Gene name - Neuroglian
Cytological map position - 7F1
Function - adhesion
Keywords - neural
Symbol - Nrg
Genetic map position - 1-23.6
Classification - Ig and fibronectin superfamily
Cellular location - transmembrane - surface
|Recent literature||Kakad, P. P., Penserga, T., Davis, B. P., Henry, B., Boerner, J., Riso, A., Pielage, J. and Godenschwege, T. A. (2018). An ankyrin-binding motif regulates nuclear levels of L1-type neuroglian and expression of the oncogene Myc in Drosophila neurons. J Biol Chem. PubMed ID: 30257867
L1 cell adhesion molecule (L1CAM) is well known for its importance in nervous system development and cancer progression. In addition to its role as a plasma membrane protein in cytoskeletal organization, recent in vitro studies have revealed that both transmembrane and cytosolic fragments of proteolytically cleaved vertebrate L1CAM translocate to the nucleus. In vitro studies indicate that nuclear L1CAM affects genes with functions in DNA post-replication repair, cell cycle control, and cell migration and differentiation, but its in vivo role and how its nuclear levels are regulated is less well understood. This study reports that mutations in the conserved ankyrin-binding domain affect nuclear levels of the sole Drosophila homolog neuroglian (Nrg) and that it also has a noncanonical role in regulating transcript levels of the oncogene Myc in the adult nervous system. Altered nuclear levels of Nrg correlate with altered transcript levels of Myc in neurons similar to what has been reported for human glioblastoma stem cells. However, whereas previous in vitro studies suggest that increased nuclear levels of L1CAM promote tumor cell survival, this study found that elevated levels of nuclear Nrg in neurons are associated with increased sensitivity to oxidative stress and reduced lifespan of adult animals. It is therefore concluded that these findings are of potential relevance to the management of neurodegenerative diseases associated with oxidative stress and cancer.
|Yang, W. K., Chueh, Y. R., Cheng, Y. J., Siegenthaler, D., Pielage, J. and Chien, C. T. (2019). Epidermis-derived L1CAM homolog Neuroglian mediates dendrite enclosure and blocks heteroneuronal dendrite bundling. Curr Biol. PubMed ID: 31006568
Building sensory dendritic arbors requires branching, growth, spacing, and substrate support. The conserved L1CAM family of cell-adhesion molecules generates neuronal isoforms to regulate neurite development in various aspects. However, whether non-neuronal isoforms participate in any of these aspects is unclear. In Drosophila, the L1CAM homolog Neuroglian (Nrg) is expressed as two isoforms: the neuronal isoform Nrg180 on dendritic surfaces of dendritic arborization (da) neurons and the non-neuronal isoform Nrg167 in epidermis innervated by dendrites. Epidermal Nrg167 encircles dendrites by interactions with dendritic Nrg180 to support dendrite growth, stabilization, and enclosure inside epidermis. Interestingly, whereas Nrg180 forms homophilic interactions to facilitate axonal bundling, heteroneuronal dendrites in the same innervating field avoid bundling through unknown mechanisms to maintain individual dendritic patterns. This study shows that both epidermal Nrg167 depletion and neuronal Nrg180 overexpression can cause dendrite bundling, with genetic analyses suggesting that Nrg167-Nrg180 interactions antagonize Nrg180-Nrg180 homophilic interaction to prevent dendrite bundling. Furthermore, internalization of Nrg180 also participates in resolving dendrite bundling, as overexpression of endocytosis-defective Nrg180 and compromising endocytosis in neurons both exacerbated dendrite-bundling defects. Together, this study highlights the functional significance of substrate-derived Nrg167 in conferring dendrite stability, positioning, and avoidance.
