Differential functional interaction of two Vesl/Homer protein isoforms with ryanodine receptor type 1: a novel mechanism for control of intracellular calcium signaling

Reciprocal Homer1a and Homer2 Isoform Expression Is a Key Mechanism for Muscle Soleus Atrophy in Spaceflown Mice

The molecular mechanisms of skeletal muscle atrophy under extended periods of either disuse or microgravity are not yet fully understood. The transition of Homer isoforms may play a key role during neuromuscular junction (NMJ) imbalance/plasticity in space. Here, we investigated the expression pattern of Homer short and long isoforms by gene array, qPCR, biochemistry, and laser confocal microscopy in skeletal muscles from male C57Bl/N6 mice (n = 5) housed for 30 days in space (Bion-flight = BF) compared to muscles from Bion biosatellite on the ground-housed animals (Bion ground = BG) and from standard cage housed animals (Flight control = FC). A comparison study was carried out with muscles of rats subjected to hindlimb unloading (HU). Gene array and qPCR results showed an increase in Homer1a transcripts, the short dominant negative isoform, in soleus (SOL) muscle after 30 days in microgravity, whereas it was only transiently increased after four days of HU. Conversely, Homer2 long-form was downregulated in SOL muscle in both models. Homer immunofluorescence intensity analysis at the NMJ of BF and HU animals showed comparable outcomes in SOL but not in the extensor digitorum longus (EDL) muscle. Reduced Homer crosslinking at the NMJ consequent to increased Homer1a and/or reduced Homer2 may contribute to muscle-type specific atrophy resulting from microgravity and HU disuse suggesting mutual mechanisms.

Effects of two inhibitors of metabolic glutamate receptor 5 on expression of endogenous homer scaffold protein 1 in the auditory cortex of mice with tinnitus

Tinnitus is deemed as the result of abnormal neural activities in the brain, and Homer proteins are expressed in the brain that convey nociception. The expression of Homer in tinnitus has not been studied. We hypothesized that expression of Homer in the auditory cortex was altered after tinnitus treatment. Mice were injected with sodium salicylate to induce tinnitus. Expression of Homer was detected by quantitative real-time polymerase chain reaction, western blotting, and immunohistochemistry assays. We found that Homer1 expression was upregulated in the auditory cortex of mice with tinnitus, while expression of Homer2 or Homer3 exhibited no significant alteration. Effects of two inhibitors of metabolic glutamate receptor 5 (mGluR5), noncompetitive 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) and competitive α-methyl-4-carboxyphenylglycine (MCPG), on the tinnitus scores of the mice and on Homer1 expression were detected. MPEP significantly reduced tinnitus scores and suppressed Homer1 expression in a concentration dependent manner. MCPG had no significant effects on tinnitus scores or Homer1 expression. In conclusion, Homer1 expression was upregulated in the auditory cortex of mice after tinnitus, and was suppressed by noncompetitive mGluR5 inhibitor MPEP, but not competitive mGluR5 inhibitor MCPG.

The Role of Stress-Induced Changes of Homer1 Expression in Stress Susceptibility

Stress negatively affects processes of synaptic plasticity and is a major risk factor of various psychopathologies such as depression and anxiety. HOMER1 is an important component of the postsynaptic density: constitutively expressed long isoforms HOMER1b and HOMER1c bind to group I metabotropic glutamate receptors MGLUR1 (GRM1) and MGLUR5 and to other effector proteins, thereby forming a postsynaptic protein scaffold. Activation of the GLUR1-HOMER1b,c and/or GLUR5-HOMER1b,c complex regulates activity of the NMDA and AMPA receptors and Ca2+ homeostasis, thus modulating various types of synaptic plasticity. Dominant negative transcript Homer1a is formed as a result of activity-induced alternative termination of transcription. Expression of this truncated isoform in response to neuronal activation impairs interactions of HOMER1b,c with adaptor proteins, triggers ligand-independent signal transduction through MGLUR1 and/or MGLUR5, leads to suppression of the AMPA- and NMDA-mediated signal transmission, and thereby launches remodeling of the postsynaptic protein scaffold and inhibits long-term potentiation. The studies on animal models confirm that the HOMER1a-dependent remodeling most likely plays an important part in the stress susceptibility, whereas HOMER1a itself can be regarded as a neuroprotector. In this review article, we consider the effects of different stressors in various animal models on HOMER1 expression as well as impact of different HOMER1 variants on human behavior as well as structural and functional characteristics of the brain.

Interaction of the Homer1 EVH1 domain and skeletal muscle ryanodine receptor

The skeletal muscle ryanodine receptor (RyR1) proteins are intracellular calcium (Ca²⁺) release channels on the membrane of the sarcoplasmic reticulum (SR) and required for skeletal muscle excitation-contraction coupling. Homer (Vesl) is a family of scaffolding proteins that modulate target proteins including RyRs (ryanodine receptors), mGluRs (group 1 metabotropic glutamate receptors) and IP₃Rs (inositol-1,4,5-trisphosphate receptors) through a conserved EVH1 (Ena/VASP homology 1) domain. Here, we examined the interaction between Homer1 EVH1 domain and RyR1 by co-immunoprecipitation, continuous sucrose density-gradient centrifugation, and bio-layer interferometry binding assay at different Ca²⁺ concentrations. Our results show that there exists a high-affinity binding between the Homer1 EVH1 domain and RyR1, especially at 1 mM of Ca²⁺. Based on our data and the known structures of Homer1 EVH1 domain and RyR1, we found two consensus proline-rich sequences in the structure of RyR1, PPHHF and FLPPP, and proposed two corresponding binding models to show mechanisms of recognition different from those used by other proline-rich motifs. The side proline residues of two proline-rich motifs from RyR1 are away from the hydrophobic surface of Homer1 EVH1, rather than buried in this hydrophobic surface. Our results provide evidence that Homer1 regulates RyR1 by direct interaction.

Control of Neuronal Ryanodine Receptor-Mediated Calcium Signaling by Calsenilin

Calsenilin is a calcium ion (Ca²⁺)-binding protein involved in regulating the intracellular concentration of Ca²⁺, a second messenger that controls multiple cellular signaling pathways. The ryanodine receptor (RyR) amplifies Ca²⁺ signals entering the cytoplasm by releasing Ca²⁺ from endoplasmic reticulum (ER) stores, a process termed calcium-induced calcium release (CICR). Here, we describe a novel mechanism, in which calsenilin controls the activity of neuronal RyRs. We show calsenilin co-localized with RyR2 and 3 in the ER of mouse hippocampal and cortical neurons using immunocytochemistry. The underlying protein-protein interaction between calsenilin and the RyR was determined in mouse central nervous system (CNS) neurons using immunoprecipitation studies. The functional relevance of this interaction was assayed with single-channel electrophysiology. At low physiological Ca²⁺ concentrations, calsenilin binding to the cytoplasmic face of neuronal RyRs decreased the RyR's open probability, while calsenilin increased the open probability at high physiological Ca²⁺ concentrations. This novel molecular mechanism was studied further at the cellular level, where faster release kinetics of caffeine-induced Ca²⁺ release were measured in SH-SY5Y neuroblastoma cells overexpressing calsenilin. The interaction between calsenilin and neuronal RyRs reveals a new regulatory mechanism and possibly a novel pharmacological target for the control of Ca²⁺ release from intracellular stores.

