Coupling Activation of Pro-Apoptotic Caspases With Autophagy in the Meckel´s Cartilage

Abstract

Mammalian Meckel´s cartilage is a temporary structure associated with mandible development. Notably, its elimination is not executed by apoptosis, and autophagy was suggested as the major mechanism. Simultaneous reports point to pro-apoptotic caspases as novel participants in autophagic pathways in general. The aim of this research was to find out whether activation of pro-apoptotic caspases (-2, -3, -6, -7, -8 and -9) was associated with autophagy of the Meckel´s cartilage chondrocytes. Active caspases were examined in serial histological sections of mouse mandible using immunodetection and were correlated with incidence of autophagy based on Beclin-1 expression. Caspase-2 and caspase-8 were found in Beclin-1 positive regions, whereas caspase-3, -6, -7 and -9 were not present. Caspase-8 was further correlated with Fas/FasL and HIF-1alpha, potential triggers for its activation. Some Fas and FasL positivity was observed in the chondrocytes but caspase-8 activation was found also in FasL deficient cartilage. HIF-1alpha was abundantly present in the hypertrophic chondrocytes. Taken together, caspase-8 activation in the Meckel´s cartilage was demonstrated for the first time. Caspase-8 and caspase-2 were the only pro-apoptotic caspases detected in the Beclin-1 positive segment of the cartilage. Activation of caspase-8 appears FasL/Fas independent but may be switched on by HIF-1alpha.

Caspase Inhibition Affects the Expression of Autophagy-Related Molecules in Chondrocytes

“…39 This pathway is associated with growth factor mediated processes 42 and FGF signaling was recently suggested as a regulator of bone growth through autophagy. 11 Caspase-8 is one of the key apoptotic components shown to be involved in autophagy 43 and is present also in nonapoptotic chondrocytes, 20 including the growth plate. 19 Inhibition of caspase-8 in micromass culture caused a 3-fold increase in Tnfsf10 (Trail) expression.…”

“…17 Notably, activation of proapoptotic caspases was recently demonstrated to occur also in non-apoptotic growth plate chondrocytes, 19 and in developing Meckel's cartilage. 20 In keeping with this non-apoptotic role, caspase inhibition in vitro changed expression of osteogenic markers in developing chondrocytes. 19,21 This is in agreement with caspases having novel non-apoptotic roles in addition to their role in cell death.…”

Diverse Fate of an Enigmatic Structure: 200 Years of Meckel’s Cartilage

“…However, in addition to their engagement in cell death ( Bohensky et al, 2014 ), autophagy has been identified also as a survival mechanism in MC ( Song et al, 2017 ; Luo et al, 2019 ). Notably, caspase-2 and -8 were found to be activated in the Beclin1 positive regions suggesting a role of these two regulators in autophagy ( Bilikova et al, 2019 ) and indicating that these pro-apoptotic caspases may be acting in a non-canonical manner in MC.…”