Toll-Like Receptor 3 Expression in Glia and Neurons Alters in Response to White Matter Injury in Preterm Infants

Abstract

Toll-like receptors (TLRs) are members of the pattern recognition receptor family that detect components of foreign pathogens or endogenous molecules released in response to injury. Recent studies demonstrate that TLRs also have a functional role in regulating neuronal proliferation in the developing brain. This study investigated cellular expression of TLR3 using immunohistochemistry on human brain tissue. The tissue sections analysed contained anterior and lateral periventricular white matter from the frontal and parietal lobes in post-mortem neonatal cases with a postmenstrual age range of 23.6-31.4 weeks. In addition to preterm brains without overt pathology (control), preterm pathology cases with evidence of white matter injuries (WMI) were also examined. In order to identify TLR-positive cells, we utilized standard double-labelling immunofluorescence co-labelling techniques and confocal microscopy to compare co-expression of TLR3 with a neuronal marker (NeuN) or with glial markers (GFAP for astrocytes, Iba-1 for microglia and Olig2 for oligodendrocytes). We observed an increase in the neuronal (28 vs. 17%) and astroglial (38 vs. 21%) populations in the WMI group compared to controls in the anterior regions of the periventricular white matter in the frontal lobe. The increase in neurons and astrocytes in the WMI cases was associated with an increase in TLR3 immunoreactivity. This expression was significantly increased in the astroglia. The morphology of the TLR3 signal in the control cases was globular and restricted to the perinuclear region of the neurons and astrocytes, whilst in the cases of WMI, both neuronal, axonal and astroglial TLR3 expression was more diffuse (i.e., a different intracellular distribution) and could be detected along the extensions of the processes. This study demonstrates for the first time that neurons and glial cells in human neonatal periventricular white matter express TLR3 during development. The patterns of TLR3 expression were altered in the presence of WMI, which might influence normal developmental processes within the immature brain. Identifying changes in TLR3 expression during fetal development may be key to understanding the reduced volumes of grey matter and impaired cortical development seen in preterm infants.

Toll-Like Receptor-3 Activation Increases the Vulnerability of the Neonatal Brain to Hypoxia-Ischemia

“…with Poly I:C alters fetal brain development in an IL-6-dependent manner (Smith et al, 2007). However, in our experiments, the findings are correlative, and additional studies, using for example relevant knock-out animals or cytokine stimulation in TRIF KO mice, are needed to determine the specific roles of these inflammatory mediators in TLR-3-induced brain injury.…”

“…The direct influence of IL-6 on the development of brain injury is more controversial. Although studies have consistently demonstrated elevated IL-6 levels in asphyxiated infants (Sävman et al, 1998) and mice overexpressing IL-6 develop severe neurologic syndromes (Campbell et al, 1997), others find that mice deficient in IL-6 develop more severe brain injuries (Penkowa and Hidalgo, 2000). Interestingly, maternal immune activation ϩhigh -expressing and a CD11b ϩlow -expressing population, which were both affected by HI.…”

“…In the developing brain, TLR-3 activation impairs neural progenitor cell proliferation (Lathia et al, 2008), and maternal immune activation with poly inosinic:poly cytidylic acid (Poly I:C), a synthetic TLR-3 agonist, alters fetal brain development in an IL-6-dependent manner (Smith et al, 2007). Furthermore, maternal exposure to Poly I:C results in long-lasting effects in the offspring, including deficiencies in memory, learning tasks, and attention (Ratnayake et al, 2012;Vuillermot et al, 2012).…”

“…Furthermore, maternal exposure to Poly I:C results in long-lasting effects in the offspring, including deficiencies in memory, learning tasks, and attention (Ratnayake et al, 2012;Vuillermot et al, 2012). In neonatal animals, exposure to Poly I:C increases seizure susceptibility later during adulthood (Galic et al, 2009), suggesting that TLR-3 activation during brain development has detrimental effects that may affect vulnerability to later insults.…”

Perinatal Brain Injury As a Consequence of Preterm Birth and Intrauterine Inflammation: Designing Targeted Stem Cell Therapies

“…Ongoing TLR activation on immune cells is associated with increased immune cell infiltrates, microglial activation and cytokine release (Cai et al, 2000). It is suggested that, in particular, TLR3 protein expression in the brain is upregulated in cases of white matter injury and invading pathogens in the preterm brain (Vontell et al, 2013). TLR3 is implicated in normal neural cell differentiation and proliferation.…”