miRNAs as Biomarkers in Diabetes: Moving towards Precision Medicine

Diabetes mellitus (DM) is a complex metabolic disease with many specifically related complications. Early diagnosis of this disease could prevent the progression to overt disease and its related complications. There are several limitations to using existing biomarkers, and between 24% and 62% of people with diabetes remain undiagnosed and untreated, suggesting a large gap in current diagnostic practices. Early detection of the percentage of insulin-producing cells preceding loss of function would allow for effective therapeutic interventions that could delay or slow down the onset of diabetes. MicroRNAs (miRNAs) could be used for early diagnosis, as well as for following the progression and the severity of the disease, due to the fact of their pancreatic specific expression and stability in various body fluids. Thus, many studies have focused on the identification and validation of such groups or "signatures of miRNAs" that may prove useful in diagnosing or treating patients. Here, we summarize the findings on miRNAs as biomarkers in diabetes and those associated with direct cellular reprogramming strategies, as well as the relevance of miRNAs that act as a bidirectional switch for cell therapy of damaged pancreatic tissue and the studies that have measured and tracked miRNAs as biomarkers in insulin resistance are addressed.

The role of DNA demethylation in liver to pancreas transdifferentiation

Background

Insulin producing cells generated by liver cell transdifferentiation, could serve as an attractive source for regenerative medicine. The present study assesses the relationship between DNA methylation pTFs induced liver to pancreas transdifferentiation.

Results

The transdifferentiation process is associated with DNA demethylation, mainly at gene regulatory sites, and with increased expression of these genes. Active inhibition of DNA methylation promotes the pancreatic transcription factor-induced transdifferentiation process, supporting a causal role for DNA demethylation in this process.

Conclusions

Transdifferentiation is associated with global DNA hypomethylation, and with increased expression of specific demethylated genes. A combination of epigenetic modulators may be used to increase chromatin accessibility of the pancreatic transcription factors, thus promoting the efficiency of the developmental process.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03159-6.

Phenotypic assessment of liver-derived cell cultures during in vitro expansion

Background: Liver cells represent an attractive source of cells for autologous regenerative medicine. The present study assesses the liver cells' stability during in vitro expansion, as a prerequisite for therapeutic use. Results: The human liver cell cultures in this study were propagated efficiently in vitro for at least 12 passages. No significant changes in morphology, intracellular ultrastructures and characteristic markers expression were found during in vitro expansion of cells from all analyzed donors. However, expanded cells derived from male donors of >60 years old, lost the Y chromosome. Conclusion: Liver-derived cell cultures adopt a proliferative, stable mesenchymal phenotype, through an epithelial to mesenchymal transition process. The molecular and phenotypic changes of the cells during propagation are uniform, despite the heterogeneity of the different donors. Loss of Y chromosome occurs after cells' propagation in elder male donors.

The effect of liver donors' age, gender and metabolic state on pancreatic lineage activation

Autologous cells replacement therapy by liver to pancreas transdifferentiation (TD) allows diabetic patients to be also the donors of their own therapeutic tissue. Aim: To analyze whether the efficiency of the process is affected by liver donors' heterogeneity with regard to age, gender and the metabolic state. Materials & methods: TD of liver cells derived from nondiabetic and diabetic donors at different ages was characterized at molecular and cellular levels, in vitro. Results: Neither liver cells proliferation nor the propagated cells TD efficiency directly correlate with the age (3-60 years), gender or the metabolic state of the donors. Conclusion: Human liver cells derived from a wide array of ages and metabolic states can be used for autologous cells therapies for diabetics.

Liver to Pancreas Transdifferentiation

PURPOSE OF THE REVIEW

Here, we review recent findings in the field of generating insulin-producing cells by pancreatic transcription factor (pTF)-induced liver transdifferentiation (TD). TD is the direct conversion of functional cell types from one lineage to another without passing through an intermediate stage of pluripotency. We address potential reasons for the restricted efficiency of TD and suggest modalities to overcome these challenges, to bring TD closer to its clinical implementation in autologous cell replacement therapy for insulin-dependent diabetes.

