The multifunctional role of Semaphorin6A during brain development and disease: Moving forward with reverse signaling

The brain is a complex and versatile organ that undergoes various phases of development. This thesis focuses on the moment when neurons migrate from their birthplace to different brain areas and start to make connections with other cells to form neural circuits. During this phase, axon guidance molecules play an important role in establishing proper neural organization and connectivity. The principle of axon guidance is that growing axons and migrating cells rely on intermediate targets and guidance cues presented over short or long distances along their paths in order to establish neuronal connections. Growing axons extend their growth cones to scan the environment and respond to axon guidance cues that provide direction towards or away from specific regions. Semaphorins are axon guidance molecules that are well-known for their function as attractants or repellents. Although only a limited set of semaphorins has been identified, accumulating evidence indicates that distinct molecular mechanisms act to diversify the effects of semaphorins, making semaphorins multifunctional molecules that control a wide variety of cellular events. Chapter 2 comprises an extended introduction and description of general features and novel aspects of semaphorin function and signaling, including the role of their receptors; plexins and neuropilins. The focus is on the contribution of these molecules to the regulation of cell migration and the formation of neuronal connections. Semaphorin6A (Sema6A) is an example of a member of the semaphorin protein family that plays a key role in the nervous system, yet we know very little about its multifunctional role and diverse signaling mechanisms. In this thesis, the aim is to determine and specify these Sema6A signaling events during the development of the brain. Throughout the subsequent chapters of the thesis, three topics will be addressed: 1. Sema6A reverse signaling in vivo (Chapters 3 and 4) 2. Sema6A cis interaction and the specific structural domains involved in this interaction (Chapter 5) and 3. Novel functions of Sema6A in the diseased brain (Chapter 6). This thesis provides an overview of recent developments in the field of axon guidance research focusing on semaphorins, including the role of their interactors; plexins and neuropilins (Chapter 2). Furthermore, it focuses on Sema6A, which is known to play important roles during the development of the nervous system. The overall aim of this thesis is to study the multifunctional character of Sema6A by selectively exploring functions of the intracellular domain (Chapter 3) and reverse signaling events (Chapter 4), interactors and regulators involved in Sema6A signaling pathways (Chapter 4), cis interaction domains (Chapter 5) and to specify novel roles during puberty onset and neuronal disease (Chapter 6). These previously unexplored functions help to understand the complexity of semaphorin signaling pathways and provide an extensive and in-depth overview of Sema6A signaling during brain development. The findings described in this thesis contribute to the knowledge of complex and dynamic cellular and molecular events that help to understand brain development and disorders

SEMA6A drives GnRH neuron-dependent puberty onset by tuning median eminence vascular permeability.

Funder: Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Netherlands Organisation for Scientific Research); doi: https://doi.org/10.13039/501100003246Innervation of the hypothalamic median eminence by Gonadotropin-Releasing Hormone (GnRH) neurons is vital to ensure puberty onset and successful reproduction. However, the molecular and cellular mechanisms underlying median eminence development and pubertal timing are incompletely understood. Here we show that Semaphorin-6A is strongly expressed by median eminence-resident oligodendrocytes positioned adjacent to GnRH neuron projections and fenestrated capillaries, and that Semaphorin-6A is required for GnRH neuron innervation and puberty onset. In vitro and in vivo experiments reveal an unexpected function for Semaphorin-6A, via its receptor Plexin-A2, in the control of median eminence vascular permeability to maintain neuroendocrine homeostasis. To support the significance of these findings in humans, we identify patients with delayed puberty carrying a novel pathogenic variant of SEMA6A. In all, our data reveal a role for Semaphorin-6A in regulating GnRH neuron patterning by tuning the median eminence vascular barrier and thereby controlling puberty onset