By Mary L. Vo, MD, PharmD
Assistant Professor of Neurology, Weill Cornell Medical College
Dr. Vo reports no financial relationships relevant to this field of study.
SYNOPSIS: Specific and objective measures of improvement after carpal tunnel decompression (i.e., electrodiagnostic testing, somatosensory function, and intraepidermal nerve fiber density) also correlated with upregulation of ADCYAP1/PACAP genes. These genes may be potential therapeutic targets in the future.
SOURCE: Baskozos G, Sandy-Hindmarch O, Clark AJ, et al. Molecular and cellular correlates of human nerve regeneration: ADCYAP1/PACAP enhance nerve outgrowth. Brain 2020; July 1. doi: 10.1093/brain/awaa163. [Online ahead of print].
Despite extensive preclinical studies highlighting potential targets of peripheral nerve regeneration, human studies elucidating molecular and cellular factors important to nerve regeneration and clinical outcomes are severely limited. In recent years, skin biopsies have been used to gain perspective into the regeneration of human sensory nerves, myelination of large fiber nerves, and histological changes during nerve regeneration.1
Preclinical literature has highlighted the function of adenylate cyclase-activating polypeptide 1 (ADCYAP1)/pituitary adenylate cyclase-activating peptide (PACAP) in neural regeneration, but its effect in human neurites is not understood. PACAP is a highly conserved protein involved in neuronal survival following injury and neurite outgrowth. PACAP binds with high affinity to the polypeptide type 1 (PAC1) receptor, expressed in human sensory afferents both in human skin as well as in human-induced pluripotent stem cell-derived (hiPSCd) sensory neurons in vitro. In this study, the authors combined histological and molecular analysis with traditional clinical outcome measures to evaluate the role of the ADCYAP1/PCAP in human nerve regeneration.
Baskozos et al studied 60 patients with clinically and electrodiagnostically confirmed carpal tunnel syndrome (CTS) from the Oxford University Hospitals NHS Foundation Trust prior to and six months following surgical decompression. Twenty matched healthy controls also were included. Patients with osteoarthritis, cervical radiculopathy, trauma, pregnancy, diabetes, and prior CTS surgery were excluded. All subjects underwent electrodiagnostic testing, quantitative sensory testing, and completed a Boston Carpal Tunnel Questionnaire. Skin biopsies were taken from the ventrolateral aspect of the proximal phalanx of the index finger at baseline and were repeated six months following surgical decompression.
Samples were evaluated for measures of myelinated fiber integrity, including Meissner corpuscles per millimeter, dermal nerve bundles, and nodal length. Histological analysis included measurements of intraepidermal nerve fiber density and subepidermal plexus nerve fiber length. Molecular analysis of target tissue entailed ribonuculeic acid (RNA) sequencing validated with droplet digital polymerase chain reaction (PCR). hiPSCd sensory neurons were derived from two control lines and differentiated into primary sensory neurons, where they were treated with different doses of PACAP protein.
Pre- and postsurgical data were analyzed using two-sided independent t-tests or Mann Whitney U-tests. Spearman’s correlation analysis was used to determine the magnitude of clinical recovery and objective measures of neural regeneration. Clinical recovery was determined as a two-point improvement on a global rating scale focused on hand symptoms and function. Following surgery, 83.3% of subjects reported significant improvement in symptoms (≥ 5 points on a global rating scale), including improvements in both symptom subscore (pre: 2.8 [0.7], post: 1.5 [0.5], t [59] = 13.8, P < 0.0001) and functional subscore (pre: 2.2 [0.8], post: 1.5 [0.5], t [59] = 7.00, P < 0.00001). Although the majority of patients had notable improvement, 46.6% of postoperative subjects continued to have pain and 38.3% had paresthesia, albeit much milder than baseline.
Similarly, the majority of subjects were found to have objective improvements on electrodiagnostic testing, somatosensory function, and intraepidermal nerve fiber density during postoperative testing, but they failed to reach the levels of healthy controls. Follow-up electrodiagnostic testing showed improvement measured by a graded neurophysiological CTS rating scale (2.0 [2.0], z [55] = -5.62, P < 0.0001). Postoperative somatosensory function measured by thermal and mechanical thresholds also showed improvement relative to the control group (t [78] > 2.30, P < 0.015). Intraepidermal nerve fiber density improved following surgery relative to controls (preoperative mean: 4.20 [2.83]; controls: 8.03 [2.08], t [77] = 5.3, P < 0.00001; post: 5.35 [3.34], t [57] = 3.5, P = 0.001). No differences in nerve fiber length density in subepidermal plexus were seen at follow-up. Median nodal length and percent of elongated nodes increased in patients and remained higher in healthy controls.
RNA sequencing from 47 patients showed 31 differentially expressed genes in the CTS group, with ADCYAP1 showing the most significant degree if dysregulation. ADCYAP1 encodes for PACAP protein, which was increased in postoperative studies. Further, in vitro hiPSCd sensory neurons exposed to different levels of PACAP protein resulted in dose-dependent neurite outgrowth.
COMMENTARY
The authors demonstrate a histologic and molecular signature in nerve recovery, including strong upregulation of ADCAP1/PACAP that parallels recovery of intraepidermal nerve fibers and correlates with traditional symptomatic and functional outcome metrics. These findings provide valuable insight into the complex determinants of sensory nerve regeneration. Additional research is needed to assess the role of PACAP as a therapeutic target in nerve regeneration.
REFERENCE
- Schmid AB, Bland JD, Bhat MA, Bennett, DL. The relationship of nerve fibre pathology to sensory function in entrapment neuropathy. Brain 2014;137:3186-3199.
