In prabhakarlab/Banksy: Spatial transcriptomic clustering

knitr::opts_chunk$set(
    collapse = TRUE,
    comment = "#>",
    fig.path = "figures/",
    dpi = 36
)

Here, we demonstrate a grid search of clustering parameters with a mouse hippocampus VeraFISH dataset. BANKSY currently provides four algorithms for clustering the BANKSY matrix with clusterBanksy: Leiden (default), Louvain, k-means, and model-based clustering. In this vignette, we run only Leiden clustering. See ?clusterBanksy for more details on the parameters for different clustering methods.

Loading the data

start.time <- Sys.time()

The dataset comprises gene expression for 10,944 cells and 120 genes in 2 spatial dimensions. See ?Banksy::hippocampus for more details.

# Load libs
library(Banksy)

library(SummarizedExperiment)
library(SpatialExperiment)
library(scuttle)

library(scater)
library(cowplot)
library(ggplot2)

# Load data
data(hippocampus)
gcm <- hippocampus$expression
locs <- as.matrix(hippocampus$locations)

Here, gcm is a gene by cell matrix, and locs is a matrix specifying the coordinates of the centroid for each cell.

head(gcm[,1:5])
head(locs)

Initialize a SpatialExperiment object and perform basic quality control. We keep cells with total transcript count within the 5th and 98th percentile:

se <- SpatialExperiment(assay = list(counts = gcm), spatialCoords = locs)
colData(se) <- cbind(colData(se), spatialCoords(se))

# QC based on total counts
qcstats <- perCellQCMetrics(se)
thres <- quantile(qcstats$total, c(0.05, 0.98))
keep <- (qcstats$total > thres[1]) & (qcstats$total < thres[2])
se <- se[, keep]

Next, perform normalization of the data.

# Normalization to mean library size
se <- computeLibraryFactors(se)
aname <- "normcounts"
assay(se, aname) <- normalizeCounts(se, log = FALSE)

Parameters

BANKSY has a few key parameters. We describe these below.

AGF usage

For characterising neighborhoods, BANKSY computes the weighted neighborhood mean (H_0) and the azimuthal Gabor filter (H_1), which estimates gene expression gradients. Setting compute_agf=TRUE computes both H_0 and H_1.

k-geometric

k_geom specifies the number of neighbors used to compute each H_m for m=0,1. If a single value is specified, the same k_geom will be used for each feature matrix. Alternatively, multiple values of k_geom can be provided for each feature matrix. Here, we use k_geom[1]=15 and k_geom[2]=30 for H_0 and H_1 respectively. More neighbors are used to compute gradients.

We compute the neighborhood feature matrices using normalized expression (normcounts in the se object).

k_geom <- c(15, 30)
se <- computeBanksy(se, assay_name = aname, compute_agf = TRUE, k_geom = k_geom)

computeBanksy populates the assays slot with H_0 and H_1 in this instance:

se

lambda

The lambda parameter is a mixing parameter in [0,1] which determines how much spatial information is incorporated for downstream analysis. With smaller values of lambda, BANKY operates in cell-typing mode, while at higher levels of lambda, BANKSY operates in domain-finding mode. As a starting point, we recommend lambda=0.2 for cell-typing and lambda=0.8 for zone-finding. Here, we run lambda=0 which corresponds to non-spatial clustering, and lambda=0.2 for spatially-informed cell-typing. We compute PCs with and without the AGF (H_1).

lambda <- c(0, 0.2)
se <- runBanksyPCA(se, use_agf = c(FALSE, TRUE), lambda = lambda, seed = 1000)

runBanksyPCA populates the reducedDims slot, with each combination of use_agf and lambda provided.

reducedDimNames(se)

Clustering parameters

Next, we cluster the BANKSY embedding with Leiden graph-based clustering. This admits two parameters: k_neighbors and resolution. k_neighbors determines the number of k nearest neighbors used to construct the shared nearest neighbors graph. Leiden clustering is then performed on the resultant graph with resolution resolution. For reproducibiltiy we set a seed for each parameter combination.

k <- 50
res <- 1
se <- clusterBanksy(se, use_agf = c(FALSE, TRUE), lambda = lambda, k_neighbors = k, resolution = res, seed = 1000)

clusterBanksy populates colData(se) with cluster labels:

colnames(colData(se))

Comparing cluster results

To compare clustering runs visually, different runs can be relabeled to minimise their differences with connectClusters:

se <- connectClusters(se)

Visualise spatial coordinates with cluster labels.

cnames <- colnames(colData(se))
cnames <- cnames[grep("^clust", cnames)]
cplots <- lapply(cnames, function(cnm) {
    plotColData(se, x = "sdimx", y = "sdimy", point_size = 0.1, colour_by = cnm) +
        coord_equal() +
        labs(title = cnm) +
        theme(legend.title = element_blank()) +
        guides(colour = guide_legend(override.aes = list(size = 2)))
})

plot_grid(plotlist = cplots, ncol = 2)

Compare all cluster outputs with compareClusters. This function computes pairwise cluster comparison metrics between the clusters in colData(se) based on adjusted Rand index (ARI):

compareClusters(se, func = "ARI")

or normalized mutual information (NMI):

compareClusters(se, func = "NMI")

See ?compareClusters for the full list of comparison measures.

Session information

Vignette runtime:

Sys.time() - start.time

sessionInfo()

prabhakarlab/Banksy documentation built on May 17, 2024, 12:26 a.m.