Bernd Fritzsch

Precise regulation of progenitor identity is essential for the formation of functional auditory circuits. Cochlear nuclei arise from two major progenitor populations: Atoh1-expressing progenitors generate excitatory glutamatergic neurons, while Ptf1a-expressing progenitors generate inhibitory GABAergic and glycinergic neurons. Lmx1a and Lmx1b, expressed in the hindbrain roof plate, maintain Atoh1 expression in lower rhombic lip progenitors that give rise to excitatory neurons of the cochlear nuclei. Detailed analysis using dye tracing of cochlear and vestibular central projections revealed an overall reduction of input in both Lmx1a knockout mice and Lmx1a knockout mice with additonal loss of one copy of Lmx1b, although projections to the dorsal cochlear nucleus (DCN) remain relatively preserved. Dye insertion at the crossing of the acoustic stria demonstrate a progressive loss of the ventral acoustic stria (VAS) and its altered trajectory between the trigeminal and restiform body in Lmx1a-/- and Lmx1a-/-;Lmx1b+/- mice. Together, these data show that the anteroventral cochlear nucleus (AVCN) is most severely affected at both embryonic and postnatal stages, showing loss and reduction of central projections. Ultimately, these findings identify Lmx1-dependent regulatory activity as a critical checkpoint for the proper development and connectivity of auditory pathways.

The evolution of Osteichthyes began with a split into two major lineages: Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes). In one lineage—sarcopterygians—some groups evolved robust internal bones and limb-like fins and ultimately gave rise to semi- and fully terrestrial tetrapods; the other lineage—actinopterygians—remained primarily aquatic and later radiated into the diverse teleosts. Repeated mass extinction events and ongoing genetic divergence allowed novel functions and new niches to be exploited, a pattern especially evident in recent analyses of teleost diversification. Lobe-finned fishes characteristically possess an endoskeleton fin architecture, whereas ray-finned fishes bear dermal fin rays built on a different structural plan. Primitive Osteichthyes also show an early origin of paired air-spaces (lungs), but many derived actinopterygians modified this ancestral condition into a dorsal swim bladder. Imagining a world without sarcopterygians or tetrapods highlights how teleosts might have convergently colonized many terrestrial-associated niches; although significant developmental and structural hurdles would have made such a transition challenging, this thought experiment underscores the cascading ecological consequences that the loss of a major clade can produce. Ecosystems thrive on diversity and adaptability, and episodes of environmental upheaval—such as the Silurian and Devonian extinctions—often catalyze rapid evolutionary change.

Lmx1a/b paralogous genes, which arose from the invertebrate Lmx1b-like gene, are critical for hearing in multiple vertebrate species, and mutations in these genes cause hearing deficits in humans. While the unique and redundant functions of Lmx1a/b in the inner ear are well established, their contribution to the development of the cochlear nuclei, which process and relay auditory information to the brain, is poorly understood. Since cochlear nuclei maturate postnatally, here we analyzed Lmx1a+/−;Lmx1b+/−, Lmx1a−/−, and Lmx1a−/−;Lmx1b+/− mice that survive past birth. Loss of Lmx1a reduced distinct populations of excitatory neurons in dorsal (DCN) and ventral (VCN) cochlear nuclei and their innervation from the inner ear. Additional loss of one Lmx1b copy made Lmx1a−/− phenotypes more severe, revealing that Lmx1b acts redundantly with Lmx1a. Unlike Lmx1a−/− mice, excitatory neurons were not affected in Lmx1a+/−;Lmx1b+/− mice. Thus, while cochlear nuclei are sensitive to Lmx1a/b gene dosage, these genes are not completely equivalent, and Lmx1a has a more profound role in cochlear nuclei development. Lmx1a−/− and especially Lmx1a−/−;Lmx1b+/− embryos had fewer Atoh1+ progenitors that produce excitatory neurons of the cochlear nuclei, and reduced Bmp6 expression in the roof plate, the signaling center that induces these progenitors via Bmp signaling. We found that Lmx1a is the primary regulator of Bmp6, whereas Lmx1b contributes only in the absence of Lmx1a. Thus, Lmx1a plays a major role in the formation of the mature structure and connectivity of both the DCV and VCN, and Lmx1b acts redundantly to Lmx1a but only partially compensates for Lmx1a loss.

