Permittivity of Healthy and Diseased Skeletal Muscle

Abstract

A better understanding of the permittivity of skeletal muscle is essential for the development of new diagnostic tools and approaches for neuromuscular evaluation. However, there remain important knowledge gaps in our understanding of this property in healthy and diseased skeletal muscle, which hinder its translation into clinical application. The aim of this project is to provide a platform for the researchers to contribute and share the dissemination of this property in healthy and diseased muscle. Ultimately, the normative data reported will offer the scientific community the opportunity to improve the accuracy of existing techniques as well as developing new diagnostic tools and therapies.

Background

Electromagnetism constitutes a basic physical principle widely used in the field of biomedical engineering, designed to monitor and treat a broad spectrum of conditions including Parkinson’s disease and brain tumors. Understanding how different biological tissues and fluids interact with electromagnetic fields is essential for improving the accuracy of existing analytical techniques as well as developing new diagnostic tools and therapies.

In electromagnetism, permittivity is one fundamental material parameter affecting the propagation of electromagnetic fields. When exposed to an electromagnetic field, the dipole moment of the material's molecules opposes the external electric field and so the net electric field is reduced within the material. In other words, the permittivity is a measure of the ability to store an electric charge in the polarization of the material.

Basic and applied scientific endeavors have reported the permittivity property for well over 100 years in a collective effort to understand the propagation of electromagnetic fields in the human body. However, a recent meta-analysis revealed that major gaps in the knowledge of the frequency-dependence of the permittivity in many tissues still exist, especially for those tissues such as skeletal muscle, in which this property is directionally dependent. In addition, some previous studies of the permittivity of biological specimens did not specify the state of the tissue examined, even though it is known that the permittivity values change postmortem and with temperature, nor did they specify the extent of disease, if any, present, and some did not include healthy control tissue for comparison.  Other noteworthy factors that have not been exhaustively evaluated include the variation of tissues’ permittivity with age, gender, and disease progression. This missing information highlights critical gaps in our understanding of the factors that affect the permittivity property of biological tissues required to aid in identification of clinically abnormal results in pathological tissue.

Methods

Animal experimentation: Animal experimentation was the same for all animals. All animal procedures were carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the NIH and approved by the Institutional Animal Care and Use Committee at Beth Israel Deaconess Medical Center (BIDMC, Protocol #087–2016). Mice were given ad libitum access to food (Formulab Diet 5008, LabDiet, MO, USA) and water. Animals were allowed to acclimate at least 72 h prior to testing. Prior to measurements, animals were humanely euthanized with CO₂.

Ex vivo analysis of excised gastrocnemius muscle: Following euthanasia via CO₂ inhalation, the gastrocnemius muscle was excised at its proximal extent just below the knee and distally by cutting the gastrocnemius tendon at its insertions, after removing the bicep femoris muscle. Each gastrocnemius muscle was trimmed with a scalpel to 5 mm (width) × 5 mm (length) centered at the belly of the muscle in order to fit into the dielectric measuring cell. Muscle impedance measurements were made immediately after with a heating pad under the dielectric cell to maintain a constant temperature 37 °C. Unless otherwise stated, the animals were obtained from Jackson Laboratories (JAX, Bar Harbor, ME, USA).

Dielectric cell: the dielectric cell was made of two flat plate stainless steel electrodes for applying electrical current side to side of the slab of muscle. The voltage electrodes were two monopolar EMG needles in contact the top surface of the muscle (Carefusion #902-DMG50-TP). The geometrical dimensions of the dielectric cell were 5 mm (width) × 5 mm (length), the muscle height measured with a caliper. The distance between the voltage electrodes was 4 mm. The slab of muscle was inserted into the cell first with the muscle fibers oriented parallel (longitudinal) and then perpendicular (transverse) to the current electrodes. The geometric factor K was determined from measurements of saline solution with the corresponding height of each tissue sample.

Four-electrode impedance measurements: Muscle resistance and reactance were collected from 9 to 862 kHz using a commercial impedance analyzer and then the permittivity calculated.

Spinal muscular atrophy (SMA) mice: Five spinal muscular atrophy (SMA) Model (“Smn 2B/2B-Neo”) (B6.129-Smn1tm1.1Cdid/tm1Cdid) mice were generated by intercrossing SMN 2B mice with SMN 2B-Neo mice. The official name of the Smn 2B allele is B6.129-Smn1tm1.1Cdid. 2B mice were generated from the progenitor line Smn 2B-Neo (B6.129-Smn1tm1Cdid) through removal of the flox-neo cassette. Germline mice were subsequently crossed to C57BL/6J mice (JAX stock #000664) for at least 3 generations prior to use in these studies. The median survival of 2B/2B-Neo mice is ~13 months for males and 24 months for females. Mice were studied at 40 weeks of age. Five wild type littermates served as controls.

