mXK and mKell are co-expressed in erythroid tissues, and mXK, but probably not mKell, is expressed in different regions of brain. The mXK expression pattern in brain indicates that it is mainly present in presumptive neuronal cells. The fact that both mXK and mKell are co-expressed in the spleen, liver (in newborn mouse) and bone marrow cells, and that they are covalently linked in red cells, suggests that they may play complementary roles in a hematopoietic function. However, in non-erythroid tissues, such as neurons and bladder epithelial cells, where mKell is not expressed, mXK may have a separate neuronal or epithelial function. To understand XK's physiological roles it is necessary to find the substrate for its presumptive transport function.



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Fig. 1 Expression of mXK and mKell mRNA. (a) Anatomical view of newborn mouse whole-body sagittal section stained with hematoxylin as a reference for the ISHH of the adjacent similar section (b and e). (b-d) Expression of mXK mRNA: (b) x-Ray film autoradiography following hybridization with the mXK antisense riboprobe after 5 days exposure time showing heterogeneous pattern of mXK mRNA distribution throughout several structures including the brain and spinal cord, liver, small and large intestine, bladder and spleen. (c) Lateral-most section of newborn mouse displaying spleen tissue. (d) Sense control results for (b). (e-g) Expression of mKell mRNA: (e) Adjacent similar section (a) subjected to ISHH with antisense probe following x-ray film autoradiography showing a presence of mKell mRNA in the liver and spleen. (f) Lateral-most section of newborn mouse displaying mKell labeled spleen tissue. (g) Sense control result for (e). [Bl, bladder; BM, bone marrow; Br, brain; Cb, cerebellum; H, heart; K, kidney; Li, liver; Lin, large intestine; Mol, molar tooth; Ret, retina; Sin, small intestine; SM, submaxillary gland; Spc, spinal core; Spl, spleen; St, stomach; Th, thymus]. (Reproduced, with permission, from [15].)

Acknowledgments We thank Dr. Colvin M. Redman, Emeritus member of the New York Blood Center, for valuable discussions in preparation of the manuscript and Dr. Ruth H. Walker, James J. Peters Veterans Affairs Medical Center, Bronx, NY and Mount Sinai School of Medicine, New York, NY, whose expertise in the McLeod syndrome guided our selection of brain sections for the ISHH study. This study was supported in part by an NIH Specialized Center of Research (SCOR) grant in Transfusion Biology and Medicine, (HL54459), and by an NIH grant (RO1 HL075716).

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