Role of matrix metalloproteinase-9 in neurodevelopmental disorders and experience-dependent plasticity in Xenopus tadpoles

Matrix metalloproteinase-9 (MMP-9) is a secreted endopeptidase targeting extracellular matrix proteins, creating permissive environments for neuronal development and plasticity. Developmental dysregulation of MMP-9 is also associated with neurodevelopmental disorders (ND). Here we test the hypothesis that chronically elevated MMP-9 activity during early neurodevelopment is responsible for neural circuit hyperconnectivity observed after early exposure to valproic acid (VPA), a known teratogen associated with autism spectrum disorder in humans. In Xenopus tadpoles, VPA exposure results in excess local synaptic connectivity, disrupted social behavior and increased seizure susceptibility. We found that overexpressing MMP-9 in the brain copies effects of VPA on synaptic connectivity, and blocking MMP-9 activity pharmacologically or genetically reverses effects of VPA on physiology and behavior. We further show that during normal neurodevelopment MMP-9 levels are tightly regulated by neuronal activity and required for structural plasticity. These studies show a critical role for MMP-9 in both normal and abnormal development.


Introduction
Autism spectrum disorder (ASD) encompasses a highly heterogeneous set of neurodevelopmental conditions characterized by a complex behavioral phenotype and deficits in both social and cognitive functions.Over 100 gene mutations have been associated with ASD suggesting very different underlying etiologies (Betancur, 2011;Betancur & Coleman, 2013;Butler, 2018;Croft et al., 2011;Grove et al., 2019).This complex neurodevelopmental disorder, which is influenced by both genetic and environmental factors, results from profound changes in brain function and connectivity.The pathophysiology of ASD is often attributed to abnormal synaptic maturation and plasticity, defects in microcircuitry organization, disruption of the brain excitation to inhibition balance, local overconnectivity and long-range underconnectivity throughout the brain (Kumar et al., 2019;Markram & Markram, 2010).Even though some aspects of the etiology of ASD have been studied, how ASD-related behaviors arise from synaptic dysfunction and abnormal neural circuit development remain elusive.
Studies in both humans (Bromley et al., 2013;Christensen et al., 2013) and animal models (Markram et al., 2008;Rodier et al., 1996;Roullet et al., 2010;Schneider et al., 2006;Schneider & Przewłocki, 2005) have shown that prenatal exposure to valproic acid (VPA), a common antiepileptic drug, results in a higher incidence of ASD in humans or ASD-like deficits in animals.In rodents, in utero exposure to VPA results in autistic-like behaviors in offspring; namely decreased socialization, increased repetitive behaviors, and hypersensitivity to sensory stimuli (Kataoka et al., 2013;Kim et al., 2011;Mehta et al., 2011;Moldrich et al., 2013).These behavioral deficits are accompanied by corresponding deficits in brain physiology, including increased local recurrent connectivity, synaptic activity, and decreased intrinsic neuronal excitability (Markram & Markram, 2010).Xenopus tadpoles when exposed to VPA have abnormal sensory-motor and schooling behavior which is accompanied by hyper-connected neural networks in the optic tectum, increased excitatory and inhibitory synaptic drive, elevated levels of (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
Consistent with these findings, VPA treated tadpoles also have increased seizure susceptibility and decreased acoustic startle habituation (James et al., 2015).VPA, a known histone-deacetylase inhibitor, causes these effects by hyperacetylation of gene promoters, and thus, by altering gene expression of various targets (Ghodke-Puranik et al., 2013;Terbach & Williams, 2009).Preliminary studies in Xenopus using microarrays identified MMP-9 upregulation as a potential downstream effector of VPA exposure, suggesting that dysregulation of this protein may be important for mediating effects of VPA.
MMP-9 is a secreted zinc-dependent extracellular endopeptidase that can cleave ECM and several cell surface receptors facilitating synaptic and circuit level reorganization (Ethell & Ethell, 2007;Niedringhaus et al., 2012;Reinhard et al., 2015).Early formation of proper synaptic connections and neural circuits is heavily regulated by the pericellular environment, and dysregulation of proteins important for extracellular matrix function could disrupt this process resulting in circuit abnormalities associated with ASD (Choi, 2018;Ferrer-Ferrer & Dityatev, 2018).During development, experiencedependent activity during critical periods can drive neural plasticity and MMP-9 plays an important role in this critical period plasticity (Murase et al., 2017;Oliveira-Silva et al., 2007;Reinhard et al., 2015;Spolidoro et al., 2012).Furthermore, MMP-9 is thought to play a role in adult neurogenesis and synaptic plasticity in the adult brain, and has been implicated in neural circuit formation (Fujioka et al., 2012;Peixoto et al., 2012;Pielecka-Fortuna et al., 2015;Wang et al., 2008).
Previous studies from the lab have utilized the Xenopus laevis tadpole CNS as a model for understanding the synaptic origin of neurodevelopmental disorders (James et al., 2015;Pratt & Khakhalin, 2013;Truszkowski et al., 2016).The optic tectum, a region in Xenopus CNS, receives converging inputs from multiple sensory modalities and is responsible for carrying out multisensory (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
The copyright holder for this preprint this version posted August 13, 2020.; https://doi.org/10.1101/2020.05.29.123661 doi: bioRxiv preprint integration(Deeg & Aizenman, 2011;Pratt & Aizenman, 2007, 2009).The retinotectal circuit and local circuits within the optic tectum are known to refine during development in an activity-dependent manner and mediate robust visually-guided behaviors that are very sensitive to abnormal circuit development (Dong et al., 2009;R. H. Lee et al., 2010), providing a strong link between neural circuit dysfunction and behavior and provide a robust model for understanding neurodevelopmental disorders at various levels of organization -from synapses to behavior.The present work investigates (1) whether upregulation of MMP-9 by whole-brain electroporation mimics electrophysiological effects induced by VPA, (2) whether downregulation of MMP-9 using a pharmacological agent or an antisense morpholino directed against MMP-9 rescues VPA-induced electrophysiological and behavioral effects, and (3) whether MMP-9 is required for activity dependent structural plasticity.

