P12.07.B Getting to the core of microglia versus bone marrow-derived macrophages in glioblastoma

Background

Microglia and bone marrow-derived macrophages (BMDMs) are two ontogenetically distinct myeloid populations present within glioblastoma that can comprise 30-50% of the tumour mass. Historically, these cell types have been conflated and studied as a single population of ‘tumour-associated macrophages’. Recent advances in single-cell omics have allowed population delineation, suggesting microglia and BMDMs may play different roles within the tumour and subsequently differentially affect tumour progression. Despite building evidence for the unique functions of these cells within glioblastoma, the inherent heterogeneity of the tumour landscape has complicated such studies. Indeed, macrophages exist as phenotypically and functionally diverse populations that are polarised in a context-dependent manner. Hence, to understand the differences between microglia and BMDMs within glioblastoma, both ontogeny and spatial location must be considered.

Material and Methods

To elucidate the functional roles of microglia and BMDMs across the tumour landscape, a publicly available RNAseq dataset was utilised to classify myeloid cells into four populations based on spatial location and ontogeny. These were tumour core BMDMs and microglia, or tumour periphery BMDMs and microglia. Differential gene analysis was then performed to identify significant differentially expressed genes (DEGs) between classified myeloid populations. Tumour core DEGs were then compared against the Ivy Glioblastoma Atlas to define their expression across anatomical tumour regions. Finally, myeloid DEGs were validated at the protein level on human glioblastoma tissue through immunohistochemistry.

Results

Microglia and BMDMs showed different spatial distributions across the tumour landscape and displayed distinct functional expression profiles. Microglia held a more chemotactic and pro-inflammatory profile, whereas BMDMs held a more pro-tumoural profile. However, a comparison of microglia between the tumour core and periphery revealed that tumour microglia upregulate many pro-tumoural genes, including multiple genes that have previously been defined as ‘BMDM-enriched’. Moreover, we found myeloid DEGs identified within the tumour core cluster to distinct spatial tumour regions such as the vascular or hypoxic niche. Immunohistochemical staining reflected these spatial expression profiles, identifying a distinct population of phagocytic macrophages within the hypoxic niche.

Conclusion

Although microglia and BMDMs represent two ontogenetically distinct myeloid populations within glioblastoma, both cell types can adopt similar functional expression profiles within the tumour core, particularly within tumour niches. This indicates that myeloid cell function is strongly influenced by the tumour microenvironment, rather than ontogeny alone.

Neuroprotective potential of CB1 receptor agonists in an in vitro model of Huntington's disease

BACKGROUND AND PURPOSE

The therapeutic potential of cannabinoids in Huntington's disease (HD) has been investigated by several groups with complex and sometimes contrasting results. We sought to examine key points of intersection between cannabinoid receptor 1 (CB(1)) signalling, survival and the formation of mutant huntingtin aggregates in HD.

EXPERIMENTAL APPROACH

Using a simplified pheochromocytoma (PC12) cell model of HD expressing exon 1 of wild-type or mutant huntingtin, we assayed cell death and aggregate formation using high-throughput cytotoxicity and image-based assays respectively.

KEY RESULTS

CB(1) activation by HU210 conferred a small but significant level of protection against mutant huntingtin-induced cell death. Pertussis toxin uncoupled HU210 from the inhibition of cAMP, preventing rescue of cell death. Phosphorylation of extracellular signal-regulated kinase (ERK) was also critical to CB(1)-mediated rescue. Conversely, treatments that elevated cAMP exacerbated mutant huntingtin-induced cell death. Despite opposing effects on HD cell survival, both HU210 and compounds that elevated cAMP increased the formation of mutant huntingtin aggregates. The increase in aggregation by HU210 was insensitive to Pertussis toxin and UO126, suggesting a G-protein alpha subtype s (G(s))-linked mechanism.

CONCLUSIONS AND IMPLICATIONS

We suggest that the CB(1) receptor, through G-protein alpha subtype i/o (G(i/o))-linked, ERK-dependent signal transduction, is a therapeutic target in HD. However the protective potential of CB(1) may be limited by promiscuous coupling to G(s), the stimulation of cAMP formation and increased aggregate formation. This may underpin the poor therapeutic efficacy of cannabinoids in more complex model systems and suggest that therapies that are selective for the G(i/o), ERK pathway may be of most benefit in HD.