RECENT FINDINGS

Liver to pancreas TD is restricted to cells that are a priori predisposed to undergo the developmental process. In vivo, the predisposition of liver cells is affected by liver zonation and hepatic regeneration. The TD propensity of liver cells is related to permissive epigenome which could be extended to TD-resistant cells by specific soluble factors. An obligatory role for active Wnt signaling in continuously maintaining a "permissive" epigenome is suggested. Moreover, the restoration of the pancreatic niche and vasculature promotes the maturation of TD cells along the β cell function. Future studies on liver to pancreas TD should include the maturation of TD cells by 3D culture, the restoration of vasculature and the pancreatic niche, and the extension of TD propensity to TD-resistant cells by epigenetic modifications. Liver to pancreas TD is expected to result in the generation of custom-made "self" surrogate β cells for curing diabetes.

The role of the vasculature niche on insulin-producing cells generated by transdifferentiation of adult human liver cells

Background

Insulin-dependent diabetes is a multifactorial disorder that could be theoretically cured by functional pancreatic islets and insulin-producing cell (IPC) implantation. Regenerative medicine approaches include the potential for growing tissues and organs in the laboratory and transplanting them when the body cannot heal itself. However, several obstacles remain to be overcome in order to bring regenerative medicine approach for diabetes closer to its clinical implementation; the cells generated in vitro are typically of heterogenic and immature nature and the site of implantation should be readily vascularized for the implanted cells to survive in vivo. The present study addresses these two limitations by analyzing the effect of co-implanting IPCs with vasculature promoting cells in an accessible site such as subcutaneous. Secondly, it analyzes the effects of reconstituting the in vivo environment in vitro on the maturation and function of insulin-producing cells.

Methods

IPCs that are generated by the transdifferentiation of human liver cells are exposed to the paracrine effects of endothelial colony-forming cells (ECFCs) and human bone marrow mesenchymal stem cells (MSCs), which are the “building blocks” of the blood vessels. The role of the vasculature on IPC function is analyzed upon subcutaneous implantation in vivo in immune-deficient rodents. The paracrine effects of vasculature on IPC maturation are analyzed in culture.

Results

Co-implantation of MSCs and ECFCs with IPCs led to doubling the survival rates and a threefold increase in insulin production, in vivo. ECFC and MSC co-culture as well as conditioned media of co-cultures resulted in a significant increased expression of pancreatic-specific genes and an increase in glucose-regulated insulin secretion, compared with IPCs alone. Mechanistically, we demonstrate that ECFC and MSC co-culture increases the expression of CTGF and ACTIVINβα, which play a key role in pancreatic differentiation.

Conclusions

Vasculature is an important player in generating regenerative medicine approaches for diabetes. Vasculature displays a paracrine effect on the maturation of insulin-producing cells and their survival upon implantation. The reconstitution of the in vivo niche is expected to promote the liver-to-pancreas transdifferentiation and bringing this cell therapy approach closer to its clinical implementation.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1157-5) contains supplementary material, which is available to authorized users.

The Wnt/β-catenin pathway determines the predisposition and efficiency of liver-to-pancreas reprogramming

Transdifferentiation (TD) is the direct reprogramming of adult cells into cells of alternate fate and function. We have previously shown that liver cells can be transdifferentiated into beta-like, insulin-producing cells through ectopic expression of pancreatic transcription factors (pTFs). However, the efficiency of the process was consistently limited to <15% of the human liver cells treated in culture. The data in the current study suggest that liver-to-pancreas TD is restricted to a specific population of liver cells that is predisposed to undergo reprogramming. We isolated TD-predisposed subpopulation of liver cells from >15 human donors using a lineage tracing system based on the Wnt response element, part of the pericentral-specific promoter of glutamine synthetase. The cells, that were propagated separately, consistently exhibited efficient fate switch and insulin production and secretion in >60% of the cells upon pTF expression. The rest of the cells, which originated from 85% of the culture, resisted TD. Both populations expressed the ectopic pTFs with similar efficiencies, followed by similar repression of hepatic genes. Our data suggest that the TD-predisposed cells originate from a distinct population of liver cells that are enriched for Wnt signaling, which is obligatory for efficient TD. In TD-resistant populations, Wnt induction is insufficient to induce TD. An additional step of chromatin opening enables TD of these cells.

