Plasma Membrane Targeting of Protocadherin 15 Is Regulated by the Golgi-Associated Chaperone Protein PIST.

Protocadherin 15 (PCDH15) is a core component of hair cell tip-links and crucial for proper function of inner ear hair cells. Mutations of PCDH15 gene cause syndromic and nonsyndromic hearing loss. At present, the regulatory mechanisms responsible for the intracellular transportation of PCDH15 largely remain unknown. Here we show that PIST, a Golgi-associated, PDZ domain-containing protein, interacts with PCDH15. The interaction is mediated by the PDZ domain of PIST and the C-terminal PDZ domain-binding interface (PBI) of PCDH15. Through this interaction, PIST retains PCDH15 in the trans-Golgi network (TGN) and reduces the membrane expression of PCDH15. We have previously showed that PIST regulates the membrane expression of another tip-link component, cadherin 23 (CDH23). Taken together, our finding suggests that PIST regulates the intracellular trafficking and membrane targeting of the tip-link proteins CDH23 and PCDH15.

Sorting nexin 9 (SNX9) is not essential for development and auditory function in mice.

Sorting nexins are a large family of evolutionarily conserved proteins that play fundamental roles in endocytosis, endosomal sorting and signaling. As an important member of sorting nexin family, sorting nexin 9 (SNX9) has been shown to participate in coordinating actin polymerization with membrane tubulation and vesicle formation. We previously showed that SNX9 is expressed in mouse auditory hair cells and might regulate actin polymerization in those cells. To further examine the physiological role of SNX9, we generated Snx9 knockout mice using homologous recombination method. Unexpectedly, Snx9 knockout mice have normal viability and fertility, and are morphologically and behaviorally indistinguishable from control mice. Further investigation revealed that the morphology and function of auditory hair cells are not affected by Snx9 inactivation, and Snx9 knockout mice have normal hearing threshold. In conclusion, our data revealed that Snx9-deficient mice do not show defects in development as well as auditory function, suggesting that SNX9 is not essential for mice development and hearing.

FCHSD1 and FCHSD2 are expressed in hair cell stereocilia and cuticular plate and regulate actin polymerization in vitro.

Mammalian FCHSD1 and FCHSD2 are homologous proteins containing an amino-terminal F-BAR domain and two SH3 domains near their carboxyl-termini. We report here that FCHSD1 and FCHSD2 are expressed in mouse cochlear sensory hair cells. FCHSD1 mainly localizes to the cuticular plate, whereas FCHSD2 mainly localizes along the stereocilia in a punctuate pattern. Nervous Wreck (Nwk), the Drosophila ortholog of FCHSD1 and FCHSD2, has been shown to bind Wsp and play an important role in F-actin assembly. We show that, like its Drosophila counterpart, FCHSD2 interacts with WASP and N-WASP, the mammalian orthologs of Drosophila Wsp, and stimulates F-actin assembly in vitro. In contrast, FCHSD1 doesn't bind WASP or N-WASP, and can't stimulate F-actin assembly when tested in vitro. We found, however, that FCHSD1 binds via its F-BAR domain to the SH3 domain of Sorting Nexin 9 (SNX9), a well characterized BAR protein that has been shown to promote WASP-Arp2/3-dependent F-actin polymerization. FCHSD1 greatly enhances SNX9's WASP-Arp2/3-dependent F-actin polymerization activity. In hair cells, SNX9 was detected in the cuticular plate, where it colocalizes with FCHSD1. Our results suggest that FCHSD1 and FCHSD2 could modulate F-actin assembly or maintenance in hair cell stereocilia and cuticular plate.

Overexpression of p204 leads to abnormal embryos and osteogenesis in zebrafish.

p204, an inteferon-inducible protein, is known to play an important role in modulating cell proliferation, cell cycling, and the differentiation of various tissues, including osteoblasts. In order to determine the role of p204 during development in vivo, the teleost zebrafish (Danio rerio), an established vertebrate model for developmental studies, was employed. p204 cDNA was introduced into zebrafish by microinjection, and p204 was ectopically expressed throughout the whole embryo during the early stages of zebrafish embryogenesis, then its expression gradually decreased, mainly in ventrally located cells and retina capsules. Importantly, overexpression of p204 in zebrafish resulted in striking malformations such as bent spine and expanded belly. Furthermore, the expressions of some genes (vent, runx2b, osn) involved in dorsoventral patterning and osteogenesis were significantly upregulated after p204 injection. This study provides not only the in vivo evidences demonstrating the role of p204 during embryonic development, but also new insights into the molecular mechanism by which p204 mediate osteogenesis.

Heat stress induces Cdc2 protein decrease prior to mouse spermatogenic cell apoptosis.

Both mitosis and meiosis are driven by M-phase promoting factor (MPF), a complex with Cdc2 and Cyclin B. The concentration of Cdc2 remains relatively constant during the cell cycle, while the concentration of Cyclin B fluctuates periodically. Many studies have demonstrated high expression levels of Cdc2 and Cyclin B in the testis. In some gene knock-out mice insufficient amounts of MPF blocked the spermatocytes at the G2/M transition and this was followed by spermatocyte apoptosis. In this study, we examined the expression and the alteration of Cdc2 in testis during the spermatocyte apoptosis process induced by transient heat stress. The results showed that the spermatogenic cell apoptosis was detectable by the TUNEL assay at 4h post-treatment. At 10h, almost all spermatocytes began apoptosis. In situ hybridization and immunohistochemistry indicated that cdc2 was primarily expressed in spermatocytes. Neither the distribution nor the amount of cdc2 mRNA was significantly influenced by the heat stress. In contrast, the amount of Cdc2 protein decreased significantly at 3h post-treatment, which was detectable before apoptosis. This indicated that Cdc2 was susceptible to heat stress in the testis. Cdc2 levels remained low until 8h post-treatment. It was possible that the swift decline in Cdc2 and the resulting lack of MPF blocked the spermatocytes at G2/M transition. Meiosis in the spermatocytes was disrupted leading to the initiation of apoptosis. The results provide evidence that the lack of Cdc2 might induce spermatocyte apoptosis after transient heat stress.

Multiple splicing results in at least two p203 proteins that are expressed in the liver and down-regulated during liver regeneration.

The interferon inducible p200 family proteins are expressed in a variety of cells and tissues. Intensive studies showed that they were involved in the regulation of cell proliferation and differentiation based on their ability to bind and modulate the activities of multiple transcription activators and inhibitors. Among p200 proteins, p203 has received the least attention and its function is unknown. In the present study, four multiple splicing isoforms of Ifi203, named temporally as Ifi203a/b-1, Ifi203a/b-2, Ifi203 a/b-3, and Ifi203a/b-4, were cloned from the interferon induced 10T1/2 cells. Anti-p203 antiserum was prepared and it could immunodetect a 46 kD and a 51 kD p203 proteins in a variety of cell lines. Unlike other p200 proteins, p203 was exclusively expressed in the liver in the adult C129/SvJ and C57BL/6 inbred strain mice. During the liver regeneration following a standard partial hepatectomy in C57BL/6 mice, the level of p203 decreased significantly in 6-24 h post-operation prior to the cell cycle progression through the G1/S transition.