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Keywords:

  • cognition;
  • erythropoietin;
  • neuroprotection;
  • oxydative stress;
  • schizophrenia

Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. DISCUSSION
  5. ACKNOWLEDGMENT
  6. REFERENCES

This systematic review summarizes and critically appraises the literature on the effect of erythropoietin (EPO) in schizophrenia patients and the pathophysiological mechanisms that may explain the potential of its use in this disease. EPO is mainly known for its regulatory activity in the synthesis of erythrocytes and is frequently used in treatment of chronic anemia. This cytokine, however, has many other properties, some of which may improve the symptoms of psychiatric illness. The review follows the preferred reporting items for systematic reviews and meta-analysis (PRISMA) statement guidelines. Three databases (Medline, Web of Science, and Cochrane) were searched combining the search terms ‘erythropoietin AND (psychotic disorders OR schizophrenia)’. Seventy-eight studies were included in qualitative synthesis, a meta-analytic approach being prohibited. The findings suggest that several EPO cerebral potential properties may be relevant for schizophrenia treatment, such as neurotransmission regulation, neuroprotection, modulation of inflammation, effects on blood–brain barrier permeability, effects on oxidative stress and neurogenesis. Several potentially detrimental side-effects of EPO therapy, such as increased risk of thrombosis, cancer, increased metabolic rate and mean arterial blood pressure leading to cerebral ischemia could severely limit or halt the use of EPO. Overall, because the available data are inconclusive, further efforts in this field are warranted.

ERYTHROPOIETIN (EPO) IS approved by the Food and Drug Administration for the treatment of anemia, but a recent body of work has found that EPO is not only required for erythropoiesis, but also functions in other organs and tissues, such as the brain, heart, and vascular system.1–10 EPO and its derivatives could be a treatment of choice in the negative symptoms of schizophrenia, a clinically heterogeneous psychotic illness, the etiology of which remains poorly understood. We propose here a systematic review on the effect of EPO in schizophrenia patients and the pathophysiological mechanisms underlying its potential therapeutic role in this disease. The purpose of this paper was not to make an extensive review of the work and fundamental data published since 1977 on EPO, but after a few reminders on the general and cerebral physiology of EPO (part 1), to extract the data that seem relevant for understanding its potential therapeutic role in schizophrenia (part 2), and to present a short overview of preliminary studies on the effects of EPO in patients with chronic schizophrenia (part 3).

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. DISCUSSION
  5. ACKNOWLEDGMENT
  6. REFERENCES

The review follows the PRISMA (preferred reporting items for systematic reviews and meta-analysis) statement guidelines. Methods of the analysis and inclusion criteria were specified in advance.

Eligibility criteria

All animal and human studies concerning effects of EPO in schizophrenia (or animal models of schizophrenia) were included. English and French language were imposed. There were no publication date or publication status limitations.

Studies were identified by searching electronic databases, scanning reference lists of articles, and consultation with experts in the field. This search was applied to Medline (1966–present) and Web of Science (1975–present). The last search was run on 31 October 2011.

The search paradigm was: ‘erythropoietin AND (psychotic disorders OR schizophrenia)’. The reference lists of the selected manuscripts were scrutinized for additional manuscripts. The search was conducted by two independent researchers (G.F and D.C). Disagreements were resolved by consensus among all authors. The search of Medline and Web of science provided a total of 23 + 26 = 49 citations (26 after withdrawing duplicates). No unpublished relevant studies were obtained. Seventy-eight studies were included in qualitative synthesis.

Short overview on physiology of EPO

General physiology

While the regulation of the synthesis of red blood cells by a humoral factor was demonstrated in the beginning of the 20th century, EPO was not discovered until 1977 due to its low plasma concentration in humans. EPO is a 30.4-kDa glycoprotein of 165 amino acids, a member of the superfamily of cytokines type I, with approximately half of its molecular weight derived from carbohydrates that can vary among species. It has long been thought that the only function of EPO was to maintain optimal oxygenation of tissues by regulating the number of erythrocytes via a system of negative feedback between the kidneys and the bone marrow. It is now proven, however, that EPO developed as part of an ancient immune system, conserved during evolution, the function of which is to limit damage to the heart, nervous system and other tissues in response to inflammatory, ischemic or hypoxic aggression. During the production and secretion of EPO, a 166-amino acid peptide is initially generated following the cleavage of a 27-amino acid hydrophobic secretory leader at the amino-terminal. In addition, a carboxy-terminal arginine in position 166 is removed both in the mature human and recombinant human EPO (rhEPO), resulting in a circulatory mature protein of 165 amino acids. The glycosylated chains are important for the biological activity of EPO and can protect EPO from oxygen radical degradation. The presence of the carbohydrates is important in the control of the metabolism of EPO, because EPO molecules with high sialic acid content can easily be cleared by the body through specific binding in the liver.11 The biological activity of EPO also relies upon two disulfide bonds formed between cysteines in positions 7 and 160, 29 and 33.12 The necessity of these disulfide bridges has been demonstrated by the observation that the reduction of these bonds results in the loss of the biologic activity of EPO. Alkylation of the sulfhydryl groups results in the irreversible loss of biological activity of EPO. Re-oxidization of EPO after reduction by guanidine restores 85% of the biological activity of EPO. Cysteine 33 replacement with proline also reduces the biological function of EPO. At physiological doses, clearance of the EPO hormone is done through a receptor-mediated endocytosis in the bone marrow. At pharmacological doses, however, it is eliminated via the liver and the kidneys.

