Axonal regeneration proceeds through specific axonal fusion in transected C. elegans neurons

Abstract

Functional neuronal recovery following injury arises when severed axons reconnect with their targets. In Caenorhabditis elegans following laser-induced axotomy, the axon still attached to the cell body is able to regrow and reconnect with its separated distal fragment. Here we show that reconnection of separated axon fragments during regeneration of C. elegans mechanosensory neurons occurs through a mechanism of axonal fusion, which prevents Wallerian degeneration of the distal fragment. Through electron microscopy analysis and imaging with the photoconvertible fluorescent protein Kaede, we show that the fusion process re-establishes membrane continuity and repristinates anterograde and retrograde cytoplasmic diffusion. We also provide evidence that axonal fusion occurs with a remarkable level of accuracy, with the proximal re-growing axon recognizing its own separated distal fragment. Thus, efficient axonal regeneration can occur by selective reconnection and fusion of separated axonal fragments beyond an injury site, with restoration of the damaged neuronal tract. Developmental Dynamics 240:1365-1372,

C. elegansas a genetic model to identify novel cellular and molecular mechanisms underlying nervous system regeneration

“…In some cases, the regenerating axon can fuse with the distal segment, preventing it from degeneration. 1 Based on studies in a variety of organisms, it was found that although the capacity of neuronal regeneration should be universal, in C. elegans as a genetic model to identify novel cellular and molecular mechanisms underlying nervous system regeneration Caenorhabditis elegans has emerged as a powerful model for studying axon regeneration since the invention of the femtosecond laser system opens up the possibilities for high throughput genetic screens to identify novel genes involved in regulating axon regeneration. 2 In this review, we first discuss the general view about nerve regeneration and the advantages of using C. elegans as a model system to study axon regeneration.…”

The cellular and molecular mechanisms of axonal maintenance and regeneration

“…We speculated that the process of axonal fusion might resemble that of apoptosis 7 , in which the dying cell exposes "eat-me" signals that mediate recognition and engulfment by surrounding phagocytes. During apoptosis, plasma membrane phospholipid asymmetry is lost, causing phosphatidylserine (PS), which is normally restricted to the cytoplasmic leaflet, to be externalized to the exoplasmic leaflet where it enables recognition by phagocytic cells [8][9][10][11] .…”

“…While axon fusion is conserved across a range of species [1][2][3][4][5][6][7] , EFF-1 is not 26 . In order to identify other conserved components required for this process, we undertook a candidate screen on molecules previously identified to mediate biological events that we predicted might share open arrowheads show fusion site; scale bar 25 μm.…”

“…This process, referred to as axonal fusion, occurs when the proximal axon that is still attached to the cell body regrows towards, then reconnects and fuses with its separated distal fragment. Previously, we and others have demonstrated that axonal fusion in the C. elegans mechanosensory neurons is a highly specific process that occurs through a fusion event, with both membrane and cytoplasmic continuity re-established 4,7 . However, the genetic components that mediate axonal fusion remain largely unknown, with the nematode-specific fusogen, Epithelial Fusion Failure 1 (EFF-1), the only protein previously found to be involved 4 .…”

“…Although extensive knowledge has been gained regarding the intrinsic and extrinsic modifiers of axonal regeneration, insight into how a neuron can rejoin with its target is lacking. A highly efficient means to re-establish the connection between an injured neuron and its target tissue occurs spontaneously in several invertebrate species [1][2][3][4][5][6][7] . This process, referred to as axonal fusion, occurs when the proximal axon that is still attached to the cell body regrows towards, then reconnects and fuses with its separated distal fragment.…”