Axonal signals in the assembly of neural circuitry

Wrapping Glial Morphogenesis and Signaling Control the Timing and Pattern of Neuronal Differentiation in the Drosophila Lamina

Various regions of the developing brain coordinate their construction so that the correct types and numbers of cells are generated to build a functional network. We previously discovered that wrapping glia in the Drosophila visual system are essential for coordinating retinal and lamina development. We showed that wrapping glia, which ensheath photoreceptor axons, respond to an epidermal growth factor cue from photoreceptors by secreting insulins. Wrapping glial insulins activate the mitogen-activated protein kinase (MAPK) pathway downstream of insulin receptor in lamina precursors to induce neuronal differentiation. The signaling relay via wrapping glia introduces a delay that allows the lamina to assemble the correct stoichiometry and physical alignment of precursors before differentiating and imposes a stereotyped spatiotemporal pattern that is relevant for specifying the individual lamina neuron fates. Here, we further describe how wrapping glia morphogenesis correlates with the timing of lamina neuron differentiation by 2-photon live imaging. We also show that although MAPK activity in lamina precursors drives neuronal differentiation, the upstream receptor driving MAPK activation in lamina precursors and the ligand secreted by wrapping glia to trigger it differentially affect lamina neuron differentiation. These results highlight differences in MAPK signaling properties and confirm that communication between photoreceptors, wrapping glia, and lamina precursors must be precisely controlled to build a complex neural network.

Prevention of medulla neuron dedifferentiation by Nerfin-1 requires inhibition of Notch activity

The Drosophila larval central nervous system comprises the central brain, ventral nerve cord and optic lobe. In these regions, neuroblasts (NBs) divide asymmetrically to self-renew and generate differentiated neurons or glia. To date, mechanisms of preventing neuron dedifferentiation are still unclear, especially in the optic lobe. Here, we show that the zinc-finger transcription factor Nerfin-1 is expressed in early-stage medulla neurons and is essential for maintaining their differentiation. Loss of Nerfin-1 activates Notch signaling, which promotes neuron-to-NB reversion. Repressing Notch signaling largely rescues dedifferentiation in nerfin-1 mutant clones. Thus, we conclude that Nerfin-1 represses Notch activity in medulla neurons and prevents them from dedifferentiation.

Dendrosomatic Sonic Hedgehog Signaling in Hippocampal Neurons Regulates Axon Elongation

The presence of Sonic Hedgehog (Shh) and its signaling components in the neurons of the hippocampus raises a question about what role the Shh signaling pathway may play in these neurons. We show here that activation of the Shh signaling pathway stimulates axon elongation in rat hippocampal neurons. This Shh-induced effect depends on the pathway transducer Smoothened (Smo) and the transcription factor Gli1. The axon itself does not respond directly to Shh; instead, the Shh signal transduction originates from the somatodendritic region of the neurons and occurs in neurons with and without detectable primary cilia. Upon Shh stimulation, Smo localization to dendrites increases significantly. Shh pathway activation results in increased levels of profilin1 (Pfn1), an actin-binding protein. Mutations in Pfn1's actin-binding sites or reduction of Pfn1 eliminate the Shh-induced axon elongation. These findings indicate that Shh can regulate axon growth, which may be critical for development of hippocampal neurons.

SIGNIFICANCE STATEMENT

Although numerous signaling mechanisms have been identified that act directly on axons to regulate their outgrowth, it is not known whether signals transduced in dendrites may also affect axon outgrowth. We describe here a transcellular signaling pathway in embryonic hippocampal neurons in which activation of Sonic Hedgehog (Shh) receptors in dendrites stimulates axon growth. The pathway involves the dendritic-membrane-associated Shh signal transducer Smoothened (Smo) and the transcription factor Gli, which induces the expression of the gene encoding the actin-binding protein profilin 1. Our findings suggest scenarios in which stimulation of Shh in dendrites results in accelerated outgrowth of the axon, which therefore reaches its presumptive postsynaptic target cell more quickly. By this mechanism, Shh may play critical roles in the development of hippocampal neuronal circuits.

