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from patchy hyperpigmentation in the macular region to complete loss of retinal
pigmentation (Rummelt et al. 1993 ; Smith et al. 1999 ) . Atrophic photoreceptor
outer segments are commonly detected in MELAS but normal photoreceptor cells
dominate in the periphery (Rummelt et al. 1993 ). The electroretinogram of one
MELAS patient was characterized by markedly reduced scotopic and photopic
b-wave amplitudes (Latkany et al. 1999 ) .
One of the characteristic features of taurine-deficient cats is a retinopathy. Indeed,
the retinopathy was the first reported pathological symptom of taurine deficiency
(Hayes et al. 1975 ). Like MELAS, the amplitude of the electroretinogram b-wave of
the taurine transporter knockout mouse and the taurine-depleted cat is severely
diminished (Heller-Stilb et al. 2002 ; Schmidt et al. 1977 ) . However, abnormal elec-
troretinograms have also been seen in patients undergoing long-term total parenteral
nutrition, an effect normalized by taurine treatment but may be more closely tied to
changes in zinc and vitamin E levels (Geggel et al. 1985 ; Vinton et al. 1990 ) .
Ultrastructural changes of the photoreceptor outer segments vary from mild to
severe disorganization, with cones affected more than rods (Heller-Stilb et al. 2002 ;
Imaki et al. 1987 ). Severe disruption of the tapetum, a unique feline cell involved in
the reflection of light, is also a characteristic feature of taurine deficiency in cats
(Wen et al. 1979 ). Heller-Stilb et al. ( 2002 ) proposed that apoptosis is a likely con-
tributor to retinal degeneration, although no apoptotic cells have been detected in
deficient cats. Also contributing to opthalmic dysfunction are the loss of optic nerve
fibers (Lake et al. 1988 ) and atrophy of the olfactory bulb (Anderson et al. 1979 ) .
While changes in the index of visual function have been attributed in part to oxida-
tive stress, further investigation of the causes of the taurine deficiency-mediated
retinopathy is warranted.
16.2.9
Renal Defects in MELAS and Taurine De fi ciency
The most common renal defects reported in MELAS patients are proximal tubulopathy,
proteinuria, and focal segment glomerulosclerosis (Hirano et al. 2002 ; Sproule and
Kaufmann 2008 ). According to a study by Hirano et al. ( 2002 ) , proteinuria was pres-
ent in all cases of MELAS studied. Focal segment glomerulosclerosis was also pres-
ent in most cases of MELAS, particularly at an early stage of the nephropathy. At
later stages of the nephropathy, diffuse global glomerulosclerosis with marked tubu-
lointerstitial changes is observed (Hirano et al. 2002 ). To date, the mechanism link-
ing mitochondrial dysfunction to renal defects has not been established, indicating
the need for further research.
Taurine functions as a free radical scavenger, a cation transport modulator, and
an osmoregulator in the kidney (Han and Chesney 2012 ). In accordance with tau-
rine's role as an osmolyte, excretion of water and salt is restricted in the taurine-
deficient rat, with the excretion of a hypotonic NaCl load being particularly
diminished by taurine deficiency. In the uninephrectomized adult rat and the fetal/
prenatal rat, taurine depletion accelerates the onset of hypertension (Mozaffari et al.
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