Entry into CNS is tightly

Entry into CNS is tightly regulated by highly dynamic barriers, namely, the BBB, the blood-CSF barrier, and the arachnoid epithelium (Abbott et al., 2010; Yasuda et al., 2013). Similar barriers also exist within the CNS including the ependymal barrier as well as the cellular membrane barriers of the neurons and glia (Cavanagh and Warren, 1985; Lee et al., 2001; Whish et al., 2015). In addition to the physical and cellular structures, the prominent feature of all these barriers are numerous efflux and uptake transporters regulating the bidirectional movement of mediators of physiologic and pathologic processes as well as drugs within the CNS, and between the CNS and the periphery. The significance of transporters on these barriers in health, disease, and pharmacotherapy of the CNS is well-established; years of study have been dedicated to understanding and characterizing the expression, distribution, localization, regulation and nature of substrates of transporters in the CNS. These have been reviewed extensively in both health (Lee et al., 2001; Kusuhara and Sugiyama, 2005; Loscher and Potschka, 2005b; Neuwelt et al., 2011; Chan et al., 2013; Geier et al., 2013; Qosa et al., 2015; Stieger and Gao, 2015; Mahringer and Fricker, 2016) and CNS disease including epilepsy, stroke and ischemic injury, ionophore tumors and multiple sclerosis, as well as chronic neurodegenerative diseases such as Alzheimer's Disease, amyotrophic lateral sclerosis, and Parkinson's Disease (Ronaldson, 2014; Henderson and Piquette-Miller, 2015; Qosa et al., 2015).
Membrane transporters in TBI A summary of membrane transporters investigated for their role in TBI is presented in Table 1 and Table 2. There are three ways transporters may impact outcomes in TBI. First, transporters play a key role in the clearance of endogenous biological mediators following TBI. Second, genetic association studies suggest transporters may be important in the transition of TBI from acute brain injury to a chronic neurological disease. Finally, transporters dynamically control the brain penetration of many drugs and their distribution within the brain, contributing to pharmacoresistance and possibly in some cases pharmacosensitivity. Understanding the nature of drugs or candidate drugs in development for TBI with respect to whether they are a transporter substrate is relevant to understanding whether they distribute to therapeutic targets in sufficient concentrations. Evidence supporting the role of particular membrane transporters in each of these processes is presented below.
Specific considerations in pediatrics: the ontogeny of membrane transporters in the brain A unified pattern of transporter expression with age does not exist. Brouwer and colleagues reviewed transporter ontogeny in highly metabolic organs including the intestines, liver, and kidneys. They found that among the most prominently expressed transporters, expression patterns and ontogeny vary across organs and time with no consistent pattern (Brouwer et al., 2015). Readers are directed to the review by Strazielle and colleagues for a broader discussion of brain transporter ontogeny (Strazielle and Ghersi-Egea, 2015). Daood and colleagues studied the protein expression and localization of prominent ABC transporters (ABCB1, ABCG2, and ABCC1) in human embryos at 22 weeks gestational age, through newborns and adults. They found expression of ABCG2 and ABCB1 in capillary endothelial cells of the BBB, and ABCC1 in the choroid plexus and Purkinje cells of the cerebellum. They also reported that ABCB1 expression increases throughout gestation, and that ABCG2 and ABCC1 expression was stable postnatally (Daood et al., 2008). Multiple rat studies have shown Abcb1 mRNA and protein expression in brain tissue and microvessels as early as postnatal day (PND) 2, with increasing expression with age (Harati et al., 2013; Soares et al., 2016; Adams et al., 2018b). Harati and colleagues found that Abcg2 mRNA and protein increase early in rat development with stabilization by PND 21 (Harati et al., 2013). ABCC1 expression was also not found at any age on human cerebral microvessel endothelial cells (Daood et al., 2008), but Abcc1 mRNA is expressed at consistent levels throughout the brain and brain microvessel endothelium in rats at all ages (Soares et al., 2016; Adams et al., 2018b), and ABCC1 appears to be upregulated on brain endothelium after TBI in humans (Willyerd et al., 2016). Studies in rats have shown particular complex patterns of expression in SLC transporters that are specific to tissue (e.g. hippocampus, cortex) and the relevant barrier (e.g. blood-CSF vs. BBB) (Harati et al., 2013; Strazielle and Ghersi-Egea, 2015; Soares et al., 2016; Adams et al., 2018b). Much remains to be learned related to developmental aspects of transporter maturation and the impact of TBI on transporter expression and function. We are exploring this using a pediatric TBI model, CCI in PND 17 rats. We found a common pattern of decreased transporter mRNA expression in the first 72 h following injury, with some transporters showing a delayed increase in expression following a spike in expression of inflammatory cytokines and transcription factors including Il-6 and Nfe2l2 (Adams et al., 2018b). Interestingly, a reduction in Abcb1 is observed in brains from adult rats that underwent CCI two months earlier at PND 17 (Pop et al., 2013), a time of development similar to a two-year old human in terms of brain vulnerability and synaptogenesis (Harris et al., 1992; Rice and Barone Jr., 2000). A better understanding of age-dependent differences in baseline transporter expression and function, and age-dependent differences in the response to TBI, appear necessary to better estimate changes in brain exposure to xenobiotics and endogenous substrates important in the pathophysiology and treatment of pediatric TBI.