Biological and methodological features of the measurement of S100B, a putative marker of brain injury

Abstract

The S100B astroglial protein is widely used as a parameter of glial activation and/or death in several conditions of brain injury. Cerebrospinal fluid and serum S100B variations have been proposed to evaluate clinical outcomes in these situations. Here, we briefly broach some aspects, commonly not sufficiently valorized, concerning the biology and measurements of this protein. S100B has molecular targets and activities in and outside of astrocytes, and variations of intra and extracellular content are not necessarily coupled. We discuss the extracellular origin of this protein in brain tissue, as well as extracerebral sources of this protein in serum, comparing it with other available protein markers of brain damage. The superestimation of the heterodimer S100A1-B in the current clinical literature is also analyzed. We affirm that poor dualistic views that consider S100B elevation as “bad” or “good” simplify clinical practice and delay our comprehension of the role of this protein, both in physiological conditions and in brain disorders.

Introduction

The search for specific peripheral biochemical markers of brain tissue activity and injury has increased over the last decade. One putative marker is the calcium-binding protein, S100B, predominantly expressed and secreted by astrocytes in vertebrate brain [1]. S100B is a calcium-binding protein, belonging to the S100 family proteins that are characterized by their high solubility and, currently, comprises 21 members that are expressed in a cell-specific manner (see [2] for a review). Like most other members, S100B has a homodimeric structure, where each beta monomer is approximately 10.5 kDa. Each monomer has two EF hand sites for Ca²⁺-binding and also independent sites for Zn²⁺-binding. S100B has two disulphide bridges, but the dimeric structure is maintained independently of this aspect. This protein has many putative intracellular targets (see Fig 1), it is also secreted and has autocrine and paracrine effects on glia, neurons and microglia. Some proposed intracellular targets of S100B include proteins of the cytoskeleton (e.g. GFAP and CapZ), modulators of the cell cycle (e.g. p53 and Ndr kinase), protein kinase C and phosphatase 2B (calcineurin). The extracellular effect of S100B, observed in neural cultures, depends on its concentration, since it is neurotrophic at pico and nanomolar levels and apoptotic at micromolar levels [2], [27], [28]. This effect involves activation of signaling pathways such as ERK and NF-κB.

S100B mRNA and protein levels (intra and extracellular) have been used as a parameter of astrocyte activation and/or death in several situations of brain injury [29], [30], [31]. Brain disorders associated with peripheral increments of S100B include traumatic brain damage [32], [33], brain ischemia [34], [35], [36], neurodegenerative diseases [37], [38], [39], [40], [41] and psychiatric disorders [42], [43], [44].

There are many interesting and provocative reviews about the role of this protein and its clinical usefulness [2], [4], [5], [27], [31]. In fact, recent studies correlate S100B variations with neuroimage alterations, intracerebral pressure measurements and neurological signals (e.g. [45]). Moreover, experimental findings have suggested the importance of this protein not only as a marker, but also as of therapeutic potential, particularly in traumatic brain injury [46]. The significance of serum S100B variations, however, is often unknown and debatable due to the possible extra-brain sources of S100B or methodological problems surrounding the measurement of S100B. Reviewing the literature, it may be noted that a number of controversial aspects exist with regard to the use of S100B as a marker of brain damage, since several biochemical features of this protein and its physiological and pathological variations are not well characterized; furthermore, possible methodological pitfalls in immunoassays could contribute to these discrepancies. In this short review, we will discuss (i) the meaning and usefulness of S100B as a marker of brain injury; (ii) some current uncertainties and misunderstandings regarding peripheral S100B protein; and (iii) methodological aspects for S100B measurement, based on our experience and the current literature, not exclusively limited to the disorders of central nervous system (CNS).