Brain derived neurotrophic factor-BDNF-and Autism- II-Impact of xenobiotics

Now, we know some about the roles and importance of BDNF.
Time to some questions
What gene/ receptor activated produces BDNF?
What sequence of reactions produces BDNF?
What affects BDNF production?

This is from IMFAR 2007

“SELECTIVE INDUCTION OF BRAIN-DERIVED NEUROTROPHIC FACTOR (BDNF) SECRETION BY ASTROCYTES CHALLENGED WITH A TOLL-LIKE RECEPTOR (TLR) 9 AGONIST: POSSIBLE LINK BETWEEN PERINATAL INFECTION AND ELEVATED BDNF LEVELS IN AUTISM
List of Authors
D. S. Spinner, E. J. Marchi, D. J. Kerr, E. Babcock, G. LaFauci, G. S. Merz, R. J. Kascsak, W. T. Brown
Background: Elevation of the neurotrophic factor BDNF in serum and glial cell activation in the brain are both common features of autism. Viral infection during the perinatal period such as rubella in humans, or influenza and Borna disease in rodents are also associated with autism or autism-like neurodevelopmental abnormalities. Since viral and other infections trigger innate immune responses by activating members of the Toll-like receptor (TLR) family (TLR1-13), we investigated whether TLR ligation increases secretion of autism-associated neurotrophic factors by glial cells.

Objectives: To determine whether activation of TLR signaling in microglia and astrocytes influences the secretion of neurotrophic factors.

Methods: Microglia and astrocytes in culture were exposed to agonists for TLR2 (bacterial lipopeptide pam3CSK4), TLR3 (poly(I:C) double-stranded RNA), TLR4 (lipopolysaccharide), or TLR9 (unmethylated CpG DNA) at 10 ug/ml for 48 hrs. Levels of BDNF in culture medium were quantified by a Luminex-based immunoassay.

Results: BDNF secretion in astrocytes was substantially increased upon exposure to the TLR9 agonist CpG DNA, but not following exposure to agonists for TLR2, 3, or 4. In microglia, however, BDNF remained undetectable under all experimental conditions. Both astrocytes and microglia were found to express all TLRs tested (TLR2, 3, 4, and 9).
Conclusions: Activation of TLR9 signaling in astrocytes specifically upregulates BDNF secretion. These findings provide links bridging infection, gliosis, and biochemical features of autism, and thus have significant implications regarding a role for innate immune activation in the etiology of autism.”

Therefore I want to introduce TLR-9. TLR are Toll-like Receptors.
TLR-9

There are TLR on the cell surfaces and TLR on the membranes that are used to degrade patogens.
You will find this description
“TLR-9* binds unmethylated cytosine-guanine dinucleotide sequences (CpG DNA) found in bacterial and viral genomes”
The point is that the cytosine-guanine dinucleotide sequence is NOT present in mammals, therefore this is a “radar” to detect non- self sequences .

TLR
In all, thousands of genes are activated by TLR signaling, and collectively, the TLRs constitutes one of the most powerful and important gateways for gene modulation.

Exp Dermatol. 2006 May;15(5):331-41.
Immune response modifiers--mode of action.Schiller M, Metze D, Luger TA, Grabbe S, Gunzer M.
The innate immune system governs the interconnecting pathways of microbial recognition, inflammation, microbial clearance, and cell death. A family of evolutionarily conserved receptors, known as the Toll-like receptors (TLRs), is crucial in early host defense against invading pathogens. Upon TLR stimulation, nuclear factor-kappaB activation and the interferon (IFN)-regulatory factor 3 pathway initiate production of pro-inflammatory cytokines, such as interleukin-1 and tumor necrosis factor-alpha, and production of type I IFNs (IFN-alpha and IFN-beta), respectively. The innate immunity thereby offers diverse targets for highly selective therapeutics, such as small molecular synthetic compounds that modify innate immune responses. The notion that activation of the innate immune system is a prerequisite for the induction of acquired immunity raised interest in these immune response modifiers as potential therapeutics for viral infections and various tumors…The induction of predominately T helper (Th)1-type cytokine profiles by TLR agonists such as imiquimod might have further benefits by shifting the dominant Th2-type response in atopic diseases such as asthma and atopic dermatitis to a more potent Th1 response.

