Astrocyte ensheathment of calyx-forming axons of the auditory brainstem precedes accelerated expression of myelin genes and myelination by oligodendrocytes.

Early postnatal brain development involves complex interactions among maturing neurons and glial cells that drive tissue organization. We previously analyzed gene expression in tissue from the mouse medial nucleus of the trapezoid body (MNTB) during the first postnatal week to study changes that surround rapid growth of the large calyx of Held (CH) nerve terminal. Here, we present genes that show significant changes in gene expression level during the second postnatal week, a developmental timeframe that brackets the onset of airborne sound stimulation and the early stages of myelination. Gene Ontology analysis revealed that many of these genes are related to the myelination process. Further investigation of these genes using a previously published cell type-specific bulk RNA-Seq data set in cortex and our own single-cell RNA-Seq data set in the MNTB revealed enrichment of these genes in the oligodendrocyte lineage (OL) cells. Combining the postnatal day (P)6-P14 microarray gene expression data with the previously published P0-P6 data provided fine temporal resolution to investigate the initiation and subsequent waves of gene expression related to OL cell maturation and the process of myelination. Many genes showed increasing expression levels between P2 and P6 in patterns that reflect OL cell maturation. Correspondingly, the first myelin proteins were detected by P4. Using a complementary, developmental series of electron microscopy 3D image volumes, we analyzed the temporal progression of axon wrapping and myelination in the MNTB. By employing a combination of established ultrastructural criteria to classify reconstructed early postnatal glial cells in the 3D volumes, we demonstrated for the first time that astrocytes within the mouse MNTB extensively wrap the axons of the growing CH terminal prior to OL cell wrapping and compaction of myelin. Our data revealed significant expression of several myelin genes and enrichment of multiple genes associated with lipid metabolism in astrocytes, which may subserve axon wrapping in addition to myelin formation. The transition from axon wrapping by astrocytes to OL cells occurs rapidly between P4 and P9 and identifies a potential new role of astrocytes in priming calyceal axons for subsequent myelination.

Transcriptional profiling reveals roles of intercellular Fgf9 signaling in astrocyte maturation and synaptic refinement during brainstem development.

Neural tissue maturation is a coordinated process under tight transcriptional control. We previously analyzed the kinetics of gene expression in the medial nucleus of the trapezoid body (MNTB) in the brainstem during the critical postnatal phase of its development. While this work revealed timed execution of transcriptional programs, it was blind to the specific cells where gene expression changes occurred. Here, we utilized single-cell RNA-Seq to determine transcriptional profiles of each major MNTB cell type. We discerned directional signaling patterns between neuronal, glial, and vascular-associated cells for VEGF, TGFß, and Delta-Notch pathways during a robust period of vascular remodeling in the MNTB. Furthermore, we describe functional outcomes of the disruption of neuron-astrocyte fibroblast growth factor 9 (Fgf9) signaling. We used a conditional KO (cKO) approach to genetically delete Fgf9 from principal neurons in the MNTB, which led to an early onset of glial fibrillary acidic protein (Gfap) expression in astrocytes. In turn, Fgf9 cKO mice show increased levels of astrocyte-enriched brevican (Bcan), a component of the perineuronal net matrix that ensheaths principal neurons in the MNTB and the large calyx of Held terminal, while levels of the neuron-enriched hyaluronan and proteoglycan link protein 1 (Hapln1) were unchanged. Finally, volumetric analysis of vesicular glutamate transporters 1 and 2 (Vglut1/2), which serves as a proxy for terminal size, revealed an increase in calyx of Held volume in the Fgf9 cKO. Overall, we demonstrate a coordinated neuron-astrocyte Fgf9 signaling network that functions to regulate astrocyte maturation, perineuronal net structure, and synaptic refinement.

Truncating RAX Mutations: Anophthalmia, Hypopituitarism, Diabetes Insipidus, and Cleft Palate in Mice and Men.

