Characterization of age-related changes in the testis stem cell niche

Project Summary
The objectives of this application are to characterize how age-related changes (1) in gene expression in niche
cells and (2) in the niche microenvironment impact the process of spermatogenesis. It is known that decreased
stem cell function with age is due at least in part to a decline in niche cell function and number. However, niche
cell aging is poorly understood in most tissues. This proposal uses the adult Drosophila testis as a model for
identifying age-related intrinsic and extrinsic processes in niche cell function. The work will capitalize upon the
powerful genetics available in flies, as well as the ability to unequivocally identify all cells in the testis. The fly
testis stem cell niche is formed by 12 quiescent niche cells (termed “hub” cells), which supports germline stem
cells and somatic cyst stem cells (CySCs). The number of hub cells significantly declines during aging as they
lose quiescence and transdifferentiate into fully-functional CySCs. The PI's published work shows that Activin
signaling controls hub cell quiescence (and hence hub cell number) during aging. In young testes, Activin are
inhibited by the secreted antagonist Follistatin (Fs). In older testes, there are more Activin mRNAs and less Fs
mRNAs, leading to an increase in free local Activin proteins, which trigger the Activin receptor on hub cells.
Autonomous Activin signaling causes hub cells to exit quiescence and transdifferentiate into new CySCs,
resulting in the age-related decline of hub cells. These results support the model that age-related changes in
Activin-responsive genes induce the cell cycle in aged hub cells. State-of-the-art transcriptomic techniques will
be used in Aim 1 to identify genes differentially expressed in young and aged hub cells and genes whose
expression is dependent on Activin signaling. Validated genes will be used to test the model that the differentially-
upregulated genes causes hub cell proliferation, hub-to-CySC conversion, and, ultimately, hub cell loss.
Other published work from the PI's lab shows an age-dependent change in the niche microenvironment: an
extracellular matrix (ECM) surrounds the niche in aged but not young testes. When this ECM is inhibited, there
is a significant increase in the number of hub cells in aged testes. These results support the model that the ECM
in aged testes binds Activin, increasing the chance of Activin binding to its receptor. However, there are no
reagents to monitor Activin or Fs proteins or pathway activity in vivo in flies. To overcome this barrier, new tools
to monitor Activin components and signaling in flies will be developed in Aim 2. They will be used to test whether
suppressing the ECM alters Activin and Fs distribution in aged testes and whether the ECM increases Activin
near the surface of hub cells, resulting in more pathway activation in aged testes. The studies in this proposal
will increase the knowledge base about how aging affects niche cell function and number in more complex
systems and will foster new avenues of research into mechanisms and treatments for age-related male infertility.

Grant Number: 1R21AG086723-01A1
NIH Institute/Center: NIH
Principal Investigator: Erika Bach