Super-resolution imaging pinpoints the periodic ultrastructure at the human node of Ranvier and its disruption in patients with polyneuropathy

Contactin -Associated protein1 Regulates Autophagy by Modulating the PI3K/AKT/mTOR Signaling Pathway and ATG4B Levels in Vitro and in Vivo

Contactin-associated protein1 (Caspr1) plays an important role in the formation and stability of myelinated axons. In Caspr1 mutant mice, autophagy-related structures accumulate in neurons, causing axonal degeneration; however, the mechanism by which Caspr1 regulates autophagy remains unknown. To illustrate the mechanism of Caspr1 in autophagy process, we demonstrated that Caspr1 knockout in primary neurons from mice along with human cell lines, HEK-293 and HeLa, induced autophagy by downregulating the PI3K/AKT/mTOR signaling pathway to promote the conversion of microtubule-associated protein light chain 3 I (LC3-I) to LC3-II. In contrast, Caspr1 overexpression in cells contributed to the upregulation of this signaling pathway. We also demonstrated that Caspr1 knockout led to increased LC3-I protein expression in mice. In addition, Caspr1 could inhibit the expression of autophagy-related 4B cysteine peptidase (ATG4B) protein by directly binding to ATG4B in overexpressed Caspr1 cells. Intriguingly, we found an accumulation of ATG4B in the Golgi apparatuses of cells overexpressing Caspr1; therefore, we speculate that Caspr1 may restrict ATG4 secretion from the Golgi apparatus to the cytoplasm. Collectively, our results indicate that Caspr1 may regulate autophagy by modulating the PI3K/AKT/mTOR signaling pathway and the levels of ATG4 protein, both in vitro and in vivo. Thus, Caspr1 can be a potential therapeutic target in axonal damage and demyelinating diseases.

Deep learning-driven automated high-content dSTORM imaging with a scalable open-source toolkit

Super-resolution microscopy offers the ability to visualize molecular structures in biological samples with unprecedented detail. However, the full potential of these techniques is often hindered by a lack of automated, user-independent workflows. Here, we present an open-source toolkit that automates dSTORM super-resolution microscopy using deep learning for segmentation and object detection. This standalone program enables reliable segmentation of diverse biomedical images, even in low-contrast samples, surpassing existing solutions. Integrated into the imaging pipeline, it rapidly processes high-content data in minutes, reducing manual labor. Demonstrated by biological examples, such as microtubules in cell culture and the βII-spectrin in nerve fibers, our approach makes super-resolution imaging faster, more robust, and easy to use, even by nonexperts. This broadens its potential applications in biomedicine, including high-throughput experimentation.

Super-resolution of nodal and paranodal disruption in anti-pan-neurofascin-associated autoimmune nodopathy

Introduction

In autoimmune nodopathies, autoantibodies target the nodes of Ranvier, impairing saltatory nerve conduction. Understanding the impact of autoantibody binding on protein assembly is crucial for gaining insights into the pathogenicity of different autoantibodies. We investigated nodal, paranodal, and cytoskeletal axonal proteins in teased fibers from a sural nerve biopsy of a patient with anti-pan-neurofascin autoantibodies. Conventional diagnostic tools, including fluorescence microscopy, often miss subtle alterations at the ultrastructural level.

Methods

We utilized direct stochastic optical reconstruction microscopy (dSTORM), a super-resolution fluorescence imaging technique, to assess the nanoscale architecture of nodal, paranodal, and cytoskeletal axonal proteins.

Result

While conventional fluorescence microscopy revealed severe paranodal and nodal damage in 14% of the nodes, with 86% appearing normal at first glance, the super-resolved images revealed a decreased neurofascin-155 and Caspr-1 density, but preserved colocalization of these adhesion proteins in paranodes that initially seemed normal. At the nodes, sodium channel density and distribution remained intact, but neurofascin-186 density was reduced. Axonal beta-IV spectrin was altered only in severely damaged nodes. This indicates that axonal integrity is largely preserved, with a potentially reversible decrease in paranodal and nodal adhesion proteins in patients with nodopathy revealing subtle alterations in nodal integrity that are not apparent with conventional imaging.

Discussion

These likely reversible changes may explain the rapid recovery seen in patients with anti-pan-neurofascin autoantibodies following autoantibody depletion. Conversely, the small percentage of severely and axonally damaged nodes may account for the residual symptoms experienced by most patients.

The node of Ranvier influences the in vivo axonal transport of mitochondria and signaling endosomes

Efficient long-range axonal transport is essential for maintaining neuronal function, and perturbations in this process underlie severe neurological diseases. Nodes of Ranvier (NoR) are short, specialized unmyelinated axonal domains with a unique molecular and structural composition. Currently, it remains unresolved how the distinct molecular structures of the NoR impact axonal transport dynamics. Using intravital time-lapse microscopy of sciatic nerves in live, anesthetized mice, we reveal (1) similar morphologies of the NoR in fast and slow motor axons, (2) signaling endosomes and mitochondria accumulate specifically at the distal node, and (3) unique axonal transport profiles of signaling endosomes and mitochondria transiting through the NoR. Collectively, these findings provide important insights into the fundamental physiology of peripheral nerve axons, motor neuron subtypes, and diverse organelle dynamics at the NoR. Furthermore, this work has relevance for several pathologies affecting peripheral nerves and the NoR.

The iron maiden: Oligodendroglial metabolic dysfunction in multiple sclerosis and mitochondrial signaling

Multiple sclerosis (MS) is an autoimmune disease, governed by oligodendrocyte (OL) dystrophy and central nervous system (CNS) demyelination manifesting variable neurological impairments. Mitochondrial mechanisms may drive myelin biogenesis maintaining the axo-glial unit according to dynamic requisite demands imposed by the axons they ensheath. The promotion of OL maturation and myelination by actively transporting thyroid hormone (TH) into the CNS and thereby facilitating key transcriptional and metabolic pathways that regulate myelin biogenesis is fundamental to sustain the profound energy demands at each axo-glial interface. Deficits in regulatory functions exerted through TH for these physiological roles to be orchestrated by mature OLs, can occur in genetic and acquired myelin disorders, whereby mitochondrial efficiency and eventual dysfunction can lead to profound oligodendrocytopathy, demyelination and neurodegenerative sequelae. TH-dependent transcriptional and metabolic pathways can be dysregulated during acute and chronic MS lesion activity depriving OLs from critical acetyl-CoA biochemical mechanisms governing myelin lipid biosynthesis and at the same time altering the generation of iron metabolism that may drive ferroptotic mechanisms, leading to advancing neurodegeneration.