Hypersocialization, Social Blindness and Motor Deficits: the Role of PSD95 and PSD93

Rebecca Rocco

PSD95 and PSD93 are both membrane-associated guanylate kinase (MAGUK) proteins which are prominent and essential to glutaminergic postsynaptic terminal efficacy and function. Mutations of these proteins have been linked to the development of synaptopathies such as schizophrenia and autism (Yang et al., 2019, Coley et al., 2019). While PSD95 knockout animal models have shown that lack of PSD95 results in strange behaviours such as immobility, repetitive grooming, and heightened anxiety (Zhang et al., 2014), overall behaviour of these animal models has been difficult to assess due to motor impairment. In addition, the majority of genetic mutations within humans are in the heterozygous form, where a deficiency of a gene product is observed rather than a complete absence. Therefore, Winkler et al. (2018) investigated modified behaviour of PSD95 deficient mice. Furthermore, PSD93 knockout and deficient mice also had their behaviour investigated in order to assess whether PSD93 expression impairment results in similar behavioural phenotypes as in PSD95 deficiency, pointing to a potential shared functional and/or biological redundancy of PSD93/ PSD95. PSD95 deficient and wildtype C57BL/6J mice were raised and then subjected to a wide range of behavioural tests. PSD93 wildtype, deficient and knock-out mice were also investigated using a similar battery of behavioural tests. Mice were taken from each cohort to confirm genetic makeup and expression of PD genes using qPCR methods. This study found that PSD95 deficient mice showed hypersocial behaviours, only mild motor impairment and increased PSD93 expression within the hippocampus; males had increased aggression responding to foreign mice and females had increased vocalization responding to an anesthetized mouse. PSD93 knock-out mice showed comparable hypersocial behaviour to that of PSD95 deficient mice, as well as severe motor impairment. These results suggest that both PSD95 and PSD93 are implicated in social processing and behaviour. As PSD protein impairment has been implicated in the development of neuropsychiatric diseases (Gao et al., 2013), the use of PSD95 deficient mice as a model to study neuropsychiatric diseases which have social disinhibition symptoms associated with them may be beneficial. Key Words: Membrane-associated guanylate kinases (MAGUKs), Post synaptic density, PSD95, PSD93, schizophrenia, autism, synaptopathies, hippocampus, synaptic plasticity, genetic redundancy, social behaviour, social processing, social disinhibition, social blindness, hypersocialization

Introduction The neuronal membrane-associated guanylate kinase (MAGUK) family of proteins are essential scaffolding proteins which act as central building blocks of the glutaminergic postsynaptic density (PSD) framework (Won et al., 2017; Zheng et al., 2011;). The PSD is a congregation of proteins at the post-synaptic terminal - its structure and its composition allows transmission through glutaminergic excitatory processes to be effective (Scannevin and Huganir, 2000). MAGUKs serve to organize and structurally uphold this PSD framework by organizing protein complexes within the PSD, clustering, moving and modifying glutamate receptors, linking these receptors to intracellular mediators, as well as dynamically enforcing synaptic maturation (Bustos et al., 2017; Chen et al., 2015). Alteration of MAGUK function has been implicated with synaptopathies, such as schizophrenia, autism and Alzheimer’s disease (Coley and Gao, 2018; Feyder et al., 2010; Proctor et al., 2010). density framework. PSD95 has been implicated in synapse maturation, plasticity, as well as in shaping the protein composition within glutaminergic post-synaptic terminals (Mardones et al., 2019). These proteins have been implicated in learning and consolidation of memory as well. (Li et al., 2018; Bustos et al., 2017; Zheng et al., 2011) Expression of PSD95 dynamically changes throughout the life cycle of dendritic spines (Lambert et al., 2017). It has also been suggested that the presence of PSD95 is essential for later stages of synaptic development and neurogenesis rather than early on (McGee et al., 2001). PSD95 overexpression studies have shown that overabundance of PSD95 leads to increased postsynaptic clustering, increased number and size of dendritic spines and activity of glutamate receptors within the hippocampus (El-Husseini et al., 2000). Due to this, it is strongly believed that PSD95 plays a large role in the stabilization of synapses as well as in synaptic plasticity. Genetic studies have associated MAGUK impairment, such as that of PSD95, within schizophrenia and autism (Yang et al., 2019; Coley and Gao, 2019). Genome sequencing has shown that psychiatric patients show impairment of PSD constituents, such as PSD95 and animal studies have shown that disruption of PSD95 leads to cognition and learning impairment (Coley and Gao, 2018). However, prior behavioural studies have shown that complete PSD95 knockout within mice has resulted in very strange phenotypes, such as severe motor impairment, limb clasping, variable social preferences, in addition to learning and working memory impairment (Coley and Gao, 2019; Zhang et al., 2014). Severe motor phenotypes have made behavioural deduction regarding these knockout models difficult to investigate and interpret. In addition, humans tend to be heterozygous for genetic mutations, leading to a deficiency of protein product rather than a complete lack thereof (Winkler et al. 2018). Due to this, the authors of this study decided to investigate the social behaviour and social processing of PSD95 mutant (+/-) mice relative to PSD95 wildtype (+/+) mice. PSD93 is a closely related protein to PSD95. Both arose from the duplication and independent alteration of a common ancestral gene. Both of these genes play roles within the maintenance of the PSD (Favaro et al., 2018; Won et al., 2017). In contrast with PSD95, PSD93 accumulates quicker within developing dendritic spines (Lambert et al., 2017). In addition, PSD93 knockout within Purkinje neurons, where PSD93 is the only MAGUK expressed, has been shown to cause no structural or functional abnormalities, nor changes within the localization of PSD93 interacting proteins (McGee et al, 2001). Both PSD95 and PSD93 had been thought to play similar roles in the maturation of glutaminergic synapses, however, recent studies have argued against this (Favaro et al., 2018). Studies which have investigated PSD95 and PSD93 knockout mice found that glutamate receptors, AMPARs and NMDARs, had reduced synaptic transmission as well as postsynaptic density (PSD) congregation, in addition to an increased number of silent synapses and a sharp reduction in the size of PSD with no significant changes to the pre/post synaptic membrane (Chen et al., 2015). PSD93 knockout (-/-), deficient (+/-) and wildtype (+/+) had their social behaviour investigated.

