A cardinal feature of the auditory pathway is frequency selectivity, represented in a tonotopic map from the cochlea to the cortex. The molecular determinants of the auditory frequency map are unknown. Here, we discovered that the transcription factor ISL1 regulates the molecular and cellular features of auditory neurons, including the formation of the spiral ganglion and peripheral and central processes that shape the tonotopic representation of the auditory map. We selectively knocked out Isl1 in auditory neurons using Neurod1Cre strategies. In the absence of Isl1, spiral ganglion neurons migrate into the central cochlea and beyond, and the cochlear wiring is profoundly reduced and disrupted. The central axons of Isl1 mutants lose their topographic projections and segregation at the cochlear nucleus. Transcriptome analysis of spiral ganglion neurons shows that Isl1 regulates neurogenesis, axonogenesis, migration, neurotransmission-related machinery, and synaptic communication patterns. We show that peripheral disorganization in the cochlea affects the physiological properties of hearing in the midbrain and auditory behavior. Surprisingly, auditory processing features are preserved despite the significant hearing impairment, revealing central auditory pathway resilience and plasticity in Isl1 mutant mice. Mutant mice have a reduced acoustic startle reflex, altered prepulse inhibition, and characteristics of compensatory neural hyperactivity centrally. Our findings show that ISL1 is one of the obligatory factors required to sculpt auditory structural and functional tonotopic maps. Still, upon Isl1 deletion, the ensuing central plasticity of the auditory pathway does not suffice to overcome developmentally induced peripheral dysfunction of the cochlea.

• Keywords
• auditory behavior, auditory maps, auditory nuclei, inferior colliculus, spiral ganglion neurons,
• MeSH
• Spiral Ganglion * enzymology   MeSH
• Cochlea embryology   innervation   MeSH
• Mice MeSH
• Neurogenesis * genetics   MeSH
• Cochlear Nucleus * embryology   MeSH
• LIM-Homeodomain Proteins * genetics   physiology   MeSH
• Auditory Pathways * embryology   MeSH
• Transcription Factors * genetics   physiology   MeSH
• Hair Cells, Auditory * physiology   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Names of Substances
• insulin gene enhancer binding protein Isl-1 MeSH Browser
• LIM-Homeodomain Proteins * MeSH
• Transcription Factors * MeSH

We review the molecular basis of three related basic helix-loop-helix (bHLH) genes (Neurog1, Neurod1, and Atoh1) and upstream regulators Eya1/Six1, Sox2, Pax2, Gata3, Fgfr2b, Foxg1, and Lmx1a/b during the development of spiral ganglia, cochlear nuclei, and cochlear hair cells. Neuronal development requires early expression of Neurog1, followed by its downstream target Neurod1, which downregulates Atoh1 expression. In contrast, hair cells and cochlear nuclei critically depend on Atoh1 and require Neurod1 and Neurog1 expression for various aspects of development. Several experiments show a partial uncoupling of Atoh1/Neurod1 (spiral ganglia and cochlea) and Atoh1/Neurog1/Neurod1 (cochlear nuclei). In this review, we integrate the cellular and molecular mechanisms that regulate the development of auditory system and provide novel insights into the restoration of hearing loss, beyond the limited generation of lost sensory neurons and hair cells.

• Keywords
• bHLH genes, cochlea development, cochlear nuclei projections, neuronal differentiation,
• Publication type
• Journal Article MeSH
• Review MeSH