CONTEXT: The transcription factor RAX is a paired-type homeoprotein that plays a critical role in eye and forebrain development of vertebrate species. RAX knockout mice have anophthalmia, cleft palate, and an abnormal hypothalamus and display perinatal lethality. In humans, homozygous or compound heterozygous RAX mutations have been reported to cause bilateral microphthalmia or anophthalmia without consistent associated features. Congenital hypopituitarism can be associated with various eye or craniofacial anomalies; however, the co-occurrence of congenital hypopituitarism, anophthalmia, cleft palate, and diabetes insipidus has been very rare. RESULTS: We report the case of a child with anophthalmia, congenital hypopituitarism, diabetes insipidus, and bilateral cleft lip and palate who had a homozygous frameshift truncating mutation c.266delC (p.Pro89Argfs*114) in exon 1 of the RAX gene. Rax knockout mice show loss of ventral forebrain structures, pituitary, and basosphenoid bone and palate and a misplaced anterior pituitary gland along the roof of the oral cavity. CONCLUSIONS: Our patient's phenotype was more severe than that reported in other patients. Although most of the previously reported patients with RAX mutations showed either a missense or some less severe mutation in at least one of their RAX alleles, our patient was homozygous for truncating mutations that would yield a severe, null protein phenotype. The severity of the genetic defect, the precise match between the knockout mouse and the patient's endocrine phenotypes, and the prominent roles of RAX in eye and pituitary development and diencephalic patterning suggest that the RAX null mutations could fully account for the observed phenotype.

Glial Cell Expansion Coincides with Neural Circuit Formation in the Developing Auditory Brainstem.

Neural circuit formation involves maturation of neuronal, glial and vascular cells, as well as cell proliferation and cell death. A fundamental understanding of cellular mechanisms is enhanced by quantification of cell types during key events in synapse formation and pruning and possessing qualified genetic tools for cell type-specific manipulation. Acquiring this information in turn requires validated cell markers and genetic tools. We quantified changing proportions of neurons, astrocytes, oligodendrocytes, and microglia in the medial nucleus of the trapezoid body (MNTB) during neural circuit development. Cell type-specific markers, light microscopy and 3D virtual reality software, the latter developed in our laboratory, were used to count cells within distinct cell populations at postnatal days (P)3 and P6, bracketing the period of nerve terminal growth and pruning in this system. These data revealed a change from roughly equal numbers of neurons and glia at P3 to a 1.5:1 ratio of glia to neurons at P6. PCNA and PH3 labeling revealed that proliferation of oligodendrocytes contributed to the increase in glial cell number during this timeframe. We next evaluated Cre driver lines for selectivity in labeling cell populations. En1-Cre was specific for MNTB neurons. PDGFRα-Cre and Aldh1L1-Cre, thought to be mostly specific for oligodendrocyte lineage cells and astrocytes, respectively, both labeled significant numbers of neurons, oligodendrocytes, and astrocytes and are non-specific genetic tools in this neural system.

Embryonic markers of cone differentiation.

PURPOSE: Photoreceptor cells are born in two distinct phases of vertebrate retinogenesis. In the mouse retina, cones are born primarily during embryogenesis, while rod formation occurs later in embryogenesis and early postnatal ages. Despite this dichotomy in photoreceptor birthdates, the visual pigments and phototransduction machinery are not reactive to visual stimulus in either type of photoreceptor cell until the second postnatal week. Several markers of early cone formation have been identified, including Otx2, Crx, Blimp1, NeuroD, Trß2, Rorß, and Rxrγ, and all are thought to be involved in cellular determination. However, little is known about the expression of proteins involved in cone visual transduction during early retinogenesis. Therefore, we sought to characterize visual transduction proteins that are expressed specifically in photoreceptors during mouse embryogenesis. METHODS: Eye tissue was collected from control and phosducin-null mice at embryonic and early postnatal ages. Immunohistochemistry and quantitative reverse transcriptase-PCR (qPCR) were used to measure the spatial and temporal expression patterns of phosducin (Pdc) and cone transducin γ (Gngt2) proteins and transcripts in the embryonic and early postnatal mouse retina. RESULTS: We identified the embryonic expression of phosducin (Pdc) and cone transducin γ (Gngt2) that coincides temporally and spatially with the earliest stages of cone histogenesis. Using immunohistochemistry, the phosducin protein was first detected in the retina at embryonic day (E)12.5, and cone transducin γ was observed at E13.5. The phosducin and cone transducin γ proteins were seen only in the outer neuroblastic layer, consistent with their expression in photoreceptors. At the embryonic ages, phosducin was coexpressed with Rxrγ, a known cone marker, and with Otx2, a marker of photoreceptors. Pdc and Gngt2 mRNAs were detected as early as E10.5 with qPCR, although at low levels. CONCLUSIONS: Visual transduction proteins are expressed at the earliest stages in developing cones, well before the onset of opsin gene expression. Given the delay in opsin expression in rods and cones, we speculate on the embryonic function of these G-protein signaling components beyond their roles in the visual transduction cascade.