Sequential axon-derived signals couple target survival and layer specificity in the Drosophila visual system

Neural circuit formation relies on interactions between axons and cells within the target field. While it is well established that target-derived signals act on axons to regulate circuit assembly, the extent to which axon-derived signals control circuit formation is not known. In the Drosophila visual system, anterograde signals numerically match R1-R6 photoreceptors with their targets by controlling target proliferation and neuronal differentiation. Here we demonstrate that additional axon-derived signals selectively couple target survival with layer specificity. We show that Jelly belly (Jeb) produced by R1-R6 axons interacts with its receptor, anaplastic lymphoma kinase (Alk), on budding dendrites to control survival of L3 neurons, one of three postsynaptic targets. L3 axons then produce Netrin, which regulates the layer-specific targeting of another neuron within the same circuit. We propose that a cascade of axon-derived signals, regulating diverse cellular processes, provides a strategy for coordinating circuit assembly across different regions of the nervous system.

Evidence for tissue-specific Jak/STAT target genes in Drosophila optic lobe development

The evolutionarily conserved JAK/STAT pathway plays important roles in development and disease processes in humans. Although the signaling process has been well established, we know relatively little about what the relevant target genes are that mediate JAK/STAT activation during development. Here, we have used genome-wide microarrays to identify JAK/STAT targets in the optic lobes of the Drosophila brain and identified 47 genes that are positively regulated by JAK/STAT. About two-thirds of the genes encode proteins that have orthologs in humans. The STAT targets in the optic lobe appear to be different from the targets identified in other tissues, suggesting that JAK/STAT signaling may regulate different target genes in a tissue-specific manner. Functional analysis of Nop56, a cell-autonomous STAT target, revealed an essential role for this gene in the growth and proliferation of neuroepithelial stem cells in the optic lobe and an inhibitory role in lamina neurogenesis.

Role of JAK/STAT signaling in neuroepithelial stem cell maintenance and proliferation in the Drosophila optic lobe

During Drosophila optic lobe development, proliferation and differentiation must be tightly modulated to reach its normal size for proper functioning. The JAK/STAT pathway plays pleiotropic roles in Drosophila development and in the larval brain, has been shown to inhibit medulla neuroblast formation. In this study, we find that JAK/STAT activity is required for the maintenance and proliferation of the neuroepithelial stem cells in the optic lobe. In loss-of-function JAK/STAT mutant brains, the neuroepithelial cells lose epithelial cell characters and differentiate prematurely while ectopic activation of this pathway is sufficient to induce neuroepithelial overgrowth in the optic lobe. We further show that Notch signaling acts downstream of JAK/STAT to control the maintenance and growth of the optic lobe neuroepithelium. Thus, in addition to its role in suppression of neuroblast formation, the JAK/STAT pathway is necessary and sufficient for optic lobe neuroepithelial growth.

100 years of Drosophila research and its impact on vertebrate neuroscience: a history lesson for the future

Discoveries in fruit flies have greatly contributed to our understanding of neuroscience. The use of an unparalleled wealth of tools, many of which originated between 1910–1960, has enabled milestone discoveries in nervous system development and function. Such findings have triggered and guided many research efforts in vertebrate neuroscience. After 100 years, fruit flies continue to be the choice model system for many neuroscientists. The combinational use of powerful research tools will ensure that this model organism will continue to lead to key discoveries that will impact vertebrate neuroscience.

