The Structure and Function of the Honeybee Blood-Brain Barrier

Many animals possess a blood-brain barrier, which is a layer of cells that restricts the passage of molecules into the central nervous system. The primary function of the blood-brain barrier is to preserve ionic homeostasis within the brain; however, it is also responsible for selectively importing an array of nutritional and signaling molecules to support brain function and for exporting metabolic waste. Across the species in which it has been studied, the structure and function of the blood-brain barrier dynamically regulates the interaction between the brain and peripheral physiological systems. Honeybee (Apis mellifera) workers are a firmly established neurobiological model which can be utilized to answer questions about the physiological and environmental mechanisms that regulate central nervous system health and behavior. It is likely that the honeybee blood-brain barrier plays an important role mediating the interactions between the brain and its environment, however, the blood-brain barrier is largely unconsidered in the realm of honeybee neurobiological research. In this dissertation, I provide the first in depth characterizations of the structure and function of the honeybee blood-brain barrier. First, I characterized the ultrastructural organization of the honeybee blood-brain barrier. The results of this study demonstrate its structural heterogeneity, including how this heterogeneity compares between two age groups. Next, I assessed two dimensions of blood-brain barrier permeability among three honeybee age groups and among honeybees exposed to varying amounts of infestation with the parasitic mite Varroa destructor. This study demonstrated that paracellular permeability has greater resilience than transcellular permeability, the latter of which is particularly increased by a high parasitic load. Finally, I developed a novel technique combining stable isotope labelling and Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) to demonstrate that the large, pro-social protein vitellogenin is able to cross the honeybee blood-brain barrier into the brain. Together, these studies represent the first in-depth analysis of the honeybee blood-brain barrier, establishing new directions for understanding the regulation of honeybee health, disease, and behavior.