Temporal patterns of gene expression during calyx of held development.

Relating changes in gene expression to discrete developmental events remains an elusive challenge in neuroscience, in part because most neural territories are comprised of multiple cell types that mature over extended periods of time. The medial nucleus of the trapezoid body (MNTB) is an attractive vertebrate model system that contains a nearly homogeneous population of neurons, which are innervated by large glutamatergic nerve terminals called calyces of Held (CH). Key steps in maturation of CHs and MNTB neurons, including CH growth and competition, occur very quickly for most cells between postnatal days (P)2 and P6. Therefore, we characterized genome-wide changes in this system, with dense temporal sampling during the first postnatal week. We identified 541 genes whose expression changed significantly between P0-6 and clustered them into eight groups based on temporal expression profiles. Candidate genes from each of the eight profile groups were validated in separate samples by qPCR. Our tissue sample permitted comparison of known glial and neuronal transcripts and revealed that monotonically increasing or decreasing expression profiles tended to be associated with glia and neurons, respectively. Gene ontology revealed enrichment of genes involved in axon pathfinding, cell differentiation, cell adhesion and extracellular matrix. The latter category included elements of perineuronal nets, a prominent feature of MNTB neurons that is morphologically distinct by P6, when CH growth and competition are resolved onto nearly all MNTB neurons. These results provide a genetic framework for investigation of general mechanisms responsible for nerve terminal growth and maturation.

Embryonic origins of the mouse superior olivary complex.

Many areas of the central nervous system are organized into clusters of cell groups, with component cell groups exhibiting diverse but related functions. One such cluster, the superior olivary complex (SOC), is located in the ventral auditory brainstem in mammals. The SOC is an obligatory contact point for most projection neurons of the ventral cochlear nucleus and plays central roles in many aspects of monaural and binaural information processing. Despite their important interrelated functions, little is known about the embryonic origins of SOC nuclei, due in part to a paucity of developmental markers to distinguish individual cell groups. In this report, we present a collection of novel markers for the developing SOC nuclei in mice, including the transcription factors FoxP1, MafB, and Sox2, and the lineage-marking transgenic line En1-Cre. We use these definitive markers to examine the rhombic lip and rhombomeric origins of SOC nuclei and demonstrate that they can serve to uniquely identify SOC nuclei and subnuclei in newborn pups. The markers are also useful in identifying distinct nuclear domains within the presumptive SOC as early as embryonic day (E) 14.5, well before morphological distinction of individual nuclei is evident. These findings indicate that the mediolateral and dorsoventral position of SOC nuclei characteristic of the adult brainstem is established during early neurogenesis.

Rax is a selector gene for mediobasal hypothalamic cell types.