CONCLUSION

Liver-to-pancreas TD occurs in defined predisposed cells. These cells' predisposition is maintained by Wnt signaling that endows the cells with the plasticity needed to alter their transcriptional program and developmental fate when triggered by ectopic pTFs. These results may have clinical implications by drastically increasing the efficacy of TD in future clinical uses. (Hepatology 2018).

Reprogramming of liver cells into insulin-producing cells

Tissue replacement is a promising direction for the treatment of diabetes, which will become widely available only when islets or insulin-producing cells that will not be rejected by the diabetic recipients are available in unlimited amounts. The present review addresses the research in the field of generating functional insulin-producing cells by transdifferentiation of adult liver cells both in vitro and in vivo. It presents recent knowledge of the mechanisms which underlie the process and assesses the challenges which should be addressed for its efficient implementation as a cell based replacement therapy for diabetics.

The temporal and hierarchical control of transcription factors-induced liver to pancreas transdifferentiation

Lineage-specific transcription factors (TFs) display instructive roles in directly reprogramming adult cells into alternate developmental fates, in a process known as transdifferentiation. The present study analyses the hypothesis that despite being fast, transdifferentiation does not occur in one step but is rather a consecutive and hierarchical process. Using ectopic expression of Pdx1 in human liver cells, we demonstrate that while glugacon and somatostatin expression initiates within a day, insulin gene expression becomes evident only 2–3 days later. To both increase transdifferentiation efficiency and analyze whether the process indeed display consecutive and hierarchical characteristics, adult human liver cells were treated by three pancreatic transcription factors, Pdx1, Pax4 and Mafa (3pTFs) that control distinct hierarchical stages of pancreatic development in the embryo. Ectopic expression of the 3pTFs in human liver cells, increased the transdifferentiation yield, manifested by 300% increase in the number of insulin positive cells, compared to each of the ectopic factors alone. However, only when the 3pTFs were sequentially supplemented one day apart from each other in a direct hierarchical manner, the transdifferentiated cells displayed increased mature β-cell-like characteristics. Ectopic expression of Pdx1 followed by Pax4 on the 2^nd day and concluded by Mafa on the 3^rd day resulted in increased yield of transdifferentiation that was associated by increased glucose regulated c-peptide secretion. By contrast, concerted or sequential administration of the ectopic 3pTFs in an indirect hierarchical mode resulted in the generation of insulin and somatostatin co-producing cells and diminished glucose regulated processed insulin secretion. In conclusion transcription factors induced liver to pancreas transdifferentiation is a progressive and hierarchical process. It is reasonable to assume that this characteristic is general to wide ranges of tissues. Therefore, our findings could facilitate the development of cell replacement therapy modalities for many degenerative diseases including diabetes.

Ectopic PDX-1 expression directly reprograms human keratinocytes along pancreatic insulin-producing cells fate

BACKGROUND

Cellular differentiation and lineage commitment have previously been considered irreversible processes. However, recent studies have indicated that differentiated adult cells can be reprogrammed to pluripotency and, in some cases, directly into alternate committed lineages. However, although pluripotent cells can be induced in numerous somatic cell sources, it was thought that inducing alternate committed lineages is primarily only possible in cells of developmentally related tissues. Here, we challenge this view and analyze whether direct adult cell reprogramming to alternate committed lineages can cross the boundaries of distinct developmental germ layers.

METHODOLOGY/PRINCIPAL FINDINGS

We ectopically expressed non-integrating pancreatic differentiation factors in ectoderm-derived human keratinocytes to determine whether these factors could directly induce endoderm-derived pancreatic lineage and β-cell-like function. We found that PDX-1 and to a lesser extent other pancreatic transcription factors, could rapidly and specifically activate pancreatic lineage and β-cell-like functional characteristics in ectoderm-derived human keratinocytes. Human keratinocytes transdifferentiated along the β cell lineage produced processed and secreted insulin in response to elevated glucose concentrations. Using irreversible lineage tracing for KRT-5 promoter activity, we present supporting evidence that insulin-positive cells induced by ectopic PDX-1 expression are generated in ectoderm derived keratinocytes.

