Familial Neurogenic Diabetes Insipidus: a disease caused by a traffic jam?

By
Marga Nijenhuis, Robbert Zalm and J. Peter H. Burbach,
The University of Utrecht, The Netherlands.

The riddle of familial neurogenic diabetes insipidus.

Familial neurogenic diabetes insipidus is the inheritable form of central diabetes insipidus. It is caused by a defect in vasopressin synthesis and its secretion. The disease is very rare. Until now only 45 families with familial neurogenic DI have been identified. It has been demonstrated that the underlying cause of the disease is a mutation in the vasopressin gene, resulting in a defective vasopressin precursor protein. This vasopressin precursor protein is the protein that contains and normally gives rise to vasopressin. The disease is inherited in a dominant manner, meaning that there is a 50% chance that children of an affected parent will also have the mutation and develop the disease. In addition, this dominant inheritance indicates that the presence of one mutated vasopressin gene suffices to cause the disease. This is very surprising, because people with one mutated vasopressin gene also have an intact gene (originating from the other parent). So, one would expect that vasopressin can normally be synthesized from the normal gene. In this case, vasopressin levels in the affected persons would not be expected to be less than half the normal level. In a rat strain with familial neurogenic DI, this is indeed the case: rats containing only one mutated vasopressin gene are not diabetic. The rats have only diabetes insipidus, when both genes are mutated. However, in humans with one mutated vasopressin gene, vasopressin levels decline gradually after birth and after a certain period vasopressin synthesis stops and the child becomes diabetic. Apparently, after a while the mutant vasopressin gene also disturbs the synthesis of vasopressin from the normal gene. In our department at the Utrecht University, we investigated what the mutant vasopressin precursor is doing to the cell that expresses it, and why it affects vasopressin synthesis. Normally the vasopressin precursor is expressed in a small group of nerve cells at the bottom of the brain. We used cultured cells for our study.

Familial Neurogenic DI mutant vasopressin precursor are not transported.

After the normal vasopressin precursor has been made, it is rapidly transported to special granules which are stored at the end of nerve terminals and which rapidly release vasopressin when the kidney needs to retain water. On its way to this granules, the vasopressin precursor follows a fixed route, in which it passes several compartments within the cell. These compartments can be compared to train stations, which you pass going from one place to the other. However, the first compartment (cq. the train station where you depart) has a special function. This compartment checks whether the proteins that want to leave, are in proper shape. If the proteins are not ‘in shape’, the transport of these proteins is blocked. Or in terms of a train trip: if you don’t buy a ticket at the departure station, you will not be allowed to travel to the next station. In our experiments we could see that the normal vasopressin precursor rapidly left the first compartment. However, the transport of the familial neurogenic DI causing vasopressin precursors was largely blocked. This indicates, that although the changes in the familial neurogenic DI causing vasopressin precursors are small (in most cases only 0.7% of the total precursor is changed), cells can recognize them as being ‘wrong’ proteins.

The transport block of the familial neurogenic DI causing vasopressin precursors does not really explain its apparent effect on the normal vasopressin precursor in humans. Most mutated proteins are rapidly degraded upon block of their transport from the first compartment (cq. chased out of the train station in the case of people who do not want to buy a ticket). This avoids that the ‘wrong’ proteins disturb cells in performing their normal function. Indeed, the cells we cultured in our flasks, did not seem to be disturbed by production of the familial neurogenic DI causing vasopressin precursors. They kept on growing happily and seemed healthy. However, normally the vasopressin precursor is produced by large nerve cells which are located in a brain region that is called the hypothalamus. They have very large extensions, which are located in the pituitary gland. This is the place where they release vasopressin into the blood if water reabsorption in the kidney is needed. One way in which these large nerve cells differ from our cells in flasks, is that they produce enormous quantities of the vasopressin precursor. We investigated what would happen if we produced much higher levels of the familial neurogenic DI vasopressin precursor in our cultured cells. What we saw is that the first compartment got totally filled with the familial neurogenic DI vasopressin precursor, even so that it totally changed shape. Whereas it is normally a fine tubular network, it now was very much extended and had very much broadened tubes. So, it seemed as if the departure train station got so overfilled with people without a ticket, that they started to jam the station and prevented the people who wanted to buy a ticket from coming in and doing so. In this way, the people without a ticket would block transport for everyone. Our hypothesis is, that this is the case also for the familial neurogenic DI vasopressin precursor. Apparently, when very much of the familial neurogenic DI vasopressin precursor is produced by the cell (as is the case in the large nerve cells that produces this precursor in the human body), the degradation machinery can not keep up with degrading this enormous amount of ‘wrong’ protein and the transport route in the cell gets totally jammed. There are indications that ultimately the vasopressin-synthesizing nerve cells disappear: due to the continuing problems the railway company decides to close the station.

