Dscam-mediated self- versus non-self-recognition by individual neurons

DSCAM-mediated control of dendritic and axonal arbor outgrowth enforces tiling and inhibits synaptic plasticity

Mature mammalian neurons have a limited ability to extend neurites and make new synaptic connections, but the mechanisms that inhibit such plasticity remain poorly understood. Here, we report that OFF-type retinal bipolar cells in mice are an exception to this rule, as they form new anatomical connections within their tiled dendritic fields well after retinal maturity. The Down syndrome cell-adhesion molecule (Dscam) confines these anatomical rearrangements within the normal tiled fields, as conditional deletion of the gene permits extension of dendrite and axon arbors beyond these borders. Dscam deletion in the mature retina results in expanded dendritic fields and increased cone photoreceptor contacts, demonstrating that DSCAM actively inhibits circuit-level plasticity. Electrophysiological recordings from Dscam-/- OFF bipolar cells showed enlarged visual receptive fields, demonstrating that expanded dendritic territories comprise functional synapses. Our results identify cell-adhesion molecule-mediated inhibition as a regulator of circuit-level neuronal plasticity in the adult retina.

A large family of Dscam genes with tandemly arrayed 5' cassettes in Chelicerata

Drosophila Dscam1 (Down Syndrome Cell Adhesion Molecules) and vertebrate clustered protocadherins (Pcdhs) are two classic examples of the extraordinary isoform diversity from a single genomic locus. Dscam1 encodes 38,016 distinct isoforms via mutually exclusive splicing in D. melanogaster, while the vertebrate clustered Pcdhs utilize alternative promoters to generate isoform diversity. Here we reveal a shortened Dscam gene family with tandemly arrayed 5' cassettes in Chelicerata. These cassette repeats generally comprise two or four exons, corresponding to variable Immunoglobulin 7 (Ig7) or Ig7-8 domains of Drosophila Dscam1. Furthermore, extraordinary isoform diversity has been generated through a combination of alternating promoter and alternative splicing. These sDscams have a high sequence similarity with Drosophila Dscam1, and share striking organizational resemblance to the 5' variable regions of vertebrate clustered Pcdhs. Hence, our findings have important implications for understanding the functional similarities between Drosophila Dscam1 and vertebrate Pcdhs, and may provide further mechanistic insights into the regulation of isoform diversity.

Novel axon projection after stress and degeneration in the Dscam mutant retina

The Down syndrome cell adhesion molecule gene (Dscam) is required for normal dendrite patterning and promotes developmental cell death in the mouse retina. Loss-of-function studies indicate that Dscam is required for refinement of retinal ganglion cell (RGC) axons in the lateral geniculate nucleus, and in this study we report and describe a requirement for Dscam in the maintenance of RGC axon projections within the retina. Mouse Dscam loss of function phenotypes related to retinal ganglion cell axon outgrowth and targeting have not been previously reported, despite the abundance of axon phenotypes reported in Drosophila Dscam1 loss and gain of function models. Analysis of the Dscam deficient retina was performed by immunohistochemistry and Western blot analysis during postnatal development of the retina. Conditional targeting of Dscam and Jun was performed to identify factors underlying axon-remodeling phenotypes. A subset of RGC axons were observed to project and branch extensively within the Dscam mutant retina after eye opening. Axon remodeling was preceded by histological signs of RGC stress. These included neurofilament accumulation, axon swelling, axon blebbing and activation of JUN, JNK and AKT. Novel and extensive projection of RGC axons within the retina was observed after upregulation of these markers, and novel axon projections were maintained to at least one year of age. Further analysis of retinas in which Dscam was conditionally targeted with Brn3b or Pax6α Cre indicated that axon stress and remodeling could occur in the absence of hydrocephalus, which frequently occurs in Dscam mutant mice. Analysis of mice mutant for the cell death gene Bax, which executes much of Dscam dependent cell death, identified a similar axon misprojection phenotype. Deleting Jun and Dscam resulted in increased axon remodeling compared to Dscam or Bax mutants. Retinal ganglion cells have a very limited capacity to regenerate after damage in the adult retina, compared to the extensive projections made in the embryo. In this study we find that DSCAM and JUN limit ectopic growth of RGC axons, thereby identifying these proteins as targets for promoting axon regeneration and reconnection.

A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata

Technical limitations have hindered comprehensive studies of highly variable immune response molecules that are thought to have evolved due to pathogen-mediated selection such as fibrinogen-related proteins (FREPs) from Biomphalaria glabrata. FREPs combine upstream immunoglobulin superfamily (IgSF) domains with a C-terminal fibrinogen-related domain (FreD) and participate in reactions against trematode parasites. From RNAseq data we assembled a de novo reference transcriptome of B. glabrata to investigate the diversity of FREP transcripts. This study increased over two fold the number of bonafide FREP subfamilies and revealed important sequence diversity within FREP12 subfamily. We also report the discovery of related molecules that feature one or two IgSF domains associated with different C-terminal lectin domains, named C-type lectin-related proteins (CREPs) and Galectin-related protein (GREP). Together, the highly similar FREPs, CREPs and GREP were designated VIgL (Variable Immunoglobulin and Lectin domain containing molecules).

