A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

Nine insights from internet engineering that help us understand brain network communication

Philosophers have long recognized the value of metaphor as a tool that opens new avenues of investigation. By seeing brains as having the goal of representation, the computer metaphor in its various guises has helped systems neuroscience approach a wide array of neuronal behaviors at small and large scales. Here I advocate a complementary metaphor, the internet. Adopting this metaphor shifts our focus from computing to communication, and from seeing neuronal signals as localized representational elements to seeing neuronal signals as traveling messages. In doing so, we can take advantage of a comparison with the internet's robust and efficient routing strategies to understand how the brain might meet the challenges of network communication. I lay out nine engineering strategies that help the internet solve routing challenges similar to those faced by brain networks. The internet metaphor helps us by reframing neuronal activity across the brain as, in part, a manifestation of routing, which may, in different parts of the system, resemble the internet more, less, or not at all. I describe suggestive evidence consistent with the brain's use of internet-like routing strategies and conclude that, even if empirical data do not directly implicate internet-like routing, the metaphor is valuable as a reference point for those investigating the difficult problem of network communication in the brain and in particular the problem of routing.

Autophagy collaborates with apoptosis pathways to control myelination specificity and function

Oligodendrocytes are the sole myelin producing cells in the central nervous system. Oligodendrocyte numbers are tightly controlled across diverse brain regions to match local axon type and number, but the underlying mechanisms and functional significance remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under canonical settings, elicits premyelinating oligodendrocyte apoptosis during development and regulates critical aspects of nerve pulse propagation. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy acts in the TFEB-Bax/Bak pathway and elevates PUMA mRNA levels to trigger premyelinating oligodendrocyte apoptosis cell-autonomously. Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath numbers and fine-tune nerve pulse propagation. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrocyte number.

HIGHLIGHTS

Autophagy flux increases in the premyelinating and myelinating oligodendrocytesAutophagy promotes premyelinating oligodendrocyte (pre-OL) apoptosis to control myelination location and timing Autophagy acts in the TFEB-PUMA-Bax/Bak pathway and elevates PUMA mRNA levels to determine pre-OL fate Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath thickness and finetune nerve pulse propagation.

Improved post-stroke spontaneous recovery by astrocytic extracellular vesicles

Spontaneous recovery after a stroke accounts for a significant part of the neurological recovery in patients. However limited, the spontaneous recovery is mechanistically driven by axonal restorative processes for which several molecular cues have been previously described. We report the acceleration of spontaneous recovery in a preclinical model of ischemia/reperfusion in rats via a single intracerebroventricular administration of extracellular vesicles released from primary cortical astrocytes. We used magnetic resonance imaging and confocal and multiphoton microscopy to correlate the structural remodeling of the corpus callosum and striatocortical circuits with neurological performance during 21 days. We also evaluated the functionality of the corpus callosum by repetitive recordings of compound action potentials to show that the recovery facilitated by astrocytic extracellular vesicles was both anatomical and functional. Our data provide compelling evidence that astrocytes can hasten the basal recovery that naturally occurs post-stroke through the release of cellular mediators contained in extracellular vesicles.

Human Cerebral Organoid Implantation Alleviated the Neurological Deficits of Traumatic Brain Injury in Mice

Traumatic brain injury (TBI) causes a high rate of mortality and disability, and its treatment is still limited. Loss of neurons in damaged area is hardly rescued by relative molecular therapies. Based on its disease characteristics, we transplanted human embryonic stem cell- (hESC-) derived cerebral organoids in the brain lesions of controlled cortical impact- (CCI-) modeled severe combined immunodeficient (SCID) mice. Grafted organoids survived and differentiated in CCI-induced lesion pools in mouse cortical tissue. Implanted cerebral organoids differentiated into various types of neuronal cells, extended long projections, and showed spontaneous action, as indicated by electromyographic activity in the grafts. Induced vascularization and reduced glial scar were also found after organoid implantation, suggesting grafting could improve local situation and promote neural repair. More importantly, the CCI mice's spatial learning and memory improved after organoid grafting. These findings suggest that cerebral organoid implanted in lesion sites differentiates into cortical neurons, forms long projections, and reverses deficits in spatial learning and memory, a potential therapeutic avenue for TBI.

