Evidence for an Expansion-Based Temporal Shh Gradient in Specifying Vertebrate Digit Identities

The zone of polarizing activity (ZPA) in the posterior limb bud produces Sonic Hedgehog (Shh) protein, which plays a critical role in establishing distinct fates along the anterior-posterior axis. This activity has been modeled as a concentration-dependent response to a diffusible morphogen. Using recombinase base mapping in the mouse, we determine the ultimate fate of the Shh-producing cells. Strikingly, the descendants of the Shh-producing cells encompass all cells in the two most posterior digits and also contribute to the middle digit. Our analysis suggests that, while specification of the anterior digits depends upon differential concentrations of Shh, the length of time of exposure to Shh is critical in the specification of the differences between the most posterior digits. Genetic studies of the effects of limiting accessibility of Shh within the limb support this model, in which the effect of the Shh morphogen is dictated by a temporal as well as a spatial gradient.

BMP2 activity, although dispensable for bone formation, is required for the initiation of fracture healing

Adult bones have a notable regenerative capacity. Over 40 years ago, an intrinsic activity capable of initiating this reparative response was found to reside within bone itself, and the term bone morphogenetic protein (BMP) was coined to describe the molecules responsible for it. A family of BMP proteins was subsequently identified, but no individual BMP has been shown to be the initiator of the endogenous bone repair response. Here we demonstrate that BMP2 is a necessary component of the signaling cascade that governs fracture repair. Mice lacking the ability to produce BMP2 in their limb bones have spontaneous fractures that do not resolve with time. In fact, in bones lacking BMP2, the earliest steps of fracture healing seem to be blocked. Although other osteogenic stimuli are still present in the limb skeleton of BMP2-deficient mice, they cannot compensate for the absence of BMP2. Collectively, our results identify BMP2 as an endogenous mediator necessary for fracture repair.

The RNaseIII enzyme Dicer is required for morphogenesis but not patterning of the vertebrate limb

The RNaseIII-containing enzyme Dicer is believed to be required for the processing of most, if not all, microRNAs (miRNAs) and for processing long dsRNA into small interfering RNAs. Because the complete loss of Dicer in both zebrafish and mice results in early embryonic lethality, it has been impossible to determine what role, if any, Dicer has in patterning later tissues in the developing vertebrate embryo. To bypass the early requirement of Dicer in development, we have created a conditional allele of this gene in mice. Using transgenes to drive Cre expression in discrete regions of the limb mesoderm, we find that removal of Dicer results in the loss of processed miRNAs. Phenotypically, developmental delays, in part due to massive cell death as well as disregulation of specific gene expression, lead to the formation of a much smaller limb. Thus, Dicer is required for the formation of normal mouse limbs. Strikingly, however, we did not detect defects in basic patterning or in tissue-specific differentiation of Dicer-deficient limb buds.

Genetic analysis of the roles of BMP2, BMP4, and BMP7 in limb patterning and skeletogenesis

Bone morphogenetic protein (BMP) family members, including BMP2, BMP4, and BMP7, are expressed throughout limb development. BMPs have been implicated in early limb patterning as well as in the process of skeletogenesis. However, due to complications associated with early embryonic lethality, particularly for Bmp2 and Bmp4, and with functional redundancy among BMP molecules, it has been difficult to decipher the specific roles of these BMP molecules during different stages of limb development. To circumvent these issues, we have constructed a series of mouse strains lacking one or more of these BMPs, using conditional alleles in the case of Bmp2 and Bmp4 to remove them specifically from the limb bud mesenchyme. Contrary to earlier suggestions, our results indicate that BMPs neither act as secondary signals downstream of Sonic Hedghog (SHH) in patterning the anteroposterior axis nor as signals from the interdigital mesenchyme in specifying digit identity. We do find that a threshold level of BMP signaling is required for the onset of chondrogenesis, and hence some chondrogenic condensations fail to form in limbs deficient in both BMP2 and BMP4. However, in the condensations that do form, subsequent chondrogenic differentiation proceeds normally even in the absence of BMP2 and BMP7 or BMP2 and BMP4. In contrast, we find that the loss of both BMP2 and BMP4 results in a severe impairment of osteogenesis.

