Protection and regrowth of neurons
Hypoxic brain injury, peripheral denervation, myelin loss - here are ways to recover?
Time to circle back to Neuroscience news and today we look at emerging understandings of the injury and recovery process for the nervous system.
Glial Amino Acid Transport and ROS Injury
When the brain is hit with hypoxic injury from a stroke or trauma, one of the first consequences is loss of the Blood Brain Barrier - it gets leaky. And lets in a flood of blood components, including the non excitatory amino acids which are most prevalent. The brain cells transport them inside along with Na+. The sodium osmotically pulls in water, expanding cells. Astrocytes, then protect themselves by opening ion channels to discharge the excess water and molecules. The excitatory amino acid glutamate is released, too, which overstimulates NMDA receptors, leading to overwhelming calcium release, swelling and cell death.
Fluxes of Na+ and Ca2+ are co-ordinated by cationic channels and plasmalemmal and mitochondrial Na+–Ca2+ exchangers. Steep transmembrane Na+ gradients, in addition, provide the energy for plasmalemmal transport of many molecules, including other ions, neurotransmitters and amino acids. The astroglial swelling induced by accumulation of non-excitatory amino acid and release of the excitotoxins through antiporters and Volume Regulated Anion Channels (VRAC) exacerbates this hypoxia-induced neuronal injury.
Abstract. In ischemic stroke and post-traumatic brain injury (TBI), blood–brain barrier disruption leads to leaking plasma amino acids (AA) into cerebral parenchyma. Bleeding in hemorrhagic stroke and TBI also release plasma AA. Although excitotoxic AA were extensively studied, little is known about non-excitatory AA during hypoxic injury. Hypoxia-induced synaptic depression in hippocampal slices becomes irreversible with non-excitatory AA, alongside their intracellular accumulation and increased tissue electrical resistance. Four non-excitatory AA (l-alanine, glycine, l-glutamine, l-serine: AGQS) at plasmatic concentrations were applied to slices from mice expressing EGFP in pyramidal neurons or astrocytes during normoxia or hypoxia. Two-photon imaging, light transmittance (LT) changes, and electrophysiological field recordings followed by electron microscopy in hippocampal CA1 st. radiatum were used to monitor synaptic function concurrently with cellular swelling and injury. During normoxia, AGQS-induced increase in LT was due to astroglial but not neuronal swelling. LT raise during hypoxia and AGQS manifested astroglial and neuronal swelling accompanied by a permanent loss of synaptic transmission and irreversible dendritic beading, signifying acute damage. Neuronal injury was not triggered by spreading depolarization which did not occur in our experiments. Hypoxia without AGQS did not cause cell swelling, leaving dendrites intact. Inhibition of NMDA receptors prevented neuronal damage and irreversible loss of synaptic function. Deleterious effects of AGQS during hypoxia were prevented by alanine-serine-cysteine transporters (ASCT2) and volume-regulated anion channels (VRAC) blockers. Our findings suggest that astroglial swelling induced by accumulation of non-excitatory AA and release of excitotoxins through antiporters and VRAC may exacerbate the hypoxia-induced neuronal injury. - I Álvarez-Merz et al
If the alanine-serine-cysteine transporters (ASCT2/SLC1A5) and volume-regulated anion channels (VRAC) were blocked with antagonist ligands, the bad effects of hypoxia were prevented. These molecules may have therapeutic potential when added to established treatment protocols.
