Green fluorescent protein as a tracer in chimeric tissues: the power of vapor fixation

Evidence for a phenotypic switch in corneal afferents after lacrimal gland excision

Dry eye is a common cause of ocular pain. The aim of this study was to investigate corneal innervation, ongoing pain, and alterations in corneal afferent phenotypes in a mouse model of severe aqueous tear deficiency. Chronic dry eye was produced by ipsilateral excision of the extra- and intraorbital lacrimal glands in male and female mice. Tearing was measured using a phenol thread and corneal epithelial damage assessed using fluorescein. Changes in corneal ongoing ocular pain was evaluated by measuring palpebral opening ratio. Corneal axons were visualized using Nav1.8-Cre;tdTomato reporter mice. Immunohistochemistry was performed to characterize somal expression of calcitonin gene-related peptide (CGRP), the capsaicin sensitive transient receptor potential vanilloid 1 (TRPV1), and activating transcription factor-3 (ATF-3) in tracer labeled corneal neurons following lacrimal gland excision (LGE). LGE decreased tearing, created severe epithelial damage, and decreased palpebral opening, indicative of chronic ocular irritation, over the 28-day observation period. Corneal axon terminals exhibited an acute decrease in density after LGE, followed by a regenerative process over the course of 28 days that was greater in male animals. Corneal neurons expressing CGRP, TRPV1, and ATF3 increased following injury, corresponding to axonal injury and regeneration processes observed during the same period. CGRP and TRPV1 expression was notably increased in IB4-positive cells following LGE. These results indicate that dry eye-induced damage to corneal afferents can result in alterations in IB4-positive neurons that may enhance neuroprotective mechanisms to create resiliency after chronic injury.

VEGF alleviates lower limb ischemia in diabetic mice by altering muscle fiber types

Lower limb ischemia caused by diabetic foot (DF) is one of the most serious complications of diabetes. The therapeutic role of VEGF in DF is well documented. However, the mechanism for action of VEGF is still not clear. The present study aimed to explore the effects of VEGF-mediated skeletal muscle fiber type switch in angiogenesis and the treatment of DF. C57BL/6 mice housed in cages equipped with a voluntary running wheel were used to access VEGF protein level and citrate synthase activity (by ELISA) as well as muscle fiber type changes (by immunofluorescence) in the gastrocnemius muscle. C57BL/6 mice were fed on a high-fat diet for 6 weeks and then injected with streptozocin to induce diabetic lower limb ischemia model. Control adenovirus (Ad-GFP) or Ad-VEGF-GFP were then injected into the left gastrocnemius of the ischemic diabetic mice. Blood flow perfusion was examined by laser Doppler imaging at 1, 3, 7 and 14 days after adenovirus transduction. On day 14, all mice were anesthetized and sacrificed. VEGF expression levels, citrate synthase activity and muscle fiber type changes in the gastrocnemius muscle were assayed by ELISA and immunofluorescence analysis of myosin heavy chain IIa (MHCIIa) expression, respectively. Transwell assays were performed to determine whether VEGF-treated C2C12 myotubes played a role on tubule formation and migration of HUVECs. It was found that VEGF levels and citrate synthase activity were upregulated after voluntary exercise, along with the increased frequency of oxidized muscle fibers. Notably, adenovirus-mediated VEGF overexpression in the muscle also increased the frequency of oxidized (MHCIIa-positive) muscle fibers, enhanced citrate synthase activity and ameliorated lower limb ischemia in diabetic mice. VEGF treatment enhanced the phosphorylation of PI3K, Akt and AMPK (assayed by western blotting), as well as glucose consumption and metabolism (assayed by western blotting and glucose uptake assay), in the C2C12 myotubes. Interestingly, VEGF-treated C2C12 myotubes promoted the migration and tubule formation of HUVEC cells. The present findings suggest that skeletal muscle fiber conversion might be a potential approach for VEGF-mediated angiogenesis and disease treatment, which may provide new options for the prevention and treatment of DF.

Optimized protocol for immunostaining of experimental GFP-expressing and human hearts

Morphological and histochemical analysis of the heart is fundamental for the understanding of cardiac physiology and pathology. The accurate detection of different myocardial cell populations, as well as the high-resolution imaging of protein expression and distribution, within the diverse intracellular compartments, is essential for basic research on disease mechanisms and for the translatability of the results to human pathophysiology. While enormous progress has been made on the imaging hardware and methods and on biotechnological tools [e.g., use of green fluorescent protein (GFP), viral-mediated gene transduction] to investigate heart cell structure and function, most of the protocols to prepare heart tissue samples for analysis have remained almost identical for decades. We here provide a detailed description of a novel protocol of heart processing, tailored to the simultaneous detection of tissue morphology, immunofluorescence markers and native emission of fluorescent proteins (i.e., GFP). We compared a variety of procedures of fixation, antigen unmasking and tissue permeabilization, to identify the best combination for preservation of myocardial morphology and native GFP fluorescence, while simultaneously allowing detection of antibody staining toward sarcomeric, membrane, cytosolic and nuclear markers. Furthermore, with minimal variations, we implemented such protocol for the study of human heart samples, including those already fixed and stored with conventional procedures, in tissue archives or bio-banks. In conclusion, a procedure is here presented for the laboratory investigation of the heart, in both rodents and humans, which accrues from the same tissue section information that would normally require the time-consuming and tissue-wasting observation of multiple serial sections.

Use of a novel collagen matrix with oriented pore structure for muscle cell differentiation in cell culture and in grafts

Tissue engineering of skeletal muscle from cultured cells has been attempted using a variety of synthetic and natural macromolecular scaffolds. Our study describes the application of artificial scaffolds (collagen sponges, CS) consisting of collagen-I with parallel pores (width 20–50 μm) using the permanent myogenic cell line C₂C₁₂. CS were infiltrated with a high-density cell suspension, incubated in medium for proliferation of myoblasts prior to further culture in fusion medium to induce differentiation and formation of multinucleated myotubes. This resulted in a parallel arrangement of myotubes within the pore structures. CS with either proliferating cells or with myotubes were grafted into the beds of excised anterior tibial muscles of immunodeficient host mice. The recipient mice were transgenic for enhanced green fluorescent protein (eGFP) to determine a host contribution to the regenerated muscle tissue. Histological analysis 14–50 days after surgery showed that donor muscle fibres had formed in situ with host contributions in the outer portions of the regenerates. The function of the regenerates was assessed by direct electrical stimulation which resulted in the generation of mechanical force. Our study demonstrated that biodegradable CS with parallel pores support the formation of oriented muscle fibres and are compatible with force generation in regenerated muscle.