A group of related signaling molecules called bone morphogenetic proteins (BMPs) are known to play important roles in the formation of the structures such as the limbs. However, because different members of this group often have similar effects on target cells and are produced in overlapping regions of the embryo and hence can be redundant with one another, removal of any single member of the BMP family may not reveal the full extent of the roles they play during development. We have therefore improved on this type of analysis by removing pairs of these factors (BMP2 and BMP4 or BMP2 and BMP7) specifically from the developing limb. Although some have speculated that these signals play an early role in organizing or “patterning” the different tissues of the limb, we find no evidence for such a role. We do find, however, that a minimal amount of BMP signal is required to form cartilage, and hence some cartilaginous elements fail to form in limbs deficient in both BMP2 and BMP4. Moreover, in the absence of these two BMP family members, there is a severe impairment in the development of bone tissue, resulting in severely deformed limbs. This study gives important new insight into the roles of these BMP signals in making skeletal tissues in the embryo.

DNA mismatch repair and genetic instability

Mismatch repair (MMR) systems play a central role in promoting genetic stability by repairing DNA replication errors, inhibiting recombination between non-identical DNA sequences and participating in responses to DNA damage. The discovery of a link between human cancer and MMR defects has led to an explosion of research on eukaryotic MMR. The key proteins in MMR are highly conserved from bacteria to mammals, and this conservation has been critical for defining the components of eukaryotic MMR systems. In eukaryotes, there are multiple homologs of the key bacterial MutS and MutL MMR proteins, and these homologs form heterodimers that have discrete roles in MMR-related processes. This review describes the genetic and biochemical approaches used to study MMR, and summarizes the diverse roles that MMR proteins play in maintaining genetic stability.

Conditional loss of Dicer disrupts cellular and tissue morphogenesis in the cortex and hippocampus

To investigate the role of Dicer and microRNAs in the mammalian CNS, we used mice in which the second RNase III domain of Dicer was conditionally floxed. Conditional Dicer mice were bred with mice expressing an alpha-calmodulin kinase II Cre to selectively inactivate Dicer in excitatory forebrain neurons in vivo. Inactivation of Dicer results in an array of phenotypes including microcephaly, reduced dendritic branch elaboration, and large increases in dendritic spine length with no concomitant change in spine density. Microcephaly is likely caused by a 5.5-fold increase in early postnatal apoptosis in these animals as determined by active caspase-3 and TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling) staining in the cortex. Loss of Dicer function had no measurable effect on cortical lamination as determined by in situ hybridization, suggesting that microcephaly is not caused by defects in neuronal migration. Together, these results illustrate the in vivo significance of Dicer and miRNAs in the mammalian CNS and provide additional support for previous in vitro studies indicating that misregulation of this pathway may result in gross abnormalities in cell number and function that may contribute to a variety of neurological disorders.

Expression of anoctamin 1/TMEM16A by interstitial cells of Cajal is fundamental for slow wave activity in gastrointestinal muscles

Interstitial cells of Cajal (ICC) generate pacemaker activity (slow waves) in gastrointestinal (GI) smooth muscles, but the mechanism(s) of pacemaker activity are controversial. Several conductances, such as Ca(2+)-activated Cl() channels (CaCC) and non-selective cation channels (NSCC) have been suggested to be involved in slow wave depolarization. We investigated the expression and function of a new class of CaCC, anoctamin 1 (ANO1), encoded by Tmem16a, which was discovered to be highly expressed in ICC in a microarray screen. GI muscles express splice variants of the Tmem16a transcript in addition to other paralogues of the Tmem16a family. ANO1 protein is expressed abundantly and specifically in ICC in all regions of the murine, non-human primate (Macaca fascicularis) and human GI tracts. CaCC blocking drugs, niflumic acid and 4,4-diisothiocyano-2,2-stillbene-disulfonic acid (DIDS) reduced the frequency and blocked slow waves in murine, primate, human small intestine and stomach in a concentration-dependent manner. Unitary potentials, small stochastic membrane depolarizations thought to underlie slow waves, were insensitive to CaCC blockers. Slow waves failed to develop by birth in mice homozygous for a null allele of Tmem16a (Tmem16a(tm1Bdh)(/tm1Bdh)) and did not develop subsequent to birth in organ culture, as in wildtype and heterozygous muscles. Loss of function of ANO1 did not inhibit the development of ICC networks that appeared structurally normal as indicated by Kit antibodies. These data demonstrate the fundamental role of ANO1 in the generation of slow waves in GI ICC.

