Date Published: May 16, 2015
Publisher: Springer Berlin Heidelberg
Author(s): Madhubanti Basu, Mahismita Paichha, Banikalyan Swain, Saswati S. Lenka, Samarpal Singh, Rina Chakrabarti, Mrinal Samanta.
Toll-like receptors (TLRs) and nucleotide binding and oligomerization domain (NOD) receptors are pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play crucial role in innate immunity. In addition to PAMPs, PRRs recognize endogenous molecules released from damaged tissue or dead cells [damage-associated molecular patterns (DAMPs)] and activate signaling cascades to induce inflammatory processes. In the aquatic environment, large variation in seasonal and diurnal water temperature causes heat and cold stresses in fish, resulting in tissue injury and mortality of fish. In the Indian subcontinent, catla (Catla catla) is an economically important freshwater fish species and is prone to thermal stresses. To investigate the response of pattern recognition receptors in thermal stress, we analyzed TLRs (TLR2, TLR4 and TLR5) and NOD (NOD1 and NOD2) receptors gene expression in catla following heat and cold stress. Analysis of tissue samples (gill, liver, kidney and blood) of the thermal stressed and control fish by quantitative real-time PCR (qRT-PCR) assay revealed significant (p < 0.05) induction of TLR2, TLR4 and NOD2 gene expression in majority of the tested tissues of the treated fish as compared to the control. The expression of TLR5 and NOD1 gene was also induced in the heat and cold stressed fish, but mostly restricted in the blood. The downstream signaling molecule of TLR and NOD signaling pathway viz., MyD88 (myeloid differentiation primary response gene 88) and RICK (receptor interacting serine-threonine protein kinase-2) was also induced in the thermal stressed fish suggesting the engagement of TLR and NOD signaling pathway during thermal stress.
The success of aquaculture depends on providing an optimum and congenial environment to fish which subsequently helps to achieve their higher survival rate and growth (Boyd and Tucker 1998). The health status of aquatic animals is uniquely influenced by their immediate surroundings viz., pH, salinity, temperature, ambient light intensity, presence of contaminants, dissolved oxygen concentration etc. (Tort 2011). Among these, temperature is one the most important abiotic factors that plays a critical role in the life of poikilothermic animals like fish (Fry 1971; Brett 1971; Stewart et al. 2002). Various fish species differ in requirement of their optimal temperature (Sharma et al. 2014). Beyond that limit, fishes experience the thermal stress resulting in tissue injury and are prone to be infected by opportunistic pathogens (Das et al. 2004; Gordon 2005; Dalvi et al. 2009). To defend against pathogenic invasion, fish primarily depend upon non-specific or innate immunity contributed by pattern recognition receptors (PRRs) like toll-like receptors (TLRs) and nucleotide binding and oligomerization domain (NOD) receptors. In addition to the PAMPs (pathogen-associated molecular patterns) recognition, TLRs sense DAMPs (damage-associated molecular patterns) which are endogenous host molecules viz., fibronectin (Okamura et al. 2001), heparin sulfate (Johnson et al. 2002), biglycan (Schaefer et al. 2005), fibrinogen (Smiley et al. 2001), oligosaccharides of hyaluronan breakdown products (Jiang et al. 2005; Taylor et al. 2004, 2007), heat shock proteins (Yu et al. 2010), high mobility group box1 (HMGB1) (Tang et al. 2011), tenascin-C (Midwood et al. 2009), cardiac myosin (Zhang et al. 2009), S100 proteins, thioredoxin-interacting protein (TXNIP) (Yu et al. 2010) etc. Physiologically, these endogenous ligands are localized under different cellular compartment, but under stress they are either released passively from the injured tissues/dying cells or actively secreted by activated cells via non-conventional lysosomal route (Pollanen et al. 2009). Endogenous TLR ligands act as alarmins and may serve as early warning signals to innate and adaptive immunity (Matzinger 2002; Seong and Matzinger 2004). Recognition of DAMPs by PRRs activates signaling cascade resulting in the induction of cytokines, recruitment of more immune cells and repair of damaged tissue (Medzhitov 2008). In addition to TLRs, NOD-like receptors (NLRs) have also been shown to respond to both microbial components (Franchi et al. 2012) and endogenous ligands derived from tissue/cellular injuries (Ting et al. 2008; Tschopp and Schroder 2010; Krishnaswamy et al. 2013; Monie 2013).
This article demonstrates TLR2, TLR4, TLR5, NOD1 and NOD2 gene expression in catla fish during the early developmental stages, and this is the first report. The inductive expression of TLR and NOD receptor genes along with their downstream molecules MyD88 and RICK, respectively, suggests the release of DAMPs during thermal stress in fish. The data in this study may help in investigating the greater role of TLR and NOD receptors in repairing the damage tissues and pathology of fish.