Gene name - faint sausage
Cytological map position - 50B7--50B9
Function - mRNA splicing protein
Symbol - fas
Genetic map position - 2-
Classification - Ig superfamily
Cellular location - secreted
|Recent literature||Sauerwald, J., Soneson, C., Robinson, M.D. and
Luschnig, S. (2017). Faithful
mRNA splicing depends on the Prp19 complex subunit faint sausage
and is required for tracheal branching morphogenesis in Drosophila.
Development [Epub ahead of print]. PubMed ID: 28087625
Morphogenesis requires the dynamic regulation of gene expression, including transcription, mRNA maturation and translation. Dysfunction of the general mRNA splicing machinery can cause surprisingly specific cellular phenotypes, but the basis for these effects is not clear. This study shows that the Drosophila faint sausage (fas) locus, implicated in epithelial morphogenesis and previously reported to encode a secreted immunoglobulin domain protein, in fact encodes a subunit of the spliceosome-activating Prp19 complex, which is essential for efficient pre-mRNA splicing. Loss of zygotic fas function globally impairs the efficiency of splicing, and is associated with widespread retention of introns in mRNAs and dramatic changes in gene expression. Surprisingly, despite these general effects, zygotic fas mutants show specific defects in tracheal cell migration during mid-embryogenesis when maternally supplied splicing factors have declined. The study proposes that tracheal branching, which relies on dynamic changes in gene expression, is particularly sensitive for efficient spliceosome function. These results reveal an entry point to study requirements of the splicing machinery during organogenesis and provide a better understanding of disease phenotypes associated with mutations in general splicing factors.
Although significant progress has been made in understanding the mechanisms of axon outgrowth and guidance during Drosophila neurogenesis, little is known about the control of neural progenitor cell segregation and migration. To address this problem, existing collections of mutations were screened for defects in neural progenitor segregation and neuronal migration. This assay took embryos from each mutant line and stained them with the antibody marker mAb22C10, which stains sensory neurons, thereby allowing for the detection of abnormalities in the number, position and/or shape of sensory neurons. Any such abnormalities would mirror changes that had taken place during the specification, division and migration of sensory neural progenitors. One of the genes identified in this screen has been called faint sausage (fas) (Lekven, 1998).
One of several striking aspects of the phenotype resulting from loss of fas function provides evidence that sensory neurons of the peripheral nervous system (PNS) do not move normally out of the epidermal layer; this results in defective sensillum differentiation. Further, neurons of the central nervous system (CNS) do not migrate to their proper locations during the phase of germ band retraction and CNS condensation, which leads to gross abnormalities in cell shape and position; subsequently, patterning defects in axonal pathfinding occur. Because fas codes for a member of the immunoglobulin superfamily, it is presumed that the Fas protein functions by modulating intercellular adhesion (Lekven, 1998).
The Drosophila CNS develops from an invariant population of progenitor cells (neuroblasts) that segregate from the ectoderm and divide in a reproducible pattern to form a multilayer of neurons. At the approximate time when neurons start differentiating (stages 12-14), the CNS primordium undergoes a dramatic compaction. First, the germband (including the CNS primordium) retracts [Images], leading to a more than 50% reduction in CNS length. After germband retraction (stages 13-16) the CNS shortens further, until it measures only about 30% of its original, maximally extended length. The reduction in length of the CNS is mainly compensated for by an increase in the length of the dorsoventral axis and a higher cell-packing density, both for neuroblasts and neurons. Thus, cells that in the early embryo are arranged one behind the other, come to lie closer or even above and/or beside one another in the CNS of mature embryos. The CNS primordium of fas null mutant embryos develops normally until the late extended germ band stage; then, towards the end of germ band retraction (stage 12), defects in CNS morphogenesis and the expression of differentiation markers become apparent. Thus, markers for subsets of neuroblasts and their progeny (seven up, even-skipped, engrailed) do not show detectable abnormalities in the neuroblast map of early fas mutant embryos. However, the shortening and thickening of the CNS that normally take place during stages 13-16 fails to occur. At the same time, the fas mutant CNS remains flattened (along the dorsoventral axis) and widened (along the mediolateral axis). There does not appear in fas mutant embryos an obviously decreased number of neurons or glial cells, as visualized with antibodies against the Elav and Repo proteins, respectively. Also, markers for specific subsets of neurons are expressed in fas mutants (e.g., Even skipped, expressed in the aCC/pCC pair of neurons, and Engrailed, expressed in the progeny of the MNB neuroblast). However, the repositioning of neurons and glia that normally takes place during CNS condensation fails to occur. For example, longitudinal glial cells (LGCs) remain in segmental clusters, separated by gaps; they also stay at a more lateral level than their wild-type counterparts. Midline cells, such as the MNB progeny, do not form the regular, dense clusters typical for the wild-type embryo. Thus, the correct specification of NBs does not require fas, but the later morphogenetic movements of nerve cord condensation and consequent cell positioning do require fas. In summary, loss of faint sausage is associated with severe defects in tissue organization and cell movement in the epidermis and CNS. These defects, which first become manifest in the stages during and after germ band retraction, with the cell rearrangements that accompany this phase of development, result in generally abnormal shape and position of neurons. Subsequently, patterning defects in axonal pathfinding occur, possibly as a secondary consequence of the abnormal cell body positions, since Fas is not expressed on the surface of the axons themselves (Lekven, 1998).
