TLT-1s, Alternative Transcripts of Triggering Receptor Expressed on Myeloid Cell-like Transcript-1 (TLT-1), Inhibits the Triggering Receptor Expressed on Myeloid Cell-2 (TREM-2)-mediated Signaling Pathway during Osteoclastogenesis

Triggering receptor expressed on myeloid cells (TREM)-like transcript-1 (TLT-1) is an immunoreceptor tyrosine-based inhibitory motif (ITIM)-baring TREM family protein. In this study, we identified an alternative transcript form of TLT-1, namely TLT-1s, which has very short extracellular immunoglobulin domain consisting of only 202 amino acids. TLT-1s was mainly expressed in macrophages and osteoclast precursor cells. Upon receptor activator of nuclear factor-κB ligand stimulation, TLT-1s mRNA and protein levels were gradually decreased in BMMs. We also showed the TLT-1s is localized to the cytoplasmic membrane in osteoclast precursor cells. TLT-1s silencing strongly enhanced the formation and resorption activity of osteoclast. In addition, forced expression of TLT-1s showed reduced formation of osteoclast. Because ITIM-baring proteins inhibit immunoreceptor tyrosine-based activation motif (ITAM)-mediated receptor signaling, we tested whether TLT-1s physically interacted with TREM-2, the ITAM-associated co-stimulatory receptor essential for osteoclast differentiation. We showed that TLT-1s is associated with TREM-2 in osteoclast precursor cells. TLT-1s is also associated with tyrosine Src homology 2 domain-containing phosphatase-1 and SH2 domain-containing inositol phosphatase-1 and recruited them to the TREM2-ITAM signaling complex. In addition, knockdown of TLT-1s markedly elevated the intracellular calcium concentration and oscillation in osteoclast precursor cells. In addition, calcium-mediated induction of nuclear factor of activated T cells was also increased by TLT-1s silencing. Furthermore, TREM-2-mediated Akt activation and proliferation of osteoclast precursor cells were also enhanced in TLT-1s silenced cells. In this paper, we found the noble ITIM-baring inhibitory membrane protein; TLT-1s, which regulates ITAM-mediated signaling on osteoclastogenesis. Background: The triggering receptor expressed on myeloid cell (TREM)-mediated signaling is essential for osteoclastogenesis. Results: The alternative transcripts of triggering receptor expressed on myeloid cell-like transcript-1 (TLT-1s) inhibits osteoclast formation by counteracting the TREM-2 signaling pathway. Conclusion: TLT-1s is a negative regulator of osteoclastogenesis, by constitutively associating with SHP-1 and SHIP-1 phosphatases, abrogating the TREM-2 signaling upon RANKL stimulation. Significance: We discovered that an alternative transcript of TLT-1, namely TLT-1s, negatively regulat