Patterns of olfactory bulb neurogenesis in the adult zebrafish are altered following reversible deafferentation

Disparate progenitor cell populations contribute to maintenance and repair neurogenesis in the zebrafish olfactory epithelium

Olfactory sensory neurons (OSNs) undergo constant turnover under physiological conditions but also regenerate efficiently following tissue injury. Maintenance and repair neurogenesis in the olfactory epithelium (OE) have been attributed to the selective activity of globose (GBCs) and horizontal basal cells (HBCs), respectively. In zebrafish, cells with GBC-like properties are localized to the peripheral margins of the sensory OE and contribute to OSN neurogenesis in the intact OE, while cells that resemble HBCs at the morphological and molecular level are more uniformly distributed. However, the contribution of these cells to the restoration of the injured OE has not been demonstrated. Here, we provide a detailed cellular and molecular analysis of the tissue response to injury and show that a dual progenitor cell system also exists in zebrafish. Zebrafish HBCs respond to the structural damage of the OE and generate a transient population of proliferative neurogenic progenitors that restores OSNs. In contrast, selective ablation of OSNs by axotomy triggers neurogenic GBC proliferation, suggesting that distinct signaling events activate GBC and HBC responses. Molecular analysis of differentially expressed genes in lesioned and regenerating OEs points toward an involvement of the canonical Wnt/β-catenin pathway. Activation of Wnt signaling appears to be sufficient to stimulate mitotic activity, while inhibition significantly reduces, but does not fully eliminate, HBC responses. Zebrafish HBCs are surprisingly active even under physiological conditions with a strong bias toward the zones of constitutive OSN neurogenesis, suggestive of a direct lineage relationship between progenitor cell subtypes.

Diving into the streams and waves of constitutive and regenerative olfactory neurogenesis: insights from zebrafish

The olfactory system is renowned for its functional and structural plasticity, with both peripheral and central structures displaying persistent neurogenesis throughout life and exhibiting remarkable capacity for regenerative neurogenesis after damage. In general, fish are known for their extensive neurogenic ability, and the zebrafish in particular presents an attractive model to study plasticity and adult neurogenesis in the olfactory system because of its conserved structure, relative simplicity, rapid cell turnover, and preponderance of neurogenic niches. In this review, we present an overview of the anatomy of zebrafish olfactory structures, with a focus on the neurogenic niches in the olfactory epithelium, olfactory bulb, and ventral telencephalon. Constitutive and regenerative neurogenesis in both the peripheral olfactory organ and central olfactory bulb of zebrafish is reviewed in detail, and a summary of current knowledge about the cellular origin and molecular signals involved in regulating these processes is presented. While some features of physiologic and injury-induced neurogenic responses are similar, there are differences that indicate that regeneration is not simply a reiteration of the constitutive proliferation process. We provide comparisons to mammalian neurogenesis that reveal similarities and differences between species. Finally, we present a number of open questions that remain to be answered.

Decoding the role of zebrafish neuroglia in CNS disease modeling

Neuroglia, including microglia and astrocytes, is a critical component of the central nervous system (CNS) that interacts with neurons to modulate brain activity, development, metabolism and signaling pathways. Thus, a better understanding of the role of neuroglia in the brain is critical. Complementing clinical and rodent data, the zebrafish (Danio rerio) is rapidly becoming an important model organism to probe the role of neuroglia in brain disorders. With high genetic and physiological similarity to humans and rodents, zebrafish possess some common (shared), as well as some specific molecular biomarkers and features of neuroglia development and functioning. Studying these common and zebrafish-specific aspects of neuroglia may generate important insights into key brain mechanisms, including neurodevelopmental, neurodegenerative, neuroregenerative and neurological processes. Here, we discuss the biology of neuroglia in humans, rodents and fish, its role in various CNS functions, and further directions of translational research into the role of neuroglia in CNS disorders using zebrafish models.

