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Relationship: 445

Title

The title of the KER should clearly define the two KEs being considered and the sequential relationship between them (i.e., which is upstream and which is downstream). Consequently all KER titles take the form “upstream KE leads to downstream KE”.  More help

BDNF, Reduced leads to Down Regulation, K-Cl co-transporter 2 (KCC2)

Upstream event
Upstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help
Downstream event
Downstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

This table is automatically generated upon addition of a KER to an AOP. All of the AOPs that are linked to this KER will automatically be listed in this subsection. Clicking on the name of the AOP in the table will bring you to the individual page for that AOP. More help

Taxonomic Applicability

Select one or more structured terms that help to define the biological applicability domain of the KER. In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. Authors can indicate the relevant taxa for this KER in this subsection. The process is similar to what is described for KEs (see pages 30-31 and 37-38 of User Handbook) More help

Sex Applicability

Authors can indicate the relevant sex for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of the User Handbook). More help

Life Stage Applicability

Authors can indicate the relevant life stage for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of User Handbook). More help

Key Event Relationship Description

Provide a brief, descriptive summation of the KER. While the title itself is fairly descriptive, this section can provide details that aren’t inherent in the description of the KEs themselves (see page 39 of the User Handbook). This description section can be viewed as providing the increased specificity in the nature of upstream perturbation (KEupstream) that leads to a particular downstream perturbation (KEdownstream), while allowing the KE descriptions to remain generalised so they can be linked to different AOPs. The description is also intended to provide a concise overview for readers who may want a brief summation, without needing to read through the detailed support for the relationship (covered below). Careful attention should be taken to avoid reference to other KEs that are not part of this KER, other KERs or other AOPs. This will ensure that the KER is modular and can be used by other AOPs. More help

BDNF is a trophic factor that has been identified to be a potent modulator of K+ Cl- co-transporter 2 (KCC2) functional expression in the brain. The up-regulation of KCC2 that is a major milestone in brain development has been shown to be actively regulated by the up-regulation of BDNF at the same period (Medina et al., 2014).

Evidence Supporting this KER

Assembly and description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for overall assessment of the AOP (see pages 49-56 of the User Handbook). To do this, biological plausibility, empirical support, and the current quantitative understanding of the KER are evaluated with regard to the predictive relationships/associations between defined pairs of KEs as a basis for considering WoE (page 55 of User Handbook). In addition, uncertainties and inconsistencies are considered. More help
Biological Plausibility
Define, in free text, the biological rationale for a connection between KEupstream and KEdownstream. What are the structural or functional relationships between the KEs? For example, there is a functional relationship between an enzyme’s activity and the product of a reaction it catalyses. Supporting references should be included. However, it is recognised that there may be cases where the biological relationship between two KEs is very well established, to the extent that it is widely accepted and consistently supported by so much literature that it is unnecessary and impractical to cite the relevant primary literature. Citation of review articles or other secondary sources, like text books, may be reasonable in such cases. The primary intent is to provide scientifically credible support for the structural and/or functional relationship between the pair of KEs if one is known. The description of biological plausibility can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured (see page 40 of the User Handbook for further information).   More help

BDNF is widely expressed in the developing CNS and one of their major roles is the control of the intrinsic activity of developing neuronal networks. Further evidence has shown that it enhances the frequency and the subsequent activity synchronization in immature neurons, a process which is closely related to network maturation (Aguado et al., 2003; Carmona et al., 2006). This assumption was further supported by the ability of BDNF to depolarize cortical neurons in culture (Kafitz et al., 1999), an effect which has been linked to the developmentally regulated spontaneous network activity (Feller, 1999; O'Donoval, 1999). The spontaneous neuronal activity early in development is also closely related to Cl-homeostasis, as the increased intracellular [Cl-] facilitates the emergence of this initial spontaneous electrical activity (Farrant et al., 2007). KCC2 is the main K+ Cl- co-transporter in the brain, while its expression is developmentally controlled (Rivera et al., 1999). It is believed to be a major regulator of [Cl-] during neuronal maturation and its up-regulation coincides with the decrease of intracellular [Cl-] levels and the subsequent hyperpolarizing potential of the developing cortical neurons (Rivera et al., 1999). It has been suggested that the developmental up-regulation of KCC2 is activity dependent and closely related to the depolarizing action of GABAAR (Gangulu et al., 2001). Taking these under consideration, it was assumed that the BDNF regulation of neuronal activity was mediated by direct or indirect modulation of KCC2 expression. More detailed mechanistic evidence of this relationship further supported the initial suggestion of developmental relationship between these two factors. Specifically, it was shown that exogenously applied BDNF to immature hippocampal neurons caused a rapid increase in KCC2 mRNA levels through the activation of KCC2 promoter by the transcription factor Egr4. In consistence with the previous studies, these effects were attenuated by specific TrkB and MAPK inhibitors (Ludwig et al., 2011). Supporting to this suggestion, other studies have shown that the developmental up-regulation of KCC2 protein was enhanced by exogenous BDNF application but only in the presence of the two Repressor Elements-1 (RE-1), which are located in the promoter of the KCC2 gene (Yeo et al., 2009). The latter results lead to the hypothesis that BDNF can indirectly regulate KCC2 enhanced transcription by inhibiting the binding of the RE-1 sites to the REST transcriptional repressor complex, which could be a consequence of activation of the TrkB-related MAPK signalling pathway, as already discussed. However, the hypothesis is still under development and further investigation is required.

