Infants Uniquely Express High Levels of RBM3 and Other Cold-Adaptive Neuroprotectant Proteins in the Human Brain

RBM3 Promotes Anti-inflammatory Responses in Microglia and Serves as a Neuroprotective Target of Ischemic Stroke

Ischemic stroke is a major cause of death and disability in adults. Hypothermic treatment is successful in treating neonatal cerebral ischemia, but its application is restricted in adult patients due to complex management strategies and severe adverse effects. Two homologous RNA-binding proteins, RBM3 and CIRP, are the only known cold-inducible proteins in vertebrates, and their expression levels are robustly elevated by mild to moderate hypothermia. In previous studies, we and others have demonstrated that both RBM3 and CIRP mediate the neuroprotective and neurogenic effects of hypothermia in cell and animal models. However, CIRP can also be detrimental to neurons by triggering neuroinflammatory responses, complicating its post-stroke functions. In this study, we compared the properties of the two cold-inducible RNA-binding proteins after ischemic stroke. Our results indicated that RBM3 expression was stimulated in the ischemic brain of stroke patients, while CIRP expression was not. In an experimental model, RBM3 can ameliorate ischemic-like insult by promoting neuronal survival and eliciting anti-inflammatory responses in activated microglia, while the impact of CIRP was intriguing. Collectively, our data supported the notion that RBM3 may be a more promising therapeutic target than CIRP for treating ischemic stroke. We further demonstrated that zr17-2, a small molecule initially identified to target CIRP, can specifically target RBM3 but not CIRP in microglia. zr17-2 demonstrated anti-inflammatory and neuroprotective effects after ischemic stroke both in vitro and in vivo, suggesting its potential therapeutic value.

Retarded astrogliogenesis in response to hypoxia is facilitated by downregulation of CIRBP

The adverse impacts of chronic hypoxia on maternal and infant health at high altitudes warrant significant attention. However, effective protective measures against the resultant growth restrictions and neurodevelopmental disorders in infants and young children are still lacking. This study investigated the neurodevelopment of mice offspring under hypoxic conditions by exposing pregnant mice to a hypobaric oxygen chamber that simulated the hypobaric hypoxia at an altitude of 4000 m until 28 days after delivery. Our findings suggested that prolonged exposure to hypoxia might result in emotional abnormalities and social disorders in offspring. The significant reduction in astrogliogenesis was a characteristic feature associated with neurodevelopmental disorders induced by hypoxia. Further studies demonstrated that cold-induced RNA-binding protein (CIRBP) was a key transcriptional regulator in astrogliogenesis, which downregulated astrocytic differentiation under hypoxia through its crosstalk with the NFIA. Our study emphasized the crucial role of CIRBP in regulating astrogliogenesis and highlighted its potential as a promising target for therapeutic interventions in neurodevelopmental disorders associated with hypoxia.

RBM3 Ameliorates Acute Brain Injury-induced Inflammation and Oxidative Stress by Stabilizing GAS6 mRNA Through Nrf2 Signaling Pathway

RNA-binding motif protein 3 (RBM3), as a cold-inducible protein, exhibits neuroprotective function in brain disorders. This study was conducted to investigate the effects of RBM3 on acute brain injury (ABI) and its underlying mechanism. The cerebral injury (CI) rat model and oxygen-glucose deprivation (OGD) cell model were established. The neurological severity score, wire-grip score, morris water maze test, and Y-maze test were used to detect the neurological damage, vestibular motor, learning, and memory functions. Cerebral injury, apoptosis, oxidative stress, and inflammatory level were evaluated by hematoxylin-eosin and TUNEL staining and specific kits. Flow cytometry was used to analyze the apoptosis rate. The relationship between RBM3 and growth arrest specific (GAS) 6 was analyzed by RNA immunoprecipitation assay. The results indicated that RBM3 recovered of neurological function and behaviour impairment of CI rats. Additionally, RBM3 reversed the increased oxidative stress, inflammatory level, and apoptosis induced by CI and OGD. RBM3 interacted with GAS6 to activate the Nrf2 signaling pathway, thus playing neuroprotection on ABI. Besides, the results of RBM3 treatment were similar to those of mild hypothermia treatment. In summary, RBM3 exerted neuroprotection and ameliorated inflammatory levels and oxidative stress by stabilizing GAS6 mRNA through the Nrf2 signaling pathway, suggesting that RBM3 might be a potential therapeutic candidate for treating ABI.

