Marine Plasmalogens: A Gift from the Sea with Benefits for Age-Associated Diseases

Aging increases oxidative and inflammatory stress caused by a reduction in metabolism and clearance, thus leading to the development of age-associated diseases. The quality of our daily diet and exercise is important for the prevention of these diseases. Marine resources contain various valuable nutrients, and unique glycerophospholipid plasmalogens are found abundantly in some marine invertebrates, including ascidians. One of the major classes, the ethanolamine class (PlsEtn), exists in a high ratio to phospholipids in the brain and blood, while decreased levels have been reported in patients with age-associated diseases, including Alzheimer’s disease. Animal studies have shown that the administration of marine PlsEtn prepared from marine invertebrates improved PlsEtn levels in the body and alleviated inflammation. Animal and human studies have reported that marine PlsEtn ameliorates cognitive impairment. In this review, we highlight the biological significance, relationships with age-associated diseases, food functions, and healthcare materials of plasmalogens based on recent knowledge and discuss the contribution of marine plasmalogens to health maintenance in aging.


Introduction
Aging decreases metabolic, clearance, and immune functions and increases oxidative and inflammatory stress [1,2].For example, a decrease in the clearance of amyloid-β (Aβ) from the brain by aging is hypothesized to develop into Alzheimer's disease (AD) via chronic oxidation and inflammation [3].As human life expectancy is high in contemporary society, it is necessary to reduce the risk of age-associated diseases.
At the beginning of life, the sea is still mysterious, and marine resources possess various valuable nutrients that are not found in land resources.Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) as n-3 polyunsaturated fatty acids (PUFAs) contained in marine food contribute to health maintenance in current diets containing excessive n-6 PUFAs and saturated fatty acids [4].Algae-derived xanthophylls, including fucoxanthin and astaxanthin, have been reported to exhibit not only antioxidative activity but also various food functions [5].
Plasmalogens are glycerophospholipids (GPLs) with a vinyl ether bond at the sn-1 position.In particular, the ethanolamine class (PlsEtn) is abundantly distributed in the nervous system, including the brain, and PlsEtn is thought to exhibit nerve protection as an endogenous antioxidant.In some marine invertebrates, high concentrations of PlsEtn exist in the muscles and viscera because of lower neural differentiation than that in vertebrates [6][7][8].Reportedly, PlsEtn levels in the brain and blood are altered by aging and some age-associated diseases, including AD [9,10], and the administration of PlsEtn from marine invertebrates (marine PlsEtn) alleviates the impairments caused by aging [11,12].In this review, we discuss and summarize the biological significance of plasmalogens and their relationship with age-associated diseases, food functions, and healthcare materials.
In mammals, plasmalogens are ubiquitous in the body (Tables 1 and 2).However, their ratio to phospholipids depends on the organ.The ratio of PlsEtn to EtnGpl is high in the nervous system, including the brain (50-90%; especially high ratio in the myelin sheath), heart (25-75%), blood plasma (approximately 50%), and red blood cells (RBCs; 45-65%), whereas it is low in the liver (<10%) [13][14][15].The ratio of PlsCho to ChoGpl is basically lower than that of PlsEtn, with the highest being 5-40% in the heart.Tissues having high levels of plasmalogens (i.e., the brain and heart) indicate low expression of lyso-plasmalogenase, which hydrolyzes the vinyl ether bond of lyso-plasmalogens, while tissues having low levels, like the liver, exhibit high expression [16].Guinea pig Whole brain 0.9 [21] The composition of this table was primarily created by referring to [14,15] with their permission.For references [20,22], the values displayed in the table were converted from the data in the literature.Average molecular weight of glycerophospholipids used was 775.ChoGpl, choline glycerophospholipid; EtnGpl, ethanolamine glycerophospholipid; HDL, high-density lipoprotein; LDL, low-density lipoprotein; PL, phospholipid; PlsCho, choline plasmalogen; PlsEtn, ethanolamine plasmalogen; VLDL, very-low-density lipoprotein.
Table 2. Predominant plasmalogen species found in different mammalian organs and tissues.

Species Organs and Tissues
Predominant Plasmalogen Species Refs.

