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The Role of sema2a in the Neural Compensatory Growth of Gryllus bimaculatus
Ana García-Moreno, Class of 2017
Nervous system growth and development is an intricate process and not yet fully
understood. Because many factors that influence the growth of neurons are well conserved across
a broad range of species, in learning about the more complicated nervous systems found in
humans and other mammals, it can be useful to study the simpler systems found in insects and
other animals. A better understanding of nervous system development can help scientists to learn
how nervous systems react to and recover from injury. Considering the degree to which nervous
system damage can affect people’s lives, a clearer perception of how simple nervous systems
react to injury could help us to deduce the mechanisms responsible for general neural recovery,
and hopefully one day this knowledge will elucidate ways in which we can promote the recovery
of damaged human nervous systems.
The nervous system consists largely of neurons that pass information from one to the
other, ultimately resulting in an exchange of information between the brain and the body. Where
two neurons meet, the dendrites of one neuron receive messages from the axons of other neurons.
If these dendrites lose contact with their affiliated axons, this cessation of input often causes cell
death. The Horch lab studies a nervous system that displays an exception to this pattern: the
auditory system of the cricket Gryllus bimaculatus. In this species, cutting off the connection
between nerve 5, which carries auditory information, and AN-2, an auditory interneuron
receiving this information, in a process known as deafferentation, does not result in the death of
the AN-2 neuron (as might be expected) but rather in the growth of the AN-2 dendrites. Where
the AN-2 dendrites once avoided the cricket’s midline, they grow to cross it in order to form
functional connections with the axons of the auditory nerve on the cricket’s opposite side, which
also experience compensatory growth in the form of increased axonal branching.
Proteins play a key role in neuronal growth and development, affecting dendritic and
axonal growth by acting as guidance cues or as receptors for these guidance cues. By looking at
how the levels of different proteins are altered upon auditory system injury and recovery, we may
begin to understand the role various proteins play in post-injury dendritic and axonal growth. In
looking at protein levels, it is important to look at the genes that code for these proteins and the
levels at which these genes are expressed. One gene that the Horch lab has found to be underexpressed post-deafferentation is the sema2a gene. This gene codes for a type of protein known
as a semaphorin. Semaphorins are known guidance molecules that influence the growth of axons,
dendritic development, midline crossing, and the manner in which neural systems react to injury.
In attempting to illuminate just how the level of semaphorins might play a role in this
post-deafferentation compensatory growth, we performed a genetic knockdown of sema2a by
injecting non-deaffarented crickets with double stranded RNA (dsRNA) targeting two regions of
the sema2a gene. This knockdown should have, like deafferentation, lowered sema2a levels in
the cricket’s auditory system. In order to verify knockdown success, quantitative PCR will be
performed on stored tissue from injected crickets to measure levels of sema2a mRNA. To
visualize the effects of the knockdown, we filled AN-2 and nerve 5 of injected crickets with dye
in order to view their dendritic and axonal arbors under the confocal microscope. Because this
experiment mimicked sema2a levels upon deafferentaion, we expect to see some dendritic and/or
axonal growth within the auditory system similar to that induced by deafferentation.
Faculty Mentor: Hadley Horch
Funded by the NIH and INBRE
Horch, Hadley Wilson, et al. "Bilateral consequences of chronic unilateral deafferentation
in the auditory system of the cricket Gryllus bimaculatus." Developmental neuroscience 33.1
(2011): 21-37.
Horch, Hadley W., et al. "Differential gene expression during compensatory sprouting of
dendrites in the auditory system of the cricket Gryllus bimaculatus." Insect molecular biology
18.4 (2009): 483-496.
Pfister, Alexandra, et al. "Quantification of dendritic and axonal growth after injury to the
auditory system of the adult cricket Gryllus bimaculatus." Frontiers in physiology 3 (2012).
Chong, M. (2015). Semaphorin 1 in the compensatory growth of the cricket auditory
system. Bowdoin College.