The Drosophila homolog of APP promotes Dscam expression to drive axon terminal growth, revealing interaction between Down syndrome genes

Down syndrome (DS) is caused by triplication of human chromosome 21 (HSA21). Although several HSA21 genes have been found to be responsible for aspects of DS, whether and how HSA21 genes interact with each other is poorly understood. DS patients and animal models present with a number of neurological changes, including aberrant connectivity and neuronal morphology. Previous studies have indicated that amyloid precursor protein (APP) and Down syndrome cell adhesion molecule (DSCAM) regulate neuronal morphology and contribute to neuronal aberrations in DS. Here, we report the functional interaction between the Drosophila homologs of these two genes, Amyloid precursor protein-like (Appl) and Dscam (Dscam1). We show that Appl requires Dscam to promote axon terminal growth in sensory neurons. Moreover, Appl increases Dscam protein expression post-transcriptionally. We further demonstrate that regulation of Dscam by Appl does not require the Appl intracellular domain or second extracellular domain. This study presents an example of functional interactions between HSA21 genes, providing insights into the pathogenesis of neuronal aberrations in DS.

Regulated alternative splicing of Dscam2 is required for somatosensory circuit wiring

Axon and dendrite placement and connectivity is guided by a wide range of secreted and surface molecules in the developing nervous system. Nevertheless, the extraordinary complexity of connections in the brain requires that this repertoire be further diversified to precisely and uniquely regulate cell-cell interactions. One important mechanism for molecular diversification is alternative splicing. Drosophila Down syndrome cell adhesion molecule (Dscam2) undergoes cell type-specific alternative splicing to produce two isoform-specific homophilic binding proteins. Regulated alternative splicing of Dscam2 is important for dendrite and axon patterning, but how this translates to circuit wiring and animal behavior is not well understood. Here, we examined the role of cell-type specific expression of Dscam2 isoforms in regulating synaptic partner selection in the larval somatosensory system. We found that synaptic partners in the nociceptive circuit express different Dscam2 isoforms. Forcing synaptic partners to express a common isoform resulted in nociceptive axon patterning defects and attenuated nocifensive behaviors, indicating that a role for Dscam2 alternative splicing is to ensure that synaptic partners do not express matching isoforms. These results point to a model in which regulated alternative splicing of Dscam2 across populations of neurons restricts connectivity to specific partners and prevents inappropriate synaptic connections.

Dscam1 overexpression impairs the function of the gut nervous system in Drosophila

BACKGROUND

Down syndrome (DS) patients have a 100-fold increase in the risk of Hirschsprung syndrome of the colon and rectum (HSCR), a lack of enteric neurons in the colon. The leading DS candidate gene is trisomy of the Down syndrome cell adhesion molecule (DSCAM).

RESULTS

We find that Dscam1 protein is expressed in the Drosophila enteric/stomatogastric nervous system (SNS). Axonal Dscam1 phenotypes can be rescued equally by diverse isoforms. Overexpression of Dscam1 resulted in frontal and hindgut nerve overgrowth. Expression of dominant negative Dscam1-ΔC led to a truncated frontal nerve and increased branching of the hindgut nerve. Larval locomotion is influenced by feeding state, and we found that the average speed of larvae with Dscam1 SNS expression was reduced, whereas overexpression of Dscam1-ΔC significantly increased the speed. Dscam1 overexpression reduced the efficiency of food clearance from the larval gut.

CONCLUSION

Our work demonstrates that overexpression of Dscam1 can perturb gut function in a model system.