Homer1 (VesL-1) in the rat esophagus: focus on myenteric plexus and neuromuscular junction

Homer1, a scaffolding protein of the postsynaptic density (PSD), enriched at excitatory synapses is known to anchor and modulate group I metabotropic glutamate receptors (mGluRs) and different channel- and receptor-proteins. Homer proteins are expressed in neurons of different brain regions, but also in non-neuronal tissues like skeletal muscle. Occurrence and location of Homer1 and mGluR5 in myenteric plexus and neuromuscular junctions (NMJ) of rat esophagus have yet not been characterized. We located Homer1 and mGluR5 immunoreactivity (-iry) in rat esophagus and focused on myenteric neurons, intraganglionic laminar endings (IGLEs) and NMJs, using double- and triple-label immunohistochemistry and confocal laser scanning microscopy. Homer1-iry was found in a subpopulation of vesicular glutamate transporter 2 (VGLUT2) positive IGLEs and cholinergic varicosities within myenteric ganglia, but neither in nitrergic nor cholinergic myenteric neuronal cell bodies. Homer1-iry was detected in 63% of esophageal and, for comparison, in 35% of sternomastoid NMJs. Besides the location in the PSD, Homer1-iry colocalized with cholinergic markers, indicating a presynaptic location in coarse VAChT/CGRP/NF200- immunoreactive (-ir) terminals of nucleus ambiguus neurons supplying striated esophageal muscle. mGluR5-iry was found in subpopulations of myenteric neuronal cell bodies, VGLUT2-ir IGLEs and cholinergic varicosities within the myenteric neuropil and NMJs of esophagus and sternomastoid muscles. Thus, Homer1 may anchor mGluR5 at presynaptic sites of cholinergic boutons at esophageal motor endplates, in a small subpopulation of VGLUT2-ir IGLEs and cholinergic varicosities within myenteric ganglia possibly modulating Ca²⁺-currents and neurotransmitter release.

Emerging Synaptic Molecules as Candidates in the Etiology of Neurological Disorders

Synapses are complex structures that allow communication between neurons in the central nervous system. Studies conducted in vertebrate and invertebrate models have contributed to the knowledge of the function of synaptic proteins. The functional synapse requires numerous protein complexes with specialized functions that are regulated in space and time to allow synaptic plasticity. However, their interplay during neuronal development, learning, and memory is poorly understood. Accumulating evidence links synapse proteins to neurodevelopmental, neuropsychiatric, and neurodegenerative diseases. In this review, we describe the way in which several proteins that participate in cell adhesion, scaffolding, exocytosis, and neurotransmitter reception from presynaptic and postsynaptic compartments, mainly from excitatory synapses, have been associated with several synaptopathies, and we relate their functions to the disease phenotype.

Effects of Vesl/Homer Proteins on Intracellular Signaling

The clustering of signaling molecules at specialized cellular sites allows cells to effectively convert extracellular signals into intracellular signals and to produce a concerted functional output with specific temporal and spatial patterns. A prime example for these molecules and their effects on cellular signaling are the postsynaptic density proteins of the central nervous system. Recently, one group of these proteins, the Vesl/Homer protein family has received increased attention because of its unique molecular properties that allow both the clustering end functional modulation of a plethora of different binding Proteins. Within multlprotein signaling complexes, Vesl/Homer Proteins influence proteins as diverse as metabotropic glutamate receptors; transient receptor potential channels; intracellular calcium channels; the scaffolding protein, Shank; small GTPases; transcription factors; and cytoskeletal proteins. Furthermore, interaction with such functionally relevant proteins also links Vesl/Homer proteins indirectly to an even larger group of cellular effector proteins, putting the Vesl/Homer Proteins at the crossroads of several critical intracellular signaling processes. In addition to the initial reports of Vesl/Homer protein expression in the central nervous system, members of this protein family have now been identified in other excitable cells in various muscle types and in a large number of nonexcitable cells. The widespread expression of Vesl/Homer proteins in different organs and their functional importance in cellular protein signaling complexes is further evidenced by their conservation in organisms from Drosoohila to humans.

Upregulation of Homer1a Promoted Retinal Ganglion Cell Survival After Retinal Ischemia and Reperfusion via Interacting with Erk Pathway

Retinal ischemia and reperfusion (I/R) is extensively involved in ocular diseases, causing retinal ganglion cell (RGCs) death resulting in visual impairment and blindness. Homer1a is considered as an endogenous neuroprotective protein in traumatic brain injury. However, the roles of Homer1a in RGCs I/R injury have not been elucidated. The present study investigated the changes in expression and effect of Homer1a in RGCs both in vitro and in vivo after I/R injury using Western blot, TUNEL assay, gene interference and overexpression, and gene knockout procedures. The levels of Homer1a and phosphorylated Erk (p-Erk) increased in RGCs and retinas after I/R injury. Upregulation of Homer1a in RGCs after I/R injury decreased the level of p-Erk, and mitigated RGCs apoptosis. Conversely, downregulation of Homer1a increased the level of p-Erk, and augmented RGCs apoptosis. Furthermore, inhibition of the p-ERK reduced RGCs apoptosis, and increased the expression of Homer 1a after I/R injury. Finally, the retinas of Homer1a KO mice treated with I/R injury had significantly less dendrites and RGCs, compared with Homer1a WT mice. These findings demonstrated that Homer1a may contribute to RGCs survival after I/R injury by interacting with Erk pathway.

mTOR signaling in mice with dysfunctional cardiac ryanodine receptor ion channel

Simultaneous substitution of three amino acid residues in the calmodulin binding domain (W3587A/L3591D/F3603A, ADA) of the cardiac ryanodine receptor ion channel (RyR2) impairs calmodulin inhibition of RyR2 and causes cardiac hypertrophy and early death of Ryr2^ADA/ADA mice. To determine the physiological significance of growth promoting signaling molecules, the protein and phosphorylation levels of Ser/Thr kinase mTOR and upstream and downstream signaling molecules were determined in hearts of wild-type and Ryr2^ADA/ADA mice. Phosphorylation of mTOR at Ser-2448, and mTOR downstream targets p70S6 kinase at Thr-389, S6 ribosomal protein at Ser-240/244, and 4E-BP1 at Ser-65 were increased. However, there was no increased phosphorylation of mTOR upstream kinases PDK1 at Ser-241, AKT at Thr-308, AMPK at Thr-172, and ERK1/2 at Thr-202/Tyr204. To confirm a role for mTOR signaling in the development of cardiac hypertrophy, rapamycin, an inhibitor of mTOR, was injected into wild-type and mutant mice. Rapamycin decreased mouse heart-to-body weight ratio, improved cardiac performance, and decreased phosphorylation of mTOR and downstream targets p70S6K and S6 in 10-day-old Ryr2^ADA/ADA mice but did not extend longevity. Taken together, the results link a dysfunctional RyR2 to an altered activity of signaling molecules that regulate cardiac growth and function.