Biological Role
Plasmalogens play various roles in biological membranes.Plasmalogens are closer to the sn-1 and sn-2 chains than those in diacyl-types; therefore, plasmalogens strengthen lipid membranes and lower their fluidity, both of which are related to cellular functions [56,57].Plasmalogens scavenge reactive oxygen species (ROS), such as singlet oxygen ( 1 O 2 ) and superoxide (O 2 − ), at their alkenyl (vinyl ether) linkages [58,59].Plasmalogens also store PUFAs at the sn-2 position, and the PUFAs are released by phospholipase A 2 (PLA 2 ), including the plasmalogen-selective PLA 2 [60].The released PUFAs can be metabolized into eicosanoids and docosanoids, which exhibit various bioactivities, including the promotion and inhibition of inflammation [61].PLA 2 isoforms differ in specificity depending on the linkage, length, and unsaturation of the carbon chains at the sn-1 and sn-2 positions; therefore, plasmalogens and PLA 2 can control biological reactions [62].
Plasmalogens are synthesized in the peroxisomes and endoplasmic , and the initial reaction is an acylation of dihydroxyacetone phosphate (DHAP) by acyltransferase (DHAP-AT).Peroxisome biogenesis disorders, which are congenital dysfunctions of the peroxisome, impair the metabolism of lipids and ROS (e.g., low plasmalogen levels and promotion of inflammation), thus resulting in various symptoms, including hypomyelination.Zellweger syndrome and rhizomelic chondrodysplasia punctata (RCDP) are severe types of peroxisome biogenesis disorders, and most patients with their diseases die during infancy.Mutant cells deficient in plasmalogen synthesis exhibit low resistance to oxidation, and the addition of alkylglycerol as a plasmalogen precursor increases plasmalogen levels to enhance oxidative resistance [63].Plasmalogen levels in cells are tightly administered; as feedback inhibition, excessive levels of plasmalogens reduce the levels of fatty acyl-CoA reductase 1 (Far1), which provides fatty alcohols in the synthesis, by promoting the degradation of the protein to control the plasmalogen levels [64].Cells, mice, and humans deficient in peroxisomes have decreased PlsEtn levels but increased PtdEtn levels alternatively, and the total EtnGpl levels are maintained [65].
The significance of plasmalogens in the brain and nerves has been extensively investigated.Because PlsEtn easily forms an inverse hexagonal phase at body temperature compared with PtdEtn, PlsEtn is involved in membrane fusion during synaptic transmission [66].In plasmalogen-deficient mice (knockout of the DHAP-AT gene), myelination in the spinal cord and optic nerves was improved by intraperitoneal (i.p.) injection of alkyl glycerol [67].The knockdown of the DHAP-AT gene in the hippocampus of adult mice decreases memory function and neurogenesis [68].According to a recent study, a decrease in PlsEtn levels reduces cholesterol synthesis; because the brain does not receive cholesterol from the blood, low PlsEtn levels in the brain lead to a shortage of 24-hydroxycholesterol as a cholesterol metabolite and a ligand of the liver X receptor (LXR), which inhibits myelin formation via LXR [64].Additionally, plasmalogens are reported to be related to ferroptosis, a type of cell death caused by the accumulation of phospholipid hydroperoxides.Saturated fatty acids, which are metabolized to fatty alcohols by Far1, enhance the effects of ferroptosis inducers [69].Because Far1 levels receive feedback inhibition by plasmalogens, such as those described above, a decrease in the brain levels of plasmalogens may induce neurocyte death.
In terms of the action of plasmalogens on the heart and blood, there are reports on their antioxidant properties [70][71][72].In macrophages, which are derived from monocytes and are responsible for the immune response, PlsEtn affects the signal transduction of inflammation by providing ARA as a source of eicosanoids [73] and by controlling the number and size of lipid rafts [54].Because plasmalogens in the heart and blood plasma possess the same levels of PlsCho as PlsEtn, whereas those in other organs are predominantly PlsEtn, PlsCho may play important roles in the heart and blood [74].Mice deficient in mitochondrial transacylase tafazzin, a model of Barth syndrome, had decreased cardiac levels of PlsCho but not PlsEtn, whereas a significant decrease in PlsEtn levels was observed in the brain, kidney, and liver [45,75].However, the biological significance of plasmalogens in the organs remains unclear.

Aging
Aging is associated with cellular functional impairments, including mitochondrial dysfunction, that induce chronic oxidative and inflammatory stress [1,2].For example, in human plasma and RBCs, the levels of GPL peroxides (PLOOHs), the first oxides of GPLs, increase depending on aging, and hyperlipidemia enhances the accumulation of PLOOHs with aging [76][77][78].Reportedly, ChoGpl peroxides (PCOOHs) evoke monocyte adherence to the arterial wall during the initiation of atherosclerosis [79], and aged RBCs, which accumulate PLOOHs, reduce their ability to transport oxygen to the organs.Therefore, aging synergistically increases the risk of various diseases caused by oxidative and inflammatory stress.
In the human brain, plasmalogen levels peak in middle age and tend to decrease in old age [80,81], along with myelin (rich in PlsEtn) [82].Free aldehydes and α-hydroxyaldehydes derived from the sn-1 position of plasmalogen oxides are markedly increased in the older adult human brain [81].In human plasma and RBCs, plasmalogen levels decrease with aging [9].This is because aging reduces peroxisomal functions [83] and promotes the degradation of the vinyl ether bond in plasmalogens by cytochrome c and oxidative stress [84].A decrease in the levels of the antioxidant GPL plasmalogens reduces oxidative resistance and induces membrane dysfunction, including that of lipid rafts.

