Plasma neurodegeneration biomarker concentrations associate with glymphatic and meningeal lymphatic measures in neurological disorders

Traumatic brain injury and sleep in military and veteran populations: A literature review

BACKGROUND

Traumatic brain injury (TBI) is a hallmark of wartime injury and is related to numerous sleep wake disorders (SWD), which persist long term in veterans. Current knowledge gaps in pathophysiology have hindered advances in diagnosis and treatment.

OBJECTIVE

We reviewed TBI SWD pathophysiology, comorbidities, diagnosis and treatment that have emerged over the past two decades.

METHODS

We conducted a literature review of English language publications evaluating sleep disorders (obstructive sleep apnea, insomnia, hypersomnia, parasomnias, restless legs syndrome and periodic limb movement disorder) and TBI published since 2000. We excluded studies that were not specifically evaluating TBI populations.

RESULTS

Highlighted areas of interest and knowledge gaps were identified in TBI pathophysiology and mechanisms of sleep disruption, a comparison of TBI SWD and post-traumatic stress disorder SWD. The role of TBI and glymphatic biomarkers and management strategies for TBI SWD will also be discussed.

CONCLUSION

Our understanding of the pathophysiologic underpinnings of TBI and sleep health, particularly at the basic science level, is limited. Developing an understanding of biomarkers, neuroimaging, and mixed-methods research in comorbid TBI SWD holds the greatest promise to advance our ability to diagnose and monitor response to therapy in this vulnerable population.

Neurosurgery and the glymphatic system

The discovery of the glymphatic system has fundamentally altered our comprehension of cerebrospinal fluid transport and the removal of waste from brain metabolism. In the past decade, since its initial characterization, research on the glymphatic system has surged exponentially. Its potential implications for central nervous system disorders have sparked significant interest in the field of neurosurgery. Nonetheless, ongoing discussions and debates persist regarding the concept of the glymphatic system, and our current understanding largely relies on findings from experimental animal studies. This review aims to address several key inquiries: What methodologies exist for evaluating glymphatic function in humans today? What is the current evidence supporting the existence of a human glymphatic system? Can the glymphatic system be considered distinct from the meningeal-lymphatic system? What is the human evidence for glymphatic-meningeal lymphatic system failure in neurosurgical diseases? Existing literature indicates a paucity of techniques available for assessing glymphatic function in humans. Thus far, intrathecal contrast-enhanced magnetic resonance imaging (MRI) has shown the most promising results and have provided evidence for the presence of a glymphatic system in humans, albeit with limitations. It is, however, essential to recognize the interconnection between the glymphatic and meningeal lymphatic systems, as they operate in tandem. There are some human studies demonstrating deteriorations in glymphatic function associated with neurosurgical disorders, enriching our understanding of their pathophysiology. However, the translation of this knowledge into clinical practice is hindered by the constraints of current glymphatic imaging modalities.

Ultrasonic cerebrospinal fluid clearance improves outcomes in hemorrhagic brain injury models

Impaired clearance of the byproducts of aging and neurologic disease from the brain exacerbates disease progression and severity. We have developed a noninvasive, low intensity transcranial focused ultrasound protocol that facilitates the removal of pathogenic substances from the cerebrospinal fluid (CSF) and the brain interstitium. This protocol clears neurofilament light chain (NfL) - an aging byproduct - in aged mice and clears red blood cells (RBCs) from the central nervous system in two mouse models of hemorrhagic brain injury. Cleared RBCs accumulate in the cervical lymph nodes from both the CSF and interstitial compartments, indicating clearance through meningeal lymphatics. Treating these hemorrhagic brain injury models with this ultrasound protocol reduced neuroinflammatory and neurocytotoxic profiles, improved behavioral outcomes, decreased morbidity and, importantly, increased survival. RBC clearance efficacy was blocked by mechanosensitive channel antagonism and was effective when applied in anesthetized subjects, indicating a mechanosensitive channel mediated mechanism that does not depend on sensory stimulation or a specific neural activity pattern. Notably, this protocol qualifies for an FDA non-significant risk designation given its low intensity, making it readily clinically translatable. Overall, our results demonstrate that this low-intensity transcranial focused ultrasound protocol clears hemorrhage and other harmful substances from the brain via the meningeal lymphatic system, potentially offering a novel therapeutic tool for varied neurologic disorders.