The brain plays a central role in controlling energy, glucose, and lipid homeostasis, with specialized neurons within nuclei of the mediobasal hypothalamus, namely the arcuate (ARC) and ventromedial (VMH), tasked with proper signal integration. Exactly how the exquisite cytoarchitecture and underlying circuitry becomes established within these nuclei remains largely unknown, in part because hypothalamic developmental programs are just beginning to be elucidated. Here, we demonstrate that the Retina and anterior neural fold homeobox (Rax) gene plays a key role in establishing ARC and VMH nuclei in mice. First, we show that Rax is expressed in ARC and VMH progenitors throughout development, consistent with genetic fate mapping studies demonstrating that Rax+ lineages give rise to VMH neurons. Second, the conditional ablation of Rax in a subset of VMH progenitors using a Shh::Cre driver leads to a fate switch from a VMH neuronal phenotype to a hypothalamic but non-VMH identity, suggesting that Rax is a selector gene for VMH cellular fates. Finally, the broader elimination of Rax throughout ARC/VMH progenitors using Six3::Cre leads to a severe loss of both VMH and ARC cellular phenotypes, demonstrating a role for Rax in both VMH and ARC fate specification. Combined, our study illustrates that Rax is required in ARC/VMH progenitors to specify neuronal phenotypes within this hypothalamic brain region. Rax thus provides a molecular entry point for further study of the ontology and establishment of hypothalamic feeding circuits.

Phosphorylation of phosducin accelerates rod recovery from transducin translocation.

PURPOSE: In rods saturated by light, the G protein transducin undergoes translocation from the outer segment compartment, which results in the uncoupling of transducin from its innate receptor, rhodopsin. We measured the kinetics of recovery from this adaptive cellular response, while also investigating the role of phosducin, a phosphoprotein binding transducin ßγ subunits in its de-phosphorylated state, in regulating this process. METHODS: Mice were exposed to a moderate rod-saturating light triggering transducin translocation, and then allowed to recover in the dark while free running. The kinetics of the return of the transducin subunits to the outer segments were compared in transgenic mouse models expressing full-length phosducin, and phosducin lacking phosphorylation sites serine 54 and 71, using Western blot analysis of serial tangential sections of the retina. RESULTS: In mice expressing normal phosducin, transducin α and ßγ subunits returned to the outer segments with a half-time (t(1/2)) of ∼24 and 29 minutes, respectively. In the phosducin phosphorylation mutants, the transducin α subunit moved four times slower, with t(1/2) ∼95 minutes, while the movement of transducin ßγ was less affected. CONCLUSIONS: We demonstrate that the recovery of rod photoreceptors from the ambient saturating levels of illumination, in terms of the return of the light-dispersed transducin subunits to the rod outer segments, occurs six times faster than reported previously. Our data also support the notion that the accumulation of transducin α subunit in the outer segment is driven by its re-binding to the transducin ßγ dimer, because this process is accelerated significantly by phosducin phosphorylation.

Maturation of synaptic partners: functional phenotype and synaptic organization tuned in synchrony.

Maturation of principal neurons of the medial nucleus of the trapezoid body (MNTB) was assessed in the context of the developmental organization and activity of their presynaptic afferents, which grow rapidly to form calyces of Held and to establish mono-innervation between postnatal days (P)2 and 4. MNTB neurons and their inputs were studied from embryonic day (E)17, when the nucleus was first discernable, until P14 after the onset of hearing. Using a novel slice preparation containing portions of the cochlea, cochlear nucleus and MNTB, we determined that synaptic inputs form onto MNTB neurons at E17 and stimulation of the cochlear nucleus can evoke action potentials (APs) and Ca(2+) signals. We analysed converging inputs onto individual MNTB neurons and found that competition among inputs was resolved quickly, as a single large input, typically larger than 4 nA, emerged from P3-P4. During calyx growth but before hearing onset, MNTB cells acquired their mature, phasic firing property and quantitative real-time PCR confirmed a coincident increase in low threshold K(+) channel mRNA. These events occurred in concert with an increase in somatic surface area and a 7-fold increase in the current threshold (30 to >200 pA) required to evoke action potentials, as input resistance (R(in)) settled from embryonic values greater than 1 GΩ to approximately 200 MΩ. We postulate that the postsynaptic transition from hyperexcitability to decreased excitability during calyx growth could provide a mechanism to establish the mature 1:1 innervation by selecting the winning calyceal input based on synaptic strength. By comparing biophysical maturation of the postsynaptic cell to alterations in presynaptic organization, we propose that maturation of synaptic partners is coordinated by synaptic activity in a process that is likely to generalize to other neural systems.

Does the brain connect before the periphery can direct? A comparison of three sensory systems in mice.

