Microgliosis is a hallmark of Alzheimer’s disease (AD) and is associated with inflammatory activation of microglia. Discovery of new molecules capable of mitigating the inflammatory response may provide a viable means to treat AD. Somatostatin receptor subtype-4 (SST₄) shows particular promise as a treatment target for AD given its microglia and neuronal localization in the brain. A SST₄ agonist has the potential to lessen microglia inflammatory activity. This research evaluated the novel SST₄ agonist SM-I-26 in its ability to modulate microglia inflammatory responses and enhance mechanisms affiliated with the breakdown of amyloid-beta peptide (Aβ).

The selective and high affinity SST₄ agonist SM-I-26 was evaluated in BV2 mouse microglia cells concurrent with inflammatory challenge via lipopolysaccharide (LPS) treatment. After plating for 24 hrs., cells were treated for another 24 hrs. with SM-I-26 (0, 10, 1000 nM) against LPS (0, 10, 100 ng/mL). Cell viability/proliferation (alamarBlue assay), nitrite (surrogate of nitric oxide, Griess assay) output in media, and mRNA expression (RT-qPCR) of genes associated with SST₄, inflammation, Aβ phagocytosis, and Aβ degrading enzymes were measured. Two-way ANOVAs with Tukey post-hoc tests were used to determine significance (α= 0.05).

SM-I-26 decreased BV-2 cell viability/proliferation, while LPS increased viability/proliferation. A significant interaction between LPS and SM-I-26 was found on mean nitrite. Within no LPS, mean nitrite was similar across all concentrations of SM-I-26. With 10 ng/ml LPS, nitrite output trended towards a decrease with both 10 and 1000 nM SM-I-26 compared to vehicle. Within 100 ng/ml LPS, both 10 and 1000 nM SM-I-26 significantly decreased nitrite compared to vehicle control.

The mRNA expression of key genes associated with microglia inflammation, SST4, function, and Aβ clearance were next evaluated. A significant interaction was found between SM-I-26 and LPS for Sst₄. Sst₄ mRNA expression increased with SM-I-26 treatment under both basal and inflammatory conditions.

The Aβ degrading enzymes Neprilysin and insulin degrading enzyme showed no changes in mRNA expression across treatments. However, SM-I-26 treatment significantly increased angiotensin converting enzyme mRNA expression under basal conditions at highest concentration of SM-I-26. Similar upregulation pattern was observed for endothelin converting enzyme-1, while endothelin converting enzyme-2 showed a decline with treatment.

Interactions were found for macrophage scavenger receptor-1 (Msr1) and triggering receptor expressed on myeloid cells-1 (Trem1). Increased expression of Msr1 by SM-I-26 under non-inflammatory and 10 ng/mL LPS-stimulated conditions implicate enhanced capacity to clear Aβ. The SM-I-26 dose-dependently downregulated Trem1, identifying potential beneficial effects on anti-inflammatory pathways. SM-I-26 treatment increased catalase with and without LPS, but not their interactions. This overexpression of catalase could be neuroprotective against Aβ toxicity.

Significant interactions were found between SM-I-26 and LPS for tumor necrosis factor-α (Tnf-α) and interleukin-1β (IL1β). The 1000 nM SM-I-26 significantly decreased Tnf-α mRNA expression under basal and inflammatory conditions, with 10 nM SM-I-26 decreasing Tnf-α only with the highest LPS concentration. The 10 and 1000 nM SM-I-26 decreased IL-1β expression at the highest LPS concentration. The 10 nM SM-I-26 decreased interleukin-6 with low and high dose LPS treatment. The anti-inflammatory cytokine interleukin-10 was upregulated by 10 nM SM-I-26 under both basal and inflammatory conditions, but not at the higher 1000 nM concentration. Data indicates a dose-range viability respective to effects on cytokines.

These results show treatment with SST₄ agonist SM-I-26 exerts an anti-inflammatory and antioxidant activity in microglia with capacity to modulate cell surface proteins associated with Aβ phagocytosis. Treatment with SST₄ agonist SM-I-26 generally shifted the activation state of the LPS challenged BV-2 microglia from a pro-inflammatory M1 state to anti-inflammatory/pro-phagocytic M2 state. Data support potential use of SST₄ agonist for AD treatment.