Cholesterol synthesis inhibition promotes axonal regeneration in the injured central nervous system

Integrated multi-omics analysis reveals molecular changes associated with chronic lipid accumulation following contusive spinal cord injury

Functional and pathological recovery after spinal cord injury (SCI) is often incomplete due to the limited regenerative capacity of the central nervous system (CNS), which is further impaired by several mechanisms that sustain tissue damage. Among these, the chronic activation of immune cells can cause a persistent state of local CNS inflammation and damage. However, the mechanisms that sustain this persistent maladaptive immune response in SCI have not been fully clarified yet. In this study, we integrated histological analyses with proteomic, lipidomic, transcriptomic, and epitranscriptomic approaches to study the pathological and molecular alterations that develop in a mouse model of cervical spinal cord hemicontusion. We found significant pathological alterations of the lesion rim with myelin damage and axonal loss that persisted throughout the late chronic phase of SCI. This was coupled by a progressive lipid accumulation in myeloid cells, including resident microglia and infiltrating monocyte-derived macrophages. At tissue level, we found significant changes of proteins indicative of glycolytic, tricarboxylic acid cycle (TCA), and fatty acid metabolic pathways with an accumulation of triacylglycerides with C16:0 fatty acyl chains in chronic SCI. Following transcriptomic, proteomic, and epitranscriptomic studies identified an increase of cholesterol and m6A methylation in lipid-droplet-accumulating myeloid cells as a core feature of chronic SCI. By characterizing the multiple metabolic pathways altered in SCI, our work highlights a key role of lipid metabolism in the chronic response of the immune and central nervous system to damage.

P-aminobenzoic acid promotes retinal regeneration through activation of Ascl1a in zebrafish

JOURNAL/nrgr/04.03/01300535-202408000-00040/figure1/v/2023-12-16T180322Z/r/image-tiff The retina of zebrafish can regenerate completely after injury. Multiple studies have demonstrated that metabolic alterations occur during retinal damage; however to date no study has identified a link between metabolites and retinal regeneration of zebrafish. Here, we performed an unbiased metabolome sequencing in the N-methyl-D-aspartic acid-damaged retinas of zebrafish to demonstrate the metabolomic mechanism of retinal regeneration. Among the differentially-expressed metabolites, we found a significant decrease in p-aminobenzoic acid in the N-methyl-D-aspartic acid-damaged retinas of zebrafish. Then, we investigated the role of p-aminobenzoic acid in retinal regeneration in adult zebrafish. Importantly, p-aminobenzoic acid activated Achaetescute complex-like 1a expression, thereby promoting Müller glia reprogramming and division, as well as Müller glia-derived progenitor cell proliferation. Finally, we eliminated folic acid and inflammation as downstream effectors of PABA and demonstrated that PABA had little effect on Müller glia distribution. Taken together, these findings show that PABA contributes to retinal regeneration through activation of Achaetescute complex-like 1a expression in the N-methyl-D-aspartic acid-damaged retinas of zebrafish.

MircoRNA-25-3p in skin precursor cell-induced Schwann cell-derived extracellular vesicles promotes axon regeneration by targeting Tgif1

Nerve injury often leads to severe dysfunction because of the lack of axon regeneration in adult mammal. Intriguingly a series of extracellular vesicles (EVs) have the obvious ability to accelerate the nerve repair. However, the detailed molecular mechanisms to describe that EVs switch neuron from a transmitter to a regenerative state have not been elucidated. This study elucidated the microRNA (miRNA) expression profiles of two types of EVs that promote nerve regeneration. The functions of these miRNAs were screened in vitro. Among the 12 overlapping miRNAs, miR-25-3p was selected for further analysis as it markedly promoted axon regeneration both in vivo and in vitro. Furthermore, knockdown experiments confirmed that PTEN and Klf4, which are the major inhibitors of axon regeneration, were the direct targets of miR-25-3p in dorsal root ganglion (DRG) neurons. The utilization of luciferase reporter assays and functional tests provided evidence that miR-25-3p enhances axon regeneration by targeting Tgif1. Additionally, miR-25-3p upregulated the phosphorylation of Erk. Furthermore, Rapamycin modulated the expression of miR-25-3p in DRG neurons. Finally, the pro-axon regeneration effects of EVs were confirmed by overexpressing miR-25-3p and Tgif1 knockdown in the optic nerve crush model. Thus, the enrichment of miR-25-3p in EVs suggests that it regulates axon regeneration, proving a potential cell-free treatment strategy for nerve injury.