Chelicerata sDscam isoforms combine homophilic specificities to define unique cell recognition

Thousands of Down syndrome cell adhesion molecule (Dscam1) isoforms and ∼60 clustered protocadhrein (cPcdh) proteins are required for establishing neural circuits in insects and vertebrates, respectively. The strict homophilic specificity exhibited by these proteins has been extensively studied and is thought to be critical for their function in neuronal self-avoidance. In contrast, significantly less is known about the Dscam1-related family of ∼100 shortened Dscam (sDscam) proteins in Chelicerata. We report that Chelicerata sDscamα and some sDscamβ protein trans interactions are strictly homophilic, and that the trans interaction is meditated via the first Ig domain through an antiparallel interface. Additionally, different sDscam isoforms interact promiscuously in cis via membrane proximate fibronectin-type III domains. We report that cell-cell interactions depend on the combined identity of all sDscam isoforms expressed. A single mismatched sDscam isoform can interfere with the interactions of cells that otherwise express an identical set of isoforms. Thus, our data support a model by which sDscam association in cis and trans generates a vast repertoire of combinatorial homophilic recognition specificities. We propose that in Chelicerata, sDscam combinatorial specificity is sufficient to provide each neuron with a unique identity for self-nonself discrimination. Surprisingly, while sDscams are related to Drosophila Dscam1, our results mirror the findings reported for the structurally unrelated vertebrate cPcdh. Thus, our findings suggest a remarkable example of convergent evolution for the process of neuronal self-avoidance and provide insight into the basic principles and evolution of metazoan self-avoidance and self-nonself discrimination.

Revisiting Dscam diversity: lessons from clustered protocadherins

The complexity of neuronal wiring relies on the extraordinary recognition diversity of cell surface molecules. Drosophila Dscam1 and vertebrate clustered protocadherins (Pcdhs) are two classic examples of the striking diversity from a complex genomic locus, wherein the former encodes more than 10,000 distinct isoforms via alternative splicing, while the latter employs alternative promoters to attain isoform diversity. These structurally unrelated families show remarkably striking molecular parallels and even similar functions. Recent studies revealed a novel Dscam gene family with tandemly arrayed 5' cassettes in Chelicerata (e.g., the scorpion Mesobuthus martensii and the tick Ixodes scapularis), similar to vertebrate clustered Pcdhs. Likewise, octopus shows a more remarkable expansion of the Pcdh isoform repertoire than human. These discoveries of Dscam and Pcdh diversification reshape the evolutionary landscape of recognition molecule diversity and provide a greater understanding of convergent molecular strategies for isoform diversity. This article reviews new insights into the evolution, regulatory mechanisms, and functions of Dscam and Pcdh isoform diversity. In particular, the convergence of clustered Dscams and Pcdhs is highlighted.

Roles of the Laodelphax striatellus Down syndrome cell adhesion molecule in Rice stripe virus infection of its insect vector

The arthropod Down syndrome cell adhesion molecule (Dscam) mediates pathogen-specific recognition via an extensive protein isoform repertoire produced by alternative splicing. To date, most studies have focused on the subsequent pathogen-specific immune response, and few have investigated the entry into cells of viruses or endosymbionts. In the present study, we cloned and characterized the cDNA of Laodelphax striatellus Dscam (LsDscam) and investigated the function of LsDscam in rice stripe virus (RSV) infection and the influence on the endosymbiont Wolbachia. LsDscam displayed a typical Dscam domain architecture, including 10 immunoglobulin (Ig) domains, six fibronectin type III domains, one transmembrane domain and a cytoplasmic tail. Alternative splicing occurred at the N-termini of the Ig2 and Ig3 domains, the complete Ig7 domain, the transmembrane domain and the C-terminus, comprising 10, 51, 35, two and two variable exons, respectively. Potentially LsDscam could encode at least 71 400 unique isoforms and 17 850 types of extracellular regions. LsDscam was expressed in various L. striatellus tissues. Knockdown of LsDscam mRNA via RNA interference decreased the titres of both RSV and Wolbachia, but did not change the numbers of the extracellular symbiotic bacterium Acinetobacter rhizosphaerae. Specific Dscam isoforms may play roles in enhancing the infection of vector-borne viruses or endosymbionts.