Redox regeneration
Astrocytes are also key elements of the CNS antioxidative system. Neuronal energy metabolism, generates high amounts of reactive oxygen species (ROS), which need to be scavenged to avoid cellular damage. The antioxidant system of the CNS mainly relies on glutathione and ascorbic acid. Glutathione scavenges ROS in a nonenzymatic direct reaction or else it acts as an electron donor for glutathione peroxidase. Neuronal synthesis of glutathione requires either cysteine or glutamylcysteine precursors, both being provided by astroglia. Astrocytes accumulate cystine through the Sxcgluta-mate/cystine exchanger; subsequently, cystine is reduced to cysteine, which is further converted to glutamyl-cysteine (CysGlu). Astrocytes release cysteine and glutathione, the latter being converted into CysGlu by ectoenzyme-glutamyl transpeptidase (GT). Both cysteine and CysGlu are accumulated into neurons for glutathione synthesis, with cysteine being transported by neuronal EAAT2/3 transporters. In the presence of astrocytes in vitro, neurons sustain high levels of glutathione. But if astroglia are removed from culture or GT inhibited, ROS toxicity prevails. A single astrocyte can protect up to 20 neurons.
Astroglial antioxidant capacity is also mediated by ascorbic acid. Astrocytes can accumulate the dehydroascorbic acid derived from ROS oxidation and released by neurons, then reduce it (using glutathione) back to ascorbic acid. Neuronal activity (probably through release of glutamate) stimulates astroglial release of ascorbic acid. This release can be mediated through anion channels such as VRAC.
In the peripheral nervous system, cutaneous wound healing depends on axon reinnervation of the skin. Experiments with zebrafish time-lapse imaging show a dual role for the redox enzymes NADPH oxidases in axon processes of de- and regeneration. Hydrogen peroxide generation is an important signal in this process.
Abstract Tissue wounding induces cutaneous sensory axon regeneration via hydrogen peroxide (H2O2) that is produced by the epithelial NADPH oxidase, Duox1. Sciatic nerve injury instead induces axon regeneration through neuronal uptake of the NADPH oxidase, Nox2, from macrophages. We therefore reasoned that the tissue environment in which axons are damaged stimulates distinct regenerative mechanisms. Here, we show that cutaneous axon regeneration induced by tissue wounding depends on both neuronal and keratinocyte-specific mechanisms involving H2O2 signaling. Genetic depletion of H2O2 in sensory neurons abolishes axon regeneration, whereas keratinocyte-specific H2O2 depletion promotes axonal repulsion, a phenotype mirrored in duox1 mutants. Intriguingly, cyba mutants, deficient in the essential Nox subunit, p22Phox, retain limited axon regenerative capacity but display delayed Wallerian degeneration and axonal fusion, observed so far only in invertebrates. We further show that keratinocyte-specific oxidation of the epidermal growth factor receptor (EGFR) at a conserved cysteine thiol (C797) serves as an attractive cue for regenerating axons, leading to EGFR-dependent localized epidermal matrix remodeling via the matrix-metalloproteinase, MMP-13. Therefore, wound-induced cutaneous axon de- and regeneration depend on the coordinated functions of NADPH oxidases mediating distinct processes following injury. - Antonio Cadiz Diaz et al.
Myelin Regrowth in MS
Keeping with this month’s theme of thyroid mimetic therapeutics, here we give news on using thyroid hormones’ to mend myelin in degeneration diseases. The thyroid hormones induce oligodendrocyte progenitor cells (OPC) differentiation in developmental myelination and enhance myelin repair in experimental demyelination models.
The thyromimetic sobetirome was developed and trialed for lipid control. It was examined for CNS myelin repair. However, it had limited ability to cross the blood-brain barrier. So scientists added a chemical tag to the molecule, creating Sob-AM2, to eliminate a negative electrical charge that keeps sobetirome from crossing the barrier. After crossing BBB, enzymatic cleavage reverts it back to free sobetirome, resulting in a tenfold increase in CNS levels.
