Effect of thyroid deficiency on Go alpha-subunit isoforms in developing rat cerebral cortex

Developmentally-induced hypothyroidism alters the expression of Egr-1 and Arc genes and the sensitivity to cannabinoid agonists in the hippocampus. Possible implications for memory and learning

We analyzed the role of the cannabinoid system in the cognitive deficits caused by developmentally-induced hypothyroidism. We studied in control and hypothyroid rats the effect of a cannabinoid agonist on spatial memory, hippocampal phosphorylation of CREB and expression of early genes. Our results show that, 1-basal hippocampal expression of early genes and spatial learning are decreased in hypothyroid rats; 2-hypothyroid rats are very sensitive to cannabinoid agonists. Low dose of cannabinoid agonist ineffective in controls altered spatial memory, CREB's phosphorylation and early gene expression in hypothyroids. These effects are not due a change in CB1 receptor (CB1R) content. 3-Treatment of hypothyroid rats with thyroid hormones normalized the biochemical and behavioral responses to cannabinoid agonists but did not correct the low basal levels of early gene transcripts and the deficits in spatial learning. All these data suggest that the hippocampal deregulation of early genes expression could play an important role in the basal cognitive deficits of hypothyroid rats.

Thyroid hormone regulates Galphai1 gene expression in the rat cerebellar cortex during post-natal development

Thyroid hormone regulates the expression of G protein in tissues such as fat and heart. In the brain, very little information is available relative to the regulation by thyroid hormone of G proteins. Here, we show that the expression of the Galphai1 gene is induced by thyroid hormones in the rat cerebellum during development. Hence, the levels of Galphai1 transcripts and protein were decreased in the cerebellum of hypothyroid neonates. In situ hybridization studies showed that the neurons of the cerebellar cortex, particularly Purkinje cells, were affected. Surprisingly, and in contrast with the in vivo stimulatory effect described above, thyroid hormone repressed the activity of the rat Galphai1 promoter in vitro, suggesting that the effect of this hormone in the cerebellum is indirect. In this regard, we present data suggesting that the transcription factor C/EBPbeta could be implicated. First, there are active CEBP binding sites in the Galphai1 promoter. Second, we have found a diminished DNA binding activity of hypothyroid nuclear proteins to a Galphai1 promoter sequence containing a C/EBP binding site. Third, this complex is likely to contain C/EBPbeta protein as it is displaced by specific anti-C/EBPbeta antibodies. Finally, there is a significant decrease in the C/EBPbeta protein content in the hypothyroid cerebellar cortex.

Increased baclofen-stimulated G protein coupling and deactivation in rat brain cortex during development

The number and affinity of GABA(B) receptors (assayed by the specific antagonist [(3)H]CGP54626A) was unchanged when compared in carefully washed cerebrocortical membranes from young (12-day-old) and adult (90-day-old) rats. In contrast, high-affinity GTPase activity, both basal and baclofen-stimulated was significantly higher (by 45% and 56%, respectively) in adult than in young rats. Similar results were obtained by concomitant determination of agonist (baclofen)-stimulated GTP gamma S binding. Under standard conditions, baclofen-stimulated GTPase activity was further considerably enhanced by exogenously added regulator of G protein function, RGS1, but not by RGS16. RGS16 was able to affect agonist-stimulated GTPase activity only in the presence of markedly increase substrate (GTP) concentrations. RGS1 alone slightly increased GTPase activity in adult rats, but neither RGS1 nor RGS16 influenced GTPase activity in membrane preparations isolated from young animals. These findings indicate increasing functional activity of trimeric G protein(s) involved in GABAergic transmission in the developing rat brain cortex and suggest a high potential of RGS1 in regulation of high-affinity GTPase activity.

