Bone and cartilage physiology
Research in our group is aimed at understanding the molecular mechanisms underlying the physiological regulation of bone and cartilage homeostasis.
Björn Barenius, MD, PhD, Head of Section for Arthroplasty and Arthroscopy, Department of Orthopaedic Surgery, Södersjukhuset.
- Eva Lindgren (research engineer)
- Giuseppe Di Lernia (master student, Karolinska Institutet)
- Rinat Gizatulin (visiting researcher)
- Sima Rahbar Zare, (master student, University of Skövde, Sweden, 2012-2013)
- Oskar Ljungkvist (summer student, Karolinska Summer School, 2012)
- Frida Kozma (summer student, Karolinska Summer School, 2012)
- Amanda Demir (summer student, Karolinska Summer School, 2013)
- Klas Walldén (summer student, Karolinska Summer School, 2013)
- Savas Ay (medical student from Dicle University, Turkey, supported by Erasmus program, summer 2013)
Project 1: G-protein pathways involved in the growth plate fusion
Humans do not grow during the entire length of their life-time. We stop growing due to fusion of the so-called growth plate or epiphyseal plate - a thin layer of chondrocytes located between the epiphysis and the metaphysis of long bones. Fusion of the growth plate normally occurs during puberty and results in cessation of longitudinal growth. However, mechanisms of the growth plate fusion are completely unknown.
It was recently shown that ablation of the parathyroid hormone (PTH) receptor (PTHR1) in chondrocytes induces fusion of growth plate in mice. The main mediator of PTHR1 action is the alpha-subunit of the stimulatory G-protein (Gsa). However, we found that Gsa does not mediate fusion of the growth plate entirely and there are additional g-proteins involved in this process. It was reported that Gq/11 and Ga12/13 g-protein families can be activated by PTH.
To test the role of g-proteins in the growth plate fusion we genetically inactivate g-proteins in chondrocytes and monitor bone growth and behavior of the growth plate.
These experiments will reveal factors involved in the growth plate fusion. The obtained knowledge could lead to the development of new drugs, targets and strategies regulating fusion of the growth plate. This, in turn, can help children with growth abnormalities, for example extremely tall stature.
Please see KI News: New findings explain why we stop growing at one point
Project 2: Role of autophagy in longitudinal bone growth, chondrocyte survival and hypertrophy
The main endocrine regulator of longitudinal growth is the growth hormone/insulin-like growth factor (GH/IGF1) system. In many cellular systems IGF1 is signaling via PI3K/Akt/mTOR pathway. Mammalian Target of Rapamycin (mTOR) kinase is the main regulator of autophagy. Autophagy is promoted primarily by inhibition of mTOR, followed by phosphorylation of the Atg proteins responsible for autophagosome formation. Autophagy is a cellular process of catabolic degradation of damaged organelles and long-lived protein complexes. In addition to providing building blocks and energy, autophagy is involved in heart hypertrophy, neurodegeneration, immunological responses and bone turnover. A recent genome-wide association studies demonstrated that the regulation-of-autophagy pathway has the strongest association with bone size.
We hypothesize that autophagy plays an important role in chondrocyte physiology, contributing to cell survival and chondrocyte hypertrophy, as well as participating in endocrine and nutritional regulation of bone growth.
To explore the contribution of autophagy to longitudinal bone growth we utilize both ex vivo and in vivo models. For ex vivo experiments we use our recent model of postnatal mouse bone cultures. We treat bones with pharmacological modulators of autophagy and monitor their growth in vitro.
To modulate activity of autophagy in mice in a time- and tissue-specific manner we are inactivating either Atg5 or Atg7 genes utilizing Cre-lox approach. Inactivation of either Atg5 or Atg7 abrogates autophagy. We employ the following transgenic mice: Atg5 floxed, Atg7 floxed, cartilage specific Collagen-2-driven Cre (Col2-Cre), and tamoxifen-inducible Col2-Cre. Upon breeding Cre and floxed strains, progenies have no autophagy specifically in their chondrocytes. We thereafter characterize the growth plate phenotype of the obtained mice and their responsiveness to hormones and nutrition.
Understanding of mechanisms regulating longitudinal bone growth could lead to development of new drugs to modulate linear growth for children with growth disorders.
Project 3: Mechanisms of cell size increase
Children’s long bones grow from narrow cartilage organs located at the ends of the bones called growth plates. Cells in the growth plate, called chondrocytes, increase in size by up to fifteen times their volume which determines 60-80 % of overall bone growth. Very little is known about the mechanisms within the chondrocytes, which controls this cellular enlargement, called hypertrophy, either at the basic research or at the clinical level. This situation is emphasized by the fact that the usual treatment for children suffering from extreme tall or short stature is invasive surgery or hormone therapy, respectively. These procedures are suboptimal, expensive, and moreover they do not target the cause of the problem directly. If we are to treat these patients more rationally and efficiently, further research of the downstream signalling pathways coordinating growth plate development is needed. Our research goal is to discover how chondrocyte hypertrophy is coordinated at the cellular level. This project has the possibility to uncover the molecular mechanisms and therefore reveal pharmaceutical approaches to treat children with extremely tall or short stature, which would vastly improve the current treatments.
Project 4: Osteoarthritis
Osteoarthritis is an age-dependent degradation of articular (joint) cartilage. This is a very common problem affecting more than 50 % of Swedish people above 65 years of age. Etiology of this disease is poorly known, with many potential underlying mechanisms including genetics, trauma, aging etc.
In our laboratory we are exploring mechanisms of cartilage protection and underlying causes of osteoarthritis development.
- Karolinska Institutet
- Swedish Society for Medical Research
- HKH Kronprinsessan Lovisas förening för barnasjukvård/Stiftelsen Axel Tielmans minnesfond
- The Lars Hierta Memorial Foundation
- Åke Wibergs stiftelse
- Swedish Research Council
- Jeanssons stiftelser
- Sällskapet Barnavård
See also KI News: New findings explain why we stop growing at one point