Cerebral physiology

Tan et al. identified mRNA production of EPO in brain tissue in 1992.13 Investigations later showed that neurons and astrocytes were able to synthesize a specific (i.e. non-hematopoietic) EPO receptor (EPOR) that is part of the type 1 super-family of cytokine receptors. When EPO binds EPOR, it causes homodimerization and activation of the receptor and autophosphorylation of Janus-tyrosine-kinase-2 (JAK-2).13–15 EPO modulates intracellular calcium in neurons and thus directly influences neurotransmitter release and neuronal activity.16 The synthesis of EPO and its receptor in the brain suggests that EPO can function as an autocrine as well as a paracrine substance. The major sites of EPO production and secretion in the nervous system are hippocampus, internal capsule, cortex, midbrain, cerebral endothelial cells and astrocytes.17–20

EPO cerebral potential properties that may be relevant for schizophrenia treatment

Excitotoxicity and neurotransmission regulation

Erythropoietin reduces calcium-induced glutamate release from cultured cerebellar granule neurons and confers protection very quickly by inhibiting presynaptic functions.21 EPO stimulates dopamine release and tyrosine hydroxylase activity in PC12 cells, through the activation of calcium channels, induces membrane depolarization, stimulates mitogen-activated protein kinase activity and increases nitric oxide (NO) synthesis (especially in the vessels).16 EPO stimulates the release of dopamine and acetylcholine from hippocampal and striatal slices in rats, but this occurs independently of NO production.16 It is interesting to note that intense neuronal depolarization (as during a electroconvulsive therapy session) is a metabolic stress condition that triggers EPO synthesis.22

Neuroprotection

Erythropoietin has shown a neuroprotective activity, for example in limitation of apoptosis in tissues adjacent to injury.23 In an in vitro model of cerebral ischemia, consisting of hypoxia and glucose deprivation, EPO protects cultured neurons, but not astrocytes, from death.18

Erythropoietin acts at multiple levels, including limiting the production of tissue-injuring molecules such as reactive oxygen species and glutamate, reversal of vasospasm, stimulation of angiogenesis, attenuation of apoptosis, modulation of inflammation and recruitment of stem cells.24

Cytoprotection by EPO is further related to the maintenance of mitochondrial membrane potential (DCm). EPO has the capacity to prevent the depolarization of the mitochondrial membrane that also affects the release of cytochrome C.3,7

Modulation of inflammation

It is well known that among various environmental and genetic factors, inflammatory immune processes are implicated in the etiology and pathology of schizophrenia.25–29 Maternal infection, obstetric complications, neonatal hypoxia and brain injury all recruit cytokines that mediate inflammatory processes and thus influence brain development.

The role of EPO during cellular inflammation seems to be of equal importance to its role in the functional preservation of cells. EPO blocks primary microglial activation and proliferation that may lead to cellular damage through the generation of reactive oxygen species during inflammation and through the production of cytokines.20 EPO can directly inhibit several pro-inflammatory cytokines, such as interleukin-6, tumor necrosis factor-α, and monocyte chemoattractant protein 1 as well as reduce leukocyte inflammation.17

Effects on blood–brain barrier permeability

Both EPO and schizophrenia may be implied in blood–brain barrier (BBB) integrity and/or permeability. EPO antagonizes the permeability of the BBB induced by vascular endothelial growth factor or inflammation.30 In contrast, P-glycoprotein, a major efflux pump in the BBB, has a profound effect on entry of drugs, peptides and other substances into the central nervous system (CNS) and may be influenced by inflammatory mediators playing a role in schizophrenia.31