We review the molecular basis of several transcription factors (Eya1, Sox2), including the three related genes coding basic helix-loop-helix (bHLH; see abbreviations) proteins (Neurog1, Neurod1, Atoh1) during the development of spiral ganglia, cochlear nuclei, and cochlear hair cells. Neuronal development requires Neurog1, followed by its downstream target Neurod1, to cross-regulate Atoh1 expression. In contrast, hair cells and cochlear nuclei critically depend on Atoh1 and require Neurod1 expression for interactions with Atoh1. Upregulation of Atoh1 following Neurod1 loss changes some vestibular neurons' fate into "hair cells", highlighting the significant interplay between the bHLH genes. Further work showed that replacing Atoh1 by Neurog1 rescues some hair cells from complete absence observed in Atoh1 null mutants, suggesting that bHLH genes can partially replace one another. The inhibition of Atoh1 by Neurod1 is essential for proper neuronal cell fate, and in the absence of Neurod1, Atoh1 is upregulated, resulting in the formation of "intraganglionic" HCs. Additional genes, such as Eya1/Six1, Sox2, Pax2, Gata3, Fgfr2b, Foxg1, and Lmx1a/b, play a role in the auditory system. Finally, both Lmx1a and Lmx1b genes are essential for the cochlear organ of Corti, spiral ganglion neuron, and cochlear nuclei formation. We integrate the mammalian auditory system development to provide comprehensive insights beyond the limited perception driven by singular investigations of cochlear neurons, cochlear hair cells, and cochlear nuclei. A detailed analysis of gene expression is needed to understand better how upstream regulators facilitate gene interactions and mammalian auditory system development.

• Keywords
• bHLH genes, cochlea hair cells, cochlear nuclei, neuronal differentiation, spiral ganglion neurons, transcription factors,
• MeSH
• Cochlea cytology   metabolism   MeSH
• Humans MeSH
• Neurogenesis genetics   physiology   MeSH
• Basic Helix-Loop-Helix Transcription Factors genetics   metabolism   MeSH
• Transcription Factors genetics   metabolism   MeSH
• Hair Cells, Auditory metabolism   MeSH
• Gene Expression Regulation, Developmental MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Basic Helix-Loop-Helix Transcription Factors MeSH
• Transcription Factors MeSH

This review provides an up-to-date source of information on the primary auditory neurons or spiral ganglion neurons in the cochlea. These neurons transmit auditory information in the form of electric signals from sensory hair cells to the first auditory nuclei of the brain stem, the cochlear nuclei. Congenital and acquired neurosensory hearing loss affects millions of people worldwide. An increasing body of evidence suggest that the primary auditory neurons degenerate due to noise exposure and aging more readily than sensory cells, and thus, auditory neurons are a primary target for regenerative therapy. A better understanding of the development and function of these neurons is the ultimate goal for long-term maintenance, regeneration, and stem cell replacement therapy. In this review, we provide an overview of the key molecular factors responsible for the function and neurogenesis of the primary auditory neurons, as well as a brief introduction to stem cell research focused on the replacement and generation of auditory neurons.

• Keywords
• auditory pathways, cochlea, genetic mutations, single-cell RNAseq, transcription factor,
• MeSH
• Spiral Ganglion embryology   physiology   MeSH
• Induced Pluripotent Stem Cells cytology   MeSH
• Cochlea embryology   physiology   MeSH
• Humans MeSH
• Brain Stem MeSH
• Mutation MeSH
• Mice MeSH
• Neurogenesis MeSH
• Neurons physiology   MeSH
• Cochlear Nucleus embryology   physiology   MeSH
• Hearing Loss, Sensorineural physiopathology   MeSH
• Regenerative Medicine methods   MeSH
• Base Sequence MeSH
• Evoked Potentials, Auditory, Brain Stem MeSH
• Hair Cells, Auditory physiology   MeSH
• Ear, Inner embryology   physiology   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Review MeSH

Ear development requires the transcription factors ATOH1 for hair cell differentiation and NEUROD1 for sensory neuron development. In addition, NEUROD1 negatively regulates Atoh1 gene expression. As we previously showed that deletion of the Neurod1 gene in the cochlea results in axon guidance defects and excessive peripheral innervation of the sensory epithelium, we hypothesized that some of the innervation defects may be a result of abnormalities in NEUROD1 and ATOH1 interactions. To characterize the interdependency of ATOH1 and NEUROD1 in inner ear development, we generated a new Atoh1/Neurod1 double null conditional deletion mutant. Through careful comparison of the effects of single Atoh1 or Neurod1 gene deletion with combined double Atoh1 and Neurod1 deletion, we demonstrate that NEUROD1-ATOH1 interactions are not important for the Neurod1 null innervation phenotype. We report that neurons lacking Neurod1 can innervate the flat epithelium without any sensory hair cells or supporting cells left after Atoh1 deletion, indicating that neurons with Neurod1 deletion do not require the presence of hair cells for axon growth. Moreover, transcriptome analysis identified genes encoding axon guidance and neurite growth molecules that are dysregulated in the Neurod1 deletion mutant. Taken together, we demonstrate that much of the projections of NEUROD1-deprived inner ear sensory neurons are regulated cell-autonomously.

