Axonal Degeneration during Aging and Its Functional Role in Neurodegenerative Disorders

Do oligodendrocytes regulate axonal glucose uptake and consumption?

Neurons have high energy demands. In a recent study, Looser et al. identified oligodendrocyte Kir4.1 as the activity-dependent driver of oligodendrocyte glycolysis that ensures that lactate is supplied to active neurons. Given that oligodendrocyte Kir4.1 also influenced axonal glucose consumption and uptake, oligodendrocytes may play a broader role in neuronal metabolic regulation.

Neuroprotective Efficacy of the Glucocorticoid Receptor Modulator PT150 in the Rotenone Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder worldwide. Current treatments for PD largely center around dopamine replacement therapies and fail to prevent the progression of pathology, underscoring the need for neuroprotective interventions. Approaches that target neuroinflammation, which occurs prior to dopaminergic neuron (DAn) loss in the substantia nigra (SN), represent a promising therapeutic strategy. The glucocorticoid receptor (GR) has been implicated in the neuropathology of PD and modulates numerous neuroinflammatory signaling pathways in the brain. Therefore, we investigated the neuroprotective effects of the novel GR modulator, PT150, in the rotenone mouse model of PD, postulating that inhibition of glial inflammation would protect DAn and reduce accumulation of neurotoxic misfolded ⍺-synuclein protein. C57Bl/6 mice were exposed to 2.5 mg/kg/day rotenone by intraperitoneal injection for 14 days, immediately followed by oral treatment with 30 mg/kg/day or 100 mg/kg/day PT150 in the 14-day post-lesioning incubation period, during which the majority of DAn loss and α-synuclein (α-syn) accumulation occurs. Our results indicate that treatment with PT150 reduced both loss of DAn and microgliosis in the nigrostriatal pathway. Although morphologic features of astrogliosis were not attenuated, PT150 treatment promoted potentially neuroprotective activity in these cells, including increased phagocytosis of hyperphosphorylated α-syn. Ultimately, PT150 treatment reduced the loss of DAn cell bodies in the SN, but not the striatum, and prohibited intra-neuronal accumulation of α-syn. Together, these data indicate that PT150 effectively reduced SN pathology in the rotenone mouse model of PD.

Regulated cell death and its role in Alzheimer's disease and amyotrophic lateral sclerosis

Despite considerable research efforts, it is still not clear which mechanisms underlie neuronal cell death in neurodegenerative diseases. During the last 20 years, multiple pathways have been identified that can execute regulated cell death (RCD). Among these RCD pathways, apoptosis, necroptosis, pyroptosis, ferroptosis, autophagy-related cell death, and lysosome-dependent cell death have been intensively investigated. Although RCD consists of numerous individual pathways, multiple common proteins have been identified that allow shifting from one cell death pathway to another. Another layer of complexity is added by mechanisms such as the endosomal machinery, able to regulate the activation of some RCD pathways, preventing cell death. In addition, restricted axonal degeneration and synaptic pruning can occur as a result of RCD activation without loss of the cell body. RCD plays a complex role in neurodegenerative processes, varying across different disorders. It has been shown that RCD is differentially involved in Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS), among the most common neurodegenerative diseases. In AD, neuronal loss is associated with the activation of not only necroptosis, but also pyroptosis. In ALS, on the other hand, motor neuron death is not linked to canonical necroptosis, whereas pyroptosis pathway activation is seen in white matter microglia. Despite these differences in the activation of RCD pathways in AD and ALS, the accumulation of protein aggregates immunoreactive for p62/SQSTM1 (sequestosome 1) is a common event in both diseases and many other neurodegenerative disorders. In this review, we describe the major RCD pathways with clear activation in AD and ALS, the main interactions between these pathways, as well as their differential and similar involvement in these disorders. Finally, we will discuss targeting RCD as an innovative therapeutic concept for neurodegenerative diseases, such as AD and ALS. Considering that the execution of RCD or "cellular suicide" represents the final stage in neurodegeneration, it seems crucial to prevent neuronal death in patients by targeting RCD. This would offer valuable time to address upstream events in the pathological cascade by keeping the neurons alive.

Influences of amyloid-β and tau on white matter neurite alterations in dementia with Lewy bodies

Dementia with Lewy bodies (DLB) is a neurodegenerative condition often co-occurring with Alzheimer's disease (AD) pathology. Characterizing white matter tissue microstructure using Neurite Orientation Dispersion and Density Imaging (NODDI) may help elucidate the biological underpinnings of white matter injury in individuals with DLB. In this study, diffusion tensor imaging (DTI) and NODDI metrics were compared in 45 patients within the dementia with Lewy bodies spectrum (mild cognitive impairment with Lewy bodies (n = 13) and probable dementia with Lewy bodies (n = 32)) against 45 matched controls using conditional logistic models. We evaluated the associations of tau and amyloid-β with DTI and NODDI parameters and examined the correlations of AD-related white matter injury with Clinical Dementia Rating (CDR). Structural equation models (SEM) explored relationships among age, APOE ε4, amyloid-β, tau, and white matter injury. The DLB spectrum group exhibited widespread white matter abnormalities, including reduced fractional anisotropy, increased mean diffusivity, and decreased neurite density index. Tau was significantly associated with limbic and temporal white matter injury, which was, in turn, associated with worse CDR. SEM revealed that amyloid-β exerted indirect effects on white matter injury through tau. We observed widespread disruptions in white matter tracts in DLB that were not attributed to AD pathologies, likely due to α-synuclein-related injury. However, a fraction of the white matter injury could be attributed to AD pathology. Our findings underscore the impact of AD pathology on white matter integrity in DLB and highlight the utility of NODDI in elucidating the biological basis of white matter injury in DLB.

Telomere shortening induces aging-associated phenotypes in hiPSC-derived neurons and astrocytes

Telomere shortening is a well-established hallmark of cellular aging. Telomerase reverse transcriptase (TERT) plays a crucial role in maintaining the length of telomeres, which are specialised protective caps at the end of chromosomes. The lack of in vitro aging models, particularly for the central nervous system (CNS), has impeded progress in understanding aging and age-associated neurodegenerative diseases. In this study, we aimed to explore the possibility of inducing aging-associated features in cell types of the CNS using hiPSC (human induced pluripotent stem cell) technology. To achieve this, we utilised CRISPR/Cas9 to generate hiPSCs with a loss of telomerase function and shortened telomeres. Through directed differentiation, we generated motor neurons and astrocytes to investigate whether telomere shortening could lead to age-associated phenotypes. Our findings revealed that shortened telomeres induced age-associated characteristics in both motor neurons and astrocytes including increased cellular senescence, heightened inflammation, and elevated DNA damage. We also observed cell-type specific age-related morphology changes. Additionally, our study highlighted the fundamental role of TERT and telomere shortening in neural progenitor cell (NPC) proliferation and neuronal differentiation. This study serves as a proof of concept that telomere shortening can effectively induce aging-associated phenotypes, thereby providing a valuable tool to investigate age-related decline and neurodegenerative diseases.

Optical coherence tomography as a potential surrogate marker of dopaminergic modulation across the life span

The retina has been considered a "window to the brain" and shares similar innervation by the dopaminergic system with the cortex in terms of an unequal distribution of D1 and D2 receptors. Here, we provide a comprehensive overview that Optical Coherence Tomography (OCT), a non-invasive imaging technique, which provides an "in vivo" representation of the retina, shows promise to be used as a surrogate marker of dopaminergic neuromodulation in cognition. Overall, most evidence supports reduced retinal thickness in individuals with dopaminergic dysregulation (e.g., patients with Parkinson's Disease, non-demented older adults) and with poor cognitive functioning. By using the theoretical framework of metacontrol, we derive hypotheses that retinal thinning associated to decreased dopamine (DA) levels affecting D1 families, might lead to a decrease in the signal-to-noise ratio (SNR) affecting cognitive persistence (depending on D1-modulated DA activity) but not cognitive flexibility (depending on D2-modulated DA activity). We argue that the use of OCT parameters might not only be an insightful for cognitive neuroscience research, but also a potentially effective tool for individualized medicine with a focus on cognition. As our society progressively ages in the forthcoming years and decades, the preservation of cognitive abilities and promoting healthy aging will hold of crucial significance. OCT has the potential to function as a swift, non-invasive, and economical method for promptly recognizing individuals with a heightened vulnerability to cognitive deterioration throughout all stages of life.

