Mechanistic and functional analysis of Slit-Robo proteins

Zakrys, Linas (2012) Mechanistic and functional analysis of Slit-Robo proteins. PhD thesis, University of Glasgow.

Abstract

Slits are large secreted proteins which mediate their functions by binding to single pass transmembrane receptors called Robo. This signalling axis was first identified as a pivotal guidance mechanism in the development of the nervous system. A repulsive activity of Slit proteins controls the projection and movement of Robo expressing neurons in Drosophila and vertebrates. Slit and Robo expression is not limited to the nervous system or development only, they were found to be expressed in many different tissues both in embryo and adult organism. Correspondingly, the Slit-Robo signalling axis now is implicated in many other biological and pathological processes such as angiogenesis, cancer and tissue remodelling. This study sought to investigate two different aspects of the Slit-Robo signalling system: Robo1 transmembrane signalling mechanism and possible Slit-Robo roles within the immune system.
Robos and other type I transmembrane receptors are unable to transmit signal across the membrane within a single molecule because a single transmembrane α-helix restricts propagation of conformational changes between extracellular and intracellular parts of the protein. Therefore, the type I receptors usually act as homo- or heterooligomers, formation or dissociation of which is controlled by ligand binding. Based on indirect evidence a similar transmembrane signalling mechanism was suggested for Robo receptors. It was hypothesized that Slit binding induces changes in oligomeric state of Robo which in turn initiates downstream signalling events. In order to test this hypothesis and determine details of the transmembrane signalling mechanism, the oligomeric state of Robo1 receptor was assessed in live cells using two different fluorescence resonance energy transfer (FRET) based methods. A strong FRET signal was observed between differently tagged Robo1 proteins, indicating that the receptor forms oligomers in the resting state. However, contrary to the initial hypothesis, the addition of Slit2 protein did not have an observable effect on the FRET signal and thus on receptor oligomeric state. Moreover, Robo1 proteins were found to form higher density domains within the cell membrane. This property could be abolished by removing the intracellular part of the protein indicating constant Robo1 association with intracellular structures. These data show that the Robo1 transmembrane signalling mechanism might be more complicated than initially expected and likely involves structural changes within the Robo1 oligomer or Robo1 complexes with other proteins.
The Slit-Robo signalling axis was linked to the immune system when it was demonstrated that Slit2 is able to inhibit chemotactic leukocyte migration and reduce inflammatory responses in mice. However, the precise role of these proteins remained unknown. In order to gain further insight into possible Slit-Robo functions within the immune system promoter analysis was performed with the aim to identify transcription factors responsible for the control of these proteins. Among many putative transcription factors discovered, Foxn1 was the most prominent as its binding site was identified both in Slit2 and Slit3 promoters. Development and functions of thymus, a primary lymphoid organ responsible for T cell development, is regulated by Foxn1 and since thymic activity includes active and abundant leukocyte movement into, out-of and within the organ, a hypothesis was suggested that Slits contribute to regulation of these processes. Quantitative Slit2-Slit3 expression studies and assessment of embryonic thymus cellular composition in Slit2 knockout mouse were employed to test this hypothesis. Quantitative PCR revealed that both Slit2 and Slit3 are downregulated four-fold in thymic epithelial cells starting embryonic stage E13, however no differences in cellular composition of embryonic thymi were detected between wild type and Slit2 knockout animals using flow cytometry. Possible Slit2 and Slit3 redundancy might be the reason for the lack of observable effects, unfortunately a Slit3 knockout model was unavailable for these studies. Interestingly, flow cytometry also revealed that in addition to thymic epithelial cells, Slit2 is expressed by pericyte type cells surrounding the thymic vasculature. It seems that Slit proteins have intricate and regulated expression patterns within the thymus, however their precise role remains an open question.
In summary, data collected during this study illuminates two different aspects of the Slit-Robo signalling system. Both of them, despite Slits being secreted proteins, hint at short-range, possibly even juxtacrine, ligand – receptor interactions. Given Robo receptor evolutionary connections to cell adhesion molecules and Slit affinity to heparan sulphate it is not a completely unexpected finding.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Slit, Robo, transmembrane signaling, thymus
• Subjects: Q Science > Q Science (General)
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Infection & Immunity; College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
• Supervisor's Name: Milligan, Prof. Graeme and Graham, Prof. Gerard
• Date of Award: 2012
• Depositing User: Mr Linas Zakrys
• Unique ID: glathesis:2012-3672
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 07 Nov 2012
• Last Modified: 10 Dec 2012 14:09
• URI: https://theses.gla.ac.uk/id/eprint/3672