Alternate splicing in human Na+-MI cotransporter gene yields differentially regulated transport isoforms

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• myo-Inositol is vital for cellular function, acting as an osmolyte and phosphoinositide precursor.
• Na+-myo-inositol cotransporters (SMIT) maintain millimolar intracellular myo-inositol concentrations.
• SMIT activity and transcript expression show variability across mammalian tissues and species.

Purpose of the Study:

• To investigate the molecular basis for observed heterogeneity in SMIT structure and regulation.
• To identify novel mechanisms generating SMIT diversity.
• To explore the functional implications of alternative splicing in SMIT gene expression.

Main Methods:

• Functional cloning and expression of canine SMIT cDNA in Xenopus oocytes.
• Analysis of SMIT transcript arrays in various mammalian cells and species.
• Investigation of alternative splicing events in human SMIT gene expression using human retinal pigment epithelial cells.

Main Results:

• A canine SMIT cDNA predicted a 718-amino acid peptide homologous to Na+-glucose cotransporters, with potential phosphorylation sites.
• Osmotic induction of diverse SMIT transcripts (1.0-13.5 kb) was observed in mammalian cells.
• Alternative splicing, including the use of a novel splice donor site within the coding region, generates SMIT isoforms with altered C-termini and potential phosphorylation sites.

Conclusions:

• Alternative splicing within the SMIT coding region can generate novel isoforms with distinct intracellular C-termini.
• These novel isoforms possess unique phosphorylation sites (Protein Kinase A and C), potentially explaining functional heterogeneity.
• The findings provide insight into the structural and regulatory diversity of Na+-myo-inositol cotransporters.