GABAA/benzodiazepine receptor gamma 2 subunit gene expression in developing normal and mutant mouse cerebellum

Transient change in GABA(A) receptor subunit mRNA expression in Lurcher cerebellar nuclei during Purkinje cell degeneration

Background

Lurcher mice suffer from a complete Purkinje cell (PC) loss in the first four postnatal weeks. Parallel to this degeneration, GABAergic synapses in the deep cerebellar nuclei (DCN), the major recipient of the inhibitory PC projection, increase synaptic conductance. Here, we further investigated this phenomenon, using real-time RT-PCR to assess GABA_A receptor subunit gene expression during PC degeneration.

Results

We observed a specific reduction in γ2 subunit gene expression, while α1–5, β1–2, γ1,3 and δ subunits were unaffected. We made two further specific findings. First, the difference in gene expression was shown in tissue from DCN only. Neither the hippocampus nor coronal sections through the forebrain showed such effects. Furthermore, the involvement of different levels of corticosterone, a possible humeral trigger for differences in gene expression, could be excluded. Second, like the known potentiation of GABAergic synapses, the γ2 down-regulation was present only after the onset of degeneration at p14. The difference in γ2 mRNA expression, however, appeared transient, since it was no longer detectable in adult Lurcher mice.

Conclusion

In conclusion, the down-regulation of γ2 subunits may be related to differences in synaptic efficacy and, as such, may reflect the initial phase of adaptive responses of DCN tissue to massive GABAergic deafferentation. Its transient course, however, does not support the idea that modulations in GABAergic transmission are at the basis of the well-known DCN-based functional benefit of Lurcher mice present throughout their life.

Normal formation of the postsynaptic elements of GABAergic synapses in the reeler cerebellum

Synaptic transmission mediated by gamma-amino butyric acid (GABA) plays an important role in inhibition of glutamatergic excitatory transmission and expression of higher brain functions, such as memory, learning and anxiety. To elucidate mechanisms underlying formation of the postsynaptic elements for GABAergic transmission, we employed the reeler mutant mice in this study. In the reeler cerebellum, abnormal cytoarchitecture and an aberrant environment affect the formation of neural networks and maturation of neurons. We examined the expression and localization of GABA(A) receptor alpha subunits in the reeler cerebellum and determined whether various abnormalities in the reeler mice affected formation of the postsynaptic elements. In situ hybridization analysis revealed that the specific expression of alpha subunit mRNAs in each neuronal type was preserved. Abnormal expression of alpha subunits was not detected, although GABAergic networks were altered and neuronal maturation was severely disturbed. Immunohistochemistry for the alpha1 and alpha6 subunits, which were expressed abundantly in the reeler cerebellum, revealed that both subunit proteins accumulated at positions adjacent to GABAergic terminals. These results, taken together, suggested that expression of the GABA(A) receptor subunits in postsynaptic neurons might be genetically determined, but trafficking and accumulation of the subunit proteins at the GABAergic synapse may be induced by GABAergic innervation.

Harmaline-induced changes in gamma aminobutyric acid(A) receptor subunit mRNA expression in murine olivocerebellar nuclei

Increased CNS activity in the form of electrically or chemically induced seizures is known to alter the properties of GABA(A) receptors. The tremorgen, harmaline, causes a bursting pattern of activity in inferior olivary neurons, the effects of which are transmitted throughout the olivocerebellar circuit to other regions of the CNS. In situ hybridization was used to determine the effect of this increased activity on gamma aminobutyric acid(A) (GABA(A)) receptor subunit gene expression in the cerebellar Purkinje cell layer, deep cerebellar nuclei and inferior olivary complex of adult mice. In Purkinje cells, the expression of alpha(1), beta(2), and gamma(2) mRNAs was increased only slightly (<5%) by harmaline administration, while in deep cerebellar neurons, beta(2) transcript levels were initially elevated (26%), but dropped to control levels immediately thereafter. The expression of alpha(2), alpha(4), beta(3) and gamma(1) mRNAs in olivary neurons was affected differentially by harmaline administration. The alpha(4) transcript was increased, reaching >60% above control levels at 6 h post-injection. A smaller increase was observed for alpha(2) mRNA, while beta(3) and gamma(1) transcripts dropped below control levels during the same period. The expression of corticotropin-releasing factor mRNA was also elevated in the olivary complex. These data indicate that while Purkinje cells and deep cerebellar neurons are only minimally affected, harmaline induced changes in cellular properties may result in increased numbers of alpha(4)-containing, diazepam-insensitive, GABA(A) receptors in olivary neurons.

