Tetrachlorodibenzo-p-dioxin exposure alters radial arm maze performance and hippocampal morphology in female AhR +/- mice

Effects of Low-Dose Gestational TCDD Exposure on Behavior and on Hippocampal Neuron Morphology and Gene Expression in Mice

BACKGROUND

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a persistent and toxic environmental pollutant. Gestational exposure to TCDD has been linked to cognitive and motor deficits, and increased incidence of autism spectrum disorder (ASD) traits in children. Most animal studies of these neurodevelopmental effects involve acute TCDD exposure, which does not model typical exposure in humans.

OBJECTIVES

The aim of the study was to establish a dietary low-dose gestational TCDD exposure protocol and performed an initial characterization of the effects on offspring behavior, neurodevelopmental phenotypes, and gene expression.

METHODS

Throughout gestation, pregnant C57BL/6J mice were fed a diet containing a low dose of TCDD (9  ng TCDD/kg body weight per day) or a control diet. The offspring were tested in a battery of behavioral tests, and structural brain alterations were investigated by magnetic resonance imaging. The dendritic morphology of pyramidal neurons in the hippocampal Cornu Ammonis (CA)1 area was analyzed. RNA sequencing was performed on hippocampi of postnatal day 14 TCDD-exposed and control offspring.

RESULTS

TCDD-exposed females displayed subtle deficits in motor coordination and reversal learning. Volumetric difference between diet groups were observed in regions of the hippocampal formation, mammillary bodies, and cerebellum, alongside higher dendritic arborization of pyramidal neurons in the hippocampal CA1 region of TCDD-exposed females. RNA-seq analysis identified 405 differentially expressed genes in the hippocampus, enriched for genes with functions in regulation of microtubules, axon guidance, extracellular matrix, and genes regulated by SMAD3.

DISCUSSION

Exposure to 9  ng TCDD/kg body weight per day throughout gestation was sufficient to cause specific behavioral and structural brain phenotypes in offspring. Our data suggest that alterations in SMAD3-regulated microtubule polymerization in the developing postnatal hippocampus may lead to an abnormal morphology of neuronal dendrites that persists into adulthood. These findings show that environmental low-dose gestational exposure to TCDD can have significant, long-term impacts on brain development and function. https://doi.org/10.1289/EHP7352.

Loss of Tiparp Results in Aberrant Layering of the Cerebral Cortex

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase (TIPARP) is an enzyme that adds a single ADP-ribose moiety to itself or other proteins. Tiparp is highly expressed in the brain; however, its function in this organ is unknown. Here, we used Tiparp^–/– mice to determine Tiparp’s role in the development of the prefrontal cortex.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase (TIPARP) is an enzyme that adds a single ADP-ribose moiety to itself or other proteins. Tiparp is highly expressed in the brain; however, its function in this organ is unknown. Here, we used Tiparp^–/– mice to determine Tiparp’s role in the development of the prefrontal cortex. Loss of Tiparp resulted in an aberrant organization of the mouse cortex, where the upper layers presented increased cell density in the knock-out mice compared with wild type. Tiparp loss predominantly affected the correct distribution and number of GABAergic neurons. Furthermore, neural progenitor cell proliferation was significantly reduced. Neural stem cells (NSCs) derived from Tiparp^–/– mice showed a slower rate of migration. Cytoskeletal components, such as α-tubulin are key regulators of neuronal differentiation and cortical development. α-tubulin mono-ADP ribosylation (MAR) levels were reduced in Tiparp^–/– cells, suggesting that Tiparp plays a role in the MAR of α-tubulin. Despite the mild phenotype presented by Tiparp^–/– mice, our findings reveal an important function for Tiparp and MAR in the correct development of the cortex. Unravelling Tiparp’s role in the cortex, could pave the way to a better understanding of a wide spectrum of neurological diseases which are known to have increased expression of TIPARP.

