Neurofilaments and pathologies of the Peripheral Nervous System
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The efficiency of signal propagation in the peripheral nervous system (PNS) is maximized by myelination and axon diameter. Myelination induces axonal expansion through radial growth. Radial growth is dependent on neurofilaments (NFs) that can be made up of the neurofilament light (NF-L) subunit in association with either the neurofilament medium (NF-M) or neurofilament heavy (NF-H) subunit. Myelin thickness and length (internodal) are established proportional to axon diameter for optimal conduction velocity. Myelin thickness is regulated by total neuregulin I type III (Nrg1 type III) levels present on the axon whereas the mechanisms that control the establishment of internodal length are less understood. My work expands on previous data demonstrating that myelin thickness does not respond to alterations in axonal diameter. In contrast to the previous study, my work describes myelin thickness in the context of decreased axonal diameters. NF subunit mutants that result in varying degrees of altered axonal diameter were used as a tool to study the response of myelin thickness to larger reductions in axonal diameter. At two and six months, g-ratios corresponded to the degree of axonal diameter change. At two months, the size of axons arranged into the following order: wild type > NF-H[superscript [[delta]]Tail] > NF-M[superscript [[delta]]Tail]>NF-(M/H) [superscript [[delta]]Tail]. Correspondingly, g-ratios arranged into the same order indicating the larger the decrease in axon diameter, the greater the proportional increase in myelin thickness. At six months, axon diameters grouped into "wild type" sizes and "NFM?Tail" sizes. Similarly, g-ratios grouped into "wild type" ratios and "NF-M?Tail" ratios indicating that myelin thickness did not respond to increased radial growth. At six months NF-M?Tail mice demonstrated decreased internodal length suggesting that internodal length responded to alterations in axon diameter. My work provides the first evidence of the consequence of altered myelin thickness in isolation. Mice with hypomyelination, alone, demonstrated reduced swing speed and stride length in all limbs. Mutations in proteins specific to myelin result CMT1 that display uniform slowing of conduction velocity. In contrast, CMT2E arises from mutations to axonal proteins resulting in non-uniform slowing of conduction velocity. We generated a mouse model of CMT2E by expressing a hNF-L[superscript E397K] transgene. hNF-L[superscript E397K] expression causes inherent defects to the neurofilament network. As a result, our CMT2E model demonstrates altered myelin thickness in motor and sensory nerves and unilateral gait alterations that include decreased stride length, increased foot drags, and altered coordination of coupled limbs. The correlation between defects observed in our hypomyelination model and our CMT2E model suggest that altered myelin thickness may play a role in CMT2E phenotype. NF accumulations first appear at the NMJs of the diaphragm in SMA?7 mice. Motor axon loss and decreased axonal diameter is observed in the cervical spinal cord which is responsible for innervating the diaphragm. Taken together, these data suggest that inherent NF defects may be present in SMA?7 mice. My work provides a comprehensive analysis of the NF network in a cell, sciatic nerve, where analyses wouldn't be confounded by axonal loss. My analyses demonstrated that total NF levels, trafficking, and deposition were unaffected in SMA?7 mice suggesting that the NF network was uncompromised. Therefore, NF accumulations at the NMJ are most likely due to local alterations to NF dynamics. Furthermore, my work demonstrates that alterations to the transport of retrograde motors and anterograde transport of vital synaptic vesicle proteins coincide with the appearance of NF accumulations.
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