Non-linear relationship between serum cholesterol levels and cognitive change among older people in the preclinical and prodromal stages of dementia: a retrospective longitudinal study in Taiwan

BACKGROUND

Adverse effects of rigorously lowering low-density lipoprotein cholesterol on cognition have been reported; therefore, we aimed to study the contribution of serum cholesterol in cognitive decline in older people with or without dementia.

METHODS

Cognitive function was assessed by the Cognitive Abilities Screening Instrument (CASI). We investigated associations between serum cholesterol with cognitive decline using multiple regressions controlling for the effects of demographics, vascular risk factors, and treatments.

RESULTS

Most associations between cholesterol and CASI scores could be explained by non-linear and inverted U-shaped relationships (R2 = 0.003-0.006, p < 0.016, Šidákcorrection). The relationships were most evident between changes in cholesterol and CASI scores in older people at the preclinical or prodromal stages of dementia (R2 = 0.02-0.064, p values < 0.016). There were no differences in level of changes in CASI scores between individuals in 1st decile and 10th decile groups of changes in cholesterol (p = 0.266-0.972). However, individuals in the 1st decile of triglyceride changes and with stable and normal cognitive functions showed significant improvement in CASI scores compared to those in the 10th decile (t(202) = 2.275, p values < 0.05).

CONCLUSION

These findings could implicate that rigorously lowering cholesterol may not be suitable for the prevention of cognitive decline among older people, especially among individuals in preclinical or prodromal stages of dementia.

Traumatic Optic Neuropathy: Update on Management

Traumatic optic neuropathy is one of the causes of visual loss caused by blunt or penetrating head trauma and is classified as both direct and indirect. Clinical history and examination findings usually allow for the diagnosis of traumatic optic neuropathy. There is still controversy surrounding the management of traumatic optic neuropathy; some physicians advocate observation alone, while others recommend steroid therapy, surgery, or both. In this entry, we tried to highlight traumatic optic neuropathy's main pathophysiologic mechanisms with the most available updated treatment. Recent research suggests future therapies that may be helpful in traumatic optic neuropathy cases.

Delayed administration of elezanumab, a human anti-RGMa neutralizing monoclonal antibody, promotes recovery following cervical spinal cord injury

Spinal cord injury (SCI) elicits a cascade of degenerative events including cell death, axonal degeneration, and the upregulation of inhibitory molecules which limit repair. Repulsive guidance molecule A (RGMa) is an axon growth inhibitor which is also involved in neuronal cell death and differentiation. SCI causes upregulation of RGMa in the injured rodent, non-human primate, and human spinal cord. Recently, we showed that delayed administration of elezanumab, a high affinity human RGMa-specific monoclonal antibody, promoted neuroprotective and regenerative effects following thoracic SCI. Since most human traumatic SCI is at the cervical level, and level-dependent anatomical and molecular differences may influence pathophysiological responses to injury and treatment, we examined the efficacy of elezanumab and its therapeutic time window of administration in a clinically relevant rat model of cervical impact-compression SCI. Pharmacokinetic analysis of plasma and spinal cord tissue lysate showed comparable levels of RGMa antibodies with delayed administration following cervical SCI. At 12w after SCI, elezanumab promoted long term benefits including perilesional sparing of motoneurons and increased neuroplasticity of key descending pathways involved in locomotion and fine motor function. Elezanumab also promoted growth of corticospinal axons into spinal cord gray matter and enhanced serotonergic innervation of the ventral horn to form synaptic connections caudal to the cervical lesion. Significant recovery in grip and trunk/core strength, locomotion and gait, and spontaneous voiding ability was found in rats treated with elezanumab either immediately post-injury or at 3 h post-SCI, and improvements in specific gait parameters were found when elezanumab was delayed to 24 h post-injury. We also developed a new locomotor score, the Cervical Locomotor Score, a simple and sensitive measure of trunk/core and limb strength and stability during dynamic locomotion.

