William M. Brown

Synthesis of the foetal protein fetuin by early developing neurons in the immature neocortex

SummaryThe presence of the foetal protein fetuin has previously been demonstrated by immunocytochemistry to be specifically confined to the primordial plexiform layer, the early cortical plate and subplate zone cells in the developing neocortex of a number of species. In order to investigate its origin there, we have appliedin situ hybridization in paraffin sections of Bouins fixed foetal sheep brain, using a short anti-sense oligonucleotide probe. The distribution of fetuin mRNA has been compared with that of the protein by using anti-fetuin antibodies and immunocytochemistry. This allowed us to confirm that fetuin is synthesised initially in cells of the primordial plexiform layer and subsequently cortical plate and subplate cells. On the other hand, cells in the ventricular zone that are fetuin (protein) positive do not contain detectable fetuin mRNA. The time course of fetuin mRNA expression in the developing neocortex follows closely the previously described pattern of fetuin (protein) distribution in the sheep brain, apart from its absence from the ventricular zone where its origin is probably by uptake from cerebrospinal fluid.

Fetuin: an acute phase protein in cattle

SummaryFetuin is a plasma protein present in high concentrations during fetal development in animals of the order Artiodactyla. Its role is not known. The human homologue of fetuin — α2HS glycoprotein — has been shown to be a negative acute phase protein in adult plasma. In the present study, the concentration of fetuin was measured in the serum of healthy cattle (Bovis bovis) and in animals with various injuries and inflammatory disorders. The levels were decreased by 30% in pregnancy but increased up to 10-fold in some trauma cases. A significant negative correlation between the concentrations of fetuin and albumin has also been found. Thus, fetuin appears to be a positive acute phase protein in cattle.

Structure of the Fetuins

The fetuins are a group of closely related proteins belonging to the cystatin superfamily (see Introduction and Fig. 1.1). Full cDNA sequences of seven fetuins are known, and partial amino acid data on two more are available (see below). In this chapter we will describe in detail what is known about fetuin’s structure based on their amino acid and cDNA sequences and on biochemical and molecular studies performed over the last 50 years. Both bovine and human fetuins will be reviewed with regard to the physicochemical and biochemical data available, and this will be followed by a discussion of molecular data (cDNA and genomic sequence) on the other known fetuins. Finally, we will discuss the posttranslational modification of fetuins.

Fetuin in Plasma and Cerebrospinal Fluid

Although fetuin levels in plasma change in response to acute phase stimuli, and in some pathological conditions (see chapter 4), the most characteristic trait of fetuin, which led to its discovery by Pedersen in 1944,1 is the high concentration of this protein in the plasma of fetal animals belonging to the order Artiodactyla (cow, sheep, pig, goat etc).

Functions of Fetuin

No review of fetuin could possibly avoid listing the claimed, counterclaimed and reclaimed possible functions of fetuin. Since its discovery just 50 years ago,1 a whole plethora of papers claiming specific functions for fetuin has appeared in the literature. There have been several excellent reviews on fetuin, especially in tissue culture applications (see refs. 2 and 3 for most recent and comprehensive), so we will not dwell on this aspect of the field for too long.

Future and Conclusions

In the preceding six chapters we tried to summarize our present knowledge of fetuin structure (including its place in the newly formed family of cystatins) and gene organization, to describe fetuin’s role and possible functions as claimed by many studies, and to illustrate the distribution of fetuin in tissue development, with special emphasis on the nervous system. During our research of the material for this book we were struck by how little fetuin revealed about itself from so much work and knowledge gained about the protein.

Fetuin in Tissue Development

Fetuin is present in the fetus not only in blood plasma (see chapter 5) but also in many developing organs. In fact fetuin seems to be the most widely intracellulary distributed plasma protein in the developing animals studied thus far. Extensive information on fetuin, however, is only available for the sheep fetus which has been our main animal model for many years (see below). Some information on fetuin in the human, and more recently, rat fetuses is now available and will also be discussed.

Fetuin and the Cystatins

As mentioned in chapter 1, Elzanowski and colleagues were the first to make a real connection between the fetuins, the kininogens and the other members of the cystatin superfamily.1 The idea of a cystatin superfamily was born when it was realized that repeated cystatin-like sequences were present in the kininogens2 and that the stefins were quite closely related to both the cystatins and the repeats in the kininogens.2 Nawa et al3 and Ohkubo et ale reported sequence duplications within the bovine and human kininogen protein sequences, respectively. Ohkubo et al further noted that the repeats identified showed homology to a number of cystatin and stefin sequences.2 Later analysis led Salvesen et al to propose that there were, in fact, three cystatin-like repeats within the kininogen sequence, the first being rather less well conserved4 (Fig. 3.1).