Researchers at Australia's Garvan Institute of Medical Research have for the first time mapped the genetic profile of the skeleton's master regulator cells, known as osteocytes.

Published in the May 5, 2021, issue of Nature Communications, the study outlines the genes that are switched on or off in osteocytes, a type of bone cell that controls how other types of cells make or break down parts of the skeleton to maintain strong and healthy bones.

The group started out with the hypothesis that the genes that are going to affect the skeleton are going to be expressed there, lead study author Scott Youlten told BioWorld Science.

"Without a complete blueprint of the genes and molecular pathways – the gene interactions that are occurring in the skeleton – we're limited in our capacity to define whether or not a gene is likely to impact the skeleton.

"That becomes relevant if you're looking at a patient that has a skeletal disease of unknown origin and you're looking through the variants in their DNA and trying to find the gene that's causing that," said Youlten, who is a research officer in the Bone Biology Lab and a computational biologist.

"This new information provides a kind of genetic shortlist we can look to when diagnosing bone diseases that have a genetic component," he said. "Identifying this unique genetic pattern will also help us find new therapies for bone disease and better understand the impacts of current therapies on the skeleton."

First look at osteocyte transcriptome

The skeleton is a dynamic structure that changes shape and composition over a person's life. Osteocytes are the most abundant cell type in bone but have proved difficult to study because they are embedded within the hard mineral structure of the skeleton.

Osteocytes are cells that are defined by where they are, starting out as bone-building cells on the bone surface as osteoblasts, and then as they secrete the bony tissue, they become trapped and turn into osteocytes. It's their physical context that determines what these cells are, Youlten said.

It has been challenging to study osteocytes because they're buried in the hardest tissue in the body. To get them out, they have to be subjected to techniques that take them away from the space that defines them.

Inside the bone, osteocytes form a network similar in scale and complexity to the neurons in the brain (with more than 23 trillion connections between 42 billion osteocytes) that monitor bone health and responds to aging and damage by signaling other cells to build more bone or break down old bone. Diseases such as osteoporosis and rare genetic skeletal disorders arise from an imbalance in these processes.

To understand what genes are involved in controlling bone build-up or breakdown, the researchers isolated bone samples from different skeletal sites of experimental models to measure the average gene activity in osteocytes. Through this, they mapped a comprehensive osteocyte signature of 1,239 genes that are switched on in osteocytes and that distinguish them from other cells. In fact, 77% of these genes have no previously known role in the skeleton and many were completely novel and only found in these critical cells.

"The idea that more than 70% of the genes we found had no known function in the skeleton before shows that our understanding of the skeleton is poorly defined and lags behind other organs or conditions," Youlten said.

However, this is disproportionate to the burden of disease because the skeleton is one of the most extensive organ systems in the body and often neglected.

"One of the clearest signals that came out early in the investigation was the differential patterning that occurs in osteocytes from different parts of the body," he said. "That was one of the first indicators of the real quality of the data that gave us the confidence to explore some of these other areas."

"Many of the genes we saw enriched in osteocytes are also found in neurons, which is interesting given these cells share similar physical characteristics and may suggest they are more closely related than we previously thought."

"When you see a family of related genes like that all moving in the same direction and telling you the same thing, you get a real sense that you're seeing something important.

"The osteocytes are a beautiful cell type in the way they form these complex interconnected networks, and at the point they do that, that's the point they start to engage these signaling patterns that are typically associated with the formation of other cellular networks like the neuronal network."

The big picture view

Co-senior author Peter Croucher, deputy director of the Garvan Institute and Head of the Bone Biology Lab, says that "the osteocyte transcriptome map gives researchers a picture of the whole landscape of genes that are switched on in osteocytes for the first time, rather than just a small glimpse."

"The majority of genes that we've found to be active within osteocytes had no previously known role in bones. This discovery will help us understand what controls the skeleton, which genes are important in rare and common skeletal diseases and help us identify new treatments that can stop development of bone disease and also restore lost bone."

"Our list is a way of finding genes that have the capacity to directly or specifically target the skeleton and so they present an opportunity to say if we manipulate that gene we may have a more on-target effect and the potential have less diverse effect on the body," Youlten added.

A comparison of the osteocyte signature genes with human genetic association studies of osteoporosis identify new genes that maybe associated with susceptibility to this common skeleton disease. Furthermore many of these osteocyte genes were also shown to cause rare bone diseases.

"Mapping the osteocyte transcriptome could help clinicians and researchers more easily establish whether a rare bone disease has a genetic cause, by looking through the list of genes known to play an active role in controlling the skeleton," Youlten said.

Researchers will now try to better understand the role of the differential patterning occurring in the skeleton, he said.

The group will also begin to screen patients with undiagnosed skeletal disorders for mutations and try to identify those genes using the list as a filter. Rare diseases often provide clues into unrelated conditions, he said.

"We're starting to see how these things come together to form a coherent picture."