The overall body of data is now leading to an appreciation of these cells as a critical component in the formation of neural circuitry and as a potential target for developmental disruption. The contributory role of microglia to the developing vascular and axonal networks has been recently reviewed (Pont-Lezica et al., 2011
) and includes a discussion of results from Paolicelli et al. (2011)
showing that microglial CX3CR1 is required for spine elimination during a specific developmental period in the juvenile hippocampus. Similarly, microglial expression of CSF-1R, the receptor for IL-34, in the postnatal mouse occurs at times that coincide with peak expansion of microglial cell numbers. The expression of this receptor is essential for appropriate brain development, most notably in the cortex and olfactory bulb (Erblich et al., 2011
). The phagocytic action of microglia on amyloid-β (Aβ) fibrils exhibits age dependence in that brain microglia cultured from PND 0 brain, but not from PND 180 brain, phagocytose Aβ in vitro
(Floden and Combs, 2011
). This phagocytic action appears to involve IL-34 interactions with microglia (Mizuno et al., 2011
). Depletion of microglia in the transcriptional factor, PU.1 did not affect NPC survival or neurogenesis in vitro
, but did result in inhibited precursor cell proliferation and astrogenesis (Antony et al., 2011
). Overall, the data suggests that microglial actions may be most critical during postnatal brain maturation rather than during embryonic stages of development.
The contributions of microglia during development can be impacted by changes in experience during the postnatal period. Using a model of early life stress, Chocyk et al. (2011)
demonstrated that maternal separation resulted in an increase in active caspase 9+
microglia within the substantia nigra and the ventral tegmental area of juvenile male rats, potentially resulting in a decrease in microglia in these regions in the adult. Peak expression of the lipopolysaccharide binding protein (LBP) occurs at 2–3 weeks postnatally in close proximity to both the post-synapse and microglial processes. Wei et al. (2011)
reported that early life stress induces a loss of LBP and results in an increased anxiety level and impaired memory in the adult. However, prenatal stress exposure leads to slightly increased anxiety-like behavior in the adult (Kohman et al., 2010
) with no deficits in learning and memory performance observed with or without LPS challenge. Early handling of male rat pups daily between PND 2 and PND 10 produced a 3-fold increase in mRNA levels for the anti-inflammatory cytokine, IL-10, within the nucleus accumbens (Schwarz et al., 2011
). These authors concluded that neonatal handling can shift glia into a predominantly anti-inflammatory state and that this will serve to block morphine-induced glial activation. Further work showed a 5-fold decrease in relative methylation of the IL-10 gene in microglia of handled rats suggesting an early-life epigenetic programming of IL-10 expression that would manifest upon challenge in the adult (Schwarz et al., 2011
). Prior to our current knowledge of epigenetic programming, this feature would have been considered as a long-term preconditioning.
Translation of the changes in microglia in the adult brain to the developing brain may not be easy. In the adult, the normal response examined is a shift in morphology and an elevation in pro-inflammatory cytokines. In the immature brain, the microglia either continue to display an amoeboid phenotype and maintain high levels of cyotkine production, the roles are different than what occurs with adult injury. In addition, as in the adult brain, determining whether any observed microglia change represents a direct effect upon microglia as an underlying mechanism or rather if the cells are simply responding to their environment. One interesting study examined the morphological differences in microglia as a function of thyroid hormone status. Alterations in thyroid hormone (TH) signaling during development are associated with detrimental effects on the brain, including changes in synapse formation and myelination. Lima et al (2001)
reported that isolated rat microglia express TH receptors (TRα1 and TRβ1), and that changes in thyroid status, either a deficit or over expression, significantly alter the maturation of microglia (Flavia et al., 2001
). Within the first 1–2 weeks of postnatal life, hypothyroid animals showed a deficit in the density of microglia and a delay in process extension. In contrast, hyperthyroidism accelerated the extension of microglial processes and increased the density of cortical microglial cell bodies. How this altered timing for the shift of microglia from the fetal amoeboid morphology to process bearing affects microglia function and/ or their ability to respond remains unknown. What this data does demonstrate is that subtle changes in the neural environment, in the absence of a cell death response, can shift the morphology of developing microglia.