Homer3 regulates the establishment of neutrophil polarity

Most chemoattractants rely on activation of the heterotrimeric G-protein Gαi to regulate directional cell migration, but few links from Gαi to chemotactic effectors are known. Through affinity chromatography using primary neutrophil lysate, we identify Homer3 as a novel Gαi2-binding protein. RNA interference-mediated knockdown of Homer3 in neutrophil-like HL-60 cells impairs chemotaxis and the establishment of polarity of phosphatidylinositol 3,4,5-triphosphate (PIP3) and the actin cytoskeleton, as well as the persistence of the WAVE2 complex. Most previously characterized proteins that are required for cell polarity are needed for actin assembly or activation of core chemotactic effectors such as the Rac GTPase. In contrast, Homer3-knockdown cells show normal magnitude and kinetics of chemoattractant-induced activation of phosphoinositide 3-kinase and Rac effectors. Chemoattractant-stimulated Homer3-knockdown cells also exhibit a normal initial magnitude of actin polymerization but fail to polarize actin assembly and intracellular PIP3 and are defective in the initiation of cell polarity and motility. Our data suggest that Homer3 acts as a scaffold that spatially organizes actin assembly to support neutrophil polarity and motility downstream of GPCR activation.

Store-operated calcium entry: unveiling the calcium handling signalplex

Store-operated Ca(2+) entry (SOCE) is an important mechanism for Ca(2+) influx in non-excitable cells, also present in excitable cells. The activation of store-operated channels (SOCs) is finely regulated by the filling state of the intracellular agonist-sensitive Ca(2+) compartments, and both, the mechanism of sensing the Ca(2+) stores and the nature and functional properties of the SOCs, have been a matter of intense investigation and debate. The identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and both Orai1, as the pore-forming subunit of the channels mediating the Ca(2+)-selective store-operated current, and the members of the TRPC subfamily of proteins, as the channels mediating the cation-permeable SOCs, has shed new light on the underlying events. This review summarizes the initial hypothesis and the current advances on the mechanism of activation of SOCE.

Macro advances in microRNAs and myocardial regeneration

PURPOSE OF REVIEW

Myocardial injury and disease often result in heart failure, a major cause of death worldwide. To achieve myocardial regeneration and foster development of efficient therapeutics for cardiac injury, it is essential to uncover molecular mechanisms that will promote myocardial regeneration. In this review, we examine the latest progress made in elucidation of the roles of small non-coding RNAs called microRNAs (miRs) in myocardial regeneration.

RECENT FINDINGS

Promising progress has been made in studying cardiac regeneration. Several miRs, which include miR-590, miR-199a, miR-17-92 cluster, miR-199a-214 cluster, miR-34a, and miR-15 family, have been recently shown to play an essential role in myocardial regeneration by regulating different processes during cardiac repair, including cell death, proliferation, and metabolism. For example, miR-590 promotes cardiac regeneration through activating cardiomyocyte proliferation, whereas miR-34a inhibits cardiac repair through inducing apoptosis.

SUMMARY

These recent findings shed new light on our understanding of myocardial regeneration and suggest potential novel therapeutic targets to treat cardiac disease.

The emerging role of dopamine-glutamate interaction and of the postsynaptic density in bipolar disorder pathophysiology: Implications for treatment

Aberrant synaptic plasticity, originating from abnormalities in dopamine and/or glutamate transduction pathways, may contribute to the complex clinical manifestations of bipolar disorder (BD). Dopamine and glutamate systems cross-talk at multiple levels, such as at the postsynaptic density (PSD). The PSD is a structural and functional protein mesh implicated in dopamine and glutamate-mediated synaptic plasticity. Proteins at PSD have been demonstrated to be involved in mood disorders pathophysiology and to be modulated by antipsychotics and mood stabilizers. On the other side, post-receptor effectors such as protein kinase B (Akt), glycogen synthase kinase-3 (GSK-3) and the extracellular signal-regulated kinase (Erk), which are implicated in both molecular abnormalities and treatment of BD, may interact with PSD proteins, and participate in the interplay of the dopamine-glutamate signalling pathway. In this review, we describe emerging evidence on the molecular cross-talk between dopamine and glutamate signalling in BD pathophysiology and pharmacological treatment, mainly focusing on dysfunctions in PSD molecules. We also aim to discuss future therapeutic strategies that could selectively target the PSD-mediated signalling cascade at the crossroads of dopamine-glutamate neurotransmission.

Differences in Brain Waves of Normal Persons and Stroke Patients during Action Observation and Motor Imagery

[Purpose] The purpose of this study was to examine the changes in brain waves between action observation and motor imagery of stroke patients and normal subjects, and to compare them. [Methods] Twelve stroke patients and twelve normal persons participated in this research. Each group executed action observation and the motor imagery training for 3 minutes, and before and during each intervention the brain waves were measured for 3 minutes, and the relative alpha power and relative beta power analyzed. [Results] Both normal persons and stroke patients showed significant differences in relative alpha power during action observation, but no significant difference in relative alpha power was found during motor imagery. The relative beta power increased similarly in both groups but it was more significantly different during action observation than during motor imagery. [Conclusion] Both action observation and motor imagery can be used as a therapeutic method for motor learning. However, action observation induces stronger cognitive activity, so for the stroke patients who have difficulty with fine motor representation, action observation might be a more effective therapy.

Differential up-regulation of Vesl-1/Homer 1 protein isoforms associated with decline in visual performance in a preclinical glaucoma model

Glaucoma is a multifactorial progressive ocular pathology, clinically presenting with damage to the retina and optic nerve, ultimately leading to blindness. Retinal ganglion cell loss in glaucoma ultimately results in vision loss. Vesl/Homer proteins are scaffolding proteins that are critical for maintaining synaptic integrity by clustering, organizing and functionally regulating synaptic proteins. Current anti-glaucoma therapies target IOP as the sole modifiable clinical parameters. Long-term pharmacotherapy and surgical treatment do not prevent gradual visual field loss as the disease progresses, highlighting the need for new complementary, alternative and comprehensive treatment approaches. Vesl/Homer expression was measured in the retinae of DBA/2J mice, a preclinical genetic glaucoma model with spontaneous mutations resulting in a phenotype reminiscent of chronic human pigmentary glaucoma. Vesl/Homer proteins were differentially expressed in the aged, glaucomatous DBA/2J retina, both at the transcriptional and translational level. Immunoreactivity for the long Vesl-1L/Homer 1c isoform, but not of the immediate early gene product Vesl-1S/Homer 1a was increased in the synaptic layers of the retina. This increased protein level of Vesl-1L/Homer 1c was correlated with phenotypes of increased disease severity and a decrease in visual performance. The increased expression of Vesl-1L/Homer 1c in the glaucomatous retina likely results in increased intracellular Ca(2+) release through enhancement of synaptic coupling. The ensuing Ca(2+) toxicity may thus activate neurodegenerative pathways and lead to the progressive loss of synaptic function in glaucoma. Our data suggest that higher levels of Vesl-1L/Homer 1c generate a more severe disease phenotype and may represent a viable target for therapy development.

