Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

PSD-95 (Postsynaptic Density Protein-95)

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101786

Synonyms

Historical Background

Postsynaptic protein-95 (PSD-95), the major scaffolding and hub protein found in excitatory chemical synapses, was originally discovered in 1992 when Mary B. Kennedy et al. observed a protein containing a guanylate kinase domain with a similar sequence to the Drosophila discs large (Dlg) 1 tumor suppressor protein in the rat brain (Cho et al. 1992). During the 1990s, Kennedy and her research team found that PSD-95 resides in the postsynaptic density (PSD) fraction, where it performs its primary function: stabilizing and organizing the array situated beneath the postsynaptic membrane (Hunt et al. 1996) that contains different proteins, ion channels, and synaptic receptors to promote normal synaptic transmission and function (Kornau et al. 1995; Zhang et al. 1999). PSD-95 remains a target of high interest due to its involvement in the regulation of synapse formation, the stabilization of chemical synaptic transmission, and its function in the central nervous system.

Expression and Physiological Functions of PSD-95 in the Central Nervous System

The PSD is an electron-dense region attached to the postsynaptic membrane of a postsynaptic neuron, and changes in its composition regulate the strength and plasticity of excitatory synaptic neurotransmission. PSD-95, the expression of which has been reported to depend on synaptic activation, is principally located within the PSD and is involved in several synaptic functions, including synapse formation, promoting the clustering of synaptic signaling molecules, and interactions with proteins, such as cell adhesion molecules, glutamatergic receptors, and other actin cytoskeleton-related elements (Table 1).
PSD-95 (Postsynaptic Density Protein-95), Table 1

Physiological synaptic functions of PSD-95

 

Functions

References

Synapse

Maintenance of PSD stabilization and organization

Hunt et al. (1996)

Maturation of glutamatergic synapses

El-Husseini et al. (2000)

Formation of the synapse through the clustering of synaptic signaling molecules and synaptogenic ligands

Farías et al. (2009) and Varela-Nallar et al. (2010)

Dendritic spines

Formation of new dendritic spines

Takahashi et al. (2003)

Maturation of dendritic spines

Ampuero et al. (2016)

Regulation of dendritic branching and outgrowth

Bustos et al. (2014)

Cytoskeleton

Organization of the cytoskeletal-signaling complex through interaction with proteins associated with microtubule dynamics

Naisbitt et al. (2000)

This table summarizes the different functions performed by PSD-95 at the synaptic and dendritic levels and the functions related to dendritogenesis

PSD-95 is present in nascent spines and is required for the formation of new spines. Once a stable pool of actin filaments composed of drebrin (an actin-binding protein) and actin has been formed in the developing dendritic spine, PSD-95 and glutamate receptors are translocated from the shaft to the spine and accumulate in clusters to produce mature spines with organized PSD structures (Takahashi et al. 2003). One of the well-known functions of PSD-95 independent of its roles in synaptic function is its ability to regulate the maturation of dendritic spines and to maintain neuronal structure (Ampuero et al. 2016).

Moreover, PSD-95 is known to participate in the regulation of dendritic branching and outgrowth. Several publications have indicated the involvement of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) subunits NR2A and NR2B in dendritogenesis, but the specific mechanism by which they act was unclear at that time. Several years ago, Bustos et al. found that the synaptic expression of PSD-95 increases during the development of immature neurons, thereby obstructing the synaptic clustering of NR2B-NMDARs. In turn, the reactivation of dendritic branching is restricted, the number of dendritic spines increases, and synaptic plasticity is impeded (Bustos et al. 2014). However, the neuronal developmental period during which PSD-95 is expressed should be considered. PSD-95 is a crucial protein in mature hippocampal neurons that can confer synaptic stability via the generation of stable synaptic contacts for synaptic transmission.

As previously mentioned, PSD-95 plays a role in dendritogenesis by suppressing or inhibiting the neuronal arborization of mature neurons. This mechanism may be induced by the interaction of PSD-95 with proteins that are associated with the microtubule dynamics that regulate dendritic growth and with proteins that affect the neuronal cytoskeleton. It has been reported that microtubule irruption in dendritic spines is required to anchor PSD-95 in the postsynaptic membrane within these spines. For example, the organization of the cytoskeletal-signaling complex is regulated by PSD-95. Trafficking of the PSD-95 complex to postsynaptic sites is mediated by specific motor proteins, such as myosin-V and cytoplasmic dynein, which both interact indirectly with PSD-95 through guanylate kinase-associated protein (GKAP) (Naisbitt et al. 2000).