2,3,7,8-Tetrachlorodibenzo-p-dioxin modifies alternative splicing in mouse liver

Alternative splicing is a co-transcriptional mechanism that generates protein diversity by including or excluding exons in different combinations, thereby expanding the diversity of protein isoforms of a single gene. Abnormalities in this process can result in deleterious effects to human health, and several xenobiotics are known to interfere with splicing regulation through multiple mechanisms. These changes could lead to human diseases such as cancer, neurological disorders, autoimmune diseases, and developmental disorders. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant generated as a byproduct of various industrial activities. Exposure to this dioxin has been linked to a wide range of pathologies through the alteration of multiple cellular processes. However, the effects of TCDD exposure on alternative splicing have not yet been studied. Here, we investigated whether a single po. dose of 5 μg/kg or 500 μg/kg TCDD influence hepatic alternative splicing in adult male C57BL/6Kou mouse. We identified several genes whose alternative splicing of precursor messenger RNAs was modified following TCDD exposure. In particular, we demonstrated that alternative splicing of Cyp1a1, Ahrr, and Actn1 was significantly altered after TCDD treatment. These findings show that the exposure to TCDD has an impact on alternative-splicing, and suggest a new avenue for understanding TCDD-mediated toxicity and pathogenesis.

Tyroxine Hydroxylase-Positive Neuronal Cell Population is Increased by Temporal Dioxin Exposure at Early Stage of Differentiation from Human Embryonic Stem Cells

Background: The neurological effects of short-term dioxin exposure during the fetal period is an important health risk in humans. Here, we investigated the effects of dioxin on neural differentiation using human embryonic stem cells (hESCs) to evaluate human susceptibility to dioxin. Methods: Using an enzymatic bulk passage, neural differentiation from human ESCs was carried out. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was added to various stages of culture. The expression levels of the neuronal markers microtubule-associated protein 2 (MAP2) and thyroxine hydroxylase (TH) were measured by RT-qPCR and image analysis of immunostaining. Results: Although early-stage neuronal cells are quite resistant to TCDD, the numbers of neural rosettes and increases in mRNA expression levels and the number of cells positive for MAP2 and TH were significant by temporal exposure at embryoid body stage (Day9-exposure group). In contrast, the TCDD exposures against ESCs (Day0-exposure group) and differentiated neural cells (Day35-exposure group) were not affected at all. The increment was similarly observed by continuous exposure of TCDD from Day9 through Day60. Conclusions: These results indicated that dioxin exposure during the early stage of differentiation from hESCs increases the contents of neuronal cells, especially TH-positive neuronal cells. Regulations of aryl hydrocarbon receptor (AHR) signaling in an early stage of embryogenesis should be investigated extensively to understand the mechanism underlying the increase in neuronal cell populations and to apply the knowledge to regenerative medicine.

Sex-specific responses in neuroanatomy of hatchling American kestrels in response to embryonic exposure to the flame retardants bis(2-ethylhexyl)-2,3,4,5-tetrabromophthalate and 2-ethylhexyl-2,3,4,5-tetrabromobenzoate

Bis(2-ethylhexyl)-2,3,4,5-tetrabromophthalate (BEH-TEBP) and 2-ethylhexyl-2,3,4,5-tetrabromobenzoate (EH-TBB), flame retardant components of FireMaster 550® and 600® have been detected in tissues of wild birds. To address the paucity of information regarding potential impacts of flame retardants on the brain, brain volume regions of hatchling American kestrels (Falco sparverius) were evaluated following in ovo injection at embryonic day 5 with safflower oil or to 1 of 3 doses of either BEH-TEBP (12, 60, or 107 ng/g egg) or EH-TBB (11, 55, or 137 ng/g egg). The doses for both chemicals reflected concentrations reported in wild birds. The volumes of the hippocampus and telencephalon and volumetric differences between left and right hemispheres were measured in hatchlings (embryonic day 28). A sex-specific effect of BEH-TEBP on relative hippocampus volume was evident: the hippocampus was significantly enlarged in high-dose females compared to control females but smaller in low-dose females than the other females. There was no significant effect of EH-TBB on hippocampus volume in female kestrel hatchlings or of either chemical in male hatchlings and no effects of these concentrations of EH-TBB or BEH-TEBP on telencephalon volume or the level of symmetry between the hemispheres of the brain. In sum, embryonic exposure of female kestrels to these BEH-TEBP concentrations altered hippocampus volume, having the potential to affect spatial memory relating to ecologically relevant behavior such as prey capture, predator avoidance, and migration. Environ Toxicol Chem 2018;37:3032-3040. © 2018 SETAC.

