Expression of carbonic anhydrases IX and XII during mouse embryonic development

Background Of the thirteen active carbonic anhydrase (CA) isozymes, CA IX and XII have been linked to carcinogenesis. It has been suggested that these membrane-bound CAs participate in cancer cell invasion, which is facilitated by an acidic tumor cell environment. Since active cell migration is a characteristic feature of embryonic development, we set out to explore whether these isozymes are expressed in mouse embryos of different ages. The studies were focused on organogenesis stage. Results Immunohistochemistry demonstrated that both CA IX and XII are present in several tissues of the developing mouse embryo during organogenesis. Staining for CA IX revealed a relatively wide distribution pattern with moderate signals in the brain, lung, pancreas and liver and weak signals in the kidney and stomach. The expression pattern of CA XII in the embryonic tissues was also relatively broad, although the intensity of immunostaining was weak in most tissues. The CA XII-positive tissues included the brain, where the most prominent staining was seen in the choroid plexus, and the stomach, pancreas, liver and kidney. Conclusion Membrane-bound CA isozymes IX and XII are expressed in various tissues during mouse organogenesis. These enzymes may regulate ion and pH homeostasis within the developing embryo.

Of the thirteen active isozymes, CA IX and XII have been linked to neoplastic invasion [12,13]. Both are transmembrane proteins. CA IX is composed of four domains: an N-terminal proteoglycan domain, a CA catalytic domain, a transmembrane region and a short cytoplasmic tail [14]. It is a highly active enzyme, and its activity can be efficiently inhibited by sulfonamides [15][16][17][18][19]. In addition to its enzyme activity and role in pH control, CA IX is a cell adhesion molecule and may also contribute to cell proliferation [20][21][22]. The distribution of CA IX has been studied in adult human, rat and mouse tissues [5,23]. The most abundant expression of CA IX was observed in the human alimentary tract, particularly in the mucosa of the stomach and gallbladder, and it was also detected in the ileum, colon, liver and pancreas. In mouse tissues, the highest immunoreactivity for CA IX was reported in the gastric mucosa, while moderate signals were also seen in the colon and brain and lower expression in some other tissues, including the pancreas and various segments of the small intestine. CA IX is ectopically expressed at relatively high levels and with a high prevalence in some tumor tissues whose normal counterparts do not contain this protein, e.g. carcinomas of the cervix uteri, esophagus, kidney, lung and breast [24][25][26][27][28][29]. On the other hand, tumors originating from tissues with high natural CA IX expression, such as the stomach and gallbladder, often lose some or all of their CA IX upon conversion to carcinomas [30][31][32].
CA XII contains an N-terminal extracellular domain, a putative transmembrane α-helix and a small intracellular C-terminal segment with potential phosphorylation sites [6,14,33]. Its expression has been demonstrated by immunohistochemistry in the adult human kidney, colon, prostate, pancreas, ovary, testis, lung and brain [34,35], and the enzyme has been localized to the basolateral plasma membranes of the epithelial cells [36][37][38]. In the human kidney, CA XII is confined to the proximal and distal tubules and the principal cells of the collecting duct [39]. In mouse tissues it is most abundant in the kidney [40] and the surface epithelial cells of the colon [41]. CA XII expression also shows a clear association with certain tumors, being overexpressed in renal cancer cells, for example [6].
One characteristic feature of embryonic development is active cell migration from one place to another. Although this clearly represents a benign process, it has some mechanistic similarities to cancer cell invasion [42,43], e.g. the fact that the moving cells invade through the extracellular matrix. Since CA IX and XII probably participate in neoplastic invasion, we set out to explore how these isozymes are expressed during embryonic development.

