mutant mice show long-term, incomplete retention of some afferent inner ear innervation

Abstract
Background
Ears of Brn3c null mutants develop immature hair cells, identifiable only by certain molecular markers, and undergo apoptosis in neonates. This partial development of hair cells could lead to enough neurotrophin expression to sustain sensory neurons through embryonic development. We have therefore investigated in these mutants the patterns of innervation and of expression of known neurotrophins.

Results
At birth there is a limited expression of BDNF and NT-3 in the mutant sensory epithelia and DiI tracing shows no specific reduction of afferents or efferents that resembles neurotrophin null mutations. At postnatal day 7/8 (P7/8), innervation is severely reduced both qualitatively and quantitatively. 1% of myosin VIIa-positive immature hair cells are present in the mutant cochlea, concentrated in the base. Around 20% of immature hair cells exist in the mutant vestibular sensory epithelia. Despite more severe loss of hair cells (1% compared to 20%), the cochlea retains many more sensory neurons (46% compared to 15%) than vestibular epithelia. Even 6 months old mutant mice have some fibers to all vestibular sensory epithelia and many more to the cochlear apex which lacks MyoVIIa positive hair cells. Topologically organized central cochlea projections exist at least until P8, suggesting that functional hair cells are not required to establish such projections.

Conclusion
The limited expression of neurotrophins in the cochlea of Brn3c null mice suffices to support many sensory neurons, particularly in the cochlea, until birth. The molecular nature of the long term survival of apical spiral neurons remains unclear.

Background
Brn3c is a POU domain factor that is crucial for inner ear hair cell development. Targeted null Brn3c mutants have no mature hair cells [1,2]. Close examination has revealed that some 'immature' hair cells form in Brn3c null mutants and express cellular markers such as Myosin VI and VIIa, calretinin and parvalbumin [3]. Furthermore, these immature hair cells of Brn3c null mutants undergo apoptosis in neonates [3]. Consistent with an apparent absence of mature hair cells, initial work suggested that all vestibular and most spiral ganglion cells are lost by postnatal day 14 (P14; [1]). However, more detailed quantification by others [3] reported that at P4 about 77% of vestibular neurons and only 29% of spiral neurons are lost. It was suggested that there is possibly a complete loss in adults [3]. Other than these preliminary statements, no data exists concerning the detailed pattern of loss of innervation in Brn3c null mutants.
The initial development and partial differentiation of hair cells in Brn3c mutants could possibly lead to some neurotrophin expression in these cells to sustain sensory neurons through embryonic development and beyond. Data on various single and compound neurotrophin null mutants have shown that the loss of a specific neurotrophin leads to topologically restricted loss of sensory neurons in the embryonic ear [4,5]. Such selective loss in Brn3c null mutants would therefore indicate reduction of a specific neurotrophin in immature hair cells. Moreover, recent work shows that in embryos NT-3 is primarily expressed in supporting cells, moving only around birth into hair cells [6,7]. In fact, the selective loss of vestibular as compared to cochlear sensory neurons (77% versus 29%; [3]) suggests that NT-3 expression may be less downregulated in Brn3c null mutants than BDNF [6,8,9], provided that at least some differentiation of supporting cells takes place. In the ear [4] as well as elsewhere [10] neurotrophins are progressively downregulated in postnatal mammals and possibly replaced by other factors [11].
We have investigated in detail the pattern of innervation in the Brn3c mutants, as well as the expression of NT-3 and BDNF. We want to evaluate a possible correlation between the topology of sensory neuron loss and absence of a specific neurotrophin or topological loss of hair cells at birth and in older animals. This information could be important for an in-depth evaluation of the human deafness related to the Pou4f3 gene, DFNA15 [12].
We report here long term retention of cochlear sensory neurons for at least 6 months, in particular in the cochlear apex, in Brn3c null mutant mice. This retention of afferents and efferents is unrelated to hair cell differentiation as not even immature hair cells can be detected at early postnatal stages with MyoVII immunocytochemistry in this part of the cochlea. This retention of apical spiral neurons is also largely unrelated to neurotrophins which are known to be reduced in their expression in neonatal rodents [4].

Results
To appreciate the effects of the Brn3c null mutation on the pattern of the inner ear innervation, we first want to present the effects of BDNF and NT-3 null mutations at birth [6,13,14]. Null mutants of BDNF or its receptor trkB lose all innervation to the semicircular canals and have a reduced innervation to the utricle, saccule and apical turn of the cochlea. In contrast, null mutations of either NT-3 or its receptor trkC result in loss of spiral neurons in the basal turn with formation of an inner spiral bundle of afferents extending to the basal tip. Our null hypothesis for this study would be that Brn3c null mice show severe compromised production of these neurotrophins and should therefore show a comparable pattern of nerve fiber loss.

