Arthritis, Alzheimer’s, diabetes, cardiovascular disease, osteoporosis, cancer, etc. are all diseases of cellular metabolism and secretion. What goes on inside cells and on their surfaces explains a lot about health and why we get sick. Cells feed off of what’s around them, use some of those materials to replicate and package up cell-made materials for export. Eat, replicate and secrete. Symptoms of disease result if those processes are compromised.
Cell that make Cartilage, Eat Cartilage
The connective tissue that makes up the cartilage of tendons and the non-mineral parts of bones, as well as a layers of skin, is made up of proteins (collagen) and polysaccharides (glycosaminoglycans, GAGs), e.g. heparan sulfate, hyaluronan and chondroitin sulfate, produced by chondrocytes or fibroblasts. These proteins and polysaccharides are synthesized and then secreted by cells. This process goes on continuously, since the connective tissue is alive and literally crawling with cells that make the cartilage. To keep the connective tissue healthy, the old tissue has to be digested, so that new material can replace it. Thus, the cells that live in cartilage also eat cartilage. These cells get all of their nutrients, e.g. protein and carbs, from eating cartilage. They don’t get glucose and amino acids, or even oxygen (they ferment), from the blood, because there are no blood vessels in cartilage. The photomicrograph at left shows the red chondrocytes surrounded by a light capsule of heparan sulfate as they burrow through the purple cartilage. The next micrograph shows the cytoskeleton of actin filaments (stained with a red fluorescent dye, that lies under the cytoplasm of a chondrocyte. Motor proteins move other proteins, such as syndecans, the proteins to which the heparan sulfate chains are attached, through the cell membrane (see the animations below.) The last micrograph shows the green stained microtubule network on which vesicles move to carry heparan sulfate products from one end of the cell to the other (under the actin and past the orange-dyed nucleus) during synthesis and digestion.
Chondrocytes Burrow Through Cartilage
Chondrocytes are the cells that eat and make cartilage, but all of this eating and making goes on at the same time that the cartilage is also holding everything together, i.e. it is still strong. If cartilage is cut and the cut ends are held tightly together, the chondrocytes will knit the cartilage together and it will become as strong as it was.
Heparan Sulfate Circulates over the Surface of Cells
Chondrocytes are not actually rigidly embedded in the cartilage, but rather maintain a capsule of heparan sulfate around themselves. Thus, they continue to secrete a mixture of heparan sulfate, chondroitin sulfate and collagen, but the heparan sulfate is recycled through the capsule and the other molecules merge into the existing cartilage. Thus, the heparan sulfate is a kind of carrier that keeps the cartilage from “setting up” while it is being made and transported. Other cells of the body, such as neurons, don’t make cartilage, but they still have heparan sulfate (HS) circulation that is intimately involved in many other processes, such as the action of hormones. Disruption of HS circulation causes the symptoms of Alzheimer’s or type 1 diabetes, for example, since amyloids assemble as filaments on threads of HS, and the amyloid filaments jam essential HS circulation. Plaque in atherosclerotic vessels is high in HS content. HS is also a major component surrounding vessels to form the blood brain barrier and the barrier to protein loss from kidneys into urine or loss into the gut lumin. Heparin (fragments of HS) is continually released from mast cells in the lining of the gut to prevent pathogens from binding to cell HSPGs.
HS Sweep the Cell Surface
There is a constant flow of heparan sulfate proteoglycans (HSPGs) through the cell membrane from the rear of the chondrocyte to the front where the HS is digested again and the protein that was embedded in the membrane, syndecan, is recycled to the Golgi for another trip. HSPGs (animation to left with blue protein and yellow HS) are attached to motor proteins that propel them through the membrane along microfilaments of actin that form the cyctoskeleton just under the membrane in the cortical region of the cell. Thus, the heparan sulfate of the HSPGs stick out like hair from the cell surface and sweep continuously from the back to the front of the cell. At the front of the cell, the HS sweeps through the intact cartilage and reverses the process of cartilage assembly. The chondroitin sulfate, collagen and HSPGs are dragged into the cell and digested. The protein parts of the HSPGs are transported to the Golgi and the HS is synthesized along with other cartilage components and moved in vesicles along microtubules before it is secreted.
HS is Secreted at One End and Eaten at the Other
The animation left shows 1) the initial digestion of the cartilage proteins and polysaccharides on the left. These cartilage components of amino acids and sugars, are used by the chondrocytes as their sole nutrients 2), and to produce new proteoglycans 3) HS and chondroitin sulfate proteoglycans, in the Golgi, are 4) packaged into secretory vesicles and are 5) secreted on the right. The HS chains, attached to proteins, are 6) swept through the membrane (see the first animation above) toward the front of the cell, leaving the collagen and chondroitin sulfate for form cartilage behind. In the process, the heparan sulfate proteoglycans 7) disrupt and solublilize old cartilage ahead as the chondrocytes 8) move through the connective tissue like moles digging through soil.
Other Cell Processes Involving Heparan Sulfate:
- Amyloids of Alzheimer’s and type I diabetes assemble bound to HS.
- Hormones bind to receptors wrapped around HS.
- Blood clotting is controlled by HS.
- Complement is controlled by HS.
- Blood brain barrier is composed of HS.
- Kidney protein barrier is composed of HS.
- Inflammation blocks HS synthesis and promotes heparanase synthesis.
- GAGs are animal soluble fiber when eaten and feed gut flora.
- Pathogens bind to HS.
- HIV-TAT is transported between cells by HS circulation.
- Heparin is made by heparanase fragmentation of HSPG in mast cells and is secreted along with histamine.
- NFkB activation inhibits HSPG production and stimulates heparanase production.
- Heparan sulfate proteoglycans organize nerve synapses and acetylcholine esterase binds to HS.
- Gastric proteases cleave around heparin binding domains of proteins, e.g. milk, consist of clusters of basic amino acids. Peptides with heparin binding domain are antimicrobial; all of the heparin binding peptides are subsequently degraded by pancreatic proteases.
- Heparanase is initially secreted inactive and bound to HSPGs, but it remains bound and is internalized again along with the recycling HSPGs, and is activated before being secreted again.
- Allergens and autoantigens are unusual proteins with sequences of three adjacent basic amino acids (arginine or lysine) that require HSPG circulation for presentation of the immune system. Nuclear proteins that interact with nucleic acids have sequences of four basic amino acids, the nuclear translocation signal, and are therefore common antinuclear auto antigens.