A hypothesis for the role of axon demyelination in seizure generation

Loss of myelin and altered oligodendrocyte distribution in the cerebral cortex are commonly observed both in postsurgical tissue derived from different focal epilepsies (such as focal cortical dysplasias and tuberous sclerosis) and in animal models of focal epilepsy. Moreover, seizures are a frequent symptom in demyelinating diseases, such as multiple sclerosis, and in animal models of demyelination and oligodendrocyte dysfunction. Finally, the excessive activity reported in demyelinated axons may promote hyperexcitability. We hypothesize that the extracellular potassium rise generated during epileptiform activity may be amplified by the presence of axons without appropriate myelin coating and by alterations in oligodendrocyte function. This process could facilitate the triggering of recurrent spontaneous seizures in areas of altered myelination and could result in further demyelination, thus promoting epileptogenesis.

Splice-dependent trans-synaptic PTPδ-IL1RAPL1 interaction regulates synapse formation and non-REM sleep

Alternative splicing regulates trans‐synaptic adhesions and synapse development, but supporting in vivo evidence is limited. PTPδ, a receptor tyrosine phosphatase adhering to multiple synaptic adhesion molecules, is associated with various neuropsychiatric disorders; however, its in vivo functions remain unclear. Here, we show that PTPδ is mainly present at excitatory presynaptic sites by endogenous PTPδ tagging. Global PTPδ deletion in mice leads to input‐specific decreases in excitatory synapse development and strength. This involves tyrosine dephosphorylation and synaptic loss of IL1RAPL1, a postsynaptic partner of PTPδ requiring the PTPδ‐meA splice insert for binding. Importantly, PTPδ‐mutant mice lacking the PTPδ‐meA insert, and thus lacking the PTPδ interaction with IL1RAPL1 but not other postsynaptic partners, recapitulate biochemical and synaptic phenotypes of global PTPδ‐mutant mice. Behaviorally, both global and meA‐specific PTPδ‐mutant mice display abnormal sleep behavior and non‐REM rhythms. Therefore, alternative splicing in PTPδ regulates excitatory synapse development and sleep by modulating a specific trans‐synaptic adhesion.

Benefits of physical exercise on cognition and glial white matter pathology in a mouse model of vascular cognitive impairment and dementia

White matter (WM) pathology is a clinically predictive feature of vascular cognitive impairment and dementia (VCID). Mice overexpressing transforming growth factor-β1 (TGF) with an underlying cerebrovascular pathology when fed a high cholesterol diet (HCD) develop cognitive deficits (VCID mice) that we recently found could be prevented by physical exercise (EX). Here, we further investigated cognitive and WM pathology in VCID mice and examined the cellular substrates of the protective effects of moderate aerobic EX focusing on WM alterations. Six groups were studied: Wild-type (WT) and TGF mice (n = 20-24/group) fed standard lab chow or a 2% HCD, with two HCD-fed groups given concurrent access to running wheels. HCD had a significant negative effect in TGF mice that was prevented by EX on working and object recognition memory, the latter also altered in WT HCD mice. Whisker-evoked increases in cerebral blood flow (CBF) were reduced in HCD-fed mice, deficits that were countered by EX, and baseline WM CBF was similarly affected. VCID mice displayed WM functional deficits characterized by lower compound action potential amplitude not found in EX groups. Moreover, there was an increased number of collapsing capillaries, galectin-3-expressing microglial cells, as well as a reduced number of oligodendrocytes in the WM of VCID mice; all of which were prevented by EX. Our findings indicate that a compromised cerebral circulation precedes reduced WM vascularization, enhanced WM inflammation and impaired oligodendrogenesis that all likely account for the increased susceptibility to memory impairments in VCID mice, which can be prevented by EX. MAIN POINTS: A compromised cerebral circulation increases susceptibility to anatomical and functional white matter changes that develop alongside cognitive deficits when challenged with a high cholesterol diet; preventable by a translational regimen of exercise.

