The Cre/loxP system and gene targeting in the kidney

Examining a punishment-related brain circuit with miniature fluorescence microscopes and deep learning

In humans experiencing substance use disorder (SUD), abstinence from drug use is often motivated by a desire to avoid some undesirable consequence of further use: health effects, legal ramifications, etc. This process can be experimentally modeled in rodents by training and subsequently punishing an operant response in a context-induced reinstatement procedure. Understanding the biobehavioral mechanisms underlying punishment learning is critical to understanding both abstinence and relapse in individuals with SUD. To date, most investigations into the neural mechanisms of context-induced reinstatement following punishment have utilized discrete loss-of-function manipulations that do not capture ongoing changes in neural circuitry related to punishment-induced behavior change. Here, we describe a two-pronged approach to analyzing the biobehavioral mechanisms of punishment learning using miniature fluorescence microscopes and deep learning algorithms. We review recent advancements in both techniques and consider a target neural circuit.

Generation of floxed Spag6l mice and disruption of the gene by crossing to a Hprt-Cre line

Mouse sperm-associated antigen 6 like (SPAG6L) is an axoneme central apparatus protein, essential for the normal function of the ependymal cell and lung cilia, and sperm flagella. Accumulated evidence has disclosed multiple biological functions of SPAG6L, including ciliary/flagellar biogenesis and polarization, neurogenesis, and neuronal migration. Conventional Spag6l knockout mice died of hydrocephalus, which impedes further investigation of the function of the gene in vivo. To overcome the limitation of the short lifespan of conventional knockout mice, we developed a conditional allele by inserting two loxP sites in the genome flanking exon 3 of the Spag6l gene. By crossing the floxed Spag6l mice to a Hrpt-Cre line which expresses Cre recombinase ubiquitously in vivo, mutant mice that are missing SPAG6L globally were obtained. Homozygous mutant Spag6l mice showed normal appearance within the first week after birth, but reduced body size was observed after 1 week, and all developed hydrocephalus and died within 4 weeks of age. The phenotype mirrored that of the conventional Spag6l knockout mice. The newly established floxed Spag6l model provides a powerful tool to further investigate the role of the Spag6l gene in individual cell types and tissues.

Generation of Conditional KO Mice of CCN2 and Its Function in the Kidney

CCN2 has been shown to be closely involved in the progression of renal fibrosis, indicating the potential of CCN2 inhibition as a therapeutic target. Although the examination of the renal disease phenotypes of adult CCN2 knockout mice has yielded valuable scientific insights, perinatal death has limited studies of CCN2 in vivo. Conditional knockout technology has become widely used to delete genes in the target cell populations or time points using cell-specific Cre recombinase-expressing mice. Therefore, several lines of CCN2-floxed mice have been developed to assess the functional role of CCN2 in adult mice.CCN2 levels are elevated in renal fibrosis and proliferative glomerulonephritis, making them suitable disease models for assessing the effects of CCN2 deletion on the kidney. Renal fibrosis is characterized by glomerulosclerosis and tubulointerstitial fibrosis and transforming growth factor-β. CCN2 is increased in fibrosis and modulates a number of downstream signaling pathways involved in the fibrogenic properties of TGF-β. Unilateral ureteral obstruction is one of the most widely used models of renal tubulointerstitial fibrosis. In addition, anti-glomerular basement membrane antibody glomerulonephritis has become the most widely used model for evaluating the effect of increased renal CCN2 expression. Herein, we describe the construction of CCN2-floxed mice and inducible systemic CCN2 conditional knockout mice and methods for the operation of unilateral ureteral obstruction and the induction of anti-glomerular basement membrane antibody glomerulonephritis.

CRISPR applications in ophthalmologic genome surgery

PURPOSE OF REVIEW

The present review seeks to summarize and discuss the application of clustered regularly interspaced short palindromic repeats (CRISPR)-associated systems (Cas) for genome editing, also called genome surgery, in the field of ophthalmology.

