SIGLEC16 encodes a DAP12‐associated receptor expressed in macrophages that evolved from its inhibitory counterpart SIGLEC11 and has functional and non‐functional alleles in humans

Abstract

Sialic acid binding immunoglobulin-like lectins (Siglec) are important components of immune recognition. The organization of Siglec genes in different species is consistent with rapid selection imposed by pathogens. We studied SIGLEC11 genes in human, rodent, dog, cow and non-human primates. The lineages of SIGLEC11 genes in these species have undergone dynamic gene duplication and conversion, forming a potential inhibitory (SIGLEC11)/activating (SIGLEC16) receptor pair in chimpanzee and humans. A cDNA encoding human Siglec-16, currently classed as a pseudogene in the databases (SIGLECP16), is expressed in various cell lines and tissues. A polymorphism screen for the two alleles (wild type and four-base pair deletion, 4bpDelta) of SIGLEC16 found their frequencies to be 50% amongst the UK population. A search for donor sequences for SIGLEC16 revealed a subfamily of activating Siglec with charged transmembrane domains predicted to associate with ITAM-encoding adaptor proteins. In support of this, Siglec-16 was expressed at the cell surface in the presence of DAP12, but not the FcRgamma chain. Using antisera specific to the cytoplasmic tail of Siglec-16, we identified Siglec-16 expression in CD14(+) tissue macrophages and in normal human brain, cancerous oesophagus and lung. This is the first activating human Siglec receptor found to have functional and non-functional alleles within the population.

Alleviation of Neurotoxicity by Microglial Human Siglec-11

“…The Siglec-16 protein shares a high percentage (ϳ99%) of amino acid identity with Siglec-11 especially at the extracellular part, but its transmembrane domain contains a charged lysine residue and it has a short cytoplasmic tail without ITIM. The Siglec-16 protein has a molecular weight of ϳ58 kDa (Cao et al, 2008). Although our immunohistochemistry could not discriminate between Siglec-11 and Siglec-16 since the antibody was raised against the highly homologous extracellular part of Siglec-11, the RT-PCR products obtained by using Siglec-11-specific primers and the molecular weight of the Western blot analysis firmly supported the expression of Siglec-11 in human brain tissue.…”

“…The protein size of Siglec-11 detected by Western blotting in human brain tissue samples was identical to the Siglec-11 protein expressed in 293 cells after transduction with the gene Siglec-11 splice variant 2. Recently, Siglec-16 was found and described as being evolved from Siglec-11 as a DAP12-associated receptor expressed in macrophages (Cao et al, 2008). The Siglec-16 protein shares a high percentage (ϳ99%) of amino acid identity with Siglec-11 especially at the extracellular part, but its transmembrane domain contains a charged lysine residue and it has a short cytoplasmic tail without ITIM.…”

“…A striking feature of the CD33-related Siglecs is that they were evolving very rapidly. In humans, there are 10 CD33-related Siglecs and 1 Siglec-like protein, whereas there are only 5 CD33-related Siglecs in mice, which seem to have undergone wholesale loss of CD33-related Siglec genes (Angata et al, , 2007Crocker et al, 2007;Cao et al, 2008Cao et al, , 2009Varki, 2009). Although CD33-related Siglecs have unique expression profiles, they are predominantly found on leukocytes.…”

Expression and preliminary functional analysis of Siglec-F on mouse macrophages

“…The majority of Siglec family members contain ITIMs within cytoplasmic domains (Angata et al, 2006;Cao et al, 2008). Typically, receptors with ITIMs function as inhibitory receptors and suppress activation signals that emanate from receptors associated with immunoreceptor tyrosine-based activation motifs (ITAMs) through recruitment of tyrosine and inositol phosphatases.…”

Role of activating-type Siglecs on myeloid cell function

“…In the case of Siglec-14, there is an allele (SIGLEC14-null allele) in which the SIGLEC14 gene is fused with downstream SIGLEC5, resulting in the loss of Siglec-14-coding segment and concomitant emergence of a fusion gene (SIGLEC14/5) that encodes a protein with the same amino acid sequence as Siglec-5 10) . As for Siglec-16, an allele of SIGLEC16 gene has a 4-nucleotide deletion in exon 2 (encoding the first immunoglobulin-like domain), resulting in premature termination (SIGLEC16-null allele) 11) . Furthermore, the frequencies of these null alleles are high (exceeding 0.5 in some populations).…”