Researchers Grow Retinal Ganglion Cells in the Laboratory


Researchers from laboratory of Donald Zack at The Johns Hopkins University in Baltimore, Maryland have used genome editing methods to efficiently differentiate human pluripotent stem cells into retinal ganglion cells. Retinal ganglion cells are found in the retina that and helps transmit visual signals from the eye to the brain. Abnormalities or death of ganglion cells can cause vision loss, and conditions such as glaucoma and multiple sclerosis can wreak havoc on ganglion cells.

“Our work could lead not only to a better understanding of the biology of the optic nerve, but also to a cell-based human model that could be used to discover drugs that stop or treat blinding conditions,” said Zack, who is the Guerrieri Family Professor of Ophthalmology at the Johns Hopkins University School of Medicine. “And, eventually it could lead to the development of cell transplant therapies that restore vision in patients with glaucoma and MS.”

Published in the journal Scientific Reports, Zack and his team genetically modified a line of human embryonic stem cells so that they would fluoresce once they differentiated into retinal ganglion cells. Then they used these cells to develop new differentiation methods and characterize the resulting cells.

To genetically modify their cells, Zack and others used the CRISPR-Cas9 system. CRISPR stands for “clustered regularly interspaced short palindromic repeats” and these are short segments DNA, which are found in bacteria, contain short repeated sequences. Following each repeated sequence is a short spacer that usually comes from previous exposures to a bacterial virus or plasmid. Bacteria use the CRISPR/Cas system as a kind of immune system that prevents cells from being invaded by foreign DNA. CRISPRs are found in approximately 40% of sequenced bacterial genomes and 90% of sequenced archaeal genomes.

When bacteria are invaded by a virus, the particular Cas nucleases capture the viral DNA, cut it and insert it into the CRISPR array. When the bacterial cell is infected by a virus, an RNA is transcribed from the CRISPR array called the crRNA. This crRNA then hybridizes with the invading DNA or RNA and the double-stranded RNA or DNA/RNA hybrid is degraded by Cas proteins.

The CRISPR/Cas system is a useful laboratory tool for gene editing or adding, disrupting or changing the sequences of particular genes. If Cas9 and the appropriate crRNA are delivered into cells, you can cut a genome almost anywhere. CRISPR has a huge number of potential applications.

Zack and his group used the CRISPR/Cas system to insert a fluorescent protein gene into the DNA of their stem cells line. This red fluorescent protein would be expressed if a gene called BRN3B (POU4F2) was also expressed. BRN3B is expressed by mature retinal ganglion cells. Therefore, once these cells differentiated into retinal ganglion cells, they would glow red when viewed with a fluorescence microscope.

After differentiating their cells, Zack and his coworkers used a technique called fluorescence-activated cell sorting to isolate fully differentiated cells from other cells. The pure cell culture contained cells that displayed the biological and physical properties observed in retinal ganglion cells produced naturally, according to Zack.

As an added bonus, Valentin Sluch, a former graduate student in Zack’s laboratory, and her colleagues discovered that soaking the pluripotent stem cells in a chemical called “forskolin” at the commencement of the differentiation protocol significantly improved the efficiency of differentiation. Forskolin is a labdane diterpene found in the roots of the Indian Coleus plant (Coleus forskohlii), which belongs to the mint family.  It is used by some people as a weight loss supplement by some people.

“By the 30th day of culture, there were obvious clumps of fluorescent cells visible under the microscope,” said Sluch, who is now a postdoctoral scholar working at Novartis. Sluch continued, “I was very excited when it first worked. I just jumped up from the microscope and ran [to get a colleague]. It seems we can now isolate the cells and study them in a pure culture, which is something that wasn’t possible before.”

“We really see this as just the beginning,” adds Zack. In follow-up studies using CRISPR, his lab is looking to find other genes that are important for ganglion cell survival and function. “We hope that these cells can eventually lead to new treatments for glaucoma and other forms of optic nerve disease.”

To use these cells to develop new treatments for Multiple Sclerosis, Zack is collaborating with Dr. Peter Calabresi, professor of neurology and director of the Johns Hopkins Multiple Sclerosis Center.

Growing Skeletal Muscle in the Laboratory


Skeletal muscle – that type of voluntary muscle that allows movement – has proven difficult to grow in the laboratory. While particular cells can be differentiated into skeletal muscle cells, forming a coherent, structurally sound skeletal muscle is a tough nut to crack from a research perspective.

