Thyroxin regulates BDNF expression to promote survival of injured neurons

Plastic Events of the Vestibular Nucleus: the Initiation of Central Vestibular Compensation

Vestibular compensation is a physiological response of the vestibular organs within the inner ear. This adaptation manifests during consistent exposure to acceleration or deceleration, with the vestibular organs incrementally adjusting to such changes. The molecular underpinnings of vestibular compensation remain to be fully elucidated, yet emerging studies implicate associations with neuroplasticity and signal transduction pathways. Throughout the compensation process, the vestibular sensory neurons maintain signal transmission to the central equilibrium system, facilitating adaptability through alterations in synaptic transmission and neuronal excitability. Notable molecular candidates implicated in this process include variations in ion channels and neurotransmitter profiles, as well as neuronal and synaptic plasticity, metabolic processes, and electrophysiological modifications. This study consolidates the current understanding of the molecular events in vestibular compensation, augments the existing research landscape, and evaluates contemporary therapeutic strategies. Furthermore, this review posits potential avenues for future research that could enhance our comprehension of vestibular compensation mechanisms.

Investigating the Predictive Value of Thyroid Hormone Levels for Stroke Prognosis

Given the expansion of life expectancy, the aging of the population, and the anticipated rise in the number of stroke survivors in Europe with severe neurological consequences in the coming decades, stroke is becoming the most prevalent cause of functional disability. Therefore, the prognosis for a stroke must be timely and precise. Two databases (MEDLINE and Scopus) were searched to identify all relevant studies published between 1 January 2005 and 31 December 2022 that investigated the relationship between thyroid hormone levels and acute stroke severity, mortality, and post-hospital prognosis. Only full-text English-language articles were included. This review includes Thirty articles that were traced and incorporated into the present review. Emerging data regarding the potential predictive value of thyroid hormone levels suggests there may be a correlation between low T3 syndrome, subclinical hypothyroidism, and poor stroke outcome, especially in certain age groups. These findings may prove useful for rehabilitation and therapy planning in clinical practice. Serum thyroid hormone concentration measurement is a non-invasive, relatively harmless, and secure screening test that may be useful for this purpose.

Thyroid Axis and Vestibular Physiopathology: From Animal Model to Pathology

A recent work of our group has shown the significant effects of thyroxine treatment on the restoration of postural balance function in a rodent model of acute peripheral vestibulopathy. Based on these findings, we attempt to shed light in this review on the interaction between the hypothalamic-pituitary-thyroid axis and the vestibular system in normal and pathological situations. Pubmed database and relevant websites were searched from inception through to 4 February 2023. All studies relevant to each subsection of this review have been included. After describing the role of thyroid hormones in the development of the inner ear, we investigated the possible link between the thyroid axis and the vestibular system in normal and pathological conditions. The mechanisms and cellular sites of action of thyroid hormones on animal models of vestibulopathy are postulated and therapeutic options are proposed. In view of their pleiotropic action, thyroid hormones represent a target of choice to promote vestibular compensation at different levels. However, very few studies have investigated the relationship between thyroid hormones and the vestibular system. It seems then important to more extensively investigate the link between the endocrine system and the vestibule in order to better understand the vestibular physiopathology and to find new therapeutic leads.

Pathophysiology and Management Possibilities of Thyroid-Associated Depression

Thyroid hormones and the hypothalamic-pituitary-thyroid (HPA) axis are responsible for multiple metabolic processes and psychological well-being. Hypothyroidism can induce mood changes, depressive symptoms and even major depressive disorder. This review is focused on the pathophysiology and mechanisms through which the low level of thyroid hormones may affect the brain function, causing the characteristic symptoms of depression. Key pathways of hypothyroid-associated depressive states include: morphological changes in some brain areas (mainly in the hippocampus – a reduction in its volume); a significant reduction of the cerebral blood flow (incl. hippocampus), and lowered levels of neurotrophic factors (e. g. BDNF – brain-derived neurotrophic factor), which are regulated by the thyroid hormones. An adequate and timely thyroid hormone replacement and treatment with conventional antidepressants often can reverse the psychological symptoms.

L-Thyroxine Improves Vestibular Compensation in a Rat Model of Acute Peripheral Vestibulopathy: Cellular and Behavioral Aspects

Unilateral vestibular lesions induce a vestibular syndrome, which recovers over time due to vestibular compensation. The therapeutic effect of L-Thyroxine (L-T4) on vestibular compensation was investigated by behavioral testing and immunohistochemical analysis in a rat model of unilateral vestibular neurectomy (UVN). We demonstrated that a short-term L-T4 treatment reduced the vestibular syndrome and significantly promoted vestibular compensation. Thyroid hormone receptors (TRα and TRβ) and type II iodothyronine deiodinase (DIO2) were present in the vestibular nuclei (VN), supporting a local action of L-T4. We confirmed the T4-induced metabolic effects by demonstrating an increase in the number of cytochrome oxidase-labeled neurons in the VN three days after the lesion. L-T4 treatment modulated glial reaction by decreasing both microglia and oligodendrocytes in the deafferented VN three days after UVN and increased cell proliferation. Survival of newly generated cells in the deafferented vestibular nuclei was not affected, but microglial rather than neuronal differentiation was favored by L-T4 treatment.

Distinct Actions of the Thyroid Hormone Transporters Mct8 and Oatp1c1 in Murine Adult Hippocampal Neurogenesis

Inactivating mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) result in Allan-Herndon-Dudley Syndrome, a severe form of psychomotor retardation, while inactivating mutations in another TH transporter, organic anion transporting polypeptide 1c1 (OATP1C1), are linked to juvenile neurodegeneration. These diseases point to essential roles for TH transporters in CNS function. We recently defined the presence of Mct8 in adult hippocampal progenitors and mature granule cell neurons and unraveled cell-autonomous and indirect requirements for Mct8 in adult hippocampal neurogenesis. Here, we investigated whether Oatp1c1 is involved in the hippocampal neurogenic process in concert with Mct8. We detected Oatp1c1 gene expression activity and transcripts in subsets of progenitors, neurons and niche cells in the dentate gyrus. Absence of Oatp1c1 resulted in increased neuroblast and reduced immature neuron numbers in 6-month-old Oatp1c1ko and Mct8/Oatp1c1 double knockout (M/Odko) mice. Reduced EdU-label retention in Mct8ko and M/Odko mice confirmed the impact of Mct8 on neuron formation. In contrast, no significant effect of Oatp1c1 loss on granule cell neuron production and anxiety-like behavior in the open field arena were seen. Together, our results reinforce that distinct actions of each TH transporter are required at multiple stages to ensure proper adult hippocampal neurogenesis.