Modulation of GABA(A) receptor subunit mRNA levels in olivocerebellar neurons of purkinje cell degeneration and weaver mutant mice

In olivocerebellar circuits, changes in the subunit composition of GABA(A) receptors occur at a time of extensive synaptic remodeling. In the deep cerebellar nuclei, GABA(A) receptor alpha1, beta2, and gamma2 subunit mRNA expression increases throughout neonatal development, whereas in the inferior olivary complex, the perinatal combination of alpha3, alpha5, beta3, and gamma2 mRNAs switches to the adult combination of alpha2, alpha4, beta3 and gamma1 during postnatal week 2. In situ hybridization was used to examine changes in subunit expression in the olivocerebellar nuclei of Purkinje cell degeneration and weaver mutant mice. In Purkinje cell degeneration, subunit transcripts decreased below control levels in olivary neurons; however, alpha1, beta2, and gamma2 transcript levels were slightly increased in the medial nucleus of the deep cerebellar nuclei. In weaver olivary neurons, although the switch from early- to late-onset subunit mRNAs occurred as in normal mice, transcript levels were differentially modulated by the mutation. Our studies indicate that major alterations in synaptic connectivity do not prevent developmentally programmed switches in GABA(A) receptor gene expression but can modulate the timing and level of transcript expression in afferent and efferent neurons.

Novel receptor protein tyrosine phosphatase (RPTPrho) and acidic fibroblast growth factor (FGF-1) transcripts delineate a rostrocaudal boundary in the granule cell layer of the murine cerebellar cortex

We have identified a novel receptor-like protein tyrosine phosphatase (RPTPrho) transcript whose expression in the cerebellar cortex is restricted to the granule cell layer of lobules 1-6. Acidic fibroblast growth factor (FGF-1) mRNA follows a similar cerebellar expression pattern. Together, the two markers define a sharp boundary in lobule 6, slightly caudal to the primary fissure. Anterior and posterior compartments became discernible only during postnatal weeks two and six, for RPTPrho and FGF-1, respectively. A rostrocaudal boundary in lobule 6 of the murine cerebellar cortex has also been identified morphologically by the effects of the meander tail mutation. The position of the RPTPrho and FGF-1 boundary on the rostrocaudal axis of the cerebellar cortex was close to, but not coincident with, the caudal extent of the disorganized anterior lobe of meander tail and the rostral extent of Otx-2 expression. The restricted pattern of FGF-1 and RPTPrho implies that these molecules may have specific signaling roles in the tyrosine phosphorylation/dephosphorylation pathway in the anterior compartment of the adult cerebellar cortex.

Cerebellar Grafts Partially Reverse Amino Acid Receptor Changes Observed in the Cerebellum of Mice with Hereditary Ataxia: Quantitative Autoradiographic Studies

We used quantitative autoradiography of [3H]CNQX (200 nM), [3H]muscimol (13 nM), and [3H]flunitrazepam (10 nM) binding to study the distribution of non-NMDA and GABAA receptors in the cerebellum of pcd mutant mice with unilateral cerebellar grafts. Nonspecific binding was determined by incubation with 1 mM Glu, 200 μM GABA, or 1 μM clonazepam, respectively. Saturation parameters were defined in wild-type and mutant cerebella. In mutants, non-NMDA receptors were reduced by 38% in the molecular layer and by 47% in the granule cell layer. The reduction of non-NMDA receptors in the pcd cerebellar cortex supports their localization on Purkinje cells. [3H] CNQX binding sites were visualized at higher density in grafts that had migrated to the cerebellar cortex of the hosts (4.1 and 11.0 pmol/mg protein, respectively, at 23 and 37 days after grafting) than in grafts arrested intraparen-chymally (2.6 and 6.2 pmol/mg protein, respectively, at 23 and 37 days after grafting). The pattern of expression of non-NMDA receptors in cortical vs. parenchymal grafts suggests a possible regulation of their levels by transacting elements from host parallel fibers. GABAA binding levels in the grafts for both ligands used were similar to normal molecular layer. Binding was increased in the deep cerebellar nuclei of pcd mutants: the increase in [3H]muscimol binding over normal was 215% and the increase in [3H]flunitrazepam binding was 89%. Such increases in the pcd deep cerebellar nuclei may reflect a denervation-induced supersensitivity subsequent to the loss of Purkinje axon terminal innervation. In the deep nuclei of pcd mutants with unilateral cerebellar grafts, [3H]muscimol binding was 31% lower in the grafted side than in the contralateral nongrafted side at 37 days after transplantation; [3H]fluni-trazepam binding was also lower in the grafted side by 15% compared to the nongrafted side. Such changes in GABAA receptors suggest a significant, albeit partial, normalizing trend of cerebellar grafts on the state of postsynaptic supersensitive receptors in the host cerebellar nuclei.

