Date of Award
Master of Science
Molecular Cellular and Systemic Physiology
The importance of activity during the development of central components of the nervous system such as the visual system has long been recognized (Wiesel & Hubel 1963) and it is beginning to be understood that sensory experience and motor behavior are equally important for neuromuscular development (Brumley et al. 2015; Sharp & Bekoff 2015). The chick embryo model has proven to be especially useful in studying the relationships among motor behavior, sensory experience, and neuromuscular development (Oppenheim et al. 1978; Sharp & Bekoff 2001) due to its accessibility and early onset of movement behavior. Traditionally, neuromuscular blockers have been used to broadly study the role of neural activity and muscle activity during development (Oppenheim et al. 1978; Ding et al. 1983). In order to noninvasively alter neural activity in specific populations of cells, the Sharp lab has developed an optogenetic approach that allows the expression of ChIEF, a variant of channelrhodopsin-2, in the spinal cord of living chick embryos (Sharp & Fromherz 2011). In order to better understand the unique role that muscle activity plays in neuromuscular development, it would be advantageous to directly and noninvasively control muscle activity through light-activation of ChIEF expressed in muscle fibers. Therefore, the primary objective of this thesis research was to achieve ChIEF expression in the plasma membrane of myotubes in living chick embryos. Initial attempts to express ChIEF in chick muscle resulted in low success rates. The CAG promoter in pPB-ChIEF-Tom, the plasmid vector that encodes ChIEF, was likely hindering expression of ChIEF in muscle tissue. Therefore, standard molecular cloning techniques were used to replace the CAG promoter with the myosin light chain promoter which was known to drive transgene expression in chick muscle (Wang et al. 2011). The new DNA construct that resulted from modifying pPB-ChIEF-Tom was identified as pPB-MLC-ChIEF-Tom (mChIEF). ChIEF was successfully expressed in hindlimb muscles of chick embryos via somite electroporation of mChIEF and observed between E7 and E18. Expression patterns corresponded with the current understanding of muscle progenitor contributions of somites to hindlimb muscles (Rees et al. 2003). ChIEF was located in the outer membrane of muscle fibers on E9, E14, and E18 when tissue was histologically examined in conjunction with myosin heavy chain immunofluorescence. Importantly, light-activation of ChIEF in the hindlimb muscle of living chick embryos resulted in muscle contraction and light-evoked hindlimb movements. In addition to demonstrating the functionality of ChIEF expression, an effort was made to characterize the effects of altered parameters of light stimuli on light-evoked movement and determine whether light-evoked muscle contraction could be used to imitate normal, neuronal muscle control. Light intensity was directly related to amplitude and rate of light-evoked movement. Light duration was directly related to amplitude and latency of peak movement. Unfused and fused tetanus were observed when bursts of short duration light pulses with varying interpulse intervals were used to activate ChIEF. This thesis research strongly suggests that light-activation of ChIEF expressed in living, chick embryo hindlimb muscle results in muscle contractions in manner similar to normal, neurally-driven muscle contraction.
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