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Neural circuits are formed with great precision during development. Accumulated evidence over the past three decades has demonstrated that growing axons are navigated toward their targets by the combined actions of attractants and repellents together with their receptors. It has long been known that proteoglycans, glycosylated proteins possessing covalently attached glycosaminoglycans, play a critical role in axon guidance; however, the molecular mechanisms by which proteoglycans regulate axon behaviors remain largely unknown. Glycosaminoglycans such as heparan sulfate and chondroitin sulfate are large linear polysaccharides composed of repeating disaccharide units that are highly modified by specific sulfation and epimerization. Recent biochemical and molecular biological studies have identified the enzymes that are involved in the biosynthesis of glycosaminoglycans. Interestingly, many mutants lacking glycosaminoglycan-synthesizing enzymes or proteoglycans in several model organisms show defects in specific nerve tract formation. In parallel, detailed biochemical studies have identified the molecular interactions between axon guidance molecules and glycosaminoglycans that have specific modification in their sugar chains. This review summarizes the structure and function of axon guidance molecules and glycosaminoglycans, and then tries to combine the knowledge from these studies to understand the role of proteoglycans from a new vantage point. Deciphering the sugar code is important for understanding the complicated nature of proteoglycans in axon guidance. Neural circuits are formed by the combined actions of axon guidance molecules. Proteoglycans play critical roles in regulating axon guidance through the interaction between signaling molecules and glycosaminoglycan chains attached to the core protein. This paper summarizes the structure and functions of axon guidance molecules and glycosaminoglycans and reviews the molecular mechanisms by which proteoglycans regulate axon guidance from a new vantage point. This article is part of the 60th Anniversary special issue., (© 2016 International Society for Neurochemistry.)

In cell transplantation therapy for the treatment of neurodegenerative disorders, encapsulation of implanted cells in a semipermeable membrane is a promising approach to protect the implanted cells from host immune rejection and inhibit the invasion of tumor into surrounding tissue if the implanted cells form a tumor after transplantation. However, implanted neurons isolated by capsules could not build connections with host neurons, preventing the implanted neurons from responding to stimuli from host neurons. In the present study, we focused on the passage of neurites and axons navigated by axon guidance molecules through membrane pores to enable encapsulated neurons and host neurons to form connections. The type of matrix coated on membranes and the pore size of the membranes greatly affected the successful passage of PC12 neurites through membrane pores. PC12 neurites preferably passed through collagen-coated membranes with pores greater than 0.8 μm in diameter, but the neurites did not pass through albumin- or fibronectin-coated membranes or membranes with pores less than 0.1 μm in diameter. We could navigate the direction of commissural neural axon extensions by utilizing the axon guidance molecules secreted from floor plate and make guided axons pass through the membrane pores. These results suggest the feasibility of building connections between encapsulated neurons and host neurons by encapsulating the implanted neurons and axon guidance molecules, which attract the axons of host neurons into the capsule, in the porous membranes with suitable pore size and matrix coating.

Cortical excitatory neurons migrate from their origin in the ventricular zone (VZ) toward the pial surface. During migration, these neurons exhibit a stellate shape in the intermediate zone (IZ), transform into bipolar cells, and then initiate radial migration, extending a trailing process, which may lead to an axon. Here we examined the role of neuropilin 1 (NRP1) in these developmental events. Both NRP1 mRNA and protein were highly expressed in the IZ, where stellate-shaped cells were located. DiI labeling experiments showed that neuronal migration occurred normally in Nrp1 mutant mice up to embryonic day (E) 14.5, the latest day to which the mutant survives, with only subtle axonal defasciculation. However, interference with Nrp1 signaling at a later stage caused pathfinding errors: when a dominant negative form of Nrp1 was electroporated into the cortical VZ cells at E12.5 or E15.5 and examined perinatally, guidance errors were found in tangential axonal extension toward the midline. In contrast, no significant effect was noted on the migration of cortical excitatory neurons. These findings indicate that NRP1 plays an important role in the guidance of callosal axons originating from cortical excitatory neurons but does not support a role in their migration. Moreover, insofar as radial axonal extension within the cortical plate was unaffected, the present findings imply that molecular mechanisms for the axonal extension of excitatory neurons within the cortical plate are distinct from those in the white matter.

The transcriptional corepressor SnoN is a critical regulator of axonal morphogenesis, but how SnoN drives axonal growth is unknown. Here, we report that gene-profiling analyses in cerebellar granule neurons reveal that the large majority of genes altered upon SnoN knockdown are surprisingly downregulated, suggesting that SnoN may activate transcription in neurons. Accordingly, we find that the transcriptional coactivator p300 interacts with SnoN, and p300 plays a critical role in SnoN-induced axon growth. We also identify the gene encoding the signaling scaffold protein Ccd1 as a critical target of SnoN in neurons. Ccd1 localizes to the actin cytoskeleton, is enriched at axon terminals in neurons, and activates the axon growth-promoting kinase JNK (c-Jun N-terminal protein kinase). Knockdown of Ccd1 in neurons reduces axonal length and suppresses the ability of SnoN to promote axonal growth. Importantly, Ccd1 knockdown in rat pups profoundly impairs the formation of granule neuron parallel fiber axons in the rat cerebellar cortex in vivo. These findings define a novel SnoN-Ccd1 link that promotes axonal growth in the mammalian brain, with important implications for axonal development and regeneration.

The corticospinal tract (CST) plays an important role in controlling voluntary movement. Because the CST has a long trajectory throughout the brain toward the spinal cord, many axon guidance molecules are required to navigate the axons correctly during development. Previously, we found that double-knockout (DKO) mouse embryos lacking the heparan sulfate endosulfatases, Sulf1 and Sulf2 , showed axon guidance defects of the CST owing to the abnormal accumulation of Slit2 protein on the brain surface. However, postnatal development of the CST, especially the pyramidal decussation and spinal cord projection, could not be assessed because DKO mice on a C57BL/6 background died soon after birth. We recently found that Sulf1/2 DKO mice on a mixed C57BL/6 and CD-1/ICR background can survive into adulthood and therefore investigated the anatomy and function of the CST in the adult DKO mice. In Sulf1/2 DKO mice, abnormal dorsal deviation of the CST fibers on the midbrain surface persisted after maturation of the CST. At the pyramidal decussation, some CST fibers located near the midline crossed the midline, whereas others located more laterally extended ipsilaterally. In the spinal cord, the crossed CST fibers descended in the dorsal funiculus on the contralateral side and entered the contralateral gray matter normally, whereas the uncrossed fibers descended in the lateral funiculus on the ipsilateral side and entered the ipsilateral gray matter. As a result, the CST fibers that originated from 1 side of the brain projected bilaterally in the DKO spinal cord. Consistently, microstimulation of 1 side of the motor cortex evoked electromyogram responses only in the contralateral forelimb muscles of the wild-type mice, whereas the same stimulation evoked bilateral responses in the DKO mice. The functional consequences of the CST defects in the Sulf1/2 DKO mice were examined using the grid-walking, staircase, and single pellet-reaching tests, which have been used to evaluate motor function in mice. Compared with the wild-type mice, the Sulf1/2 DKO mice showed impaired performance in these tests, indicating deficits in motor function. These findings suggest that disruption of Sulf1/2 genes leads to both anatomical and functional defects of the CST. [ABSTRACT FROM AUTHOR]