The somatropic axis regulates growth in vertebrates

Our growth during development is controlled by a complex brain-body axis called the somatotropic axis. Put simply as in the photo below, the hypothalamus in the brain signals the pituitary to release growth hormone into the body where it stimulates the liver to produce two forms of insulin-like growth facts (IGF1 and IGF2). Both growth hormone and IGF have different effects on growth of muscle and bone. While we rely on mouse models to study how IGF might impact human development, it turns out that the relative secretion of IGF1 and IGF2 over the course of life is quite different in the two species. In fact, we know little about IGF production and signaling in non-mammals.

Expanded view of the somatotropic axis to show receptors and binding proteins. From Yakar et al. (2018).

Abby Beatty, from Tonia Schwartz’s lab at Auburn University, set out to determine the developmental pattern of the somatotropic axis in brown anoles. Of course, the axis is much more complex than the diagram above, including receptors in various tissues, binding proteins that carry the signals around the body (see below), and proteins in cells that cause responses (IRSs). Abby studied expression of IGF1, IGF2, and five binding proteins during brown anole development, from embryo to hatchling to adult. She expected to find that IGF1 and 2 would be expressed differentially and that the expression patterns would differ across life stages. That’s exactly what she found. IGF1 and 2 were both low and similar in expression early in development, but at hatching IGF increased with IGF2 expressed more than IGF in adults. Surprisingly this is more like a human pattern than a mouse is!

As for the binding proteins, expression was similar for all of them in the brain, gonads, and liver, but BP3 was expressed less in the heart. It’s still unclear what these patterns in binding proteins mean for brown anole development, but they make for some excellent future research questions! Indeed, these results add to the already-long list of things that makes anoles good model systems.

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