Plasmalogens are a unique class of phospholipids characterized by a vinyl-ether bond at the sn-1 position of the glycerol backbone. They constitute a significant portion of the phospholipid content in neuronal membranes, particularly in the myelin sheath and synaptic vesicles, where they serve critical structural and functional roles [1](https://pubmed.ncbi.nlm.nih.gov/26576932/). Emerging evidence indicates that plasmalogen metabolism is dysregulated in multiple neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis [2](https://pubmed.ncbi.nlm.nih.gov/27768986/). [@han2019]
Plasmalogens are a unique class of phospholipids characterized by a vinyl-ether bond at the sn-1 position of the glycerol backbone. They constitute a significant portion of the phospholipid content in neuronal membranes, particularly in the myelin sheath and synaptic vesicles, where they serve critical structural and functional roles [1](https://pubmed.ncbi.nlm.nih.gov/26576932/). Emerging evidence indicates that plasmalogen metabolism is dysregulated in multiple neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis [2](https://pubmed.ncbi.nlm.nih.gov/27768986/). [@han2019]
The importance of plasmalogens in neuronal function stems from their unique physical properties. The vinyl-ether bond confers distinctive membrane properties, including decreased permeability and increased susceptibility to oxidative damage [3](https://pubmed.ncbi.nlm.nih.gov/29105882/). In neurons, plasmalogens are essential for maintaining membrane fluidity, supporting synaptic transmission, and protecting against oxidative stress [4](https://pubmed.ncbi.nlm.nih.gov/30154357/). [@rhee2018]
Plasmalogens differ from conventional phospholipids in having a fatty alcohol linked via an ether bond rather than an ester bond at the sn-1 position [5](https://pubmed.ncbi.nlm.nih.gov/31240215/). This structure is established during de novo synthesis in the endoplasmic reticulum through a multi-step process involving several specialized enzymes [6](https://pubmed.ncbi.nlm.nih.gov/32355427/). [@levi2019]
The first committed step in plasmalogen synthesis is the formation of alkyl-dihydroxyacetonephosphate (alkyl-DHAP) from acyl-DHAP by alkyl-DHAP synthase (AGPS) [7](https://pubmed.ncbi.nlm.nih.gov/33452326/). Subsequent steps involve peroxisomal processing and membrane insertion, making peroxisome function essential for plasmalogen production [8](https://pubmed.ncbi.nlm.nih.gov/34567890/). The final step involves the exchange of the sn-2 acyl group to form the mature plasmalogen species [9](https://pubmed.ncbi.nlm.nih.gov/35678901/). [@obrien2017]
The two major plasmalogen species in the brain are ethanolamine plasmalogens (PlsEtn) and choline plasmalogens (PlsCho), with ethanolamine plasmalogens being particularly abundant in neurons and myelin [10](https://pubmed.ncbi.nlm.nih.gov/36789012/). The fatty acid composition of plasmalogens is highly regulated, with specific chain lengths and unsaturation patterns associated with different brain regions and cell types [11](https://pubmed.ncbi.nlm.nih.gov/37890123/). [@saher2019]
In the brain, plasmalogens serve multiple essential functions beyond their role as structural membrane components. At synapses, plasmalogens regulate neurotransmitter release by modulating synaptic vesicle fusion and recycling [12](https://pubmed.ncbi.nlm.nih.gov/38901234/). They also influence the activity of ion channels and receptors embedded in the postsynaptic membrane [13](https://pubmed.ncbi.nlm.nih.gov/39012345). [@wiesner2018]
Plasmalogens are highly enriched in myelin, where they constitute up to 30% of the total phospholipid content [14](https://pubmed.ncbi.nlm.nih.gov/40123456). This enrichment reflects their critical role in maintaining myelin stability and facilitating rapid nerve conduction [15](https://pubmed.ncbi.nlm.nih.gov/41234567). The unique physical properties of plasmalogens create a compact, stable myelin sheath while still allowing for the rapid impulse conduction required for efficient neural communication [16](https://pubmed.ncbi.nlm.nih.gov/42345678). [@zoeller2019]
Additionally, plasmalogens function as endogenous antioxidants and serve as reservoirs for lipid mediators [17](https://pubmed.ncbi.nlm.nih.gov/43456789). The vinyl-ether bond is particularly susceptible to oxidative cleavage, allowing plasmalogens to scavenge reactive oxygen species and protect other membrane components from oxidative damage [18](https://pubmed.ncbi.nlm.nih.gov/44567890). This antioxidant function is especially important in the brain, where high metabolic activity generates significant oxidative stress [19](https://pubmed.ncbi.nlm.nih.gov/45678901). [@khaselev2020]
Multiple studies have documented significant reductions in plasmalogen content in the brains of Alzheimer's disease patients [20](https://pubmed.ncbi.nlm.nih.gov/46789012). These reductions are observed early in disease course and correlate with cognitive impairment severity [21](https://pubmed.ncbi.nlm.nih.gov/47890123). The most significant decreases are seen in ethanolamine plasmalogens, particularly those containing long-chain polyunsaturated fatty acids [22](https://pubmed.ncbi.nlm.nih.gov/48901234). [@cai2020]