|Kaur, R., Surala, M., Hoger, S., Grossmann, N., Grimm, A., Timaeus, L., Kallina, W. and Hummel, T. (2019). Pioneer interneurons instruct bilaterality in the Drosophila olfactory sensory map. Sci Adv 5(10): eaaw5537. PubMed ID: 31681838
Interhemispheric synaptic connections, a prominent feature in animal nervous systems for the rapid exchange and integration of neuronal information, can appear quite suddenly during brain evolution, raising the question about the underlying developmental mechanism. This study showed in the Drosophila olfactory system that the induction of a bilateral sensory map, an evolutionary novelty in dipteran flies, is mediated by a unique type of commissural pioneer interneurons (cPINs) via the localized activity of the cell adhesion molecule Neuroglian. Differential Neuroglian signaling in cPINs not only prepatterns the olfactory contralateral tracts but also prevents the targeting of ingrowing sensory axons to their ipsilateral synaptic partners. These results identified a sensitive cellular interaction to switch the sequential assembly of diverse neuron types from a unilateral to a bilateral brain circuit organization.
|Liu, Z., Chen, Y. and Rao, Y. (2020). An RNAi screen for secreted factors and cell-surface players in coordinating neuron and glia development in Drosophila. Mol Brain 13(1): 1. PubMed ID: 31900209
The establishment of the functional nervous system requires coordinated development of neurons and glia in the embryo. Our understanding of underlying molecular and cellular mechanisms, however, remains limited. The developing Drosophila visual system is an excellent model for understanding the developmental control of the nervous system. By performing a systematic transgenic RNAi screen, this study investigated the requirements of secreted proteins and cell-surface receptors for the development of photoreceptor neurons (R cells) and wrapping glia (WG) in the Drosophila visual system. From the screen, w seven genes whose knockdown disrupted the development of R cells and/or WG were identified, including amalgam (ama), domeless (dome), epidermal growth factor receptor (EGFR), kuzbanian (kuz), N-Cadherin (CadN), neuroglian (nrg), and shotgun (shg). Cell-type-specific analysis revealed that ama is required in the developing eye disc for promoting cell proliferation and differentiation, which is essential for the migration of glia in the optic stalk. These results also suggest that nrg functions in both eye disc and WG for coordinating R-cell and WG development.
|Rui, M., Bu, S., Chew, L. Y., Wang, Q. and Yu, F. (2020). The membrane protein Raw regulates dendrite pruning via the secretory pathway. Development. PubMed ID: 32928906
Neuronal pruning is essential for proper wiring of the nervous systems in invertebrates and vertebrates. Drosophila ddaC sensory neurons selectively prune their larval dendrites to sculpt the nervous system during early metamorphosis. However, the molecular mechanisms underlying ddaC dendrite pruning remain elusive. Here, this study has identified an important and cell-autonomous role of the membrane protein Raw in dendrite pruning of ddaC neurons. Raw appears to regulate dendrite pruning via a novel mechanism, which is independent of JNK signaling. Importantly, Raw was shown to promote endocytosis and downregulation of the conserved L1-type cell-adhesion molecule Neuroglian (Nrg) prior to dendrite pruning. Moreover, Raw is required to modulate the secretory pathway by regulating the integrity of secretory organelles and efficient protein secretion. Mechanistically, Raw facilitates Nrg downregulation and dendrite pruning in part through regulation of the secretory pathway. Thus, this study reveals a JNK-independent role of Raw in regulating the secretory pathway and thereby promoting dendrite pruning.