Thyroid function in the subacute phase of traumatic brain injury: a potential predictor of post-traumatic neurological and functional outcomes

PURPOSE

That thyroid hormones exert pleiotropic effects and have a contributory role in triggering seizures in patients with traumatic brain injury (TBI) can be hypothesized. We aimed at investigating thyroid function tests as prognostic factors of the development of seizures and of functional outcome in TBI.

METHODS

This retrospective study enrolled 243 adult patients with a diagnosis of mild-to-severe TBI, consecutively admitted to our rehabilitation unit for a 6-month neurorehabilitation program. Data on occurrence of seizures, brain imaging, injury characteristics, associated neurosurgical procedures, neurologic and functional assessments, and death during hospitalization were collected at baseline, during the workup and on discharge. Thyroid function tests (serum TSH, fT4, and fT3 levels) were performed upon admission to neurorehabilitation.

RESULTS

Serum fT3 levels were positively associated with an increased risk of late post-traumatic seizures (LPTS) in post-TBI patients independent of age, sex and TBI severity (OR = 1.85, CI 95% 1.22-2.61, p < 0.01). Measured at admission, fT3 values higher than 2.76 pg/mL discriminated patients with late post-traumatic seizures from those without, with a sensitivity of 74.2% and a specificity of 60.9%. Independently from the presence of post-traumatic epilepsy and TBI severity, increasing TSH levels and decreasing fT3 levels were associated with worse neurological and functional outcome, as well as with higher risk of mortality within 6 months from the TBI event.

CONCLUSIONS

Serum fT3 levels assessed in the subacute phase post-TBI are associated with neurological and functional outcome as well as with the risk of seizure occurrence. Further studies are needed to investigate the mechanisms underlying these associations.

In vitro differentiation of human bone marrow stromal cells into neural precursor cells using small molecules

BACKGROUND

Neurogenic differentiation of human marrow stromal stem cells (hMSCs) into neural precursor cells (NPCs) offers new hope in many neurological diseases. Stromal cells can be differentiated into NPCs using small molecules acting as chemical inducers. The aim of this study is to formulate an efficient, direct, fast and safe protocol to differentiate hMSCs into NPCs using different inducers: b-mercaptoethanol (BME), triiodothyronine (T3), and curcumin (CUR). NEW METHOD: hMSCs were subjected to either 1 mM BME, 0.5 µM T3, or 5 µM CUR. Neurogenic differentiation was determined by assessing the protein expression of PAX6, SOX2, DLX2, and GAP-43 with flow cytometry and immunofluorescence, along with Nissl staining of differentiated cells.

RESULTS AND COMPARISON WITH EXISTING METHOD

It was revealed that T3 and CUR are 70-80% better than BME in terms of efficiency and safety, and surprisingly BME was a good promoting factor for cell preconditioning with limited effects on neural trans-differentiation related to its toxic effects on cell viability.

CONCLUSION

Reprogramming of bone marrow stromal cells into neural cells gives hope for treating different neurological disorders. Our study shows that T3 and CUR were effective in generation of NPCs from hMSCs with preservation of cell viability. BME was a good promoting factor for cell preconditioning with limited effects on neural transdifferentiation related to its toxic effects on cell viability.

The critical period: neurochemical and synaptic mechanisms shared by the visual cortex and the brain stem respiratory system

The landmark studies of Wiesel and Hubel in the 1960's initiated a surge of investigations into the critical period of visual cortical development, when abnormal visual experience can alter cortical structures and functions. Most studies focused on the visual cortex, with relatively little attention to subcortical structures. The goal of the present review is to elucidate neurochemical and synaptic mechanisms common to the critical periods of the visual cortex and the brain stem respiratory system in the normal rat. In both regions, the critical period is a time of (i) heightened inhibition; (ii) reduced expression of brain-derived neurotrophic factor (BDNF); and (iii) synaptic imbalance, with heightened inhibition and suppressed excitation. The last two mechanisms are contrary to the conventional premise. Synaptic imbalance renders developing neurons more vulnerable to external stressors. However, the critical period is necessary to enable each system to strengthen its circuitry, adapt to its environment, and transition from immaturity to maturity, when a state of relative synaptic balance is attained. Failure to achieve such a balance leads to neurological disorders.

[Hormonal influence on hearing]

BACKGROUND

Hearing loss leads to impairments in communication, social interactions, and cognitive functions. This renders early treatment particularly important. A causal therapy is not yet available. Human and animal studies have shown that certain hormones can have a positive effect on hearing.

OBJECTIVE

This review provides an overview of the effects of various hormones on hearing and describes the potential benefit for future therapeutic approaches.

MATERIALS AND METHODS

A systematic literature review of reviews dealing with the effects of various hormones on hearing in humans and animals published in PubMed between 2015 and 2020 was conducted.

RESULTS

Hormones may mediate antiapoptotic effects on structure-relevant cells of the cochlea and auditory pathway, and may influence hair cell functionality or the electrolyte balance of the endo- and perilymph. Current research focuses on glucocorticoids; the mineral corticoid aldosterone; the sex hormones estrogen, progesterone, and testosterone; the growth hormones GH (growth hormone) and IGF‑1 (insulin-like growth factor 1); thyroid hormones; and insulin. Study results are still inconsistent at this time, but various hormones appear to represent a possible future treatment option for acute hearing loss. Long-term hormone treatment, which would be necessary particularly in the case of age-related hearing loss, does not currently represent a sensible course of action due to the side effect profile of the systemic treatment/lack of practicable topical application options.

CONCLUSION

The mode of action of hormones is complex. Whether they can be used in the future for individualized treatment of patients with acute hearing impairment requires further investigation.

