Phosphatidylserine (PS) is the phospholipid that is the primary regulator of all membrane protein function in the neuronal membrane — it is the most abundant acidic phospholipid in the neuronal membrane, it constitutes approximately 15-20% of the phospholipids of the synaptic membrane, and it is the cofactor for the function of the proteins that are most critical for neuronal communication, including the neurotransmitter receptors, the ion channels, the signal transduction enzymes, and the synaptic vesicle fusion proteins. PS is synthesised in the neuronal membrane from phosphatidylcholine and from serine by the phosphatidylserine synthase enzyme, and it is maintained at high concentrations in the inner leaflet of the neuronal membrane by the ATP-dependent phosphatidylserine flippase enzyme. The unique PS-dependent proteins of the neuronal membrane — including the acetylcholine receptors, the NMDA receptors, the sodium channels, the calcium channels, and the protein kinase C (PKC) enzyme — require PS as a cofactor for their function, and without adequate PS, the activity of these proteins is impaired, neuronal communication is disrupted, and cognitive function declines. This PS-dependent vulnerability of neuronal membrane protein function is the foundation of the age-related decline in cognitive function that is associated with the decline in neuronal PS levels that occurs with normal ageing.
PS and Neurotransmitter Receptor Function
The neurotransmitter receptors — including the acetylcholine receptors, the NMDA glutamate receptors, the AMPA glutamate receptors, the GABA-A receptors, and the dopamine receptors — are the membrane proteins that mediate neuronal communication at the synapse, and they are all dependent on PS for their function. The acetylcholine receptors (both the nicotinic and the muscarinic subtypes) require PS as a cofactor for their ligand-binding and for their ion channel function — the binding of acetylcholine to the nicotinic receptor induces a conformational change that opens the ion channel, and this conformational change requires the PS molecules that are adjacent to the receptor in the membrane. The NMDA receptors are particularly sensitive to PS levels — the NMDA receptor is the glutamate receptor that is most critical for memory formation and for synaptic plasticity, and its function is directly dependent on the PS content of the synaptic membrane. When PS levels decline with age, the function of the NMDA receptor is impaired, and the NMDA receptor-dependent mechanisms of memory formation (including long-term potentiation, or LTP) are disrupted — this is the primary mechanism of the age-related decline in memory and in the capacity for synaptic plasticity.
The clinical importance of PS for memory function is demonstrated by the consistent benefits of PS supplementation in older adults with age-related cognitive decline. A meta-analysis of 10 RCTs in 1,514 older adults with age-related cognitive decline found that PS supplementation at 300mg daily significantly improved memory, attention, and the speed of information processing compared to placebo. The benefits were most pronounced in people with the most severe cognitive impairment (MMSE scores below 24), and they were sustained for the duration of the supplementation period (up to 12 months in the longest trials). The mechanism of this cognitive benefit is thought to involve the restoration of NMDA receptor function (which is dependent on PS), the enhancement of synaptic vesicle fusion (which requires PS for the function of the synaptophysin and synaptobrevin proteins), and the stimulation of the signal transduction pathways that are required for memory formation (including the PKC pathway and the MAPK pathway).
PS and the Cortisol-Stress Response
PS has also been shown to modulate the cortisol response to acute stress — it reduces the cortisol spike that is produced by the hypothalamic-pituitary-adrenal (HPA) axis in response to a physical or psychological stressor. The mechanism involves the PS-dependent modulation of the corticotropin-releasing hormone (CRH) neurons of the hypothalamus and of the ACTH-secreting cells of the anterior pituitary gland, both of which are regulated by the glucocorticoid receptor (GR) in a PS-dependent manner. The reduction of the cortisol response to stress is important for cognitive function because cortisol (at chronically elevated levels) is neurotoxic — it impairs memory formation, reduces the volume of the hippocampus, disrupts the blood-brain barrier, and promotes the accumulation of the amyloid-beta plaques that are the pathological hallmark of Alzheimer disease. The PS-induced reduction of the cortisol response to stress is therefore one of the most important mechanisms by which PS supports cognitive function in older adults.
Practical Application
For general PS supplementation, the evidence-based dose is 100-300mg of phosphatidylserine daily (as PS from bovine cortex, soy lecithin, or sunflower lecithin — the sunflower and soy sources are preferred for people who want to avoid animal products or who are concerned about the bovine spongiform encephalopathy risk). For age-related cognitive decline and for the management of the cortisol response to stress, the evidence-based dose is 200-300mg daily, taken in the morning (for the modulation of the morning cortisol peak) or 30-60 minutes before a stressful event. PS is generally well-tolerated with no significant adverse effects at doses up to 300mg daily, though very high doses may produce gastrointestinal symptoms. For comprehensive cognitive support, PS pairs well with acetyl-L-carnitine (which supports neuronal mitochondrial function and which has complementary cognitive benefits), with the omega-3 fatty acids (which provide the EPA and DHA that are incorporated into the neuronal membrane phospholipids and which support the synaptic membrane function), with Ginkgo biloba extract (which supports cerebral blood flow and which has complementary cognitive benefits), and with the Bacopa monnieri extract (which supports memory formation through a different mechanism).
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