Homer-1a immediate early gene expression correlates with better cognitive performance in aging

The molecular mechanisms underlying cognitive decline during healthy aging remain largely unknown. Utilizing aged wild-type C57BL/6 mice as a model for normal aging, we tested the hypothesis that cognitive performance, memory, and learning as assessed in established behavioral testing paradigms are correlated with the differential expression of isoforms of the Homer family of synaptic scaffolding proteins. Here we describe a loss of cognitive and motor function that occurs when Homer-1a/Vesl-1S protein levels drop during aging. Our data describe a novel mechanism of age-related synaptic changes contributing to loss of biological function, spatial learning, and memory formation as well as motor coordination, with the dominant negative uncoupler of synaptic protein clustering, Homer-1a/Vesl-1S, as a potential target for the prophylaxis and treatment of age-related cognitive decline.

Presenilins regulate the cellular activity of ryanodine receptors differentially through isotype-specific N-terminal cysteines

Presenilins (PS), endoplasmic reticulum (ER) transmembrane proteins, form the catalytic core of γ-secretase, an amyloid precursor protein processing enzyme. Mutations in PS lead to Alzheimer's disease (AD) by altering γ-secretase activity to generate pathologic amyloid beta and amyloid plaques in the brain. Here, we identified a novel mechanism where binding of a soluble, cytosolic N-terminal domain fragment (NTF) of PS to intracellular Ca(2+) release channels, ryanodine receptors (RyR), controls Ca(2+) release from the ER. While PS1NTF decreased total RyR-mediated Ca(2+) release, PS2NTF had no effect at physiological Ca(2+) concentrations. This differential function and isotype-specificity is due to four cysteines absent in PS1NTF, present, however, in PS2NTF. Site-directed mutagenesis targeting these cysteines converted PS1NTF to PS2NTF function and vice versa, indicating differential RyR binding. This novel mechanism of intracellular Ca(2+) regulation through the PS-RyR interaction represents a novel target for AD drug development and the treatment of other neurodegenerative disorders that critically depend on RyR and PS signaling.

The closing and opening of TRPC channels by Homer1 and STIM1

Influx of Ca(2+) is a central component of the receptor-evoked Ca(2+) signal. A ubiquitous form of Ca(2+) influx comes from Ca(2+) channels that are activated in response to depletion of the endoplasmic reticulum Ca(2+) stores and are thus named the store-operated Ca(2+) -influx channels (SOCs). One form of SOC is the transient receptor potential canonical (TRPC) channels. A major question in the field of Ca(2+) signalling is the molecular mechanism that regulates the opening and closing of these channels. All TRPC channels have a Homer-binding ligand and two conserved negative charges that interact with two terminal lysines of the stromal interacting molecule 1 (STIM1). The Homer and STIM1 sites are separated by only four amino acid residues. Based on available results, we propose a molecular mechanism by which Homer couples TRPC channels to IP(3) receptors (IP(3) Rs) to keep these channels in the closed state. Dissociation of the TRPCs-Homer-IP(3) Rs complex allows STIM1 access to the TRPC channels negative charges to gate open these channels.

Scaffold proteins at the postsynaptic density

Scaffold proteins are abundant and essential components of the postsynaptic density (PSD). They play a major role in many synaptic functions including the trafficking, anchoring, and clustering of glutamate receptors and adhesion molecules. Moreover, they link postsynaptic receptors with their downstream signaling proteins and regulate the dynamics of cytoskeletal structures. By definition, PSD scaffold proteins do not have intrinsic enzymatic activities but are formed by modular and specific domains deputed to form large protein networks. Here, we will discuss the latest findings regarding the structure and functions of major PSD scaffold proteins. Given that scaffold proteins are central components of PSD architecture, it is not surprising that deletion or mutations in their human genes cause severe neuropsychiatric disorders including autism, mental retardation, and schizophrenia. Thus, their dynamic organization and regulation are directly correlated with the essential structure of the PSD and the normal physiology of neuronal synapses.

Microarray analysis of gene expression changes in the brains of NR2B-induced memory-enhanced mice

Glutamate N-methyl-D-aspartate (NMDA) receptors play a pivotal role in different forms of memory. The dysfunction of NMDA receptors contributes to the pathology of central nervous system (CNS) disorders. To further investigate the role of the NMDA receptors in brain processes, we analyzed and compared the gene expression profiles in the hippocampus of NR2B overexpression-induced memory-enhanced mice (Tg mice) with those of their wild-type littermates. Results reveal that 249 genes, which are mainly involved in neurotransmission, signal transduction, cytoskeletal structure, hormone activity, and transcription, were significantly affected in Tg mice. Interestingly, the intracellular calcium channel proteins ryanodine receptor (RyR) 1 and 3, as well as functionally related proteins such as the histidine-rich calcium-binding protein and triadin 2, were upregulated. The Homer-1c protein was also increased in Tg mice and formed a complex with the RyR protein in the mouse brain, suggesting that Homer-1c is an important modulator in both intracellular calcium signaling and overall neuronal signaling by simultaneously interacting with the NMDA receptors and RyR. Western blot and real-time PCR results show that the expression of phospho-CREB, c-fos, and the immediate-early genes Egr2 and Egr4 were also upregulated in Tg mice. The current study demonstrates that a chronic increase in the activation of NMDA receptors affected the expression of a large number of genes and may provide important clues for a better understanding of the molecular mechanism of NMDA receptor-modulated learning and memory, as well as of CNS disorders.