Muscular dystrophy (MDX) mice: The D2.B10 (DBA/2-congenic) Dmd^mdx mouse (also referred in the literature as DBA/2J-mdx or D2-mdx mice) was chosen as a model of Duchenne muscular dystrophy model as it recapitulates several of the human characteristics of DMD myopathology including lower hind limb muscle weight, fewer myofibers, increased fibrosis and fat accumulation, and muscle weakness relative to strains with this mutant allele on other genetic backgrounds. These genetically altered mice develop the disease without additional intervention and live at least one year. Fifteen male D2.B10-Dmd^mdx/J mice hemizygous for Dmd^mdx (6–9 weeks of age, JAX strain #013141) and studied at various ages from 6 to 43 weeks. Fifteen male wild type mice (DBA/2J, JAX strain #000671) served as controls (5 mice per time point).

Obese/Diabetic mice: The DB/DB mouse (BKS.Cg-Dock7m +/+ Leprdb/J) was chosen as a model of diabetes type II and obesity. Mice homozygous for the diabetes spontaneous mutation (Leprdb) become obese at approximately three to four weeks of age. Elevations of plasma insulin begin at 10 to 14 days and elevations of blood sugar at four to eight weeks. Homozygous mutant mice are polyphagic, polydipsic, and polyuric. These mice are well known for their obesity and for developing substantial intramuscular fat deposition by approximately 8 weeks of age. Ten male mice (5 weeks, JAX strain #000642) were studied at 6 and 20 weeks (5 mice per time point). Ten WT type C57BLKS/J (JAX strain #000662) served as controls (5 mice per time point).

Amyotrophic lateral sclerosis (ALS) mice: Breeding pairs of ALS B6SJL-Tg(SOD1*G93A)1Gur/J mice (JAX strain #002726) were obtained and bred to obtain 37 animals (approximately half female and half male). To study varying fiber size, animals were studied at various ages ranging from 8–18 weeks (approximately 6–7 animals per fortnight, at 8, 10, 12, 14, 16, and 18 weeks).

Mice with myofiber hypertrophy: Twenty male wild type mice (C57BL/6J, 8 weeks of age, JAX strain #000664) were obtained. Starting at 9 weeks of age, mice were divided randomly into two groups of 10 mice per group. Mice were treated twice weekly with subcutaneous injections of either phosphate-buffered saline (PBS) or the myostatin ligand trap ActRIIB-mFc (Acceleron Pharma, Cambridge, MA, USA) at a dose of 3.3 mg/kg. ActRIIB-mFc (also termed RAP-031) is a protein comprised of a form of the extracellular domain of ActRIIB fused to a mouse Fc that acts as a ligand trap to inhibit myostatin signaling. Animals were weighed weekly with an analytical balance (AS64, Adventurer SL, Ohaus Corporation, Pine Brook, NJ, USA) to ensure correct dosing throughout the course of the study. All procedures were performed after 5 weeks of treatment with ActRIIB-mFc.

mtDNA mutator mice: Homozygous knock-in mtDNA mutator mice (PolgA^D257A/D257A) and wild type (WT) littermates (PolgA^+/+) were bred and maintained at BIDMC Animal Facility as previously published.^18,19 The presence of the PolG knock-in mutation was confirmed in this line as previously published.^9,18,19 PolG and WT mice (male and female) were treated with either normal drinking water or drinking water containing NMN (obtained from David Sinclair, PhD, Harvard Medical School, Boston, MA) at 300 mg/kg/day for 20 weeks starting at 40 weeks of age. A total of 30 WT (water n = 13, NMN n= 17) and 24 mtDNA mutator gastrocnemii (water n= 16, NMN n= 8) were available and used in this study.  Mice were harvested at ~ 60 weeks of age. Body mass was recorded at harvest.

Data Description

The format of the data files is the same for healthy and diseased muscle conditions. The anisotropic permittivity property measured in longitudinal and transverse directions is organized in columns while the frequency dependence is organized in rows. For each time point measured, which also is organized in columns, the permittivity average value and the standard error of the mean are reported.

Usage Notes

The csv data files begin with WT for healthy wild-type, or Disease for diseased. They contain mean and standard error of the mean measurements of conductivity and relative permittivity, for both longitudinal and transverse measurements. The mice may also be grouped by age. For more details, we suggest referring to the accompanying papers by Nagy et al and Clark‐Matott et al [1.2].

Release Notes

- mtDNA mutator mice permittivity data.

Conflicts of Interest

Dr. Sanchez serves as a consultant to Myolex, Inc., and Impedimed, Inc., a company that develop impedance technology for research and clinical use. Dr. Sanchez is Co-Founder of Haystack Diagnostics, Inc., a company that commercializes needle impedance technology. Haystack Diagnostics, Inc., has the option to license patented needle impedance technology of which Dr. Sanchez is named inventor.

References

1. Nagy, J. A., DiDonato, C. J., Rutkove, S. B., and Sanchez, B. Permittivity of ex vivo healthy and diseased murine skeletal muscle from 10 kHz to 1 MHz. Scientific Data, 2019, 6:37
2. Clark‐Matott, J., Nagy, J. A., Sanchez, B., Taylor, R., Riveros, D., Abraham, N. A., Simon D. K., Rutkove, S. B. Altered muscle electrical tissue properties in a mouse model of premature aging. 2019. doi: 10.1002/mus.26714