Characterization of a novel mRNA expressed by neurons in mature brain

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

Auxiliary subunits of Shaker-type potassium channels

Voltage-gated potassium channels are important determinants of membrane excitability. This family of ion channels is composed of several classes of proteins, including the pore-forming Kvalpha subunits and the recently identified auxiliary Kvbeta subunits. A combination of a large number of genes that encode various alpha subunits and beta subunits and the selective formation of alpha-alpha and alpha-beta heteromultimeric channels provides rich molecular diversity that allows for regulated functional heterogeneity in both excitable tissues and nonexcitable tissues. Because the Kvbeta subunits can either upregulate or downregulate potassium currents, depending on the specific subunit combination, it is essential to understand their function at the molecular level. Furthermore, targeted changes of the Kvbeta expression or disruption of certain alpha-beta interactions could serve as a molecular basis for designing drugs and therapy to regulate excitability clinically.

Latent herpes simplex virus infection of sensory neurons alters neuronal gene expression

The persistence of herpes simplex virus (HSV) and the diseases that it causes in the human population can be attributed to the maintenance of a latent infection within neurons in sensory ganglia. Little is known about the effects of latent infection on the host neuron. We have addressed the question of whether latent HSV infection affects neuronal gene expression by using microarray transcript profiling of host gene expression in ganglia from latently infected versus mock-infected mouse trigeminal ganglia. (33)P-labeled cDNA probes from pooled ganglia harvested at 30 days postinfection or post-mock infection were hybridized to nylon arrays printed with 2,556 mouse genes. Signal intensities were acquired by phosphorimager. Mean intensities (n = 4 replicates in each of three independent experiments) of signals from mock-infected versus latently infected ganglia were compared by using a variant of Student's t test. We identified significant changes in the expression of mouse neuronal genes, including several with roles in gene expression, such as the Clk2 gene, and neurotransmission, such as genes encoding potassium voltage-gated channels and a muscarinic acetylcholine receptor. We confirmed the neuronal localization of some of these transcripts by using in situ hybridization. To validate the microarray results, we performed real-time reverse transcriptase PCR analyses for a selection of the genes. These studies demonstrate that latent HSV infection can alter neuronal gene expression and might provide a new mechanism for how persistent viral infection can cause chronic disease.

Developmental expression of voltage-gated potassium channel beta subunits

Expression of potassium channel beta subunits (Kvbeta) was determined in the developing mouse CNS using an antiserum against an amino acid sequence present in the C-terminus of Kvbeta1, Kvbeta2, and Kvbeta3. Using the anti-Kvbeta antiserum, we determined that Kvbeta expression is restricted to the spinal cord and dorsal root ganglia in the embryonic CNS. At birth, Kvbeta expression is detected in brainstem and midbrain nuclei, but was not detected in the hippocampus, cerebellum or cerebral cortex. During the first postnatal week, Kvbeta expression is present in hippocampal and cortical pyramidal cells and in cerebellar Purkinje cells. Expression of Kvbeta subunits reaches adult levels by the third postnatal week in all of the brain regions examined. A rabbit antiserum directed against a unique peptide sequence in the N-terminus of the Kvbeta1 protein demonstrates that this subunit displays a novel expression pattern in the developing mouse brain. Kvbeta1 expression is high at birth in all brain regions examined and decreases with age. In contrast, Kvbeta2 expression is low at birth and increases with age to reach adult levels by the third postnatal week. These findings support the notion that the differential regulation of distinct potassium channel beta subunits, in the developing mouse nervous system, may confer the functional diversity required to mediate both neuronal survival and maturation.