CONCLUSIONS/SIGNIFICANCE

These findings constitute the first demonstration of human ectoderm cells to endoderm derived pancreatic cells transdifferentiation. The study represents a proof of concept which suggests that transcription factors induced reprogramming is wider and more general developmental process than initially considered. These results expanded the arsenal of adult cells that can be used as a cell source for generating functional endocrine pancreatic cells. Directly reprogramming somatic cells into alternate desired tissues has important implications in developing patient-specific, regenerative medicine approaches.

NKX6.1 promotes PDX-1-induced liver to pancreatic β-cells reprogramming

Reprogramming adult mammalian cells is an attractive approach for generating cell-based therapies for degenerative diseases, such as diabetes. Adult human liver cells exhibit a high level of developmental plasticity and have been suggested as a potential source of pancreatic progenitor tissue. An instructive role for dominant pancreatic transcription factors in altering the hepatic developmental fate along the pancreatic lineage and function has been demonstrated. Here we analyze whether transcription factors expressed in mature pancreatic β-cells preferentially activate β-cell lineage differentiation in liver. NKX6.1 is a transcription factor uniquely expressed in β-cells of the adult pancreas, its potential role in reprogramming liver cells to pancreatic lineages has never been analyzed. Our results suggest that NKX6.1 activates immature pancreatic markers such as NGN-3 and ISL-1 but not pancreatic hormones gene expression in human liver cells. We hypothesized that its restricted capacity to activate a wide pancreatic repertoire in liver could be related to its incapacity to activate endogenous PDX-1 expression in liver cells. Indeed, the complementation of NKX6.1 by ectopic PDX-1 expression substantially and specifically promoted insulin expression and glucose regulated processed hormone secretion to a higher extent than that of PDX-1 alone, without increasing the reprogrammed cells. This may suggest a potential role for NKX6.1 in promoting PDX-1 reprogrammed cells maturation along the β-cell-like lineage. By contrast, NKX6.1 repressed PDX-1 induced proglucagon gene expression. The individual and concerted effects of pancreatic transcription factors in adult extra-pancreatic cells, is expected to facilitate developing regenerative medicine approaches for cell replacement therapy in diabetics.

Exendin-4 promotes liver cell proliferation and enhances the PDX-1-induced liver to pancreas transdifferentiation process

Over the last few years, evidence has accumulated revealing the unexpected potential of committed mammalian cells to convert to a different phenotype via a process called transdifferentiation or adult cell reprogramming. These findings may have major practical implications because this process may facilitate the generation of functional autologous tissues that can be used for replacing malfunctioning organs. An instructive role for transcription factors in diverting the developmental fate of cells in adult tissues has been demonstrated when adult human liver cells were induced to transdifferentiate to the pancreatic endocrine lineage upon ectopic expression of the pancreatic master regulator PDX-1 (pancreatic and duodenal homeobox gene 1). Since organogenesis and lineage commitment are affected also by developmental signals generated in response to environmental triggers, we have now analyzed whether the hormone GLP-1 (glucogen-like peptide-1) documented to play a role in pancreatic beta cell differentiation, maturation, and survival, can also increase the efficiency of liver to pancreas transdifferentiation. We demonstrate that the GLP-1R agonist, exendin-4, significantly improves the efficiency of PDX-1-mediated transdifferentiation. Exendin-4 affects the transdifferentiation process at two distinct steps; it increases the proliferation of liver cells predisposed to transdifferentiated in response to PDX-1 and promotes the maturation of transdifferentiated cells along the pancreatic lineage. Liver cell reprogramming toward the pancreatic beta cell lineage has been suggested as a strategy for functional replacement of the ablated insulin-producing cells in diabetics. Understanding the cellular and molecular basis of the transdifferentiation process will allow us to increase the efficiency of the reprogramming process and optimize its therapeutic merit.