Cirbp suppression compromises DHODH-mediated ferroptosis defense and attenuates hypothermic cardioprotection in an aged donor transplantation model

Hypothermia is commonly used to protect donor hearts during transplantation. However, patients transplanted with aged donor hearts still have severe myocardial injury and decreased survival rates, but the underlying mechanism remains unknown. Because aged hearts are not considered suitable for donation, the number of patients awaiting heart transplants is increasing. In this study, we examined whether hypothermic cardioprotection was attenuated in aged donor hearts during transplantation and evaluated potential therapeutic targets. Using a rat heart transplantation model, we found that hypothermic cardioprotection was impaired in aged donor hearts but preserved in young donor hearts. RNA-Seq showed that cold-inducible RNA-binding protein (Cirbp) expression was decreased in aged donor hearts, and these hearts showed severe ferroptosis after transplantation. The young donor hearts from Cirbp-KO rats exhibited attenuated hypothermic cardioprotection, but Cirbp overexpression in aged donor hearts ameliorated hypothermic cardioprotection. Cardiac proteomes revealed that dihydroorotate dehydrogenase (DHODH) expression was significantly decreased in Cirbp-KO donor hearts during transplantation. Consequently, DHODH-mediated ubiquinone reduction was compromised, thereby exacerbating cardiac lipid peroxidation and triggering ferroptosis after transplantation. A cardioplegic solution supplemented with CIRBP agonists improved hypothermic cardioprotection in aged donor hearts, indicating that this method has the potential to broaden the indications for using aged donor hearts in transplantation.

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.

Hypothermia increases cold-inducible protein expression and improves cerebellar-dependent learning after hypoxia ischemia in the neonatal rat

BACKGROUND

Hypoxic ischemic encephalopathy remains a significant cause of developmental disability.1,2 The standard of care for term infants is hypothermia, which has multifactorial effects.3-5 Therapeutic hypothermia upregulates the cold-inducible protein RNA binding motif 3 (RBM3) that is highly expressed in developing and proliferative regions of the brain.6,7 The neuroprotective effects of RBM3 in adults are mediated by its ability to promote the translation of mRNAs such as reticulon 3 (RTN3).8 METHODS: Hypoxia ischemia or control procedure was conducted in Sprague Dawley rat pups on postnatal day 10 (PND10). Pups were immediately assigned to normothermia or hypothermia at the end of the hypoxia. In adulthood, cerebellum-dependent learning was tested using the conditioned eyeblink reflex. The volume of the cerebellum and the magnitude of cerebral injury were measured. A second study quantified RBM3 and RTN3 protein levels in the cerebellum and hippocampus collected during hypothermia.

RESULTS

Hypothermia reduced cerebral tissue loss and protected cerebellar volume. Hypothermia also improved learning of the conditioned eyeblink response. RBM3 and RTN3 protein expression were increased in the cerebellum and hippocampus of rat pups subjected to hypothermia on PND10.

CONCLUSIONS

Hypothermia was neuroprotective in male and female pups and reversed subtle changes in the cerebellum after hypoxic ischemic.

IMPACT

Hypoxic ischemic produced tissue loss and a learning deficit in the cerebellum. Hypothermia reversed both the tissue loss and learning deficit. Hypothermia increased cold-responsive protein expression in the cerebellum and hippocampus. Our results confirm cerebellar volume loss contralateral to the carotid artery ligation and injured cerebral hemisphere, suggesting crossed-cerebellar diaschisis in this model. Understanding the endogenous response to hypothermia might improve adjuvant interventions and expand the clinical utility of this intervention.

The Impact of Mmu17 Non-Hsa21 Orthologous Genes in the Ts65Dn Mouse Model of Down Syndrome: The Gold Standard Refuted

BACKGROUND

Despite successful preclinical treatment studies to improve neurocognition in the Ts65Dn mouse model of Down syndrome, translation to humans has failed. This raises questions about the appropriateness of the Ts65Dn mouse as the gold standard. We used the novel Ts66Yah mouse that carries an extra chromosome and the identical segmental Mmu16 trisomy as Ts65Dn without the Mmu17 non-Hsa21 orthologous region.