Effects on oxidative stress

Oxidative stress represents a significant mechanism for the destruction of cells, and can involve apoptotic cell injury and neuronal or vascular degeneration.11 Apoptosis-induced oxidative stress in conjunction with processes of mitochondrial dysfunction can contribute to a variety of diseases such as diabetes, general cognitive loss, Alzheimer's disease, and schizophrenia.32,33 In particular, oxidative damage to lipids, proteins and DNA as observed in schizophrenia is known to impair cell viability and function. Evidence currently available points towards an alteration in the activities of enzymatic and non-enzymatic antioxidant systems in schizophrenia. In fact, experimental models have demonstrated that oxidative stress induces behavioral and molecular anomalies similar to those observed in schizophrenia.11 These findings suggest that oxidative stress is intimately linked to a variety of pathophysiological processes, such as inflammation, oligodendrocyte abnormalities, mitochondrial dysfunction, hypoactive N-methyl-d-aspartate receptors and the impairment of fast-spiking γ-aminobutyric acid interneurons.11 Such self-sustaining mechanisms may progressively worsen, producing the functional and structural consequences associated with schizophrenia. Supporting this hypothesis are recent clinical studies that have shown antioxidant treatment to be effective in ameliorating schizophrenia symptoms.32

Neurogenesis

It appears that the long-latency effects of EPO may be due to its modulation of neurogenesis: EPO may play a part in the control of proliferation and differentiation of neuronal stem cells. Cultured neural stem cells under hypoxia produce two–threefold more neurons and this is associated with an elevation in EPO mRNA expression.18 It has also been demonstrated that EPO is a mediator for dopaminergic neuron differentiation from CNS precursors under low oxygen conditions.34 EPO mediates ascorbate-induced dopaminergic differentiation from embryonic mesencephalic precursors.35 Furthermore, EPO has a trophic effect on brain cholinergic neurons in vitro and in vivo.36

Adult neurogenesis (AN) is one of the most rapidly growing areas in neuroscience research and there is great interest in its potential role in the pathophysiology of psychiatric illness. In schizophrenia there is evidence of abnormal mechanisms of neurogenesis and abnormal expression of developmental genes.37 The recent discovery of molecular markers transiently expressed in newborn neurons within adult neurogenic brain regions could be used to probe whether neurogenesis is disturbed in schizophrenia. The functional relevance of disturbed AN may encompass erroneous temporal encoding of new memory traces, thereby contributing to cognitive deficits observed in schizophrenia.38 This AN hypothesis of schizophrenia is supported by neuroimaging, as well as by several genetically modified rodent models, for example reelin and NPAS3 knockout mice.38 Furthermore, several genes impacting on AN, including NPAS3, were also found to be associated with schizophrenia in case–control studies.37

Potential use of EPO in chronic schizophrenia cognitive impairment

EPO as a neuroprotective agent

EPO meets the four criteria for a potential neuroprotective agent in schizophrenia.39

  • 1) 
    When given artificially in rats and humans, rhEPO reaches the brain even through an intact BBB.39 In addition it was shown that routine use of EPO at appropriate doses allows EPO to cross the BBB sufficiently for neuroprotection.
  • 2) 
    EPO penetration in the brain is increased in healthy subjects, and more so in schizophrenia patients when expression of EPOR was found in the hippocampus and cerebral cortex.39 In addition, post-mortem brains of schizophrenia patients showed that the synthesis of EPOR is increased compared to healthy controls.39
  • 3) 
    The in vitro neuronal death induced by haloperidol is reduced in the presence of rhEPO.39
  • 4) 
    The peripheral use of rhEPO increased cognitive functioning in mice in the context of a task involving cortical and subcortical areas presumed affected in schizophrenia (see part 3).39
Rodent studies

According to the results of rodent models, EPO seems also a very promising molecule in improving learning and memory. A conditioned aversion task among mice was measured by determining the reduction in drinking upon subsequent exposure to an illness-producing solution.39 After a 5-day recovery, water-deprived animals were presented with the same solution. Animals that received an injection of rhEPO showed a virtually complete aversion to the fluid, in spite of being water deprived, and tolerated a water deficit approximately twice longer than controls.

EPO treatment among schizophrenia patients

There is unfortunately no schizophrenia animal model to date, especially for negative symptoms of the disease.40 Studies of brain morphology of schizophrenia patients show that there is a neurotoxic component in this illness: brain imaging indeed shows the progressive loss of cortical gray matter during the illness progression.41 These data have led to the virtual rebirth of Kraeplin's dementia praecox concept and to new concepts for therapeutic interventions, such as neuroprotection therapy.39

Elevated EPO concentrations during infant maturation have been correlated with increased Mental Development Index scores, therefore it has been suggested that EPO may provide developmental cognitive support in humans.42 Genes that are important in learning and memory are also upregulated during neural repair after a stroke, including the stathmin family genes SCLIP and SCG-10,43 the membrane-associated phosphoproteins GAP43 and MARCKS,44,45 the transcription factor c-jun46 and the cell adhesion molecule L1.47

