Research paperGene expression profiles of autophagy-related genes in multiple sclerosis
Introduction
Multiple sclerosis (MS) is the most common disease of the central nervous system characterized by inflammation and neurodegeneration, affecting especially young adults (Compston and Coles, 2002).While the cause is not clear, the underlying mechanism is considered to be destruction by the immune system (T cell-mediated demyelination) or failure of the myelin-producing cells (Nakahara et al., 2012). Proposed causes involve genetic susceptibility and environmental factors such as infections (Compston and Coles, 2002, Ascherio and Munger, 2007). However, increased T cell proliferation and prolonged T cell survival have been linked to MS (Alirezaei et al., 2009). The best defining feature of the disease is formation of the sclerotic plaques, which is the end stage of several processes such as inflammation caused by recognition of myelin as a foreign antigen by T cells, demyelination and remyelination, oligodendrocyte depletion and astrocytosis, and neurodegeneration (Compston and Coles, 2002).
Macroautophagy, hereafter referred to simply as autophagy, is a conserved intracellular process which provides instruments for the toxic, aggregation-prone proteins and damaged organelles, such as mitochondria; to be degraded (Yang and Klionsky, 2010) Autophagy is initiated with a double-membrane structure formation called autophagosome. Yeast has approximately 20 different autophagy genes (Atg) that are necessary for autophagosome formation and the further stages of autophagy (Yang and Klionsky, 2010). Autophagy is greatly up-regulated by prolonged starvation or fasting (Levine and Kroemer, 2008). However, it was discovered that autophagy also has a significant role in the immune responses, and autophagy is not limited to the conditions under the stimulus of starvation (Beau et al., 2011). Autophagy is responsible for constitutive protein turnover even under nutrient-rich conditions.
In autophagy process, the cargos are packaged into autophagasomes and shipped to the lysosome for degradation. Since autophagosomes are non-degradative vacuoles, they just seclude the cytoplasmic material (Beau et al., 2011). The structure known as phagophore or the isolation membrane is derived from a single membrane and elongates around the cellular cargo (Yang and Klionsky, 2010). Autophagy begins with the construction of the complex that initiates phagophore membrane formation, including Beclin-1 (BECN1, ATG6), vacuolar protein sorting 34 (Vps34), and class III phosphoinositide 3-kinase, plus autophagy genes (Beau et al., 2011, Yordy et al., 2012). Following the formation of the phagophore membrane, construction of the autophagosome continues with the elongation and closure of the membrane. In mammalians, while Atg9, ULK1 (Unc51-Like Kinase 1) complex, and the PI3K (Phosphatidylinositol 3-Kinase) complex initiate the autophagy under certain circumstances (Beau et al., 2011), Atg 14 has the critical role in the endoplasmic reticulum (ER) targeting of the PI3K complex (Matsunaga et al., 2010). To form the autophagosome, Atg12-Atg5, Atg16 and LC3-II (Microtubule-Associated Protein 1, Light Chain 3) act to extend the phagophore membrane (Nakatogawa et al., 2009). The mitochondrial outer membrane (Hailey et al., 2010), the plasma membrane (Ravikumar et al., 2010) and also post-Golgi compartments (Mari et al., 2010) may be another sources of the phagophore membrane. After closure, except the LC3-II, the Atg proteins are reacquired from the autophagosome membrane (Yang and Klionsky, 2010).
Malfunction of autophagy contributes to the pathogenesis of certain diseases such as cancer, infectious diseases, cardiovascular and neurodegenerative disorders (Ravikumar et al., 2010). Inhibiton of autophagy significantly accelerates the development of neurodegeneration (Hara et al., 2006). In neurodegenerative disorders such as amyotrophic lateral sclerosis, Parkinson's and Huntington's diseases, the lack of autophagy is associated with neurodegeneration even in the absence of harmful proteins (Komatsu et al., 2006).
As known, the cells that play roles in the initiation and progression of inflammation are CD4 + T cells, CD8 + T cells, gamma/delta T cells, B cells, antibodies and cells of innate immune system such as microglia and macrophages (Friese and Fugger, 2005). Macrophages use autophagy to clear microbial pathogens (Pua et al., 2007). It is now known that autophagy has a role in the development and function of adaptive immune system (Pua and He, 2007).
The discovery of autophagy-related genes has highly improved our understanding and established our perspective about the mechanism responsible for the fate of autophagy from autophagosome formation to maturation and lysosomal degradation. The regulation of autophagy by protein kinases and transcription factors involving in nuclear regulation of autophagy genes is now better known. Based on the possible role of autophagy in neurodegenerative disorders and adaptive immune system, in this study, we aimed to understand whether there might be an association between autophagy and multiple sclerosis pathogenesis by monitoring the gene expression levels of several autophagy-related genes (Table 1) involving in autophagy machinery in different stages as discussed in Discussion section.
Section snippets
Blood samples
Our study group consisted of a total of 95 unrelated patients (aged 17–65 years, 37 male and 58 female) diagnosed as MS by Department of Neurology and 95 healthy controls. 74 of patients had relapsing–remitting MS, 7 had primary progressive MS, 14 had secondary progressive MS. The clinical data of patients is shown in Table 2. The blood samples were collected by the Department of Neurology of the University of Gaziantep.
The study was approved by the local Ethics Committee (Ethical Approval
Results
The expression levels of several genes involved in the autophagy pathway were analyzed in this study using Fluidigm BioMark™ HD System 96.96 Dynamic Array among MS patients and healthy controls.
We selected 78 genes (Table 1) regarding especially the autophagosome formation and subsequent induction of autophagy. When t-test was run between patient and healthy control samples, it was seen that 29 of 78 genes have significant differences in expression levels between these groups with
Discussion
(In this section, depending on our findings, the genes studied and found as overexpressed are represented by bold characters and the genes low-expressed are represented underlined).
Recently, in many reviews, autophagy has been covered in detail (Beau et al., 2011, Mehrpour et al., 2010). Autophagy is a multistep degradation process involved in the turnover of organelles and long-lived proteins and has pleiotropic functions in development, differentiation, cellular homeostasis, and survival (
Conflict of interest
The authors declare that they have no conflict of interest.
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