Karen Atkinson-Leadbeater

Serotonin mediates food-odor associative learning in the nematode Caenorhabditis elegans

We demonstrate that Caenorhabditis elegans is able to form an association between the presence of the odorant benzaldehyde and the food content of its environment. When exposed to 100% benzaldehyde for 1 h in the absence of food the naive attractive response is reduced, and we have found that this olfactory adaptation is attenuated by the presence of food. Contrary to nonassociative (single stimulus) learning theory, this response is not a function of the total time of exposure to benzaldehyde but rather an associative function of the ability of benzaldehyde to predict a nutrient-deficient environment. Genetic and pharmacological evidence revealed that the effects of food in this learning paradigm are mediated by serotonergic signaling.

Targeting of Retinal Axons Requires the Metalloproteinase ADAM10

The role of extrinsic cues in guiding developing axons is well established; however, the means by which the activity of these extrinsic cues is regulated is poorly understood. A disintegrin and metalloproteinase (ADAM) enzymes are Zn-dependent proteinases that can cleave guidance cues or their receptors in vitro. Here, we identify the first example of a metalloproteinase that functions in vertebrate axon guidance in vivo. Specifically, ADAM10 is required for formation of the optic projection by Xenopus retinal ganglion cell (RGC) axons. Xadam10 mRNA is expressed in the dorsal neuroepithelium through which RGC axons extend. Pharmacological or molecular inhibition of ADAM10 within the brain each resulted in a failure of RGC axons to recognize their target. In contrast, molecular inhibition of ADAM10 within the RGC axons themselves had no effect. These data argue strongly that in the dorsal brain ADAM10 acts cell non-autonomously to regulate the guidance of RGC axons.

Dynamic Expression of Axon Guidance Cues Required for Optic Tract Development Is Controlled by Fibroblast Growth Factor Signaling

Axons are guided to their targets by molecular cues expressed in their environment. How is the presence of these cues regulated? Although some evidence indicates that morphogens establish guidance cue expression as part of their role in patterning tissues, an important question is whether morphogens are then required to maintain guidance signals. We found that fibroblast growth factor (FGF) signaling sustains the expression of two guidance cues, semaphorin3A (xsema3A) and slit1 (xslit1), throughout the period of Xenopus optic tract development. With FGF receptor inhibition, xsema3A and xslit1 levels were rapidly diminished, and retinal ganglion cell axons arrested in the mid-diencephalon, before reaching their target. Importantly, direct downregulation of XSema3A and XSlit1 mostly phenocopied this axon guidance defect. Thus, FGFs promote continued presence of specific guidance cues critical for normal optic tract development, suggesting a second later role for morphogens, independent of tissue patterning, in maintaining select cues by acting to regulate their transcription.

Voltage‐gated potassium channels regulate the response of retinal growth cones to axon extension and guidance cues

Xenopus retinal ganglion cell growth cones express various voltage‐gated potassium (Kv) channels. We showed previously that 4‐aminopyridine and tetraethylammonium have different effects on the outward currents of embryonic Xenopus retinal ganglion cells. Therefore, we asked whether these Kv channel inhibitors differentially regulate the response of retinal ganglion cell growth cones to extrinsic cues. First, we tested the role of Kv channels in axon extension mediated by a substrate bound cue and found that 4‐aminopyridine blocked, whereas tetraethylammonium enhanced basal extension on laminin. Yet, when the growth cones were stimulated to extend with application of soluble growth factors, both inhibitors resulted in a return to the basal extension rates observed in the presence of laminin alone. Second, we asked if Kv channels modulate the response of retinal ganglion cell growth cones to a guidance cue, the chemorepellent fibroblast growth factor‐2. When presented in a gradient to one side of the growth cone, fibroblast growth factor‐2 repulsed retinal ganglion cell growth cones in the presence of 4‐aminopyridine but not tetraethylammonium. These data argue that tetraethylammonium‐ and 4‐aminopyridine‐sensitive Kv channels differ in the manner by which they regulate the response of retinal ganglion cell axons to extension and guidance cues. Non‐ratiometric calcium imaging indicated that differences in the ability of tetraethylammonium‐ and 4‐aminopyridine‐sensitive Kv channels to regulate calcium activity within the growth cone may underlie their unique modulation of growth cone behaviour.

A genetic dissociation of learning and recall in Caenorhabditis elegans.

A learning event can be dissociated into 3 components: acquisition, storage, and recall. When the laboratory wild-type strain of Caenorhabditis elegans (N2 strain) is exposed to benzaldehyde in the absence of food, the worms display a reduction of their attractive response to this volatile odorant. This results from the association between benzaldehyde and a nutrient-deficient environment. Another wild-type isolate, the CB4856 strain, fails to display this decreased response to benzaldehyde after exposure to benzaldehyde in the absence of food. However, like the N2 strain, when tested to isoamyl alcohol after benzaldehyde conditioning, the CB4856 strain displays a decreased isoamyl alcohol response. Therefore, the CB4856 strain does not have an acquisition deficit, but it suffers from a recall deficit specific to benzaldehyde.