MicroRNA-responsive 'sensor' transgenes uncover Hox-like and other developmentally regulated patterns of vertebrate microRNA expression

MicroRNAs (miRNAs) are a class of short ( approximately 22-nt) noncoding RNA molecules that downregulate expression of their mRNA targets. Since their discovery as regulators of developmental timing in Caenorhabditis elegans, hundreds of miRNAs have been identified in both animals and plants. Here, we report a technique for visualizing detailed miRNA expression patterns in mouse embryos. We elucidate the tissue-specific expression of several miRNAs during embryogenesis, including two encoded by genes embedded in homeobox (Hox) clusters, miR-10a and miR-196a. These two miRNAs are expressed in patterns that are markedly reminiscent of those of Hox genes. Furthermore, miR-196a negatively regulates Hoxb8, indicating that its restricted expression pattern probably reflects a role in the patterning function of the Hox complex.

Distinct stem cell populations regenerate the follicle and interfollicular epidermis

The regeneration of the skin and its appendages is thought to occur by the regulated activation of a dedicated stem cell population. A population of cells in the bulge region of the hair follicle has been identified as the putative stem cell of both the follicle and the interfollicular epidermis. While this assertion is supported by a variety of surrogate assays, there has been no direct confirmation of the normal contribution of these cells to the regeneration of structures other than the cycling portion of the hair follicle. Here, we report lineage analysis revealing that the follicular epithelium is derived from cells in the epidermal placode that express Sonic hedgehog. This analysis also demonstrates that the stem cells resident in the follicular bulge that regenerate the follicle are neither the stem cells of the epidermis nor the source of the stem cells of the epidermis in the absence of trauma.

Dicer function is essential for lung epithelium morphogenesis

DICER is a key enzyme that processes microRNA and small interfering RNA precursors into their short mature forms, enabling them to regulate gene expression. Only a single Dicer gene exists in the mouse genome, and it is broadly expressed in developing tissues. Dicer-null mutants die before gastrulation. Therefore, to study Dicer function in the later event of lung formation, we inactivated it in the mouse lung epithelium using a Dicer conditional allele and the Sonic Hedgehogcre (Shhcre) allele. Branching arrests in these mutant lungs, although epithelial growth continues in distal domains that are expanded compared with normal samples. These defects result in a few large epithelial pouches in the mutant lung instead of numerous fine branches present in a normal lung. Significantly, the initial phenotypes are apparent before an increase in epithelial cell death is observed, leading us to propose that Dicer plays a specific role in regulating lung epithelial morphogenesis independent of its requirement in cell survival. In addition, we found that the expression of Fgf10, a key gene involved in lung development, is up-regulated and expanded in the mesenchyme of Dicer mutant lungs. Previous studies support the hypothesis that precise localization of FGF10 in discrete sites of the lung mesenchyme serves as a chemoattractant for the outgrowth of epithelial branches. The aberrant Fgf10 expression may contribute to the Dicer morphological defects. However, the mechanism by which DICER functions in the epithelium to influence Fgf10 expression in the mesenchyme remains unknown.

The microRNA miR-196 acts upstream of Hoxb8 and Shh in limb development

MicroRNAs (miRNAs) are an abundant class of gene regulatory molecules (reviewed in refs 1, 2). Although computational work indicates that miRNAs repress more than a third of human genes, their roles in vertebrate development are only now beginning to be determined. Here we show that miR-196 acts upstream of Hoxb8 and Sonic hedgehog (Shh) in vivo in the context of limb development, thereby identifying a previously observed but uncharacterized inhibitory activity that operates specifically in the hindlimb. Our data indicate that miR-196 functions in a fail-safe mechanism to assure the fidelity of expression domains that are primarily regulated at the transcriptional level, supporting the idea that many vertebrate miRNAs may function as a secondary level of gene regulation.