Single-Nucleus RNA-Seq Characterizes the Cell Types Along the Neuronal Lineage in the Adult Human Subependymal Zone and Reveals Reduced Oligodendrocyte Progenitor Abundance with Age

The subependymal zone (SEZ), also known as the subventricular zone (SVZ), constitutes a neurogenic niche that persists during postnatal life. In humans, the neurogenic potential of the SEZ declines after the first year of life. However, studies discovering markers of stem and progenitor cells highlight the neurogenic capacity of progenitors in the adult human SEZ, with increased neurogenic activity occurring under pathological conditions. In the present study, the complete cellular niche of the adult human SEZ was characterized by single-nucleus RNA sequencing, and compared between four youth (age 16-22) and four middle-aged adults (age 44-53). We identified 11 cellular clusters including clusters expressing marker genes for neural stem cells (NSCs), neuroblasts, immature neurons, and oligodendrocyte progenitor cells. The relative abundance of NSC and neuroblast clusters did not differ between the two age groups, indicating that the pool of SEZ NSCs does not decline in this age range. The relative abundance of oligodendrocyte progenitors and microglia decreased in middle-age, indicating that the cellular composition of human SEZ is remodeled between youth and adulthood. The expression of genes related to nervous system development was higher across different cell types, including NSCs, in youth as compared with middle-age. These transcriptional changes suggest ongoing central nervous system plasticity in the SEZ in youth, which declined in middle-age.

Oligodendrocyte-axon metabolic coupling is mediated by extracellular K+ and maintains axonal health

The integrity of myelinated axons relies on homeostatic support from oligodendrocytes (OLs). To determine how OLs detect axonal spiking and how rapid axon-OL metabolic coupling is regulated in the white matter, we studied activity-dependent calcium (Ca2+) and metabolite fluxes in the mouse optic nerve. We show that fast axonal spiking triggers Ca2+ signaling and glycolysis in OLs. OLs detect axonal activity through increases in extracellular potassium (K+) concentrations and activation of Kir4.1 channels, thereby regulating metabolite supply to axons. Both pharmacological inhibition and OL-specific inactivation of Kir4.1 reduce the activity-induced axonal lactate surge. Mice lacking oligodendroglial Kir4.1 exhibit lower resting lactate levels and altered glucose metabolism in axons. These early deficits in axonal energy metabolism are associated with late-onset axonopathy. Our findings reveal that OLs detect fast axonal spiking through K+ signaling, making acute metabolic coupling possible and adjusting the axon-OL metabolic unit to promote axonal health.

Bioenergetics of Axon Integrity and Its Regulation by Oligodendrocytes and Schwann Cells

Axons are long slender portions of neurons that transmit electrical impulses to maintain proper physiological functioning. Axons in the central nervous system (CNS) and peripheral nervous system (PNS) do not exist in isolation but are found to form a complex association with their surrounding glial cells, oligodendrocytes and Schwann cells. These cells not only myelinate them for faster nerve impulse conduction but are also known to provide metabolic support. Due to their incredible length, continuous growth, and distance from the cell body (where major energy synthesis takes place), axons are in high energetic demand. The stability and integrity of axons have long been associated with axonal energy levels. The current mini-review is thus focused on how axons accomplish their high energetic requirement in a cell-autonomous manner and how the surrounding glial cells help them in maintaining their integrity by fulfilling their energy demands (non-cell autonomous trophic support). The concept that adjacent glial cells (oligodendrocytes and Schwann cells) provide trophic support to axons and assist them in maintaining their integrity comes from the conditional knockout research and the studies in which the metabolic pathways controlling metabolism in these glial cells are modulated and its effect on axonal integrity is evaluated. In the later part of the mini-review, the current knowledge of axon-glial metabolic coupling during various neurodegenerative conditions was discussed, along with the potential lacunae in our current understanding of axon-glial metabolic coupling.

Impaired neuron differentiation in GBA-associated Parkinson's disease is linked to cell cycle defects in organoids

The mechanisms underlying Parkinson's disease (PD) etiology are only partially understood despite intensive research conducted in the field. Recent evidence suggests that early neurodevelopmental defects might play a role in cellular susceptibility to neurodegeneration. To study the early developmental contribution of GBA mutations in PD we used patient-derived iPSCs carrying a heterozygous N370S mutation in the GBA gene. Patient-specific midbrain organoids displayed GBA-PD relevant phenotypes such as reduction of GCase activity, autophagy impairment, and mitochondrial dysfunction. Genome-scale metabolic (GEM) modeling predicted changes in lipid metabolism which were validated with lipidomics analysis, showing significant differences in the lipidome of GBA-PD. In addition, patient-specific midbrain organoids exhibited a decrease in the number and complexity of dopaminergic neurons. This was accompanied by an increase in the neural progenitor population showing signs of oxidative stress-induced damage and premature cellular senescence. These results provide insights into how GBA mutations may lead to neurodevelopmental defects thereby predisposing to PD pathology.

Phosphorylated SARM1 is involved in the pathological process of rotenone-induced neurodegeneration

Sterile alpha and Toll/interleukin receptor motif-containing protein 1 (SARM1) is a NAD+ hydrolase that plays a key role in axonal degeneration and neuronal cell death. We reported that c-Jun N-terminal kinase (JNK) activates SARM1 through phosphorylation at Ser-548. The importance of SARM1 phosphorylation in the pathological process of Parkinson's disease (PD) has not been determined. We thus conducted the present study by using rotenone (an inducer of PD-like pathology) and neurons derived from induced pluripotent stem cells (iPSCs) from healthy donors and a patient with familial PD PARK2 (FPD2). The results showed that compared to the healthy neurons, FPD2 neurons were more vulnerable to rotenone-induced stress and had higher levels of SARM1 phosphorylation. Similar cellular events were obtained when we used PARK2-knockdown neurons derived from healthy donor iPSCs. These events in both types of PD-model neurons were suppressed in neurons treated with JNK inhibitors, Ca2+-signal inhibitors, or by a SARM1-knockdown procedure. The degenerative events were enhanced in neurons overexpressing wild-type SARM1 and conversely suppressed in neurons overexpressing the SARM1-S548A mutant. We also detected elevated SARM1 phosphorylation in the midbrain of PD-model mice. The results indicate that phosphorylated SARM1 plays an important role in the pathological process of rotenone-induced neurodegeneration.

Microglia-mediated demyelination protects against CD8+ T cell-driven axon degeneration in mice carrying PLP defects

Axon degeneration and functional decline in myelin diseases are often attributed to loss of myelin but their relation is not fully understood. Perturbed myelinating glia can instigate chronic neuroinflammation and contribute to demyelination and axonal damage. Here we study mice with distinct defects in the proteolipid protein 1 gene that develop axonal damage which is driven by cytotoxic T cells targeting myelinating oligodendrocytes. We show that persistent ensheathment with perturbed myelin poses a risk for axon degeneration, neuron loss, and behavioral decline. We demonstrate that CD8+ T cell-driven axonal damage is less likely to progress towards degeneration when axons are efficiently demyelinated by activated microglia. Mechanistically, we show that cytotoxic T cell effector molecules induce cytoskeletal alterations within myelinating glia and aberrant actomyosin constriction of axons at paranodal domains. Our study identifies detrimental axon-glia-immune interactions which promote neurodegeneration and possible therapeutic targets for disorders associated with myelin defects and neuroinflammation.

Alzheimer's disease: an axonal injury disease?

Alzheimer's disease (AD) is the primary cause of dementia and is anticipated to impose a substantial economic burden in the future. Over a significant period, the widely accepted amyloid cascade hypothesis has guided research efforts, and the recent FDA approval of an anti- amyloid-beta (Aβ) protofibrils antibody, believed to decelerate AD progression, has further solidified its significance. However, the excessive emphasis placed on the amyloid cascade hypothesis has overshadowed the physiological nature of Aβ and tau proteins within axons. Axons, specialized neuronal structures, sustain damage during the early stages of AD, exerting a pivotal influence on disease progression. In this review, we present a comprehensive summary of the relationship between axonal damage and AD pathology, amalgamating the physiological roles of Aβ and tau proteins, along with the impact of AD risk genes such as APOE and TREM2. Furthermore, we underscore the exceptional significance of axonal damage in the context of AD.

