Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

C286, an orally available retinoic acid receptor β agonist drug, regulates multiple pathways to achieve spinal cord injury repair

Retinoic acid receptor β2 (RARβ2) is an emerging therapeutic target for spinal cord injuries (SCIs) with a unique multimodal regenerative effect. We have developed a first-in-class RARβ agonist drug, C286, that modulates neuron-glial pathways to induce functional recovery in a rodent model of sensory root avulsion. Here, using genome-wide and pathway enrichment analysis of avulsed rats' spinal cords, we show that C286 also influences the extracellular milieu (ECM). Protein expression studies showed that C286 upregulates tenascin-C, integrin-α9, and osteopontin in the injured cord. Similarly, C286 remodulates these ECM molecules, hampers inflammation and prevents tissue loss in a rodent model of spinal cord contusion C286. We further demonstrate C286's efficacy in human iPSC-derived neurons, with treatment resulting in a significant increase in neurite outgrowth. Additionally, we identify a putative efficacy biomarker, S100B, which plasma levels correlated with axonal regeneration in nerve-injured rats. We also found that other clinically available retinoids, that are not RARβ specific agonists, did not lead to functional recovery in avulsed rats, demonstrating the requirement for RARβ specific pathways in regeneration. In a Phase 1 trial, the single ascending dose (SAD) cohorts showed increases in expression of RARβ2 in white blood cells correlative to increased doses and at the highest dose administered, the pharmacokinetics were similar to the rat proof of concept (POC) studies. Collectively, our data suggests that C286 signalling in neurite/axonal outgrowth is conserved between species and across nerve injuries. This warrants further clinical testing of C286 to ascertain POC in a broad spectrum of neurodegenerative conditions.

Early embryogenesis in CHDFIDD mouse model reveals facial clefts and altered cranial neurogenesis

CDK13-related disorder, also known as congenital heart defects, dysmorphic facial features and intellectual developmental disorder (CHDFIDD) is associated with mutations in the CDK13 gene encoding transcription-regulating cyclin-dependent kinase 13 (CDK13). Here, we focused on the development of craniofacial structures and analyzed early embryonic stages in CHDFIDD mouse models, with one model comprising a hypomorphic mutation in Cdk13 and exhibiting cleft lip/palate, and another model comprising knockout of Cdk13, featuring a stronger phenotype including midfacial cleft. Cdk13 was found to be physiologically expressed at high levels in the mouse embryonic craniofacial structures, namely in the forebrain, nasal epithelium and maxillary mesenchyme. We also uncovered that Cdk13 deficiency leads to development of hypoplastic branches of the trigeminal nerve including the maxillary branch. Additionally, we detected significant changes in the expression levels of genes involved in neurogenesis (Ache, Dcx, Mef2c, Neurog1, Ntn1, Pou4f1) within the developing palatal shelves. These results, together with changes in the expression pattern of other key face-specific genes (Fgf8, Foxd1, Msx1, Meis2 and Shh) at early stages in Cdk13 mutant embryos, demonstrate a key role of CDK13 in the regulation of craniofacial morphogenesis.

Netrin-1 as A neural guidance protein in development and reinnervation of the larynx

Neural guidance proteins participate in motor neuron migration, axonal projection, and muscle fiber innervation during development. One of the guidance proteins that participates in axonal pathfinding is Netrin-1. Despite the well-known role of Netrin-1 in embryogenesis of central nervous tissue, it is still unclear how the expression of this guidance protein contributes to primary innervation of the periphery, as well as reinnervation. This is especially true in the larynx where Netrin-1 is upregulated within the intrinsic laryngeal muscles after nerve injury and where blocking of Netrin-1 alters the pattern of reinnervation of the intrinsic laryngeal muscles. Despite this consistent finding, it is unknown how Netrin-1 expression contributes to guidance of the axons towards the larynx. Improved knowledge of Netrin-1's role in nerve regeneration and reinnervation post-injury in comparison to its role in primary innervation during embryological development, may provide insights in the search for therapeutics to treat nerve injury. This paper reviews the known functions of Netrin-1 during the formation of the central nervous system and during cranial nerve primary innervation. It also describes the role of Netrin-1 in the formation of the larynx and during recurrent laryngeal reinnervation following nerve injury in the adult.

Quantitative Biofabrication Platform for Collagen-Based Peripheral Nerve Grafts with Structural and Chemical Guidance

Owing to its crucial role in the human body, collagen has immense potential as a material for the biofabrication of tissues and organs. However, highly refined fabrication using collagen remains difficult, primarily because of its notably soft properties. A quantitative biofabrication platform to construct collagen-based peripheral nerve grafts, incorporating bionic structural and chemical guidance cues, is introduced. A viscoelastic model for collagen, which facilitates simulating material relaxation and fabricating collagen-based neural grafts, achieving a maximum channel density similar to that of the native nerve structure of longitudinal microchannel arrays, is established. For axonal regeneration over considerable distances, a gradient printing control model and quantitative method are developed to realize the high-precision gradient distribution of nerve growth factor required to obtain nerve grafts through one-step bioprinting. Experiments verify that the bioprinted graft effectively guides linear axonal growth in vitro and in vivo. This study should advance biofabrication methods for a variety of human tissue-engineering applications requiring tailored cues.

