BIOLOGICAL OVERVIEW

FAS1 is a novel protein, functioning as a homophilic cell adhesion molecule (Elkins, 1990). The protein does not span the membrane as do many other cell surface adhesive molecules, rather it is attached to the surface of cells via a lipid link. FAS1 is expressed in the central nervous system where it functions in axon guidance, the process in which axons find their way from the neural cells that generate them to other target neurons or muscle cells.

Alternative splicing, generates multiple messenger RNA molecules from a coding sequence. These are translated into FAS1 molecules that may have highly specific roles in determining the specificity of axon guidance, due to their different self recognition sequences (McAllister, 1992b).

Fas1 mutations on their own do not cause an observable phenotype. However, when combined with mutations in the abl tyrosine kinase (a cytoplasmic signal transduction protein) defective axon tracts arise (Elkins, 1990). Is this interaction with abl accidental, or are fas1 signals transmitted into the cell and subsequently carried to the nucleus by ABL? Since Fas1 is lipid linked to the membrane, how are its signals transduced to ABL?

cDNA clone length - 3 kb. A total of six Fas1 mRNAs have been detected. They differ primarily in their 3' UTRs. The major length difference can be accounted for by the use of different polyadenylation sites giving rise to 31 additional untranslated sequences (McAllister, 1992b).

Bases in 5' UTR - 124

Exons - 15 , distributed over 14 kb of DNA

Bases in 3' UTR - 1049

PROTEIN STRUCTURE

Multiple Fas1 mRNAs show a highly conserved sequence at the end of the second domain that can be altered by the addition of three or six amino acids, encoded by two alternatively spliced 9 base-pair micro-exons, developmentally regulated (McAllister, 1992b).

Amino Acids - 638

Structural Domains

FAS1 has no transmembrane domain but does have a signal peptide which functions to facilitate secretion. The protein is a lipid linked cell surface glycoprotein, tightly associated with the lipid bilayer by a phosphatidylinositol lipid moiety (Hortsch, 1990). A large fraction is in a secreted form with no membrane anchor (Hortsch, 1990). The mature protein has four novel homologous extracellular domains, primarily alpha-helical, of approximately 150 amino acids each (Zinn, 1988). Fas1 structure is different from other known cell adhesion molecules that are predicted to be elongated beta-sheet structures (Wang, 1993).

Evolutionary Homologs

A cell adhesion molecule of Volvox is homologous to FAS1 (Huber, 1994), as is human beta ig-h3, a gene induced in an adenocarcinoma cell line (Skonier, 1992). Fas1 of grasshopper, which was isolated first, shows alternative splice patterns differing only slightly from those of Drosophila (McAllister, 1992b).

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).

Drosophila Fasciclin I is the prototype of a family of vertebrate and invertebrate proteins that mediate cell adhesion and signaling. The midline fasciclin gene (FlyBase ID: FBgn0024211) encodes a second Drosophila member of the Fasciclin I family. Midline fasciclin largely consists of four 150 amino acid repeats characteristic of the Fasciclin I family of proteins. Hydrophobicity plot analysis of the protein sequence suggests that Mfas is secreted and/or associated with the cell surface. The N-terminus of Mfas has a positively charged arginine followed by a stretch of 16 predominantly hydrophobic residues, a feature that resembles a membrane signal sequence. There are 9 potential glycosylation sites scattered throughout the second half of the protein. Immunostaining and biochemical analysis using antibodies to Midline fasciclin indicates that the protein is membrane-associated, although the sequence does not reveal a transmembrane domain. The gene is expressed in a dynamic fashion during embryogenesis in the blastoderm, central nervous system midline cells, and trachea, suggesting it plays multiple developmental roles. Protein localization studies indicate that Midline fasciclin is found within cell bodies of midline neurons and glia, and on midline axons. Initial cellular analysis of a midline fasciclin loss-of-function mutation reveals only weak defects in axonogenesis. However, embryos mutant for both midline fasciclin and the abelson nonreceptor tyrosine kinase, show more severe defects in axonogenesis that resemble fasciclin I abelson double mutant phenotypes (Hu, 1998).

DEVELOPMENTAL BIOLOGY

Embryonic

Fasciclin I is expressed in several distinct patterns at different stages of development. In blastoderm embryos it is briefly localized in a graded pattern. During the germ band extended period its expression evolves through two distinct phases. Fasciclin I mRNA and protein are initially localized in a 14-stripe pattern that corresponds to segmentally repeated patches of neuroepithelial cells and neuroblasts. Expression then becomes confined to the CNS and peripheral sensory (PNS) neurons. Fasciclin I is expressed on all PNS neurons, and this expression is stably maintained for several hours. In the CNS, fasciclin I is initially expressed on all commissural axons, but then becomes restricted to specific axon bundles. The early commissural expression pattern is not observed in grasshopper embryos, but the later bundle-specific pattern is very similar to that seen in grasshopper. The existence of an initial phase of expression on all commissural bundles helps to explain the loss-of-commissures phenotype in embryos lacking expression of both fasciclin I and D-abl tyrosine kinase. Fasciclin I is also expressed in several nonneural tissues in the embryo (McAllister, 1992).

