Inflammation is increasingly recognized as being a critical contributor to both normal development and injury outcome in the immature brain. The focus of this Review is to highlight important differences in innate and adaptive immunity in immature versus adult brain, which support the notion that the consequences of inflammation will be entirely different depending on context and stage of CNS development. Perinatal brain injury can result from neonatal encephalopathy and perinatal arterial ischaemic stroke, usually at term, but also in preterm infants. Inflammation occurs before, during and after brain injury at term, and modulates vulnerability to and development of brain injury. Preterm birth, on the other hand, is often a result of exposure to inflammation at a very early developmental phase, which affects the brain not only during fetal life, but also over a protracted period of postnatal life in a neonatal intensive care setting, influencing critical phases of myelination and cortical plasticity. Neuroinflammation during the perinatal period can increase the risk of neurological and neuropsychiatric disease throughout childhood and adulthood, and is, therefore, of concern to the broader group of physicians who care for these individuals.

Highlight► A unique catalogue of phenotype markers and neuronotoxic effects of polarised primary microglia, as a comparative tool to screen neurotherapies.

Fetal growth restriction (FGR) is a significant complication of pregnancy describing a fetus that does not grow to full potential due to pathological compromise. FGR affects 3–9% of pregnancies in high‐income countries, and is a leading cause of perinatal mortality and morbidity. Placental insufficiency is the principal cause of FGR, resulting in chronic fetal hypoxia. This hypoxia induces a fetal adaptive response of cardiac output redistribution to favour vital organs, including the brain, and is in consequence called brain sparing. Despite this, it is now apparent that brain sparing does not ensure normal brain development in growth‐restricted fetuses. In this review we have brought together available evidence from human and experimental animal studies to describe the complex changes in brain structure and function that occur as a consequence of FGR. In both humans and animals, neurodevelopmental outcomes are influenced by the timing of the onset of FGR, the severity of FGR, and gestational age at delivery. FGR is broadly associated with reduced total brain volume and altered cortical volume and structure, decreased total number of cells and myelination deficits. Brain connectivity is also impaired, evidenced by neuronal migration deficits, reduced dendritic processes, and less efficient networks with decreased long‐range connections. Subsequent to these structural alterations, short‐ and long‐term functional consequences have been described in school children who had FGR, most commonly including problems in motor skills, cognition, memory and neuropsychological dysfunctions.

The relative contributions of apoptosis and necrosis in brain injury have been a matter of much debate. Caspase-3 has been identified as a key protease in the execution of apoptosis, whereas calpains have mainly been implicated in excitotoxic neuronal injury. In a model of unilateral hypoxia-ischemia in 7-day-old rats, caspase-3-like activity increased 16-fold 24 h postinsult, coinciding with cleavage of the caspase-3 proenzyme and endogenous caspase-3 substrates. This activation was significantly decreased by pharmacological calpain inhibition, using CX295, a calpain inhibitor that did not inhibit purified caspase-3 in vitro. Activation of caspase-3 by m-calpain, but not -calpain, was facilitated in a dose-dependent manner in vitro by incubating cytosolic fractions, containing caspase-3 proform, with calpains. This facilitation required the presence of some active caspase-3 and could be abolished by including the specific calpain inhibitor calpastatin. This indicates that initial cleavage of caspase-3 by m-calpain, producing a 29-kDa fragment, facilitates the subsequent cleavage into active forms. This is the first report to our knowledge suggesting a direct link between the early, excitotoxic, calcium-mediated activation of calpain after cerebral hypoxia-ischemia and the subsequent activation of caspase-3, thus representing a tentative pathway of "pathological apoptosis."The relative contributions of necrosis and apoptosis to the injury that develops after cerebral hypoxia-ischemia (HI) 1 has been a matter of much debate (1). Recent studies suggest that cell death after HI is different from developmentally regulated cell death in most cases and cannot appropriately be described as apoptotic (2-5). Nevertheless, HI cell death shares important morphological and biochemical features with apoptotic cell death, such as activation of caspases and nucleosomal DNA fragmentation (6 -17). Caspases, a family of cysteine proteases with an unusual substrate specificity, requiring an aspartate residue in the P1 position, have been identified as key executors of apoptosis (18). Calpains, another family of cysteine proteases, are calcium-activated and are proposed to participate in the turnover of cytoskeletal proteins and regulation of kinases, transcription factors, and receptors (19,20). Calpains have mainly been implicated in excitotoxic neuronal injury and necrosis (21-23). Pharmacological inhibitors of calpains and caspases exert cerebroprotective effects (9, 14 -16, 24 -26). A growing body of literature has emerged, demonstrating functional connections between calpains and caspases (27). Common substrate proteins have been identified, such as fodrin (28 -31), calpastatin (32, 33), actin (34), PARP (35), and tau (36). There are reports demonstrating calpain-mediated cleavage of caspase-3 (35, 37) and caspase-7 (38, 39) as well as caspase-8 and -9 (39). Furthermore, the proapoptotic protein Bax was cleaved by calpain during drug-induced apoptosis of HL-60 cells (40), and calpain may be responsible for cleaving the loop r...

Inflammation is increasingly recognized as being of both physiological and pathological importance in the immature brain. The rationale of this review is to present an update on this topic with focus on long-term consequences of inflammation during childhood and in adults. The immature brain can be exposed to inflammation in connection with viral or bacterial infection during pregnancy or as a result of sterile central nervous system (CNS) insults. Through efficient anti-inflammatory and reparative processes, inflammation may resolve without any harmful effects on the brain. Alternatively, inflammation contributes to injury or enhances CNS vulnerability. Acute inflammation can also be shifted to a chronic inflammatory state and/or adversely affect brain development. Hypothetically, microglia are the main immunocompetent cells in the immature CNS, and depending on the stimulus, molecular context, and timing, these cells will acquire various phenotypes, which will be critical regarding the CNS consequences of inflammation. Inflammation has long-term consequences and could speculatively modify the risk of a variety of neurological disorders, including cerebral palsy, autism spectrum disorders, schizophrenia, multiple sclerosis, cognitive impairment, and Parkinson disease. So far, the picture is incomplete, and data mostly experimental. Further studies are required to strengthen the associations in humans and to determine whether novel therapeutic interventions during the perinatal period can influence the occurrence of neurological disease later in life.