Among many proposed environmental factors, epidemiological data support an association between maternal infections and neuropsychiatric disorders (Mednick et al., 1988
; Brown, 2006
; Ellman and Susser, 2009
). Here, we present evidence that maternal infection, mimicked by injections of LPS, and subsequent foetal inflammation during mid–late gestation in rats (equivalent to mid-second trimester in human) considerably perturbed brain development, shortly after the inflammatory response was triggered in the foetus, by interfering with signalling pathways involved in neuronal cell distribution pattern. Enhanced microglia activation, reactive astroglia and increased expression of pro-inflammatory cytokines were detected in the foetal brain after maternal exposure to LPS, suggesting that the observed effects might, at least in part, be generated by these immune mediators. Previous studies have reported increased levels of pro-inflammatory cytokines in the maternal serum as well as at the maternal/foetal interface (amniotic fluid, placenta) after stimulation of the maternal immune system with different agents [LPS, poly(I:C), etc.] (Boksa, 2010
). Altogether these findings advocate for a role of these immune mediators in the morphological and neurobehavioural changes in offspring exposed to prenatal inflammation.
Studies in humans as well as in accepted animal models of maternal infection have described a series of abnormalities in adult brain cytoarchitecture, including decreased dendritic arborization and aberrant neuronal migration (Fatemi and Folsom, 2009
; Deutsch et al., 2010
), pointing at a neurodevelopmental origin of severe psychiatric disorders and implying that certain defects are present before the onset of the disorder. Alterations in normal cortical development caused by small disturbances of neurogenesis and neuronal migration may elicit maldevelopment of these areas, affecting the formation of neuronal networks and resulting in the neuropathological defects described in the post-mortem brain of persons affected by schizophrenia and other neurodevelopmental disorders. Perhaps some pathways active in the adult brain are already dysregulated during foetal life by maternal conditions leading to the formation of malfunctioning neuronal circuits in the adolescent and adult brain. In the present study, an enlarged CP and abnormal expression of markers for immature neurons were observed in the neocortex of LPS-exposed foetuses at GD18, i.e. 2 days after LPS exposure, in comparison with the brains of age-matched control foetuses. In addition, a number of important molecules, including reelin, GLAST and Arc, were decreased in foetal brains following LPS exposure. Finally, at P1, the cerebral cortex of LPS-exposed animals was significantly larger than in age-matched control offspring and the cells appeared to be more compact, resulting in a reduction of the space these cells have available to extend processes compared with control.
Our results suggest an effect on the cleavage of reelin triggered by inflammation and the consequent cascade of events. In the developing mammalian brain, reelin has a pivotal role in cortical layer formation by regulating neuronal migration (Fatemi, 2005
; Forster et al., 2010
). In adult brain, this glycoprotein participates in synaptic plasticity and memory formation and is considered a susceptibility gene for neuropsychiatric disorders, such as schizophrenia and autism (Fatemi, 2005
; Forster et al., 2010
). Proteolytic cleavage of reelin produces five fragments, among them the central fragment was shown to mimic reelin functions in vitro
. The function(s) of the full-length and shorter fragments of reelin has hitherto not been completely elucidated, albeit it was reported that inhibition of reelin processing in vivo
prevents signalling and hampers development in cortical embryonic slices (Jossin et al., 2004
). Cleavage appears to be required for reelin to be released in the intercellular space and to bind to its receptors on receptive cells. In reeler Orleans (relnorl/orl) mice, lack of reelin signalling is due to abnormal protein processing and expression of a truncated, non-releasable reelin fragment (de Bergeyck et al., 1997
; Derer et al., 2001
). In contrast with the C-terminus fragments, both the N-terminus and the central fragments have been detected in the CP and are considered important for reelin function. Moreover, Jossin et al. (2007)
showed that, while the larger fragments as well as the full length could be only detected in the proximity of the MgZ, antibodies, raised against the N-terminal region or the central fragment, detected reelin among the cells of the CP. Hence, reelin processing seems important for his diffusion, which might be influenced by the fragments' size (ranging from 100 to 330 kDa), i.e. the smaller N-terminal and central fragments may diffuse farther into the CP and influence late born neurons which are at a greater distance from the MgZ. These differences would create a gradient of reelin throughout the developing cortex (Zhao and Frotscher, 2010
). The decreased levels of the 180 kDa isoform reported in this study suggest an impairment of reelin processing, which could, at least in part, explain the increased presence of doublecortin and βIII-tubulin positive cells in the IZ in the LPS-exposed foetuses compared with controls.
During brain development, proper synaptic activity must activate a cascade of genes involved in transforming immature neuronal connections into functional circuits. IEGs are highly expressed during CNS development and have important roles in the adult brain. Their expression is developmentally regulated and influenced by exogenous signals such as neurotransmitters and second messenger signalling pathways. IEG activation in response to neural activity is fundamental for controlling expression of downstream genes and their products, which are involved in specification and maturation of neural progenitors in the cerebral cortex as well as in other brain areas (Herdegen and Leah, 1998
; Kaufmann and Worley, 1999
). Interference with the early expression and activity of IEGs, such as Arc
, likely plays a role in the brain maldevelopment found in individuals with schizophrenia. As a consequence, synaptic transmission and plasticity are affected in young adulthood, when refinement of synaptic connections requires higher activity potentially leading to a loss of synaptic plasticity (Fatemi and Folsom, 2009
These findings portray an intricate process by which foetal inflammation perturbs neuronal patterning and cortical development contributing to cognitive and/or psychotic manifestations later in adulthood. Such a process acts upon a number of different pathways, a number of which then have additional roles in mediating some of the experience-dependent plasticity in the adult brain. Based on these results, we surmise that the formation of neuronal networks in offspring from LPS-injected dams is altered, and such abnormalities may represent a major underlying pathophysiology of psychiatric disorders with a neurodevelopmental origin.
The LPS model used in this study does not fully recapitulate the events triggered by bacterial pathogens and their toxins in the foetal brain, and reproduces only part of the inflammation-mediated effects. Nonetheless, our study has set the stage to unravel the sequelae of events that underlie the neurobehavioural deficits reported in animals exposed to an antenatal insult.