Glia Keep Synapse Distribution under Wraps

Abstract

The wiring of the nervous system requires that axons navigate to the correct targets and maintain their correct positions during developmental growth. In this issue, Shao et al. (2013) now reveal a crucial new role for glia in preserving correct synaptic connectivity during developmental growth.

Neural circuits are largely established during embryonic development and many of these synaptic connections are maintained throughout life. The preservation of embryonically-derived synaptic connections amidst dramatic developmental growth represents a major challenge for the nervous system. The human brain grows 4-fold to reach its adult volume, whereas other species such as Drosophilia grow 200-fold in volume during their larval stages. How do neurons conserve the specificity of their connections during this enormous degree of growth? In this issue of Cell, Shao et al. (2013) report that glia play a crucial role in preserving synaptic connectivity during growth. They show that interaction between epidermal cells and glia ensures the appropriate positioning of glial processes next to axons during growth, which in turn maintains the correct distribution of synapses.

Previous studies have already found that the signals that maintain nervous system architecture throughout adult life are distinct from those that establish the developing nervous system (Bénard and Hobert, 2009). The normal function of these maintenance signals is to counteract the changes exerted on the nervous system during postnatal growth. For instance, several independent genetic screens in the invertebrate C. elegans have previously identified a series of adhesion molecules which are required for the maintenance of axon positions during growth, including IgCAMs (Sasakura et al., 2005), ZIG proteins (Aurelio et al., 2002), DIG proteins (Bénard and Hobert, 2009), F-spondin (Woo et al., 2008), and a specific isoform of the FGF receptor EGL-15(5A) (Bülow et al., 2004). These maintenance signals build adhesive complexes that anchor neurons and their projections to their appropriate environment. Additionally, these molecules may regulate direct cell-cell interactions among neurons, between neurons and neighboring cells, or with the extracellular matrix. To date, only a few synaptic maintenance signals have been identified, and little is known about the roles nonneuronal cells, such as glia, play in maintaining synaptic distribution during growth.

To identify the signals that control synaptic connectivity maintenance, Shao et al. performed a genetic screen to identify mutations that affect the well-defined pattern of synaptic connectivity of the AIY neurons in C. elegans, a pair of bilaterally symmetric neurons in the nematode nerve ring. These authors uncovered the requirement of an SLC17 transporter protein CIMA-1 for maintaining presynaptic distribution during growth in C. elegans. The neurons in cima-1 loss-of-function mutants develop normally and are indistinguishable from wild-type animals; in adulthood, however, cima-1 mutants display profound defects in the distribution of presynaptic sites. These data suggest that cima-1 is required for preserving the presynaptic distribution and inhibiting ectopic presynaptic site formation during growth (Figure 1). Surprisingly, the ectopic presynaptic sites found in cima-1 mutants are not accompanied by the formation of ectopic postsynaptic sites, indicating that cima-1 is not required for the maintenance of post-synaptic sites during growth. To further examine the relationship between cima-1 expression and the maintenance of presynaptic distribution, the authors investigated the role of cima-1 expression in mutant animals with abnormally long or abnormally short body lengths. They found that in double-mutant animals lacking cima-1 and with the long phenotype, the distribution of presynaptic sites is more severely disrupted following growth, when compared with single cima-1 mutants. In contrast, in double mutants lacking cima-1 with the short phenotype, no defects in presynaptic distribution following growth could be observed. Together, these findings demonstrate that CIMA-1 is a maintenance signal that regulates synaptic positioning during growth.

Figure 1. Glial Processes Maintain Presynaptic Site Distribution during Growth.

(A) In wild-type animals, an SLC17 transporter CIMA-1 negatively regulates EGL-15(5A) expression, by an unknown mechanism, to reduce epidermalglia adhesion and prevent glial endfoot extension during growth. This interaction maintains the glial cell morphology, which in turn preserves the correct presynaptic site distribution during growth.

(B) In animals with cima-1 loss-of-function mutations, or in animals in which egl-15(5A) is overexpressed, the interaction between the epidermal cells and the glia is misregulated. This results in the inappropriate extension of the glial endfeet, and the formation of ectopic presynaptic sites.

To understand the mechanism by which cima-1 suppresses the formation of ectopic presynaptic sites during growth, Shao et al., examined the expression pattern of this gene. Surprisingly, they found that cima-1 is not expressed by neurons or glia but that cima-1 is primarily expressed by epidermal cells. The cell-specific expression of cima-1 in epidermal cells raises the question of how cima-1 functions in epidermal cells to maintain presynaptic patterning during growth. The authors find that the ventral cephalic sheath glial cells, which are morphologically similar to vertebrate astrocytes, are located between the epidermal cells and the neurons, and that they send slender processes (known as glial end feet) which ensheath synapses within the C. elegans nerve ring. Inspection of the location of the glial processes in adult cima-1 mutants reveals that the position and morphology of the glia is profoundly changed, with these cells extending their processes into more posterior zones of the animal (Figure 1). Moreover, the authors determined that the position of the glial endfeet could be correlated with the emergence of ectopic synaptic sites in cima-1 mutants. This finding indicates that cima-1 acts in epidermal cells to maintain glial cell morphology during growth and that the correct maintenance of glial morphology is required for preventing the emergence of ectopic presynaptic sites. Consistent with this idea, ablation of the glial cells significantly suppresses the formation of ectopic presynaptic sites cima-1 mutants.

How does CIMA-1 expression in epidermal cells regulate glial morphology and presynaptic site position during growth? Shao et al., discovered that CIMA-1 acts in epidermal cells to negatively regulate the protein levels of EGL-15(5A), the only FGF receptor in C. elegans (Figure 1). Consistent with this, overexpression of egl-15(5A) in epidermal cells of wild-type animals results in abnormal glial cell morphology and ectopic presynaptic sites, similar to the phenotype observed in cima-1 mutants. This requirement of egl-15(5A) in epidermal cells for the maintenance of axon position is in accordance with previous studies (Bülow et al., 2004). Together, these findings clearly demonstrate a genetic interaction between cima-1 and egl-15(5A); however, the precise mechanism by which cima-1 regulates EGL-15(5A) protein levels remains to be determined. Given that CIMA-1 is a member of the SLC17 family that includes Sialin, the authors propose that CIMA-1 could act by transporting acidic monosaccharides that could modify and regulate the protein levels of EGL-15(5A) (Shao et al., 2013). Although, the main findings of this study support this model, given that mutations in egl-15(5A) do not completely suppress the cima-1 phenotype, it is likely that cima-1 also controls glial cell morphology by other as yet unidentified mechanisms.

These new findings of Shao et al. add new insight into the important synaptic functions of glia. In the mammalian nervous system, glia, including astrocytes and Schwann cells, have already been shown to powerfully control the formation, maturation, and function of synapses through various secreted and contact-mediated signals (Clarke and Barres, 2013). Similarly, in C. elegans, glia promote synaptogenesis and also delimit shape changes of sensory neuron receptive endings (Colón-Ramos et al., 2007; Procko et al., 2011). This study highlights an important new role for glia in maintaining the location of synaptic distribution and architecture during growth. Given that C. elegans synapses share many of the features of mammalian synapses, perhaps similar mechanisms exist in mammals to preserve the neural circuitry during growth. Indeed, a previous time-lapse study of glial interactions with synapses in living mice found that presynaptic terminals remain colocalized with glia as the glial nuclei gradually changed position over several months (Pomeroy and Purves, 1988). A better understanding of the mechanisms that exist to maintain the neural architecture may uncover a role for these factors in neurodevelopmental and neurodegenerative diseases.