Caenorhabditis elegans β-G Spectrin Is Dispensable for Establishment of Epithelial Polarity, but Essential for Muscular and Neuronal Function

Abstract

The Caenorhabditis elegans genome encodes one α spectrin subunit, a β spectrin subunit (β-G), and a β-H spectrin subunit. Our experiments show that the phenotype resulting from the loss of the C. elegans α spectrin is reproduced by tandem depletion of both β-G and β-H spectrins. We propose that α spectrin combines with the β-G and β-H subunits to form α/β-G and α/β-H heteromers that perform the entire repertoire of spectrin function in the nematode. The expression patterns of nematode β-G spectrin and vertebrate β spectrins exhibit three striking parallels including: (1) β spectrins are associated with the sites of cell–cell contact in epithelial tissues; (2) the highest levels of β-G spectrin occur in the nervous system; and (3) β spec-trin-G in striated muscle is associated with points of attachment of the myofilament apparatus to adjacent cells. Nematode β-G spectrin associates with plasma membranes at sites of cell–cell contact, beginning at the two-cell stage, and with a dramatic increase in intensity after gastrulation when most cell proliferation has been completed. Strikingly, depletion of nematode β-G spectrin by RNA-mediated interference to undetectable levels does not affect the establishment of structural and functional polarity in epidermis and intestine. Contrary to recent speculation, β-G spectrin is not associated with internal membranes and depletion of β-G spectrin was not associated with any detectable defects in secretion. Instead β-G spectrin-deficient nematodes arrest as early larvae with progressive defects in the musculature and nervous system. Therefore, C. elegans β-G spectrin is required for normal muscle and neuron function, but is dispensable for embryonic elongation and establishment of early epithelial polarity. We hypothesize that heteromeric spectrin evolved in metazoans in response to the needs of cells in the context of mechanically integrated tissues that can withstand the rigors imposed by an active organism.

ELF a β-spectrin is a neuronal precursor cell marker in developing mammalian brain; structure and organization of the elf/β-G spectrin gene

“…Nestin is a neurofilament protein expressed in neuronal precursor cells, and is also expressed in radial glial cells (Dahlstrand et al, 1995), as well as in neurites and growth cones of primary cerebellar granule cells, supportive of its role in growth cone guidance during axon elongation. Our studies on ELF also support recent evidence that bspectrin is essential in growth cones for axon outgrowth, where it has been disrupted in C. elegans (Moorthy et al, 2000;Hammarlund et al, 2000). Mutant animals appeared to have the normal complement of neurons, yet these neurons did not extend axons to their targets (Sobue and Kanda, 1989;Sobue, 1990).…”

“…Its striking localization to the axon hillock and axonal projection is consistent with its known involvement in axonal guidance as well as cell polarization (Moorthy et al, 2000;Hammarlund et al, 2000;Dubreuil et al, 2000). ELF also appears to be a marker of purkinje cell precursors in the cerebellum.…”

TGF-β, Neuronal Stem Cells and Glioblastoma

“…Its striking localization to the axon hillock and axonal projection is consistent with its known involvement in axonal guidance as well as cell polarization (Dubreuil et al, 2000;Hammarlund et al, 2000;Moorthy et al, 2000).…”

Ankyrin-B Targets β2-Spectrin to an Intracellular Compartment in Neonatal Cardiomyocytes

“…It is important to emphasize that our results were obtained using cultured neonatal cardiomyocytes and may not necessarily extend to neonatal heart tissue or to adult cardiomyocytes. It is of interest in this regard that loss of the sole ␤ 2 -spectrin ortholog in C. elegans is compatible with normal embryonic morphogenesis but not with survival after hatching (38,39). Moreover, mice homozygous for a null mutation in ␤ 2 -spectrin display cardiac abnormalities as well as other defects and die in midgestation, consistent with a critical role for ␤ 2 -spectrin in the heart (40).…”