Since the discovery of Latimeria chalumnae, coelacanths have provided a critical comparative framework for reconstructing ancestral sarcopterygian anatomy. However, the function of several anatomical features in both extant and fossil coelacanths remains unresolved. Among these, the presence of large ossified chambers in the body cavity of fossil coelacanths has remained enigmatic, with different studies proposing respiratory or auditory functions. Here, we examine lung and inner ear anatomy based on new observations from synchrotron phase-contrast microCT scans of two 240-million-year-old latimerioid coelacanths, alongside multiple developmental stages of the extant L. chalumnae. These data, combined with archival histological sections of L. chalumnae and 3D reconstructions of a Devonian coelacanth, suggest that extinct coelacanths possessed an ossified lung capable of transmitting sound pressure to auditory sensory epithelia in the inner ear via a perilymphatic system. We propose that the lung of extinct coelacanths supported both respiratory and auditory functions.

Vestibular and spiral ganglion neurons (VGNs and SGNs) developed in the inner ear, where they extend fibers to innervate the vestibular and cochlear hair cells and project centrally to the vestibular and cochlear nuclei. This review focuses on representative molecular factors that regulate key processes in the development of inner ear neurons, including their specification, differentiation, axon targeting, and functional diversification. A temporal regulatory cascade defines the initial precursors through factors such as Smarca4, Six1, Eya1, followed by Sox2. While Sox2 deletion abolishes hair cell formation, a subset of inner ear neurons transiently develops but undergoes apoptosis before birth. In contrast, Neurog1 deletion eliminates all ear-derived neurons but results in differential reductions in cochlear and vestibular hair cells. The development and survival of inner ear neurons depend on TrkB and TrkC signaling. Although deletion of TrkB and TrkC results in a complete loss of neurons, each shows distinct effects on VGN and SGN survival and innervation. Downstream of early transcriptional regulators, Neurod1 and Isl1 promote neuronal differentiation, survival, migration, and the formation of peripheral and central projections. The development of VGNs depends on at least two progenitor populations that give rise to three neuronal subtypes that differ in their innervation of vestibular hair cells but show incomplete segregation in the vestibular nuclei. In contrast, SGNs develop later and exhibit sequential segregation into four neuronal subtypes, corresponding to the two types of cochlear hair cells, with tonotopically organized projections to both the cochlea and cochlear nuclei.

Dicer is crucial for the generation of microRNAs (miRNAs), which are essential for regulating gene expression and keeping neuronal health. Dicer's conditional deletion cuts all spiral ganglion neurons but spares a small fraction of vestibular ganglion neurons, innervating the utricle and part of the saccule. Hair cells develop in the utricle, saccule, posterior crista, and the cochlea in
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. Cochlear hair cells develop at the base and expand the OHC and IHC in the middle, or split into a base/middle and the apex. In contrast,
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cuts all canal cristae and cochlea hair cells, leaving a reduced utricle and an exceedingly small saccule. Likewise,
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shows no cochlear hair cells and is absent in the horizontal and reduced in the posterior crista. In contrast, the utricle, saccule, and anterior crista are nearly normal, underscoring the intricate regulatory networks involved in hair cell and neuronal development. The central projections have been described as the topology of various null deletions. Still, without spiral ganglion neurons, fibers from Dicer null mice navigate to the cochlear nuclei and expand into the vestibular nuclei to innervate the caudal brainstem. Beyond a ramification around the CN, no fibers expand to reach the cerebellum, likely due to
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that cut these neurons. Genetic alterations, such as Dicer deletion, can lead to hearing loss and impairments in auditory signal processing, illustrating the critical role of microRNAs in the development and function of auditory and vestibular neurons. Further studies on this topic could help in understanding potential therapeutic targets for hearing loss associated with neuronal degradation of miRNA.