PSD95 is a MAGUK and a major constituent of the postsynaptic This paper (Winkler et al., 2018), aimed to investigate the social behaviour and processing in PSD95(+/-) partial knock out mice. In addition, PSD93(-/-) knockout, PSD93(+/-) partial knock out and PSD93(+/+) wildtype mice had their social behaviour and phenotypes investigated in order to assess whether PSD95/ PSD93 are functionally redundant and whether loss of PSD93 results in similar behavioural consequences as seen in PSD95 knockouts. All male and female mice were kept in different cabinets, where they were group housed or single housed depending on what was required for testing conditions. Mice underwent an extensive battery of tests; sexes were always tested separately, and significant results were prioritized for extensive publication. Observational tests included: dyadic social interactions test, resident/intruder test, ultrasonic vocalization test, the chimney test and behaviour in home cage as well as in the open field. Prior to testing, some mice cohort members had their genotypes verified at 4 weeks of age where hippocampal samples were taken, and their PSD95/93 protein level deficiency or normalcy was verified using qPCR methods. Main findings of this study included: Both male and female PSD95(+/-) mice demonstrated hypersocialization behaviour within the dyadic social interaction tests. Non-social learning and processing of non-social stimuli appeared to be unaffected in these mice. Expression of NMDA and AMPA glutamate receptor were found to be normal. Male PSD95(+/-) mice were found to be more aggressive than PSD95(+/+) mice within resident/intruder testing. Female PSD95(+/-) mice had increased vocalization when exposed to an anesthetized mouse. PSD95 (+/-) mice moderately showed hypoactivity within the open field, corresponding to mild motor impairment. PSD93(-/-) knockout mice showed comparable hypersocialization behaviour with that of PSD95(+/-). This may indicate that PSD93 is also essential for the processing of social stimuli, but less so than PSD95, as PSD93 heterozygosity does not result in hypersocialization as seen in PSD95 mutants. PSD93(-/-) mice showed a severe motor deficit phenotype. In addition, PSD93 also had increased expression within the hippocampus of PSD95(+/-) mice, which may be evidence for a partial substitution effect of PSD93 for PSD95 deficiency. These results suggest

that both PSD95 and PSD93 are involved in social processing and behaviour. In addition, PSD95 deficient mice, which have a milder motor deficit phenotype associated with them, could potentially be used to study neuropsychiatric disorders which feature social disinhibition or “social blindness” as a symptom.