Expression and regulation of Homer in human skeletal muscle during neuromuscular junction adaptation to disuse and exercise

Protein calcium sensors of the Homer family have been proposed to modulate the activity of various ion channels and nuclear factor of activated T cells (NFAT), the transcription factor modulating skeletal muscle differentiation. We monitored Homer expression and subcellular localization in human skeletal muscle biopsies following 60 d of bedrest [Second Berlin Bedrest Study (BBR2-2)]. Soleus (SOL) and vastus lateralis (VL) biopsies were taken at start (pre) and at end (end) of bedrest from healthy male volunteers of a control group without exercise (CTR; n=9), a resistive-only exercise group (RE; n=7), and a combined resistive/vibration exercise group (RVE; n=7). Confocal analysis showed Homer immunoreactivity at the postsynaptic microdomain of the neuromuscular junction (NMJ) at bedrest start. After bedrest, Homer immunoreactivity decreased (CTR), remained unchanged (RE), or increased (RVE) at the NMJ. Homer2 mRNA and protein were differently regulated in a muscle-specific way. Activated NFATc1 translocates from cytoplasm to nucleus; increased amounts of NFATc1-immunopositive slow-type myonuclei were found in RVE myofibers of both muscles. Pulldown assays identified NFATc1 and Homer as molecular partners in skeletal muscle. A direct motor nerve control of Homer2 was confirmed in rat NMJs by in vivo denervation. Homer2 is localized at the NMJ and is part of the calcineurin-NFATc1 signaling pathway. RVE has additional benefit over RE as countermeasure preventing disuse-induced neuromuscular maladaptation during bedrest.

Homer-1 polymorphisms are associated with psychopathology and response to treatment in schizophrenic patients

The HOMER 1 protein plays a crucial role in mediating glutamatergic neurotransmission. It has previously shown to be a candidate gene for etiology and pathophysiology of different psychiatric diseases such as schizophrenia. To identify genes involved in response to antipsychotics, subgroups of animals were treated with haloperidol (1 mg/kg, n = 11) or saline (n = 12) for one week. By analyzing microarray data, we replicated the observed increase of Homer 1 gene expression. Furthermore, we genotyped 267 schizophrenic patients, who were treated monotherapeutically with different antipsychotics within randomized-controlled trials. Psychopathology was measured weekly using the PANSS for a minimum of four and a maximum of twelve weeks. Correlations between PANSS subscale scores at baseline and PANSS improvement scores after four weeks of treatment and genotypes were calculated by using a linear model for all investigated SNP's. We found an association between two HOMER 1 polymorphisms (rs2290639 and rs4704560) and different PANSS subscales at baseline. Furthermore all seven investigated polymorphisms were found to be associated with therapy response in terms of a significant correlation with different PANSS improvement subscores after four weeks of antipsychotic treatment. Most significant associations have been shown between the rs2290639 HOMER 1 polymorphism and PANSS subscales both at baseline conditions and after four weeks of antipsychotic treatment. This is the first study which shows an association between HOMER 1 polymorphisms and psychopathology data at baseline and therapy response in a clinical sample of schizophrenic patients. Thus, these data might further help in detecting differential therapy response in individuals with schizophrenia.

Homer 1a gates the induction mechanism for endocannabinoid-mediated synaptic plasticity

At hippocampal excitatory synapses, endocannabinoids (eCBs) mediate two forms of retrograde synaptic inhibition that are induced by postsynaptic depolarization or activation of metabotropic glutamate receptors (mGluRs). The homer family of molecular scaffolds provides spatial organization to regulate postsynaptic signaling cascades, including those activated by mGluRs. Expression of the homer 1a (H1a) immediate-early gene produces a short homer protein that lacks the domain required for homer oligomerization, enabling it to uncouple homer assemblies. Here, we report that H1a differentially modulates two forms of eCB-mediated synaptic plasticity, depolarization-induced suppression of excitation (DSE) and metabotropic suppression of excitation (MSE). EPSCs were recorded from cultured hippocampal neurons and DSE evoked by a 15 s depolarization to 0 mV and MSE evoked by a type I mGluR agonist. Expression of H1a enhanced DSE and inhibited MSE at the same synapse. Many physiologically important stimuli initiate H1a expression including brain-derived neurotrophic factor (BDNF). Treating hippocampal cultures with BDNF increased transcription of H1a and uncoupled homer 1c-GFP (green fluorescent protein) clusters. BDNF treatment blocked MSE and enhanced DSE. Thus, physiological changes in H1a expression gate the induction pathway for eCB-mediated synaptic plasticity by uncoupling mGluR from eCB production.

The unliganded long isoform of estrogen receptor beta stimulates brain ryanodine receptor single channel activity alongside with cytosolic Ca2+

Ca(2+) release from intracellular stores mediated by endoplasmic reticulum membrane ryanodine receptors (RyR) plays a key role in activating and synchronizing downstream Ca(2+)-dependent mechanisms, in different cells varying from apoptosis to nuclear transcription and development of defensive responses. Recently discovered, atypical "nongenomic" effects mediated by estrogen receptors (ER) include rapid Ca(2+) release upon estrogen exposure in conditions implicitly suggesting involvement of RyRs. In the present study, we report various levels of colocalization between RyR type 2 (RyR2) and ER type beta (ER beta) in the neuronal cell line HT-22, indicating a possible functional interaction. Electrophysiological analyses revealed a significant increase in single-channel ionic currents generated by mouse brain RyRs after application of the soluble monomer of the long form ER beta (ER beta 1). The effect was due to a strong increase in open probability of RyR higher open channel sublevels at cytosolic [Ca(2+)] concentrations of 100 nM, suggesting a synergistic action of ER beta 1 and Ca(2+) in RyR activation, and a potential contribution to Ca(2+)-induced Ca(2+) release rather than to basal intracellular Ca(2+) concentration level at rest. This RyR/ER beta interaction has potential effects on cellular physiology, including roles of shorter ER beta isoforms and modulation of the RyR/ER beta complexes by exogenous estrogens.

Prefrontal cortex Homer expression in an animal model of attention-deficit/hyperactivity disorder

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is a pervasive neurobehavioral disorder affecting approximately 5% of children and adolescents and 3% of adults, and the prefrontal cortex (PFC) may play the most critical role in the expression of ADHD. Converging previous studies indicate a potential role of Homer--a scaffolding protein family localized at the postsynaptic density (PSD) of glutamatergic excitatory synapses--in behavioral pathologies associated with neuropsychiatric disorders. Accordingly, we speculate that these Homer isoforms might contribute to the etiology and development of ADHD.

METHOD

We investigated the differential mRNA and protein expressions of several Homer isoforms in the PFC of the spontaneous hypertensive rat/Wistar-Kyoto rats (SHR/WKY), the most frequently used animal model of ADHD, using RT-PCR and western blotting. Furthermore, we examined the effects of methylphenidate (MPH) exposure on the behaviors and the expression of different Homer isoforms in the PFC of SHR, using Làt maze, RT-PCR and western blotting, respectively.

RESULTS

Homer 1a and Homer 2a/b, but not Homer 1b/c, were expressed at a significantly lower levels in the PFC of SHR compared with WKY. MPH exposure decreased the locomotor activity and non-selective attention of SHR, and it up regulated the expression of Homer 1a and Homer 2a/b, but not Homer 1b/c, in the PFC of SHR.

CONCLUSION

It is plausible that Homer 1a and Homer 2a/b may be involved in the etiology and pathogenesis of ADHD. Future work will focus on elucidating the specific mechanisms of Homer 1a and Homer 2a/b in ADHD.