Functional alterations in bipartite network of white and grey matters during aging

The effects of normal aging on functional connectivity (FC) within various brain networks of gray matter (GM) have been well-documented. However, the age effects on the networks of FC between white matter (WM) and GM, namely WM-GM FC, remains unclear. Evaluating crucial properties, such as global efficiency (GE), for a WM-GM FC network poses a challenge due to the absence of closed triangle paths which are essential for assessing network properties in traditional graph models. In this study, we propose a bipartite graph model to characterize the WM-GM FC network and quantify these challenging network properties. Leveraging this model, we assessed the WM-GM FC network properties at multiple scales across 1,462 cognitively normal subjects aged 22-96 years from three repositories (ADNI, BLSA and OASIS-3) and investigated the age effects on these properties throughout adulthood and during late adulthood (age ≥70 years). Our findings reveal that (1) heterogeneous alterations occurred in region-specific WM-GM FC over the adulthood and decline predominated during late adulthood; (2) the FC density of WM bundles engaged in memory, executive function and processing speed declined with age over adulthood, particularly in later years; and (3) the GE of attention, default, somatomotor, frontoparietal and limbic networks reduced with age over adulthood, and GE of visual network declined during late adulthood. These findings provide unpresented insights into multi-scale alterations in networks of WM-GM functional synchronizations during normal aging. Furthermore, our bipartite graph model offers an extendable framework for quantifying WM-engaged networks, which may contribute to a wide range of neuroscience research.

C-NODDI: a constrained NODDI model for axonal density and orientation determinations in cerebral white matter

Purpose

Neurite orientation dispersion and density imaging (NODDI) provides measures of neurite density and dispersion through computation of the neurite density index (NDI) and the orientation dispersion index (ODI). However, NODDI overestimates the cerebrospinal fluid water fraction in white matter (WM) and provides physiologically unrealistic high NDI values. Furthermore, derived NDI values are echo-time (TE)-dependent. In this work, we propose a modification of NODDI, named constrained NODDI (C-NODDI), for NDI and ODI mapping in WM.

Methods

Using NODDI and C-NODDI, we investigated age-related alterations in WM in a cohort of 58 cognitively unimpaired adults. Further, NDI values derived using NODDI or C-NODDI were correlated with the neurofilament light chain (NfL) concentration levels, a plasma biomarker of axonal degeneration. Finally, we investigated the TE dependence of NODDI or C-NODDI derived NDI and ODI.

Results

ODI derived values using both approaches were virtually identical, exhibiting constant trends with age. Further, our results indicated a quadratic relationship between NDI and age suggesting that axonal maturation continues until middle age followed by a decrease. This quadratic association was notably significant in several WM regions using C-NODDI, while limited to a few regions using NODDI. Further, C-NODDI-NDI values exhibited a stronger correlation with NfL concentration levels as compared to NODDI-NDI, with lower NDI values corresponding to higher levels of NfL. Finally, we confirmed the previous finding that NDI estimation using NODDI was dependent on TE, while NDI derived values using C-NODDI exhibited lower sensitivity to TE in WM.

Conclusion

C-NODDI provides a complementary method to NODDI for determination of NDI in white matter.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

Axonal energy metabolism, and the effects in aging and neurodegenerative diseases

Human studies consistently identify bioenergetic maladaptations in brains upon aging and neurodegenerative disorders of aging (NDAs), such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and Amyotrophic lateral sclerosis. Glucose is the major brain fuel and glucose hypometabolism has been observed in brain regions vulnerable to aging and NDAs. Many neurodegenerative susceptible regions are in the topological central hub of the brain connectome, linked by densely interconnected long-range axons. Axons, key components of the connectome, have high metabolic needs to support neurotransmission and other essential activities. Long-range axons are particularly vulnerable to injury, neurotoxin exposure, protein stress, lysosomal dysfunction, etc. Axonopathy is often an early sign of neurodegeneration. Recent studies ascribe axonal maintenance failures to local bioenergetic dysregulation. With this review, we aim to stimulate research in exploring metabolically oriented neuroprotection strategies to enhance or normalize bioenergetics in NDA models. Here we start by summarizing evidence from human patients and animal models to reveal the correlation between glucose hypometabolism and connectomic disintegration upon aging/NDAs. To encourage mechanistic investigations on how axonal bioenergetic dysregulation occurs during aging/NDAs, we first review the current literature on axonal bioenergetics in distinct axonal subdomains: axon initial segments, myelinated axonal segments, and axonal arbors harboring pre-synaptic boutons. In each subdomain, we focus on the organization, activity-dependent regulation of the bioenergetic system, and external glial support. Second, we review the mechanisms regulating axonal nicotinamide adenine dinucleotide (NAD+) homeostasis, an essential molecule for energy metabolism processes, including NAD+ biosynthetic, recycling, and consuming pathways. Third, we highlight the innate metabolic vulnerability of the brain connectome and discuss its perturbation during aging and NDAs. As axonal bioenergetic deficits are developing into NDAs, especially in asymptomatic phase, they are likely exaggerated further by impaired NAD+ homeostasis, the high energetic cost of neural network hyperactivity, and glial pathology. Future research in interrogating the causal relationship between metabolic vulnerability, axonopathy, amyloid/tau pathology, and cognitive decline will provide fundamental knowledge for developing therapeutic interventions.

Longitudinal in vivo monitoring of axonal degeneration after brain injury

Traumatic brain injury (TBI)-induced axonal degeneration leads to acute and chronic neuropsychiatric impairment, neuronal death, and accelerated neurodegenerative diseases of aging, including Alzheimer's and Parkinson's diseases. In laboratory models, axonal degeneration is traditionally studied through comprehensive postmortem histological evaluation of axonal integrity at multiple time points. This requires large numbers of animals to power for statistical significance. Here, we developed a method to longitudinally monitor axonal functional activity before and after injury in vivo in the same animal over an extended period. Specifically, after expressing an axonal-targeting genetically encoded calcium indicator in the mouse dorsolateral geniculate nucleus, we recorded axonal activity patterns in the visual cortex in response to visual stimulation. In vivo aberrant axonal activity patterns after TBI were detectable from 3 days after injury and persisted chronically. This method generates longitudinal same-animal data that substantially reduces the number of required animals for preclinical studies of axonal degeneration.

Cytotoxic CNS-associated T cells drive axon degeneration by targeting perturbed oligodendrocytes in PLP1 mutant mice

Myelin defects lead to neurological dysfunction in various diseases and in normal aging. Chronic neuroinflammation often contributes to axon-myelin damage in these conditions and can be initiated and/or sustained by perturbed myelinating glia. We have previously shown that distinct PLP1 mutations result in neurodegeneration that is largely driven by adaptive immune cells. Here we characterize CD8⁺ CNS-associated T cells in myelin mutants using single-cell transcriptomics and identify population heterogeneity and disease-associated changes. We demonstrate that early sphingosine-1-phosphate receptor modulation attenuates T cell recruitment and neural damage, while later targeting of CNS-associated T cell populations is inefficient. Applying bone marrow chimerism and utilizing random X chromosome inactivation, we provide evidence that axonal damage is driven by cytotoxic, antigen specific CD8⁺ T cells that target mutant myelinating oligodendrocytes. These findings offer insights into neural-immune interactions and are of translational relevance for neurological conditions associated with myelin defects and neuroinflammation.

STAT1/3 signaling suppresses axon degeneration and neuronal cell death through regulation of NAD+-biosynthetic and consuming enzymes

Nicotinamide adenine dinucleotide (NAD)⁺-biosynthetic and consuming enzymes are involved in various intracellular events through the regulation of NAD⁺ metabolism. Recently, it has become clear that alterations in the expression of NAD⁺-biosynthetic and consuming enzymes contribute to the axonal stability of neurons. We explored soluble bioactive factor(s) that alter the expression of NAD⁺-metabolizing enzymes and found that cytokine interferon (IFN)-γ increased the expression of nicotinamide nucleotide adenylyltransferase 2 (NMNAT2), an NAD⁺-biosynthetic enzyme. IFN-γ activated signal transducers and activators of transcription 1 and 3 (STAT1/3) followed by c-Jun N-terminal kinase (JNK) suppression. As a result, STAT1/3 increased the expression of NMNAT2 at both mRNA and protein levels in a dose- and time-dependent manner and, at the same time, suppressed activation of sterile alpha and Toll/interleukin receptor motif-containing 1 (SARM1), an NAD⁺-consuming enzyme, and increased intracellular NAD⁺ levels. We examined the protective effect of STAT1/3 signaling against vincristine-mediated cell injury as a model of chemotherapy-induced peripheral neuropathy (CIPN), in which axonal degeneration is involved in disease progression. We found that IFN-γ-mediated STAT1/3 activation inhibited vincristine-induced downregulation of NMNAT2 and upregulation of SARM1 phosphorylation, resulting in modest suppression of subsequent neurite degradation and cell death. These results indicate that STAT1/3 signaling induces NMNAT2 expression while simultaneously suppressing SARM1 phosphorylation, and that both these actions contribute to suppression of axonal degeneration and cell death.