Influence of fat-hippo and notch signaling on the proliferation and differentiation of Drosophila optic neuroepithelia

The Drosophila optic lobe develops from neuroepithelial cells, which function as symmetrically dividing neural progenitors. We describe here a role for the Fat-Hippo pathway in controlling the growth and differentiation of Drosophila optic neuroepithelia. Mutation of tumor suppressor genes within the pathway, or expression of activated Yorkie, promotes overgrowth of neuroepithelial cells and delays or blocks their differentiation; mutation of yorkie inhibits growth and accelerates differentiation. Neuroblasts and other neural cells, by contrast, appear unaffected by Yorkie activation. Neuroepithelial cells undergo a cell cycle arrest before converting to neuroblasts; this cell cycle arrest is regulated by Fat-Hippo signaling. Combinations of cell cycle regulators, including E2f1 and CyclinD, delay neuroepithelial differentiation, and Fat-Hippo signaling delays differentiation in part through E2f1. We also characterize roles for Jak-Stat and Notch signaling. Our studies establish that the progression of neuroepithelial cells to neuroblasts is regulated by Notch signaling, and suggest a model in which Fat-Hippo and Jak-Stat signaling influence differentiation by their acceleration of cell cycle progression and consequent impairment of Delta accumulation, thereby modulating Notch signaling. This characterization of Fat-Hippo signaling in neuroepithelial growth and differentiation also provides insights into the potential roles of Yes-associated protein in vertebrate neural development and medullablastoma.

Analyses of mental dysfunction-related ACSl4 in Drosophila reveal its requirement for Dpp/BMP production and visual wiring in the brain

Long-chain acyl-CoA synthetases (ACSLs) convert long-chain fatty acids to acyl-CoAs, the activated substrates essential in various metabolic and signaling pathways. Mutations in ACSL4 are associated with non-syndromic X-linked mental retardation (MRX). However, the developmental functions of ACSL4 and how it is involved in the pathogenesis of MRX remain largely unknown. The Drosophila ACSL-like protein is highly homologous to human ACSL3 and ACSL4, and we designate it as dAcsl. In this study, we demonstrate that dAcsl and ACSL4 are highly conserved in terms of ACSL4's ability to substitute the functions of dAcsl in organismal viability, lipid storage and the neural wiring in visual center. In neurodevelopment, decapentaplegic (Dpp, a BMP-like molecule) production diminished specifically in the larval brain of dAcsl mutants. Consistent with the Dpp reduction, the number of glial cells and neurons dramatically decreased and the retinal axons mis-targeted in the visual cortex. All these defects in Drosophila brain were rescued by the wild-type ACSL4 but not by the mutant products found in MRX patients. Interestingly, expression of an MRX-associated ACSL4 mutant form in a wild-type background led to the lesions in visual center, suggesting a dominant negative effect. These findings validate Drosophila as a model system to reveal the connection between ACSL4 and BMP pathway in neurodevelopment, and to infer the pathogenesis of ACSL4-related MRX.

Focal adhesion kinase controls morphogenesis of the Drosophilaoptic stalk

Photoreceptor cell axons (R axons) innervate optic ganglia in the Drosophila brain through the tubular optic stalk. This structure consists of surface glia (SG) and forms independently of R axon projection. In a screen for genes involved in optic stalk formation, we identified Fak56D encoding a Drosophila homolog of mammalian focal adhesion kinase (FAK). FAK is a main component of the focal adhesion signaling that regulates various cellular events, including cell migration and morphology. We show that Fak56D mutation causes severe disruption of the optic stalk structure. These phenotypes were completely rescued by Fak56D transgene expression in the SG cells but not in photoreceptor cells. Moreover, Fak56D genetically interacts with myospheroid, which encodes an integrin β subunit. In addition,we found that CdGAPr is also required for optic stalk formation and genetically interacts with Fak56D. CdGAPr encodes a GTPase-activating domain that is homologous to that of mammalian CdGAP, which functions in focal adhesion signaling. Hence the optic stalk is a simple monolayered structure that can serve as an ideal system for studying glial cell morphogenesis and the developmental role(s) of focal adhesion signaling.