Alzheimer's Disease
AD is the most common form of dementia worldwide.One of the pathological characteristics of AD is the progressive aggregation and accumulation of Aβ peptide in the senile plaques of the human brain.Because brain Aβ, particularly its fibril form, is highly neurotoxic, the progressive aggregation of Aβ is a critical step in AD pathogenesis [85].Aging promotes chronic inflammation and Aβ accumulation, increasing the risk of developing and advancing AD [86,87].
Plasma Aβ is hypothesized to readily contact RBCs and impair their functions in the circulating human blood [88,89].Our group and other researchers investigated this hypothesis and found that Aβ induces oxidative injury in RBCs by binding to them and causing the accumulation of PLOOHs [90,91].Additionally, Aβ induces the binding of RBCs to endothelial cells and decreases endothelial viability, possibly by generating oxidative and inflammatory stress [92].Moreover, RBC Aβ and PCOOH levels increase with age in healthy subjects, and RBC PCOOH levels increase in patients with AD [93,94].
By contrast, the levels of PlsEtn and PlsCho have been reported to decrease specifically in the brain and serum of patients with AD, depending on the severity of the disease [10, 50,95,96].We also analyzed the levels of Aβ, PCOOHs, and PlsEtn in the blood of patients with AD and their spouses (healthy subjects) (Figure 1) [97].Evidently, plasma from patients with AD exhibited lower concentrations of PlsEtn species, particularly PlsEtn bearing a DHA moiety.Additionally, lower PlsEtn and higher PCOOH levels were observed in the RBCs of patients with AD.In both the AD and control blood samples, RBC PCOOH levels were correlated with plasma levels of Aβ 40 , particularly in patients with AD, and each PlsEtn species exhibited different correlations with plasma Aβ.Furthermore, PlsEtn bearing DHA suppressed the formation and disruption of Aβ fibrils in vitro, whereas the PtdEtn and PtdCho species were not affected (Figure 2) [97].Together with the toxicities of Aβ fibrils [85][86][87][88][89][90][91][92], a decrease in PlsEtn levels is believed to weaken the blood and brain, thus advancing AD symptoms.
neurotoxic, the progressive aggregation of Aβ is a critical step in AD pathogenesis [85].Aging promotes chronic inflammation and Aβ accumulation, increasing the risk of developing and advancing AD [86,87].
Plasma Aβ is hypothesized to readily contact RBCs and impair their functions in the circulating human blood [88,89].Our group and other researchers investigated this hypothesis and found that Aβ induces oxidative injury in RBCs by binding to them and causing the accumulation of PLOOHs [90,91].Additionally, Aβ induces the binding of RBCs to endothelial cells and decreases endothelial viability, possibly by generating oxidative and inflammatory stress [92].Moreover, RBC Aβ and PCOOH levels increase with age in healthy subjects, and RBC PCOOH levels increase in patients with AD [93,94].
By contrast, the levels of PlsEtn and PlsCho have been reported to decrease specifically in the brain and serum of patients with AD, depending on the severity of the disease [10, 50,95,96].We also analyzed the levels of Aβ, PCOOHs, and PlsEtn in the blood of patients with AD and their spouses (healthy subjects) (Figure 1) [97].Evidently, plasma from patients with AD exhibited lower concentrations of PlsEtn species, particularly PlsEtn bearing a DHA moiety.Additionally, lower PlsEtn and higher PCOOH levels were observed in the RBCs of patients with AD.In both the AD and control blood samples, RBC PCOOH levels were correlated with plasma levels of Aβ40, particularly in patients with AD, and each PlsEtn species exhibited different correlations with plasma Aβ.Furthermore, PlsEtn bearing DHA suppressed the formation and disruption of Aβ fibrils in vitro, whereas the PtdEtn and PtdCho species were not affected (Figure 2) [97].Together with the toxicities of Aβ fibrils [85][86][87][88][89][90][91][92], a decrease in PlsEtn levels is believed to weaken the blood and brain, thus advancing AD symptoms.

Parkinson's Disease
Parkinson's disease (PD) is characterized by motor dysfunction caused by the loss of dopaminergic neurons in the substantia nigra pars compacta and the depletion of dopa-

Parkinson's Disease
Parkinson's disease (PD) is characterized by motor dysfunction caused by the loss of dopaminergic neurons in the substantia nigra pars compacta and the depletion of dopamine in the nigrostriatal pathway.Aging increases PD morbidity, which is responsible for mitochondrial dysfunction and chronic oxidative and inflammatory stress [98].Furthermore, in the advanced stages of PD, some patients develop dementia, and Aβ deposition, a pathological feature of AD, is observed in the brain [99].In the PD brain, plasmalogens were lower than those in the control brain, but not significantly; however, in lipid rafts from the PD brain, the levels of plasmalogens were significantly lower [95,100].Lipid rafts are strongly associated with signal transduction.It has been suggested that a reduction in plasmalogen levels decreases membrane strength and the levels of eicosanoids and docosanoids from PUFAs, which affect signal transduction.Additionally, PlsEtn levels in the plasma and RBCs of patients with PD are significantly lower than those in control subjects [101,102].The lipid composition of blood is thought to reflect that of the organs [103].In contrast, the reduction in blood PlsEtn rich in PUFAs may induce impairment of brain blood flow, which is associated with PD [104].It would be interesting to determine whether, in patients with PD, blood PlsEtn levels fall earlier than those of brain lipid rafts.