Results and discussion
Immunohistochemical staining of CA IX revealed a relatively wide distribution pattern, although the signal intensity most often remained low or moderate. The E7.5 embryos, representing a gastrulation stage, were completely negative ( Figure 1). CA IX expression in the various tissues during organogenesis is summarized in Table 1. The protein was present in the developing brain at all ages Immunostaining of CA IX and CA XII in the embryos at E7.5 Figure 1 Immunostaining of CA IX and CA XII in the embryos at E7.5. No immunoreaction is detected for either CA IX (A) or CA XII (B). Original magnifications: × 400. studied ( Figure 2). The brain tissue was stained moderately, and some positivity was occasionally observed in cells present in the mesenchyme beneath the developing brain (data not shown). Moderate staining was also seen in the nerve ganglia and choroid plexus ( Figure 2). No immunoreaction for CA IX was detected in the kidney at E11.5, whereas a weak positive signal appeared at E12.5 ( Figure 3). The developing pancreas showed a moderate positive reaction at E12.5, which was primarily seen in the basolateral plasma membrane and intracellular compartment of the epithelial cells ( Figure 4). Weak staining for CA IX was present in the stomach at all ages studied (Figure 5). This is in accordance with the finding that CA IX is functionally important for a normal gastric histological structure [44]. The liver also showed positive immunostaining in scattered cells ( Figure 5). Positive labeling was seen in the lung and heart, tissues not expressing the protein in the adult mouse (data not shown). It is notable, however, that the adult heart tissue also gave a slight positive signal with the automated immunostaining method, even though it has been previously considered negative for CA IX [23].
The expression pattern of CA XII in embryonic tissues was also relatively broad, although the staining intensity was weak in most tissues. The E7.5 embryos showed no immunoreaction ( Figure 1). Results at later stages are summarized in Table 2. CA XII protein was expressed in the brain and nerve ganglia at every subsequent age during organogenesis (Figure 2), most prominently in the choroid plexus at E12.5 and E13.5 (Figure 2), i.e. at the time when the developing choroid plexus usually becomes visible. Interestingly, a weak signal for CA XII was detected in several embryonic tissues, including the stomach ( Figure 5), pancreas ( Figure 4) and liver ( Figure  5), which are all negative in adult mice [41]. No staining was detected in the stomach at E11.5, while a weak positive signal appeared there at E12.5. The liver showed weak or moderate staining for CA XII during organogenesis. It is notable that even though CA XII is highly expressed in the adult mouse kidney, the embryonic kidney showed only a weak signal (Figure 3). Weak immunostaining was also seen in the pancreas, where just a few of the developing ducts were positive ( Figure 4). In the heart, the staining became stronger during mouse development (data not shown), but as with CA IX, the specificity of CA XII immunostaining is questionable in this particular organ. However, the control stainings using normal rabbit serum instead of the anti-CA IX or anti-CA XII serum gave no positive signals.
CA IX and XII are distinct CA isozymes in that they are overexpressed in certain tumors and subjected to regulation by the von Hippel Lindau tumor suppressor protein/ hypoxia pathway [35,45]. In developing embryo, the expression patterns of CA IX and CA XII may also be related to the presence of hypoxia, which is considered essential for proper morphogenesis of various tissues [46]. Hypoxia appears important particularly for development of the brain, myocardial vascularization, lung branching morphogenesis, formation of mesoderm and establishment of various progenitor cells [47][48][49].
The high catalytic activities of CA IX and XII support their role in acidification of the tumor microenvironment, which in turn facilitates the migration of tumor cells through the extracellular matrix [12,13]. The question is whether CA IX and XII also participate in cell migration during embryonic development. Although the present results provide no functional evidence that CA IX or XII is involved in cell migration during embryogenesis, they do indicate that several cell types in the mouse embryo express these isozymes. Interestingly, both isozymes were present though at quite low level in some embryonic tissues whose adult counterparts do not express these particular proteins or the expression is very low. These findings contrast with prior studies on the developmental regulation of CA IV. This isozyme, like many of the cytosolic isozymes, is expressed at much lower levels in most tissues of the embryo than are found in the adult [50,51].

Conclusion
Membrane-bound CA isozymes IX and XII are expressed in several tissues of developing mouse embryo. As membrane-bound CAs with an extracellular active site, CA IX and XII represent key enzymes in the maintenance of an appropriate pH in the extracellular milieu. Future studies should therefore be focused on exploring how strictly pH homeostasis is regulated in a developing embryo and what are the possible structural or functional consequences if this homeostasis is disrupted. Immunostaining of CA IX and CA XII in embryonic and adult mouse nervous tissues Figure 2 Immunostaining of CA IX and CA XII in embryonic and adult mouse nervous tissues. All embryos are aged E12.5 except the choroid plexus for CA XII, which is aged E13.5. CA IX shows moderate staining in the embryonic brain (A), with the signal mainly located in the neurons. CA IX is also present in the trigeminal ganglion (B) and the choroid plexus (C). Panel G shows strong positive staining for CA IX in the adult brain. CA XII gives weak staining in the embryonic brain (D), but panel E shows moderate staining in the trigeminal ganglion. The strongest immunoreaction is located in the chroid plexus (F). No specific signal for CA XII is detectable in the adult brain (H) except for the choroid plexus (data not shown). Control immunostaining of the embryonic brain with normal rabbit serum is negative (I). Manual PAP staining in panels A-E and I, automated immunostaining in panels F-H. Original magnifications: A-E, I × 400, F × 630, G-H × 200.

Antibodies
Polyclonal rabbit antibodies to mouse CA IX and CA XII have been described earlier [40,44]. Non-immune normal rabbit serum (NRS) was used in the control stainings instead of the specific antisera.

Immunohistochemistry
Mouse embryos were obtained by mating male and female NMRI mice. The procedures were approved by the animal care committees of Helsinki University and Tampere University. Noon on the day on which the copulation plug was found was considered to represent 0.5 days Immunostaining of CA IX and CA XII in the kidney of E12.5 mouse embryos and in the adult mouse kidney p.c. 7.5 (n = 2), 11.5 (n = 3), 12.5 (n = 4) and 13.5 (n = 2) p.c. embryos with or without extraembryonic tissues were briefly washed with PBS, fixed with 4% paraformaldehyde and embedded in paraffin. Sections were cut at 5-8 μm and placed on SuperFrost ® Plus microscope slides (Menzel; Braunschweig, Germany). Tissue samples from the stomach, heart, brain, liver, kidney and pancreas of an adult NMRI mouse were obtained for control purposes. Immunoperoxidase staining was performed using an automated Lab Vision Autostainer 480 (ImmunoVision Technologies Co., Brisbane, CA, USA). As this automated immunostaining method produced some nonspecific Immunostaining of CA IX and CA XII in the embryonic (E12.5) and adult mouse pancreas labeling of the nuclei in the embryonal tissues, immunostaining was repeated using a less sensitive but more specific peroxidase-antiperoxidase complex method (manual PAP) to confirm the validity of the results. Immunostaining of CA IX and CA XII in the embryonic (E12.5) mouse stomach and liver