Reply to: "Letter to the Editor by Derkinderen and Colleagues"

We are pleased to know that LRRK2 expression is not limited to immune cells in the lamina propria but is also observed in enteric neurons of the submucosal and myenteric plexus of gastrointestinal tracts, as described by Maekawa et al.1 In our recently published study,2 human colonic tissues were sampled by routine biopsy, which was usually shallow to the level of the submucosal layer, and rarely to the muscular level. Therefore, the LRRK2 expression observed in our routine colonic biopsies was mainly in the lamina propria and did not cover either the submucosal or myenteric plexus. Notably, in our study, most of the colonic biopsies were performed in the transverse or descending colon as part of the follow-up for colonic polyps in a limited patient sample.2 We did not have sigmoid colonic biopsy samples, as Derkinderen et al showed in their letter to the editor and their recent study.3 Since the innervation system differs between the descending colon and sigmoid,4 we hypothesize that LRRK2 expression may not be evenly distributed throughout the gastrointestinal tract. Further studies, enrolling more participants with multiple biopsies from different regions of the gastrointestinal tract, are needed to elucidate the distribution of LRRK2 in the gut. We agree with Derkinderen and colleagues that additional investigations are needed to delineate the role of LRRK2 in the enteric nervous system—including studies using multiplex immunohistochemistry/immunofluorescence staining to determine the specific localization of LRRK2 in individual cell types, such as ChAT-positive, VIP-secreting neurons, sympathetic TH-positive neurons, and enteric glia. Furthermore, as we have addressed,2 LRRK2 G2385R is a polymorphism that may cause a partial loss-of-function mutation instead of increasing LRRK2 kinase activity.5 Our findings from transgenic LRRK2 G2019S mice should be verified by a large cohort study enrolling patients with the LRRK2 G2019S mutation. Finally, we agree with Derkinderen and colleagues that it is insufficient to measure total LRRK2 expression level, and necessary to also examine the levels of phospho-LRRK2 and its substrates, such as phospho-Rab10. However, phosphorylated proteins are liable to lose their antigenicity in paraffin-embedded tissue samples.6 One study demonstrates that several phosphorylated proteins, including phospho-AKT, phosphor-ERK1/2, and phospho-tyrosine, lost their antigenicity within 2 hours of cold ischemic time during tissue preparation for the paraffin-embedding process.6 Therefore, the expression of phospho-LRRK2 in paraffin-embedded biopsied colonic tissues should be cautiously interpreted. Future prospective studies, in which fresh biopsied gut tissues are collected in a follow-up study design, are needed to elucidate LRRK2 expression and activity in the gastrointestinal tract during the Parkinson's disease process. We are grateful for the technical assistance provided by the First and Second Core Labs in the National Taiwan University College of Medicine, Taipei, Taiwan. Nothing to report. (1) Research project: A. Conception, B. Organization, C. Data Acquisition, D. Data Analysis and Interpretation; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript: A. Writing of the First Draft, B. Review and Critique; (4) A. Obtained Funding, B. Study Supervision. C.H.L.: 1A, 1C, 1D, 2B, 3A, 3B, 4A, 4B H.Y.L.: 1C, 1D, 2B E.P.H.: 1C, 1D, 2B Y.C.K.: 1C, 1D, 2B M.F.C.: 1C, 1D, 2B C.Y.S.: 1C C.H.W.: 1C P.H.L.: 1C A.Y.H.H.: 1C L.A.C.: 1C, 1D, 4A Y.D.L.: 1C Y.H.L.: 1C G.T.W.: 1C Y.C.T.: 1C, 4A C.T.S.: 1C H.M.C.: 1C M.S.W.: 1C, 4A