Nuclear Phospholipids in Genome Regulation: Structure, Signaling And Emerging Functional Roles

Varied class of hydrophobic and amphiphilic chemicals, lipids are crucial for cellular structure, signaling, and energy storage. Phospholipids are a large class of complex lipids that have rather wide-ranging biological importance. Amphipathic compounds are those that have a phosphate group in addition to two hydrophobic fatty acid chains and a hydrophilic head group. The structural variation of phospholipids is influenced by factors such as their length, the degree of acyl chain saturation, and the type of polar head groups they contain. Sphingophospholipids  and glycerophospholipids are the two main types of phospholipids. Because phospholipids are amphipathic, they spontaneously organize into micelles in an aqueous environment, forming the lipid bilayer, the fundamental structure of biological membranes[1]. Membranes define the organelles within eukaryotic cells and divide the external and intracellular environments. In addition to these structural tasks, phospholipids are directly and through their metabolites engaged in a number of other biological processes, including gene expression, cellular signaling, and chromatin modeling. The 1960s saw the discovery of phospholipids in the cell nucleus[2, 3]. Although their presence and significance in this compartment were previously believed to be unfounded or anecdotal, nuclear phospholipids play crucial roles in maintaining nuclear architecture and function.

This is mostly due to the fact that nuclear phospholipids are now recognized to be functionally active not only in the nuclear envelope but also within the nucleoplasm. With an emphasis on their functional activities within the nuclear membrane and nucleoplasm in relation to their impacts on the genome, we examine the varied biology of nuclear phospholipids in this review[4,5].

Phospholipids are special and adaptable compounds. Both naturally occurring and essential, they make up the bulk of cellular membranes. A lipid bilayer composed of phospholipids is essential for the formation and function of cellular membranes. Their hydrophilic head group and lipophilic/hydrophobic tail make them amphiphilic[6].

Figure 1: Cell Membrane Structure

 illustrates the molecular blueprint of a phospholipid, as shown by 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and the different possible headgroups that vary with the alcohol type: phosphatidylinositol (PI) with inositol, phosphatidylserine (PS) with serine, phosphatidylglycerol (PG) with glycerolamine (PE) with ethanolamine, and phosphatidylinositol (PI) with inositol.

2. STRUCTURE AND COMPOSITION

Plasma membranes are extremely adaptable structures composed of a phospholipid bilayer that separates the intracellular and extracellular fluid compartments. Even while other proteins buried in the membrane perform a variety of unique functions, cholesterol is primarily responsible for the membrane's fluidity. A phospholipid molecule is composed of two parts: the head and the tail. In contrast to the two neighbouring fatty acid chains that compose the tail, the head consists of a hydrophilic (negatively charged) phosphate group. Here in the bilayer, the polarised head allows for interactions with water on both the internal and external membranes. In contrast, lipid tails are hydrophobic because they are uncharged or non-polar. This membrane's fluidity is also influenced by the mixture of unsaturated and saturated fatty acid tails. As a result, the phospholipids that make up the bilayer are referred to as amphipathic [7,8].

Figure 2 shows the phospholipid bilayer structure, which includes consecutive layers and the orientation of the lipid tails of the molecules facing inward (Figure 2). Consequently, the heads are pointing outward, with one set of cells facing the inside of the cell and the other set facing the outside. This enables the bilayer's interior hydrophobic portion to be kept out of the intracellular and extracellular compartments. Because they "float" in or across the membrane, membrane proteins and other lipids, such as cholesterol, also pass through this region and give the cell membrane additional activities that contribute to cell homeostasis without compromising the fluidity of the bilayer[9].

Figure 2 :Structure Of Phospholipid Molecule

The "head" of a phospholipid molecule is a polar phosphate and is hydrophilic; the "tail" is a non-polar lipid and is hydrophobic. Hydrophobic tails become twisted as a consequence of unsaturated fatty acids.

Figure 3 :  Phospholipid bilayer

Two adjacent phospholipid sheets, organised tail-to-tail, make up the phospholipid bilayer. Interacting with each other, the hydrophobic tails construct the membrane's interior. Both the intracellular and extracellular fluids are touched by the polar heads.