Major Results PSD95(+/-) Mice: increased socialization, aggression and vocalization Both male and female PSD95(+/-) mice showed increased socialization behaviour within dyadic social interaction testing. Under this test, same sex/genotype pairs were introduced into a neutral testing cage and were allowed to interaction for 10 minutes. Socialization was then quantified in review of interaction tapes via the display of socially motivated actions within tion within mice (Winkler et al., 2018). Female PSD95 mutants then introduced to an anesthetized female mouse. The number

the pairings, such as: time in contact [Fig. 1, A-E], chasing, as well as snout-snout and anogenital sniffing behaviour. PSD95 (+/-) mouse pairs displayed significantly higher bouts of social interaction than that of PSD95(+/+) pairings. The social behavioural profile of PSD95 mutants are shown to be distinct from PSD95 knockout mice. Knockouts have been shown to have impaired motor performance, strange grooming behaviours, increased anxiety-related responses with a variable preference for social stimuli (Coley and Gao, 2019; Feyder et al., 2010). This study shows that while PSD95 mutants display mild motor impairment, they display fairly normal behaviour in non-social testing conditions, but distinct altered behaviours within social situations. This may be due to the fact that PSD95 is still at sigmales were (I) quicker to attack a foreign intruder and (J) more likely to attack overall compared to PSD95 wildtype males. Female PSD95 underwent an ultrasonic vocalization test, after being presented with a foreign anesthetized female. PSD95 mutant females (K) called out more and (L) took less time to express an initial call when presented with the incapacitated female, relative to PSD95 wildtype females. (Winkler et al., 2018) PSD93(-/-) Mice: increased socialization and motor impairment

nificant levels within PSD95 mutants to allow for normal functioning in non-socially motivated circuits.

Figure 1. Time spent in contact within dyadic social interaction in contact with their same sex, same genotype partner more

testing, same sex/genotype. Both PSD95(+/-) (A) male pairs and (E) female pairs spent significantly more time in contact with one another than PSD95(+/+) pairs. (Winkler et al., 2018) Male PSD95 mutants also showed heightened aggression towards foreign males within the resident-intruder test. Within this test, previously group housed PSD95 mutant males were housed in a single cage for four weeks. The cage was heated in order to increase the basal level of aggression as C57BL/6 mice have a low standard of inter-male aggression (Winkler et al., 2018). Intruder males were introduced into the cage, and the pairing was observed for 10 minutes or until first sign of attack (biting). Latency of attack as well as number of attack events of PSD95 mice were recorded [Fig. 2, I-J]. Male PSD95 mutants were significantly more likely to attack quicker and attack more social processing and behaviour may be impaired within PSD95 mice. Female PSD95 mutants showed increased vocalization when faced with an anesthetized female mouse within an ultrasonic vocalization test. This test has been known to induce vocalizawere individually housed for 3 days prior to the test and were of vocalizations/calls as well as the time it took to initiate vocalization when faced with the anesthetized mouse was recorded [Fig. 2, K-L]. PSD95 mutant females were significantly faster to begin calling out when faced with the incapacitated mouse, and they also expressed more calls in total relative to PSD95 wildtype females. This is also evidence that normal social processing and social behaviour is impaired within PSD95 deficient mice.

Figure 2. Male PSD95 mice underwent a resident/intruder test where inter-male aggression was investigated; PSD95 mutant PSD93 knockout mice, rather than PSD93 mutants, were found to display hypersocial behaviour comparable to that of PSD95 mutants within this study. PSD93 knockout mice tended to be relative to PSD95 wildtype mice. This is evidence that normal

often within testing relative to PSD93 mutants and PSD93 wildtype pairs [Fig. 3, A]. PSD93 knockout mice also displayed a prominent motor deficit phenotype which made observations regarding their behaviour more difficult. The finding that PSD93 knockout mice display motor deficits comes in contrast with the findings of a prior study, which stated that PSD93 knockout results in no structural or functional abnormalities, including motor impairment within mice (McGee et al., 2001). The results of the originally reviewed paper potentially serve as evidence that both PSD95 and PSD93 are proteins implicated in the processing of social stimuli.

