Genes, Manganese, and Zinc in Formation of Otoconia: Labeling, Recovery, and Maternal Effects

Published studies indicate that genes and dietary manganese deficiency cause vestibular defects and ataxic behaviors. Manganese deficiency during development causes otoconial defects in mice, rats, guinea pigs, and chicks. Mutant genes cause otoconial defects in mice, mink, and poultry. Manganese supplementation prevents the otoconial defects in some mutant mice and mink. Manganese is essential, before crystallization of the otoconia, for synthesis of mucopolysaccharides and otoconial matrix. Such defects can be induced, after otoconia are crystallized during fetal development, by dietary zinc deficiency and sulfonamide treatment (inhibits carbonic anhydrase, a zinc-requiring enzyme). Manganese and/or zinc supplementation ameliorates otoconial defects in pallid and lethal-milk (zinc-deficient) mice. Studies herein show that: 1) Developing otoconia can be quantitatively labeled with 45 Ca. This may provide a means for studying calcium metabolism in otoconia over a prolonged period of time and for determining the possible effects of diet, drugs, and other factors on otoconia. 2) Otoconial defects, induced after otoconia form in the fetus, were observed in newborn mice, but disappeared by two days after birth. Conditions of the inner ear may contribute to the calcification of otoconia. 3) Manganese and zinc supplementation of pallid mice via acidified drinking water is more effective than dietary supplementation in preventing otoconial defects. The effectiveness of zinc but not of manganese is related to ma tern al genotype (+/pa vs. pa/pa). The effect of supplementation of the dams with zinc but not with manganese increases over successive litters. These studies indicate the potential for interaction of genes and trace minerals on otoconial formation and maintenance

Mammalian Otolin: A Multimeric Glycoprotein Specific to the Inner Ear that Interacts with Otoconial Matrix Protein Otoconin-90 and Cerebellin-1

The mammalian otoconial membrane is a dense extracellular matrix containing bio-mineralized otoconia. This structure provides the mechanical stimulus necessary for hair cells of the vestibular maculae to respond to linear accelerations and gravity. In teleosts, Otolin is required for the proper anchoring of otolith crystals to the sensory maculae. Otoconia detachment and subsequent entrapment in the semicircular canals can result in benign paroxysmal positional vertigo (BPPV), a common form of vertigo for which the molecular basis is unknown. Several cDNAs encoding protein components of the mammalian otoconia and otoconial membrane have recently been identified, and mutations in these genes result in abnormal otoconia formation and balance deficits.Here we describe the cloning and characterization of mammalian Otolin, a protein constituent of otoconia and the otoconial membrane. Otolin is a secreted glycoprotein of ∼70 kDa, with a C-terminal globular domain that is homologous to the immune complement C1q, and contains extensive posttranslational modifications including hydroxylated prolines and glycosylated lysines. Like all C1q/TNF family members, Otolin multimerizes into higher order oligomeric complexes. The expression of otolin mRNA is restricted to the inner ear, and immunohistochemical analysis identified Otolin protein in support cells of the vestibular maculae and semi-circular canal cristae. Additionally, Otolin forms protein complexes with Cerebellin-1 and Otoconin-90, two protein constituents of the otoconia, when expressed in vitro. Otolin was also found in subsets of support cells and non-sensory cells of the cochlea, suggesting that Otolin is also a component of the tectorial membrane.Given the importance of Otolin in lower organisms, the molecular cloning and biochemical characterization of the mammalian Otolin protein may lead to a better understanding of otoconial development and vestibular dysfunction