Expression of novel splice variants of the G protein subunit, Go alpha, is tissue-specific and age-dependent in the rat

Heterotrimeric G proteins play an essential role in coupling numerous surface membrane receptors to intracellular signal transduction pathways. Relatively little is known about the splice variants of G proteins, including whether they undergo differential expression as a function of aging. We screened for splice variants of the alpha subunit of the dominant inhibitory G protein family member G(o) in a variety of tissues from rat and examined the expression of these splice variants during development. The splice variants were characterized using specific primers for G(o)alpha1 and G(o)alpha2 in conjunction with reverse transcription-polymerase chain reaction, and subsequently sequenced. G(o)alpha1 expression dominated over G(o)alpha2 in all neuronal tissues screened, including cerebral cortex, pituitary, spinal cord, colon myenteric plexus, dorsal root ganglion, and prenatal cortex. The sequence data of G(o)alpha1 supports the presence of three splice variants: G(o)alpha1a, G(o)alpha1b, G(o)alpha1c. The G(o)alpha1a variant was reported previously [J. Biol. Chem. 262 (1987) 14241], whereas G(o)alpha1b and G(o)alpha1c represent novel variants. The G(o)alpha1b splice variant demonstrates a 94 bp deletion using a cryptic donor site in exon 10. The G(o)alpha1c variant demonstrates a complete deletion of exon 10. A protein product with a molecular weight of approximately 34 kDa consistent with that expected for G(o)alpha1c was identified using Western blot analysis and two-dimensional gel electrophoresis. The expression of G(o)alpha1a decreased postnatally, supporting a potential physiological role during fetal development, whereas G(o)alpha1c expression increased postnatally. The age-dependent and tissue-specific expression of the G(o)alpha1 splice variants presage a broader functional role than has been observed historically with G(o).

Opposing changes of trimeric G protein levels during ontogenetic development of rat brain

Developmental changes in the distribution of guanine nucleotide-binding regulatory proteins (G proteins) were investigated in the rat brain during postnatal development. Using a standard or high-resolution urea-SDS-PAGE and specific polyclonal antipeptide antibodies oriented against G(i)alpha1/G(i)alpha2, G(i)alpha3, G(s)alpha, G(o)alpha1/G(o)alpha2, G(q)alpha/G(11)alpha and Gbeta subunit, all these proteins were determined by quantitative immunoblotting in homogenates prepared from cortex, thalamus, hippocampus and pituitary of 1-, 7-, 12-, 18-, 25- and 90-day-old animals. The levels of the majority of G protein alpha subunits, namely G(i)alpha1, G(i)alpha2, G(i)alpha3, G(o)alpha1, G(o)alpha2, G(q)alpha, G(11)alpha and Gbeta, were high already at birth. Whereas the short variant of G(s)alpha, G(s)alphaS, rose sharply in all tested brain regions between postnatal day (PD) 1 and 90, the long variant of G(s)alpha, G(s)alphaL, was unchanged in cortex and thalamus and slightly increased in hippocampus. An increase was observed also in expression of G(i)alpha1/G(i)alpha2 and G(o)alpha1 protein, while G(o)alpha2 remained constant. Minority protein G(o)alpha* dramatically increased in cortex and thalamus, was unchanged in hippocampus and not detectable in pituitary. By contrast, the highest levels of G(i)alpha3 and G(q)alpha/G(11)alpha were detected as early as at PD 1. During the next 90 days, the immunological signal of G(i)alpha3 almost disappeared and G(q)alpha/G(11)alpha continuously declined to the levels corresponding to 50% of the levels determined at birth. Expression of Gbeta subunit was basically unchanged during postnatal development. Our present analysis indicates that G(s)alpha, G(i)alpha/G(o)alpha and G(q)alpha/G(11)alpha proteins are differently expressed in the course of brain development. Differential expression of the individual alpha subunits of trimeric G proteins during postnatal development suggests their different roles in maturation of the brain tissue.

