-out mice. For example, in the figure below (taken from PMID: 21250545) shows a comparison of the Suv39 double mutant (as described in Figure 1 of Peters et al alluded to by #2) that is viable albeit runted and the HP1beta null mutant that is perinatal lethal. ![file] Lethality of HP1 beta null mutant is more severe than the Suv(3)9h1 h2 double mutant mouse, indicating that the essential function of HP1u03b2 lies outside its interaction with the heterochromatic H3K9me3 determinant of the u201chistone codeu201d imposed by the Suv(3)9h1 h2 HMTases. The situation for HP1alpha is that the null mutants are viable and fertile (see https: pubpeer.com publications E0F04DC414EAF3D4C8B22967A10470#3 for details), indicating that its function is essentially redundant. This begs the question, is the work on HP1alpha and phase separation a  house-of-cards ? A response by the authors would be appreciated. , 
 #3 I think that this paper is consistent with HP1alpha null mice being viable and fertile, as it argues that HP1alpha is dispensable for chromocenter compaction and that HP1alpha does not undergo LLPS in their cells. They reference the Suv39 double null mice for still having compact chromocenters in the absence of H3K9me3 HP1, not for arguing that HP1alpha is highly important because of the phenotype in these mice. The bottom line is that HP1alpha is maybe not so important for heterochromatin at the end. ,