MMP-9 overexpression increases spontaneous synaptic activity and network connectivity, an effect also observed after VPA exposure
Exposure to VPA increases spontaneous synaptic activity, network connectivity and excitability in the developing tectum in Xenopus (James et al., 2015).Previous microarray studies from the lab have shown that VPA exposure results striking alterations in gene expression, including a greater than two-fold increase in matrix metalloproteinase 9 (MMP-9).Here we validated this finding by performing qPCR on tectal samples of VPA-exposed tadpoles and found a several-fold increase in mRNA levels for MMP9 [increase in MMP9 mRNA: 2.7±0.3 fold increase over control, n=3 replicates].This suggested the hypothesis that chronically elevated levels of MMP-9 may mediate some of the effects of VPA by promoting excess plasticity leading to local hyperconnectivity.Thus, we first tested whether overexpression of MMP-9 mimics the VPA-induced effects on synaptic connectivity and excitability in (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
The copyright holder for this preprint this version posted August 13, 2020.; https://doi.org/10.1101/2020.05.29.123661 doi: bioRxiv preprint more information about the tectal connectivity and excitability, we counted the number of spontaneous recurrent barrages occurring in MMP-9 overexpressing tectal cells as well as from GFP control cells.
Increased number of spontaneous recurrent barrages were observed in both transfected and nontransfected cells from MMP-9 overexpressing animals, compared to GFP controls, indicating persistent local connectivity which usually is pruned during development (Pratt et al., 2008) [Mean number of barrages/min : GFP control,0.1 ± 0.1,n = 11;1.13 ± 0.37,n = 15;1.10 ± 0.40,n = 19] (Fig. 1D).The increased amount of barrage activity in non-transfected neurons, indicates that a small number of MMP-9-overexpressing cells are also sufficient to drive enhanced activity in the whole tectal network.
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Morpholino targeted against MMP-9 rescues VPA-induced effects
As an alternative manipulation, we genetically downregulated levels of MMP-9 using an antisense morpholino oligo (MO) targeted against MMP-9 to test whether we could reverse VPA-induced effects.
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Inhibition of MMP-9 reverses VPA-induced behavioral effects
The results so far suggest that increasing levels of MMP-9 produces VPA-like hyperconnected excitable tectal networks while downregulation of MMP-9 rescues these effects.One consequence resulting from increased recurrent circuitry within the tectum, and possibly other brain regions, is that this circuitry can promote the generation of behaviors associated with hyperexcitability like increased seizure susceptibility or abnormal startle habituation response (James et al., 2015).We tested whether inhibition of MMP-9 could reverse these behavioral effects of VPA.To measure seizure susceptibility, tadpoles were exposed to 5mM of the convulsant PTZ, over a period of 20 minutes, during which seizure activity was characterized.One-way ANOVA revealed significant differences for seizure frequency across different treatment groups [seizure frequency (events/minute): p = 0.01; time to first seizure (s): p = 0.006].Consistent with prior work, VPA-reared tadpoles exhibited more frequent pharmacologically induced seizures compared to their matched controls, while animals reared in VPA+SB-3CT showed no increased seizure frequency, that is, a reversal of the VPA-induced effect [seizure frequency (events/minute): control, 0.72 ± 0.04, n = 48; VPA, 0.90 ± 0.04, n = 45; VPA+SB-3CT, 0.8 ± 0.05, n = 34; p (control and VPA) = 0.004; p (VPA and VPA+SB-3CT) = 0.12] (Fig. 4A).This indicates that pharmacologically inhibiting MMP-9 during VPA exposure prevents VPA-induced changes in seizure susceptibility.As a second behavioral test, we measured acoustic startle habituation responses by presenting a repeated acoustic stimuli over six bouts.Typically, tadpoles show habituation during each bout and across bouts (James et al., 2015).A repeated measure ANOVA revealed significant differences in startle speed across experimental groups [p < 0.001], bouts [p < 0.001 ] and group*bouts [p < 0.001] interaction.Control tadpoles showed decreased startle speed over repeated bouts; indicating habituation (Fig. 4B).In VPA reared tadpoles, the animals showed reduced habituation.Co-exposure of SB-3CT and VPA reversed VPA-induced effects; although the startle habituation was significantly, but only (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
The copyright holder for this preprint this version posted August 13, 2020.; https://doi.org/10.1101/2020.05.29.123661 doi: bioRxiv preprint partially restored relative to control levels.The sixth bout was measured with a 15 minute delay between bouts, indicating long-term habituation, and this was also similarly affected across experimental groups.

