This blog, as you will have noticed, does rather meander through science of autism. As a result there are some gaps and unanswered questions.
The blog talks a lot about the neurotransmitter GABA and the excitatory/inhibitory imbalance. We have ended up with some therapies based on this that do seem to help many people.
The opposing (excitatory) neurotransmitter is glutamate which affects the NMDA, AMP and mGlu receptors.
It appears that in autism there is an unusually high level of glutamate, but another issue looks likely to be at specific receptors, for example mGluR5
This does get very complicated and lacks any immediate therapies.
One very interesting insight was that you can repurpose the existing cheap generic GABAB drug Baclofen to treat NMDAR-hypofunction.
This seems to work really well at low doses with many people with Asperger’s. People with more severe autism do not seem to respond to low doses, however some do to higher doses. The more potent version R Baclofen is a research drug.
Reduced N-methyl-D-aspartate-receptor (NMDAR) signaling has been associated with schizophrenia, autism and intellectual disability. NMDAR-hypofunction is thought to contribute to social, cognitive and gamma (30–80 Hz) oscillatory abnormalities, phenotypes common to these disorders.
Constitutive NMDAR-hypofunction caused a loss of E/I balance, with an increase in intrinsic pyramidal cell excitability and a selective disruption of parvalbumin-expressing interneurons. Disrupted E/I coupling was associated with deficits in auditory-evoked gamma signal-to-noise ratio (SNR). Gamma-band abnormalities predicted deficits in spatial working memory and social preference, linking cellular changes in E/I signaling to target behaviors. The GABAB-receptor agonist baclofen improved E/I balance, gamma-SNR and broadly reversed behavioral deficits.
Excitotoxicity
We have touched on this subject on a few occasions but today, excitotoxicity is the focus of this post.
Excitotoxicity looks likely to be present in much autism and helps to connect all the various dysfunctions that we can read about in the literature.
It is a little scary because you cannot know to what extent this process is reversible. It looks like in milder cases it should be treatable, whereas in extreme cases damage will be irreversible.
Unfortunately you can trigger glutamate excitotoxity via a dysfunction in GABAA receptors.
For example if you severely inhibit GABAA receptors you kill brain cells, but it was the reaction in glutamate signaling that did the damage. GABA is supposed to be inhibitory; in some autism it is not and then Glutamate gets out of balance. This does lead to excess firing of neurons, which seems to degrade cognition, but it will tend towards glutamate excitotoxity.
When you see the cascade of events triggered by glutamate excitotoxity you will see how this really helps to explain biological finding in autism, even mitochondrial dysfunctions.
You can then trace this all back to the faulty GABA switch caused by too little KCC2 and too much NKCC1.
Then you can look at other neurological conditions that feature glutamate excitotoxity, like traumatic brain injury and neuropathic pain, and you see that the research shows low expression of KCC2.
This then suggests that much of autism would have been prevented if you could increase KCC2. You would not just fix the E/I imbalance but you would avoid all the damage done by excitotoxity.
Just how early you would have to correct KCC2 expression is not clear. For sure it is a case of better late than never, but how much damage caused by excitotoxicity is reversible?
Good News
The good news is that because KCC2 underexpression is a feature of many conditions there is plenty of research money being spent looking for answers. When they find a solution for increasing KCC2 to treat neuropathic pain, or spinal cord injury (SCI), the drug can be simply re-purposed for autism.
The French government is funding research into increasing KCC2 to treat SCI. They are starting with serotin 5-HT2A receptor agonists. Regular readers without any memory loss may recall that back in the 1960 Lovaas was giving LSD to people with autism at UCLA. LSD is a potent 5-HT2A receptor agonist. The French are also looking at BDNF to upregulate KCC2 and then they plan to have a blind test where they try all the chemicals they have in their library. The French are of course doing their trials in test tubes.
When I looked at this subject a while back, I looked for existing therapies that are known to be safe and should be effective.
My conclusion then was that intranasal insulin was the best choice.