Arteriosclerosis
Aging and obesity induce chronic inflammation and dysfunction of the lipid metabolism, leading to arteriosclerosis and cardiovascular disease [100].The oxidized low-density lipoprotein (LDL) containing PCOOHs induces epidemic apoptosis [101,102] and promotes monocyte adherence to the arterial walls [79].Plasma PCOOH levels increase during aging, particularly during hyperlipidemia [76].By contrast, the addition of plasmalogens in vitro increased the oxidative stability of LDL, which is dependent on its content [71].Patients with hyperlipidemia, particularly those with high levels of both triglycerides and cholesterol, exhibit lower serum plasmalogen levels than healthy subjects; serum plasmalogen levels are negatively correlated with serum triglyceride levels [103].Patients with familial hypercholesterolemia indicate lower plasmalogen levels in plasma and LDL compared with healthy subjects; the supplementation of α-tocopherol as a lipophilic antioxidant increases their levels [32].Serum levels of plasmalogens, particularly PlsCho bearing oleic acid, exhibit correlations with atherosclerosis-related factors (positive with protective functions and negative with promotion factors) [104,105].Patients with coronary artery disease have lower levels of plasma PlsCho [106,107] and RBC PlsEtn [31].Animal models of ischemia show a decrease in plasmalogen levels and an increase in lipid oxides, including α-hydroxyaldehyde as a plasmalogen oxide, in the heart and spinal cord [108][109][110].Together with their antioxidant properties [70][71][72], it is thought that blood plasmalogen levels are easily influenced by oxidative stress and abnormal lipid metabolism, and a decrease in plasmalogen levels reduces resistance to their stress.Therefore, maintaining blood levels may be important for health of the circulatory system.

Cancer
Cancers are accentuated by DNA damage, apoptosis, dysregulation of cell homeostasis, and dedifferentiation [111], which are caused by chronic inflammation from aging as well as a dangerous lifestyle [1,112].Reportedly, cancers alter the metabolism of GPLs, including plasmalogens [113].Many studies have reported that higher levels of ether lipids (i.e., plasmalogens and alkyl-GPLs) are observed in cancer cells and tumors than in normal cells and tissues.This is due to the upregulation of biosynthesis enzymes of ether lipids in malignant cells and ether lipids' contribution to the promotion of cancer progression via the generation of signaling lipids [114].Hypoxia, which is a factor in cancer progression, increases plasmalogen levels in in vitro colon cancer cells by activating the re-acylation of lyso-plasmalogens, not plasmalogen synthesis [115].Patients with gastric carcinoma show higher plasma levels of PlsEtn [116].In contrast, some studies reported that cancers decreased plasmalogen levels, esophageal tumors decreased PlsEtn and other phospholipids [117], and patients with pancreatic cancer exhibited lower serum levels of PlsEtn than control subjects [118].Lyso-PlsEtn and n-3 PUFAs, provided from PlsEtn by epidermal secretory PLA 2 (sPLA 2 -IID), promote skin cancer hypergrowth and reduce antitumor immunity, respectively [119].Additionally, ferroptosis, an attractive target for anticancer therapy [120], is associated with plasmalogens.Plasmalogens containing PUFAs accelerate ferroptosis via lipid peroxidation [121], while plasmalogens with non-PUFAs suppress ferroptosis enhanced by saturated fatty acids [69].The gene expression of transmembrane protein 189 (TMEM189), which catalyzes the desaturation of PakEtn to PlsEtn, has been correlated with resistance to ferroptosis inducers in 654 cancer cell lines [69].The administration of the plasmalogen precursor alkylglycerol decreased the growth, vascularization, and dissemination of Lewis lung carcinoma in a mouse model that used grafted tumors and reduced the plasmalogen content in the tumors [122].

Resources
Although the bovine brain is used as a resource for complex lipids, such as PlsEtn, its use has become difficult owing to outbreaks of bovine spongiform encephalopathy.Similarly, because eating the central nervous system of mammals poses anthropozoonotic risks, resources from different parts or different phyla are required.
Marine invertebrates, which exhibit lower neural differentiation than mammals, reportedly contain high levels of PlsEtn [6].Accordingly, we investigated the contents of EtnGpl subclasses in marine invertebrates [7].PlsEtn content in marine invertebrates varies widely (200-5000 nmol/g wet weight).The marine invertebrates examined, excluding prawns, squids, and octopuses, exhibited a high ratio of PlsEtn to EtnGpl in the muscle and viscera (65-98 mol%) (Figure 3).Sea anemones and starfish, which are not or are rarely used as food, had the highest content and ratio of PlsEtn (>2500 nmol/g wet weight, >95 mol%).The edible parts of ascidians, sea urchins, and shellfish have varying PlsEtn contents (200-4000 nmol/g wet weight) and a higher ratio of PlsEtn than mammalian brains (>65 mol%).PlsEtn from marine invertebrates is abundant in DHA and EPA at the sn-2 position; for example, the PlsEtn bearing DHA (18:0/22:6-PlsEtn) ratio of ascidian muscle and viscera is 9 mol% and 31 mol% of the total PlsEtn, respectively, whereas the ratio of land meat (leg muscle of beef, pig, and chicken) is <1 mol% (Figure 4) [8].
Chicken skin and large cervid meat (caribou and moose) have also been reported to have high levels of PlsEtn (1000 nmol and 5000 nmol/g wet weight, respectively; 75 mol% and 50 mol%, respectively), and the fatty acid composition of PlsEtn is predominantly ARA [123,124].Bovine heart is often used as a PlsCho resource [125,126] and is also abundant in PakCho.Large cervid meat, beef, lamb, tuna, and scallops have been reported to contain high levels of PlsCho [124,127].Some shellfish have high levels of serine-type plasmalogens, similar to those of PlsEtn [128].Plasmalogens have not been found in plants and fungi, whereas those with unusual fatty chains and head groups have been found in anaerobic bacteria [129,130].
muscle and viscera is 9 mol% and 31 mol% of the total PlsEtn, respectively, whereas the ratio of land meat (leg muscle of beef, pig, and chicken) is <1 mol% (Figure 4) [8].Chicken skin and large cervid meat (caribou and moose) have also been reported to have high levels of PlsEtn (1000 nmol and 5000 nmol/g wet weight, respectively; 75 mol% and 50 mol%, respectively), and the fatty acid composition of PlsEtn is predominantly ARA [123,124].Bovine heart is often used as a PlsCho resource [125,126] and is also abundant in PakCho.Large cervid meat, beef, lamb, tuna, and scallops have been reported to contain high levels of PlsCho [124,127].Some shellfish have high levels of serine-type plasmalogens, similar to those of PlsEtn [128].Plasmalogens have not been found in plants and fungi, whereas those with unusual fatty chains and head groups have been found in anaerobic bacteria [129,130].

