Expression of tyrosine hydroxylase in cerebellar Purkinje cells of ataxic mutant mice:its relation to the onset and/or development of ataxia

Expression of tyrosine hydroxylase in cerebellar Purkinje cells of ataxic
mutant mice:its relation to the onset and/or development of ataxia

Kazuhiko Sawada and Yoshihiro Fukui

Department of Anatomy, The University of Tokushima School of Medicine, Tokushima Japan

Abstract: This report describes recent studies on tyrosine hydroxylase (TH) expression in Purkinje cells of the cerebellum of ataxic mutant mice. An increased expression of TH in some Purkinje cells has been observed in two allelic groups of mutant mice, tottering and dilute. TH-positive Purkinje cells appeared preceding the onset of ataxia. Northern blot analysis revealed2.1kb of TH mRNA in the mutant cerebella, and the size was identical to that of TH transcripts in other brain regions. However, TH in Purkinje cells did not seem to participate in catecholamine biosynthesis. In vitro studies showed that cultured non-catecholaminergic neurons expressed the TH transcripts following Ca2+ influx. Therefore, abnormal TH expression in the mutant Purkinje cells may indicate neuronal dysfunction caused by misregulation of intracellular Ca2+concentrations. J. Med. Invest. 48:5-10, 2001

Keywords:Purkinje cells;ataxia;tyrosine hydroxylase;calcium channel;tottering;rolling mouse Nagoya; dilute-lethal mouse

INTRODUCTION
Tyrosine hydroxylase (TH), the first step enzyme for catecholamine (CA) synthesis, is mainly expressed in CAergic neurons in the brain. TH was also expressed in some non-CAergic neurons of various brain regions during development (1-5),in ataxic mutant mice (6-9), and in the experimental conditions such as organotypic tissue culture(10,11) and transplantation (12). This report describes recent studies on TH expression in cerebellar Purkinje cells of ataxic mutant mice and its relation to the onset and/or development of the ataxia.

TH EXPRESSION IN CEREBELLA OF ATAXIC MUTANT MICE
Normal mice expressed TH gene (6,7) and immunoreactivity (9,13,14) in Purkinje cells at low levels. TH-positive Purkinje cells transiently increased from the first to second weeks of postnatal life (6,13). Then the number of TH-positive Purkinje cells decreased, maintained a low level, and increased again by 11 months of age (13).
Hess and Wilson first reported increased expressions of both TH mRNA and immunoreactivity in some Purkinje cells of adult tottering and leaner mice (6). Our recent study also exhibited an enhanced TH immunoreactivity in some Purkinje cells of rolling mouse Nagoya (RMN) on day 14 and thereafter (9). These three mutant mice carry recessive mutant alleles of the tottering (Cchl1a4) locus on chromosome 8 (15-17). They commonly express ataxia, whereas varying degrees of several abnormal neurological phenotypes have been exhibited (Table1). Thus, abnormal TH expression in Purkinje cells appears in all three mutants of the tottering allelic group.
We have further reported an increased TH immunoreactivity in some Purkinje cells of dilute-lethal mice (DL)(9). DL carry the mutation allele in the dilute (Myo5a) locus on chromosome 9 (18),and show the ataxia and opisthotonic seizures (Table1). Therefore, abnormal TH expression in Purkinje cells may not be specific to the allelic group, but rather related to the ataxic symptoms.
In RMN, TH-positive Purkinje cells were distributed in all lobules of the cerebellum and were arranged into parasagittal bands running through the vermis and hemispheres (9). Five narrow bands were observed in the anterior vermis, distributed symmetrically on each side of the midline band (Fig.1B). The bands widened in the posterior vermis (Fig.1D). Consistent results have been obtained in tottering and leaner mice (7,8). The topology of TH-positive Purkinje cells in the mutant cerebella corresponded to Zebrin-positive Purkinje cells (8).DL also showed similar banding pattern of TH-positive Purkinje cells in lobules IX and X of the vermis (9).
The relationship between TH expression in Purkinje cells and the onset of ataxic symptoms has been reported clearly in DL (19). DL walked normally by day8. The ataxic symptom was exhibited by about 20% of DL on day 9 and by all DL by day 10. TH-positive Purkinje cells began to appear in lobules IX and X of the vermis of either ataxic or non-ataxic DL on day9 (Fig.2B), and they drastically increased between days 9 and 10 (Fig.2C). We concluded that ataxia in DL may appear immediately following abnormal TH expression in the Purkinje cells.