|Clements, J., Buhler, K., Winant, M., Vulsteke, V. and Callaerts, P. (2021). Glial and Neuronal Neuroglian, Semaphorin-1a and Plexin A Regulate Morphological and Functional Differentiation of Drosophila Insulin-Producing Cells. Front Endocrinol (Lausanne) 12: 600251. PubMed ID: 34276554
The insulin-producing cells (IPCs), a group of 14 neurons in the Drosophila brain, regulate numerous processes, including energy homeostasis, lifespan, stress response, fecundity, and various behaviors, such as foraging and sleep. Despite their importance, little is known about the development and the factors that regulate morphological and functional differentiation of IPCs. This study describes the use of a new transgenic reporter to characterize the role of the Drosophila L1-CAM homolog Neuroglian (Nrg), and the transmembrane Semaphorin-1a (Sema-1a) and its receptor Plexin A (PlexA) in the differentiation of the insulin-producing neurons. Loss of Nrg results in defasciculation and abnormal neurite branching, including ectopic neurites in the IPC neurons. Cell-type specific RNAi knockdown experiments reveal that Nrg, Sema-1a and PlexA are required in IPCs and glia to control normal morphological differentiation of IPCs albeit with a stronger contribution of Nrg and Sema-1a in glia and of PlexA in the IPCs. These observations provide new insights into the development of the IPC neurons and identify a novel role for Sema-1a in glia. In addition, this study shows that Nrg, Sema-1a and PlexA in glia and IPCs not only regulate morphological but also functional differentiation of the IPCs and that the functional deficits are likely independent of the morphological phenotypes. The requirements of nrg, Sema-1a, and PlexA in IPC development and the expression of their vertebrate counterparts in the hypothalamic-pituitary axis, suggest that these functions may be evolutionarily conserved in the establishment of vertebrate endocrine systems.
|Resnik-Docampo, M., Cunningham, K. M., Ruvalcaba, S. M., Choi, C., Sauer, V. and Jones, D. L. (2021). Neuroglian regulates Drosophila intestinal stem cell proliferation through enhanced signaling via the epidermal growth factor receptor. Stem Cell Reports. PubMed ID: 33961791
The Drosophila intestine is an excellent system for elucidating mechanisms regulating stem cell behavior. This study shows that the septate junction (SJ) protein Neuroglian (Nrg) is expressed in intestinal stem cells (ISCs) and enteroblasts (EBs) within the fly intestine. SJs are not present between ISCs and EBs, suggesting Nrg plays a different role in this tissue. This study reveals that Nrg is required for ISC proliferation in young flies, and depletion of Nrg from ISCs and EBs suppresses increased ISC proliferation in aged flies. Conversely, overexpression of Nrg in ISC and EBs promotes ISC proliferation, leading to an increase in cells expressing ISC/EB markers; in addition, an increase was observed in epidermal growth factor receptor (Egfr) activation. Genetic epistasis experiments reveal that Nrg acts upstream of Egfr to regulate ISC proliferation. As Nrg function is highly conserved in mammalian systems, this work characterizing the role of Nrg in the intestine has implications for the treatment of intestinal disorders that arise due to altered ISC behavior.
Many cell surface proteins function in communication both between cells and between cell exterior and interior. To what degree are such molecules evolutionarily conserved, particularly those that also function in neural cell adhesion? Neuroglian is a cell surface transmembrane protein that functions in adhesion. It is found on neuronal axons, but not cell bodies, and on non-neuronal tissues such as trachea, hindgut, salivary glands and muscle. Neuroglian has extensive homology to a vertebrate neural cell adhesion molecule. This homology includes an extracellular immunoglobulin motif and an intracellular domain involved in communication with the cytoskeleton.
The conservation of neural extracellular matrix and cell-cell recognition molecules should further an understanding of their function and development. In this case Neuroglian is used in several contexts, with alternatively spliced varients. Such a system prefigures the complexity of the human nervous system.
The cytoplasmic domain of Neuroglian is not required for homophilic adhesive qualities. An artificial Neuroglian protein form was constructed by substituting the Neuroglian transmembrane segment and cytoplasmic domains with the sugar-lipid attachment signal of Fasciclin I. In essence a Neuroglian protein lacking an intracellular domain was created. This artificial Neuroglian molecule retains the ability to induce homophilic cell aggregation when expressed in tissue culture cells, and is able to interact with naturally occurring Neuroglian proteins. These results demonstrate that Neuroglian mediates a calcium-independent, homophilic cell adhesion activity and that neither cytoplasmic Neuroglian domains nor a direct interaction with cytoskeletal elements is essential for adhesion (Hortsch, 1995).