Enhanced visual activity increases MMP-9 levels and MMP-9 inhibition prevents visual-activity induced dendritic growth
Our data suggest that while inhibiting MMP-9 reverses the effect of VPA, this inhibition has no effect on control animals, suggesting that MMP-9 levels are basaly low.One possibility is that during normal development, MMP-9 levels are tightly regulated by neural activity and are elevated transiently when there is a need for structural plasticity.Thus, periods of increased MMP-9 activity may correlate with (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
experience-dependent synaptic reorganization during critical periods in the development of an organism (Murase et al., 2017).We used an assay for experience-dependent plasticity in the tectum, to test this hypothesis.Exposure of freely-swimming tadpoles to enhanced-visual stimulation for several hours is known to promote tectal neuron dendritic growth and synaptogenesis (Aizenman & Cline, 2007;Sin et al., 2002).We first tested whether this manipulation elevated MMP-9 levels in the brain.We exposed a group of tadpoles to enhanced visual activity in a custom-built chamber for 4 hours, another group was dark-exposed and a third group was left in normal light/dark rearing conditions, and then measured brain MMP-9 levels using a Western blot.We found that exposure to enhanced visual activity resulted in significantly increased MMP-9 levels compared to dark and normal rearing conditions [Ratio of MMP-9 levels: L/D, 1.34 ± 0.10, n = 5; L/N, 1.35 ± 0.10, n = 5; D/N, 1.04 ± 0.14, n = 5; p (L/D) = 0.03; p (L/N) = 0.03; p (D/N) = 0.79) (Figure 5A, B).We then compared the effect of MMP-9 inhibition on visual-stimulation induced dendritic growth.We used electroporation to express GFP in single tectal neurons in vivo (Bestman et al., 2006), and then imaged and reconstructed dendritic arbors to compare dendritic growth rate during a 3 hour dark-exposure period vs. growth rate after 3 hours of enhanced visual stimulation.We observed that in the control group, enhanced visual stimulation results in an increase in the total dendritic branch length compared to the dark period [Growth of total dendritic length (percent): Control/Dark,97.35 ± 5.76,n = 9;Control/Light,128.32 ± 7.25,n = 9] (Fig. 5C).In the presence of SB-3CT, the enhanced visual stimuli does not increase total dendritic branch length [Growth of total dendritic length (%): SB-3CT/Dark, 107.30 ± 6.61, n = 9; SB-3CT/Light, 96.88 ± 7.88, n =9].Thus, the change in dendritic growth rate was significantly decreased in the SB-3CT reared tadpoles in comparison to the matched control animals [Change in growth rate (%): Control,136.61 ± 12.29,n = 9;91.27 ± 6.42,n = 9; p = 0.009] (Fig. 5D, E).Taken together, these results suggest that enhanced visual stimulation increased MMP-9 levels, and that this increase is necessary for (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
The copyright holder for this preprint proper activity-driven dendritic arborization.By extension, chronically elevated levels of MMP-9 present in VPA treated tadpoles would result in unregulated plasticity, leading to hyperconnectivity and hyperexcitability.

