Christine A. Fargeas

Prominin: A Story of Cholesterol, Plasma Membrane Protrusions and Human Pathology

Prominin is the first identified member of a novel family of polytopic membrane proteins conserved throughout the animal kingdom. It has an unusual membrane topology, containing five transmembrane domains and two large glycosylated extracellular loops. In mammals, prominin is expressed in various embryonic and adult epithelial cells, as well as in nonepithelial cells, such as hematopoietic stem cells. At the subcellular level, prominin is selectively localized in microvilli and other plasma membrane protrusions, irrespective of cell type. At the molecular level, prominin specifically interacts with membrane cholesterol and is a marker of a novel type of cholesterol‐based lipid ‘raft’. A frameshift mutation in the human prominin gene, which results in a truncated protein that is no longer transported to the cell surface, is associated with retinal degeneration. Given that prominin is concentrated in the plasma membrane evaginations at the base of the outer segment of rod photoreceptor cells, which are essential precursor structures in the biogenesis of photoreceptive disks, it is proposed that prominin has a role in the generation of plasma membrane protrusions, their lipid composition and organization and their membrane‐to‐membrane interactions.

CD133 as a biomarker for putative cancer stem cells in solid tumours: limitations, problems and challenges.

The cancer stem cell (CSC) hypothesis, despite the limitations of the currently available models and assays, has ushered in a new era of excitement in cancer research. The development of novel strategies for anti‐tumour therapy relies on the use of biomarkers to identify, enrich, and/or isolate the cell population(s) of interest. In this context, various cell characteristics and antigen expression profiles are discussed as surrogate markers. The cell surface expression of the human prominin‐1 (CD133) antigen, in particular of the AC133 epitope, is among those that have been most frequently studied in solid cancers, although no mechanism has yet been proposed to link CD133 expression with the CSC phenotype. Some inconsistencies between published data can be ascribed to different analytical tools as well as methodological limitations and pitfalls, highlighted in the present review. Therefore, a comprehensive overview on the current state of knowledge in this growing and exciting field with an emphasis on the most recent studies is presented. We highlight the link between the tumour microenvironment, tumour cell plasticity, and CD133 expression, and evaluate the utility of CD133 expression as a prognostic marker. Copyright

New insights into the cell biology of hematopoietic progenitors by studying prominin-1 (CD133).

Prominin-1 (alias CD133) has received considerable interest because of its expression by several stem and progenitor cells originating from various sources, including the neural and hematopoietic systems. As a cell surface marker, prominin-1 is now used for somatic stem cell isolation. Its expression in cancer stem cells has broadened its clinical value, as it might be useful to outline new prospects for more effective cancer therapies by targeting tumor-initiating cells. Cell biological studies of this molecule have demonstrated that it is specifically concentrated in various membrane structures that protrude from the planar areas of the plasmalemma. Prominin-1 binds to the plasma membrane cholesterol and is associated with a particular membrane microdomain in a cholesterol-dependent manner. Although its physiological function is not yet determined, it is becoming clear that this cell surface protein, as a unique marker of both plasma membrane protrusions and membrane microdomains, might reveal new aspects of the cell biology of rare stem and cancer stem cells. The aim of this review is to outline the recent discoveries regarding the dynamic reorganization of the plasma membrane of rare CD133+ hematopoietic progenitor cells during cell migration and division.

AC133 Antigen, CD133, Prominin‐1, Prominin‐2, Etc.: Prominin Family Gene Products in Need of a Rational Nomenclature

Correspondence: Denis Corbeil, Ph.D., Medical Clinic and Polyclinic I, Technical University Dresden, Fetscherstrasse 74,D-01307 Dresden, Germany. Telephone: 49-351-210-1488; Fax: 49-351-210-1489; e-mail: corbeil@mpi-cbg.deReceived March 3, 2003; accepted for publication April 28, 2003. ©AlphaMed Press 1066-5099/2003/

Identification of novel Prominin-1/CD133 splice variants with alternative C-termini and their expression in epididymis and testis.

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Prominin-1 (CD133): from progenitor cells to human diseases

Prominin-1/CD133 is a five-membrane-span glycoprotein that is thought to act as an organizer of plasma-membrane protrusions. Here, we report the molecular and cell-biological characterization of four novel prominin-1 splice variants isolated from a mouse testis cDNA library and referred to as prominin-1.s3 to prominin-1.s6. Compared with kidney-derived prominin-1.s1, the s3, s4 and s5 variants contain a distinct cytoplasmic C-terminal domain. The s4 and s5 variants bear, in addition, two and one inframe deletion(s), respectively, in the extracellular domains. The s6 variant displays a truncated C-terminal domain caused by a premature termination resulting from intron retention. Upon their ectopic expression in Chinese hamster ovary cells, the s3 and s6 variants were found to be concentrated in plasma-membrane protrusions, whereas the s4 and s5 variants did not reach the cell surface. Biochemical analyses suggest that most of the prominin-1 in the adult male reproductive system is expressed as the s6 variant. Immunohistological and electron microscopic analyses show that prominin-1 is: (1) confined to the apical surface of the epithelium all along the epididymal duct, with the exception of the initial segment; (2) concentrated in stereocilia of the epididymal duct epithelium; and (3) found on the tail of developing spermatozoa in seminiferous tubules. Our data suggest that prominin-1 is involved in the formation and/or stabilization of epididymal stereocilia and the tail of spermatozoa, and hence might play a dual role in the biogenesis of spermatozoa.