TLR-9 is related to GI issues
Homeostatic effects of TLR9 signaling in experimental colitis

The Pathway of TLR-9 and other TLR

Int Rev Immunol. 2006 May-Aug;25(3-4):155-81.
TLR9 in health and disease.Vollmer J.
Coley Pharmaceutical, GmbH, Langenfeld, Germany.

Toll-like receptor 9 (TLR9) is specialized for the recognition of pathogenic nucleic acids. TLR9 is expressed in intracellular compartments where it responds specifically to pathogen DNA. Several factors contribute to the ability of TLR9 to discriminate between self and foreign DNA. Regulatory mechanisms of the innate and adaptive immune system exist that balance the immune responses mediated by TLR9. Short synthetic CpG oligodeoxynucleotides -the ones I mentioned above-are used to induce controlled and directed TLR9-dependent stimulation and are effective immune modulators in preclinical and clinical studies. This review will summarize the interplay between TLR9-dependent opposing stimulatory and regulatory effects in innate and adaptive immunity.
Therefore TLR-9 is important not only because of the substrates that bind to it, but also because of the consecuences of the binding (such as BDNF elevation)

Now, let´s go to present some published clues on the impact on BDNF and neurotrophic factors in general of the environment

BDNF

*Regulates glycinergic and Gabaergic synaptogenesis in developing spinal neurons
Link
*Triggers Calcium peaks in developìng dendrites and Calcium signalling with other neurotrophic factors
Link-1

*Modulates the function of the Serotonin transporter function
Link
Elevated platelet serotonin (5-HT) in 20%-25% of cases and efficacy of selective 5-HT reuptake inhibitors (SSRIs) in treating anxiety, depression, and repetitive behaviors points to the 5-HT transporter (5-HTT; SERT) as a strong candidate gene.
mounting evidence for genetic linkage of autism to the chromosome 17q11.2 region that harbors the SERT locus (SLC6A4) supports a genetic effect at or near this gene. We confirm recent reports of sex-biased genetic effects in 17q by showing highly significant linkage driven by families with only affected males. Association with common alleles fails to explain observed linkage; therefore, we hypothesized that preferential transmission of multiple alleles does explain it

Am J Hum Genet. 2005 Aug;77(2):265-79.
Allelic heterogeneity at the serotonin transporter locus (SLC6A4) confers susceptibility to autism and rigid-compulsive behaviors.Sutcliffe JS, Delahanty RJ, Prasad HC, McCauley JL, Han Q, Jiang L, Li C, Folstein SE, Blakely RD.

What if there is a combination of SLC6A4 polymorphism plus high BDNF?

*Affects the disease progression of MeCp2 mutant mice
Link

*Up regulates the alpha7 nicotinic acethylcholine receptor levels in hippocampal neurons
Link
In neurons the alpha-7 nicotinic receptor activates PI3 kinase, activating the anti-apoptotic kinase AKT.

*Is higher in fullterm than in preterm non-autistic children
Link
Not only neonatal BDNF levels, but also levels during the first week of life are important. In Preterm neonates day 4 the levels was much lower that in Preterm day 1. It would be interesting to see how BDNF changes during development in autistics.

*It is related to thymocytes activatioin
Link
a complex network involving the neurotrophin BDNF and its specific receptors may have a role in sustaining thymocyte precursor survival and supporting the thymocyte differentiation process. All together, our results suggest that the thymus may be the site of integration of different neuroimmune networks that are potentially involved in the regulation of thymocyte survival and/or differentiation

* It is related to transient receptor potencial C
Link
*It is expressed in several parts of the (rat) developing brain
Link
*It is less efficiently processed than NGF
Link
*It is very much involved in synaptic plasticity
Link
Our data provide striking evidence for a rapid inhibitory effect of p75(NTR) on NMDA-R currents that antagonizes TrkB-mediated NMDA-R potentiation. These opposing mechanisms might be present in a large proportion of forebrain synapses and may contribute importantly to synaptic plasticity.
Impact of diet
*BDNF Modulates the outcome of a fat saturated diet on synaptic plasticity and cognition
Link
*BDNF´s receptors are localized to enteric ganglion cells and glia (TrkB). TrkA also share same localization ( but not TrkC).
Link

In postnatal intestine, BDNF immunoreactivity was primarily localized to enteric ganglion cells, with NT-3 localized to enteric plexuses, intermuscular basal lamina, and along or between circular and longitudinal smooth muscle cells. CONCLUSIONS: These data indicate that neurotrophic influences may be involved in ENS development and survival, with potential importance in functional differentiation disorders of the intestinal ENS.

