Cerebrospinal Fluid Surrogates

The immunocytochemical method for detecting APP is one of the most sensitive methods for detecting axonal damage; however, this method is confined only to fatal cases. Therefore, the question arises: How can we examine axonal injury during life and in patients who eventually recover from their disease? Electrophysiological and neuropsycho-logical tests and brain imaging can help identify parenchy-mal abnormalities, although imaging often underestimates the abnormalities later seen at pathologic examination (Patankar et al., 2002) and is not cell-type specific.

One method that is gaining popularity is the measurement by enzyme-linked immunosorbent assays (ELISA) of markers of axonal damage released into the cerebrospinal fluid (CSF) or blood of patients with various neurological disorders. A marker that is often used is t, a phosphorylated microtubule-associated protein, considered to be important for maintaining the stability of axonal microtubules involved in the mediation of fast axonal transport of synap-tic constituents (Green et al., 2000; Jimenez-Jimenez et al., 2002). Another surrogate marker for degenerated axons that can be measured in CSF is the light subunit of the neurofilament protein (NFL) (Hagberg et al., 2000).

Although immunocytochemistry for APP and ELISA for t or NFL give a measure of axonal damage the results do not provide completely analogous information (Table 1). For example, the CSF markers only give an indication of acute, irreversible, axonal degeneration, as the cytoskeletal components must be released into the extracellular fluid and gain access to the CSF, which has a turnover time of approximately 6 hours. Serial CSF measurements can be made during the course of patients' disease but performing multiple lumbar punctures on ill patients may not be clinically or ethically appropriate. In comparison, APP immunohistochem-istry allows the detection of a spectrum of axonal injury ranging from mild, reversible damage to irreversible degeneration. Immunoreaction in damaged axons for APP

Figure I Brain sections from severe malaria patients stained for the b-amyloid precursor protein (b-APP). (A-D) Different staining patterns of P-APP: single axons (arrow heads) in close association with a vessel containing parasitized erythrocytes (arrow) (A) Linear arrays with lesion boundary not well defined. (B) Focal area containing swollen and club-shaped axons. (C) Axonal bulbs. (D). (E-I) Serial sections stained for either P-APP to visualize areas of axonal damage (E, G) or CD68 to identify microglia (F, I) or Luxol Fast Blue Cresyl Violet to identify demyelination (H). (E-F) Axonal damage (E) and microglial response (F) to a ring hemorrhage. (G-I) Two focal areas of axonal damage (G). The right foci of axonal damage is associated with a larger area of demyelination (H) and the left focus of axonal damage is associated with a stronger microglial response (I).

Table 1

Immunocytochemistry on brain tissue

Biochemical analysis of CSF

Markers

P-APP

T, neurofilament

Patient

Fatal cases only

Fatal cases and

survivors

Time course

No

Yes

measurements

Spatial information

Yes

No

Type of axonal injury

Spectrum—mild

Acute, axonal

reversible to

degeneration only

irreversible

degeneration

Window of marker

Months

Hours

detection

becomes positive within hours after the insult and may remain positive for at least a month (see previous discussion). P-APP immunohistochemistry gives spatial information, whereas the CSF measurements provide a crude measure of the total extent and degree of axonal damage during life and may reflect preferentially those axons closest to the subarachnoid space. This last point must be emphasized as lesion location is probably more relevant to functional impairment than the total amount of damaged axons.

Imaging surrogates of axonal damage and loss are discussed in other chapters of this book (see Chapters 13, 14, and 15).

0 0

Post a comment