From signals to circuits: The layered role of Semaphorin-6A in brain development and disease

The human brain is built through a highly coordinated developmental process in which neurons are generated in limited regions, migrate to precise locations, and then form extensive networks. A central question in developmental neurobiology is how these neurons reach their correct destinations and establish appropriate connections. Axon guidance molecules play a key role in this process. Initially identified as cues directing axons, these proteins are now known to regulate multiple aspects of neural development, including neuronal migration, dendrite formation, and synapse establishment. Disruptions in these processes are associated with neurodevelopmental disorders such as autism, epilepsy, and schizophrenia. Despite the identification of several dozen guidance molecules, it remains unclear how this relatively small set can orchestrate the development of the complex brain. This thesis focuses on Semaphorin-6A (SEMA6A), an axon guidance molecule with diverse functions in brain development. While Semaphorins are classically described as ligands, SEMA6A also contains an intracellular domain, suggesting it may function as a receptor. The extent to which these dual signalling roles contribute to brain development has remained largely unexplored. Using mouse models, this thesis investigates how SEMA6A regulates neurodevelopment through distinct signalling mechanisms.

Chapter 2 provides a conceptual framework by reviewing how classical guidance molecules regulate neuronal migration, highlighting the importance of developmental context, timing, and interactions between signalling pathways. In Chapter 3, we demonstrate that SEMA6A functions as a receptor during brain development. We develop a mouse line that lacks SEMA6A receptor signalling, which exhibits a variety of pronounced structural brain abnormalities. In the cerebral cortex, neuronal mispositioning arises not from defects in neurons themselves, but from altered function of radial glial cells, which provide the scaffold for migration. This identifies a previously unrecognised, non-cell-autonomous role for SEMA6A reverse signalling in brain development. Chapter 4 further dissects this mechanism by analysing radial glial cells at the molecular level. Proteomic and functional approaches reveal candidate mediators of SEMA6A signalling, and their disruption leads to altered glial morphology and impaired neuronal migration, establishing a pathway through which SEMA6A indirectly controls cortical organisation. Chapter 5 examines the consequences of neuronal misplacement in the cortex. Although misplaced neurons retain their molecular identity, they show abnormal dendritic development and reduced interhemispheric connectivity, indicating that migration defects may have lasting effects on circuit formation and cortical function. In Chapter 6, we identify a distinct role for SEMA6A in the GnRH system. While GnRH neurons migrate normally, their projections towards the median eminence fail to extend properly, leading to delayed puberty and reduced fertility. This phenotype is linked to an unexpected role of SEMA6A in regulating blood–brain barrier permeability, indirectly affecting neuronal outgrowth. Given that SEMA6A mutations are associated with delayed puberty in humans, this mechanism is likely conserved across species. Finally, Chapter 7 places these findings in a broader context, highlighting that axon guidance molecules operate through multiple, context-dependent signalling pathways. Together, this work demonstrates how a single molecule can exert diverse functions during brain development and provides insight into mechanisms that may underlie neurological disorders.