Bend It Like Occam: Ductal Origin of New Islet Cells in Human Pancreas After Injury Free

Diabetes results from an inadequate mass of functional insulin-producing β-cells. Thus, β-cell replacement is being pursued as a therapy, both by supplying exogenous insulin-producing cells (cadaveric or stem cell–derived islets) and by stimulating expansion of endogenous ones. The latter has been reported to occur by replication of preexisting β-cells (self-duplication) (1) and differentiation from stem/progenitor cells (neogenesis), most commonly described in the pancreatic ducts (2). However, the concept that β-cell neogenesis may occur after birth remains controversial despite the general agreement that islets arise from ducts during embryonic development and the many indications that replication and ductal cell–mediated neogenesis (Fig. 1) are not mutually exclusive (3,4).

While early lineage tracing of insulin-expressing cells in mice (1) seemingly excluded any potential contribution of newly differentiated cells, lineage tracing of ductal cells has had variable results, with some reports demonstrating neogenesis (5,6,7) and others showing no postnatal ductal contribution to either acinar or islet tissue (8,9). These discrepancies have led to the prevailing view that studies supporting self-replication as the only mechanism of β-cell growth and regeneration were somehow more rigorous. However, none of the reports supporting the exclusivity of β-cell duplication explicitly disproved the potential contribution of progenitors to endocrine regeneration. At most, they show an absence of evidence, not evidence of absence. Importantly, using improved lineage-tracing mice and long chase times, Gribben et al. (10) recently showed dilution of insulin-traced cells with age and the concordant increase in labeled islet cells in an inducible HNF1β tracing experiment in adult mice, providing solid evidence of a small (0.66–0.68%/week) but significant contribution of newly formed islet cells derived from pancreatic ducts.

This study prompted a strong rebuttal by the deniers of the ductal origin of new β-cells after birth (11). Categorically, they affirmed that “hormone-producing cells that line ducts are most likely stable duct residents rather than neogenic endocrine cells derived dynamically from the duct.” In response, Gribben et al. (12) countered the complaints of the specifics of their study as well as the generality of the concept of neogenesis.

The lack of labeled acinar cells in the negative studies using ductal promoters has been puzzling, because the massive growth of the pancreas during the neonatal period cannot be accounted for by replication alone (13). Suggested explanations for the negative results of lineage tracing in mice have included low recombination frequency and that transcription factors such as Sox9, Hnf1β, and Hes1, used as the drivers, are highly expressed only in some pancreatic duct cells, so that the cells most likely to undergo recombination would be those expressing the highest levels of these factors and the least likely to differentiate into islet or acinar cells (14). This cellular heterogeneity of the various pancreatic compartments (reviewed in Domínguez-Bendala et al. [4]) underscores one of the major limitations of lineage tracing, a strategy that is predicated on the notion that cell fates are cast in stone. A lineage-tracing promoter that is active one day in one particular situation may not be so in a different one, confounding the interpretation of the data. Consistent with this, we have shown that ductal Sox9 and HNF1β expression were dynamic and transiently decreased after replication and that the ducts are highly proliferative before weaning (15,16). Thus, a lack of high-expressing duct cells in the perinatal period (the time of tamoxifen administration in many of the studies with negative findings) may have resulted in little initial duct labeling and the negative findings in postnatal islets and acini (15,16).

While the controversy around lineage tracing of murine β-cell regeneration still goes on, it is important not to ignore that different species can retain developmental pathways to different degrees, so the ultimate question is whether neogenesis does occur in humans. The potential origin of new β-cells from human pancreatic ductal cells is supported by data identifying specialized progenitor cells within the adult ducts (17–19), increasing insulin-positive cells in cultured purified human duct tissue (20,21), the widespread expression of insulin within the ductal tree of immunologically stressed pancreata (22), and even the real-time observations of conversion of ductal cells into functional insulin-expressing cells in live pancreatic slices from donors with type 1 diabetes (23).

In this issue of Diabetes, Lee et al. (24) provide an innovative pipeline for studying ductal remodeling in the adult human pancreas. Importantly, they present striking and powerful images on how the ductal epithelium and local β-cell populations interact in the early stages of pancreatic microcyst (∼1-mm diameter) formation, a rather unexplored model of pancreatic remodeling. The authors explain that microcysts are common benign lesions in the pancreas that appear spontaneously, especially with age. Microcysts involve epithelial remodeling probably in response to local injury. The main finding of their article is illustrated in a series of spectacular microphotographs and videos in which one can see, in all the superresolution splendor of three-dimensional/Airyscan imaging, individual β-cells lining the basal domain of the ductal epithelium, with CK7⁺ ductal cells intercalated in the apical domain. The projection images stunningly show abnormality of the ductal structures that suggests an underlying expansion of the ducts rather than a loss of acinar tissue due to upstream ductal blockage.

While these may sound like extremely specific observations, the implications are far-reaching and bring a readily citable human reference to a controversial field where information so systematically collected and reported is often missing. The authors acknowledge that the images only represent a snapshot in time and, while strongly suggestive of dynamic processes, fall short of explaining how this nuanced arrangement of ductal cells and β-cells came to be. Still, the sheer power of these images, revealed in the highest possible degree of spatial resolution, presents a biological picture that is difficult to reconcile with the notion that these exquisitely organized interfaces between hormone-positive cells and the ductal epithelium are just the product of chance or active delamination/migration from preexisting islets. If there is any truth to the principle of Occam’s razor, the simpler alternative that β-cells just developed from the ductal epithelium in response to a local injury milieu is not only more likely but also strongly aligned with everything we know about ductal responses to stress.

In summary, descriptive and admittedly limited in scope as they may be, the findings reported by Lee et al. (24) are no less important and impactful. At the very least, they may help shift the focus from the ever-increasing complexity of murine lineage tracing designs to the study of β-cell neogenesis in humans and to whether the stimulation of this program could be used therapeutically. In situ lineage-tracing in humans are not possible, but future studies using genetic duct lineage reporters in transplanted human islets, ex vivo pancreatic slices, and/or organoids may provide a close approximation and are manipulable systems.