METHODS

Forebrains from embryonic day 18.5 Ts66Yah and Ts65Dn mice, along with euploid littermate controls, were used for gene expression and pathway analyses. Behavioral experiments were performed in neonatal and adult mice. Because male Ts66Yah mice are fertile, parent-of-origin transmission of the extra chromosome was studied.

RESULTS

Forty-five protein-coding genes mapped to the Ts65Dn Mmu17 non-Hsa21 orthologous region; 71%-82% are expressed during forebrain development. Several of these genes are uniquely overexpressed in Ts65Dn embryonic forebrain, producing major differences in dysregulated genes and pathways. Despite these differences, the primary Mmu16 trisomic effects were highly conserved in both models, resulting in commonly dysregulated disomic genes and pathways. Delays in motor development, communication, and olfactory spatial memory were present in Ts66Yah but more pronounced in Ts65Dn neonates. Adult Ts66Yah mice showed milder working memory deficits and sex-specific effects in exploratory behavior and spatial hippocampal memory, while long-term memory was preserved.

CONCLUSIONS

Our findings suggest that triplication of the non-Hsa21 orthologous Mmu17 genes significantly contributes to the phenotype of the Ts65Dn mouse and may explain why preclinical trials that used this model have unsuccessfully translated to human therapies.

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

OBJECTIVE

Fibroblast Growth Factor 21 (FGF21) and Growth Differentiation Factor-15 (GDF-15) are putative neuroprotective cold stress hormones (CSHs) provoked by cold exposure that may be age-dependent. We sought to characterize serum FGF21 and GDF-15 levels in pediatric cardiac arrest (CA) patients and their association with use of therapeutic hypothermia (TH).

METHODS

Secondary analysis of serum samples from clinical trials. We measured FGF21 and GDF-15 levels in pediatric patients post-CA and compared levels to both pediatric intensive care (PICU) and healthy controls. Post-CA, we compared normothermia (NT) vs TH (33 °C for 72 h) treated cohorts at < 24 h, 24 h, 48 h, 72 h, and examined the change in CSHs over 72 h. We also assessed association between hospital mortality and initial levels.

RESULTS

We assessed 144 samples from 68 patients (27 CA [14 TH, 13 NT], 9 PICU and 32 healthy controls). Median initial FGF21 levels were higher post-CA vs. healthy controls (392 vs. 40 pg/mL, respectively, P < 0.001). Median GDF-15 levels were higher post-CA vs. healthy controls (7,089 vs. 396 pg/mL, respectively, P < 0.001). In the CA group, the median change in FGF21 from PICU day 1-3 (after 72 h of temperature control), was higher in TH vs. NT (231 vs. -20 pg/mL, respectively, P < 0.05), with no difference in GDF-15 over time. Serum GDF-15 levels were higher in CA patients that died vs. survived (19,450 vs. 5,337 pg/mL, respectively, P < 0.05), whereas serum FGF21 levels were not associated with mortality.

CONCLUSION

Serum levels of FGF21 and GDF-15 increased after pediatric CA, and FGF21 appears to be augmented by TH.

Hypoxia-ischemia-mediated effects on neurodevelopmentally regulated cold-shock proteins in neonatal mice under strict temperature control

BACKGROUND

Neonates have high levels of cold-shock proteins (CSPs) in the normothermic brain for a limited period following birth. Hypoxic-ischemic (HI) insults in term infants produce neonatal encephalopathy (NE), and it remains unclear whether HI-induced pathology alters baseline CSP expression in the normothermic brain.

METHODS

Here we established a version of the Rice-Vannucci model in PND 10 mice that incorporates rigorous temperature control.

RESULTS

Common carotid artery (CCA)-ligation plus 25 min hypoxia (8% O2) in pups with targeted normothermia resulted in classic histopathological changes including increased hippocampal degeneration, astrogliosis, microgliosis, white matter changes, and cell signaling perturbations. Serial assessment of cortical, thalamic, and hippocampal RNA-binding motif 3 (RBM3), cold-inducible RNA binding protein (CIRBP), and reticulon-3 (RTN3) revealed a rapid age-dependent decrease in levels in sham and injured pups. CSPs were minimally affected by HI and the age point of lowest expression (PND 18) coincided with the timing at which heat-generating mechanisms mature in mice.

CONCLUSIONS

The findings suggest the need to determine whether optimized therapeutic hypothermia (depth and duration) can prevent the age-related decline in neuroprotective CSPs like RBM3 in the brain, and improve outcomes during critical phases of secondary injury and recovery after NE.

