The roles of miRNAs and gene regulatory networks during the development and maintenance of insulin-producing pancreatic β-cells.

keywords: Insulin, Stem Cells, Embryonic Development, microRNA, Transcription Factor, Mouse Models, Beta-cell Biology, Sox9, Math6, Differentiation, Progenitor Cells, Pancreas, Islets, Endocrine.

Diabetes mellitus results from dysfunction, damage or loss or of pancreatic β-cells. These cells reside in small endocrine clusters, called the islets of Langerhans, which are interspersed throughout the pancreas and secrete the hormones insulin and glucagon in response to changes in blood glucose. In order to ameliorate and eventually cure both forms of diabetes, β-cells will need to be functionally restored, regenerated, or replaced. Islet and pancreas transplantation have demonstrated the promise of β-cell replacement, but a short supply of transplantable tissue limits the applicability of these approaches in broadly curing diabetes mellitus.

Our group is interested in understanding the mechanisms that regulate the formation of islet β-cells from pancreatic stem or progenitor cells during solid organ formation. We focus on the gene regulatory networks at play in the progenitor cells and how these networks change during differentiation to mature endocrine cells and in the long-term maintenance of the β-cell.

We believe that a greater understanding of these genetic mechanisms and pathways will refine cell-based approaches for preventing and reversing the β-cell deterioration and loss that occur with diabetes.

Transcriptional Control of β-cell genesis
The central dogma of molecular biology posits that nuclear genomic DNA is transcribed into RNA, which is then translated into protein. These processes are highly regulated and dynamic changes in gene expression are necessary for normal development to occur. The classical model of gene regulation relies upon sequence-specific interactions of nuclear proteins called transcription factors with the promoter regions of genes. The gene regulatory outcome of transcription factor binding to DNA is dependent on both the intrinsic properties of the factor and the regulatory or promoter context. Transcription factors have an indispensable role during all the stages of β-cell differentiation.

Our past work has focussed on two transcription factors that are important for β-cell develcpment: Sox9 and Math6. Sox9, an SRY/HMGbox transcription factor, is expressed in the progenitor cells within the developing pancreas and is downregulated during β-cell differentiation. We have demonstrated that Sox9 plays a bifunctional role in these cells: maintaining undifferentiated characteristics and positively regulating the pro-endocrine factor Neurogenin3. We do not currently understand what factors are necessary for switching between these two roles and this is an area of future research. Math6 is a basic-helix-loop-helix factor that is expressed downstream of Neurogenin3 and modulates the endocrine differentiation program possibly through regulating Neurogenin3 expression. We have generated both germline and conditional null Math6 mice and are currently trying to further understand its role in the formation of β-cells.

Post-transcriptional Control of β-cell genesis
Once transcribed there are further opportunities for gene regulation, including regulation of mRNA stability and regulation of the translation of mRNA into protein. These forms of regulation are known as post-transcriptional regulation and play crucial roles in both normal physiology and organismal development. Recently a novel class of genes, known as microRNAs (miRNAs), has been described that can post-transcriptionally regulate gene expression.

We have demonstrated that microRNAs are necessary for β-cell formation and play a vital role in the Sox9-expressing progenitor cells prior to activation of Neurogenin3. Future work is focussed on understanding: 

1. Which microRNAs are important for normal β-cell genesis
2. How specific microRNAs impinge on the β-cell developmental program
3. How micrcRNAs regulate normal β-cell function
4. If specific microRNAs can drive or enhance β-cell differentiation from human embryonic stem cells.

Smith, S.B., Qu, H-Q., Taleb, N., Kishimoto, N., Scheel, D.W., Lu, Y., Patch, A.M., Grabs, R., Wang, J., Lynn, F.C., Miyatsuka, T., Mitchell, J., Seerke, R., Désir, J., Vanden Ejnden, S., Abramowicz, M., Kacet, N., Weill, J., Renard, M-E., Gentile, M., Hansen, I., Dewar, K., Hattersley, A.T., Wang, R., Wilson, M.E., Johnson, J.D., Polychronakos, C. & German, M.S. (2009). Endoderm Factor Rfx6 Directs Islet Formation and Insulin Production in Mice and Humans. Nature 463, 775-780.