Epidermal stem cells arise from the hair follicle after wounding

During normal development, the epidermis and hair follicle are distinct lineage compartments maintained by independent stem cell populations. Both epidermal and follicular keratinocytes are recruited to participate in epidermal repair in response to injury. However, it is generally thought that follicular cells contribute to the wound epidermis only transiently and are ultimately replaced by the progeny of stem cells derived from the original epidermal compartment prior to wounding. Here we use inducible and constitutive cre recombinase expressed from the Sonic hedgehog locus (Shh) for in vivo lineage tracing. This analysis confirms that follicular cells participate in the initial resurfacing of the wound but also reveals that their progeny persist in wound epidermis for months after the wound is healed. It further demonstrates that Shh is not induced in keratinocytes during the wound healing process. We conclude that follicular cells can undergo reprogramming to become long-term repopulating epidermal progenitors following wounding.

MicroRNA expression is required for pancreatic islet cell genesis in the mouse

OBJECTIVE

The generation of distinct cell types during the development of the pancreas depends on sequential changes in gene expression. We tested the hypothesis that microRNAs (miRNAs), which limit gene expression through posttranscriptional silencing, modulate the gene expression cascades involved in pancreas development.

RESEARCH DESIGN AND METHODS

miRNAs were cloned and sequenced from developing pancreata, and expression of a subset of these genes was tested using locked nucleic acid in situ analyses. To assess the overall contribution of miRNAs to pancreatic development, Dicer1, an enzyme required for miRNA processing, was conditionally deleted from the developing pancreas.

RESULTS

Sequencing of small RNAs identified over 125 miRNAs, including 18 novel sequences, with distinct expression domains within the developing pancreas. To test the developmental contribution of these miRNAs, we conditionally deleted the miRNA processing enzyme Dicer1 early in pancreas development. Dicer-null animals displayed gross defects in all pancreatic lineages, although the endocrine cells, and especially the insulin-producing beta-cells, were most dramatically reduced. The endocrine defect was associated with an increase in the notch-signaling target Hes1 and a reduction in the formation of endocrine cell progenitors expressing the Hes1 target gene neurogenin3.

CONCLUSIONS

The expression of a unique profile of miRNAs is required during pancreas development and is necessary for beta-cell formation.

MicroRNAs in vertebrate development

The vertebrate genome contains hundreds of small non-coding 'microRNAs' that have been implicated in controlling the expression of potentially thousands of target genes. Presently, only a handful of these targets have been characterized. Recent reports of microRNA 'sensors', microRNA microarrays and the creation of vertebrates that lack all microRNA activity will aid in determining the roles played by microRNAs, and the genes that they regulate, during vertebrate development.

Degeneration and regeneration of the intervertebral disc: lessons from development

Degeneration of the intervertebral discs, a process characterized by a cascade of cellular, biochemical, structural and functional changes, is strongly implicated as a cause of low back pain. Current treatment strategies for disc degeneration typically address the symptoms of low back pain without treating the underlying cause or restoring mechanical function. A more in-depth understanding of disc degeneration, as well as opportunities for therapeutic intervention, can be obtained by considering aspects of intervertebral disc development. Development of the intervertebral disc involves the coalescence of several different cell types through highly orchestrated and complex molecular interactions. The resulting structures must function synergistically in an environment that is subjected to continuous mechanical perturbation throughout the life of an individual. Early postnatal changes, including altered cellularity, vascular regression and altered extracellular matrix composition, might set the disc on a slow course towards symptomatic degeneration. In this Perspective, we review the pathogenesis and treatment of intervertebral disc degeneration in the context of disc development. Within this scope, we examine how model systems have advanced our understanding of embryonic morphogenesis and associated molecular signaling pathways, in addition to the postnatal changes to the cellular, nutritional and mechanical microenvironment. We also discuss the current status of biological therapeutic strategies that promote disc regeneration and repair, and how lessons from development might provide clues for their refinement.