Figure 4 : Cell membrane

Phospholipid bilayers make up the cell membrane, which contains a wide variety of molecular components such as proteins, cholesterol, and various carbohydrate groups.

The physicochemical properties of the cell membrane cause it to be selectively permeable. The size and solubility of the substance determine its capacity to pass across the membrane. Oxygen, carbon dioxide, and lipid-soluble compounds can all flow through with ease. Transmembrane proteins or channels are necessary for the free passage of water-soluble glucose and other charged molecules, such ions, into and out of the cell [7].

3.Membrane proteins

Integral and peripheral proteins make up the bulk of the proteins found in the plasma membrane's lipid bilayer. Integral proteins are anchored in the membrane (Figure 3).

Examples of integral proteins are:

• Channel or transport proteins: they selectively permit substances, like ions, to pass across the plasma membrane.

• Receptors: often referred to as cell recognition proteins, which enable the selective attachment of molecules external to the cell.  The cell's second messenger systems are typically activated when a molecule binds. For other cells and proteins to detect cells, these receptors function similarly to cell labels.

• Glycoproteins: The extracellular matrix is accessible through the carbohydrate groups that are connected to these proteins.  The carbs have 'tags' on them, which helps in cell-to-cell recognition as well. Additionally, they are an integral component of the glycocalyx, a crucial mechanism that incorporates lipids from outside the cell membrane [7].

The glycocalyx has many roles that include:

• Receptors for hormone binding

• Cell-to-cell binding

• Enzymatic breakdown of nutrient.

4. Membrane Specialization:

Parts of the plasma membrane give the membranes of specialized cells particular functions:

The microvilli, which are extensions of the cell membrane that resemble fingers, enhance the surface area of the cell, allowing for maximum absorption. They are present in small intestinal cells.

A unique class of proteins called junctions (Figure 4) firmly packs and joins cells. There are three primary kinds:

• Tight junctions: prevent anything from moving between cells by holding neighboring cells firmly together.

• Desmosomes, also known as anchoring junctions, are fibers that hold cells together while permitting some mobility without separation.

• Gap junction: These proteins allow specific chemicals to move between cells across gap junctions, which are open spaces in the plasma membrane between two cells [1].

5. Ion Permeability:

Because of selective ion transport and the resulting difference in concentrations (electrical charge) on either side of the cell membrane, a potential difference can be established across the membrane because of the semipermeable nature of the membrane. Since a charge transfer is theoretically possible, the bilayer can double as a capacitor. The resting membrane potential of cells is typically 70–80 mV lower than that of extracellular fluid [8, 10].

The cell is permeable to potassium, so it can leave the cell at will along a concentration gradient. Thus, the resting membrane potential drops, balancing the electrical and chemical gradients and preventing additional potassium loss from the cell.

Other cations cannot pass through the cell, although Na+ does gradually seep down its concentration gradient [7].

GAP JUNCTION

ANCHORING JUNCTION

Figure  5 : Three major types of junctions

• There are three sodium ions released by the cell's pump, and
• Two potassium ions are injected into the cell.

6. Membrane potential

As indicated earlier, the resting membrane potential is created by the negative charge that permeable potassium produces as it moves along its own diffusion gradient, which counteracts the diffusion of potassium. For these two forces to be considered in equilibrium, they must be equal [7,10].

If the ion in issue is permeable and present across the membrane in electrochemical equilibrium at changing quantities, the potential difference that the ion produces can be calculated using the Nernst equation.

If the potential difference of one permeable ion across a membrane is described by the Nernst equation, then the true membrane potential must be determined by considering allions with different permeabilities across the same membrane. The Goldman-Hodgkin-Katz equation does this. If the potential difference of a single permeable ion across a membrane can be explained by the Nernst equation, then the real membrane potential must be found by taking into consideration allions with varying permeabilities across those same membranes. To do this, the Goldman-Hodgkin-Katz equation is employed [9,11].

