Horrer, M. | Search bioRxiv

bioRxiv2021

The HUSH complex controls brain architecture and protocadherin fidelity

AbstractFine-tuning of neural connectivity is important for cerebral functions and brain evolution. Protocadherins provide barcodes for neuronal identity as well as synapse formation and expansion of protocadherin cluster genes has been linked to advanced cognitive functions. The tightly controlled stochastic and combinatorial expression of the different protocadherin isoforms in individual neurons provides the molecular basis for neuronal diversity, neuronal network complexity and function of the vertebrate brain. How protocadherins are epigenetically controlled has not yet been fully elucidated. Here we show that the HUSH (human silencing hub) complex containing H3K9me3 binding protein M-phase phosphoprotein 8 (MPP8) and Microrchidia CW-type zinc finger protein 2 (MORC2), critically controls the fidelity of protocadherin expression. MPP8 and MORC2A are highly expressed in the murine brain and exclusively found in neurons. Genetic inactivation of Mphosph8 (coding for MPP8) or Morc2a in the nervous system of mice leads to increased brain size, altered brain architecture, and behavioral changes. Mechanistically, MPP8 and MORC2A precisely and selectively suppress the repetitive-like protocadherin gene cluster on mouse chromosome 18 in a H3K9me3-dependent manner, thereby affecting synapse formation. Moreover, we demonstrate that individual MPHOSPH8- or MORC2-deficient neurons in human cerebral organoids express increased numbers of clustered protocadherin isoforms. Our data identify the HUSH complex, previously linked to silencing of repetitive transposable elements, as a key epigenetic regulator of protocadherin expression in the nervous system and thereby brain development and neuronal individuality in mice and humans.

neuroscience↗

bioRxiv2021

FIBCD1 is a Conserved Receptor for Chondroitin Sulphate Proteoglycans of the Brain Extracellular Matrix and a Candidate Gene for a Complex Neurodevelopmental Disorder

The brain extracellular matrix (ECM) is enriched in chondroitin sulphate proteoglycans (CSPGs) with variable sulphate modifications that intimately participate in brain maturation and function. Very little is known about how the changing biophysical properties of the CSPGs are signalled to neurons. Here, we report Fibrinogen C Domain Containing 1 (FIBCD1), a known chitin-binding receptor of the innate immune system, to be highly expressed in the hippocampus and to specifically bind CSPGs containing 4-O sulphate modification (CS-4S). Cultured Fibcd1 knockout (KO) neurons lack phenotypic and transcriptomic responses to CSPG stimulation. Further, Fibcd1 KO mice exhibit accumulation of CS-4S, likely resulting in deficits of hippocampal-dependent learning tasks and abrogated synaptic remodelling, a phenotype rescued by enzymatic digestion of CSPGs. Likewise, neuronal specific knockdown of a Fibcd1 orthologue in flies results in neuronal morphological changes at the neuromuscular junctions and behavioural defects. Finally, we report two undiagnosed patients with a complex neurodevelopmental disorder with deleterious variants in FIBCD1, strongly implicating FIBCD1 in the development of the disease. Taken together, our results demonstrate that FIBCD1 is a novel, evolutionarily conserved component of ECM sulphation recognition that is crucial for neuronal development and function.

neuroscience↗