The construction and refinement of early neuronal circuits is fundamentally relevant to adult brain function, developmental disorders, and learning and plasticity, but only a small part of this process is well understood (Cang and Feldheim, 2013; Ebert and Greenberg, 2013). A significant and long-standing debate in the field concerns the relative contributions of "hardwired" genetic and molecular determinants versus "plastic" environment and activity-driven alterations (Cline, 2003). While circuits are largely hardwired in most insect and invertebrate species (Hiesinger, 2006), the mammalian nervous system appears to rely on a combination of early molecular cues and later periods of activity-driven plasticity to refine circuits. An interesting middle ground in this process is a period during which circuits have begun to form and propagate activity, but do not yet function in an adult state (Huberman, 2008; Kirkby, 2013; Wong, 1999). At this time, multiple sensory systems are believed to experience "spontaneous" activity patterns that may help to refine circuits, but the form, relevance, and even existence of this activity is under debate (Cang and Feldheim, 2013). Similarly, relatively little is known about the molecules that drive these later stages of synapse and neuronal arbor formation, and the relation that they might have to available activity patterns (Feldheim and O'Leary, 2010).
In this thesis, I first describe experiments using in vivo calcium imaging in both retinal ganglion cell (RGC) axons and visual midbrain and cortex neuronal cell bodies that confirm the existence of patterned spontaneous activity ("retinal waves") throughout the early postnatal mouse visual system. In a second series of experiments with a genetic knockout believed to disrupt retinal waves in vivo, I find that both the frequency and pattern of waves are drastically disrupted in krrodcout mice relative to controls. Interestingly, I also find that downstream "spontaneous" activity patterns are "de-coupled" from retinal wave activity in this knockout. Conditional knockout experiments further revealed that retinal waves are required in a region-specific manner to drive circuit refinement, thus confirming the necessity of spontaneous activity to development in the visual system. Subsequent rescue experiments demonstrated that the properties of retinal waves are differentially relevant to separate visual circuits, implying that normal wave activity is likely optimized to refine multiple circuits concurrently. A final set of experiments was designed to investigate the role of Down syndrome cell adhesion molecule (DSCAM) in visual circuit development at ages when activity-dependent refinement is now believed to predominate. These results revealed that DSCAM knockout disrupts multiple visual circuits at a surprisingly late age, and in surprising ways. A striking "barrel" phenotype in the retinotopic map of germline and retina-specific conditional knockout mice strongly implies that loss of this cell-adhesion molecule can act on both axon-specific and non cell-autonomous levels during later ages when axon, synapse, and dendrite elaboration is currently believed to be primarily driven by spontaneous activity.
Together, these results depict visual system development as a process that initially relies on graded molecular cues to establish rough circuit guidelines, and then uses finely tuned patterns of spontaneous activity along with synaptic adhesion molecules to induce synapse and arbor elaboration and refinement. While there are likely a great deal of redundant and homeostatic mechanisms to ensure correct formation of such fundamental circuits, the defects induced by our manipulations were highly penetrant and often persisted into late adulthood, implying the existence of critical periods that will likely prove relevant to future studies of plasticity and developmental disorders. Overall, this work describes a system that relies on a complex synchronization of all available information to ensure the correct development of evolutionarily-relevant circuits within a short period of time.
|School Location:||United States -- Connecticut|
|Source:||DAI-B 76/11(E), Dissertation Abstracts International|
|Subjects:||Neurosciences, Developmental biology|
|Keywords:||Cell Adhesion Molecule, Circuit Development, Down Syndrome, Retinal Waves, Spontaneous Activity, Visual Plasticity|
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