Possible explanation for the dominance and delayed onset of familial neurogenic DI.

Jamming of the transport route in the cell by large amounts of familial neurogenic DI vasopressin precursor would explain both the dominant inheritance and the delayed onset of this disease. Jamming of the transport route within the large vasopressin synthesizing nerve cells, will also abolish transport of the normal vasopressin precursor present in these cells, so that one mutated vasopressin gene will ultimately abolish synthesis of all vasopressin. In addition, it will take some time before enough of the ‘wrong’ precursor is synthesized to jam the whole transport route, explaining the synthesis of vasopressin early in life. It has been demonstrated that the onset of familial neurogenic DI can vary between different members of one family or between different families with the same mutation. This could for instance be due to a different activity of degrading enzymes in these members or to a different level of synthesis of the vasopressin precursor. Because synthesis of vasopressin and its precursor increases when the body needs to retain water, this synthesis will be dependent on for instance drinking habits, environmental temperature and salt intake.

In addition, the mechanism of disease development we propose, explains the difference observed between familial neurogenic DI in the rat and in humans. The diabetic rats have a mutation, that strongly alters the produced vasopressin precursor. It is even altered so much, that the cells can not produce large amounts of this rat familial neurogenic DI vasopressin precursor and in addition rapidly degrade this ‘wrong’ precursor. Consequently, there is never enough familial neurogenic DI vasopressin precursor within the cell to jam the transport route and transport of the normal vasopressin precursor is not inhibited. Hence, these rats are only diabetic if both vasopressin precursors are mutant.

Whereas 44 of the 45 identified families with familial neurogenic DI have the dominant inherited form, recently one family was identified with a recessively inherited form. It would be interesting to see, whether the mutation present in this family also abolishes efficient production of the familial neurogenic DI vasopressin precursor (as in the rat) or whether this particular familial neurogenic DI vasopressin precursor escapes the quality control in the first transport compartment and thus is normally transported out of this compartment.

In summary.

By analyzing the transport of familial neurogenic DI vasopressin precursors within cultured cells, we discovered that these precursors are not able to leave the first transport compartment within the cells. Moreover, when the cells synthesize very high levels of these familial neurogenic DI vasopressin precursors, the whole first transport compartment fills up with this ‘wrong’ protein. We believe that this blocks all transport within the cell, also that of the normal vasopressin precursor. This hypothesis can explain both the dominant inheritance and delayed onset of familial neurogenic DI. However, although this is a likely mechanism for the formation of familial neurogenic DI, it will only be proven when someone can demonstrate that this traffic jamming not only occurs in cultured cells, but also in diabetic humans or animals.

Dr. Marga Nijenhuis,  a post-doctoral fellow, has many years experience in studying the transport of proteins within the cell.

Dr. J. Peter H. Burbach, a  professor at the Utrecht University, has longstanding experience in studying the function of vasopressin produced and secreted within the brain by other nerve cells than the magnocellurlar nerve cells that produce vasopressin for secretion into the blood stream, and the synthesis of vasopressin.

Robbert Zalm ,  a young and enthusiastic technician,  has aided in many of the experiments.