The development of peripheral to central neural connections within the auditory, visual, and olfactory systems of mice is reviewed to address whether peripheral signaling may play an instructive role during initial synapse formation. For each sensory system, developmental times of histogenesis and the earliest ages of innervation and function are considered for peripheral and selected central relays. For the auditory and visual system, anatomical and functional reports indicate that central connections may form prior to synapse formation in the periphery. However, evidence from the olfactory system suggests that the peripheral olfactory sensory neurons form synaptic connections before more central olfactory connections are established. We find that significant gaps in knowledge exist for embryonic development of these systems in mice and that genetic tools have not yet been systematically directed to address these issues.

Otitis media with effusion: a possible role for Helicobacter pylori?

OBJECTIVES: To compare Helicobacter pylori prevalence rates in the nasopharynx of pediatric patients with and without otitis media with effusion (OME). STUDY DESIGN: Prospective, controlled. METHODS: The study group consisted of patients undergoing adenoidectomy for persistent OME. A control group of patients with no history of OME undergoing adenoidectomy was simultaneously enrolled. As a substudy, middle ear effusion samples were also analyzed for Helicobacter pylori. Polymerase chain reaction assays were run by using primers against the Helicobacter pylori gene. RESULTS: Helicobacter pylori was detected in the adenoids of 10 of 45 study group patients, and 6 of 37 controls (P = 0.49). Thirty-two percent of middle ear aspirates were positive for Helicobacter pylori. CONCLUSION: The current study confirms the presence of Helicobacter pylori in the nasopharynx and middle ear space, but our results do not support a role for this bacterium in the pathogenesis of otitis media with effusion.

Molecular guidance cues necessary for axon pathfinding from the ventral cochlear nucleus.

During development, multiple guidance cues direct the formation of appropriate synaptic connections. Factors that guide developing axons are known for various pathways throughout the mammalian brain; however, signals necessary to establish auditory connections are largely unknown. In the auditory brainstem the neurons whose axons traverse the midline in the ventral acoustic stria (VAS) are primarily located in the ventral cochlear nucleus (VCN) and project bilaterally to the superior olivary complex (SOC). The circumferential trajectory taken by developing VCN axons is similar to that of growing axons of spinal commissural neurons. Therefore, we reasoned that netrin-DCC and slit-robo signaling systems function in the guidance of VCN axons. VCN neurons express the transcription factor, mafB, as early as embryonic day (E) 13.5, thereby identifying the embryonic VCN for these studies. VCN axons extend toward the midline as early as E13, with many axons crossing by E14.5. During this time, netrin-1 and slit-1 RNAs are expressed at the brainstem midline. Additionally, neurons within the VCN express RNA for DCC, robo-1, and robo-2, and axons in the VAS are immunoreactive for DCC. VCN axons do not reach the midline of the brainstem in mice mutant for either the netrin-1 or DCC gene. VCN axons extend in pups lacking netrin-1, but most DCC-mutant samples lack VCN axonal outgrowth. Stereological cell estimates indicate only a modest reduction of VCN neurons in DCC-mutant mice. Taken together, these data show that a functional netrin-DCC signaling system is required for establishing proper VCN axonal projections in the auditory brainstem.

Synaptogenesis of the calyx of Held: rapid onset of function and one-to-one morphological innervation.

Synaptogenesis during early development is thought to follow a canonical program whereby synapses increase rapidly in number and individual axons multiply-innervate nearby targets. Typically, a subset of inputs then out-competes all others through experience-driven processes to establish stable, long-lasting contacts. We investigated the formation of the calyx of Held, probably the largest nerve terminal in the mammalian CNS. Many basic functional and morphological features of calyx growth have not been studied previously, including whether mono-innervation, a hallmark of this system in adult animals, is established early in development. Evoked postsynaptic currents, recorded from neonatal mice between postnatal day 1 (P1) and P4, increased dramatically from -0.14 +/- 0.04 nA at P1 to -6.71 +/- 0.65 nA at P4 with sharp jumps between P2 and P4. These are the first functional assays of these nascent synapses for ages less than P3. AMPA and NMDA receptor-mediated currents were prominent across this age range. Electron microscopy (EM) revealed a concomitant increase, beginning at P2, in the prevalence of postsynaptic densities (16-fold) and adhering contacts (73-fold) by P4. Therefore, both functional and structural data showed that young calyces could form within 2 d, well before the onset of hearing around P8. Convergence of developing calyces onto postsynaptic targets, indicative of competitive processes that precede mono-innervation, was rare (4 of 29) at P4 as assessed using minimal stimulation electrophysiology protocols. Serial EM sectioning through 19 P4 cells further established the paucity (2 of 19) of convergence. These data indicate that calyces of Held follow a noncanonical program to establish targeted innervation that occurs over a rapid time course and precedes auditory experience.