The Olfactory System of Zebrafish as a Model for the Study of Neurotoxicity and Injury: Implications for Neuroplasticity and Disease

The olfactory system, composed of the olfactory organs and the olfactory bulb, allows organisms to interact with their environment and through the detection of odor signals. Olfaction mediates behaviors pivotal for survival, such as feeding, mating, social behavior, and danger assessment. The olfactory organs are directly exposed to the milieu, and thus are particularly vulnerable to damage by environmental pollutants and toxicants, such as heavy metals, pesticides, and surfactants, among others. Given the widespread occurrence of olfactory toxicants, there is a pressing need to understand the effects of these harmful compounds on olfactory function. Zebrafish (Danio rerio) is a valuable model for studying human physiology, disease, and toxicity. Additionally, the anatomical components of the zebrafish olfactory system are similar to those of other vertebrates, and they present a remarkable degree of regeneration and neuroplasticity, making it an ideal model for the study of regeneration, reorganization and repair mechanisms following olfactory toxicant exposure. In this review, we focus on (1) the anatomical, morphological, and functional organization of the olfactory system of zebrafish; (2) the adverse effects of olfactory toxicants and injury to the olfactory organ; and (3) remodeling and repair neuroplasticity mechanisms following injury and degeneration by olfactory toxicant exposure.

Reversible deafferentation of the zebrafish olfactory bulb with wax plug insertion

BACKGROUND

Deafferentation of the zebrafish olfactory bulb allows investigation of neuroplasticity in a particularly dynamic brain region of a popular model animal known for its regenerative abilities. Current methods to remove sensory input to the zebrafish olfactory bulb differ in the extent of deafferentation and potential for recovery.

NEW METHOD

We present a novel method of olfactory bulb deafferentation using continuous wax plug insertions into the nasal cavity of zebrafish. Wax plugs were placed in the nasal cavity and replaced if needed over 1wk or 3wk survival periods. Wax plugs were removed from fish after 1wk of occlusion to analyze the potential recovery of the olfactory organ and bulb.

RESULTS

Wax plug insertions caused a dramatic reduction in olfactory organ size and structure and significantly reduced afferent input to the olfactory bulb after 1wk and 3wk. Removal of the wax plugs after 1wk allowed for recovery of the olfactory organ and subsequent reinnervation of the olfactory bulb.

COMPARISONS WITH EXISTING METHODS

Chemical ablation with detergent causes partial, temporary deafferentation of the olfactory bulb. Cautery ablation causes complete, permanent deafferentation of the olfactory bulb. Wax plug insertions cause nearly complete, temporary deafferentation, allowing both significant deafferentation and the potential for reinnervation of the olfactory bulb.

CONCLUSIONS

The wax plug insertion method of deafferentation described here is unique in that it destroys almost completely the structure of the olfactory organ and removes almost completely sensory input to the olfactory bulb, yet the organ returns to its typical morphology and afferent innervation returns.

Deafferentation-induced alterations in mitral cell dendritic morphology in the adult zebrafish olfactory bulb

The removal of afferent input to the olfactory bulb by both cautery and chemical olfactory organ ablation in adult zebrafish results in a significant decrease in volume of the ipsilateral olfactory bulb. To examine the effects of deafferentation at a cellular level, primary output neurons of the olfactory bulb, the mitral cells, were investigated using retrograde tract tracing with fluorescent dextran using ex vivo brain cultures. Morphological characteristics including the number of major dendritic branches, total length of dendritic branches, area of the dendritic arbor, overall dendritic complexity, and optical density of the arbor were used to determine the effects of deafferentation on mitral cell dendrites. Following 8 weeks of permanent deafferentation there were significant reductions in the total length of dendritic branches, the area of the dendritic arbor, and the density of fine processes in the dendritic tuft. With 8 weeks of chronic, partial deafferentation there were significant reductions in all parameters examined, including a modified Sholl analysis that showed significant decreases in overall dendritic complexity. These results show the plasticity of mitral cell dendritic structures in the adult brain and provide information about the response of these output neurons following the loss of sensory input in this key model system.