Otoconin-90, the mammalian otoconial matrix protein, contains two domains of homology to secretory phospholipase A ₂

Abstract

The ability to sense orientation relative to gravity requires dense particles, called otoconia, which are localized in the vestibular macular organs. In mammals, otoconia are composed of proteins (otoconins) and calcium carbonate crystals in a calcite lattice. Little is known about the mechanisms that regulate otoconial biosynthesis. To begin to elucidate these mechanisms, we have partially sequenced and cloned the major protein component of murine otoconia, otoconin-90 (OC90). The amino acid sequence identified an orphan chimeric human cDNA. Because of its similarity to secretory phospholipase A 2 (sPLA 2 ), this gene was referred to as PLA 2 -like (PLA2L) and enabled the identification of human Oc90. Partial murine cDNA and genomic clones were isolated and shown to be specifically expressed in the developing mouse otocyst. The mature mouse OC90 is composed of 453 residues and contains two domains homologous to sPLA 2 . The cloning of Oc90 will allow an examination of the role of this protein in otoconial biosynthesis and in diseases that affect the vestibular system.

Interfacial Kinetic and Binding Properties of the Complete Set of Human and Mouse Groups I, II, V, X, and XII Secreted Phospholipases A2

“…Interestingly, these enzymes are still myotoxic or cytotoxic or able to inhibit human immunodeficiency virus type 1 (68). Besides venoms, other catalytically inactive sPLA 2 s such as otoconin-22 (69), otoconin-95 (7,8), and a putative zebrafish group XII-like inactive sPLA 2 (41) have been found, but the functions of these are not yet identified. It is generally stated that sPLA 2 s require submillimolar to millimolar concentrations of calcium as a catalytic cofactor.…”

“…Many of these sPLA 2 s were identified in the past few years, and studies are under way to determine their physiological functions. A catalytically inactive sPLA 2 -like protein called otoconin-95 was also identified in mice and humans (7,8). The different sPLA 2 paralogs are not closely related isoforms because the amino acid identity between any two is in the range of Ͻ15-50%.…”

Osteopontin is not Critical for Otoconia Formation or Balance Function

“…Deletion of OPN protein in the null mice was previously validated (Liaw et al 1998) and was confirmed in the present study by Western blotting of otoconia extracts (data not shown). With Oc90 comprising nearly 90% of the otoconial protein content (Pote and Ross 1986;Wang et al 1998;Ornitz et al 1998), OPN is present at only a low level in wt otoconia (Takemura et al 1994;Sakagami 2000;Zhao et al 2007).…”

“…Both types of matrices contain sulfated glycosaminoglycans (as GAGs or proteoglycans), osteopontin (OPN), fetuin-A, and members of the collagen X family (collagen X in bone and otolin in otoconia; Lundberg et al 2006;Zhao et al 2007). The fourth known otoconia component protein, Oc90, is the predominant otoconial matrix protein (Wang et al 1998;Verpy et al 1999) whose role in bone formation has yet to be studied.…”

Mixing model systems: Using zebrafish and mouse inner ear mutants and other organ systems to unravel the mystery of otoconial development

“…Oc90 accounts for 90% of the total protein of the otoconial core and is expressed by the nonsensory epithelium of the inner ear (Verpy et al, 1999;Wang et al, 1998). Knockout mice for Oc90 initially develop giant otoconia, suggesting that Oc90 is essential for normal otoconial formation (Y. Lundberg, personal communication).…”