Calcium-binding protein secretagogin-expressing neurones in the human hippocampus are largely resistant to neurodegeneration in Alzheimer's disease.

The pathological findings in Alzheimer's disease (AD) are partly attributed to alterations in calcium-binding protein (CBP) functions. We showed previously that immunoreactivity of secretagogin, a recently cloned CBP, in the human hippocampus is restricted to pyramidal neurones and that the amount of immunoreactive neurones does not differ between AD cases and controls. In this study we investigate the influence of hippocampal tau pathology on secretagogin expression in more details. The study group consisted of 26 cases with different degrees of neuropathologically confirmed AD pathology. Sections were incubated separately with secretagogin- and tau-specific antibodies, respectively. The amount of immunoreactive neurones and integral optical densities were assessed. In addition, double immunofluorescence for both secretagogin and tau was performed. No difference with respect to secretagogin immunoreactivity was observed in different stages of AD pathology, and similarly no significant associations were seen between the amount of secretagogin and tau immunoreactivity in the different hippocampal subfields. Double immunofluorescence revealed that both proteins rarely colocalize because only 5.3% of tau and 2.9% of secretagogin immunoreactive neurones, respectively, showed colocalization. Because there are no differences in the amount of hippocampal secretagogin expression between AD cases and controls (as we have shown previously), the lack of an association between the amount of secretagogin expression and tau burden together with the low frequency of colocalization of tau and secretagogin in the human hippocampus, suggest that secretagogin-expressing neurones are largely resistant to neurodegeneration in AD.