“HUSH” Protein Complex Shapes Brain Architecture and Development

Recently, scientists identified the importance of the human silencing hub, or HUSH, complex for silencing repetitive genetic elements in mammals. This complex comprises proteins like MPP8, which binds the histone modification mark H3K9me3, and recruits proteins such as MORC2, mutations of which are associated with axonal neuropathy and neurodevelopmental disorders. Though teams have uncovered a lot about this complex, much remains unknown about how MPP8 and MORC2 affect brain health. That’s why researchers from the Institute of Molecular Biotechnology of the Austrian Academy of Sciences (IMBA) uncovered in vivo targets and physiological functions of HUSH.

The team, whose work was published in Science Advances, was led by Astrid Hagelkruys, Senior Research Associate in the Penninger group at IMBA. Their work utilized a comprehensive in vivo approach, using behavioral, motor, genetic, developmental, and transcriptomic experiments. Ultimately, their findings suggested that MPP8 and MORC2A (the mouse ortholog of human MORC2) were highly expressed in the brain, exclusively within neurons. 

“We demonstrated that MPP8 and MORC2A play a role in normal brain development, the specification of neuronal identity and connectivity of neurons, as well as mouse behavior,” says first author Astrid Hagelkruys.

The researchers observed that deletions of MPP8 or MORC2A in the nervous system of their mouse models led to increased brain size and altered brain architecture, with noticeable changes in the mice’s motor functions and behavior. “Hence, surprisingly in a living animal, we showed that MPP8 and MORC2A act beyond transposable element regulation,” says Hagelkruys.

Even with these observations in their animal models, the team also noted that the brain’s architecture changed without major alterations to transposable element expression. “We showed that MPP8 and MORC2A suppressed the protocadherin gene clusters in an H3K9me3-dependent manner. At the protein level, these protocadherin gene clusters form neuronal surface proteins that mediate contact with other neurons. Although protocadherins are not transposable elements, some are expressed in the central nervous system as ‘repetitive-like’ gene clusters,” explains Hagelkruys.

Within the mouse models, MPP8 and MORRC2a silenced the protocadherin cluster on mouse chromosome 18. Deleting these proteins led to more synaptic formation for neurons, which may coincide with impairment of neuronal individuality. This means that the neurons’ ability to distinguish themselves from other cells is impaired.

By expressing different combinations of clustered protocadherins, neurons acquire an identifying marker, or “barcode” that allows them to control the formation of synaptic connections with other neurons. By targeting these clustered protocadherins, MPP8 and MORC2A may ensure that neurons require the right “barcode” and only form synapses with correct counterparts.

The scientists also investigated the effects of MPP8 and MORC2 in human brain organoids. They found similar results – the absence of MPP8 or MORC2 led to increased numbers of clustered protocadherins expressed in organoid neurons at the single-cell level. This suggests that the lack of these two proteins disrupted neuronal identity, just like that observed in mice.

“The interest of these findings on the essential function of the HUSH complex in the brain lies in the implication of protocadherins in neuronal fidelity and brain evolution. However, how this is regulated remained largely unknown. The dysregulation of clustered protocadherins has been associated with various neurological and neurodevelopmental diseases, but also multiple mental disorders in humans. Hence, our findings might help us better understand the epigenetic regulation mechanisms governing these diseases and provide a new way to study brain evolution,” concludes senior author Josef Penninger, group leader at IMBA.