Tissue-Targeted Transcriptomics Reveals SEMA3D Control of Hypoglossal Nerve Projection to Mouse Tongue Primordia

The mouse hypoglossal nerve originates in the occipital motor nuclei at embryonic day (E)10.5 and projects a long distance, reaching the vicinity of the tongue primordia, the lateral lingual swellings, at E11.5. However, the details of how the hypoglossal nerve correctly projects to the primordia are poorly understood. To investigate the molecular basis of hypoglossal nerve elongation, we used a novel transcriptomic approach using the ROKU method. The ROKU algorithm identified 3825 genes specific for lateral lingual swellings at E11.5, of which 34 genes were predicted to be involved in axon guidance. Ingenuity Pathway Analysis-assisted enrichment revealed activation of the semaphorin signaling pathway during tongue development, and quantitative PCR showed that the expressions of Sema3d and Nrp1 in this pathway peaked at E11.5. Immunohistochemistry detected NRP1 in the hypoglossal nerve and SEMA3D as tiny granules in the extracellular space beneath the epithelium of the tongue primordia and in lateral and anterior regions of the mandibular arch. Fewer SEMA3D granules were localized around hypoglossal nerve axons and in the space where they elongated. In developing tongue primordia, tissue-specific regulation of SEMA3D might control the route of hypoglossal nerve projection via its repulsive effect on NRP1.

Improving Schwann Cell Differentiation from Human Adipose Stem Cells with Metabolic Glycoengineering

Schwann cells (SCs) are myelinating cells that promote peripheral nerve regeneration. When nerve lesions form, SCs are destroyed, ultimately hindering nerve repair. The difficulty in treating nerve repair is exacerbated due to SC's limited and slow expansion capacity. Therapeutic use of adipose-derived stem cells (ASCs) is emerging in combating peripheral nerve injury due to these cells' SC differentiation capability and can be harvested easily in large numbers. Despite ASC's therapeutic potential, their transdifferentiation period typically takes more than two weeks. In this study, we demonstrate that metabolic glycoengineering (MGE) technology enhances ASC differentiation into SCs. Specifically, the sugar analog Ac5ManNTProp (TProp), which modulates cell surface sialylation, significantly improved ASC differentiation with upregulated SC protein S100β and p75NGFR expression and elevated the neurotrophic factors nerve growth factor beta (NGFβ) and glial cell-line-derived neurotrophic factor (GDNF). TProp treatment remarkably reduced the SC transdifferentiation period from about two weeks to two days in vitro, which has the potential to improve neuronal regeneration and facilitate future use of ASCs in regenerative medicine.

Enteric Neural Network Assembly Was Promoted by Basic Fibroblast Growth Factor and Vitamin A but Inhibited by Epidermal Growth Factor

Extending well beyond the original use of propagating neural precursors from the central nervous system and dorsal root ganglia, neurosphere medium (NSM) and self-renewal medium (SRM) are two distinct formulas with widespread popularity in enteric neural stem cell (ENSC) applications. However, it remains unknown what growth factors or nutrients are crucial to ENSC development, let alone whether the discrepancy in their components may affect the outcomes of ENSC culture. Dispersed enterocytes from murine fetal gut were nurtured in NSM, SRM or their modifications by selective component elimination or addition to assess their effects on ENSC development. NSM generated neuriteless neurospheres, whereas SRM, even deprived of chicken embryo extract, might wire ganglia together to assemble neural networks. The distinct outcomes came from epidermal growth factor, which inhibited enteric neuronal wiring in NSM. In contrast, basic fibroblast growth factor promoted enteric neurogenesis, gangliogenesis, and neuronal wiring. Moreover, vitamin A derivatives might facilitate neuronal maturation evidenced by p75 downregulation during ENSC differentiation toward enteric neurons to promote gangliogenesis and network assembly. Our results might help to better manipulate ENSC propagation and differentiation in vitro, and open a new avenue for the study of enteric neuronal neuritogenesis and synaptogenesis.

The regulatory landscape of neurite development in Caenorhabditis elegans

Neuronal communication requires precise connectivity of neurite projections (axons and dendrites). Developing neurites express cell-surface receptors that interpret extracellular cues to enable correct guidance toward, and connection with, target cells. Spatiotemporal regulation of neurite guidance molecule expression by transcription factors (TFs) is critical for nervous system development and function. Here, we review how neurite development is regulated by TFs in the Caenorhabditis elegans nervous system. By collecting publicly available transcriptome and ChIP-sequencing data, we reveal gene expression dynamics during neurite development, providing insight into transcriptional mechanisms governing construction of the nervous system architecture.