Alteration during Storage and Cooking
Alterations in the plasmalogen levels during storage and cooking have also been reported.GPLs in oysters are enzymatically hydrolyzed mainly to lyso-forms and free fatty acids during storage, depending on time and temperature, whereas PlsEtn hydrolysis is lower than that of other GPLs [131].Similarly, plasmalogens in fish are lost during storage in a time-and temperature-dependent manner, and ethanolamine-and PUFA-type plasmalogen species can easily undergo degradation and oxidation [132].During the postmortem aging (wet aging) of beef and venison, PlsEtn species are hardly hydrolyzed compared to PtdEtn bearing ARA [133].Overall, GPLs in raw foods during storage and postmortem aging at low temperatures are altered by phospholipase hydrolysis (i.e., PLA2, PLC, and PLD) and oxidation.Plasmalogens hardly undergo enzymatic hydrolysis because of their substrate specificities, whereas PlsEtn with PUFAs is easily oxidized owing to its active methylene and amino groups compared to the species having saturated and

Alteration during Storage and Cooking
Alterations in the plasmalogen levels during storage and cooking have also been reported.GPLs in oysters are enzymatically hydrolyzed mainly to lyso-forms and free fatty acids during storage, depending on time and temperature, whereas PlsEtn hydrolysis is lower than that of other GPLs [131].Similarly, plasmalogens in fish are lost during storage in a time-and temperature-dependent manner, and ethanolamine-and PUFA-type plasmalogen species can easily undergo degradation and oxidation [132].During the postmortem aging (wet aging) of beef and venison, PlsEtn species are hardly hydrolyzed compared to PtdEtn bearing ARA [133].Overall, GPLs in raw foods during storage and postmortem aging at low temperatures are altered by phospholipase hydrolysis (i.e., PLA 2 , PLC, and PLD) and oxidation.Plasmalogens hardly undergo enzymatic hydrolysis because of their substrate specificities, whereas PlsEtn with PUFAs is easily oxidized owing to its active methylene and amino groups compared to the species having saturated and monounsaturated fatty acids and PlsCho.
Among the cooking processes, boiling results in a lower loss of plasmalogens in beef compared with roasting and frying, which reduce plasmalogen levels depending on time and temperature [134].The surface plasmalogen loss during frying was higher than the core loss, and the surface loss was reduced by butter coating.This indicates that, in roasting and frying processes, plasmalogens may be lost easily by dripping and oxidation.Additionally, it is thought that combinations with others affect plasmalogen conditions; for example, cooking and reducing sugar at the same time may modify EtnGpl, including PlsEtn, to Maillard reaction compounds (e.g., amadori-EtnGpl) [135].

Absorption and Metabolism
To utilize the dietary functions of plasmalogens effectively, their digestion, absorption, and metabolism must be understood.Because the vinyl ether bond at the sn-1 position of plasmalogen is acid-hydrolyzed, plasmalogen degradation by gastric acid is thought to occur; however, plasmalogens contained in diets are hardly degraded because of the buffering action of the diets [136,137].In rats, ingested plasmalogens are absorbed into the lymph but not the portal vein [125].PlsEtn is hydrolyzed to lyso-PlsEtn, which does not bear fatty acids at the sn-2 position, and is re-esterized after absorption; interestingly, PlsEtn, even though it hardly bears ARA like oyster-and ascidian-derived products, is preferentially re-esterized with ARA and partly base-converted to PlsCho during absorption, and structural changes occur in the intestinal mucosa [125,138,139].Lyso-phosphatidylcholine acyltransferase 3, which plays a major role in incorporating ARA into phospholipids, is highly expressed in the intestine [140].When PlsCho was administered, PlsCho levels in the lymph were four-fold higher than PlsEtn levels when PlsEtn was administered, and the absorbed PlsCho was slightly base-converted to PlsEtn [125].After marine PlsEtn administration, the peak levels of PlsEtn containing PUFAs (i.e., DHA, EPA, and ARA) were almost the same in blood plasma and decreased early [138,139], suggesting that each level of PlsEtn with PUFAs in plasma is strictly controlled and rapidly transferred to the RBCs and organs [141].By contrast, continuous diets containing bovine-brain-derived phospholipids (100 g of phospholipids/kg diets; 22 mol% plasmalogens in phospholipids) for 7 days increased the plasma and liver levels of plasmalogens but not RBCs [137].