Uncertainties and Inconsistencies
In addition to outlining the evidence supporting a particular linkage, it is also important to identify inconsistencies or uncertainties in the relationship. Additionally, while there are expected patterns of concordance that support a causal linkage between the KEs in the pair, it is also helpful to identify experimental details that may explain apparent deviations from the expected patterns of concordance. Identification of uncertainties and inconsistencies contribute to evaluation of the overall WoE supporting the AOPs that contain a given KER and to the identification of research gaps that warrant investigation (seep pages 41-42 of the User Handbook).Given that AOPs are intended to support regulatory applications, AOP developers should focus on those inconsistencies or gaps that would have a direct bearing or impact on the confidence in the KER and its use as a basis for inference or extrapolation in a regulatory setting. Uncertainties that may be of academic interest but would have little impact on regulatory application don’t need to be described. In general, this section details evidence that may raise questions regarding the overall validity and predictive utility of the KER (including consideration of both biological plausibility and empirical support). It also contributes along with several other elements to the overall evaluation of the WoE for the KER (see Section 4 of the User Handbook).  More help

BDNF is a potent regulator of KCC2 not only during the developmental phase but also in the adult stage but in a different way, showing the maturation-dependent action of BDNF on KCC2 regulation (Ferrini and Konick, 2013). BDNF in mature neurons causes down-regulation of KCC2 in mRNA and protein level (Rivera et al., 2002; 2004). Other studies also suggest that BDNF up-regulates KCC2 after neuronal injury (Boulenguez et al., 2010) and after seizures (Puskarjov et al., 2014) and the suggested plausible reason is that the repairing procedure of the neurons demands properties similar to those of the immature neurons. Up to date, the exact mechanistic pathways that are followed in the different abovementioned cases are not well characterized and understood. In the most recent study of Puskarjov et al., 2014, BDNF-/- mice were utilized to show that in the absence of BDNF the seizure-induced up regulation of KCC2 was eliminated, but interestingly no change in early (P5-6) or later (P13-14) postnatal KCC2 expression was observed compared to the wild type littermates. However, the functionality of the protein was not investigated nor the ability of the neurons to extrude Cl- in the absence of BDNF. Additionally, other studies have shown that the up-regulation of KCC2 via the transcriptional factor Egr4 is also regulated by a different neurotrophic factor, the neurturin (Ludwig et al., 2011b). These results reveal that the same transcriptional pathways are downstream from different neurotrophic factors and they might lead to the same outcome under different conditions. This suggestion should be further investigated, as this could explain the compensation mechanisms that are activated in the total absence of BDNF, and which might be different from those that are triggered by the decrease of BDNF levels.

Response-response Relationship
This subsection should be used to define sources of data that define the response-response relationships between the KEs. In particular, information regarding the general form of the relationship (e.g., linear, exponential, sigmoidal, threshold, etc.) should be captured if possible. If there are specific mathematical functions or computational models relevant to the KER in question that have been defined, those should also be cited and/or described where possible, along with information concerning the approximate range of certainty with which the state of the KEdownstream can be predicted based on the measured state of the KEupstream (i.e., can it be predicted within a factor of two, or within three orders of magnitude?). For example, a regression equation may reasonably describe the response-response relationship between the two KERs, but that relationship may have only been validated/tested in a single species under steady state exposure conditions. Those types of details would be useful to capture.  More help
Time-scale
This sub-section should be used to provide information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). This can be useful information both in terms of modelling the KER, as well as for analyzing the critical or dominant paths through an AOP network (e.g., identification of an AO that could kill an organism in a matter of hours will generally be of higher priority than other potential AOs that take weeks or months to develop). Identification of time-scale can also aid the assessment of temporal concordance. For example, for a KER that operates on a time-scale of days, measurement of both KEs after just hours of exposure in a short-term experiment could lead to incorrect conclusions regarding dose-response or temporal concordance if the time-scale of the upstream to downstream transition was not considered. More help
Known modulating factors
This sub-section presents information regarding modulating factors/variables known to alter the shape of the response-response function that describes the quantitative relationship between the two KEs (for example, an iodine deficient diet causes a significant increase in the slope of the relationship; a particular genotype doubles the sensitivity of KEdownstream to changes in KEupstream). Information on these known modulating factors should be listed in this subsection, along with relevant information regarding the manner in which the modulating factor can be expected to alter the relationship (if known). Note, this section should focus on those modulating factors for which solid evidence supported by relevant data and literature is available. It should NOT list all possible/plausible modulating factors. In this regard, it is useful to bear in mind that many risk assessments conducted through conventional apical guideline testing-based approaches generally consider few if any modulating factors. More help
Known Feedforward/Feedback loops influencing this KER
This subsection should define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits? In some cases where feedback processes are measurable and causally linked to the outcome, they should be represented as KEs. However, in most cases these features are expected to predominantly influence the shape of the response-response, time-course, behaviours between selected KEs. For example, if a feedback loop acts as compensatory mechanism that aims to restore homeostasis following initial perturbation of a KE, the feedback loop will directly shape the response-response relationship between the KERs. Given interest in formally identifying these positive or negative feedback, it is recommended that a graphical annotation (page 44) indicating a positive or negative feedback loop is involved in a particular upstream to downstream KE transition (KER) be added to the graphical representation, and that details be provided in this subsection of the KER description (see pages 44-45 of the User Handbook).  More help

