Protein tyrosine phosphatases (PTPases) are important regulatory proteins that, together with protein tyrosine kinases, determine the phosphotyrosine levels in cell signalling proteins. By PCR amplification of mouse brain cDNA fragments encoding the catalytic domains of these enzymes, we identified three novel members of the PTPase gene family. Northern-blot analysis showed that two of these novel clones represent brain-specific PTPases, whereas the third originates from a large-sized mRNA that is more ubiquitously expressed. A full-length cDNA encoding one of the brain-specific PTPases, PTP-SL, was isolated. Sequence analysis revealed a transmembrane PTPase containing a single catalytic phosphatase domain that has 45% homology to a rat cytoplasmic brain-specific PTPase named STEP. This suggests a role for PTP-SL in cell-cell signalling processes in the brain.

The importance of growth factors acting via receptor-type protein-tyrosine kinases in the continuous renewal of the epidermis from the keratinocyte stem cell population has been well established. Protein-tyrosine phosphatases (PTPases), which dephosphorylate phosphotyrosine-containing proteins, may therefore be expected to play an equally important role in the control of epidermal growth and differentiation. In this study, we have made an inventory of the various PTPases that are expressed during mouse keratinocyte proliferation and maturation. A panel of 13 different PTPases probes was obtained by combining a set of PTPase cDNAs previously cloned from mouse brain and a set of PTPase probes obtained from a normalized keratinocyte PTPase cDNA library. This PTPase cDNA panel, spanning probes for receptor-type as well as cytoplasmic-type family members, was used to monitor RNA expression levels in keratinocyte fractions isolated from murine epidermis and in keratinocyte cell cultures. No overt changes were observed in PTPase mRNA levels in all strata of mouse epidermis, but comparison of cultured cells with freshly isolated keratinocytes revealed several conspicuous differences. In the cultured Balb/MK cell line, absence of PTP delta expression and upregulation of PTP kappa and, to a lesser extent, PTP gamma mRNA ratios were observed compared to the freshly isolated cells. These results provide a basis for further research on the impact of PTPase activity on epidermal growth control.

Using degenerate oligodeoxynucleotide primers we previously obtained cDNA fragments from ten different murine protein-tyrosine phosphatases (PTPases). Employing this same primer set, a method was developed to assess the expression levels of these PTPase family members in a fast and simple way. RT-PCR products of several cell types and tissue samples were used as probes on dot-blots containing the ten different PTPase fragments in equimolar amounts. Hybridization intensities at the various dots reflect the relative expression levels of the corresponding PTPases in the starting material. In this way expression of PTPases during mouse brain development could be monitored. Expression of PTP delta was found to be absent in embryonic stem cells but high in fetal and adult brain. PTP epsilon expression is shown to gradually increase in brain during maturation. Our method is generally applicable to gene families of which the transcripts can be detected with a single degenerate primer pair and is especially useful in situations where only limited amounts of RNA can be obtained.