Unraveling Axonal Transcriptional Landscapes: Insights from iPSC-Derived Cortical Neurons and Implications for Motor Neuron Degeneration

Neuronal function and pathology are deeply influenced by the distinct molecular profiles of the axon and soma. Traditional studies have often overlooked these differences due to the technical challenges of compartment specific analysis. In this study, we employ a robust RNA-sequencing (RNA-seq) approach, using microfluidic devices, to generate high-quality axonal transcriptomes from iPSC-derived cortical neurons (CNs). We achieve high specificity of axonal fractions, ensuring sample purity without contamination. Comparative analysis revealed a unique and specific transcriptional landscape in axonal compartments, characterized by diverse transcript types, including protein-coding mRNAs, RNAs encoding ribosomal proteins (RPs), mitochondrial-encoded RNAs, and long non-coding RNAs (lncRNAs). Previous works have reported the existence of transcription factors (TFs) in the axon. Here, we detect a set of TFs specific to the axon and indicative of their active participation in transcriptional regulation. To investigate transcripts and pathways essential for central motor neuron (MN) degeneration and maintenance we analyzed KIF1C-knockout (KO) CNs, modeling hereditary spastic paraplegia (HSP), a disorder associated with prominent length-dependent degeneration of central MN axons. We found that several key factors crucial for survival and health were absent in KIF1C-KO axons, highlighting a possible role of these also in other neurodegenerative diseases. Taken together, this study underscores the utility of microfluidic devices in studying compartment-specific transcriptomics in human neuronal models and reveals complex molecular dynamics of axonal biology. The impact of KIF1C on the axonal transcriptome not only deepens our understanding of MN diseases but also presents a promising avenue for exploration of compartment specific disease mechanisms.

Neural regeneration in the human central nervous system-from understanding the underlying mechanisms to developing treatments. Where do we stand today?

Mature neurons in the human central nervous system (CNS) fail to regenerate after injuries. This is a common denominator across different aetiologies, including multiple sclerosis, spinal cord injury and ischemic stroke. The lack of regeneration leads to permanent functional deficits with a substantial impact on patient quality of life, representing a significant socioeconomic burden worldwide. Great efforts have been made to decipher the responsible mechanisms and we now know that potent intra- and extracellular barriers prevent axonal repair. This knowledge has resulted in numerous clinical trials, aiming to promote neuroregeneration through different approaches. Here, we summarize the current understanding of the causes to the poor regeneration within the human CNS. We also review the results of the treatment attempts that have been translated into clinical trials so far.

Chronic demyelination of rabbit lesions is attributable to failed oligodendrocyte progenitor cell repopulation

The failure of remyelination in the human CNS contributes to axonal injury and disease progression in multiple sclerosis (MS). In contrast to regions of chronic demyelination in the human brain, remyelination in murine models is preceded by abundant oligodendrocyte progenitor cell (OPC) repopulation, such that OPC density within regions of demyelination far exceeds that of normal white matter (NWM). As such, we hypothesized that efficient OPC repopulation was a prerequisite of successful remyelination, and that increased lesion volume may contribute to the failure of OPC repopulation in human brain. In this study, we characterized the pattern of OPC activation and proliferation following induction of lysolecithin-induced chronic demyelination in adult rabbits. The density of OPCs never exceeded that of NWM and oligodendrocyte density did not recover even at 6 months post-injection. Rabbit OPC recruitment in large lesions was further characterized by chronic Sox2 expression in OPCs located in the lesion core and upregulation of quiescence-associated Prrx1 mRNA at the lesion border. Surprisingly, when small rabbit lesions of equivalent size to mouse were induced, they too exhibited reduced OPC repopulation. However, small lesions were distinct from large lesions as they displayed an almost complete lack of OPC proliferation following demyelination. These differences in the response to demyelination suggest that both volume dependent and species-specific mechanisms are critical in the regulation of OPC proliferation and lesion repopulation and suggest that alternate models will be necessary to fully understand the mechanisms that contribute to failed remyelination in MS.