Distinct subtypes of somatostatin-containing neocortical interneurons revealed in transgenic mice.

GABA-releasing inhibitory interneurons in the cerebral cortex can be classified by their neurochemical content, firing patterns, or axonal targets, to name the most common criteria, but whether classifications using different criteria converge on the same neuronal subtypes, and how many such subtypes exist, is a matter of much current interest and considerable debate. To address these issues, we generated transgenic mice expressing green fluorescent protein (GFP) under control of the GAD67 promoter. In two of these lines, named X94 and X98, GFP expression in the barrel cortex was restricted to subsets of somatostatin-containing (SOM+) GABAergic interneurons, similar to the previously reported "GIN" line (Oliva et al., 2000), but the laminar distributions of GFP-expressing (GFP+) cell bodies in the X94, X98, and GIN lines were distinct and nearly complementary. We compared neurochemical content and axonal distribution patterns of GFP+ neurons among the three lines and analyzed in detail electrophysiological properties in a dataset of 150 neurons recorded in whole-cell, current-clamp mode. By all criteria, there was nearly perfect segregation of X94 and X98 GFP+ neurons, whereas GIN GFP+ neurons exhibited intermediate properties. In the X98 line, GFP expression was found in infragranular, calbindin-containing, layer 1-targeting ("Martinotti") cells that had a propensity to fire low-threshold calcium spikes, whereas X94 GFP+ cells were stuttering interneurons with quasi fast-spiking properties, residing in and targeting the thalamo-recipient neocortical layers. We conclude that much of the variability previously attributed to neocortical SOM+ interneurons can be accounted for by their natural grouping into distinct subtypes.

Detection of fungi in the sinus mucosa using polymerase chain reaction.

OBJECTIVE: To compare the presence of fungi in the sinus mucosa of patients with and without chronic rhinosinusitis. STUDY DESIGN AND SETTING: Prospective observational study using polymerase chain reaction and conventional culture to detect fungi in the sinus mucosa. Middle meatus mucosal samples were collected from 31 patients with chronic rhinosinusitis and 14 control subjects. RESULTS: Fungi were detected in 6.5% of subjects with chronic rhinosinusitis and in none of the control subjects using polymerase chain reaction. Fungi were detected in 29% of subjects with the combination of inhalant allergies, nasal polyposis, and asthma. Fungi were detected in none of the subjects without the combination of these three comorbidities (P = 0.03). CONCLUSION: Polymerase chain reaction assay appears to be able to detect fungi in chronic rhinosinusitis. SIGNIFICANCE: Fungi may not be implicated in the pathogenesis of most chronic rhinosinusitis. EBM RATING: B-3b.

Conditional alleles for activation and inactivation of the mouse Rx homeobox gene.

The Rx homeobox gene is a transcriptional regulator indispensable for development of the eye and ventral forebrain. Rx-null homozygotes lack optic pits, which are the earliest ocular structures. To study the roles Rx may play at various stages of eye and brain development, we generated an allelic series at the Rx locus. The targeted allele, Rx(neo), is a severely hypomorphic or null allele. This Rx(neo) allele is converted via FLP-mediated recombination to the Rx(flox) allele, which is phenotypically identical to the wildtype allele. Cre-mediated conversion of Rx(flox) generates the RxDelta2 allele, which, when homozygous, results in an Rx-null phenotype that includes perinatal lethality, anophthalmia, and anterior neural and craniofacial defects. Mice carrying these alleles allow both Cre-mediated inactivation and FLP-mediated activation of Rx gene activity on a conditional basis and will be useful in examining Rx function at different developmental stages and in distinct tissue environments.