The Aryl Hydrocarbon Receptor and the Nervous System

The aryl hydrocarbon receptor (or AhR) is a cytoplasmic receptor of pollutants. It translocates into the nucleus upon binding to its ligands, and forms a heterodimer with ARNT (AhR nuclear translocator). The heterodimer is a transcription factor, which regulates the transcription of xenobiotic metabolizing enzymes. Expressed in many cells in vertebrates, it is mostly present in neuronal cell types in invertebrates, where it regulates dendritic morphology or feeding behavior. Surprisingly, few investigations have been conducted to unravel the function of the AhR in the central or peripheral nervous systems of vertebrates. In this review, we will present how the AhR regulates neural functions in both invertebrates and vertebrates as deduced mainly from the effects of xenobiotics. We will introduce some of the molecular mechanisms triggered by the well-known AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which impact on neuronal proliferation, differentiation, and survival. Finally, we will point out the common features found in mice that are exposed to pollutants, and in AhR knockout mice.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces alterations in myogenic differentiation of C2C12 cells

Dioxin-induced toxicities that affect the development of the motor system have been proposed since many years. However, cellular evidence and the molecular basis for the effects are limited. In this study, a cultured mouse myoblast cell line, C2C12, was utilized to examine the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on myogenic differentiation and expression of acetylcholinesterase (AChE), a neuromuscular transmission-related gene. The results showed that TCDD exposure at 10^-10 M repressed the myotube formation of C2C12 cells by disturbing the fusion process and suppressing the expression of myosin heavy chain, a myobute structural protein, and not by induction of cytotoxicity. Furthermore, TCDD dose dependently suppressed the transcriptional expression and enzymatic activity of AChE during the myogenic differentiation, particularly in the middle stage. However, the administration of aryl hydrocarbon receptor antagonists, CH223191 and alpha-naphthoflavone, did not completely reverse the TCDD-induced downregulation of muscular AChE during myogenic differentiation. These findings suggest that low dose exposure to dioxin may result in disturbances of muscle differentiation and neuromuscular transmission.

Dioxin induces expression of hsa-miR-146b-5p in human neuroblastoma cells

Dioxin can cause a series of neural toxicological effects. MicroRNAs (miRs) play important roles in regulating nervous system function and mediating cellular responses to environmental pollutants, such as dioxin. Hsa-miR-146b-5p appears to be involved in neurodegenerative diseases and brain tumors. However, little is known about effects of dioxin on the expression of hsa-miR-146b-5p. We found that the hsa-miR-146b-5p expression and its promoter activity were significantly increased in dioxin treated SK-N-SH cells, a human-derived neuroblastoma cell line. Potential roles of hsa-miR-146b-5p in mediating neural toxicological effects of dioxin may be due to the regulation of certain target genes. We further confirmed that hsa-miR-146b-5p significantly suppressed acetylcholinesterase (AChE) activity and targeted the 3'-untranslated region of the AChE T subunit, which has been down-regulated in dioxin treated SK-N-SH cells. Functional bioinformatic analysis showed that the known and predicted target genes of hsa-miR-146b-5p were involved in some brain functions or cyto-toxicities related to known dioxin effects, including synapse transmission, in which AChE may serve as a responsive gene for mediating the effect.