Essential role for Dicer during skeletal muscle development

microRNAs (miRNAs) regulate gene expression post-transcriptionally by targeting mRNAs for degradation or by inhibiting translation. Dicer is an RNase III endonuclease which processes miRNA precursors into functional 21-23 nucleotide RNAs that are subsequently incorporated into the RNA-induced silencing complex. miRNA-mediated gene regulation is important for organogenesis of a variety of tissues including limb, lung and skin. To gain insight into the roles of Dicer and miRNAs in mammalian skeletal muscle development, we eliminated Dicer activity specifically in the myogenic compartment during embryogenesis. Dicer activity is essential for normal muscle development during embryogenesis and Dicer muscle mutants have reduced muscle miRNAs, die perinatally and display decreased skeletal muscle mass accompanied by abnormal myofiber morphology. Dicer mutant muscles also show increased apoptosis and Cre-mediated loss of Dicer in Myod-converted myoblasts results in enhanced cell death. These observations demonstrate key roles for Dicer in skeletal muscle and implicate miRNAs as critical components required for embryonic myogenesis.

Podocyte-specific deletion of dicer alters cytoskeletal dynamics and causes glomerular disease

MicroRNAs (miRNAs) regulate gene expression by binding the 3' untranslated region of mRNAs. To define their role in glomerular function, miRNA biogenesis was disrupted in mouse podocytes using a conditional Dicer allele. Mutant mice developed proteinuria by 3 wk after birth and progressed rapidly to end-stage kidney disease. Podocyte pathology included effacement, vacuolization, and hypertrophy with crescent formation. Despite normal expression of WT1, podocytes underwent dedifferentiation, exemplified by cytoskeletal disruption with early transcriptional downregulation of synaptopodin. These abnormalities differed from Cd2ap(-/-) mice, indicating they were not a general consequence of glomerular disease. Glomerular labeling of ezrin, moesin, and gelsolin was altered at 3 wk, but expression of nestin and alpha-actinin was unchanged. Abnormal cell proliferation or apoptosis was not responsible for the glomerular injury. Mutant podocytes were incapable of synthesizing mature miRNA, as revealed by their loss of miR-30a. In contrast, expression of glomerular endothelial and mesangial cell miRNAs (miR-126 and miR-145, respectively) was unchanged. These findings demonstrate a critical role for miRNA in glomerular function and suggest a pathway that may participate in the pathogenesis of kidney diseases of podocyte origin. The unique architecture of podocytes may make them especially susceptible to cytoskeletal alterations initiated by aberrant miRNA dynamics.

TMEM16A induces MAPK and contributes directly to tumorigenesis and cancer progression

Frequent gene amplification of the receptor-activated calcium-dependent chloride channel TMEM16A (TAOS2 or ANO1) has been reported in several malignancies. However, its involvement in human tumorigenesis has not been previously studied. Here, we show a functional role for TMEM16A in tumor growth. We found TMEM16A overexpression in 80% of head and neck squamous cell carcinoma (SCCHN), which correlated with decreased overall survival in patients with SCCHN. TMEM16A overexpression significantly promoted anchorage-independent growth in vitro, and loss of TMEM16A resulted in inhibition of tumor growth both in vitro and in vivo. Mechanistically, TMEM16A-induced cancer cell proliferation and tumor growth were accompanied by an increase in extracellular signal-regulated kinase (ERK)1/2 activation and cyclin D1 induction. Pharmacologic inhibition of MEK/ERK and genetic inactivation of ERK1/2 (using siRNA and dominant-negative constructs) abrogated the growth effect of TMEM16A, indicating a role for mitogen-activated protein kinase (MAPK) activation in TMEM16A-mediated proliferation. In addition, a developmental small-molecule inhibitor of TMEM16A, T16A-inh01 (A01), abrogated tumor cell proliferation in vitro. Together, our findings provide a mechanistic analysis of the tumorigenic properties of TMEM16A, which represents a potentially novel therapeutic target. The development of small-molecule inhibitors against TMEM16A may be clinically relevant for treatment of human cancers, including SCCHN.

