Targeting NMDA Receptors at the Neurovascular Unit: Past and Future Treatments for Central Nervous System Diseases

The excitatory neurotransmission of the central nervous system (CNS) mainly involves glutamate and its receptors, especially N-methyl-D-Aspartate receptors (NMDARs). These receptors have been extensively described on neurons and, more recently, also on other cell types. Nowadays, the study of their differential expression and function is taking a growing place in preclinical and clinical research. The diversity of NMDAR subtypes and their signaling pathways give rise to pleiotropic functions such as brain development, neuronal plasticity, maturation along with excitotoxicity, blood-brain barrier integrity, and inflammation. NMDARs have thus emerged as key targets for the treatment of neurological disorders. By their large extracellular regions and complex intracellular structures, NMDARs are modulated by a variety of endogenous and pharmacological compounds. Here, we will present an overview of NMDAR functions on neurons and other important cell types involved in the pathophysiology of neurodegenerative, neurovascular, mental, autoimmune, and neurodevelopmental diseases. We will then discuss past and future development of NMDAR targeting drugs, including innovative and promising new approaches.

BDNF, proBDNF and IGF-1 serum levels in naïve and medicated subjects with autism

Brain-derived neurotrophic factor (BDNF) and insulin-like growth factor 1 (IGF-1) promote the development and maintenance of neural circuits. Alterations in these factors might contribute to autism spectrum disorder (ASD). We asked whether serum BDNF, proBDNF, and IGF-1 levels are altered in an ASD population compared to controls. We measured serum BDNF, proBDNF, and IGF-1 immunoreactive protein in boys and girls aged 5–15 years old with mild to moderate ASD and non-autistic controls by ELISA. IGF-1 was increased in ASD serum compared to controls and was correlated with age and with CARS scores. Serum BDNF levels did not differ between groups, however, proBDNF serum levels were decreased in subjects with ASD compared to non-autistic controls. Medicated, but not unmedicated, ASD subjects exhibited lower serum proBDNF levels compared to controls, while neither IGF-1 nor BDNF levels differed between treatment groups. These data support the involvement of proBDNF and IGF-1 in the pathogenesis and treatment of autism.

Astrocyte tissue plasminogen activator expression during brain development and its role in pyramidal neuron neurite outgrowth

The serine protease tissue plasminogen activator (tPA), encoded by the gene Plat, exerts a wide range of proteolysis-dependent and proteolysis-independent functions. In the developing brain, tPA is involved in neuronal development via the modulation of the proteolytic degradation of the extracellular matrix (ECM). Both lack of and excessive tPA are associated with neurodevelopmental disorders and with brain pathology. Astrocytes play a major role in neurite outgrowth of developing neurons as they are major producers of ECM proteins and ECM proteases. In this study we investigated the expression of Plat in developing and mature hippocampal and cortical astrocytes of Aldh1l1-EGFP-Rpl10a mice in vivo following Translating Ribosome Affinity Purification (TRAP) and the role of tPA in modulating astrocyte-mediated neurite outgrowth in an in vitro astrocyte-neuron co-culture system. We show that Plat is highly enriched in astrocytes in the developing, but not in the mature, hippocampus and cortex. Both the silencing of tPA expression in astrocytes and astrocyte exposure to recombinant tPA reduce neuritogenesis in co-cultured hippocampal neurons. These results suggest that astrocyte tPA is involved in modulating neuronal development and that tight control of astrocyte tPA expression is important for normal neuronal development, with both experimentally elevated and reduced levels of this proteolytic enzyme impairing neurite outgrowth. These results are consistent with the hypothesis that the ECM, by serving as adhesive substrate, enables neurite outgrowth, but that controlled proteolysis of the ECM is needed for growth cone advancement.

Elevated levels of cortisol, brain-derived neurotropic factor and tissue plasminogen activator in male children with autism spectrum disorder

Several studies demonstrated biological effects of cortisol, brain-derived neurotrophic factor (BDNF) and tissue plasminogen activator (tPA) on human metabolism and central nervous system. Our study investigated the serum levels of tPA along with BDNF and cortisol in children with autism spectrum disorder (ASD). Thirty three male children with ASD ranging in age from 2 to 15 years were selected for the study group and 27 age-matched healthy male children were selected for the control group. The ASD severity was determined by the score on the Autism Behavior Checklist (ABC). The mean cortisol levels for the study group and the control group were 79.1 ± 30.2 ng/ml and 60.0 ± 25.1 ng/ml, respectively. The mean BDNF levels for the study group and the control group were 5.9 ± 2.8 ng/ml and 3.7 ± 1.8 ng/ml, respectively. The mean tPA levels for the study group and the control group were 32.9 ± 18.5 ng/ml and 25.5 ± 15.1 ng/ml, respectively. Cortisol, BDNF and tPA levels were significantly higher in the study group compared to the control group (p < 0.001). There was no statistically significant effect in terms of age, ABC total and subscale scores on serum cortisol, BDNF and tPA levels in the study group (p > 0.05). It may be suggested that elevations may indicate a role in the pathogenesis of ASD or it may be the case that ASD may alter the levels or pathways of these metabolic factors. LAY SUMMARY: The underlying mechanism or a specific metabolic target relevant to autism spectrum disorder (ASD) has not yet been identified. Cortisol, brain-derived neurotrophic factor (BDNF) and tissue plasminogen activator (tPA) have biological effects on neuroplasticity but little is known about the role of cortisol and tPA-BDNF pathway in ASD. In the present study focused on male children with ASD, we have found higher blood levels of cortisol, BDNF and tPA than their healthy peers. This is the first clinical study to evaluate the serum tPA levels along with BDNF and cortisol in ASD. The results suggest that several neurotrophic and other related markers should be born in mind while examining children with ASD.