RECENT FINDINGS

Precision medicine is an emerging approach for disease treatment and prevention that takes into account the variability of an individual's genetic sequence. Various groups have used CRISPR-Cas genome editing to make significant progress in mammalian preclinical models of eye disease, the basic science of eye development in zebrafish, the in vivo modification of ocular tissue, and the correction of stem cells with therapeutic applications. In addition, investigators have creatively used the targeted mutagenic potential of CRISPR-Cas systems to target pathogenic alleles in vitro.

SUMMARY

Over the past year, CRISPR-Cas genome editing has been used to correct pathogenic mutations in vivo and in transplantable stem cells. Although off-target mutagenesis remains a concern, improvement in CRISPR-Cas technology and careful screening for undesired mutations will likely lead to clinical eye therapeutics employing CRISPR-Cas systems in the near future.

Aquaporin 2-labeled cells differentiate to intercalated cells in response to potassium depletion

The mammalian renal collecting duct consists of principal cells (PCs) and intercalated cells (ICs). Both PCs and ICs are involved in potassium (K(+)) homeostasis, PCs through their role in K(+) secretion and ICs through their ability to facilitate K(+) resorption. We previously hypothesized that PCs may differentiate into ICs upon K(+) depletion. However, no direct evidence has yet been obtained to conclusively demonstrate that PCs differentiate into ICs in response to K(+) depletion. Here, we present direct evidence for the differentiation of PCs into ICs by cell lineage tracing using aquaporin 2 (AQP2)-Cre mice and R26R-EYFP transgenic mice. In control mice, AQP2-EYFP(+) cells exhibited mainly a PC phenotype (AQP2-positive/H(+)-ATPase-negative). Interestingly, some AQP2-EYFP(+) cells exhibited an IC phenotype (H(+)-ATPase-positive/AQP2-negative); these cells accounted for 1.7 %. After K(+) depletion, the proportion of AQP2-EYFP(+) cells with an IC phenotype was increased to 4.1 %. Furthermore, some AQP2-EYFP(+) cells exhibited a "null cell" phenotype (AQP2-negative/H(+)-ATPase-negative) after K(+) depletion. Collectively, our data demonstrate that AQP2-labeled cells can differentiate into ICs, as well as null cells, in response to K(+) depletion. This finding indicates that some of AQP2-labeled cells possess properties of progenitor cells and that they can differentiate into ICs in the adult mouse kidney.

Renal collecting duct NOS1 maintains fluid-electrolyte homeostasis and blood pressure

Nitric oxide is a pronatriuretic and prodiuretic factor. The highest renal NO synthase (NOS) activity is found in the inner medullary collecting duct. The collecting duct (CD) is the site of daily fine-tune regulation of sodium balance, and led us to hypothesize that a CD-specific deletion of NOS1 would result in an impaired ability to excrete a sodium load leading to a salt-sensitive blood pressure phenotype. We bred AQP2-CRE mice with NOS1 floxed mice to produce flox control and CD-specific NOS1 knockout (CDNOS1KO) littermates. CDs from CDNOS1KO mice produced 75% less nitrite, and urinary nitrite+nitrate (NOx) excretion was significantly blunted in the knockout genotype. When challenged with high dietary sodium, CDNOS1KO mice showed significantly reduced urine output, sodium, chloride, and NOx excretion, and increased mean arterial pressure relative to flox control mice. In humans, urinary NOx is a newly identified biomarker for the progression of hypertension. These findings reveal that NOS1 in the CD is critical in the regulation of fluid-electrolyte balance, and this new genetic model of CD NOS1 gene deletion will be a valuable tool to study salt-dependent blood pressure mechanisms.

beta1 integrin is necessary for ureteric bud branching morphogenesis and maintenance of collecting duct structural integrity

The kidney collecting system develops from branching morphogenesis of the ureteric bud (UB). This process requires signaling by growth factors such as glial cell line derived neurotrophic factor (GDNF) and fibroblast growth factors (FGFs) as well as cell extracellular matrix interactions mediated by integrins. The importance of integrin signaling in UB development was investigated by deleting integrin beta1 at initiation (E10.5) and late (E18.5) stages of development. Deletion at E10.5 resulted in a severe branching morphogenesis phenotype. Deletion at E18.5 did not alter renal development but predisposed the collecting system to severe injury following ureteric obstruction. beta1 integrin was required for renal tubular epithelial cells to mediate GDNF- and FGF-dependent signaling despite normal receptor localization and activation in vitro. Aberrations in the same signaling molecules were present in the beta1-null UBs in vivo. Thus beta1 integrins can regulate organ branching morphogenesis during development by mediating growth-factor-dependent signaling in addition to their well-defined role as adhesion receptors.

