Age and Alzheimer's Disease-Related Oligodendrocyte Changes in Hippocampal Subregions

Navigating oligodendrocyte precursor cell aging in brain health

Oligodendrocyte precursor cells (OPCs) comprise 5-8 % of the adult glial cell population and stand out as the most proliferative cell type in the central nervous system (CNS). OPCs are responsible for generating oligodendrocytes (OLs), the myelinating cells of the CNS. However, OPC functions decline as we age, resulting in impaired differentiation and inadequate remyelination. This review explores the cellular and molecular changes associated with OPC aging, and their impact on OPC differentiation and functionality. Furthermore, it examines the impact of OPC aging within the context of multiple sclerosis and Alzheimer's disease, both neurodegenerative conditions wherein aged OPCs exacerbate disease progression by impeding remyelination. Moreover, various pharmacological interventions targeting pathways related to senescence and differentiation are discussed as potential strategies to rejuvenate aged OPCs. Enhancing our understanding of OPC aging mechanisms holds promise for developing new therapies to improve remyelination and repair in age-related neurodegenerative disorders.

Dynamics of myelin deficits in the 5xFAD mouse model for Alzheimer's disease and the protective role of BDNF

Recent findings highlight myelin breakdown as a decisive early event in Alzheimer's Disease (AD) acting as aggravating factor of its progression. However, it is still unclear whether myelin loss is attributed to increased oligodendrocyte vulnerability, reduced repairing capacity or toxic stimuli. In the present study, we sought to clarify the starting point of myelin disruption accompanied with Oligodendrocyte Progenitor Cell (OPC) elimination in the brain of the 5xFAD mouse model of AD at 6 months of age in Dentate Gyrus of the hippocampus in relation to neurotrophin system. Prominent inflammation presence was detected since the age of 6 months playing a key role in myelin disturbance and AD progression. Expression analysis of neurotrophin receptors in OPCs was performed to identify new targets that could increase myelination in health and disease. OPCs in both control and 5xFAD mice express TrkB, TrkC and p75 receptors but not TrkA. Brain-derived neurotrophic factor (BDNF) that binds to TrkB receptor is well-known about its pro-myelination effect, promoting oligodendrocytes proliferation and differentiation, so we focused our investigation on its effects in OPCs under neurodegenerative conditions. Our in vitro results showed that BDNF rescues OPCs from death and promotes their proliferation and differentiation in presence of the toxic Amyloid-β 1-42. Collectively, our results indicate that BDNF possess an additional neuroprotective role through its actions on oligodendrocytic component and its use could be proposed as a drug-based myelin-enhancing strategy, complementary to amyloid and tau centered therapies in AD.

The Effects and Mechanism of Scutellaria baicalensis Georgi Stems and Leaves Flavonoids on Myelin Sheath Degeneration Induced by Composite Aβ in Rats

BACKGROUND

Alzheimer's disease is a degenerative disease of the central nervous system, and its characteristic pathological changes are closely associated with Aβ deposition and neurofibrillary tangles. Many studies have found that malignant changes in the myelin sheath and oligodendrocyte (OL) are accompanied by the occurrence and development of AD. Therefore, any method that can resist myelin sheath and OL disorders may be a potential strategy for AD.

OBJECTIVE

To investigate the effects and mechanism of Scutellaria baicalensis Georgi stem and leaf flavonoids (SSFs) on the myelin sheath degeneration induced by Aβ25-35 combined with AlC13 and RHTGF-β1 (composite Aβ) in rats.

METHODS

A rat AD model was established by intracerebroventricular injection of composite Aβ. The Morris water maze was used to screen the memory impairment rat model. The successful model rats were divided into the model group and the 35, 70, and 140 mg/kg SSFS groups. The myelin sheath changes in the cerebral cortex were observed with an electron microscope. The expression of the oligodendrocyte- specific protein claudin 11 was detected with immunohistochemistry. The protein expression levels of myelin oligodendrocyte glycoprotein (MOG), myelin-associated glycoprotein (MAG) and myelin basic protein (MBP), sphingomyelin synthase-1 (SMS1), and sphingomyelinase-2 (SMPD2) were assayed by Western blotting.

RESULTS

The intracerebroventricular injection of composite Aβ caused degeneration of the myelin sheath structure and was accompanied by the decreased claudin 11, MOG, MAG, MBP, and SMS1, and increased SMPD2 protein expression in the cerebral cortex. However, 35, 70, and 140 mg/kg SSFs can differentially ameliorate the above abnormal changes induced by composite Aβ.

CONCLUSION

SSFs can alleviate myelin sheath degeneration and increase the protein expression of claudin 11, MOG, MAG, and MBP, and the effective mechanism may be related to the positive regulation of SMS1 and SMPD2 activities.

Secondary Central Nervous System Demyelinating Disorders in the Elderly: A Narrative Review

Secondary demyelinating diseases comprise a wide spectrum group of pathological conditions and may either be attributed to a disorder primarily affecting the neurons or axons, followed by demyelination, or to an underlying condition leading to secondary damage of the myelin sheath. In the elderly, primary demyelinating diseases of the central nervous system (CNS), such as multiple sclerosis, are relatively uncommon. However, secondary causes of CNS demyelination may often occur and in this case, extensive diagnostic workup is usually needed. Infectious, postinfectious, or postvaccinal demyelination may be observed, attributed to age-related alterations of the immune system in this population. Osmotic disturbances and nutritional deficiencies, more commonly observed in the elderly, may lead to conditions such as pontine/extrapontine myelinolysis, Wernicke encephalopathy, and demyelination of the posterior columns of the spinal cord. The prevalence of malignancies is higher in the elderly, sometimes leading to radiation-induced, immunotherapy-related, or paraneoplastic CNS demyelination. This review intends to aid clinical neurologists in broadening their diagnostic approach to secondary CNS demyelinating diseases in the elderly. Common clinical conditions leading to secondary demyelination and their clinical manifestations are summarized here, while the current knowledge of the underlying pathophysiological mechanisms is additionally presented.

Myelin in Alzheimer's disease: culprit or bystander?

Alzheimer's disease (AD) is a neurodegenerative disorder with neuronal and synaptic losses due to the accumulation of toxic amyloid β (Αβ) peptide oligomers, plaques, and tangles containing tau (tubulin-associated unit) protein. While familial AD is caused by specific mutations, the sporadic disease is more common and appears to result from a complex chronic brain neuroinflammation with mitochondriopathies, inducing free radicals' accumulation. In aged brain, mutations in DNA and several unfolded proteins participate in a chronic amyloidosis response with a toxic effect on myelin sheath and axons, leading to cognitive deficits and dementia. Αβ peptides are the most frequent form of toxic amyloid oligomers. Accumulations of misfolded proteins during several years alters different metabolic mechanisms, induce chronic inflammatory and immune responses with toxic consequences on neuronal cells. Myelin composition and architecture may appear to be an early target for the toxic activity of Aβ peptides and others hydrophobic misfolded proteins. In this work, we describe the possible role of early myelin alterations in the genesis of neuronal alterations and the onset of symptomatology. We propose that some pathophysiological and clinical forms of the disease may arise from structural and metabolic disorders in the processes of myelination/demyelination of brain regions where the accumulation of non-functional toxic proteins is important. In these forms, the primacy of the deleterious role of amyloid peptides would be a matter of questioning and the initiating role of neuropathology would be primarily the fact of dysmyelination.