Figure 3. (A) PSD93(-/-) knockout mice showed increased social interaction within same sex/genotype pairs. PSD93 knockout pairings spent more time in contact relative to both PSD93(+/-) mutants and PSD93(+/+) wildtype mice. (Winkler et al., 2018) PSD95(+/-): increased hippocampal PSD93 expression PSD95 mutant mice were found to have increased expression of PSD93 within the hippocampus. At 4 weeks of age, some PSD95 cohort mice were taken and had their relative PSD95 and PSD93 protein expression quantified using hippocampal enrichment preparations and qPCR. PSD95 mutants had a greater level of PSD93 protein expressed within the hippocampus relative to PSD95 wildtype mice [Fig. 4, B]. This could be potential evidence of functional or biological redundancy concerning PSD93 under PSD95 deficient conditions. Prior literature has argued that because these related proteins share common structural domains, they may operate in a similar fashion (McGee et al., 2001). Many more recent studies have shown, however, that PSD93 and PSD95 play some complimentary, but opposing roles, within the regulation and development of synapses (Favaro et al., 2018).

Figure 4. (B) Increased PSD93 protein expression within PSD95 (+/-) mutants relative to PSD95 wildtype mice. Hippocampal enrichment preparations were from PSD95 mutant mice at 4 GAPDH. (Winkler et al., 2018) Conclusions This study found that PSD95(+/-) mice had increased socialization behaviour as well as increased PSD93 expression within the hippocampus. Male PSD95 mutants showed increased aggression towards foreign intruders and females showed increased vocalization when faced with an anesthetized mouse relative to PSD95 wildtypes. PSD93 knockout mice showed comparable increased socialization when compared to PSD95 mutants, but in addition to severe motor deficits. These results suggest that both PSD95 and PSD93 are involved in the processing and coordination of social behaviour. PSD95 mutants were shown to demonstrate hypersocial behaviour in the form of reduced social distance, which could be described as “social blindness”- these mice appeared to act more familiar with conspecifics, act more aggressively towards foreign intruders and vocalize more frequently when exposed to incapacitated mice. These mice may have the inability to process specific social signals and react appropriately with regard to other mice. According to the authors, social recognition and memory were affected in PSD95 mutant mice, but non-social learning, general memory and behaviour towards non-social stimuli were found to be unaffected. Due to this, the authors argue that this evidence points to PSD95 as a prominent component of the processing of social stimuli and control of social behaviour. Much of the literature concerns itself with the observation of PSD95 knockout mice and the change of biomarkers within the brain in addition to severe learning and motor phenotypes. The investigation into PSD95 deficiency with regard to social control and behaviour seen within this study is novel in this respect. This study also highlights the relative sensitivity of PSD95 levels within the brain as both PSD95(+/-) mice and PSD95(-/-) show distinct behavioural profiles. PSD93 knockout mice displayed similar hypersocial behaviour to that of PSD95 mutants; indicating that PSD93 may have a similar role with regard to the processing of social stimuli as PSD95. They found that PSD93 knockouts also had impaired motor movement, which is at odds with the findings of other studies which stated that PSD93 knockout mice showed no motor coordination problems (McGee et al., 2001). PSD95 and PSD93 may show some level of functional and/or biological redundancy for one another, as PSD93 protein expression is increased within PSD95 mutant mice. The authors have argued that the increase of PSD93 expression within the hippocampus of PSD95 deficient mice may be due to PSD93 trying to compensate for PSD95 function. They do state however, that PSD95 and PSD93 are likely both implicated in the processing of social stimuli in their own unique ways. Increased PSD93 expression within PSD95 mutants probably cannot take over for all of PSD95’s roles, and overexpression of PSD93 may actually impair social neuronal circuits more. This falls in line with the current thinking, that although PSD93 and PSD95 are

weeks of age, PSD93 protein expression was normalized to structurally very similar, greater evidence supports that these proteins act very differently in many contexts (Favaro et al., 2018). The authors conclude that this study’s findings show that PSD95(+/-) and PSD93 (-/-) mice demonstrate behavioural phenotypes which are the complete opposite as those seen in mouse models of monogenic autism. Within these models of autism, mice display reduced social interaction, reduced intermale aggression, reduced vocalization and mild motor impairment. PSD95 mutant mice in particular, show increased social

