UC Santa Cruz

Abstract

James A. Shanks

Cortical-Thalamic Axons are Required for Retinal Ganglion Cell Targeting to the Mouse Dorsal Lateral Geniculate Nucleus

The human brain contains over 85 billion neurons, which make trillions of synapses in a very ordered and stereotypical manner (Williams and Herrup, 1988). The human cerebral cortex, which contains over 20 percent of the total number of neurons within the brain, is responsible for critical functions such as memory, attention, perception, awareness, language, thought, and consciousness, and it dedicates over 30 percent of the total area to the process of vision, 8 percent for touch, and 3 percent for hearing (Grady et al., 1993). One major question in the field of neuroscience is: how do neurons navigate the extremely complex environment of the developing brain that contains an endless amount of possible synaptic partners and continually, precisely, and stereotypically find their appropriate synaptic targets?

Many disorders that affect the human population can be attributed to the loss of the precise wiring within the brain, including mental retardation, schizophrenia, and autism (Tye and Bolton, 2013; Kana et al., 2011; Huttenlocker, 1991; Buchmann et al., 2014). Recent studies that utilized fMRI to investigate brain activity in people who suffer from schizophrenia, a disorder that renders the inflicted an inability to perceive what is real, show a distinct loss of cortical activity in the visual thalamus resulting from a loss of thalamo-cortical circuitry. Elucidating the mechanisms responsible for the proper wiring of the brain is of paramount importance, and could lead to possible remedies for afflictions such as schizophrenia.

The mouse visual system has become an increasingly popular model to study the development of neural circuits given the vast genetic tools and the ease with which the output neurons of the retina, retinal ganglion cells (RGCs), can be labeled. Multiple distinct targets of RGCs have been well characterized and many behaviors are associated with these nuclei that can be easily assayed to detect for dysfunction (Sweeney et al., 2014; Osterhout et al., 2015). It has been demonstrated that activity is necessary for the refinement of RGCs in the dLGN during development and the proper placement of RGCs within eye specific domains is reliant on specific receptor tyrosine kinases (Rossi et al., 1988; Pfeifenberger et al., 2005). Mounting evidence suggests that specific transmembrane adhesion molecules are necessary for the proper targeting of RGCs to specific visual nuclei responsible for multiple non-image forming behaviors, including circadian rhythms, the pupillary light reflex, and image stabilization (Osterhout et al., 2011; Osterhout et al., 2015; Sun et al., 2015; Su et al., 2011). Molecules necessary for the proper targeting of image forming nuclei including the dLGN and the superior colliculus (SC), a midbrain target necessary for reflexive head and eye movements, remain elusive.

While the retina sends projections to specific subcortical nuclei, many of these same nuclei also receive cortical input from either layer 5 or layer 6 of the visual cortex (V1). The timing and patterns of innervation of cortical layer 6 neurons in the dLGN has recently been elucidated (Seabrook et al., 2013). These data indicate that the removal of RGC axons from the dLGN during development allows cortical neurons to innervate the dLGN at earlier time points, suggesting RGC axons control the timing of innervation. Whether cortical neurons affect the innervation patterns of RGCs is not well understood.

In order to better understand the role of V1 input during the development of the mouse visual system, I utilized a transgenic mouse harboring a floxed conditional allele for a splicing factor that when removed with a cre-recombinase driver line, only expressed in cortical tissue, results in the almost complete loss of the mouse neo-cortex. Surprisingly, the animal is viable and thus allowed me to investigate the necessity of V1 innervation during the development of the mouse visual system including innervation patterns of RGCs within multiple subcortical nuclei. I was also able to investigate well known non-image forming behaviors, including the pupillary light reflex, to test for the necessity of cortical layer 5 input, and circadian rhythms to better understand if the cortex modulates sleep wake cycles. Also, I tested whether mice lacking cortical input and almost complete loss of RGC inputs to the dLGN can perceive light and whether this perception can be used to locate a cue in a complex environment.