Cellular and Molecular Analysis of Neuronal Structural Plasticity in the Mammalian Cortex

Cellular and Molecular Analysis of Neuronal Structural Plasticity in the Mammalian Cortex by Wei-Chung Allen Lee S.B., Chemical Engineering California Institute of Technology, 1998 A.B., Biochemistry and Government Bowdoin College, 1998 Submitted to the Department of Brain and Cognitive Sciences in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Neuroscience at the Massachusetts Institute of Technology June 2006 © 2006 Massachusetts Institute of Technology. All rights reserved. Signature of Author............................................................................................................................ Department of Brain and Cognitive Sciences May 15, 2006 Certified by........................................................................................................................................ Elly Nedivi Fred and Carol Middleton Associate Professor of Neurobiology Thesis Supervisor Accepted by....................................................................................................................................... Matthew Wilson Picower Scholar & Professor of Neurobiology Chair, Committee for Graduate Students Cellular and Molecular Analysis of Neuronal Structural Plasticity in the Mammalian Cortex by Wei-Chung Allen Lee Submitted to the Department of Brain and Cognitive Sciences on May 15, 2006 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Neuroscience ABSTRACT Despite decades of evidence for functional plasticity in the adult brain, the role of structural plasticity in its manifestation remains unclear. cpg15 is an activity-regulated gene encoding a membrane-bound ligand that coordinately regulates growth of apposing dendritic and axonal arbors and the maturation of their synapses. Here we compare cpg15 expression during normal development of the rat visual system, with that seen in response to dark rearing, monocular retinal action potential blockade, or monocular deprivation. Our results show that: (1) cpg15 expression in visual cortex correlates with the electrophysiologically mapped critical period for development of eye-specific preference in the primary visual cortex. (2) Dark rearing elevates adult levels of expression. (3) A component of cpg15 expression is activity-dependent after the peak of the critical period. (4) At the peak of the critical period, monocular deprivation decreases cpg15 expression more than monocular TTX blockade. And (5) cpg15 expression is robust and regulated by light in the superficial layers of the adult visual cortex. This suggests that cpg15 is an excellent molecular marker for the visual system’s capacity for plasticity and predicts that neural remodeling normally occurs in the extragranular layers of the adult visual cortex. To examine the extent of neuronal remodeling that occurs in the brain on a daily basis, we used a multi-photon based microscopy system for chronic in vivo imaging and reconstruction of entire neurons in the superficial layers of the rodent cerebral cortex. Here, we show the first unambiguous evidence of dendrite growth and remodeling in adult neurons. Over a period of months, neurons could be seen extending and retracting existing branches, and in rare cases adding new branch tips. Neurons exhibiting dynamic arbor rearrangements were GABA positive non-pyramidal interneurons, while pyramidal cells remained stable. These results are consistent with the idea that dendritic structural remodeling is a substrate for adult plasticity and suggest that circuit rearrangement in the adult cortex is restricted by cell type-specific rules. Thesis Supervisor: Elly Nedivi Title: Fred and Carol Middleton Associate Professor of Neurobiology 2 Acknowledgements Tremendous thanks to my advisor Elly Nedivi for teaching, nurturing, and having faith in a fledgling interested in the science of how the brain works. I would also like to thank the past and present members of the Nedivi Lab, especially Marnie Phillips, Tadahiro Fujino, Corey Harwell, Ulrich Putz and Jeff Cottrell for their colleagueship, friendship, and for the unique, invigorating research environment they helped to create. I would next like to thank my committee, Mark Bear, Martha Constantine-Paton, and Peter So, for their guidance. I would also like to thank my collaborators, Josh Sanes, Guoping Feng, Emery Brown, and particularly Peter So and Hayden Huang, whose assistance was invaluable. I would like to thank my parents, Han-San and Ming-Tzu Lee, for their life-long support and dedication to my scholarship. Finally, this would not have been possible without the unwavering support, patience, love and understanding of my wife, Jenny. 3 Contents Abstract..........................................................................................................................................2 Acknowledgements........................................................................................................................3 Table of Contents...........................................................................................................................4 Chapter 1: Introduction................................................................................................................6 Chapter 2: Extended plasticity of visual cortex in dark-reared animals may result from prolonged expression of cpg15-like genes......................................................................14 Chapter 3: Dynamic remodeling of dendritic arbors in GABAergic interneurons of adult visual cortex......................................................................................................................44 Chapter 4: Conclusion.................................................................................................................75 Appendix.......................................................................................................................................84 References.....................................................................................................................................87 4 But the functional specialization of the brain imposed on the neurones two great lacunae; proliferative inability and irreversibility of intraprotoplasmic differentiation. It is for this reason that, once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In adult centres the nerve paths are something fixed, ended, immutable. Everything may die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree. −Santiago Ramon y Cajal 5 Chapter 1 Introduction Developmental and adult plasticity Within the astounding complexity of the brain, fundamental yet unknown mechanisms sub serve learning, memory, perception, behavior, reason, and consciousness. How the outside world influences the brain likely changes the mind. The capacity of the central nervous system (CNS) to modify connections in response to activity is termed plasticity. Plasticity is a prominent feature of CNS development (Constantine-Paton et al., 1990; Shatz, 1990; Goodman and Shatz, 1993), and in the adult, putatively underlies learning and memory (Bailey and Kandel, 1993; Bliss and Collingridge, 1993) and adaptive reorganization of primary sensory maps (Merzenich et al., 1988; Kaas et al., 1990; Allard et al., 1991; Kaas, 1991; Gilbert and Wiesel, 1992; Recanzone et al., 1993; Zarzecki et al., 1993; Armstrong-James et al., 1994; Diamond et al., 1994). Anatomical and physiological studies of developmental plasticity in the visual system have provided valuable insight into how activity drives the final patterning of neuronal connections (Constantine-Paton et al., 1990; Shatz, 1990; Katz and Shatz, 1996). The critical period for susceptibility to environmental influences is one of the most influential models to emerge from studies of visual system development. In highly binocular animals such as cats, primates, and ferrets, electrophysiological plasticity during the critical period for development of eye specific preference in the primary visual cortex is accompanied by activity-driven segregation of geniculocortical afferents into ocular dominance columns (Hubel et al., 1977; Shatz and Stryker, 1978; Issa et al., 1999). There exist, however, many instances where no apparent anatomical plasticity is detectable when physiological plasticity clearly exists. Electrophysiological studies of the rodent visual system show that cortical neurons develop well 6 defined functional properties relating to eye preference and orientation selectivity (Parnavelas et al., 1981; Maffei et al., 1992; Fagiolini et al., 1994; Gordon and Stryker, 1996; Antonini et al., 1999) and are capable of undergoing shifts in eye preference during a critical period in development (Drager, 1978; Fagiolini et al., 1994; Gordon and Stryker, 1996; Guire et al., 1999; Frenkel and Bear, 2004); however, they are not anatomically segregated according to these properties akin to the feline and primate visual system. Since there is no segregation into eye- specific zones, anatomical changes in response to monocular deprivation (MD) during the critical period can be seen only with single-cell labeling and reconstruction (Antonini et al., 1999). The capacity for change in response to manipulation of patterned visual input is normally restricted to a defined age window during postnatal development (Hubel and Wiesel, 1970; Hubel et al., 1977). However, in kittens or

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