The role of autophagy in cartilage physiology and metabolism : implications for growth and ageing
Vuppalapati, Karuna Kumari
Lecture Hall, Pharmacology Library, Nanna Svartz Väg 2, Karolinska Institutet, Solna
Inst för fysiologi och farmakologi / Dept of Physiology and Pharmacology
Cartilage is the main constituent of the embryonic skeleton. At the ends of long bones cartilage forms
a growth plate consisting of chondrocytes in distinct stages of differentiation and arranged into three
zones. These chondrocytes mediate linear bone growth through synchronized proliferation,
differentiation, and production of matrix. The cartilage lining the articulating surfaces of bones also
contains chondrocytes arranged in different layers that secrete extracellular matrix and preserve
cartilage integrity. Articular cartilage is relatively permanent, whereas the growth plate is transient.
Although each of these cartilaginous structures has a unique structure and function, one fundamental
similarity is that the chondrocytes in both are exposed to little blood and, thereby, low levels of
oxygen and nutrients.
Autophagy is an intracellular pathway of lysosomal degradation that protects cells from both internal
and external stressors and promotes cell viability when nutrition is limited. The protein kinase
mTORC1 is a negative regulator of autophagy and its activity is, in turn, governed by various stimuli
such as nutrition and growth factors, depletion of which inhibits mTORC1 and activates autophagy.
Attenuated autophagy leads to various developmental and ageing-associated degenerative diseases.
Therefore, our primary hypothesis was that autophagy promotes chondrocyte survival, so, that
inhibition of this process may impair the linear growth of bones and promote the development of agerelated
osteoarthritis. Our second hypothesis was that autophagy improves metabolic parameters
during long-term intermittent caloric restriction.
First, we studied the role of autophagy in the chondrocytes of mouse metatarsal bones and in C5.18
cells by blocking this process with the lysosomal inhibitors bafilomycin A1 and chloroquine. We
found that mTORC1 activity in chondrocytes was increased by blocking lysosomal V-ATPase
enzymes. This effect is chondrocyte-specific and in contrast to well-accepted dogma. At the same
time, inhibition of lysosomal activity stimulated the linear growth of mouse metatarsal bones by
enhancing chondrocyte hypertrophy. Moreover, chondrocytes with impaired autophagy showed
similar responses (Paper I).
Subsequently, to investigate the effects of autophagy on linear bone growth (Paper II) and ageassociated
osteoarthritis (Paper III) directly we abrogated autophagy in chondrocytes by conditional
deletion of the autophagy related Atg5 or Atg7 gene. We observed reduced axial and appendicular
bone growth due to attenuated chondrocyte proliferation and elevated cell death in both cases.
Moreover, chondrocyte viability in the human growth plate and mouse metatarsal bones was reduced
by treatment with 3-methyladenine or bafilomycin A1, inhibitors of autophagy (Paper II).
Fibrillations and proteoglycan loss in the articular cartilage of aged mice without a functional Atg5
gene was elevated indicating the development of osteoarthritis (Paper III). These impaired bone
growth and degenerative changes in articular cartilage are the consequences of enhanced apoptosis
mediated by activation of caspases-3 and -9 (Paper II and III). Furthermore, release of cytochrome C
initiated the cleavage of caspases even in the absence of autophagy (Paper II).
Finally, we examined the role of autophagy in metabolism during intermittent caloric restriction
(according to a 5:2 diet) in obese individuals and with and without type II diabetes. We observed
improvements in anthropometric and metabolic parameters in both our diabetic and non-diabetic
subjects. Moreover, in diabetic subjects whose insulin sensitivity was improved by caloric restriction,
autophagy also increased (Paper IV).
In conclusion, the observations from our in vitro and in vivo studies confirm that autophagy is
essential for the survival and homeostasis of chondrocytes in the growth plate and articular cartilage.
At the same time, mTORC1 activation is chondrocyte-specific and independent of autophagy. In
addition, autophagy improves metabolic parameters during intermittent caloric restriction in humans.
Elucidating mTOR induced autophagy in greater detail will provide further insights in to disorders of
linear growth, cartilage degeneration and metabolism, there by opening up novel approaches to