Plasma messengers are molecules circulating in the bloodstream. They function as a complex communication network, carrying information between cells and organs throughout the body. These messengers facilitate communication without directly entering the cell, instead using a cascade of events to transform the signal into a cellular change. This system plays a role in maintaining the body’s balance and coordinating various biological processes.
The Many Types of Plasma Messengers
Plasma, the liquid component of blood, transports various substances, including chemical messengers. These messengers come in diverse forms, each with unique characteristics. One category includes cell-free DNA (cfDNA), which are fragments of DNA released from cells into the bloodstream. Different types of RNA, such as messenger RNA (mRNA), microRNA (miRNA), and long non-coding RNA (lncRNA), also circulate as plasma messengers, carrying genetic information or regulating gene expression.
Proteins, including hormones and enzymes, represent another broad class of plasma messengers, directly influencing cellular functions. Extracellular vesicles (EVs), like exosomes and microvesicles, are tiny sacs released by cells. They contain proteins, lipids, and nucleic acids, transporting complex biological messages. Hydrophobic molecules like diacylglycerol and phosphatidylinositols are membrane-associated and signal within cell membranes, while hydrophilic molecules such as cyclic AMP (cAMP) and calcium ions (Ca2+) are water-soluble and signal within the cytosol. Gases like nitric oxide (NO) and carbon monoxide (CO) can diffuse through both the cytosol and cellular membranes to transmit signals.
The Role of Plasma Messengers in Communication
Plasma messengers act as biological couriers, transmitting signals from one cell or tissue to distant targets. These signals are relayed through signal transduction, converting extracellular signals into intracellular signals. This allows cells to respond to changes in their environment or to signals from other parts of the body. For instance, hormones can bind to receptors on the cell surface, initiating a cascade of events inside the cell through the generation of second messengers like cAMP or calcium ions.
These messengers regulate various bodily processes, including immune responses, metabolism, and maintaining cellular balance, known as homeostasis. When a ligand binds to a single receptor on the cell surface, it can lead to significant changes in biochemical activities within the cell due to the amplification of the signal by second messengers. This communication system ensures cells and organs work in concert to maintain physiological stability and respond to stimuli.
Plasma Messengers: Clues to Health and Disease
Changes in the quantity, type, or cargo of plasma messengers can indicate health status or the presence of disease. This makes them valuable biomarkers for early disease detection, monitoring disease progression, and assessing treatment effectiveness. For example, specific plasma biomarkers are associated with the activity and severity of fibrosis in nonalcoholic fatty liver disease (NAFLD).
In neurodegenerative diseases like Alzheimer’s disease (AD), small extracellular vesicles (sEVs) carry pathological proteins, RNAs, and lipids, allowing for diagnostic applications. For instance, profiling specific proteins like amyloid-beta and tau, and microRNAs within sEVs can help identify prodromal AD. Plasma phosphorylated tau (p-tau) levels also show high accuracy in distinguishing individuals with amyloid pathology, indicating their potential for non-invasive diagnosis and monitoring of conditions like AD.