It is crucial to remember that this equation, in contrast to the Nernst equation, ignores valency and that the concentration of negative ions is the opposite of that of positive ions [1]

7. Action potential:

Both chemical (mediated by cholinergic receptors, which open Na+ channels) and physical (mediated by pressure receptors, which deform Na+ channels) mechanisms allow excitable cells to change their membrane potential in reaction to stimuli. Therefore, the membrane potential is raised (becoming more negative) when the excitable cell is activated because Na+ permeability is increased. When the threshold potential is satisfied, an action potential is generated. It is at this voltage point, typically -55 mV that fast Na+ channels, also called voltage-gated Na+ channels, open. An all-or-nothing effect results from this. Sodium and potassium are essential components of cell membranes that generate an action potential [7,9,12 ] .

There are three states of the Na+ channels that cause depolarization:

1. Closed and Na+ impermeable.

2. When threshold probable is reached, it becomes open and permeable to Na+.

3. Immediately upon entering the open state, it becomes inactivated and impermeable to Na+. This state lasts until the potential falls below -50 mV.

The slow opening of voltage-gated potassium channels during depolarisation reduces the membrane potential and increases potassium permeability; these channels are crucial for repolarisation.

By diffusing across the cell membrane and raising the membrane potential above the resting potential in neighbouring cells, an increase in sodium in one region causes the action potential to spread. Afterward, the cell experiences a depolarisation and repolarisation wave due to the opening of fast Na+ channels. The maintenance of cell membrane potentials and the importance of the sodium-potassium ATPase pump within cell membranes are highlighted by this process [7 ]

8.Transport cell membrane

The cellular membrane allows for the regulation of intracellular material concentrations. The cell membrane maintains a stable concentration of ions such as Na+, K+, Ca+2, and Cl-as well as waste products such as carbon dioxide, carbs, fatty acids, amino acids, and ions. As a result of their hydrophobic interior and dense packing, phospholipids give the first degree of control within the phospholipid bilayer. Because of this structure, the selective permeability property is described. Oxygen, carbon monoxide, lipids, and alcohol are examples of small, non-polar molecules that meet the criteria for selective permeability and can therefore flow through the membrane unimpeded. However, because the hydrophobic phospholipid tails reject water-soluble molecules, they require aid to cross the membrane[7 ,10 11,13 ].

There are two main ways in which molecules might penetrate a membrane: passively and actively.

By utilising concentration gradients and diffusion, molecules can be transported to areas with lower concentrations in this manner.

A variety of processes contribute to diffusion across cell membranes, including:

• simple diffusion
• Osmosis
• Facilitated diffusion(uniporters)

Diffusion rate and intensity are affected by gradients caused by hydrostatic pressure, concentration of solutes, and electrical charg.

• Simple diffusion:

The permeability of the membrane determines whether a chemical with a concentration gradient can travel across it via simple diffusion. Some substances are very fluid, such as gases such as oxygen and carbon dioxide. Since cell uses oxygen quickly, its internal concentrations are often substantially lower, resulting in a concentration gradient that permits continuous diffusion into the cell. In contrast, high intracellular amounts of carbon dioxide migrate from the cytoplasm to the interstitial fluid compartment, where the concentration is lower (Figure 6) [7].

Figure 6 : Simple diffusion across the cell (plasma) membrane

Lipid bilayer architecture restricts diffusion across cell membranes to tiny, non-polar molecules like carbon dioxide and oxygen, which can move down concentration gradients.

• Osmosis :

As shown in Figure 6, osmosis allows water to diffuse across a semipermeable barrier. Water can diffuse across all cell membranes by either penetrating protein channels or sliding between the lipid tails of the membrane. When the concentration of solutes across the cell membrane is unequal, osmosis takes place. The concentration gradient across membranes, etc., determines how much water moves through a solution. The osmotic pressure that the solutes in the solution generate is essentially what pulls water, as illustrated in the figure below. The effect is concentration-dependent; an increase in osmotic pressure results from a more concentrated solution. Isotonicity ensures the structural and functional integrity of the cell and maintains homeostasis in the cellular environment, both of which are dependent on this effect. Maintaining homeostasis is an important function of the kidneys [7].

Figure 7 : Osmosis

Water diffuses through a semipermeable membrane following a concentration gradient; this process is called osmosis. When a membrane allows water to pass through but not a solute, the water will diffuse to the side with a lower concentration, bringing its own concentration level to the same as the solute's. The hypertonic solution can be seen on the right side of the membrane in the left-hand beaker.