interaction, increased inter-male aggression and increased vocalization. In contrast, studies investigating complete knockout of PSD95 within mice, have found that PSD95(-/-) mice show strange behaviours that emulate autistic-like behaviour, such as those mentioned previously (Coley and Gao et al., 2019, Feyder et al., 2010). The mechanisms which underlie the strong behavioural distinction between PSD95 mutant and PSD95 knockout mice are still unknown. Nonetheless, the authors conclude that PSD95 mutant mice, which feature only mild motor impairment, may have the potential to be used as model systems for the investigation of neuropsychiatric conditions which feature symptoms such as hypersocialization and issues with social cue processing. Critical Analysis This study featured a wide array of behavioural tests, which were impeccably done. Every test was performed multiple times, which supported the validity of their results. Statistics of all the tests performed were featured within the article document. Visualization methods were used in order to summarize the primary findings beautifully. Comparisons between knockout animals, mutant animals and wildtype animals were concisely stated. Genetic testing done prior to the commencement of behavioural tests ensured that PSD proteins were causally implicated within the results. Throughout the paper, the authors rely on other reports of PSD95 knockout behaviour in order to compare the behaviour of PSD95(+/-) mutants. The authors should have investigated PSD95 knockout animals in this study as well. This is because, reports regarding the social likability of PSD95 mice vary. Feyder et al. (2010), argues that PSD95 knockout animals show normal social interactions in pair testing, but increased preference for social stimuli. Coley and Gao (2019), a more recent study published after the study under review, operates under the assumption that PSD95 knockout mice display a complete lack of sociability. If the authors had included PSD95 knockout animals within the study, comparing the behavioural profiles of PSD95 mutant and PSD95 knockout mice may have been more valid. The results of this study also feature conflict. McGee et al (2001), found that the knockout of PSD93 within Purkinje neurons resulted in no structural or functional abnormalities, and no motor coordination issues within these mice. The paper under review however, states that PSD93 knockout animals within this study demonstrated distinct motor impairment phenotypes - so much so, that PSD93 knockout animals were unable to complete every test within the study. In addition, observing animals and discussing their behaviour can only further science so far. Causal mechanisms explaining how PSD95 or PSD93 may be implicated in social processing networks must be elucidated. This paper did not emphasize how they believe these proteins contribute to the processing of social cues or the processing of behaviour. The authors merely stated that causal mechanisms can only be speculated upon. Furthermore, one of the main conclusions of this paper featured this idea that because PSD95 (+/-) demonstrated hypersocialization, apparent social blindness and only mild motor deficits, these mice would make adequate models in order to study neuropsychiatric conditions which feature similar symptoms. The authors should hold off this recommendation prior to understanding functionally how PSD95 deficiency may cause behaviours emulating social blindness within mice. After reviewing this paper, experiments which look at the social processing functionality of PSD95 and PSD93 should be done. Experiments which look at localization of PSD proteins within the development of young mice into adulthood should be looked at. Objective biomarkers should be used in order to describe the dynamic nature and function of the PSD proteins under study. By understanding the native function of PSD95 and PSD93, impairments of their function can be better understood, and resulting behavioural symptoms can be better treated. Future Directions In order to further this area of research, the biological mechanisms which account for hypersocialization within PSD95 mutant and PSD93 knockout mice should be investigated. During qPCR testing, the researchers of this study found that expression of the glutamate receptor NMDA subunits NR1, NR2A, and NR2B, along with that of AMPA GluR1 within PSD95 mutants were unaffected. Other research has also found that reduction of PSD95 levels does not impact NMDA expression, rather, it impacts NMDA modulation and clustering. (Chen et al., 2015; McGee et al., 2001; Scannevin and Huganir, 2000). Reduction in glutamate receptor congregation within the postsynaptic density framework has been associated with an increased number of silent synapses within the brain of PSD95 and PSD93 knockout mice (Chen et al., 2015). Visualization studies, which would be tailored to the region of the brain under investigation, could be done to observe the congregation of NMDA and AMPA receptors in glutaminergic areas associated with the government of social behaviour. If congregation of receptors is impaired within PSD95(+/-) or PSD93(-/-) mice, this may explain the malfunction of social processing as synaptic connections overall are impaired. If congregation of NMDA and AMPA receptors are normal, then this could mean that the PSD95 levels within PSD (+/- ) or PSD93 levels within PSD93(-/-) may be sufficient in order to allow for normal congregation of these essential receptors. Age of the mice may also be a confounding factor: presence of PSD95/93 and their role in synaptic plasticity and development varies through development (Lambert et al., 2017; McGee et al., 2001). In addition, research produced within the same year as the original review article’s publication, supports a causal pathway between PSD95 and conditioned fear. Li et al. (2018), found that NMDAR stimulation can cause an increase in nitric oxide (NO), which must link with PSD95, in order for fear memory formation to occur. Nitric oxide formed must link to PSD95, as PSD95 is associated with neuronal oxide synthase(nOS), which is implicated further on in the pathway. If this NO/PSD95 interaction is impaired, then fear memory is selectively reduced. Reduction in the available PSD95 to engage with this key interaction may explain why PSD95(+/-) mice display social blindness and hypersocialization features: their social fear consolidation may be impaired. Visualization studies could also be done in order to see the relationship between nitric oxide and PSD95 within the amygdala, which is a key part of the brain associated