Aryl hydrocarbon receptor mediates both proinflammatory and anti-inflammatory effects in lipopolysaccharide-activated microglia

The aryl hydrocarbon receptor (AhR) regulates peripheral immunity; but its role in microglia-mediated neuroinflammation in the brain remains unknown. Here, we demonstrate that AhR mediates both anti-inflammatory and proinflammatory effects in lipopolysaccharide (LPS)-activated microglia. Activation of AhR by its ligands, formylindolo[3,2-b]carbazole (FICZ) or 3-methylcholanthrene (3MC), attenuated LPS-induced microglial immune responses. AhR also showed proinflammatory effects, as evidenced by the findings that genetic silence of AhR ameliorated the LPS-induced microglial immune responses and LPS-activated microglia-mediated neurotoxicity. Similarly, LPS-induced expressions of tumor necrosis factor α (TNFα) and inducible nitric oxide synthase (iNOS) were reduced in the cerebral cortex of AhR-deficient mice. Intriguingly, LPS upregulated and activated AhR in the absence of AhR ligands via the MEK1/2 signaling pathway, which effects were associated with a transient inhibition of cytochrome P450 1A1 (CYP1A1). Although AhR ligands synergistically enhance LPS-induced AhR activation, leading to suppression of LPS-induced microglial immune responses, they cannot do so on their own in microglia. Chromatin immunoprecipitation results further revealed that LPS-FICZ co-treatment, but not LPS alone, not only resulted in co-recruitment of both AhR and NFκB onto the κB site of TNFα gene promoter but also reduced LPS-induced AhR binding to the DRE site of iNOS gene promoter. Together, we provide evidence showing that microglial AhR, which can be activated by LPS, exerts bi-directional effects on the regulation of LPS-induced neuroinflammation, depending on the availability of external AhR ligands. These findings confer further insights into the potential link between environmental factors and the inflammatory brain disorders.

Executive function deficits and social-behavioral abnormality in mice exposed to a low dose of dioxin in utero and via lactation

An increasing prevalence of mental health problems has been partly ascribed to abnormal brain development that is induced upon exposure to environmental chemicals. However, it has been extremely difficult to detect and assess such causality particularly at low exposure levels. To address this question, we here investigated higher brain function in mice exposed to dioxin in utero and via lactation by using our recently developed automated behavioral flexibility test and immunohistochemistry of neuronal activation markers Arc, at the 14 brain areas. Pregnant C57BL/6 mice were given orally a low dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at a dose of either 0, 0.6 or 3.0 µg/kg on gestation day 12.5. When the pups reached adulthood, they were group-housed in IntelliCage to assess their behavior. As a result, the offspring born to dams exposed to 0.6 µg TCDD/kg were shown to have behavioral inflexibility, compulsive repetitive behavior, and dramatically lowered competitive dominance. In these mice, immunohistochemistry of Arc exhibited the signs of hypoactivation of the medial prefrontal cortex (mPFC) and hyperactivation of the amygdala. Intriguingly, mice exposed to 3.0 µg/kg were hardly affected in both the behavioral and neuronal activation indices, indicating that the robust, non-monotonic dose-response relationship. In conclusion, this study showed for the first time that perinatal exposure to a low dose of TCDD in mice develops executive function deficits and social behavioral abnormality accompanied with the signs of imbalanced mPFC-amygdala activation.

Effects of environmentally relevant mixtures of persistent organic pollutants on the developmental neurobiology in rats