Effects of neonatal hypothyroidism on the expressions of growth cone proteins and axon guidance molecules related genes in the hippocampus

During critical periods of development, hypothyroidism causes abnormalities of the central nervous system such as incomplete maturation of neuronal and glial cells, reduction in synaptic densities and myelin deficits. In this study expression of development regulated genes, ie transcription of beta-actin, sema 3F, CRMP 1 to 4, GAP-43, G alpha o1, G alpha o2 and translation of beta-actin, G alpha o, G alpha o1, CRMP-2, CRMP-4 genes were examined in the hippocampus of neonatal methimazole induced hypothyroid rats at the age of day 16. All CRMPs mRNA levels were significantly higher in the hypothyroid rats. Significant higher CRMP-2 protein but not CRMP-4 protein was found in the hypothyroid rats. The neonatal experimental hypothyroid states did not affect the protein levels of beta-actin but up-regulate its mRNA. Transcription of CRMP 1 to 4, GAP-43, G alpha o1 but not G alpha o2 and sema 3F was altered by the neonatal treatment. The only sex difference in gene expression was found in the transcription of CRMP-2 gene.

Characterization of the major bovine brain Go alpha isoforms. Mapping the structural differences between the alpha subunit isoforms identifies a variable region of the protein involved in receptor interactions

Go is the major G protein in bovine brain, with at least three isoforms, GoA, GoB, and GoC. Whereas alphaoA and alphaoB arise from a single Goalpha gene as alternatively spliced mRNAs, alphaoA and alphaoC are thought to differ by covalent modification. To test the hypothesis that alphaoA and alphaoC have different N-terminal lipid modifications, proteolytic fragments of alphao isoforms were immunoprecipitated with an N terminus-specific antibody and analyzed by matrix-assisted laser desorption ionization mass spectrometry. The major masses observed in immunoprecipitates were the same for all three alphao isoforms and corresponded to the predicted mass of a myristoylated N-terminal fragment. Structural differences between alphaoA and alphaoC were also compared before and after limited tryptic proteolysis using SDS-polyacrylamide gel electrophoresis containing 6 M urea. Based upon the alphao subunit fragments produced under activating and nonactivating conditions, differences between alphaoA and alphaoC were localized to a C-terminal fragment of the protein. This region, involved in receptor and effector interactions, implies divergent signaling roles for these two alphao proteins. Finally, the structural difference between alphaoA and alphaoC is associated with a difference of at most 2 daltons based upon measurements by electrospay ionization mass spectrometry.

Expression of G protein alpha subunits in normal rat colon and in azoxymethane-induced colonic neoplasms

BACKGROUND & AIMS

Heterotrimeric G proteins are important in growth-regulating signal transduction. The aim of this study was to characterize the relative expression of G protein alpha subunits in rat colonocytes, colonocyte antipodal plasma membranes, and colonic neoplasms.

METHODS

Antipodal plasma membranes were prepared from isolated colonocytes. Azoxymethane was administered to rats to induce colonic neoplasms. K-ras mutations in the neoplasms were determined by oligonucleotide hybridization and confirmed by primer mediated-restriction fragment length polymorphism. Colonocyte and tumor homogenates or membranes were probed for Galpha subunits by Western blotting with isoform-specific antibodies.

RESULTS

The expressions of Galphai2, alphai3, and alphaq/11 were significantly enriched in the basolateral compared with brush border fraction of colonic antipodal plasma membranes. In neoplasms without K-ras mutations, the expression of Galphai2 increased 4-fold, Galphas(long) increased 2.5-fold, and Galphai3 increased 1.5-2-fold. Expression did not differ among tumor grades. K-ras mutations were associated with lowered expression of G proteins, especially Galphao.

CONCLUSIONS

In colonocytes, Galpha subunits are localized primarily in basolateral plasma membranes. The increased expressions of Galphai2 and, to a lesser degree, Galphai3 and Galphas(long) in tumors was independent of tumor grade but was modulated by the presence of K-ras mutations.