Prabhakara P. Reddi,
Associate Professor of
Transcriptional Regulation of
The long term goal of our research is
to contribute to a better understanding of male infertility for which
currently there are limited treatment options. During spermatogenesis
the male germ cells must express a series of differentiation markers in
a spatiotemporal manner following a precise transcriptional program;
failure to do so will result in maturation arrest and male infertility.
Thus, a significant question to ask is how gene transcription is
controlled during spermatogenesis. The question of how testis-specific
genes remain silent in the somatic tissues is equally important because
many cancers aberrantly express spermatogenic differentiation markers.
We have utilized a candidate gene approach to understand the mechanisms
of testis-specific gene transcription. The mouse acrv1 gene, which
codes for the evolutionarily conserved acrosomal protein SP-10 and is
transcribed exclusively in the round spermatids, served as a model for
promoter analysis studies in transgenic mice. Our main findings are: 1)
a chromatin insulator located in the proximal promoter of the acrv1
gene tethers it to the nuclear lamina within the somatic tissues and
silences the gene by sequestering the promoter from the transcriptional
machinery. 2) This tethering is released in the male germ line ahead of
the formation of round spermatids and pausing of RNA Pol II at the
promoter provides further fine-tuning for the maintenance of
spatiotemporal expression. Our studies suggest that remodeling of the
nuclear lamina-chromatin interaction plays a key role in the
differentiation of the spermatocytes and spermatids by releasing a
number of differentiation markers for transcription.
The acrv1 insulator is novel because it
lacks the CpG dinucleotides and CTCF binding sites characteristic of
the prototypical HS4 and H19 ICR vertebrate insulators. The mechanism
of insulator function, i.e., by tethering to the nuclear lamina, is
similar to that of other insulators. We have identified the DNA/RNA
binding protein TDP-43 as acrv1 promoter binding protein. TDP-43 is
expressed ubiquitously. Gal4 recruitment studies showed that TDP-43
functions as a transcriptional repressor and in vivo chromatin
immunoprecipitation showed promoter occupancy consistent with such a
role in acrv1 transcription in a physiological context. Mutation
studies indicate that TDP-43 occupancy of the acrv1 promoter is
required for prevention of premature expression in spermatocytes. Taken
together, we propose that one function of TDP-43 is to represses gene transcription by maintaining RNA Pol
II in a paused state. We are testing the hypothesis that TDP-43 is
essential for spermatogenesis.
Link to neurodegenerative
A prominent role for TDP-43 has emerged
in the field of neurodegenerative disease in the recent years. It turns
out that TDP-43, which is normally a nuclear protein, becomes
mislocalized, aberrantly phosphorylated, ubiquitylated and aggregated
in the motor neurons and glial cells of patients with a variety of
neurodegenerative disorders including frontotemporal dementia, ALS, and
Alzheimer’s disease. TDP-43 appears to be linked to the familial form
of ALS. Currently it is not clear whether the disease is caused due to
the loss of function or toxic gain of function, owing in part to
insufficient understanding of the function of TDP-43. Based on our
studies using spermatogenesis as a model system, we hypothesize that in
the neuronal cells TDP-43 coordinately regulates the transcription of a
number of genes including those which must be in a ready-to-respond
state (with a paused polymerase at the promoter) and that its
mislocalization to the cytoplasm leads to loss of function of motor
neurons leading to disease.
1: Divya Saro Varghese, Uma Chandran, Soumya A, Sathy M. Pillai,
Jayakrishnan K, Reddi PP, and Pradeep G Kumar.
Aberrant expression of TAR DNA binding protein-43 is associated with
spermatogenic disorders in human males. Reproduction, Fertility, and
Development. September 2014. “In Press”.
2: Osuru HP, Monroe JE, Chebolu AP, Akamune J, Pramoonjago P,
Ranpura SA, Reddi PP. The acrosomal
protein SP-10 (Acrv1) is an ideal marker for staging of the cycle of
seminiferous epithelium in the mouse. Mol Reprod Dev. 2014 Aug 26.
doi: 10.1002/mrd.22358. [Epub ahead of print]
3: Lalmansingh AS, Urekar CJ, Reddi PP. TDP-43 is a
transcriptional repressor: the testis-specific mouse acrv1 gene is a
TDP-43 target in vivo. J Biol Chem. 2011 Apr 1;286(13):10970-82. Epub
2011 Jan 20. PubMed PMID: 21252238; PubMed Central PMCID:
4: Reddi PP, Urekar CJ, Abhyankar MM, Ranpura SA.
Role of an insulator in testis-specific gene transcription. Ann N Y
Acad Sci. 2007 Dec;1120:95-103. Review. PubMed PMID: 18184912.
5: Ranpura SA, Deshmukh U, Reddi PP. NF45 and NF90
in murine seminiferous epithelium: potential role in SP-10 gene
transcription. J Androl. 2008 Mar-Apr;29(2):186-97. Epub 2007 Oct 17.
PubMed PMID: 17942973.
6: Abhyankar MM, Urekar C, Reddi PP. A novel
CpG-free vertebrate insulator silences the testis-specific SP-10 gene
in somatic tissues: role for TDP-43 in insulator function. J Biol Chem.
2007 Dec 14;282(50):36143-54. Epub 2007 Oct 11. PubMed PMID:
7: Acharya KK, Govind CK, Shore AN, Stoler MH, Reddi
PP. cis-requirement for the maintenance of round
spermatid-specific transcription. Dev Biol. 2006 Jul 15;295(2):781-90.
Epub 2006 May 3. PubMed PMID: 16730344.
8: Reddi PP, Shore AN, Shapiro JA, Anderson A,
Stoler MH, Acharya KK. Spermatid-specific
promoter of the SP-10 gene functions as an insulator in somatic
cells. Dev Biol. 2003 Oct 1;262(1):173-82. PubMed PMID:14512027