with fear-based memory construction. If this pathway is implicated in the hypersocialization behaviour seen within PSD95 mutant mice, then the NO/PSD95 relationship should be investigated to see if it struggles, or malfunctions, under reduced PSD95 conditions. This could explain the social behaviour of PSD95 mutant within the original study under review. If the interaction between NO and PSD95 appear to be normal, other members of the pathway may be implicated instead, such as nOS which is directly associated with PSD95.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15. Bustos, F. J., Ampuero, E., Jury, N., Aguilar, R., Falahi, F., Toledo, J., Ahumada, J., Lata, J., Cubillos, P., Henríquez, B., Guerra, M. V., Stehberg, J., Neve, R. L., Inestrosa, N. C., Wyneken, U., Fuenzalida, M., Härtel, S., Sena-Esteves, M., Varela-Nallar, L., … van Zundert, B. (2017). Epigenetic editing of the Dlg4/PSD95 gene improves cognition in aged and Alzheimer’s disease mice. Brain: A Journal of Neurology, 140(12), 3252–3268. https://doi.org/10.1093/brain/awx272 Chen, X., Levy, J. M., Hou, A., Winters, C., Azzam, R., Sousa, A. A., Leapman, R. D., Nicoll, R. A., & Reese, T. S. (2015). PSD95 family MAGUKs are essential for anchoring AMPA and NMDA receptor complexes at the postsynaptic density. Proceedings of the National Academy of Sciences of the United States of America, 112(50), E6983-6992. https://doi.org/10.1073/ pnas.1517045112 Coley, A. A., & Gao, W.-J. (2018). PSD95: A synaptic protein implicated in schizophrenia or autism? Progress in NeuroPsychopharmacology & Biological Psychiatry, 82, 187–194. https://doi.org/10.1016/j.pnpbp.2017.11.016 Coley, A. A., & Gao, W.-J. (2019). PSD-95 deficiency disrupts PFC-associated function and behavior during neurodevelopment. Scientific Reports, 9(1), 9486. https://doi.org/10.1038/s41598-019-45971-w El-Husseini, A. E., Schnell, E., Chetkovich, D. M., Nicoll, R. A., & Bredt, D. S. (2000). PSD-95 involvement in maturation of excitatory synapses. Science (New York, N.Y.), 290(5495), 1364–1368. Favaro, P. D., Huang, X., Hosang, L., Stodieck, S., Cui, L., Liu, Y.-Z., Engelhardt, K.-A., Schmitz, F., Dong, Y., Löwel, S., & Schlüter, O. M. (2018). An opposing function of paralogs in balancing developmental synapse maturation. PLoS Biology, 16 (12), e2006838. https://doi.org/10.1371/journal.pbio.2006838 Feyder, M., Karlsson, R.-M., Mathur, P., Lyman, M., Bock, R., Momenan, R., Munasinghe, J., Scattoni, M. L., Ihne, J., Camp, M., Graybeal, C., Strathdee, D., Begg, A., Alvarez, V. A., Kirsch, P., Rietschel, M., Cichon, S., Walter, H., Meyer-Lindenberg, A., … Holmes, A. (2010). Association of Mouse Dlg4 (PSD-95) Gene Deletion and Human DLG4 Gene Variation With Phenotypes Relevant to Autism Spectrum Disorders and Williams’ Syndrome. American Journal of Psychiatry, 167(12), 1508–1517. https://doi.org/10.1176/appi.ajp.2010.10040484 Gao, C., Tronson, N. C., & Radulovic, J. (2013). MODULATION OF BEHAVIOR BY SCAFFOLDING PROTEINS OF THE POSTSYNAPTIC DENSITY. Neurobiology of Learning and Memory, 105, 3–12. https://doi.org/10.1016/j.nlm.2013.04.014 Lambert, J. T., Hill, T. C., Park, D. K., Culp, J. H., & Zito, K. (2017). Protracted and asynchronous accumulation of PSD95- family MAGUKs during maturation of nascent dendritic spines. Developmental Neurobiology, 77(10), 1161–1174. https:// doi.org/10.1002/dneu.22503 Li, L.-P., Dustrude, E. T., Haulcomb, M. M., Abreu, A. R., Fitz, S. D., Johnson, P. L., Thakur, G. A., Molosh, A. I., Lai, Y., & Shekhar, A. (2018). PSD95 and nNOS interaction as a novel molecular target to modulate conditioned fear: Relevance to PTSD. Translational Psychiatry, 8(1), 155. https://doi.org/10.1038/s41398-018-0208-5 Mardones, M. D., Jorquera, P. V., Herrera-Soto, A., Ampuero, E., Bustos, F. J., van Zundert, B., & Varela-Nallar, L. (2019). PSD95 regulates morphological development of adult-born granule neurons in the mouse hippocampus. Journal of Chemical Neuroanatomy, 98, 117–123. https://doi.org/10.1016/j.jchemneu.2019.04.009 McGee, A. W., Topinka, J. R., Hashimoto, K., Petralia, R. S., Kakizawa, S., Kauer, F., Aguilera-Moreno, A., Wenthold, R. J., Kano, M., & Bredt, D. S. (2001). PSD-93 Knock-Out Mice Reveal That Neuronal MAGUKs Are Not Required for Development or Function of Parallel Fiber Synapses in Cerebellum. Journal of Neuroscience, 21(9), 3085–3091. https:// doi.org/10.1523/JNEUROSCI.21-09-03085.2001 Proctor, D. T., Coulson, E. J., & Dodd, P. R. (2010). Reduction in post-synaptic scaffolding PSD-95 and SAP-102 protein levels in the Alzheimer inferior temporal cortex is correlated with disease pathology. Journal of Alzheimer’s Disease: JAD, 21 (3), 795–811. https://doi.org/10.3233/JAD-2010-100090 Scannevin, R. H., & Huganir, R. L. (2000). Postsynaptic organisation and regulation of excitatory synapses. Nature Reviews Neuroscience, 1(2), 133–141. https://doi.org/10.1038/35039075 Winkler, D., Daher, F., Wüstefeld, L., Hammerschmidt, K., Poggi, G., Seelbach, A., Krueger-Burg, D., Vafadari, B., Ronnenberg, A., Liu, Y., Kaczmarek, L., Schlüter, O. M., Ehrenreich, H., & Dere, E. (2018). Hypersocial behavior and biological redundancy in mice with reduced expression of PSD95 or PSD93. Behavioural Brain Research, 352, 35–45. https:// doi.org/10.1016/j.bbr.2017.02.011

