ABSTRACT OF THE DISSERTATION
Role of Glia in the Development and Shaping of Connections Within the Mammalian Auditory Brainstem
Minhan Lynn Dinh
Doctor of Philosophy in Biological Sciences
University of California, Irvine, 2016
Professor Karina S. Cramer, Chair
The formation and maturation of precise synapses are a central question in the field of developmental neuroscience. This dissertation focuses on investigating the mechanisms underlying appropriate synapse growth within the mammalian brainstem. This region is comprised of several precisely wired connections that allows an organism to process auditory stimuli and to perform sound localization. This dissertation investigates glial mechanisms important for mediating development of the pathway from the ventral cochlear nucleus (VCN) to the medial nucleus of the trapezoid body (MNTB), a contralateral projection that terminates in the calyx of Held. Glial mechanisms are considered both in normal developmental and in lesion-induced plasticity.
In Chapter 2, we found that several glial markers are present in the auditory brainstem regions of VCN and MNTB during early postnatal development. In particular, astrocytes and microglia were found in direct apposition to the developing calyx. Following cochlear removal, in which ectopic calyces form from the intact VCN, microglia and astrocytes can be found within close proximity of the new calyces.
The subsequent chapters further investigate the role of microglia in shaping the VCN-MNTB circuitry during normal development (Chapter 3). Here, two microglial mutant mouse lines (Cx3cr1 and Csf1R) were used. Both mutant mice exhibited a decreased calyx size, decreased MNTB area, in addition to pruning deficits. High-resolution imaging demonstrated that mutant Cx3cr1 mice had decreased levels of VGLUT1/2, suggesting that microglia may mediate overall amount of synaptic proteins. Additionally, we observed that activation of microglia activity, through LPS injections, resulted in an increase in VGLUT1/2 levels, further supporting our data.
Finally, we tested the role of microglia in lesion-induced synaptic plasticity in the brainstem (Chapter 4). Following unilateral cochlear removal, we observed that microglia were associated with newly formed calyces that formed following injury. In addition, mutant Cx3cr1 mice exhibited decreased plasticity compared to their littermate controls.
Collectively, these results point to the importance of non-neuronal mechanisms that help form and maintain synapses within the brainstem and support the overarching hypothesis that both astrocytes and microglia are important for mediating development of the VCN-MNTB pathway in development and in lesion-induced plasticity.