Dscam and pancrustacean immune memory - a review of the evidence

Evidence is accumulating for a memory-like phenomenon in the immune defence of invertebrates. Down syndrome cell adhesion molecule (Dscam) has been proposed as a key candidate for a somatically diversified receptor system in the crustaceans and insects (Pancrustacea) that could enable challenge-specific protection. However, what is the evidence for an involvement of Dscam in pancrustacean immune memory, and in particular specificity? Here we review the current state of the art, and discuss hypotheses of how Dscam could be involved in immunity. We conclude that while there is increasing evidence for the involvement of Dscam in pancrustacean immunity, crucial experiments to address whether it plays a role in specificity upon secondary encounter with a pathogen still remain to be done.

WSSV-induced crayfish Dscam shows durable immune behavior

One of the major gaps in our understanding of arthropod specific immune priming concerns the mechanism[s] by which the observed long-term (>2 weeks) protective effects might be mediated. Hypervariable Dscam (Down syndrome cell adhesion molecule) might support arthropod innate immunity with specificity for more extended periods. We show here that, in the relatively long-lived arthropod Cherax quadricarinatus, CqDscam does not behave like a typical, immediately-acting, short-lived innate immune factor: CqDscam was not induced within hours after challenge with a lethal virus, but instead was only up-regulated after 2-5 days. This initial response faded within ∼ 2 weeks, but another maximum was reached ∼ 1 month later. At around 2 months after the initial challenge, the virus-induced CqDscam bound to the virus virion and acted to neutralize the virus However, although CqDscam helped crayfish to survive during persistent infection, it nevertheless failed to provide any enhanced protection against a subsequent WSSV challenge. Thus, CqDscam is capable of supporting extended anti-virus immune memory in arthropods. Also, during a persistent virus infection, the balance of "immune firepower" in crayfish appears to be altered such that the general immune factors become depleted while CqDscam becomes relatively predominant.

Properties of Litopenaeus vannamei Dscam (LvDscam) isoforms related to specific pathogen recognition

Arthropod Down syndrome cell adhesion molecules (Dscam) may sometimes function as hypervariable pathogen recognition receptors. They consist of an extracellular region and a cytoplasmic tail, both of which are highly variable. In shrimp, tail-less Dscam proteins (Dscams) have recently been identified, and these appear to be unlike other arthropod extracellular Dscams that are released from the cell membrane by proteolytic cleavage. Here we investigate the properties of these unique shrimp proteins and show that they can be directly secreted from transfected cells. We also investigate the diverse cytoplasmic tail variants of membrane-bound shrimp Dscams, and show that elements E1A and E3 seem to be related to Dscam immune function. Challenge with Vibrio harveyi not only enhanced total Dscam and the immune-related cytoplasmic tail variants, but also induced expression of certain Ig2 + Ig3 combinations. A pathogen binding assay with these Ig2 + Ig3 extracellular variants showed that both the V. harveyi-induced Dscams and Dscams induced by buffer injection could be either pathogen-specific or specific only for Gram-negative pathogens, while other "general" Dscam variants were sensitive to a wide range of pathogens. The same assay also suggested that shrimp Dscam isoforms show a stronger response to the host's natural pathogens.

Tyrosine phosphorylation is essential for DSCAML1 to promote dendrite arborization of mouse cortical neurons

Dendritic self-avoidance is critical for appropriate dendrite arborization. We herein examined the role of Down syndrome cell adhesion molecule like-1 (DSCAML1) in regulating dendritic self-avoidance and that of tyrosine phosphorylation in mediating the effects of DSCAML1. Knocking down DSCAML1 in newborn mouse cortical neurons compromised dendritic self-avoidance as evidenced by dendritic fasciculation and increased dendritic self-crossing. Introduction of a DSCAML1(Y1808F) mutant into the DSCAML1-knocked down neurons failed to reverse the abnormal dendritic arborization. These results suggest that DSCAML1 promotes dendritic self-avoidance in cortical neurons, and that phosphorylation at Y1808 is essential in mediating the effects of DSCAML1.