Identification of nucleus pulposus precursor cells and notochordal remnants in the mouse: implications for disk degeneration and chordoma formation

A classically identified "notochordal" cell population in the nucleus pulposus is thought to regulate disk homeostasis. However, the embryonic origin of these cells has been under dispute for >60 years. Here we provide the first direct evidence that all cell types in the adult mouse nucleus pulposus are derived from the embryonic notochord. Additionally, rare isolated embryonic notochord cells remained in the vertebral column and resembled "notochordal remnants," which in humans have been proposed to give rise to a rare type of late-onset cancer called chordoma. Previously, this cell type had not been identified in the mouse model system. The development and characterization of a mouse model that can be used to fate map nucleus pulposus precursor cells in any mutant background will be useful for uncovering the cellular and molecular mechanisms of disk degeneration. In addition, the identification of notochordal remnants in mice is the first step towards generating an in vivo model of chordoma.

Members of the miRNA-200 family regulate olfactory neurogenesis

MicroRNAs (miRNAs) are highly expressed in vertebrate neural tissues, but the contribution of specific miRNAs to the development and function of different neuronal populations is still largely unknown. We report that miRNAs are required for terminal differentiation of olfactory precursors in both mouse and zebrafish but are dispensable for proper function of mature olfactory neurons. The repertoire of miRNAs expressed in olfactory tissues contains over 100 distinct miRNAs. A subset, including the miR-200 family, shows high olfactory enrichment and expression patterns consistent with a role during olfactory neurogenesis. Loss of function of the miR-200 family phenocopies the terminal differentiation defect observed in absence of all miRNA activity in olfactory progenitors. Our data support the notion that vertebrate tissue differentiation is controlled by conserved subsets of organ-specific miRNAs in both mouse and zebrafish and provide insights into control mechanisms underlying olfactory differentiation in vertebrates.

Transmembrane protein 16A (TMEM16A) is a Ca2+-regulated Cl- secretory channel in mouse airways

For almost two decades, it has been postulated that calcium-activated Cl(-) channels (CaCCs) play a role in airway epithelial Cl(-) secretion, but until recently, the molecular identity of the airway CaCC(s) was unknown. Recent studies have unequivocally identified TMEM16A as a glandular epithelial CaCC. We have studied the airway bioelectrics of neonatal mice homozygous for a null allele of Tmem16a (Tmem16a(-/-)) to investigate the role of this channel in Cl(-) secretion in airway surface epithelium. When compared with wild-type tracheas, the Tmem16a(-/-) tracheas exhibited a >60% reduction in purinoceptor (UTP)-regulated CaCC activity. Other members of the Tmem16 gene family, including Tmem16f and Tmem16k, were also detected by reverse transcription-PCR in neonatal tracheal epithelium, suggesting that other family members could be considered as contributing to the small residual UTP response. TMEM16A, however, appeared to contribute little to unstimulated Cl(-) secretion, whereas studies with cystic fibrosis transmembrane conductance regulator (CFTR)-deficient mice and wild-type littermates revealed that unstimulated Cl(-) secretion reflected approximately 50% CFTR activity and approximately 50% non-Tmem16a activity. Interestingly, the tracheas of both the Tmem16a(-/-) and the CFTR(-/-) mice exhibited similar congenital cartilaginous defects that may reflect a common Cl(-) secretory defect mediated by the molecularly distinct Cl(-) channels. Importantly, the residual CaCC activity in Tmem16a(-/-) mice appeared inadequate for normal airway hydration because Tmem16a(-/-) tracheas exhibited significant, neonatal, lumenal mucus accumulation. Our data suggest that TMEM16A CaCC-mediated Cl(-) secretion appears to be necessary for normal airway surface liquid homeostasis.