Discussion
The results of this study suggest that chronic MMP-9 upregulation during early exposure to VPA, is responsible for mediating its effects on neural circuit connectivity and behavior.Exposure to VPA during a key developmental period in Xenopus produces hyperconnected and hyperexcitable neural networks responsible for producing abnormal behaviors such as enhanced seizure susceptibility and decreased habituation (James et al., 2015).Similarly, we found that upregulation of MMP-9 via overexpression resulted in increased frequency of sEPSCs, sIPSCs and barrages suggesting increased tectal connectivity and excitability.Downregulation of MMP-9 not only reversed VPA-induced synaptic connectivity and excitability in developing Xenopus tectum but also rescued VPA-induced behavioral effects.This suggests that chronically elevated levels of MMP-9 may produce abnormally high and sustained plasticity, leading to overconnectivity.Sensory experience during critical periods of development is crucial for proper maturation of sensory networks, and may involve transient, activitydependent increases in MMP-9.In Xenopus, acute exposure to enhanced visual activity results in increased plasticity and dendritic growth in optic tectal neurons (Aizenman & Cline, 2007;Sin et al., 2002).We found that following enhanced visual stimulation, MMP-9 levels were elevated and that increased MMP-9 was necessary for experience-dependent dendritic growth, suggesting that during normal development, MMP-9 levels are low but transiently elevated to facilitate activity-induced structural plasticity.
MMP-9 plays a paramount role in establishing synaptic connections during development and in restructuring of synaptic networks in the adult brain (Reinhard et al., 2015).Recent studies show that MMP-mediated proteolysis is important for remodelling of synaptic structure and function required for processes such as learning and memory, and action of MMPs on various extracellular matrix components, cell surface receptors and growth factors is crucial for modulating neurogenesis and synaptic remodelling; a hallmark of various neurodevelopmental disorders (Huntley, 2012).In particular, dysregulation of MMP-9 is known to influence cognitive processes (Beroun et al., 2019) and increased MMP-9 levels are associated with several neurodevelopmental disorders including ASD (Abdallah et al., 2012;Yoo et al., 2016), fragile X syndrome (Dziembowska et al., 2013;Sidhu et al., 2014), epilepsy (Konopka et al., 2013), bipolar disorder (Rybakowski et al., 2013) and schizophrenia.(Yamamori et al., 2013).Consistent with these studies, our data showed that elevated MMP-9 activity results in neurodevelopmental deficits in Xenopus by creating a hyperplastic state which leads to local overconnectivity.
Early life sensory experience during critical periods of development modifies functional circuits in the brain (Chklovskii et al., 2004;Hensch & Fagiolini, 2005).Periods of high MMP-9 activity correlate with structural and functional synaptic reorganization during critical period plasticity (CPP) and developmental windows across CNS regions, suggesting a role for MMP-9 in determining the developmental time course of CPP (Kaliszewska et al., 2012;Oliveira-Silva et al., 2007;Spolidoro et al., 2012).The results from our study are consistent with these observations in that MMP-9 levels increase in response to enhanced visual activity during a key period of development.Furthermore, this increase in MMP-9 correlates with visual activity induced plasticity and the inhibition of MMP-9 using a pharmacological inhibitor prevents visual-activity induced dendritic growth.The secretion and activation of MMP-9 is tightly regulated by a wide variety of factors and once activated, it can act on multiple targets including various components of the ECM, cell adhesion molecules, cell surface receptors, cytokines, growth factors, and other proteases, each of which can have differential effects in creating permissive environments for structural plasticity in the brain( Van den Steen et al., 2002; This central role for MMP-9 is further supported by recent studies in adult mice, in which upon light reintroduction, intense MMP-9 activity is observed surrounding excitatory synapses in the binocular visual cortex suggesting that MMP-9 can modulate plasticity even after closure of critical periods (Murase et al., 2017(Murase et al., , 2019)).Enzymatic activity of MMP-9 has been known for inducing and maintaining long term potentiation and causes elongation and thinning of dendritic spines in the hippocampal neurons (Huntley, 2012;Michaluk et al., 2011).MMP-9 knock-out mice and overexpressing models show significantly reduced synaptic plasticity suggesting that appropriate levels of MMP-9 are needed for structural and functional reorganization of the brain (Wiera et al., 2013).Local activation of MMP-9 at the level of individual synapses can induce the maturation of spine heads (Szepesi et al., 2014).MMP-9 can act on various cell surface receptors like integrins, BDNF, ICAMs, NGL, all of which are known to be important for developmental and adult plasticity (Fiore et al., 2002;H. Lee et al., 2014;Michaluk et al., 2009;Yamamori et al., 2013).MMP-9, through activation of cell surface and cell adhesion molecules, is implicated not only in active dendritic spine remodeling and stabilization but also in pre-and postsynaptic receptor dynamics; consolidation of LTP; myelination and synaptic pruning (Reinhard et al., 2015).Perineuronal nets (PNNs), which are extracellular matrix components, are often associated with inhibitory interneurons and regulate their development and functions.Recent studies have shown that MMP-9 can rapidly degrade the PNNs surrounding various neurons enabling synaptic and network reorganization during experience dependent plasticity, and could also provide a mechanism by which dysregulation of MMP-9 can lead to abnormal development of neural circuitry (Murase et al., 2017(Murase et al., , 2019;;Wen et al., 2018).Thus, understanding the precise downstream targets and functions of MMP-9 will be critical for understanding various cellular and molecular mechanisms underlying neurodevelopmental (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
disorders.Our study highlights the importance of MMP-9 in the etiology of neurodevelopmental disorders and future studies will focus on investigating the downstream molecular targets of MMP-9.