A C-terminal motif targets Hedgehog to axons, coordinating assembly of the Drosophila eye and brain

The developmental signal Hedgehog is distributed to two receptive fields by the photoreceptor neurons of the developing Drosophila retina. Delivery to the retina propagates ommatidial development across a precursor field. Transport along photoreceptor axons induces the development of postsynaptic neurons in the brain. Hedgehog is composed of N-terminal and C-terminal domains that dissociate in an autoproteolytic reaction that attaches cholesterol to the N-terminal cleavage product. Here, we show that the N-terminal domain is targeted to the retina when synthesized in the absence of the C-terminal domain. In contrast to studies that have focused on cholesterol as a determinant of subcellular localization, we find that the C-terminal domain harbors a conserved motif that overrides retinal localization, sending most of the autocleavage products into vesicles bound for growth cones or synapses. Competition between targeting signals at the opposite ends of Hedgehog apparently controls the match between eye and brain development.

The highly ordered assembly of retinal axons and their synaptic partners is regulated by Hedgehog/Single-minded in the Drosophila visual system

During development of the Drosophila visual center, photoreceptor cells extend their axons (R axons) to the lamina ganglion layer, and trigger proliferation and differentiation of synaptic partners (lamina neurons) by delivering the inductive signal Hedgehog (Hh). This inductive mechanism helps to establish an orderly arrangement of connections between the R axons and lamina neurons, termed a retinotopic map because it results in positioning the lamina neurons in close vicinity to the corresponding R axons. We found that the bHLH-PAS transcription factor Single-minded (Sim) is induced by Hh in the lamina neurons and is required for the association of lamina neurons with R axons. In sim mutant brains, lamina neurons undergo the first step of differentiation but fail to associate with R axons. As a result, lamina neurons are set aside from R axons. The data reveal a novel mechanism for regulation of the interaction between axons and neuronal cell bodies that establishes precise neuronal networks.

Nonvesicular release of acetylcholine is required for axon targeting in the Drosophila visual system

We report evidence for a developmental role of acetylcholine in axon pathfinding in the Drosophila visual system. Acetylcholine was detected on photoreceptor axons during their navigation to target sites in the brain, a time well before the formation of functional synapses. The pattern of photoreceptor axon projections was severely disrupted when acetylcholine synthesis or metabolism was altered or eliminated, or when transgenic alpha-bungarotoxin, a nicotinic acetylcholine receptor antagonist, was expressed in the developing eye or brain. The requirement for acetylcholine signaling exists before photoreceptor neurons form synaptic connections and does not require the function of vesicular acetylcholine transporter protein. That this early effect of acetylcholine is mediated through nonvesicular release is further supported by the observation that transgenic expression of tetanus toxin, a blocker of neurotransmitter release via synaptic vesicles, did not cause similar photoreceptor axon projection defects. These observations support the notion that a form of acetylcholine secretion mediates the behavior of growth cones during axon pathfinding.

Retinal ganglion cell-derived sonic hedgehog signaling is required for optic disc and stalk neuroepithelial cell development

The development of optic stalk neuroepithelial cells depends on Hedgehog (Hh) signaling, yet the source(s) of Hh protein in the optic stalk is unknown. We provide genetic evidence that sonic hedgehog (Shh) from retinal ganglion cells (RGCs) promotes the development of optic disc and stalk neuroepithelial cells. We demonstrate that RGCs express Shh soon after differentiation, and cells at the optic disc in close proximity to the Shh-expressing RGCs upregulate Hh target genes, which suggests they are responding to RGC-derived Shh signaling. Conditional ablation of Shh in RGCs caused a complete loss of optic disc astrocyte precursor cells, resulting in defective axon guidance in the retina, as well as conversion of the neuroepithelial cells in the optic stalk to pigmented cells. We further show that Shh signaling modulates the size of the Pax2(+) astrocyte precursor cell population at the optic disc in vitro. Together, these data provide a novel insight into the source of Hh that promotes neuroepithelial cell development in the mammalian optic disc and stalk.

