Dysfunctions of Human Muscle Stem Cells in Sepsis

Severe critical illness is often complicated by Intensive Care Unit - Acquired Weakness (ICU-AW), which is clinically characterized by bilateral and symmetrical limb weakness and is related to a myopathy and/or axonal polyneuropathy. ICU-AW affects between 25% to 60% mechanically ventilated patients more than 7 days and is associated with increased in and post-ICU mortality, with delayed weaning from mechanical ventilation and with long-term functional disability. Most patients who develop an ICU-AW have been admitted for a sepsis episode and the main risk factors of ICU-AW include the severity of critical illness, immobilization, hyperglycemia and the use of some medications including steroids and neuromuscular agents, although this is somewhat controversial.

The pathophysiology of critical illness myopathy is thought to involve the following mechanisms: 1) impairment of muscular membrane excitability, secondary to an dysregulation of sodium channel gating, 2) mitochondrial dysfunction leading to bioenergetic failure and oxidative stress and 3) proteolysis, mainly related an activation of the ubiquitin-proteasome pathway. These mechanisms can be triggered by various factors, notably systemic inflammatory mediators, endocrine dysfunction, immobilization, some drugs and electrolyte disturbances. The protracted functional consequences of ICU-AW indicate that muscle regeneration is also impaired. Surprisingly, muscle regeneration, which essentially depends on the muscle stem cells (also called satellite cells), has not been well investigated in the context of critical illness. The satellite cells (SC), that are located at the periphery of the muscle fiber, are activated in response to any muscle injury and then proliferate and differentiate to repair or replace the damaged fibers, but also self-renew to replenish the muscle stem cells reservoir.

It was recently demonstrated in a murine model of polymicrobial peritonitis that SC activation, proliferation and expression of myogenic markers were impaired after sepsis, leading to an impaired muscle regeneration, but that post-sepsis intramuscular administration of exogenous Mesenchymal Stem Cells (MSCs) could reverse this SC dysfunction. MSC treatment significantly improved the post-injury muscle recovery with decreasing necrosis and fibrosis but also increased the force of isolated single fibers.

The objectives of this translational research are to identify the mechanisms of Human SC dysfunction in patients with severe sepsis, to describe in vitro the effects of MSCs on this SC dysfunction, and to study hematopoietic cells and their damage in the blood and muscle consecutively to a sepsis, and their interaction with muscle stem cells.