Structural adaption of axons during de- and remyelination in the Cuprizone mouse model

Multiple Sclerosis is an autoimmune disorder causing neurodegeneration mostly in young adults. Thereby, myelin is lost in the inflammatory lesions leaving unmyelinated axons at a high risk to degenerate. Oligodendrocyte precursor cells maintain their regenerative capacity into adulthood and are able to remyelinate axons if they are properly activated and differentiate. Neuronal activity influences the success of myelination indicating a close interplay between neurons and oligodendroglia. The myelination profile determines the distribution of voltage-gated ion channels along the axon. Here, we analyze the distribution of the sodium channel subunit Nav1.6 and the ultrastructure of axons after cuprizone-induced demyelination in transgenic mice expressing GFP in oligodendroglial cells. Using this mouse model, we found an increased number of GFP-expressing oligodendroglial cells compared to untreated mice. Analyzing the axons, we found an increase in the number of nodes of Ranvier in mice that had received cuprizone. Furthermore, we found an enhanced portion of unmyelinated axons showing vesicles in the cytoplasm. These vesicles were labeled with VGlut1, indicating that they are involved in axonal signaling. Our results highlight the flexibility of axons towards changes in the glial compartment and depict the structural changes they undergo upon myelin removal. These findings might be considered if searching for new neuroprotective therapies that aim at blocking neuronal activity in order to avoid interfering with the process of remyelination.

Distinctive functional deficiencies in axonal conduction associated with two forms of cerebral white matter injury

Aims

This study determines whether assessment with compound action potentials (CAPs) can distinguish two different forms of cerebral white matter injury at the functional levels.

Methods

A pure demyelination model was induced in C57/BL6 adult mice by dietary supplementation of cuprizone (0.2%) for 6 weeks. Callosal L‐N5‐(1‐Iminoethyl) ornithine (L‐NIO) hydrochloride (27 mg/mL) was injected into the corpus callosum (CC) to induce a focal white matter stroke (WMS), resulting in both demyelination and axonal injury. White matter integrity was assessed by performing CAP recording, electron microscopy, and immunohistological and luxol fast blue (LFB) staining.

Results

Immunohistological and electron microscopic analyses confirmed the induction of robust demyelination in CC with cuprizone, and mixed demyelination and axonal damage with L‐NIO. Electrophysiologically, cuprizone‐induced demyelination significantly reduced the amplitude of negative peak 1 (N1), but increased the amplitude of negative peak 2 (N2), of the CAPs compared to the sham controls. However, cuprizone did not affect the axonal conduction velocity. In contrast, the amplitude and area of both N1 and N2 along with N1 axonal conduction velocity were dramatically decreased in L‐NIO‐induced WMS.

Conclusions

Concertedly, parameters of the CAPs offer a novel functional assessment strategy for cerebral white matter injury in rodent models.

Protease-independent action of tissue plasminogen activator in brain plasticity and neurological recovery after ischemic stroke

Emerging evidence suggests that tissue plasminogen activator (tPA), currently the only FDA-approved medication for ischemic stroke, exerts important biological actions on the CNS besides its well-known thrombolytic effect. In this study, we investigated the role of tPA on primary neurons in culture and on brain recovery and plasticity after ischemic stroke in mice. Treatment with recombinant tPA stimulated axonal growth in culture, an effect independent of its protease activity and achieved through epidermal growth factor receptor (EGFR) signaling. After permanent focal cerebral ischemia, tPA knockout mice developed more severe sensorimotor and cognitive deficits and greater axonal and myelin injury than wild-type mice, suggesting that endogenously expressed tPA promotes long-term neurological recovery after stroke. In tPA knockout mice, intranasal administration of recombinant tPA protein 6 hours poststroke and 7 more times at 2 d intervals mitigated white matter injury, improved axonal conduction, and enhanced neurological recovery. Consistent with the proaxonal growth effects observed in vitro, exogenous tPA delivery increased poststroke axonal sprouting of corticobulbar and corticospinal tracts, which might have contributed to restoration of neurological functions. Notably, recombinant mutant tPA-S478A lacking protease activity (but retaining the EGF-like domain) was as effective as wild-type tPA in rescuing neurological functions in tPA knockout stroke mice. These findings demonstrate that tPA improves long-term functional outcomes in a clinically relevant stroke model, likely by promoting brain plasticity through EGFR signaling. Therefore, treatment with the protease-dead recombinant tPA-S478A holds particular promise as a neurorestorative therapy, as the risk for triggering intracranial hemorrhage is eliminated and tPA-S478A can be delivered intranasally hours after stroke.