Thyroid hormone regulation of adult neural stem cell fate: A comparative analysis between rodents and primates

Thyroid hormone (TH) signaling, a highly conserved pathway across vertebrates, is crucial for brain development and function throughout life. In the adult mammalian brain, including that of humans, multipotent neural stem cells (NSCs) proliferate and generate neuronal and glial progenitors. The role of TH has been intensively investigated in the two main neurogenic niches of the adult mouse brain, the subventricular and the subgranular zone. A key finding is that T3, the biologically active form of THs, promotes NSC commitment toward a neuronal fate. In this review, we first discuss the roles of THs in the regulation of adult rodent neurogenesis, as well as how it relates to functional behavior, notably olfaction and cognition. Most research uncovering these roles of TH in adult neurogenesis was conducted in rodents, whose genetic background, brain structure and rate of neurogenesis are considerably different from that of humans. To bridge the phylogenetic gap, we also explore the similarities and divergences of TH-dependent adult neurogenesis in non-human primate models. Lastly, we examine how photoperiodic length changes TH homeostasis, and how that might affect adult neurogenesis in seasonal species to increase fitness. Several aspects by which TH acts on adult NSCs seem to be conserved among mammals, while we only start to uncover the molecular pathways, as well as how other in- and extrinsic factors are intertwined. A multispecies approach delivering more insights in the matter will pave the way for novel NSC-based therapies to combat neurological disorders.

Protective Effects of PGC-1α Activators on Ischemic Stroke in a Rat Model of Photochemically Induced Thrombosis

The pharmacological induction and activation of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), a key regulator of ischemic brain tolerance, is a promising direction in neuroprotective therapy. Pharmacological agents with known abilities to modulate cerebral PGC-1α are scarce. This study focused on the potential PGC-1α-modulating activity of Mexidol (2-ethyl-6-methyl-3-hydroxypyridine succinate) and Semax (ACTH(4-7) analog) in a rat model of photochemical-induced thrombosis (PT) in the prefrontal cortex. Mexidol (100 mg/kg) was administered intraperitoneally, and Semax (25 μg/kg) was administered intranasally, for 7 days each. The expression of PGC-1α and PGC-1α-dependent protein markers of mitochondriogenesis, angiogenesis, and synaptogenesis was measured in the penumbra via immunoblotting at Days 1, 3, 7, and 21 after PT. The nuclear content of PGC-1α was measured immunohistochemically. The suppression of PGC-1α expression was observed in the penumbra from 24 h to 21 days following PT and reflected decreases in both the number of neurons and PGC-1α expression in individual neurons. Administration of Mexidol or Semax was associated with preservation of the neuron number and neuronal expression of PGC-1α, stimulation of the nuclear translocation of PGC-1α, and increased contents of protein markers for PGC-1α activation. This study opens new prospects for the pharmacological modulation of PGC-1α in the ischemic brain.

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl^-), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl^- currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development.

Thyroid Hormones in the Brain and Their Impact in Recovery Mechanisms After Stroke

Thyroid hormones are of fundamental importance for brain development and essential factors to warrant brain functions throughout life. Their actions are mediated by binding to specific intracellular and membranous receptors regulating genomic and non-genomic mechanisms in neurons and populations of glial cells, respectively. Among others, mechanisms include the regulation of neuronal plasticity processes, stimulation of angiogenesis and neurogenesis as well modulating the dynamics of cytoskeletal elements and intracellular transport processes. These mechanisms overlap with those that have been identified to enhance recovery of lost neurological functions during the first weeks and months after ischemic stroke. Stimulation of thyroid hormone signaling in the postischemic brain might be a promising therapeutic strategy to foster endogenous mechanisms of repair. Several studies have pointed to a significant association between thyroid hormones and outcome after stroke. With this review, we will provide an overview on functions of thyroid hormones in the healthy brain and summarize their mechanisms of action in the developing and adult brain. Also, we compile the major thyroid-modulated molecular pathways in the pathophysiology of ischemic stroke that can enhance recovery, highlighting thyroid hormones as a potential target for therapeutic intervention.

T3 level may be a helpful marker to predict disease prognosis of acute central nervous system viral infections

AIM OF THE STUDY

Acute central nervous system viral infections are progressive and inflammatory diseases with inflammatory cells infiltrating into the central nervous system (CNS), and thyroid hormone (TH) level is associated with the oxidative and antioxidant status. Variations in oxidative stress and antioxidant status are related to the pathogenesis of inflammatory diseases. Our study aimed to investigate the possible correlation between viral infections in CNS and TH levels of thyroid stimulating hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3).

MATERIALS AND METHODS

We measured serum concentrations of TSH, fT4, and fT3 in 206 individuals, including 59 viral meningitis (VM) patients, 60 viral encephalitis (VE) patients, and 87 healthy controls.

RESULTS

Our findings showed that VE and VM patients had lower levels of fT3 and higher levels of fT4 compared with healthy controls, whether male or female. Moreover, levels of TSH and fT3 in patients with viral infections in CNS were inversely correlated with disease prognosis measured by the Glasgow Outcome Scale.

CONCLUSIONS

Variations in TH level may represent the oxidative status and are surrogate biomarkers for disease prognosis of acute central nervous system viral infections.

Thyroid hormone and the brain: Mechanisms of action in development and role in protection and promotion of recovery after brain injury

Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5'-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neural death. In clinical studies, central hypothyroidism, due to hypothalamic and pituitary dysfunction, has been found in some individuals after brain injury. TH has been shown, in animal models, to be protective for the damage incurred from brain injury and may have a role to limit injury and promote recovery. Although clinical trials have not yet been reported, findings from in vitro and in vivo models inform potential treatment strategies utilizing TH for protection and promotion of recovery after brain injury.

