CLC-5 AND THE PROXIMAL TUBULE ENDOCYTOSIS OF LOW
MOLECULAR WEIGHT PROTEINS: WHAT THE KONOCKOUT MICE CAN TELL
US ABOUT IT. 1,2Jackson Souza-Menezes, 1Marcelo M. Morales, 2Deepali N.
Tukaye, 2Sandra E. Guggino and 2William B. Guggino. (1) Instituto de Biofísica
Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ,
Brazil and (2) The Johns Hopkins University, School of Medicine, Baltimore, MD,
USA.
The ClC-5 protein belongs to a family of voltage-gated chloride channels
and has 746-amino acids. During the past 9 years ClC-5 was consider a chloride
channel but now there are evidences showing that ClC-5 could work as H+/Clexchanger.
In the kidney, the ClC-5 is highly expressed in proximal tubules and also in
collecting ducts (α and β intercalary cells). In the proximal tubules, ClC-5 is located
in apical endosomes membrane together with the electrogenic V-type H+-ATPases
where it has complementary function in the acidification of the endosomes. ClC-5
helps to create an electrical shunt in order to minimize the generation of the
voltage across the vesicular membrane that would inhibit further pumping of the
H+-ATPase.
The ClC-5 physiological relevance in renal function was emphasized when
mutations in ClCN5 gene, that encodes the ClC-5 protein, were associated with an
X-linked renal tubular disorder called Dent’s disease. This disorder is characterized
by low molecular weight proteinuria (LMWP), hypercalciuria, nephrocalcinosis,
nephrolithiasis, aminoaciduria, phosphaturia, kaliuresis, uricosuria, glycosuria and,
eventually, renal failure. Although the clinical symptoms of the Dent´s disease
patients are variable, the LMWP is always found, and, for that reason this sign has
been accepted as a common phenotype of Dent´s disease. LMWP was suggested,
by studies in ClC-5 knockout (KO) mice, to be secondary to the intra-cellular
impaired vesicular acidification that leads to proximal tubular endocytosis
dysfunction. This proximal tubule abnormalities could be the cause of the decrease
in megalin content in renal proximal tubules of ClC-5 KO mice and also in the urine
of Dent’s disease patients.
Those findings suggest that the impaired endosomal acidification, observed
in the absence of the ClC-5, reduce the recycling of megalin back to the apical
brush border membrane impairing its important role in the renal proximal tubular
endocitosis of low molecular weight protein and other peptides such as
parathormone and insulin. We have shown, in ClC-5 KO mice, that there is
increase in the renal excretion of insulin secondary to decrease in megalinmediated endocytosis of insulin.
Although the LMW proteinuria had been emphasized in Dent´s disease, the
mechanism(s) underlying glycosuria in patients and ClC-5 KO mice is not very well
understood. Glycosuria is usually found in patients with hyperglycemia as seen in
Diabetes Mellitus. Although this is the most common cause, genetic syndromes,
such as Fanconi-Bickel syndrome caused by mutation in the facilitated glucose
transporter (GLUT2) also lead to glycosuria.
We have shown that the glycosuria in ClC-5 KO mice occurs as a result of
decreased renal re-absorption of glucose in proximal tubules cells associated with
a reduction in GLUT2 protein expression and by a decrease in plasma insulin
levels. The low levels of plasma insulin lead to an impaired glucose homeostasis
associated with glucose intolerance and no peripheral tissue insulin resistance.
The reduction in plasma insulin concentration could be secondary to decreased
secretion of insulin by  cells of pancreas and not just because of increased urinary
loss as mentioned above. Further studies need to be done to uncover this
mechanism.
The ClC-5 KO mouse has almost all symptoms observed in patients and its
development was essential to understand the genesis of the low molecular weight
proteinuria and other symptoms observed in Dent´s disease.