Based on the expression pattern of the Fas protein and the unfolding of the fas phenotype, the following model is proposed for Fas function. Fas is expressed in the somata of undifferentiated neurons. It promotes adhesion during the phase of nervous system condensation when neurons undergo compaction and the complex relocalization movements that are required for later phases of neuronal development, such as axonal pathfinding. Thus, condensation and relocalization create contacts between cells that were formerly separated. Neurons, such as the pioneers aCC/pCC, are brought into direct contact with the substratum on which their axons grow. In the absence of fas function, condensation and neuronal relocalization fail to occur. As a consequence, neurons differentiate at abnormal positions, resulting in profound abnormalities in axonal outgrowth and pathfinding. This model explains the consistent pathfinding defect observed in fas mutant embryos. The pioneers of the longitudinal tracts are not in contact with the LGCs, which remain in lateral positions; also, they do not form continuous longitudinal 'tracks,' as in wild type. This could cause pCC and MP axons to grow laterally, instead of longitudinally. Later forming axons follow this course, so that longitudinal axon tracts and a regular neuropile are never formed. The grossly abnormal pattern and reduced packing density of the CNS may also be responsible for the generally reduced rate of axon formation in older fas mutant embryos. Thus, if neurons and/or glia secrete axonal growth-promoting molecules, the concentration of these molecules at any given position could be dependent on neuronal and glial packing density. Since the density of neurons and glia is reduced in fas mutants, concentrations of axonal growth promoting molecules may also be reduced; consequently, the result would be a decrease in axon formation. A direct involvement of Fas in growth cone guidance, that is, Fas providing a direct interaction between axons and cell bodies, cannot be ruled out; however, this would require that Fas function through heterophilic interactions, since Fas could not be detected on axons or growth cones. Additionally, if Fas were directly involved in growth cone guidance, one might expect that a null phenotype would result in a random directional outgrowth of pioneer axons, which was not observed. Thus, this possibility is considered unlikely (Lekven, 1998).
The fas locus spans at least 110 kb pairs, and possibly more than 150 kb pairs (Lekven, 1998)
Sequence analysis of FAS cDNAs reveals that the Fas protein product has characteristics of an extracellular protein and that it is a novel member of the immunoglobulin (Ig) superfamily. Five putative Ig domains have been found, starting from approximately amino acid 250. The Fas protein has a predicted molecular mass of approximately 90 kDa in the absence of post-translational processing. The N-terminal 30 amino acids have the characteristics of a signal peptide; therefore, Fas is predicted to be extracellular. Because the predicted Fas protein does not contain any sequence that would qualify as a transmembrane domain, Fas is either secreted or anchored to the external plasma membrane via the addition of a glycophosphatidyl inositol (GPI) anchor. In seeming support of a GPI-anchored form of Fas, the C-terminal amino acids of the predicted Fas protein show a short stretch of hydrophobic residues preceded by amino acids conforming to a consensus GPI-addition sequence. No determination has yet been made as to whether Fas is found in vivo in only one or in both forms, although immunohistochemical stainings suggest that at least a membrane anchored form exists. The predicted Fas protein sequence was compared to proteins in the NCBI protein databases using the BLAST program; this yielded significant scores from many members of the Ig superfamily. The highest scoring matches include RAGE (Receptor for Advanced Glycosylation Endproducts) (Neeper, 1992); Cell-CAM 105 (Aurivillius, 1990); Contactin (Ranscht, 1988), and IrreC (Ramos, 1993). Inspection of the Fas amino acid sequence shows that it contains five putative Ig domains, each of which is characterized by two highly conserved cysteine residues and a tryptophan residue. Ig domains are grouped into three categories: V, C1 and C2, the main differences being that V type domains have two more beta-strands than the others, and C1 type domains are only found in proteins of the immune system. Comparison of the five Fas Ig domains to the Ig consensus sequence suggests that domains 1, 3, 4 and 5 are likely to adopt the V type and domain 2 the C2 type domain topology. Comparisons between Fas and the proteins listed above using the FastA program show that Fas shares high degrees of similarity with all of these, over limited ranges. However, none of the proteins show more than 30% identity to Fas. Presumably, the significant similarities between the proteins reflect similarities in the structures of their respective Ig domains; these other proteins are unlikely to be orthologs of Fas. Thus, Fas represents a novel Drosophila member of the Ig superfamily (Lekven, 1998).
date revised: 10 August 98
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