Excitoxicity in Autism
Autism is a debilitating neurodevelopment disorder characterized by stereotyped interests and behaviours, and abnormalities in verbal and non-verbal communication. It is a multifactorial disorder resulting from interactions between genetic, environmental and immunological factors. Excitotoxicity and oxidative stress are potential mechanisms, which are likely to serve as a converging point to these risk factors. Substantial evidence suggests that excitotoxicity, oxidative stress and impaired mitochondrial function are the leading cause of neuronal dysfunction in autistic patients. Glutamate is the primary excitatory neurotransmitter produced in the CNS, and overactivity of glutamate and its receptors leads to excitotoxicity. The over excitatory action of glutamate, and the glutamatergic receptors NMDA and AMPA, leads to activation of enzymes that damage cellular structure, membrane permeability and electrochemical gradients. The role of excitotoxicity and the mechanism behind its action in autistic subjects is delineated in this review
The influx of intracellular calcium triggers the induction of inducible nitric oxide (iNOS) and phosphorylation of protein kinase C. Increased iNOS enhances nitric oxide (NO•) production in excess, whereas protein kinase C activates phospholipase A2 which in turn results in the generation of pro-inflammatory molecules The subsequent generation of free radicals can inhibit oxidative phosphorylation and damage mitochondrial enzymes involved in the electron transport chain, which mitigate energy production .
Reactive intermediates such as peroxynitrates and other peroxidation products hamper the normal function of mitochondrial enzymes by impairing oxidative phosphorylation and inhibiting complex II of the electron transport chain. Moreover, lipid peroxidation products, such as 4-hydroxynonenal (4-HNE) can interact with synaptic protein and impair transport of glucose and glutamate, thereby decreasing energy production and increasing excitotoxic sensitivity
Overstimulation of the glutamate receptors, NMDA and AMPA, leads to the release of other excitotoxins resulting in the accumulation of glutamate. Indeed, excess glutamate concentrations results in an increase in calcium levels in the cytosol. This effect is attributed to the fact that excessive glutamate allows calcium channel to open for longer periods of time, leading to increased influx of calcium into cells. Calcium triggers inducible nitric oxide and protein kinase C that produce free radicals, ROS and arachidonic aid. Generation of these oxidants results in mitochondrial dysfunction and accumulation of pro-inflammatory molecules and finally cell death. Free radicals interact with the mitochondrial and cellular membrane to form lipid peroxidation. 4-HNE is a major destructive product of this process. Lipid peroxidation prevents the dephosphorylation of excessively phosphorylated tau protein, significantly interfering with microtubule function. It has also been shown to inhibit glutathione reductase needed to convert oxidised glutathione to its functional reduced form
The mechanism responsible for excitotoxicity and neuronal cell death is diverse. Experimental studies have shown that the apoptotic and/or necrotic cell death may be due to the severity of NMDA damage or can be dependent on receptor subunit composition of neurons (Bonfoco et al. 1995; Portera-Cailliau et al. 1997). Pathological events related to this mode of action can be loss of cellular homoeostasis with acute mitochondrial dysfunction leading to hindrance in ATP production. Moreover, glutamatergic insults can cause cell death by the action of one or more molecular pathways which involves the action of signaling molecules such as cysteine proteases, mitochondrial endonucleases, peroxynitrite, PARP-1 and GAPDH in the excitotoxic neurodegeneration pathway.
Intracellular calcium levels also rely on voltage-dependent calcium channels and Na exchangers . The Na?/Ca2? exchanger is a bi-directional membrane ion transporter, which during membrane depolarisation or the opening of the gated sodium channels, transports sodium out of the cell and calcium into the cell. AMPA-type glutamate receptors are highly permeable to calcium and its over expression can lead to excitotoxicity. The Ca2? permeability capability of AMPA-type glutamate receptors relies on the presence or the absence of the GluR2 subunit in the receptor complex. Reduced GluR2 expression permits the construction of AMPA receptors with high Ca2? permeability and contributes to neuronal defect and excitotoxicity. Another mechanism is the release of calcium from internal stores such as the endoplasmic reticulum and mitochondria. It results in mitochondrial dysfunction, reduction in ATP synthesis and ROS generation.
