Using a bioinformatics strategy to screen for genes predominantly expressed in the hippocampus and located in linkage regions for LOAD, we identified CALHM1
on chromosome 10 (). CALHM1
was found to encode an integral membrane glycoprotein with key characteristics of a Ca2+
channel. CALHM1 controls cytosolic Ca2+
levels, homomultimerizes, and shares important sequence similarities with the predicted selectivity filter of NMDAR ( and ). Significantly, we have also demonstrated that CALHM1 contains a functionally important N residue at position 72 that aligns with the Q/R/N site of the NMDAR selectivity filter (). Thus, NMDAR and CALHM1 share important structural similarities at the sequence level in a region that was previously described as a critical determinant of Ca2+
selectivity and permeability in glutamate receptor ion channels (Wollmuth and Sobolevsky, 2004
). The potential role of CALHM1 in ion permeability was further investigated by voltage-clamping using two different cell models. This approach demonstrated that expression of CALHM1 generates a novel constitutive Ca2+
selective cation current at the plasma membrane. Additional studies that will examine the topology of CALHM1 and more precisely the organization of the region containing the critical N72 residue, will help us to clearly identify the role of CALHM1 in ion permeation.
In the present report, we have provided compelling evidence that the rs2986017 SNP in CALHM1
, which results in the P86L substitution, is associated with both an increased risk for LOAD and a significant dysregulation of Ca2+
homeostasis and APP metabolism ( and ). Specifically, we have shown that the P86L polymorphism impairs plasma membrane Ca2+
permeability, reduces cytosolic Ca2+
levels, affects sAPPα production, and concomitantly derepresses the effect of CALHM1 on Aβ accumulation. A large body of literature supports the notion that a deranged intracellular Ca2+
signaling occurs in AD and may be involved in the deregulation of APP processing and neurodegeneration (Khachaturian, 1989
; LaFerla, 2002
). APP metabolism involves a complex series of events and the direct influence of Ca2+
signaling on this process is still poorly understood (LaFerla, 2002
). The present work provides strong support for the Ca2+
hypothesis of AD and is also an important step towards understanding the potential pathological cross talk between Ca2+
signaling disturbances and pathways of Aβ accumulation. Moreover, the identification of CALHM1 as a key modulator of Ca2+
homeostasis will allow us to further dissect the precise mechanism by which cytosolic Ca2+
modulates APP metabolism.
Screening the human genome for genes predominantly expressed in the hippocampus successfully prioritized CALHM1 among the many genes found in LOAD loci and thus demonstrates the utility of tissue expression profiling in the identification a novel candidate genes for LOAD. Candidate genes located in LOAD regions are often considered based on their potential implication in known AD biology (e.g., IDE). The strategy used in this study can therefore complement these approaches and suggest candidates, including those of unknown function, worthy of consideration. Supporting this notion, recent data have shown that tissue expression profiles can be used to effectively prioritize candidate genes in another neurodegenerative genetic disorder (F. Campagne, Program No. 446.12, San Diego, CA: Society for Neuroscience, 2007).
In summary, we propose that CALHM1 is a pore component of a novel cerebral ion channel family and that variants in the CALHM genes may constitute robust risk factors for LOAD. These results not only provide important new insights into the pathophysiology of Ca2+ homeostasis and APP metabolism in the central nervous system but also represent a strong genetic evidence of a channelopathy contribution to AD etiology. Finally, given its cell surface ion channel properties and its restricted expression, our work further establish CALHM1 as a potentially important molecular target for an anti-amyloid therapy in AD.