Voltage-gated K+ channel beta subunits: expression and distribution of Kv beta 1 and Kv beta 2 in adult rat brain

Recent cloning of K+ channel beta subunits revealed that these cytoplasmic polypeptides can dramatically alter the kinetics of current inactivation and promote efficient glycosylation and surface expression of the channel-forming alpha subunits. Here, we examined the expression, distribution, and association of two of these beta subunits, Kv beta 1 and Kv beta 2, in adult rat brain. In situ hybridization using cRNA probes revealed that these beta-subunit genes are heterogeneously expressed, with high densities of Kv beta 1 mRNA in the striatum, CA1 subfield of the hippocampus, and cerebellar Purkinje cells, and high densities of Kv beta 2 mRNA in the cerebral cortex, cerebellum, and brainstem. Immunohistochemical staining using subunit-specific monoclonal and affinity-purified polyclonal antibodies revealed that the Kv beta 1 and Kv beta 2 polypeptides frequently co-localize and are concentrated in neuronal perikarya, dendrites, and terminal fields, and in the juxtaparanodal region of myelinated axons. Immunoblot and reciprocal co-immunoprecipitation analyses indicated that Kv beta 2 is the major beta subunit present in rat brain membranes, and that most K+ channel complexes containing Kv beta 1 also contain Kv beta 2. Taken together, these data suggest that Kv beta 2 is a component of almost all K+ channel complexes containing Kv 1 alpha subunits, and that individual channels may contain two or more biochemically and functionally distinct beta-subunit polypeptides.

Genomic organization and genetic mapping of the neuroimmune gene I2rf5 to mouse chromosome 4

The nervous and immune systems share many functional and molecular similarities, including shared surface antigens, secretions of soluble factors, and cross-modulatory effects. We have identified previously a novel mRNA termed F5, which is expressed only in activated T lymphocytes and mature, postmitotic neurons. Tissue specificity and sequence conservation suggest an important function for F5 in T-lymphocyte proliferation and neuronal maturation. The F5 gene product is an evolutionarily conserved, cytoskeletal-associated phosphoprotein. A full-length mouse genomic clone has been isolated. The protein coding region of the F5 gene is approximately 16 kb in length and is composed of 13 coding exons. The gene encoding F5, termed I2rf5, was mapped using interspecies mouse crosses in close proximity to a number of genes associated with neuronal defects on distal chromosome 4.

Subcellular localization of the F5 protein to the neuronal membrane-associated cytoskeleton

F5 was identified originally as an interleukin-2-regulated gene in the murine helper T-lymphocyte clone L2. Subsequent studies demonstrated high levels of F5 mRNA and protein in mature neurons in adult mouse central and peripheral nervous systems. The F5 protein was present in dendrites and perikarya but not in axons. In the present studies, the intracellular localization of the F5 protein in adult mouse brain was determined by subcellular fractionation and Western blotting. Although the deduced F5 sequence predicts a soluble protein, virtually no F5 immunoreactivity was found in the cytosol. The F5 protein was restricted to the P2 crude mitochondrial and P3 crude microsomal particulate fractions. Within the P2 fraction, F5 protein was enriched in the P2B synaptosomal subfraction. The results of temperature-dependent phase separation with Triton X-114 and alkaline extraction with sodium carbonate of the P2 and P3 fractions were consistent with the F5 protein being an extrinsic membrane-associated protein. Although essentially all of the F5 protein in the P3 fraction was membrane-associated, a substantial proportion of P2-associated F5 protein and nearly all of the synaptosomal F5 protein was detergent-insoluble. Direct isolation and subfractionation of brain cytoskeleton confirmed colocalization of F5 immunoreactivity with the membrane-associated cytoskeleton and postsynaptic densities. These studies suggest that the F5 protein, which has a large number of potential phosphorylation sites, plays a role in membrane-cytoskeletal interactions and in dynamic aspects of synaptic structure or function.