Impact on Neurodegeneration
Some scientific studies have reported that the administration of plasmalogens improves cognitive deficits and lipid composition in rodent models and humans with neurodegenerative disorders, including AD.The administration of scallop-derived plasmalogens tends to improve cognitive function in patients with mild cognitive impairment (MCI), AD, and PD and increase PlsEtn levels in plasma and RBCs (Table 3) [11,[142][143][144].Ascidianderived plasmalogens tended to improve memory function in subjects with MCI and mild forgetfulness [12,145].Chicken-derived plasmalogens reportedly indicated similar tendencies in healthy subjects with mild forgetfulness [146,147].As noted above, the administration of plasmalogens (0.5-100 mg/day) is expected to improve cognitive impairment.By contrast, the supplementation of DHA and EPA as n-3 PUFAs (0.2-2.0 g/day) is reported to attenuate the symptoms of MIC, mild AD, and PD [148].Considering supplementation with other GPLs, 0.6-1.0g/day of PtdCho and 0.3 g/day of phosphatidylserine are reported to improve the cognitive impairment associated with cerebrovascular disease and elderly memory impairment, respectively [149].It is difficult to reference the advantages of marine plasmalogens for humans compared to those of other lipids because of the differences in dose, duration, evaluation, daily diet, and race of the subjects in the reports above.In experiments using rodent models of AD, the oral administration of marine plasmalogens from ascidians, scallops, and sea cucumbers improves the cognitive function and biochemical characteristics of AD (Table 4).In aged mice, the oral administration of marine plasmalogens improves cognitive function and promotes neurogenesis [150].Neuroinflammation induced by Aβ accumulation, lipopolysaccharides (LPSs), and a high-fat diet impairs peroxisome function, including DHAP-AT, and subsequently reduces PlsEtn levels [151,152].A reduction in brain PlsEtn levels leads to suppression of neurogenesis by reducing the expression of the brain-derived neurotrophic factor (BDNF) [68] and the promotion of neuronal death by raising the expression of the p75 neurotrophin receptor (p75NTR) and protein kinase Cδ (PKCδ) [151,153].p75NTR plays a critical role in the production of Aβ, neuronal death, neurite degeneration, and tau hyperphosphorylation; an increase in PKCδ expression is observed in human brains with AD, and the activation of PKCδ enhances toll-like receptor 4 (TLR4)-mediated pro-inflammatory signaling.In contrast, marine PlsEtn administration suppresses brain Aβ accumulation and tau hyperphosphorylation to attenuate neuroinflammation and apoptosis by restoring tropomyosin receptor kinase A/p75NTR signaling and attenuating PKCδ expression [151, 153,154].
The oral and intraperitoneal administration of plasmalogens prepared from chicken breast muscle also attenuates inflammatory stress and enhances memory [68,152,155].Although chicken-derived PlsEtn promotes BDNF expression by activating ERK and AKT signaling to induce neurogenesis in young mice, marine PlsEtn enhances memory function better [68].This may be because marine PlsEtn is richer in DHA and EPA than the chicken derivative.Although EPA-rich PtdEtn also exhibits neuroprotective effects, these effects are weaker than those of marine PlsEtn [154].
The beneficial effects are thought to occur because the administration of plasmalogens maintains brain PlsEtn levels [68,155,156].The levels of PlsEtn bearing DHA in the cerebral cortex correlate with working-memory-related learning ability in AD rats [156].PlsEtn containing DHA exhibits stronger suppressive effects on neuronal inflammation, neuronal apoptosis, and Aβ aggregation in vitro than other PlsEtn species [97,157,158].PlsEtn bearing DHA strongly reduces γ-secretase activity and reduces Aβ production in vitro [159].Small-sized liposomes can deliver content to the brain through the blood-brain barrier [160], and intravenous injection of liposomes containing PlsEtn with DHA has been reported to increase its concentration in the prefrontal cortex and locomotor activity in normal rats [161].
The utilization of plasmalogen precursors and analogs has also been attempted to treat neurodegenerative disorders.These oral administrations improve the decreased content of dopamine-related substances in the brain and intestine and improve behavior in mouse models of PD and RCDP [162][163][164].Their administration markedly increases PlsEtn levels in the serum or plasma but does not significantly increase brain levels.Because a slight but significant increase in brain PlsEtn levels was observed in an experiment using a labeled precursor [49], plasmalogens and/or precursors can pass through the blood-brain barrier, but in very small quantities [165].In contrast to AD and aging, RCDP possesses plasmalogen synthesis defects, and PD does not markedly decrease brain PlsEtn levels [95,162].Therefore, in PD and RCDP, plasmalogens may help patients recover from peripheral impairments and improve central function.
Hence, the ingestion of plasmalogens, particularly marine derivatives rich in DHA and EPA, is expected to attenuate the origin and development of cognitive impairments associated with aging.As described above, plasmalogen ingestion can rescue brain PlsEtn levels observed in AD models [155,156], and the mechanism suggests that the suppression of chronic inflammation in the entire body improves plasmalogen synthesis and degradation in the brain to increase plasmalogen levels rather than transfer from circulating plasmalogens.