Furthermore, it has been reported that PSD-95 is necessary for the maturation of glutamatergic synapses in hippocampal neurons because it induces spine stability to promote postsynaptic clusters of glutamate receptors and maturation of the presynaptic region (El-Husseini et al. 2000). In fact, after initial phases of long-term potentiation (LTP), sufficient levels of PSD-95 are essential for activity-dependent synapse stabilization, a process associated with the incorporation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) at the synapse. Moreover, overexpression of PSD-95 stimulates the development of excitatory synapses, thus enhancing the clustering of AMPARs. In contrast, the disruption of PSD-95 triggers an increase in LTP and impairs long-term depression (LTD), thereby altering synaptic plasticity. Thus, PSD-95 plays an important role in synaptic plasticity to stabilize synaptic changes during LTP (Meyer et al. 2014).

PSD-95 is involved in several signaling pathways that control synaptic physiology. For instance, it has been reported that activation of the Wnt signaling pathway triggers the mobilization of PSD-95 to the synaptic region of dendritic spines. Specifically, it has been observed that the induction of the noncanonical Wnt pathway promotes an enhancement in dendritic spine density and activity due to a simultaneous increase in PSD-95 clustering (Farías et al. 2009; Varela-Nallar et al. 2010).

Molecular Interactions of PSD-95: The PDZ Domains

PSD-95 is encoded by the gene Dlg4, which is approximately 30 kb long, is composed of 22 exons, and is located on chromosome band 17p13.1 in humans. PSD-95 belongs to the membrane-associated guanylate kinase (MAGUK) family of synaptic proteins that also includes PSD-93, SAP-102, and SAP-97. All MAGUK proteins are expressed in different submembrane domains and are molecularly arranged in independent modular domains that stabilize the PSD and contribute to organizing its macromolecular components (Sturgill et al. 2009). These domains are connected in series by flexible polypeptide linkers and include three PDZ domains [PSD-95, Dlg1, and zonula occludens-1 proteins (zo-1)] and one Src homology 3-guanylate kinase (SH3-GK) module, and these domains have recently been classified as two independent supramodules: PDZ1-PDZ2 and PDZ3-SH3-GK.

Due to their high levels of conformational plasticity, these PDZ domains are specialized to bind to signaling molecules or glutamate receptors through a C-terminal binding sequence or via PDZ-PDZ interactions (Vallejo et al. 2016) (Fig. 1). The NMDAR subunits NR2A and NR2B interact preferentially with PDZ1 and PDZ2, although it has been reported that NR2B occasionally binds the PDZ3 domain. AMPARs, through transmembrane AMPA receptor regulatory protein (TARP) stargazin, are able to interact with all PDZ domains to trigger the recruitment of additional AMPARs and the subsequent potentiation of synaptic transmission. The cell adhesion molecule neuroligin 1 (NL-1) has shown a preference for the PDZ3 domain of PSD-95, whereas other postsynaptic components, such as Ca2+/calmodulin-dependent protein kinase II (CaMKII) and neural nitric oxide synthase (nNOS), bind to PDZ1 and PDZ2, respectively. Cypin (cytosolic PSD-95 interactor), a protein that acts as a regulator of dendritic arborization and dendritogenesis, shows a preference for PDZ1 and PDZ2, as well as the Shaker and inward rectifier K+ channels. The synaptic Ras GTPase-activating protein (SynGAP), an important postsynaptic component involved in the organization of synaptic structure, is a clear example of the high promiscuity of PSD-95; it is able to interact with all three PDZ domains. Moreover, microtubule-associated protein-1a (MAP1a) and GKAP are examples of proteins that exhibit a preference for other modular domains of PSD-95, such as the SH3-GK module.
PSD-95 (Postsynaptic Density Protein-95), Fig. 1

PDZ domains of PSD-95 and their interactions with other synaptic components. The illustration schematically presents the structural molecular organization of PSD-95 through a representation of its conformationally independent modular domains. PSD-95 consists of three PDZ domains (PDZ1, PDZ2, and PDZ3) and the SH3-GK module. Glutamate receptors, proteins, ion channels, and signaling enzymes that preferentially bind to specific PDZ domains are also depicted

Localization of PSD-95 Is Regulated by Posttranslational Modifications

In addition to making PSD-95 a target of multiple proteins and synaptic elements, the independent PDZ modular domains that provide the conformational structure of PSD-95 confer this protein with a high susceptibility to modification that affects its postsynaptic localization within the PSD of dendritic spines. Posttranslational modification is a well-defined mechanism involving different enzymes and residues, and it contributes to the synaptic localization of PSD-95. Modifications affecting PSD-95 include phosphorylation, palmitoylation, S-nitrosylation, ubiquitination, and the lesser-known neddylation (Fig. 2). Notably, posttranslational protein modifications are able to induce the alteration of several signaling pathways that determine the synaptic fate of PSD-95 (Vallejo et al. 2016) (Table 2).
PSD-95 (Postsynaptic Density Protein-95), Fig. 2