FGF receptor dependent regulation of Lhx9 expression in the developing nervous system.

LIM‐homeodomain (LIM‐hd) proteins form a multifunctional family of transcription factors that plays critical roles in the development of progenitor and post‐mitotic cells. Considerable work has focused on what regulates their expression post‐mitotically in the spinal cord. However, little is known about what regulates LIM‐hd genes at earlier developmental stages. To address this question, we explored the role of fibroblast growth factor (FGF) signalling in regulating the expression of a Xenopus laevis Lhx9 orthologue (XLhx9). XLhx9 is first expressed in the eye field and hindbrain, and when FGF receptor (FGFR) activation was inhibited prior to its onset, both brain and eye field expression was diminished. However, when FGFRs were inhibited after XLhx9 onset, retinal expression remained strong and brain expression was again diminished. These data suggest that while FGF signalling initiates and maintains brain XLhx9 expression, in the eye primordium the requirement of FGFs for expression is rapidly lost. Developmental Dynamics 238:367–375, 2009.

EGCG stabilizes growth cone filopodia and impairs retinal ganglion cell axon guidance

Background: Antioxidants such as the green tea polyphenol epigallocatechin gallate (EGCG) are neuroprotective under many conditions in mature nervous systems; however, their impact has rarely been explored in developing nervous systems, in which a critical step is the formation of connections between neurons. Axons emerge from newly formed neurons and are led by a dynamic structure found at their tip called a growth cone. Here we explore the impact of EGCG on the development of retinal ganglion cell (RGC) axons, which connect the eye to the brain. Results: EGCG acts directly on RGC axons to increase the number of growth cone filopodia, fingerlike projections that respond to extrinsic signals, in vitro and in vivo. Furthermore, EGCG exposure leads to a dramatic defect in the guided growth of RGC axons where the axons fail to make a key turn in the mid‐diencephalon required to reach their target. Intriguingly, at guidance points where RGCs do not show a change in direction, EGCG has no influence on RGC axon behavior. Conclusions: We propose that EGCG stabilizes filopodia and prevents normal filopodial dynamics required for axons to change their direction of outgrowth at guidance decision points. Developmental Dynamics 245:667–677, 2016.

Fgfr signaling is required as the early eye field forms to promote later patterning and morphogenesis of the eye

Background: A major step in eye morphogenesis is the transition from optic vesicle to optic cup, which occurs as a ventral groove forms along the base of the optic vesicle. A ventral gap in the eye, or coloboma, results when this groove fails to close. Extrinsic signals, such as fibroblast growth factors (Fgfs), play a critical role in the development and morphogenesis of the vertebrate eye. Whether these extrinsic signals are required throughout eye development, or within a defined critical period remains an unanswered question. Results: Here we show that an early Fgf signal, required as the eye field is first emerging, drives eye morphogenesis. In addition to triggering coloboma, inhibition of this early Fgf signal results in defects in dorsal‐ventral patterning of the neural retina, particularly in the nasal retina, and development of the periocular mesenchyme (POM). These processes are unaffected by inhibition of Fgfr signaling at later time points. Conclusions: We propose that Fgfs act within an early critical period as the eye field forms to promote development of the neural retina and POM, which subsequently drive eye morphogenesis. Developmental Dynamics 243:663–675, 2014.

Extrinsic factors as multifunctional regulators of retinal ganglion cell morphogenesis

Neurons acquire a unique cell‐type dependent morphology during development that is critical for their function in a neural circuit. The process involves a neuron sending out an axon that grows in a directed fashion to its target, and the elaboration of multiple, branched dendrites. The ultimate morphology of the neuron is sculpted by factors in the environment that act directly or indirectly to influence the behavior of the growing axon and dendrites. The output neuron of the retina, the retinal ganglion cell (RGC), has served as a useful model for the identification of molecular signals that control neuronal morphogenesis, because the entire development of the neuron, from the initiation of neurites to the establishment of synapses, is accessible for experimental manipulation and visualization. In this review we discuss data which argue that the visual system uses a limited number of signals to control RGC morphogenesis, with single molecules being reused multiple times to control distinct events in axon and dendrite outgrowth.

Interaction and developmental activation of two neuroendocrine systems that regulate light‐mediated skin pigmentation

Lower vertebrates use rapid light‐regulated changes in skin colour for camouflage (background adaptation) or during circadian variation in irradiance levels. Two neuroendocrine systems, the eye/alpha‐melanocyte‐stimulating hormone (α‐MSH) and the pineal complex/melatonin circuits, regulate the process through their respective dispersion and aggregation of pigment granules (melanosomes) in skin melanophores. During development, Xenopus laevis tadpoles raised on a black background or in the dark perceive less light sensed by the eye and darken in response to increased α‐MSH secretion. As embryogenesis proceeds, the pineal complex/melatonin circuit becomes the dominant regulator in the dark and induces lightening of the skin of larvae. The eye/α‐MSH circuit continues to mediate darkening of embryos on a black background, but we propose the circuit is shut down in complete darkness in part by melatonin acting on receptors expressed by pituitary cells to inhibit the expression of pomc, the precursor of α‐MSH.