Pancreatic and duodenal homeobox gene 1 induces hepatic dedifferentiation by suppressing the expression of CCAAT/enhancer-binding protein beta

It is believed that adult tissues in mammals lack the plasticity needed to assume new developmental fates because of the absence of efficient pathways of dedifferentiation. However, the well-documented ability of the transcription factor pancreatic and duodenal homeobox gene 1 (PDX-1) to activate pancreatic lineage development and insulin production following ectopic expression in liver suggests a surprising degree of residual plasticity in adult liver cells. This study seeks a mechanistic explanation for the capacity of PDX-1 to endow liver cells with pancreatic characteristics and function. We demonstrate that PDX-1, previously shown to play an essential role in normal pancreatic organogenesis and pancreatic beta-cell function and to possess the potential to activate multiple pancreatic markers in liver, can also direct hepatic dedifferentiation. PDX-1 represses the adult hepatic repertoire of gene expression and activates the expression of the immature hepatic marker alpha-fetoprotein. We present evidence indicating that PDX-1 triggers hepatic dedifferentiation by repressing the key hepatic transcription factor CCAAT/enhancer-binding protein beta. Hepatic dedifferentiation is necessary though not sufficient for the activation of the mature pancreatic repertoire in liver.

CONCLUSION

Our study suggests a dual role for PDX-1 in liver: inducing hepatic dedifferentiation and activating the pancreatic lineage. The identification of dedifferentiation signals may promote the capacity to endow mature tissues in mammals with the plasticity needed for acquiring novel developmental fates and functions to be implemented in the field of regenerative medicine.

Ectopic PDX-1 expression in liver ameliorates type 1 diabetes

Type 1 diabetes mellitus (T1DM) results from a specific autoimmune mediated destruction of the pancreatic beta-cells. PDX-1 induced developmentally redirected liver cells were suggested to restore the ablated pancreatic function in chemically induced diabetes. However, developmentally redirected liver cells, may have acquired along with the desired beta-cell characteristics and functions, also undesired sensitivity to autoimmune attack and therefore may be inefficient in ameliorating T1DM. This study analyzes whether subjects with beta-cell autoimmunity could benefit from Ad-CMV-PDX-1 gene therapy. Using the model of cyclophosphamide-accelerated diabetes in non-obese diabetic (CAD-NOD) mice, we report that recombinant adenovirus mediated PDX-1 gene therapy, ameliorates hyperglycemia in CAD-NOD mice. Our data demonstrate that 43% of the overtly diabetic CAD-NOD mice treated with Ad-CMV-PDX-1 became normoglycemic and maintained a stable body weight. Ectopic PDX-1 expression induced pancreatic gene expression and insulin production in the mice livers. The amelioration of hyperglycemia, in PDX-1 treated diabetic mice was associated with an immune modulation manifested by Th1 to Th2 shift in the autoimmune T-cell response to antigens associated with NOD diabetes. Thus, liver-to-pancreas transdifferentiation ameliorates T1DM in a process which is associated with a concomitant modulation of the autoimmune attack. Our findings suggest a beneficial therapeutic effect of the PDX-1 gene therapy for treating autoimmune type 1 diabetes mellitus (T1DM).

Regenerative medicine: using liver to generate pancreas for treating diabetes

Recent advances in pancreatic islet transplantation emphasize the potential of this approach for the long-term control of blood glucose levels as treatment of diabetes. To overcome the organ shortage for cell replacement therapy, efforts are being invested in generating new and abundant sources of insulin-producing cells from embryonic or adult stem cells. We review recent evidence documenting the surprising capacity of the mature liver to serve as a potential source of tissue for generating functional endocrine pancreas. The process of liver-to-pancreas developmental redirection is induced by ectopic expression of pancreatic transcription and differentiation factors. This approach may allow the diabetic patient to be the donor of his or her own therapeutic tissue, thus alleviating both the need for allotransplantations and the subsequent immune suppression.

