The thyroid gland in the human body is responsible for making thyroid hormones. There are two thyroid hormones: T3 (L-triiodothyronine) and T4 (L-Thyroxine). Concentrations of T3 and T4 within blood plasma regulate the energy-yielding metabolism within our bodies (Lehninger, 752). Basically, T3 and T4 bind to specific membrane receptor molecules that stimulate transcription enzymes. Without proper regulation of these essential hormones, our body's growth and metabolism could be severely altered.
The hypothalamus gland controls the complex system of hormone secretion from the thyroid. The hypothalamus secretes a specific releasing factor called TRH (Thyrotropin-Releasing Hormone). This TRH signals the anterior pituitary gland to produce TSH (Thyrotropin Stimulating Hormone). Figure 1 shows normal interaction within the thyroid cycle.
The goal of TSH, as it is secreted from the anterior pituitary gland, is to reach the thyroid gland in order to signal it. When the thyroid receives the signal from the anterior pituitary, it starts taking in I- from the circulating blood in order to bind it to a protein (biological pathway is shown in fig. 1). Once this is accomplished, the thyroid gland releases the protein-iodine complex into the blood stream. This protein bound I- is what we earlier referred to as T3 and T4. The Na+/I- symporter sits within the cell membrane of the thyroid cells. It acts as a gate way for I- into the thyroid cells. It directly controls the flow of I- into in and out. In a sense, the symporter determines whether or not there will be hormone production, and to what extent the production will go.
The four hormones TRH, TSH, T3, and T4 work in an inverse way. Normal thyroid secretion depends on TSH. High concentrations of TSH stimulate the thyroid, and as a result, T3 and T4 levels increase. If T3 and T4 concentrations increase then TRH and TSH concentrations decrease. The balanced negative feed back regulation of these four hormones keeps our bodies in a normal metabolic state.
There is also a direct relationship between TRH and TSH. If the hypothalamus produces an excess of TRH, then the anterior pituitary produces more TSH. TSH cannot inhibit the hypothalamus. However, high concentrations of T3 and T4 do.
Knowledge of hormone interaction is necessary in order to understand defects and diseases within the thyroid system. Here are some examples of problematic situations for people diagnosed with certain thyroid diseases. First, if the thyroid gland is defective or is infected with a disease, it will most likely produce far below the average levels of T3 and T4 at any given TSH level. Hypothyroidism is the usual diagnosis for this condition. Doctors generally prescribe oral supplements of T3 and T4 to compensate for the inadequate thyroid production. The most difficult decision is determining how much supplemental T3 and T4 are needed. Doctors begin by giving the patient a low dosage of hormone and then periodically checking to see if the levels are acceptable. If the hormone levels are not acceptable, the dosage is increased until the right concentration is reached. The procedure is fairly simple, but cumbersome. The hormone concentrations can be easily determined by an analysis of the patient's blood sample. However, it may take months to determine the correct dosage as every person's biological activity and reactivity differs, to the hormones. One patient will need very little supplemental hormone, whereas another patient may need substantially more in order to have metabolism that is close to normal. For this reason doctors may have trouble determining the correct dosage within a short period of time.
If a patient has a hypothalamic or pituitary disease, the TRH and TSH levels will be low. This is only because the source of the hormones is defective. However, in some situations the levels could be high due to the secretion of biologically inactive hormones (Surks, p. 1688). In this situation the hypothalamus and the pituitary glands are working normally, but the problem lies within the hormones ability to signal the thyroid. The TSH is defective and cannot signal the thyroid. If TSH cannot stimulate the thyroid, then it cannot induce T3 and T4 production. Further, as already seen, the pituitary responds to this by increasing the TSH secretion thinking there is not enough, when in actuality TSH is inactive. Since, blood plasma concentrations of TSH are generally low, an accurate diagnosis of this patient would be hyperthyroidism. In both hypothyroidism and hyperthyroidism the problems primarily stem from the glands. Therefore, if any of the hormones in the cycle fail to be effective, disruption can occur.