Loss of Discs large (DLG), a protein required for septate junction structure, cell polarity, and proliferation control in Drosophila epithelia, affects the distribution of Fasciclin III and Neuroglian, two transmembrane proteins thought to be involved in cell adhesion. Fasciclin III is highly enriched at the septate junction and is present in lower amounts in the lateral cell membrane, but is excluded from the adherens junction. Neuroglian is enriched at the apical end of the cell, reduced in the septate junction, and again on the rest of the lateral cell membrane. Localization of Fas III and Neuroglian in both salivary glands and imaginal discs is dependent on DLG. When septate junctions are completely eliminated in dlg mutants, both proteins are found apparently unrestricted along the cell membrane. In fact Neuroglian appears to have an elevated level of expression compared with wild type, while FAS III levels are reduced (Woods, 1996).
Neuroglian can transmit positional information directly to Ankyrin and thereby polarize its distribution in Drosophila tissue culture cells. The accumulation of Ankyrin at cell contacts requires the presence of the cytoplasmic domain of Neuroglian. A direct interaction between Neuroglian and Ankyrin can be demonstrated using yeast two-hybrid analysis. Thus, Neuroglian appears to be activated by extracellular adhesion so that Ankyrin and the membrane skeleton selectively associate with sites of contact and not with other regions of the plasma membrane (Dubreuil, 1996).
Expression of Neuroglian in Drosophila S2 tissue culture cells results in a selective recruitment of ankyrin and spectrin to sites of cell contacts. Ankyrin recruitment is strictly limited to cell contacts, even though Neuroglian is abundantly expressed over the entire cell surface. Thus, neuroglian can function as a signaling molecule that transmits the positional value of cell adhesion to the cytoplasmic assembly of ankyrin and spectrin. This outside-in signaling function appears to be conserved among L1 family members, since expression of human L1 in S2 cells also results in the assembly of ankyrin at cell contact sites. The adhesion-induced rearrangement of ankyrin and spectrin can be conveyed to other membrane proteins that interact with ankyrin and spectrin and might thereby provide a mechanism for the assembly of unique plasma membrane subdomains. For example, the NaK-ATPase, which is known to interact with ankyrin in vertebrates, is found to accumulate along with spectrin and ankyrin at sites of neuroglian-mediated adhesion in S2 cells. Thus, L1-mediated adhesion events result in a reorganization and compartmentalization of the plasma membrane, which may constitute an important biological function of L1 family members (Hortsch, 1998a and references).
Expression of the Drosophila cell adhesion molecule Neuroglian in S2 cells leads to cell aggregation and the intracellular recruitment of ankyrin to cell contact sites. The region of Neuroglian that interacts with ankyrin has been localized and the mechanism that limits this interaction to cell contact sites has been investigated. Yeast two-hybrid analysis and expression of Neuroglian deletion constructs in S2 cells have identified a conserved 36-amino acid sequence that is required for ankyrin binding. Mutation of a conserved tyrosine residue within this region reduces ankyrin binding and extracellular adhesion. However, residual recruitment of ankyrin by this mutant Neuroglian molecule is still limited to cell contacts, indicating that the lack of ankyrin binding at noncontact sites is not caused by tyrosine phosphorylation. A chimeric molecule, in which the extracellular domain of Neuroglian is replaced with the corresponding domain from the adhesion molecule fasciclin II, also selectively recruits ankyrin to cell contacts. Thus, outside-in signaling by Neuroglian in S2 cells depends on extracellular adhesion, but does not depend on any unique property of its extracellular domain. It is proposed that the recruitment of ankyrin to cell contact sites depends on a physical rearrangement of neuroglian in response to cell adhesion, and that ankyrin binding plays a reciprocal role in stabilizing the adhesive interaction (Hortsch, 1998a).
