Pathophysiology of a sickle cell trait mouse model: human alpha(beta)(S) transgenes with one mouse beta-globin allele

As a potential model for sickle cell trait (AS), we examined mice containing one normal mouse beta-globin allele in combination with a human hemoglobin S (h(alpha)beta(S)) transgene (m(beta)/hS). The mice segregated into two subpopulations containing low and high proportions of hemoglobin S (m(beta)/hS1 and m(beta)/hS2, respectively) that was associated with one or two human h(alpha)beta(S) transgenes. We noted striking kidney pathology (cortical cysts, hyperplastic tubules, and glomerulonephritis), increasing with age and with greater severity in m(beta)/hS1. mBeta/hS2 animals were largely tolerant to 5% O(2) for 1 h, whereas 80% of m(beta)/hS1 mice died, exhibiting acute sequestration of erythrocytes in spleen, liver, and heart. These pathologies appear to result from a decreased oxygen affinity of the hybrid (human alpha/mouse beta) hemoglobins with a mild beta-thalassemia phenotype. Thus, these mouse models of sickle trait seem to manifest their renal pathology and sensitivity to hypoxia by mechanisms related to low tissue oxygen delivery and are different from the human syndrome. Analyses of parameters such as P(50), red cell indices, and genetic background are necessary in establishing potential relevance of any mouse model of the sickle cell syndromes.

Stereoelectronic activation of methylenetetrahydrofolate by thymidylate synthase: resonance Raman spectroscopic evidence

Resonance Raman (RR) spectra are reported for the ternary complex of Escherichia coli thymidylate synthase with the cofactor 5,10-methylenetetrahydrofolate (CH2-H4-folate) and the inhibitor 5-fluoro-2'-deoxyuridylate, excited at 337 or 356 nm, in resonance with perturbed absorption bands of the p-aminobenzoylglutamate (PABA-Glu) portion of the cofactor. For comparison, RR spectra were obtained with 260 nm excitation for PABA-Glu in various solvents, and for CH2H4-folate and H4-folate in aqueous solution. These reference spectra are assigned to modes of PABA-Glu in its benzenoid form. The ternary complex RR spectra are very different, however, and are assigned, with the aid of isotopic data, to the PABA-Glu in a predominantly quinoid form. Similar spectra were obtained for the ternary complexes of the E58Q and K48Q mutants, indicating that neither Glu58 nor Lys48 are essential for maintaining the quinoid structure, even though their side chains complement the dipolar charge distribution of the quinoid form of PABA-Glu. Since these are the only charged residues in the PABA-Glu vicinity, electrostatic stabilization is not essential to maintenance of the quinoid structure. It is proposed that quinoid formation results from steric forces, probably resulting from the protein conformation change known to accompany cofactor binding, which enforce coplanarity of the PABA-Glu ring and substituents. This stereoelectronic change activates the cofactor by opening the methylene bridge. A second RR spectrum of the ternary complex, previously proposed to reflect an alternate structure, is shown to result instead from irreversible formation of a laser-induced photoproduct.

The method of constant stimuli is inefficient

Simpson (1988) has argued that the method of constant stimuli is as efficient as adaptive methods of threshold estimation, and has supported this claim with simulations. We show that Simpson's simulations are not a reasonable model of the experimental process, and that more plausible simulations confirm that adaptive methods are much more efficient than the method of constant stimuli.