Impact on Arteriosclerosis
Plasmalogen precursors and marine PlsEtn suppress the development of arteriosclerosis in rodent models.A plasmalogen precursor (butyl alcohol) suppressed the formation of atherosclerotic plaques in ApoE-or ApoE/glutathione peroxidase-1 (GPx1)-deficient mice fed a high-fat diet [72].Additionally, precursor intake increases the plasma and cardiac levels of plasmalogens and lyso-phospholipids, which lose sn-1 fatty alcohols from plasmalogens by oxidation.In particular, in ApoE/GPx1-deficient mice, which possess lower resistance to oxidation, the precursor suppressed plasma levels of cholesterol and LDL and decreased oxidative stress and the expression of the vascular cellular adhesion molecule-1 in the aorta.
Sea-cucumber-derived PlsEtn intake attenuated the increase in plasma cholesterol and LDL levels to suppress the formation of atherosclerotic plaques in ApoE-or LDLreceptor-deficient mice fed a high-fat diet [167,168].PlsEtn rich in EPA facilitated the synthesis and excretion of bile acids from excess cholesterol, whereas no effects were observed with the intake of EPA-rich PtdCho [167].Likewise, in hamsters fed a high-fat diet, the administration of sea-urchin-derived PlsEtn and PakCho, which predominantly bear DHA and EPA, attenuated atherosclerotic lesions and hepatic steatosis by downregulating adipogenesis genes and upregulating lipid β-oxidation genes and bile acid biosynthesis genes in the liver; PlsEtn further increased hepatic sterol metabolism and serum levels of lipids bearing PUFAs compared to PakCho [169].Additionally, dietary plasmalogens reduced hepatic cholesterol levels in rodents fed normal diets but not plasma cholesterol levels [137,170].A decrease in the hepatic levels of PlsEtn and PlsCho bearing DHA and DHAP-AT expression was observed in a mouse model of nonalcoholic steatohepatitis (NASH), and cholesterol accumulation was induced in the livers of heterozygous DHAP-AT gene-deficient mice [171].
Overall, plasmalogens are responsible for the resistance to oxidation found in the vascular endothelium and hepatic sterol metabolism, and they act as antioxidants and LDL modulators in the vasculature to prevent arteriosclerosis.The structures of both the vinyl ether bond and EPA are important for suppressing arteriosclerosis.

Impact on Colon Impairments
The intestine digests food, absorbs nutrients and water, and is deeply implicated in the maintenance of human health via the immune and nervous systems [172].However, intestinal impairments, such as colon cancer and inflammatory bowel disease (IBD), are becoming increasingly devastating diseases in all sexes worldwide, despite advances in diagnosis and treatment [173,174].The incidence of IBD is increasing in younger generations and is thought to be caused by lifestyle factors, including diet, rather than aging.Aging and IBD increase the risk of developing colon cancer [175,176].GPLs, particularly PtdCho, increase in colon cancer tumors by reducing their turnover [25].Recently, PlsEtn metabolism was reported to be considerably dysregulated in dedifferentiated colon mucosa [177].PlsCho has also been reported to inhibit cancer cell proliferation [178].The authors indicated that plasmalogens might play a key role in protecting the normal colon mucosa from transitioning to hyper-proliferative and adenomatous polyps.
Marine plasmalogens suppress the formation of aberrant crypt foci (ACF) and intestinal inflammation in mouse models.A mouse model of ACF with precancerous colonic lesions was established via i.p. injection of 1,2-dimethylhydrazine (DMH).Diets containing ascidianderived PlsEtn ameliorated DMH-induced ACF formation and oxidative and inflammatory stress in the colon [170].A mouse model of IBD was established using drinking water containing sodium dextran sulfate (DSS).Dietary-ascidian-derived PlsEtn alleviated DSSinduced colon injury and neutrophil infiltration [179].DMH and DSS treatments tended to decrease the PlsEtn species, particularly those bearing ARA, whereas ascidian-derived PlsEtn rescued their levels.In an in vitro intestinal tract model, ascidian-derived PlsEtn inhibited LPS-induced inflammation to protect intestinal cells [180].Additionally, dietary PlsEtn improved cecal levels of short-chain fatty acids, which are metabolites of enteric bacteria and parameters of intestinal health and were decreased by DSS treatment [179].Dietary PlsEtn rich in EPA altered enteric bacteria flora with a high-fat diet [168].
The in vivo and in vitro studies used fish oil and porcine-liver-derived PtdEtn for comparison.Marine PlsEtn performed better on intestinal inflammation caused by aging and other factors [170,179,180].Therefore, the beneficial effects in the intestine are specific to PlsEtn bearing DHA and EPA.

Plasmalogens as Healthcare Materials
Among the materials used for the diagnosis, treatment, and management of human health and disease, plasmalogens have attractive functionalities as "healthcare materials".In this section, the role of plasmalogens in the field of healthcare materials will be explained and examples of their use presented.

