Phenotype of the Obese Koletsky (f) Rat Due to Tyr763Stop Mutation in the Extracellular Domain of the Leptin Receptor (Lepr): Evidence for Deficient Plasma-to-CSF Transport of Leptin in Both the Zucker and Koletsky Obese Rat

Abstract

The obese phenotypes of the diabetes (db) mouse and fatty (fa) rat are due to functional null mutations of the leptin receptor (Lepr). The recessive mutation in the Koletsky (f) obese rat maps to the same genetic intervals as db and fa and fails to complement the fa mutation. Comparison of the sequence of brain Lepr cDNA from +/+ and f/f animals reveals a T2349A transversion resulting in a Tyr763Stop nonsense mutation in the gene just before the transmembrane domain. Virtual absence of Lepr mRNA in whole brain from /^animals is consistent with the presence of a null mutation. The predicted reduced cerebrospinal fluid (CSF) transport of leptin in both f/f and fa/fa mutants is reflected in thẽ 10-fold lower ratio of CSF/plasma leptin concentration in the obese versus lean animals. However, equivalent CSF leptin concentration between lean and obese rats (fa/fa, f/f) indicates that leptin can enter the CSF through a non-Lepr-mediated mechanism, which may be saturated at normal physiological plasma leptin concentration. Diabetes 46:513-518, 1997 T he obese (ob) and diabetes (db) mutations have recently been shown to be in the Lep (leptin) and Lepr (leptin receptor) genes, respectively (1,2). Leptin is secreted from adipose tissue and is a ligand for the receptor whose five or more splice variants are expressed in a tissue-specific fashion (3,4). Because leptin (16 kDa) is produced only in adipose tissue and is too large to From the Laboratory of Human Behavior and Metabolism (X.S.W.-R, S.C.C., cross the blood-brain barrier, it is thought to enter the brain parenchyma via a saturable transport system, possibly constituted by one or more of the various membrane bound splice variants of Lepr.The identical phenotypes of ob and db mutant micewhich include early onset, severe obesity, insulin resistance, and strain-dependent diabetes-and the results of cross circulation experiments between ob and db animals provided the initial evidence that these were mutations in a signaling pathway for the size of body fat stores (5). Rats with a similar autosomal recessive phenotype were described first by Zucker et al. (fatty =/a) (6) and then by Koletsky (Koletsky =f) (7) in two different rodent colonies and rat strains. Based on genetic mapping experiments (8,9), it was suspected (and subsequently proven) that the Zucker fatty mutation was in the same gene as the db mutation (10). The high likelihood that fa and/are mutations in the same gene was demonstrated by the obese phenotype offa/f compound mutants (11-13) and the recent molecular mapping of/to the same genomic interval as/a (13).The recently cloned Lepr gene encodes a single transmembrane-spanning protein that resembles gpl30, a member of the class I cytokine receptor superfamily (1,14). The db and db Pas mutations have been shown to be, respectively, novel splice donor (3) and gene duplication (9) mutations in Lepr. The fa mutation is a Gln269Pro transversion in Lepr that impairs intracytoplasmic transport of the protein (10,15).In this report, we show that / is a nonsense ...

Differential Expression and Regulation of Leptin Receptor Isoforms in the Rat Brain: Effects of Fasting and Oestrogen

“…This may be of importance for the understanding of leptin physiology. It also suggests that leptin-R S in the CP may be an attractive drug target as the transport of leptin across the BBB appears to be an important site for the development of leptin resistance and that the leptin-R, at least in part, participates in this transport [16, 18]. …”

“…However, leptin-R gene expression seems to differ between different strains of rodents with defects in the leptin system. The ob/ob mouse has increased expression of leptin-R in the brain [40], whereas the obese Koletsky rat has very low levels of leptin-R transcripts [18]. …”

“…The biological role of leptin-R S is not understood, but recent studies suggest that it may modulate the activity of leptin-R L [14, 15], act as a transport protein for leptin [11, 16] and may have some signalling capacity [17]. Defects in the leptin-R have been identified in the diabetes (db/db) mouse, the Koletsky (f/f) rat and in the Zucker (fa/fa) rat, all of which display phenotypes similar to that of the ob/ob mouse [18, 19, 20]. Serum leptin levels appear to reflect body fat mass under steady-state conditions [3] indicating that regulation of transport of leptin across the blood-brain barrier (BBB) and sensitivity to leptin in the brain may be of importance for the regulation of body fat mass under physiological and pathophysiological conditions [4].…”

Regulation of Hypothalamic Proopiomelanocortin by Leptin in Lean and Obese Rats

“…Rats used in these experiments were the result of serial backcrosses (N10 equivalent) of the mutation Lepr fa-f (also known as ‘Koletsky’, fa f ,f or cp ) to the inbred rat strain, LA/N [28]. Male 3- to 4-month-old lean (+/+, +/ fa f ) and obese (fa f /fa f ) rats weighed 357 ± 4 and 529 ± 12 g, respectively.…”

“…Homozygosity for this autosomal point mutation that produces a premature termination codon in the extracellular domain of the leptin receptor results in early-onset severe obesity, insulin resistance, and strain-dependent diabetes mellitus [27, 28]. We compared the levels of POMC mRNA in the MBH of lean (+/+, +/ fa f ) and obese (fa f /fa f ) rats using a sensitive solution hybridization assay.…”