diagnostics Article Neurogranin as a Reliable Biomarker for Synaptic Dysfunction in Alzheimer’s Disease Luisa Agnello 1 , Bruna Lo Sasso 1,2 , Matteo Vidali 3 , Concetta Scazzone 1 , Tommaso Piccoli 4 , Caterina Maria Gambino 1 , Giulia Bivona 1 , Rosaria Vincenza Giglio 1 , Anna Maria Ciaccio 5 , Vincenzo La Bella 6 and Marcello Ciaccio 1,2, *   Citation: Agnello, L.; Lo Sasso, B.; Vidali, M.; Scazzone, C.; Piccoli, T.; Gambino, C.M.; Bivona, G.; Giglio, R.V.; Ciaccio, A.M.; La Bella, V.; et al. Neurogranin as a Reliable Biomarker for Synaptic Dysfunction in Alzheimer’s Disease. Diagnostics 2021, 11, 2339. https://doi.org/ 10.3390/diagnostics11122339 Academic Editors: Ludmilla Morozova-Roche and Panteleimon Giannakopoulos Received: 16 September 2021 Accepted: 10 December 2021 Published: 12 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Biomedicine, Neurosciences and Advanced Diagnostics, Institute of Clinical Biochemistry, Clinical Molecular Medicine and Clinical Laboratory Medicine, University of Palermo, 90127 Palermo, Italy; luisa.agnello@unipa.it (L.A.); bruna.losasso@unipa.it (B.L.S.); concetta.scazzone@unipa.it (C.S.); cmgambino@libero.it (C.M.G.); giulia.bivona@unipa.it (G.B.); giglio.rosaria.vincenza@gmail.com (R.V.G.) 2 Department of Laboratory Medicine, Azienda Ospedaliera Universitaria Policlinico “P. Giaccone”, 90127 Palermo, Italy 3 Foundation IRCCS Ca’ Granda Ospedale Maggiore Policlinico, 20122 Milan, Italy; matteo.vidali@gmail.com 4 Unit of Neurology, Department of Biomedicine, Neurosciences and Advanced Diagnostics, University of Palermo, 90127 Palermo, Italy; tommaso.piccoli@unipa.it 5 Unit of Clinical Biochemistry, University of Palermo, 90127 Palermo, Italy; annamaria.ciaccio@unipa.it 6 ALS Clinical Research Center, Department of Biomedicine, Neuroscience and Advanced Diagnostics, University of Palermo, 90129 Palermo, Italy; vincenzo.labella@unipa.it * Correspondence: marcello.ciaccio@unipa.it; Tel.: +39-0916553296 Abstract: (1) Background: Neurogranin is a post-synaptic protein expressed in the neurons of the hippocampus and cerebral cortex. It has been recently proposed as a promising biomarker of synaptic dysfunction, especially in Alzheimer’s disease (AD). However, more efforts are needed before intro- ducing it in clinical practice, including the definition of its reference interval (RI). The aim of the study was to establish the RI of cerebrospinal fluid (CSF) neurogranin levels in controls and individuals with non-neurodegenerative neurological diseases; (2) We included a total of 136 individuals that were sub-grouped as follows: AD patients (n = 33), patients with non-neurodegenerative neurological diseases (n = 70) and controls (33). We measured CSF neurogranin levels by a commercial ELISA kit. CSF RI of neurogranin was calculated by a robust method; (3) Results: AD patients showed increased levels of neurogranin. We also found that neurogranin was significantly correlated with T-tau, P-tau and mini mental state examination in AD patients. The lower and upper reference limits of the RI were 2.9 (90%CI 0.1–10.8) and 679 (90%CI 595–779), respectively; (4) Conclusion: This is the first study establishing the RI of CSF neurogranin. Keywords: RC3; biomarkers; neurodegeneration; controls; diagnosis; prognosis; tau 1. Introduction Neurogranin is a calmodulin-binding protein discovered in 1990, but only in recent years it gained attention as a potential biomarker of neurodegeneration [1]. The term “neurogranin” refers to its expression in granule-like structures within excitatory neurons of the hippocampus and cerebral cortex [2]. It is a post-synaptic protein with a pivotal role in regulating synaptic plasticity and function [3,4]. Neurogranin knockout mice display a decrease in long-term potentiation (LTP) induction and cognition, while upregulation promotes LTP and improves cognitive performance [5,6]. In the last few decades, research has focused on the possible role of neurogranin as a biomarker for synaptic dysfunction in neurodegenerative diseases, such as Alzheimer’s disease (AD) [7,8]. Synaptic dysfunction, indeed, represents an important phenomenon in AD pathophysiology, which occurs early in the disease course, and leads to reduced cognitive function [9]. Neurogranin levels have been found to be markedly reduced in Diagnostics 2021, 11, 2339. https://doi.org/10.3390/diagnostics11122339 https://www.mdpi.com/journal/diagnostics