Effect of knocking out mouse Slc44a4 on colonic uptake of the microbiota-generated thiamine pyrophosphate and on colon physiology

Humans/mammals obtain vitamin B1 from dietary and gut microbiota sources. Considerable amount of the microbiota generated vitamin exists in the form of thiamine pyrophosphate (TPP), and colonocytes are capable of absorbing TPP via a specific carrier-mediated process that involves the colonic TPP transporter (cTPPT; encoded by SLC44A4). Little is known about the relative contribution of SLC44A4 toward total colonic carrier-mediated TPP uptake, and its role in colon physiology. To address these issues, we generated an Slc44a4 knockout (KO) mouse model (by Cre-Lox recombination) and found a near complete inhibition in colonic carrier-mediated 3H-TPP uptake in the Slc44a4 KO compared to Wild-type-littermates (WT). We also observed a significant reduction in KO mice body weight and a shortening of their colon compared to WT. Using RNAseq and Ingenuity Pathway Analysis (IPA) approaches, we found that knocking out the colonic Slc44a4 to lead to changes in level of expression of many genes, including up-regulation in those associated with intestinal inflammation/colitis. Finally, we found that the Slc44a4 KO mice to be more susceptible to the effect of the colitogenic dextran sodium sulfate (DSS) compared to WT animals, a finding that lends support to the recent prediction by multiple genome-wide association studies (GWAS) that the SLC44A4 is a possible colitis susceptibility gene. In summary, results of these investigations show that the Slc44a4 is the predominant/only transporter involved in colonic uptake of TPP, that the transporter is important for colon physiology, and that its deletion increases susceptibility to inflammation.

Changes in the innate immune response to SARS-CoV-2 with advancing age in humans

BACKGROUND

Advancing age is a major risk factor for respiratory viral infections. The infections are often prolonged and difficult to resolve resulting hospitalizations and mortality. The recent COVID-19 pandemic has highlighted this as elderly subjects have emerged as vulnerable populations that display increased susceptibility and severity to SARS-CoV-2. There is an urgent need to identify the probable mechanisms underlying this to protect against future outbreaks of such nature. Innate immunity is the first line of defense against viruses and its decline impacts downstream immune responses. This is because dendritic cells (DCs) and macrophages are key cellular elements of the innate immune system that can sense and respond to viruses by producing inflammatory mediators and priming CD4 and CD8 T-cell responses.

RESULTS

We investigated the changes in innate immune responses to SARS-CoV-2 as a function of age. Our results using human PBMCs from aged, middle-aged, and young subjects indicate that the activation of DCs and monocytes in response to SARS-CoV-2 is compromised with age. The impairment is most apparent in pDCs where both aged and middle-aged display reduced responses. The secretion of IL-29 that confers protection against respiratory viruses is also decreased in both aged and middle-aged subjects. In contrast, inflammatory mediators associated with severe COVID-19 including CXCL-8, TREM-1 are increased with age. This is also apparent in the gene expression data where pathways related host defense display an age dependent decrease with a concomitant increase in inflammatory pathways. Not only are the inflammatory pathways and mediators increased after stimulation with SARS-CoV-2 but also at homeostasis. In keeping with reduced DC activation, the induction of cytotoxic CD8 T cells is also impaired in aged subjects. However, the CD8 T cells from aged subjects display increased baseline activation in accordance with the enhanced baseline inflammation.

CONCLUSIONS

Our results demonstrate a decline in protective anti-viral immune responses and increase in damaging inflammatory responses with age indicating that dysregulated innate immune responses play a significant role in the increased susceptibility of aged subjects to COVID-19. Furthermore, the dysregulation in immune responses develops early on as middle-aged demonstrate several of these changes.