Placental mediated mechanisms of perinatal brain injury

The establishment and maintenance of pregnancy relies on delicate homeostasis. Indeed, physiologic equilibrium throughout the maternal-placental-fetal axis is a critical determinant of health for the pregnant woman and developing fetus, including maturation of the central nervous system (CNS). Of critical importance is placental homeostasis, encompassing stability, function, and a precise balance between pro- and anti-inflammatory factors. While the successful delivery of an infant requires multiple signaling pathways, cells, and molecules central to inflammation, the expression, timing, and location of these factors are key. When homeostatic regulation of inflammatory cells and signal transduction is perturbed, pathologies common in pregnancy emerge, including preterm birth and intrauterine growth restriction. Numerous clinical and preclinical studies support the role that the placental-fetal brain axis plays in neurological development, with alterations or disruptions in this axis leading to brain injury.(Dammann and Leviton, 1997; Dammann and Leviton, 2014; Fant et al., 2014) Notably, chorioamnionitis, hypoxia-ischemia, placental insufficiency, prenatal exposure to opioids and alcohol, and certain genetic conditions are linked, as described throughout this Special Issue, through catalyzing pathophysiology defined by inflammation and oxidative injury in the placenta.(Dudink et al., 2022; Ruyak et al., 2022; Ruffaner-Hanson et al., 2022; Gutherz et al., 2022; Zucker and Burd, 2022) The resultant effect is stress throughout the maternal-placental-fetal axis and a toxic microenvironment for CNS development. Significantly, the common denominator of pro-inflammatory signal transduction and activation of toxic networks throughout the maternal-placental-fetal-brain axis, links many neurodevelopmental disorders and acquired brain injuries to acute or chronic in utero events, and widespread immune and placental dysfunction.(Muthukumar et al., 2022; Chin et al., 2022; Reiss et al., 2022; Redline, 2022; Tarui et al., 2022)

The health of the placenta and the subsequent relationship to the developing brain and maturing neural-immune system, provides clues to CNS health and function throughout the lifespan. Changes in the uterine microenvironment and subsequent perinatal events also trigger injury to the developing immune system. The abnormal development of the immune system in turn impacts CNS development, and precipitates perinatal brain injury (PBI) and increasing susceptibility to chronic neural-immune sequelae later in life.(Gluckman et al., 2008; O’Shea et al., 2009) Molecular and cellular inflammatory mediators, including cytokines, chemokines, resident glial cells, and leukocytes, play essential roles in the pathophysiology of PBI. Perinatal brain injury is defined broadly and diversely secondary to hypoxic-ischemic encephalopathy, cerebral palsy, fetal alcohol syndrome, posthemorrhagic hydrocephalus, encephalopathy of prematurity, and prenatal opioid exposure as discussed throughout this Issue. Certainly, this list is not all encompassing. PBI reflects a multitude of initiating insults and diagnoses culminating in neural cell injury, deficits in local, anatomical, and functional cerebral connectivity, and pathological hallmarks of CNS injury at molecular, cellular, biochemical, structural, and ultrastructural levels. Systemic inflammation and neuroinflammation each contribute uniquely to the pathophysiology of PBI and subsequent elaboration of neurodevelopmental impairment, including cerebral palsy, sensory impairment, and deficits of socialization, cognition, and executive function. Indeed, the adverse effects of an abnormal intrauterine environment on the developing immune system and CNS extend far beyond the immediate symptoms revealed as pregnancy complications and during the first weeks of newborn life. Accordingly, as highlighted here by the authors in this Special Issue, there is an urgent need to define the full spectrum of adverse maternal, fetal, infant and childhood outcomes associated with placental abnormalities and inflammation. Their data stress the cumulative importance of inflammation itself in homeostatic regulation of the placental microenvironment and subsequent risk for PBI.