Nuclear hormone receptors in demyelinating diseases

Demyelination results from the pathological loss of myelin and is a hallmark of many neurodegenerative diseases. Despite the prevalence of demyelinating diseases, there are no disease modifying therapies that prevent the loss of myelin or promote remyelination. This review aims to summarize studies in the field that highlight the importance of nuclear hormone receptors in the promotion and maintenance of myelination and the relevance of nuclear hormone receptors as potential therapeutic targets for demyelinating diseases. These nuclear hormone receptors include the estrogen receptor, progesterone receptor, androgen receptor, vitamin D receptor, thyroid hormone receptor, peroxisome proliferator-activated receptor, liver X receptor, and retinoid X receptor. Pre-clinical studies in well-established animal models of demyelination have shown a prominent role of these nuclear hormone receptors in myelination through their promotion of oligodendrocyte maturation and development. The activation of the nuclear hormone receptors by their ligands also promotes the synthesis of myelin proteins and lipids in mouse models of demyelination. There are limited clinical studies that focus on how the activation of these nuclear hormone receptors could alleviate demyelination in patients with diseases such as multiple sclerosis (MS). However, the completed clinical trials have reported improved clinical outcome in MS patients treated with the ligands of some of these nuclear hormone receptors. Together, the positive results from both clinical and pre-clinical studies point to nuclear hormone receptors as promising therapeutic targets to counter demyelination.

Tracking the evolution of CNS remyelinating lesion in mice with neutral red dye

Animal models of central nervous system (CNS) demyelination, including toxin-induced focal demyelination and immune-mediated demyelination through experimental autoimmune encephalomyelitis (EAE), have provided valuable insights into the mechanisms of neuroinflammation and CNS remyelination. However, the ability to track changes in transcripts, proteins, and metabolites, as well as cellular populations during the evolution of a focal lesion, has remained challenging. Here, we developed a method to label CNS demyelinating lesions by the intraperitoneal injection of a vital dye, neutral red (NR), into mice before killing. We demonstrate that NR-labeled lesions can be easily identified on the intact spinal cord in both lysolecithin- and EAE-mediated demyelination models. Using fluorescence microscopy, we detected NR in activated macrophages/microglia and astrocytes, but not in oligodendrocytes present in lesions. Importantly, we successfully performed RT-qPCR, Western blot, flow cytometry, and mass spectrometry analysis of precisely dissected NR-labeled lesions at 5, 10, and 20 d postlesion (dpl) and found differential changes in transcripts, proteins, cell populations, and metabolites in lesions over the course of remyelination. Therefore, NR administration is a simple and powerful method to track and analyze the detailed molecular, cellular, and metabolic changes that occur within the lesion microenvironment over time following CNS injury. Furthermore, this method can be used to identify molecular and metabolic pathways that regulate neuroinflammation and remyelination and facilitate the development of therapies to promote repair in demyelinating disorders such as multiple sclerosis.

Regulation of Myelination by Exosome Associated Retinoic Acid Release from NG2-Positive Cells

In the CNS, oligodendrocytes are responsible for myelin formation and maintenance. Following spinal cord injury, oligodendrocyte loss and an inhibitory milieu compromise remyelination and recovery. Here, we explored the role of retinoic acid receptor-beta (RARβ) signaling in remyelination. Using a male Sprague Dawley rat model of PNS-CNS injury, we show that oral treatment with a novel drug like RARβ agonist, C286, induces neuronal expression of the proteoglycan decorin and promotes myelination and differentiation of oligodendrocyte precursor cells (NG2⁺ cells) in a decorin-mediated neuron–glia cross talk. Decorin promoted the activation of RARα in NG2⁺ cells by increasing the availability of the endogenous ligand RA. NG2⁺ cells synthesize RA, which is released in association with exosomes. We found that decorin prevents this secretion through regulation of the EGFR–calcium pathway. Using functional and pharmacological studies, we further show that RARα signaling is both required and sufficient for oligodendrocyte differentiation. These findings illustrate that RARβ and RARα are important regulators of oligodendrocyte differentiation, providing new targets for myelination.

SIGNIFICANCE STATEMENT This study identifies novel therapeutic targets for remyelination after PNS-CNS injury. Pharmacological and knock-down experiments show that the retinoic acid (RA) signaling promotes differentiation of oligodendrocyte precursor cells (OPCs) and remyelination in a cross talk between neuronal RA receptor-beta (RARβ) and RARα in NG2⁺ cells. We show that stimulation of RARα is required for the differentiation of OPCs and we describe for the first time how oral treatment with a RARβ agonist (C286, currently being tested in a Phase 1 trial, ISRCTN12424734) leads to the endogenous synthesis of RA through retinaldehyde dehydrogenase 2 (Raldh2) in NG2 cells and controls exosome-associated-RA intracellular levels through a decorin–Ca²⁺ pathway. Although RARβ has been implicated in distinct aspects of CNS regeneration, this study identifies a novel function for both RARβ and RARα in remyelination.