We report the developmental neuropathology for rat pups at postnatal day (PND) 37 and PND 77 and the molecular biomarkers for PND 35, 75, and 350 after perinatal exposure to a reconstituted mixture of persistent organochlorine pollutants (POPs) based on the blood profiles of people living in the Great Lake Basin. The developmental neuropathology included routine histopathology evaluation, quantification of cell proliferation and death in the subventricular zone, linear morphometric measurements, and transcriptional analysis. No histopathological, structural, or stereological changes were observed in animals treated with the POPs or Aroclor 1254, on PND 37 or PND 77. While no transcriptional changes were found in Arcolor-treated animals, significant transcriptional changes were observed on PND 350 in female offspring perinatally exposed to 0.13 mg/kg of the POP mixture. Markers of the cholinergic system including acetylcholinesterase and the muscarinic receptors (subtypes M1-M5) were downregulated 2- to 6-fold. In addition, structural genes including neurofilaments (NFLs) and microtubule-associated protein (MAP-2) were downregulated at least 2-fold or greater. Our results support that in utero and lactational exposure to the chemical mixture of POPs lead to developmental changes in adult rat brains.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces apoptosis in neural growth factor (NGF)-differentiated pheochromocytoma PC12 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent toxicant that alters normal brain development and produces cognitive disability and motor dysfunction. However, after decades of intense study, the molecular mechanisms of TCDD-induced neurotoxicity, the signaling pathways involved and its molecular targets in neurons still remain unknown. TCDD acts as an exogenous ligand of the aryl hydrocarbon receptor (AhR) that becomes a key signaling molecule in the regulation of the toxic and carcinogenic properties of TCDD. We have used NGF-differentiated pheochromocytoma (dPC12) cells to determine the type of cell death that takes place by TCDD toxicity. TCDD induced cell death in dPC12 cultures with an EC(50) of 218+/-24 nM, similar to that obtained in undifferentiated PC12 cells, 171+/-31 nM. Nuclear fragmentation was observed after TCDD incubation in parallel to an increase in caspase-3 activity. Staurosporine, which readily induced apoptosis in dPC12 cells, showed a similar increase in caspase-3 activity and the characteristic pattern of nuclear fragmentation. Flow cytometry measurements showed that dPC12 cells in the presence of TCDD were positive for annexin V labeling but negative for propidium iodide staining. In addition, TCDD increased the area of the peak corresponding to hypodiploid (apoptotic) DNA content. All together these results support the hypothesis that TCDD toxicity in dPC12 cells takes place mainly through an apoptotic process.

Organohalogen contaminants and metabolites in cerebrospinal fluid and cerebellum gray matter in short-beaked common dolphins and Atlantic white-sided dolphins from the western North Atlantic

Concentrations of several congeners and classes of organohalogen contaminants (OHCs) and/or their metabolites, namely organochlorine pesticides (OCs), polychlorinated biphenyls (PCBs), hydroxylated-PCBs (OH-PCBs), methylsulfonyl-PCBs (MeSO(2)-PCBs), polybrominated diphenyl ether (PBDE) flame retardants, and OH-PBDEs, were measured in cerebrospinal fluid (CSF) of short-beaked common dolphins (n = 2), Atlantic white-sided dolphins (n = 8), and gray seal (n = 1) from the western North Atlantic. In three Atlantic white-sided dolphins, cerebellum gray matter (GM) was also analyzed. The levels of OCs, PCBs, MeSO(2)-PCBs, PBDEs, and OH-PBDEs in cerebellum GM were higher than the concentrations in CSF. 4-OH-2,3,3',4',5-pentachlorobiphenyl (4-OH-CB107) was the only detectable OH-PCB congener present in CSF. The sum (Sigma) OH-PCBs/Sigma PCB concentration ratio in CSF was approximately two to three orders of magnitude greater than the ratio in cerebellum GM for dolphins.

AhR-mediated gene expression in the developing mouse telencephalon

We hypothesize that TCDD-induced developmental neurotoxicity is modulated through an AhR-dependent interaction with key regulatory neuronal differentiation pathways during telencephalon development. To test this hypothesis we examined global gene expression in both dorsal and ventral telencephalon tissues in E13.5 AhR-/- and wildtype mice exposed to TCDD or vehicle. Consistent with previous biochemical, pathological and behavioral studies, our results suggest TCDD initiated changes in gene expression in the developing telencephalon are primarily AhR-dependent, as no statistically significant gene expression changes are evident after TCDD exposure in AhR-/- mice. Based on a gene regulatory network for neuronal specification in the developing telencephalon, the present analysis suggests differentiation of GABAergic neurons in the ventral telencephalon is compromised in TCDD exposed and AhR-/- mice. In addition, our analysis suggests Sox11 may be directly regulated by AhR based on gene expression and comparative genomics analyses. In conclusion, this analysis supports the hypothesis that AhR has a specific role in the normal development of the telencephalon and provides a mechanistic framework for neurodevelopmental toxicity of chemicals that perturb AhR signaling.