Fas1 is expressed briefly in a graded pattern. During germ band extention [Image], protein and mRNA are initially localized in a 14-stripe pattern which corresponds to segmentally repeated patches of neuroepithelial cells and neuroblasts. Later expression becomes confined to the ventral nervous system (CNS). In the CNS, Fas1 is initially expressed on all commissural axons, but then becomes restricted to specific axon bundles (McAllister, 1992b). At 11-14 hours, FAS1 is almost entirely confined to the CNS. FAS1 is found on a single large fascicle in the posterior commissure and on a smaller fascicle in the anterior commissure (Zinn, 1988).

All peripheral sensory axons express Fas1, as do glia surrounding the peripheral nerves. FAS1 is also found in cell bodies and axons of sensory structures of the head, particularly the dorsal terminal organs of the antennomaxillary complex (Zinn, 1988).

Fasciclin I (Fas I) is expressed in motor nerve axons and terminals that innervate the body-wall muscles of Drosophila larvae. Immunohistochemical analysis of these motor nerve terminals has revealed that nerve terminal arborization, quantified by the numbers of the nerve terminal branches and varicosities, is enhanced in the null mutant fas ITE. In contrast, the number of branches and varicosities are reduced in larvae that overexpress the Fas I molecule resulting from additional copies of the fas I transgene. Although arborization is altered, the overall stereotypical pattern of nerve terminal innervation of the body-wall muscle fibers appears to be preserved in Fas I mutants. Voltage-clamp analysis of excitatory junctional currents (ejcs) at the neuromuscular junction indicates that the amplitude of ejcs is reduced in fas ITE, but increased in P(fas I+) and Dp(fas I), as compared to that in wild-type larvae. Further electrophysiological analysis shows that the quantal content and the evoked frequency-dependent response are affected in these mutants, indicating a defective presynaptic function in addition to the anatomical abnormality. Therefore, the cell adhesion molecule Fas I may not be essential for target recognition and synaptogenesis at the larval neuromuscular junction, but may play a role in fine-tuning nerve terminal arborization, and possibly in modifying (directly or indirectly), development of presynaptic functions (Zhong, 1995).

Effects of mutation or deletion

There is a loss of commisures in embryos lacking expression of both Fas1 and the Drosophila Abl tyrosine kinase proto-oncogene homolog. Double mutants display major defects in CNS axons pathways (Elkins, 1990). Similarly, Fas2 interacts with Abl.

REFERENCES

Desai, C. J., Popova, E. and Zinn, K. (1994). A Drosophila receptor tyrosine phosphatase expressed in the embryonic CNS and larval optic lobes is a member of the set of proteins bearing the "HRP" carbohydrate epitope. J. Neurosci. 14: 7272-7283

Elkins, T., Zinn, K., McAllister, L., Hoffmann, F.M. and Goodman, C.S. (1990). Genetic analysis of a Drosophila neural cell adhesion molecule: interaction of Fasciclin I and Abelson tyrosine kinase mutations. Cell 60(4): 565-575

Hortsch, M. and Goodman, C.S. (1990). Drosophila fasciclin I, a neural cell adhesion molecule, has a phosphatidylinositol lipid membrane anchor that is developmentally regulated. J. Biol. Chem. 265(25): 15104-09

Hu, S., Sonnenfeld, M., Stahl, S. and Crews, S. T. (1998). Midline Fasciclin: a Drosophila Fasciclin-I-related membrane protein localized to the CNS midline cells and trachea. J. Neurobiol. 35(1): 77-93.

Huber, O. and Sumper, M. (1994). Algal-CAMs: isoforms of a cell adhesion molecule in embryos of the alga Volvox with homology to Drosophila fasciclin I. EMBO J. 13(18): 4212-4222

McAllister, L., Goodman, C. S. and Zinn, Z. (1992a). Dynamic expression of the cell adhesion molecule fasciclin I during embryonic development in Drosophila. Development 115: 267-76.

McAllister, L., Rehm, E.J., Goodman, G.S. and Zinn, K. (1992b). Alternative splicing of micro-exons creates multiple forms of the insect cell adhesion molecule fasciclin I. J. Neurosci. 12(3): 895-905

Skonier, J., Neubauer, M., Madisen, L., Bennett, K., Plowman, G.D. and Purchio, A.F. (1992). cDNA cloning and sequence analysis of beta ig-h3, a novel gene induced in a human adenocarcinoma cell line after treatment with transforming growth factor-beta. DNA Cell Biol. 11(7): 511-522

Wang, W.C., Zinn, K. and Bjorkman, P.J. (1993). Expression and structural studies of fasciclin I, an insect cell adhesion molecule. J. Biol. Chem. 268(2): 1448-1455

Zinn, K., McAllister, L. and Goodman, C.S. (1988). Sequence analysis and neuronal expression of Fasciclin I in grasshopper and Drosophila. Cell 53: 577-587

Zhong, Y. and Shanley, J. (1995). Altered nerve terminal arborization and synaptic transmission in Drosophila mutants of cell adhesion molecule fasciclin I. J. Neurosci. 15: 6679-6687

date revised: 6 feb  96 

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