Mutations that alter an Arg-Gly-Asp (RGD) sequence in the adenovirus type 2 penton base protein abolish its cell-rounding activity and delay virus reproduction in flat cells

The adenovirus penton base protein has a cell rounding activity and may lyse endosomes during virus entry into the cytoplasm. We found that penton base that was expressed in Escherichia coli also caused cell rounding and that cells adhered to polystyrene wells that were coated with the protein. Mutant analysis showed that both properties required an Arg-Gly-Asp (RGD) sequence at residues 340 to 342 of penton base. In flat adherent cells, virus mutants with amino acid substitutions in the RGD sequence were delayed in virus reproduction and in the onset of viral DNA synthesis. In nonadherent or poorly spread cells, the kinetics of mutant virus reproduction were similar to those of wild-type adenovirus type 2. Expression of the mutant phenotype exclusively in the flat cells that we tested supports a model in which penton base interacts with an RGD-directed cell adhesion molecule during adenovirus uptake or uncoating.

Loss of TMEM16A causes a defect in epithelial Ca2+-dependent chloride transport

Molecular identification of the Ca(2+)-dependent chloride channel TMEM16A (ANO1) provided a fundamental step in understanding Ca(2+)-dependent Cl(-) secretion in epithelia. TMEM16A is an intrinsic constituent of Ca(2+)-dependent Cl(-) channels in cultured epithelia and may control salivary output, but its physiological role in native epithelial tissues remains largely obscure. Here, we demonstrate that Cl(-) secretion in native epithelia activated by Ca(2+)-dependent agonists is missing in mice lacking expression of TMEM16A. Ca(2+)-dependent Cl(-) transport was missing or largely reduced in isolated tracheal and colonic epithelia, as well as hepatocytes and acinar cells from pancreatic and submandibular glands of TMEM16A(-/-) animals. Measurement of particle transport on the surface of tracheas ex vivo indicated largely reduced mucociliary clearance in TMEM16A(-/-) mice. These results clearly demonstrate the broad physiological role of TMEM16A(-/-) for Ca(2+)-dependent Cl(-) secretion and provide the basis for novel treatments in cystic fibrosis, infectious diarrhea, and Sjöegren syndrome.

Dicer inactivation leads to progressive functional and structural degeneration of the mouse retina

MicroRNAs (miRNAs) are small, highly conserved molecules that have been shown to regulate the expression of genes by binding to specific target mRNAs. Dicer, an RNase III endonuclease, is essential for the production and function of mature miRNAs, and removal of Dicer has been shown to disrupt many developmental processes. In this study, Dicer was removed specifically from the retina using a floxed Dicer conditional allele and the retinal Chx10Cre transgene. Retinal Dicer knock-out mice displayed a reproducible inability to respond to light. In addition, morphological defects were observed with the formation of photoreceptor rosettes at postnatal day 16, which progressed to more general cellular disorganization and widespread degeneration of retinal cell types as the animals aged. This was accompanied by concomitant decrease in both scotopic and photopic electroretinogram (ERG) responses. Interestingly, removing a single allele of Dicer resulted in ERG deficits throughout life but not to morphological abnormalities. Northern blot analysis of Dicer-depleted retinas showed a decrease in several miRNAs. The observation that progressive retinal degeneration occurred after removal of Dicer raises the possibility that miRNAs are involved in retinal neurodegenerative disorders.

Dicer1 is required for differentiation of the mouse male germline

MicroRNAs (miRNAs) are small noncoding RNAs that posttranscriptionally regulate gene expression. Hundreds of miRNAs are expressed in mammals; however, their functions are just starting to be uncovered. MicroRNAs are processed from a long hairpin mRNA transcript, down to a approximately 23-nucleotide duplex. The enzyme Dicer1 is required for miRNA processing, and mouse knockouts of Dicer1 are embryonic lethal before 7.5 days postcoitus. To examine the function of miRNAs specifically in the germline, we used a mouse model that expresses Cre recombinase from the TNAP locus and a floxed Dicer1 conditional allele. Removal of Dicer1 from germ cells resulted in male infertility. Germ cells were present in adult testes, but few tubules contained elongating spermatids. Germ cells that did differentiate to elongating spermatids exhibited abnormal morphology and motility. Rarely, sperm lacking Dicer1 could fertilize wild-type eggs to generate viable offspring. These results show that Dicer1 and miRNAs are essential for proper differentiation of the male germline.