Pou3f4-expressing otic mesenchyme cells promote spiral ganglion neuron survival in the postnatal mouse cochlea.

During inner ear development, primary auditory neurons named spiral ganglion neurons (SGNs) are surrounded by otic mesenchyme cells, which express the transcription factor Pou3f4. Mutations in Pou3f4 are associated with DFNX2, the most common form of X-linked deafness and typically include developmental malformations of the middle ear and inner ear. It is known that interactions between Pou3f4-expressing mesenchyme cells and SGNs are important for proper axon bundling during development. However, Pou3f4 continues to be expressed through later phases of development, and potential interactions between Pou3f4 and SGNs during this period had not been explored. To address this, we documented Pou3f4 protein expression in the early postnatal mouse cochlea and compared SGNs in Pou3f4 knockout mice and littermate controls. In Pou3f4y/- mice, SGN density begins to decline by the end of the first postnatal week, with approximately 25% of SGNs ultimately lost. This period of SGN loss in Pou3f4y/- cochleae coincides with significant elevations in SGN apoptosis. Interestingly, this period also coincides with the presence of a transient population of Pou3f4-expressing cells around and within the spiral ganglion. To determine if Pou3f4 is normally required for SGN peripheral axon extension into the sensory domain, we used a genetic sparse labeling approach to track SGNs and found no differences compared with controls. We also found that Pou3f4 loss did not lead to changes in the proportions of Type I SGN subtypes. Overall, these data suggest that otic mesenchyme cells may play a role in maintaining SGN populations during the early postnatal period.

Current concepts in cochlear ribbon synapse formation.

In mammals, hair cells and spiral ganglion neurons (SGNs) in the cochlea together are sophisticated "sensorineural" structures that transduce auditory information from the outside world into the brain. Hair cells and SGNs are joined by glutamatergic ribbon-type synapses composed of a molecular machinery rivaling in complexity the mechanoelectric transduction components found at the apical side of the hair cell. The cochlear hair cell ribbon synapse has received much attention lately because of recent and important findings related to its damage (sometimes termed "synaptopathy") as a result of noise overexposure. During development, ribbon synapses between type I SGNs and inner hair cells form in the time window between birth and hearing onset and is a process coordinated with type I SGN myelination, spontaneous activity, synaptic pruning, and innervation by efferents. In this review, we highlight new findings regarding the diversity of type I SGNs and inner hair cell synapses, and the molecular mechanisms of selective hair cell targeting. Also discussed are cell adhesion molecules and protein constituents of the ribbon synapse, and how these factors participate in ribbon synapse formation. We also note interesting new insights into the morphological development of type II SGNs, and the potential for cochlear macrophages as important players in protecting SGNs. We also address recent studies demonstrating that the structural and physiological profiles of the type I SGNs do not reach full maturity until weeks after hearing onset, suggesting a protracted development that is likely modulated by activity.

Plakophilin3 increases desmosome assembly, size and stability by increasing expression of desmocollin2.

Previous reports show that the desmosomal plaque protein plakophilin3 (PKP3) is essential for desmosome formation. Here, we report that PKP3 over-expression decreases calcium dependency for de novo desmosome formation and makes existing cell-cell adhesion junctions more resilient in low calcium medium due to an increase in desmocollin2 expression. PKP3 overexpression increases the stability of other desmosomal proteins independently of the increase in DSC2 levels and regulates desmosome formation and stability by a multimodal mechanism affecting transcription, protein stability and cell border localization of desmosomal proteins.

14-3-3σ Gene Loss Leads to Activation of the Epithelial to Mesenchymal Transition Due to the Stabilization of c-Jun Protein.

Loss of 14-3-3σ has been observed in multiple tumor types; however, the mechanisms by which 14-3-3σ loss leads to tumor progression are not understood. The experiments in this report demonstrate that loss of 14-3-3σ leads to a decrease in the expression of epithelial markers and an increase in the expression of mesenchymal markers, which is indicative of an induction of the epithelial to mesenchymal transition (EMT). The EMT was accompanied by an increase in migration and invasion in the 14-3-3σ(-/-) cells. 14-3-3σ(-/-) cells show increased stabilization of c-Jun, resulting in an increase in the expression of the EMT transcription factor slug. 14-3-3σ induces the ubiquitination and degradation of c-Jun in an FBW7-dependent manner. c-Jun ubiquitination is dependent on the presence of an intact nuclear export pathway as c-Jun is stabilized and localized to the nucleus in the presence of a nuclear export inhibitor. Furthermore, the absence of 14-3-3σ leads to the nuclear accumulation and stabilization of c-Jun, suggesting that 14-3-3σ regulates the subcellular localization of c-Jun. Our results have identified a novel mechanism by which 14-3-3σ maintains the epithelial phenotype by inhibiting EMT and suggest that this property of 14-3-3σ might contribute to its function as a tumor suppressor gene.

14-3-3γ-Mediated transport of plakoglobin to the cell border is required for the initiation of desmosome assembly in vitro and in vivo.

The regulation of cell-cell adhesion is important for the processes of tissue formation and morphogenesis. Here, we report that loss of 14-3-3γ leads to a decrease in cell-cell adhesion and a defect in the transport of plakoglobin and other desmosomal proteins to the cell border in HCT116 cells and cells of the mouse testis. 14-3-3γ binds to plakoglobin in a PKCµ-dependent fashion, resulting in microtubule-dependent transport of plakoglobin to cell borders. Transport of plakoglobin to the border is dependent on the KIF5B-KLC1 complex. Knockdown of KIF5B in HCT116 cells, or in the mouse testis, results in a phenotype similar to that observed upon 14-3-3γ knockdown. Our results suggest that loss of 14-3-3γ leads to decreased desmosome formation and a decrease in cell-cell adhesion in vitro, and in the mouse testis in vivo, leading to defects in testis organization and spermatogenesis.

Filamin A stimulates cdc25C function and promotes entry into mitosis.

The activity of the dual specificity phosphatase cdc25C is required for mitotic progression though the mechanisms by which cdc25C is activated prior to mitosis in human cells remain unclear. The data presented herein show that the actin binding protein Filamin A forms a complex with cdc25C in vivo and binds preferentially to the mitotic form of cdc25C. Co-expression of Filamin A with cdc25C results in an increase in PCC induced by cdc25C, while knocking down Filamin A expression reduces the levels of PCC induced by cdc25C overexpression. Further, only a Filamin A fragment that forms a complex with both cdc25C and cyclin B1 and retains the dimerization domain can stimulate the ability of cdc25C to induce PCC. These results suggest that Filamin A provides a platform for the assembly of the cyclin B1-cdk1- cdc25C complex resulting in cdk1 activation and mitotic progression.