Another problem dogging muscle research is the difficulty growing new muscle in patients with muscle diseases such as muscular dystrophy or other types of disorders that weaken and degrade skeletal muscle.

Now research groups at the Boston Children’s Hospital Stem Cell Program have reported that they can boost the muscle mass and even reverse the disease of mice that suffer from a type of murine muscular dystrophy. To do this, this group use a combination of three different compounds that were identified in a rapid culture system.

This ingenious rapid culture system uses the cells of zebrafish (Danio rerio) embryos to screen for these muscle-inducing compounds. These single cells are placed into the well of a 96-well plate, and then treated with various compounds to determine if those chemical induce the muscle formation. To facilitate this process, the zebrafish embryo cells express a very special marker that consists of the myosin light polypeptide 2 gene fused to a red-colored protein called “cherry.” When cells become muscle, they express the myosin light polypeptide 2 gene at high levels. Therefore, any embryo cell that differentiates into muscle should glow a red color.

(A) myf5-GFP;mylz2-mCherry double-transgenic expression recapitulates expression of the endogenous genes. myf5-GFP is first detected at the 11-somite stage. mylz2-mCherry expression is not observed until 32 hpf. Scale bars represent 200 mm. (B) myf5-GFP;mylz2-mCherry embryos were dissociated at the oblong stage and cultured in zESC medium. Images were taken 48 hr after plating. Scale bars represent 250 mm.
(A) myf5-GFP;mylz2-mCherry double-transgenic expression recapitulates expression of the endogenous genes. myf5-GFP is first detected at the 11-somite
stage. mylz2-mCherry expression is not observed until 32 hpf. Scale bars represent 200 mm.
(B) myf5-GFP;mylz2-mCherry embryos were dissociated at the oblong stage and cultured in zESC medium. Images were taken 48 hr after plating. Scale bars
represent 250 mm.

Once a cocktail of muscle-inducing chemicals were identified in this assay, those same chemicals were used to treat induced pluripotent stem cells made from cells taken from patients with muscular dystrophy.  Those iPSCs were treated with the combination of chemicals identified in the zebrafish embryo screen as muscle inducing agents.

Zebrafish embryo culture system

The results were outstanding.  Leonard Zon from the Division of Hematology/Oncology, Children’s Hospital Boston and Dana-Farber Cancer Institute and his colleagues showed that a combination of basic Fibroblast Growth Factor, an  adenylyl cyclase activator called forskolin, and the GSK3β inhibitor BIO induced skeletal muscle differentiation in human induced pluripotent stem cells (iPSCs).  Furthermore, these muscle cells produced engraftable myogenic progenitors that contributed to muscle repair when implanted into mice with a rodent form of muscular dystrophy.

Representative hematoxylin and eosin staining (H&E) images and immunostaining on TA sections of preinjured NSG mice injected with 1 3 105 iPSCs at day 14 of differentiation. Muscles injected with BJ, 00409, or 05400 iPSC-derived cells stain positively for human d-Sarcoglycan protein (red). Fibers were counterstained with Laminin (green). No staining is observed in PBS-injected mice or when 00409 fibroblast cells were transplanted. Because the area of human cell engraftment could not be specifically distinguished on H&E stained sections, which must be processed differently from sections for immunostaining, the H&E images shown do not represent the same muscle region as that shown in immunofluorescence images. Scale bars represent 100 mm, n = 3 per sample.
Representative hematoxylin and eosin staining
(H&E) images and immunostaining on TA sections
of preinjured NSG mice injected with 1 3 105
iPSCs at day 14 of differentiation. Muscles injected
with BJ, 00409, or 05400 iPSC-derived cells
stain positively for human d-Sarcoglycan protein
(red). Fibers were counterstained with Laminin
(green). No staining is observed in PBS-injected
mice or when 00409 fibroblast cells were transplanted.
Because the area of human cell engraftment
could not be specifically distinguished on
H&E stained sections, which must be processed
differently from sections for immunostaining, the
H&E images shown do not represent the same
muscle region as that shown in immunofluorescence
images. Scale bars represent 100 mm, n = 3
per sample.

Zon hopes that clinical trials can being soon in order to translate these remarkable results into patients with muscle loss within the next several years.  Zon and his co-workers are also screening compounds to address other types of disorders beyond muscular dystrophy.