*BDNF expression is related to diet

Link

*Two months on the HFS diet were sufficient to reduce hippocampal level of BDNF and spatial learning performance
Link

Allergies
*BDNF is upregulated during allergic inflammation. Within the inflammed tissue neurons, structural cells and invading immune cells are sources and targets of neurotrophins.
Link
*BDNF is altered in asthma
Link
Furthermore, immune cells themselves can produce neurotrophins under certain conditions, and the levels of neurotrophins, as well as neurotrophic activities, are strongly upregulated in allergic conditions.

The role of neurotrophins in bronchial asthma: contribution of the pan-neurotrophin receptor p75
Nerve growth factor: an important molecule in allergic inflammation and tissue remodelling
*BDNF levels can be a marker of atopic dermatitis
Link
*BDNF selectively enhances specific allergen IgE production
Link

Xenobiotic impact through oxidative stress

*In the cellular answer to oxidative stress ( mediated by H2O2) sustained activation of all three MAPK subfamilies: extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 was found, inducing apoptosis. NF-kappaB is related to both the ERK and JNK/SAPK signalling. Being simple, oxidative stress affects/induces cell death.
Necrosis is different from apoptosis.Apoptosis may occur in a single cell surrounded by viable cells;necrosis involves a group of cells simultaneously.

*Apoptosis can be related to caspases activation
Link
*This MAPK can activate a lot of reactions
Link
Free radical scavengers N-acetyl-L-cysteine (NAC), or glutathione (GSH), inhibited ERK2 activation and, to a much lesser extent, JNK1 activation by BHA/tBHQ, implicating the role of oxidative stress.
Several ribosome-directed toxicants have the capacity to damage 28S ribosomal RNA and/or interfere with its functioning, thus compromising protein synthesis. This can lead to what has been called "ribotoxic stress," a response that stimulates MAP kinase signaling
Link

Toxic Elements
*BDNF modulates PKC activation. PKC can be affected by heavy metals
Link
*Mercury in different forms abolish activation of different kinase pathways
Link-1
Link-2
Link-3
Or causes necrosis in certain cell lines as thimerosal
Link-4

Control levels of fragmented DNA were similar in both the presence and absence of NGF. With and without NGF, thimerosal caused elevated levels of fragmented DNA appearing at 0.01 microM (apoptosis) to decrease at concentrations >1 microM (necrosis). These data demonstrate that thimerosal could alter NGF-induced signaling in neurotrophin-treated cells at concentrations lower than those responsible for cell death.

*Lead affects diferent kinase pathways
Link-1
These results indicate that the phosphorylation of hippocampal ERK1/2 and p38(MAPK) is stimulated by lead in a period of rapid brain development, an effect that may underlie, at least in part, the neurotoxicty elicited by this metal.
Link-2
Link-3
Link-4

*Aluminium induces apoptosis
Link-1
Our study demonstrates that aluminum can induce the apoptosis of cortical neurons and SAPK/JNK signal transduction pathway may play an important role in the apoptosis
Link-2

Essential elements
*Zn defficiency induces cell death activates Trk signalink pathway

Link-1
Link-2
Link-3
and BDNF expression
Link-4

Recent work has shown that zinc is involved in the developmental regulation of neurotrophins and N-methyl-D-aspartate (NMDA) receptors, controlling use of glutamate as a neurotransmitter in the central nervous system (CNS). This is particularly important in the hippocampus, a region of the brain involved in learning and memory, and is an intriguing link to the role of zinc in neuropsychological development.

Nutr Rev. 2006 Sep;64(9):428-32.
Regulation of the NMDA receptor: implications for neuropsychological development.Levenson CW.

*Iron and amyloid beta peptide neurotoxicity are related to proapoptotic signaling
Link-1
Please remember than in a subgroup of autistics high levels of amyloid beta peptide have been found in a recent manuscript.
Link-2
Link-3


*Calcium channels are of paramount importance here.