Homer and the ryanodine receptor

Homer proteins have recently been identified as novel high-affinity ligands that modulate ryanodine receptor (RyR) Ca(2+) release channels in heart and skeletal muscle, through an EVH1 domain which binds to proline-rich regions in target proteins. Many Homer proteins can also self-associate through a coiled-coil domain that allows their multimerisation. In other tissues, especially neurons, Homer anchors proteins embedded in the surface membrane to the Ca(2+) release channel in the endoplasmic reticulum and can anchor membrane or cytosolic proteins to the cytoskeleton. Although this anchoring aspect of Homer function has not been extensively investigated in muscle, there are consensus sequences for Homer binding in the RyR and on many of the proteins that it interacts with in the massive RyR ion channel complex. In this review we explore the potential of Homer to contribute to a variety of cell processes in muscle and neurons that also involve RyR channels.

In vitro modulation of the cardiac ryanodine receptor activity by Homer1

The Homer protein family allows clustering and/or functional modulation of many proteins from different calcium signalling complexes including those formed by the ryanodine receptor (RyR) Ca(2+) release channel in skeletal muscle and the heart. Homer1b/c and the cardiac RyR (RyR2) are strongly expressed in the heart and neurons where their interaction with each other may modulate Ca(2+) signalling. However, functional interactions between Homer1b and RyR2 have been poorly defined. Our preliminary data and similar consensus binding sites for Homer in RyR2 and skeletal RyR (RyR1) proteins, led to the hypothesis that Homer may similarly regulate both RyR isoforms. Single-channel and [(3)H]ryanodine binding data showed that RyR2 and RyR1 activity increased to a maximum with ~50-100 nM Homer1b and fell with Homer1b > 200 nM. Homer1b (50 nM) activated RyR2 and RyR1 at all cytosolic [Ca(2+)]; estimated EC(50) value of RyR2 diminished from ~2.8 microM Ca(2+) (control) to ~1.9 microM Ca(2+) in the presence of 50 nM Homer1b. Short Homer1 (lacking the coiled-coil multimerisation domain) and Homer1b similarly modulated RyR2, indicating an action through ligand binding, not mutimerisation. These actions of Homer were generally similar in RyR2 and RyR1. The strong functional interactions suggest that Homer1 is likely to be an endogenous modulator of RyR channels in the heart and neurons as well as in skeletal muscle.

Requirement of the immediate early gene vesl-1S/homer-1a for fear memory formation

Background

The formation of long-term memory (LTM) and the late phase of long-term potentiation (L-LTP) depend on macromolecule synthesis, translation, and transcription in neurons. vesl-1S (VASP/Ena-related gene upregulated during seizure and LTP, also known as homer-1a) is an LTP-induced immediate early gene. The short form of Vesl (Vesl-1S) is an alternatively spliced isoform of the vesl-1 gene, which also encodes the long form of the Vesl protein (Vesl-1L). Vesl-1L is a postsynaptic scaffolding protein that binds to and modulates the metabotropic glutamate receptor 1/5 (mGluR1/5), the IP₃ receptor, and the ryanodine receptor. Vesl-1 null mutant mice show abnormal behavior, which includes anxiety- and depression-related behaviors, and an increase in cocaine-induced locomotion; however, the function of the short form of Vesl in behavior is poorly understood because of the lack of short-form-specific knockout mice.

Results

In this study, we generated short-form-specific gene targeting (KO) mice by knocking in part of vesl-1L/homer-1c cDNA. Homozygous KO mice exhibited normal spine number and morphology. Using the contextual fear conditioning test, we demonstrated that memory acquisition and short-term memory were normal in homozygous KO mice. In contrast, these mice showed impairment in fear memory consolidation. Furthermore, the process from recent to remote memory was affected in homozygous KO mice. Interestingly, reactivation of previously consolidated fear memory attenuated the conditioning-induced freezing response in homozygous KO mice, which suggests that the short form plays a role in fear memory reconsolidation. General activity, emotional performance, and sensitivity to electrofootshock were normal in homozygous KO mice.

Conclusion

These results indicate that the short form of the Vesl family of proteins plays a role in multiple steps of long-term, but not short-term, fear memory formation.

Factors that contribute to the misidentification of tyrosine nitration by shotgun proteomics

The high selectivity and throughput of tandem mass spectrometry allow for rapid identification and localization of various posttranslational protein modifications from complex mixtures by shotgun approaches. Although sequence database search algorithms provide necessary support to process the potentially enormous quantity of MS/MS spectra generated from large scale tandem mass spectrometry experiments, false positive identifications of peptide modifications may exist even after implementation of stringent identification criteria. In this report, we describe factors that lead to misinterpretation of MS/MS spectra as well as common chemical and experimental artifacts that generate false positives using the proteomics-based identification of tyrosine nitration as an example. In addition to the proposed manual validation criteria, the importance of peptide synthesis and subsequent MS/MS characterization for validation of peptide nitration demonstrated by several examples from earlier publications is also presented.

Proteomic analysis of mouse brain microsomes: identification and bioinformatic characterization of endoplasmic reticulum proteins in the mammalian central nervous system

The endoplasmic reticulum (ER) is the main source for the storage and release of intracellular calcium in neurons and, thus, contributes to the functionality of a diverse set of pathways that control critical aspects of central nervous system function including but not limited to gene expression, neurotransmission, learning, and memory. ER-derived proteins obtained after subcellular fractionation of mouse brain homogenate were digested with trypsin and the corresponding peptides fractionated by strong cation exchange chromatography followed by LC-MS/MS analysis on a hybrid linear ion trap--Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. A comprehensive catalogue representing 1914 proteins was generated from this particular proteomic analysis using identification criteria that corresponded to a false positive identification rate of 0.4%. Various molecular functions and biological processes relevant to the ER were identified upon gene ontology (GO)-based analysis including pathways associated with molecular transport, protein trafficking and localization, and cell signaling. Comparison of the 2D-LC-MS/MS results with those obtained from shotgun LC-MS/MS analyses demonstrated that most molecular functions and biological processes were represented via GO analysis using either methodology. Results from this comparison as well as a focused investigation into components of calcium-mediated signaling in the mouse brain ER are also presented.