Brn3c null mutants at birth (P0)
Vestibular ganglia are smaller in Brn3c null mutants (Fig. 1b) than in control littermates (Fig. 1a), but larger than in BDNF or trkB null mutants of the same age [13,15]. The reduction in apparent size of the vestibular ganglia is in agreement with quantitative data published previously [3,13]. These data suggest a loss of 80–85% of vestibular sensory neurons in BDNF and trkB null mutants [13] and of 77% of vestibular sensory neurons in P4 Brn3c null mutants [3]. Thus, the size reduction in the vestibular sensory ganglion could be compatible with a loss of BDNF production in the immature hair cells.
Figure 1  Size variations of vestibular ganglia in control and mutant littermates labeled with DiI. In newborn animals, the vestibular ganglion shows a dramatic reduction in Brn3c null mutants (b) compared to control littermates (a). Abbreviations for this and other figures: AC, anterior crista; ggl, ganglion; c, spiral capillary; Genic. ggl., geniculate ganglion; GER, greater epithelial ridge; HC, horizontal crista; HaC, vestibular hair cell; IHC, inner hair cell; iHC, immature hair cell; IGSB, intraganglionic spiral bundle; OHC, outer hair cell; PC, posterior crista; S, saccule; SG, spiral ganglion; TM, tectorial membrane; U, utricle; VCN, ventral cochlear nucleus; VG, vestibular ganglion. Bar indicates 1000 μm. However, the Brn3c null mutants show only a reduced density of afferent and efferent fibers to all vestibular sensory epithelia. There is no specific loss of all afferent and efferent innervation to the canal crista (Fig. 2a), a hallmark of both BDNF and trkB null mutations [13,15]. In fact, the reduction of fibers seems to be rather uniform throughout a given sensory epithelium with the crista innervation being qualitatively no more reduced than the innervation of the utricle and saccule. No loss in specific areas, comparable to that in BDNF null mutant mice, is apparent in the saccule or utricle of Brn3c null mice. Similar patterns of innervation are obtained using acteylated tubulin immunocytochemistry (Fig. 2b).
Figure 2  Innervation of Brn3c null and control ears are shown for newborn mice. There is no specific loss of fibers to any vestibular endorgan, as visualized by DiI labeling (a) or acetylated tubulin immunoreactivity (b). No major differences in pattern of projection through radial fibers are found in the cochlea of Brn3c null mutants (d,f) as compared to control littermates (c). Note, however, the lack of orderly fiber outgrowth to the outer hair cells (c, d). Efferent fibers to the ear show a well developed intraganglionic spiral bundle (IGSB) with no detectable differences compared to controls (e). Bar indicates 100 μm. Consistent with the finding of Xiang et al. [3] of only a 29% loss of spiral sensory neurons at P4, our data show little difference in the pattern of innervation of the cochlea in P0 Brn3c null mutants (Fig. 2c,2d). No selective loss of spiral neurons is observed in Brn3c null mutants in the basal turn, a feature of either NT-3 or trkC loss [6,9,14]. Likewise, the innervation of the apex (Fig. 2f) shows no detectable abnormality in overall pattern of innervation compared to control animals (data not shown), an indication that BDNF could be expressed in the apex [6]. In addition, the pattern of efferent innervation shows no deviation from normal either (Fig. 2e), whereas they show the same pattern of loss as afferent fibers in neurotrophin null mutants [16]. These data suggest that the spiral sensory neurons develop qualitatively normal at least until P0 and therefore allow normal pathfinding of efferents. Most interestingly, there is no increase in radial fiber spacing in the apex, a specific problem of BDNF null mutants [6,13].
However, there is one qualitative difference not recognized in any single neurotrophin null mutant. Afferents reach all three rows of outer hair cells in the basal turn of control wildtype littermates (Fig. 2c), but both afferent and efferent outgrowth is disorganized to outer hair cells in Brn3c null mutants (Fig. 2d) and does not show any clear organization into three distinct longitudinal fiber bundles paralleling the three rows in the outer hair cell region. These data suggest that fiber organization in the outer hair cell region is partly disrupted in Brn3c null mutant.

Brn3c null mutants