Intraarticular induction of interleukin-1beta expression in the adult mouse, with resultant temporomandibular joint pathologic changes, dysfunction, and pain

OBJECTIVE

To examine the effects of intraarticular induction of interleukin-1beta (IL-1beta) expression in adult mice.

METHODS

We used somatic mosaic analysis in a novel transgenic mouse with an inducible IL-1beta transcription unit. Transgene activation was induced by Cre recombinase in the temporomandibular joints (TMJs) of adult transgenic mice (conditional knockin model). The effects of intraarticular IL-1beta induction were subsequently evaluated at the cellular, histopathologic, and behavioral levels.

RESULTS

We developed transgenic mice capable of germline transmission of a dormant transcription unit consisting of the mature form of human IL-1beta as well as the reporter gene beta-galactosidase driven by the rat procollagen 1A1 promoter. Transgene activation by a feline immunodeficiency virus Cre vector resulted in histopathologic changes, including articular surface fibrillations, cartilage remodeling, and chondrocyte cloning. We also demonstrated up-regulation of genes implicated in arthritis (cyclooxygenase 2, IL-6, matrix metalloproteinase 9). There was a lack of inflammatory cells in these joints. Behavioral changes, including increased orofacial grooming and decreased resistance to mouth opening, were used as measures of nociception and joint dysfunction, respectively. The significant increase in expression of the pain-related neurotransmitter calcitonin gene-related peptide (CGRP) in the sensory ganglia as well as the auxiliary protein CGRP receptor component protein of the calcitonin-like receptor in the brainstem further substantiated the induction of pain.

CONCLUSION

Induction of IL-1beta expression in the TMJs of adult mice led to pathologic development, dysfunction, and related pain in the joints. The somatic mosaic model presented herein may prove useful in the preclinical evaluation of existing and new treatments for the management of joint pathologic changes and pain, such as in osteoarthritis.

UT-A urea transporter promoter, UT-Aalpha, targets principal cells of the renal inner medullary collecting duct

The urea transporters, UT-A1 and UT-A3, two members of the UT-A gene family, are localized to the terminal portion of the inner medullary collecting duct (IMCD). In this manuscript, we demonstrate that 4.2 kb of the 5'-flanking region of the UT-A gene (UT-Aalpha promoter) is sufficient to drive the IMCD-specific expression of a heterologous reporter gene, beta-galactosidase (beta-Gal), in transgenic mice. RT-PCR, immunoblotting, and immunohistochemistry demonstrate that within the kidney, transgene expression is confined to the terminal portion of the IMCD. Colocalization studies with aquaporin-2 show that expression is localized to the principal cells of the IMCD2 and IMCD3 regions. Utilizing beta-Gal activity assays, we further show that within the kidney, the beta-Gal transgene can be regulated by both water restriction and glucocorticoids, similar to the regulation of the endogenous UT-A gene. These results demonstrate that 4.2 kb of the UT-Aalpha promoter is sufficient to drive expression of a heterologous reporter gene in a tissue-specific and cell-specific fashion in transgenic mice.

Using siRNA technique to generate transgenic animals with spatiotemporal and conditional gene knockdown

Based on the RNAi technique, we have developed a new approach that generates transgenic animals capable of mimicking human genetic diseases. The new system is a combination of siRNA with Cre-loxP and tetracycline-on. It has the characteristics of being stable, inheritable, and inducible, with the siRNA able to be transcribed tissue specifically. To support the ability of this new method to generate a model for a disease, we created an ABCA1-deficient mouse line that mimics Tangier disease under controlled conditions. Thus, it should now be possible to rapidly establish human genetic diseases as a whole animal model without the use of embryonic stem cell and gene targeting. This system also provides a tool for pathological and pharmacological studies of aspects peculiar to particular human genetic diseases.

