LKB1-AMPK signalling pathway drives the hypoxic ventilatory response by regulating brainstem nuclei but not the carotid body
Mahmoud, Amira Dia
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Ventilatory drive is mediated by respiratory central pattern generators that are located in the brainstem, which are continuously modulated by specialised peripheral and central chemoreceptors to adjust ventilatory patterns according to changes in arterial PO2. These specialised oxygen-sensing chemoreceptors are activated in response to acute reductions in arterial PO2 and ultimately trigger a respiratory response that acts to restore oxygen-levels. However, the molecular mechanism by which mammals are able to regulate their breathing pattern in such a manner during hypoxia remains controversial. Therefore, the studies performed in this thesis aimed to investigate the possibility that this process may be mediated by the liver kinase B 1 (LKB1)/ AMP-activated protein kinase (AMPK) signalling pathway, which is central to cellular adaptations to metabolic stress. This first involved the development of transgenic mice in which Lkb1 or AMPK were deleted. Global knockout of Lkb1 (Sakamoto, 2006) or AMPK activity (Viollet et al., 2009) are embryonic lethal. Thus, the Cre/loxP system was used to develop transgenic mice that had either Lkb1 or both isoforms of the AMPK catalytic α- subunit (α1 and α2) conditionally knocked out in catecholaminergic cells (including therein hypoxia-activated cells of the brainstem and carotid body) by driving Cre expression through a tyrosine-hydroxylase-specific promoter region. The consequent effects on the ventilatory response to hypoxia were then examined using unrestrained whole-body plethysmography. This demonstrated that, in contrast to the hyperventilation evoked in controls, increased ventilation was virtually abolished in the Lkb1 and AMPK α1 and α2 double knockouts during hypoxia. Both knockout mice also exhibited periods of hypoventilation with frequent apnoeas during hypoxia. Additionally, studies on single AMPK α1 and AMPK α2 knockouts identified that the ventilatory dysfunction in AMPK α1 and α2 double knockouts was primarily caused by AMPK α1 deletion. In contrast, the severe ventilatory abnormalities exhibited during hypoxia following the deletion of Lkb1 and AMPK in catecholaminergic cells were mostly reversed upon exposure of mice to hypoxia with hypercapnia. Also, the ventilatory response to hypercapnia alone was without any major effect as a result of Lkb1 deletion or the dual-deletion of AMPK α1 and α2 catalytic subunits in catecholaminergic cells. This thesis therefore demonstrates, for the first time, that the LKB1-AMPK signalling pathway is key to respiratory adaptations during hypoxia, by regulating catecholaminergic oxygen-sensing cells, thus protecting against hypoventilation and apnoeas. The LKB1-AMPK signaling pathway can thereby determine oxygen and energy supply at both a cellular and whole-body level.