The evolution of insulin in vertebrates — including humans — has encountered a roadblock, limiting its ability to adapt to obesity and thereby rendering most people vulnerable to Type 2 diabetes, a significant research has claimed.

Scientists from Indiana University (IU), University of Michigan and Case Western Reserve University determined that the sequence of insulin has become entrenched at the edge of impaired production — an intrinsic vulnerability unmasked by rare mutations in the insulin gene causing diabetes in childhood.

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Insulin is produced by a series of highly specific processes that occur in specialised cells called beta cells.

A key step is the folding of a biosynthetic precursor, called proinsulin, to achieve the hormone’s functional three-dimensional structure.

Past studies have suggested that impaired biosynthesis could be the result of diverse mutations that hinder the foldability of proinsulin.

“Biological processes ordinarily evolve to be robust, and this protects us in the majority of cases from birth defects and diseases. Yet, diabetes seems to be an exception,” said Michael Weiss, Distinguished Professor at IU School of Medicine.

The group discovered that even the slightest variation of the insulin-sequencing process not only impairs insulin folding (and eventual insulin secretion) but also induces cellular stress that leads to beta cell dysfunction and eventually permanent damage.

The study, published in the Proceedings of the National Academy of Sciences, highlights the importance of folding efficiency as a critical but hidden factor in the evolution of insulin over the past 540 million years.

Humans have evolved to be vulnerable to diverse mutations in the insulin gene and that this vulnerability underlies a rare monogenic form of diabetes and provides an evolutionary backdrop to the present obesity-related diabetes pandemic.

“The authors highlight the fact that the insulin gene has been susceptible throughout evolution to mutations that impair insulin’s function or stress beta cells,” said Barbara Kahn from Harvard Medical School.

“As we approach the 100th anniversary of the discovery of insulin, these elegant observations might lead to a better understanding of the pathogenesis of Type 2 diabetes”.

The group will work to fully define the sequence determinants that make proinsulin foldable in beta cells.