Developmental expression of the actin depolymerizing factor ADF in the mouse inner ear and spiral ganglia

Abstract

Hair cells, the inner ear's sensory cells, are characterized by tens to hundreds of actin-rich stereocilia that form the hair bundle apparatus necessary for mechanoelectrical transduction. Both the number and length of actin filaments are precisely regulated in stereocilia. Proper cochlear and vestibular function also depends on actin filaments in nonsensory supporting cells. The formation of actin filaments is a dynamic, treadmill-like process in which actin-binding proteins play crucial roles. However, little is known about the presence and function of actin binding molecules in the inner ear, which set up, and maintain, actin-rich structures and regulate actin turnover. Here we examined the expression and subcellular location of the actin filament depolymerizing factor (ADF) in the cochlea and vestibular organs. By means of immunocytochemistry and confocal microscopy, we analyzed whole-mount preparations and cross-sections in fetal and postnatal mice (E15-P26). We found a transient ADF expression in immature hair cells of the organ of Corti, the utricle, and the saccule. Interestingly, the stereocilia were not labeled. By P26, ADF expression was restricted to supporting cells. In addition, we localized ADF in presynaptic terminals of medio-olivocochlear projections after hearing onset. A small population of spiral ganglion neurons strongly expressed ADF. Based on their relative number, peripheral location within the ganglion, smaller soma size, and coexpression of neurofilament 200, we identified these cells as Type II spiral ganglion neurons. The developmentally regulated ADF expression suggests a temporally restricted function in the stereocilia and, thus, a hitherto undescribed role of ADF.

N-Cofilin Can Compensate for the Loss of ADF in Excitatory Synapses

“…By immunocytochemistry and immuno-EM, we found that ADF is present in virtually all excitatory synapses of hippocampal pyramidal cells and located in presynaptic as well as postsynaptic structures. As previously described for outer hair cell-innervating medio-olivocochlear projections [24], we found an enrichment of ADF in presynaptic terminals of hippocampal pyramidal cells, whereas the other ADF/cofilin family member, n-cofilin, reportedly is enriched in postsynaptic spines [23]. The complementary distribution pattern of ADF and n-cofilin implies that these two molecules fulfill diverse functions in excitatory synapses.…”

Withdrawal and restoration of central vagal afferents within the dorsal vagal complex following subdiaphragmatic vagotomy

“…Polyclonal anti-synaptophysin I antibody recognizes a single band at 35 kDa on western blots of rat brain samples (Wimmer et al, 2006). The specificity of the antibody has been confirmed in previous immunohistochemical studies using rodent brain tissue (Herde et al, 2010;Gazula et al, 2010). The rabbit antisynapsin 1 polyclonal antiserum recognizes two bands of 77 and 80 kDa on western blots of mouse brain extracts, corresponding to the expected size of synapsin 1a and 1b, respectively.…”

A Corticosteroid-Responsive Transcription Factor, Promyelocytic Leukemia Zinc Finger Protein, Mediates Protection of the Cochlea from Acoustic Trauma

“…Maintenance of cochlear stereocilia appears to be a very dynamic process with association and disassociation of actin filaments under active control (Rzadzinska et al, 2004; Belyantseva et al, 2009; Peng et al, 2009; Herde et al, 2010; Zheng et al, 2010). Stereocilia damage becomes more severe with increasing time or intensity of acoustic exposure, a finding that might be interpreted as an inability of these dynamic processes to keep up with an increased demand associated with acoustic overstimulation.…”