Cerebellar Hypoplasia in Jaundiced Gunn Rats

Abstract

A mutant strain of Wistar rats with hereditary acholuric jaundice, now known as“Gunn rat, ”was first reported by C. K. Gunn in 1938. The trait of jaundice has been characterized as autosomal recessive inheritance and evidenced to be due to a virtually complete deficiency of an enzyme in hepatic bilirubin metabolism, bilirubin : UDP-glucuronyltransferase. From these points of view, homozygous Gunn rats are generally regarded as an animal model comparable to a human inborn error of bilirubin metabolism, Crigler-Najjar syndrome Type I, which is frequently accompanied by nuclear jaundice (kernicterus) in the neonatal period with rare exceptions.In this communication, we describe our recent studies on the pathobiochemical aspect of bilirubin-induced cerebellar hypoplasia, another notable disorder of the central nervous system, in homozygous Gunn rats of Sprague-Dawley strain, which have been bred in our laboratory for several years. Heterozygous Gunn rats, which were apparently normal in every respect including cerebellar development, were used as a control. While the concentration of serum bilirubin in heterozygotes remained low throughout (0.5 mg/dl or less), its level in homozygotes was increased up to approximately 7 mg/dl as early as 24 hours after birth and reached maximum (17-18 mg/dl) at postnatal days 17-20, followed by a gradual decrease to about 8 mg/dl at day 30. The cerebellar underdevelopment of homozygotes began at day 5, and the increase of cerebellar wet weight virtually ceased after day 10. Photoirradiation of homozygous newborns from b to 10 days of life could effectively prevent their cerebellum from hypoplasia.Analyses of cerebellar proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed characteristic changes of two proteins associated with cerebellar hypoplasia, i.e., marked decrease and increase of 250 k and 50 k-dalton proteins, respectively. The former protein called GR-250 showed very similar properties, except for its molecular weight, to P400, a protein reportedly characteristic of cerebellar Purkinje cells. The latter protein designated GR-50 was unequivocally identified by using a newly developed immunoblotting method as glial fibrillary acidic protein, which was known to be localized in astrocytes.It was found that in the hypoplastic cerebellum of homozygotes the number of lysosomes with different sizes was increased, and lysosomal enzymes were activated to various degrees, among which β-glucuronidase activity was most enhanced-more than 8 times compared with the control. From Percoll density gradient centrifugation analyses and the degree of activation of cerebellar lysosomal enzymes, it was strongly suggested that there were at least three types of lysosomes with functional heterogeneity, and also that a particular type of lysosomes or enzymes was specifically activated accompanied with bilirubin encephalopathy.