Untraditional methods for targeting the kidney in transgenic mice

With the completion of the human genome project and the sequencing of many genomes of experimental models, there is a pressing need to determine the physiological relevance of newly identified genes. Gene-targeting approaches have become an important tool in our arsenal to dissect the significance of genes expressed in many tissues. A wealth of experimental models has been made to assess the role of gene expression in renal function and development. The development of new and informative models is presently limited by the anatomic complexity of the kidney and the lack of cell-specific promoters to target the numerous diverse cell types in that organ. Because of this, new approaches may have to be developed. In this review, we will discuss several untraditional methods to target gene expression to the kidney. These approaches should provide some additional tricks and tools to help in developing additional informative models for studying renal physiology.

Genomics and homeostasis

The Cannon lecture this year illustrates how knowledge of DNA sequences of complex living organisms is beginning to shape the landscape of physiology in the 21st century. Enormous challenges and opportunities now exist for physiologists to relate the galaxy of genes to normal and pathological functions. The first extensive genomic systems biology map for cardiovascular and renal function was completed last year as well as a new hypothesis-generating tool ("physiological profiling") that enables us to hypothesize relationships between specific genes responsible for the regulation of regulatory pathways. Techniques of chromosomal substitution (consomic and congenic rats) are beginning to confirm statistical results from linkage analysis studies, narrow the regions of genetic interest for positional cloning, and provide genetically well-defined control strains for physiological studies. Patterns of gene expression identified by microarray and mapping of expressed genes to chromosomal sites are adding to the understanding of systems physiology. The previously unimaginable goal of connecting approximately 36,000 genes to the complex functions of mammalian systems is indeed well underway.

Differential, inducible gene targeting in renal epithelia, vascular endothelium, and viscera of Mx1Cre mice

The Cre/loxP transgenic system may be used to achieve temporally and/or spatially regulated gene deletion. The Mx1Cre mouse expresses Cre recombinase under control of the IFN-inducible Mx1 promoter. Mx1Cre mice were crossed with a reporter strain (ROSA26tm1Sor) in which beta-galactosidase activity is expressed only after Cre-mediated recombination to determine the cellular pattern of Cre-mediated genetic recombination in the kidney and other tissues. Widespread recombination was observed in vascular endothelium as well as in the liver and spleen. Recombination was restricted to subsets of stromal cells in uterus, duodenum, colon, aorta, and kidney. In the cortex, chi-galactosidase activity was detected in a subset of tubules and all glomerular cells, including endothelium, mesangium, and podocytes. No chi-galactosidase activity was detected in proximal tubules. Costaining of kidneys with segment-specific markers demonstrated induction of chi-galactosidase activity in collecting duct, with sporadic labeling of the thick ascending limb but no significant labeling of distal convoluted tubules. We conclude that Mx1-driven gene recombination is spatially as well as temporally restricted. The Mx1Cre transgene should prove a useful reagent to achieve temporally regulated recombination in endothelial, glomerular, and distal renal epithelia in mice.

Manipulating the mammalian genome by homologous recombination

Gene targeting in mammalian cells has proven invaluable in biotechnology, in studies of gene structure and function, and in understanding chromosome dynamics. It also offers a potential tool for gene-therapeutic applications. Two limitations constrain the current technology: the low rate of homologous recombination in mammalian cells and the high rate of random (nontargeted) integration of the vector DNA. Here we consider possible ways to overcome these limitations within the framework of our present understanding of recombination mechanisms and machinery. Several studies suggest that transient alteration of the levels of recombination proteins, by overexpression or interference with expression, may be able to increase homologous recombination or decrease random integration, and we present a list of candidate genes. We consider potentially beneficial modifications to the vector DNA and discuss the effects of methods of DNA delivery on targeting efficiency. Finally, we present work showing that gene-specific DNA damage can stimulate local homologous recombination, and we discuss recent results with two general methodologies--chimeric nucleases and triplex-forming oligonucleotides--for stimulating recombination in cells.