Hepatoma-derived growth factor (HDGF) belongs to a polypeptide family containing five additional members called HDGF related proteins 1–4 (HRP-1 to -4) and Lens epithelial derived growth factor. Whereas some family members such as HDGF and HRP-2 are expressed in a wide range of tissues, the expression of others is very restricted. HRP-1 and -4 are only expressed in testis, HRP-3 only in the nervous system. Here we investigated the expression of HDGF, HRP-2 and HRP-3 in the central nervous system of adult mice on the cellular level by immunohistochemistry. In addition we performed Western blot analysis of various brain regions as well as neuronal and glial cell cultures.
HDGF was rather evenly expressed throughout all brain regions tested with the lowest expression in the substantia nigra. HRP-2 was strongly expressed in the thalamus, prefrontal and parietal cortex, neurohypophysis, and the cerebellum, HRP-3 in the bulbus olfactorius, piriform cortex and amygdala complex. HDGF and HRP-2 were found to be expressed by neurons, astrocytes and oligodendrocytes. In contrast, strong expression of HRP-3 in the adult nervous system is restricted to neurons, except for very weak expression in oligodendrocytes in the brain stem. Although the majority of neurons are HRP-3 positive, some like cerebellar granule cells are negative.
The coexpression of HDGF and HRP-2 in glia and neurons as well as the coexpression of all three proteins in many neurons suggests different functions of members of the HDGF protein family in cells of the central nervous system that might include proliferation as well as cell survival. In addition the restricted expression of HRP-3 point to a special function of this family member for neuronal cells.