Coordinated epigenetic regulation of Engrailed-1 by the chromatin remodelers Smarca1 and Smarca5 mediates cerebellar morphogenesis

Background

Morphological patterning of the cerebellum requires precise changes in Engrailed homeotic gene expression yet the mechanisms controlling this process remain elusive. Here, we show that the Iswi chromatin remodeling proteins, Smarca5 and Smarca1, are required for the dynamic regulation of Engrailed-1 (En1). Conditional Smarca5-null mice display abnormal cerebellar foliation, ataxia-like symptoms and young mortality. Postnatal granule neuron progenitor expansion and Purkinje cell (PC) development are compromised and attributed to loss of En1 expression. Mutants survive to early adulthood via upregulation of Smarca1 and restoration of En1 expression in PCs, while ablation of both Iswi genes results in lethality at birth. During late cerebellar development, we observe co-binding of the Iswi proteins at the En1 locus and an altered H2AZ/H3.3 chromatin profile that accompanies changes in En1 expression.

Materials and methods

Through mouse ES cell homologous recombination, we targeted exon 5 of the Smarca5 gene that encodes for part of the helicase domain, thereby allowing us to conditionally ablate Smarca5 expression in the central nervous system (CNS). We ablated Smarca5 expression in CNS progenitors using the Nestin-Cre driver, as well as in Purkinje neurons of the cerebellum using the PCP2-Cre driver mouse line. We also ablated expression of both mammalian Iswi proteins by breeding to a Smarca1-null mouse that we have previously characterized [1].

Conclusions Our results support an epigenetic mechanism in which Iswi-mediated histone variant exchange modulates En1 expression levels to subsequently control cerebellar morphogenesis.