Background Knockin mouse models expressing calbindin (Calb1), calretinin (Calb2), and peripherin (Prph) exhibit changes in hair cells (HCs), spiral ganglion neurons (SGN), vestibular ganglion neurons (VGNs), and their central projections.Results Developing cristae HCs show strong Calb1-positive expression, but adult HCs are mainly Calb2-positive. Utricle and saccule initially have Calb2-positive HCs and later develop Calb1-positive HCs in the striola region. Inner hair cells (IHCs) and outer hair cells (OHCs) in the cochlea express Calb2 early on. Calb1 expression in OHCs overlaps with Calb2; the expression of Myo7a, Calb1, and Calb2 reaches the apex later. SGNs and VGNs exhibit distinct Calb1 and Calb2 patterns but include a subpopulation with mixed expression. Central fibers are Calb1- and Calb2-positive early in the developing cochlear nuclei (CN) and vestibular nuclei (VN) but remain highly Prph-positive. VGNs innervate the lateral and VN, which are positive for Calb2 and Prph. Distinct Calb1-positive neurons overlap with the anterior (A) and ventral (V) cochlear nuclei (AVCN, PVCN) with Calb2, while the dorsal cochlear nucleus (DCN) shows segregation of Calb2 and Calb1.Conclusion We offer insights into the timing of how neuronal identity and connectivity are regulated in the auditory and vestibular systems, as shown by the expression of Calb1, Calb2, and Prph.

Basic helix–loop–helix (bHLH) transcription factors, such as those in the atonal family, are important in cellular fate determination. The expression of the sponge ortholog of the atonal bHLH gene family, AmqbHLH1, in Xenopus laevis previously resulted in the formation of ectodermal ectopic neurons. However, the extent to which these neurons persist through development and the effects on the inner ear and lateral line, which require a critical level and timing of bHLH genes, remains unexplored. To test these long-term effects, we injected various concentrations of AmqbHLH1 mRNA into X. laevis embryos and assessed neurosensory development at developmental stages coinciding with fully developed neurosensory structures. The expression of AmqbHLH1 mRNA in X. laevis resulted in a dose-dependent reduction in or loss of ears and the lateral line system without eliminating ectopic neurons. At the lowest concentrations examined, we found that inner ear neurosensory development consisted sometimes of only a few scattered hair cells in a single-layer epithelium. Furthermore, low concentrations of AmqbHLH1 mRNA affected inner ear afferent guidance. Our data suggest that the AmqbHLH1 gene has some anti-neurosensory abilities in frogs and that the overexpression of a single gene may not be sufficient for stable long-term transdifferentiation in cells.

The transcription factor Lmx1a is widely expressed during early inner ear development, and mice lacking Lmx1a expression exhibit fusion of cochlear and vestibular hair cells and fail to form the ductus reuniens and the endolymphatic sac. Lmx1a dreher ( Lmx1a dr/dr ), a recessive null mutation, results in non-functional Lmx1a expression, which expands from the outer sulcus to the stria vascularis and Reissner’s membrane. In the absence of Lmx1a , we observe a lack of proteins specific to the stria vascularis, such as BSND and KCNQ1 in marginal cells and CD44 in intermediate cells. Further analysis of the superficial epithelial cell layer at the expected stria vascularis location shows that the future intermediate cells migrate during embryonic development but subsequently disappear. Using antibodies against pendrin ( Slc26a4 ) in Lmx1a knockout (KO) mice, we observe an expansion of pendrin expression across the stria vascularis and Reissner’s membrane. Moreover, in the absence of Lmx1a expression, no endocochlear potential is observed. These findings highlight the critical role of Lmx1a in inner ear development, particularly in the differentiation of cochlear and vestibular structures, the recruitment of pigment cells, and the expression of proteins essential for hearing and balance.