Proliferation capacity of the renal proximal tubule involves the bulk of differentiated epithelial cells

Abstract

We investigated the proliferative capacity of renal proximal tubular cells in healthy rats. Previously, we observed that tubular cells originate from differentiated cells. We now found 1) by application of bromo-deoxyuridine (BrdU) for 14 days and costaining for BrdU, and the G 1-phase marker cyclin D1 that the bulk of cells in the S3 segment of juvenile rats were involved in proliferation; 2) that although the proliferation rate was about 10-fold higher in juvenile rats compared with adult rats, roughly 40% of S3 cells were in G 1 in both groups; 3) that after a strong mitotic stimulus (lead acetate), proliferation was similar in juveniles and adults; 4) that there was a high incidence of cyclin D1-positive cells also in the healthy human kidney; and 5) by labeling dividing cells with BrdU for 2 days before the application of lead acetate and subsequent costaining for BrdU and cell cycle markers, that, although a strong mitotic stimulus does not abolish the period of quiescence following division, it shortens it markedly. Thus the capacity of the proximal tubule to rapidly recruit cells into division relies on a large reserve pool of cells in G 1 and on the shortening of the obligatory period of quiescence that follows division. kidney; cyclin D1; bromodeoxyuridine; stem cells; regeneration HOW THE KIDNEY maintains the homeostasis of tubular epithelial cell number has been a fundamental question for years. Since differentiated cells in most tissues, including renal tubules, have only a limited life span, it is crucial for these cells to be replaced to maintain organ function. Also a variation in tissue mass as seen during physiological adaptation encompasses the production of specialized cells. Furthermore, after injury, the generation of new cells is a prerequisite to restore the normal function of the injured organ.The source of newly formed renal epithelial cells is debated. Some studies show that novel cells are derived from divisions of differentiated cells (1,10,27), and others claim that a subpopulation in the renal tubular epithelial cells function as progenitor cells (5,9,16,19). In addition, hematopoietic stem cells (12,15,21,22) and mesenchymal stem cells (18) have been shown to be able to repopulate the renal tubular system; however, under physiological conditions tubular cells originating from extrarenal stem cells are not detectable in the kidney, and even after severe organ injury these cells constitute only a small population (7).Our previous studies revealed that tubular cell proliferation in healthy rats rely on the division of differentiated cells (25). Furthermore, we were unable to detect any slow-cycling stem cells or rapidly cycling transit amplifying (TA) cells to support a "classic" stem cell system (26).In the present study we address the question whether the capacity to divide is restricted to a small subpopulation of the differentiated cells in S3 or whether it is broadly distributed among the cells in that segment. To that aim we investigated the cycling behavior of tubular cells in...

Origin of regenerating tubular cells after acute kidney injury

“…In early descriptive studies, it has been proposed that regeneration of proximal tubular cells occurred from any surviving tubular cell following mild to moderate injury (5)(6)(7). In addition, stainings for cyclin D1, Ki-67, and BrdU suggested that fully differentiated tubular cells were either growth arrested or had progressed to the G1 phase, potentially acting as a reserve to rapidly reenter the cell cycle upon injury (6).…”

Renal organogenesis

“…23 What is uncontested is the increase in mitotic activity within damaged tubules. 24 It has also been suggested that cells within the proximal tubule rest in G 1 rather than G 0 , potentially priming them for a rapid proliferative response to injury. 24 Despite these data, more recently, several other possibilities have been proposed including (1) recruitment of stem cells from distant sites in the body (e.g., bone marrow), (2) the presence of a persistent stem/progenitor within the postnatal renal epithelium and/or (3) the recruitment of a stem/progenitor population from a non-epithelial compartment of the kidney.…”

“…24 It has also been suggested that cells within the proximal tubule rest in G 1 rather than G 0 , potentially priming them for a rapid proliferative response to injury. 24 Despite these data, more recently, several other possibilities have been proposed including (1) recruitment of stem cells from distant sites in the body (e.g., bone marrow), (2) the presence of a persistent stem/progenitor within the postnatal renal epithelium and/or (3) the recruitment of a stem/progenitor population from a non-epithelial compartment of the kidney. We will not discuss the literature pertaining to the recruitment of bone marrow cells to the kidney in response to injury here other than to reference that it has been extensively studied, and while this can occur, the evidence would suggest that integration of such cells into the renal parenchyma is very rare, with the repair process largely involving cell populations within the kidney itself.…”

“…While the kidney is not regarded as a regenerative organ, unlike the intestine, skin or blood, it has long been appreciated that this organ does have a capacity to respond to particularly acute damage. Such repair responses have been regarded as involving the proliferation of existing terminally differentiated tubular epithelial cells, 23,24 allowing the recovering of denuded basement membranes. The survival of any given nephron then depends upon the efficiency of this process vs. the number of cells lost.…”

Pathfinder Cells Provide A Novel Therapeutic Intervention For Acute Kidney Injury

“…31 The rationale for this is that there is a large reserve of these cells in the G 1 phase that enables them to elicit a proliferative response when activated. [31][32][33] More recently, it has been reported that the repair of renal proximal tubules is facilitated by self-duplication of differentiated epithelial cells and not by intra-tubular progenitor cells, as previously suggested. This same study also reports that the cells most likely to be involved in the repair process are the result of dedifferentiation/redifferentiation events.…”