The importance of inflammatory pathophysiology in the development of PBI can be understood using chorioamnionitis as platform for study. The role of inflammatory cytokines in impaired white matter development is well defined in children with cerebral palsy and cognitive delay.(Hoon Jr. et al., 2009) Histologically, chorioamnionitis is sub-classified by the amount and distribution of inflammatory infiltrate within the chorion, amnion and umbilical compartments.(Redline et al., 2002; Redline, 2012) Concomitant inflammatory infiltrate affects placental circulation, and the degree of placental hypoxia-ischemia. (Redline, 2009) Stressed placental cells release Danger Associated Molecular Signals (DAMPS) that activate toll-like receptors (TLRs). This signaling induces cytokine production.(Kim et al., 2015; Nadeau-Vallée et al., 2016; Galinsky et al., 2013) These cytokines precipitate the expression of chemokines that recruit neutrophils, monocytes, and macrophages to propagate intrauterine inflammation.(Kim et al., 2015; Paton et al., 2017; Cappelletti et al., 2020) Cumulatively, this process affects the entire maternal-placental-fetal axis, increases inflammatory cellular and molecular trafficking to the developing brain and mediates neural-immune injury.

A significant connection between the placenta and the developing brain is the fetal circulation. Depending on its severity, placental inflammation can pass through the umbilical cord and bloodstream to catalyze a fetal inflammatory response syndrome (FIRS). After delivery, this can develop into systemic inflammatory response syndrome (SIRS) in the neonate.(Andrews et al., 2006; Gotsch et al., 2007) FIRS and SIRS are both defined by elevated levels of pro-inflammatory cytokines and activation of immune cells that alter neuro-immune crosstalk and increase the risk of later neurodevelopmental disorders.(Yoon et al., 1996; Kaukola et al., 2006) Specifically, increased circulating inflammatory cytokines cross the developing blood-brain barrier and trigger neuroinflammatory responses through activation of microglia and astrocytes.(McAdams and Juul, 2012; Schmidt et al., 2016) The accumulation, infiltration and function of T cells, neutrophils, monocytes, and infiltrating macrophages follow and mirror this evolving inflammatory state, transforming the quality with diversification of the cellular and molecular milieu.

Emerging evidence suggests systemic inflammation initiated by intrauterine insults typically fails to completely self-resolve.(Lin et al., 2010; Taher et al., 2021; Zareen et al., 2021) Improved understanding of immune memory and priming through the placental-fetal-brain axis is required. New knowledge is also needed regarding communication between resident glial cells and peripheral inflammatory cells throughout pregnancy and in the early postnatal period.(Sharma et al., 2022) Investigations utilizing multi-parameter and multi-dimensional flow cytometry to interrogate T cell, monocytes and macrophages, concomitant with their metabolism and function long-term, and improved understanding of their compartment specific developmental trajectory is required. Similarly, evolution to include novel microRNAs and extracellular vesicles as central players in inflammatory networks beyond cytokines may prove useful. Within this framework, sterile inflammation, and inflammation secondary to infections are equally important to the pathophysiology of brain injury commencing in utero. The role of inflammation in the pathophysiology of PBI secondary to prenatal alcohol and opioid exposure is an essential future research direction, especially as related to receptor mediated- and receptor independent inflammatory signal transduction catalyzed by different opioids and alcohol metabolites. Assessment of the preclinical literature reveals a dearth of knowledge on the direct effects of opioids on developing immune system and simultaneously maturing neural circuitry. Opioids affect placental integrity and function leading to reduced nutrient and oxygen delivery to the fetus.(Almario et al., 2009; Martin et al., 2006; Serra et al., 2017) The presence of microenvironmental toxins, such as LPS and opioids, are recognized in the fetomaternal compartment by TLRs.(Chin et al., 2016) TLRs, including TLR4 activation, facilitate the production of numerous proinflammatory cytokines and chemokines, but the role of modulation of these pathways in protection from neonatal opioid withdrawal syndrome (NOWS) or the white matter injury commonly observed after opioid exposure, is unknown. It is well established, however, that risk of brain injury and immune dysfunction is increased in infants who have persistent and/or recurrent elevations of pro-inflammatory proteins.(Kuban et al., 2015) Specifically, the increasing breadth of early neonatal inflammation, indexed by the number of protein elevations or the number of functional protein classes elevated, is associated with increased structural and functional brain injury.(Kuban et al., 2015; Leviton et al., 2011a; Leviton et al., 2011b; O’Shea et al., 2012; Dammann et al., 2016; Leviton et al., 2016)