The Retinol Circulating Complex Releases Hormonal Ligands During Acute Stress Disorders

Intensive care workers actively participate in very hot debates aiming at defining the true metabolic, hormonal and nutritional requirements of critically ill patients, the contributory roles played by thyroid and retinoid ligands being largely underestimated. The present article makes up for redressing the balance on behalf of these last hormonal compounds. The retinol circulating complex is transported in the bloodstream in the form of a trimolecular edifice made up of transthyretin (TTR), retinol-binding protein (RBP) and its retinol ligand. TTR reflects the size of the lean body mass (LBM) and is one of the 3 carrier-proteins of thyroid hormones whereas RBP is the sole conveyor of retinol in human plasma. In acute inflammatory disorders, both TTR and RBP analytes experience abrupt cytokine-induced suppressed hepatic synthesis whose amplitude is dependent on the duration and severity of the inflammatory burden. The steep drop in TTR and RBP plasma values releases thyroxine and retinol ligands in their physiologically active forms, creating free pools estimated to be 10-20 times larger than those described in healthy subjects. The peak endocrine influence is reached on day 4 and the freed ligands undergo instant cellular overconsumption and urinary leakage of unmetabolized fractions. As a result of these transient hyperthyroid and hyperretinoid states, helpful stimulatory and/or inhibitory processes are set in motion, operating as second frontlines fine-tuning the impulses primarily initiated by cytokines. The data explain why preexisting protein malnutrition, as assessed by subnormal LBM and TTR values, impairs the development of appropriate recovery processes in critically ill patients. These findings have survival implications, emphasizing the need for more adapted therapeutic strategies in intensive care units.

Stimulating effect of thyroid hormones in peripheral nerve regeneration: research history and future direction toward clinical therapy

Injury to peripheral nerves is often observed in the clinic and severe injuries may cause loss of motor and sensory functions. Despite extensive investigation, testing various surgical repair techniques and neurotrophic molecules, at present, a satisfactory method to ensuring successful recovery does not exist. For successful molecular therapy in nerve regeneration, it is essential to improve the intrinsic ability of neurons to survive and to increase the speed of axonal outgrowth. Also to induce Schwann cell phenotypical changes to prepare the local environment favorable for axonal regeneration and myelination. Therefore, any molecule that regulates gene expression of both neurons and Schwann cells could play a crucial role in peripheral nerve regeneration. Clinical and experimental studies have reported that thyroid hormones are essential for the normal development and function of the nervous system, so they could be candidates for nervous system regeneration. This review provides an overview of studies devoted to testing the effect of thyroid hormones on peripheral nerve regeneration. Also it emphasizes the importance of combining biodegradable tubes with local administration of triiodothyronine for future clinical therapy of human severe injured nerves. We highlight that the local and single administration of triiodothyronine within biodegradable nerve guide improves significantly the regeneration of severed peripheral nerves, and accelerates functional recovering. This technique provides a serious step towards future clinical application of triiodothyronine in human severe injured nerves. The possible regulatory mechanism by which triiodothyronine stimulates peripheral nerve regeneration is a rapid action on both axotomized neurons and Schwann cells.

Thyroid hormone- and estrogen receptor interactions with natural ligands and endocrine disruptors in the cerebellum

Although the effects of phytoestrogens on brain function is widely unknown, they are often regarded as "natural" and thus as harmless. However, the effects of phytoestrogens or environmental pollutants on brain function is underestimated. Estrogen (17beta-estradiol, E2) and thyroid hormones (THs) play pivotal roles in brain development. In the mature brain, these hormones regulate metabolism on cellular and organismal levels. Thus, E2 and THs do not only regulate the energy metabolism of the entire organism, but simultaneously also regulate important homeostatic parameters of neurons and glia in the CNS. It is, therefore, obvious that the mechanisms through which these hormones exert their effects are pleiotropic and include both intra- and intercellular actions. These hormonal mechanisms are versatile, and the experimental investigation of simultaneous hormone-induced mechanisms is technically challenging. In addition, the normal physiological settings of metabolic parameters depend on a plethora of interactions of the steroid hormones. In this review, we discuss conceptual and experimental aspects of the gonadal and thyroid hormones as they relate to in vitro models of the cerebellum.

Brain Derived Neurotrophic Factor Modification of Epileptiform Burst Discharges in a Temporal Lobe Epilepsy Model

INTRODUCTION

Transforming Growth Factor-Beta 1 (TGF-β1) is a pleiotropic cytokine with potent anti-inflammatory property, which has been considered as an essential risk factor in the inflammatory process of Ischemic Stroke (IS), by involving in the pathophysiological progression of hypertension, atherosclerosis, and lipid metabolisms. -509C/T TGF-β1 gene polymorphism has been found to be associated with the risk of IS. The aim of this meta-analysis was to provide a relatively comprehensive account of the relation between -509C/T gene polymorphisms of TGF-β1 and susceptibility to IS.

METHODS

Male Wistar rats were divided into sham (receiving phosphate buffered saline within dorsal hippocampus), pilocarpine (epileptic model of TLE), single injection BDNF (epileptic rats which received single high dose of BDBF within dorsal hippocampus), and multiple injections BDNF (epileptic rats which received BDNF in days 10, 11, 12, and 13 after induction of TLE) groups. Their electrocorticogram was recorded and amplitude, frequency, and duration of spikes were evaluated.

RESULTS

Amplitude and frequency of epileptiform burst discharges were significantly decreased in animals treated with BDNF compared to pilocarpine group.

CONCLUSION

Our findings suggested that BDNF may modulate the epileptic activity in the animal model of TLE. In addition, it may have therapeutic effect for epilepsy. More studies are necessary to clarify the exact mechanisms of BDNF effects.

The K(+)-Cl(-) Cotransporter KCC2 and Chloride Homeostasis: Potential Therapeutic Target in Acute Central Nervous System Injury

The K(+)-Cl(-) cotransporter-2 (KCC2) is a well-known member of the electroneutral cation-chloride cotransporters with a restricted expression pattern to neurons. This transmembrane protein mediates the efflux of Cl(-) out of neurons and exerts a critical role in inhibitory γ-aminobutyric acidergic (GABAergic) and glycinergic neurotransmission. Moreover, KCC2 participates in the regulation of various physiological processes of neurons, including cell migration, dendritic outgrowth, spine morphology, and dendritic synaptogenesis. It is important to note that down-regulation of KCC2 is associated with the pathogenesis of multiple neurological diseases, which is of particular relevance to acute central nervous system (CNS) injury. In this review, we aim to survey the pathogenic significance of KCC2 down-regulation under the condition of acute CNS injuries. We propose that further elucidation of the molecular mechanisms regarding KCC2 down-regulation after acute CNS injuries is necessary because of potential promising avenues for prevention and treatment of acute CNS injury.