16.

17.

18.

19. Won, S., Levy, J. M., Nicoll, R. A., & Roche, K. W. (2017). MAGUKs: Multifaceted synaptic organizers. Current Opinion in Neurobiology, 43, 94–101. https://doi.org/10.1016/j.conb.2017.01.006 Yang, Y., Geng, Y., Jiang, D., Ning, L., Kim, H. J., Jeon, N. L., Lau, A., Chen, L., & Lin, M. Z. (2019). Kinase pathway inhibition restores PSD95 induction in neurons lacking fragile X mental retardation protein. Proceedings of the National Academy of Sciences of the United States of America, 116(24), 12007–12012. https://doi.org/10.1073/pnas.1812056116 Zhang, J., Saur, T., Duke, A. N., Grant, S. G. N., Platt, D. M., Rowlett, J. K., Isacson, O., & Yao, W.-D. (2014). Motor Impairments, Striatal Degeneration, and Altered Dopamine-Glutamate Interplay in Mice Lacking PSD-95. Journal of Neurogenetics, 28(1–2), 98–111. https://doi.org/10.3109/01677063.2014.892486 Zheng, C.-Y., Seabold, G. K., Horak, M., & Petralia, R. S. (2011). MAGUKs, synaptic development, and synaptic plasticity. The Neuroscientist: A Review Journal Bringing Neurobiology, Neurology and Psychiatry, 17(5), 493–512. https:// doi.org/10.1177/1073858410386384