As identified by the Authors and Experts featured in this Special Issue, a broader and more encompassing, understanding of the placental-mediated mechanisms of PBI is required to fully move the diagnosis and treatment of infants and children with brain injury forward.(Dudink et al., 2022; Zucker and Burd, 2022; Reiss et al., 2022; Redline, 2022; Sharma et al., 2022) There is an immense gap in knowledge regarding the various molecular (i.e., ATP receptors), cellular (i.e., microglia) and extracellular (i.e. exosomes) pathophysiological actors of in utero inflammation and how the subsequent dysregulation of the fetal immune system impairs neural-immune maturation, causes aberrant anatomical connectivity, and fragments neural networks.(Gall et al., 2022) While these studies take the first step in identifying how placental pathology, whether driven through exosomes, leukocytes, or broader hypothalamic-pituitary-adrenal axis dysfunction, determines brain structure and function in adulthood, they also shed light on the type of targeted interventions that may be required to improve long-term outcomes in countless of children born preterm, with cerebral palsy, and/or exposed to alcohol and/or opioids in utero. Specifically, the authors here draw attention to precision medicine and how it is needed to guide the treatment of injury.(Gutherz et al., 2022; Zucker and Burd, 2022; Muthukumar et al., 2022; Redline, 2022; Gall et al., 2022; Andescavage and Limperopoulos, 2022) Defined biomarkers in neonates could provide estimation of extent of immune system abnormalities and CNS injury, and provide pharmacodynamic support to guide timing, duration, or degree of emerging treatments and therapeutic approaches.

With respect to brain injury, minimizing the impact of insults with substantial placental-fetal-brain axis involvement requires identification of critical pathways underlying the developmental program shared by both the placenta and developing brain. It also relies on the continued development of precision placental, liquid, neuroimaging and biobehavioral biomarkers for diagnosis of dysfunction. This Special Issue highlights many rigorous, quantifiable, and clinically translatable outcome measures of immune system and CNS structure and function. The use of touchscreen technology to assess cognition and executive function is sophisticated and cutting-edge.(Muthukumar et al., 2022) These assays are directly translatable to humans and complement clinical biomarker platforms, lymphocyte stimulation assays, quantitative maps of functional connectivity, connectome lesion mapping, and anatomical connectivity analyses. As described by Kang and colleagues and Andescavage and colleagues, MRI and photoacoustic imaging are promising avenues for non-invasive, individualized approaches. (Andescavage and Limperopoulos, 2022; Kang et al., 2022) Not only could these biomarkers inform diagnosis and future clinical trial design aimed at improving long-term outcomes, but they could also provide estimation of extent of CNS injury around the time of delivery. In this respect, a rigorous, complimentary, and coordinated approach with precision medicine is needed to transform the care of children with brain injury commencing in utero. In this context, emphasis on inflammatory networks, instead of individual cytokines and chemokines in isolation, maybe clinically relevant. Similarly, durable changes in peripheral blood mononuclear cell (PBMC) reactivity may prove effective as a biomarker.(Taher et al., 2021; Zareen et al., 2021; Lin et al., 2010; Huggard et al., 2020; Melo et al., 2021) PBMCs as a biomarker may have high clinical utility given the relative access ease of access to these cells and well-defined stimulation protocols for patients of all ages. Undoubtedly, future study on the metabolism of myeloid cells and therapeutic response may prove useful.

In conclusion, perinatal brain injury is a complex pathophysiological entity encompassing the entirety of the maternal-placental-fetal brain axis. PBI is not a single injury or a fixed injury with static consequences. Rather, it is a constellation of intricate pathophysiological processes that change the trajectory of CNS development starting with, most often, changes in the placenta. More research is needed to establish the molecular signatures of intrauterine inflammation. In this way, studies incorporating spatial RNAseq, snRNAseq, ATACseq, miRNAseq and other transcriptomic evaluations in both the placenta and brain are required, including placental, endothelial, choroid plexus, and cerebral blood vessels. These studies will not only aid in identification of novel immunoregulatory signals, but could also shed important light on vesicle trafficking, cell mobility metrics, and context dependent maps of neural-immune interactions. Such studies will also provide foundational support for investigation of neuroimmunomodulation for neurorepair, and continue to aid in answering the questions: Which infants need to be treated and require neurorepair? What combination of biomarkers are needed for accurate diagnosis and precise prognosis? And finally: Is placental pathology the key to understanding perinatal brain injury and treatment success?