• Keywords
• Axon guidance, Central projections, Ear neurosensory development, Neuronal differentiation, bHLH genes,
• MeSH
• Apoptosis genetics   MeSH
• Axons metabolism   MeSH
• Models, Biological MeSH
• Cell Differentiation genetics   MeSH
• Organ of Corti pathology   MeSH
• Gene Deletion MeSH
• Epithelium metabolism   MeSH
• Spiral Ganglion metabolism   MeSH
• Mutation genetics   MeSH
• Mice, Knockout MeSH
• Nerve Fibers metabolism   MeSH
• Nerve Tissue Proteins genetics   metabolism   MeSH
• Gene Expression Regulation MeSH
• Gene Expression Profiling MeSH
• Basic Helix-Loop-Helix Transcription Factors genetics   metabolism   MeSH
• SOXB1 Transcription Factors metabolism   MeSH
• Hair Cells, Auditory metabolism   pathology   ultrastructure   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Atoh1 protein, mouse MeSH Browser
• Neurogenic differentiation factor 1 MeSH Browser
• Nerve Tissue Proteins MeSH
• Basic Helix-Loop-Helix Transcription Factors MeSH
• SOXB1 Transcription Factors MeSH

SOX2 is essential for maintaining neurosensory stem cell properties, although its involvement in the early neurosensory development of cranial placodes remains unclear. To address this, we used Foxg1-Cre to conditionally delete Sox2 during eye, ear, and olfactory placode development. Foxg1-Cre mediated early deletion of Sox2 eradicates all olfactory placode development, and disrupts retinal development and invagination of the lens placode. In contrast to the lens and olfactory placodes, the ear placode invaginates and delaminates NEUROD1 positive neurons. Furthermore, we show that SOX2 is not necessary for early ear neurogenesis, since the early inner ear ganglion is formed with near normal central projections to the hindbrain and peripheral projections to the undifferentiated sensory epithelia of E11.5-12.5 ears. However, later stages of ear neurosensory development, in particular, the late forming auditory system, critically depend on the presence of SOX2. Our data establish distinct differences for SOX2 requirements among placodal sensory organs with similarities between olfactory and lens but not ear placode development, consistent with the unique neurosensory development and molecular properties of the ear.

• Keywords
• Eye, Inner ear, Neuronal projections, Olfactory system, Placode development,
• MeSH
• Apoptosis MeSH
• Mice, Knockout MeSH
• Mice MeSH
• Neurogenesis * MeSH
• Nasal Mucosa embryology   metabolism   MeSH
• Lens, Crystalline embryology   metabolism   MeSH
• SOXB1 Transcription Factors genetics   metabolism   MeSH
• Ear, Inner cytology   embryology   metabolism   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Names of Substances
• Sox2 protein, mouse MeSH Browser
• SOXB1 Transcription Factors MeSH

The molecular mechanisms regulating sympathetic innervation of the heart during embryogenesis and its importance for cardiac development and function remain to be fully elucidated. We generated mice in which conditional knockout (CKO) of the Hif1a gene encoding the transcription factor hypoxia-inducible factor 1α (HIF-1α) is mediated by an Islet1-Cre transgene expressed in the cardiac outflow tract, right ventricle and atrium, pharyngeal mesoderm, peripheral neurons, and hindlimbs. These Hif1aCKO mice demonstrate significantly decreased perinatal survival and impaired left ventricular function. The absence of HIF-1α impaired the survival and proliferation of preganglionic and postganglionic neurons of the sympathetic system, respectively. These defects resulted in hypoplasia of the sympathetic ganglion chain and decreased sympathetic innervation of the Hif1aCKO heart, which was associated with decreased cardiac contractility. The number of chromaffin cells in the adrenal medulla was also decreased, indicating a broad dependence on HIF-1α for development of the sympathetic nervous system.