Materials and Methods
All animal experiments were performed in accordance with and approved by Brown University Institutional Animal Care and Use Committee standards and guidelines (Protocol number 19-05-0016).

Experimental animals
All experimental comparisons were done with matched controls from the same clutch, to account for variability in responses over time and across experimental groups.

Drug treatments
Tadpoles were raised in Steinberg's rearing media on a 12 h light/dark cycle at 18-21°C for 7-8 d, until they reached developmental stage 42 (Nieuwkoop & Faber, 1958).They were then transferred to either (i) control rearing media, (ii) 1 mM solution of VPA in Steinberg's solution, (iii) 1 mM VPA + 3 uM SB-3CT (a pharmacological inhibitor of MMP-9) or (iv)3 uM SB-3CT alone, depending on the experiment, and raised at temperatures ranging from 18°C to 21°C until they reached developmental stages of either 47-48.Developmental stages of tadpoles were determined according to Nieuwkoop and Faber (1958).For all the electrophysiological and behavioral experiments the same concentration of VPA and SB-3CT were used.Previous experiments utilizing 1 mM VPA elicited distinct electrophysiological and behavioral effects (James et al., 2015).The rearing medium was renewed every 3 d.Animals of either sex were used because, at these developmental stages, tadpoles of either sex are phenotypically indistinguishable.

Plasmid and antisense constructs:
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MMP-9 overexpression construct:
The MMP-9 overexpression plasmid contained a backbone of pCMV-SPORT6 plasmid with CMV promoter driving the expression of downstream cloned genes -MMP-9 and a GFP reporter.GFP control construct: The GFP control plasmid consisted of a backbone of pCMV-SPORT6 plasmid with CMV promoter driving the expression of only a GFP reporter.Each working solution contained 4ug/ul of appropriate overexpression plasmid + 0.3uL Fast Green.
Reduction of matrix metalloproteinase 9 gene expression was accomplished through electroporation of a morpholino antisense oligonucleotide (MO) (Bestman & Cline, 2014).Lissamine tagged MOs were designed and ordered from GeneTools.Sequences used for the MMP-9 MO and control MO were 5' AGACTAAAACTCCCACCCTACCCAT 3' and 5' CCTCTTACCTCAGTTACAATTTATA 3', respectively, and were adopted from the methods of Faulkner et al. (2014).The control MO was a scrambled MO not known to align elsewhere.Working solution was prepared by diluting stock MO using endotoxin free molecular grade water and adding 1% Fast Green stock solution to make up 6% of the working solution, with a final concentration of 0.1mM of MO.