The reverse transcriptase inhibitor 3TC protects against age-related cognitive dysfunction

Aging is the primary risk factor for most neurodegenerative diseases, including Alzheimer's disease. Major hallmarks of brain aging include neuroinflammation/immune activation and reduced neuronal health/function. These processes contribute to cognitive dysfunction (a key risk factor for Alzheimer's disease), but their upstream causes are incompletely understood. Age-related increases in transposable element (TE) transcripts might contribute to reduced cognitive function with brain aging, as the reverse transcriptase inhibitor 3TC reduces inflammation in peripheral tissues and TE transcripts have been linked with tau pathology in Alzheimer's disease. However, the effects of 3TC on cognitive function with aging have not been investigated. Here, in support of a role for TE transcripts in brain aging/cognitive decline, we show that 3TC: (a) improves cognitive function and reduces neuroinflammation in old wild-type mice; (b) preserves neuronal health with aging in mice and Caenorhabditis elegans; and (c) enhances cognitive function in a mouse model of tauopathy. We also provide insight on potential underlying mechanisms, as well as evidence of translational relevance for these observations by showing that TE transcripts accumulate with brain aging in humans, and that these age-related increases intersect with those observed in Alzheimer's disease. Collectively, our results suggest that TE transcript accumulation during aging may contribute to cognitive decline and neurodegeneration, and that targeting these events with reverse transcriptase inhibitors like 3TC could be a viable therapeutic strategy.

Intracavernous injection of platelet-rich plasma reverses erectile dysfunction of chronic cavernous nerve degeneration through reduction of prostate hyperplasia evidence from an aging-induced erectile dysfunction rat model

Age-induced erectile dysfunction (ED) is a convoluted medical condition, and restoring erectile function (EF) under geriatric conditions is highly complicated. Platelet-rich plasma (PRP) treatment is an inexpensive cell-based therapeutic strategy. We have aimed to restore EF in aged-ED rats with PRP as a therapeutic tool. Male rats were grouped into aged and young according to age. The young rats were considered as normal control (NC) and treated with saline. Aged were further divided into 2 groups and treated with intracavernous (IC) PRP and saline. Treatment was scheduled at the 9th and 10th week for NC and 41th and 42th week for aged-ED rats, with EF analysis scheduled on the 12th week for NC and 44th week for aged-ED rats, respectively. Erectile response, immunofluorescence staining, and electron microscopic analyses were performed. IC PRP treatment effectively reduced prostate hyperplasia (PH). EF response indicated a significant increase in crucial EF parameters in PRP-treated aged-ED rats. Histological evidence denoted a rigid and restored development of tunica adventitia of the dorsal artery, decreased vacuolation of the dorsal penile nerve, and structural expansion of the epineurium. Masson's trichrome and immunostaining results affirmed an elevated expression of α-smooth muscle actin (α-SMA) in the corpus cavernosum (CC). Ultrastructure findings revealed that PRP effectively rejuvenated degenerating nerves, preserved endothelium and adherent junctions of corporal smooth muscle, and restored the axonal scaffolds by upregulating neurofilament-H (NF-H) expression. Finally, PRP enhanced neural stability by enhancing the axonal remyelination processes in aged-ED rats. Hence, PRP treatment was proven to restore EF in aged-ED rats, which was considered a safe, novel, cost-effective, and hassle-free strategy for EF restoration in geriatric patients.

A multi-regression framework to improve diagnostic ability of optical coherence tomography retinal biomarkers to discriminate mild cognitive impairment and Alzheimer’s disease

Background

Diagnostic performance of optical coherence tomography (OCT) to detect Alzheimer’s disease (AD) and mild cognitive impairment (MCI) remains limited. We assessed whether compensating the circumpapillary retinal nerve fiber layer (cpRNFL) thickness for multiple demographic and anatomical factors as well as the combination of macular layers improves the detection of MCI and AD.

Methods

This cross-sectional study of 62 AD (n = 92 eyes), 108 MCI (n = 158 eyes), and 55 cognitively normal control (n = 86 eyes) participants. Macular ganglion cell complex (mGCC) thickness was extracted. Circumpapillary retinal nerve fiber layer (cpRNFL) measurement was compensated for several ocular factors. Thickness measurements and their corresponding areas under the receiver operating characteristic curves (AUCs) were compared between the groups. The main outcome measure was OCT thickness measurements.

Results

Participants with MCI/AD showed significantly thinner measured and compensated cpRNFL, mGCC, and altered retinal vessel density (p < 0.05). Compensated RNFL outperformed measured RNFL for discrimination of MCI/AD (AUC = 0.74 vs 0.69; p = 0.026). Combining macular and compensated cpRNFL parameters provided the best detection of MCI/AD (AUC = 0.80 vs 0.69; p < 0.001).

Conclusions and relevance

Accounting for interindividual variations of ocular anatomical features in cpRNFL measurements and incorporating macular information may improve the identification of high-risk individuals with early cognitive impairment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-00982-0.

Autophagy protein ULK1 interacts with and regulates SARM1 during axonal injury

Autophagy is a cellular catabolic pathway generally thought to be neuroprotective. However, autophagy and in particular its upstream regulator, the ULK1 kinase, can also promote axonal degeneration. We examined the role and the mechanisms of autophagy in axonal degeneration using a mouse model of contusive spinal cord injury (SCI). Consistent with activation of autophagy during axonal degeneration following SCI, autophagosome marker LC3, ULK1 kinase, and ULK1 target, phospho-ATG13, accumulated in the axonal bulbs and injured axons. SARM1, a TIR NADase with a pivotal role in axonal degeneration, colocalized with ULK1 within 1 h after SCI, suggesting possible interaction between autophagy and SARM1-mediated axonal degeneration. In our in vitro experiments, inhibition of autophagy, including Ulk1 knockdown and ULK1 inhibitor, attenuated neurite fragmentation and reduced accumulation of SARM1 puncta in neurites of primary cortical neurons subjected to glutamate excitotoxicity. Immunoprecipitation data demonstrated that ULK1 physically interacted with SARM1 in vitro and in vivo and that SAM domains of SARM1 were necessary for ULK1-SARM1 complex formation. Consistent with a role in regulation of axonal degeneration, in primary cortical neurons ULK1-SARM1 interaction increased upon neurite damage. Supporting a role for autophagy and ULK1 in regulation of SARM1 in axonal degeneration in vivo, axonal ULK1 activation and accumulation of SARM1 were both decreased after SCI in Becn1^+/- autophagy hypomorph mice compared to wild-type (WT) controls. These findings suggest a regulatory crosstalk between autophagy and axonal degeneration pathways, which is mediated through ULK1-SARM1 interaction and contributes to the ability of SARM1 to accumulate in injured axons.

Multi-omic profiling reveals the ataxia protein sacsin is required for integrin trafficking and synaptic organization

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset cerebellar ataxia caused by mutations in SACS, which encodes the protein sacsin. Cellular ARSACS phenotypes include mitochondrial dysfunction, intermediate filament disorganization, and progressive death of cerebellar Purkinje neurons. It is unclear why the loss of sacsin causes these deficits or why they manifest as cerebellar ataxia. Here, we perform multi-omic profiling in sacsin knockout (KO) cells and identify alterations in microtubule dynamics and mislocalization of focal adhesion (FA) proteins, including multiple integrins. Deficits in FA structure, signaling, and function can be rescued by targeting PTEN, a negative regulator of FA signaling. ARSACS mice possess mislocalization of ITGA1 in Purkinje neurons and synaptic disorganization in the deep cerebellar nucleus (DCN). The sacsin interactome reveals that sacsin regulates interactions between cytoskeletal and synaptic adhesion proteins. Our findings suggest that disrupted trafficking of synaptic adhesion proteins is a causal molecular deficit in ARSACS.

