Joseph T. Greene

Microscale engagement of the hemi-immunoreceptor tyrosine-based activation motif-containing receptor Dectin-1 by fungal particles activates Src-family kinases (SFKs) and Syk, drives second-messenger generation, and induces downstream Erk and Akt signaling and proinflammatory responses in macrophages. To avoid inappropriate activation in the absence of a pathogenic threat, macrophages restrict signaling in response to low-valency ligands. To examine how SFKs regulate this sensitivity threshold, we compared signaling induced by pharmacological SFK activation with signaling triggered by depleted zymosan, a high-valency β-glucan particle that engages Dectin-1 to form a phagocytic synapse. We found that particulate engagement of Dectin-1 protected the inhibitory ITIM-associated phosphatase SHIP1 from phosphorylation by SFKs, allowing robust activation of Erk and Akt and proinflammatory induction. In contrast, receptor-independent SFK activation induced phosphorylation of SHIP1 and failed to amplify signaling downstream of PLCγ2 and PI3K. Although multiple SFKs could phosphorylate SHIP1, Lyn uniquely maintained the basal set-point of SHIP1 phosphorylation, thereby keeping PIP3 levels low and suppressing basal Erk and Akt signaling. This Lyn-dependent regulation was essential for suppressing Akt activation and balancing signaling through the Erk and Akt pathways in the absence of a phagocytic synapse. In contrast, antimicrobial responses to particulate stimuli, including second-messenger signaling, Erk/Akt, and proinflammatory outputs, did not strictly require Lyn expression. These findings highlight the unique role of Lyn in limiting spurious proinflammatory signaling and shed light on a mechanism by which macrophages selectively respond to high-valency particulate ligands that override this basal inhibitory program.

Toll-like receptor (TLR) signaling is vital for antimicrobial macrophage function, and its dysregulation is associated with diseases such as lupus, multiple sclerosis, pulmonary fibrosis, and cancer. The Src-family kinase Lyn may have net activating or inhibitory effects on TLR signaling, yet distinct functions of the Lyn splice variants LynA and LynB in TLR signaling have not been investigated. We used isoform-specific Lyn knockout mice (LynAKO and LynBKO) to interrogate the contribution of each isoform to TLR signaling in bone-marrow-derived macrophages. Bulk RNA sequencing and cytokine analyses revealed that complete Lyn deficiency (LynKO) dampened TLR4- and TLR7-induced inflammatory gene expression and production of tumor necrosis factor but enhanced the expression of genes responsible for synthesizing the extracellular matrix and promoting proliferation. Despite reduced expression of total Lyn in single-isoform-knockout bone-marrow-derived macrophages, expression of either LynA or LynB alone was sufficient to preserve a wild-type-like transcriptome at steady state and after treatment with the TLR7 agonist R848. However, LynAKO and LynBKO macrophages did have impaired production of tumor necrosis factor in response to the TLR4 agonist lipopolysaccharide. Additionally, LynAKO and LynBKO macrophages were as hyperproliferative as LynKO cells. These data suggest that Lyn promotes macrophage activation in response to TLR signaling and restrains aberrant proliferation and matrix deposition in a dose-dependent rather than isoform-specific manner.

Toll-like receptor (TLR) signaling is vital to antimicrobial macrophage function, and its dysregulation is associated with many disease states, including lupus, multiple sclerosis, pulmonary fibrosis, and cancer. The Src-family kinase Lyn plays activating and inhibitory roles downstream of TLRs, yet distinct functions of the Lyn splice variants LynA and LynB in TLR signaling had not been investigated. We used isoform-specific Lyn knockout mice (LynA ^KO and LynB ^KO ) to interrogate the contribution of each isoform to TLR signaling in bone marrow-derived macrophages. Bulk RNA sequencing and cytokine analyses revealed that complete Lyn deficiency (Lyn ^KO ) dampens TLR4- and TLR7-induced inflammatory gene expression and TNF production, but enhances the expression of genes responsible for synthesizing the extracellular matrix and promoting proliferation. Despite a reduction in total Lyn levels, the expression of either LynA or LynB alone was sufficient to preserve wild-type transcriptional responses and TNF production in response to the TLR7 agonist R848. However, LyA ^KO and LynB ^KO macrophages did have partially impaired TNF production in response to the TLR4 agonist lipopolysaccharide. Additionally, LynA ^KO and LynB ^KO macrophages were hyperproliferative, like Lyn ^KO cells. These data suggest that Lyn promotes macrophage activation downstream of TLRs and restrains aberrant proliferation and matrix deposition in a dose-dependent rather than isoform-specific manner. Summary Sentence: RNA sequencing and functional assays demonstrate that both LynA and LynB restrict macrophage proliferation and drive TLR-induced ECM-remodeling and inflammatory cytokine production.

