Ángulo de caída

Several bacteria and viruses have been shown to induce ceramide formation through activation of ASM. Staphylococcus aureus known to cause sepsis, pneumonia and wound

infections was shown to cause apoptosis of human endothelial cells. The mechanism of induction of apoptosis was shown to be result of activation of ASM and ceramide generation leading to subsequent stimulation of JNK signaling, cellular caspases and release of cytochrome c (Esen et al., 2001). CEACAM receptor mediated phagocytosis of Neisseria gonorrhoeae into human neutrophils led to activation of ASM and pharmacological inhibition of ASM activity inhibited its phagocytosis, further reconstitution of ceramide in ASM inhibited cells restored

internalization of N. gonorrhoeae. They also showed that CEACAM receptor-initiated stimulation of src-like tyrosine kinases and Jun N-terminal kinases required ASM activity

(Hauck et al., 2000). Pseudomonas aeruginosa infection of human nasal epithelial cells or murine tracheal epithelial cell triggered ASM activation, its translocation from intracellular compartment to the extracellular leaflet and generation of ceramide for the formation of platforms within 5-10 minutes of infection which disappeared in next 30 min and this was required to internalize P.

aeruginosa and also induce apoptosis (Grassme et al., 2003b).

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Acid sphingomyelinase and ceramide-enriched membrane platforms are also been shown to be involved in the infection of human cells with pathogenic rhinoviruses. Rhinovirus infection triggers rapid activation of acid sphingomyelinase with microtubule- and

microfilament-mediated translocation of ASM to plasma membrane and generation of ceramide platforms that colocalize with rhinovirus for their uptake into the cells (Grassmé et al., 2005). Measles virus binding to the pattern recognition receptor on dendritic cells causes activation of neutral and acid sphingomyelinases leading to accumulation of ceramide in plasma membrane. This process of activation of ASM and recruitment of ASM to plasma membrane also induced efficient recruitment of CD150 (which is the uptake receptor for measles virus) from the intracellular Lamp1+ storage compartment shared with ASM. This process promotes receptor and signalosome co-segregation into ceramide enriched microdomains and provide favorable environment for membrane fusion and pathogen uptake (Avota et al., 2011). Ebolavirus was found to be associated with sphingomyelin enriched lipid rafts and its binding led to recruitment of lysosomal ASM to the cell surface. The activity of ASM was shown to be required for the attachment and entry of ebolavirus into the cells (Miller et al., 2012). HIV infection was shown to be inhibited by ASM generated ceramide platforms that restrict the lateral diffusion of CD4 (cellular receptor for HIV) but not the coreceptor CCR5 leading to inhibition of fusion process primarily resulting from clustering of CD4 molecules (Finnegan et al., 2007). Trypanosoma cruzi, an intracellular protozoan invades large number of different cell types without the

requirement of host actin rearrangement (Schenkman et al., 1991) but required recruitment and fusion of host lysosomes to its entry site (Tardieux et al., 1992). The entry of T. cruzi into host cells was shown to mimic a process of plasma membrane injury and repair that involves Ca²⁺ -dependent exocytosis of lysosomes that delivers ASM to the outer leaflet of plasma membrane.

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The ASM on the outer leaflet leads to generation of ceramide that activates endocytosis and internalizes the membrane lesion along with internalization of T. cruzi. Inhibition or depletion of lysosomal ASM blocked plasma membrane repair and markedly reduced the susceptibility of host cells to T. cruzi invasion (Fernandes et al., 2011).

ASM regulates vesicular fusion process by modifying the steric conformation of cellular membranes and this has been shown to have implications in phago-lysosomal fusion in

macrophages infected with Listeria monocytogenes, in exocytosis of secretory lysosomes of lymphocytic choriomeningitis virus-specific cytotoxic T cells and in generation of

multinucleated giant cells in granuloma of mice infected with Mycobacterium avium.

ASM knock-out mice were highly susceptible to L. monocytogenes with bacterial loads in liver and spleen exceeding 100-10,000 folds of wild-type mice. The kinetics of phagosomal

maturation and subsequent fusion with lysosomes is critical in control of L. monocytogenes infection. In ASM knock-out macrophages a prolonged localization of L. monocytogenes in late phagosomes and localization of L. monocytogenes with Lamp1+ lysosomal compartments occurred with reduced frequency, suggesting phagosomal maturation defects in ASM knock-out macrophages as a results of perturbation in lysosomal fusion process. The inability of ASM knock-out mice to restrict the growth of L. monocytogenes was attributed to the impaired transfer of lysosomal hydrolases to late phagosomes in ASM knock-out mice that was required for killing of L. monocytogenes in phago-lysosomes before it can escape from the phagosome into the cytosol (Utermöhlen et al., 2008; Utermöhlen et al., 2003). ASM was found to be critical for fusion of secretory lysosomes with the plasma membrane and formation of fusion pore in cytotoxic T lymphocytes and as a result of this, the final degranulation to release cytotoxic effector molecules from granules was impaired in ASM knock-out cytotoxic T lymphocytes

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leading to delayed elimination of Lymphocytic choriomeningitis virus in a ASM knock-out mice (Herz et al., 2009; Utermöhlen et al., 2008). ASM localization to the outer leaflet of the plasma membrane was shown to be important in the formation of multinucleated giant cells by fusion of macrophages in granuloma of mice infected with Mycobacterium avium. M. avium infected ASM knock-out mice survived for up to 120 days without any clinical signs while wild-type mice died between 70-80 days post infection. The bacterial loads in liver and spleen was comparable in wild-type and ASM knock-out mice but histologically wild-type mice showed large

multinucleated giant cells with infiltrates of hypertrophic macrophages harboring masses of mycobacteria but in ASM knock-out mice only small granulomas were seen. The formation of giant cells which requires ASM activity is the site for replication of M. avium are not formed in ASM knock-out mice restricting the growth of M. avium in them (Utermöhlen et al., 2008).