Next, the team introduced a mutation into the tagged human insulin gene that causes the proinsulin molecule to fold incorrectly. This allowed them to see what happened when the misfolded human proinsulin and the normal rat proinsulin were produced together inside the same cell.
What they saw was misfolded fluorescent proinsulin getting stuck in the endoplasmic reticulum, so it could not move along normal conveyor belt to make insulin. Simultaneously, this blocked the traffic of the normal proinsulin in the same cells. This protein mess in the endoplasmic reticulum directly inhibits insulin production in the beta cells, even including insulin production that comes from the otherwise normal rat proinsulin. The beta cells begin to suffer from this, and they ultimately die.
The Arvan lab is also collaborating with other groups to identify new mutations in the proinsulin gene of people with congenital diabetes, and to understand how these mutations may cause a similar protein mess.
These mutations are apparently the second most common genetic cause of congenital diabetes, which is a relatively rare genetic illness. Congenital diabetes differs from Type 1 diabetes because congenital diabetes is not caused by an attack by the immune system on the bodys own beta cells, and because it is passed down from parent to child. Arvan and his team suspect that congenital diabetes in babies mirrors the proinsulin misfolding seen in their new study, and in a strain of mice known as Akita mice, which develop diabetes spontaneously after birth.
The big question -- still to be determined -- is how much of the more common forms of diabetes also involve proinsulin misfolding in beta cells that are stressed to the max to make all the insulin they can, Arvan notes. This is a question
|Contact: Kara Gavin|
University of Michigan Health System