Post-guidance signaling by extracellular matrix-associated Slit/Slit-N maintains fasciculation and position of axon tracts in the nerve cord

Abstract

Axon-guidance by Slit-Roundabout (Robo) signaling at the midline initially guides growth cones to synaptic targets and positions longitudinal axon tracts in discrete bundles on either side of the midline. Following the formation of commissural tracts, Slit is found also in tracts of the commissures and longitudinal connectives, the purpose of which is not clear. The Slit protein is processed into a larger N-terminal peptide and a smaller C-terminal peptide. Here, I show that Slit and Slit-N in tracts interact with Robo to maintain the fasciculation, the inter-tract spacing between tracts and their position relative to the midline. Thus, in the absence of Slit in post-guidance tracts, tracts de-fasciculate, merge with one another and shift their position towards the midline. The Slit protein is proposed to function as a gradient. However, I show that Slit and Slit-N are not freely present in the extracellular milieu but associated with the extracellular matrix (ECM) and both interact with Robo1. Slit-C is tightly associated with the ECM requiring collagenase treatment to release it, and it does not interact with Robo1. These results define a role for Slit and Slit-N in tracts for the maintenance and fasciculation of tracts, thus the maintenance of the hardwiring of the CNS.

Slit2 is necessary for optic axon organization in the zebrafish ventral midline

“…Functions in fasciculation have previously been attributed to the Slit-Robo signaling pathway in Drosophila (Bhat, 2017) and to Slit2 acting through Robo1 and Robo2 in mice (Jaworski and Tessier-Lavigne, 2012). The observed expression of slit2 in cells closely surrounding the optic nerves from early stages strongly supports the idea that a channeling mechanism could be in place, delineating the path for RGC axons at the zebrafish optic nerve.…”

GM604 regulates developmental neurogenesis pathways and the expression of genes associated with amyotrophic lateral sclerosis

“…We identified 77 genes linked to ALS by multiple database sources that were also regulated by GM6 at one or more time points (e.g., TUBA4A, NEFL, NEDD4L, FGFR1, RET ). Taken together, our findings support the hypothesis that GM6 enables neuron survival by restoring an embryonic-stage gene expression program [ 19 ], while additionally strengthening cell adhesion and an extracellular matrix scaffold supporting the central and peripheral nervous systems [ 67 , 68 ]. This multi-target mechanism of action is unique among existing ALS drug candidates and may provide therapeutic benefit for ALS and multiple other diseases characterized by progressive neuron loss (e.g., Alzheimer’s, Huntington’s and Parkinson’s diseases) [ 20 ].…”

“…The extracellular matrix provides a scaffold and microenvironment that supports neurons and has an active role in directing axon extension and growth [ 67 , 68 ]. An unexpected finding from this study was that prolonged GM6 treatment (24–48 h) increased expression of genes encoding collagen ( COL1A1 , COL6A2 , COL26A1 ) and other proteins localized to the ECM or functioning in cell adhesion ( TFIP2 , MMP17 , AGRN , MCAM ).…”

“…These and other findings have supported an ECM abnormality affecting multiple tissues in ALS patients, although the contribution of this to the disease pathogenesis is not understood [ 82 , 83 ]. Nonetheless, the up-regulation of genes contributing to ECM and cell adhesion proteins was a significant effect of GM6 in the current study, which would be expected to bolster the scaffold supporting axon growth [ 67 , 68 ] or may otherwise influence underlying collagen metabolism deficits in ALS patients [ 84 , 85 ].…”

Decoupled epineurial and axonal deformation in mouse median and ulnar nerves

“…Highly tortuous axons have been hypothesized to emerge through growth‐cone motility or by local guidance of axonal segments by filopodia . On the other hand, mutations in the Trembler‐J mouse cause hypertortuous axons in peripheral nerves, and a variety of extracellular signaling pathways, including the Draxin/Netrin and Slit‐Robo pathways, and guide axonal fasciculation and positioning developmentally. Such studies suggested that axonal architecture may be controlled intrinsically by cytoskeletal dynamics, and also through external influences, including myelination and adhesion …”