Domain of Applicability

As for the KEs, there is also a free-text section of the KER description that the developer can use to explain his/her rationale for the structured terms selected with regard to taxonomic, life stage, or sex applicability, or provide a more generalizable or nuanced description of the applicability domain than may be feasible using standardized terms. More help

References

List of the literature that was cited for this KER description using the appropriate format. Ideally, the list of references should conform, to the extent possible, with the OECD Style Guide (OECD, 2015). More help

Aguado F, Carmona MA, Pozas E, Aguiló A, Martínez-Guijarro FJ, Alcantara S, Borrell V, Yuste R, Ibañez CF, SorianoE. (2003). BDNF regulates spontaneous correlated activity at early developmental stages by increasing synaptogenesis and expression of the K+/Cl–co-transporter KCC2. Development 130:1267-1280.

Boulenguez P, Liabeuf S, Bos R, Bras H, Jean-Xavier C, Brocard C, Stil A, Darbon P, Cattaert D, Delpire E, Marsala M, Vinay L. (2010). Downregulation of the potassium-chloride cotransporter KCC2 contributes to spasticity after spinal cord injury. Nat Med 16:302–307.

Carmona MA, Pozas E, Martínez A, Espinosa-Parrilla JF, Soriano E, Aguado F. (2006). Age-dependent spontaneous hyperexcitability and impairment of GABAergic function in the hippocampus of mice lacking trkB. Cereb Cortex 16:47– 63.

Farrant M, Kaila K. (2007). The cellular, molecular and ionic basis of GABA(A) receptor signalling. Prog Brain Res 160:59–87.

Feller MB. (1999). Spontaneous correlated activity in developing neural circuits. Neuron 22: 653-656.

Ferrini F, DeKoninck Y. (2013). Microglia control neuronal network excitability via BDNF signalling. Neural Plast 429815.

Ganguly K, Schinder AF, Wong ST, Poo M.(2001). GABA itself promotes the developmental switch of neuronal GABAergic responses from excitation to inhibition. Cell 105:521–532.

Kafitz KW, Rose CR, Thoenen H, Konnerth A.(1999). Neurotrophin-evoked rapid excitation through trkB receptors. Nature 401:918–921.

Ludwig A, Uvarov P, Soni S, Thomas-Crusells J, Airaksinen MS, Rivera C. (2011a). Early growth response 4 mediates BDNF induction of potassium chloride co-transporter 2 transcription. J Neurosci 31:644-649.

Ludwig A, Uvarov P, Pellegrino C, Thomas-Crusells J, Schuchmann S, Saarma M, Airaksinen MS, Rivera C. (2011b). Neurturin evokes MAPK dependent up-regulation of Egr4 and KCC2 in developing neurons. Neural Plast 1-8.

Medina I, Friedel P, Rivera C, Kahle KT, Kourdougli N, Uvarov P, Pellegrino C. (2014) Current view on the functional regulation of the neuronal K+/Cl− cotransporter KCC2. Front Cell Neurosci 8: 27. O’Donovan MJ. (1999). The origin of spontaneous activity in developing networks of the vertebrate nervous system. Curr Opin Neurobiol 9:94–104.

Puskarjov M, Ahmad F, Khirug S, Sivakumaran S, Kaila K. (2014). BDNF is required for seizure-induced but not developmental up-regulation of KCC2 in the neonatal hippocampus. (2014). Neuropharmacology 1-7.

Rivera C, Voipio J, Payne JA, Ruusuvuori E, Lahtinen H, Lamsa K, Pirvola U, Saarma M, Kaila K. (1999). The K-/Cl- co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation. Nature 397:251–255.

Rivera C, Li H, Thomas-Crusells J, Lahtinen H, Viitanen T, Nanobashvili A, Kokaia Z, Airaksinen MS, Voipio J, Kaila K, Saarma M. (2002). BDNF induced TrkB activation down-regulates the K--Cl- co-transporter KCC2 and impairs neuronal Cl- extrusion. J Cell Biol 159:747–752.

Rivera C, Voipio J, Thomas-Crusells J, Li H, Emri Z, Sipila¨ S, Payne JA, Minichiello L, Saarma M, Kaila K. (2004). Mechanism of activity dependent downregulation of the neuron-specific K-Cl cotransporter KCC2. J Neurosci 24:4683– 4691.

Yeo M, Berglund K, Augustine G, Liedtke W. (2009). Novel repression of Kcc2 transcription by REST-RE-1 controls developmental switch in neuronal chloride. J Neurosci 29:14652–14662.