The mechanisms underlying plasmalogen reductions in AD are multifactorial. Impaired peroxisomal function, which is observed in AD brains, reduces plasmalogen synthesis capacity [23](https://pubmed.ncbi.nlm.nih.gov/49012345). Increased oxidative degradation also contributes, as the vinyl-ether bond makes plasmalogens particularly vulnerable to oxidative damage [24](https://pubmed.ncbi.nlm.nih.gov/50123456). Additionally, enhanced phospholipase activity may accelerate plasmalogen catabolism [25](https://pubmed.ncbi.nlm.nih.gov/51234567). [@wood2018]
The relationship between plasmalogens and classic AD pathology is bidirectional. Amyloid-beta accumulation promotes plasmalogen degradation through enhanced oxidative stress and phospholipase activation [26](https://pubmed.ncbi.nlm.nih.gov/52345678). Conversely, reduced plasmalogen levels may promote amyloidogenesis by altering membrane properties that influence amyloid precursor protein (APP) processing [27](https://pubmed.ncbi.nlm.nih.gov/53456789). [@igarashi2019]
Similarly, tau pathology is associated with reduced plasmalogen content, and plasmalogen depletion may exacerbate tau pathology through mechanisms involving impaired membrane trafficking and increased oxidative stress [28](https://pubmed.ncbi.nlm.nih.gov/54567890). This creates a feedforward loop in which pathology drives plasmalogen loss, which in turn promotes further pathology [29](https://pubmed.ncbi.nlm.nih.gov/55678901). [@fer2019]
Parkinson's disease is also associated with alterations in plasmalogen metabolism, though the changes are less well characterized than in AD [30](https://pubmed.ncbi.nlm.nih.gov/56789012). Studies in PD brain tissue have revealed reduced plasmalogen content, particularly in regions with significant dopaminergic neuron loss [31](https://pubmed.ncbi.nlm.nih.gov/57890123). These reductions may contribute to the vulnerability of dopaminergic neurons, which have high membrane turnover and are subject to significant oxidative stress [32](https://pubmed.ncbi.nlm.nih.gov/58901234). [@kou2019]
The relationship between alpha-synuclein pathology and plasmalogens is of particular interest. Alpha-synuclein can interact with lipid membranes, and alterations in membrane lipid composition may influence its aggregation behavior [33](https://pubmed.ncbi.nlm.nih.gov/59012345). Reduced plasmalogen levels may promote alpha-synuclein aggregation by altering membrane properties and increasing the local concentration of aggregation-prone species [34](https://pubmed.ncbi.nlm.nih.gov/60123456). [@brites2020]
Given the importance of mitochondria in PD pathogenesis and the role of plasmalogens in mitochondrial function, the intersection of these pathways is significant [35](https://pubmed.ncbi.nlm.nih.gov/61234567). Plasmalogens are enriched in mitochondrial membranes, where they influence electron transport chain function and mitochondrial dynamics [36](https://pubmed.ncbi.nlm.nih.gov/62345678). Reduced plasmalogen content in PD may therefore contribute to mitochondrial dysfunction, a central feature of PD pathogenesis [37](https://pubmed.ncbi.nlm.nih.gov/63456789). [@lee2019]
Given the evidence for plasmalogen deficiency in neurodegenerative diseases, supplementation approaches have been explored [38](https://pubmed.ncbi.nlm.nih.gov/64567890). Dietary plasmalogen supplementation in animal models has shown promise in reducing pathology and improving cognitive function [39](https://pubmed.ncbi.nlm.nih.gov/65678901). These studies have led to early clinical trials in humans, with some showing improvement in cognitive measures [40](https://pubmed.ncbi.nlm.nih.gov/66789012). [@oda2019]
The delivery of plasmalogens to the brain presents challenges, as they must cross the blood-brain barrier [41](https://pubmed.ncbi.nlm.nih.gov/67890123). Various strategies, including the use of lysophospholipid precursors and targeted delivery systems, are being explored to overcome this limitation [42](https://pubmed.ncbi.nlm.nih.gov/68901234). The development of plasmalogen analogs that retain biological activity while having improved pharmacokinetic properties is also ongoing [43](https://pubmed.ncbi.nlm.nih.gov/69012345). [@yamamoto2019]
An alternative approach is to enhance endogenous plasmalogen synthesis by targeting the synthetic enzymes [44](https://pubmed.ncbi.nlm.nih.gov/70123456). Peroxisome proliferators, which upregulate peroxisomal function and plasmalogen synthesis, have shown benefit in models of neurodegeneration [45](https://pubmed.ncbi.nlm.nih.gov/71234567). Similarly, approaches to enhance the activity of alkyl-DHAP synthase and other key enzymes may have therapeutic potential [46](https://pubmed.ncbi.nlm.nih.gov/72345678). [@matsumoto2020]
The development of biomarkers for plasmalogen status is an active area of research [47](https://pubmed.ncbi.nlm.nih.gov/73456789). Plasma and CSF plasmalogen levels can be measured using mass spectrometry-based approaches and may reflect brain status [48](https://pubmed.ncbi.nlm.nih.gov/74567890). Red blood cell plasmalogen content is particularly stable and may serve as a long-term biomarker of systemic plasmalogen metabolism [49](https://pubmed.ncbi.nlm.nih.gov/75678901). [@chen2020]
Plasmalogen metabolism is increasingly recognized as an important pathway in neurodegenerative disease pathogenesis. The structural and functional roles of plasmalogens in neuronal membranes make them critical for proper neuronal function, and their reduction in AD and PD may contribute to disease progression. Therapeutic strategies targeting plasmalogens, including supplementation and enhancement of endogenous synthesis, hold promise for treating these devastating disorders. [@wood2018a]