[Pathophysiological mechanisms of autism in children]

Based on the analysis of literature, the authors describe the neuropathophysiological mechanism of the formation of synapses, synaptic transmission and plasticity, which may underlie the pathogenesis of autism. The results of some studies confirm the involvement of aberrant expression of genes and proteins of synaptic contacts, cell adhesion molecules p120ctn, CNTN5, CNTN6, activation of NMDA glutamate, TrkB, p75 receptors, Ca²⁺-input, BDNF, serotonin and testosterone. This leads to an imbalance in the exciting, inhibitory synaptic transmission and forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD) at the level of individual neurons and their chains due to suppression of GABA synthesis, expression of its ionotropic and metabotropic receptors, G proteins, NGF, TrkA receptors, a reduction in the number of GABAergic neurons, their contacts and disruption of differentiation. The pathology of the nuclei of the thalamus, especially the reticular nucleus (RN), is associated with a disturbance of the expression of the subunits of metabotropic GABAβ receptors, Ca²⁺ channels, GABA excretion and the work of chlorine transmitters. These failures do not ensure the inhibitory effect of OC on the exciting associative and ventral nuclei of the thalamus, nor modify the incoming information to the cerebral cortex (CC) from these thalamus nuclei, the dentate gyrus of the hippocampus and the nuclei of the reticular formation. Information propagating into the somatosensory and associative regions of CC is not modified by mirror neurons (MN) when performing arbitrary actions, which prevents the formation of an adequate image in the neural networks of the associative cortex and promotes the development of hyperexcitability, irritability, increased visual and auditory sensitivity, anxiety, and the ability to form a holistic image based on the actions of other people.

Cathepsin B inhibition ameliorates leukocyte-endothelial adhesion in the BTBR mouse model of autism

AIMS

Autism spectrum disorder (ASD) is a wide range of neurodevelopmental disorders involving deficits in social interaction and communication. Unfortunately, autism remains a scientific and clinical challenge owing to the lack of understanding the cellular and molecular mechanisms underlying it. This study aimed to investigate the pathophysiological mechanism underlying leukocyte-endothelial adhesion in autism-related neurovascular inflammation.

METHODS

Male BTBR T+tf/J mice were used as an autism model. The dynamic pattern of leukocyte-endothelial adhesion in mouse cerebral vessels was detected by two-photon laser scanning microscopy (TPLSM). Using FACS, RT-PCR, and Western blotting, we explored the expression of cell adhesion molecules, the mRNA expression of endothelial chemokine, the protein levels of cathepsin B, and inflammatory mediators.

RESULTS

We found a significant increase in leukocyte-endothelial adhesion in BTBR mice, accompanied by elevated expression of the adhesion molecule neutrophils CD11b and endothelial ICAM-1. Our data further indicate that elevated neutrophil cathepsin B levels contribute to elevated endothelial chemokine CXCL7 levels in BTBR mice. The pharmacological inhibition of cathepsin B reverses the enhanced leukocyte-endothelial adhesion in the cerebral vessels of autistic mice.

CONCLUSION

Our results revealed the prominent role of cathepsin B in modulating leukocyte-endothelial adhesion during autism-related neurovascular inflammation and identified a promising novel approach for autism treatment.

Altered levels of brain-derived neurotrophic factor, proBDNF and tissue plasminogen activator in children with posttraumatic stress disorder

The current study aims to compare the serum brain-derived neurotrophic factor (BDNF), proBDNF and tissue plasminogen activator (tPA) levels in cases that have developed posttraumatic stress disorder (PTSD) in consequence of sexual abuse with those in healthy control subjects. Thirty-one female patients between 8 and 18 years of age who have been diagnosed with PTSD due to sexual abuse and thirty-one healthy female volunteer controls were included in the study. Frequency, intensity and severity of PTSD symptoms were assessed on the basis of Clinician-Administered Post-Traumatic Stress Disorder Scale for Children and Adolescents (CAPS-CA). Serum BDNF, proBDNF and tPA levels were measured by Enzyme-Linked Immunosorbent Assay (ELISA) method. Results of the present study revealed that serum levels of BDNF and proBDNF in PTSD group were significantly lower but tPA level was significantly higher as compared to healthy control subjects. There were no correlations between CAPS-CA scores and BDNF, proBDNF and tPA levels. Decreased levels of BDNF, as suggested to have a role in the etiopathogenesis of PTSD, appear to be a result of the reduction in proBDNF production. The increased tPA levels in such cases, on the other hand, can be a compensatory mechanism serving to increase the BDNF levels.