Figure 8 : Facilitated diffusion

(a) Proteins called channel proteins and carrier proteins help chemicals diffuse across the cell membrane, also known as the plasma membrane. Typically, channel proteins only slightly differentiate between their cargo according to size and charge, making them less selective than carrier proteins. (b) Carrier proteins are notoriously picky, letting through far fewer molecules than other types.

• Facilitated diffusion:

Help is needed for substances whose physicochemical characteristics (size, shape, polarity, etc.) prevent them from passing through the cell membrane. One common example of a chemical that requires enhanced diffusion to enter the cell and produce ATP is glucose. Enhanced inward diffusion of glucose requires a specialised carrier protein called the glucose transporter (Figure 7). Glucose is too large and polar to pass through the cell membrane alone, even when there is a concentration gradient. Amino acids and some metabolic byproducts can't be transported without these carrier proteins. Crucially, assisted diffusion does not require ATP because it is a passive process[7,10, 11,13 ].

9. Active diffusion :

To overcome a concentration gradient, molecules must undergo active transport, which uses ATP for energy. The following are examples of active transport mechanisms:

• Material movement is the main form of active travel.
• Connections between substances and the active movement of other ions against concentration gradients allow them to migrate along electrochemical gradients, a process known as secondary active transport. Common examples include ions of hydrogen and sodium.
• Sodium and amino acids are two examples of chemicals that can be transported across a membrane by carrier proteins called co-transporters or symporters.
• A process is considered passive if the energy required to lower one substance's concentration gradient is greater than the energy required to raise the other substance's concentration gradient.
• Counter-transporters, also known as antiporters, use carrier proteins to carry two chemicals in opposite direction.

Figure 9 : Sodium-potassium pump

Many cell membranes contain the sodium-potassium pump. The sodium-potassium ion pump works by transferring ions in the opposite direction of their concentration gradients, using ATP as its power source. A single pump cycle causes the cell to expel three sodium ions and import two potassium ions.

The most common example of cellular active transport is the sodium-potassium pump, sometimes called Na+/K+ ATPase (Figure 9). This pump, found in the membranes of many different kinds of cells, is responsible for the intracellular transport of two potassium ions and the extracellular release of three sodium ions. This pump is extensively dispersed in nerve cells to keep an electrical gradient across the membrane of the cell. This electrical charge difference accounts for most of the ATP consumption in tissues; it generates an intracellular charge that is relatively negative (-70 mV), which is essential for the proper functioning of nerve cells. This pump helps maintain cellular homeostasis among its many other functions [7,10,13,14].

The pump has numerous different roles, one of which is maintaining cellular homeostasis.

• The net loss of osmoles keeps the cell volume constant.
• It has been shown in the previous situation that the potential difference across the membrane is preserved.
• Making chemical gradients that can be used in S-GLUT and other secondary active transport systems.

10. Endocytosis and exocytosis :

In order to move materials inside cells, these activities entail the creation of a vesicle.

As shown in Figure 9, endocytosis occurs when a material is absorbed into a cell by means of the cell membrane invaginating around it, which is then pinched off such that the membrane portion and its contents form a distinct intracellular vesicle. Two subtypes of endocytosis are phagocytosis and pinocytosis.

Figure 10 : Three forms of endocytosis

Phagocytosis is the process by which larger particles are taken up into the cell. By generating a vacuole inside the leukocyte, immune cells absorb germs and subsequently consume them. When substances that contain fluid are endocytosed but do not generate a vacuole, this process is called pinocytosis. Such two types of endocytosis do not necessarily "ingest" very selective large extracellular particles. However, receptor-mediated endocytosis is a mechanism that certain cells employ to regulate this process. After binding with specific substances, receptor-ligand complexes are able to regulate the substance's entrance into the cell through endocytosis. This is the standard mechanism for regulating iron entry into RBCs [7,10].

Exocytosis is an active calcium-dependent process that occurs when vesicles containing contents merge with the plasma membrane (Figure 10). The pancreatic and stomach cells, which secrete digestive enzymes and hormones, provide an example of this.

Figure 11: Exocytosis