Posttranslational modifications of PSD-95. The figure represents the structural molecular organization of PSD-95 and the corresponding polypeptide areas or domains at which different posttranslational modifications occur to alter its postsynaptic localization. Palmitoylation, S-nitrosylation (orange spectrum), ubiquitination (green), phosphorylation (red), and neddylation (blue) zones are depicted following a color code

PSD-95 (Postsynaptic Density Protein-95), Table 2

Role of different posttranslational modifications of PSD-95 in synaptic function

Posttranslational modification

Effector agent

Location

Function

References

Phosphorylation

c-Abl kinase

Tyr533

PSD-95 clustering and synapse formation

de Arce et al. (2010)

CaMKII

Ser73

Destabilization of PSD-95 in the PSD and disruption of spine growth and synaptic plasticity

Gardoni et al. (2006)

GSK-3β

Thr19

Nelson et al. (2013)

Palmitoylation

DHHC2

Cys3, Cys5

Clustering and stabilization of PSD-95 in the PSD to promote synaptic transmission

El-Husseini et al. (2002)

S-nitrosylation

NO

Cys3, Cys5

Disruption of synaptic plasticity by maintaining PSD-95 in the depalmitoylated form

Ho et al. (2011)

Ubiquitination

Mdm2

Lys10, Lys403, Lys544, Lys672, and Lys679

Disruption of glutamatergic synapses, synaptic strength, and plasticity

Colledge et al. (2003) and Bianchetta et al. (2011)

Neddylation

Nedd8

Lys202

Promotion of PSD-95 clustering and the development, stabilization, and maturation of the spine

Vogl et al. (2015)

This table summarizes the roles of different posttranslational modifications in the postsynaptic localization of PSD-95. Here, several effector agents that act on specific residues of PSD-95 are described along with their corresponding synaptic alterations

Several posttranslational modifications, such as palmitoylation and neddylation, lead to the accumulation of PSD-95 and a corresponding increase in its clustering within the PSD, thereby enhancing synaptic transmission. In contrast, the S-nitrosylation and ubiquitination processes destabilize PSD-95 within the PSD and, in some cases, induce the translocation of PSD-95 from the active spine to the dendritic shaft disrupting synaptic plasticity and spine growth. However, several kinases phosphorylate PSD-95 at specific residues, altering its clustering in the synapse; thus, depending on the residue modified, PSD-95 may remain stable in the PSD or may be translocated out of the spine. For example, the PSD-95 phosphorylation at Tyr533 by c-Abl kinase increases the PSD-95 clustering at postsynaptic sites and consequently enhances the number of synapses (de Arce et al. 2010), whereas the PSD-95 phosphorylation by CaMKII and glycogen synthase kinase-3β (GSK-3)β at Ser73 (Gardoni et al. 2006) and Thr19 (Nelson et al. 2013), respectively, induces the destabilization of PSD-95.

The palmitoylating enzyme DHHC2, located in the dendritic spine, palmitoylates PSD-95 at the Cys3 and Cys5 residues to maintain the balance between palmitoylated PSD-95 and nonpalmitoylated cytosolic PSD-95 in dendritic shafts. In addition, the palmitate turnover on PSD-95 is dynamically regulated by glutamate receptor activity. In fact, glutamate-induced synaptic activity leads to the depalmitoylation of PSD-95, possibly via a putative palmitoyl protein thioesterase and ultimately triggers the selective loss of synaptic AMPARs. In contrast, blockade of glutamatergic activity increases the palmitoylation of PSD-95 and its subsequent accumulation at synaptic sites, thereby promoting the synaptic transmission mediated by the recruitment of synaptic AMPARs (El-Husseini et al. 2002). Nitric oxide (NO) production results from the interaction of nNOS with calmodulin triggered by an influx of calcium via NMDA ion channels, and it is primarily responsible for the PSD-95 S-nitrosylation at Cys3 and Cys5, the same residues at which DHHC2 palmitoylates PSD-95. Thus, NO competes with palmitoylation to maintain PSD-95 in the depalmitoylated state; conversely, endogenous palmitoylation decreases the level of S-nitrosylated PSD-95 (Ho et al. 2011). One well-described posttranslational modification affecting the synaptic localization of PSD-95 is mediated by the ubiquitin-proteasome pathway. Although synaptic ubiquitination has been reported to have a nonproteolytic function involving cyclin-dependent kinase 5 (CDK5) activity and PSD-95 level, it is also known that murine double minute 2 (Mdm2) ubiquitinates PSD-95 at five different lysine residues (Bianchetta et al. 2011) in response to NMDAR activation. Consequently, PSD-95 is removed from synaptic sites and is degraded in a proteasome-dependent manner to trigger the endocytosis of AMPARs (Colledge et al. 2003). Neddylation is a posttranslational modification similar to ubiquitination that involves the conjugation of neural precursor cell-expressed developmentally downregulated gene 8 (Nedd8), which is highly expressed in hippocampal cells, with substrates such as PSD-95, which undergoes neddylation at Lys202 (Vogl et al. 2015). This mechanism promotes the scaffolding function of PSD-95, thereby playing an important role in the development of dendritic spines during maturation and their subsequent stability in excitatory synapses, which confers stability to PSD-95 clustering within the PSD.