Although no molecule seems yet to have clearly demonstrated positive cognitive effects in schizophrenia,48 repeated EPO treatment seems to improve neurocognitive function in healthy subjects and schizophrenia patients:49 in a double-blind, placebo-controlled, randomized, multicenter trial, 39 male patients aged 25–50 years with chronic schizophrenia stabilized deficit (= last hospitalization >10 years previously and last acute episode >6 months previously) received EPO (40000 IU i.v.) versus placebo (15-min infusion) weekly for 12 weeks. Bleeding was performed when the hematocrit exceeded 50% for two consecutive weeks. Cognitive functions were tested at 2, 4 and 12 weeks using the Repeatable Battery for the Assessment of Status Neuropsychological,50 the Wisconsin Card-Sorting Test-6451 and a premorbid intelligence test. The Positive and Negative Syndrome Scale (PANSS),52 the Subjective Well-Being under Neuroleptic Treatment,53 the Disability Assessment Schedule54 were undertaken at 2, 4 and 12 weeks. No significant improvement in PANSS scores could be attributed to treatment with EPO, but EPO-treated patients had significantly increased scores on all the cognitive tests, suggesting that EPO may be proposed as an adjuvant treatment in stabilized schizophrenia patients with major cognitive impairment.

DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. DISCUSSION
  5. ACKNOWLEDGMENT
  6. REFERENCES

Many concerns should be addressed in the practical use of EPO. First, the cognitive deficit in schizophrenia is not an acute response, therefore EPO should be given by injection at regular intervals for the whole life long. In the Ehrenreich et al. study an infusion (40 000 IU rhEPO) was given weekly for 3 months.49 That was the only double-blind study to adopt EPO in human schizophrenia (n = 39 for schizophrenia patients, n = 19 for normal controls), so that no one can deny that the improved effect on cognitive batteries is only temporary. This is partly because no following study was performed after the initial study. Second, unlike renal anemia, it is not expected that hemoglobin could be an indicator protein for cognitive performance. Therefore we will not know exactly when this injection should be stopped, or whether the quantity is appropriate or not.

Moreover, one limitation of the use of EPO is that this cytokine, as an autocrine–paracrine hormone, triggers both the neurological and the general pathways (such as the stimulation of bone marrow).30

The potential progression of cancer has been a significant concern raised with EPO use.17,55 EPO may promote the growth of malignant tumors sensitive to EPO both in vitro and in vivo.30 Not only have both EPO and its receptor expression been found in tumor specimens, but under some conditions EPO has been suggested to block tumor cell apoptosis through Akt,56 enhance tumor progression and increase metastatic disease.57 In studies of patients with head and neck cancer, EPO decreased disease progression-free survival and overall survival.58 Similar results were reported in trials with metastatic breast cancer.59 It should be noted, however, that the potential risk of EPO in either initiating tumor growth or inducing tumor progression is not entirely understood.

In addition to the concerns outlined in patients with cancer, other important considerations for EPO exist such as problems associated with EPO abuse and gene doping.60–62 EPO has also been correlated with the alteration of red cell membrane properties leading to a cognitive decrease in rodent animal models.17,20 Finally, several potentially detrimental vascular side-effects of EPO therapy, such as increased risk of thrombosis, increased metabolic rate and mean arterial blood pressure leading to cerebral ischemia could severely limit or halt the use of EPO, especially in neurovascular diseases.30,63

Strategies for future research

New studies are investigating the role of targeted bioavailability for EPO such as in bone marrow stromal cells genetically engineered to secrete EPO64 and controlled release of EPO from encapsulated cells.65 The passage of EPO into the CNS continues to attract significant interest, as does the use of novel intranasal routes for EPO.66 Other pathways of interest are the development of derivations of EPO with lesser erythropoietic activity and therefore less potential vascular complications.67 This research has led to the discovery of asialoEPO, generated by total enzymatic desialylation of rhEPO.68

Yet, these lines of investigation are not without limitations, because chemical derivatives of EPO can become devoid of clinical effectiveness,17,20 as well as possibly lose the ability to promote sustainable cytoprotective effects, such as neurogenesis.69

Erythropoietin appears as a cytokine of interest in the treatment of cognitive symptoms of schizophrenia, but the molecule still has some side-effects that do not currently allow its use in clinical practice, and the long-term effects are not known as yet. Research in the development of new molecules and the study of its physiological mechanisms may open new therapeutic leads for many psychiatric illnesses and especially schizophrenia with chronic cognitive impairment. In addition, based on the aforementioned data, preventive treatment ought to be considered as early as the first episode of the disease, in order to halt the neurodegenerative process.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. DISCUSSION
  5. ACKNOWLEDGMENT
  6. REFERENCES