DIG-1, a novel giant protein, non-autonomously mediates maintenance of nervous system architecture

Abstract

Dedicated mechanisms exist to maintain the architecture of an animal's nervous system after development is completed. To date, three immunoglobulin superfamily members have been implicated in this process in the nematode Caenorhabditis elegans: the secreted two-Ig domain protein ZIG-4, the FGF receptor EGL-15 and the L1-like SAX-7 protein. These proteins provide crucial information for neuronal structures, such as axons, that allows them to maintain the precise position they acquired during development. Yet, how widespread this mechanism is throughout the nervous system, and what other types of factors underlie such a maintenance mechanism, remains poorly understood. Here, we describe a new maintenance gene, dig-1, that encodes a predicted giant secreted protein containing a large number of protein interaction domains. With 13,100 amino acids, the DIG-1 protein is the largest secreted protein identifiable in any genome database. dig-1 functions post-developmentally to maintain axons and cell bodies in place within axonal fascicles and ganglia. The failure to maintain axon and cell body position is accompanied by defects in basement membrane structure, as evidenced by electron microscopy analysis of dig-1 mutants. Expression pattern and mosaic analysis reveals that dig-1 is produced by muscles to maintain nervous system architecture, demonstrating that dig-1 functions non-autonomously to preserve the proper layout of neural structures. We propose that DIG-1 is a component of the basement membrane that mediates specific contacts between cellular surfaces and their environment through the interaction with a cell-type specific set of other maintenance factors.

Neural Maintenance Roles for the Matrix Receptor Dystroglycan and the Nuclear Anchorage Complex in Caenorhabditis elegans

“…The locations of neuronal soma, axons, and dendrites must be maintained to ensure proper nervous system function during the addition and removal of neurons and synapses and in response to mechanical stresses associated with body growth and movement. The factors maintaining nervous system architecture are often distinct from those involved in its establishment during development, a division of labor which likely allows flexibility to cope with the stresses involved in remodeling, growth, and movement.The involvement of extracellular matrix components, cell adhesion molecules, and cytoskeletal proteins in previously reported neural maintenance activities demonstrate that adhesive cell-matrix and possibly cell-cell interactions play a critical role (Aurelio et al 2002;Bulow et al 2004;Sasakura et al 2005;Wang et al 2005;Benard et al 2006Burket et al 2006;Pocock et al 2008;Woo et al 2008; Zhou et al 2008). Although previously unreported, factors controlling nuclear position in neurons may also be predicted to play key roles in positional maintenance of neuronal soma.…”

“…The involvement of extracellular matrix components, cell adhesion molecules, and cytoskeletal proteins in previously reported neural maintenance activities demonstrate that adhesive cell-matrix and possibly cell-cell interactions play a critical role (Aurelio et al 2002;Bulow et al 2004;Sasakura et al 2005;Wang et al 2005;Benard et al 2006Burket et al 2006;Pocock et al 2008;Woo et al 2008; Zhou et al 2008). Although previously unreported, factors controlling nuclear position in neurons may also be predicted to play key roles in positional maintenance of neuronal soma.…”

“…We find that dgn-1 null mutants display a progressive displacement of lumbar neuron cell bodies, indicative of a defect in neuroanatomical maintenance. This neural role of dystroglycan likely involves interaction with extracellular partners, including the matrix component DIG-1 previously implicated in neural maintenance (Benard et al 2006;Burket et al 2006).…”

“…We find that dgn-1 null mutants display a progressive displacement of lumbar neuron cell bodies, indicative of a defect in neuroanatomical maintenance. This neural role of dystroglycan likely involves interaction with extracellular partners, including the matrix component DIG-1 previously implicated in neural maintenance (Benard et al 2006;Burket et al 2006).A forward genetic screen for other mutations producing dgn-1-like lumbar neuron maintenance defects identified a second novel maintenance factor: the KASH domain protein ANC-1, a nesprin/Syne protein known to mediate anchorage of nuclei and mitochondria Starr and Han 2002). Mutants in the KASH protein UNC-83 (Starr et al 2001) and in the ANC-1-and UNC-83-binding SUN domain protein UNC-84 (Malone et al 1999) also display lumbar maintenance defects, supporting the prediction that factors controlling nuclear position and migration are indeed critical in maintenance of neuron cell body location.…”

“…These factors may thus play roles in neural maintenance outside of the lumbar ganglion. The secreted factor DIG-1, which is expressed from embryonic comma stage through the adult stage in either intestine or mesodermal tissue, has been previously reported to function in positional maintenance of a number of neuronal cell bodies and axons, possibly through a role in stabilizing the basement membrane (Benard et al 2006).The cell specificity within the lumbar ganglion for dependence of soma positional maintenance on the factors we have characterized is intriguing and may reflect unique aspects of the immediate environment of each neuron and the repertoire of stresses to which it is subjected. PVQ may be particularly sensitive in this regard, because it is characteristically the most anterior cell body in the ganglion and thus more easily displaced forward away from the more sterically constrained neurons located posterior of it in the ganglion.…”