Hydroxysafflor Yellow A Attenuates Neuron Damage by Suppressing the Lipopolysaccharide-Induced TLR4 Pathway in Activated Microglial Cells

Microglia activation initiates a neurological deficit cascade that contributes to substantial neuronal damage and impairment following ischemia stroke. Toll-like receptor 4 (TLR4) has been demonstrated to play a critical role in this cascade. In the current study, we tested the hypothesis that hydroxysafflor yellow A (HSYA), an active ingredient extracted from Flos Carthami tinctorii, alleviated inflammatory damage, and mediated neurotrophic effects in neurons by inducing the TLR4 pathway in microglia. A non-contact Transwell co-culture system comprised microglia and neurons was treated with HSYA followed by a 1 mg/mL lipopolysaccharide (LPS) stimulation. The microglia were activated prior to neuronal apoptosis, which were induced by increasing TLR4 expression in the activated microglia. However, HSYA suppressed TLR4 expression in the activated microglia, resulting in less neuronal damage at the early stage of LPS stimulation. Western blot analysis and immunofluorescence indicated that dose-dependently HSYA down-regulated TLR4-induced downstream effectors myeloid differentiation factor 88 (MyD88), nuclear factor kappa b (NF-κB), and the mitogen-activated protein kinases (MAPK)-regulated proteins c-Jun NH2-terminal protein kinase (JNK), protein kinase (ERK) 1/2 (ERK1/2), p38 MAPK (p38), as well as the LPS-induced inflammatory cytokine release. However, HSYA up-regulated brain-derived neurotrophic factor (BDNF) expression. Our data suggest that HSYA could exert neurotrophic and anti-inflammatory functions in response to LPS stimulation by inhibiting TLR4 pathway-mediated signaling.

Prenatal Hypoxia-Ischemia Induces Abnormalities in CA3 Microstructure, Potassium Chloride Co-Transporter 2 Expression and Inhibitory Tone

Infants who suffer perinatal brain injury, including those with encephalopathy of prematurity, are prone to chronic neurological deficits, including epilepsy, cognitive impairment, and behavioral problems, such as anxiety, inattention, and poor social interaction. These deficits, especially in combination, pose the greatest hindrance to these children becoming independent adults. Cerebral function depends on adequate development of essential inhibitory neural circuits and the appropriate amount of excitation and inhibition at specific stages of maturation. Early neuronal synaptic responses to γ-amino butyric acid (GABA) are initially excitatory. During the early postnatal period, GABA_AR responses switch to inhibitory with the upregulation of potassium-chloride co-transporter KCC2. With extrusion of chloride by KCC2, the Cl⁻ reversal potential shifts and GABA and glycine responses become inhibitory. We hypothesized that prenatal hypoxic–ischemic brain injury chronically impairs the developmental upregulation of KCC2 that is essential for cerebral circuit formation. Following late gestation hypoxia–ischemia (HI), diffusion tensor imaging in juvenile rats shows poor microstructural integrity in the hippocampal CA3 subfield, with reduced fractional anisotropy and elevated radial diffusivity. The loss of microstructure correlates with early reduced KCC2 expression on NeuN-positive pyramidal neurons, and decreased monomeric and oligomeric KCC2 protein expression in the CA3 subfield. Together with decreased inhibitory post-synaptic currents during a critical window of development, we document for the first time that prenatal transient systemic HI in rats impairs hippocampal CA3 inhibitory tone. Failure of timely development of inhibitory tone likely contributes to a lower seizure threshold and impaired cognitive function in children who suffer perinatal brain injury.

Thyroid hormone in the frontier of cell protection, survival and functional recovery

Thyroid hormone (TH) exerts important actions on cellular energy metabolism, accelerating O2consumption with consequent reactive oxygen species (ROS) generation and redox signalling affording cell protection, a response that is contributed by redox-independent mechanisms. These processes underlie genomic and non-genomic pathways, which are integrated and exhibit hierarchical organisation. ROS production led to the activation of the redox-sensitive transcription factors nuclear factor-κB, signal transducer and activator of transcription 3, activating protein 1 and nuclear factor erythroid 2-related factor 2, promoting cell protection and survival by TH. These features involve enhancement in the homeostatic potential including antioxidant, antiapoptotic, antiinflammatory and cell proliferation responses, besides higher detoxification capabilities and energy supply through AMP-activated protein kinase upregulation. The above aspects constitute the molecular basis for TH-induced preconditioning of the liver that exerts protection against ischemia-reperfusion injury, a strategy also observed in extrahepatic organs of experimental animals and with other types of injury, which awaits application in the clinical setting. Noteworthy, re-adjusting TH to normal levels results in several beneficial effects; for example, it lengthens the cold storage time of organs for transplantation from brain-dead donors; allows a superior neurological outcome in infants of <28 weeks of gestation; reduces the cognitive side-effects of lithium and improves electroconvulsive therapy in patients with bipolar disorders.

Decreased thyroid hormone signaling accelerates the reinnervation of the optic tectum following optic nerve crush in adult zebrafish

The regenerative capacity of the adult mammalian central nervous system (CNS) is poor and finding ways to stimulate long distance axonal regeneration in humans remains a challenge for neuroscientists. Thyroid hormones, well known for their key function in CNS development and maturation, more recently also emerged as molecules influencing regeneration. While several studies investigated their influence on peripheral nerve regeneration, in vivo studies on their role in adult CNS regeneration remain scarce. We therefore investigated the effect of lowering T3 signaling on the regeneration of the optic nerve (ON) following crush in zebrafish, a species where full recovery occurs spontaneously. Adult zebrafish were exposed to iopanoic acid (IOP), which lowered intracellular 3,5,3'-triiodothyronine (T3) availability, or to the thyroid hormone receptor β antagonist methylsulfonylnitrobenzoate (C1). Both treatments accelerated optic tectum (OT) reinnervation. At 7days post injury (7dpi) there was a clear increase in the biocytin labeled area in the OT following anterograde tracing as well as an increased immunostaining of Gap43, a protein expressed in outgrowing axons. This effect was attenuated by T3 supplementation to IOP-treated fish. ON crush induced very limited cell death and proliferation at the level of the retina in control, IOP- and C1-treated fish. The treatments also had no effect on the mRNA upregulation of the regeneration markers gap43, tub1a, and socs3b at the level of the retina at 4 and 7dpi. We did, however, find a correlation between the accelerated OT reinnervation and a more rapid resolution of microglia/macrophages in the ON and the OT of IOP-treated fish. Taken together these data indicate that lowering T3 signaling accelerates OT reinnervation following ON crush in zebrafish and that this is accompanied by a more rapid resolution of the inflammatory response.