Glymphatic-lymphatic coupling: assessment of the evidence from magnetic resonance imaging of humans

The discoveries that cerebrospinal fluid participates in metabolic perivascular exchange with the brain and further drains solutes to meningeal lymphatic vessels have sparked a tremendous interest in translating these seminal findings from animals to humans. A potential two-way coupling between the brain extra-vascular compartment and the peripheral immune system has implications that exceed those concerning neurodegenerative diseases, but also imply that the central nervous system has pushed its immunological borders toward the periphery, where cross-talk mediated by cerebrospinal fluid may play a role in a range of neoplastic and immunological diseases. Due to its non-invasive approach, magnetic resonance imaging has typically been the preferred methodology in attempts to image the glymphatic system and meningeal lymphatics in humans. Even if flourishing, the research field is still in its cradle, and interpretations of imaging findings that topographically associate with reports from animals have yet seemed to downplay the presence of previously described anatomical constituents, particularly in the dura. In this brief review, we illuminate these challenges and assess the evidence for a glymphatic-lymphatic coupling. Finally, we provide a new perspective on how human brain and meningeal clearance function may possibly be measured in future.

Sleep and APOE-ε4 have a synergistic effect on plasma biomarkers and longitudinal cognitive decline in older adults

BACKGROUND

Sleep disorders are prevalent among patients with Alzheimer's disease (AD), and the APOE ε4 genotype is a key genetic risk factor for sporadic AD. However, the combined effect of the genotype and sleep disorders on cognitive decline remains uncertain.

METHODS

A total of 972 participants were drawn from the SILCODE cohort, comprising 655 without the ε4 allele (APOE-) and 317 with ε4 allele (APOE+). Data were collected, including neuropsychological assessments, sleep measurements, plasma biomarkers, and PET imaging. A Sleep Composite Index (SCI) was created, categorizing participants into high risk (Sleep+) and low risk (Sleep-).

RESULTS

Significant predictions of dementia risk associated with plasma p-tau181, neurofilament light chain (NfL), and SCI. Individuals with both Sleep+ and APOE+ had a higher risk of dementia compared to those with Sleep-. The Sleep+/APOE+ group had higher plasma NfL levels than the Sleep-/APOE- group. Similar trends emerged in plasma NfL levels among the Aβ PET-positive subgroup. Plasma NfL levels explained 23% of the relationship between SCI and cognitive impairment.

CONCLUSION

Our study highlights sleep disorder was associated with cognitive decline, with plasma NfL playing a partial mediating role. These findings explain how sleep disorders affect cognitive function and emphasize the importance of healthy sleep for older adults.

Imaging of brain barrier inflammation and brain fluid drainage in human neurological diseases

The intricate relationship between the central nervous system (CNS) and the immune system plays a crucial role in the pathogenesis of various neurological diseases. Understanding the interactions among the immunopathological processes at the brain borders is essential for advancing our knowledge of disease mechanisms and developing novel diagnostic and therapeutic approaches. In this review, we explore the emerging role of neuroimaging in providing valuable insights into brain barrier inflammation and brain fluid drainage in human neurological diseases. Neuroimaging techniques have enabled us not only to visualize and assess brain structures, but also to study the dynamics of the CNS in health and disease in vivo. By analyzing imaging findings, we can gain a deeper understanding of the immunopathology observed at the brain-immune interface barriers, which serve as critical gatekeepers that regulate immune cell trafficking, cytokine release, and clearance of waste products from the brain. This review explores the integration of neuroimaging data with immunopathological findings, providing valuable insights into brain barrier integrity and immune responses in neurological diseases. Such integration may lead to the development of novel diagnostic markers and targeted therapeutic approaches that can benefit patients with neurological disorders.