Mice lacking Homer 1 exhibit a skeletal myopathy characterized by abnormal transient receptor potential channel activity

Transient receptor potential (TRP) channels are nonselective cation channels, several of which are expressed in striated muscle. Because the scaffolding protein Homer 1 has been implicated in TRP channel regulation, we hypothesized that Homer proteins play a significant role in skeletal muscle function. Mice lacking Homer 1 exhibited a myopathy characterized by decreased muscle fiber cross-sectional area and decreased skeletal muscle force generation. Homer 1 knockout myotubes displayed increased basal current density and spontaneous cation influx. This spontaneous cation influx in Homer 1 knockout myotubes was blocked by reexpression of Homer 1b, but not Homer 1a, and by gene silencing of TRPC1. Moreover, diminished Homer 1 expression in mouse models of Duchenne's muscular dystrophy suggests that loss of Homer 1 scaffolding of TRP channels may contribute to the increased stretch-activated channel activity observed in mdx myofibers. These findings provide direct evidence that Homer 1 functions as an important scaffold for TRP channels and regulates mechanotransduction in skeletal muscle.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Dynamic regulation of ryanodine receptor type 1 (RyR1) channel activity by Homer 1

Homer, a family of scaffolding proteins originally identified in neurons, is also expressed in skeletal muscle. Previous studies showed that splice variants of Homer 1 (H1) amplify the gain of the ryanodine receptor type 1 (RyR1) channel complex. Using [3H]ryanodine ([3H]Ry) to probe the conformational state of RyR1, the actions of long- and short-forms of H1 are examined singly and in combination. At < or =200 nM, H1 long-forms (H1b or H1c possessing coiled-coil (CC) domains) and short-forms (H1a or H1EVH1 lacking CC domains) enhance specific [3H]Ry binding to RyR1. However, at a concentration > 200 nM, either H1 form completely inhibited [3H]Ry binding. Importantly, the combinations of H1c+H1EVH1, or H1b+H1a acted in an additive manner to enhance or inhibit [3H]Ry-binding activity. H1a and H1c individually or in combination produced the same dynamic pattern in regulating purified RyR1 channels reconstituted in planar lipid bilayers. In combination, their net action on RyR1 channels depends on total concentrations of H1. These data provide a mechanism by which constitutively and transiently expressed H1 forms can tightly regulate RyR1 channel activity in response to changing levels of expression and degradation of H1 proteins.

Homer proteins in Ca2+ signaling by excitable and non-excitable cells

Homers are scaffolding proteins that bind Ca(2+) signaling proteins in cellular microdomains. The Homers participate in targeting and localization of Ca(2+) signaling proteins in signaling complexes. However, recent work showed that the Homers are not passive scaffolding proteins, but rather they regulate the activity of several proteins within the Ca(2+) signaling complex in an isoform-specific manner. Homer2 increases the GAP activity of RGS proteins and PLCbeta that accelerate the GTPase activity of Galpha subunits. Homer1 gates the activity of TRPC channels, controls the rates of their translocation and retrieval from the plasma membrane and mediates the conformational coupling between TRPC channels and IP(3)Rs. Homer1 stimulates the activity of the cardiac and neuronal L-type Ca(2+) channels Ca(v)1.2 and Ca(v)1.3. Homer1 also mediates the communication between the cardiac and smooth muscle ryanodine receptor RyR2 and Ca(v)1.2 to regulate E-C coupling. In many cases the Homers function as a buffer to reduce the intensity of Ca(2+) signaling and create a negative bias that can be reversed by the immediate early gene form of Homer1. Hence, the Homers should be viewed as the buffers of Ca(2+) signaling that ensure a high spatial and temporal fidelity of the Ca(2+) signaling and activation of downstream effects.

Ca2+ signaling in microdomains: Homer1 mediates the interaction between RyR2 and Cav1.2 to regulate excitation-contraction coupling

Excitation-contraction (E-C) coupling and Ca(2+)-induced Ca(2+) release in smooth and cardiac muscles is mediated by the L-type Ca(2+) channel isoform Ca(v)1.2 and the ryanodine receptor isoform RyR2. Although physical coupling between Ca(v)1.1 and RyR1 in skeletal muscle is well established, it is generally assumed that Ca(v)1.2 and RyR2 do not directly communicate either passively or dynamically during E-C coupling. In the present work, we re-examined this assumption by studying E-C coupling in the detrusor muscle of wild type and Homer1(-/-) mice and by demonstrating a Homer1-mediated dynamic interaction between Ca(v)1.2 and RyR2 using the split green fluorescent protein technique. Deletion of Homer1 in mice (but not of Homer2 or Homer3) resulted in impaired urinary bladder function, which was associated with higher sensitivity of the detrusor muscle to muscarinic stimulation and membrane depolarization. This was not due to an altered expression or function of RyR2 and Ca(v)1.2. Most notably, expression of Ca(v)1.2 and RyR2 tagged with the complementary C- and N-terminal halves of green fluorescent protein and in the presence and absence of Homer1 isoforms revealed that H1a and H1b/c reciprocally modulates a dynamic interaction between Ca(v)1.2 and RyR2 to regulate the intensity of Ca(2+)-induced Ca(2+) release and its dependence on membrane depolarization. These findings define the molecular basis of a "two-state" model of E-C coupling by Ca(v)1.2 and RyR2. In one state, Ca(v)1.2 couples to RyR2 by H1b/c, which results in reduced responsiveness to membrane depolarization and in the other state H1a uncouples Ca(v)1.2 and RyR2 to enhance responsiveness to membrane depolarization. These findings reveal an unexpected and novel mode of interaction and communication between Ca(v)1.2 and RyR2 with important implications for the regulation of smooth and possibly cardiac muscle E-C coupling.

Polycystin-1 can interact with homer 1/Vesl-1 in postnatal hippocampal neurons

Polycystin-1 (PC-1) has been identified as critical to development of the nervous system, but the significance of PC-1 expression in neurons remains undefined, and little is known of its roles outside the kidney, where mutation results in autosomal dominant polycystic kidney disease (ADPKD). In kidney, PC-1 interacts with cadherins, catenins, and its cognate calcium channel polycystin-2 (PC-2), which in turn interacts with a number of actin-regulatory proteins. Because some of the proteins that interact with PC-1 in kidney also participate in synaptic remodeling and plasticity in the hippocampus, we decided to test PC-1's potential to interact with a recently discovered type of plasticity-associated protein (homer 1a/Vesl-1S) in postnatal mouse hippocampus. Homer 1a/Vesl-1S is an activity-induced protein believed to participate in synaptic remodeling/plasticity responses to temporal lobe seizure and learning. Here we report the following. 1) PC-1 contains a homer-binding motif (PPxxF), which lies within its purported cytoplasmic domain. 2) Immunoreactivity for PC-1 (PC-1-ir) is highly colocalized with homer 1a immunoreactivity (H1a-ir) in primary cultured hippocampal neurons. 3) PC-1-ir and H1a-ir are present and appear to be colocalized in mouse hippocampus but not cortex on postnatal day 2 (P2), when higher frequencies of spontaneous activity are normal for hippocampus compared with cortex. 4) An endogenous PC-1-ir band with the correct size for the reported C-terminal G-protein-sensitive domain cleavage product of PC-1 (approximately 150 kDa) coimmunoprecipitates with endogenous homer 1/Vesl-1 proteins from mouse brain, suggesting that PC-1 can interact with homer 1/Vesl-1 proteins in postnatal hippocampal neurons.