IMPACT

The rapid decrease in endogenous neuroprotective cold-shock proteins (CSPs) in the normothermic cortex, thalamus, and hippocampus from postnatal day (PND) 11-18, coincides with the timing of thermogenesis maturation in neonatal mice. Hypoxia-ischemia (HI) has a minor impact on the normal age-dependent decline in brain CSP levels in neonates maintained normothermic post-injury. HI robustly disrupts the expected correlation in RNA-binding motif 3 (RBM3) and reticulon-3 (RTN3). The potent neuroprotectant RBM3 is not increased 1-4 days after HI in a mouse model of neonatal encephalopathy (NE) in the term newborn and in which rigorous temperature control prevents the manifestation of endogenous post-insult hypothermia.

RNA Binding Motif 5 Gene Deletion Modulates Cell Signaling in a Sex-Dependent Manner but not Hippocampal Cell Death

RNA-binding motif 5 (RBM5) is a pro-death tumor suppressor gene in cancer cells. It remains to be determined if it is neurotoxic in the brain or rather if it plays a fundamentally different role in the central nervous system (CNS). Brain-specific RBM5 knockout (KO) mice were given a controlled cortical impact (CCI) traumatic brain injury (TBI). Markers of acute cellular damage and repair were measured in hippocampal homogenates 48 h post-CCI. Hippocampal CA1/CA3 cell counts were assessed 7 days post-CCI to determine if early changes in injury markers were associated with histological outcome. No genotype-dependent differences were found in the levels of apoptotic markers (caspase 3, caspase 6, and caspase 9). However, KO females had a paradoxical increase in markers of pro-death calpain activation (145/150-spectrin and breakdown products [SBDP]) and in DNA repair/survival markers. (pH2A.x and pCREB). CCI-injured male KOs had a significant increase in phosphorylated calcium/calmodulin-dependent protein kinase II (pCaMKII). Despite sex/genotype-dependent differences in KOs in the levels of acute cell signaling targets involved in cell death pathways, 7 day hippocampal neuronal survival did not differ from that of wild types (WTs). Similarly, no differences in astrogliosis were observed. Finally, gene analysis revealed increased estrogen receptor α (ERα) levels in the KO hippocampus in females and may suggest a novel mechanism to explain sex-dimorphic effects on cell signaling. In summary, RBM5 inhibition did not affect hippocampal survival after a TBI in vivo but did modify targets involved in neural signal transduction/Ca²⁺ signaling pathways. Findings here support the view that RBM5 may serve a purpose in the CNS that is dissimilar from its traditional pro-death role in cancer.

Associations between RNA-Binding Motif Protein 3, Fibroblast Growth Factor 21, and Clinical Outcome in Patients with Stroke

Background: RNA-binding motif protein 3 (RBM3) is a cold-induced marker of good functional outcome of ischemic stroke that is promising as a protective target. Fibroblast growth factor 21 (FGF21) is an obesity- and temperature-related hormone that upregulates the expression of RBM3, which is beneficial as a recombinant treatment and has been tested under different experimental pathological conditions, including stroke. However, the interaction between RBM3 and FGF21 has not yet been tested for clinical stroke conditions. Methods: In a sample of 66 stroke patients, we analyzed the associations between the FGF21 and RBM3 serum concentrations on admission and at 72 h, body weight, maximum temperature during the first 24 h, and the outcome of patients at 3 months. We also analyzed their association with biomarkers of obesity (adiponectin and leptin) and inflammation (interleukin-6 (IL-6) and interleukin (IL-10)). Results: Higher concentrations of FGF21 on admission and RBM3 at 72 h were associated with good outcomes. Serum FGF21 and RBM3 were directly related to body mass index and inversely related to the maximum temperature during the first 24 h. We found a positive association between the FGF21 concentrations in obese patients with leptin and a negative correlation with adiponectin in non-obese participants. Conclusions: This clinical study demonstrates the association between RBM3 and FGF21 levels and the outcome of stroke patients. Although further investigations are required, these data support the pharmacological induction of RBM3 as a promising protective therapy.