Cholesterol synthesis inhibition or depletion in axon regeneration

Cholesterol is biosynthesized by all animal cells. Beyond its metabolic role in steroidogenesis, it is enriched in the plasma membrane where it has key structural and regulatory functions. Cholesterol is thus presumably important for post-injury axon regrowth, and this notion is supported by studies showing that impairment of local cholesterol reutilization impeded regeneration. However, several studies have also shown that statins, inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase, are enhancers of axon regeneration, presumably acting through an attenuation of the mevalonate isoprenoid pathway and consequent reduction in protein prenylation. Several recent reports have now shown that cholesterol depletion, as well as inhibition of cholesterol synthesis per se, enhances axon regeneration. Here, I discussed these findings and propose some possible underlying mechanisms. The latter would include possible disruptions to axon growth inhibitor signaling by lipid raft-localized receptors, as well as other yet unclear neuronal survival signaling process enhanced by cholesterol lowering or depletion.

Traumatic optic neuropathy: a review of current studies

Traumatic optic neuropathy (TON) is a serious complication of craniofacial trauma that directly or indirectly damages the optic nerve and can cause severe vision loss. The incidence of TON has been gradually increasing in recent years. Research on the protection and regeneration of the optic nerve after the onset of TON is still at the level of laboratory studies and which is insufficient to support clinical treatment of TON. And, due to without clear guidelines, there is much ambiguity regarding its diagnosis and management. Clinical interventions for TON include observation only, treatment with corticosteroids alone, or optic canal (OC) decompression (with or without steroids). There is controversy in clinical practice concerning which treatment is the best. A review of available studies shows that the visual acuity of patients with TON can be significantly improved after OC decompression surgery (especially endoscopic transnasal/transseptal optic canal decompression (ETOCD)) with or without the use of corticosteroids. And new findings of laboratory studies such as mitochondrial therapy, lipid change studies, and other studies in favor of TON therapy have also been identified. In this review, we discuss the evolving perspective of surgical treatment and experimental study.

Multi-Dimensional Transcriptome Analysis Reveals Modulation of Cholesterol Metabolism as Highly Integrated Response to Brain Injury

Zebrafish is an attractive model to investigate regeneration of the nervous system. Despite major progress in our understanding of the underlying processes, the transcriptomic changes are largely unknown. We carried out a computational analysis of the transcriptome of the regenerating telencephalon integrating changes in the expression of mRNAs, their splice variants and investigated the putative role of regulatory RNAs in the modulation of these transcriptional changes. Profound changes in the expression of genes and their splice variants engaged in many distinct processes were observed. Differential transcription and splicing are important processes in response to injury of the telencephalon. As exemplified by the coordinated regulation of the cholesterol synthesizing enzymes and transporters, the genome responded to injury of the telencephalon in a multi-tiered manner with distinct and interwoven changes in expression of enzymes, transporters and their regulatory molecules. This coordinated genomic response involved a decrease of the mRNA of the key transcription factor SREBF2, induction of microRNAs (miR-182, miR-155, miR-146, miR-31) targeting cholesterol genes, shifts in abundance of splice variants as well as regulation of long non-coding RNAs. Cholesterol metabolism appears to be switched from synthesis to relocation of cholesterol. Based on our in silico analyses, this switch involves complementary and synergistic inputs by different regulatory principles. Our studies suggest that adaptation of cholesterol metabolism is a key process involved in regeneration of the injured zebrafish brain.