Lynn, F.C. (2009). Meta-regulation: microRNA regulation of glucose and lipid metabolism. Trends Endocrinol Metab 20, 452-9.

Wang, Y-C., Gallego, E., Iezza, C, Yuan, X., Matli, M,R., Choo, S-P., Zuraek, M.B., Gogia, R., Lynn, EC., German, M.S., Bergsland, E.K., Doneer, D.B., Wareen, R.S., Nakakura, E.K. (2008). Homeodomain transcription factor Nkx2.2 functions in immature cells to control enteroendocnne differentiation and is expressed in gastrointestinal neuroendocrine tumors. Endocrine-Related Cancer Nov 5 [Epub ahead of Printj]

Reid, j.G.*, Nagaraja, A.K. , Lynn, F.C.*, Drabek, R.B., Muzny, D,M., SHAW, C.A., Weiss, M.K,, Naghavi, A.O., Khan, M., Zhu, H., Tennakoon, J., Gunaratne, GH., Corry, Dli, Miller, J., McManus, M.T, German, M.S., Gibbs, R.A., Matzuk, MM., Gunaratne, P.H. (2008). Mouse let-? miRNA populations exhibit RNA editing that is constrained in the 5’- seed/clevage/anchor regions and stabilize predicted mmu-let-7a:mRNA duplexes. Genome Research 18, 1571-81. *equal contribution

Gasa, R., Mrejen, C., Sanchez L., Yang, K., Lynn, F.C., Gomis, R., Tapscott, S. & German, M.S. (2008). The Neurogenin..NeuroDl/Beta2 cascade during pancreatic development: involvement of additional NeuroD factors. Differentiation 76, 381-391.

Lynn, F.C., Skewes-Cox, P., Kosaka, Y., McManus, M.T., Harfe, B.D., & German, M.S. (2007). MicroRNA Expression is required for pancreatic islet cell genesis in the mouse. Diabetes 56, 2938-2945.

Piteau, S., Olver, A., Kim, S-J., Winter, K., Pospisilik, J.A., Lynn, F.C., Manhart, S., Demuth, H-U., Speck, M., Pederson, R.A., McIntosh, C.H.S., (2007). Reversal of islet GIP receptor down-regulation and resistance to GIP by reducing hyperglycaernia in the Zucker rat. Biochem Biophys Res Commun 362, 1007-1012.

Lynn, F.C.*, Smith, S.B.*, Wilson, M.E., Yang, K.Y., Nekrep, N., German, M.S., (2007). Sox9 coordinates a transcriptional network in pancreatic progenitor cells. Proc Nati Acad Sci USA 104, 10500-10505. *equal contribution

Wilson, M.E., Yang, KY., Kalousova, A., Lau, J., Kosaka, Y., Lynn, F.C., Wang, J., Mrejen, C., Episkopou, V., Clevers, H.C., & German, M.S. (2005). The HMG box transcription factor SOX4 contributes to the development of the endocrine pancreas. Diabetes 54, 3402-3409.

Lynn, F. C., Thompson, S. A., Pospisilik, J. A., Ehses, J. A., Hinke, S. A., Pamir, N., McIntosh, C. H. & Pederson, R. A. (2003). A novel pathway for regulation of glucose-dependent insulinotropic polypeptide (GIP) receptor expression in beta cells. Faseb J 17, 91-93.

Pospisilik, J. A., MARTIN, J., DOTY, T., Ehses, J. A., Pamir, N., Lynn, F. C., Piteau, S., Demuth, H. U., McIntosh, C. H. & Pederson, R. A. (2003). Dipeptidyl peptidase IV inhibitor treatment stimulates beta-cell survival and islet neogenesis in streptozotocin-induced diabetic rats. Diabetes 52, 741-750.

American Diabetes Association Annual Scientific Meeting Travel Grant —2008

Juvenile Diabetes Research Foundation International, Advanced Postdoctoral Fellowship — 2007-Present

Juvenile Diabetes Research Foundation International, Postdoctoral Fellowship —2004

Accepting Grad Students and Postdoctoral Fellows