Characterization of the 1B promoter of fibroblast growth factor 1 and its expression in the adult and developing mouse brain

The present study elucidates the molecular structure of a murine fibroblast growth factor 1 (FGF-1) promoter and describes its distribution in the adult and developing mouse brain. A cDNA clone coding for FGF-1 was isolated from a mouse brain cDNA library. Nucleotide sequence analysis revealed that the clone contained, in addition to the protein coding region, an untranslated exon (FGF-1B) 34 base pairs upstream of the translation start codon ATG. The mouse cDNA clone corresponded to the sole FGF-1 transcript in the brain. An RNase protection assay was used to map the transcription start site of the 1B promoter. The sequences upstream from the major transcription initiation site lacked consensus TATA or CAAT boxes. In situ hybridization with cRNA probes specific for the 1B transcript showed the message to be restricted largely to sensory and motor nuclei in the brainstem, and to the ventral spinal cord and cerebellum. Although occasional brainstem nuclei were labeled at low levels by embryonic day 18, the majority of nuclei became detectable autoradiographically during postnatal weeks 1 and 2, and adult levels of grain density were reached during the 3rd and 4th postnatal weeks. FGF-1B mRNA was expressed in phylogenetically older brain regions, which are involved primarily in processing information from exteroceptive sensory mechanoreceptors and in motor control. The relatively late developmental expression suggests a role for FGF-1 in neuronal maturation, rather than in neurogenesis.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.

The cerebellum: a model system for studying GABAA receptor diversity

The basic unsolved questions concerning GABAA receptors are: "How many receptor subtypes exist?", "What subtypes are used by which types of neuron and where are they located on the cell?", and "What are the functions of the different subtypes?" As described in this Review, the cerebellum is an ideal vertebrate brain region for investigating these issues.

Developmental role of fatty acid-binding proteins in mouse brain

While the functions of the cytoplasmic fatty acid-binding proteins (FABPs) are not well defined, one possibility in neural tissue is in establishing and maintaining the high levels of polyunsaturated fatty acids in membrane lipids characteristic of this tissue and thought essential for normal function. We investigated the reactivity of a protein in developing mouse brain to antiserum prepared against rat heart (H)-FABP. By immunoblot analysis, levels of H-FABP in brain were nearly undetectable until fetal day 17-19, after which levels increased until at least postnatal day 14. Levels of H-FABP were lower in the adult mouse brain, suggesting a function for the protein during differentiation of neural tissue. In immunohistochemical studies with postnatal day 14 mouse brain, the most intensely stained area was the choroid plexus. H-FABP also localized to regions of the somatosensory cortex and to the spinal trigeminal nucleus. In addition, H-FABP was present in the thalamus, entorhinal and piriform cortex, and throughout the pontine and medullary nuclei. Tracts related to the auditory system, including ventral cochlear nucleus and lateral lemniscus, also were H-FABP-positive. In cerebellum, the molecular layer was heavily labeled in cells and processes; in the granule cell layer, there was punctate staining suggestive of mossy fiber terminals. Small cells adjacent to Purkinje cells were intensely stained, while the Purkinje cells were negative. We conclude that H-FABP in brain participates in neurite formation and synapse maturation, and may be related to the similar pattern of expression of GABA related markers.

GABAA receptor subunit mRNA expression in the weaver cerebellum: modulation by cell-cell interactions

Recent studies have suggested that the developmental expression of GABA(A) receptor subunit mRNAs in cerebellar Purkinje neurons is modulated by cell--cell interactions [correction of interacactions]. In this population, the levels of mRNAs encoding the alpha1, beta2, and gamma2 subunits increase simultaneously during the second week of postnatal ontogeny, a period temporally coincident with cerebellar maturation and synapse formation. To determine the importance of cell--cell interactions in modulating receptor gene expression, the levels of GABA(A) receptor subunit mRNAs in Purkinje neurons of weaver mice and littermate controls were examined by quantitative in situ hybridization histochemistry. In the weaver mutant most granule neurons die early in postnatal development, thus eliminating the major source of excitatory input to Purkinje cells. Despite this loss, the three subunit mRNAs were expressed in all Purkinje neurons. However, the levels of expression were generally lower in the mutants than in the littermate controls. These results suggest that the onset of GABA(A) receptor gene expression in cerebellar Purkinje neurons occurs in the absence of extensive synapse formation by mechanisms which may be intrinsic to the neurons. In contrast, the absolute level of transcript expression attained appears to be modulated by cell-cell interactions or by other extrinsic cues present in the cerebellar environment.