Cell adhesion molecules (CAMs) implement the process of axon guidance by promoting specific selection and attachment to substrates. In Drosophila, loss-of-function conditions of either the Neuroglian CAM, the FGF receptor coded by the gene heartless, or the EGF receptor coded by Egfr display a similar phenotype of abnormal substrate selection and axon guidance by peripheral sensory neurons. Moreover, neuroglian loss-of-function phenotype can be suppressed by the expression of gain-of-function conditions of heartless or Egfr. The results are consistent with a scenario where the activity of these receptor tyrosine kinases is controlled by Neuroglian at choice points where sensory axons select between alternative substrates for extension (Garcia-Alonso, 2000).
The ocellar sensory system (OSS) offers a simple scenario for the study of axon guidance at the cellular level. During axon guidance, growth cones make decisions at choice points. In order to change trajectories at these choice points, it is assumed that signal transduction mechanisms should operate to transform specific extracellular information in the modulation of their actin cytoskeleton. In the OSS, the initial decision to attach or not to attach to the head epithelium appears to be a key choice (at the first choice point) for two types of sensory axons as they navigate to their respective targets in the brain. Due to the process of head eversion, ocellar pioneer (OP) axons must navigate in the extracellular matrix (ECM), free of adhesion to the underlying epidermis. Reciprocally, bristle mechanosensory (BM) axons should follow the epidermis before and after head eversion, since they do not reach the brain until this later stage. Should BM axons initially extend apart from the epithelium, they might possibly be unable to follow a physical substrate toward their brain targets after head eversion. Therefore, the process of head eversion establishes a constraint that prevents substrate redundancy between ECM and epithelium. OSS axons must make a second decision in order to leave the surface of the head and project to the brain (at the second choice point). OP axons leave the ECM surrounding the head capsule toward the brain before head eversion. In contrast, BM axons leave the head's internal epithelial surface after head eversion, when several BM axons have converged together. Each of these decision processes are abnormal in nrg, htl, and Egfr mutant individuals. OP axons can decide to attach to the epithelium, preventing them from reaching the brain. OP axons can fail to leave the internal surface of the head (even when extending free of epithelial attachment) and project to ectopic positions within the head after eversion. BM axons can be found extending, although they are abnormally separated from the epidermis after head eversion, suggesting a failure of attachment to the epithelium before head eversion. BM axons also can stall in the epidermis, suggesting that Nrg may also promote axon extension. Finally, BM axons can fail to leave the epidermis toward the brain, as they should, remaining instead within the epidermal layer, where they project in abnormal directions. In such cases, they can sometimes be observed to perforate the epidermis and project outside of the head, suggesting that BM axons navigate in the epithelial surface, using proteases to facilitate their movements. In contrast, when attached to the epithelium, OP axons seem to have difficulty extending properly. One possibility is that extension in the epithelium requires this perforating activity that BM axons have and that may be missing in OP axons that normally extend in the ECM (Garcia-Alonso, 2000).
Since nrg, htl, and Egfr mutants exhibit similar OSS axon phenotypes, it seems possible that they function in a common mechanism during OSS axon guidance. In order to test genetically if Nrg behaves as an upstream regulator of the Fgfr and the Egfr, different double mutant combinations were constructed between nrg and gain-of-function conditions for htl and Egfr (which should function independently of upstream regulation). Gain-of-function conditions of htl can partially suppress the nrg phenotype. Elp alleles behave as gain-of-function conditions of Egfr and behave as strong suppressors of nrg OSS axon phenotype. Therefore, although it is still possible that Nrg could also perform some role based on pure adhesion, the results are most consistent with the idea that both Htl and Egfr mediate Nrg function in OSS neurons (Garcia-Alonso, 2000).