Role of thyroid hormones in different aspects of nervous system regeneration in vertebrates

Spontaneous functional recovery from injury in the adult human nervous system is rare and trying to improve recovery remains a clinical challenge. Nervous system regeneration is a complicated sequence of events involving cell death or survival, cell proliferation, axon extension and remyelination, and finally reinnervation and functional recovery. Successful recovery depends on the cell-specific and time-dependent activation and repression of a wide variety of growth factors and guidance molecules. Thyroid hormones (THs), well known for their regulatory role in neurodevelopment, have recently emerged as important modulators of neuroregeneration. This review focuses on the endogenous changes in the proteins regulating TH availability and action in different cell types of the adult mammalian nervous system during regeneration as well as the impact of TH supplementation on the consecutive steps in this process. It also addresses possible differences in TH involvement between different vertebrate classes, early or late developmental stages and peripheral or central nervous system. The available data show that THs are able to stimulate many signaling pathways necessary for successful neurogeneration. They however also suggest that supplementation with T4 and/or T3 may have beneficial or detrimental influences depending on the dose and more importantly on the specific phase of the regeneration process.

Changes in thyroid hormone receptors after permanent cerebral ischemia in male rats

Thyroid hormones (TH) and receptors (TRs) may play an important role in the pathophysiology of acute cerebral ischemia. In the present study, we sought to determine whether serum triodothyronine (T3)/thyroxine (T4) and brain TRs (TRα1, TRβ1) might change after experimental stroke. Male adult Wistar rats were subjected to permanent middle cerebral artery occlusion (group P) and compared to sham-operated controls (group S). Animals were followed clinically for 14 days until brain collection for Western blot (WB) or neuropathological analysis of TRs in three different brain areas (infarcted tissue, E1; noninfarcted ipsilateral hemisphere, E2; and contralateral hemisphere, E3). Analysis of serum TH levels showed a reduction of T4 in group P (p = 0.002) at days 2 to 14, while half of the animals also displayed "low T3" values (p = 0.012) on day 14. This T4 reduction was inversely correlated to the clinical severity of stroke and the concomitant body weight loss (p < 0.005). WB analysis of TRα1 and TRβ1 protein expression showed heterogenic responses at day 14: total and nuclear TRα1 were similar between the two groups, while total TRβ1 decreased 7.5-fold within E1 (p ≤ 0.001) with a concomitant 1.8-fold increase of nuclear TRβ1 in E2 area (p = 0.03); TRβ1 expression did not differ in E3. Neuropathological analysis revealed that activated macrophages/microglia exclusively expressed nuclear TRα1 within the infarct core. Astrocytes mildly expressed nuclear TRα1 in and around the infarct, along with a prominent TRβ nuclear signal restricted in the astrocytic scar. Neurons around the infarct expressed mainly TRα1 and, to a milder degree, TRβ. Surprisingly enough, we detected for the first time a TRβ expression in the paranodal region of Ranvier nodes, of unknown significance so far. Our data support that cerebral ischemia induces a low TH response, associated with significant and heterogenic changes in brain TR expression. These findings could imply an important role of TH signaling in cerebral ischemia.

Current view on the functional regulation of the neuronal K(+)-Cl(-) cotransporter KCC2

In the mammalian central nervous system (CNS), the inhibitory strength of chloride (Cl(-))-permeable GABAA and glycine receptors (GABAAR and GlyR) depends on the intracellular Cl(-) concentration ([Cl(-)]i). Lowering [Cl(-)]i enhances inhibition, whereas raising [Cl(-)]i facilitates neuronal activity. A neuron's basal level of [Cl(-)]i, as well as its Cl(-) extrusion capacity, is critically dependent on the activity of the electroneutral K(+)-Cl(-) cotransporter KCC2, a member of the SLC12 cation-Cl(-) cotransporter (CCC) family. KCC2 deficiency compromises neuronal migration, formation and the maturation of GABAergic and glutamatergic synaptic connections, and results in network hyperexcitability and seizure activity. Several neurological disorders including multiple epilepsy subtypes, neuropathic pain, and schizophrenia, as well as various insults such as trauma and ischemia, are associated with significant decreases in the Cl(-) extrusion capacity of KCC2 that result in increases of [Cl(-)]i and the subsequent hyperexcitability of neuronal networks. Accordingly, identifying the key upstream molecular mediators governing the functional regulation of KCC2, and modifying these signaling pathways with small molecules, might constitute a novel neurotherapeutic strategy for multiple diseases. Here, we discuss recent advances in the understanding of the mechanisms regulating KCC2 activity, and of the role these mechanisms play in neuronal Cl(-) homeostasis and GABAergic neurotransmission. As KCC2 mediates electroneutral transport, the experimental recording of its activity constitutes an important research challenge; we therefore also, provide an overview of the different methodological approaches utilized to monitor function of KCC2 in both physiological and pathological conditions.