This paper represents the application of shear and utter genius.  However, there is one caveat.  The mice into which the muscles were injected were immunodeficient mice whose immune systems are unable to reject transplanted tissues.  In human patients with muscular dystrophy, an immune response against dystrophin, the defective protein, has been an enduring problem (for a review of this, see T. Okada and S. Takeda, Pharmaceuticals (Basel). 2013 Jun 27;6(7):813-836).  While there have been some technological developments that might circumvent this problem, transplanting large quantities of muscle cells might be beyond the pale.  Muscular dystrophy results from disruption of an important junction between the muscle and substratum to which the muscle is secured.  This connection is mediated by the “dystrophin-glycoprotein complex.”  Structural disruptions of this complex (shown below) lead to unanchored muscle that cannot contract properly, and eventually atrophies and degrades.

Dystrophin-glycoprotein complex. Molecular structure of the dystrophin-glycoprotein complex and related proteins superimposed on the sarcolemma and subsarcolemmal actin network (redrawn from Yoshida et al. [5], with modifications). cc, coiled-coil motif on dystrophin (Dys) and dystrobrevin (DB); SGC, sarcoglycan complex;SSPN, sarcospan; Syn, syntrophin; Cav3, caveolin-3; N and C, the N and C termini, respectively; G, G-domain of laminin; asterisk indicates the actin-binding site on the dystrophin rod domain; WW, WW domain.
Dystrophin-glycoprotein complex. Molecular structure of the dystrophin-glycoprotein complex and related proteins superimposed on the sarcolemma and subsarcolemmal actin network (redrawn from Yoshida et al. [5], with modifications). cc, coiled-coil motif on dystrophin (Dys) and dystrobrevin (DB); SGC, sarcoglycan complex;SSPN, sarcospan; Syn, syntrophin; Cav3, caveolin-3; N and C, the N and C termini, respectively; G, G-domain of laminin; asterisk indicates the actin-binding site on the dystrophin rod domain; WW, WW domain.
This is a remarkable advance, but until the host immune response issue is satisfactorily addressed, it will remain a problem.

Making Induced Pluripotent Stem Cells With Small Molecules


A Journal article in the August 9th edition of Science Magazine features work from the laboratories of Yang Zhao and Hongkui Deng, both of whom are from the College of Life Sciences and Peking-Tsinghua Center for Life Sciences at Peking University in Beijing, China. Zhao and Deng and colleagues used small molecules to transform adult cells into induced pluripotent stem cells.

To review, induced pluripotent stem cells are derived from adult cells by genetically engineering the adult cells to express a cocktail of four genes (OCT4, Klf4, Sox2, and c-Myc). To introduce these genes into cells, viruses are normally used, but other techniques are also available. The resultant cells look and act like embryonic stem cells, but they do not require the death of embryos.

In this paper, Deng and colleagues took mouse embryonic fibroblasts (skin cells cultured from mouse embryos) and used them to screen over 10,000 small molecules for their ability to substitute for the OCT4 gene in the production of iPSCs. If this sounds labor intensive, that’s because it is. To conduct the screen, they used mouse embryonic fibroblasts that were infected with viruses that expressed Sox2, Klf4, and c-Myc. These genes are not enough to convert adult cells into iPSCs. However, with these chemicals, these three genes could produce iPSCs from mouse embryonic fibroblasts (MEFs). They identified at least three molecules; Forskolin, 2-methyl-5-hydroxytryptamine and a synthetic molecule called D4476, that could substitute for OCT4.

Thus, by using chemicals, they could get away from using one of the genes required to de-differentiate adult cells into iPSCs. Could they whittle down the number of genes even further? Previously, Deng and Zhao published a paper in which a chemical cocktail was used to substitute for the other three genes so that conversion into iPSCs was achieved by introducing only the OCT4 gene into cells (Li, YQ et al., CELL RESEARCH 21(1): 196-204. They called this cocktail “VC6T.” Therefore, they used VC6T and Forskolin, on their MEFs and the beginnings of de-differentiation occurred, but not much else.

Could chemicals be identified that would take the cells the rest of the way to iPSCs? Another chemical screen examined this possibility. In this test, the MEFs were rigged so that they expressed OCT4 when the cells were treated with the antibiotic doxycycline. By giving the cells doxycycline for 4-8 days, and then testing chemicals to take the cells the rest of the way, they identified a slew of compounds that, when given to the OCT4-expressing MEFs, they became iPSCs.