Monoallelic expression of the human FOXP2 speech gene

The recent descriptions of widespread random monoallelic expression (RMAE) of genes distributed throughout the autosomal genome indicate that there are more genes subject to RMAE on autosomes than the number of genes on the X chromosome where X-inactivation dictates RMAE of X-linked genes. Several of the autosomal genes that undergo RMAE have independently been implicated in human Mendelian disorders. Thus, parsing the relationship between allele-specific expression of these genes and disease is of interest. Mutations in the human forkhead box P2 gene, FOXP2, cause developmental verbal dyspraxia with profound speech and language deficits. Here, we show that the human FOXP2 gene undergoes RMAE. Studying an individual with developmental verbal dyspraxia, we identify a deletion 3 Mb away from the FOXP2 gene, which impacts FOXP2 gene expression in cis. Together these data suggest the intriguing possibility that RMAE impacts the haploinsufficiency phenotypes observed for FOXP2 mutations.

Development of the embryonic and larval peripheral nervous system of Drosophila

The peripheral nervous system (PNS) of embryonic and larval stage Drosophila consists of diverse types of sensory neurons positioned along the body wall. Sensory neurons, and associated end organs, show highly stereotyped locations and morphologies. Many powerful genetic tools for gene manipulation available in Drosophila make the PNS an advantageous system for elucidating basic principles of neural development. Studies of the Drosophila PNS have provided key insights into molecular mechanisms of cell fate specification, asymmetric cell division, and dendritic morphogenesis. A canonical lineage gives rise to sensory neurons and associated organs, and cells within this lineage are diversified through asymmetric cell divisions. Newly specified sensory neurons develop specific dendritic patterns, which are controlled by numerous factors including transcriptional regulators, interactions with neighboring neurons, and intracellular trafficking systems. In addition, sensory axons show modality specific terminations in the central nervous system, which are patterned by secreted ligands and their receptors expressed by sensory axons. Modality-specific axon projections are critical for coordinated larval behaviors. We review the molecular basis for PNS development and address some of the instances in which the mechanisms and molecules identified are conserved in vertebrate development.

DSCAM and DSCAML1 function in self-avoidance in multiple cell types in the developing mouse retina

DSCAM and DSCAM-LIKE1 (DSCAML1) serve diverse neurodevelopmental functions, including axon guidance, synaptic adhesion, and self-avoidance, depending on the species, cell type, and gene family member studied. We examined the function of DSCAM and DSCAML1 in the developing mouse retina. In addition to a subset of amacrine cells, Dscam was expressed in most retinal ganglion cells (RGCs). RGCs had fasciculated dendrites and clumped cell bodies in Dscam(-/-) mice, suggesting a role in self-avoidance. Dscaml1 was expressed in the rod circuit, and mice lacking Dscaml1 had fasciculated rod bipolar cell dendrites and clumped AII amacrine cell bodies, also indicating a role in self-avoidance. Neurons in Dscam or Dscaml1 mutant retinas stratified their processes appropriately in synaptic laminae in the inner plexiform layer, and functional synapses formed in the rod circuit in mice lacking Dscaml1. Therefore, DSCAM and DSCAML1 function similarly in self-avoidance, and are not essential for synaptic specificity in the mouse retina.

More than just glue: the diverse roles of cell adhesion molecules in the Drosophila nervous system

Cell adhesion is the fundamental driving force that establishes complex cellular architectures, with the nervous system offering a striking, sophisticated case study. Developing neurons adhere to neighboring neurons, their synaptic partners, and to glial cells. These adhesive interactions are required in a diverse array of contexts, including cell migration, axon guidance and targeting, as well as synapse formation and physiology. Forward and reverse genetic screens in the fruit fly Drosophila have uncovered several adhesion molecules that are required for neural development, and detailed cell biological analyses are beginning to unravel how these factors shape nervous system connectivity. Here we review our current understanding of the most prominent of these adhesion factors and their modes of action.

Efficient inefficiency: biochemical "junk" may represent molecular bridesmaids awaiting emergent function as a buffer against environmental fluctuation

The biochemical function of many parts of the genome, transcriptome, proteome, and interactome remain largely unknown. We propose that portions of these fundamental building blocks of life have no current biochemical function per se. Rather, sections of these "omes" may contribute to an inventory of biochemical parts and circuits that participate in the development of emergent functions. Low fidelity deoxyribonucleic acid replication, transcription, translation, and post-translational modification all represent potential mechanisms to produce an inventory of parts. Stochastic processes that influence the conformations of ribonucleic acid molecules and proteins may also contribute to potential biochemical inventory. Some components of the biochemical inventory may enable future adaptations, some may produce disease, and some may remain useless. The function of many of these components await discovery, not by science, but by evolution. While carrying such purposeless biochemical units may appear to dilute fitness by exacting a thermodynamic cost, we argue that net fitness becomes enhanced when considering the value for potential future innovations. One can envision components that intermingle, interact, and act out mock pathways, but in most cases remain molecular bridesmaids. Given sufficiently low thermodynamic cost, such stochastic cycling may persist until a markedly advantageous or cataclysmically disadvantageous trait emerges. Maladaptive screening and utilization of inventory content can lead to disease phenotypes, a process buffered and regulated in part by the heat shock protein and stress response network. Whereas failure of the ubiquitin pathway to recycle misfolded proteins has become increasingly recognized as a source of disease, protein misfolding may itself represent one step in a process that maximizes functional innovation through increasing proteomic diversity. Fractal correlates of these processes occur at the organizational level of cells and organisms. That the abnormal accumulation of units induces local collapse may serve to limit the extension of damage to the greater system at large. The immune and cognitive systems that selectively sample and prune environmental content may serve as additional portals for innovation.