Expression Of Carps 241 mRNA Expression

mRNA expressions of CA-RPs VIII, X and XI have been studied in a panel of human and mouse vital organs by Northern blot analyses (Fujikawa-Adachi et al. 1999; Okamoto et al. 2001; Taniuchi et al. 2002; Akisawa et al. 2003; Bellingham et al. 1998), and the results are summarized in Table 2.3. CA-RPs X and XI show a similar expression pattern in humans and mice, with intense signals being observed in the brain. Notable differences in humans and mice are observed in the heart and kidney. Human kidney shows CA-RP X expression but mouse kidney does not, whereas mouse heart shows a positive signal for CA-RP XI but human heart does not. However, the message levels detected in these organs are much weaker than those in the brain. In this regard, mRNA expressions of CA-RPs X and XI appear to be relatively specific to the brain.

Northern blot analysis shows single transcripts of human CA-RP X (2.8 kb; Okamoto et al. 2001), human CA-RP XI (1.5 kb; Fujikawa-Adachi et al. 1999; Bellingham et al. 1998) and mouse CA-RP XI (1.5 kb; Taniuchi et al. 2003), all of which corresponded to the apparently full-length cDNAs (Table 2.1). For murine CA-RP X mRNA expression, however, two sizes of transcripts (2.6 and 3.0 kb), possibly generated by alternative splicing, are observed (Okamoto et al. 2001). Although a number of expression sequence tags of CA-RP X from the human placenta have been deposited in GenBank (Hewett-Emmett 2000), only a shorter transcript (~2.0 kb) is detected on the Northern blot, and the molecular nature of this shorter transcript remains uncertain at present (Okamoto et al. 2001).

In contrast to CA-RPs X and XI, there is much more variety in the transcript size and distribution of CA-RP VIII mRNA expression (Taniuchi et al. 2002; Akisawa et al. 2003). Both human and mouse CA-RPs VIII show broad distribution in a panel of vital organs (Table 2.3). A distinct difference between humans and mice is observed in the lung and liver: there is no detectable signal for a CA-RP VIII message in the human liver and lung, whereas mouse lung and liver as well as the brain show the strongest signals. There is no acceptable explanation for this discrepancy other than species differences.

The blots for mRNA expression of both human and mouse CA-RPs VIII show multiple transcripts: five transcripts with different sizes (2.4, 3.0, 4.0, 4.5 and 7.0 kb) for human CA-RP VIII (Akisawa et al. 2003) and six transcripts (1.4, 1.6, 2.4, 3.4, 3.8 and 4.9 kb) for mouse CA-RP VIII (Taniuchi et al. 2002). Among these transcripts, the 2.4-kb band shows the strongest intensity and is commonly observed in organs that show positive signals. Kato has also reported multiple transcripts of mouse CA-RP VIII (Kato 1990). The different-sized RNAs might be generated by alternative splicing or by the multiple poly(A) signals found in CA-RP VIII mRNA (Akisawa et al. 2003). These findings indicate that CA-RP VIII is diversely expressed not only in the brain but also in various peripheral tissues.

2.4.2 Immunohistochemical Localization in the Brain

Taniuchi et al. (2000, 2002) have produced monoclonal antibodies to all three human CA-RPs that are cross-reactive with mouse homologues and have studied their regional and cellular distribution in the brain by immunohistochemical analysis. Table 2.4 summarizes their results. CA-RPs VIII and XI are consistently expressed in neural cells, astrocytes and neurites of most parts of human and mouse brains (Figure 2.3, left). Lakkis et al. (1997b) have also shown mRNA expression of mouse CA-RP VIII in cerebral neurons and Purkinje cells by an in situ hybridization method. Epithelial cells of the choroid plexus and pia arachnoid also express CA-RPs VIII and XI.

It is of great interest that CA-RP X is expressed in the myelin sheath (Figure 2.3, right). This expression has been confirmed by using brain tissue sections under two unique pathological conditions. One is acute disseminated encephalomyelitis, a disease in which there is focal demyelinization in the human brain. In a demyelinized lesion of a patient's brain, the axons were clearly shown by lucsol fast blue stain,

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