Then came the real test – make iPSCs with just chemicals and no introduced genes. Could it be done? When they gave the MEFs some of the chemicals identified in the last screen (they called it DZNep), plus VC6T, the expression of OCT4 went up, but the cells simply did not look like iPSCs. So, they changed the culture medium to a “2i” culture system that inhibits some key regulatory proteins in the cells. When they used this same chemical cocktail in a 2i culture system, it worked and iPSCs were produced. Deng and Zhao called these stem cells “chemically induced pluripotent stem cells” or CiPSCs.

(A and B) Numbers of iPSC colonies induced from MEFs infected by SKM (A) or SK (B) plus chemicals or Oct4. Error bars, mean ± SD (n = 3 biological repeat wells). (C) Morphology of MEFs for chemical reprogramming on day 0 (D0) and a GFP-positive cluster generated using VC6TF on day 20 (D20) after chemical treatment. (D) Numbers of GFP-positive colonies induced after DZNep treatment on day 36. Error bars, mean ± SD (n = 2 biological repeat wells). (E to G) Morphology of a compact, epithelioid, GFP-positive colony on day 32 (D32) after treatment (E), a primary CiPSC colony on day 40 (D40) after treatment (F), and passaged CiPSC colonies (G). (H) Schematic diagram illustrating the process of CiPSC generation. Scale bars, 100 μm. For (D), cells for reprogramming were replated on day 12.
(A and B) Numbers of iPSC colonies induced from MEFs infected by SKM (A) or SK (B) plus chemicals or Oct4. Error bars, mean ± SD (n = 3 biological repeat wells). (C) Morphology of MEFs for chemical reprogramming on day 0 (D0) and a GFP-positive cluster generated using VC6TF on day 20 (D20) after chemical treatment. (D) Numbers of GFP-positive colonies induced after DZNep treatment on day 36. Error bars, mean ± SD (n = 2 biological repeat wells). (E to G) Morphology of a compact, epithelioid, GFP-positive colony on day 32 (D32) after treatment (E), a primary CiPSC colony on day 40 (D40) after treatment (F), and passaged CiPSC colonies (G). (H) Schematic diagram illustrating the process of CiPSC generation. Scale bars, 100 μm. For (D), cells for reprogramming were replated on day 12.

Next, they optimized the dosages of these chemicals in order to increase the efficiency of iPSC production. They were able to increase the efficiency of iPSC production to 5% (1 of every 20 colonies of cells), which is respectable. They also identified yet another small molecule that beefed up iPSC production by another 40-fold. Also, this chemical cocktail was able to make iPSCs from mouse adult fibroblasts, fat-derived stem cells, and fibroblasts from newly born mice.

When the CiPSC lines were characterized, they made all the right genes to be designated as pluripotent stem cells, and they had normal numbers of normal-looking chromosomes all the way through 13 passages.

When injected into mice with dysfunctional immune systems, the CiPSCs made tumors that were mixtures of tissues of all over the body. When they were transferred into early mouse embryos, they could contribute to the bodies of developing mice, and they could even contribute to the production of eggs and sperm, When baby mice were completely made from CiPSCs, those mice were fertile and had babies of their own. This is the ultimate test of pluripotency and the CiPSCs passed it with flying colors.

A) Hematoxylin and eosin staining of CiPSC-derived teratoma (clone CiPS-30). (B to D) Chimeric mice (B, clone CiPS-34), germline contribution of CiPSCs in testis, (C, clone CiPS-45) and F2 offspring (D, clone CiPS-34). Scale bars, 100 μm. (E) Genomic PCR analyzing pOct4-GFP cassettes in the tissues of chimeras. (F) Survival curves of chimeras. n, total numbers of chimeras studied.
A) Hematoxylin and eosin staining of CiPSC-derived teratoma (clone CiPS-30). (B to D) Chimeric mice (B, clone CiPS-34), germline contribution of CiPSCs in testis, (C, clone CiPS-45) and F2 offspring (D, clone CiPS-34). Scale bars, 100 μm. (E) Genomic PCR analyzing pOct4-GFP cassettes in the tissues of chimeras. (F) Survival curves of chimeras. n, total numbers of chimeras studied.

Other experiments in this paper examined why these chemicals induced pluripotency in adult cells, but these experiments, though interesting, are lost in the fact that this research group has generated iPSCs without using any viruses, or genetic engineering technology. These CiPSCs are true pluripotent stem cells and they were generated without killing any embryos or introducing genes that might drive cells to become abnormal.

If this can be replicated with human cells, it would be earth-shattering for regenerative medicine.