Characterization of the neuroimmune protein F5: localization to the dendrites and perikarya of mature neurons and the basal aspect of choroid plexus epithelial cells

F5 was identified originally as an interleukin-2-regulated gene in L2 cells, a murine helper T-lymphocyte clone. In adult mouse, F5 mRNA was expressed at a modest level in lymphoid tissues, at a high level in mature neurons in the nervous system, but not in other tissues. Although the F5 sequence is highly conserved over evolution, the function of the F5 protein is unknown. In the present studies, the putative F5 protein-coding region was translated in vitro using a reticulocyte lysate system and in Escherichia coli, yielding a protein with the predicted molecular weight of 42 kDa. Polyclonal rabbit anti-F5 antibody was generated against a synthetic peptide corresponding to the C-terminus of the F5 protein. This antibody specifically recognized recombinant F5 protein. Western blot studies demonstrated a strongly-reactive 42-kDa band and a faint 39-kDa band in extracts of adult mouse brain regions, the levels of which paralleled F5 mRNA expression. Immunoperoxidase studies of adult mouse brain demonstrated F5 immunoreactivity in neuronal perikarya and dendrites but not axons. Neurons expressing the highest levels of F5 protein corresponded to those with the highest levels of F5 mRNA. Choroid plexus epithelial cells also exhibited strong reactivity localized to their basal aspect. These observations suggest that the F5 protein, expression of which appears to be regulated predominantly at the RNA level, may be involved in the maintenance of the functional or anatomic polarity of neurons and choroid plexus epithelial cells.

Neurofilament mRNA is reduced in Parkinson's disease substantia nigra pars compacta neurons

Lewy bodies are filamentous neuronal inclusions characteristic of Parkinson's disease, and neurofilament triplet proteins are the major components of the filaments in Lewy bodies. Since the neurofilament proteins found in Lewy bodies are abnormally phosphorylated and partially degraded, the formation of Lewy bodies may be due to the defective metabolism of these proteins, and this could lead to impairments in the structure and function of neurofilament rich neuronal processes (i.e., large caliber axons). To gain further insights into the metabolism of neurofilaments in Parkinson's disease, we evaluated neurofilament mRNA levels by semi-quantitative in situ hybridization histochemistry in postmortem tissues from Parkinson's disease and control subjects. Substantia nigra pars compacta neurons were examined with digoxigenin-UTP labeled cRNA probes to the heavy and light neurofilament mRNAs. The relative abundance of these mRNAs was measured by videodensitometric image analysis of chromogenic reaction product. Using this approach, we demonstrated that the levels of both heavy and light neurofilament mRNAs were reduced in Parkinson's disease substantia nigra pars compacta neurons. Additionally, the levels of heavy neurofilament mRNA were lowest in Lewy body containing neurons in the Parkinson's disease cases. These results suggest that the formation of neurofilament-rich Lewy bodies in substantia nigra pars compacta neurons is associated with reduced levels of the heavy and light neurofilament mRNAs in Parkinson's disease. Thus, it is possible that the accumulation of abnormal neurofilament proteins in Lewy bodies and diminished neurofilament mRNAs contribute to the degeneration of substantia nigra pars compacta neurons in Parkinson's disease.

Expression of the T-lymphocyte activation gene, F5, by mature neurons

F5 was first identified as an mRNA expressed by activated but not resting T-lymphocytes. Subsequent studies suggested that it also is selectively expressed by mature neurons. Although the F5 protein coding sequence is highly conserved, the function of the F5-encoded protein is unknown. The present studies were undertaken to define the anatomic distribution, cellular specificity, and developmental pattern of F5 mRNA expression in the mouse nervous system, addressing specifically the question of whether the expression pattern of F5 corresponds to that of known ligand-receptor or signal-transduction systems. The use of a nonradioactive in situ hybridization method and paraffin-embedded sections provided excellent morphological preservation and a high degree of cellular resolution. F5 mRNA was detected in the central nervous system, peripheral nervous system, and retina in cells having the location and morphological features of neurons. Combined in situ hybridization histochemistry for F5 mRNA and immunofluorescence staining for cell-specific markers confirmed that neurons expressed F5 mRNA but astrocytes did not. The neuronal expression of F5 mRNA had two interesting features. First, the level of expression appeared to correlate directly with the size of the neuronal perikarya, the length of the axonal projection, or the extent of dendritic arborization. Second, F5 mRNA appeared late in post-natal development. These observations are of interest because of preliminary data suggesting that F5 may function as a substrate for protein kinase C, which demonstrates a similar expression pattern in the nervous system.