Defective autophagy is a key feature of cerebral cavernous malformations

Abstract

Cerebral cavernous malformation (CCM) is a major cerebrovascular disease affecting approximately 0.3–0.5% of the population and is characterized by enlarged and leaky capillaries that predispose to seizures, focal neurological deficits, and fatal intracerebral hemorrhages. Cerebral cavernous malformation is a genetic disease that may arise sporadically or be inherited as an autosomal dominant condition with incomplete penetrance and variable expressivity. Causative loss-of-function mutations have been identified in three genes, KRIT1 (CCM1), CCM2 (MGC4607), and PDCD10 (CCM3), which occur in both sporadic and familial forms. Autophagy is a bulk degradation process that maintains intracellular homeostasis and that plays essential quality control functions within the cell. Indeed, several studies have identified the association between dysregulated autophagy and different human diseases. Here, we show that the ablation of the KRIT1 gene strongly suppresses autophagy, leading to the aberrant accumulation of the autophagy adaptor p62/SQSTM1, defective quality control systems, and increased intracellular stress. KRIT1 loss-of-function activates the mTOR-ULK1 pathway, which is a master regulator of autophagy, and treatment with mTOR inhibitors rescues some of the mole-cular and cellular phenotypes associated with CCM. Insufficient autophagy is also evident in CCM2-silenced human endothelial cells and in both cells and tissues from an endothelial-specific CCM3-knockout mouse model, as well as in human CCM lesions. Furthermore, defective autophagy is highly correlated to endothelial-to-mesenchymal transition, a crucial event that contributes to CCM progression. Taken together, our data point to a key role for defective autophagy in CCM disease pathogenesis, thus providing a novel framework for the development of new pharmacological strategies to prevent or reverse adverse clinical outcomes of CCM lesions.

Oxidative stress and inflammation in cerebral cavernous malformation disease pathogenesis: Two sides of the same coin

“…On the other hand, there is compelling evidence that antioxidant compounds, such as those indicated above, can induce autophagy, including statins (Andres et al, 2014, Wei et al, 2013, Zhang et al, 2013a), fasudil (Iorio et al, 2010), sulindac derivatives (Chiou et al, 2011, Gurpinar et al, 2013), and vitamin D3 (Hoyer-Hansen et al, 2010, Kim et al, 2012, Lisse and Hewison, 2011, Wu and Sun, 2011). Together with recent findings suggesting that the interplay between defective autophagy and redox imbalance may be integral to the development and progression of CCM lesions by sensitizing endothelial cells to local oxidative stress events (Marchi et al, 2015), these observations point to autophagy as a major redox-sensitive mechanism that justifies the reported effectiveness of the different potential therapeutic compounds described so far (Marchi et al, 2016a, Marchi et al, 2016b). …”

“…Recent findings have also linked defective autophagy to enhanced endothelial cell sensitivity to oxidative stress, which suggests a pathogenetic mechanism that reconciles both the pleiotropic functions of CCM proteins and the distinct therapeutic approaches proposed so far (Marchi et al, 2015, Marchi et al, 2016a, Marchi et al, 2016b). Indeed, it is well established that autophagy inducers limit ROS accumulation and oxidative stress by stimulating the autophagic degradation of ROS-generating mitochondria (Lee et al, 2012, Scherz-Shouval and Elazar, 2011).…”

“…In this intricate mechanistic scenario, a further level of complexity is added by the discovery that CCM proteins modulate distinct redox-sensitive signaling pathways and mechanisms, including pro-oxidant and antioxidant pathways and autophagy (Gibson et al, 2015, Goitre et al, 2010, Goitre et al, 2014, Guazzi et al, 2012, Marchi et al, 2015), and are implicated in molecular and cellular responses to oxidative stress and inflammatory stimuli (Corr et al, 2012, Goitre et al, 2014). However, besides adding mechanistic complexity, these innovative research findings have also provided a potential integrative explanation for the pleiotropic effects of CCM protein dysfunctions, shedding new light on the mechanisms of CCM pathogenesis and opening new perspectives for disease prevention and treatment ( see Sections below for details ).…”

“…In particular, the discovery that defective autophagy is a common feature of loss-of-function of the three known CCM genes and may underlie the major phenotypic hallmarks of CCM disease suggests a potential unifying pathogenic mechanism that reconciles both the broad spectrum of signaling abnormalities and consequent pathologic effects linked to CCM protein dysfunctions, and the distinct therapeutic approaches proposed so far (Marchi et al, 2015, Marchi et al, 2016a, Marchi et al, 2016b). Indeed, most of the reported pleiotropic functions and dysfunctional effects of CCM proteins are directly or indirectly related to autophagy and its tight interconnection with redox homeostasis and signaling (Filomeni et al, 2015, Marchi et al, 2016b, Scherz-Shouval and Elazar, 2011), suggesting that the modulation of autophagy may represent an underlying and unifying mechanism for CCM protein physio-pathological functions.…”

“…Consistently, autophagy is a converging point of multiple physiological and pathological pathways and may exert pleiotropic effects on several molecular and cellular processes (Choi et al, 2013). Specifically, autophagy plays a pivotal role in various signaling pathways linked to CCM proteins, including pathways involving Sirt1/FoxO1 (Liu et al, 2015, Zhao et al, 2010), JNK/c-Jun (Yogev and Shaulian, 2010), RhoA/ROCK (Mleczak et al, 2013), β-catenin (Lin et al, 2015), TGF-β (Marchi et al, 2015), MEKK3 (Linares et al, 2015), and KLF2/4 (Guixe-Muntet et al, 2016, Liao et al, 2015). Moreover, most phenotypic hallmarks of CCM disease can be linked to autophagic dysfunctions, including enhanced ROS production (Huang et al, 2011, Marchi et al, 2015) and EndMT (Marchi et al, 2015), as well as altered cell adhesion and angiogenic potential (Kim et al, 2013a, Kumar et al, 2013, Tuloup-Minguez et al, 2013), and enhanced endothelial cell sensitivity to oxidant-induced injury (Liu et al, 2015).…”