Chemokines and cytokines on the neuroimmunoaxis: Inner ear neurotrophic cytokines in development and disease. Prospects for repair?

The earliest stages of neuronal and sensory cell development in vertebrate sensory organs depend on "inflammatory" immune system neurotrophic cytokines/chemokines. Although classical nerve growth factors, brain-derived neurotrophic factors and glial growth factors play critical roles at various stages, the earliest directive roles belong to immune system cytokines. In frogs, fishes, birds and mammals, macrophage migration inhibitory factor (MIF), monocyte chemoattractant protein 1 (MCP1) and RANTES, components of the otocyst-derived factor, are involved in sorting, morphogenesis, providing directional neuronal outgrowth cues as well as survival factors for both neurons and sensory cells. In this review we discuss their roles in the vertebrate inner ear.

Macrophage migration inhibitory factor acts as a neurotrophin in the developing inner ear.

This study is the first to demonstrate that macrophage migration inhibitory factor (MIF), an immune system 'inflammatory' cytokine that is released by the developing otocyst, plays a role in regulating early innervation of the mouse and chick inner ear. We demonstrate that MIF is a major bioactive component of the previously uncharacterized otocyst-derived factor, which directs initial neurite outgrowth from the statoacoustic ganglion (SAG) to the developing inner ear. Recombinant MIF acts as a neurotrophin in promoting both SAG directional neurite outgrowth and neuronal survival and is expressed in both the developing and mature inner ear of chick and mouse. A MIF receptor, CD74, is found on both embryonic SAG neurons and adult mouse spiral ganglion neurons. Mif knockout mice are hearing impaired and demonstrate altered innervation to the organ of Corti, as well as fewer sensory hair cells. Furthermore, mouse embryonic stem cells become neuron-like when exposed to picomolar levels of MIF, suggesting the general importance of this cytokine in neural development.

Embryonic inner ear cells use migratory mechanisms to establish cell patterns in vitro.

The hair cells of the sensory epithelium in the inner ear are among the most precisely organized cells in vertebrates. The mechanisms that lead to this orderly arrangement are only beginning to be understood. It has been suggested that hair cells use migratory mechanisms to help achieve their final position in the organ of Corti. The small size and complex organization of the intact inner ear have made it difficult to monitor changes in hair cell location over time in vivo. In the present study, an established in vitro assay of dissociated, embryonic inner ear cells was used to monitor how hair cells reorganize over time. The hair cell specific marker myosin-VI demonstrated that hair cell precursors from both cochlear and vestibular regions reorganized into specific patterns between 3-24 hr in vitro. In contrast to the unlabeled cells, the myosin-VI-positive cells extended processes while establishing the hair cell patterning within an aggregate. These studies support the hypothesis that hair cell precursors actively migrate to help achieve final patterning within the inner ear sensory epithelium.

Immortalized mouse inner ear cell lines demonstrate a role for chemokines in promoting the growth of developing statoacoustic ganglion neurons.

The target-derived factors necessary for promoting initial outgrowth from the statoacoustic ganglion (SAG) to the inner ear have not been fully characterized. In the present study, conditioned medium from embryonic Immortomouse inner ear cell lines that maintain many characteristics of developing inner ear sensory epithelia were screened for neurite-promoting activity. Conditioned medium found to be positive for promoting SAG neurite outgrowth and neuronal survival was then tested for the presence of chemokines, molecules that have not previously been investigated for promoting SAG outgrowth. One candidate molecule, monocyte chemotactic protein 1 (MCP-1), was detected in the conditioned medium and subsequently localized to mouse hair cells by immunocytochemistry. In vitro studies demonstrated that function-blocking MCP-1 antibodies decreased the amount of SAG neurite outgrowth induced by the conditioned medium and that subsequent addition of MCP-1 protein was able to promote outgrowth when added to the antibody-treated conditioned medium. The use of the Immortomouse cell lines proved valuable in identifying this candidate cofactor that promotes outgrowth of early-stage SAG nerve fibers and is expressed in embryonic hair cells.

Lac z Histochemistry and immunohistochemistry reveal ephrin-B ligand expression in the inner ear.

Immunostaining in transgenic mice carrying the lac z gene can be used to map gene and protein distribution in a single tissue. In this study, we examined inner ears from ephrin-B3 homozygous and ephrin-B2 heterozygous mice. Ephrin-B3 lac z expression was limited in these mice. However, immunostaining revealed ephrin-B3 throughout cochlear and vestibular regions. Immunoreactivity was absent in ephrin-B3-homozygous null mutants, demonstrating the specificity of the antibody. Ephrin-B2 lac z reactivity was detected in a limited number of cells in cochlear and vestibular regions. Different immunostaining patterns were found with different antibodies. Comparison with lac z expression indicated which antibody was specific for the transmembrane-bound ephrin-B2 ligand.

Embryonic inner ear cells reaggregate into specific patterns in vitro.

The sensory epithelia of the mammalian inner ear consist of a highly precise pattern of sensory hair cells and supporting cells. The mechanisms regulating this patterning are only beginning to be determined. The present study describes a method for culturing dissociated embryonic inner ear cells and the resulting patterning that occurs in these cultures. The results indicate that developing inner ear cells aggregate into precise patterns on a two-dimensional substrate, suggesting that intrinsic patterning mechanisms remain active in vitro. Using antibodies and scanning electron microscopy to detect hair cells and nonsensory cells, it was determined that only a subset of aggregates contained sensory hair cells. The hair cells were organized into specific patterns and surrounded by supporting cells, similar to the in vivo pattern. Additionally, hair cells increased their immunoreactivity and number of stereocilia over time, suggesting that hair cells continue to mature in vitro. Thus, the study reveals that the cells of the developing inner ear provide the necessary signals that direct sensory hair cells and supporting cells to reassociate into very precise patterns in vitro and that these patterns are reminiscent of the patterning that occurs in vivo.

EphB receptors influence growth of ephrin-B1-positive statoacoustic nerve fibers.

The Eph family of receptors and ligands has been implicated in a variety of developmental processes, including the provision of inhibitory guidance cues to developing nerve fibers. A unique property of the B class of receptors is that they are able to phosphorylate ephrin-B ligands, allowing for bi-directional, or reverse signalling. While most of the studies to date have focused on central nerve fibers, little is known about the role of Eph molecules in guiding nerve fibers of the peripheral nervous system. In the present study, ephrin-B1 was found to be highly expressed on developing peripheral nerve fibers including auditory and vestibular (statoacoustic) and dorsal root ganglion nerve fibers. In vitro assays revealed that EphB-Fc receptors inhibited further growth of statoacoustic nerve fibers. In contrast, EphA7-Fc and ephrin-B2-Fc did not prevent further growth of SAG. Together, these results suggest a role for EphB receptors in providing guidance signals to ephrin-B1-positive SAG nerve fibers.