Cell-replacement therapy for diabetes: Generating functional insulin-producing tissue from adult human liver cells

Shortage in tissue availability from cadaver donors and the need for life-long immunosuppression severely restrict the large-scale application of cell-replacement therapy for diabetic patients. This study suggests the potential use of adult human liver as alternate tissue for autologous beta-cell-replacement therapy. By using pancreatic and duodenal homeobox gene 1 (PDX-1) and soluble factors, we induced a comprehensive developmental shift of adult human liver cells into functional insulin-producing cells. PDX-1-treated human liver cells express insulin, store it in defined granules, and secrete the hormone in a glucose-regulated manner. When transplanted under the renal capsule of diabetic, immunodeficient mice, the cells ameliorated hyperglycemia for prolonged periods of time. Inducing developmental redirection of adult liver offers the potential of a cell-replacement therapy for diabetics by allowing the patient to be the donor of his own insulin-producing tissue.

New organs from our own tissues: liver-to-pancreas transdifferentiation

Recent advances in pancreatic islet transplantation emphasize the potential of this approach for the long-term control of blood glucose levels in diabetic patients. However, tissue-replacement therapy will become widely available as a treatment for diabetes only when new sources of islets and insulin-producing cells are found. Here, we review recent evidence that documents the potential of mature liver as a source of tissue for generating a functional endocrine pancreas, by ectopic expression of pancreatic transcription and differentiation factors. When key events in the transconversion process have been identified, using the liver as a source of pancreatic tissue might provide a valuable approach for replacing impaired beta cell function in diabetics.

Functional, persistent, and extended liver to pancreas transdifferentiation

Pancreatic and duodenal homeobox gene-1 (PDX-1) regulates pancreas development during embryogenesis, whereas in the adult it controls beta-cell function. Here we analyze whether PDX-1 functions as a pancreatic differentiation factor and a bona fide master regulator when ectopically expressed in mature fully differentiated liver in vivo. By ectopic and transient PDX-1 expression in liver in vivo, using the first generation recombinant adenoviruses, we demonstrate that PDX-1 induces in liver a wide repertoire of both exocrine and endocrine pancreatic gene expression. Moreover, PDX-1 induces its own expression (auto-induction), which in turn may explain the long lasting nature of the "liver to pancreas" transdifferentiation. Insulin as well glucagon-producing cells are mainly located in the proximity of hepatic central veins, possibly allowing direct hormone release into the bloodstream, without affecting normal hepatic function. Importantly, we demonstrate that hepatic insulin production triggered by Ad-CMV-PDX-1 recombinant adenovirus administration is functional and prevents streptozotocin-induced hyperglycemia in Balb/c mice even 8 months after the initial treatment. We conclude that PDX-1 plays an important instructive role in pancreas differentiation, not only from primitive gut endoderm but also from mature liver. Transconversion of liver to pancreas may serve as a novel approach for generating endocrine-pancreatic tissue that can replace malfunctioning beta-cells in diabetics.

Up-regulation of neutral glycosphingolipid synthesis upon long term inhibition of ceramide synthesis by fumonisin B1

In a previous study we observed that long term (5 days) incubation with fumonisin B1 (FB1), an inhibitor of acylation of sphingoid long chain bases to (dihydro)ceramide, resulted in morphological and biochemical changes in 3T3 fibroblasts (Meivar-Levy, I., Sabanay, H., Bershadsky, A. D., and Futerman, A. H. (1997) J. Biol. Chem. 272, 1558-1564). Among these were changes in the profile of synthesis of sphingolipids (SLs) and glycosphingolipids (GSLs). Whereas [3H]globotriaosylceramide ([3H]Gb3) comprised 1.9% of the total [3H]SLs and [3H]GSLs synthesized in control cells, it comprised 16. 5% in FB1-treated cells. We now demonstrate by in vitro analysis that inhibition of ceramide synthesis by FB1 for 5 days results in up-regulation of the activities of three enzymes in the pathway of Gb3 synthesis, namely glucosylceramide, lactosylceramide, and Gb3 synthases; up-regulation is due to an increase in Vmax, with no change in Km values toward lipid substrates. Moreover, molecular analysis (reverse transcriptase-polymerase chain reaction) of glucosylceramide synthase indicated that this enzyme is up-regulated at the transcriptional level. No changes in either the Vmax or Km values of sphingomyelin or of GM3 synthase were detected after FB1 treatment. Analysis of SL and GSL synthesis in cultured cells using [4,5-3H]sphinganine as a metabolic precursor demonstrated that at low substrate concentrations, Gb3 synthesis is favored over GM3 synthesis and glucosylceramide synthesis is favored over sphingomyelin synthesis, whereas the opposite is true at high substrate concentrations. These data demonstrate that GSL synthesis and in particular Gb3 synthesis are tightly regulated in fibroblasts, presumably so as to maintain constant levels of Gb3 on the cell surface.