Exogenous Neural Precursor Cell Transplantation Results in Structural and Functional Recovery in a Hypoxic-Ischemic Hemiplegic Mouse Model

Cerebral palsy (CP) is a common pediatric neurodevelopmental disorder, frequently resulting in motor and developmental deficits and often accompanied by cognitive impairments. A regular pathobiological hallmark of CP is oligodendrocyte maturation impairment resulting in white matter (WM) injury and reduced axonal myelination. Regeneration therapies based on cell replacement are currently limited, but neural precursor cells (NPCs), as cellular support for myelination, represent a promising regeneration strategy to treat CP, although the transplantation parameters (e.g., timing, dosage, mechanism) remain to be determined. We optimized a hemiplegic mouse model of neonatal hypoxia-ischemia that mirrors the pathobiological hallmarks of CP and transplanted NPCs into the corpus callosum (CC), a major white matter structure impacted in CP patients. The NPCs survived, engrafted, and differentiated morphologically in male and female mice. Histology and MRI showed repair of lesioned structures. Furthermore, electrophysiology revealed functional myelination of the CC (e.g., restoration of conduction velocity), while cylinder and CatWalk tests demonstrated motor recovery of the affected forelimb. Endogenous oligodendrocytes, recruited in the CC following transplantation of exogenous NPCs, are the principal actors in this recovery process. The lack of differentiation of the transplanted NPCs is consistent with enhanced recovery due to an indirect mechanism, such as a trophic and/or “bio-bridge” support mediated by endogenous oligodendrocytes. Our work establishes that transplantation of NPCs represents a viable therapeutic strategy for CP treatment, and that the enhanced recovery is mediated by endogenous oligodendrocytes. This will further our understanding and contribute to the improvement of cellular therapeutic strategies.

Tissue plasminogen activator promotes white matter integrity and functional recovery in a murine model of traumatic brain injury

Recombinant tissue plasminogen activator (tPA) is a Food and Drug Administration-approved thrombolytic treatment for ischemic stroke. tPA is also naturally expressed in glial and neuronal cells of the brain, where it promotes axon outgrowth and synaptic plasticity. However, there are conflicting reports of harmful versus neuroprotective effects of tPA in acute brain injury models. Furthermore, its impact on white matter integrity in preclinical traumatic brain injury (TBI) has not been thoroughly explored, although white matter disruption is a better predictor of long-term clinical outcomes than focal lesion volumes. Here we show that the absence of endogenous tPA in knockout mice impedes long-term recovery of white matter and neurological function after TBI. tPA-knockout mice exhibited greater asymmetries in forepaw use, poorer sensorimotor balance and coordination, and inferior spatial learning and memory up to 35 d after TBI. White matter damage was also more prominent in tPA knockouts, as shown by diffusion tensor imaging, histological criteria, and electrophysiological assessments of axon conduction properties. Replenishment of tPA through intranasal application of the recombinant protein in tPA-knockout mice enhanced neurological function, the structural and functional integrity of white matter, and postinjury compensatory sprouting in corticofugal projections. tPA also promoted neurite outgrowth in vitro, partly through the epidermal growth factor receptor. Both endogenous and exogenous tPA protected against white matter injury after TBI without increasing intracerebral hemorrhage volumes. These results unveil a previously unappreciated role for tPA in the protection and/or repair of white matter and long-term functional recovery after TBI.

The effect of age-related risk factors and comorbidities on white matter injury and repair after ischemic stroke

White matter injury is a crucial component of human stroke, but it has often been neglected in preclinical studies. Most human stroke is associated with one or more comorbidities, including aging, hypertension, diabetes and metabolic syndrome including hyperlipidemia. The purpose of this review is to examine how age and hypertension impact stroke-induced white matter injury as well as white matter repair in both human stroke and preclinical models. It is essential that comorbidities be examined in preclinical trials as they may impact translatability to the clinic. In addition, understanding how comorbidities impact white matter injury and repair may provide new therapeutic opportunities for patients with those conditions.