One Raft to Guide Them All, and in Axon Regeneration Inhibit Them

Central nervous system damage caused by traumatic injuries, iatrogenicity due to surgical interventions, stroke and neurodegenerative diseases is one of the most prevalent reasons for physical disability worldwide. During development, axons must elongate from the neuronal cell body to contact their precise target cell and establish functional connections. However, the capacity of the adult nervous system to restore its functionality after injury is limited. Given the inefficacy of the nervous system to heal and regenerate after damage, new therapies are under investigation to enhance axonal regeneration. Axon guidance cues and receptors, as well as the molecular machinery activated after nervous system damage, are organized into lipid raft microdomains, a term typically used to describe nanoscale membrane domains enriched in cholesterol and glycosphingolipids that act as signaling platforms for certain transmembrane proteins. Here, we systematically review the most recent findings that link the stability of lipid rafts and their composition with the capacity of axons to regenerate and rebuild functional neural circuits after damage.

The Role of Lipids, Lipid Metabolism and Ectopic Lipid Accumulation in Axon Growth, Regeneration and Repair after CNS Injury and Disease

Axons in the adult mammalian nervous system can extend over formidable distances, up to one meter or more in humans. During development, axonal and dendritic growth requires continuous addition of new membrane. Of the three major kinds of membrane lipids, phospholipids are the most abundant in all cell membranes, including neurons. Not only immature axons, but also severed axons in the adult require large amounts of lipids for axon regeneration to occur. Lipids also serve as energy storage, signaling molecules and they contribute to tissue physiology, as demonstrated by a variety of metabolic disorders in which harmful amounts of lipids accumulate in various tissues through the body. Detrimental changes in lipid metabolism and excess accumulation of lipids contribute to a lack of axon regeneration, poor neurological outcome and complications after a variety of central nervous system (CNS) trauma including brain and spinal cord injury. Recent evidence indicates that rewiring lipid metabolism can be manipulated for therapeutic gain, as it favors conditions for axon regeneration and CNS repair. Here, we review the role of lipids, lipid metabolism and ectopic lipid accumulation in axon growth, regeneration and CNS repair. In addition, we outline molecular and pharmacological strategies to fine-tune lipid composition and energy metabolism in neurons and non-neuronal cells that can be exploited to improve neurological recovery after CNS trauma and disease.

Selective axonal translation of the mRNA isoform encoding prenylated Cdc42 supports axon growth

The small Rho-family GTPase Cdc42 has long been known to have a role in cell motility and axon growth. The eukaryotic Ccd42 gene is alternatively spliced to generate mRNAs with two different 3' untranslated regions (UTRs) that encode proteins with distinct C-termini. The C-termini of these Cdc42 proteins include CaaX and CCaX motifs for post-translational prenylation and palmitoylation, respectively. Palmitoyl-Cdc42 protein was previously shown to contribute to dendrite maturation, while the prenyl-Cdc42 protein contributes to axon specification and its mRNA was detected in neurites. Here, we show that the mRNA encoding prenyl-Cdc42 isoform preferentially localizes into PNS axons and this localization selectively increases in vivo during peripheral nervous system (PNS) axon regeneration. Functional studies indicate that prenyl-Cdc42 increases axon length in a manner that requires axonal targeting of its mRNA, which, in turn, needs an intact C-terminal CaaX motif that can drive prenylation of the encoded protein. In contrast, palmitoyl-Cdc42 has no effect on axon growth but selectively increases dendrite length. Together, these data show that alternative splicing of the Cdc42 gene product generates an axon growth promoting, locally synthesized prenyl-Cdc42 protein. This article has an associated First Person interview with one of the co-first authors of the paper.