• Keywords
• cardiac innervation, coronary artery branching, hypoxia, sympathetic neurons, tyrosine hydroxylase,
• MeSH
• Coronary Vessel Anomalies embryology   MeSH
• Chromaffin Cells MeSH
• Adrenal Medulla embryology   innervation   MeSH
• Hypoxia-Inducible Factor 1, alpha Subunit physiology   MeSH
• Coronary Vessels embryology   MeSH
• Mice, Knockout MeSH
• Mice, Transgenic MeSH
• Mice MeSH
• Heart embryology   innervation   MeSH
• Ganglia, Sympathetic embryology   growth & development   MeSH
• Sympathetic Nervous System enzymology   growth & development   MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Mice MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Hypoxia-Inducible Factor 1, alpha Subunit MeSH
• Hif1a protein, mouse MeSH Browser

Hearing depends on extracting frequency, intensity, and temporal properties from sound to generate an auditory map for acoustical signal processing. How physiology intersects with molecular specification to fine tune the developing properties of the auditory system that enable these aspects remains unclear. We made a novel conditional deletion model that eliminates the transcription factor NEUROD1 exclusively in the ear. These mice (both sexes) develop a truncated frequency range with no neuroanatomically recognizable mapping of spiral ganglion neurons onto distinct locations in the cochlea nor a cochleotopic map presenting topographically discrete projections to the cochlear nuclei. The disorganized primary cochleotopic map alters tuning properties of the inferior colliculus units, which display abnormal frequency, intensity, and temporal sound coding. At the behavioral level, animals show alterations in the acoustic startle response, consistent with altered neuroanatomical and physiological properties. We demonstrate that absence of the primary afferent topology during embryonic development leads to dysfunctional tonotopy of the auditory system. Such effects have never been investigated in other sensory systems because of the lack of comparable single gene mutation models.SIGNIFICANCE STATEMENT All sensory systems form a topographical map of neuronal projections from peripheral sensory organs to the brain. Neuronal projections in the auditory pathway are cochleotopically organized, providing a tonotopic map of sound frequencies. Primary sensory maps typically arise by molecular cues, requiring physiological refinements. Past work has demonstrated physiologic plasticity in many senses without ever molecularly undoing the specific mapping of an entire primary sensory projection. We genetically manipulated primary auditory neurons to generate a scrambled cochleotopic projection. Eliminating tonotopic representation to auditory nuclei demonstrates the inability of physiological processes to restore a tonotopic presentation of sound in the midbrain. Our data provide the first insights into the limits of physiology-mediated brainstem plasticity during the development of the auditory system.

• Keywords
• Neurod1 mutation, auditory pathway, cochlear nucleus, inferior colliculus, plasticity, sensory topographical map,
• MeSH
• Behavior, Animal physiology   MeSH
• Inferior Colliculi anatomy & histology   physiology   MeSH
• Spiral Ganglion cytology   physiology   MeSH
• Brain Mapping MeSH
• Mesencephalon embryology   physiology   MeSH
• Mice, Knockout MeSH
• Mice MeSH
• Cochlear Nucleus anatomy & histology   physiology   MeSH
• Hearing physiology   MeSH
• Auditory Perception genetics   physiology   MeSH
• Pregnancy MeSH
• Basic Helix-Loop-Helix Transcription Factors genetics   physiology   MeSH
• Reflex, Startle genetics   physiology   MeSH
• Vestibule, Labyrinth anatomy & histology   physiology   MeSH
• Pitch Perception physiology   MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Mice MeSH
• Pregnancy MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Names of Substances
• Neurod1 protein, mouse MeSH Browser
• Basic Helix-Loop-Helix Transcription Factors MeSH