Autism is a severe neurodevelopmental disorder characterized by problems in communication, social skills, and repetitive behavior. Many areas of the brain in autism show abnormalities including decreased Purkinje cell counts in cerebellar hemispheres and vermis, loss of granule cells and Purkinje cell atrophy. Recent studies suggest that apoptotic mechanisms may contribute to the pathogenesis of this disorder. Cathepsin D is the predominant lysosomal protease and is abundantly expressed in the brain. It plays an important role in regulation of cellular apoptosis and has been shown to mediate apoptosis induced by inflammatory cytokines TNF-α and IFN-γ.
The aim of this study is to determine whether cathepsin D activities are altered in the brain of autistic subjects and whether the alterations are associated with the pathogenic apoptotic processes in autistic brain.
Frozen human brain tissue (frontal cerebral cortex and cerebellum) of 7 autistic subjects (mean age 8.1±2.1 years) and 7 age-matched control subjects (mean age 7.7±1.9 years) were obtained from the NICHD Brain and Tissue Bank for Developmental Disorders. Donors with autism fit the diagnostic criteria of the Diagnostic and Statistical Manual-IV, as confirmed by the Autism Diagnostic Interview-Revised. Participants were excluded from the study if they had a diagnosis of fragile X syndrome, epileptic seizures, obsessive–compulsive disorder, affective disorders, or any additional psychiatric or neurological diagnoses. This study was approved by the Institutional Review Board of the NY State Institute of Basic Research. In this study, Western Blot Analyses were used to detect the expression levels of cathepsin D, Bcl2 and caspase-3 in the brain homogenates. Immunohistochemistry studies were used to examine the expression of cathepsin D in the brain sections.
We found that cathepsin D protein expression was significantly increased in the frontal cortex, in pyramidal and granule cells of the hippocampus, and in cerebellar neurons in autistic subjects as compared to controls. In addition, we found that the expression of the anti-apoptotic protein Bcl2 was significantly decreased, while caspase-3, a critical executioner of apoptosis, was increased in the cerebellum of autistic subjects. Previously our studies have shown that Bcl2 expression is decreased and the BDNF-Akt-Bcl2 pathway is compromised in the frontal cortex of autistic subjects, which suggested that increased apoptosis may be involved in the pathogenesis of autism. Our current finding of decreased Bcl2 and increased capase-3 in the cerebellum of autistic subjects further supports this suggestion.
Conclusions: Our findings suggest that cathepsin D may play an important role in the pathogenesis of autism through its regulation of apoptosis.