Tectal cell transfection:
To transfect cells in the tectum, stage 43-44 animals were anesthetized in a solution of 0.01% MS-222, plasmids or antisense morpholino constructs were injected into the midbrain ventricle, and voltage pulses were applied across the midbrain using platinum electrodes to electroporate cells lining the tectal ventricle (Haas et al., 2002).Tadpoles were then screened for appropriate transfection by checking for fluorescence under a fluorescence microscope.

Behavioral assays
Tadpoles that were used for seizures and acoustic startle response habituation protocols were not used again for other experiments.Immediately before behavioral experiments, tadpoles were transferred to Steinberg's media and left for 1 h to recover from acute action of the drug.

Seizures
For seizure experiments, when tadpoles reached stage 47, they were transferred into individual wells in a six-well plate (Corning), each filled with 7 ml of 5 mm pentylenetetrazol (PTZ) solution in Steinberg's media.The plate was diffusely illuminated from below and an overhead SCB 2001 color camera (Samsung) imaged tadpoles at 30 frames/s.Tadpole positions were acquired using Noldus EthoVision XT (Noldus Information Technology) and processed offline in a custom MATLAB program (MathWorks).Seizure events were defined as periods of rapid and irregular movement, interrupted by periods of immobility (Bell et al., 2011) The onset of regular seizures happened on average 3.9 ± 1.3 min into the recording and were detected automatically using swimming speed thresholding at a level of half of the maximal swimming speed.Number, frequency and duration of seizure events were measured across 5 min intervals of a 20-min-long recording.

Startle habituation response
A six-well plate fixed between two audio speakers (SPA2210/27; Philips), and mechanically connected to diaphragms of these speakers by inflexible plastic struts acted as an experimental arena for the startle habituation response experiment.Stage 49 tadpoles were placed in the experimental arena and acoustic stimuli (one period of a 200 Hz sine wave, 5 ms long) were presented every 5 s which evoked a reliable startle response.The observed startle response is likely due to a combination of inner ear-mediated and lateral line-mediated inputs, and, for the purposes of this study, we did not attempt to differentiate between the two.The amplitude of stimuli for habituation experiments was set at two times above the (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
The copyright holder for this preprint this version posted August 13, 2020.; https://doi.org/10.1101/2020.05.29.123661 doi: bioRxiv preprint startle threshold.Each train of stimuli lasted for 2 min; trains 1 to 5 were separated by 5 min gaps, whereas train 6 was separated from train 5 by a 15 min gap.In order to compensate for possible variation in stimulus delivery across the wells, the location of treatment (1 mM VPA and 1 mM VPA + 3 uM SB-3CT) tadpoles and matched controls within the six-well plate was alternated across experiments.An overhead camera was used to acquire videos of the tadpoles, which were then tracked in EthoVision and processed offline in a custom MATLAB script.Peak speed of each startle response was measured across a 2 s interval after stimulus delivery.To quantify habituation of startle responses at different timescales, we adopted the nomenclature from previous studies (Eddins et al., 2010) (Roberts et al., 2011).Startle speeds were averaged across 1-min-long periods of acoustic stimulation and compared across periods.For rapid habituation, we compared responses during minutes 1 and 2 of the first 2-min train of stimuli.

