Our mission is to understand the differential regenerative capacity of mammalian organs so that someday we can confer regenerative ability to any organ in the setting of disease.
To make this possible, we develop tools to perform high-throughput functional genomics directly within a living mouse.
The loss of parenchymal cells in organs such as the heart, brain, and pancreas underlies the lifelong morbidity of numerous diseases including heart attack, stroke, and diabetes. In these organs, parenchymal cell loss begets permanent organ impairment because these tissues have no means of replacing lost cells and restoring organ function. There are no stem cell populations to replace the lost cells and the remaining parenchymal cells permanently exited the cell cycle as part of their differentiation process. A means of restoring proliferative capacity to these differentiated cells could enable these organs to regenerate after injury and provide desperately needed cures for numerous diseases.
Although cellular differentiation and proliferative capacity are typically mutually exclusive, the liver, and its parenchymal hepatocytes, exist as remarkable and informative exceptions. Unlike any other solid organ, the liver can completely regenerate itself after injury. Moreover, this regeneration is driven not by a stem cell population but rather by proliferation of the differentiated hepatocytes. In the uninjured liver, most hepatocytes have exited the cell cycle and execute the liver’s diverse metabolic functions. However, if the liver is ever injured, these cells will immediately re-enter the cell cycle and regenerate the organ in a matter of days. By virtue of this ability to re-enter the cell cycle, hepatocytes are considered to be quiescent rather than permanently arrested. This unique property of hepatocytes demonstrates that cell cycle arrest is not an obligate consequence of differentiation and that organ regeneration is possible without a stem cell population.
Bringing high-throughput functional genomics into the organism
Our ability to understand mammalian physiology and disease has long been limited by the availability of tools for high-throughput genetic dissection within a living organism. Interrogating gene function in living mammals has largely been restricted to single-gene knockout mice, effectively precluding unbiased and comprehensive investigation of physiology and disease. To overcome this technical barrier, we recently established genome-wide CRISPR screening in the mouse liver (Keys and Knouse 2022). Although genome-wide screening in the liver alone can offer novel insights into diverse phenomena, the full potential of high-throughput screening in the organism rests on expending this technology to other organs and CRISPR applications. We are now working to establish efficient and stable transgene delivery and genome-scale CRISPR screening in any cell type of interest in the living mouse. We aspire to bring the experimental tractability once restricted to cell culture throughout the entire animal, enabling unprecedented insight into mammalian physiology and disease.
What are the molecular requirements for organ regeneration?
The liver’s remarkable regenerative capacity has at its core a fundamental distinction of cell cycle states: the reversible dormancy that defines quiescent cells as opposed to the permanent cell cycle arrest that defines most other differentiated cell types. Importantly, this critical functional difference between quiescence and permanent arrest lacks a molecular explanation. We are leveraging our genome-wide screening platform to identify the genes required for quiescent hepatocytes to re-enter the cell cycle and regenerate the liver. We are also working to expand our genome-wide screening tools to identify barriers to regeneration in other tissues. By defining the molecular rules governing permissive and restrictive contexts for regeneration, we will bring our goal of unlocking regenerative ability across tissues into the realm of possibility.
How does the organism monitor organ size and function?
Critical for effective regeneration is cell cycle re-entry occurring only when and as long as needed. Although the signaling pathways involved in the initiation and completion of liver regeneration have been identified, it is unclear how these pathways sense liver size or function. We are working to understand how liver size and function are transduced at the molecular level to ensure that regeneration commences and ceases at the proper times. By understanding how the organism monitors organ size and function, we will improve our capacity to modulate organ regeneration for therapeutic purposes.
From regeneration to cancer
The ability of cells to re-enter the cell cycle after prolonged periods of dormancy is true not only for regeneration but also for disseminated cancer cells. The latter can lie dormant in a distant tissue and evade traditional chemotherapies for years before re-entering the cell cycle to form a metastatic tumor. As we uncover the molecular rules governing permissive and restrictive contexts for regeneration, we will leverage this knowledge to identify new therapeutic approaches for these opposing disease states. We aim to identify pathways to unlock regenerative capacity in non-renewing tissues and uncover therapeutic vulnerabilities for dormant cancer cells.