Voltage gated channels found in dendrites and cell bodies of neurons modulate neuronal excitability and calcium-regulated signaling cascades (Dolmetsch et al. 2001; Catterall et al. 2005). Point mutations in the gene encoding the L-type voltage-gated channels Ca v1.2 (CACNA1C) and Ca v1.4. (CACNA1F) prevent voltage-dependent inactivation of these genes. This causes the channel to open for longer time, leading to excessive influx of calcium.
Conclusion
Autism is a multifactorial disorder characterized by neurobehavioral and neurological dysfunction. Excitotoxicity is the major neurobiological mechanism that modulates diverse risk factors associated with autism. It is triggered by potential mutation in ion channels and signalling pathways, viral and bacterial pathogens, toxic metals and free radical generation. Over expression of glutamate receptors and increased glutamate levels leads to increased calcium influx and oxidative stress and progressive cellular degeneration and cell death. Genetic defect, such as mutation in voltage gated or ligand channels that regulate neuronal excitability leads to defect in synaptic transmission and excitotoxic condition in autism. Mutation in BKCa and Ca v1.2 channels also results in excess calcium influx Sodium, potassium and chloride channels also play important roles in maintaining homoeostasis of neuronal cells, and decreased channel activity leads to destabilization of membrane potential and excitotoxicity. Moreover, over expression of BDNF results in hyperexcitability. Excessive BDNF and NMDA receptor activity increases the neurotransmitter release and excitotoxic vulnerability. Given that autism is a multifaceted disorder with multiple risk factors, more precise studies are needed to explore the signalling pathways that influence emergence of excitotoxicity in ASDs.
Some relevant reading for those interested:-
Abstract
Background
Autism spectrum disorder (ASD) is characterized by three core behavioral domains: social deficits, impaired communication, and repetitive behaviors. Glutamatergic/GABAergic imbalance has been found in various preclinical models of ASD. Additionally, autoimmunity immune dysfunction, and neuroinflammation are also considered as etiological mechanisms of this disorder. This study aimed to elucidate the relationship between glutamatergic/ GABAergic imbalance and neuroinflammation as two recently-discovered autism-related etiological mechanisms.
Methods
Twenty autistic patients aged 3 to 15 years and 19 age- and gender-matched healthy controls were included in this study. The plasma levels of glutamate, GABA and glutamate/GABA ratio as markers of excitotoxicity together with TNF-α, IL-6, IFN-γ and IFI16 as markers of neuroinflammation were determined in both groups.
Results
Autistic patients exhibited glutamate excitotoxicity based on a much higher glutamate concentration in the autistic patients than in the control subjects. Unexpectedly higher GABA and lower glutamate/GABA levels were recorded in autistic patients compared to control subjects. TNF-α and IL-6 were significantly lower, whereas IFN-γ and IFI16 were remarkably higher in the autistic patients than in the control subjects.
Conclusion
Multiple regression analysis revealed associations between reduced GABA level, neuroinflammation and glutamate excitotoxicity. This study indicates that autism is a developmental synaptic disorder showing imbalance in GABAergic and glutamatergic synapses as a consequence of neuroinflammation.
Keywords: Autism, Glutamate excitotoxicity, Gamma aminobutyric acid (GABA), Glutamate/GABA, Tumor necrosis factor-α, Interleukin-6, Interferon-gamma, Interferon-gamma-inducible protein 16
CONCLUSIONS:
Subjects with autism may have specific abnormalities in the AMPA-type glutamate receptors and glutamate transporters in the cerebellum. These abnormalities may be directly involved in the pathogenesis of the disorder.