The type II class of RAF inhibitors currently in clinical trials paradoxically activate BRAF at subsaturating concentrations. Activation is mediated by induction of BRAF dimers, but why activation rather than inhibition occurs remains unclear. Using biophysical methods tracking BRAF dimerization and conformation we built an allosteric model of inhibitor-induced dimerization that resolves the allosteric contributions of inhibitor binding to the two active sites of the dimer, revealing key differences between type I and type II RAF inhibitors. For type II inhibitors the allosteric coupling between inhibitor binding and BRAF dimerization is distributed asymmetrically across the two dimer binding sites, with binding to the first site dominating the allostery. This asymmetry results in efficient and selective induction of dimers with one inhibited and one catalytically active subunit. Our allosteric models quantitatively account for paradoxical activation data measured for 11 RAF inhibitors. Unlike type II inhibitors, type I inhibitors lack allosteric asymmetry and do not activate BRAF homodimers. Finally, NMR data reveal that BRAF homodimers are dynamically asymmetric with only one of the subunits locked in the active αC-in state. This provides a structural mechanism for how binding of only a single αC-in inhibitor molecule can induce potent BRAF dimerization and activation.

Spondweni virus (SPONV) is the closest known relative of Zika virus (ZIKV). SPONV pathogenesis resembles that of ZIKV in pregnant mice, and both viruses are transmitted by Aedes aegypti mosquitoes. We aimed to develop a translational model to further understand SPONV transmission and pathogenesis. We found that cynomolgus macaques ( Macaca fascicularis ) inoculated with ZIKV or SPONV were susceptible to ZIKV but resistant to SPONV infection. In contrast, rhesus macaques ( Macaca mulatta ) supported productive infection with both ZIKV and SPONV and developed robust neutralizing antibody responses. Crossover serial challenge in rhesus macaques revealed that SPONV immunity did not protect against ZIKV infection, whereas ZIKV immunity was fully protective against SPONV infection. These findings establish a viable model for future investigation into SPONV pathogenesis and suggest that the risk of SPONV emergence is low in areas with high ZIKV seroprevalence due to one-way cross-protection between ZIKV and SPONV.

The type II class of RAF inhibitors currently in clinical trials paradoxically activate BRAF at subsaturating concentrations. Activation is mediated by induction of BRAF dimers, but why activation rather than inhibition occurs remains unclear. Using biophysical methods tracking BRAF dimerization and conformation we built an allosteric model of inhibitor-induced dimerization that resolves the allosteric contributions of inhibitor binding to the two active sites of the dimer, revealing key differences between type I and type II RAF inhibitors. For type II inhibitors the allosteric coupling between inhibitor binding and BRAF dimerization is distributed asymmetrically across the two dimer binding sites, with binding to the first site dominating the allostery. This asymmetry results in efficient and selective induction of dimers with one inhibited and one catalytically active subunit. Our allosteric models quantitatively account for paradoxical activation data measured for 11 RAF inhibitors. Unlike type II inhibitors, type I inhibitors lack allosteric asymmetry and do not activate BRAF homodimers. Finally, NMR data reveal that BRAF homodimers are dynamically asymmetric with only one of the subunits locked in the active αC-in state. This provides a structural mechanism for how binding of only a single αC-in inhibitor molecule can induce potent BRAF dimerization and activation.