It is supposed that this is kind of a feedback.NMDA receptor stimulation induces the influx of extracelular Ca+2 that may evoke the release of BDNF and the activation of TRkB. TrkA is the receptor of NGF and TrkB is the receptor of BDNF. Interferon gamma (IFN-gamma) increased NFG expression, down regulated BDNF expression The chronic blocking of NMDA receptors has a secondary effect the inhibition of BDNF synthesis in the hippocampus and may impair neuronal development. Brain specific phosphorylation of ME CP2 regulates BDNF transcription, dendritic growth and spine maturity.

Link-1

Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations…

* Recent studies confirm the importance of Zinc
Link

Calcium homeostasis in the Central Nervous System – implications for brain development and autism

About other environmental stressors:
1-Formaldehyde and neurotrophins
Toxicology. 2004 Apr 1;197(1):1-13
Differential immunogenic and neurogenic inflammatory responses in an allergic mouse model exposed to low levels of formaldehyde.
Fujimaki H, Kurokawa Y, Kunugita N, Kikuchi M, Sato F, Arashidani K.
...These results provide the first experimental evidence that low levels of long-term formaldehyde inhalation can induce differential immunogenic and neurogenic responses in allergic mice...
2- Sci Total Environ. 2001 Apr 10;270(1-3):113-21. Induction of the imbalance of helper T-cell functions in mice exposed to diesel exhaust. Fujimaki H, Ushio H, Nohara K, Ui N.
Administration of diesel exhaust particles (DEP) increases antigen-specific IgE production and IgE-secreting cells, and induces Th2-type cytokine profiles in the airway in mice and humans. To determine the early effects of diesel exhaust (DE) inhalation on the cytokine production profile, BALB/c mice were exposed to 0 (controls) and 1.0 mg/m3 DE inhalation for 4 weeks. Intraperitoneal sensitization with ovalbumin (OVA) was conducted immediately before DE inhalation. Mice were treated with anti-CD4 or anti-CD8 mAb 1 day before and after the sensitization. On day 21, these mice were boosted with OVA and blood; bronchoalveolar lavage (BAL) fluid, and spleens were collected on day 28. In BAL fluid, both TNFalpha and IL-10 production in DE-exposed and control mice remained basically the same. IL-6 production in the anti-CD4 treatment group of DE-exposed mice, however, significantly increased compared with that of the controls. In vitro antigen-stimulated interleukin-4 (IL-4) and -10 (IL-10) production in spleen cells of exposed mice were not affected by low-dose DE inhalation. In vitro interferon (IFN)-gamma production in the anti-CD4 treated group of exposed mice decreased markedly. Although anti-OVA IgE production in the plasma of sham-treated mice exposed to DE was the same level as for controls, anti-CD4 mAb treatment in DE-exposed mice significantly reduced IgE production compared to controls. In anti-OVA IgG1 production, anti-CD4 or anti-CD8 mAb treatment in DE-exposed groups also significantly reduced. Anti-OVA IgG2a production was reduced by treatment with anti-CD4 mAb, but increased by anti-CD8 mAb treatment in DE-exposed mice. Low dose DE inhalation is thus shown to adversely affect the cytokine and antibody production in mice by altering CD4+ and CD8+ T-cell functions.
3-The effect of chlorpyrifos and chlorpyrifos-oxon on brain cholinesterase, muscarinic receptor binding, and neurotrophin levels in rats following early postnatal exposure
4-Developmental neurotoxicity of industrial chemicals
A few industrial chemicals (eg, lead, methylmercury, polychlorinated biphenyls [PCBs], arsenic, and toluene) are recognised causes of neurodevelopmental disorders and subclinical brain dysfunction. Exposure to these chemicals during early fetal development can cause brain injury at doses much lower than those affecting adult brain function. Recognition of these risks has led to evidence-based programmes of prevention, such as elimination of lead additives in petrol. Although these prevention campaigns are highly successful, most were initiated only after substantial delays. Another 200 chemicals are known to cause clinical neurotoxic effects in adults…
The toxic effects of such chemicals in the developing human brain are not known and they are not regulated to protect children. The two main impediments to prevention of neurodevelopmental deficits of chemical origin are the great gaps in testing chemicals for developmental neurotoxicity and the high level of proof required for regulation. New, precautionary approaches that recognise the unique vulnerability of the developing brain are needed for testing and control of chemicals.

Therefore imbalances in toxic/essential elements affects strongly BDNF expression, levels and receptors number and function. Even more, the kinase cascade that BDNF begins can be strongly affected by xenobiotics and oxidative stress- including ribotoxic stress.

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