Coupling organelle inheritance with mitosis to balance growth and differentiation

Abstract

INTRODUCTION Adult tissues must balance growth and differentiation to develop and maintain homeostasis. Excessive differentiation can lead to aging and poor wound healing. Too much growth is observed in hyperproliferative disorders and cancers. How tissue imbalances arise in disease states is poorly understood. Skin is an excellent system for understanding the importance of this balance. Essential for keeping harmful microbes out and retaining body fluids, the skin barrier is maintained by an inner layer of proliferative basal progenitors, which generate a constant outward flux of terminally differentiating cells. It is known that when epidermal progenitors accumulate mutations that will give rise to malignancy, they change their program of gene expression. However, the extent to which cancer progression involves a gain of proliferation versus a loss of differentiation is unclear. A detailed molecular knowledge of how normal basal epidermal progenitors transition from a proliferative, undifferentiated state to a terminally differentiated state allows us to investigate how this process goes awry in a tumorigenic state. We use a genetic screen to identify which of the gene changes that occur in both early cell commitment and cancer are integral to maintaining the balance between growth and differentiation. RATIONALE Epithelial cancers are among the most prevalent and life-threatening cancers worldwide. Despite intensive research, the mechanisms by which these cancers evade regulatory systems working to balance differentiation and proliferation remain poorly understood. To provide new insights into how malignancies arise and how this might be exploited in advancing cancer therapeutics, we tackled this problem in the developing skin where these regulatory systems are established. RESULTS To understand how the balance between growth and differentiation is controlled, we first devised a strategy to transcriptionally profile epidermal stem cells and their terminally differentiating progeny. Using this method, we defined the earliest molecular events associated with the commitment of epidermal progenitors to their differentiation program. Of the many changes that occur, we focused on the cohort of genes that are also mutated in human epithelial cancers. To sift through which of these genes are functional drivers in cancers and how they perturb homeostasis, we conducted an in vivo epidermal RNA interference (RNAi) screen to identify candidates that are selectively enriched or depleted in proliferative progenitors relative to their differentiating progeny. We focused on PEX11b, a protein associated with peroxisomes, organelles involved in fatty acid and energy metabolism. PEX11b deficiency compromised epidermal terminal differentiation and barrier formation. Without PEX11b, peroxisomes functioned but failed to localize and therefore segregate properly during mitosis. Probing deeper, we discovered that in normal cells, peroxisomes take on stereotyped positions during mitosis. However, after depletion of PEX11b, peroxi...

An explanation for the mysterious distribution of melanin in human skin: a rare example of asymmetric (melanin) organelle distribution during mitosis of basal layer progenitor keratinocytes

“…Importantly, there is increasing evidence that elements of the mitotic machinery, including the centrioles, participate in regulating the fate decision of which daughter cell inherits different age organelles . Moreover, it was recently shown that ‘inappropriate peroxisome segregation’ during KC mitosis can influence cell division, skew daughter fate and disturb epidermal balance . Thus, we aimed to see if there was any correlation between the filiation of the centrosome and the amount of melanin inherited by a daughter KC; in other words, whether mother and daughter centrosomes were always associated with one type of daughter KC.…”

A role for Mitochondrial Rho GTPase 1 (MIRO1) in motility and membrane dynamics of peroxisomes

“…The shape and number of peroxisomes is controlled by PEX11β, a peroxisomal membrane protein, which induces elongation and remodelling of the peroxisomal membrane and acts as a GTPase activating protein on the large fission GTPase DNM1L 13, 14, 15. Loss of PEX11β was recently linked to spindle misorientation and peroxisome mislocalisation in mitosis causing imbalances in epidermal differentiation 16. These findings underline the importance of peroxisome multiplication, distribution and inheritance for cell fate decisions.…”

Skin and Its Regenerative Powers: An Alliance between Stem Cells and Their Niche

“…When the epidermis is mature, a shift to largely parallel divisions occurs, although basal cells still appear to partition NOTCH signaling differentially to their daughters (Clayton et al, 2007) (Figure 3). KRT14/KRT5 basal cells that dually express differentiation markers, e.g., KRT10, INVOLUCRIN, and NOTCH, appear to be fated to move upward and terminally differentiate (Asare et al, 2017; Clayton et al, 2007; Mascre et al, 2012; Watt and Green, 1982). …”

“…The data support a simple model where both basal daughters have an equal chance to proliferate or differentiate. That said, in this study, morphology and location were used to distinguish proliferative versus differentiating, and these criteria overlook basal daughters expressing dual differentiation markers (Asare et al, 2017), which may still have some proliferative potential but have been referred to as “committed progenitors” (Mascre et al, 2012). …”

“…Another key point is that basal location is not synonymous with progenitor status. Indeed, when fluorescence-activated cell sorting was recently used to purify dually positive KRT14/KRT10 cells from Krt14-eGFP and Krt10-RFP mice, this cohort comprised some cells that showed polarized integrins to substantiate their basal layer identity but also displayed a transcriptional profile reflective of a differentiation-poised state (Asare et al, 2017). …”