Aberrant Compartment Formation by HSPB2 Mislocalizes Lamin A and Compromises Nuclear Integrity and Function

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Small heat shock proteins (HSPBs) possess intrinsically disordered regions (IDRs) with unknown functions.
• The role of HSPB2 and HSPB3 in cellular processes, particularly during differentiation, remains largely unexplored.

Purpose of the Study:

• To investigate the function of intrinsically disordered regions in HSPB2.
• To elucidate the role of HSPB2 phase separation in mammalian cells, specifically during myoblast differentiation.
• To determine the involvement of HSPB3 in regulating HSPB2 phase separation and its implications in myopathy.

Main Methods:

• Utilized mammalian cell cultures to observe HSPB2 behavior.
• Investigated the requirement of HSPB2's C-terminal domain for phase separation.
• Analyzed the interaction between nuclear HSPB2 compartments and lamin A during myoblast differentiation.
• Examined the effect of HSPB3 mutations on HSPB2 phase separation.

Main Results:

• HSPB2 undergoes liquid-like phase separation in the nucleus, mediated by its disordered C-terminal domain.
• Nuclear HSPB2 compartments sequester lamin A in differentiating myoblasts.
• Aberrant nuclear compartments and mislocalization of lamin A and chromatin occur with increased HSPB2 levels, impairing nuclear integrity.
• HSPB3 regulates HSPB2 phase separation, and loss-of-function mutations in HSPB3 disrupt this regulation, correlating with myopathy.

Conclusions:

• HSPB2 phase separation is a key mechanism for nucleoplasm reorganization during myoblast differentiation.
• Aberrant HSPB2 phase separation, caused by HSPB3 mutations, is implicated as a contributing factor to myopathy.
• The findings highlight the functional significance of IDRs in small heat shock proteins and their role in nuclear organization and disease pathogenesis.