Mechanisms behind changes of neurodegeneration biomarkers in plasma induced by sleep deprivation

Acute sleep deprivation has been shown to affect cerebrospinal fluid and plasma concentrations of biomarkers associated with neurodegeneration, though the mechanistic underpinnings remain unknown. This study compared individuals who, for one night, were either subject to total sleep deprivation or free sleep, (i) examining plasma concentrations of neurodegeneration biomarkers the morning after sleep deprivation or free sleep and (ii) determining how overnight changes in biomarkers plasma concentrations correlate with indices of meningeal lymphatic and glymphatic clearance functions. Plasma concentrations of amyloid-β 40 and 42, phosphorylated tau peptide 181, glial fibrillary acid protein and neurofilament light were measured longitudinally in subjects who from Day 1 to Day 2 either underwent total sleep deprivation (n = 7) or were allowed free sleep (n = 21). The magnetic resonance imaging contrast agent gadobutrol was injected intrathecally, serving as a cerebrospinal fluid tracer. Population pharmacokinetic model parameters of gadobutrol cerebrospinal fluid-to-blood clearance were utilized as a proxy of meningeal lymphatic clearance capacity and intrathecal contrast-enhanced magnetic resonance imaging as a proxy of glymphatic function. After one night of acute sleep deprivation, the plasma concentrations of amyloid-β 40 and 42 were reduced, but not the ratio, and concentrations of the other biomarkers were unchanged. The overnight change in amyloid-β 40 and 42 plasma concentrations in the sleep group correlated significantly with indices of meningeal lymphatic clearance capacity, while this was not seen for the other neurodegeneration biomarkers. However, overnight change in plasma concentrations of amyloid-β 40 and 42 did not correlate with the glymphatic marker. On the other hand, the overnight change in plasma concentration of phosphorylated tau peptide 181 correlated significantly with the marker of glymphatic function in the sleep deprivation group but not in the sleep group. The present data add to the evidence of the role of sleep and sleep deprivation on plasma neurodegeneration concentrations; however, the various neurodegeneration biomarkers respond differently with different mechanisms behind sleep-induced alterations in amyloid-β and tau plasma concentrations. Clearance capacity of meningeal lymphatics seems more important for sleep-induced changes in amyloid-β 40 and 42 plasma concentrations, while glymphatic function seems most important for change in plasma concentration of phosphorylated tau peptide 181 during sleep deprivation. Altogether, the present data highlight diverse mechanisms behind sleep-induced effects on concentrations of plasma neurodegeneration biomarkers.

Plasma and cerebrospinal fluid concentrations of neurofilament light protein correlate in patients with idiopathic normal pressure hydrocephalus

BACKGROUND

Neurofilament light chain protein (NFL), a marker of neuronal axonal degeneration, is increased in cerebrospinal fluid (CSF) of patients with idiopathic normal pressure hydrocephalus (iNPH). Assays for analysis of NFL in plasma are now widely available but plasma NFL has not been reported in iNPH patients. Our aim was to examine plasma NFL in iNPH patients and to evaluate the correlation between plasma and CSF levels, and whether NFL levels are associated with clinical symptoms and outcome after shunt surgery.

METHODS

Fifty iNPH patients with median age 73 who had their symptoms assessed with the iNPH scale and plasma and CSF NFL sampled pre- and median 9 months post-operatively. CSF plasma was compared with 50 healthy controls (HC) matched for age and gender. Concentrations of NFL were determined in plasma using an in-house Simoa method and in CSF using a commercially available ELISA method.

RESULTS

Plasma NFL was elevated in patients with iNPH compared to HC (iNPH: 45 (30-64) pg/mL; HC: 33 (26-50) (median; Q1-Q3), p = 0.029). Plasma and CSF NFL concentrations correlated in iNPH patients both pre- and postoperatively (r = 0.67 and 0.72, p < 0.001). We found only weak correlations between plasma or CSF NFL and clinical symptoms and no associations with outcome. A postoperative NFL increase was seen in CSF but not in plasma.

CONCLUSIONS

Plasma NFL is increased in iNPH patients and concentrations correlate with CSF NFL implying that plasma NFL can be used to assess evidence of axonal degeneration in iNPH. This finding opens a window for plasma samples to be used in future studies of other biomarkers in iNPH. NFL is probably not a very useful marker of symptomatology or for prediction of outcome in iNPH.