Cerebellar granule cells in vitro recapitulate the in vivo pattern of GABAA-receptor subunit expression

GABAA-receptor heterogeneity is based on the combinatorial assembly of a family of 15 subunits (alpha 1-6, beta 1-3, gamma 1-3, delta, rho 1-2) into multiple receptor subtypes. The factors regulating the differential expression of GABAA-receptor subtypes in the CNS are largely unknown. In the present study, we have used primary cultures of rat cerebellar granule cells as model system to analyze to which extent the expression, subunit composition, and subcellular localization of GABAA-receptors depend on signals available in the cerebellum in vivo, in particular GABAergic input which is lost in vitro. GABAA-receptor subunits were visualized immunohistochemically with subunit-specific antibodies and their subcellular localization was investigated by confocal laser microscopy with selective markers for dendritic proteins (microtubule-associated protein 2, non-phosphorylated neurofilaments) and synaptic vesicles (synaptophysin). Triple immunofluorescence staining experiments revealed that dissociated granule cells taken from 7-day-old rats and maintained in culture for 9 days co-expressed the subunits alpha 1 alpha 6 beta 2,3 gamma 2 delta, a subunit repertoire which is identical to that seen in vivo in the adult cerebellum. GABAA-receptor subunits were localized selectively in granule cell somata and in proximal neurites immunopositive for MAP-2. No staining was detected in distal neurites immunopositive for neurofilaments. GABAA-receptor subunits frequently were aggregated in clusters closely apposed to synaptophysin-immunoreactive varicosities, suggesting a post-synaptic localization. Thus, major functional determinants of GABAA-receptors in granule cells (subunit repertoire, subcellular segregation and clustering in post-synaptic sites) develop in vitro, indicating that they are regulated to a large extent by intrinsic factors, independently of GABAergic input.

Developmental changes in the expression of gamma-aminobutyric acidA/benzodiazepine receptor subunit mRNAs in the murine inferior olivary complex

The pharmacological and physiological properties of ligand-gated ion channels are dependent on their subunit composition; spontaneously occurring changes in subunit composition during neuronal development may result in dramatic functional differences between embryonic and adult forms of the receptor complex. In the present study, in situ hybridization with antisense cRNA probes was used to examine the subunit composition of the gamma-aminobutyric acidA/benzodiazepine (GABAA/BZ) receptor in the developing inferior olivary complex. This receptor is thought to be a pentameric chloride channel comprised of selected alpha, beta, gamma, delta, and rho subunits, the majority of which have several isoforms: alpha 1-6, beta 1-4, gamma 1-4, and rho 1,2. Among the 13 subunit variants present in the mammalian central nervous system, alpha 2-5, beta 3, and gamma 1,2 mRNAs are expressed at significant levels in the inferior olivary complex. Two clearly different temporal patterns of GABAA/BZ receptor subunit mRNA expression were observed: The expression of alpha 3, alpha 5, beta 3, and gamma 2 mRNAs was at a peak during embryonic and early postnatal development followed by rapid down-regulation thereafter. Conversely, alpha 2, alpha 4, and gamma 1 mRNA expression was very low or absent during early development, and a pronounced increase was observed at the end of postnatal week 1. These studies suggest that there are developmental changes in the subunit composition of the GABAA/BZ receptor in inferior olivary neurons. These changes in subunit expression, which occur during a period of major alterations in afferent and efferent synaptic connections, may subserve a change in the role of GABA from its function as a neurotrophic factor to that of an inhibitory neurotransmitter.

Differential expression of the short and long forms of the gamma 2 subunit of the GABAA/benzodiazepine receptors

The distribution of the mRNAs encoding the gamma 2S and gamma 2L subunits of the GABAA receptor in the rat brain has been revealed by in situ hybridization, northern blot and dot blot analysis using specific antisense oligonucleotides. In addition, the quantitative distribution of the gamma 2S and gamma 2L subunit peptides participating in the fully assembled GABAA receptors/benzodiazepine receptors has been mapped by immunoprecipitation with specific anti-gamma 2S and anti-gamma 2L antibodies. Several neuronal types and brain regions are enriched in gamma 2L such as neurons of the layer II of striate cortex and cerebellar Purkinje cells as well as the inferior colliculus, superior colliculus, deep cerebellar nuclei, medulla and pons. Other neuronal types and regions are enriched in gamma 2S such as the mitral cells of the olfactory bulb, pyramidal neurons of the pyriform cortex, layer VI of the neocortex, granule cells of the dentate gyrus and pyramidal cells of the hippocampus. Other cortical areas and cerebellar granule cells express both gamma 2S and gamma 2L in comparable amounts. There is a good correlation between the relative expression of gamma 2S and gamma 2L mRNAs and the relative presence of these protein subunits in fully assembled and mature receptors in the studied brain regions. The differential distribution of gamma 2S and gamma 2L might result in differential ethanol sensitivity of the neurons expressing these GABAA receptor subunits.