Insulin stimulates atypical protein kinase C-mediated phosphorylation of the neuronal adaptor FE65 to potentiate neurite outgrowth by activating ARF6-Rac1 signaling

Neurite outgrowth is a fundamental process in neurons that produces extensions and, consequently, neural connectivity. Neurite damage and atrophy are observed in various brain injuries and disorders. Understanding the intrinsic pathways of neurite outgrowth is essential for developing strategies to stimulate neurite regeneration. Insulin is a pivotal hormone in the regulation of glucose homeostasis. There is increasing evidence for the neurotrophic functions of insulin, including the induction of neurite outgrowth. However, the associated mechanism remains elusive. Here, we demonstrate that insulin potentiates neurite outgrowth mediated by the small GTPases ADP-ribosylation factor 6 (ARF6) and Ras-related C3 botulinum toxin substrate 1 (Rac1) through the neuronal adaptor FE65. Moreover, insulin enhances atypical protein kinase Cι/λ (PKCι/λ) activation and FE65 phosphorylation at serine 459 (S459) in neurons and mouse brains. In vitro and cellular assays show that PKCι/λ phosphorylated FE65 at S459. Consistently, insulin potentiates FE65 S459 phosphorylation only in the presence of PKCι/λ. Phosphomimetic studies show that an FE65 S459E mutant potently activates ARF6, Rac1, and neurite outgrowth. Notably, this phosphomimetic mutation enhances the FE65-ARF6 interaction, a process that promotes ARF6-Rac1-mediated neurite outgrowth. Likewise, insulin treatment and PKCι/λ overexpression potentiate the FE65-ARF6 interaction. Conversely, PKCι/λ knockdown suppresses the stimulatory effect of FE65 on ARF6-Rac1-mediated neurite outgrowth. The effect of insulin on neurite outgrowth is also markedly attenuated in PKCι/λ knockdown neurons, in the presence and absence of FE65. Our findings reveal a novel mechanism linking insulin with ARF6-Rac1-dependent neurite extension through the PKCι/λ-mediated phosphorylation of FE65.

Fingolimod effects on the brain are mediated through biochemical modulation of bioenergetics, autophagy, and neuroinflammatory networks

Fingolimod (FTY720) is an oral drug approved by the Food and Drug Administration (FDA) for management of multiple sclerosis (MS) symptoms, which has also shown beneficial effects against Alzheimer's (AD) and Parkinson's (PD) diseases pathologies. Although an extensive effort has been made to identify mechanisms underpinning its therapeutic effects, much remains unknown. Here, we investigated Fingolimod induced proteome changes in the cerebellum (CB) and frontal cortex (FC) regions of the brain which are known to be severely affected in MS, using a tandem mass tag (TMT) isobaric labeling-based quantitative mass-spectrometric approach to investigate the mechanism of action of Fingolimod. This study identified 6749 and 6319 proteins in CB and FC, respectively, and returned 2609 and 3086 differentially expressed proteins in mouse CB and FC, respectively, between Fingolimod treated and control groups. Subsequent bioinformatics analyses indicated a metabolic reprogramming in both brain regions of the Fingolimod treated group, where oxidative phosphorylation was upregulated while glycolysis and pentose phosphate pathway were downregulated. In addition, modulation of neuroinflammation in the Fingolimod treated group was indicated by upregulation of retrograde endocannabinoid signaling and autophagy pathways, and downregulation of neuroinflammation related pathways including neutrophil degranulation and the IL-12 mediated signaling pathway. Our findings suggest that Fingolimod may exert its protective effects on the brain by inducing metabolic reprogramming and neuroinflammation pathway modulation.

GPBAR1 preserves neurite and synapse of dopaminergic neurons via RAD21-OPCML signaling: Role in preventing Parkinson's disease in mouse model and human patients

Parkinson's disease (PD) exhibits systemic impacts on the metabolism, while metabolic alteration contributes to the risk and progression of PD. Bile acids (BA) metabolism disturbance has been linked to PD pathology. Membrane-bound G protein-coupled bile acid receptor 1 (GPBAR1) is expressed in the brain and thought to be neuroprotective; however, the role of GPBAR1 in PD remains unknown. The current study aimed to explore the effect of GPBAR1 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice with dopaminergic (DA) neuron-specific Gpbar1 knockdown or central GPBAR1 activation. The underlying mechanisms were investigated using mesencephalic primary neurons analyzed. Our study found that GPBAR1 was reduced in the substantia nigra of PD patients and MPTP-PD mice, and its expression was negatively correlated with the severity of PD-related features. Genetic downregulation of Gpbar1 in mouse mesencephalic DA neurons exacerbated MPTP-induced neurobehavioral and neuropathological deficits, whereas activation of central GPBAR1 with INT-777 (INT) relieved it. Moreover, in vivo and in vitro experiments showed the neurite- and synapse-protective effects of GPBAR1 activation in PD model. Mechanistically, by promoting the nuclear localization of cohesin subunit RAD21, GPBAR1 activation increased opioid-binding cell adhesion molecule (Opcml) expression, thereby inhibiting neurite and synapse degeneration of DA neurons in PD model. Collectively, our findings demonstrate that GPBAR1 is implicated in PD pathogenesis and activation of central GPBAR1 with INT antagonizes neurodegenerative pathology in PD model. This neuroprotection, at least in part, is attributed to the RAD21-OPCML signaling in neurons. Hence, GPBAR1 may serve as a promising candidate target for PD treatment.

Translocation of Distinct Alpha Synuclein Species from the Nucleus to Neuronal Processes during Neuronal Differentiation

Alpha synuclein (aSyn) and its aggregation are crucial for the neurodegeneration of Parkinson’s disease (PD). aSyn was initially described in the nucleus and presynaptic nerve terminals. However, the biology of nuclear aSyn and the link of aSyn between subcellular compartments are less understood. Current knowledge suggests the existence of various aSyn species with distinct structural and biochemical properties. Here, we identified a C-terminal-targeting aSyn antibody (Nu-aSyn-C), which has a high immunoaffinity towards aSyn in the nucleus. Comparing the Nu-aSyn-C antibody to aSyn antibodies developed against phosphorylated or aggregated forms, we observed that nuclear aSyn differs from cytosolic aSyn by an increased phosphorylation and assembly level in proliferating cells. Employing Nu-aSyn-C, we characterized aSyn distribution during neuronal differentiation in midbrain dopaminergic neurons (mDANs) derived from human-induced pluripotent stem cells (hiPSCs) and Lund human mesencephalic cells, and in primary rat hippocampal neurons. We detected a specific translocation pattern of aSyn during neuronal differentiation from the nucleus to the soma and finally to neuronal processes. Interestingly, a remarkable shift of Nu-aSyn-C-positive species towards neurites was detected in hiPSC mDANs from a PD patient carrying aSyn gene duplication. Together, our results reveal distinct nuclear and cytosolic aSyn species that redistribute during neuronal differentiation—a process that is altered in PD-derived neurons.

Inhibition of Wnt/β-catenin signaling attenuates axonal degeneration in models of Parkinson's disease

There are currently no treatments to delay or prevent Parkinson's disease (PD), and protective treatments require early administration. Targeting axonal degeneration in early PD could have an important clinical effect; however, the underlying molecular mechanisms controlling axonal degeneration in PD are not fully understood. Here, we studied the role of Wnt/β-catenin signaling in axonal degeneration induced by 6-hydroxydopamine (6-OHDA) or overexpression of alpha-synuclein (α-Syn) in vitro and in vivo. We found that the levels of both β-catenin and p-S9-glycogen synthase kinase-3β (GSK-3β) increased and the levels of phosphorylated β-catenin (p-β-catenin) decreased during 6-OHDA-induced axonal degeneration and that the inhibitors of the Wnt/β-catenin pathway IWR-1 and Dickkopf-1 (DKK-1) attenuated the degenerative process in primary neurons in vitro. Furthermore, IWR-1 enhanced the increase of LC3-II levels and the decrease of p62 triggered by 6-OHDA treatment, whereas the autophagy inhibitor 3-Methyladenine (3-MA) alleviated the protective effect of IWR-1 on axons in vitro. Consistent with the in vitro findings, both β-catenin and p-S9-GSK-3β were upregulated in a 6-OHDA-induced rat PD model, and blocking the Wnt/β-catenin pathway with DKK-1 attenuated the degeneration of dopaminergic axons at an early time point in vivo. The protective effect of inhibition of Wnt/β-catenin signaling was further confirmed in an α-Syn overexpression-induced animal models of PD. Taken together, these data indicate that the Wnt/β-catenin pathway is involved axonal degeneration in PD, and suggest that Wnt/β-catenin pathway inhibitors have the therapeutic potential for the prevention of PD.

The Role of Spastin in Axon Biology

Neurons are highly polarized cells with elaborate shapes that allow them to perform their function. In neurons, microtubule organization—length, density, and dynamics—are essential for the establishment of polarity, growth, and transport. A mounting body of evidence shows that modulation of the microtubule cytoskeleton by microtubule-associated proteins fine tunes key aspects of neuronal cell biology. In this respect, microtubule severing enzymes—spastin, katanin and fidgetin—a group of microtubule-associated proteins that bind to and generate internal breaks in the microtubule lattice, are emerging as key modulators of the microtubule cytoskeleton in different model systems. In this review, we provide an integrative view on the latest research demonstrating the key role of spastin in neurons, specifically in the context of axonal cell biology. We focus on the function of spastin in the regulation of microtubule organization, and axonal transport, that underlie its importance in the intricate control of axon growth, branching and regeneration.

