The Drosophila fussel gene is required for bitter gustatory neuron differentiation acting within an Rpd3 dependent chromatin modifying complex

Members of the Ski/Sno protein family are classified as proto-oncogenes and act as negative regulators of the TGF-ss/BMP-pathways in vertebrates and invertebrates. A newly identified member of this protein family is fussel (fuss), the Drosophila homologue of the human functional Smad suppressing elements (fussel-15 and fussel-18). We and others have shown that Fuss interacts with SMAD4 and that overexpression leads to a strong inhibition of Dpp signaling. However, to be able to characterize the endogenous Fuss function in Drosophila melanogaster, we have generated a number of state of the art tools including anti-Fuss antibodies, specific fuss-Gal4 lines and fuss mutant fly lines via the CRISPR/Cas9 system. Fuss is a predominantly nuclear, postmitotic protein, mainly expressed in interneurons and fuss mutants are fully viable without any obvious developmental phenotype. To identify potential target genes or cells affected in fuss mutants, we conducted targeted DamID experiments in adult flies, which revealed the function of fuss in bitter gustatory neurons. We fully characterized fuss expression in the adult proboscis and by using food choice assays we were able to show that fuss mutants display defects in detecting bitter compounds. This correlated with a reduction of gustatory receptor gene expression (Gr33a, Gr66a, Gr93a) providing a molecular link to the behavioral phenotype. In addition, Fuss interacts with Rpd3, and downregulation of rpd3 in gustatory neurons phenocopies the loss of Fuss expression. Surprisingly, there is no colocalization of Fuss with phosphorylated Mad in the larval central nervous system, excluding a direct involvement of Fuss in Dpp/BMP signaling. Here we provide a first and exciting link of Fuss function in gustatory bitter neurons. Although gustatory receptors have been well characterized, little is known regarding the differentiation and maturation of gustatory neurons. This work therefore reveals Fuss as a pivotal element for the proper differentiation of bitter gustatory neurons acting within a chromatin modifying complex. Author summary Ski/Sno proteins have been discovered as proto-oncogenes transforming chicken fibroblasts into cancer cells. They have been found to be ubiquitously expressed in embryonic and adult tissues and to interfere with TGF-ss/BMP signaling. More recently, a group of proteins has been discovered which belongs to the same protein family, the functional Smad suppressing elements (Fussel). They have a highly restricted, mainly neuronal expression pattern suggesting different functional importance compared to Ski/Sno. We have used Drosophila as a model organism to characterize the highly specific neuronal expression pattern and created knock-out mutations within the Drosophila fuss gene. Surprisingly, fuss mutants are fully viable, but they show defects in bitter taste perception, and indeed, we could prove that Fuss is expressed specifically in bitter sensing neurons, where it affects their terminal differentiation making these cells insensitive for bitter compounds. To understand the molecular process involved in Fuss function we started protein interaction studies and could show, that Fuss forms part of a chromatin modifying complex, which seems to be important for the proper differentiation of neurons in the adult nervous system, therefore, assigning Drosophila as an indispensable model to study the molecular function of the Fuss protein family.