Prominin-2 is a cholesterol-binding protein associated with apical and basolateral plasmalemmal protrusions in polarized epithelial cells and released into urine

Prominin-1 (CD133) is a pentaspan membrane glycoprotein that binds to plasma membrane cholesterol and concentrates selectively in plasma membrane protrusions. In recent years, this molecule has received considerable interest due to its expression in various progenitors, including those derived from the neural and hematopoietic system, as well as in cancer originating from these systems. Prominin-1 is also the target of mutations leading to retinal degeneration. In the future, prominin-1-positive progenitor cells might become clinically significant, particularly with regard to tissue engineering, and prominin-1 itself might reveal some fundamental cell biological aspects concerning the self-renewal capacity of somatic stem cells. Biomedical research has recently been focusing on the biological characterization of stem and progenitor cells. Understanding, and eventually controlling, the self-renewal capacity of these cells as well as their ability to differentiate into mature cells could lead to the development of new cell-based therapeutic strategies. A considerable research effort is thus being made in order to identify the key players responsible for the proliferation, self-renewal and differentiation of stem and progenitor cells. The current strategy to isolate these rare cell subpopulations is based mainly on monoclonal antibodies directed against specific cell surface markers. Searching for such stem cell surface markers, a

Expression of distinct splice variants of the stem cell marker prominin-1 (CD133) in glial cells.

Prominin-2 is a pentaspan membrane glycoprotein structurally related to the cholesterol-binding protein prominin-1, which is expressed in epithelial and non-epithelial cells. Although prominin-1 expression is widespread throughout the organism, the loss of its function solely causes retinal degeneration. The finding that prominin-2 appears to be restricted to epithelial cells, such as those found in kidney tubules, raises the possibility that prominin-2 functionally substitutes prominin-1 in tissues other than the retina and provokes a search for a definition of its morphological and biochemical characteristics. Here, we have investigated, by using MDCK cells as an epithelial cell model, whether prominin-2 shares the biochemical and morphological properties of prominin-1. Interestingly, we have found that, whereas prominin-2 is not restricted to the apical domain like prominin-1 but is distributed in a non-polarized fashion between the apical and basolateral plasma membranes, it retains the main feature of prominin-1, i.e. its selective concentration in plasmalemmal protrusions; prominin-2 is confined to microvilli, cilia and other acetylated tubulin-positive protruding structures. Similar to prominin-1, prominin-2 is partly associated with detergent-resistant membranes in a cholesterol-dependent manner, suggesting its incorporation into membrane microdomains, and binds directly to plasma membrane cholesterol. Finally, prominin-2 is also associated with small membrane particles that are released into the culture media and found in a physiological fluid, i.e. urine. Together, these data show that all the characteristics of prominin-1 are shared by prominin-2, which is in agreement with a possible redundancy in their role as potential organizers of plasma membrane protrusions.

Prominin-1 (CD133) is not restricted to stem cells located in the basal compartment of murine and human prostate

Prominin‐1 (CD133) is a cholesterol‐interacting pentaspan membrane glycoprotein specifically associated with plasma membrane protrusions. Prominin‐1 is expressed by various stem and progenitor cells, notably neuroepithelial progenitors found in the developing embryonic brain. Here, we further investigated its expression in the murine brain. Biochemical analyses of brain membranes at early stages of development revealed the expression of two distinct splice variants of prominin‐1, s1 and s3, which have different cytoplasmic C‐terminal domains. The relative abundance of the s3 variant increased toward adulthood, whereas the opposite was observed for the s1 variant. Our combined in situ hybridization and immunohistochemistry revealed the expression of prominin‐1 in a subpopulation of Olig‐2‐positive oligodendroglial cells present within white matter tracts of postnatal and adult brain. Furthermore, immunohistological and biochemical characterization suggested strongly that the s3 variant is a novel component of myelin. Consistent with this, the expression of prominin‐1.s3 was significantly reduced in the brain of myelin‐deficient mice. Finally, oligodendrocytes expressed selectively the s3 variant whereas GFAP‐positive astrocytes expressed the s1 variant in primary glial cell cultures derived from embryonic brains. Collectively, our data demonstrate a complex expression pattern of prominin‐1 molecules in developing adult brain. Given that prominin‐1 is thought to act as an organizer of plasma membrane protrusions, they further suggest that a specific prominin‐1 splice variant might play a role in morphogenesis and/or maintenance of the myelin sheath.

Differential expression of Prominin-1 (CD133) and Prominin-2 in major cephalic exocrine glands of adult mice

Rodent and human prominin‐1 are expressed in numerous adult epithelia and somatic stem cells. A report has shown that human PROMININ‐1 carrying the AC133 epitope can be used to identify rare prostate basal stem cells (Richardson et al., J Cell Sci 2004; 117:3539–3545). Here we re‐investigated its general expression in male reproductive tract including mouse and human prostate and in prostate cancer samples using various anti‐prominin‐1 antibodies.