Electrophysiology experiments
For whole-brain recordings, tadpole brains were prepared as described by Wu et al. (1996) and Aizenman et al. (2003).In brief, tadpoles were anesthetized in 0.02% tricainemethane sulfonate (MS-222).To access the ventral surface of the tectum, brains were filleted along the dorsal midline and dissected in HEPES-buffered extracellular saline [in mm: 115 NaCl, 2 KCl, 3 Cacl2, 3 MgCl2, 5 HEPES, 10 glucose, pH 7.2 (osmolarity, 255 mOsm)].Brains were then pinned to a submerged block of Sylgard in a recording chamber and maintained at room temperature (24°C).To access tectal cells, the ventricular membrane surrounding the tectum was carefully removed using a broken glass pipette.For evoked synaptic response experiments, a bipolar stimulating electrode (FHC) was placed on the optic chiasm to activate RGC axons.
Whole-cell voltage-clamp and current-clamp recordings were performed using glass micropipettes (8-12 MΩ) filled with K-gluconate intracellular saline [in mm: 100 K-gluconate, 8 KCl, 5 NaCl, 1.5 MgCl2, (which was not certified by peer review) is the author/funder.All rights reserved.No reuse allowed without permission.
The copyright holder for this preprint this version posted August 13, 2020.; https://doi.org/10.1101/2020.05.29.123661 doi: bioRxiv preprint 20 HEPES, 10 EGTA, 2 ATP, and 0.3 GTP, pH 7.2 (osmolarity, 255 mOsm)].Recordings were restricted consistently to retinorecipient neurons in the middle one-third of the tectum, thus avoiding any developmental variability existing along the rostrocaudal axis (Wu et al., 1996;Khakhalin and Aizenman, 2012;Hamodi and Pratt, 2014).Electrical signals were measured with a Multiclamp 700B amplifier (Molecular Devices), digitized at 10 kHz using a Digidata 1440 analog-to-digital board, and acquired using pClamp 10 software.Leak subtraction was done in real time using the acquisition software.Membrane potential in the figures was not adjusted to compensate for a predicted 12 mV liquid junction potential.Data were analyzed using AxographX software.Spontaneous synaptic events were collected and quantified using a variable amplitude template (Clements and Bekkers, 1997).
Spontaneous EPSCs (sEPSCs) were recorded at −45 mV (the reversal for chloride ions) whereas sIPSCs were collected in control media at 5 mV (the reversal for glutamatergic currents).For each cell, 60 s of spontaneous activity was recorded.For evoked synaptic response experiments, a bipolar stimulating electrode (FHC) was placed on the optic chiasm to activate RGC axons.Synaptic stimulation experiments were conducted by collecting EPSCs evoked by stimulating the optic chiasm at a stimulus intensity that consistently evoked maximal amplitude EPSCs.Polysynaptic stimulation experiments were performed by collecting EPSCs evoked by stimulating the optic chiasm at a stimulus intensity that evoked the maximal amplitude EPSC.Quantification of polysynaptic activity was calculated by measuring the total change in current over 100 ms time bins beginning at the onset of the evoked response.A spontaneous barrage was defined as a change in holding current of 10 or 20 pA intervals for a period of >200 ms (James et al., 2015).The sEPSC recordings were scanned carefully manually to detect and quantify the number of barrages observed for each of the experimental groups.Graphs show median and interquartile ranges (IQRs) as error bars, and data in the text shows the averages and SEs.
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After three brief washes with 0.1% Tween-20 in TBS, membranes were transferred to goat anti-rabbit HRP-conjugated secondary antibody (BioRad), diluted in blocking solution for an hour at room temperature.The Pierce ECL western blot substrate (Thermo Fisher Scientific, 32209) was used to visualize labeling.For quantification analysis, different exposure periods were used for the same blots to avoid saturation, and total protein normalization using Ponceau S as a loading control (Romero-Calvo et al., 2010).The densitometric analysis of western blot and the total protein was performed using Gel Analyzer of ImageJ, and the data of MMP-9 protein expression were normalized against the total protein as a loading control.Staining using anti-MMP-9 antibodies often resulted in labelling some non-specific bands in addition to the 75 kDa band.Thus, MMP-9-flag overexpression was used in combination with anti-flag antibodies to confirm specificity of the MMP-9 MO and confirm location of MMP-9 band.