Materials for Functional Membrane of Biosensors
In recent years, "liquid biopsy" has gained attention as an important technique in the medical field for the early detection and diagnosis of biomarkers in the blood, including proteins, exosomes, small molecules, and circulating cells [181][182][183].For the detection of biomarkers in liquid biopsy, low-cost and highly sensitive biosensors using semiconductor devices have been used.Such a biosensor adopts a functional membrane system that uses PtdCho.This system simulates the cell membrane and enables the evaluation of biomarkers in the blood and the assessment of compound permeability and adhesion to the cell membrane.Among these, the use of plasmalogens in functional membranes has garnered attention because it allows for the incorporation of detection sites such as antibodies, enzymes, and receptor molecules that cannot be incorporated with conventional PtdCho (Figure 5A) [184,185].
Molecules 2023, 28, 6328 22 of 30 drugs, bioactive substances, and other molecules to exert their effects in the cytoplasm.
There are several ways to make nanoparticles achieve endosome escape, such as by using cationic amphiphiles, pH-sensitive polymers, and cell-penetrating peptides.During endocytosis, cationic amphiphiles attach and fuse with the anionic endosomal membrane, facilitating the release of bioactive compounds encapsulated in the nanoparticles into the cytoplasm (Figure 5C) [191].Fay et al. reported that the transfection efficiency of mouse macrophage cell line (RAW 264.7) with plasmid DNA using polymeric nanoparticles modified with a cationic surfactant (dodecyltrimethylammonium bromide (DDAB)) was 1000 times higher compared to the commonly used lipofectamine [192].Lipid nanoparticles can exhibit endosomal escape functionality by incorporating cationic lipids, such as plasmalogens [191,193].Among the various available approaches, lipid nanoparticles are considered effective carriers for in vivo siRNA delivery [194].Therefore, plasmalogens serve as valuable healthcare materials that provide endosomal escape functionality.

Conclusions
In this review, we discussed the beneficial effects of marine plasmalogens on agingassociated diseases, including cognitive impairment.Aging causes many metabolic impairments and increases oxidative and inflammatory stress, which decreases plasmalogen levels by suppressing biosynthesis and promoting degradation.A decrease in plasmalo-

Materials for Light-Activated Liposomes
Bioactive compounds administered to the body via oral or intravenous administration achieve their activities after reaching the target organs or sites of inflammation.However, depending on the type of bioactive compound administered, its effectiveness may be limited because of instability or the body's recognition of xenobiotic substances, raising concerns about the occurrence of unforeseen side effects [160].One approach to solving such problems is to use a drug delivery system [186].Drug delivery systems encapsulate bioactive components in formulated health materials such as liposomes and nanoparticles.This enhances therapeutic efficacy and reduces side effects.As introduced in Section 2 of this review, plasmalogens are characterized by the presence of an alkenyl (vinyl ether) linkage at the sn-1 position.Plasmalogens with this moiety have been reported to be characterized by their ability to alter membrane permeability in the process associated with the cleavage to lyso-phospholipids by photosensitized stimulation with laser light (Figure 5B) [187,188].This mechanism allows the light-responsive release of bioactive compounds from plasmalogen-based liposomes.To date, the use of azobenzene to impart light sensitivity to liposomes has been limited in its clinical utility due to its high level of toxicity.On the other hand, plasmalogens exist in the body and have chemical structures that are more suitable and less toxic for healthcare use.Since ultraviolet and visible light reach only the superficial 1 cm layer of the skin but deeply into the eye, delivery studies targeting the retina are progressing using plasmalogen-based liposomes [189].

Materials for Nanoparticles with Endosomal Escape Capabilities
Depending on their size, surface structure, and charge, nanoparticles are taken up by cells via endocytosis and are subsequently degraded in the acidic environment of endosomes (pH < 6.5) [160,190].The ability to escape from the endosome is essential for mRNA, drugs, bioactive substances, and other molecules to exert their effects in the cytoplasm.There are several ways to make nanoparticles achieve endosome escape, such as by using cationic amphiphiles, pH-sensitive polymers, and cell-penetrating peptides.During endocytosis, cationic amphiphiles attach and fuse with the anionic endosomal membrane, facilitating the release of bioactive compounds encapsulated in the nanoparticles into the cytoplasm (Figure 5C) [191].Fay et al. reported that the transfection efficiency of mouse macrophage cell line (RAW 264.7) with plasmid DNA using polymeric nanoparticles modified with a cationic surfactant (dodecyltrimethylammonium bromide (DDAB)) was 1000 times higher compared to the commonly used lipofectamine [192].Lipid nanoparticles can exhibit endosomal escape functionality by incorporating cationic lipids, such as plasmalogens [191,193].Among the various available approaches, lipid nanoparticles are considered effective carriers for in vivo siRNA delivery [194].Therefore, plasmalogens serve as valuable healthcare materials that provide endosomal escape functionality.

Conclusions
In this review, we discussed the beneficial effects of marine plasmalogens on agingassociated diseases, including cognitive impairment.Aging causes many metabolic impairments and increases oxidative and inflammatory stress, which decreases plasmalogen levels by suppressing biosynthesis and promoting degradation.A decrease in plasmalogen levels worsens aging-associated diseases via abnormal lipid signaling and reduced resistance to stress.The ingestion of marine plasmalogens, which are abundant in DHA and EPA, contributes to the maintenance or increase in plasmalogen levels in the body and intestines.Alterations in plasmalogen levels are due to exogenesis from ingested plasmalogens or endogenesis caused by reducing stress.Possessing the structures of both the vinyl ether bond and n-3 PUFAs is important for suppressing certain impairments.Plasmalogens possess chemical structures that are attractive for use as healthcare materials.We believe that the chronic administration of marine plasmalogens lowers the risk of age-associated diseases and has a unique capacity to profoundly improve quality of life.

Table 1 .
Composition of plasmalogen in different mammalian organs and tissues.

Table 3 .
Effects of plasmalogen administration on age-associated diseases, including cognitive impairments, in clinical studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.Plasma, RBC, liver, and cerebral cortex levels of PlsEtn species

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.

Table 4 .
Effects of administration of plasmalogens on age-associated cognitive impairments in animal studies.