Molecular mechanisms of teneurin function in transcriptional regulation and cell adhesion

Abstract

Teneurins are large transmembrane glycoproteins that are well-conserved across phyla and show their strongest expression in the developing central nervous system (CNS). The large extracellular domain (ECD) includes several structural features like an NHL repeat domain, a predicted beta-propeller, which is responsible for homophilic rather than heterophilic interactions. It has been shown that the homophilic interaction of the ECD leads to its release, and the subsequent regulated intramembrane proteolysis (RIP) of the intracellular domain (ICD). RIP cleaves the ICD at the membrane, after which it translocates to the nucleus, where it is known to affect transcriptional regulation. The first part of my thesis discusses the evolution of teneurins. Teneurins are ancient proteins that are well-conserved across phyla from unicellular eukaryotic organisms like the choanoflagellate Monosiga brevicollis to higher multicellular organisms like vertebrates. The study suggests that teneurins may have evolved from a choanoflagellate via horizontal gene transfer from a prokaryote. It also describes the structural features of teneurins in detail, and identifies splice variants of chicken and human teneurin ICDs. The second part of my thesis describes a novel molecular mechanism in transcriptional regulation for the intracellular domain of human teneurin-1 (TEN1-ICD). We identified several new interaction partners of the TEN1-ICD in a yeast-2 hybrid screen. Concurrently, we performed a whole transcriptome analysis of a glioblastoma cell line engineered for inducible overexpressing of the TEN1-ICD comparing induced to non-induced cells, to determine potential target genes. Results included several microphthalmia-associated transcription factor (MITF) target genes. Interestingly, MITF is directly inhibited at the promoter by transcriptional repressor histidine-triad nucleotide binding protein 1 (HINT1), one of the novel TEN1-ICD interaction partners. Further experiments show that the TEN1-ICD competes for HINT1 binding to positively regulate MITF-dependent transcription of target gene GPNMB. The third part of my thesis discusses the NHL repeat domain, located in the ECD of teneurins. Since this predicted beta-propeller is responsible for homophilic, but not heterophilic interactions in chicken teneurins-1 and -2, we were interested to learn more about the structure of the domain. For this, we started by purifying the NHL repeat domain of chicken teneurin-2 and set up drops for crystallization studies, to resolve the structure by X-ray crystallography. We have set up the purification protocol, but have not yet determined the ‘right’ conditions for crystallization of the protein.