Fetal cholinergic anti-inflammatory pathway and necrotizing enterocolitis: the brain-gut connection begins in utero

Abstract

Necrotizing enterocolitis (NEC) is an acute neonatal inflammatory disease that affects the intestine and may result in necrosis, systemic sepsis and multisystem organ failure. NEC affects 5-10% of all infants with birth weight ≤ 1500 g or gestational age less than 30 weeks. Chorioamnionitis (CA) is the main manifestation of pathological inflammation in the fetus and is strong associated with NEC. CA affects 20% of full-term pregnancies and upto 60% of preterm pregnancies and, notably, is often an occult finding. Intrauterine exposure to inflammatory stimuli may switch innate immunity cells such as macrophages to a reactive phenotype ("priming"). Confronted with renewed inflammatory stimuli during labour or postnatally, such sensitized cells can sustain a chronic or exaggerated production of proinflammatory cytokines associated with NEC (two-hit hypothesis). Via the cholinergic anti-inflammatory pathway, a neurally mediated innate anti-inflammatory mechanism, higher levels of vagal activity are associated with lower systemic levels of proinflammatory cytokines. This effect is mediated by the α7 subunit nicotinic acetylcholine receptor (α7nAChR) on macrophages. The gut is the most extensive organ innervated by the vagus nerve; it is also the primary site of innate immunity in the newborn. Here we review the mechanisms of possible neuroimmunological brain-gut interactions involved in the induction and control of antenatal intestinal inflammatory response and priming. We propose a neuroimmunological framework to (1) study the long-term effects of perinatal intestinal response to infection and (2) to uncover new targets for preventive and therapeutic intervention.

Keywords: chorioamnionitis; inflammation; intestines; necrotizing enterocolitis; neuroimmunology; preterm birth; prevention; vagus nerve.

Figure 1

Structures and factors controlling paracellular permeability. Tight junctions are luminal structures that filter the passage of ions and macromolecules (A). The intercellular structural proteins, claudins and occludins, are anchored to the cytoskeleton actinomyosin apical ring through intermediary proteins (ZO-1, -2, -3, AF6). The phosphorylation of myosin light chain causing contraction is associated with opening of tight junctions and junction protein deformations that favour macromolecule passage and bacterial translocation (B). The phosphorylation/dephosphorylation of myosin light chain is controlled by myosin light chain kinase (MLCK) and myosin phosphatase (MP). PAR-2 activation increases MLCK activity through external and internal calcium (Ca++) mobilization and calmodulin binding (C). (With permission from John Wiley and Sons) (Bueno and Fioramonti, 2008).

Figure 2

Wiring of the cholinergic anti-inflammatory pathway (CAP), which balances cytokine production. Pathogens as well as ischemia and other forms of injury activate cytokine production, which normally restores health. If the cytokine response is excessive, however, then these same mediators can cause disease. Efferent signals from the vagus nerve inhibit cytokine production via pathways dependent on the α7 subunit of the acetylcholine receptor (AChR) on macrophages and other cells. Efferent vagus nerve activity also increases instantaneous heart rate variability (HRV). A cholinergic brain network that is responsive to M1 agonists can increase the activity of CAP and also increase instantaneous HRV. Afferent signals carried in the vagus nerve can activate an efferent response that inhibits cytokine release, termed the inflammatory reflex (With permission from American Society for Clinical Investigation) (Tracey, 2007).

Figure 3

Cholinergic signals derived from vagus nerve stimulation inhibit the release of TNF-α, IL-1, HMGB1, and other cytokines by transducing a cellular signal that inhibits the nuclear activity of NF-κB. TNFR stands for TNF receptor (Wang et al., ; Tracey, 2007). The cytokine producing cell can be a macrophage, among others. The nAChR family are ligand-gated ion channels that mediate diverse physiological functions and were originally identified in the nervous system. They consist of different subtypes formed by the specific assembly of five polypeptide subunits including α1-10, β1-4, γ, δ, and ε. The subunits fall into two groups: neuronal nicotinic receptors (consisting of α2–10 and β2–4) and muscle nicotinic receptors (consisting of α1, β1, γ, δ, and ε). Functional neuronal nAChR subtypes are either homomeric (consisting of 5 identical α-subunits, as in α7- or α9 nAChR) or heteromeric (consisting of combinations of the α- and β-subunits, such as α3β2nAChR) (Yeboah et al., 2008). Remarkably, it is specifically the α7nAChR that is required for CAP’s effects on peripheral innate immune cells (Figures 2,3) (With permission from American Society for Clinical Investigation) (Tracey, 2007).

Figure 4

The set point function of the immune response is defined by the magnitude of innate immune responses relative to the infection or injury stimulus. Increasing the set point or shifting the curve to the left increases the chance that tissue damage will occur from the response to infection or injury. Decreasing the set point or shifting the curve to the right reduces the probability that tissue damage will occur. CAP is the neural circuit that provides acute compensatory input to adjust the magnitude of the immune response relative to the set point (With permission from Nature Publishing Group) (Tracey, 2009).