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Oncomodulin is expressed exclusively by outer hair cells in the organ of Corti
N Sakaguchi1, M T Henzl, I Thalmann
1Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
Abstract:
Oncomodulin (OM) is a small, acidic calcium-binding protein first discovered in a rat hepatoma and later found in placental cytotrophoblasts, the pre-implantation embryo, and in a wide variety of neoplastic tissues. OM was considered to be exclusively an oncofetal protein until its recent detection in extracts of the adult guinea pig's organ of Corti. Here we report that light and electron microscopic immunostaining of gerbil, rat, and mouse inner ears with a monoclonal antibody against recombinant rat OM localizes the protein exclusively in cochlear outer hair cells (OHCs). At the ultrastructural level, high gold labeling density was seen overlying the nucleus, cytoplasm, and the cuticular plate of gerbil OHCs. Few, if any, gold particles were present over intracellular organelles and the stereocilia. Staining of a wide range of similarly processed gerbil organs failed to detect immunoreactive OM in any other adult tissues. The mammalian genome encodes one alpha- and one beta-isoform of parvalbumin (PV). The widely distributed alpha PV exhibits a very high affinity for Ca2+ and is believed to serve as a Ca2+ buffer. By contrast, OM, the mammalian beta PV, displays a highly attenuated affinity for Ca2+, consistent with a Ca2+-dependent regulatory function. The exclusive association of OM with cochlear OHCs in mature tissues is likely to have functional relevance. Teleological considerations favor its involvement in regulating some aspect of OHC electromotility. Although the fast electromotile response of OHCs does not require Ca2+, its gain and magnitude are modulated by efferent innervation. Therefore, OM may be involved in mediation of intracellular responses to cholinergic stimulation, which are known to be Ca2+ regulated. (J Histochem Cytochem 46:29-39, 1998)
Insights
Oncomodulin (OM), a calcium-binding protein, is exclusively found in the cochlear outer hair cells (OHCs) of adult mammals. This suggests OM plays a role in OHC function, potentially regulating their electromotility.
Area of Science:
- Cell Biology
- Neuroscience
- Histology
Background:
- Oncomodulin (OM) is an acidic calcium-binding protein initially identified as an oncofetal antigen.
- Previously thought to be exclusively oncofetal, OM was recently detected in the adult guinea pig organ of Corti.
Purpose of the Study:
- To investigate the precise localization of Oncomodulin (OM) in the adult mammalian inner ear.
- To explore the potential functional significance of OM's presence in cochlear outer hair cells (OHCs).
Main Methods:
- Immunohistochemistry using a monoclonal antibody against recombinant rat OM was performed on gerbil, rat, and mouse inner ear tissues.
- Light and electron microscopy were employed to visualize OM localization at the ultrastructural level.
- Extensive screening of other adult gerbil tissues was conducted to confirm OM's exclusivity to the inner ear.
Main Results:
- Oncomodulin (OM) was exclusively localized to cochlear outer hair cells (OHCs) in the adult gerbil, rat, and mouse inner ear.
- Ultrastructural analysis revealed high OM concentrations in the nucleus, cytoplasm, and cuticular plate of gerbil OHCs, but not in organelles or stereocilia.
- No immunoreactive OM was detected in any other adult gerbil tissues examined.
Conclusions:
- The exclusive presence of Oncomodulin (OM) in cochlear outer hair cells (OHCs) suggests a specialized function within these cells.
- Given OM's calcium-binding properties and OHC function, OM may be involved in regulating OHC electromotility, possibly mediating intracellular responses to cholinergic stimulation.