Secreted Acid Phosphatase (SapM) of Mycobacterium tuberculosis Is Indispensable for Arresting Phagosomal Maturation and Growth of the Pathogen in Guinea Pig Tissues

Abstract

Tuberculosis (TB) is responsible for nearly 1.4 million deaths globally every year and continues to remain a serious threat to human health. The problem is further complicated by the growing incidence of multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB), emphasizing the need for the development of new drugs against this disease. Phagosomal maturation arrest is an important strategy employed by Mycobacterium tuberculosis to evade the host immune system. Secretory acid phosphatase (SapM) of M.tuberculosis is known to dephosphorylate phosphotidylinositol 3-phosphate (PI3P) present on phagosomes. However, there have been divergent reports on the involvement of SapM in phagosomal maturation arrest in mycobacteria. This study was aimed at reascertaining the involvement of SapM in phagosomal maturation arrest in M.tuberculosis. Further, for the first time, we have also studied whether SapM is essential for the pathogenesis of M.tuberculosis. By deleting the sapM gene of M.tuberculosis, we demonstrate that MtbΔsapM is defective in the arrest of phagosomal maturation as well as for growth in human THP-1 macrophages. We further show that MtbΔsapM is severely attenuated for growth in the lungs and spleen of guinea pigs and has a significantly reduced ability to cause pathological damage in the host when compared with the parental strain. Also, the guinea pigs infected with MtbΔsapM exhibited a significantly enhanced survival when compared with M.tuberculosis infected animals. The importance of SapM in phagosomal maturation arrest as well as in the pathogenesis of M.tuberculosis establishes it as an attractive target for the development of new therapeutic molecules against tuberculosis.

Bacterial Virulence Factors: Secreted for Survival

“…The DKO mutant was also found to be susceptible to oxidants, indicating towards a role of SapM in repressing the oxidative burst as seen for acid phosphatases from other intracellular pathogens [19][20][21]. Similar results using MtbDsapM mutant were obtained by Puri et al [26]. The study was taken a step further and conducted in an animal model and guinea pigs infected with MtbDsapM were shown to have an enhanced survival in comparison to Mtb infected animals.…”

“…This study implicated that Mtb SapM would somehow block the activation and recruitment of DCs thus helping the bacteria lower down the severity of adaptive immune response. The studies which followed [25,26] however, unequivocally indicated toward a role of Mtb SapM in phagosome maturation arrest thereby substantiating the results obtained by Vergne et al [22]. In an attempt to create Mtb derived vaccine strain with a potential to undergo phagosome lysosome fusion, Saikolappan et al [25] generated a double knock out (DKO) strain, by deleting the genes fbpA and sapM in Mtb (DfbpADsapM).…”

“…The study was taken a step further and conducted in an animal model and guinea pigs infected with MtbDsapM were shown to have an enhanced survival in comparison to Mtb infected animals. It was observed that SapM was indispensable for Mtb growth and that the infection with MtbDsapM mutant caused a significant reduction in pathological damage in the host [26]. The same group carried out another study to understand the effect of vaccination with the triple knockout mutant MtbDmptpADmptpBDsapM (MtbDmms) versus BCG on the survival and pathogenesis of Mtb [28].…”

Virulence Factors and Pathogenicity of Mycobacterium

“…SapM is a secretory phosphatase that dephosphorylates phosphatidylinositol 3-phosphate (PI3P) on the phagosome membrane [87]. PI3P is essential for phagosomes to acquire lysosomal constituents; it is involved in the docking of rab effector proteins early endosomal autoantigen 1 (EEA1) and hepatocyte growth factor-regulated tyrosine kinase (HRS) substrate, which are important for phagosome maturation [89]. Disruption of sapM in MTB resulted in a highly attenuated strain with an impaired ability to grow in the THP-1 macrophages as well as in the guinea pig tissues [33].…”

Antioxidants: Friend or foe for tuberculosis patients

“…Such maturation arrest of phagosomes containing Mycobacterium tuberculosis is thought due to defective recruitment of rab proteins in such phagosomes [11]. Mycobacterial factor like secreted acid phosphatase is also thought to be important for inhibition of phagolysosome biogenesis [12]. As a result the tuberculosis bacterium resides inside the phagosome without experiencing the adverse environment produced by lysosomal acid hydrolases.…”