A Strength Endurance Exercise Paradigm Mitigates Deficits in Hypoglossal-Tongue Axis Function, Strength, and Structure in a Rodent Model of Hypoglossal Motor Neuron Degeneration

The tongue plays a crucial role in the swallowing process, and impairment can lead to dysphagia, particularly in motor neuron diseases (MNDs) resulting in hypoglossal-tongue axis degeneration (e.g., amyotrophic lateral sclerosis and progressive bulbar palsy). This study utilized our previously established inducible rodent model of dysphagia due to targeted degeneration of the hypoglossal-tongue axis. This model was created by injecting cholera toxin B conjugated to saporin (CTB-SAP) into the genioglossus muscle of the tongue base for retrograde transport to the hypoglossal (XII) nucleus via the hypoglossal nerve, which provides the sole motor control of the tongue. Our goal was to investigate the effect of high-repetition/low-resistance tongue exercise on tongue function, strength, and structure in four groups of male rats: (1) control + sham exercise (n = 13); (2) control + exercise (n = 10); (3) CTB-SAP + sham exercise (n = 13); and (4) CTB-SAP + exercise (n = 12). For each group, a custom spout with adjustable lick force requirement for fluid access was placed in the home cage overnight on days 4 and 6 post-tongue injection. For the two sham exercise groups, the lick force requirement was negligible. For the two exercise groups, the lick force requirement was set to ∼40% greater than the maximum voluntary lick force for individual rats. Following exercise exposure, we evaluated the effect on hypoglossal-tongue axis function (via videofluoroscopy), strength (via force-lickometer), and structure [via Magnetic Resonance Imaging (MRI) of the brainstem and tongue in a subset of rats]. Results showed that sham-exercised CTB-SAP rats had significant deficits in lick rate, swallow timing, and lick force. In exercised CTB-SAP rats, lick rate and lick force were preserved; however, swallow timing deficits persisted. MRI revealed corresponding degenerative changes in the hypoglossal-tongue axis that were mitigated by tongue exercise. These collective findings suggest that high-repetition/low-resistance tongue exercise in our model is a safe and effective treatment to prevent/diminish signs of hypoglossal-tongue axis degeneration. The next step is to leverage our rat model to optimize exercise dosing parameters and investigate corresponding treatment mechanisms of action for future translation to MND clinical trials.

Targeting Differential Roles of Tumor Necrosis Factor Receptors as a Therapeutic Strategy for Glaucoma

Glaucoma is an irreversible sight-threatening disorder primarily due to elevated intraocular pressure (IOP), leading to retinal ganglion cell (RGC) death by apoptosis with subsequent loss of optic nerve fibers. A considerable amount of empirical evidence has shown the significant association between tumor necrosis factor cytokine (TNF; TNFα) and glaucoma; however, the exact role of TNF in glaucoma progression remains unclear. Total inhibition of TNF against its receptors can cause side effects, although this is not the case when using selective inhibitors. In addition, TNF exerts its antithetic roles via stimulation of two receptors, TNF receptor I (TNFR1) and TNF receptor II (TNFR2). The pro-inflammatory responses and proapoptotic signaling pathways predominantly mediated through TNFR1, while neuroprotective and anti-apoptotic signals induced by TNFR2. In this review, we attempt to discuss the involvement of TNF receptors (TNFRs) and their signaling pathway in ocular tissues with focus on RGC and glial cells in glaucoma. This review also outlines the potential application TNFRs agonist and/or antagonists as neuroprotective strategy from a therapeutic standpoint. Taken together, a better understanding of the function of TNFRs may lead to the development of a treatment for glaucoma.

Plasma neurofilament light levels associate with muscle mass and strength in middle-aged and older adults: findings from GenoFit

BACKGROUND

Efforts to enhance diagnostic measures for sarcopenia have led to an increased focus on the screening utility of blood-based biomarkers. In this regard, circulating neurofilament light chain (NfL) levels are a potent indicator of axonal damage and have been linked with several neurological disorders. However, despite the strong neurogenic contribution to skeletal muscle health, no studies have explored the relevance of NfL concentrations to sarcopenia. With that in mind, this study aimed to examine the association between plasma NfL concentration and sarcopenic domains.

METHODS

Three hundred adults aged between 50 and 83 years participated to this study (male participants, n = 150; mean age: 64.2 ± 8.7 years and female participants, n = 150; mean age: 63.9 ± 8.3 years). Body composition was assessed using dual-energy X-ray absorptiometry, and a skeletal muscle index (SMI) was calculated. Muscle strength was assessed with hand dynamometry. Sarcopenia was classified using the European Working Group on Sarcopenia in Older People criteria. Plasma NfL concentration was determined using a highly sensitive, enzyme-linked immunosorbent assay.

RESULTS

Neurofilament light chain levels were associated with grip strength and SMI (P = 0.005 and P = 0.045, respectively) and were significantly elevated in sarcopenic individuals, compared with non-sarcopenic participants (P < 0.001). Individuals with pre-sarcopenia (either low grip strength or low SMI) had significantly higher NfL levels, compared with healthy controls (P = 0.001 and P = 0.006, respectively). Male participants with either low grip strength or low SMI had significantly raised NfL levels (P = 0.006 and P = 0.002, respectively), while in female participants, NfL concentrations were significantly elevated only in those with low grip strength (P = 0.049). NfL concentration displayed acceptable diagnostic accuracy for sarcopenia (area under the curve = 0.726, P < 0.001).

CONCLUSIONS

Our study clearly demonstrates the indicative pertinence of circulating NfL levels to sarcopenic domains, supporting its potential use as a biomarker of sarcopenia. More studies are needed, however, to further illuminate the diagnostic value of circulating NfL. Future research should explore whether NfL levels are more powerfully linked with muscle strength than mass and whether sex mediates the relevance of NfL concentrations to sarcopenic phenotypes.

An adult-stage transcriptional program for survival of serotonergic connectivity

Neurons must function for decades of life, but how these non-dividing cells are preserved is poorly understood. Using mouse serotonin (5-HT) neurons as a model, we report an adult-stage transcriptional program specialized to ensure the preservation of neuronal connectivity. We uncover a switch in Lmx1b and Pet1 transcription factor function from controlling embryonic axonal growth to sustaining a transcriptomic signature of 5-HT connectivity comprising functionally diverse synaptic and axonal genes. Adult-stage deficiency of Lmx1b and Pet1 causes slowly progressing degeneration of 5-HT synapses and axons, increased susceptibility of 5-HT axons to neurotoxic injury, and abnormal stress responses. Axon degeneration occurs in a die back pattern and is accompanied by accumulation of α-synuclein and amyloid precursor protein in spheroids and mitochondrial fragmentation without cell body loss. Our findings suggest that neuronal connectivity is transcriptionally protected by maintenance of connectivity transcriptomes; progressive decay of such transcriptomes may contribute to age-related diseases of brain circuitry.

Senescence-Associated Cell Transition and Interaction (SACTAI): A Proposed Mechanism for Tissue Aging, Repair, and Degeneration

Aging is a broad process that occurs as a time-dependent functional decline and tissue degeneration in living organisms. On a smaller scale, aging also exists within organs, tissues, and cells. As the smallest functional unit in living organisms, cells “age” by reaching senescence where proliferation stops. Such cellular senescence is achieved through replicative stress, telomere erosion and stem cell exhaustion. It has been shown that cellular senescence is key to tissue degradation and cell death in aging-related diseases (ARD). However, senescent cells constitute only a small percentage of total cells in the body, and they are resistant to death during aging. This suggests that ARD may involve interaction of senescent cells with non-senescent cells, resulting in senescence-triggered death of non-senescent somatic cells and tissue degeneration in aging organs. Here, based on recent research evidence from our laboratory and others, we propose a mechanism—Senescence-Associated Cell Transition and Interaction (SACTAI)—to explain how cell heterogeneity arises during aging and how the interaction between somatic cells and senescent cells, some of which are derived from aging somatic cells, results in cell death and tissue degeneration.

Preclinical evaluation of a microtubule PET ligand [11C]MPC-6827 in tau and amyotrophic lateral sclerosis animal models

BACKGROUND

Microtubules are abundant in brain and their malfunctioning occurs in the early-to-advanced stages of neurodegenerative disorders. At present, there is no in vivo test available for a definitive diagnosis of most of the neurodegenerative disorders. Herein, we present the microPET imaging of microtubules using our recently reported Positron Emission Tomography (PET) tracer, [¹¹C]MPC-6827, in transgenic mice models of tau pathology (rTg4510) and amyotrophic lateral sclerosis pathology (SOD1*G93A) and compared to corresponding age-matched controls.

