Therapeutic use of the SMR1 protein and active derivatives thereof

A method for screening ligand molecules that specifically bind to a target receptor for an XQHNPR peptide, comprising the steps of:

• a) preparing a confluent target cell culture monolayer or preparing a target organ specimen or a tissue sample;
• b) adding a candidate molecule to be tested in competition with a half-saturating concentration of labeled peptide;
• c) incubating the cell culture, organ specimen or tissue sample of step a) in the presence of the labeled candidate molecule for a time sufficient for specific binding to take place;
• and
• d) quantifying the label specifically bound to the target cell culture, organ specimen or tissue sample.

A method for determining an affinity of ligand molecules that specifically bind to a target receptor for a XQHNPR peptide, comprising the steps of:

• a) preparing a confluent target cell culture monolayer or preparing a target organ specimen or a tissue sample;
• b) adding a candidate molecule to be tested that has been previously labeled with a radioactive or a nonradioactive label;
• c) incubating the cell culture, organ specimen or tissue sample of step a) in the presence of the labeled candidate molecule for a time sufficient for specific binding to take place;
• d) quantifying the label specifically bound to the target cell culture, organ specimen or tissue sample.

A method for screening candidate ligand molecules that possess an agonist or an antagonist biological activity on a target receptor of an XQHNPR peptide, comprising the steps of:

• a) preparing a biological material comprising a confluent target cell culture monolayer, a target organ specimen or a tissue sample in cryosections or slices;
• b) incubating the cell culture, organ specimen or tissue sample of step a) in the presence of (i) 10−10-10−5 M of a candidate molecule and (ii) a submaximal concentration of QHNPR for a time sufficient for an adenylate cyclase activation to take place;
• and
• c) quantifying adenylate cyclase activity present in the biological material of step a), respectively in the presence or in the absence of the candidate ligand molecule and in the presence or in the absence of a submaximal concentration of QHNPR.

A method for determining the amount of XQHNPR or one of its biologically active derivatives to be administered to a patient suffering from a mineral ion imbalance, comprising the steps of:

• a) incubating a labeled XQHNPR peptide with a polyclonal or a monoclonal antibody directed against the same peptide;
• b) bringing into contact immune complexes formed with a biological sample from a patient to be tested suspected to contain an endogenous non-labeled XQHNPR peptide;
• c) detecting monoclonal or polyclonal antibody-bound labeled peptides that have not been displaced by the endogenous non-labelled XQHNPR peptide contained in the biological sample in order to determine a concentration of said endogenous peptide that is contained in the biological sample;
• d) comparing the concentration of the XQHNPR peptide formed at step c) with the concentration of the XQHNPR peptide normally found in a healthy individual;
• and
• e) calculating the amount of a therapeutic composition, comprising a pharmaceutically active amount of an SMR1 protein, or a maturation product of the SMR1 protein or of one of its biologically active derivatives, necessary in order to supply the defect of the XQHNPR peptide in body fluids and tissues.

A composition comprising a pharmaceutically effective amount of:

• an SMR1 protein;
• an SMR1 maturation product;
• or
• a biologicaly active derivative of the above;
• in combination with a pharmaceutically effective amount of a molecule involved in regulation of mineral ion balance in the body.

A method for producing an SMR1 protein maturation product which comprises the steps of:

• a) optionally amplifying a nucleic acid coding for a desired polypeptide using a pair of primers specific for an SMR1 genomic or cDNA sequence;
• b) inserting a nucleic acid coding for SMR1 protein inserted in an appropriate vector;
• c) inserting a nucleic acid coding for furin in a suitable vector, said vector being the vector of step b) or said vector being a vector different form the vector of step b).
• d) culturing, in an appropriate culture medium devoid of serum, a cell host previously transformed or transfected with the recombinant vector of step b) and c);
• e) harvesting the culture medium and the cell host;
• f) separating or purifying, from said culture medium or from a pellet of a resultant host cell lysate, the thus produced polypeptide of interest;
• g) characterizing the produced polypeptide of interest;
• and
• h) optionally assaying for specific recognition of said peptide by a polyclonal or a monoclonal antibody directed against a WQHNPR peptide.