Axonal transport and neuronal transcytosis of trophic factors, tracers, and pathogens

Neurons can specifically internalize macromolecules, such as trophic factors, lectins, toxins, and other pathogens. Upon internalization in terminals, proteins can move retrogradely along axons, or, upon internalization at somatodendritic domains, they can move into an anterograde axonal transport pathway. Release of internalized proteins from neurons after either retrograde or anterograde axonal transport results in transcytosis and trafficking of proteins across multiple synapses. Recent studies of binding properties of several such proteins suggest that pathogens and lectins may utilize existing transport machineries designed for trafficking of trophic factors. Specific pathways may protect trophic factors, pathogens, and toxins from degradation after internalization and may target the trophic or pathogenic cargo for transcytosis after either retrograde or anterograde transport along axons. Elucidating the molecular mechanisms of sorting steps and transport pathways will further our understanding of trophic signaling and could be relevant for an understanding and possible treatment of neurological diseases such as rabies, Alzheimer's disease, and prion encephalopathies. At present, our knowledge is remarkably sparse about the types of receptors used by pathogens for trafficking, the signals that sort trophins or pathogens into recycling or degradation pathways, and the mechanisms that regulate their release from somatodendritic domains or axon terminals. This review intends to draw attention to potential convergences and parallels in trafficking of trophic and pathogenic proteins. It discusses axonal transport/trafficking mechanisms that may help to understand and eventually treat neurological diseases by targeted drug delivery.

ETS gene Pea3 controls the central position and terminal arborization of specific motor neuron pools

The projection of developing axons to their targets is a crucial step in the assembly of neuronal circuits. In the spinal cord, the differentiation of specific motor neuron pools is associated with the expression of ETS class transcription factors, notably PEA3 and ER81. Their initial expression coincides with the arrival of motor axons in the vicinity of muscle targets and depends on limb-derived signals. We show that in Pea3 mutant mice, the axons of specific motor neuron pools fail to branch normally within their target muscles, and the cell bodies of these motor neurons are mispositioned within the spinal cord. Thus, the induction of an intrinsic program of ETS gene expression by peripheral signals is required to coordinate the central position and terminal arborization of specific sets of spinal motor neurons.

Anterograde axonal transport, transcytosis, and recycling of neurotrophic factors: the concept of trophic currencies in neural networks

Traditional views of neurotrophic factor biology held that trophic factors are released from target cells, retrogradely transported along their axons, and rapidly degraded upon arrival in cell bodies. Increasing evidence indicates that several trophic factors such as brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF-2), glial cell-line derived neurotrophic factor (GDNF), insulin-like growth factor (IGF-I), and neurotrophin-3 (NT-3), can move anterogradely along axons. They can escape the degradative pathway upon internalization and are recycled for future uses. Internalized ligands can move through intermediary cells by transcytosis, presumably by endocytosis via endosomes to the Golgi system, by trafficking of the factor to dendrites or by sorting into anterograde axonal transport with subsequent release from axon terminals and uptake by second- or third-order target neurons. Such data suggest the existence of multiple "trophic currencies," which may be used over several steps in neural networks to enable nurturing relationships between connected neurons or glial cells, not unlike currency exchanges between trading partners in the world economy. Functions of multistep transfer of trophic material through neural networks may include regulation of neuronal survival, differentiation of phenotypes and dendritic morphology, synapse plasticity, as well as excitatory neurotransmission. The molecular mechanisms of sorting, trafficking, and release of trophic factors from distinct neuronal compartments are important for an understanding of neurotrophism, but they present challenging tasks owing to the low levels of the endogenous factors.

The EGF receptor defines domains of cell cycle progression and survival to regulate cell number in the developing Drosophila eye

The number of cells in developing organs must be controlled spatially by extracellular signals. Our results show how cell number can be regulated by cell interactions controlling proliferation and survival in local neighborhoods in the case of the Drosophila compound eye. Intercellular signals act during the second mitotic wave, a cell cycle that generates a pool of uncommitted cells used for most ommatidial fates. We find that G1/S progression to start the cell cycle requires EGF receptor inactivity. EGF receptor activation is then required for progression from G2 to M phase of the same cells, and also prevents apoptosis. EGF receptor activation depends on short-range signals from five-cell preclusters of photoreceptor neurons not participating in the second mitotic wave. Through proliferation and survival control, such signals couple the total number of uncommitted cells being generated to the neural patterning of the retina.