Pharmacological approaches to intervention in hypomyelinating and demyelinating white matter pathology

White matter disease afflicts both developing and mature central nervous systems. Both cell intrinsic and extrinsic dysregulation result in profound changes in cell survival, axonal metabolism and functional performance. Experimental models of developmental white matter (WM) injury and demyelination have not only delineated mechanisms of signaling and inflammation, but have also paved the way for the discovery of pharmacological approaches to intervention. These reagents have been shown to enhance protection of the mature oligodendrocyte cell, accelerate progenitor cell recruitment and/or differentiation, or attenuate pathological stimuli arising from the inflammatory response to injury. Here we highlight reports of studies in the CNS in which compounds, namely peptides, hormones, and small molecule agonists/antagonists, have been used in experimental animal models of demyelination and neonatal brain injury that affect aspects of excitotoxicity, oligodendrocyte development and survival, and progenitor cell function, and which have been demonstrated to attenuate damage and improve WM protection in experimental models of injury. The molecular targets of these agents include growth factor and neurotransmitter receptors, morphogens and their signaling components, nuclear receptors, as well as the processes of iron transport and actin binding. By surveying the current evidence in non-immune targets of both the immature and mature WM, we aim to better understand pharmacological approaches modulating endogenous oligodendroglia that show potential for success in the contexts of developmental and adult WM pathology. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

The role of immune cells, glia and neurons in white and gray matter pathology in multiple sclerosis

Multiple sclerosis is one of the most common causes of chronic neurological disability beginning in early to middle adult life. Multiple sclerosis is idiopathic in nature, yet increasing correlative evidence supports a strong association between one's genetic predisposition, the environment and the immune system. Symptoms of multiple sclerosis have primarily been shown to result from a disruption in the integrity of myelinated tracts within the white matter of the central nervous system. However, recent research has also highlighted the hitherto underappreciated involvement of gray matter in multiple sclerosis disease pathophysiology, which may be especially relevant when considering the accumulation of irreversible damage and progressive disability. This review aims at providing a comprehensive overview of the interplay between inflammation, glial/neuronal damage and regeneration throughout the course of multiple sclerosis via the analysis of both white and gray matter lesional pathology. Further, we describe the common pathological mechanisms underlying both relapsing and progressive forms of multiple sclerosis, and analyze how current (as well as future) treatments may interact and/or interfere with its pathology. Understanding the putative mechanisms that drive disease pathogenesis will be key in helping to develop effective therapeutic strategies to prevent, mitigate, and treat the diverse morbidities associated with multiple sclerosis.

Daam2-PIP5K is a regulatory pathway for Wnt signaling and therapeutic target for remyelination in the CNS

Wnt signaling plays an essential role in developmental and regenerative myelination of the CNS; however, contributions of proximal regulators of the Wnt receptor complex to these processes remain undefined. To identify components of the Wnt pathway that regulate these processes, we applied a multifaceted discovery platform and found that Daam2-PIP5K comprise a novel pathway regulating Wnt signaling and myelination. Using dorsal patterning of the chick spinal cord we found that Daam2 promotes Wnt signaling and receptor complex formation through PIP5K-PIP2. Analysis of Daam2 function in oligodendrocytes (OLs) revealed that it suppresses OL differentiation during development, after white matter injury (WMI), and is expressed in human white matter lesions. These findings suggest a pharmacological strategy to inhibit Daam2-PIP5K function, application of which stimulates remyelination after WMI. Put together, our studies integrate information from multiple systems to identify a novel regulatory pathway for Wnt signaling and potential therapeutic target for WMI.

Current status of myelin replacement therapies in multiple sclerosis

Multiple sclerosis is an autoimmune disease of the human central nervous system characterized by immune-mediated myelin and axonal damage, and chronic axonal loss attributable to the absence of myelin sheaths. There are two aspects to the treatment of MS-first, the prevention of damage by suppressing the maladaptive immune system, and second, the long-term preservation of axons by the promotion of remyelination, a regenerative process in which new axons are restored to demyelinated axons. Medicine has made significant progress in the first of these in recent years-there is an increasing number of ever more effective disease-modifying immunomodulatory interventions. However, there are currently no widely used regenerative therapies in MS. Conceptually, there are two approaches to remyelination therapy-transplantation of myelinogenic cells and promotion of endogenous remyelination mediated by myelinogenic cells present within the diseased tissue. In this chapter, in addition to describing why remyelination therapies are important, we review both these approaches, outlining their current status and future developments.