Neuronal morphology
To label single tectal cells, Stage 44-45 tadpoles were electroporated using the pCALNL-GFP plus pCAG-Cre plasmids (courtesy Ed Ruthazer, McGill University, Montreal, Quebec, Canada;Ruthazer et al., 2013) at a ratio of 5000:1 to limit coexpression to a very small subset of neurons.Approximately 3 days after electroporation (around developmental stages 48-49) the animals were screened to check for successful fluorescence.Tadpoles were selected for those in which a single neuron in the tectum was expressing GFP and could be imaged clearly.A Zeiss LSM 800 Confocal Laser Scanning Microscope was used to image tadpoles containing labeled neurons.Prior to the experiment tadpoles were either immersed in control media or media containing SB3CT.Animals were then anesthetized in 0.02% MS-222 and imaged under the water immersion objective.For each animal, the clearest, most isolated and identifiable cell in the tectum was imaged.Initially, a z-stack encompassing the cell body and the entire length of all neurites was taken for each animal (as the reference image).After these reference images were taken, the tadpoles were transferred to the rearing media (Control or 3uM SB3CT) and incubated in dark for 3 hours.The same cell that had been previously imaged was identified for each animal and imaged after the 3-hour dark exposure.The tadpoles were then put back into their respective wells and subjected to enhanced visual stimuli using a "light box" for 3 hours.The light box consisted of a chamber with 4x3 array of green LEDs attached to the ceiling.Each row of LEDs flashed for one second and was then followed by the adjacent row, until all four rows had flashed and a one second pause occurred.The cycle then started over, and was presented over a 3 hour period.After visual simulation, a third image was taken.Dendritic arbors of imaged neurons were reconstructed in 3D and analyzed using Neutube and Fiji software (respectively).The total path length of each image was measured.Then, using the series of three images taken for each cell, growth rate was calculated.

Statistical analyses
For all the data, averages and SEs are given in the text, whereas median values and interquartile range (IQR) are shown in the figures.Sample sizes were based on power analyses and known variability from prior work in our experimental system.No outliers were removed from the data analysis, however, data points that were above 1.5 times IQR may be indicated as outliers in some graphs for data presentation purposes and increased clarity.One-way ANOVA was used to compare differences between overall Figure 2: Pharmacological inhibition of MMP-9 rescues VPA-induced effects on synaptic transmission.Representative sEPSCs (left) and evoked response (right) form control, VPA and VPA + SB-3CT treated animals.B. Boxplots showing frequency of sEPSCs for control, VPA, VPA + SB-3CT and SB-3CT alone groups.Frequency of sEPSCs are significantly enhanced in VPA group, an effect rescued by addition of SB-3CT.SB-3CT alone does not have any significant effect on sEPSC frequency.Data are represented as median ± IQR. C. Boxplots showing sEPSCs amplitude for each group.sEPSC amplitude does not change significantly between control and treatment animals.Data are represented as median ± IQR.D. Boxplot showing total charge from evoked synaptic response over a 100 ms window for each group.Total charge from evoked synaptic response is significantly higher in VPA treated animals and this effect is rescued in the presence of SB-3CT.Data are represented as median ± IQR.E. Histogram showing relative barrage frequency for each group.Higher number of barrages are observed for VPA treated animals than in any other group.* p < 0.05, ** p < 0.005.
Figure 3: Morpholino targeted against MMP-9 rescues VPA-induced effects.A. Western blot image (left) indicating MMP-9 levels (75kD band) in control (scrambled) MO and MMP-9 MO transfected groups.Boxplot (right) shows relative MMP-9 levels for control MO and MMP MO transfected tadpoles.MMP-9 MO group consistently shows relatively reduced MMP-9 levels compared to controls.The dotted line represents a ratio of 1 (no difference).The 63kDa band is a non-specific band labelled by MMP-9 antibody that shows no change.B. Boxplots showing sEPSC frequency, amplitude and evoked synaptic response from VPA + Control MO and VPA + MMP-9 MO groups.Frequency of sEPSCs and not amplitude are significantly enhanced in VPA + control MO group.Total charge from evoked synaptic response is significantly higher in the VPA + Control MO compared to VPA + MMP-9 MO treated animals.Data are represented as median ± IQR. C. Boxplots showing sEPSC frequency, amplitude and evoked synaptic response from Control MO and MMP-9 MO alone.Control MO and MMP-9 MO alone does not differ significantly in sEPSC frequency or amplitude.MMP-9 MO alone does not show any significant effect on evoked response.Data are represented as median ± IQR.D. Histogram showing relative barrage frequency for each group.Higher number of barrages are observed for VPA + control MO, and this effect is rescued by addition of MMP-9 MO. ** p < 0.005, *** p < 0.0001.

Figure
Figure 5: Enhanced visual activity increases MMP-9 levels and MMP-9 inhibition prevents visual-activity induced dendritic growth.