Thyroid hormone-dependent development of the GABAergic pre- and post-synaptic components in the rat hippocampus

Thyroid hormone (TH) plays essential roles in normal brain development mainly by regulating gene expression through binding to specific nuclear receptors which serve as transcription factors. Previous studies showed that perinatal deficiency of TH or impairment of its signaling severely affect brain development, especially the development of the γ-aminobutyric acid (GABA) system, but cellular and molecular targets of the hormone are only partly uncovered. In the present study, we focused on the developing rat hippocampus which was confirmed to be one of the regions highly sensitive to TH status, and found two new targets of the hormone among the pre- and post-synaptic components of the GABAergic system. One was glutamic acid decarboxylase 65 (GAD65), the protein level of which was reduced to less than 50% of control in the hippocampus of hypothyroid rats (obtained by administering 0.025% methimazole in drinking water to pregnant dams from gestational day 15 until 4 weeks postpartum) and recovered to control levels by daily thyroxine-replacement after birth. Reduction in GAD65 protein was correlated immunohistochemically with a 37% reduction in the number of GAD65-positive cells as well as a reduction in GAD65-positive processes. In contrast, the other GAD isotype, GAD67, was not affected by TH status. A subpopulation of GABAergic neurons containing parvalbumin was also confirmed to be highly dependent on TH status. The second target of thyroid hormone was neuron-specific K(+)/Cl(-) co-transporter, KCC2, which is responsible for switching of GABA action from excitatory to inhibitory. In the euthyroid hippocampus, a sharp rise of kcc2 expression was observed at postnatal day (PND)10 which was followed by a large increase in KCC2 protein at PND15. This transient rise in kcc2 expression was completely suppressed by hypothyroidism, resulting in nearly 80% reduction in KCC2 protein at PND15. These results indicate that the development of GABAergic terminals and the excitatory to inhibitory maturation of GABA signaling are strongly dependent on TH.

Thyroid hormone and tissue repair: new tricks for an old hormone?

Although the role of thyroid hormone during embryonic development has long been recognized, its role later in adult life remains largely unknown. However, several lines of evidence show that thyroid hormone is crucial to the response to stress and to poststress recovery and repair. Along this line, TH administration in almost every tissue resulted in tissue repair after various injuries including ischemia, chemical insults, induction of inflammation, or exposure to radiation. This novel action may be of therapeutic relevance, and thyroid hormone may constitute a paradigm for pharmacologic-induced tissue repair/regeneration.

Developmental thyroid hormone insufficiency and brain development: a role for brain-derived neurotrophic factor (BDNF)?

Thyroid hormones (TH) are essential for normal brain development. Even modest degrees of TH disruption experienced in utero can result in neuropsychological deficits in children despite normal thyroid status at birth. Neurotrophins have been implicated in a host of brain cellular functions, and in particular, brain-derived neurotrophic factor (BDNF) has a well documented role in development and function of the nervous system. A number of laboratories have reported the effects of TH administration or severe deprivation on neurotrophin expression in brain. This review provides an overview and update of recent developments in the thyroid field as they relate to the nervous system. Secondly, we describe an animal model of low level TH insufficiency that is more relevant for studying the neurological consequences associated with the modest TH perturbations of subclinical hypothyroidism, or that would be anticipated from exposure to environmental contaminants with a mode-of-action that involves the thyroid. Finally, we review the available in vivo literature on TH-mediated alterations in neurotrophins, particularly BDNF, and discuss their possible contribution to brain impairments associated with TH insufficiency. The observations of altered BDNF protein and gene expression have varied as a function of hypothyroid model, age, and brain region assessed. Only a handful of studies have investigated the relationship of neurotrophins and TH using models of TH deprivation that are not severe, and dose-response information is sparse. Differences in the models used, species, doses, regions assessed, age at assessment, and method employed make it difficult to reach a consensus. Based on the available literature, the case for a direct role for BDNF in thyroid-mediated effects in the brain is not compelling. We conclude that delineation of the potential role of neurotrophins in TH-mediated neuronal development may be more fruitful by examining additional neurotrophins (e.g., nerve growth factor), moderate degrees of TH insufficiency, and younger ages. We further suggest that investigation of BDNF invoked by synaptic activation (i.e., plasticity, enrichment, trauma) may serve to elucidate a role of thyroid hormone in BDNF-regulated synaptic function.

Ligand-induced changes in Oestrogen and thyroid hormone receptor expression in the developing rat cerebellum: A comparative quantitative PCR and Western blot study

Oestrogen (E2) and thyroid hormones (THs) are key regulators of cerebellar development. Recent reports implicate a complex mechanism through which E2 and THs influence the expression levels of each other's receptors (ERs and TRs) to precisely mediate developmental signals and modulate signal strength. We examined the modulating effects of E2 and THs on the expression levels of their receptor mRNAs and proteins in cultured cerebellar cells obtained from 7-day-old rat pups. Cerebellar granule cell cultures were treated with either E2, THs or a combination of these hormones, and resulting receptor expression levels were determined by quantitative PCR and Western blot techniques. The results were compared to non-treated controls and to samples obtained from 14-day-old in situ cerebella. Additionally, we determined the glial effects on the regulation of ER-TR expression levels. The results show that (i) ER and TR expression depends on the combined presence of E2 and THs; (ii) glial cells mediate the hormonal regulation of neuronal ER-TR expression and (iii) loss of tissue integrity results in characteristic changes in ER-TR expression levels. These observations suggest that both E2 and THs, in adequate amounts, are required for the precise orchestration of cerebellar development and that alterations in the ratio of E2/THs may influence signalling mechanisms involved in neurodevelopment. Comparison of data from in vitro and in situ samples revealed a shift in receptor expression levels after loss of tissue integrity, suggesting that such adjusting/regenerative mechanisms may function after cerebellar tissue injury as well.

The loop diuretic bumetanide blocks posttraumatic p75NTR upregulation and rescues injured neurons

Injured neurons become dependent on trophic factors for survival. However, application of trophic factors to the site of injury is technically extremely challenging. Novel approaches are needed to circumvent this problem. Here, we unravel the mechanism of the emergence of dependency of injured neurons on brain-derived neurotrophic factor (BDNF) for survival. Based on this mechanism, we propose the use of the diuretic bumetanide to prevent the requirement for BDNF and consequent neuronal death in the injured areas. Responses to the neurotransmitter GABA change from hyperpolarizing in intact neurons to depolarizing in injured neurons. We show in vivo in rats and ex vivo in mouse organotypic slice cultures that posttraumatic GABA(A)-mediated depolarization is a cause for the well known phenomenon of pathological upregulation of pan-neurotrophin receptor p75(NTR). The increase in intracellular Ca(2+) triggered by GABA-mediated depolarization activates ROCK (Rho kinase), which in turn leads to the upregulation of p75(NTR). We further show that high levels of p75(NTR) and its interaction with sortilin and proNGF set the dependency on BDNF for survival. Thus, application of bumetanide prevents p75(NTR) upregulation and neuronal death in the injured areas with reduced levels of endogenous BDNF.

