Book Description

Understanding of how neural circuits process information from the environment, produce a percept of the external stimuli, and then evoke a behavioral response has been an ultimate goal in neuroscience. To achieve this long-standing goal, we must first understand how neural circuits are established. The functionality of a mature sensory system is highly dependent on the proper formation of precise neuronal connections that convey information about environmental stimuli to specific regions of the central nervous system. The formation of neural circuitry in the developing animal occurs in a series of regulated steps. Developing neuronal precursors first undergo a series of progressive events such as cell division, cell differentiation, cell migration, neuronal extension via elongation of their axon branches, axon guidance, and establishment of synapses. During these progressive events, neurons extend an exuberant number of axons and form an excess of broad connections. The developing nervous system will subsequently undergo a series of regressive events such as cell death, synapse elimination, and pruning of axons. These regressive events refine the immature broadly patterned neural connectivity into a mature precise neural network. Perturbations of axon pruning may result in abnormal neuronal connections, which in turn can lead to deleterious behavioral effects associated with several neuropsychological disorders. The focus of my dissertation research is on the study of regulatory mechanisms underlying axon pruning during the formation of neural circuitry in the mammalian central nervous system, specifically layer 5 pyramidal corticopontine and corticospinal neurons in the visual cortex (V1). I extensively investigate the development of V1 layer 5 pyramidal neurons because both small-scale pruning of visual corticopontine terminal zone (TZ) axons and large-scale stereotyped axon pruning of visual corticospinal tract (CST) axons occur within the same cells. Surprisingly, the precise timing of development and refinement of corticopontine and corticospinal projections of V1 layer 5 pyramidal neurons are not characterized, the mechanisms for small-scale pruning of visual corticopontine TZ axons are completely unknown, and the mechanisms for large-scale stereotyped pruning of visual CST axons are incompletely determined. For my dissertation research, I investigate the development and refinement of corticopontine and corticospinal projections from V1 layer 5 pyramidal neurons. Specifically in Chapter 2, I investigate whether both small-scale pruning of visual corticopontine TZ and large-scale pruning of visual CST axons are regulated by the same set of molecular and neural activity mechanisms or differentially regulated. I will show that both small- and large-scale stereotyped pruning of V1 efferent axons are simultaneously and coordinately regulated by the same set of mechanisms: 1) Semaphorin-3F (Sema3F) signaling through Neuropilin-2 (NPN2), Plexin-A3 (PLA3), and Plexin-A4 (PLA4) co-receptors, and 2) spontaneous retinal waves, not extrinsic visually-evoked activity. By using mouse genetics and comparative approaches, I will also present several lines of evidence to demonstrate that it is the initiation of Stage 3 spontaneous retinal waves, rather than the entire duration, that is necessary for both small-scale visual corticopontine terminal axon pruning and large-scale stereotyped visual CST pruning. In Chapter 3, I investigate the development of corticospinal projections from V1 layer 5 pyramidal neurons into the spinal cord and determine whether visual CST axons descend in multiple spinal cord locations. I will demonstrate that visual CST axons initially extend into both dorsal contralateral and ventral ipsilateral spinal cord and that subsequent large-scale stereotyped pruning will eliminate visual CST axons from both spinal cord locations. I will show that subsequent large-scale stereotyped pruning of dorsal contralateral visual CST axons is regulated by both Sema3F signaling and spontaneous retinal waves interacting along the same pathway, whereas large-scale stereotyped pruning of ventral ipsilateral visual CST axons is regulated by spontaneous retinal waves but not Sema3F signaling.Thus overall, my research has elucidated molecular and neural activity mechanisms regulating axon pruning of layer 5 V1 efferent neurons to advance understanding of regulatory mechanisms underlying axon pruning during the formation of neural circuitry in the mammalian central nervous system.