General pathophysiology of traumatic brain injury
The first stages of cerebral injury after TBI are characterized by direct tissue damage and impaired regulation of CBF and metabolism. This ‘ischaemia-like’ pattern leads to accumulation of lactic acid due to anaerobic glycolysis, increased membrane permeability, and consecutive oedema formation. Since the anaerobic metabolism is inadequate to maintain cellular energy states, the ATP-stores deplete and failure of energy-dependent membrane ion pumps occurs. The second stage of the pathophysiological cascade is characterized by terminal membrane depolarization along with excessive release of excitatory neurotransmitters (i.e. glutamate, aspartate), activation of N-methyl-d-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazolpropionate, and voltage-dependent Ca2+- and Na+-channels. The consecutive Ca2+- and Na+-influx leads to self-digesting (catabolic) intracellular processes. Ca2+ activates lipid peroxidases, proteases, and phospholipases which in turn increase the intracellular concentration of free fatty acids and free radicals. Additionally, activation of caspases (ICE-like proteins), translocases, and endonucleases initiates progressive structural changes of biological membranes and the nucleosomal DNA (DNA fragmentation and inhibition of DNA repair). Together, these events lead to membrane degradation of vascular and cellular structures and ultimately necrotic or programmed cell death (apoptosis).
Excitotoxicity and oxidative stress
TBI is primarily and secondarily associated with a massive release of excitatory amino acid neurotransmitters, particularly glutamate.854 This excess in extracellular glutamate availability affects neurons and astrocytes and results in over-stimulation of ionotropic and metabotropic glutamate receptors with consecutive Ca2+, Na+, and K+-fluxes.2273 Although these events trigger catabolic processes including blood–brain barrier breakdown, the cellular attempt to compensate for ionic gradients increases Na+/K+-ATPase activity and in turn metabolic demand, creating a vicious circle of flow–metabolism uncoupling to the cell.1650
Oxidative stress relates to the generation of reactive oxygen species (oxygen free radicals and associated entities including superoxides, hydrogen peroxide, nitric oxide, and peroxinitrite) in response to TBI. The excessive production of reactive oxygen species due to excitotoxicity and exhaustion of the endogenous antioxidant system (e.g. superoxide dismutase, glutathione peroxidase, and catalase) induces peroxidation of cellular and vascular structures, protein oxidation, cleavage of DNA, and inhibition of the mitochondrial electron transport chain.31160 Although these mechanisms are adequate to contribute to immediate cell death, inflammatory processes and early or late apoptotic programmes are induced by oxidative stress.11
Non-technical summary
‘To be, or not to be’– thousands of neurons are facing this Shakespearean question in the brains of patients suffering from epilepsy or the consequences of a brain traumatism or stroke. The destiny of neurons in damaged brain depends on tiny equilibrium between pro-survival and pro-death signalling. Numerous studies have shown that the activity of the neuronal potassium chloride co-transporter KCC2 strongly decreases during a pathology. However, it remained unclear whether the change of the KCC2 function protects neurons or contributes to neuronal death. Here, using cultures of hippocampal neurons, we show that experimental silencing of endogenous KCC2 using an RNA interference approach or a dominant negative mutant reduces neuronal resistance to toxic insults. In contrast, the artificial gain of KCC2 function in the same neurons protects them from death. This finding highlights KCC2 as a molecule that plays a critical role in the destiny of neurons under toxic conditions and opens new avenues for the development of neuroprotective therapy.
A decrease in synaptic inhibition –disinhibition- appears to be an important substrate in several neuronal disorders, such as spinal cord injury (SCI), neuropathic pain... Glycine and GABA are the major inhibitory transmitters in the spinal cord. An important emerging mechanism by which the strength of inhibitory synaptic transmission can be controlled is via modification of the intracellular concentration of chloride ions ([Cl-]i) to which receptors to GABA/glycine are permeable. Briefly, a low [Cl-]i is a pre-requisite for inhibition to occur and is maintained in healthy neurons by cation-chloride co-transporters (KCC2) in the plasma membrane, which extrude Cl-. We showed recently (Nature Medicine, accepted for publication) that these transporters are down-regulated after SCI, thereby switching the action of GABA and glycine from inhibition to excitation; this can account for both SCI-induced spasticity and chronic pain. KCC2 transporters therefore appear as a new target to restore inhibition within neuronal networks in pathological conditions. The present project aims at reducing spasticity and chronic pain after SCI by up-regulating KCC2.
