MARK1 regulates dendritic spine morphogenesis and cognitive functions in vivo

Dendritic spines play a pivotal role in synaptic communication and are crucial for learning and memory processes. Abnormalities in spine morphology and plasticity are observed in neurodevelopmental and neuropsychiatric disorders, yet the underlying signaling mechanisms remain poorly understood. The microtubule affinity regulating kinase 1 (MARK1) has been implicated in neurodevelopmental disorders, and the MARK1 gene shows accelerated evolution in the human lineage suggesting a role in cognition. However, the in vivo role of MARK1 in synaptogenesis and cognitive functions remains unknown. Here we show that forebrain-specific conditional knockout (cKO) of Mark1 in mice causes defects in dendritic spine morphogenesis in hippocampal CA1 pyramidal neurons with a significant reduction in spine density. In addition, we found loss of MARK1 causes synaptic accumulation of GKAP and GluA2. Furthermore, we found that MARK1 cKO mice show defects in spatial learning in the Morris water maze and reduced anxiety-like behaviors in the elevated plus maze. Taken together, our data show a novel role for MARK1 in regulating dendritic spine morphogenesis and cognitive functions in vivo.

Role of Glutamate Receptor-related Biomarkers in the Etiopathogenesis of ADHD

Objective

: Pathways associated with glutamate receptors are known to play a role in the pathophysiology of attention-deficit hyperactivity disorder (ADHD). However, cyclin-dependent kinase 5 (CDK5), microtubule-associated protein-2 (MAP2), guanylate kinase-associated protein (GKAP), and postsynaptic density 95 (PSD95), all of which are biomarkers involved in neurodevelopmental processes closely related to glutamatergic pathways, have not previously been studied in patients with ADHD. The main purpose of this study was to evaluate the plasma levels of CDK5, MAP2, GKAP, and PSD95 in children with ADHD and investigate whether these markers have a role in the etiology of ADHD.

Methods

: Ninety-six children with ADHD between 6 and 15 years of age and 72 healthy controls were included in the study. Five milliliters of blood samples were taken from all participants. The samples were stored at -80°C until analyzed by the enzyme-linked immunosorbent assay method.

Results

: Statistically significantly lower CDK5 levels were observed in children with ADHD than in healthy controls (p = 0.037). The MAP2, GKAP, and PSD95 levels were found to be statistically significantly higher in the ADHD group than in healthy controls (p = 0.012, p = 0.009, and p = 0.024, respectively). According to binary regression analysis, CDK5 and MAP2 levels were found to be predictors of ADHD.

Conclusion

: In conclusion, we found that a close relationship existed between ADHD and glutamatergic pathways, and low levels of CDK5 and high levels of MAP2 and GKAP played a role in the etiopathogenesis of ADHD.

nArgBP2 together with GKAP and SHANK3 forms a dynamic layered structure

nArgBP2, a protein whose disruption is implicated in intellectual disability, concentrates in excitatory spine-synapses. By forming a triad with GKAP and SHANK, it regulates spine structural rearrangement. We here find that GKAP and SHANK3 concentrate close to the synaptic contact, whereas nArgBP2 concentrates more centrally in the spine. The three proteins collaboratively form biomolecular condensates in living fibroblasts, exhibiting distinctive layered localizations. nArgBP2 concentrates in the inner phase, SHANK3 in the outer phase, and GKAP partially in both. Upon co-expression of GKAP and nArgBP2, they evenly distribute within condensates, with a notable peripheral localization of SHANK3 persisting when co-expressed with either GKAP or nArgBP2. Co-expression of SHANK3 and GKAP with CaMKIIα results in phase-in-phase condensates, with CaMKIIα at the central locus and SHANK3 and GKAP exhibiting peripheral localization. Additional co-expression of nArgBP2 maintains the layered organizational structure within condensates. Subsequent CaMKIIα activation disperses a majority of the condensates, with an even distribution of all proteins within the extant deformed condensates. Our findings suggest that protein segregation via phase separation may contribute to establishing layered organization in dendritic spines.