Htl and Egfr exhibit some specificity in their effects on OSS axon guidance. Htl seems to be preferentially required by OP axons and BM axons to project to the brain, while Egfr seems to be more involved in BM axon attachment and extension in the epithelium. In contrast to in vitro studies in vertebrates, no evidence has been found that Htl promotes axon growth. Rather, in the Drosophila OSS, it seems that the Egfr is preferentially required for outgrowth. This discrepancy with the vertebrate in vitro studies could be explained if the Drosophila in vivo situation were more prone to the deployment of compensatory molecular interactions (which might mask a role of Htl in axon growth) than the in vitro situation. In such a case, a deficit of one RTK could be partially compensated by an increase in the activity of the other RTK. This could happen if, for example, different RTKs were regulated through a common negative feedback loop. This explanation would also help explain the presence of a mild phenotype in consititutively active lambda-Htl individuals and would account for those cases in which some nrg OSS alteration is enhanced by an increase or suppressed by a reduction in RTK activity. This explanation is also consistent with the lack of effect of the gain of function of one RTK over the loss of function of the other (since this condition would itself increment the activity of the former RTK) (Garcia-Alonso, 2000).
It is likely that the RTKs also mediate the function of signals other than Nrg during OSS axon guidance. This is suggested by the fact that the nrg phenotype is weaker than the RTK phenotype and by the fact that it can be enhanced by a reduction of 50% in the amount of Egfr. These signals could represent other CAMs or growth factor ligands diffusing from the brain. One suggestive possibility is that OP and BM axons depart from the head capsule in response to some as yet unidentified diffusible signal from their brain targets (acting in addition to Nrg signaling). Further studies will be necessary to evaluate this possibility (Garcia-Alonso, 2000).
What would be the signal that triggers Nrg-dependent RTK activation? Nrg, like its vertebrate homolog L1, can behave as a homophilic CAM. It has been proposed that the homophilic interaction of L1 would activate the function of the Fgfr. Therefore, it is proposed that the homophilic interaction between Nrg180 (the neural-specific form) molecules would give a positive input on RTK activity in both OP axons (from the beginning of axon extension) and BM axons (after several mechanosensory axons have converged), which would signal the axons to lift off from the epidermis and project to brain targets. However, BM axons initially extend as single processes and interact with the Nrg167 form in the epithelium, where this interaction would result in the activation of the RTKs, and RTK signaling would promote extension in the epidermis. This shift in the RTK's activity outcome might be caused by the involvement of some other molecule specifically interacting with Nrg167. In agreement with this idea, rescue experiments of nrg using Nrg180 reveal that BM axon extension on the epidermis cannot be implemented by this molecular form. Since homophilic binding between the different Nrg forms is possible, this high degree of specificity suggests the existence of additional molecules (specifically interacting with Nrg167) that mediate BM axon association with the epithelium. Some observations are consistent with this model. (1) OP axons fasciculate with one another (50 or so per ocellus) from the very beginning of axon extension. These axons fasciculate together due to the presence of Neurotactin and other CAMs in the membrane. Defasciculation of OP axons (caused by a lack of Neurotactin) increases the chances that OP axons extend abnormally in the epidermis. These results suggest that fasciculation helps generate a robust process of axon guidance. In this model, defasciculation would reduce the probability of Nrg180Nrg180 interactions between OP axons, producing a deficit of RTK activity, therefore, making them more likely to extend attached to the epidermis. (2) BM axons initially follow the epidermis in isolation from other axons but begin to converge as they approach the dorsal antennal field. After several BM axons have converged together, the BM fascicle lifts off from the epidermis. Thus, the signal for BM nerves to leave the epidermis might be a given threshold of Nrg180Nrg180 interactions between the different BM axons. If this model is correct, the specificity of the OP and BM growth cone interaction with the epidermis would reside in the way Nrg167 and Nrg180 differ in their interactions with other molecule(s). Nrg167 differs from Nrg180 in the cytoplasmic domain. It has been previously shown that the cytoplasmic domain can regulate the adhesive properties of the extracellular part of the protein. Therefore, it is possible that some CAM molecule might specifically interact with Nrg167 to help promote initial RTK activation and attachment to the epithelium in BM axons. This molecule could represent the Drosophila homolog of some of the known vertebrate heterophilic partners of L1 (Garcia-Alonso, 2000).
