Biology subjects

Sasmita, A. O.

Publications and source records attributed to Sasmita, A. O..

5 recordsLinked to original sources

bioRxiv2025

Adaptor protein complex 2 (AP2) participates in biogenesis and homeostasis of myelin sheaths in the central nervous system

Myelination of CNS axons requires oligodendrocytes to undergo extensive morphological changes by producing large amounts of myelin membrane with defined protein composition and structure. The formation of myelin sheaths thus involves efficient trafficking and sorting of future myelin constituents via vesicles that fuse with prospective myelin membranes by exocytotic mechanisms. However, the functional relevance of other trafficking steps in oligodendocytes for myelin biogenesis is largely unknown. Here, we followed the hypothesis that developmental myelination involves endocytic mechanisms. In this model, Golgi-derived vesicles fuse with the oligodendroglial plasma membrane, from which myelin constituents are retrieved by endocytosis into endosomal/lysosomal organelles before their final integration into the growing sheath. Considering that adaptor protein complex-2 subunit-{micro} (AP2M) facilitates AP2-dependent endocytosis, we recombined the Ap2m-gene in myelin-forming oligodendrocytes, causing both hypomyelination and specific changes in the myelin proteome. Most strikingly, lysosomal membrane proteins accumulate in the abaxonal (outermost) myelin layer, identifying this membrane as an active site for retrieving constituents from myelin sheaths. These data demonstrate that the AP2 complex serves a critical function in developmental myelination in vivo. Unexpectedly, we also observed pathological myelin outfoldings indicative of focal hypermyelination. Consistent with the hypothesis that this phenotype reflects impaired maintenance rather than biogenesis of myelin sheaths, recombination of the Ap2m-gene in oligodendrocytes of adult mice caused late-onset progressive focal hypermyelination. These results indicate that, in addition to astrocytic and microglial phagocytosis, oligodendrocytes cell-autonomously contribute to maintaining the structure of healthy myelin sheaths via AP2-dependent mechanisms.

neuroscience↗

bioRxiv2023

Oligodendrocytes and neurons contribute to amyloid-β deposition in Alzheimer's disease

In Alzheimers disease (AD), amyloid-{beta} (A{beta}) is thought to be of neuronal origin. However, in single-cell RNAseq datasets from mouse and human, we found transcripts of amyloid precursor protein (APP) and the amyloidogenic-processing machinery equally abundant in oligodendrocytes (OLs). By cell-type-specific deletion of Bace1 in a humanized knock-in AD model, APPNLGF, we demonstrate that almost a third of cortical A{beta} deposited in plaques is derived from OLs. However, excitatory projection neurons must provide a threshold level of A{beta} production for plaque deposition to occur and for oligodendroglial A{beta} to co-aggregate. Indeed, very few plaques are deposited in the absence of neuronally-derived A{beta}, although soluble A{beta} species are readily detected, especially in subcortical white matter. Our data identify OLs as a source of A{beta} in vivo and further underscore a non-linear relationship between cellular A{beta} production and resulting plaque formation. Ultimately, our observations are relevant for therapeutic strategies aimed at disease prevention in AD.

neuroscience↗

bioRxiv2022

Isolated catatonia-like executive dysfunction in micewith forebrain-specific loss of myelin integrity

A key feature of advanced brain aging includes structural defects of intracortical myelin that are associated with secondary neuroinflammation. A similar pathology is seen in specific myelin mutant mice that model advanced brain aging and exhibit a range of behavioral abnormalities. However, the cognitive assessment of these mutants is problematic because myelin-dependent motor-sensory functions are required for quantitative behavioral readouts. To better understand the role of cortical myelin integrity for higher brain functions, we generated mice lacking Plp1, encoding the major integral myelin membrane protein, selectively in ventricular zone stem cells of the mouse forebrain. In contrast to conventional Plp1 null mutants, subtle myelin defects were restricted to the cortex, hippocampus and underlying callosal tracts. Moreover, forebrain-specific Plp1 mutants exhibited no defect of basic motor-sensory performance at any age tested. Surprisingly, several behavioral alterations reported for conventional Plp1 null mice (Gould et al., 2018) were absent and even social interactions appeared normal. However, with novel behavioral paradigms, we determined catatonia-like symptoms and isolated executive dysfunction in both genders. This suggests that loss of myelin integrity has an impact on cortical connectivity and underlies specific defects of executive function. These observations are likewise relevant for human neuropsychiatric conditions and other myelin-related diseases.

neuroscience↗

bioRxiv2022

Axo-vascular coupling mediated by oligodendrocytes

The high energy requirements of the cortical gray matter are met by the precise cooperation of neurons, glia, and vascular cells in a process known as neurovascular coupling (NVC). In contrast, the existence and significance of NVC in white matter (WM) are still debated and basic regulatory mechanisms are unknown. We recently discovered that oligodendrocytes sense the spiking axons activity via NMDA receptors and regulate their cell surface expression of glucose transporter GLUT1 allowing an increase in glycolytic metabolism that enables lactate release to metabolically support the axons. Here, we show for the mouse optic nerve (ON), a model WM tract, that the vascular support is also dynamically controlled. Axonal spiking activity induces small vessel dilations which are sustained for more than 20 minutes upon the ending of electrical stimulation. Pharmacological inhibition shows that the electrically evoked dilation is mediated by the prostaglandin E2 receptor EP4 and can be modulated by the oxygen concentration, as has been shown in the grey matter. Importantly, we found in ONs from conditional mouse mutants that oligodendroglial NMDA receptors are required for this type of neurovascular response, demonstrating a critical role of oligodendrocytes in coupling axonal activity to pericyte function. Reminiscent of NVC in cortical slices, the "axo-vascular" response is slower and may represent a more rudimentary form of neurovascular coupling.

neuroscience↗

bioRxiv2021

Ageing-associated myelin dysfunction drives amyloid deposition in mouse models of Alzheimer's disease

The prevalence of Alzheimers disease (AD), the leading cause of dementia, shows a strict age-dependency, but why ageing constitutes the main risk factor for this disease is still poorly understood. Brain ageing affects oligodendrocytes1 and the structural integrity of myelin sheaths2, the latter associated with secondary neuroinflammation3. Since oligodendrocytes support axonal and neuronal health4-7, we hypothesised that ageing-associated loss of myelin integrity could be an upstream risk factor for neuronal amyloid-{beta} (A{beta}) deposition, the primary neuropathological hallmark of AD. Here, we show that in AD mouse models different genetically induced defects of myelin integrity or demyelinating injuries are indeed potent drivers of amyloid deposition in vivo, quantified by whole brain light sheet microscopy. Conversely, the lack of myelin in the forebrain provides protection against plaque deposition. Mechanistically, we find that myelin dysfunction causes the accumulation of the A{beta} producing machinery within axonal swellings and increases cortical amyloid precursor protein (APP) cleavage. Surprisingly, AD mice with dysfunctional myelin lack plaque-corralling microglia but show a disease-associated microglia (DAM)-like signature as revealed by bulk and single cell transcriptomics. These activated microglia, however, are primarily engaged with myelin, preventing the protective reactions of microglia to A{beta} plaques. Our data suggest a working model, in which age-dependent structural defects of myelin promote plaque formation, directly and indirectly, and are thus an upstream AD risk factor. Improving oligodendrocyte health and myelin integrity could be a promising target to delay AD.g

neuroscience↗