The kinesin Kif21b binds myosin Va and mediates changes in actin dynamics underlying homeostatic synaptic downscaling

Homeostatic synaptic plasticity adjusts the strength of synapses to restrain neuronal activity within a physiological range. Postsynaptic guanylate kinase-associated protein (GKAP) controls the bidirectional synaptic scaling of AMPA receptors (AMPARs); however, mechanisms by which chronic activity triggers cytoskeletal remodeling to downscale synaptic transmission are barely understood. Here, we report that the microtubule-dependent kinesin motor Kif21b binds GKAP and likewise is located in dendritic spines in a myosin Va- and neuronal-activity-dependent manner. Kif21b depletion unexpectedly alters actin dynamics in spines, and adaptation of actin turnover following chronic activity is lost in Kif21b-knockout neurons. Consistent with a role of the kinesin in regulating actin dynamics, Kif21b overexpression promotes actin polymerization. Moreover, Kif21b controls GKAP removal from spines and the decrease of GluA2-containing AMPARs from the neuronal surface, thereby inducing homeostatic synaptic downscaling. Our data highlight a critical role of Kif21b at the synaptic actin cytoskeleton underlying homeostatic scaling of neuronal firing.

The DLGAP family: neuronal expression, function and role in brain disorders

The neurotransmitter glutamate facilitates neuronal signalling at excitatory synapses. Glutamate is released from the presynaptic membrane into the synaptic cleft. Across the synaptic cleft glutamate binds to both ion channels and metabotropic glutamate receptors at the postsynapse, which expedite downstream signalling in the neuron. The postsynaptic density, a highly specialized matrix, which is attached to the postsynaptic membrane, controls this downstream signalling. The postsynaptic density also resets the synapse after each synaptic firing. It is composed of numerous proteins including a family of Discs large associated protein 1, 2, 3 and 4 (DLGAP1-4) that act as scaffold proteins in the postsynaptic density. They link the glutamate receptors in the postsynaptic membrane to other glutamate receptors, to signalling proteins and to components of the cytoskeleton. With the central localisation in the postsynapse, the DLGAP family seems to play a vital role in synaptic scaling by regulating the turnover of both ionotropic and metabotropic glutamate receptors in response to synaptic activity. DLGAP family has been directly linked to a variety of psychological and neurological disorders. In this review we focus on the direct and indirect role of DLGAP family on schizophrenia as well as other brain diseases.

Tale of the Good and the Bad Cdk5: Remodeling of the Actin Cytoskeleton in the Brain

Cdk5 kinase, a cyclin-dependent kinase family member, is a key regulator of cytoskeletal remodeling in the brain. Cdk5 is essential for brain development during embryogenesis. After birth, it is essential for numerous neuronal processes such as learning and memory formation, drug addiction, pain signaling, and long-term behavior changes, all of which rely on rapid alterations in the cytoskeleton. Cdk5 activity is deregulated in various brain disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and ischemic stroke, resulting in profound remodeling of the neuronal cytoskeleton, loss of synapses, and ultimately neurodegeneration. This review focuses on the "good and bad" Cdk5 in the brain and its pleiotropic contribution in regulating neuronal actin cytoskeletal remodeling. A vast majority of physiological and pathological Cdk5 substrates are associated with the actin cytoskeleton. Thus, our special emphasis is on the numerous Cdk5 substrates identified in the past two decades such as ephexin1, p27, Mst3, CaMKv, kalirin-7, RasGRF2, Pak1, WAVE1, neurabin-1, TrkB, 5-HT6R, talin, drebrin, synapsin I, synapsin III, CRMP1, GKAP, SPAR, PSD-95, and LRRK2. These substrates have unraveled the molecular mechanisms by which Cdk5 plays divergent roles in regulating neuronal actin cytoskeletal dynamics both in healthy and diseased states.

Structure of the GH1 domain of guanylate kinase-associated protein from Rattus norvegicus

Guanylate-kinase-associated protein (GKAP) is a scaffolding protein that links NMDA receptor-PSD-95 to Shank-Homer complexes by protein-protein interactions at the synaptic junction. GKAP family proteins are characterized by the presence of a C-terminal conserved GKAP homology domain 1 (GH1) of unknown structure and function. In this study, crystal structure of the GH1 domain of GKAP from Rattus norvegicus was determined in fusion with an N-terminal maltose-binding protein at 2.0 Å resolution. The structure of GKAP GH1 displays a three-helix bundle connected by short flexible loops. The predicted helix α4 which was not visible in the crystal structure associates weakly with the helix α3 suggesting dynamic nature of the GH1 domain. The strict conservation of GH1 domain across GKAP family members and the lack of a catalytic active site required for enzyme activity imply that the GH1 domain might serve as a protein-protein interaction module for the synaptic protein clustering.

