Deficiency of Transcription Factor Brn4 Disrupts Cochlear Gap Junction Plaques in a Model of DFN3 Non-Syndromic Deafness

Abstract

Brn4, which encodes a POU transcription factor, is the gene responsible for DFN3, an X chromosome–linked, non-syndromic type of hearing loss. Brn4-deficient mice have a low endocochlear potential (EP), hearing loss, and ultrastructural alterations in spiral ligament fibrocytes, however the molecular pathology through which Brn4 deficiency causes low EP is still unclear. Mutations in the Gjb2 and Gjb6 genes encoding the gap junction proteins connexin26 (Cx26) and connexin30 (Cx30) genes, respectively, which encode gap junction proteins and are expressed in cochlear fibrocytes and non-sensory epithelial cells (i.e., cochlear supporting cells) to maintain the proper EP, are responsible for hereditary sensorineural deafness. It has been hypothesized that the gap junction in the cochlea provides an intercellular passage by which K+ is transported to maintain the EP at the high level necessary for sensory hair cell excitation. Here we analyzed the formation of gap junction plaques in cochlear supporting cells of Brn4-deficient mice at different stages by confocal microscopy and three-dimensional graphic reconstructions. Gap junctions from control mice, which are composed mainly of Cx26 and Cx30, formed linear plaques along the cell-cell junction sites with adjacent cells. These plaques formed pentagonal or hexagonal outlines of the normal inner sulcus cells and border cells. Gap junction plaques in Brn4-deficient mice did not, however, show the normal linear structure but instead formed small spots around the cell-cell junction sites. Gap junction lengths were significantly shorter, and the level of Cx26 and Cx30 was significantly reduced in Brn4-deficient mice compared with littermate controls. Thus the Brn4 mutation affected the assembly and localization of gap junction proteins at the cell borders of cochlear supporting cells, suggesting that Brn4 substantially contributes to cochlear gap junction properties to maintain the proper EP in cochleae, similar to connexin-related deafness.

Age-dependent gene expression in the inner ear of big brown bats (Eptesicus fuscus)

“…Pou3f4 is a transcription factor that has been implicated in X-linked non-syndromic deafness [ 67 ]. Pou3f4 mouse mutants exhibit audiological and balance impairments, reduced coiling of the cochlea [ 68 ], and defects in gap junctions [ 101 ]. Deletion of Pou3f4 from otic mesenchyme causes defasciculation of spiral ganglion neurons [ 69 ], which could disrupt coordination of hair cell and neuronal frequencies [ 102 ].…”

On the Role of Fibrocytes and the Extracellular Matrix in the Physiology and Pathophysiology of the Spiral Ligament

“…(77) Furthermore, Pou3f4-deficiency also causes disruption of the gap junction plaques (GJPs) resulting in an abnormal morphology of cell-to-cell adhesion of SLFs. Cx26, and Cx30 expression is remarkably reduced and GJPs were significantly shorter, suggesting a degradation of the gap junctional macromolecular complex (78). Expression of Na,K/ATPase and AQP1 was also lost or misplaced in the spiral ligament.…”

“…Mutations in Brn-4/Pou3f4, encoding a POU transcription factor, have been identified as the underlying cause (77). During embryonic development, Pou3f4 is expressed in mesenchymal cells of the inner ear but is also associated with neuronal development (78).…”

Research progress of the transcription factor Brn4 (Review)

“…2), which not only benefits the survival of spiral ganglion neurons (SGNs), but also stimulates SGNs to extend axon bundles and eventually form synapses with surrounding hair cells (28). The gap junctions between cochlear cells facilitate K + transmission, thus resulting in a high resting potential for sensory hair cell excitation (29). Brn4 was reported to promote the assembly and localization of connexins at the cochlear support cell borders, thereby maintaining proper intra-cochlear lymphatic potential for the efficient transmission of sound (22).…”