Neuronal activity enhances aryl hydrocarbon receptor-mediated gene expression and dioxin neurotoxicity in cortical neurons

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor activated by dioxin and polyaromatic hydrocarbons. Recent studies have revealed that AhR activity in central neurons depends on the NMDA receptor. In this study, we investigated how the neuronal activity influence AhR-mediated dioxin-responsive gene expression and neurotoxicity. Our results show that activation of AhR by the selective agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin induced dioxin-responsive gene expression and calcium entry, which were attenuated by AhR small interfering RNA, the NMDA receptor channel blocker MK801, and the action potential blocker tetrodotoxin (TTX). In addition, AhR-mediated gene expression was enhanced in neurons during synaptogenesis (10 days in vitro) compared with younger neurons (4 days in vitro), as was sensitivity to TTX and MK801. Furthermore, TTX and MK801 differentially affected the association of AhR and its transcriptional co-activator cAMP-responsive-element binding protein with the cytochrome P450 1A1 (cyp1A1) gene enhancer. Calcium/calmodulin-dependent protein kinase IV, the cAMP-responsive-element binding protein activating enzyme, was also activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin in an activity-dependent manner. Finally, we found that neuronal susceptibility to dioxin insult was also maturation and activity-dependent. Together, the results suggest that neuronal activity may facilitate AhR-mediated calcium signaling, which in turn enhances AhR-mediated gene regulation and mediated maturation-dependent dioxin neurotoxicity.

Effects of maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on fetal brain growth and motor and behavioral development in offspring rats

The effects of maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during pregnancy on fetal brain growth and neurobehavioral development in early developmental stages were investigated using rat offspring. TCDD in corn-oil (0.1microg/kg) was orally administrated to the dams from the 9th to 19th gestational day. When TCDD effects on the fetal brain weight were analyzed on the 19th gestational day, weight ratio of the brain to the whole body, and that of the forebrain without the cerebral cortex to the whole brain were larger in the exposed group than those of the control group, suggesting premature fetal brain development. TCDD effects on motor functions were investigated using newborns in an inclined plane task. Motor development assessed by righting response on an inclination was delayed in the exposed offspring in the 8th-12th postnatal day, especially in male. Also, TCDD effects on active avoidance behavior in a shuttle box were investigated using the offspring after weaning. Latency in the active avoidance learning was longer, and locomotor activity was reduced in the exposed male offspring in the 41st-44th postnatal day. The results demonstrated that maternal TCDD exposure delayed fetal brain growth and neurodevelopment of the offspring in early stage, especially in male rats.

The aryl hydrocarbon receptor sans xenobiotics: endogenous function in genetic model systems

For more than 30 years, the aryl hydrocarbon receptor [Ah receptor (AHR)] has been extensively scrutinized as the cellular receptor for numerous environmental contaminants, including polychlorinated dioxins, dibenzofurans, and biphenyls. Recent evidence argues that this description is incomplete and perhaps myopic. Ah receptor orthologs have been demonstrated to mediate diverse endogenous functions in our close vertebrate relatives as well as our distant invertebrate ancestors. Moreover, these endogenous functions suggest that xenobiotic toxicity may be best understood in the context of intrinsic AHR physiology. In this literature review, we survey the emerging picture of endogenous AHR biology from work in the vertebrate and invertebrate model systems Mus musculus, Caenorhabditis elegans, and Drosophila melanogaster.

The bHLH-PAS protein Spineless is necessary for the diversification of dendrite morphology of Drosophila dendritic arborization neurons

Dendrites exhibit a wide range of morphological diversity, and their arborization patterns are critical determinants of proper neural connectivity. How different neurons acquire their distinct dendritic branching patterns during development is not well understood. Here we report that Spineless (Ss), the Drosophila homolog of the mammalian aryl hydrocarbon (dioxin) receptor (Ahr), regulates dendrite diversity in the dendritic arborization (da) sensory neurons. In loss-of-function ss mutants, class I and II da neurons, which are normally characterized by their simple dendrite morphologies, elaborate more complex arbors, whereas the normally complex class III and IV da neurons develop simpler dendritic arbors. Consequently, different classes of da neurons elaborate dendrites with similar morphologies. In its control of dendritic diversity among da neurons, ss likely acts independently of its known cofactor tango and through a regulatory program distinct from those involving cut and abrupt. These findings suggest that one evolutionarily conserved role for Ahr in neuronal development concerns the diversification of dendrite morphology.