Crystallization and preliminary X-ray crystallographic analysis of the C-terminal domain of guanylate kinase-associated protein from Rattus norvegicus

Guanylate kinase-associated protein (GKAP) is a scaffolding protein that plays a role in protein-protein interactions at the synaptic junction such as linking the NMDA receptor-PSD-95 complex to the Shank-Homer complex. In this study, the C-terminal helical domain of GKAP from Rattus norvegicus was purified and crystallized by the vapour-diffusion method. To improve the diffraction quality of the GKAP crystals, a flexible loop in GKAP was truncated and an MBP (maltose-binding protein)-GKAP fusion was constructed in which the last C-terminal helix of MBP is fused to the N-terminus of the GKAP domain. The MBP-GKAP crystals diffracted to 2.0 Å resolution using synchrotron radiation. The crystal was orthorhombic, belonging to space group P2₁2₁2, with unit-cell parameters a=99.1, b=158.7, c=65.5 Å. The Matthews coefficient was determined to be 2.44 Å3 Da(-1) (solvent content 49.5%) with two molecules in the asymmetric unit. Initial attempts to solve the structure by molecular replacement using the MBP structure were successful.

Circuit-selective striatal synaptic dysfunction in the Sapap3 knockout mouse model of obsessive-compulsive disorder

BACKGROUND

Synapse-associated protein 90/postsynaptic density protein 95-associated protein 3 (SAPAP3) is an excitatory postsynaptic protein implicated in the pathogenesis of obsessive-compulsive behaviors. In mice, genetic deletion of Sapap3 causes obsessive-compulsive disorder (OCD)-like behaviors that are rescued by striatal expression of Sapap3, demonstrating the importance of striatal neurotransmission for the OCD-like behaviors. In the striatum, there are two main excitatory synaptic circuits, corticostriatal and thalamostriatal. Neurotransmission defects in either or both of these circuits could potentially contribute to the OCD-like behaviors of Sapap3 knockout (KO) mice. Previously, we reported that Sapap3 deletion reduces corticostriatal alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid-type glutamate receptor-mediated synaptic transmission.

METHODS

Whole-cell electrophysiological recording techniques in acute brain slices were used to measure synaptic transmission in the corticostriatal and thalamostriatal circuits of Sapap3 KO mice and littermate control animals. Transgenic fluorescent reporters identified striatopallidal and striatonigral projection neurons. SAPAP isoforms at corticostriatal and thalamostriatal synapses were detected using immunostaining techniques.

RESULTS

In contrast to corticostriatal synapses, thalamostriatal synaptic activity is unaffected by Sapap3 deletion. At the molecular level, we find that another SAPAP family member, SAPAP4, is present at thalamostriatal, but not corticostriatal, synapses. This finding provides a molecular rationale for the functional divergence we observe between thalamic and cortical striatal circuits in Sapap3 KO mice.

CONCLUSIONS

These findings define the circuit-level neurotransmission defects in a genetic mouse model for OCD-related behaviors, focusing attention on the corticostriatal circuit for mediating the behavioral abnormalities. Our results also provide the first evidence that SAPAP isoforms may be localized to synapses according to circuit-selective principles.

Stress and trauma: BDNF control of dendritic-spine formation and regression. Prog Neurobiol. 2014;112:80-99. [PMID: 24211850 DOI: 10.1016/j.pneurobio.2013.10.005]

Chronic restraint stress leads to increases in brain derived neurotrophic factor (BDNF) mRNA and protein in some regions of the brain, e.g. the basal lateral amygdala (BLA) but decreases in other regions such as the CA3 region of the hippocampus and dendritic spine density increases or decreases in line with these changes in BDNF. Given the powerful influence that BDNF has on dendritic spine growth, these observations suggest that the fundamental reason for the direction and extent of changes in dendritic spine density in a particular region of the brain under stress is due to the changes in BDNF there. The most likely cause of these changes is provided by the stress initiated release of steroids, which readily enter neurons and alter gene expression, for example that of BDNF. Of particular interest is how glucocorticoids and mineralocorticoids tend to have opposite effects on BDNF gene expression offering the possibility that differences in the distribution of their receptors and of their downstream effects might provide a basis for the differential transcription of the BDNF genes. Alternatively, differences in the extent of methylation and acetylation in the epigenetic control of BDNF transcription are possible in different parts of the brain following stress. Although present evidence points to changes in BDNF transcription being the major causal agent for the changes in spine density in different parts of the brain following stress, steroids have significant effects on downstream pathways from the TrkB receptor once it is acted upon by BDNF, including those that modulate the density of dendritic spines. Finally, although glucocorticoids play a canonical role in determining BDNF modulation of dendritic spines, recent studies have shown a role for corticotrophin releasing factor (CRF) in this regard. There is considerable improvement in the extent of changes in spine size and density in rodents with forebrain specific knockout of CRF receptor 1 (CRFR1) even when the glucocorticoid pathways are left intact. It seems then that CRF does have a role to play in determining BDNF control of dendritic spines.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.