Calcium Phosphate Transfection of Primary Hippocampal Neurons

Summary

Calcium phosphate precipitation is a convenient and economical method for transfection of cultured cells. With optimization, it is possible to use this method on hard-to-transfect cells like primary neurons. Here we describe our detailed protocol for calcium phosphate transfection of hippocampal neurons cocultured with astroglial cells.

Abstract

Calcium phosphate precipitation is a convenient and economical method for transfection of cultured cells. With optimization, it is possible to use this method on hard-to-transfect cells like primary neurons. Here we describe our detailed protocol for calcium phosphate transfection of hippocampal neurons cocultured with astroglial cells.

Introduction

Primary neurons are one of the hardest cell types to transfect as they are postmitotic and are very sensitive to micro-environmental changes. There are four commonly used types of methods for expression of exogenous genes and short-hairpin RNAs (shRNAs) in these cells¹. Each has its own advantages and disadvantages. For example, electroporation is usually performed on freshly isolated neurons², as cells must be transferred into cuvettes for transfection. Virus infection can usually achieve very high efficiency³, but is more labor-intensive and risky for operators. Many lipid-mediated transfection reagents are available commercially, with varying degrees of success in neurons and different levels of cytotoxicity.

Calcium phosphate transfection represents a convenient and economical method for introducing foreign genes into neurons. The method was first used to introduce adenovirus DNA into mammalian cells by Graham and Van Der Eb (1973)⁴. Transfection was performed by mixing calcium chloride with recombinant DNA in a phosphate buffer. This allows the formation of DNA/calcium phosphate precipitates which, when gradually dropped onto a monolayer of cells, adhere to the cell surface, are taken up by endocytosis and finally enter the nucleus⁵. This process would lead to the expression of introduced foreign genes in the target cell. Typical efficiencies of calcium phosphate transfection range between 0.5-5%^6-8. However, with careful optimization and consistent execution of the experimental protocol, it is possible to reach a transfection efficiency of almost 50%. Here we describe our detailed protocol for calcium phosphate transfection of primary hippocampal neurons, which are cocultured with astroglial cells in a sandwich format⁹.

Protocol

1. Preparing Rat Astrocyte Culture for Conditioned Media and Astrocyte-neuron Cocultures. Prepare dissection buffer (BSS, see Table 1 for recipe) and store at 4 °C until ready for use. Anesthetize neonatal rat pups (P0-P2) with isoflurane in a 500 ml beaker. When pups are immobile, spray with 70% ethanol and decapitate. Remove the brain. Hold the head firmly with a pair of Dumont #5 forceps, and use fine scissors to make a midline incision throug…

Representative Results

When the different parameters of transfection are optimized and carefully controlled from experiment to experiment, it is possible to obtain transfection efficiencies of up to 50%. Figure 1 shows a field of neurons that are transfected with GFP on DIV4. The field contains a total of 28 neurons, among which 16 were transfected. This represents an efficiency of over 50%. A sampling of other fields on the same coverslip shows the overall efficiency is around 50% (data not shown). With the astroglial cocultu…

Discussion

There are several key parameters that need to be carefully controlled for consistently successful transfections^10,11. The most critical parameter for calcium phosphate transfection is the pH value of 2x HBS, which in our hands usually varies between 7.10-7.15. We recommend making three batches of stocks with pH values in 0.05 increments to account for the difference between pH meters. Alternatively, the Clontech mammalian transfection kit provides 2x HBS that consistently yields good efficiency. Keep in m…

Materials

Dumont #5 forceps	Fine Science Tools	11251-20
Fine scissors	Fine Science Tools	14090-09	straight / sharp 8.5 cm
Vannas-Tubinger spring scissors	Fine Science Tools	15008-08	angled / sharp / 8.5 cm / 5 mm cutting edge
Standard pattern forceps	Fine Science Tools	11000-16
Student surgical scissors	Fine Science Tools	91401-14	blunt / 14.5 cm
Spring scissors	Fine Science Tools	15006-09	angled to side / 9 cm / 10 mm cutting edge
Isoflurane	Webster Veterinary	14043-225-06
Poly-L-lysine	Sigma	P2636
MEM	Sigma	M2279
100 mM Sodium pyruvate	Sigma	S8636
Glucose	Sigma	G8270
10x HBSS	Sigma	H4385
1 M HEPES, pH 7.3	Gibco/Invitrogen	15630-080
GlutaMAX	Gibco/Invitrogen	35050-061
Neurobasal Media	Gibco/Invitrogen	21130-049
B27 Supplement (50x)	Gibco/Invitrogen	17504-044
N2 Supplement	Gibco/Invitrogen	17502-048
Ovalbumin	Sigma	A5503
Penicillin/Streptomycin	Gibco/Invitrogen	15070-063
2.5% Trypsin	Gibco/Invitrogen	15090-046
DNase I	Sigma	DN-25
Cytosine arabinoside	Calbiochem/EMD Millipore	251010
Kynurenic acid	Sigma	K3375