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

Increased Homer Activity and NMJ Localization in the Vestibular Lesion het-/- Mouse soleus Muscle

We investigated the shuttling of Homer protein isoforms identified in soluble (cytosolic) vs. insoluble (membrane-cytoskeletal) fraction and Homer protein-protein interaction/activation in the deep postural calf soleus (SOL) and non-postural gastrocnemius (GAS) muscles of het-/- mice, i.e., mice with an autosomal recessive variant responsible for a vestibular disorder, in order to further elucidate a) the underlying mechanisms of disrupted vestibular system-derived modulation on skeletal muscle, and b) molecular signaling at respective neuromuscular synapses. Heterozygote mice muscles served as the control (CTR). An increase in Homer cross-linking capacity was present in the SOL muscle of het-/- mice as a compensatory mechanism for the altered vestibule system function. Indeed, in both fractions, different Homer immunoreactive bands were detectable, as were Homer monomers (~43-48 kDa), Homer dimers (~100 kDa), and several other Homer multimer bands (>150 kDA). The het-/- GAS particulate fraction showed no Homer dimers vs. SOL. The het-/- SOL soluble fraction showed a twofold increase (+117%, p ≤ 0.0004) in Homer dimers and multimers. Homer monomers were completely absent from the SOL independent of the animals studied, suggesting muscle-specific changes in Homer monomer vs. dimer expression in the postural SOL vs. the non-postural GAS muscles. A morphological assessment showed an increase (+14%, p ≤ 0.0001) in slow/type-I myofiber cross-sectional area in the SOL of het-/- vs. CTR mice. Homer subcellular immuno-localization at the neuromuscular junction (NMJ) showed an altered expression in the SOL of het-/-mice, whereas only not-significant changes were found for all Homer isoforms, as judged by RT-qPCR analysis. Thus, muscle-specific changes, myofiber properties, and neuromuscular signaling mechanisms share causal relationships, as highlighted by the variable subcellular Homer isoform expression at the instable NMJs of vestibular lesioned het-/- mice.

Distinct and Overlapping Expression Patterns of the Homer Family of Scaffolding Proteins and Their Encoding Genes in Developing Murine Cephalic Tissues

In mammals Homer1, Homer2 and Homer3 constitute a family of scaffolding proteins with key roles in Ca²⁺ signaling and Ca²⁺ transport. In rodents, Homer proteins and mRNAs have been shown to be expressed in various postnatal tissues and to be enriched in brain. However, whether the Homers are expressed in developing tissues is hitherto largely unknown. In this work, we used immunohistochemistry and in situ hybridization to analyze the expression patterns of Homer1, Homer2 and Homer3 in developing cephalic structures. Our study revealed that the three Homer proteins and their encoding genes are expressed in a wide range of developing tissues and organs, including the brain, tooth, eye, cochlea, salivary glands, olfactory and respiratory mucosae, bone and taste buds. We show that although overall the three Homers exhibit overlapping distribution patterns, the proteins localize at distinct subcellular domains in several cell types, that in both undifferentiated and differentiated cells Homer proteins are concentrated in puncta and that the vascular endothelium is enriched with Homer3 mRNA and protein. Our findings suggest that Homer proteins may have differential and overlapping functions and are expected to be of value for future research aiming at deciphering the roles of Homer proteins during embryonic development.

Expression of the GluA2 subunit of glutamate receptors is required for the normal dendritic differentiation of cerebellar Purkinje cells

Cerebellar Purkinje cells differentiate the most elaborate dendritic trees among neurons in the brain and constitute the principal part of cerebellar neuronal circuitry. In the present study, we examined the role of the GluA2 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors in the dendritic differentiation of Purkinje cells. Since mature Purkinje cells express the GluA2 subunit, AMPA receptors on them exhibit a low Ca²⁺ permeability. Does this expression of GluA2, leading to the loss of Ca²⁺ permeability of AMPA receptors, have a positive significance in the dendritic differentiation of Purkinje cells? To answer this question, we introduced GluA2 siRNA into immature Purkinje cells in cerebellar cell cultures using a single-cell electroporation technique. The dendritic elongation and branching, as well as spine formation, were inhibited by GluA2 knockdown in Purkinje cells. GluA2 knockdown augmented the elevation of intracellular Ca²⁺ concentrations and a higher incidence of oscillation of intracellular Ca²⁺ concentrations in response to glutamate. These findings suggest that excessive elevation of intracellular Ca²⁺ concentrations has a negative effect on the dendritic differentiation of Purkinje cells and that the expression of GluA2 inhibits this negative effect in the development of Purkinje cells.

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.

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.

Dendrite formation of cerebellar Purkinje cells

During postnatal cerebellar development, Purkinje cells form the most elaborate dendritic trees among neurons in the brain, which have been of great interest to many investigators. This article overviews various examples of cellular and molecular mechanisms of formation of Purkinje cell dendrites as well as the methodological aspects of investigating those mechanisms.

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.

More than just synaptic building blocks: scaffolding proteins of the post-synaptic density regulate dendritic patterning

The dendritic arbor is responsible for receiving and consolidating neuronal input. Outgrowth and morphogenesis of the arbor are complex stages of development that are poorly understood. However, recent findings have identified synaptic scaffolding proteins as novel regulators of these important events. Scaffolding proteins are enriched in the post-synaptic density where they bind and bring into close proximity neurotransmitter receptors, signaling molecules, and regulators of the actin cytoskeleton. This property is important for dendritic spine morphogenesis and maintenance in the mature neuron. Scaffolding proteins are now being described as key regulators of neurite outgrowth, dendritic development, and pattern formation in immature neurons. These proteins, which include post-synaptic-95, Shank and Densin-180, as well as many of their interacting partners, appear to regulate both the microtubule and actin cytoskeleton to influence dendrite morphology. Through a large array of protein-protein interaction domains, scaffolding proteins are able to form large macromolecular complexes that include cytoskeletal motor proteins as well as microtubule and actin regulatory molecules. Together, the new findings form a persuasive argument that scaffolding proteins deliver critical regulatory elements to sites of dendritic outgrowth and branching to modulate the formation and maintenance of the dendritic arbor.