Iodine is an important part of the thyroid system. As discussed earlier, I- modifies a protein already found in the thyroid cells (Figure 2). Figure 2
This modified protein is known in two forms, T3 and T4. Note that the formation pathway for thyroxine starts with a single amino acid, tyrosine. Tyrosine takes on two I- molecules to form diiodotyrosine (Step 1). This is done with the assistance of the enzyme peroxidase. Peroxidase oxidizes I- to one of two forms, either I0 or I+. In vitro reactions show that I- will react spontaneously with tyrosine to form the hormones. However, there is strong enough evidence to show that this reaction is facilitated with peroxidase. After sufficient formation of diiodotyrosine, two molecules spontaneously condense to form thyroxine and alanine. Similarly, a biochemical pathway is used for the formation of triiodothyronine. Tyrosine once again starts the pathway and reacts with one molecule of iodine to form monoiodotyrosine. As with the synthesis mentioned earlier, peroxidase catalyzes the reaction. Then diiodotyrosine, made in one of the previous reactions, spontaneously condenses with monoiodotyrosine to form triiodothyronine. These reactions occur in the thyroid gland and can occur only after I- is allowed into the cell. It is the responsibility of the NIS to get iodine into the cell. This is the first step in making T3 and T4. As already mentioned, the Na+/I- symporter plays an active role in this first step. This "protein gate" controls the flow of I- into the thyroid cells. Since the NIS is actually a part of the cell membrane, it can open or close the door to allow Na+ + I- to flow through the membrane. High intracellular concentrations of Na+ depress the amount of I- taken into the cell. Negatively charged molecules depolarize Na+ to further increase the flow (Martin, 747). Once I- is in the cell, then it can be used to synthesize T3 and T4. At anytime, the thyroid cells can secrete these hormones if stimulated by TSH. Once secreted into the blood stream, the enzyme known as deiodinase promotes a quick conversion of T4 to T3 (Martin, 751). T3 is the more active hormone, and hence the reason for rapid conversion.
The Na+/I- symporter plays a key role in the thyroid cycle. It possesses the ability to be a highly regulating factor in the process. This protein can completely shut off the cycle or it can highly accelerate it, depending on how much I- it lets in. Having this much influence over the thyroid cycle is what makes this symporter so important. If doctors can control this mechanism, they might be better able to continue dealing with thyroid problems using a much better method.
The gene responsible for the production of the Na+/I- symporter has been located, isolated and cloned. cRNAs made in vitro from an FRTL-5 cell cDNA library were microinjected into oocytes and assayed for NIS activity. Positive pools containing successively fewer cDNA clones were assayed until a single positive clone was identified (Dai, 458). Once the clone was identified, a high concentration of it was injected into the oocyte of a rat. Graph A below shows activity of I- uptake into the oocyte over a period of time. I- uptake increases with time. This indicates that the cDNA produces proteins that fit the match of the Na+/I- symporter. Since the data matches, it can be assumed that the segment of DNA cloned is the gene responsible for synthesis of the NIS.
The saturation is shown in graph B, where uptake increases with concentration. A graph of 1/V vs. 1/[S] yields a slope of one (Graph C).
The Km for then NIS of Iodine in rats is 36 micromoles. This value is consistent with the Km for the protein from the cDNA. Thus, the cDNA's activity seems to be able to match sensitive NIS biological activity in the oocyte (Dai, 459). This similar value links the performance of the clone to the original, indicating a successful isolation of the gene in question.
The DNA nucleotide sequence consists of a total of 2,839 base pairs with the initiation codon being ATG. The DNA begins coding with a methionine and continues to code for a main body of 618 amino acids (relative molecular mass 65,196). The secondary structure includes a large section made up of 70 amino acids that form a highly hydrophilic region and contains 12 transmembrane domains. Finally, the protein contains only 3 charged amino acids (Asp 16, Glu 79, Arg 208) inside the membrane. The rest of the charged amino acids are resting on the inner or outer parts of the cell. These characteristics make up the heart of the protein itself. The three charged amino acids within the membrane are thought to be central in the symporter activity. Further, four leucine structures, at positions 199, 206, 213, and 220 help the symporter maintain its tertiary structure. This forms what is known as the "Leucine-Zipper Motif" (Dai, 459). These characteristics and functions were able to show that the rat symporter has a 24.6% amino acid homology with that of the human symporter. This is one of the highest similarities discovered.
The Na+/I- symporter is crucial in the thyroid system. The main structure of the NIS lies within the 618 amino acids containing the 3 charged residues. These three residues are crucial because they are within the membrane of the thyroid cells. The NIS controls the thyroid function directly, and its great influence on the system makes it a prime target for research. This could not have been discovered without the work of many individuals seeking makeup of the NIS itself. This valuable information will most likely lead into new and much more responsive thyroid techniques. The end result will allow the medical community to evaluate, diagnose and treat thyroid disorders more effectively.
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