Serum levels of the cold stress hormones FGF21 and GDF-15 after cardiac arrest in infants and children enrolled in single center therapeutic hypothermia clinical trials

Characterization of Circulating Cold Shock Proteins FGF21 and RBM3 in a Multi-Center Study of Pediatric Cardiac Arrest

Fibroblast Growth Factor 21 (FGF21) is a neuroprotective hormone induced by cold exposure that targets the β-klotho co-receptor. β-klotho is abundant in the newborn brain but decreases rapidly with age. RNA-Binding Motif 3 (RBM3) is a potent neuroprotectant upregulated by FGF21 in hypothermic conditions. We characterized serum FGF21 and RBM3 levels in patients enrolled in a prospective multi-center study of pediatric cardiac arrest (CA) via a secondary analysis of samples collected to evaluate brain injury biomarkers. Patients (n = 111) with remnant serum samples available from at least two of three available timepoints (0-24, 24-48 or 48-72 hours post-resuscitation) were included. Serum samples from 20 healthy controls were used for comparison. FGF21 was measured by Luminex and internally validated enzyme-linked immunoassay (ELISA). RBM3 was measured by internally validated ELISA. Of postarrest patients, 98 were managed with normothermia, while 13 were treated with therapeutic hypothermia (TH). FGF21 increased >20-fold in the first 24 hours postarrest versus controls (681 pg/mL [200-1864] vs. 29 pg/mL [15-51], n = 99 vs. 19, respectively, p < 0.0001, median [interquartile range]) with no difference in RBM3. FGF21 did not differ by sex, while RBM3 was increased in females versus males at 48-72 hours postarrest (1866 pg/mL [873-5176] vs. 1045 pg/mL [535-2728], n = 40 vs. 54, respectively, p < 0.05). Patients requiring extracorporeal membrane oxygenation (ECMO) postresuscitation had increased FGF21 versus those who did not at 48-72 hours (6550 pg/mL [1455-66,781] vs. 1213 pg/mL [480-3117], n = 7 vs 74, respectively, p < 0.05). FGF21 and RBM3 did not correlate (Spearman's rho = 0.004, p = 0.97). We conclude that in a multi-center study of pediatric CA patients where normothermic targeted temperature management was largely used, FGF21 was markedly increased postarrest versus control and highest in patients requiring ECMO postresuscitation. RBM3 was sex-dependent. We provide a framework for future studies examining the effect of TH on FGF21 or use of FGF21 therapy after pediatric CA.

FGF21 modulates hippocampal cold-shock proteins and CA2-subregion proteins in neonatal mice with hypoxia-ischemia

BACKGROUND

Fibroblast growth factor 21 (FGF21) is a neuroprotectant with cognitive enhancing effects but with poorly characterized mechanism(s) of action, particularly in females. Prior studies suggest that FGF21 may regulate cold-shock proteins (CSPs) and CA2-marker proteins in the hippocampus but empirical evidence is lacking.

METHODS

We assessed in normothermic postnatal day (PND) 10 female mice, if hypoxic-ischemic (HI) brain injury (25 min 8% O2/92% N2) altered endogenous levels of FGF21 in serum or in the hippocampus, or its receptor β-klotho. We also tested if systemic administration of FGF21 (1.5 mg/kg) modulated hippocampal CSPs or CA2 proteins. Finally, we measured if FGF21 therapy altered markers of acute hippocampal injury.

RESULTS

HI increased endogenous serum FGF21 (24 h), hippocampal tissue FGF21 (4d), and decreased hippocampal β-klotho levels (4d). Exogenous FGF21 therapy modulated hippocampal CSP levels, and dynamically altered hippocampal CA2 marker expression (24 h and 4d). Finally, FGF21 ameliorated neuronal damage markers at 24 h but did not affect GFAP (astrogliosis) or Iba1 (microgliosis) levels at 4d.

CONCLUSIONS

FGF21 therapy modulates CSP and CA2 protein levels in the injured hippocampus. These proteins serve different biological functions, but our findings suggest that FGF21 administration modulates them in a homeostatic manner after HI.

IMPACT

Hypoxic-ischemic (HI) injury in female post-natal day (PND) 10 mice decreases hippocampal RNA binding motif 3 (RBM3) levels in the normothermic newborn brain. HI injury in normothermic newborn female mice alters serum and hippocampal fibroblast growth factor 21 (FGF21) levels 24 h post-injury. HI injury in normothermic newborn female mice alters hippocampal levels of N-terminal EF-hand calcium binding protein 2 (NECAB2) in a time-dependent manner. Exogenous FGF21 therapy ameliorates the HI-mediated loss of hippocampal cold-induced RNA-binding protein (CIRBP). Exogenous FGF21 therapy modulates hippocampal levels of CA2-marker proteins after HI.