An Amidase_3 domain-containing N-acetylmuramyl-L-alanine amidase is required for mycobacterial cell division.

Mycobacteria possess a multi-layered cell wall that requires extensive remodelling during cell division. We investigated the role of an amidase_3 domain-containing N-acetylmuramyl-L-alanine amidase, a peptidoglycan remodelling enzyme implicated in cell division. We demonstrated that deletion of MSMEG_6281 (Ami1) in Mycobacterium smegmatis resulted in the formation of cellular chains, illustrative of cells that were unable to complete division. Suprisingly, viability in the Δami1 mutant was maintained through atypical lateral branching, the products of which proceeded to form viable daughter cells. We showed that these lateral buds resulted from mislocalization of DivIVA, a major determinant in facilitating polar elongation in mycobacterial cells. Failure of Δami1 mutant cells to separate also led to dysregulation of FtsZ ring bundling. Loss of Ami1 resulted in defects in septal peptidoglycan turnover with release of excess cell wall material from the septum or newly born cell poles. We noted signficant accumulation of 3-3 crosslinked muropeptides in the Δami1 mutant. We further demonstrated that deletion of ami1 leads to increased cell wall permeability and enhanced susceptiblity to cell wall targeting antibiotics. Collectively, these data provide novel insight on cell division in actinobacteria and highlights a new class of potential drug targets for mycobacterial diseases.

Role of stretch-activated channels on the stretch-induced changes of rat atrial myocytes.

The role of stretch-activated channels (SACs) on the stretch-induced changes of rat atrial myocytes was studied using a computer model that incorporated various ion channels and transporters including SACs. A relationship between the extent of the stretch and the activation of SACs was formulated in the model based on experimental findings to reproduce changes in electrical activity and Ca(2+) transients by stretch. Action potentials (APs) were significantly changed by the activation of SACs in the model simulation. The duration of the APs decreased at the initial fast phase and increased at the late slow phase of repolarisation. The resting membrane potential was depolarised from -82 to -70 mV. The Ca(2+) transients were also affected. A prolonged activation of SACs in the model gradually increased the amplitude of the Ca(2+) transients. The removal of Ca((2+)) permeability through SACs, however, had little effect on the stretch-induced changes in electrical activity and Ca(2+) transients in the control condition. In contrast, the removal of the Na(+) permeability nearly abolished these stretch-induced changes. Plotting the peaks of the Ca((2+)) transients during the activation of the SACs along a time axis revealed that they follow the time course of the Na(i)(+) concentration. The Ca((2+)) transients were not changed when the Na(i)(+) concentration was fixed to a control value (5.4mM). These results predicted by the model suggest that the influx of Na(+) rather than Ca(2+) through SACs is more crucial to the generation of stretch-induced changes in the electrical activity and associated Ca(2+) transients of rat atrial myocytes.