# Dynamic transcriptional and epigenetic changes define postnatal tendon growth

## TL;DR

This study explores how genes and epigenetic changes guide tendon development from birth to maturity, identifying key regulators like Yap1.

## Contribution

The study identifies Yap1 and TEAD-regulated chromatin accessibility as novel regulators of early tendon extracellular matrix formation.

## Key findings

- Differentially expressed genes cluster into modules with dynamic changes in expression and chromatin accessibility.
- Yap1 regulates early tendon matrix formation but does not affect later mechanical properties.
- Accessible chromatin regions near ECM genes contain TEAD motifs, suggesting Hippo pathway involvement.

## Abstract

Tendons are dynamic structures that efficiently transmit force and enable movement. From birth, tendons undergo dramatic changes from a principally cellular tissue to a hypocellular one characterized by a dense and highly ordered extracellular matrix. During this time, tendon cells change morphology from rounded to stellate in appearance and their proliferative rates decline. Significant expansion and maturation of the extracellular matrix (ECM) grow the tendons in length and diameter and alter their biomechanical properties to sustain increased physical activities. Surprisingly, for such an important stage of tendon maturation, we understand very little about the transcriptional and epigenetic regulators that direct these processes. Here, we present a roadmap of genes that are differentially regulated during the early neonatal and postnatal time period. We find differentially expressed genes fall into specific transcriptional modules, representing expression increases, decreases, or gene sets undergoing dynamic changes over postnatal time. By pairing our transcriptomic data with epigenetic data, we performed an integrative analysis of the datasets and further defined modules with highly correlated changes in gene expression and chromatin accessibility. From this analysis, several new pathways emerge. Among them, we focus on Yap1, a transcriptional co-activator of the Hippo signaling pathway. We observe accessible regions near to differentially expressed genes, containing motifs for TEAD, the transcription factor that binds Yap to regulate transcription. Conditional loss of Yap1 at postnatal stages alters early expression of Col1a1 and matrix organization and density but does not affect gross ultrastructural and mechanical properties at later stages. Together, our analyses identify a regulator of early matrix formation and provides a rich dataset with which to interrogate transcriptional networks and pathways during this poorly understood time in tendon growth.

Tendons provide essential connections within our musculoskeletal system, transmitting the force from muscle to bone to enable movement. Concomitant with skeletal growth, tendon development continues through postnatal stages and is characterized by several major changes in the structure and organization of the tendon extracellular matrix (ECM), regenerative capacity, and mechanical strength. Remarkably, we understand very little about the molecular changes that characterize this important period of tendon growth and maturation. By examining global changes in gene expression and genome organization, we identify distinct modules of gene and chromatin regulation that change dynamically over this developmental timeframe. We specifically identify differentially regulated regions that may be controlled by components of the Hippo signaling pathway, which are nearby tendon ECM genes. We also find that loss of a transcriptional co-regulator of Hippo signaling affects early stages of tendon ECM gene expression but does not impact later tendon ECM maturation. Altogether, this work provides a rich resource with which to understand this key developmental period of tendon growth and maturation.