Demyelination of axons in the CNS is a primary feature of multiple sclerosis (MS) and a large family of genetic leukodystrophies. Mature oligodendrocytes extend and wrap processes around axons forming pro-
tective myelin sheaths. The oligodendrocyte pool arises from differentiation of oligodendrocyte progenitor cells (OPCs), which are present both during development and in the adult CNS. The final critical differentiation step becomes impaired in MS evidenced by the presence of OPCs in demyelinating lesions in MS that do not develop into myelin-producing mature oligodendrocytes. Accordingly, there is a clinical need
for therapeutic agents that can stimulate endogenous OPC differentiation, which may lead to remyelination and repair of the demyelinating lesions in MS. One approach to this goal based on promoting OPC differentiation involves exploiting thyroid hormone (TH) action in the CNS, which is critical in developmental myelination. Indeed, the active form of TH,
3,5,3′-triiodothyronine (T3), is routinely used as a positive control in OPC differentiation screening assays and has been found to enhance myelin repair in several in vivo demyelination models. However, TH has not been pursued clinically, because chronic elevated systemic TH exposure, or hyperthyroidism, adversely affects heart, bone, and skeletal muscle, thus limiting its potential as a remyelination therapy. TH agonists, or thyromimetics, are a class of compounds that mimic T3 binding at the TH receptor (TR). Sobetirome is a clinical-stage thyromimetic devoid of the adverse effects associated with hyperthyroidism and unique among thyromimetics for its ability to cross the blood-brain barrier and distribute to the CNS from a systemic dose. Sobetirome has been shown to stimulate TH actions during brain development and alter brain lipid biomarkers in a model of X-linked adrenoleukodystrophy. The increased safety margin of sobetirome compared with TH arises from a combination of differential
tissue distribution and specificity for TRβ over TRα. Thus, sobetirome appears to have the requisite properties and therapeutic index needed for stimulating myelin repair via a TH mechanism of action. Oligodendrocyte processes wrap axons to form neuroprotective myelin sheaths, and damage to myelin in disorders, such as multiple sclerosis (MS), leads to neurodegeneration and disability. There are currently no approved treatments for MS that stimulate myelin repair. During development, thyroid hormone (TH) promotes myelination through enhancing oligodendrocyte differentiation; however, TH itself is unsuitable as a remyelination therapy due to adverse systemic effects. This problem is overcome with selective TH agonists, sobetirome and a CNS-selective
prodrug of sobetirome called Sob-AM2. We show here that TH and sobetirome stimulated remyelination in standard gliotoxin models of demyelination. We then utilized a genetic mouse model of demyelination and remyelination, in which we employed motor function tests, histology,
and MRI to demonstrate that chronic treatment with sobetirome or Sob-AM2 leads to significant improvement in both clinical signs and remyelination. In contrast, chronic treatment with TH in this model inhibited the endogenous myelin repair and exacerbated disease. These results support the clinical investigation of selective CNS-penetrating TH agonists, but not TH, for myelin repair. - MD Hartley et al.
The university has licensed the technology to Llama Therapeutics Inc., which is now working toward human trials.
REFERENCES
A Verkhratsky, M Nedergaard PHYSIOLOGY OF ASTROGLIA Physiol Rev98: 239 –389, 2018 doi:10.1152/physrev.00042.2016. https://journals.physiology.org/doi/epdf/10.1152/physrev.00042.2016
Surprising culprit worsens stroke, TBI damage (2022, August 23) https://medicalxpress.com/news/2022-08-culprit-worsens-tbi.html
I Álvarez-Merz et al. Novel mechanism of hypoxic neuronal injury mediated by non-excitatory amino acids and astroglial swelling, Glia (2022). onlinelibrary.wiley.com/doi/10.1002/glia.24241
Antonio Cadiz Diaz et al, Coordinated NADPH oxidase/hydrogen peroxide functions regulate cutaneous sensory axon de- and regeneration, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2115009119
Hydrogen peroxide: A healing agent for nerve regeneration (2022, August 16) https://medicalxpress.com/news/2022-08-hydrogen-peroxide-agent-nerve-regeneration.html
MD Hartley et al. Myelin repair stimulated by CNS-selective thyroid hormone action. JCI Insight. 2019;4(8):e126329. https://doi.org/10.1172/jci.insight.126329
Does a clonic-tonic seizure also cause this type of injury?