Agonists and antagonists of the cardiac ryanodine receptor: Potential therapeutic agents?

This review addresses the potential use of the intracellular ryanodine receptor (RyR) Ca(2+) release channel as a therapeutic target in heart disease. Heart disease encompasses a wide range of conditions with the major contributors to mortality and morbidity being ischaemic heart disease and heart failure (HF). In addition there are many rare, but devastating conditions, some of which are either genetically linked to the RyR and its regulatory proteins or involve drug-induced modification of the proteins. The defects in Ca(2+) signalling vary with the nature of the heart disease and the stage in its progress and therefore specific corrections require different modifications of Ca(2+) signalling. Compounds that activate the RyR are potential inotropic agents to increase the Ca(2+) transient and strength of contraction. Compounds that reduce RyR activity are potentially useful in conditions where excess RyR activity initiates arrhythmias, or depletes the Ca(2+) store, as in end stage HF. It has recently been discovered that the cardio-protective action of the drug JTV519 can be attributed partly to its ability to stabilise the interaction between the RyR and the 12.6 kDa binding protein for the commonly used immunosuppressive drug FK506 (FKBP12.6, known as tacrolimus). This has established the credibility of the RyR as a therapeutic target. We explore the possibility that mutations causing the rare RyR-linked arrhythmias will open the door to identification of novel RyR-based therapeutic agents. The use of regulatory binding sites within the RyR complex or on its associated proteins as templates for drug design is discussed.

Alternative poly(A) site-selection regulates the production of alternatively spliced vesl-1/homer1 isoforms that encode postsynaptic scaffolding proteins

The vesl-1/homer1 gene encodes a scaffold protein that interacts with several receptors to modulate synaptic functions. The gene also encodes two shorter forms that counteract the functions of the long form of Vesl. Expression of the shorter forms is driven by neural activities such as long-term potentiation. Here we analyzed the mechanism regulating vesl-1 alternative splicing. Each functional poly(A) site was in a different part of the 3'-terminal exon, with promoter-proximal and promoter-distal sites at the end of exons corresponding to the short and long form Vesl-1, respectively. 3'-End-processing at proximal poly(A) site, specifically at the vesl-1M poly(A) site, was enhanced by extracellular stimuli, thereby switching transcription termination from promoter-distal to -proximal poly(A) site. This switch was not specifically coupled to the vesl-1 promoter and was independent of de novo protein synthesis. Analysis of transcripts from mini-genes that mimic the structure of endogenous vesl-1 revealed that the vesl-1M poly(A) region plays a crucial role in switching to the alternative pre-mRNA splicing that is triggered by extracellular stimuli. Therefore, a 3'-end-processing event regulates the neural activity-dependent alternative splicing of vesl-1. This is the first report of a gene in which alternative poly(A) site-selection regulates alternative splicing in a protein synthesis-independent manner.

Homer proteins control neuronal differentiation through IP(3) receptor signaling

Neurons expand, sustain or prune their dendritic trees during ontogenesis [Cline, H.T. (2001). Dendritic arbor development and synaptogenesis. Curr. Opin. Neurobiol. 11, 118-126; Wong, W.T. and Wong, R.O.L. (2000) Rapid dendritic movements during synapse formation and rearrangement. Curr. Opin. Neurobiol. 10, 118-124] which critically depends on neuronal activity [Wong, W.T., Faulkner-Jones, B.E., Sanes, J.R. and Wong, R.O.L. (2000) Rapid dendritic remodeling in the developing retina: dependence on neurotransmission and reciprocal regulation by Rac and Rho. J. Neurosci. 20, 5024-5036; Li, Z., Van Aelst, L. and Cline, H.T. (2000) Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo. Nat. Neurosci. 3, 217-225; Wong, W.T. and Wong, R.O.L. (2001) Changing specificity of neurotransmitter regulation of rapid dendritic remodeling during synaptogenesis. Nat. Neurosci. 4, 351-352.] and sub-cellular Ca(2+) signals [Lohmann, C., Myhr, K.L. and Wong, R.O. (2002) Transmitter-evoked local calcium release stabilizes developing dendrites, Nature 418, 177-181.]. The role of synaptic clustering proteins connecting both processes is unclear. Here, we show that expression levels of Vesl-1/Homer 1 isoforms critically control properties of Ca(2+) release from intracellular stores and dendritic morphology of CNS neurons. Vesl-1L/Homer 1c, an isoform with a functional WH1 and coiled-coil domain, but not isoforms missing these features were capable of potentiating intracellular calcium signaling activity indicating that such regulatory interactions function as a general paradigm in cellular differentiation and are subject to changes in expression levels of Vesl/Homer isoforms.

Novel regulators of RyR Ca2+ release channels: insight into molecular changes in genetically-linked myopathies

There are many mutations in the ryanodine receptor (RyR) Ca2+ release channel that are implicated in skeletal muscle disorders and cardiac arrhythmias. More than 80 mutations in the skeletal RyR1 have been identified and linked to malignant hyperthermia, central core disease or multi-minicore disease, while more than 40 mutations in the cardiac RyR2 lead to ventricular arrhythmias and sudden cardiac death in patients with structurally normal hearts. These RyR mutations cause diverse changes in RyR activity which either excessively activate or block the channel in a manner that disrupts Ca2+ signalling in the muscle fibres. In a different myopathy, myotonic dystrophy (DM), a juvenile isoform of the skeletal RyR is preferentially expressed in adults. There are two regions of RyR1 that are variably spiced and developmentally regulated (ASI and ASII). The juvenile isoform (ASI(-)) is less active than the adult isoform (ASI(+)) and its over-expression in adults with DM may contribute to functional changes. Finally, mutations in an important regulator of the RyR, the Ca2+ binding protein calsequestrin (CSQ), have been linked to a disruption of Ca2+ homeostasis in cardiac myocytes that results in arrhythmias. We discuss evidence supporting the hypothesis that mutations in each of these situations alter protein/protein interactions within the RyR complex or between the RyR and its associated proteins. The disruption of these protein-protein interactions can lead either to excess Ca2+ release or reduced Ca2+ release and thus to abnormal Ca2+ homeostasis. Much of the evidence for disruption of protein-protein interactions has been provided by the actions of a group of novel RyR regulators, domain peptides with sequences that correspond to sequences within the RyR and which compete with the endogenous residues for their interaction sites.

Homer proteins: implications for neuropsychiatric disorders

Homer proteins regulate signal transduction, synaptogenesis and receptor trafficking, in addition to maintaining and regulating extracellular glutamate levels in limbo-corticostriatal brain regions. Converging preclinical observations indicate a potential role for both immediate early gene Homer isoforms and constitutively expressed Homer isoforms in behavioral pathologies associated with neuropsychiatric disorders, such as addiction and/or alcoholism, depression, anxiety, epilepsy and schizophrenia.