HP1 proteins regulate nucleolar structure and function by secluding pericentromeric constitutive heterochromatin

Area of Science:

• Cell Biology
• Epigenetics
• Developmental Biology

Background:

• Nucleoli regulate ribosome biogenesis and cell growth.
• In embryonic stem cells (ESCs), nucleoli are spatially separated from repressed pericentromeric heterochromatin (PCH).
• Mechanisms and physiological relevance of this nuclear partitioning are unknown.

Purpose of the Study:

• To investigate the role of repressive chromatin in nucleolar structure and function.
• To elucidate the mechanisms by which heterochromatin proteins (HP1α/β) maintain nucleolar integrity.
• To understand the physiological consequences of heterochromatin loss on ESCs.

Main Methods:

• Studied ESCs with deficiencies in heterochromatin proteins (HP1α/β) and H3K9 methyltransferases (Suv39h1/2).
• Assessed PCH integrity, heterochromatin marks (H3K9me3, H4K20me3), and satellite repeat expression.
• Analyzed nucleolar morphology, cell cycle progression, proliferation, and ribosomal RNA biosynthesis.

Main Results:

• Loss of HP1α/β deforms PCH, reduces heterochromatin marks, and upregulates satellite expression.
• Derepressed PCH aberrantly associates with nucleoli, causing severe morphological defects during S/G2.
• HP1α/β deficiency impairs cell proliferation, rRNA biosynthesis, and Nucleophosmin mobility.
• Suv39h1/2 deficiency causes similar nucleolar defects, while Suv4-20h1/2 deficiency does not.

Conclusions:

• Repressive constitutive heterochromatin (PCH) marked by SUV39H/HP1 is essential for nucleolar integrity and function.
• HP1α/β proteins require recruitment to H3K9me3-marked PCH and dimerization for nucleolar maintenance.
• Defects observed resemble human ribosomopathies, highlighting the physiological relevance of this pathway.