ROLE OF TH
Northern blot analysis revealed 2.1 kb of TH mRNA in the cerebellum of tottering and leaner mice (6,7). The size was identical to TH transcripts in other brain regions (7). However, the role of TH in the development of ataxia is unclear. In RMN, GABA immunoreactivity appeared in Purkinje cells, similarly to their littermate controls (20). TH-positive Purkinje cells in tottering and leaner mice coexpressed mRNA for glutamic acid decalboxylase (GAD), the synthetic enzyme for GABA (6). Aromatic amino acid decarboxylase, the next enzyme of the catecholamine synthesis, could not be detected immunohistochemically in any Purkinje cells in tottering and leaner mice (6). By biochemical study, noradrenaline content in the RMN cerebellum was not different from that in their controls, although TH activity was higher in the RMN cerebellum (21). Thus, TH-positive Purkinje cells in the mutant mice do not seem to participate in catecholamine biosynthesis, and a phenotypic switch from GABAergic to CAergic does not occur.

MECHANISM OF TH EXPRESSION
In vitro studies showed that the Ca2+ response element was present in the TH promoter, and non-CAergic neurons expressed the TH transcripts following Ca2+ influx (22-24). These results indicate that abnormal TH expression in the mutant Purkinje cells may be caused by misregulation of intracellular Ca2+ concentration.
In DL, the smooth endoplasmic reticulum (SER), which played a crucial role for synaptic regulation as an intracellular Ca2+ store, was missing in the dendritic spine of Purkinje cells (25). Depletion of intracellular Ca2+ stores by inositol 1,4,5-triphosphate, ionomycin and an excess of EDTA induced a sustained Ca2+ entry through the plasma membrane of mast cells (26). Therefore, Ca2+ influx followed by a failure of Ca2+ mobilization from SER may cause abnormal TH expression in Purkinje cells of DL.
The presumptive mechanism by TH expression in Purkinje cells of the mutant mice belonging to the tottering allelic group is summarized in Figure3. In tottering, leaner and RMN, mutation alleles are located on the tottering (Cchl1a4) locus (15-17), which encodes the α1A subunit of the P/Q-type Ca2+ channel (27,28). The mutated Ca2+ channel α1A subunit was prominently expressed throughout cerebellar Purkinje cells (29), and the P-type Ca2+ channel currents of those neurons were reduced (28, 30-32). In compensation for altered function of the α1A subunit, expression of the α1c subunit of the L-type Ca2+ channel increased in Purkinje cells of tottering mice (33). Chronic injections of the L-type Ca2+ channel blockers decreased TH mRNA expression in the cerebellum of tottering mice (34). Double immunohistochemistry for corticotropin-releasing factor (CRF) and TH in the RMN cerebellum revealed that distribution of TH-positive Purkinje cells corresponded to terminal fields of CRF-positive climbing fibers (35). CRF potentiated Ca2+ currents through the L-type Ca2+channel (36). Therefore, an increased level of CRF in climbing fibers may facilitate Ca2+ influx through the α1c subunit of the L-type Ca2+ channel in their target Purkinje cells, and may induce abnormal TH expression in those neurons of the mutant mice belonging to the tottering allelic group.

CONCLUSIONS
As summarized in Table1, abnormal TH expression in Purkinje cells has been observed in two allelic groups of mutant mice, tottering and dilute, and appeared in connection with the onset and/or development of the ataxic symptoms. Since the transcription of the TH gene was regulated by Ca2+, TH expression in Purkinje cells of the mutant mice may indicate neural dysfunction by alteration of the intracellular Ca2+ concentrations.

ACKNOWLEDGMENTS
The authors thank Dr. S. Oda, Graduate School of Bio-Agricultural Science, Nagoya University, for kindly supplying the mutant mice, RMN and DL. Breeding and maintenance of these mutants were supported by a grant from the Japan Health Sciences Foundation. This study was supported by Grants-in-Aid for Scientific Research (09770865and 09877221) from the Ministry of Education, Science, Sports and Culture, Japan.

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Received for publication Octover 3, 2000 ; accepted November 2, 2000.

Address correspondence and reprint requests to Kazuhiko Sawada, Ph.D., Department of Anatomy, The University of Tokushima School of Medicine, Tokushima 770-8503, Japan and Fax:+81-88-633-7053.