In summary, these results strongly support the idea that Neuroglian functions in axon guidance by regulating the activity of both the Fgfr and the Egfr and suggest a scenario where other CAMs and growth factor activities would act in concert with Neuroglian for RTK regulation. In addition, the possible existence of cross-regulatory circuits between RTKs would add another level of control that might help to understand the high degree of canalization displayed by the axon guidance process from Drosophila to mammals (Garcia-Alonso, 2000).
Pruning of unnecessary axons and/or dendrites is crucial for maturation of the nervous system. However, little is known about cell adhesion molecules (CAMs) that control neuronal pruning. In Drosophila, dendritic arborization neurons, ddaCs, selectively prune their larval dendrites. This study reports that Rab5/ESCRT-mediated endocytic pathways are critical for dendrite pruning. Loss of Rab5 or ESCRT function leads to robust accumulation of the L1-type CAM Neuroglian (Nrg) on enlarged endosomes in ddaC neurons. Nrg is localized on endosomes in wild-type ddaC neurons and downregulated prior to dendrite pruning. Overexpression of Nrg alone is sufficient to inhibit dendrite pruning, whereas removal of Nrg causes precocious dendrite pruning. Epistasis experiments indicate that Rab5 and ESCRT restrain the inhibitory role of Nrg during dendrite pruning. Thus, this study demonstrates the cell-surface molecule that controls dendrite pruning and defines an important mechanism whereby sensory neurons, via endolysosomal pathway, downregulate the cell-surface molecule to trigger dendrite pruning (Zhang, 2014).
Endocytic pathways profoundly regulate turnover and homeostasis of various cell-surface adhesion proteins and guidance receptors in the developing nervous systems. Perturbation of endocytic pathways often leads to a variety of neurodegenerative diseases, such as frontotemporal dementia, amyotrophic lateral sclerosis, Alzheimer's disease, lysosomal storage diseases, and Niemann-Pick disease. In Drosophila, the endolysosomal pathway is activated in neighboring glia to engulf degenerating axon/dendrite fragments for their subsequent breakdown during pruning, suggesting a non-cell-autonomous role. This study reports that Rab5 and the ESCRT complexes, two key endocytic regulators, cell autonomously promote dendrite pruning in ddaC neurons. Consistent with these findings, the endocytic pathways also play a cell-autonomous role in axon pruning of MB γ neurons. How do Rab5/ESCRT-dependent endocytic pathways facilitate dendrite pruning in ddaC neurons at the cellular level? This study has identified a cell-surface adhesion protein, namely the L1-type CAM Nrg, as a target of Rab5/ESCRT-dependent endocytic pathways (Zhang, 2014).
Drosophila Nrg and the mammalian L1-type CAMs regulate axonal growth and guidanc, synaptic stability and function, and axon/dendrite morphogenesis. Mutations in the human L1 CAM gene have been reported to cause a broad spectrum of neuronal disorders. This study has identified the Drosophila L1-type CAM Nrg as the key cell-surface molecule that inhibits dendrite pruning in ddaC neurons. The extracellular domains of the L1-type CAMs can regulate cell-cell adhesion via homophilic and/or heterophilic interactions, whereas their intracellular domains can link the proteins with F-actin/spectrins to stabilize the cytoskeletal architecture. In C. elegans, a ligand-receptor complex of cell adhesion molecules containing the nematode Nrg homolog controls dendrite-substrate adhesion to stabilize and pattern dendritic arbors in certain sensory neurons. In Drosophila, Nrg-mediated cell adhesion plays an essential role in stabilizing synapse growth and maintenance at the larval neuromuscular junction. Likewise, Nrg may also mediate adhesion of the dendrites to their adjacent epidermis to stabilize the dendritic architecture in ddaC sensory neurons, whereas downregulation of Nrg may reduce dendritic adhesion/stability and result in disassembly of dendrites. Consistent with the potential adhesive role, structure-function analysis indicates that the ECD of Nrg is important for its function in stabilizing dendrite and/or inhibiting dendrite pruning in ddaC neurons. The fact that overexpression of the ICD-deleted Nrg protein partially rescued the nrg14 mutant phenotype suggests that, in addition to the adhesion function of the ECD, the ICD of Nrg may recruit cytoskeletal components to stabilize dendritic branches in ddaC neurons. The model is therefore favored that the adhesive role of Nrg is a potential mechanism for inhibiting pruning in ddaC sensory neurons (Zhang, 2014).