METHODS

Automated synthesis of [¹¹C]MPC-6827 was achieved in a GE-FX2MeI/FX2M radiochemistry module. In vivo PET imaging studies of [¹¹C]MPC-6827 (3.7 ± 0.8 MBq) were performed in rTg4510 and SOD1*G93A mice groups and their corresponding littermates (n = 5 per group). Dynamic PET images were acquired using a microPET Inveon system (Siemens, Germany) at 55 min for rTg4510 and 30 min for SOD1*G93A and corresponding controls. PET images were reconstructed using the 3D-OSEM algorithm and analyzed using VivoQuant version 4 (Invicro, MA). Tracer uptake in ROIs that included whole brain was measured as %ID/g over time to generate standardized uptake values (SUV) and time-activity curves (TACs).

RESULTS

[¹¹C]MPC-6827 exhibit a trend of lower tracer binding in mouse models of Alzheimer's disease (tau pathology, line rTg4510) and Amyotrophic Lateral Sclerosis (line SOD1*G93A) compared to wild-type littermates.

CONCLUSIONS

Our finding indicates a trend of loss of microtubule binding of [¹¹C]MPC-6827 in the whole brain of AD and ALS transgenic mice models compared to control mice. The pilot studies described herein show that [¹¹C]MPC-6827 could be used as a PET ligand for preclinical and human brain imaging of Alzheimer's disease, ALS, and other neurodegenerative diseases. Preclinical Evaluation of a Microtubule PET Ligand [¹¹C]MPC-6827 in Tau and Amyotrophic Lateral Sclerosis Animal Models. J. S. Dileep Kumar, Andrei Molotkov, Jongho Kim, Patrick Carberry, Sidney Idumonyi, John Castrillon, Karen Duff, Neil A. Shneider, Akiva Mintz.

Quantification of Neurite Degeneration with Enhanced Accuracy and Efficiency in an In Vitro Model of Parkinson's Disease

Neurite degeneration is associated with early stages of neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease (PD), and amyotrophic lateral sclerosis. One method that is commonly used to analyze neurite degeneration involves calculation of a Degeneration Index (DI) following utilization of the Analyze Particles tool of ImageJ to detect neurite fragments in micrographs of cultured cells. However, DI analyses are prone to several types of measurement error, can be time consuming to perform, and are limited in application. Here, we describe an improved method for performing DI analyses. Accuracy of measurements was enhanced through modification of selection criteria for detecting neurite fragments, removal of image artifacts and non-neurite materials from images, and optimization of image contrast. Such enhancements were implemented into an ImageJ macro that enables rapid and fully automated DI analysis of multiple images. The macro features operations for automated removal of cell bodies from micrographs, thus expanding the application of DI analyses to use in experiments involving dissociated cultures. We present experimental findings supporting that, compared with the conventional method, the enhanced analysis method yields measurements with increased accuracy and requires significantly less time to perform. Furthermore, we demonstrate the utility of the method to investigate neurite degeneration in a cell culture model of PD by conducting an experiment revealing the effects of c-Jun N-terminal kinase (JNK) on neurite degeneration induced by oxidative stress in human mesencephalic cells. This improved analysis method may be used to gain novel insight into factors underlying neurite degeneration and the progression of neurodegenerative disorders.

Pharmacological perturbation of CXCL1 signaling alleviates neuropathogenesis in a model of HEVA71 infection

Hand, foot and mouth disease (HFMD) caused by Human Enterovirus A71 (HEVA71) infection is typically a benign infection. However, in minority of cases, children can develop severe neuropathology that culminate in fatality. Approximately 36.9% of HEVA71-related hospitalizations develop neurological complications, of which 10.5% are fatal. Yet, the mechanism by which HEVA71 induces these neurological deficits remain unclear. Here, we show that HEVA71-infected astrocytes release CXCL1 which supports viral replication in neurons by activating the CXCR2 receptor-associated ERK1/2 signaling pathway. Elevated CXCL1 levels correlates with disease severity in a HEVA71-infected mice model. In humans infected with HEVA71, high CXCL1 levels are only present in patients presenting neurological complications. CXCL1 release is specifically triggered by VP4 synthesis in HEVA71-infected astrocytes, which then acts via its receptor CXCR2 to enhance viral replication in neurons. Perturbing CXCL1 signaling or VP4 myristylation strongly attenuates viral replication. Treatment with AZD5069, a CXCL1-specific competitor, improves survival and lessens disease severity in infected animals. Collectively, these results highlight the CXCL1-CXCR2 signaling pathway as a potential target against HFMD neuropathogenesis.

Interaction of Alpha Synuclein and Microtubule Organization Is Linked to Impaired Neuritic Integrity in Parkinson’s Patient-Derived Neuronal Cells

Parkinson’s disease (PD) is neuropathologically characterized by the loss of dopaminergic neurons and the deposition of aggregated alpha synuclein (aSyn). Mounting evidence suggests that neuritic degeneration precedes neuronal loss in PD. A possible underlying mechanism could be the interference of aSyn with microtubule organization in the neuritic development, as implied by several studies using cell-free model systems. In this study, we investigate the impact of aSyn on microtubule organization in aSyn overexpressing H4 neuroglioma cells and midbrain dopaminergic neuronal cells (mDANs) generated from PD patient-derived human induced pluripotent stem cells (hiPSCs) carrying an aSyn gene duplication (SNCA^Dupl). An unbiased mass spectrometric analysis reveals a preferential binding of aggregated aSyn conformers to a number of microtubule elements. We confirm the interaction of aSyn with beta tubulin III in H4 and hiPSC-derived mDAN cell model systems, and demonstrate a remarkable redistribution of tubulin isoforms from the soluble to insoluble fraction, accompanied by a significantly increased insoluble aSyn level. Concordantly, SNCA^Dupl mDANs show impaired neuritic phenotypes characterized by perturbations in neurite initiation and outgrowth. In summary, our findings suggest a mechanistic pathway, through which aSyn aggregation interferes with microtubule organization and induces neurite impairments.

Myotubularin-related protein protects against neuronal degeneration mediated by oxidative stress or infection

Microbial infections have been linked to the onset and severity of neurodegenerative diseases such as amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's disease, but the underlying mechanisms remain largely unknown. Here, we used a genetic screen for genes involved in protection from infection-associated neurodegeneration and identified the gene mtm-10. We then validated the role of the encoded myotubularin-related protein, MTM-10, in protecting the dendrites of Caenorhabditis elegans from degeneration mediated by oxidative stress or Pseudomonas aeruginosa infection. Further experiments indicated that mtm-10 is expressed in the AWC neurons of C. elegans, where it functions in a cell-autonomous manner to protect the dendrite degeneration caused by pathogen infection. We also confirm that the changes observed in the dendrites of the animals were not because of premature death or overall sickness. Finally, our studies indicated that mtm-10 functions in AWC neurons to preserve chemosensation after pathogen infection. These results reveal an essential role for myotubularin-related protein 10 in the protection of dendrite morphology and function against the deleterious effects of oxidative stress or infection.

Association of Plasma Neurofilament Light Chain With Glycaemic Control and Insulin Resistance in Middle-Aged Adults

AIMS

This study aimed to determine the association of plasma neurofilament light (NfL), a marker of neurodegeneration, with diabetes status and glycaemic parameters in people with normal glycaemia (NG), pre-diabetes (PD) and type 2 diabetes (T2D).

METHODS

Clinical and descriptive data for the diagnostic groups, NG (n=30), PD (n=48) and T2D (n=29), aged between 40 and 75 years were included in this cross-sectional analysis. Plasma NfL levels were analyzed using the ultra-sensitive single-molecule array (Simoa) platform.

RESULTS

A positive correlation was evident between plasma NfL and fasting glucose (r = 0.2824; p = 0.0032). Plasma NfL levels were not correlated with fasting insulin and insulin resistance. Plasma Nfl levels were significantly different across the diabetes groups (T2D >PD >NG, p=0.0046). Post-hoc analysis indicated significantly higher plasma NfL levels in the T2D [12.4 (5.21) pg/mL] group than in the PD [10.2 (4.13) pg/mL] and NG [8.37 (5.65) pg/mL] groups. The relationship between diabetes status and NfL remained significant after adjusting for age, sex, BMI, HOMA-IR and physical activity (adjusted r² = 0.271, p = 0.035).

CONCLUSIONS

These results show biomarker evidence of neurodegeneration in adults at risk or with T2D. Larger sample size and longitudinal analysis are required to better understand the application of NfL in people with risk and overt T2D.