The role of sphingolipids in the maintenance of fibroblast morphology. The inhibition of protrusional activity, cell spreading, and cytokinesis induced by fumonisin B1 can be reversed by ganglioside GM3

Previous studies demonstrated that inhibition of sphingolipid synthesis by the mycotoxin fumonisin B1 (FB1) disrupts axonal growth in cultured hippocampal neurons (Harel, R., and Futerman, A. H. (1993) J. Biol. Chem. 268, 14476-14481) by affecting the formation or stabilization of axonal branches (Schwarz, A., Rapaport, E., Hirschberg, K., and Futerman, A.H. (1995) J. Biol. Chem. 270, 10990-10998). We now demonstrate that long term incubation with FB1 affects fibroblast morphology and proliferation. Incubation of Swiss 3T3 cells with FB1 resulted in a decrease in synthesis of ganglioside GM3, the major glycosphingolipid in 3T3 fibroblasts and of sphingomyelin. The projected cell area of FB1-treated cells was approximately 45% less than control cells. FB1 had no affect on the organization of microtubules or intermediate filaments, but fewer actin-rich stress fibers were observed, and there was a loss of actin-rich lamellipodia at the leading edge. Three other processes involving the actin cytoskeleton, cytokinesis, microvilli formation, and the formation of long processes induced by protein kinase inhibitors, were all disrupted by FB1. All the effects of FB1 on cell morphology could be reversed by addition of ganglioside GM3 even in the presence of FB1, whereas the bioactive intermediates, sphinganine, sphingosine, and ceramide, were without effect. Finally, FB1 blocked cell proliferation and DNA synthesis in a reversible manner, although ganglioside GM3 could not reverse the effects of FB1 on cell proliferation. Together, these data suggest that ongoing sphingolipid synthesis is required for the assembly of both new membrane and of the underlying cytoskeleton.

Analysis of glucocerebrosidase activity using N-(1-[14C]hexanoyl)-D-erythroglucosylsphingosine demonstrates a correlation between levels of residual enzyme activity and the type of Gaucher disease

Glucosylceramide, a degradation product of complex glycosphingolipids, is hydrolysed in lysosomes by glucocerebrosidase (GlcCerase). Mutations in the human GlcCerase gene cause a reduction in GlcCerase activity and accumulation of glucosylceramide, which results in the onset of Gaucher disease, the most common lysosomal storage disease. Significant clinical heterogeneity is observed in Gaucher disease, with three main types known, but no clear correlation has been reported between the different types and levels of residual GlcCerase activity. We now demonstrate that a correlation exists by using a radioactive, short-acyl chain substrate, N-(1-[14C]hexanoyl)-D-erythro-glucosylsphingosine ([14C]hexanoyl-GlcCer). This substrate rapidly transferred into biological membranes in the absence of detergent [Futerman and Pagano (1991) Biochem. J. 280, 295-302] and was hydrolyzed to N-(1-[14C]hexanoyl)-D-erythro-sphingosine ([14C]hexanoyl-Cer) both in vitro and in situ, with an acid pH optimum. A strict correlation was observed between levels of [14C]hexanoyl-GlcCer hydrolysis and Gaucher type in human skin fibroblasts. The mean residual activity measured in vitro for 3 h incubation in type 1 Gaucher fibroblasts (the mild form of the disease) was 46.3 +/- 4.6 nmol of [14C]hexanoyl-Cer formed per mg protein (n = 9), and in type 2 and 3 fibroblasts (the neuronopathic forms of the disease) was 19.6 +/- 6.5 (n = 9). A similar correlation was observed when activity was measured in situ, suggesting that the clinical severity of a lysosomal storage disease is related to levels of residual enzyme activity.