Drosophila HP1 and Insulator Partner Protein 1 (HIPP1) is the homologue of the human co-repressor Chromodomain Y family of proteins that repress neuronal gene expression in mammals. HIPP1 was identified by its extensive co-localization with Heterochromatin Protein 1a (HP1a) in heterochromatic regions of the genome and insulator binding proteins in euchromatic regions. The majority of HIPP1 binding to euchromatin is at binding sites for Drosophila Suppressor of Hairy-wing [Su(Hw)]. Su(Hw) is a zinc finger DNA binding protein that functions as an insulator, activator, and repressor. Transcriptional regulation by Su(Hw) is essential in the ovary and testis, where Su(Hw) functions primarily as a repressor of neuronal genes. However, the mechanism of Su(Hw) dependent repression is not clear. The focus of my thesis work has been defining the role of HIPP1 in development and its contribution to Su(Hw) function and heterochromatin formation. As part of this work, CRISPR was used to generate multiple Hipp1 null alleles and a tagged derivative of the endogenous gene (Hipp1GFP). Hipp1 null flies were found to be viable. Study of HIPP1 expression revealed it is present in most tissues and restricted to the nucleus. HIPP1 showed limited colocalization with HP1a, and tests of repression of transgenes in heterochromatin suggested that HIPP1 is not required for heterochromatin formation. Investigations of HIPP1 binding revealed that Su(Hw) is responsible for the majority of HIPP1 recruitment to euchromatin. Despite this, HIPP1 was found to be dispensable for the transcriptional and insulator functions of Su(Hw) as well as for female and male fertility. These data indicate that HIPP1 is not a critical Su(Hw) cofactor. Further studies are needed to clarify the role of HIPP1 in Drosophila development.

Epigenetic regulatory mechanisms play important roles in the control of lineage-specific differentiation during development. However, mechanisms that regulate higher-order chromatin remodelling and transcription of keratinocyte-specific genes that are clustered in the genome into three distinct loci (Keratin type I/II loci and Epidermal Differentiation Complex (EDC)) during differentiation of the epidermis are poorly understood. By using 3D-Fluorescent In Situ Hybridization (FISH), we determined that in the epidermal keratinocytes, the KtyII and EDC loci are located closely to each other in the nuclear compartment enriched by the nuclear speckles. However, in KtyII locus knockout mice, EDC locus moved away from the KtyII locus flanking regions and nuclear speckles towards the nuclear periphery, which is associated with marked changes in gene expression described previously. Chromatin architectural protein Ctcf has previously been implicated in the control of long-range enhancer-promoter contacts and inter-chromosomal interactions. Ctcf is broadly expressed in the skin including epidermal keratinocytes and hair follicles. Conditional Keratin 14-driven Ctcf ablation in mice results in the increase of the epidermal thickness, proliferation, alterations of the epidermal barrier and the development of epidermal pro-inflammatory response. Epidermal barrier defects in Krt14CreER/Ctcf fl/fl mice are associated with marked changes in gene expression in the EDC and KtyII loci, which become topologically segregated in the nucleus upon Ctcf ablation. Therefore, these data suggest that Ctcf serves as critical determinant regulating higher-order chromatin organization in lineage-specific gene loci in epidermal keratinocytes, which is required for the proper control of gene expression, maintenance of the epidermal barrier and its function.

Epigenetic regulatory mechanisms play important roles in the control of lineage-specific differentiation during development. However, mechanisms that regulate higher-order chromatin remodelling and transcription of keratinocyte-specific genes that are clustered in the genome into three distinct loci (Keratin type I/II loci and Epidermal Differentiation Complex (EDC) during differentiation of the epidermis are poorly understood. By using 3D-Fluorescent In Situ Hybridization (FISH), we determined that in the epidermal keratinocytes, the KtyII and EDC loci are located closely to each other in the nuclear compartment enriched by the nuclear speckles. However, in KtyII locus knockout mice, EDC locus moved away from the KtyII locus flanking regions and nuclear speckles towards the nuclear periphery, which is associated with marked changes in gene expression described previously. Chromatin architectural protein Ctcf has previously been implicated in the control of long-range enhancer-promoter contacts and inter-chromosomal interactions. Ctcf is broadly expressed in the skin including epidermal keratinocytes and hair follicles. Conditional Keratin 14-driven Ctcf ablation in mice results in the increase of the epidermal thickness, proliferation, alterations of the epidermal barrier and the development of epidermal pro-inflammatory response. Epidermal barrier defects in Krt14CreER/Ctcf fl/fl mice are associated with marked changes in gene expression in the EDC and KtyII loci, which become topologically segregated in the nucleus upon Ctcf ablation. Therefore, these data suggest that Ctcf serves as critical determinant regulating higher-order chromatin organization in lineage-specific gene loci in epidermal keratinocytes, which is required for the proper control of gene expression, maintenance of the epidermal barrier and its function.