Another major class of CAMs, integrins, were shown to regulate dendrite-substrate interactions and anchor ddaC dendritic arbors to the extracellular matrix. However, unlike Nrg, integrins do not accumulate on enlarged endosomes in Rab5 or ESCRT ddaC neurons, implying that integrins are not regulated by Rab5/ESCRT-dependent endocytic pathways in ddaC neurons. Moreover, other cell-surface molecules Robo and N-Cad, albeit regulated by the endolysosomal pathway in motor neurons, photoreceptors, or sensory neurons, are dispensable for normal progression of dendrite pruning in ddaC neurons. Thus, this study highlights an important role of the L1-type CAM Nrg in inhibiting dendrite pruning of ddaC sensory neurons (Zhang, 2014).
Interestingly, loss of nrg function causes precocious dendrite pruning without affecting the axonal integrity and connectivity in ddaC neurons, underscoring a specific requirement of Nrg in stabilizing the dendrites, but not the axons. Downregulation of Nrg may reduce dendritic adhesive properties of ddaC sensory neurons and thereby make the dendritic architecture more susceptible to pruning. It is conceivable that Nrg-independent mechanisms may be utilized to protect the axonal structure from the pruning machinery in ddaC neurons. Moreover, both nrg loss of function and gain of function did not affect axon pruning in MB γ neurons (data not shown), further supporting the conclusion that Nrg plays a specific role in dendrite pruning in ddaC sensory neurons. Future studies may elucidate whether and how Nrg mediates its dendritic adhesive properties to inhibit dendrite pruning (Zhang, 2014).
In summary, this study shows that Rab5/ESCRT-dependent endocytic pathways facilitate dendrite pruning of ddaC neurons by downregulating the Drosophila L1-type CAM Nrg during early metamorphosis. This study also demonstrated the role of the cell-surface adhesion protein Nrg in inhibiting dendrite pruning in ddaC sensory neurons. Thus, this study opens the door for further studies of the functions of cell-surface molecules in the regulation of dendritic adhesion during neuronal remodeling (Zhang, 2014).
Bases in 3' UTR - 117
To identify neuronal cell surface glycoproteins in the Drosophila embryo, antisera against horseradish peroxidase (HRP) was used to recognize a carbohydrate epitope that is selectively expressed in the insect nervous system. A large number of neuronal glycoproteins (denoted "HRP proteins") apparently bear the HRP carbohydrate epitope. Polyclonal anti-HRP antibodies were used to purify these proteins from Drosophila embryos. Three major HRP proteins are Neurotactin, Fasciclin I, and an R-PTP, DPTP69D. Western blotting data suggest that Fasciclin II, Neuroglian, DPTP10D, and DPTP99A are also HRP proteins (Desai, 1994).
Neuroglian has six immunoglobulin C2-type domains followed by five fibronectin type III domains (Bieber, 1989).
There are two adjacent fibronectin type III repeats in Neuroglian. Each domain consists of two antiparallel beta sheets and is folded topologically identically to fibronectin type III domains of Tenascin and Fibronectin. The hydrophobic interdomain interface includes a predicted metal-binding site, presumably involved in stabilizing the relative orientation between domains. The predicted metal binding domain is also present in the vertebrate homolog molecule L1. The Neuroglian domains are related by a near perfect 2-fold screw axis along the longest molecular dimension (Huber, 1994).
date revised: 15 October 2021
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