Cell-type-dependent thyroid hormone effects on glioma tumor cell lines

Purpose. The present study investigated the potential effects of long-term T3 treatment on glioma tumor cell lines. Thyroid hormone action on cell growth, differentiation and survival during development may be of therapeutic relevance Methods and Results 1321N1 cell line, an astrocytoma grade II, and U87MG, a glioblastoma grade IV, were exposed for 2 and 4 days in medium deprived of T3 and in medium containing 1 nM T3. T3 promoted re-differentiation in both cell lines. However, T3 increased cell proliferation in 1321N1 (2 days) which declined thereafter (4 days) while in U87MG resulted in suppression of cell proliferation. At the molecular level, a 2.9 fold increase in the expression of TRα1 receptor was observed in U87MG versus 1321N1, P < 0.05. TRβ1 receptor was undetectable. These changes corresponded to a distinct pattern of T3-induced kinase signaling activation; T3 had no effect on ERK activation in both cell lines but significantly increased phospho-Akt levels in 1321N1. Conclusion. In conclusion, T3 can re-differentiate glioma tumor cells, whereas its effect on cell proliferation appears to be dependent on the type of tumor cell line with aggressive tumors being more sensitive to T3. TRα1 receptor may, at least in part, be implicated in this response.

Effect of early compared with delayed enteral nutrition on endocrine function in patients with traumatic brain injury: an open-labeled randomized trial

BACKGROUND

Traumatic brain injury (TBI) results in a hypermetabolic and hypercatabolic status in which adequate nutrition support is essential to improve clinical outcome. The endocrine system of a patient with TBI is also affected and may play a critical role in either the metabolic or the immunologic response to the trauma. In the present study, the effect of standard, delayed enteral feeding (DEF), compared with early (within 24-48 hours) enteral feeding (EEF), on the endocrine function of patients with TBI was investigated.

METHODS

This comparative, prospective, open-labeled, randomized study included TBI patients admitted to the intensive care unit (ICU). Injury severity was assessed by the Glasgow Coma Scale and predicted mortality by the Acute Physiology and Chronic Health Evaluation II. Twenty-five patients received DEF and 34 patients received EEF. The effect of the onset of nutrition on pituitary, thyroidal, gonadal, and adrenal function was investigated on days 6 and 12 after admission to the hospital.

RESULTS

Levels of thyroid-stimulating hormone, free triiodothyronine, free thyroxine, and testosterone (in males) of DEF patients declined in comparison to levels of the day of admission to the ICU. The decrease of hormonal values was less pronounced in the EEF group. Cortisol concentrations rose in the DEF group; a lesser hormonal change was found in the EEF group. Deaths during the study for the DEF group and EEF group were 2 and 3, respectively.

CONCLUSIONS

EEF may exert beneficial effects on the hormonal profile of TBI patients, possibly contributing to a better clinical outcome in this patient group.

Thyroid hormone and cardiac disease: from basic concepts to clinical application

Nature's models of regeneration provide substantial evidence that a natural healing process may exist in the heart. Analogies existing between the damaged myocardium and the developing heart strongly indicate that regulatory factors which drive embryonic heart development may also control aspects of heart regeneration. In this context, thyroid hormone (TH) which is critical in heart maturation during development appears to have a reparative role in adult life. Thus, changes in TH -thyroid hormone receptor (TR) homeostasis are shown to govern the return of the damaged myocardium to the fetal phenotype. Accordingly, thyroid hormone treatment preferentially rebuilds the injured myocardium by reactivating developmental gene programming. Clinical data provide further support to this experimental evidence and changes in TH levels and in particular a reduction of biologically active triiodothyronine (T3) in plasma after myocardial infarction or during evolution of heart failure, are strongly correlated with patients morbidity and mortality. The potential of TH to regenerate a diseased heart has now been testing in patients with acute myocardial infarction in a phase II, randomized, double blind, placebo-controlled study (the THiRST study).

Developmental thyroid hormone insufficiency reduces expression of brain-derived neurotrophic factor (BDNF) in adults but not in neonates

Brain-derived neurotrophic factor (BDNF) is a neurotrophin critical for many developmental and physiological aspects of CNS function. Severe hypothyroidism in the early neonatal period results in developmental and cognitive impairments and reductions in mRNA and protein expression of BDNF in a number of brain regions. The present study examined the impact of modest levels of developmental thyroid hormone insufficiency on BDNF protein expression in the hippocampus, cortex and cerebellum in the neonatal and adult offspring of rat dams treated throughout pregnancy and lactation. Graded levels of hormone insufficiency were induced by adding propylthiouracil (PTU, 0, 1, 2, 3 and 10 ppm) to the drinking water of pregnant dams from early gestation (gestational day 6) until weaning of the pups. Pups were sacrificed on postnatal days (PN) 14 and 21, and -PN100, and trunk blood collected for thyroid hormone analysis. Hippocampus, cortex, and cerebellum were separated from dissected brains and assessed for BDNF protein. Dose-dependent reductions in serum hormones in dams and pups were produced by PTU. Consistent with previous findings, age and regional differences in BDNF concentrations were observed. However, no differences in BDNF expression were detected in the preweanling animals as a function of PTU exposure; yet dose-dependent alterations emerged in adulthood despite the return of thyroid hormone levels to control values. Males were more affected by PTU than females, BDNF levels in hippocampus and cortex were altered but not those in cerebellum, and biphasic dose-response functions were detected in both sexes. These findings indicate that BDNF may mediate some of the adverse effects accompanying developmental thyroid hormone insufficiency, and reflect the potential for delayed impact of modest reductions in thyroid hormones during critical periods of brain development on a protein important for normal synaptic function.