Cold protection allows local cryotherapy in a clinical-relevant model of traumatic optic neuropathy

Therapeutic hypothermia (TH) is potentially an important therapy for central nervous system (CNS) trauma. However, its clinical application remains controversial, hampered by two major factors: (1) Many of the CNS injury sites, such as the optic nerve (ON), are deeply buried, preventing access for local TH. The alternative is to apply TH systemically, which significantly limits the applicable temperature range. (2) Even with possible access for 'local refrigeration', cold-induced cellular damage offsets the benefit of TH. Here we present a clinically translatable model of traumatic optic neuropathy (TON) by applying clinical trans-nasal endoscopic surgery to goats and non-human primates. This model faithfully recapitulates clinical features of TON such as the injury site (pre-chiasmatic ON), the spatiotemporal pattern of neural degeneration, and the accessibility of local treatments with large operating space. We also developed a computer program to simplify the endoscopic procedure and expand this model to other large animal species. Moreover, applying a cold-protective treatment, inspired by our previous hibernation research, enables us to deliver deep hypothermia (4 °C) locally to mitigate inflammation and metabolic stress (indicated by the transcriptomic changes after injury) without cold-induced cellular damage, and confers prominent neuroprotection both structurally and functionally. Intriguingly, neither treatment alone was effective, demonstrating that in situ deep hypothermia combined with cold protection constitutes a breakthrough for TH as a therapy for TON and other CNS traumas.

Pathophysiology of neurodegenerative diseases: An interplay among axonal transport failure, oxidative stress, and inflammation?

Neurodegenerative diseases (NDs) are heterogeneous neurological disorders characterized by a progressive loss of selected neuronal populations. A significant risk factor for most NDs is aging. Considering the constant increase in life expectancy, NDs represent a global public health burden. Axonal transport (AT) is a central cellular process underlying the generation and maintenance of neuronal architecture and connectivity. Deficits in AT appear to be a common thread for most, if not all, NDs. Neuroinflammation has been notoriously difficult to define in relation to NDs. Inflammation is a complex multifactorial process in the CNS, which varies depending on the disease stage. Several lines of evidence suggest that AT defect, axonopathy and neuroinflammation are tightly interlaced. However, whether these impairments play a causative role in NDs or are merely a downstream effect of neuronal degeneration remains unsettled. We still lack reliable information on the temporal relationship between these pathogenic mechanisms, although several findings suggest that they may occur early during ND pathophysiology. This article will review the latest evidence emerging on whether the interplay between AT perturbations and some aspects of CNS inflammation can participate in ND etiology, analyze their potential as therapeutic targets, and the urge to identify early surrogate biomarkers.

Oligodendrocytes enhance axonal energy metabolism by deacetylation of mitochondrial proteins through transcellular delivery of SIRT2

Neurons require mechanisms to maintain ATP homeostasis in axons, which are highly vulnerable to bioenergetic failure. Here, we elucidate a transcellular signaling mechanism by which oligodendrocytes support axonal energy metabolism via transcellular delivery of NAD-dependent deacetylase SIRT2. SIRT2 is undetectable in neurons but enriched in oligodendrocytes and released within exosomes. By deleting sirt2, knocking down SIRT2, or blocking exosome release, we demonstrate that transcellular delivery of SIRT2 is critical for axonal energy enhancement. Mass spectrometry and acetylation analyses indicate that neurons treated with oligodendrocyte-conditioned media from WT, but not sirt2-knockout, mice exhibit strong deacetylation of mitochondrial adenine nucleotide translocases 1 and 2 (ANT1/2). In vivo delivery of SIRT2-filled exosomes into myelinated axons rescues mitochondrial integrity in sirt2-knockout mouse spinal cords. Thus, our study reveals an oligodendrocyte-to-axon delivery of SIRT2, which enhances ATP production by deacetylating mitochondrial proteins, providing a target for boosting axonal bioenergetic metabolism in neurological disorders.

The essential role of transcription factor Pitx3 in preventing mesodiencephalic dopaminergic neurodegeneration and maintaining neuronal subtype identities during aging

Pituitary homeobox 3 (Pitx3) is required for the terminal differentiation of nigrostriatal dopaminergic neurons during neuronal development. However, whether Pitx3 contributes to the normal physiological function and cell-type identity of adult neurons remains unknown. To explore the role of Pitx3 in maintaining mature neurons, we selectively deleted Pitx3 in the mesodiencephalic dopaminergic (mdDA) neurons of Pitx3fl/fl/DATCreERT2 bigenic mice using a tamoxifen inducible CreERT2/loxp gene-targeting system. Pitx3fl/fl/DATCreERT2 mice developed age-dependent progressive motor deficits, concomitant with a rapid reduction of striatal dopamine (DA) content and a profound loss of mdDA neurons in the substantia nigra pars compacta (SNc) but not in the adjacent ventral tegmental area (VTA), recapitulating the canonical neuropathological features of Parkinson's disease (PD). Mechanistic studies showed that Pitx3-deficiency significantly increased the number of cleaved caspase-3+ cells in SNc, which likely underwent neurodegeneration. Meanwhile, the vulnerability of SNc mdDA neurons was increased in Pitx3fl/fl/DATCreERT2 mice, as indicated by an early decline in glial cell line-derived neurotrophic factor (GDNF) and aldehyde dehydrogenase 1a1 (Aldh1a1) levels. Noticeably, somatic accumulation of α-synuclein (α-syn) was also significantly increased in the Pitx3-deficient neurons. Together, our data demonstrate that the loss of Pitx3 in fully differentiated mdDA neurons results in progressive neurodegeneration, indicating the importance of the Pitx3 gene in adult neuronal survival. Our findings also suggest that distinct Pitx3-dependent pathways exist in SNc and VTA mdDA neurons, correlating with the differential vulnerability of SNc and VTA mdDA neurons in the absence of Pitx3.

Kir6.2 is essential to maintain neurite features by modulating PM20D1-reduced mitochondrial ATP generation

Kir6.2, a pore-forming subunit of the ATP-sensitive potassium (KATP) channels, regulates the functions of metabolically active tissues and acts as an ideal therapeutic target for multiple diseases. Previous studies have been conducted on peripheral kir6.2, but its precise physiological roles in the central nervous system (CNS) have rarely been revealed. In the current study, we evaluated the neurophenotypes and neuroethology of kir6.2 knockout (kir6.2^-/-) mice. We demonstrated the beneficial effects of kir6.2 on maintaining the morphology of mesencephalic neurons and controlling the motor coordination of mice. The mechanisms underlying the abnormal neurological features of kir6.2 deficiency were analyzed by RNA sequencing (RNA-seq). Pm20d1, a gene encoding PM20D1 secretase that promotes the generation of endogenous mitochondria uncouplers in vivo, was dramatically upregulated in the midbrain of kir6.2^-/- mice. Further investigations verified that PM20D1-induced increase of N-acyl amino acids (N-AAAs) from circulating fatty acids and amino acids promoted mitochondrial impairments and cut down the ATP generation, which mediated the morphological defects of the mesencephalic neurons and thus led to the behavioral impairments of kir6.2 knockout mice. This study is the first evidence to demonstrate the roles of kir6.2 in the morphological maintenance of neurite and motor coordination control of mice, which extends our understanding of kir6.2/KATP channels in regulating the neurophysiological function.

Deep Learning to Decipher the Progression and Morphology of Axonal Degeneration

Axonal degeneration (AxD) is a pathological hallmark of many neurodegenerative diseases. Deciphering the morphological patterns of AxD will help to understand the underlying mechanisms and develop effective therapies. Here, we evaluated the progression of AxD in cortical neurons using a novel microfluidic device together with a deep learning tool that we developed for the enhanced-throughput analysis of AxD on microscopic images. The trained convolutional neural network (CNN) sensitively and specifically segmented the features of AxD including axons, axonal swellings, and axonal fragments. Its performance exceeded that of the human evaluators. In an in vitro model of AxD in hemorrhagic stroke induced by the hemolysis product hemin, we detected a time-dependent degeneration of axons leading to a decrease in axon area, while axonal swelling and fragment areas increased. Axonal swellings preceded axon fragmentation, suggesting that swellings may be reliable predictors of AxD. Using a recurrent neural network (RNN), we identified four morphological patterns of AxD (granular, retraction, swelling, and transport degeneration). These findings indicate a morphological heterogeneity of AxD in hemorrhagic stroke. Our EntireAxon platform enables the systematic analysis of axons and AxD in time-lapse microscopy and unravels a so-far unknown intricacy in which AxD can occur in a disease context.