STRUCTURE AND FORMATION OF PERINEURONAL NETS: A UNIQUE FORM OF EXTRACELLULAR MATRIX IN THE CENTRAL NERVOUS SYSTEM
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AbstractThe complex organization and trajectory of development of the central nervous system (CNS) depends not only on cell-cell interactions but also on interactions of cells with the extracellular environment. The extracellular environment in the CNS contains an organized network of proteins, glycans and glycoproteins and comprises what is called the CNS extracellular matrix (ECM). Components of this ECM are dynamically regulated not just during development but are also involved in various neurobiological processes in the adult. They are implicated in normal nervous system physiology as well as in nervous system disease and pathology. A complete understanding of the CNS therefore, requires an understanding of the composition, structure and function of the neural ECM also. Our laboratory focuses on a specialized substructure of the neural ECM, called perineuronal nets (PNNs). PNNs are conspicuous structures within the neural ECM and are thought to be key regulators of neuronal plasticity. Traditionally their role has been demonstrated in regulating forms of plasticity seen during development, but recent work has implicated PNNs in other forms of learning and plasticity across various brain regions as well. Interest in these structures has been further peaked by studies linking their alterations to a variety of neurological and neuropsychiatric diseases such as Alzheimer's disease and schizophrenia. However, despite this growing interest in PNNs, the mechanisms by which they modulate neural functions are poorly understood. The limited mechanistic understanding of PNN function is derived primarily from the fact that there are no existing models, tools or techniques that specifically target them without also disrupting the surrounding neural ECM. Therefore, a rigorous investigation of PNN function to date has not been possible. Our inability to specifically target PNNs is driven by an incomplete of understanding of their molecular composition and structure. While PNNs comprise of many known ECM components which are broadly expressed in the CNS ECM, PNNs appear clearly distinct from their surroundings. They are highly ordered and stable and show a regular organization and geometry not present in the diffuse neural ECM. The exact molecular mechanisms by which various PNN components come together and aggregate on the cell surface to form these structures however, is not iii clearly known. The primary goal of our laboratory has therefore been to determine the molecular structure, composition and formation of PNNs. Through our work described in chapters 2 and 3 of this thesis, we were able to develop a new model of PNN structure. In our proposed model, PNN components are bound to the cell surface by two distinct types of interactions, one dependent on the classical HA scaffold of PNNs and the other mediated by a complex formed by the ECM glycoprotein tenascin-R (TNR) and the chondroitin sulfate proteoglycan receptor protein tyrosine phosphatase zeta (RPTPz). Our work also allowed us to provide evidence that PNN components are immobilized on the neuronal surface by a GPI-linked mechanism. We identify the GPI-linked protein contactin-1 (CNTN1) as a key receptor molecule for PNNs. To our knowledge, this is the first ever identified receptor for PNNs. We feel that our findings presented here give us important insight into PNN structure and represent the initial important steps in understanding the formation of this unique ECM subcompartment.
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