An important part will consist in identifying new compounds that increase the cell surface expression and/or the functionality of KCC2. Two strategies are considered. 1) Serotonin and BDNF will be tested on the basis of preliminary experiments and/or previous reports in other areas of the central nervous system indicating that these two compounds may affect the expression of KCC2. 2)Testing a large amount of compounds available in a library (“blind test”) to sort out KCC2-modulating molecules. This task can only be done in vitro on an assay that enables to easily visualize and quantify cell surface expression of KCC2, in response to these molecules (HEK293 cells). The few compounds isolated at the end of this task will then be tested on cultures of motoneurons (both mouse motoneurons and human motoneurons derived from induced pluripotent cells) and characterized further (potential toxicity, ability to cross the Brain Blood Barrier and effect on internalization and endocytosis of KCC2).
The selected candidate compounds will enter into the in vivo validation phase aimed at increasing the expression of KCC2 following spinal cord injury (SCI; both contusion and complete spinal cord transection). The selected hits will be applied by intrathecal injections in SCI rats and their effects on KCC2 expression in the plasma membrane of motoneurons will be tested by means of western blots and immunohistochemistry. Their efficacy in increasing the cell-surface expression of KCC2 will also be tested electrophysiologically in vitro (i.e. their ability to hyperpolarize ECl). Functionally, their efficacy in reducing both SCI-induced spasticity and chronic pain will be assessed.
Genetic tools will be used to increase the expression of KCC2 in some spinal neurons. This task will be done in collaboration with teams in the USA. Lentiviral vectors aimed at increasing KCC2 in the host cells, after parenchymal injection, have been developed in San Diego. A transgenic mouse model with a conditional tamoxifen-induced overexpression of KCC2 has been developed in Pittsburgh. The rationale for this part of the project is to use these genetic tools in the chronic phase of SCI to reduce spasticity and chronic pain.
The last part of the project will focus on more fundamental issues regarding the relationship between the SCI-induced downregulation of KCC2 and the development of spasticity and chronic pain.
The significance of the expected results goes far beyond the scope of SCI, since altered chloride homeostasis resulting from mutation or dysfunction of cation-chloride cotransporters has been implicated in various neurological disorders such as, for instance, ischemic seizures neonatal seizures and temporal lobe epilepsy.
In healthy adults, activation of γ-aminobutyric acid (GABA)(A) and glycine receptors inhibits neurons as a result of low intracellular chloride concentration ([Cl(-)](i)), which is maintained by the potassium-chloride cotransporter KCC2. A reduction of KCC2 expression or function is implicated in the pathogenesis of several neurological disorders, including spasticity and chronic pain following spinal cord injury (SCI). Given the critical role of KCC2 in regulating the strength and robustness of inhibition, identifying tools that may increase KCC2 function and, hence, restore endogenous inhibition in pathological conditions is of particular importance. We show that activation of 5-hydroxytryptamine (5-HT) type 2A receptors to serotonin hyperpolarizes the reversal potential of inhibitory postsynaptic potentials (IPSPs), E(IPSP), in spinal motoneurons, increases the cell membrane expression of KCC2 and both restores endogenous inhibition and reduces spasticity after SCI in rats. Up-regulation of KCC2 function by targeting 5-HT(2A) receptors, therefore, has therapeutic potential in the treatment of neurological disorders involving altered chloride homeostasis. However, these receptors have been implicated in several psychiatric disorders, and their effects on pain processing are controversial, highlighting the need to further investigate the potential systemic effects of specific 5-HT(2A)R agonists, such as (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2).
Conclusion
Very little is certain in autism, in great part because only about 200 brains have ever been examined post mortem. There are many theories, but very many more sub-types of autism.
GABAA dysfunction due to the faulty GABA switch never increasing KCC2 expression in the first weeks of life, triggering glutamate excitotoxicity and all that follows would go a long way to explaining my son’s type of autism. It might well explain 30+% of all autism.
Clearly other causes of excess glutamate would lead to a similar result.
