Gene That Preserves Bone Stem Cell Supply Throughout Life Identified

Researchers headed by a team at NYU Langone Health have found that the action of a key gene is required to maintain a lifelong supply of stem cells that make bone healing possible. Working with genetically engineered mice, the team identified the gene HOXA10 as a lifelong protector of the stem cell supplies, keeping them from maturing into bone-building cells called osteoblasts until needed for healing.

“Our study shows that HOXA10 protects healing capacity normally by acting as a brake on premature maturation,” said Philipp Leucht, MD, PhD, the Jyoti Menon Associate Professor of Orthopedic Surgery in the Department of Orthopedic Surgery at NYU Grossman School of Medicine. “To our knowledge, this study is the first to assign a definitive role for HOX genes in the active maintenance of adult skeletal stem cells.” The findings suggest that HOX genes may represent potential therapeutic targets for boosting bone regeneration.

Leucht, who is also the vice chair for research in the Department of Orthopedic Surgery, is senior and co-corresponding author of the researchers’ published paper in Science Advances, titled “Hoxa10 governs lifelong skeletal stem cell maintenance and bone repair,” in which they reported, “Together, these findings define a conserved Hox-dependent program that preserves skeletal stem cell identity and regenerative competence, revealing how developmental transcription factors are repurposed to sustain the adult skeleton across the life span.”

Skeletal stem and progenitor cells (SSPCs) sustain bone homeostasis and regeneration throughout life, the authors explained, but the transcriptional programs that preserve their identity aren’t well understood. Accumulating evidence suggests Hox genes may play critical roles in maintaining tissue-resident stem cells throughout the lifespan, “… serving as molecular determinants that preserve stem cell identity and function in adult tissues across different species,” the team continued. However, there are “critical gaps” in understanding of how Hox genes may regulate bona fide skeletal stem cells (SSCs) and their progeny. “This has profound implications for understanding skeletal aging, fracture healing, and potential therapeutic interventions.”

For their newly reported study the authors examined the state of bone stem cells in mice for 15 months after they deleted HOXA10 in 6-day-old pups. They found that by the end of that period, 90% of the stem cells and descendants that should have been present were gone due to a loss of self-renewal. “Most bone research asks how stem cells become bone,” said Leucht. “We asked how they remain as stem cell replacements in the first place. Without the study gene, bone burns through its reserves too early, which is what we see in aging bone.”

The researchers in addition found that mice engineered to lack HOXA10 action could not heal properly after breaking a bone. The proportion of stem cell progenitors in the callus—the temporary tissue bridge spanning a break as it begins to heal—was only 48% for the engineered mice, compared with 76% for control mice with the functional gene.

Other experiments confirmed that deleting HOXA10 causes collagen proteins that lay the early framework for rebuilding bone to become disorderly. This is a problem because the rope-like scaffold at a break site must be arranged in neat lines, rather than as a tangled mesh, for new bone to harden with calcium. “Our in vivo genetic studies demonstrate that HoxA genes are not merely correlated with skeletal stemness but are functionally necessary for maintaining this state,” the team noted. “Loss of Hox activity results in depletion of SSC pools, defective matrix organization, impaired repair after serial injuries, and compromised skeletal morphology with age.”

The researchers also obtained bone biopsies from human patients and silenced three related HOX genes at once. The human cells responded like mouse cells, only with more dramatic reductions in stem cell numbers. The human cells also began to die, with apoptosis (programmed self-destruction) up by 50%. This suggests that human stem cells may depend even more on HOX genes for long-term self-renewal, Leucht said. Commenting on their studies in human SSPCs, the authors noted, “Together, these findings demonstrate that distal Hox genes are essential for preserving the core properties of human SSPCs—including proliferation, survival, self-renewal, and inhibition of premature differentiation—thereby supporting their role in maintaining SSC pools in adult human bone.”

To carry out the study the research team genetically engineered mice to have their HOXA10-expressing stem cells tagged with a fluorescent green marker, as well as to have built-in mechanisms that the team could use to switch the genes off on command. This let them accurately track the status of the cells at all stages of life in the study mice.

The investigators learned that HOXA10-positive cells outperform other stem cells in the periosteum, the membrane that covers the surface of most bones, at colony-forming. This test, in which a single cell is prompted to generate a colony of cells by dividing and multiplying continually, is considered a standard readout of self-renewal capacity.

HOX genes have been shown by past studies to control periosteal stem and progenitor cells, or PSPCs, which play a key role in bone healing. HOX genes are known to encode proteins that act like the body’s “zip code,” specifying the position of limbs in the womb. The new study builds on another led by Leucht and published in 2023, which showed that increasing the activity of the HOXA10 gene could improve fracture healing in older mice.

“Our study identifies HOXA10 as a preserver of skeletal stem cell pools over time by promoting self-renewal, suppressing premature maturation, and lining up the protein fibers in a break so that the two ends of broken bone can knit,” said Leucht. “The findings establish HOX genes as potential therapeutic targets for enhancing bone regeneration.”

In their paper the authors concluded, “By illuminating how a single transcriptional regulator coordinates the complex behaviors necessary to maintain SSC across the life span, this work advances our understanding of stem cell biology and provides a foundation for therapeutic  approaches to improve bone healing in aging and disease.”

The post Gene That Preserves Bone Stem Cell Supply Throughout Life Identified appeared first on GEN - Genetic Engineering and Biotechnology News.