Regulation of vertebrate eye development by Rx genes.

The paired-like homeobox-containing gene Rx has a critical role in the eye development of several vertebrate species including Xenopus, mouse, chicken, medaka, zebrafish and human. Rx is initially expressed in the anterior neural region of developing embryos, and later in the retina and ventral hypothalamus. Abnormal regulation or function of Rx results in severe abnormalities of eye formation. Overexpression of Rx in Xenopus and zebrafish embryos leads to overproliferation of retinal cells. A targeted elimination of Rx in mice results in a lack of eye formation. Mutations in Rx genes are the cause of the mouse mutation eyeless (ey1), the medaka temperature sensitive mutation eyeless (el) and the zebrafish mutation chokh. In humans, mutations in Rx lead to anophthalmia. All of these studies indicate that Rx genes are key factors in vertebrate eye formation. Because these results cannot be easily reconciled with the most popular dogmas of the field, we offer our interpretation of eye development and evolution.

Mutations in the human RAX homeobox gene in a patient with anophthalmia and sclerocornea.

Anophthalmia and microphthalmia are among the most common ocular birth defects and a significant cause of congenital blindness. The etiology of anophthalmia and microphthalmia is diverse, with multiple genetic mutations associated with each of these conditions, along with potential environmental causes. Based on findings that mutations in the Rx/Rax homeobox genes in mice and fish lead to defects in retinal development and result in animal models of anophthalmia, we screened 75 individuals with anophthalmia and/or microphthalmia for mutations in the human RAX gene. We identified a single proband from this population who is a compound heterozygote for mutations in the RAX gene. This individual carries a truncated allele (Q147X) and a missense mutation (R192Q), both within the DNA-binding homeodomain of the RAX protein, and we have characterized the biochemical properties of these mutations in vitro. Parents and grandparents of the proband were found to be carriers without visible ocular defects, consistent with an autosomal recessive inheritance pattern. This is the first report of genetic mutations in the human RAX gene.

Brain-derived neurotrophic factor mediates activity-dependent dendritic growth in nonpyramidal neocortical interneurons in developing organotypic cultures.

Brain-derived neurotrophic factor (BDNF) promotes postnatal maturation of GABAergic inhibition in the cerebral and cerebellar cortices, and its expression and release are enhanced by neuronal activity, suggesting that it acts in a feedback manner to maintain a balance between excitation and inhibition during development. BDNF promotes differentiation of cerebellar, hippocampal, and neostriatal inhibitory neurons, but its effects on the dendritic development of neocortical inhibitory interneurons remain unknown. Here, we show that BDNF mediates depolarization-induced dendritic growth and branching in neocortical interneurons. To visualize inhibitory interneurons, we biolistically transfected organotypic cortical slice cultures from neonatal mice with green fluorescent protein (GFP) driven by the glutamic acid decarboxylase (GAD)67 promoter. Nearly all GAD67-GFP-expressing neurons were nonpyramidal, many contained GABA, and some expressed markers of neurochemically defined GABAergic subtypes, indicating that GAD67-GFP-expressing neurons were GABAergic. We traced dendritic trees from confocal images of the same GAD67-GFP-expressing neurons before and after a 5 d growth period, and quantified the change in total dendritic length (TDL) and total dendritic branch points (TDBPs) for each neuron. GAD67-GFP-expressing neurons growing in control medium exhibited a 20% increase in TDL, but in 200 ng/ml BDNF or 10 mm KCl, this increase nearly doubled and was accompanied by a significant increase in TDBPs. Blocking action potentials with TTX did not prevent the BDNF-induced growth, but antibodies against BDNF blocked the growth-promoting effect of KCl. We conclude that BDNF, released by neocortical pyramidal neurons in response to depolarization, enhances dendritic growth and branching in nearby inhibitory interneurons.