In addition to acting as a target of various synaptic proteins, PSD-95 is involved in different pathways that control several posttranslational modifications and is essential for maintaining the composition and architecture of the PSD. Thus, it is not surprising that changes in these signaling pathways can trigger the dysfunction of synaptic connections to lead to the emergence of neurological diseases, such as Alzheimer’s disease (AD), schizophrenia, Huntington’s disease (HD), and fragile X syndrome (FXS). In AD, the level of synaptic PSD-95 decreases in a time-dependent manner as the pathology advances, resulting in postsynaptic degeneration and suggesting that this reduction may result from the ubiquitin-proteasomal degradation of PSD-95. Moreover, in AD pathology, the interaction of PSD-95 with other synaptic proteins is disrupted, and the PSD is destabilized. Furthermore, some studies have suggested the potential relevance of posttranslational modifications in neuropathologies. For instance, reduced palmitoylation of PSD-95 has been associated with the synaptic loss observed in patients with schizophrenia and HD, whereas increased PSD-95 clustering and an excessive number of dendritic spines resulting from PSD-95 ubiquitination have been observed in FXS patients (Vallejo et al. 2016). In summary, PSD-95 is required for the formation of proper synaptic connections that allows the preservation of cognition, memory, and functional circuitry. A complete understanding of the PDS molecular network and signaling pathways underlying normal synapse function is essential to elucidate the pathological mechanisms responsible for different neurodegenerative, neuropsychiatric, and neurodevelopmental diseases.

Summary

PSD-95 is a major structural element of chemical excitatory synapses and is considered the main scaffolding postsynaptic protein located at the PSD, a lattice-like array composed of different interacting proteins that contribute to the organization and stabilization of postsynaptic components, such as ion channels, synaptic receptors, and signaling molecules. The structural characteristics of PSD-95 allow this protein to bind synaptic elements through its PDZ domains; this structural feature is common to all MAGUK proteins, the family of synaptic proteins to which PSD-95 belongs. PDZ domains contribute to the organization and stability of the PSD and are responsible for mediating the PSD-95 function and trafficking to the postsynaptic region. Among the most representative functions of PSD-95 are the regulation of dendritic branching, outgrowth, and dendritogenesis, as well as the maturation of glutamatergic synapses. Thus, PSD-95 is fundamentally involved in multiple aspects of synaptic strength and transmission, and its proper regulation is essential for accurate synaptic development and plasticity. Unfortunately, several posttranslational modifications are known to disrupt the postsynaptic localization and normal function of PSD-95 that determine the fate of individual dendritic spines in the central nervous system; in some cases, this disruption triggers the development of neurological disease. Thus, understanding the molecular mechanisms involved in synapse stabilization and synaptic transmission is crucial for the development of specific pharmacological therapies against different neurodegenerative (AD and HD), neuropsychiatric (schizophrenia), and neurodevelopmental (FXS) diseases in which the level and postsynaptic localization of PSD-95 are compromised.

Notes

Acknowledgments

This work was supported by grants from the Basal Center of Excellence in Aging and Regeneration (CONICYT-PFB 12/2007) and FONDECYT (No. 1160724) to N. C. Inestrosa. D. Vallejo was a postdoctoral fellow.

References

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Centro de Envejecimiento y Regeneración (CARE), Departamento de Biología Celular y Molecular, Facultad de Ciencias BiológicasPontificia Universidad Católica de ChileSantiagoChile
  2. 2.Centro de Excelencia en Biomedicina de Magallanes (CEBIMA)Universidad de MagallanesPunta ArenasChile