Exposures to xenobiotics during development can directly target microglia. As one example, early work in this area by Harry and co-workers demonstrated that a direct exposure to the known neurotoxicant, trimethyltin (TMT), could activate microglia in culture to produce TNFα and shift the phagocytic capability of the cells, but in the absence of an induction of iNOS (Maier et al. 1997
). While a number of compounds have been shown to stimulate microglia in culture, including mercury, the studies with TMT reveal a direct translation from in vitro
to in vivo
models. The stimulatory effects of TMT, as indicated by morphology and the elevation of pro-inflammatory cytokines were observed in vivo
when the adolescent mouse was exposed to TMT (Bruccoleri et al., 1998
; McPherson et al., 2011
). Of interest was the observation that both in vivo
and in vitro
models showed a similar temporal progression of responses and that they occurred in the presence and absence of dying neurons, respectively.
Acute alcohol exposure in adolescent rats results in morphological changes and proliferation of hippocampal microglia that persist for at least 4 weeks post-exposure. This occurred without accompanying alterations in expression of ED-1, MHC-II, or TNFα (McClain et al., 2011
). It was suggested that this response represents a "priming" of microglia, as MHC-II and TNFα levels are increased when these exposures are repeated using an intermittent paradigm (Ward et al., 2009
; Alfonso-Loeches et al., 2010
). Early postnatal ethanol exposure (PND 3–5) results in depleted microglial cell numbers and associated loss of Purkinje cells in the cerebellum that may be inhibited by PPAR-γ activation (Kane et al., 2011
). In adolescent rats, ethanol exposure, in combination with 3,4-methyleneddioxymethamphatamine (MDMA), resulted in an accumulation of CD11b+
microglia at bordering zones of the hippocampal subgranular zone (SGZ) and concomitant decreases in adult neurogenesis and memory function (Hernandez-Rabaza et al., 2010
). Sensitivity of microglia in the developing brain has also been noted following exposure to manganese (Mn). Studies by Moreno et al. (2009
demonstrated that juvenile exposures (PND 20–34) resulted in exaggerated Iba1+
morphological changes and microglial NOS2 expression, altered dopamine content, and associated evidence of nitrosative stress in the basal ganglia, as compared to adult exposures (weeks 12–20). Interestingly, mice pre-exposed as juveniles that also received adult Mn did not exhibit morphological microglial activation and had elevated striatal nitrotyrosine adducts, in contrast to those without pre-exposure (Moreno et al., 2009
). This finding reiterates the potential for microglial priming during development as a modifying factor for the subsequent response of these cells to insult. The late postnatal period also appears to represent a sensitive window for modification of microglial function following toluene exposure, as analyses at PND 21 following exposures from either GD 14–18, PND 2–6, or PND 8–12 reveal enhanced Iba1 content and pro-inflammatory cytokine signaling (e.g., NF-κB, TLR4, TNFα) in the PND 8–12 group (Tin-Tin Win-Shwe et al., 2011
). Similarly, changes in microglia following chemical-induced excitotoxicity at PND 7 was examined by Drouin-Oullet et al. (2011) and appeared to involve transient increases in microglial activation markers (Iba1, PK11195, and ED-1) and the cytokine, IL-1β. The long-term consequences were examined at PND 56, which revealed amphetamine-induced hyperactivity as well as deficits in spatial learning and social interactions, in parallel with elevated microglial mGluR5 expression. Although all of these changes were inhibited by minocycline, the potential contribution of infiltrating cells following the intra-hippocampal injections or direct effects of minocycline on the injured neurons were not evaluated. Overall, the existing data suggests a critical regulatory role for microglia in brain development that is much expanded from initial considerations of microglia in the context of their standard, immune-mediated responses.