Abstract :
We have previously reported that the dopamine D3-receptor mRNA is lost in certain cortical regions of brains obtained postmortem from long-term hospitalized patients with chronic schizophrenia. However, the expression of a truncated D3-like mRNA, named D3nf, was found to be unaltered in these anatomical regions (Schmauss et al, 1993, PNAS 90: 8942-8946). Results from in vitro splicing experiments will now be presented that demonstrate that D3nf mRNA derives from the D-encoded pre-mRNA via removal of an alternative spliceosomal intron that has an atypical 3′ splice-site and branchpoint sequence. Despite the unusualcis -acting elements of the D3nf-specific splicing activity, D3nf mRNA and protein are abundantly expressed in human brain (Liu et al, 1994, JBC 269: 29220-29226). A more quantitative analysis (with RNase protection assays) of the relative abundance of D3 and D3nf mRNA expression in the cingulate cortex of schizophrenics showed a decreased expression of D3 mRNA and an increased expression of D3nf mRNA compared to controls. This finding suggests that enhanced D3nf-specific alternative splicing of the D3-encoded pre-mRNA is the underlying reason for the observed decreased (or abolished) expression of D3-encoded mRNA in brains of patients with chronic psychosis. Furthermore, preliminary results from in vitro splicing experiments, performed with nuclear extracts of HeLa and SY5Y cells, indicate that the efficiency of D3nf-specific splicing is controlled by alternative selection of two distinct branchpoints of the alternative intron.