Cirbp-PSD95 axis protects against hypobaric hypoxia-induced aberrant morphology of hippocampal dendritic spines and cognitive deficits

Hypobaric hypoxia (HH) is a typical characteristic of high altitude environment and causes a spectrum of pathophysiological effects, including headaches, gliovascular dysfunction and cognitive retardation. Here, we sought to understand the mechanisms underlying cognitive deficits under HH exposure. Our results showed that hypobaric hypoxia exposure impaired cognitive function and suppressed dendritic spine density accompanied with increased neck length in both basal and apical hippocampal CA1 region neurons in mice. The expression of PSD95, a vital synaptic scaffolding molecule, is down-regulated by hypobaric hypoxia exposure and post-transcriptionally regulated by cold-inducible RNA-binding protein (Cirbp) through 3′-UTR region binding. PSD95 expressing alleviates hypoxia-induced dendritic spine morphology changes of hippocampal neurons and memory deterioration. Moreover, overexpressed Cirbp in hippocampus rescues HH-induced abnormal expression of PSD95 and attenuates hypoxia-induced dendritic spine injury and cognitive retardation. Thus, our findings reveal a novel mechanism that Cirbp-PSD-95 axis appears to play an essential role in HH-induced cognitive dysfunction in mice.

TrkB signaling regulates the cold-shock protein RBM3-mediated neuroprotection

Increasing levels of the cold-shock protein, RNA-binding motif 3 (RBM3), either through cooling or by ectopic over-expression, prevents synapse and neuronal loss in mouse models of neurodegeneration. To exploit this process therapeutically requires an understanding of mechanisms controlling cold-induced RBM3 expression. Here, we show that cooling increases RBM3 through activation of TrkB via PLCγ1 and pCREB signaling. RBM3, in turn, has a hitherto unrecognized negative feedback on TrkB-induced ERK activation through induction of its specific phosphatase, DUSP6. Thus, RBM3 mediates structural plasticity through a distinct, non-canonical activation of TrkB signaling, which is abolished in RBM3-null neurons. Both genetic reduction and pharmacological antagonism of TrkB and its downstream mediators abrogate cooling-induced RBM3 induction and prevent structural plasticity, whereas TrkB inhibition similarly prevents RBM3 induction and the neuroprotective effects of cooling in prion-diseased mice. Conversely, TrkB agonism induces RBM3 without cooling, preventing synapse loss and neurodegeneration. TrkB signaling is, therefore, necessary for the induction of RBM3 and related neuroprotective effects and provides a target by which RBM3-mediated synapse-regenerative therapies in neurodegenerative disorders can be used therapeutically without the need for inducing hypothermia.

Hippocampal and Prefrontal Cortical Brain Tissue Levels of Irisin and GDF15 Receptor Subunits in Children

Cold-stress hormones (CSHs) stimulate thermogenesis and have direct neuroprotective effects on the brain. The obligatory receptor components of two new CSHs (irisin and growth differentiation factor-15 [GDF15]) were recently discovered. Irisin binds integrin-αV/β5 heterodimers while GDF-15 binds to the orphan receptor glial cell-derived neurotrophic factor (GDNF) family receptor α-like (GFRAL). In addition, integrin-αV/β5 was just identified as the key receptor mediating Zika virus infection in the CNS. We measured integrin-αV, integrin-β5, and GFRAL protein levels across 78 high-quality human male/female brain tissues in infants, toddlers, preschoolers, adolescent, and adults-providing the most robust analysis to date on their levels in the human cortex and hippocampus. We report that integrin-αV was detected at all ages in the prefrontal cortex with levels greatest in adults. Integrin-αV was also detected in the hippocampus in all age groups. In contrast, integrin-β5 was detected in cortex and hippocampus largely restricted to infants. Co-expression of integrin-αV/β5 in the human infant hippocampus and cortex suggests the possibility that irisin has a more robust effect on the developing vs. the adult brain and may have implications for Zika virus infection in infants and young children.

Perspectives from the Society for Pediatric Research. Neonatal encephalopathy clinical trials: developing the future

The next phase of clinical trials in neonatal encephalopathy (NE) focuses on hypothermia adjuvant therapies targeting alternative recovery mechanisms during the process of hypoxic brain injury. Identifying infants eligible for neuroprotective therapies begins with the clinical detection of brain injury and classification of severity. Combining a variety of biomarkers (serum, clinical exam, EEG, movement patterns) with innovative clinical trial design and analyses will help target infants with the most appropriate and timely treatments. The timing of magnetic resonance imaging (MRI) and MR spectroscopy after NE both assists in identifying the acute perinatal nature of the injury (days 3-7) and evaluates the full extent and evolution of the injury (days 10-21). Early, intermediate outcome of neuroprotective interventions may be best defined by the 21-day neuroimaging, with recognition that the full neurodevelopmental trajectory is not yet defined. An initial evaluation of each new therapy at this time point may allow higher-throughput selection of promising therapies for more extensive investigation. Functional recovery can be assessed using a trajectory of neurodevelopmental evaluations targeted to a prespecified and mechanistically derived hypothesis of drug action. As precision medicine revolutionizes healthcare, it should also include the redesign of NE clinical trials to allow safe, efficient, and targeted therapeutics. IMPACT: As precision medicine revolutionizes healthcare, it should also include the redesign of NE clinical trials to allow faster development of safe, effective, and targeted therapeutics. This article provides a multidisciplinary perspective on the future of clinical trials in NE; novel trial design; study management and oversight; biostatistical methods; and a combination of serum, imaging, and neurodevelopmental biomarkers can advance the field and improve outcomes for infants affected by NE. Innovative clinical trial designs, new intermediate trial end points, and a trajectory of neurodevelopmental evaluations targeted to a prespecified and mechanistically derived hypothesis of drug action can help address common challenges in NE clinical trials and allow for faster selection and validation of promising therapies for more extensive investigation.

Robust RBM3 and β-klotho expression in developing neurons in the human brain

RNA binding motif 3 (RBM3) is a powerful neuroprotectant that inhibits neurodegenerative cell death in vivo and is a promising therapeutic target in brain ischemia. RBM3 is increased by the hormone fibroblast growth factor 21 (FGF21) in an age- and temperature-dependent manner in rat cortical neurons. FGF21 receptor binding is controlled by the transmembrane protein β-klotho, which is mostly absent in the adult brain. We discovered that RBM3/β-klotho is unexpectedly high in the human infant vs. adult brain (hippocampus/prefrontal cortex). The use of tissue homogenates in that study precluded a comparison of RBM3/β-klotho expression among different CNS cell-types, thus, omitted key evidence (i.e. confirmation of neuronal expression) that would otherwise provide a critical link to support their possible direct neuroprotective effects in humans. This report addresses that knowledge gap. High-quality fixed human hippocampus, cortex, and hypothalamic tissues were acquired from the NIH Neurobiobank (<1 yr (premature born) infants, 1 yr, 4 yr, and 34 yr). Dual labeling of cell-type markers vs. RBM3/β-klotho revealed enriched staining of targets in neurons in the developing brain. Identifying that RBM3/β-klotho is abundant in neurons in the immature brain is fundamentally important to guide protocol design and conceptual frameworks germane to future testing of these neuroprotective pathways in humans.

A New Vision for Therapeutic Hypothermia in the Era of Targeted Temperature Management: A Speculative Synthesis

Three decades of animal studies have reproducibly shown that hypothermia is profoundly cerebroprotective during or after a central nervous system (CNS) insult. The success of hypothermia in preclinical acute brain injury has not only fostered continued interest in research on the classic secondary injury mechanisms that are prevented or blunted by hypothermia but has also sparked a surge of new interest in elucidating beneficial signaling molecules that are increased by cooling. Ironically, while research into cold-induced neuroprotection is enjoying newfound interest in chronic neurodegenerative disease, conversely, the scope of the utility of therapeutic hypothermia (TH) across the field of acute brain injury is somewhat controversial and remains to be fully defined. This has led to the era of Targeted Temperature Management, which emphasizes a wider range of temperatures (33–36°C) showing benefit in acute brain injury. In this comprehensive review, we focus on our current understandings of the novel neuroprotective mechanisms activated by TH, and discuss the critical importance of developmental age germane to its clinical efficacy. We review emerging data on four cold stress hormones and three cold shock proteins that have generated new interest in hypothermia in the field of CNS injury, to create a framework for new frontiers in TH research. We make the case that further elucidation of novel cold responsive pathways might lead to major breakthroughs in the treatment of acute brain injury, chronic neurological diseases, and have broad potential implications for medicines of the distant future, including scenarios such as the prevention of adverse effects of long-duration spaceflight, among others. Finally, we introduce several new phrases that readily summarize the essence of the major concepts outlined by this review—namely, Ultramild Hypothermia, the “Responsivity of Cold Stress Pathways,” and “Hypothermia in a Syringe.”